Aryl Borane Coupling Reactions

Polystyrene-bound trialkylboranes, which can be prepared by hydroboration of support-bound alkenes with 9-BBN, undergo palladium-mediated coupling with alkyl, vinyl, and aryl iodides (Suzuki coupling Entries 1 and 2, Table 5.3 for vinylations, see Section 5.2.4). Because boranes are compatible with many functional groups and do not react with water, these coupling reactions could become a powerful tool for solid-phase synthesis. To date, however, few examples have been reported. 

Early findings by Suzuki and co-workers [109] showed that the palladium-catalyzed iminocarbonylative cross-coupling reaction between 9-alkyl-9-BBN derivatives, t-butylisocyanide, and arylhalides gives access to alkyl aryl ketones 132 after hydrolysis of the corresponding ketimine intermediates 131. Presumably, the concentration of free isocyanide is kept to a minimum by its coordination with the borane. Formation of an iminoacylpalladium(II) halide 130 by insertion of isocyanide to the newly formed arylpalladium complex followed by a transmetallation step afford the ketimine intermediates 131 (Scheme 8.52). 

Hydroboration of acetylenic selenides with 9-BBN led to the regio- and stereoselective formation of a-selanylalkenyl boranes which were then converted into Z-a, -disubstituted vinyl selenides by cross-coupling reaction with aryl bromides [80] (Scheme 58). With unsubstituted acetylenic selenides, an inversion of regioselectivity during the hydrozirconation was observed [81,82]. 

The Pd-catalyzed cross-coupling reactions of metal nucleophiles with carbon electrophiles are of considerable value for the regio- and stereocontrolled synthesis of functionalized organometalhc compounds, in particular, silanes, stannanes, and boranes, which are important reagents for Pd-catalyzed carbon-carbon cross-coupling as shown in Sects. in.2.2-in.2.4. Symmetrical bimetallic compounds such as disilanes, distannanes, and diborons are usually used as metal nucleophiles. The present metallation is applicable to aryl, benzyl, vinyl, acyl, and aUyl (Sect. V.2.3.3) electrophiles. 

In all the entries but the first two, the phosphorus atoms contain two aryl groups. Some crowded diphosphine boranes have also been reported (entries 2, 6, 10, 14 and 16-21). In general, good yields for the coupling reaction are observed, even in the latter cases. The diphosphine boranes are obtained as enantiomerically pure white solids or oils, which can be easily deboronated by amines. The diphosphinite borane of entry 1 also deserves some comment because another product could have been formed (Scheme 4.40). 

As the overall cross-coupling reaction proceeds with inversion of stereochemistry and reductive ehmination is well known to undergo retention of stereochemistry, the result imphes that transmetaUation in this reaction proceeds predominantly with retention of stereochemistry. In addition to this study, in 1998, Woerpel and Soderquist [102] independently studied the stereochemistry of transmetaUation for the Suzuki-Miyaura cross-coupling reactions of alkyl boranes with aryl or alkenyl hahdes. Their deuterium labehng study revealed that the transmetaUation of alkyl boranes 163 or 166 proceeds with retention of stereochemistry to give products 165 or 167. Soderquist proposed a closed four-membered cyclic transition state 168 to account for the retention of stereochemistry observed during the reaction. 

Scheme Z48 Suzuki-Miyaura cross-coupling reactions of activated alkyl halides with aryl boranes.

The Suzuki coupling was developed by Professor Akira Suzuki of Hokkaido University. The Suzuki coupling uses a boron compound (R-BYj) and an alkenyl, aryl, or alkynyl halide or triflate (RX) as the carbon sources, with a palladium salt as the catalyst. Bromides and iodides are the most commonly used halides chlorides are less reactive. Alkyl halides can sometimes be used but are subject to elimination. A base is also required. The boron compound can be a borane (R jB), a borate ester (R B(OR)2), or a boric acid (R B(OH)2), where R is alkyl, alkenyl, or aryl. The general reaction is shown in the following scheme, where X is halide or triflate and Y is alkyl, alkoxyl, or OH. A list of the types of components that can be used is given in Table 24.1. This reaction is one of the principal methods now used to prepare biaryls. 

Cross-coupling reactions are extremely valuable tools for the construction of complex structures also on solid support. The accessibility of appropriate building blocks, in former times the bottleneck of library syntheses, has improved since a wide variety of alkenyl-, aryl-substituted stannanes or boranes can now be purchased from commercial suppliers in a broad variety. 

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