The Catalyzed Hydroboration L Olefin binding to metal is ireversible for 1, 1-disubstituted allylic alcohol derivatives Evans Fu JoC 1990. 55. 2280 and Evans. Fu. Anderson JAcS The Catalyzed vs Uncatalyzed Hydroboration Reactions R Conditions Anti: Syn Yield (%) 9-BBN 9-BBN Me anti Si(t-Bul Rh(PPh3)3CI /CB 15: 85 65 a Complexation involves back-donation from a filled metal d orbital-I'c=c H u The EWG(alkoxy substituent) is aligned perpendicular to the olefin(T-oc-o) Rh(PPh3)3CI/CB 50: 50 a This stereoelectronic interaction lowers the energy of t a The small group is placed inside, the most sterically congested site The uncatalyzed variant Evans, Fu,& Hoveyda JACS 1988, 110, 6917 and JACS 1992, 114, 6671 Me anti K Burgess Co-workers, J. Org. Chem. 1991, 56, 1020-1027 10A-10-Cat Hydroboration 10/8/00 8: 14 PMRhLn Rh cat CB Evans & Fu JOC 1990, 55, 2280 and Evans, Fu, Anderson JACS 1992, 114, 6679. ■ Olefin↔catalyst complexation is the stereochemistry-determining step ■ Olefin binding to metal is irreversible for 1,1-disubstituted allylic alcohol derivatives Stereochemical Model RhLn The Catalyzed Hydroboration ■ Complexation involves back-donation from a filled metal d orbital→π*C=C ■ The EWG (alkoxy substituent) is aligned perpendicular to the olefin (π→σ*C—O) ■ This stereoelectronic interaction lowers the energy of π* ■ The small group is placed "inside", the most sterically congested site CB Rh cat syn anti syn anti 9-BBN El+ 9-BBN El+ The uncatalyzed variant: Complementary diastereoselectivity for the catalyzed and uncatalyzed reactions is observed for a wide range of substrates. Anti Syn 9-BBN 9-BBN 9-BBN 75 72 67 Si(t-Bu)Me2 3 : 97 95 : 5 95 : 5 96 : 4 R Conditions Anti : Syn H 50 : 50 15 : 85 68 65 77 Yield (%) THF 20 °C The Catalyzed vs Uncatalyzed Hydroboration Reactions Rh(PPh3)3Cl / CB Rh(PPh3)3Cl / CB Rh(PPh3)3Cl / CB Si(t-Bu)Ph2 Evans, Fu, & Hoveyda JACS 1988, 110, 6917 and JACS 1992, 114, 6671. K. Burgess & Co-workers, J. Org. Chem. 1991, 56, 1020-1027 i-Pr Me OR OR Me i-Pr i-Pr R R'O H C C Me H H R Me OR' OR' Me R Me OR OH C H H Me C OH R'O H R R Me OR' OH OH OH OH OR' Me R R'O H R Me C C H H R Me OR' OR' Me R C H H C Me H R OR' 10A-10-Cat Hydroboration 10/8/00 8:14 PM