Studies of Some Strained Organic Molecules: Cycloproparenes

Abstract

Cyclopropabenzene (2) is the parent and most highly strained member of the ortho-fused cycloalkabenzene series. The present study has been directed towards the imposition of additional strain upon this molecule in an attempt to establish the limits to which the benzenoid framework can be distorted. Two distinct approaches have been investigated. The first part of the study involves 2- and 3-halocyclopropabenzene synthesis and subsequent cyclopropabenzyne formation whereas the second section of the work involves alkylidenecycloproparene synthesis. 3-Halocyclopropabenzenes have been known for some time and are readily prepared by tris-dehydrohalogenation of 3,4,7,7-tetrahalobicyclo[4.l.0]heptanes, e.g, (41). In this present study the dehydrohalogenation of 2,3,7,7- tetrahalobicyclo [4.1.0]heptanes (83) and (82) has provided2-chloro- and 2-bromo-cyclopropabenzene (80) and (47) respectively, by a pathway which is shown by labeling studies to be much more complex than that pertaining to the 3-isomers; 3,4-dibromo-7,7-dichlorobicyclo [4.I.0]heptane (45) furnished a c. 2:3 mixture of cyclopropabenzenes (47) and (80. Dehydrobromination of 2- and 3-bromocyclopropabenzene (47) and (42) in the presence of furan provides 2,5-epoxY-2,5-dihydro-1H/-cyclopropa [a]naphthalene (105) and 3,6-epoxy-3.6-dihydro-1H-cyclopropa [b]naphthalene (111) respectively. The formation of these compounds provide compelling evidence for the intermediacy of cyclopropa[1,2-d]benzyne (54) and cyclopropa[l,2-c]benzyne (55) and these molecules are taken to be the most highly strained dehydroaromatics yet produced. The incorporation of, an exocyclic carbon-carbon double bond at C-1 of cyclopropabenzene (2) makes the cyclopropabenzene system wholly planar. To this end, alkylidene cycloproparenes have been synthesised by the Peterson reaction of α-silyl cycloproparenyl anions (76) and (117) with suitable aldehydes and ketones- Although highly strained, these derivatives are surprisingly stable and this is indicative of some resonance stabilisation.