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The reaction course of acyl-diphenylmethyl biradicals (α-oxo-ω,ω-diphenyl-α,ω-alkanediyl biradicals), O=C↑–(CH2)n−2–C↑Ph2 (3BR-n), generated from the Norrish type-I reaction of 2,2-diphenylcycloalkanones (CK-n) with various ring sizes in methanol, is switched from intramolecular disproportionation (n = 6, 7), giving a diphenylalkenal, to acyl-phenyl recombination (n ≥ 9), affording a cyclophane derivative. The behavior of 3BR-9 derived from CK-9 was studied in detail; the photolysis of CK-9 afforded an open-chain and a cyclic decarbonylation product together with an unsaturated aldehyde and a 4-methylene-2,5-cyclohexadienyl ketone (a pre-cyclophane). The photolysis of this ketone gave the same products that arose from the photolysis of CK-9, presenting the possibility that the decarbonylation products and a part of the aldehyde are formed as secondary products during irradiation. The magnetic field dependence of the lifetimes of 3BR-n (n = 12 and 13) generated from CK-12 and 13, respectively, was measured in methanol by means of a pulsed-laser excitation technique. The rate constants for intersystem crossing showed a maximum at a relatively low field strength, which decreased with increasing the field strength to level off to an asymptotic value at > 1.5 kG. The results show that the role of hyperfine coupling in intersystem crossing is less important in these systems, presumably because of the presence of a carbonyl oxygen and the absence of a hydrogen atom at the radical center.

The atropisomeric and conformational properties of 1,4-benzodiazepin-2-ones were investigated by freezing the conformation with a methyl group at the C9 of 1,4-benzodiazepine. It was revealed that 1,4-benzodiazepin-2-ones exist only as a pair of enantiomers [(a1 R, a2 S) and (a1 S, a2 R)], which was confirmed by X-ray analysis. The absolute configuration of each atropisomer was deduced by comparing the [α]D and CD data with those of (–)-N-methoxycarbonylmethylated 9-methyl-5-phenyl-1,4-benzodiaepin-2-one derivative. It was elucidated that the corresponding N-methylated derivative showed similar CD spectra, although the rotational direction of [α]D was opposite to that of others.

No abstract

Isomeric 6,6-diphenyl-6,7,8,9-tetrahydro-5H-benzocyclohepten-7-one and 7,7-diphenyl-6,7,8,9-tetrahydro-5H-benzocyclohepten-6-one (2) were photolyzed in methanol to give rise to unsaturated aldehydes as well as diphenyltetrahydronaphthalene, a decarbonylation product (for 2). Lifetimes of the intermediate triplet biradicals were dependent on their structures, indicating that cyclic conformations play an important role in the intersystem crossing of biradicals.