Diastereoisomeric cationic pi-allylpalladium-(P,C)-MAP and MOP complexes and their relationship to stereochemical memory effects in allylic alkylation

The axially chiral ligands 2-(diphenylphosphanyl)-2'-methoxy-1,1'-binaphtha lene (MOP; 6) and 2'-dimethylamino-2-(diphenylphosphanyl)-1,1'-binaphthalen e (MAP; 7) coordinate to a cationic allylpalladium fragment in an unusual b identate (P,C)-mode through the triarylphosphane and ipso-carbon atom (C1') . The readily prepared MAP and MOP complexes [Pd{(P,C)-(L)}(eta (3)-allyl)] [OTf] (9 (L = 7) and 10 (L = 6)) have been characterised in solution (NMR), in which two diastereoisomeric rotamers are observed. The stereochemical i dentity of the rotamers is established by one- and two-dimensional NMR spec troscopy experiments. In both the solid state and in solution, the allyl un it is shown to coordinate in a slightly distorted eta (3)-mode that results in a more alkene-like character at the allyl terminus trans to phosphane l igand, The opposite allyl terminus, which is trans to the ipso-carbon atom (C1'), is more strongly bound and the dominant allyl stereodynamic process involves C-C bond rotation in an eta (1)-allyl intermediate bound through t his carbon. Palladium complexes of MAP and MOP are very efficient catalysts for allylic alkylation of racemic cyclopentenyl pivalate with [NaCH(CO2Me) (2)] in THF. Isotopic desymmetrisation revealed that the reaction occurs wi th powerful stereochemical memory effects and consequently with low global ee values. The memory effect is suggested to arise through selective genera tion of diastereoisomeric [Pd{(P,C)-L}(eta (3)-cyclopentenyl)](+) ions (L = MAP or MOP) and subsequent capture by nucleophile before ion-pair collapse or equilibration occurs.