HETEROCYCLES

Review | Regular issue | Vol. 83, No. 2, 2011, pp. 247-273
Received, 27th September, 2010, Accepted, 11th November, 2010, Published online, 19th November, 2010.
DOI: 10.3987/REV-10-681

■ Development of Trialkyl(2-indolyl)borates as Potential Synthetic Intermediates

Minoru Ishikura*

Faculty of Pharmaceutical Sciences, Health Sciences University of Hokkaido, Ishikari-Tobetsu, Hokkaido 061-0293, Japan

Abstract

This account deals with our investigation of the reaction and synthetic use of indolylborates as versatile synthetic tools; 1) palladium-catalyzed cross-coupling reaction and its use for the synthesis of indole alkaloids and 2) intramolecular alkylmigration in indolylbotrates for the construction of highly elaborate indole derivatives.

INTRODUCTION
The unique electronic structure of boron (1s22s22p1) accounts for its peculiar chemical features. The tri-coordinate state (BR3; sp2 hybridization) is in the same plane, with the vacant orbital being perpendicular to the plane. Coordination of R- to the vacant orbital transforms the tri-coordinate state to a tetra-coordinate state (-BR4; sp3 hybridization). This ability of boron to exist in two states (tri-coordinate and tetra-coordinate) is inherently responsible for the unique nature of boron chemistry (Scheme 1).1

Remarkable advances in organoboron chemistry have produced numerous edifying synthetic applications of organoboron compounds, which have been reviewed several times.2 The palladium-catalyzed cross-coupling using organoboron compounds in the presence of a base (Suzuki-Miyaura coupling) was developed in 1979, continuing to be an indispensable synthetic methodology.3
In spite of the pronounced progress in organoboron chemistry, exploration of organoboron chemistry to heterocyclic chemistry has attracted relatively little attention.4 We have been intrigued by the direct interaction between boron and the heteroaryl moiety, as well as by the uses of boron-substituted heteroaromatic compounds as powerful reagents in organic syntheses. Accordingly, we have initiated research to elucidate the unique chemical features and the synthetic validity of boryl-substituted heterocyclic compounds as a potential synthetic tool, enabling the development of new synthetic strategies for the construction of elaborate heterocyclic compounds.
First, we prepared a series of dialkylpyridylboranes from the corresponding bromopyridines and trialkylboranes (Figure 1).5 The dialkylpyridylboranes are exceptionally stable crystals and have chemical features that differ from those of common pyridine derivatives.

Next, our attention was turned to investigate boryl-substituted π–excessive heteroaryl compounds. A few examples of π–excessive five-membered heteroaryl boron compounds have been previously reported, but their chemical properties have not been well characterized.4
Indole alkaloids and their analogues constitute a structurally diverse class of compounds that exhibit various biological activities, and many of these compounds are widely used as drugs. To date, numerous synthetic strategies have been developed for the preparation of indole derivatives.6 In the past 10 years or so, we have studied both the reactivity and the synthetic uses of trialkyl(2-indolyl)borates, which are readily generated in situ from 2-litioindoles and trialkylboranes. Based on these studies, we have developed new strategies for preparing indole derivatives (Scheme 2).

Characteristically, trialkyl(2-indolyl)borates have an activated enamine moiety because of α-anionic boryl group on the indole ring, which confers trialkyl(2-indolyl)borates with increased reactivity. Here, we disclose a brief overview of trialkyl(2-indolyl)borates as valuable synthetic tools for preparing indole derivatives.

1. PALLADIUM-CATALYZED CROSS-COUPLING REACTION
The use of tri-coordinated organoboron compounds for the palladium-catalyzed reaction in the presence of a base (such as NaOH, K2CO3, NaOEt) is a valuable synthetic tool for carbon-carbon bond formation, in which organoboronic acid derivatives are most widely employed as highly effective cross-coupling substrates.7 Recent progress in this field demonstrated that nitrogen-containing heteroarylboronic acids (such as pyridine, isoquinoline, indole) are adaptable to palladium-catalyzed cross-coupling reactions (Scheme 3).8

In our previous examination, we established that dialkylpyridylborane derivatives (dialkylpyridyl-, dialkylquinolyl- and dialkylisoquinolyl-boranes) could serve as versatile substrates in the cross-coupling reaction with aryl and alkenyl halides (Scheme 4).9

In contrast to numerous examples of cross-coupling reactions using tri-coordinated organoboron compounds, successful applications of tetra-coordinated organoboron compounds are far less common. The reaction of trialkylalkenylborates is sluggish even under forcing conditions.10 When we attempted the palladium-catalyzed cross-coupling reaction of triethyl(3-pyridyl)borate with iodobenzene, none of the desired cross-coupling products was obtained.11
Recently, an efficient cross-coupling protocol was developed that uses heteroaryltrifluoroborates: treatment of (2-indolyl)trifluoroborate 1 with 4-chlorobenzonitrile in the presence of Pd(OAc)2 (1 mol%), RuPhos (2 mol%) and Na2CO3 (2 equiv.) in EtOH at 85 °C provided coupling product 2 (Scheme 5).12

On the other hand, from screening cross-coupling reaction conditions for various heteroaryltrialkylborates, we found that triethyl(2-indolyl)borates 3 showed notable reactivity in the absence of an additional base (Table 1).13 Typically, the reaction was carried out by simply heating 3, generated in situ from the corresponding 2-lithioindoles and triethylborane, with halides or triflates in the presence of a catalytic amount of PdCl2(PPh3)2 (5 mol%) in THF under an argon atmosphere at 60 °C. The reaction of indolylborates 3a, 3b and 3c having N-Me, N-OMe and N-Boc groups produced 2-substituted indoles 4 in good yields (Runs 1-3, 7-9 and 13-15), while 3d and 3e with strongly electron withdrawing N-SO2Ph and N-SEM groups, respectively, were far less effective (Runs 4, 5, 10, 11, 15 and 16). Although the reaction of 3f with N-MOM group gave no cross-coupling products (Runs 6, 12 and 17), triethoxyindolylborate 5 reacted with ethyl 4-bromobenzoate to give 6 in 60% yield (Scheme 6).

A plausible reaction course for the cross-coupling reaction is shown in Scheme 7. The successful reaction of 3 could be explained by the coordination of electron-rich enamine system to Pd (complex A), which in turn drives the catalytic cycle in the desired direction (Scheme 7).

However, our findings suggest that an additional base is not required in the reaction using 3; this prompted us to investigate the further scope of the cross-coupling reaction.

2. PALLADIUM CATALYZED TANDEM CYCLIZATION-CROSS-COUPLING REACTION
There are several examples of tandem cross-coupling processes, in which organozinc, organomagnesium and organotin reagents are used as transfer agents owing to their high reactivity.14 However, organoboron compounds are less effective as transfer agents because of their low reactivity and low selectivity.15 To increase the synthetic applicability of the cross-coupling reaction using indolylborates 3, the feasibility of a tandem cyclization cross-coupling reaction was examined. In contrast to the majority of tetra-coordinated organoboron compounds, indolylborates 3 have been found to be highly effective to the tandem processes.16
Acetylenic substrates 7 participated in a tandem reaction when simply heated in the presence of 3 and a catalytic amount of palladium complex in THF under an argon atmosphere (Table 2). An excess of 3 (2 equiv.) was essential to obtain the coupling products 8 in good yields, while the reaction was imperfectly completed by using equimolar quantities of 3 and 7.

Treatment of 3a,b with 7 in the presence of Pd(OAc)2 gave 8 in good yields (Runs 1, 3-5). On the other hand, the production of 8 was markedly decreased in the reaction of 3a with 7a when Pd(OAc)2+2PPh3 was used (Run 2). The reaction of 3c with 7a was sluggish, affording 8 and 9 in yields of 23% and 20%, respectively (Run 6). The reaction of 7b having a bulky TMS group with 3c did not give coupling products and a substantial amount of 7b was recovered (Run 7).
The results may be interpreted by the following mechanistic scheme; 1) Pd-catalyzed cyclization of 7 leading to complex B, 2) the transmetallation between complex B and 3 possibly involving transfer of the indole ring and/or the Et group, and 3) reductive elimination leading to 8 and/or 9 (Scheme 8).

Similarly, the reaction of 3 with 10 in the presence of Pd(OAc)2, which involved the formation of 6-membered ring, afforded 11 in good yields (Scheme 9).

In exploring the advantages of the tandem reaction, this procedure was examined with olefinic substrate 12. Treatment of 3a with 12 in the presence of Pd(OAc)2+2PPh3 provided 13 in 65% yield, as well as direct cross-coupling product 14 in 10% yield. To our surprise, treatment of 3b with 12 in the presence of Pd(OAc)2 with or without 2PPh3 provided exclusively direct cross-coupling product 14 (Scheme 10).

Subjection of olefin 15 to the reaction with 3a,b in the presence of Pd(OAc)2 provided 16 as the sole product in high yields, while the reaction of 15 with 3c was sluggish, giving 16 and 17. The product 17 might be formed through homolytic cleavage of the C-Pd bond of the σ–alkyl Pd complex C (Scheme 11).

Treatment of 3a with 18 provided cross-coupling products 19 and 20, which exhibited marked propensity that the yield of 19 exceeded that of 20 in the presence of Pd(OAc)2+2PPh3 (Scheme 12).

Next, we envisioned further exploration of the tandem protocol for the synthesis of indole alkaloids by taking advantage of the one-pot construction of vinylpyridines 22 through the reaction of 3 with vinylbromides 21 as a common strategy (Scheme 13). Pyridocarbazole alkaloids (olivacine, ellipticine) can concisely be accessed through the cyclization of 22, followed by an oxidation step. On the other hand, catalytic hydrogenation of 22 to piperidine 23 allows potential access to tubifoline.

On first examination, the reaction of 3 with vinylbromides 24 in the presence of palladium complex (5 mol%) in THF produced both tandem cross-coupling product 25 and simple cross-coupling product 26 (Table 3).17 The ratio of the two products varied with changes in the palladium complex or the structure of vinylbromides 24. The use of Pd complex without Ph3P drove the reaction of 3a,b with 24 (R=H, n=2) in the direction of 25, while 26 was obtained predominately in the presence of Pd complex with Ph3P (Runs 1, 2, 3 and 7). Notably, 25 was exclusively isolated in the reaction of 3a with 24 (R=Me) in the presence of Pd complex with Ph3P (Runs 4, 5 and 6). The observed results on the reaction with 24 (R=H, n=1, 3) are associated with the relative ease of ring-closure as a function of ring size (Runs 8 and 9).
With vinylindoles 25 in hand, we then turned our attention to the cyclization of 25 (Z=Me) to pyridocarbazole derivatives 27 (Table 4). Since the 6π-electrocyclization of a hexatriene system has been carried out under various conditions, we first examined a photochemical cyclization of 25; irradiation of 25 (R=H, n=2) in benzene with a high-pressure mercury lamp afforded 27 and isomer 28 (Run 1). On the other hand, photochemical ring-closure of 25 (R=H, n=1,3; R=Me) produced exclusively 27 (Runs 2,3 and 7).

Alternatively, an acid-promoted ring-closing reaction of 25 (Z=Me) was examined. Treatment 25 (Z=Me) of with TFA or BF3·OEt2 in CH2Cl2 led to the formation of spiroindole 29 (Runs 4, 5 and 8), which can be accounted for by the reaction path shown in Scheme 14. In contrast, cyclization of 25 to give pyridocarbazoles 27 with high selectivity was performed using TiCl4 (runs 6 and 9).

The present tandem cyclization cross-coupling protocol was extended to the reaction of indolylbotate 3c with vinylbromides 26 in the presence of a catalytic amount of Pd complex, leading to vinylindoles 27 and/or simple cross-coupling products 28 (Scheme 15).

Cyclization of 30 by irradiation in benzene produced carbazoles 32. Reductive cleavage of the N-Cbz group of 32 and subsequent oxidation with MnO2 yielded 33. Finally, the removal of the N-Boc group of 33 provided pyridocabazoles 34 (R=H and R=Me; ellipticine), and the conversion of 34 (R=H) to olivacine has already been reported (Scheme 16).18

Next, we set about the synthesis of (±)-tubifoline, in which piperidylmethylindole 23 is a profitable intermediate for the further transformation to tubifoline (Scheme 13).19 The successful route to 23 involves the reduction of 22 (R=Me, Y=H) having no substituent on the olefinic carbon. First, it was assumed that the reaction of 3c with bromide 21 (R=Me, Y=H) would provide a direct route to 22, though this resulted in the formation of complex mixtures.

Thus, 35 was subjected to the reaction with 3c in the presence of Pd complex under the aforementioned conditions (in THF at 60 °C), but resulted in significant suppression of the reaction (Table 5): only trace amounts of 36 along with substantial amounts of 37 resulted (Runs 1-4). We attributed this suppression to the serious steric repulsion between the TMS group and the N-Boc group in the transmetallation step. After screening the reaction conditions, a marked improvement in the yield of 36 was obtained through the use of PdCl2[(o-tol)3P]2 in DME at 85 °C (Run 5).
The ligation of bulky (o-Tol)3P to Pd shifts the equilibrium between D and E in favor of the less crowded E, which promptly promotes the transfer of the indole ring of 3c and in turn leads to 36 through complex F. On the other hand, competitive transfer of the Et group allows the formation of 37 through complex G (Scheme 17).

The TMS group of 36 was readily removed by treatment with TBAF in THF to provide 38. Since catalytic hydrogenation of 38 led to indoline 39 as mixtures of stereoisomers, the Na-Boc group was removed and catalytic hydrogenation of 40 was performed over 10% Pd/C in EtOH, producing cis-41 along with trans-41. Removal of the Nb-Boc group of cis-41 followed by treatment with ClCH2COCl produced 42. Then, irradiation of 42 with a low-pressure mercury lamp afforded 43 and 44. Subsequently, 43 was reduced with LiAlH4, followed by oxidation with PtO2/O2, affording (±)-tubifoline (45). Currently, conversion of cis-41 to (±)-tubotaiwine (46) is also in progress (Scheme 18).

In addition, the present tandem cross-coupling reaction was applicable to the reaction of 3a with bromide 47, which yielded 48 and 49. Irradiation of 48 in benzene with high-pressure mercury lamp afforded pentacyclic carbazole 50, a core framework of the antitumor antibiotic calithroxine B.

3. CROSS-COUPLING REACTION WITH PROPARGYL CARBONATES
The addition of various metal complexes to propargyl halides and esters is a well-known process for the formation of allenylmetal complexes, whose conversion to allene derivatives has been the subject of much recent work.20 Thus, we were intrigued by the idea of using the cross-coupling reaction of 3 with an allenylpalladium complex to construct allenylindole derivatives.
Our initial experiment showed that the reaction of 3a with tert-propargyl ester 51 in the presence of a catalytic amount of palladium complex afforded hitherto unknown 2-allenylindole 50.21 Otherwise, the reaction of 3a with sec-propargyl ester 53 resulted in the formation of 2-allenylindole 54 or 2-alkyl-3-propargylindole 55, depending on the palladium complex used. A palladium complex coordinated by Ph3P favored the formation of 3-propargylindole 55 (Scheme 20).

A plausible reaction course is depicted in Scheme 21. Oxidative addition of propargyl ester 56 to the palladium complex comes to an equilibrium between the allenylpalladium complex 57 and the propargylpalladium complex 58. The transmetallation between 3a and complex 57 produces 2-allenylindole 59, while the nucleophilic attack of 3a to complex 58 (R’=H, R’’=alkyl) accompanied by alkyl migration leads to 3-propargylindole 60, which is a rare reaction manner. In the reaction of sec-propargyl ester using palladium complex coordinated by Ph3P, large sterical repulsion between the substituent (R group) and PdLX (L being Ph3P) in 57 shifts the equilibrium toward the propargylpalladium complex 58, making the nucleophilic attack path a favorable course (Scheme 21).

4. CARBONYLATIVE CROSS-COUPLING REACTION
To put the cross-coupling procedure using indolylborates 3 into proper perspective, we examined a methodology for the construction of indolyl ketones via the carbonylative cross-coupling reaction. Various kinds of organometallic compounds (organozinc and organotin compounds and tri-coordinated organoboron derivatives) have been used for this carbonylative process.22 However, it has been reported that tetra-coordinated organoboron compounds are not suitable.23
Initially, reaction of 3a with bromobenzene in the presence of a catalytic amount of palladium complex under a pressurized carbon monoxide atmosphere (10 atm) was examined, but the reaction was sluggish to give ketone 61 (Table 6; Run 1) in low yield. The reaction of 3a with iodobenzenes in xylene at 90 °C successfully gave ketones 61 (Runs 2-3). The reaction of 3 smoothly proceeded under less forcing conditions (in THF at 60 °C) with the use of triflates and alkenyl bromides to give ketones 61 (Runs 4-10).24

A common carbonylation path could account for the present reaction, in which the transmetallation step implies that increasing the positive charge on the palladium may enhance the coordination-transmetallation step to form acyl palladium complex H (Scheme 22).

The total synthesis of the bisindole alkaloid, yuehchukene (65), could be achieved based on the present carbonylation protocol.25 Reaction of 3c with cyclohexenyl triflate 62, derived from isophorone, in the presence of a catalytic amount of palladium complex in THF under carbon monoxide (10 atm) at 60 °C for 20 h gave ketone 63 in 75% yield. Cyclzation and removal of the N-Boc group was concomitantly performed by the treatment with TFA in CH2Cl2 at room temperature, leading to 64. Reduction of the carbonyl group in 64 was carried out with DIBAH, followed by reaction with indole in the presence of Eu(OTf)3, yielding yuechukene (65) in 60% yield.

Since the transmetalation between 3 and an allenylpalladium complex 57 afforded 2-allenylindole 59, our interest was turned to explore the possibility of using a carbon monoxide atmosphere to access allenylindolyl ketone. The carbonylation reaction of 3a with propargyl carbonate 66 under carbon monoxide (10 atm) in the presence of a palladium complex allowed the isolation of cyclopenta[b]indole 67 in 60% yield. Notably, the reaction of 68 with borate 3a produced a 1:1 mixture of geometrical isomers 70a and 70b. This can be interpreted by the intermediary formation of allenyl ketone 69 and subsequent cyclization (Scheme 24).26

We anticipated that the presence of an electron-withdrawing group might suppress the cyclization step, so that the anticipated allenyl ketone would be isolable. To our delight, the carbonylation reaction of 3c with propargyl carbonates 71 and 74 gave allenyl ketones 72 and 75. Treatment of 72 with TFA in CH2Cl2 produced cyclopenta[b]indole 73, while furan 76 resulted from 75 (Scheme 25).

5. INTRAMOLECULAR ANIONIC MIGRATION REACTION
Trialkylalkenylborate complexes are well-studied synthetic intermediates, and their versatility is essentially attributed to an intramolecular alkyl migration reaction from boron to carbon, which is an important class of carbon-carbon bond formation (Scheme 26).27

This anionic 1,2-migration process is also represented by the reaction of borylated heteroaryl (such as thiophene, furan, pyrrole and pyridine) derivatves through the formation of a cationic center on the α–carbon adjacent to the boron on the heteroaromatic ring (Scheme 27).28

The cycloalkano[1,2-a]indole framework represents a structural unit of various naturally occurring substances such as mitomycin, eburnamonine and strychnine. We have developed a new one-pot strategy for the construction of cycloalkano[1,2-a]indole core based on an intramolecular alkyl migration in cyclic indolylborate 78, which was envisioned to be available from trialkylborane 77 through a sequence of lithiation and subsequent cyclization (Scheme 28).29

Initially, lithiation of borane 77, generated in situ through hydrboration of olefin 80, with tert-BuLi was attempted (Scheme 29). However, this failed due to a preferential interaction of trialkylborane 77 with the base, which led us to develop a temporarily protected boryl group prior to the lithiation. We eventually developed the desired transformation from 77 to cycloalkano[1,2-a]indole 79 through the following steps: 1) the treatment of 77 with NaOMe 2) lithiation with tert-BuLi, and 3) the addition of an electrophile (E-X). In the initial step, methoxyborate 81 is likely generated, which allows the selective lithiation at C-2 and leads to 2-lithioindole 82. This suggests that the intervention of a reversible interconversion between 82 and 83 in situ slowly produces cyclic borate 78. Electrophile could then interact with 78, promoting the alkyl migration reaction to produce 79 (Scheme 29).

We next examined one-pot construction of cycloalkno[1,2-b]indole 85 from borate 84 via an intramolecular alkyl migration process, which was expected to occur by intramolecular interaction between the C-3 of the indole and the electrophilic center (Scheme 30).30

From preliminary experiments, it appeared that π–allylpalladium complex was reasonably applicable to the expected intramolecular 1,2-migration process as an intramolecular electrophilic center. Accordingly, borates 87, generated in situ from 2-lithioindole and alkylboranes 86, were heated in the presence of a catalytic amount of palladium complex at 60 °C in THF. This provided cycloalkano[1,2-b]indoles 88 through the expected intramolecular alkyl migration process involving intramolecular nucleophilic attack of the C-3 of the indole on the π–allylpalladium complex (Scheme 31).

When both isomeric borates trans-89 and cis-89 were separately subjected to the reaction, only trans-fused indole 92 was isolated, selectively. The nucleophilic attack of the indole proceeds on the π–allylpalladium complex from the opposite side of the Pd with a simultaneous alkyl migration in anti manner. The steric repulsion between the indole and the cyclohexene rings in complex 90b makes the generation of 93 via 91b unfavorable. Complex 90b was assumed to isomerize to complex 90a, leading to 92 via 91a (Scheme 32).

6. ALKYL-BORYL MIGRATION CASCADE
During studies of the intramolecular alkyl migration reaction in indolylborates 3, we found a novel cascade of alkyl-boryl migration steps in the reaction of trialkyl(N-methoxymethylindol-2-yl)borate 94. When borate 94 was treated with MeOH, 2-alkyl-N-methylindoles 95 were isolated. Moreover, addition of CD3OD to a solution of 94 (R=Et) in THF produced 96, with the incorporation of a deuterium atom into the N-Me group.31

The results can be explained by a novel class of cascade of alkyl-boryl migration sequences, which involves the intermediary formation of alkylborane 97 through alkyl migration followed by boryl migration, and subsequent protonolysis of the C-B bond of 97 (Scheme 34).

This unprecedented process was successfully applied to the assembly of more elaborate indoles in a one-pot manner from N-methoxymethylindole (Scheme 35). Borate 94 (R=Et), generated in situ in THF from N-methoxymethylindole, was treated with MeOH in the presence of benzaldehyde. This provided alcohol 98 through the intermediary formation of alkylborane 97 and subsequent C-C bond formation between the C-B bond of 97 and benzaldehyde. Heating a mixture of 94, allyl acetate, aryl aldehyde and a catalytic amount of palladium complex in THF at 60 °C for 30 min afforded trisubstituted indoles 99.

7. CONCLUSION
Here, we have disclosed that trialkyl(2-indolyl)borates are versatile synthetic intermediates for the concise assembly of highly elaborate indole derivatives and natural alkaloids. Investigations are in progress exploring further synthetic applications of trialkyl(2-indolyl)borates based on the effective interaction between the anionic boryl group and the enamine moiety.

ACKNOWLEDGEMENTS
This research was partially supported by a Grant-in-Aid for Scientific Research (09672151, 13672226, 18590011 and 22590010), a Grant-in-Aid for High Technology Research program, and ‘Academic Frontier’ Project for Private University from the Ministry of Education, Culture, Sports, Science and Technology, and a Grant-in-Aid for the Research Project of the Research Institute of Personalized Health Sciences from Health Sciences University of Hokkaido. Acknowledgement is also made to the Akiyama Life Science Foundation.

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