A Model for Diastereoselective Hydroborations Dave Evans. Chem 115. Lecture 22 Dec 14. 1993 Diastereoselective Hydroboration EXamples H202 Favored product for J. Am. Chem.Soc.1983,1053725 ThexyIBH2 80: 20 9-BBN 93:07 aA tumover in diastereofacial selectivity is sometimes observed using BH3 nBu OH :08 W. C. Still J BH3THF→nBu ok6 Assume OH (OR)=Rm and results are consistent with the model K N Houk, M. N. Paddon-Row, Co-workers 10A-08-Hydroboration Models 10/8/00 8: 13 PMK. N. Houk, M. N. Paddon-Row, & Co-workers, Tetrahedron 1984, 40, 2257-2274. Assume OH (OR') = Rm and results are consistent with the model W. C. Still & J. C. Barrish, J. Am. Chem. Soc. 1983, 105, 2487. BH3•THF 9–BBN H2O2 OR' OH OH OTMS OAc OH nBu iPr nBu nBu nBu 92 : 08 96 : 04 91 : 09 88 : 12 42 : 58 R Selectivity R2BH M. M. Midland & Co-workers, J. Am. Chem. Soc. 1983, 105, 3725. H2O2 R2BH Dave Evans, Chem 115, Lecture 22, Dec 14, 1993 BH3 H2O2 H2O2 ■ A turnover in diastereofacial selectivity is sometimes observed using BH3 Favored product for dialkyl borane reagents H H2O2 2O2 R2BH Worse Bad ■ Hydroboration of allylic alcohols (ethers) Selectivity 50 : 50 80 : 20 93 : 07 96 : 04 BH3•DMS ThexylBH2 9–BBN (Chx)2BH Diastereoselective Hydroboration Examples A Model for Diastereoselective Hydroborations OH Me RM OH RL R OR' RL RM Me B H B H H H Me RM Me C H RL H C Me C Me OR' RL R OH R RM H OR' Me OH H H RL RM H C Me C C B B H H H Me RM H RL RM RL RM Me R R R R Me RM RL OH RL RM Me OH H H H C C H H Me H RL Me H H Me Me H H Me H H OH H Me H H Me OH 10A-08-Hydroboration Models 10/8/00 8:13 PM