Physical Methods and Energy Calculations

A study of the consequences of through-bond versus through-space coupling in strained propellanes indicates the probable importance of a new kind of isomerism. The isomers are related by a simple bond-stretching process. In tricyclo[2,2,2,0 ]octane, extended Hiickel calculations show that isomers 

The possibility of isolation of the stable biradical (la) is hindered by the symmetry-allowed [ 2 + 2J fragmentation to give 1,4-dimethylenecyclo-hexane. In contrast, the fragmentation of the biradicals derived from (4)—(7) 

Ab initio studies indicate a C-1—C-3 bond distance of 1.60 A in (4) and 1.885 A in bicyclo[l,l,l]pentane. The biradical obtained by stretching the C-1—C-3 bond is predicted to be much less stable than the ground state of (4). 

Bond-stretch isomerism in (8) and (9) has been studied. In (8), stretching of the C-1—C-5 bond gives an isomeric biradical which is only weakly stabilized by orbital interaction, but in ketone (9) and markedly in cation (10) an interaction as in (11) leads to a double energy minimum and both EH and CNDO/2 calculations support the existence of bond-stretch isomerism in (9) and (10). 

Although the synthesis and reactivity of propellanes, recently reviewed, is discussed later, it may be noted that symmetry considerations and calculations have been used to rationalize the observed high reactivity of strained propellanes to acid and to radical attack but their stability to nucleophilic attack. EH calculations have been used to probe the stabilization of bridged carbonium ions by through-bond interactions. 

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