Hydrophosphination asymmetric

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

Similar catalytic reactions allowed stereocontrol at either of the olefin carbons (Scheme 5-13, Eqs. 2 and 3). As in related catalysis with achiral diphosphine ligands (Scheme 5-7), these reactions proceeded more quickly for smaller phosphine substrates. These processes are not yet synthetically useful, since the enantiomeric excesses (ee s) were low (0-27%) and selectivity for the illustrated phosphine products ranged from 60 to 100%. However, this work demonstrated that asymmetric hydrophosphination can produce non-racemic chiral phosphines [13]. 

Scheme 5-14 Stoichiometric reactions of Pt(Me-Duphos) complexes relevant to the proposed catalytic cycle for asymmetric hydrophosphination...

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 fully 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 sulfer 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. 

Asymmetric hydrophosphination has been utilized as a route for preparing chiral phosphines. The Pt° complex [(Me-DUPHOS)Pt(t/ tf/ ,s-PhCII ClIPh)] (73) brings about the catalytic P-H addition of bulky secondary phosphines to activated alkenes with modest enantioselectivity. The most promising substrate combinations for further development appear to be bulky alkenes and less bulky phosphines (Scheme 46).195... 

Kovacik, I., Wicht, D.K., Grewal, N.S., Glueck, D.S., Incarvito, C.D., Guzei, I.A., and Rheingold, A.L., Pt(Me-Duphos)-catalyzed asymmetric hydrophosphination of activated olefins enantioselective synthesis of chiral phosphines,... 

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]. 

The Duphos ligand (157) has been used to prepare organometallic complexes of palladium.385 Rhodium complexes of ligands (156), (157), or (159) are efficient catalysts for enantioselective hydrogenations337-341,343,345,386-388 and furthermore, in the case of platinum complexes with (157), as catalysts for the asymmetric hydrophosphination of activated olefins.71... 

Although no mechanism was proposed, the Pd(Me-DuPhos)-catalyzed asymmetric hydrophosphination of an alkyne with a phosphine-borane under kinetic resolution conditions (Scheme 6) presumably involves similar insertion and reductive elimination steps [14]. 

Scheme 33 Proposed mechanism for nickel-catalyzed asymmetric hydrophosphination via attack on a methacrylonitrile complex...

In 2007, Glueck s group reported a catalytic DKR process in which secondary phosphines were converted into the corresponding enantio-enriched tertiary phosphines through palladium-catalysed asymmetric hydrophosphination of aryliodides using secondary phosphines. The key intermediates were diaster-eomeric phosphide complexes with chiral ancillary ligands (L Pd PRR ). Their relative rates of P-inversion and phosphorus-carbon bond formation controlled the enantioselectivity of the prodnct formation. As shown in Scheme 2.63, the reaction allowed moderate enantioselectivities of up to 70% ee to be achieved. 

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). 

SCHEME 4.27 Use of a P-stereogenic PCP pincer ligand in asymmetric hydrophosphination reactions [20]. 

BartoU G, Bosco M, Carlone A, LocateUi M, Mazzanti A, Sambri L, Melchiorre P. Organocatalytic asymmetric hydrophosphination of lutroalkenes. Chem. Commun. 2007 722-724. 

Chiral Metal Complex-Promoted Asymmetric Hydrophosphinations... 

Asymmetric Hydrophosphination of Pyridylphosphines Access to Chiral P-N Ligands 158... 

Asymmetric Hydrophosphination Involving Other Substrate Systems and Development... 

Chiral metallacycles have been employed as auxiliaries for the promotion and control of asymmetric reactions such as Diels-Alder cycloadditions involving phospholes [19]. This synthetic methodology can be extended to the synthesis of diphosphine motifs via asymmetric hydrophosphination of vinylic and other tmsat-urated phosphine-functionalized substrates. The advantages offered by these metal complexes are listed below ... 

Scheme 4 Asymmetric hydrophosphination of diphenyl-1-propenylphosphine using PhEtPH...

The ability of the chiral auxiliary for simultaneous control of P and C chirality was also seen in the asymmetric hydrophosphination of phenyldi[(Z)-prop-l-enyl]phos-phine with high regio- and stereoselectivity under mild conditions (Scheme 6) [47]. 

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