Directed Hydroboration and Related Reactions

University Of Michigan At Ann Arbor, Ann Arbor MI

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Abstract

DESCRIPTION (provided by applicant): Hydroboration is unique among the hydrometallation technologies for the versatility of available options for stereospecific conversion of C-B bonds into C-H, C-O, C-N, C-halogen, and C-C bonds. There has been intense recent interest in hydroboration because of the rapidly growing importance of the Suzuki and Heck coupling reactions of boronic acid derivatives. It is clear that these variations of organopalladium coupling technology will be centrally important in the discovery, development, and manufacture of the next generation of pharmaceutical agents, and in the synthesis of complex structures of academic interest. Improved methodology for regiocontrolled or stereocontrolled access to boranes and boronic acids is therefore one of the key challenges for organic synthesis. Boronic acids have advantages over many organometallics in that the reagents can usually be isolated if desired, activation for bond formation occurs under a variety of mild conditions, the intermediates are resistant to water and air, toxicity problems are minimal, and much is known about controlling the regiochemistry of hydroboration based on differences in degree of alkene substitution. However, available hydroboration methodology is not well developed for alkene (or alkyne) substrates where there is little, or no difference in degree of substitution at the unsaturated carbons. Such problems can be addressed by heteroatom substituent directing effects in the substrate, and that will be the focus of the research to be performed. A new concept for nitrogen-directed hydroboration has been demonstrated using electrophilic activation of stable amine borane complexes. The new method involves generation of reactive borenium ion pair intermediates that are capable of regiocontrolled and stereocontrolled hydroboration. The procedure will be used to prepare cyclic amine boranes and to exploit these intermediates for synthesis of a variety of derivatives using palladium catalysis. Methodology for palladium-free introduction of C-C, C-N, C-O and C-X bonds will also be studied. The procedure allows amine-directed internal functionalization of alkenes and alkynes with regio-and stereocontrol. A similar concept will be applied to oxygen-directed borenium hydroboration. Highly selective oxygen-directed hydroboration methodology has been demonstrated using the same principles. A major thrust of the program will be to investigate the stereochemistry of hydroborations directed by ether or alcohol oxygen.