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Three's a charm, but four's not half bad either: A simple cobalt–diphosphine complex facilitates the synthesis of cyclobutene derivatives through the chemoselective transformation of strained five-membered unsaturated rings with internal alkynes (see scheme). This atom-efficient, intermolecular reaction generates polycyclic products in excellent yields and with excellent exo selectivity. dppp=1,3-bis(diphenylphosphanyl)propane.
Treutwein et al. (Wed,) studied this question.