Preference for β-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) with either methyl-2-butynoate or 2-butynyl methyl ether and NI(CO)(4) in methanol affords mainly two type of bicyclic compounds: 5-cycloalkenylidenecyclopent-2-enones and 5-(1-cycloalkenyl)cyclopent-2-enones. The origin for the diversion of the process towards elimination instead of alkoxycarbonylation is interpreted as the result of the mutual conformation of the two rings in the intermediates favouring either elimination through enol formation or syn beta-elimination. In this last case, the terminating step can proceed in two different modes, namely, intraannularly or interannularly. While for the acetylenic ester, the product from interannular elimination is exclusive or predominant regardless of the ring size of the starting halide, for the 2-butynyl methyl ether, the product distribution is found to strongly depend on the nature of the allyl component. Thus, in this case, intraannular elimination is exclusively found in the reaction of the cyclopentylidene halide, while products from interannular elimination are produced in the reaction of the eight-member homologue. Mixtures of the two types of compound are formed from the cyclic 6 and 7 allyl halides. (C) 1998 Elsevier Science S.A.