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Platinum Metals Rev., 1997, 41, (4), 165

Highly Active and Enantioselective Rhodium Catalyst

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The development of platinum group metals complexes with chiral ligands, and their potential uses have been discussed here recently, with enantioselective catalysis being singled out (1). Adding chiral bisphosphine ligands – BINAP being one of the best known – to catalysts has provided a range of enantioselective catalysts capable of a variety of asymmetric transformations.

Now, scientists at the Merck Research Laboratories in New Jersey have used the planar chiral bisphosphine ligand, [2.2]PHANEPHOS, (4,12-bis(diphenylphosphino)-[2.2]-paracyclophane) with the rhodium complex, bis(1,5-cyclooctadiene)rhodium(I) triflate to produce an active and highly enantioselective catalyst, [[2.2]-PHANEPHOS Rh]+OTf which can hydrogenate dehydroamino acid methyl esters under very mild conditions (2). Formation of the catalyst before substrate addition enabled complete conversions to be achieved in under 60 minutes, by bubbling hydrogen through the reaction mixture, at temperatures as low as -45°C.

Further activity of this catalyst was demonstrated by its reduction of tetrahydropyrazine to produce the HIV protease inhibitor Crixivan intermediate precursor, at -40°C and 1.5 bar in 6 hours with 100 per cent conversion and 86 per cent ee. Until now this reduction had incomplete conversions, with only moderate enanrio-selectivity, and thus required forcing conditions.

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References

  1. 1
    A. von Zelewsky, Platinum Metals Rev ., 1996, 40, ( 3 ), 102
  2. 2
    P. J. Pye,, K. Rossen,, R. A. Reamer,, N. N. Tsou,, R. P. Volante and P. J. Reider, J. Am. Chem. Soc ., 1997, 119, ( 26 ), 6207

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