Platinum-Catalyzed Acrylonitrile Hydrophosphination. P−C Bond Formation via Olefin Insertion into a Pt−P Bond
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Abstract
The acrylonitrile complexes Pt(diphos)(CH2CHCN) (diphos = dppe (1), dcpe (2); dppe = Ph2PCH2CH2PPh2, dcpe = Cy2PCH2CH2PCy2, Cy = cyclo-C6H11) are catalyst precursors and, for some substrates, resting states, during addition of P−H bonds in primary and secondary phosphines across the CC double bond of acrylonitrile (hydrophosphination). Oxidative addition of P−H bonds to related catalyst precursors gives the phosphido hydride complexes Pt(diphos)(PRR')(H) (diphos = dppe, R = H, R' = Mes* (20), R = R' = Mes (21); diphos = dcpe, R = H, R' = Mes* (22); Mes = 2,4,6-Me3C6H2, Mes* = 2,4,6-(t-Bu)3C6H2). Acrylonitrile does not insert into the Pt−H bond of these hydrides to give cyanoethyl ligands; the putative products, the phosphido complexes Pt(diphos)(CH2CH2CN)(PRR') (diphos = dppe, R = H, R' = Mes* (9), R = R' = Mes (10); diphos = dcpe, R = H, R' = Mes* (11)) were prepared independently and found to be stable to P−C reductive elimination. Instead, catalysis appears to occur by selective insertion of acrylonitrile into the Pt−P bond to yield the alkyl hydrides Pt(diphos)[CH(CN)CH2PRR'](H), followed by C−H reductive elimination and regeneration of 1 or 2. This insertion was observed directly in model methyl phosphido complexes M(dppe)(Me)(PRR') (M = Pt, R = H, R' = Mes* (12), R = R' = Mes (13); M = Pd, R = H, R' = Mes* (17)), yielding M(dppe)[CH(CN)CH2PRR'](Me), (14, 15, 18). Similarly, treatment of Pt(dcpe)(PHMes*)(H) (22) with acrylonitrile gives Pt(dcpe)[CH(CN)CH2PHMes*](H) (24) as a mixture of diastereomers; the isomeric Pt(dcpe)[PMes*(CH2CH2CN)](H) (25), which was prepared independently, was also observed during this reaction. Both 24 and 25 decompose in the presence of acrylonitrile to form Pt(dcpe)(CH2CHCN) (2) and PHMes*(CH2CH2CN) (3a). The C−H reductive elimination step was modeled by studies of Pt(dcpe)[CH(Me)(CN)](H) (26). Another isomer, Pt(dcpe)[CH(Me)(CN)](PHMes*) (29), which formally results from insertion of acrylonitrile into the Pt−H bond of 22, was formed by decomposition of complex 2 during catalysis. Complex 29 is inactive in catalysis but decomposes to partially regenerate the active catalyst 2. The cyanoethyl compounds Pt(dcpe)(CH2CH2CN)(PHMes*) (11), trans-Pt(PPh3)2(CH2CH2CN)(Br), and PMes2(CH2CH2CN) (23) were structurally characterized by X-ray crystallography.
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