Controlling stereochemistry in C-C and C-H bond formation with electronically asymmetric organometallics and chiral poisons

Air stable (cyclopentadienyl)Mo(NO)(halide)(q3-allyl) complexes add to aldehydes to yield homoallylic alcohols in high enantioselectivity and diastereoselectivity. A new strategy, chiral poisoning, is applied to asymmetric hydrogenation and the kinetic resolution of allylic alcohols using transition metal catalysts prepared from racemic bis-phosphine ligands. The activation afforded an organic moiety by complexation led to the extensive development of transition metal reagents for organic synthesis. The increased reactivity is influenced by differences in the steric and electronic nature of the metal and its ligands. Ultimately the environment at the metal can induce high stereoselectivity in reactions involving the coordinated ligands. Our aim has been the improvement of our understanding of the origins of selectivity in these reactions and the application of these principles to the rational design of reagents and catalysts. Our approach modifies the conventional emphasis on steric effects in catalyst design and focuses attention on electronically controlled selectivity. Our initial work emphasized the control which could be obtained with nucleophilic additions to chiral [CpMo(allyl)(NO)(CO)]* cations, wherein the difference in electronic influences of the carbonyl and nitrosyl ligands directs attack of the nucleophile to the terminus of the ally1 that is cis to NO. Using enantiomerically pure metal centers, this regioand stereocontrol allows the enantioselective preparation of olefins with stereogenic centers at the a position. These reagents have also been used successfully by others in more elaborate organic syntheses (ref. 3-4). In our earlier work, resolution of the stereogenic metal center was accomplished using neomenthyl-substituted cyclopentadienyl ligands (ref. 2, 5-7). x / M0 ; \ I