Mechanistic Features of Isomerizing Alkoxycarbonylation of Methyl Oleate

摘要

The weakly coordinated triflate complex [(PΛP)Pd(Otf)]~+(Otf)~- (1) (PΛP = 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 CH_3OH or CD_3OD to 1 forms the hydride species [(PΛP)PdH(CH_3OH)]~+(Otf)~- (2-CH_3OH) or the deuteride [(PΛP)PdD(CD_3OD)]~+(Otf)~- (2D-CD_3OD), respectively. Further reaction with pyridine cleanly affords the stable and isolable hydride [(PΛP)PdH-(pyridine)]~+(Otf)~- (2-pyr). This complex yields the hydride fragment free of methanol by abstraction of pyridine with BF_3-Oet_2, and thus provides an entry to mechanistic observations including intermediates reactive toward methanol. Exposure of methyl oleate (100 equiv) to 2~D-CD_3OD resulted in rapid isomerization to the thermodynamic isomer distribution, 94.3% of internal olefins, 5.5% of α,β-unsaturated ester and <0.2% of terminal olefin. Reaction of 2-pyr/BF_3-Oet_2 with a stoichiometric amount of 1-~(13)C-labeled 1-octene at -80 ℃ yields a 50:50 mixture of the linear alkyls [(PΛP)Pd~(13)CH_2(CH_2)_6CH_3]~+ and [(PΛP)PdCH_2(CH_2)_6~(13)CH_3]~+ (4a and 4b). Further reaction with ~(13)CO yields the linear acyls [(PΛP)Pd~(13)C(=O)~(12/13)CH_2(CH_2)_6~(12/13)CH_3(L)]~+ (5-L; L = solvent or ~(13)CO). Reaction of 2-pyr/ BF_3-Oet_2 with a stoichiometric amount of methyl oleate at -80 ℃ also resulted in fast isomerization to form a linear alkyl species [(PΛP)PdCH_2(CH_2)_(16)C(-O)OCH_3]+ (6) and a branched alkyl stabilized by coordination of the ester carbonyl group as a four membered chelate [(PΛP)PdCH{(CH_2)_(15)CH_3}C(=O)OCH_3]~+ (7). Addition of carbon monoxide (2.5 equiv) at -80 ℃ resulted in insertion to form the linear acyl carbonyl [(PΛP)PdC(=O)(CH_2)_(17)C(=O)OCH_3(CO)]~+ (8-CO) and the five-membered chelate [(PΛP)PdC(=O)CH{(CH_2)_(15)CH_3}C(=O)OCH_3]+ (9). Exposure of 8-CO and 9 to ~(13)CO at -50 ℃ results in gradual incorporation of the ~(13)C label. Reversibility of 7 + CO ? 9 is also evidenced by ΔG = -2.9 kcal mol~(-1) and ΔG~# = 12.5 kcal mol~(-1) from DFT studies. Addition of methanol at -80 ℃ 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 ΔG~# = 29.7 kcal mol~(-1) for the linear (8) vs ΔG~# = 37.7 kcal mol~(-1) for the branched species (9).