Platinum-Catalyzed Asymmetric Hydrophosphination

Recently, the chiral Pt(0) precatalyst Pt[(R, R)-Me-Duphos](tra s-stUbene) (11) has been used to prepare enantiomerically enriched chiral phosphines via hydrophosphination of acrylonitrile, butyl acrylate and related substrates. This chemistry is 

The reaction rate appears to be limited by the P-H oxidative addition because of tight binding of the olefin in the complexes Pt(Me-Duphos)(olefin). Increasing the reaction temperature to speed up this step, however, reduced the enantiomeric excess [13]. 

O-Bond metathesis of the Ln-alkyl with the phosphine gives a Ln-phosphido complex. (This initiation step was observed to be faster when the hydride derivative [Cp 2LnH]2 was used.) Since the reactions were zero-order in substrate, the next 

Metal-catalyzed hydrophosphination has been explored with only a few metals and with a limited array of substrates. Although these reactions usually proceed more quickly and with improved selectivity than their uncatalyzed counterparts, their potential for organic synthesis has not yet been exploited fuUy because of some drawbacks to the known reactions. The selectivity of Pt-catalyzed reactions is not sufficiently high in many cases, and only activated substrates can be used. Lanthanide-catalyzed reactions have been reported only for intramolecular cases and also suffer from the formation of by-products. Recent studies of the mechanisms of these reactions may lead to improved selectivity and rate profiles. Further work on asymmetric hydrophosphination can be expected, since it is unlikely that good stereocontrol can be obtained in radical or acid/base-catalyzed processes. 

As in the P(III) chemistry above, both late metal (Pd) and lanthanide catalysts have been used for P(V)-H additions to alkynes, alkenes, aldehydes, and imines. In addition, titanium, aluminum, and zinc catalysts have been employed. Typical P(V) substrates include dialkyl phosphites P(0R)2(0)H and phosphine oxides PR2(0)H. 

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Scheme 5-13 Platinum(Me-Duphos)-catalyzed asymmetric hydrophosphination...

The mechanistic study on the hydrophosphination of activated olefins, in conjunction with rapid inversion of the configuration at the phosphorus center, was elaborated to develop asymmetric hydrophosphination catalyzed by a chiral phosphine platinum complex although the % ee is not excitingly high yet (Scheme 9) [15]. 

Hydroxo-bridged dinuclear complexes, [Pt(P-P)(/u-OH)]2 (where P-P = DIOP, BINAP, NORPHOS, MeDUPHOS, and BPPM) as precursors have been used for the enantioselective Baeyer-Villiger oxidation of ketones (290). The platinum(O) catalyst precursor, Pt(MeDUPHOSXfrans-stylbene), catalyzes the asymmetric hydrophosphination of activated olefins (291). 

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