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 C=C 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry