Science of Synthesis

Gandon, V.; Thorimbert, S.; Malacria, M., Science of Synthesis, (2009) 46, 230.

Wilkinson’s catalyst in acetonitrile (Method A, ‌Scheme 71‌) promotes the cyclization of 1,6-enynes ‌201‌ to give 3-methylenecyclohexenes ‌202‌ [X = CH2, CMe2, C(CO2Et)2] or 3-methylene-3,6-dihydro-2H-pyrans ‌202‌ (X = O), but substitution on the terminal positions of either the alkene or the alkyne units inhibits ring closure.[‌192‌] An alternative and seemingly more effective catalyst for such reactions is obtained from a chloro(cyclooctadiene)rhodium(I) di­mer and 4 equivalents of tris(4-fluorophenyl)phosphine in dimethylformamide at 85°C (Method B; ‌Scheme 71‌).[‌193‌] Deuterium-labeled precursors have been synthesized in order to investigate the mechanisms of such reactions and it is now established that a (vinyli­dene)rhodium complex is formed first, and this then undergoes a [2 + 2] cycloaddition with the alkene unit, followed by the loss of a β-hydrogen atom to deliver the observed adducts, after reductive elimination.

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References