Synthesis of 4‐Silylcyclobut‐2‐enethiones and their Use in Cyclobutadiene Generation

Abstract

Abstract Alkynyl silyl sulfides 2 reacted with ynamines 3 to give 1:1 adducts. The structure of 4‐silylcyclobut‐2‐enethiones 4 was confirmed by X‐ray analysis of 4a . A cyclobutadiene intermediate is probably not involved in this reaction; we think that it is initiated by a silyl transfer from 2 to 3 , and that this is followed by a combination of the resulting ions to give the unsaturated thioketene 7 , which undergoes an electrocyclization to give products 4 . In the reaction of thiones 4 with trimethyloxonium tetrafluoroborate (Meerwein salt) selective methylation at sulfur was observed to give cyclobutenethionium ions 16 . Ions 16 underwent fluoride‐induced desilylation with various fluoride sources to give cyclobutadienes 17 , which could not be isolated, but trapping of 17 a‐c was possible with dimethyl acetylenedicarboxylate yielding regioisomeric benzene derivatives 20–23 . Similarly, 17 a and bis(methylthio)methylene malononitrile ( 24 ) led to hexatriene derivative 27 by a sequence of cycloaddition and two ring‐opening reactions. In contrast, silyl‐substituted cyclobutadienes 17d,e dimerized even in the presence of trapping agents to anti ‐tricyclo‐[4.2.0.0 2,5 ]octadienes 29 ; this suggests that a two‐step cycloaddition is taking place, rather than a concerted Diels‐Alder reaction. Attempts to intercept 17d, e , generated from 16d,e with cyclopentadiene ( 30 ), gave deprotonation of 30 leading to substitution of the methylthio group in 16 and finally to formation of cyclobutenes 33 through a hydrogen shift.