Mechanistic Features of Isomerizing Alkoxycarbonylation of Methyl Oleate

Abstract

The weakly coordinated triflate complex [(P^P)Pd(OTf)](+)(OTf)(-) (1) (P^P = 1,3-bis(di-tert-butylphosphino)propane) is a suitable reactive precursor for mechanistic studies of the isomerizing alkoxcarbonylation of methyl oleate. Addition of CH(3)OH or CD(3)OD to 1 forms the hydride species [(P^P)PdH(CH(3)OH)](+)(OTf)(-) (2-CH(3)OH) or the deuteride [(P^P)PdD(CD(3)OD)](+)(OTf)(-) (2(D)-CD(3)OD), respectively. Further reaction with pyridine cleanly affords the stable and isolable hydride [(P^P)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(3)OD 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 °C yields a 50:50 mixture of the linear alkyls [(P^P)Pd(13)CH(2)(CH(2))(6)CH(3)](+) and [(P^P)PdCH(2)(CH(2))(6)(13)CH(3)](+) (4a and 4b). Further reaction with (13)CO yields the linear acyls [(P^P)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 °C also resulted in fast isomerization to form a linear alkyl species [(P^P)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 [(P^P)PdCH{(CH(2))(15)CH(3)}C(═O)OCH(3)](+) (7). Addition of carbon monoxide (2.5 equiv) at -80 °C resulted in insertion to form the linear acyl carbonyl [(P^P)PdC(═O)(CH(2))(17)C(═O)OCH(3)(CO)](+) (8-CO) and the five-membered chelate [(P^P)PdC(═O)CH{(CH(2))(15)CH(3)}C(═O)OCH(3)](+) (9). Exposure of 8-CO and 9 to (13)CO at -50 °C 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 °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 ΔG(‡) = 29.7 kcal mol(-1) for the linear (8) vs ΔG(‡) = 37.7 kcal mol(-1) for the branched species (9).

Promotion of Selective Pathways in Isomerizing Functionalization of Plant Oils by Rigid Framework Substituents

“…The methanolysis of the Pd-acyl species is rate limiting for both systems. As found previously for dtbpx, [9] the methanolysis reaction pathway via a cluster of three MeOH molecules is favored for the linear Pd-acyl species ( Figure 5, a and c) while for the case of the more hindered branched Pdacyl species the reaction pathway with a single MeOH molecule is lower in energy ( Figure 5, b and d).…”

“…This high kinetic preference for the desired linear product is a result of the high steric bulk of the phosphines' tert-butyl substituents (as reflected by an opening angle at the metal center left open by the diphosphine of only 1728), [15] which enhances methanolysis and provides an increased difference in barrier (DDG 6 ) between methanolysis of the linear and branched acyl. [9] With this picture, a necessary improvement of selectivity to reduce formation of branched side products is difficult to envision as tert-butyl groups already represent an upper limit of steric congestion imposed upon the metal center. We now show this limitation can be overcome by rigid framework adamantyl substituents that do not provide a further congestion per se at the metal center, but interact specifically and at a rather remote position to the metal center in the transition states of methanolysis.…”

Rhodium-Biphephos-Catalyzed Tandem Isomerization–Hydroformylation of Oleonitrile

“…Coupling isomerization and hydroformylation reactions in the same process refers to so-called tandem reactions. Several challenges are met in this tandem process: (i) internal alkenes are thermodynamically more stable than terminal ones, which are therefore (dynamically) formed in minutes amounts [5,6]; (ii) hydroformylation and isomerization are competitive reactions, and hence various (undesired) branched aldehydes can be obtained; and (iii) rather facile competitive hydrogenation of all kinds of C=C bonds often plagues the chemoselectivity of the reaction. Rhodium-catalyzed tandem isomerization-hydroformylation of middle-chain alkenes (<10 C) has been widely studied over the last two decades, generally with bulky ligands [7,8].…”

Three step auto-tandem catalysed hydroesterification: Access to linear fruity esters from piperylene

“…This is also the initial step when using internal olefins as a starting compound. [56,57] This is followed by the actual hydroesterification of the terminal pentene to give linear hexanoic acid methyl ester 4 (step C). However, steps B and C have to be fast enough to be rather favoured than the side reaction B', which is the complete hydrogenation to pentane.…”