(656g) Quantum Chemical Characterization of Catalytic Ester Decarbonylation: Olefins from Biomass

Quantum
Chemical Characterization of Catalytic Ester Decarbonylation: Olefins from
Biomass

Synthetic
transformations of carboxylic acids into olefins are of paramount importance
since bio-mass derived carboxylic acids could then be a renewable source for polyolefin
syntheses. An important goal, then, is to develop catalytic transformations that
sustainably transform these structurally diverse acid educts into olefin
products with high selectivity. Production of olefins via
transition-metal-catalyzed decarbonylation of bio-derived carboxylic acids is
one approach that has been explored and demonstrated to be possible with low
toxicity, and tolerant of diversity.^1-2.
As a subclass, linear α-olefins (LAOs) are important commodity chemicals that
find utility in a wide range of industrial applications. LAOs are usually
generated from petroleum, which is a nonrenewable and depleting resource. A
green and sustainable approach would be to produce LAOs by decarbonylative
dehydration of fatty acids, which are inexpensive and renewable.

Direct
routes for the conversion of carboxylic acids to olefins typically proceed with
high activation energies. However, carboxylic acid derivatives such as esters
are more reactive and can serve as key precursors in carboxylic acid
transformations, and in particular transition-metal-based catalysts have been
shown capable of transforming them to high-value olefins. However, controlling selectivity
for terminal alkene formation is a major challenge. A highly selective,
palladium-catalyzed decarbonylation of p-nitrophenyl esters of fatty
acids to their corresponding LAOs has been very recently developed.³
A cooperative ligand system including phosphines and N-heterocyclic carbenes is
reported to exhibit enhanced selectivity to terminal olefins. Herein, we
investigate the decarbonylation mechanism of p-nitrophenylbutyrate to
rationalize the switching reactivity and selectivity trends based on either
individual or cooperative roles of phosphine and heterocyclic carbene ligands.
The primary goal of the project is to elucidate the origin of terminal alkene A
selectivity vs the thermodynamically more stable internal alkene B which
can be generated as an undesirable alternative product (Scheme 1).

Scheme 1. Pd-catalyzed decarbonylation of p-nitrophenylester
to α-olefins. XantPhos and IPr ligands comprise the dual ligand system.

To
gain mechanistic insight at the atomic level of detail, we performed density
functional theory (DFT) modeling. We focused on the Pd-catalyzed decarbonylation
of p-nitrophenylbutyrate in order to fully characterize reaction
coordinates for the formation of terminal A and internal B alkene
products and to explain the roles of the ligands in promoting
α-selectivity. To assess selectivity, we have thoroughly compared the
competing reaction mechanisms using three ligand systems: IPr, XantPhos, and a
mixture of IPr and XantPhos. Calculations suggest that the bulkiness of IPr
combined with the bidentate nature of XantPhos prompt a rapid decoordination of
the terminal alkene, thus precluding the alkene isomerization pathway.

References

1. Rodriguez, N.; Goossen, L. J., Decarboxylative coupling reactions: a modern strategy for C-C-bond formation. Chemical Society Reviews 2011, 40 (10), 5030-5048.
2. Le Nôtre, J.; Scott, E. L.; Franssen, M. C. R.; Sanders, J. P. M., Selective preparation of terminal alkenes from aliphatic carboxylic acids by a palladium-catalyzed decarbonylation–elimination reaction. Tetrahedron Letters 2010, 51 (29), 3712-3715.
3. Alex John, Levi T. Hogan, Marc A. Hillmyer and William B. Tolman, Olefins from biomass feedstocks: catalytic ester decarbonylation and tandem Heck-type coupling, Chem. Commun., 2015, 51, 2731.