HETEROCYCLES

Review | Regular issue | Vol. 87, No. 8, 2013, pp. 1659-1689
Received, 7th June, 2013, Accepted, 25th June, 2013, Published online, 26th June, 2013.
DOI: 10.3987/REV-13-775

■ Functionalization of Porphyrins through C-C Bond Formation Reactions with Functional Group-Bearing Organometallic Reagents

Toshikatsu Takanami*

Pharmaceutical Sciences, Meiji Pharmaceutical University, 2-522-1 Noshio, Kiyose, Tokyo 204-8588, Japan

Abstract

Interest in the chemistry of porphyrins and related tetrapyrrolic macrocycles has increased greatly in recent years because of the importance of these compounds in many areas of chemistry, biology, and material sciences. Consequently, the development of efficient synthetic strategies and intermediates for the preparation of porphyrin derivatives with a variety of peripheral substituents has become an active field of research. Functionalized porphyrins, which contain chemically reactive functional groups such as esters, amides, nitriles, and formyl groups on the porphyrin core and as peripheral substituents, are potential precursors for more complicated porphyrin derivatives. However, current methods for synthesizing functionalized porphyrins generally suffer from limitations, including tedious multi-step preparation, laborious chromatographic purification, and low yields. This review describes our recent efforts to address some of these challenges using the following strategies: (1) a palladium-catalyzed Negishi cross-coupling reaction of halogenated porphyrins with functional group-bearing organozinc reagents, and (2) the silyl¬methylation of porphyrins with silylmethyl¬lithium and magnesium reagents, where the silylmethyl groups can be used as protected analogs of various chemically reactive functionalities, such as formyl and hydroxy¬methyl groups.

CONTENTS
1. Introduction
2. Palladium-Catalyzed Cross-Couplings with Functional Group-Bearing Organozinc Reagents
2-1. Porphyrins Bearing Functional Groups on the Peripheral Alkyl Substituents
2-2. Cyanation of Porphyrins with Cyanoethylzinc Bromide as a Cyanating Agent
3. Silylmethylation of Porphyrins with Silylmethyllithium and Magnesium Reagents
3-1. meso-Formyl- and Hydroxymethylporphyrins
3-2. Unsymmetrical Bifunctionalization of 5,15-Disubstituted Porphyrins
3-3. Silylmethylporphyrins: a Multipurpose Synthon for Fabrication of Porphyrins
4. Conclusion

1. INTRODUCTION
Porphyrins are a class of chemically and biologically important heteroaromatic macrocycles that are ubiquitous in nature and have diverse applications in many areas of chemistry, biology, and material sciences.1 For example, they are used as homo­geneous catalysts,2 photosensitizers for photodynamic therapy (PDT),3 materials for non­linear optics (NLO) and solar energy conversion systems,4 and synthetic receptors for various organic and inorganic species.5 The chemical, physical, and biological properties of porphyrins can be tuned by the electronic, steric, and conformational environments of their peripheral substituents, and by the central metal ion.1 Therefore, a large number of synthetic strategies and intermediates for preparing porphyrin derivatives bearing a diverse variety of peripheral substituents have been investigated.6–8

Functionalized porphyrin A, which possesses chemically reactive functional groups such as esters, amides, nitrile, and formyl groups on the porphyrin core and the peripheral substituents, is a potential precursor for more complicated porphyrin derivatives (Figure 1). However, the synthetic utility of functionalized porphyrins is limited, partly because they are difficult to synthesize. The classic approach involves multiple condensation reactions of aldehydes with various mono­pyrroles or substituted dipyrro­methanes under acidic conditions, followed by oxidation of the resulting porphyrinogen intermediates (Scheme 1).7 Despite the significant improvements made by Lindsay and co-workers with the introduction of new reaction conditions,8 multiple condensation methods still suffer from low yields, significant side products, and tedious purifications. This has hindered the efficient preparation of functionalized porphyrin derivatives. Thus, developing a simple, practical, general approach for synthesizing functionalized porphyrins remains an important goal.

Over the last few years, our group has pursued two different strategies to achieve this goal. The first strategy involves the direct preparation of functionalized porphyrins through the palladium-catalyzed cross-coupling of preformed halogenated porphyrins with organozinc reagents, which contain chemically reactive functional groups such as esters, halogens, and pseudohalogens. The second strategy consists of the silyl­methylation of porphyrins with silylmethyl­lithium and magnesium reagents, followed by the conversion of the silylmethyl groups on the porphyrin core to other functional groups such as hydroxymethyl, formyl, alkoxymethyl, fluoromethyl, and alkenyl groups. We have used these simple strategies to prepare a variety of functionalized porphyrins, ranging from meso- and β-mono­functionalized porphyrins to unsymmetrical meso,meso-bi­functionalized porphyrins, in good yields.9 We describe the details of these strategies in the following sections.

2. PALLADIUM-CATALYZED CROSS-COUPLINGS WITH FUNCTIONAL GROUP-BEARING ORGANOZINC REAGENTS
2-1. Porphyrins Bearing Functional Groups on the Peripheral Alkyl Substituents
Transition metal-catalyzed cross-couplings of organic halides with organometallic reagents have become standard powerful carbon-carbon bond formation reactions in synthetic organic chemistry.10 In recent years, a number of different porphyrin derivatives have been constructed using metal-mediated cross-coupling reactions with preformed halogenated porphyrins as the electrophilic coupling partner for the organometallic nucleophiles.6,11–13 Most studies have focused on couplings with simple aryl and alkenyl organo­metallic reagents, particularly organoboron14 and tin15 reagents. In contrast, very few examples of transi­tion metal-catalyzed cross-coupling reactions with reactive functional group-bearing organo­metallic reagents for preparing functionalized porphyrins have been reported. Liebeskind and co-workers reported the preparation of 3-cy­clobu­tenyl-1,2-dione-substituted porphyrins Zn-3 via the Stille reaction of the zinc complex of bromoporphyrin Zn-1 with n-butyl­stannyl­cyclo­butene­dione 2 using a Pd2(dba)3/AsPh3 catalyst system (Scheme 2).16 Odobel and co-workers also reported the Stille reaction of dienyltin acetal 5 with iodoporphyrin zinc complex Zn-4 to afford the corresponding porphyrin Zn-6 with dienal substituents at the meso-positions (Scheme 3).17 Most recently, Senge and co-workers used potassium organo­trifluoro­borates 7 containing ester, nitrile, and amide groups as the nucleophilic coupling partner in the palladium-catalyzed coupling reactions of bromoporphyrins 1 to afford functionalized porphyrins 8 in moderate to good yields (Scheme 4).18 In addition to organometallic reagents, several carbon-based organic nucleophiles, such as 1,3-diketones and acrylates, can also be used as the coupling partner in transition metal-catalyzed couplings with halogenated porphyrins 9 (Scheme 5).19

Organozinc reagents show excellent functional group tolerance, and functional group-bearing organozinc reagents allow polyfunctional molecules to be constructed without the need for protection-de­pro­tec­tion strategies or functional group interconversion.20 Therefore, functionalized porphyrin A could be readily prepared by a cross-coupling reaction of preformed halogenated porphyrins and functional group-bearing organozinc reagents. When we began our work in this area in 2004, only Therien and co-workers had published metal-catalyzed cross-couplings of halogenated porphyrins with organozinc reagents. They used simple alkyl and arylzinc reagents, such as MeZnCl, n-BuZnCl, [2,5-(OMe)2C6H3]ZnCl, and C6F5ZnCl, in Pd-catalyzed Negishi cross-coupling reactions of Zn complexes of β-bromo­porphyrin Zn-12 and meso-dibromo­porphyrin Zn-13 (Scheme 6).21 This was the first example of a Pd-catalyzed Negishi cross-coupling reaction in porphyrin synthesis.
Recently, we developed a practical method for synthesizing functionalized porphyrins Zn-17 where the meso-alkyl substituents contain chemically reactive groups such as -COOR, -Cl, and -CN. The method involves the palladium-catalyzed Negishi cross-coupling of easily available brominated precursors 16 with the corresponding functional group-bearing organozinc reagents (Scheme 7).22 The reaction is catalyzed by Pd(OAc)2/Cy3P, and proceeds under mild conditions with a variety of bromoporphyrins, including their Zn(II), Ni(II), and Cu(II) complexes as well as the free bases. The reaction is suitable for a wide range of functional groups and affords the desired products in good yields. Similar reaction conditions with a different phosphine ligand, t-Bu3P·HBF4, produced bifunctionalized Zn(II) complexes Zn-18 from the corresponding free base, dibromo­porphyrin 13 (Scheme 8).

2-2. Cyanation of Porphyrins with Cyanoethylzinc Bromide as a Cyanating Agent
Cyano-substituted porphyrins are among the most useful precursors for porphyrin derivatives because the nitrile group is an intermediate for many important functional groups, such as aldehydes, amines, amides, and carboxylic acid derivatives.23 The Rosenmund-von Braun cyanation reaction is usually used to access cyano-substituted porphyrins, with stoichiometric copper(I) cyanide as the cyanating agent (Scheme 9).24 However, the Rosenmund-von Braun cyanation usually requires an elevated reaction temperature to promote the cyanation of non-metallic base porphyrins; the reactions are generally performed in a refluxing solvent such as quinoline above 200 °C. To achieve cyanation at a lower reaction temperature (~100 °C), the substrate porphyrins must be protected with metals, such as Ni(II) and Zn(II). Alternatively, the following three methods have been reported for the cyanation of porphyrins: i) a multi-step synthesis, involving Vilsmeier formylation, oxime formation, and dehydration (Scheme 10);25 ii) nucleophilic addition of cyanide ions to the π-cation radical of porphyrins (Scheme 11);26 and iii) Friedel−Crafts cyanation of porphyrins with cyanogen bromide and a Lewis acid (Scheme 12).27 These reactions involve cationic porphyrin intermediates and are not suitable for the preparation of porphyrins bearing more than one nitrile group, because the introduction of a CN group deactivates the system toward further cyanation.

The cyanation of aryl halides catalyzed by palladium and related transition metals28 offers an alternative route to cyano­porphyrins. Our method was the first to apply palladium-based cyanation to porphyrins. The main problem with the palladium-catalyzed cyanation of aryl halides is catalyst deactivation by an excess of cyanide ions in the reaction mixture. Therefore, new approaches to the development of catalysts and cyanating agents have been sought by numerous research groups.28 During our work on the palladium-catalyzed Negishi cross-couplings of bromoporphyrins 28 with functional group-bearing organozinc reagents,22 we unexpectedly found that 2-cyano­ethylzinc bromide 29 functions as an effective cyanide ion source for the cyanation of porphyrins (Scheme 13).29,30 Although details of the catalytic mechanism have not been clarified, the generation of free cyanide ions is thought to occur via a Brood-type fragmentation of zinc reagent 29 (Scheme 14). Thus, the reaction is initiated by the oxidative addition of Pd(0) to the C−Br bond of substrate 28. Subsequent metal ion insertion from zinc reagent 29 to the porphyrin core simultaneously generates the cyanide ion and ethylene. Finally, reductive elimination of Pd(II) cyanide intermediate 32 gives product 30. During this process, the molar ratio of the cyanide ions to the catalyst is 1:1, which prevents catalyst deactivation. The catalytic protocol can easily be applied to a variety of bromo­porphyrins, such as meso-mono-, meso-di-, and β-mono-bromo-substituted porphyrins, enabling the synthe­sis not only of meso- and β-mono-cyanated porphyrins (Schemes 13 and 15) but also of meso-di­cyano-substituted derivatives (Scheme 16).

3. SILYLMETHYLATION OF PORPHYRINS WITH SILYLMETHYLLITHIUM AND MAGNESIUM REAGENTS
3-1. meso-Formyl- and Hydroxymethylporphyrins
Formyl­porphyrins are one of the most useful precursors for more complicated porphyrin derivatives.31,32 The conventional method for introducing a formyl group into a porphyrin core involves the Vilsmeier formylation and related reactions.25a,33 However, this allows limited control over the site of formylation, and it works well only with Ni(II) and Cu(II) complexes which lack acid-sensitive functional groups because the formylation requires strongly acidic conditions. In addition, the demetallation of the resulting formyl-substituted metal complexes to the corresponding free bases also requires harsh acidic conditions, such as CF3CO2H in H2SO4.
In 2004, two groups reported significant advances in the preparation of formyl­porphyrins. Lindsey and co-workers demonstrated that dipyrromethane derivatives 36 bearing acetal substituents as masked formyl groups can be useful precursors for formyl-substituted free-base porphyrins 40.32a The preparation consists of a three-step procedure that involves a Lewis acid-promoted condensation of dipyrromethane derivatives 36 and 37, followed by oxidation of the resulting dihydro­porphyrin intermediate 38 with DDQ to give acetal-substituted porphyrin 39, and deprotection of its acetal group (Scheme 17). However, low yields of the desired formylporphyrins 40 are a major problem. Another substantial advance in the preparation of formyl­porphyrins was made by Senge and co-workers.34 They achieved the preparation of meso-formylporphyrins 40 by using a stepwise procedure that involves an SNAr reaction of 2-lithio-1,3-di­thiane 41 with 5,15-di­substi­tuted porphyrins 42 to generate the corresponding meso-(1,3-di­thianyl)­porphyrin 43 and the subsequent oxida­tive conversion of the dithianyl moiety into the CHO group (Scheme 18). This approach can be used under basic conditions to prepare meso-formylated Ni(II) complexes Ni-40 in good yields. However, the SNAr reaction of lithium reagent 41 with free-base porphyrins produced the free-base formyl­porphyrins 2H-40 in only low to moderate yields.

We used [bis(2-pyridyldimethylsilyl)methyl]lithium (PyMe2SiCH2Li)35 instead of 2-lithio-1,3-di­thiane as an organolithium reagent for the meso-formylation of 5,15-di­substi­tuted porphyrins 42 based on the SNAr strategy.36–38 Scheme 19 shows the simple one-pot three-step formylation of porphyrin 42 using PyMe2SiCH2Li as an organo­lithium reagent. The formylation involves the SNAr reaction of 5,15-di­substi­tuted porphyrin 42 with PyMe2SiCH2Li followed by the hydrolysis of the resulting anionic species 44 and successive oxidation reactions of dihydro­porphyrin 45 and silylmethyl- and hydroxy­methyl-substituted porphyrin 46 and 47, respectively. Although the oxidation mechanism for silylmethylporphyrin 46 and hydroxymethylporphyrin 47 is not clear, we suggest the mechanism shown in Scheme 20. Each oxidation reaction is initiated by a single-electron transfer39,40 from the respective compounds (46 and 47) to DDQ, which is a well-known electron acceptor.41,42 The one-pot protocol is suitable for a range of 5,15-diaryl- and 5,15-di­alkyl-substituted free-base porphyrins as well as their metal complexes, providing a new series of porphyrins with a formyl group at the meso position in good to high yields (Scheme 19).

Furthermore, changing the oxidizing agent from DDQ to molecular oxygen produced meso-hydroxy­methyl­porphyrin 47 in good to high yields, without further oxidation of the hydroxymethyl functionality to an aldehyde (Scheme 21).43 Using molecular oxygen as an oxidizing agent allows a wide variety of substrates to be used, including 5,15-dialkyl- and 5,15-diaryl-substituted free-base porphyrins and their metal complexes, to afford the corresponding meso-hydroxy­methyl-porphyrins in good yields. A plausible mechanism for the hydroxymethylation is shown in Scheme 22. The oxidative conversion of intermediate 45 to meso-hydroxymethylporphyrin 47 proceeds by a Fleming-Tamao oxidation mechanism,44 in which hydrogen peroxide, generated in situ from the aerobic oxidation of dihydroporphyrin 45, oxidizes the silyl group to a hydroxyl group.

3-2. Unsymmetrical Bifunctionalization of 5,15-Disubstituted Porphyrins
Unsymmetrical porphyrins bearing two distinct reactive functional groups at the meso positions are valuable building blocks for more complex porphyrin systems. Each functional group directly attached to the porphyrin core can be replaced with other functionalities. We published the first direct preparation of unsymmetrical porphyrins with two different reactive functional groups at the meso positions.45 The formylation of porphyrins with PyMe2SiCH2Li proceeds through the generation of anionic species 44 (Scheme 19).36,43 Thus, we envisaged that anionic inter­medi­ate 44 could be trapped by an electrophile (FG-X), such as an acyl chloride, to form unsymmetrical porphyrin C with a formyl group and other chemically reactive func­tion­al­ities (Scheme 23).
Thus, we examined the unsymmetrical bifunctionalization of 5,15-disubstituted porphyrins 42 with a one-pot procedure, involving an SNAr reaction with PyMe2Si­CH2Li followed by trapping the resulting anion with several electrophiles, such as acyl chlorides 48, chloroformates 49, and isocyanates 50, and then oxidation with DDQ.46 The one-pot sequential reaction using acyl chlorides as an electrophile proceeded smoothly, enabling the direct conversion of 42 into the desired meso-acyl-substituted formyl­por­phyrins 51 in good to high yields (Scheme 24). Similarly, chloro­formates produced meso-alkoxy­carbonyl-substituted formyl­porphyrins 52 (Scheme 25), and isocyanates produced meso-carbamoyl-substituted formyl­porphyrins 53 (Scheme 26).

We used α,β-unsaturated carbonyl compounds 54 as an electrophile to introduce an activated alkenyl substituent and formyl group directly onto the meso carbons of free-base 5,15-disubstituted porphyrins 42 (Scheme 27).47 The process involves a sequential SNAr reaction of free-base 5,15-disubstituted porphyrins with PyMe2SiCH2Li, followed by a conjugate addition to an enone or alkeno­ate in the presence of TMSCl, and then oxidation with DDQ. Crucially, formal direct couplings between enones or alkenoates and porphyrins at the meso positions take place during the reaction and provide meso-activated alkenyl-substituted meso-formyl­porphyrins 55, which has never been achieved by previous porphyrin functionalization methods. This simple one-pot procedure requires mild conditions, is suitable for a wide range of 5,15-diaryl- and 5,15-dialkyl-substituted porphyrins, and works for cyclic and acyclic alkenoates and enones. It allows the direct conversion of free-base 5,15-disubstituted porphyrins to the corresponding meso-activated alkenyl-substituted meso-formyl­porphyrins 55 in good yields.
We suggest that the reaction proceeds via porphodimethene derivative 56, which bears a PyMe2Si­CH2 group and an enol silyl ether moiety on the sp3 carbons of the ring (Scheme 28). Thus, anionic intermediate 44, generated from the SNAr reaction of porphyrin 42 with PyMe2SiCH2Li, undergoes conjugate addition to α,β-unsaturated carbonyl compound 54 in the presence of TMSCl to give porpho­di­methene derivative 56. The silylmethyl group, enol silyl ether moiety, and porpho­di­methene ring on 56 are oxidized with DDQ to a formyl group, an enone or alkenoate moiety, and the porphyrin core, respectively, resulting in final product 55. However, the precise order of these oxidation reactions is not yet clear.48

3-3. Silylmethylporphyrins: a Multipurpose Synthon for Fabrication of Porphyrins
Silylmethyl-substituted porphyrin, which is proposed as a reaction intermediate 46 in the formylation of porphyrins via the SNAr reaction with silylmethylithium reagents (Scheme 19),36 could be used as a multipurpose synthon for fabricating porphyrin derivatives by converting the silylmethyl groups. Silylmethyl groups are very versatile and undergo many transformations to a wide variety of functional groups, including hydroxymethyl, formyl, and alkenyl groups.49 However, the intermediate silyl­methyl­porphyrins 46 could not be isolated owing to their instability under oxidation conditions. Even using molecular oxygen instead of DDQ did not suppress the overoxidation of the silylmethyl­porphyrins 46, and hydroxy­methyl-substituted porphyrins 47 were obtained as the sole isolable products (Schemes 21 and 22).36,43
Recently, we have developed a facile, efficient method for preparing silylmethyl-substituted porphyrins using the palladium-catalyzed Kumada cross-coupling reaction of bromoporphyrins with silylmethyl Grignard reagents.50,51 Table 1 shows that the success of this reaction relies on the use of Pd2(dba)3 as the catalyst and that Ph2P(O)H was the only effective ligand (entry 11).52 The use of well-defined palladium tertiary phosphine complexes, either generated in situ or preformed, was ineffective in the coupling reaction, and afforded low yields of the silyl­methyl-substituted diphenylporphyrin 59 and the dehalogenated product 60. The palladium N-heterocyclic carbene complex, PEPPSI, which is often an effective catalyst in the Kumada reaction,53 also produced low yields (entry 3). The Kumada coupling reaction with a Pd2(dba)3/Ph2P(O)H catalyst is compatible with many meso-mono-, meso-di-, and β-bromo-substituted porphyrins and a variety of silylmethyl magnesium reagents, and generally achieves good yields (Schemes 29–31).

The resulting silylmethyl-substituted porphyrin 67 could be used as a multipurpose synthon for fabricating porphyrin derivatives through a variety of transformations of the silylmethyl groups, including the DDQ-promoted oxidative conversion to CHO, CH2OH, CH2OMe, and CH2F functionalities,54 and the fluoride ion-mediated desilylative introduction of carbon-carbon single bonds55 (Scheme 32). The DDQ-mediated formylation could also be used for the conversion of the meso-di- and β-silylmethylporphyrins 64 and 66 to the corresponding formyl derivatives 74 and 75 (Schemes 33 and 34). To our knowledge, these results are the first examples of direct, regioselective preparation of meso-di- and β-formyl-substituted free-base porphyrins under mild conditions. The installation of a carbon-carbon double bond at the carbon α to the porphyrin core via the Peterson olefination, which is one of the most valuable transformations in organo­silicon chemistry, was achieved using bis­(tri­methyl­silyl)­methyl-substituted porphyrin 76 as the substrate. The reaction of porphyrin 76 with aldehyde 77 proceeded smoothly in the presence of TBAF to furnish Peterson olefination product 78 (Scheme 35).56

4. CONCLUSION
In this review, we have described our recent efforts toward developing concise, straightforward routes to functionalized porphyrins with chemically reactive functional groups such as esters, amides, nitriles, and formyl groups on the porphyrin core and the peripheral substituents. We have directly introduced functional groups to the porphyrin core via palladium-catalyzed cross-coupling reactions with organozinc reagents. Furthermore, we developed a stepwise approach involving the silyl­methylation of porphyrins with silylmethyl­lithium and magnesium reagents, followed by converting the silylmethyl groups to other functional groups. We expect these functionalized porphyrins will be useful building blocks for the total synthesis of more complex porphyrin compounds, which will allow the step economy of the routes to be optimized. Furthermore, we hope the results described here will aid the design and construction of new porphyrin systems for potential applications in areas such as catalysis, medicine, and molecular recognition and sensing. We are currently conducting further studies, the results of which will be reported in due course.

ACKNOWLEDGEMENT
This work was partially supported by a Grant-in-Aid for Scientific Research (KAKENHI) from JSPS and a grant from the High-Tech Research Center Project, MEXT, Japan. I thank Dr. K. Suda, Professor Emeritus of Meiji Pharmaceutical University, for helpful discussions. Finally, special thanks are given to those listed as the co-authors in our paper cited here.

References

1. For reviews, see: (a) ‘The Porphyrin Handbook,’ ed. by K. M. Kadish, K. M. Smith, and R. Guilard, Academic Press, San Diego, 1999−2003; Vol. 1−20; (b) ‘Handbook of Porphyrin Science,’ ed. by K. M. Kadish, K. M. Smith, and R. Guilard, World Scientific, Singapore, 2010, Vol. 1−25; (c) A. Satake, J. Synth. Org. Chem. Jpn., 2007, 65, 298. CrossRef
2. For selected recent examples, see: (a) W. Xiao, J. Wei, C.-Y. Zhou, and C.-M. Che, Chem. Commun., 2013, 49, 4619; CrossRef (b) T. Tachinami, T. Nishimura, R. Ushimaru, R. Noyori, and H. Naka, J. Am. Chem. Soc., 2013, 135, 50; CrossRef (c) L.-M. Jin, X. Xu, H. Lu, X. Cui, L. Wojtas, and X. P. Zhang, Angew. Chem. Int. Ed., 2013, 52, 5309; CrossRef (d) W. Liu, X. Huang, M.-J. Cheng, R. J. Nielsen, W. A. Goddard III, and J. T. Groves, Science, 2012, 337, 1322; CrossRef (e) K. Fujiwara, T. Kurahashi, and S. Matsubara, J. Am. Chem. Soc., 2012, 134, 5512; CrossRef (f) Z. Cong, T. Kurahashi, and H. Fujii, J. Am. Chem. Soc., 2012, 134, 4469; CrossRef (g) S. M. Paradine and M. C. White, J. Am. Chem. Soc., 2012, 134, 2036; CrossRef (h) T. Ozawa, T. Kurahashi, and S. Matsubara, Org. Lett., 2012, 14, 3008; CrossRef (i) B. Morandi, J. Cheang, and E. M. Carreira, Org. Lett., 2011, 13, 3080; CrossRef (j) D. L. Usanov and H. Yamamoto, J. Am. Chem. Soc., 2011, 133, 1286; CrossRef (k) D. L. Usanov and H. Yamamoto, Angew. Chem. Int. Ed., 2010, 49, 8169; CrossRef (l) B. Morandi and E. M. Carreira, Angew. Chem. Int. Ed., 2010, 49, 938; CrossRef (m) K. Suda, S. Nakajima, Y. Satoh, and T. Takanami, Chem. Commun., 2009, 1255; CrossRef (n) T. Takanami and K. Suda, J. Synth. Org. Chem. Jpn., 2009, 67, 595. CrossRef
3. For selected recent examples, see: (a) A. Kamkaew, S. H. Lim, H. B. Lee, L. V. Kiew, L. Y. Chung, and K. Burgess, Chem. Soc. Rev., 2013, 42, 77; CrossRef (b) M. Ethirajan, P. Chen, T. Y. Ohulchansky, L. N. Goswami, A. Gupta, A. Srivatsan, M. P. Dobhal, J. R. Missert, P. N. Prasad, K. M. Kadish, and R. K. Pandey, Chem. Eur. J., 2013, 19, 6670; CrossRef (c) D. Kumar, B. A. Mishra, K. P. Chandra Shekar, A. Kumar, K. E. Kusaka, and T. Ito, Chem. Commun., 2013, 49, 683; CrossRef (d) C. S. Jin, J. F. Lovell, J. Chen, and G. Zheng, ACS nano, 2013, 7, 2541; CrossRef (e) P. Wyrębek and S. Ostrowski, Bull. Chem. Soc. Jpn., 2012, 85, 1167; CrossRef (f) A. P. Thomas, P. S. S. Babu, S. A. Nair, S. Ramakrishnan, D. Ramaiah, T. K. Chandrashekar, A. Srinivasan, and M. R. Pillai, J. Med. Chem., 2012, 55, 5110; CrossRef (g) C.-Y. Hsu, M.-P. Nieh, and P.-S. Lai, Chem. Commun., 2012, 48, 9343; CrossRef (h) M. Ethirajan, Y. Chen, P. Joshi, and R. K. Pandey, Chem. Soc. Rev., 2011, 40, 340. CrossRef
4. For selected recent examples, see: (a) H.-J. Son, S. Jin, S. Patwardhan, S. J. Wezenberg, N. C. Jeong, M. So, C. E. Wilmer, A. A. Sarjeant, G. C. Schatz, R. Q. Snurr, O. K. Farha, G. P. Wiederrecht, and J. T. Hupp, J. Am. Chem. Soc., 2013, 135, 862; CrossRef (b) V. Bandi, K. Ohkubo, S. Fukuzumi, and F. D’Souza, Chem. Commun., 2013, 49, 2867; CrossRef (c) R. K. Yadav, J.-O. Baeg, G. H. Oh, N.-J. Park, K.-j. Kong, J. Kim, D. W. Hwang, and S. K. Biswas, J. Am. Chem. Soc., 2012, 134, 11455; CrossRef (d) S. Patwardhan, S. Sengupta, L. D. A. Siebbeles, F. Würthner, and F. C. Grozema, J. Am. Chem. Soc., 2012, 134, 16147; CrossRef (e) S. Verma and H. N. Ghosh, J. Phys. Chem. Lett., 2012, 3, 1877; CrossRef (f) V. V. Diev, C. W. Schlenker, K. Hanson, Q. Zhong, J. D. Zimmerman, S. R. Forrest, and M. E. Thompson, J. Org. Chem., 2012, 77, 143; CrossRef (g) Y. Matano, K. Matsumoto, H. Hayashi, Y. Nakao, T. Kumpulainen, V. Chukharev, N. V. Tkachenko, H. Lemmetyinen, S. Shimizu, N. Kobayashi, D. Sakamaki, A. Ito, K. Tanaka, and H. Imahori, J. Am. Chem. Soc., 2012, 134, 1825; CrossRef (h) S.-D. Jeong, B. Min, S. Y. Cho, C. Lee, B. K. Park, K.-S. An, and J. Lim, J. Org. Chem., 2012, 77, 8329; CrossRef (i) T. Hasobe and H. Sakai, J. Porphyrins Phthalocyanines, 2011, 15, 301; CrossRef (j) J. M. Ribó, Angew. Chem. Int. Ed., 2011, 50, 12402. CrossRef
5. For selected recent examples, see: (a) H. Adams, E. Chekmeneva, C. A. Hunter, M. C. Misuraca, C. Navarro, and S. M. Turega, J. Am. Chem. Soc., 2013, 135, 1853; CrossRef (b) M. Matsumura, A. Tanatani, I. Azumaya, H. Masu, D. Hashizume, H. Kagechika, A. Muranaka, and M. Uchiyama, Chem. Commun., 2013, 49, 2290; CrossRef (c) B.-Y. Wang, T. Žujović, D. A. Turner, C. M. Hadad, and J. D. Badjić, J. Org. Chem., 2012, 77, 2675; CrossRef (d) H. Jintoku, M. Takafuji, R. Oda, and H. Ihara, Chem. Commun., 2012, 48, 4881; CrossRef (e) J. Zhang, Y. Li, W. Yang, S.-W. Lai, C. Zhou, H. Liu, C.-M. Che, and Y. Li, Chem. Commun., 2012, 48, 3602; CrossRef (f) Q.-F. Liang, J.-J. Liu, and J. Chen, Tetrahedron Lett., 2011, 52, 3987; CrossRef (g) N. Berova, G. Pescitelli, A. G. Petrovica, and G. Pronic, Chem. Commun., 2009, 5958; CrossRef (h) G. A. Hembury, V. V. Borovkov, and Y. Inoue, Chem. Rev., 2008, 108, 1; CrossRef (i) X. Li and B. Borhan, J. Am. Chem. Soc., 2008, 130, 16126; CrossRef (j) X. Li, M. Tanasova, C. Vasileiou, and B. Borhan, J. Am. Chem. Soc., 2008, 130, 1885. CrossRef
6. For reviews, see: (a) M. O. Senge, Chem. Commun., 2011, 47, 1943; CrossRef (b) B. M. J. M. Suijkerbuijk and R. J. M. Klein Gebbink, Angew. Chem. Int. Ed., 2008, 47, 7396; CrossRef (c) T. Ren, Chem. Rev., 2008, 108, 4185; CrossRef (d) S. Horn, K. Dahms, and M. O. Senge, J. Porphyrins Phthalocyanines, 2008, 12, 1053; CrossRef (e) W. M. Sharman and J. E. van Lier, J. Porphyrins Phthalocyanines, 2000, 4, 441. CrossRef
7. (a) K. M. Smith, In ‘The Porphyrin Handbook,’ ed. by K. M. Kadish, K. M. Smith, and R. Guilard, Academic Press, San Diego, 2000, Vol. 1, pp. 1-43; (b) J. S. Lindsey, In ‘The Porphyrin Handbook,’ ed. by K. M. Kadish, K. M. Smith, and R. Guilard, Academic Press, San Diego, 2000, Vol. 1, pp. 45-118.
8. For a review, see: (a) J. S. Lindsey, Acc. Chem. Res., 2010, 43, 300; CrossRef and selected examples, see: (b) Y. Terazono, E. J. North, A. L. Moore, T. A. Moore, and D. Gust, Org. Lett., 2012, 14, 1776; CrossRef (c) D. K. Dogutan, D. K. Bediako, T. S. Teets, M. Schwalbe, and D. G. Nocera, Org. Lett., 2010, 12, 1036; CrossRef (d) S.-J. Hong, S.-D. Jeong, J. Yoo, J. S. Kim, J. Yoon, and C.-H. Lee, Tetrahedron Lett., 2008, 49, 4138; CrossRef (e) M. Tanaka, S. Hayashi, S. Eu, T. Umeyama, Y. Matano, and H. Imahori, Chem. Commun., 2007, 2069; CrossRef (f) S. Tamaru, L. Yu, W. J. Youngblood, K. Muthukumaran, M. Taniguchi, and J. S. Lindsey, J. Org. Chem., 2004, 69, 765; CrossRef (g) K. Wada, T. Mizutani, and S. Kitagawa, J. Org. Chem., 2003, 68, 5123; CrossRef (h) W.-S. Cho, H.-J. Kim, B. J. Littler, M. A. Miller, C.-H. Lee, and J. S. Lindsey, J. Org. Chem., 1999, 64, 7890. CrossRef
9. The use of peripherally borylated porphyrins as key building blocks is another effective approach for functionalized porphyrin synthesis, and a number of different porphyrin derivatives with variety substituents have been prepared by applying this method. For reviews on the preparation of borylated porphyrins and their transformations, see: (a) H. Shinokubo and A. Osuka, Chem. Commun., 2009, 1011; CrossRef (b) H. Yorimitsu and A. Osuka, Asian J. Org. Chem., 2013, 2, 356; and selected examples, see: (c) Y. Mitsushige, S. Yamaguchi, B. S. Lee, Y. M. Sung, S. Kuhri, C. A. Schierl, D. M. Guldi, D. Kim, and Y. Matsuo, J. Am. Chem. Soc., 2012, 134, 16540; CrossRef (d) A. Ryan, A. Gehrold, R. Perusitti, M. Pintea, M. Fazekas, O. B. Locos, F. Blaikie, and M. O. Senge, Eur. J. Org. Chem., 2011, 5817; CrossRef (e) M. A. Bakar, N. N. Sergeeva, T. Juillard, and M. O. Senge, Organometallics, 2011, 30, 3225; CrossRef (f) N. K. S. Davis, M. Pawlicki, and H. L. Anderson, Org. Lett., 2008, 10, 3945; CrossRef (g) I. Hisaki, S. Hiroto, K. S. Kim, S. B. Noh, D. Kim, H. Shinokubo, and A. Osuka, Angew. Chem. Int. Ed., 2007, 46, 5125; CrossRef (h) N. Aratani and A. Osuka, Org. Lett., 2001, 3, 4213; CrossRef (i) T. Mizutani, K. Wada, and S. Kitagawa, J. Am. Chem. Soc., 2001, 123, 6459; CrossRef (j) P. M. Iovine, M. A. Kellett, N. P. Redmore, and M. J. Therien, J. Am. Chem. Soc., 2000, 122, 8717; CrossRef (k) A. G. Hyslop, M. A. Kellett, P. M. Iovine, and M. J. Therien, J. Am. Chem. Soc., 1998, 120, 12676. CrossRef
10. (a) ‘Cross-Coupling Reactions: Topics in Current Chemistry, Vol. 219,’ ed. by N. Miyaura, Springer-Verlag, Berlin, 2002; CrossRef (b) D. S. Surry and S. L. Buchwald, Angew. Chem. Int. Ed., 2008, 47, 6338; CrossRef (c) A. Joshi-Pangu and M. R. Biscoe, Synlett, 2012, 23, 1103. CrossRef
11. A wide variety of porpyrins that bear heteroatom substituents, such as boron- (see, ref. 9), nitrogen-, oxygen-, phosphorus-, and sulfur-derived functional groups, at the meso- and β-positions have also be prepared by transition metal-catalyzed cross-coupling reactions of halogenated porphyrins with heteroatom nucleophiles, see for selected examples: (a) A. M. V. M. Pereira, M. G. P. M. S. Neves, J. A. S. Cavaleiro, C. Jeandon, J.-P. Gisselbrecht, S. Choua, and R. Ruppert, Org. Lett., 2011, 13, 4742; CrossRef (b) Y. Y. Enakieva, A. G. Bessmertnykh, Y. G. Gorbunova, C. Stern, Y. Rousselin, A. Y. Tsivadze, and R. Guilard, Org. Lett., 2009, 11, 3842; CrossRef (c) Y. Matano, K. Matsumoto, Y. Nakao, H. Uno, S. Sakaki, and H. Imahori, J. Am. Chem. Soc., 2008, 130, 4588; CrossRef (d) G.-Y. Gao, J. V. Ruppel, K. B. Fields, X. Xu, Y. Chen, and X. P. Zhang, J. Org. Chem., 2008, 73, 4855; CrossRef (e) C. Liu, D.-M. Shen, and Q.-Y. Chen, J. Org. Chem., 2007, 72, 2732; CrossRef (f) Y. Matano, T. Shinokura, K. Matsumoto, H. Imahori, and H. Nakano, Chem. Asian J., 2007, 2, 1417; CrossRef (g) L. J. Esdaile, M. O. Senge, and D. P. Arnold, Chem. Commun., 2006, 4192; CrossRef (h) F. Atefi, J. C. McMurtrie, P. Turner, M. Duriska, and D. P. Arnold, Inorg. Chem., 2006, 25, 6479; CrossRef (i) L. J. Esdaile, J. C. McMurtrie, P. Turner, and D. P. Arnold, Tetrahedron Lett., 2005, 46, 6931; CrossRef (j) Y. Chen, G.-Y. Gao, and X. P. Zhang, Tetrahedron Lett., 2005, 46, 4965; CrossRef (k) G.-Y. Gao, Y. Chen, and X. P. Zhang, Org. Lett., 2004, 6, 1837; CrossRef (l) G.-Y. Gao, A. J. Colvin, Y. Chen, and X. P. Zhang, J. Org. Chem., 2004, 69, 8886; CrossRef (m) Y. Chen and X. P. Zhang, J. Org. Chem., 2003, 68, 4432; CrossRef (n) G.-Y. Gao, A. J. Colvin, Y. Chen, and X. P. Zhang, Org. Lett., 2003, 5, 3261; CrossRef (o) T. Takanami, M. Hayashi, F. Hino, and K. Suda, Tetrahedron Lett., 2003, 44, 7353. CrossRef
12. Recently, transition metal catalyst-free C-N and C-S bond formation reactions of porphyrins at the meso-positions have been reported, see: (a) K. Yamashita, K. Kataoka, M. S. Asano, and K. Sugiura, Org. Lett., 2012, 14, 190; CrossRef (b) D.-M. Shen, C. Liu, X.-G. Chen, and Q.-Y. Chen, J. Org. Chem., 2009, 74, 206; CrossRef (c) M. C. Balaban, C. Chappaz-Gillot, G. Canard, O. Fuhr, C. Roussel, and T. S. Balaban, Tetrahedron, 2009, 65, 3733. CrossRef
13. Mizorogi-Heck and Sonogashira coupling reactions of halogenated porphyrins with alkenes and alkynes have also reported, see for selected examples: (a) A. K. Sahoo, S. Mori, H. Shinokubo, and A. Osuka, Angew. Chem. Int. Ed., 2006, 45, 7972; CrossRef (b) T. Morotti, M. Pizzotti, R. Ugo, S. Quici, M. Bruschi, P. Mussini, and S. Righetto, Eur. J. Inorg. Chem., 2006, 1743; CrossRef (c) H. Tsuji, M. Sasaki, Y. Shibano, M. Toganoh, T. Kataoka, Y. Araki, K. Tamao, and O. Ito, Bull. Chem. Soc. Jpn., 2006, 79, 1338; CrossRef (d) O. B. Locos and D. P. Arnold, Org. Biomol. Chem., 2006, 4, 902; CrossRef (e) T.-G. Zhang, Y. Zhao, I. Asselberghs, A. Persoons, K. Clays, and M. J. Therien, J. Am. Chem. Soc., 2005, 127, 9710; CrossRef (f) C. Monnereau, E. Blart, V. Montembault, L. Fontaine, and F. Odobel, Tetrahedron, 2005, 61, 10113; CrossRef (g) K. Tomizaki, A. B. Lysenko, M. Taniguchi, and J. S. Lindsey, Tetrahedron, 2004, 60, 2011; CrossRef (h) A. Kato, K. Sugiura, H. Miyasaka, H. Tanaka, T. Kawai, M. Sugimoto, and M. Yamashita, Chem. Lett., 2004, 33, 578. CrossRef
14. For selected examples of the Suzuki reaction of halogenated porphyrins with organoboron reagents, see: (a) G. Garcia, D. Naud-Martin, D. Carrez, A. Croisy, and P. Maillard, Tetrahedron, 2011, 67, 4924; CrossRef (b) D. A. Shultz, C. P. Mussari, K. K. Ramanathan, and J. W. Kampf, Inorg. Chem., 2006, 45, 5752; CrossRef (c) L.-M. Jin, L. Chen, J.-J. Yin, J.-M. Zhou, C.-C. Guo, and Q.-Y. Chen, J. Org. Chem., 2006, 71, 527; CrossRef (d) R. Wakabayashi, Y. Kubo, O. Hirata, M. Takeuchi, and S. Shinkai, Chem. Commun., 2005, 5742; CrossRef (e) K. Kurotobi and A. Osuka, Org. Lett., 2005, 7, 1055; CrossRef (f) D.-F. Shi and R. T. Wheelhouse, Tetrahedron Lett., 2002, 43, 9341; CrossRef (g) B. Vaz, R. Alvarez, M. Nieto, A. I. Paniello, and A. R. de Lera, Tetrahedron Lett., 2001, 42, 7409; CrossRef (h) C. M. Muzzi, Cr. J. Medforth, L. Voss, M. Cancilla, C. Lebrilla, J.-G. Ma, J. A. Shelnutt, and K. M. Smith, Tetrahedron Lett., 1999, 40, 6159. CrossRef
15. For selected resent examples of the Stille reaction of halogenated porphyrins with organotin reagents, see: (a) M. J. Frampton, H. Akdas, A. R. Cowley, J. E. Rogers, J. E. Slagle, P. A. Fleitz, M. Drobizhev, A. Rebane, and H. L. Anderson, Org. Lett., 2005, 7, 5365; CrossRef (b) G. S. Wilson and H. L. Anderson, Chem. Commun., 1999, 1539; CrossRef (c) S. G. DiMagno, V. S.-Y. Lin, and M. J. Therien, J. Org. Chem., 1993, 58, 5983; CrossRef (d) S. G. DiMagno, V. S.-Y. Lin, and M. J. Therien, J. Am. Chem. Soc., 1993, 115, 2513. CrossRef
16. X. Shi and L. S. Liebeskind, J. Org. Chem., 2000, 65, 1665. CrossRef
17. F. Odobel, F. Suzenet, E. Blart, and J.-P. Quintard, Org. Lett., 2000, 2, 131. CrossRef
18. S. Horn, B. Cundell, and M. O. Senge, Tetrahedron Lett., 2009, 50, 2562. CrossRef
19. C. Liu, D.-M. Shen, and Q.-Y. Chen, J. Org. Chem., 2007, 72, 2732.
20. (a) E. Erdic, ‘Organic Reagents in Organic Synthesis,’ CRC, New York, 1996; (b) ‘Organic Reagents: A Practical Approach,’ ed. by P. Knochel and P. Jones, Oxford, New York, 1999; (c) E. Nakamura, In: ‘Organometallics in Synthesis,’ ed. by M. Schlosser, Wiley, New York, 2002, pp. 579 –664.
21. S. G. DiMagno, V. S.-Y. Lin, and M. J. Therien, J. Am. Chem. Soc., 1993, 115, 2513; CrossRef J. Org. Chem., 1993, 58, 5983. CrossRef
22. T. Takanami, M. Yotsukura, W. Inoue, N. Inoue, F. Hino, and K. Suda, Heterocycles, 2008, 76, 439. CrossRef
23. (a) Z. Rappoport, ‘The Chemistry of the Cyano Group,’ Interscience Publishers, London, 1970; CrossRef (b) R. C. Larock, ‘Comprehensive Organic Transformations,’ VCH: New York, 1988.
24. (a) H. J. Callot, Tetrahedron Lett., 1973, 4987; CrossRef (b) H. J. Callot, Bull. Soc. Chim. Fr., 1974, 1492; (c) S. Richeter, C. Jeandon, N. Kyritsakas, R. Ruppert, and H. J. Callot, J. Org. Chem., 2003, 68, 9200. CrossRef
25. (a) H. H. Inhoffen, J.-H. Fuhrhop, H. Voigt, and H. Brockmann, Jr., Liebigs Ann. Chem., 1966, 695, 133; CrossRef (b) A. W. Johnson and D. J. Oldfield, J. Chem. Soc. C, 1966, 794; (c) C.-Y. Yeh, S. E. Miller, S. D. Carpenter, and D. G. Nocera, Inorg. Chem., 2001, 40, 3646.
26. (a) K. M. Smith, G. H. Barnett, B. Evans, and Z. Martynenko, J. Am. Chem. Soc., 1979, 101, 5953; CrossRef (b) H. J. Callot, A. Louati, and M. Gross, Tetrahedron Lett., 1980, 21, 3281; CrossRef (c) G.-Z. Wu, H.-K. Leung, and W.-X. Gan, Tetrahedron, 1990, 46, 3233. CrossRef
27. K. M. Smith, D. A. Goff, and D. J. Simpson, J. Am. Chem. Soc., 1985, 107, 4946. CrossRef
28. For a review, see: P. Anbarasan, T. Schareina, and M. Beller, Chem. Soc. Rev., 2011, 40, 5049. CrossRef
29. T. Takanami, M. Hayashi, H. Chijimatsu, W. Inoue, and K. Suda, Org. Lett., 2005, 7, 3937. CrossRef
30. Recently, Balaban et al. reported palladium-catalyzed cyanation of porphyrins utilizing Zn(CN)2 as a cyanating agent; M. C. Balaban, A. Eichhöfer, G. Buth, R. Hauschild, J. Szmytkowski, H. Kalt, and T. S. Balaban, J. Phys. Chem. B, 2008, 112, 5512. CrossRef
31. For a reviews, see: (a) A. Balakumar, K. Muthukumaran, and J. S. Lindsey, J. Org. Chem., 2004, 69, 5112; CrossRef (b) G. V. Ponomarev, Chem. Heterocycl. Compd., 1994, 30, 1444. CrossRef
32. Very recently, Osuka, et al. reported regioselective β-functionalization of meso-formyl porphyrins using the CHO group as an efficient directing group, see: S. Tokuji, H. Awane, H. Yorimitsu, and A. Osuka, Chem. Eur. J., 2013, 19, 64. CrossRef
33. (a) A. W. Johnson and D. J. Oldfield, Chem. Soc. C, 1966, 794; (b) K. M. Smith, G. M. F. Bisset, and H. D. Tabba, J. Chem. Soc., Perkin. Trans. 1, 1982, 581; CrossRef (c) A. Ando, M. Yamazaki, M. Komura, Y. Sano, N. Hattori, M. Omote, and I. Kumadaki, Heterocycles, 1999, 50, 913. CrossRef
34. (a) M. O. Senge, S. S. Hatscher, A. Wiehe, K. Dahms, and A. Kelling, J. Am. Chem. Soc., 2004, 126, 13634; CrossRef (b) K. Dahms,; M. O. Senge, and M. B. Bakar, Eur. J. Org. Chem., 2007, 3833. CrossRef
35. (a) K. Itami, K. Mitsudo, and J. Yoshida, Tetrahedron Lett., 1999, 40, 5533; CrossRef (b) K. Itami, K. Mitsudo, and J. Yoshida, Tetrahedron Lett., 1999, 40, 5537; CrossRef (c) K. Itami, T. Kamei, K. Mitsudo, T. Nokami, and J. Yoshida, J. Org. Chem., 2001, 66, 3970. CrossRef
36. T. Takanami, A. Wakita, A. Sawaizumi, K. Iso, H. Onodera, and K. Suda, Org. Lett., 2008, 10, 685. CrossRef
37. Porphyrin modification based on the SNAr strategy with organolithium reagents was pioneered by Senge group, see for a review: M. O. Senge, Acc. Chem. Res., 2005, 38, 733. CrossRef
38. Recently, Osuka, et al. reported that alkynyllithium reagents can be used as a nucleophile in the SNAr modification of porphyrins, see: S. Anabuki, S. Tokuji, N. Aratani, and A. Osuka, Org. Lett., 2012, 14, 2778. CrossRef
39. For reviews on electron-transfer oxidation of organosilicon compounds, see: (a) J. Yoshida and K. Nishiwaki, J. Chem. Soc., Dalton Trans., 1998, 2589; CrossRef (b) J. Yoshida, In ‘Electrochemistry V: Topics in Current Chemistry, Vol. 170,’ ed. by E. Steckhan, Springer-Verlag, Berlin, 1994; pp. 39–81. CrossRef
40. For reviews on electron-transfer oxidation of alcohols, see: (a) T. Shono, Electroorganic Chemistry as a New Tool in Organic Synthesis; Springer-Verlag: Berlin, 1984; pp. 29–38; CrossRef (b) S. Torii, Electroorganic Synthesis; Kodansha-VCH: Tokyo, 1985; pp. 239–248.
41. R. Foster and M. L. Foreman, In ‘The Chemistry of the Quinonoid Compounds, Part 1,’ ed. by S. Patai, Wiley, New York, 1974; pp. 257–333.
42. Several groups have reported that DDQ can oxidize aryl methanols to aromatic aldehydes, see: O. Branytska and R. Neumann, Synlett, 2004, 1575. CrossRef
43. T. Takanami, J. Matsumoto, Y. Kumagai, A. Sawaizumi, and K. Suda, Tetrahedron Lett., 2009, 50, 68. CrossRef
44. L. Kurti and B. Czako, In ‘Strategic Applications of Named Reactions in Organic Synthesis,’ Elsevier, Oxford, 2005; p. 174.
45. Multi-step total syntheses of asymmetrically substituted porphyrins have been reported, see: refs 6a and 8a.
46. T. Takanami, A. Wakita, J. Matsumoto, S. Sekine, and K. Suda, Chem. Commun., 2009, 101. CrossRef
47. T. Takanami, S. Hayashi, K. Iso, J. Matsumoto, and F. Hino, Tetrahedron Lett., 2011, 52, 5345. CrossRef
48. DDQ can serve as an oxidant for the conversion of enol silyl ethers into enones, see: A. Bhattacharya, L. M. DiMichele, U.-H. Dolling, E. J. J. Grabowski, and V. J. Grenda, J. Org. Chem., 1989, 54, 6118. CrossRef
49. For reviews on organosilicon chemistry, see: (a) M. A. Brook, ‘Silicon in Organic, Organometallic, and Polymer Chemistry,’ Wiley, New York, 2000; (b) A. Hosomi and K. Miura, Bull. Chem. Soc. Jpn., 2004, 77, 835; CrossRef (c) ‘Handbook of Reagents for Organic Synthesis: Reagents for Silicon-Mediated Organic Synthesis,’ ed. by P. L. Fuchs, Wiley, Chichester, 2011.
50. N. Sugita, S. Hayashi, F. Hino, and T. Takanami, J. Org. Chem., 2012, 77, 10488. CrossRef
51. For a recent review on transition metal-catalyzed Kumada cross-coupling reactions, see: R. Jana, T. P. Pathak, and M. S. Sigman, Chem. Rev., 2011, 111, 1417. CrossRef
52. For examples of the use of sterically demanding secondary phosphine oxides, such as (1-Ad)2P(O)H and t-Bu2P(O)H, as a ligand for the palladium- or nickel-catalyzed Kumada cross-coupling, see: (a) L. Ackermann, R. Born, J. H. Spatz, and D. Meyer, Angew. Chem. Int. Ed., 2005, 44, 7216; CrossRef (b) L. Ackermann and A. Althammer, Org. Lett., 2006, 8, 3457; CrossRef (c) C. Wolf and H. Xu, J. Org. Chem., 2008, 73, 162; CrossRef (d) L. Ackermann, H. K. Potukuchi, A. R. Kapdi, and C. Schulzke, Chem. Eur. J., 2010, 16, 3300. CrossRef
53. For selected examples, see: (a) M. G. Organ, S. Avola, I. Dubovyk, N. Hadei, E. A. B. Kantchev, C. J. O’Brien, and C. Valente, Chem. Eur. J., 2006, 12, 4749; CrossRef (b) G. Manolikakes and P. Knochel, Angew. Chem. Int. Ed., 2009, 48, 205. CrossRef
54. For a review on oxidative conversions of silylmethyl groups to other functionalities, see: J. Yoshida, K. Kataoka, R. Horcajada, and A. Nagaki, Chem. Rev., 2008, 108, 2265. CrossRef
55. For selected examples of fluoride ion-mediated desilylative introduction of carbon-carbon bonds, see: (a) K. Itami, T. Kamei, and J. Yoshida, J. Am. Chem. Soc., 2001, 123, 8773; CrossRef (b) L. F. Staden, D. Gravestock, and D. J. Ager, Chem. Soc. Rev., 2002, 31, 195; CrossRef (c) T. Mita, K. Michigami, and Y. Sato, Org. Lett., 2012, 14, 3462. CrossRef
56. For selected examples of desilylative Peterson olefination of bis(silyl)methyl group-containing compounds, see: (a) N. Kano and T. Kawashima, In ‘Modern Carbonyl Olefination,’ ed. by T. Takeda, Wiley-VCH, Weinheim, 2004; pp. 18–103; (b) C. Palomo, J. M. Aizpurua, J. M. Garcia, I. Ganboa, F. P. Cossio, B. Lecea, and C. López, J. Org. Chem., 1990, 55, 2498. CrossRef