Nucleophilic substitution at an acetylenic carbon : Competing processes in the reactions of sodium methoxide with phenylbromoacetylene or phenylchloroacetylene in methanol

Abstract

Phenylhaloacetylenes are triphilic towards methoxide ion in methanol. With phenylchloroacetylene ( 1′), methoxide attacks ∼0·2% on chlorine, ∼99% on carbon 1, and ∼1% on carbon 2; with phenylbromoacetylene ( 1), methoxide attacks ∼5% on bromine, ∼83% on carbon 1, and ∼11% on carbon 2. The first detectable products, phenylacetylene, phenylmethoxyacetylene ( 2), ( E)-β-bromo-β-methoxy- styrene, and ( Z)-β-bromo-α-methoxystyrene lie on discrete mechanistic paths and are not interconvertible under the reaction conditions. Comparable amounts of alkynyl and alkenyl ethers are formed by attack on carbon 1. The element effects for attack on halogen, K(Cl)/k(Br) ⋍ 0·4, and on carbon 1, k(Cl)/L(Br) ⋍ 1·6 are consistent with the notion of independent competing processes. Phenylmethoxyacetylene is a short- lived transient; its formation and decay (with methoxide) rate constants at 78° arc k 1′2 (Cl) 0·6 × 10 −4 * or k 12 (Br) 0·5 × 10 −4 and K j 1·7 × 10 −3 M −1 sec −1. Previously, the intermediacy of alkoxyacetylenes under analogous conditions has only been surmised. In the course of this work, five of the six bromomethoxystyrenes were prepared and some of their distinctive chemistries, as well as those of the corresponding carbonyl compounds, were worked out (eq 2–4).