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Sterically protected bis(phosphinidene)cyclobutenes were prepared starting from the diphosphane and their isomerism involving a 1,6-diphosphahexa-1,2,4,5-tetraene intermediate, which is both cumulative and conjugated, was studied. X-Ray crystallographic study of the (E,Z)-phosphinidenecyclobutene shows that the molecule suffers from large steric congestion between intra- and inter-substituents.
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Yoshifuji et al. (1990) studied this question.
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