Cycloaddition Reactions of Phenylallene. Ring Closure of the Diradical Intermediate Involving the Aromatic Ring

Article abstract of DOI:10.1021/jo00360a009

Phenylallene (PHA) reacts with 1,1-dichloro-2,2-difluoroethene (1122) to produce, in part, the two benzylidene-containing cycloadducts 5 and 6, with the E isomer 5 predominating as expected.In addition, two stereoisomeric 2:1 PHA-1122 adducts of structure 8 and 9 are formed by reaction of the PHA cyclodimers 10 with 1122.The reaction of PHA with diethyl fumarate (DEF) produces, in part, the two cycloadducts 15 and 16 in which the Z isomer 16 predominates.The major product 19 is formed by ring closure at an ortho position of the aromatic ring in the E diradical intermediate, followed by an ene reaction of the intermediate with DEF.The reaction of PHA with N-phenylmaleimide (NPMI) similarly produces cycloadducts 20 and 21 in which the Z isomer 21 predominates.The major product 23 is formed by ring closure at an ortho position of the aromatic ring followed by <1.5> hydrogen sigmatropic rearrangement and loss of hydrogen.The dominant formation of the E isomer 5 with 1122 and the Z isomers 16 and 21, and 19 and 23, in the reactions with DEF and NPMI indicate a highly reversible formation of the Z diradical intermediate 27.The cycloaddition of PHA with acrylonitrile produces essentially only a normal distribution of cycloadducts, indicating that cleavage of the Z diradical intermediate does not occur.