ON THE MECHANISM OF THE HYDROCARBALKOXYLATION OF OLEFINS CATALYZED BY PALLADIUM COMPLEXES

The acyl complex PdCl(COR)(PPh3)2 (R = Et, n-Hex), isolated during the course of hydrocarbalkoxylation reactions catalyzed by the precursor system PdCl2(PPh3)2-PPh3 (95°C, P(CO) 100-120 atm; Pd: P = 1:3-4), in ethanol or higher alkanols as solvents, reacts with an alkanol R'OH in the presence of added PPh3 (Pd: P = 1:3) to yield the ester RCOOR′and a mixture of PdCl2(PPh2)2 and Pd(PPh3)3 or 4. Moreover, when it is employed as catalyst precursor (R = n-Hex; Pd: P = 1:4) in the hydrocarbalkoxylation of ethylene, it is recovered as its ethyl analog and it yields almost stoichiometric amounts of n-HexCOOR′. When the di carbomethoxy complex PdCl(COOMe)(PPh3)2, isolated in mixture with PdCl(COR)(PPh3)2 in hydrocarbomethoxylation experiments, is treated with 1-hexene in methanol (Pd: P:1-hexene : MeOH = 1:3:40:125), under nitrogen, in the absence of carbon monoxide, at 95°C, methyl heptanoate ester is not formed, and the starting complex is recovered almost quantitatively (92%). When PdCl2(PPh3)2 or PdCl(COOMe)(PPh3)2 are used as catalyst precursors for the carbonylation of 1-hexene in MeOH, in the absence of added PPh3 and in the presence of NEt3 or of carboxylic acid anions (both of them are known to favor the formation of the carbomethoxy complex), no catalytic activity is observed and the precursors are recovered as palladium(0)carbonylphosphine complexes, ultimately mixed with the carbomethoxy complex. The results suppport the view that, of the two commonly accepted mechanisms for the catalytic hydrocarbalkoxylation of olefins, involving MH or MCOOR′ addition to the olefinic double bond, only the first one, in which a key intermediate is a Pd-acyl species, is probably involved. When n-BuOH is isued as solvent the catalytic activity remains high even after 5-6 reuses of the catalyst, whereas in MeOH the activity falls significantly below its initial value because of decomposition of the catalyst into inactive palladium(0) complexes and palladium metal, probably via a carbomethoxypalladium complex. © 1990.