Zinc carbenoid-mediated reactions

DFT studies have been performed on two zinc carbenoid-mediated reactions. On (g) one hand, a mechanistic investigation of the one-carbon homologation of /3-keto-esters has revealed two competing pathways (i) formation of the donor - acceptor cyclopropane intermediate (41) and (ii) a reaction involving the cyclopropane transition state (42). A preference for the classical or the non-classical pathway is not clear... 

PRODUCTS OF ZINC CARBENOID-MEDIATED CHAIN EXTENSION REACTIONS... 

The more recently reported route to diethyl 3-oxoalkylhosphonates uses a zinc carbenoid-mediated approach, which is believed to proceed through the intermediacy of a cyclopropylzinc alkoxide. Thus, treatment of simple diethyl 2-oxoaIkylphosphonates with the Furukawa-modifled Simmons-Smith reagent provides a rapid and efficient preparation of 3-oxoalkyIphosphonates. The chain extension of simple, unfunctionalized P-ketophosphonates requires an excess (6 eq) of both Et2Zn and CH2I2 at room temperature. The presence of a-substitution on the 2-oxoalkylphosphonate does not diminish the efficiency of the reaction (see Section 7.2.3.7). 

The zinc-mediated reaction tolerates a variety of functionality in the p-keto ester. In fact, the method described above has been applied successfully to p-keto amides and p-keto phosphonates, Unsubstituted p-keto esters, amides and phosphonates have been chain-extended in yields that ranged from 58% to 98% (Table I). The primary limitation to this method is the inefficiency with which a-substituted esters and amides undergo methylene insertion. The zinc carbenoid must be employed in at least a threefold excess h... 

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