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

Abstract

AbstractAlkynyl 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‐[4.2.0.02,5]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.