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

• Published in: Coordination chemistry reviews Vol. 155; pp. 35 - 68
• Main Authors: Pregosin, Paul S., Salzmann, Renzo
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-11-1996
• Subjects: Allyl complex of Pd(II); Enantioselective allylic alkylation; Enantioselective allylic alkylation; Allyl complex of Pd(II)
• ISSN: 0010-8545; 1873-3840
• DOI: 10.1016/S0010-8545(96)90176-9

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
η
3-
η
1 isomerization can be under either electronic or steric control. The terminal allyl
13C chemical shifts in 1,3-diphenylallyl palladium(II) complexes of various chelate ligands are discussed.