Methyleneketenes and methylenecarbenes. III. Pyrolytic synthesis of arylacetylenes and their thermal rearrangements involving arylrnethylenecarbenes

Abstract

Flash vacuum pyrolysis (550-600°) of 5-benzylidene derivatives of 2,2- dimethyl-1,3-dioxan-4,6-dione(isopropylidene malonate) gives phenylacetylenes bearing m- or p-substituents (H, Me, MeO, Cl, CN)in 64-98% yield. o-Methoxy- and o-phenyl-benzylidene derivatives give the arylacetylenes even at 420°. Arylacetylenes equilibrate with arylmethylenecarbenes, ArCH=C:, at 550-720°, as shown by labelling experiments using ¹³C and by the formation of products of interaction with o-substituents. Methylenebenzocyclobutene and o-tolylacetylene each form indene at 700-720°.