Chapter 11. Alicyclic compounds

Abstract

Useful quantitative data on heats of hydrogenation and combustion have appeared ; this allows the estimation of strain energy effects for bicyclo[l,l,O]butane,' bicyclo[ 2,1,O] pentane, ' quadricyclene,l and a number of C4H6 cyclic hydrocarbons.2 The strain energy in the bridgehead olefin bicyclo[3,3,1]non-1-ene is estimated3 from the heat of acetic acid addition to be 12 kcal. per mole. Quantitative assessment of strain and conformational effects has allowed estimation of activation energies for a variety of small ring thermolyse~.~ Assumption of a non-concerted diradical mechanism shows good fit with experiment in all cases studied except those of cyclobutene ring opening and rearrangement of cis-1 -methyl-2-vinylcyclopropanes, which are much more ready and, therefore, follow a concerted path. Differential thermal analysis may be used to measure activation energies and heats of reaction simultaneously, as in the dimerisation of cyclopentadiene and the endo,exo-cyclopentadiene dimer isomerisati~n,~ and in thermolyses of hexamethyl dewar benzene and hexamethylprismane.6Reviews of fulvenes,' cyclopropanols,8 metal-catalysed reactions of norbornadiene,' 1,2-and l,.l-addition to conjugated dienes," the steric course of cyclohexenone additions," and solvolysis of norbornen-7-yl systems' have been produced. An issue of Angewandte Chernie13 is devoted to small ring chemistry.Conformational Analysis.-The sophistication and accuracy in computersimulation of energy and geometry in cyclic systems increases ; Hendrickson has presented more detailed analysis of medium rings and of methyl-substituted cycloalkenes, coupled with a full topological analysis of conformer interconversion pathways. l4 Allinger's school has refined the Westheimer-Hen-' '