Theoretical study on intramolecular allene-diene cycloadditions catalyzed by PtCl<inf>2</inf> and Au(i) complexes

The intramolecular [4C+3C] cycloaddition reaction of allenedienes catalysed by PtCl2 and several Au(i) complexes has been studied by means of DFT calculations. Overall, the reaction mechanism comprises three main steps: (i) the formation of a metal allyl cation intermediate, (ii) a [4C(4π)+3C(2π)] cycloaddition that produces a seven-membered ring and (iii) a 1,2-hydrogen migration process on these intermediates. The reaction proceeds with complete diastereochemical control resulting from a favoured exo-like cycloaddition. Allene substituents have a critical influence in the reaction outcome and mechanism. The experimental observation of [4C+2C] cycloadducts in the reaction of substrates lacking substituents at the allene terminus can be explained through a mechanism involving Pt(iv)-metallacycles. With gold catalysts it is also possible to obtain [4C+2C] cycloaddition products, but only with substrates featuring terminally disubstituted allenes, and employing π-acceptor ligands at gold. However the mechanism for the formation of these adducts is completely different to that proposed with PtCl2, and consists of the formation of a metal allyl cation, subsequent [4C+3C] cycloaddition and a 1,2-alkyl shift (ring contraction). Electronic analysis indicates that the divergent pathways are mainly controlled by the electronic properties of the gold heptacyclic species (L-Au-C 2), in particular, the backdonation capacity of the metal center to the unoccupied C2 (pπ-orbital) of the intermediate resulting from the [4C+3C] cycloaddition. The less backdonation, (i.e. using P(OR) 3Au+ complexes), the more favoured is the 1,2-alkyl shift. © 2011 The Royal Society of Chemistry.