GAS-PHASE SYNTHESIS OF REACTIVE MOLECULES USING ADSORBED REAGENTS

Base‐induced elimination reactions have found wide application in the synthesis of strained alkenes. The method is often complicated by a competing process in which the incipient alkene is trapped by nucleophilic addition of the base. Recent studies have shown that the bi‐molecular side reactions can be avoided when bases such as potassium tert‐but‐oxide are supported on an inert surface like silica gel and the reaction is carried out in vacuo. This vacuum gas‐solid reaction (VGSR) procedure has the additional advantage that very unstable species may be isolated under conditions that allow detailed studies of their physical and chemical properties. The scope of this procedure has been extended to include the use of fluoride salts supported on glass helices to effect the elimination of β‐halosilanes. The fluoride route to strained alkenes is particularly attractive since the reaction may be carried out at very moderate temperatures and the starting materials are readily available. This review describes the development of this vacuum gas‐phase procedure and its application in the synthesis and characterization of small‐ring alkenes and bicycloalkenes, including methylene‐cyclopropene, spiropentadiene, bicyclo‐propenyls, 1,2‐ and 1,3‐bridged cyclo‐propenes, and other simple cyclo‐propenes. Copyright © 1994 by VCH Verlagsgesellschaft mbH, Germany