ADONIS 206922209100034R The boron approach to asymmetric synthesis

Abstract: The unique chemistry of organoboranes has made possible a three-pronged approach to asymmetric synthesis: 1) asymmetric hydroboration, 2) asymmetric reduction, and 3) asymmetric allyl- and crotylboration. Consequently, boron chemistry makes possible not only a uniquely general synthesis of pure enantiomers (with purities approaching 100% ee) as well as more complex stereoisomers by processes that are simple and broadly applicable. Boron derived reagents have proven to be exceptionally versatile for the asymmetric reduction of the carbonyl group in a variety of derivatives to give products of optical purities approaching 100 % ee. Finally, the synthesis of chiral organoboranes containing allyl, crotyl and related groups attached to boron is providing a highly valuable synthesis via allylboration of stereoisomers in purities approaching 100 % ee. It is evident that boron chemistry is providing major new routes for asymmetric synthesis in organic chemistry. HYDROBORATION The remarkably facile addition of diborane in ether solvents to alkenes and alkynes was discovered in 1956 (ref. 1) (1). For the next decade my students and associates studied the hydroboration of alkenes, alkynes and their substitution until we felt that our understanding of hydroboration was satisfactory (ref. 2). These studies revealed promising characteristics (Fig. 1). Reactions are essentially insuolurne~us and quantitative. The addition takes place preferentially from the less hindered side. + BH,- _t 99.6 % 0.4 % I /B \ The addition proceeds in an anti-Markovnikov manner. No rearrangements of the carbon skeleton have been observed. '4 x R * ) , B- R m O H 101 _t _t The reaction involves a cis addition of the H-B bond