Angewandte
Chemie
sponding ketone, and only a small amount of fragmentation
products was formed by secondary photolysis of this
ketone.[13] The high yield and good selectivity observed for
b-O-4 cleavage of vanadium-catalyzed reaction indicates a
drastic reactivity change of the ketyl radical by coordination
to vanadium.
reactivity of this transformation combined with good selec-
tivity for a highly functionalized aryl enone demonstrates a
great potential of lignin as a chemical feedstock. Further
mechanistic studies and applications to biomass degradation
are underway.
Formation of the insoluble VIV complex 12 and only a
slightly lower turnover number under anaerobic conditions
suggests that regeneration of a VV complex by the aryloxy
radical competes with formation of an insoluble VIV aggre-
gate. Molecular oxygen probably accelerates the reaction by
increasing the effective concentration of catalytically active
species by converting insoluble 12 to soluble VV species.
Although the vanadium-catalyzed non-oxidative degra-
dation of 1 proceeds with high efficiency and selectivity in
CH3CN, it is desirable to utilize solvents without nitrogen
atoms for application to biofuel synthesis to prevent forma-
tion of NOx. When the reaction was performed in EtOAc on
1 mmol scale, the desired products were obtained with higher
yields and selectivity than reactions run in CH3CN
(Scheme 5).[14] Moreover, a more complex trimeric model
Received: March 3, 2010
Published online: April 15, 2010
À
Keywords: aryl enones · C O cleavage · lignin ·
non-oxidative cleavage · vanadium
.
[1] For recent reviews, see: a) A. Corma, S. Iborra, A. Velty, Chem.
[2] a) F. G. Sales, L. C. A. Maranhao, N. M. L. Filho, C. A. M.
Abreu, Chem. Eng. Sci. 2007, 62, 5386; b) S. Kawai, Y.
363; c) C. Crestini, M. C. Caponi, D. S. Argyropoulos, R. Salad-
compound 18[2f,15] underwent clean C O cleavage to provide
À
three monomeric compounds, demonstrating the possibility
of application of this reaction to more complex systems
(Scheme 6).[16]
[3] G. Brunow, K. Lundquist, G. Gellersted in Analytical Methods in
Wood Chemistry, Pulping, and Paper-making, Vol. 1 (Eds.: E.
Sjꢁstrꢁm, R. Alꢂn), Springer, Berlin, 1999, p. 77.
[4] Typical experimental procedure: A 4 mL vial charged with 1
(17 mg, 0.05 mmol), catalyst (10 mol%), and CD3CN (0.5 mL)
was closed with a screw cap and heated in an oil bath at 808C for
24 h. The reaction mixture was cooled to room temperature, and
internal standard (2-timethylsilylethanol, 7.2 mL, 0.05 mmol)
Scheme 5. Conversion of 1 on 1 mmol scale in a solvent without
nitrogen atoms.
1
was added before H NMR analysis. ACS grade solvents were
used as received, and no special precaution was taken to exclude
trace amounts of water from the reaction mixture.
[5] For recent examples of vanadium-catalyzed aerobic alcohol
c) V. D. Pawar, S. Bettigeri, S. S. Weng, J. Q. Kao, C. T. Chen, J.
S. Matsumura, T. Nishimura, T. Kawamura, S. Uemura, J. Org.
[6] For X-ray structure data of these compounds, see Supporting
Information.
[7] As the reaction progresses, the dark purple reaction mixture
with catalyst 9 turns clear, and grey precipitate is formed. The
reaction mixture with catalyst 11 stays as a dark purple solution.
X-ray crystal structure data shows that 9 has a dimer unit cell
unlike 11 with a monomeric unit cell in solid state.
Scheme 6. Conversion of trimeric lignin model compound 18.
In conclusion, we have demonstrated that changes in the
ancilliary ligands can divert the reactivity of vanadium–oxo
complexes from the typical alcohol oxidation to an unprece-
[8] 12 was prepared following a procedure for similar VIV com-
[9] For a recent report on the evidence of reduction of VV to VIII
under anaerobic conditions, see: S. K. Hanson, R. T. Baker, J. C.
Gordon, B. L. Scott, A. D. Sutton, D. L. Thorn, J. Am. Chem.
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dented C O bond cleavage reaction. This novel reactivity has
À
been applied to a vanadium-catalyzed non-oxidative C O
bond cleavage reactions of dimeric lignin model compounds
that produces aryl enones as unprecedented degradation
products. This transformation is proposed to proceed through
a ketyl radical generated by hydrogen atom transfer to a
vanadium(V) complex. Oxygen is not essential for the
reaction, although it increases the reaction rate. The novel
[10] A concerted mechanism is proposed based on the lower reaction
rate of threo-1. The rationalization of this rate difference is based
on the required antiperiplanar alignment of hydrogen and
aryloxy group for a concerted transition state as shown below.
Angew. Chem. Int. Ed. 2010, 49, 3791 –3794
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3793