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

The axially chiral ligands 2-(diphenylphosphanyl)-2'-methoxy1,1'-binaphthalene (MOP; 6) and 2'-dimethylamino-2-(diphenylphosphanyl)-1,1'-binaphthalene (MAP; 7) coordinate to a cationic allylpalladium fragment in an unusual bidentate (P,C)-mode through the triarylphosphane and ipso-carbon atom (C1′). The readily prepared MAP and MOP complexes [Pd{P,C)-(L)}(η³-allyl)][OTf] (9 (L = 7) and 10 (L = 6)) have been characterised in solution (NMR), in which two diastereoisomeric rotamers are observed. The stereochemical identity of the rotamers is established by one- and two-dimensional NMR spectroscopy experiments. In both the solid state and in solution, the allyl unit is shown to coordinate in a slightly distorted η³-mode that results in a more alkene-like character at the allyl terminus trans to phosphane ligand. The opposite allyl terminus, which is trans to the ipsocarbon atom (C1′), is more strongly bound and the dominant allyl stereodynamic process involves C-C bond rotation in an η¹-allyl intermediate bound through this 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 with powerful stereochemical memory effects and consequently with low global ee values. The memory effect is suggested to arise through selective generation of diastereoisomeric [Pd{(P,C)-L}(η³-cyclopentenyl)]⁺ ions (L = MAP or MOP) and subsequent capture by nucleophile before ion-pair collapse or equilibration occurs.