The use of boron chemistry for the synthesis of enantiomerically enriched organic compounds is described. Key advances towards the preparation of organoboranes with control of stereochemistry are supplemented by a discussion of the use of these chiral organoboranes in carbon-carbon bond forming reactions. Particular emphasis is given to advances in the Suzuki-Miyaura coupling of chiral secondary organoboranes and homologation reactions. Both of these reactions convert the initially generated B-C bond into a C-C bond and thus lead to a significant increase in complexity.
The addition of Lewis acids such as trispentafluoroboron as cocatalysts has been found to have a dramatic effect on the Rh-catalyzed hydroboration of olefins with pinacol borane. For example, aliphatic olefins do not react at all in noncoordinating solvents, but with the addition of 2% of B(C(6)F(5))(3), the reaction is complete in minutes. Similarly, the reaction of aromatic olefins with HBPin occurs slowly and nonselectively in the absence of B(C(6)F(5))(3), but is accelerated and occurs more selectively in its presence. Preliminary mechanistic studies suggest that the B(C(6)F(5))(3) needs to be present throughout the course of the reaction, not just at the initiation stage, and implicate this species, along with THF, in the heterolytic cleavage of the B-H bond of HBPin.
Any substituent does it: The hydroboration of vinyl arenes with pinacol borane (HBPin) and cationic rhodium complexes selectively placed the boron proximal to the aryl rather than phenyl ring, regardless of whether this ring bears electron‐donating or electron‐withdrawing substituents. In competition experiments between styrene and various vinyl arenes, preferential hydroboration also occured at the substituted arene (see scheme). Hammett plots indicate a break in the mechanism.
Various routes were examined for the synthesis of chiral biphenyl species that are substituted at the 2,2', 4,4' and 6,6' positions. Because the biaryl bond is tetrasubstituted, many coupling reactions were not suitable. The most reliable coupling reaction proved to be the Ullmann, which gave the desired product in 82% yield. The products were required as the starting point for the preparation of chiral materials using these as the monomer. For this reason, a route was required that produced large quantities of both enantiomers. The two enantiomers were resolved at the penultimate step by the use of chiral HPLC. A complicating feature proved to be the necessity to have a reactive group at the 4,4' positions, which would permit polymerization though this point. Ultimately, we employed an Ullmann coupling on a dibrominated arene, which occurred selectively at the more hindered bromine by virtue of the directing effect of an ortho ester substituent.
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