Structure and dynamics of chiral allyl complexes of Pd(II): NMR spectroscopy and enantioselective allylic alkylation

The idea of NOE 'reporter ligands', be they simple nitrogen chelates or relatively complicated bidentate phosphine compounds, is explored. These NMR experiments lead to a better understanding of subtle (allyl hybridization, phenyl-phenyl stacking) and gross (three-dimensional aspects, shape of the chiral pocket) structural features in the chemistry of chiral palladium-allyl complexes. Combined with X-ray crystallography and MM2* calculations, one can learn how the BINAP, CHIRAPHOS and JOSIPHOS families of chiral ligands use steric effects to transfer chiral information. A relatively rigid chiral pocket seems a necessary, but not sufficient, condition, since an intrusive chiral pocket leads to syn-anti isomers. Two-dimensional exchange NMR has shown unexpected selectivity with respect to allyl isomerization. The eta(3)-eta(1) isomerization can be under either electronic or steric control. The terminal allyl C-13 chemical shifts in 1,3-diphenylallyl palladium(II) complexes of various chelate ligands are discussed.