A series of d 6 transition-metal complexes of the type ML 5 (η 2 -CpH), where ML 5 = [Os II (NH 3 ) 5 ] 2+, [Ru II (NH 3 ) 5 ] 2+, and [Re I (PPh 3 )(PF 3 )(dien)] +, were synthesized as their triflate salts and combined with electrophiles (HOTf, CH 2 (OMe) 2 ) to form η 3 -allyl complexes. Treatment of these π-allyl complexes with the mild carbon nucleophile 1-methoxy-2-methyl-1-(trimethylsiloxy)propene (MMTP) followed by decomplexation affords substituted η 2 -cyclopentene derivatives with excellent regio- and stereocontrol. Deuteration and NOE studies for the π-allyl complexes along with stereochemical analysis of the organic products confirm that both electrophilic and nucleophilic addition occurs exclusively from the exo face of the ring (opposite to metal coordination) for all three systems.
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Spera et al. (1996) studied this question.