Reactions of 1,2-oxaphospholenes. 7. Anomalous low reactivity of a tertiary allylic bromide. The crystal and molecular structure of (E)-4,5-dibromo-3,5-di-tert-butyl-2-phenyl-1,2-oxaphosphol-3-ene 2-oxide

Abstract

Reaction of 2,2,6,6-tetramethgl-4-heptyn-9-ol (5) with dichlorophenylphosphine leads to (2,2,6,-tetramethyl-3,4 heptadien-3-yl)phenylphosphinic acid (7), which undergoes Ag + -catalysed cyclization to 3,5-di-tert- butyl-2-phenyl-1,2-oxaphosphol-3-ene 2-oxide (1e, 75-55% Z, 25-45%E). Electrophilic bromination of 7 affords 1f, the 4-bromo derivative of 1e, as a 75% Z 55% E diastereomer mixture from which pure (Z)-1f can be isolated. Free radical allylic bromination of 1e and 1f leads to the corresponding 5-bromo derivatives 2e and 2f, each as the single diastereoisomer