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η4-Vinylketene and/or metallacyclopentenone complexes can be considered key intermediates in the ruthenium- and rhodium-catalyzed ring-opening dimerization of cyclobutenones to give 2-pyranones as well as in the rhodium-catalyzed coupling reactions of cyclobutenones with 2-norbornene (see scheme).
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Kondo et al. (2004) studied this question.
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