P−C and C−C Bond Formation by Michael Addition in Platinum-Catalyzed Hydrophosphination and in the Stoichiometric Reactions of Platinum Phosphido Complexes with Activated Alkenes

We recently proposed a new mechanism for platinum-catalyzed hydrophosphination of activated alkenes,
in which nucleophilic attack of a phosphido ligand in the intermediate hydride complex Pt(diphos)(PR2)(H) (1) on the alkene H2CCH(X) (X = CN or CO2R) gave the zwitterion Pt(diphos)(H)(PR2CH2CHX)
(2), containing a cationic Pt center and a phosphine ligand with a pendent stabilized carbanion. Subsequent
C−H bond formation involving the Pt−H and the carbanion would yield the product R2PCH2CH2X (3)
and regenerate the catalyst, while attack of the carbanion on another alkene would yield byproducts
derived from more than one alkene, such as R2P(CH2CH(X))nCH2CH2X (7). Several tests of this mechanism
and related pathways for product and byproduct formation were investigated. Attempts to trap the proposed
carbanion with another electrophile led to the development of a Pt-catalyzed three-component coupling
of secondary phosphines, tert-butyl acrylate, and benzaldehyde, yielding the functionalized phosphines
R2PCH2CH(CO2t-Bu)(CHPh(OH)) (R2P = Ph2P (10a); R2P = Me(Is)P (10b, Is = 2,4,6-(i-Pr)3C6H2)).
Reactions of the complexes Pt(diphos)(R‘)(PR2) (diphos = (R,R)-Me-Duphos, R‘ = Me, PR2 = PPh2
(11), PPh(i-Bu) (12); R‘ = Ph, PR2 = PMeIs (13); diphos = dppe, R‘ = Me, PR2 = PPh2 (14), PPh(i-Bu)
(15)), models for 1, with tert-butyl acrylate or acrylonitrile gave mixtures of products including Pt(diphos)(R‘)(CH(X)CH2PR2) (A, X = CO2t-Bu or CN), Pt(diphos)(R‘)(CH(X)CH2CH(X)CH2PR2) (B),
R2PCH2CH2X (3), R2P(CH2CH(X))n(CH2CH2X) (7), and, in some cases, the dinuclear phosphido-bridged
cations [(Pt(diphos)(Me))2(μ-PR2)]+ (17). When tert-butanol or water was added to these reactions, more
of the phosphines 3 and 7, and less of the intermediates A and B, were formed. Decomposition of A and
B gave unidentified platinum dialkyls (C), tentatively formulated as Pt(diphos)(R‘)(CH(X)R‘ ‘). The complex
Pt(dppe)(Me)(CH(Me)CO2t-Bu) (21), a model for A, B, and C, was generated either from Pt(dppe)(Cl)(CH(Me)CO2t-Bu) (20) and ZnMe2 or from Pt(dppe)(Me)(Cl) (19) and ZnBr(CH(Me)CO2t-Bu)·THF;
complexes 20 and 21 did not react with tert-butyl acrylate. These observations are consistent with the
proposed nucleophilic mechanism for P−C and C−C bond formation.