Concerted perpendicular attack

Based on the stereochemical data, the nearly complete retention of the stereochemistry for both isomers of compound 1 can be interpreted either by a concerted perpendicular attack of the nucleophile on the 7i orbital (Scheme 32.3) or by a stepwise addition-elimination mechanism (Scheme 32.5). 

The more widely accepted mechanism for oxo-transfer involves direct substrate attack at the oxo ligand with concerted or sequential C-0 bond formation. In 1985, Groves proposed a transition state geometry for epoxidation by porphyrin complexes involving a side-on, perpendicular approach of the olefin to the metal-oxygen bond [17]. This trajectory accounted for the enhanced reactivity of cis- over frans-alkenes (4a vs. 4b) in porphyrin and other metal oxo catalyst systems (Fig. 2). This model has also helped explain the observed enantioselec-tivities in AE reactions with successful chiral catalysts [18,19]. 

Among the possible concerted mechanisms for such systems, the in-plane Sn2-type substitution was discarded, but a perpendicular nucleophilic attack on the n orbital was seriously considered. On the other hand, among the stepwise alternatives, an elimination-addition via phenylacetylene and a vinylic substitution via addition-elimination, may be operative. 

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