PREFERENCE FOR BETA-H ELIMINATION IN THE TERMINATION OF THE NI-PROMOTED CARBONYLATIVE CYCLOADDITION OF 2-HALOETHYLIDENE-CYCLOALKANES AND ALKYNES

The reaction of 2-haloethylidenecycloalkanes (ring size: 5, 6, 7, 8) w ith either methyl-2-butynoate or 2-butynyl methyl ether and NI(CO)(4) in methanol affords mainly two type of bicyclic compounds: 5-cycloalke nylidenecyclopent-2-enones and 5-(1-cycloalkenyl)cyclopent-2-enones. T he origin for the diversion of the process towards elimination instead of alkoxycarbonylation is interpreted as the result of the mutual con formation of the two rings in the intermediates favouring either elimi nation through enol formation or syn beta-elimination. In this last ca se, the terminating step can proceed in two different modes, namely, i ntraannularly or interannularly. While for the acetylenic ester, the p roduct from interannular elimination is exclusive or predominant regar dless of the ring size of the starting halide, for the 2-butynyl methy l ether, the product distribution is found to strongly depend on the n ature of the allyl component. Thus, in this case, intraannular elimina tion is exclusively found in the reaction of the cyclopentylidene hali de, while products from interannular elimination are produced in the r eaction of the eight-member homologue. Mixtures of the two types of co mpound are formed from the cyclic 6 and 7 allyl halides. (C) 1998 Else vier Science S.A.