Intramolecular [2+2+2] Cycloaddition Reactions of Yne-ene-yne and Yne-yne-ene Enediynes Catalysed by RhI: Experimental and Theoretical Mechanistic Studies

N-Tosyl-linked open-chain yne-ene-yne enediynes 1 and 2 and yne-yne-ene enediynes 3 and 4 have been satisfactorily synthesised. The [2+2+2] cycloaddition process catalysed by the Wilkinson catalyst [RhCl-(PPh3)(3)] was tested with the above-mentioned substrates resulting in the production of high yields of the cyclo-adducts. Enediynes 1 and 2 gave standard [2+2+2] cycloaddition reactions whereas enediynes 3 and 4 suffered beta-hydride elimination followed by reductive elimination of the Wilkinson catalyst to give cycloadducts, which are isomers of those that would be obtained by standard [2+2+2] cycloaddition reactions. The different reactivities of these two types of enediyne have been rationalised by density functional theory calculations.