Diastereoselective hydrogenation at indenols: Evidence for sterically and electronically unfavorable adsorption on palladium

The stereochemistry of hydrogen addition to C=C bonds was studied in the di astereoselective hydrogenation of selected indene derivatives over an alumi na-supported Pd catalyst. In ail cases hydrogenation occurred via bottom-si de syn addition of two hydrogen atoms to the chemisorbed substrate. Hydroge nation of indenol derivatives demonstrated that methyl or ethyl groups are not sufficiently bulky to hinder the adsorption and hydrogenation of the su bstrate in a sterically and electronically unfavorable ("upside down") posi tion, in which the alkyl group points toward and the OH group points away f rom the metal surface. The implications of this observation for understandi ng the mechanism of asymmetric hydrogenation over metal catalysts are discu ssed. (C) 1999 Academic Press.