Studies in the cycloproparene series: cycloaddition reactions of diarylmethylidenecycloproparenes.
Diarylmethylidenecyclopropanaphthalenes I [R= Ph, 4-MeOC6H4, 3-(CF3)C6H4] add diphenylisobenzofuran (DPIBF) and a-pyrone across the exocyclic double bond to give ring expanded products that result from subsequent relief of ring strain in the non-isolable spirocyclic intermediates. The benzene homologues II [R = Ph, 4-MeOC6H4] add DPIBF across the bridge bond to give the norcaradiene adducts. These observations match expectation based upon the loss of aromaticity in the arene moiety caused by addn. to the bridge bond. The cycloaddns have been studied also by ab initio quantum mech. calcns. at the MP2/6-31G(d)//HF/6-31G(d) and MP3 levels of theory. With acetylenic(phenyl)-iodonium triflates, formal [2+2] cycloaddn. to the exocyclic p bond of I leads to the 2,3-disubstituted naphthalenes III.TfO-.
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