Transition‐Metal‐Free Ring‐Opening Reaction of 2‐Halocyclobutanols through Ring Contraction

Abstract

Dedicated to prof. Ei-ichi Negishi for his outstanding contribution to science.The present work describes the preparation of halohydrins from 2-halocyclobutanones by means of reactions with Grignard reagents at À 78°C. We discovered that the prepared cyclobutanols underwent a thermal ring-opening reaction. Depending on the structure of the starting cyclobutanol, different products were formed. More specifically, 1-substituted 2-bromocyclobutan-1-ol was found to open to γ-substituted butyrophenones. A novel 1,3-dihydro-2H-inden-2-ylidene derivative was obtained for indene-derived cyclobutanols. Based on the outcomes of the performed experiments, a mechanism for the ring-opening of cyclobutanols can be proposed.