The solid-state photodimerisation of 2,5-dibenzylidenecyclopentanone (DBCP); a topochemical reaction that yields an amorphous product

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Abstract
When 2,5-dibenzylidenecyclopentanone (DBCP)is irradiated by u.v. light in the crystalline state the principal product formed by a [2 + 2]dimerisation is 2,9-dibenzylidene-6, 12-diphenyldispiro[4.1.4.1]-dodecene-1,8-dione (DDBCP); a subsidiary dimeric species is formed in substantial amounts at low conversion but is consumed during the course of further reaction and is absent when the total conversion approaches 90%. Crystalline (+)-2,5-dibenzylidene-3-methylcyclopentanone [DB(+)3MeCP] under the same condition is photostable. The contrasting photoreactivity of these two solids is interpreted in terms of the mutual disposition of potentially reactive olefinnic bonds on neighbouring molecules. Even when such bonds are not parallel (and for DBCP they are inclined at an angle of 56°) reaction may ensue since the distances separating the carbon atoms are approximately 3.7 Å. It is argued that the photostability of DB(+)3MeCP, where the separation distances of the potentially reactive olefinic bonds on adjacent molecules are comparable to those in DBCP and where the bonds are again not parallel, is attributable to the non-coplanarity of the two π-bonds.

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