posted on 2014-12-03, 00:00authored byPhilipp Roesle, Lucia Caporaso, Manuel Schnitte, Verena Goldbach, Luigi Cavallo, Stefan Mecking
Theoretical
studies on the overall catalytic cycle of isomerizing
alkoxycarbonylation reveal the steric congestion around the diphosphine
coordinated Pd-center as decisive for selectivity and productivity.
The energy profile of isomerization is flat with diphosphines of variable
steric bulk, but the preference for the formation of the linear Pd-alkyl
species is more pronounced with sterically demanding diphosphines. CO insertion is feasible and reversible
for all Pd-alkyl species studied and only little affected by the diphosphine.
The overall rate-limiting step associated with the highest energetic
barrier is methanolysis of the Pd-acyl species. Considering methanolysis
of the linear Pd-acyl species, whose energetic barrier is lowest within
all the Pd-acyl species studied, the barrier is calculated to be lower
for more congesting diphosphines. Calculations indicate that energy
differences of methanolysis of the linear versus branched Pd-acyls
are more pronounced for more bulky diphosphines, due to involvement
of different numbers of methanol molecules in the transition state.
Experimental studies under pressure reactor conditions showed a faster
conversion of shorter chain olefin substrates, but virtually no effect
of the double bond position within the substrate. Compared to higher
olefins, ethylene carbonylation under identical conditions is much
faster, likely due not just to the occurrence of reactive linear acyls
exclusively but also to an intrinsically favorable insertion reactivity
of the olefin. The alcoholysis reaction is slowed down for higher
alcohols, evidenced by pressure reactor and NMR studies. Multiple
unsaturated fatty acids were observed to form a terminal Pd-allyl
species upon reaction with the catalytically active Pd-hydride species.
This process and further carbonylation are slow compared to isomerizing
methoxycarbonylation of monounsaturated fatty acids, but selective.