Molybdenum-catalyzed Asymmetric Allylic Alkylations

Moberg, C.


Molybdenum-catalyzed nucleophilic substitutions of allylic acetates and carbonates proceed with high site- and stereoselectivity. Monosubstituted allylic substrates yield branched products, in contrast to what is commonly observed in reactions catalyzed by palladium complexes. High enantioselectivities are observed in the presence of pyridinylamide and oxazolinylamide ligands. The range of nucleophiles that undergo reaction is somewhat limited and includes malonates, cyano esters, α-imino esters and lactones, oxalactimes, and oxindoles.

The reactions proceed via oxidative addition of the substrate to a ligated Mo(0) species forming an asymmetric η3-allylmolybdenum(II) complex, coordination of the nucleophile to the metal, and final reductive elimination to form the product. Overall retention of configuration is observed as a result of retentive oxidative addition and retentive nucleophilic attack.

Equilibration of intermediate η3-allyl complexes is usually rapid in comparison to nucleophilic attack, which leads to the formation of a single major η3-allyl complex. As a consequence, close to equal mixtures of enantiomers and constitutional isomers are obtained from enantioenriched or racemic branched substrates as well as from linear substrates.