Mechanistic Features of Isomerizing Alkoxycarbonylation of Methyl Oleate

The weakly coordinated triflate complex [(P boolean AND P)Pd(OTf)](+)(OTf)(-) (1) (P boolean AND P = 1,3-bis(di-tert-butyl-phosphino)propane) is a suitable reactive precursor for mechanistic studies of the isomerizing alkoxcarbonylation of methyl oleate. Addition of CH3OH or CD3OD to 1 forms the hydride species [(P boolean AND P)PdH(CH3OH)](+)(OTf)(-) (2-CH3OH) or the deuteride [(P boolean AND P)PdD(CD3OD)](+)(OTf)(-) (2(D)-CD3OD), respectively. Further reaction with pyridine cleanly affords the stable and isolable hydride [(P boolean AND P)PdH-(pyridine)](+)(OTf)(-) (2-pyr). This complex yields the hydridefragment free of methanol by abstraction of pyridine with BF3 center dot OEt2, and thus provides an entry to mechanistic observations including intermediates reactive toward methanol. Exposure of methyl oleate (100 equiv) to 2(D)-CD3OD resulted in rapid isomerization to the thermodynamic isomer distribution, 94.3% of internal olefins, 5.5% of alpha,beta-unsaturated ester and <0.2% of terminal olefin. Reaction of 2-pyr/BF3 center dot OEt2 with a stoichiometric amount of 1-C-13-labeled 1-octene at -80 degrees C yields a 50:50 mixture of the linear alkyls [(P boolean AND P)(PdCH2)-C-13(CH2)(6)CH3](+) and [(P boolean AND P)PdCH2(CH2)(6)(CH3)-C-13](+) (4a and 4b). Further reaction with (CO)-C-13 yields the linear acyls [(P boolean AND P)(PdC)-C-13(=O)(CH2)-C-12/13(CH2)(6)(CH3)-C-12/13(L)](+) (5-L; L = solvent or (CO)-C-13). Reaction of 2-pyr/BF3 center dot OEt2 with a stoichiometric amount of methyl oleate at -80 degrees C also resulted in fast isomerization to form a linear alkyl species [(P boolean AND P)PdCH2(CH2)(16)C(=O)OCH3](+) (6) and a branched alkyl stabilized by coordination of the ester carbonyl group as a four membered chelate [(P boolean AND P)PdCH{(CH2)(15)CH3}C(=O)OCH3](+) (7). Addition of carbon monoxide (2.5 equiv) at -80 degrees C resulted in insertion to form the linear acyl carbonyl [(P boolean AND P)PdC(=O)(CH2)(17)C(=O)OCH3(CO)](+) (8-CO) and the five-membered chelate [(P boolean AND P)PdC(=O)CH{(CH2)(15)CH3}C(=O)OCH3](+) (9). Exposure of 8-CO and 9 to (CO)-C-13 at -50 degrees C results in gradual incorporation of the C-13 label. Reversibility of 7 + CO reversible arrow 9 is also evidenced by Delta G = -2.9 kcal mol(-1) and Delta G(double dagger) = 12.5 kcal mol(-1) from DFT studies. Addition of methanol at -80 degrees C results in methanolysis of 8-L (L = solvent) to form the linear diester, 1,19-dimethylnonadecandioate, whereas 9 does not react and no branched diester is observed. DFT yields a barrier for methanolysis of Delta G(double dagger) = 29.7 kcal mol(-1) for the linear (8) vs Delta G(double dagger) = 37.7 kcal mol(-1) for the branched species (9).