Alkenyl Borane Coupling Reactions

Miyaura, N. Yamada, K. Suginome, H. Suzuki, A. Novel and convenient method for the stereo- and regiospecific synthesis of conjugated alkadienes and alkenynes via the Pd-catalyzed cross-coupling reaction of 1-alkenyl-boranes with bromoalkenes and bromoalk-ynesj. Am. Chem. Soc. 1985, 107, 972-980. 

Conjugated enynes are of importance for themselves, as well as for the synthesis of conjugated dienes. The cross-coupling reaction of 1-alkenyl(disiamyl)boranes (3c) with 1-bromo-l-alkynes (Scheme 2-34) provides conjugated enynes in high yields [45]. 

Coupling reactions of alkyl boranes, formed by hydroboration of alkenes, with unsaturated halides (or triflates or phosphonates) is possible, and this reaction is finding increasing use in synthesis. For example, coupling of the alkyl borane derived from hydroboration (with 9-borobicyclo[3.3.1]nonane, 9-BBN) of the alkene 200 with the alkenyl iodide 201 gave the substituted cyclopentene 202, used in a synthesis of prostaglandin Ei (1.205). This type of B-alkyl Suzuki coupling reaction is very useful for the synthesis of substituted alkenes. 

Although the intrinsic reactivity of alkenyl boranes in Pd-catalyzed cross-coupUng is rather low, the use of a base ranarkably enhances their reactivity as demonstrated by Suzuki and co-workers. t Compared with other organometals containing Li, Mg, Zn, Al, Cu, and Zr, alkenylborons are very stable, even to alcohols and H2O. Thus, a wide variety of functional groups can be tolerated in their coupling reactions, as shown in Scheme 50. 

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. 

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. 

Trialkylboranes including 9-alkyl-9-BBN and alkyl(disiamyl)borane readily undergo the cross-coupling with 1-alkenyl or aryl halides or triflates [8] (Scheme 20). The reaction is limitedly used for primary alkylboranes thus, hydroboration of terminal alkenes with 9-BBN or HB(Sia)2 is the most convenient to furnish the desired boron reagents. Trialkylboranes thus obtained are... 

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