Regiochemical diversity in allylic alkylations via molybdenum catalysts

Abstract

The use of molybdenum as a template to control regioselectivity in allylic alkylations is explored. The feasibility of performing allylic alkylations with preformed π-allylmolybdenum complexes is established. As in palladium reactions, addition of excess phosphine has a profound effect on the rate of these reactions. A catalytic reaction based upon molybdenum hexacarbonyl is developed. Using malonate anion, excellent regioselectivity for attack at the more substituted end of an allyl system regardless of the positional identity of the initial leaving group exists. With β-ketoesters, substrates which possess a secondary and a primary carbon in the allyl unit lead to preferential attack at the secondary carbon. However, substrates that possess a tertiary and a primary carbon at the termini lead to attack at the primary carbon. Anions derived from substituted malonates and 1,3-diketones lead to substitution at the less substituted position of allyl systems. The presence of strongly electron withdrawing substituents has little effect on these orientational biases. Mechanistic implications of these results are discussed.