Comparative Study of Diastereoisomer Interconversion in Chiral BINOL-ate and Diamine Platinum Complexes of Conformationally Flexible NUPHOS Diphosphines
摘要:
A thorough and detailed study of diastereointerconversion in the chiral platinum complexes [(NUPHOS)Pt{(S)-BINOL}] (3a-e) has been undertaken and compared with the results of a similar study with [(BIPHEP)Pt{(S)-BINOL}]. Rate data revealed that this process obeys first-order relaxation kinetics, and rate constants for conversion of the minor to the major diastereoisomer have been obtained. Eyring analysis of the data gave DeltaH(double dagger) and DeltaS(double dagger) values of 22-25 kcal mol(-1) and -1 to -16 eu, respectively. In combination with computational analysis, these studies indicate that atropinversion most likely occurs via an on-metal pathway involving a planar seven-membered transition state. Substitution of (S)-BINOL for (S,S)-DPEN results in a marked reduction in the barrier to atropinversion; a DeltaH(double dagger) value of 17 kcal mol(-1) has been determined for the conversion of delta-[(Ph-4-NUPHOS)Pt{(S,S)-DPEN}]Cl-2 to lambda-[(Ph-4-NUPHOS)Pt{(S,S)-DPEN}]Cl-2, which could indicate that an alternative mechanism operates.
Highly Efficient Asymmetric Hetero-Diels−Alder Reactions of Carbonyl Compounds Catalyzed by Lewis Acid Platinum Complexes of Conformationally Flexible NUPHOS-Type Diphosphines
作者:Simon Doherty、Julian G. Knight、Christopher Hardacre、He−Kuan Lou、Colin R. Newman、Rakesh K. Rath、Sarah Campbell、Mark Nieuwenhuyzen
DOI:10.1021/om049392z
日期:2004.12.1
NUPHOS-type diphosphines have been resolved as their diastereopure platinum BINOLate complexes δ- and λ-[(NUPHOS)Pt(S)-BINOL}] and the corresponding enantiopure Lewis acids δ- and λ-[(NUPHOS)Pt(OTf)2], being generated by protonation with trifluoromethanesulfonic acid, act as highly efficient catalysts for the hetero-Diels−Alder reaction of nonactivated conjugated dienes with aryl glyoxals and glyoxylate
Lewis Acid Platinum Complexes of Conformationally Flexible NUPHOS Diphosphines: Highly Efficient Catalysts for the Carbonyl−Ene Reaction
作者:Simon Doherty、Peter Goodrich、Christopher Hardacre、He−Kuan Luo、Mark Nieuwenhuyzen、Rakesh K. Rath
DOI:10.1021/om0505716
日期:2005.11.1
Enantiopure Lewis acid complexes of conformationally flexible acyclic and monocyclic NUPHOS diphosphines, delta- and lambda-[(NUPHOS)Pt(OTf)(2)], are efficient catalysts for the carbonyl-ene reaction between various unsymmetrical 1,1'-disubstituted alkenes and phenylglyoxal or ethyl glyoxylate. While catalyst performance was substrate dependent, ee values as high as 95% and yields up to 90% have been obtained. In a number of cases catalysts generated from delta- and lambda-[(NUPHOS)Pt(S)-BINOL}] showed marked enhancements in enantioselectivity in ionic liquids compared with organic media. Although an enhancement in enantioselectivity was not obtained for all substrate combinations in such cases, the enantioselectivities were comparable to those obtained in dichloromethane. Furthermore, although the ee's are initially comparable in both the ionic liquid and dichloromethane, a gradual erosion of ee with time was found in the organic solvent, whereas the ee remained constant in the ionic liquid. Preliminary kinetic investigations suggest that the decrease in ee may be due to a faster racemization of the catalyst in dichloromethane compared with the ionic liquid.