The reaction COWM of biraakals, O=Cr-(CH&2-CfPh2. generated from Norrish type I reaction of 2,2-dtphenylcycloakanones with various ring sizes, IS switched from intramolecular disproportionahon (n=6.7) to acyl-phenyl recombination (r&J in methanol.
Norrish type I (photochemical a-cleavage) reactions of cyclic ketones usually proceed from ketone triplets to generate triplet biradicals, which afford, after intersystem crossing, disproportionation and/or cyclization prcducts,lz and much attention has been focused on the nature of intersystem crossing and chain dynamics of bimdicals.3 Among the triplet biradicals, 0=Cf-(CH2)n-2-CtPhz, generated from a-cleavage of n-membered 2,2diphenylcycloalkanones (CK-n), those from 2,2-diphenylcyclohexanone give exclusively an unsaturated aldehyde (A-6), as reported previously.2 Herein we wish to report switching of the reaction courses of this series of triplet biradicals depending upon the chain length R and the role of intermediate methylenecyclohaxadienyl ketones in the reaction course.
Irradiation of 2,2diphenylcycloheptanone (CK-7) with 313-nm light in methanol gave exclusively an unsaturated aldehyde (A-7) in 76% yield, but neither cyclophane-type products nor methyl ester (E-n) was detecta the photochemical behavior is very similar to that of CK-6. However, 2,2diphenylcyclododecanone (CK-12) and 2,2diphenylcyclotridecanone (CK-13) afforded paracyclophanyl ketones (PC-n, n =12 and 13; 50 and 27% yield, respectively) together with decarbonylated cyclophanes (PH-n, II =12 and 13; 27 and 37%, respectively); there are no significant amounts of aldehydes (A-n).
As previously reported for CK-6,2 the a-cleavage of CK-n takes place in the triplet state to generate triplet biradicals (SBR-n), which lead to products by way of intersystem crossing to the corresponding singlet biradicals (*BR-n). Intramolecular hydrogen transfer (disproportionation) in lBR-7 gives A-7, and cyclization between the acyl and phenyl groups followed by proton shift in the resulting methylenecyclohexadienyl ketone, P-n. gives PC-n, where n is 12 or 13. Thus, the reaction course is very much different between CK-n with a smaller ring size and those with a larger ring size. The reaction processes are proposed in Scheme 1.
The photochemistry of CK-9 and CK-11 is rather complicate but informative. Prolonged irradiation of CK-9 with 313-nm light in methanol afforded an open-chain (L-9; 23%) and a cyclic decarbonylation product (C-