Aerobic oxidation of lignin models using a base metal vanadium catalyst

Article abstract of DOI:10.1021/ic100528n

Dipicolinate vanadium(V) complexes oxidize lignin model complexes pinacol monomethyl ether (A), 2-phenoxyethanol (B), 1-phenyl-2-phenoxyethanol (C), and 1,2-diphenyl-2-methoxyethanol (D). With substrates having C-H bonds adjacent to the alcohol moiety (B-D), the C-H bond is broken in pyridine-d₅ solvent, yielding 2-phenoxyacetaldehyde from B, 2-phenoxyacetophenone from C, and benzoin methyl ether from D. In DMSO-d₆ solvent the reaction is slower, and both C-H and C-C bond cleavage products are observed for D. The vanadium(IV) products of these reactions have been identified and characterized. Catalytic oxidation of C and D has been demonstrated using air and (dipic)V(O)OⁱPr. For both substrates, the C-C bond between the alcohol and ether groups is broken in the catalytic oxidation. 1-Phenyl-2-phenoxyethanol is oxidized to a mixture of phenol, formic acid, benzoic acid, and 2-methoxyacetophenone. The products of oxidation of 1,2-diphenyl-2-methoxyethanol depend on the solvent; in DMSO benzaldehyde and methanol are the major products, while benzoic acid and methyl benzoate are the major products obtained in pyridine solvent. Phenyl substituents on the model complex facilitate the oxidation, with relative rates of oxidation D > C > B.

Full text of DOI:10.1021/ic100528n

Inorg. Chem. 2010, 49, 5611–5618 5611
DOI: 10.1021/ic100528n
Aerobic Oxidation of Lignin Models Using a Base Metal Vanadium Catalyst
Susan K. Hanson,† R. Tom Baker,*,†,§ John C. Gordon,*,† Brian L. Scott,‡ and David L. Thorn*,†
†Chemistry Division and ‡Materials Physics Applications Division, Los Alamos National Laboratory,
Los Alamos, New Mexico 87545. § Current address: Department of Chemistry, and Centre for
Catalysis Research and Innovation, University of Ottawa, Ottawa, ON Canada K1N6N5
Received March 19, 2010
Dipicolinate vanadium(V) complexes oxidize lignin model complexes pinacol monomethyl ether (A), 2-phenoxyetha-
nol (B), 1-phenyl-2-phenoxyethanol (C), and 1,2-diphenyl-2-methoxyethanol (D). With substrates having C-H bonds
adjacent to the alcohol moiety (B-D), the C-H bond is broken in pyridine-d5 solvent, yielding 2-phenoxyacetalde-
hyde from B, 2-phenoxyacetophenone from C, and benzoin methyl ether from D. In DMSO-d6 solvent the reaction is
slower, and both C-H and C-C bond cleavage products are observed for D. The vanadium(IV) products of these
reactions have been identified and characterized. Catalytic oxidation of C and D has been demonstrated using air and
(dipic)V(O)OiPr. For both substrates, the C-C bond between the alcohol and ether groups is broken in the catalytic
oxidation. 1-Phenyl-2-phenoxyethanol is oxidized to a mixture of phenol, formic acid, benzoic acid, and 2-methoxy-
acetophenone. The products of oxidation of 1,2-diphenyl-2-methoxyethanol depend on the solvent; in DMSO
benzaldehyde and methanol are the major products, while benzoic acid and methyl benzoate are the major products
obtained in pyridine solvent. Phenyl substituents on the model complex facilitate the oxidation, with relative rates of
oxidation D > C > B.
Introduction
5-(chloromethyl)furfural,6 and ethylene glycol.7 Less progress
has been made with selective transformations of lignin,8-11
which is typically treated in paper and forest industries by
kraft pulping or incineration.12,13 Currently, more than 40
million tons of lignin are produced annually worldwide,
roughly 95% of which is burned for energy.14 In coming
years, as technologies which employ fermentation or other
selective conversions of cellulose and hemicellulose are imple-
mented, production of lignin is anticipated to increase dra-
matically. The discovery of new methods to convert lignin
directly to value-added products would allow for more
efficient use of this natural resource.
Lignin is a randomized polymer containing methoxylated
phenoxy propanol units.15 A number of different linkages
occur naturally; one ofthe most prevalent isthe β-O-4 linkage
shown in Figure 1,15 containing a C-C bond with 1,2-
hydroxy ether substituents. We envisioned that a selective
oxidative cleavage ofthis carbon-carbon bond could beused
to break lignin into smaller, more utilizable components, for
example, aromatic product streams that could feed into
existing industrial markets. Air (oxygen) would be the ideal
oxidant for this transformation.
The development of alternatives to petroleum-based fuels
and chemicals is becoming increasingly urgent because of
concerns over climate change, growing world energy de-
mand, and energy security issues. Biomass is the only renew-
able carbon feedstock available, and thus much recent effort
has focused on developing technologies that convert biomass
into chemicals and fuels.1,2 The majority of non food-derived
biomass is in the form of lignocellulose, which is often not
fully utilized because of difficulties associated with breaking
down both lignin and cellulose.3 Recently, a number of
methods have been reported to transform cellulose directly
into more valuable materials such as glucose,4 sorbitol,5
*To whom correspondence should be addressed. E-mail: rbaker@
uottawa.ca (R.T.B.), jgordon@lanl.gov (J.C.G.), dthorn@lanl.gov (D.L.T.).
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J. G. Angew. Chem., Int. Ed. 2008, 47, 8510–8513.
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Liu, S. Catal. Lett. 2008, 126, 106–111.
While the oxidative C-C bond cleavage of 1,2-diols is well
established for a variety of metals, including vanadium, iron,
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(10) Yan, N.; Zhao, C.; Dyson, P. J.; Wang, C.; Liu, L.; Kou, Y.
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r
2010 American Chemical Society
Published on Web 05/21/2010
pubs.acs.org/IC

5612 Inorganic Chemistry, Vol. 49, No. 12, 2010
Hanson et al.
Figure 1. β-O-4 linkage and lignin model complexes.
˚
Figure 2. X-ray Structures of 3 and 5 (thermal ellipsoids at 50% probability, H atoms omitted for clarity). Selected bond lengths (A) for 3: V1-O1 =
1.568(2), V1-O7 = 1.775(2), V1-O6 = 2.547(2), V1-O4 = 1.929(2), V1-O2 = 1.949(2), V1-N1 = 2.060(3). For 5: V1-O1 = 1.576(2), V1-O6 =
1.784(2), V1-O7 = 2.438(2), V1-O2 = 1.925(2), V1-O4 = 1.931(2), V1-N1 = 2.053(2).
manganese, ruthenium, and polyoxometalate complexes,16-20
less is known about oxidative C-C bond cleavage of 1,2-
hydroxyethers.21 Permanganate and tribromide, both com-
mon reagents for C-C bond cleavage of vicinal diols, do not
react with 1,2-hydroxyethers to break the carbon-carbon
bond.22-25 Instead, oxidation of only the alcohol group
(yielding 1,2-carbonyl-ethers) is observed, and different me-
chanistic pathways have been proposed for the reactions of
vicinal diols and their monoethers.22-28 In contrast, cerium-
(IV) has been shown to break the C-C bond of 2-methoxy-
cyclohexanol to give adipaldehyde (1,6-hexane-dial) and
methanol.29,30 N-iodosuccinimide is also an effective reagent
for C-C bond cleavage of 1,2-hydroxy ethers (yielding
carbonyl compounds and acetals),31 but no catalytic version
of this reaction has previously been reported.32
The use of an earth-abundant (non-precious) metal cata-
lyst and air as an oxidant would represent a significant
advance in our ability to break carbon-carbon bonds in
1,2-hydroxyether compounds such as lignin. We now report
that dipicolinate vanadium complexes oxidize the lignin model
complexes pinacol monomethyl ether, 2-phenoxyethanol,
1-phenyl-2-phenoxyethanol, and 1,2-diphenyl-2-methoxy-
ethanol.33 Both C-H and C-C cleavage modes have been
observed in these substrates. Catalytic aerobic oxidative
C-C bond cleavage has been demonstrated in 1-phenyl-2-
phenoxyethanol and 1,2-diphenyl-2-methoxyethanol. To the
best of our knowledge, these are the first examples of catalytic
aerobic C-C bond cleavage of 1,2-hydroxyether com-
pounds.
(16) Riano, S.; Fernandez, D.; Fadini, L. Catal. Commun. 2008, 9, 1282–
1285.
(17) Barroso, S.; Blay, G.; Fernandez, I.; Pedro, J. R.; Ruiz-Garcia, R.;
Pardo, E.; Lloret, F.; Munoz, M. C. J. Mol. Catal. A. 2006, 243, 214–220.
(18) Takezawa, E.; Sakaguchi, S.; Ishii, Y. Org. Lett. 1999, 1, 713–715.
(19) Khenkin, A. M.; Neumann, R. J. Am. Chem. Soc. 2008, 130, 14474–
14476.
(20) Felthouse, T. R. J. Am. Chem. Soc. 1987, 109, 7566–7568.
(21) Baciocchi, E.; Bietti, M.; Putignani, L.; Steenken, S. J. Am. Chem.
Soc. 1996, 118, 5952–5960.
(22) Bhatia, I.; Banerji, K. K. J. Chem. Soc. Perkin Trans. II 1983, 1577–1580.
(23) Goswami, G.; Kothari, S.; Banerji, K. K. Proc. Indian Acad.
Sci.(Chem. Sci.) 2001, 113, 43–54.
(24) Gosain, J.; Sharma, P. K. Proc. Indian Acad. Sci. (Chem. Sci.) 2003,
115, 135–145.
(25) Bhatt, M.; Sharma, P. K.; Banerji, K. K. Indian J. Chem. 2002, 41B,
826–831.
Results and Discussion
(26) For chromic acid, Pb(OAc)4, and Co(OAc)3, different rates of
oxidation of glycols and their mono-ethers have been used to support a
pathway for glycol C-C bond cleavage involving a chelating diolate complex.
However, the products of the glycol mono-ether oxidations were not
characterized. See: Trahanovsky, W. S.; Gilmore, J. R.; Heaton, P. C.
J. Org. Chem. 1973, 38, 760–763.
(27) Littler, J. S.; Waters, W. A. J. Chem. Soc. 1960, 2767–2772.
(28) Morimoto, T.; Hirano, M. J. Chem. Soc., Perkin Trans. 2 1982, 1087–
1090.
(29) Hintþz, H. L.; Johnson, D. C. J. Org. Chem. 1967, 32, 556–564.
(30) VO2 has been reported to react with pinacol monomethyl ether, but
the organic products of this reaction were not characterized: Jones, J. R.;
Waters, W. A.; Littler, J. S. J. Chem. Soc. 1961, 630–633.
Initial investigations aimed to gain insight into the stoi-
chiometric reactivity of the 1,2-hydroxyether lignin models
(31) McDonald, C. E.; Holcomb, H.; Leathers, T.; Ampadu-Nyarko, F.;
Frommer, J. Tetrahedron Lett. 1990, 31, 6283–6286.
(32) Electrolysis of 1,2-hydroxyethers and 1,2-diethers has been reported
to give C-C bond cleavage: Shono, T.; Matsumura, Y.; Hashimoto, T.;
Hibino, K.; Hamaguchi, H.; Aoki, T. J. Am. Chem. Soc. 1975, 97, 2546–2548.
(33) A non-oxidative selective C-O bond cleavage of lignin model com-
plexes catalyzed by vanadium was very recently reported. See: Son, S.; Toste,
D. Angew. Chem., Int. Ed. 2010, 49, 3791-3794.

Article
Inorganic Chemistry, Vol. 49, No. 12, 2010 5613
with (dipic)VV. Reaction of (dipic)VV(O)OiPr (1a) or
(dipic)VV(O)OEt (1b) with pinacol monomethyl ether (A),
2-phenoxyethanol (B), 1-phenyl-2-phenoxyethanol (C), or
1,2-diphenyl-2-methoxyethanol (D)34 in acetonitrile sol-
vent yielded new vanadium(V) complexes where the alcohol-
ether ligand was bound in a chelating fashion. Complexes
(dipic)VV(O)(pinOMe) (2), (dipic)VV(O)(OPE) (3), (dipic)VV-
(O)(OPPE) (4) and (dipic)VV(O)(DPME) (5) (pinOMe = 2,3-
dimethyl-3-methoxy-2-butanoxide; OPE = 2-phenoxyethox-
ide; OPPE = 1-phenyl-2-phenoxyethoxide; DPME = 1,2-di-
phenyl-2-methoxyethoxide), were isolated in good yields
(39-88%) and characterized by NMR and IR spectroscopy,
elemental analysis, and X-ray crystallography (for 2, 3, and 5).
The X-ray structures of 2 (Supporting Information, Figure
S1), 3 (Figure 2), and 5 (Figure 2), display similar vanadium
and 1-phenyl-2-phenoxyethanol (eq 2). 2-Phenoxyacetophe-
none was prepared independently according to a published
procedure37 and spiked into the reaction mixture, con-
firming the identity of this product. Complex 5 also under-
went C-H bond cleavage when dissolved in pyr-d5 at room
temperature, reacting completely within 10 min to form 6 and
a 1:1 ratio of benzoin methyl ether (2-methoxy-2-phenylaceto-
phenone) and 1,2-diphenyl-2-methoxyethanol (eq 3)
˚
oxo bond distances of 1.573(2), 1.568(2), and 1.576(2) A,
respectively. In each case the substrate binds in a chelating
manner, similar to the previously reported vanadium(V)
pinacolate complex (dipic)V(O)(pinOH).35
However, in the absence of pyridine, conversion of 5 was
significantly slower, and in addition to C-H bond cleavage,
C-C bond cleavage was observed even for compounds
having secondary C-H bonds on the coordinated alcohol
moiety. Heating a DMSO-d6 solution of complex 5 (30 min at
100 °C) resulted in a mixture of C-H and C-C bond
cleavage products (eq 4). The organic products of this
reaction consisted of benzoin methyl ether (30%), copro-
ducts methanol and benzaldehyde (66%), and 1,2-diphenyl-
2-methoxyethanol (100%). The green vanadium product of
this reaction was the new vanadium(IV) complex
(dipic)VIV(O)(DMSO)2 (7). Complex 7 showed a VdO
stretch in the IR spectrum at 948 cm-1 and was characterized
When pinacol monomethyl ether complex 2 was heated in
pyr-d5 solution (6 h at 100 °C, or 3 weeks at 25 °C), the
previously reported vanadium(IV) complex (dipic)VIV(O)-
(pyr)2 (6),35 acetone, 2-methoxypropene, and pinacol mono-
methyl ether were formed. Yields of the organic products
1
were determined by integration of the H NMR spectrum
against an internal standard ( p-xylene); acetone, 2-methoxy-
propene, and pinacol monomethyl ether were formed in
94%, 76%, and 92% of the theoretical maximum yields
based on the oxidizing equivalents of vanadium consumed,
suggesting the stoichiometry shown in eq 1.
by X-ray crystallography (Figure 3). The VdO bond length
35
˚
˚
in 7 (1.605(2) A) is similar to that observed in 6 (1.611(2) A).
Complex 7 could be independently prepared by the thermo-
lysis of the pinacolate complex (dipic)V(O)(Hpin)35 in
DMSO solvent and was isolated in 83% yield.
In contrast to compound 2, all the compounds having
secondary C-H bonds on the coordinated alcohol moiety
underwent oxidative C-H bond cleavage under stoichio-
metric conditions in pyridine solution. Heating 2-phenoxy-
ethoxide complex 3 in pyr-d5 solution (10 min at 100 °C)
afforded 6, 2-phenoxyacetaldehyde (78%), and 2-phenoxy-
ethanol (98%), suggesting the stoichiometry shown in eq 2.
The formation of 2-phenoxyacetaldehyde was confirmed by
comparison with an authentic sample prepared according to
a literature procedure.36
The mechanism of the C-C cleavage reaction depicted in
eq 4 is currently under investigation, but one possible path-
way involves initial reaction of 5 to form benzaldehyde and a
methoxybenzyl radical. Subsequent reaction of the methox-
ybenzyl radical with water and a second equivalent of 5
would form methanol, a second equivalent of benzaldehyde,
and release 1,2-diphenyl-2-methoxyethanol.38 Similar radical
pathways have been proposed for the cleavage reactions of
When dissolved in pyridine-d5 solution, 1-phenyl-2-phen-
oxyethoxide complex 4 reacted rapidly at room temperature
(10 min) to form 6 and a 1:1 ratio of 2-phenoxyacetophenone
(37) Bu, X.; Jing, H.; Wang, L.; Chang, T.; Jin, L.; Liang, Y. J. Mol. Cat.
A. 2006, 259, 121–124.
(34) Prepared as an 85:15 mixture of u (R,S þ S,R):l (S,S þ R,R)
diastereomers by the acid-catalyzed ring opening of trans-stilbene oxide in
CH3OH (see Experimental Section for details).
(35) Hanson, S. K.; Baker, R. T.; Gordon, J. C.; Scott, B. L.; Sutton,
A. D.; Thorn, D. L. J. Am. Chem. Soc. 2009, 131, 428–429.
(36) Speranza, G.; Mueller, B.; Orlandi, M.; Morelli, C. F.; Manitto, P.;
Schink, B. Helv. Chim. Acta 2003, 86, 2629–2636.
(38) Attempts to test this hypothesis by the addition of radical traps to the
reaction were inconclusive. Complex 5 reacted directly with BHT (2,6-di-
tert-butyl-4-methylphenol) and PPh3 to give products that were not further
characterized. When 9,10-dihydroanthracene was added to the reaction
mixture, only trace (<1%) anthracene was detected, with the reaction
course being otherwise unaffected.

5614 Inorganic Chemistry, Vol. 49, No. 12, 2010
Hanson et al.
and benzoin methyl ether (13%), with the remaining pro-
ducts consisting of benzoic acid and methyl benzoate (5% or
less each) (Table 1, entry 3). Nearly complete consumption
(94%) of the starting material was observed: a total of 19
turnovers (turnover = mol 1,2-diphenyl-2-methoxyethanol con-
sumed per mol V). Using a lower catalyst loading (2.5 mol %)
resulted in a greater turnover number (37), but required a
longer reaction time (40 h).
A different product distribution was observed when the
catalytic reaction was run in pyr-d5 solvent. 1,2-Diphenyl-2-
methoxyethanol was heated with 10 mol % 1a under air at
100 °C (Table 1, entry 4). After 6 days, the starting material
had been completely consumed, and the organic products
consisted of benzoic acid (85%), methyl benzoate (84%),
benzoin methyl ether (9%), benzaldehyde (9%), and metha-
nol (6%). Benzil and benzoin were also detected as minor
products (less than 5% each). Benzoin methyl ether is a likely
intermediate in the formation of benzoic acid and methyl
benzoate; this ketone was detected in the reaction mixture in
45% overall yield at 56% conversion.
Figure 3. X-ray structure of 7 (thermal ellipsoids at 50% probability, H
˚
atoms omitted for clarity). Selected bond lengths (A) for 7: V1-O1 =
1.605(2), V1-O2 = 2.020(2), V1-O3 = 2.062(2), V1-O4 = 2.020(2),
V1-O6 = 2.030(2), V1-N1 = 2.145(2).
Model compound 2-phenoxyethanol (substrate B), having
no backbone phenyl substituents, was oxidized only very
slowly by complex 1a (Table 1, entry 1). When a DMSO-d6
solution of 2-phenoxyethanol was heatedwith 10 mol %1a at
100 °C for 1 week under air, only approximately 20% of the
starting material was consumed (integration against the
internal standard). Phenol (18%), formic acid (6%), and
several other minor unidentified products were detected in
the reaction mixture at this time. Conducting the catalytic
reaction in pyr-d5 solvent also gave poor results; only 35%
conversion had occurred after 1 week at 100 °C.
monoprotected diols by CeIV and N-iodosuccinimide men-
tioned above.29,31
After observing stoichiometric oxidation of lignin model
compounds A-D, we tested the catalytic oxidation of 2-phen-
oxyethanol, 1-phenyl-2-phenoxyethanol, and 1,2-diphenyl-
2-methoxyethanol. Oxidations were carried out using 2.5-
10 mol % (dipic)VV(O)OiPr (1a) under an atmosphere of air
in pyridine-d5 or DMSO-d6 solvents at 100 °C. In general,
phenyl substituents on the carbon backbone facilitated the
oxidation, with 1,2-diphenyl-2-methoxyethanol being the
most reactive substrate, and 2-phenoxyethanol the least.
Control reactions under air with no vanadium catalyst in
DMSO-d6 or pyr-d5 solvents showed no reaction for 2-phen-
oxyethanol or 1-phenyl-2-phenoxyethanol and only trace
oxidation (<5%) of 1,2-diphenyl-2-methoxyethanol after 1
week at 100 °C.
For the oxidations of B-D in DMSO-d6, 51V NMR
spectra of the reaction mixtures acquired at the end of the
reaction time showed one resonance at -518 ppm, consistent
with the cis-dioxo anion [(dipic)V(O)2]- (8). The identity of
this species was confirmed by comparison with spectra
obtained from authentic samples of known complexes
[(dipic)V(O)2]HPyr35 and [(dipic)V(O)2]K39 in DMSO-d6.
The cis-dioxo anion could also be formed by oxidation of
the vanadium(IV) complex 7. When a solution of 7 was
heated in DMSO-d6 solution at 100 °C under air (18 h), the
green color faded, and 8 was detected by 1H and 51V NMR
spectroscopy.
A DMSO-d6 solution of 1-phenyl-2-phenoxyethanol
(substrate C) was heated under an atmosphere of air at
100 °C with 10 mol % 1a (Table 1, entry 2). After 1 week,
1
integration of the H NMR spectrum against the internal
standard (1,3,5-tri-tert-butylbenzene) revealed that 95% of
the starting material had been consumed. The products of
this reaction consisted of formic acid (46%), benzoic acid
(81%), phenol (77%), and 2-phenoxyacetophenone (9%)
(yields expressed as a percentage of the theoretical maximum
basedon theinitialamount of substrate). At 50% conversion,
2-phenoxyacetophenone was detected in 35% overall yield,
suggesting that the ketone is formed initially and further
oxidized to benzoic acid, phenol, and formic acid. The exact
reason for the low yield of formic acid is unclear at this time
and may be due to deeper oxidation to CO2, which is difficult
to detect under the reaction conditions (air). However, a
separate experiment demonstrated that (dipic)V(O)OiPr
(10 mol %) is an inefficient catalyst for the aerobic oxidation
of formic acid, with less than 5% formic acid reacting after 3
days at 100 °C under air in DMSO-d6 solvent.
Control experiments were conducted to investigate a
possible role of the DMSO-d6 solvent in the catalysis. Under
anaerobic conditions, a solution of the vanadium(IV) com-
plex (dipic)VIV(O)(pyr)2 in DMSO-d6 showed no turnovers
with 1,2-diphenyl-2-methoxyethanol after 1 week at 100 °C,
suggesting that the DMSO solvent was not acting as the
oxidant in the catalytic reaction. Reactions under anaerobic
conditions with (dipic)VIV(O)(DMSO)2 and 2-phenoxyetha-
nol or 1-phenyl-2-phenoxyethanol also showed no reaction
after heating in DMSO-d6 at 100 °C for 1 week. No
dimethylsulfide was detected in the reaction mixtures from
the catalytic oxidation of B-D by 13C NMR spectroscopy or
GC-MS, providing further support that the DMSO solvent is
not the terminal oxidant. However, some oxidation of the
DMSO solvent does take place under the catalytic condi-
tions, as dimethylsulfone-d6 was detected in the 13C NMR
spectra and GC-MS traces of the reaction mixtures from
oxidation of B-D. Oxidation of DMSO solvent has been
For 1,2-diphenyl-2-methoxyethanol (substrate D), the
products of the catalytic oxidation differed in DMSO-d6
and pyr-d5 solvents. Heating a solution of 1,2-diphenyl-2-
methoxyethanol and 5 mol % 1a in DMSO-d6 over 20 h at
100 °C under air gave benzaldehyde (73%), methanol (69%),
(39) Wieghardt, K. Inorg. Chem. 1978, 17, 57–64.

Article
Inorganic Chemistry, Vol. 49, No. 12, 2010 5615
Table 1. Catalytic Oxidation of Lignin Models B-De
a 10 mol % (dipic)V(O)OiPr, 7 days. b 10 mol % (dipic)V(O)OiPr, 7 days. c 5 mol % (dipic)V(O)OiPr, 20 h. d 10 mol % (dipic)V(O)OiPr, 6 days.
e Conditions: All runs were carried out at 100 °C.
reported previously in the aerobic oxidation of HMF
(5-hydroxymethyl-2-furaldehyde) catalyzed by supported
vanadyl-pyridine complexes.40
catalytic conditions. The precise mechanisms of the oxida-
tions are the subject of ongoing detailed experimental and
computational investigations.
Although we previously found evidence suggesting the
involvement of vanadium(III) in the catalytic oxidation of
pinacol, reduction of vanadium(IV) to vanadium(III) does
not occur with the 1,2-hydroxyether substrates. No reaction
was observed upon heating (dipic)VIV(O)(pyr)2 (6) under
anaerobic conditions with pinacol monomethyl ether, 1-phenyl-
2-phenoxyethanol, or 1,2-diphenyl-2-methoxyethanol in pyr-
d5 at 100 °C for up to a week, in contrast with the reaction
between 6 and pinacol in pyr-d5 at 100 °C, which formed
a vanadium(III) μ-oxo dimer within 48 h.35 Although the
mechanism for reduction to vanadium(III) is not well under-
stood at this time, these results suggest that the reduction may
be facilitated by two adjacent -OH functional groups.
Using the base-metal vanadium catalyst and air as the
oxidant is a completely new method for breaking carbon-
carbon bonds in 1,2-hydroxy ether compounds. This ap-
proach affords aromatic monomers from the lignin model
complexes under mild conditions. The results described here
suggest the potential utility of dipicolinate vanadium com-
plexes in the selective oxidative disassembly of lignin.
Furthermore, the homogeneous nature of the catalyst pro-
vides new opportunities for ligand design to optimize activity
and selectivity in these reactions. Future work will focus on
the design of more active catalysts and extension of this
catalytic oxidation reaction to more complex model systems
and lignins.
Conclusions
Experimental Section
Dipicolinate vanadium(V) complexes stoichiometrically
oxidize the lignin model complexes pinacol monomethyl
ether, 2-phenoxyethanol, 1-phenyl-2-phenoxyethanol, and
1,2-diphenyl-2-methoxyethanol. Both C-H and C-C bond
cleavage products are observed in these reactions, which
proceed by reduction of the metal center to form well-defined
vanadium(IV) complexes. Successful catalytic, aerobic oxi-
dative C-C bond cleavage of 1-phenyl-2-phenoxyethanol
and 1,2-diphenyl-2-methoxyethanol has also been demon-
strated. Phenyl substitution on the carbon backbone facil-
itates the C-C bond cleavage reaction, with unsubstituted
substrate 2-phenoxyethanol reacting only very slowly under
General Considerations. Unless specified otherwise, all reac-
tions were carried out under a dry argon atmosphere using
standard glovebox and Schlenk techniques. Pyridine-d5,
DMSO-d6, and CD3CN were purchased from Cambridge Iso-
tope Laboratories. Pyridine-d5 and CD3CN were dried over
CaH2. Anhydrous grade acetonitrile, pyridine, and diethyl ether
were obtained from Acros and used as received. 1-Phenyl-2-
phenoxyethanol was purchased from Princeton Biomolecular
1
Research (Monmouth Junction, NJ). H, 13C, and 51V NMR
spectra were obtained at room temperature on a Bruker AV400
MHz spectrometer, with chemical shifts (δ) referenced to the
residual solvent signal (1H and 13C) or referenced externally to
VOCl3 (0 ppm). UV-vis spectra were obtained on a Varian
CARY 500 Scan instrument and IR spectra were obtained on a
Varian 1000 FT-IR Scimitar Series instrument. GC-MS analysis
was obtained using a Hewlett-Packard 6890 GC system
(40) Navarro, O. C.; Canos, A. C.; Chornet, S. I. Top. Catal. 2009, 52,
304–314.

5616 Inorganic Chemistry, Vol. 49, No. 12, 2010
Hanson et al.
equipped with a Hewlett-Packard 5973 mass selective detector.
Elemental analyses were performed by Midwest Microlab in
Indianapolis, IN. (dipic)VV(O)OiPr (1a),41 (dipic)VV(O)OEt (1b),42
(dipic)VV(O)OCH3, 41 (dipic)VV(O)(Hpin),35 and [(dipic)VV-
(O)2]HPyr (6)35 were prepared according to published proce-
dures.
gave a second crop of crystals (0.060 g). Total yield: 0.280 g
(61%). H NMR (py-d5, 400 MHz) δ 8.52 (t, 1H, J = 7.6 Hz,
Py), 8.42 (d, 2H, J = 7.6 Hz, Py), 2.48 (s, 3H, pin-OCH3), 1.58 (s,
1
6H, C(CH3)2), 1.04 (s, 6H, C(CH3)2). IR (thin film): νCdO
=
1699 cm-1, νVdO = 993 cm-1. Anal. Calcd for C14H18NO7V: C,
46.29; H, 4.99; N, 3.86. Found: C, 46.23; H, 4.82; N, 3.72.
(dipic)V(O)(OPE) (3). (dipic)V(O)OEt (0.298 g, 0.923 mmol)
was dissolved in CH3CN (5 mL), and the solution filtered
through glass wool/Celite into a vial containing 2-phenoxyetha-
nol (0.331 g, 2.40 mmol). The reaction mixture was allowed to
stand at room temperature. After 1 h, diethyl ether (15 mL) was
added, and the yellow solution was cooled to -15 °C overnight,
yielding yellow crystals. The supernatant was decanted, the
crystals washed with diethyl ether (2 ꢀ 4 mL), and dried under
vacuum (yielding 0.2005 g). A second crop was isolated by
addition of another 8 mL of diethyl ether to the supernatant
and cooling to -15 °C (0.059 g). Total yield: 0.2596 g (76%). 1H
NMR (py-d5, 400 MHz) δ 8.38 (t, 1H, J = 7.6 Hz, Py), 8.31 (d,
2H, J = 7.6 Hz, Py), 7.31 (t, 2H, J = 8.4 Hz, OPh), 7.13 (d, 2H,
J = 8.4 Hz, OPh), 6.99 (t, 1H, J = 7.6 Hz, OPh), 6.52 (t (broad),
2H, J = 4.4 Hz, V-OCH2), 4.51 (t, 2H, J = 4.4 Hz, V-OCH2-
CH2-OPh). IR (thin film): νCdO = 1691 cm-1, νVdO = 991
cm-1. Anal. Calcd for C15H12NO7V: C, 48.80; H, 3.28; N, 3.79.
Found: C, 48.76; H, 3.21; N, 3.79.
Pinacol monomethyl ether (A) was prepared by a modified
version of the published procedure for 1-methoxy-2-decanol.43
2,3-Dimethyl-2,3-epoxybutane (tetramethyloxirane) (1.196 g,
11.96 mmol) was dissolved in CH3OH (10 mL). Less than one
drop of sulfuric acid was carefully added by pipet, resulting in
immediate warming of the solution. After 1 h, water (10 mL) was
added, and the mixture extracted with diethyl ether (2 ꢀ 50 mL).
The ether extracts were combined, dried over MgSO4, and
decanted. The ether was removed by rotovap, and the resulting
clear oil was subjected to vacuum for 20 min. The clear oil
pinacol monomethyl ether (1.495 g, 85%) contained about 0.5
1
equiv of CH3OH (as determined by H NMR) and was used
without further purification in the synthesis of (dipic)V(O)-
1
(pinOMe). H NMR (py-d5, 400 MHz) δ 5.58 (br, s, 1H, pin-
OH), 3.63 (s, 3 H, CH3OH), 3.27 (s, 3H, pin-OCH3), 1.42 (s,
6H, C(CH3)2), 1.29 (s, 6H, C(CH3)2). 13C{1H} NMR (py-d5,
100 MHz) δ 80.38 (s, 1C, C(CH3)2), 75.55 (s, 1C, C(CH3)2),
50.36 (s, 1C, CH3OH), 50.31 (s, 1C, pin-OCH3), 26.01 (s, 2C,
C(CH3)2), 20.08 (s, 2C, C(CH3)2).
(dipic)V(O)(OPPE) (4). (dipic)V(O)OCH3 (0.073 g, 0.247
mmol) and 1-phenyl-2-phenoxyethanol were dissolved in
CH3CN (2 mL). After 1 h the solvent was removed under
vacuum, leaving an orange oil. Diethyl ether (6 mL) was added,
forming a yellow slurry, which was stirred at room temperature
(1 h). The yellow solid was allowed to settle, and the supernatant
removed by pipet. The yellow powder was washed with diethyl
ether (2 ꢀ 5 mL) and dried under vacuum. Yield: 0.0962 g
(88%). 1H NMR (CD3CN, 400 MHz) δ 8.34 (t, 1H, J = 7.6 Hz,
Py), 8.12 (d, 1H, J = 7.6 Hz, Py), 7.79 (d, 1H, J = 7.6 Hz, Py),
7.55-7.44 (m, 5H, Ph), 7.00 (t, 2H, J = 7.2 Hz, Ph), 6.89 (t, 1H,
J = 6.8 Hz, Ph), 6.60 (m (br), 1H, V-OCH), 6.50 (d, 2H, J = 8.0
Hz, Ph), 4.60 (t, 1H, J = 9.6 Hz, -CHH-), 4.35 (dd, 1H, J = 9.6
Hz, 4.0 Hz, -CHH-). 51V NMR (CD3CN, 105 MHz) -551.1 (s).
u-1,2-Diphenyl-2-methoxyethanol (D). trans-Stilbene oxide
(0.705 g, 3.60 mmol) was dissolved in CH3OH (10 mL) at room
temperature. Sulfuric acid (less than 1 drop) was carefully added
by pipet, resulting in gradual warming of the solution. After
reacting for 1 h, the solution was treated with NaHCO3
(approximately 100 mg) and allowed to stand for 30 min. The
solution was filtered, and the solvent removed under vacuum,
yielding a white solid. Yield 0.780 g (95%). The white solid
product could be recrystallized from hot methanol, but was used
without further purification in the synthesis of (dipic)V(O)-
(DPME) (5). Integration of the benzylic protons in the 1H
NMR spectrum showed that the product was a mixture of
85:15 u (R,S þ S,R): l (R,R þ S,S) diastereomers.44 Similar
diastereoselectivity has been reported for the ring-opening of
trans-stilbene oxide by Cp2ZrCl2 (85:15)45 and polymer-sup-
ported Fe and Ru catalysts (84:16).46 1H NMR (benzene-d6, 400
MHz) δ 7.24 (d, 2H, J = 6.8 Hz, Ph), 7.14-6.87 (m, 8H, Ph),
4.81 (d, 0.85H, J = 5.6 Hz, benzylic hydrogen on u dia-
stereomer), 4.73 (d, 0.15H, J = 8.4 Hz, benzylic hydrogen on l
diastereomer), 4.16 (d, 0.85H, J = 5.6 Hz, benzylic hydrogen on
u diastereomer), 3.97 (d, 0.15H, J = 8.4 Hz, benzylic hydrogen
on l diastereomer), 2.94 (s, 3H, OCH3). Anal. Calcd for
C15H16O2: C, 78.92; H, 7.06. Found: C, 79.04; H, 6.91.
IR (thin film): νCdO = 1702 cm-1, νVdO = 995 cm-1
.
(dipic)V(O)(DPME) (5). (dipic)V(O)OiPr (0.191 g, 0.544
mmol) and 1,2-diphenyl-2-methoxyethanol (0.251 g, 1.10 mmol,
an 85:15 mixture of u:l diastereomers)44 were dissolved in
CH3CN (2 mL) by stirring at room temperature for 5 min.
The solvent was immediately removed under vacuum, leaving a
yellow-orange oil. The oil was redissolved in CH3CN (2 mL),
and diethyl ether (5 mL) was added, resulting in a small amount
of a pale blue-green precipitate. The mixture was filtered
through a Teflon syringe filter, giving an orange solution, which
was immediately cooled to -15 °C. Over 2 days at -15 °C,
orange crystals formed. The supernatant was decanted, the
orange crystals washed with diethyl ether (3 ꢀ 2 mL), and dried
under vacuum. The u isomer of complex 5 crystallized prefer-
entially and was used in all subsequent reactions. Yield: 0.0966 g
(39%). 1H NMR (CD3CN, 400 MHz) δ 8.66 (t, 1H, J = 7.0 Hz,
Py), 8.35 (d, 1H, J = 7.0 Hz, Py), 8.33 (d, 1H, J = 7.0 Hz, Py),
7.18-7.08 (m, 10H, Ph), 6.74 (br s, 1H, V-OCHPh), 5.08 (d, 1H,
J = 6.0 Hz, V-OCHPh-CHPh), 2.45 (s, 3H, OCH3). 51V NMR
(CD3CN, 105 MHz) -536.3 (s). IR (thin film): νCdO = 1705
cm-1, νVdO = 1001 cm-1. Anal. Calcd for C22H18NO7V: C,
57.53; H, 3.95; N, 3.05. Found: C, 57.73; H, 3.83; N, 3.08. The
orange crystals were stored at -15 °C.
(dipic)V(O)(pinOMe) (2). Pinacol monomethyl ether (0.320 g,
2.42 mmol) and (dipic)V(O)OEt (0.406 g, 1.26 mmol) were
combined in CH3CN (10 mL). The reaction mixture was stirred
for 15 min and filtered through a Teflon syringe filter. Diethyl
ether (5 mL) was added, and the solution cooled to -15 °C
overnight, yielding yellow crystals. The supernatant was de-
canted, the crystals washed with diethyl ether (2 ꢀ 3 mL), and
dried under vacuum (yielding 0.220 g). Addition of another
5 mL diethyl ether to the supernatant and cooling to -15 °C
(41) Thorn, D. L.; Harlow, R. L.; Herron, N. Inorg. Chem. 1996, 35, 547–
548.
(42) Mimoun, H.; Chaumette, P.; Mignard, M.; Saussine, L.; Fischer, J.;
Weiss, R. Nouv. J. Chim. 1983, 7, 467–475.
Thermolysis of (dipic)V(O)(pinOMe). In a resealable Teflon-
capped NMR tube, 2 (9.3 mg, 0.026 mmol) was dissolved in
pyridine-d5 (0.6 mL) containing p-xylene as an internal standard
(3.3 mM). The solution was allowed to react at room tempera-
ture over the course of 3 weeks, resulting in a color change from
yellow to green. Integration against the internal standard re-
vealed formation of acetone (94%), 2-methoxypropene (76%),
and pinacol monomethyl ether (92%). A broad signal in the 1H
(43) Bertsch, R. J.; Ouellette, R. J. J. Org. Chem. 1974, 39, 2755–2759.
(44) The nomenclature u (unlike) and l (like) is recommended by IUPAC
to replace erythro and threo in the designation of diastereomers having two
chirality elements. See: Moss, G. P. Basic Terminology of Stereochemistry.
Pure Appl. Chem., 1996, 68, 2193-2222.
(45) Kantam, M. L.; Aziz, K.; Jeyalakshmi, K.; Likhar, P. R. Cat. Lett.
2003, 89, 95–97.
(46) Lee, S. H.; Lee, E. Y.; Yoo, D. W.; Hong, S. J.; Lee, J. H.; Kwak, H.;
Lee, Y. M.; Kim, J.; Kim, C.; Lee, J. K. New J. Chem. 2007, 31, 1579–1582.

Article
Inorganic Chemistry, Vol. 49, No. 12, 2010 5617
1
NMR spectrum (pyr-d5) at 7.39 ppm and the green color of the
solution were consistent with formation of the previously re-
ported (dipic)V(O)(pyr)2 (6). The identity of 2-methoxypropene
was confirmed by spiking the sample with the authentic com-
pound (purchased from Aldrich). Reducing the volume of the
solution to ∼0.2 mL by slow evaporation yielded green needles
of 6 (6.1 mg, 61%).
Yield: 0.110 g (83%). H NMR (DMSO-d6, 400 MHz) δ 7.31
(br, Δν1/2 =280Hz). IR(DMSO-d6 solution): νCdO = 1666 cm-1
,
1335 cm-1, νVdO = 947 cm-1. UV-vis (DMSO): λ= 384 nm, ε=
210 M-1 cm-1
.
Control Reaction of (dipic)VIV(O)(pyr)2 (6) with 1,2-Diphenyl-
2-methoxyethanol in DMSO-d6. Under argon in a resealable
Teflon-capped NMR tube, complex 6 (4.8 mg, 0.012 mmol) and
1,2-diphenyl-2-methoxyethanol (5.9 mg, 0.026 mmol, an 85:15
mixture of u:l diastereomers) were dissolved in DMSO-d6
(0.6 mL) containing p-xylene (11.8 mM) as an internal standard.
The tube was heated at 100 °C and monitored by H NMR
spectroscopy. After 1 week at 100 °C, the green color of the
solution had not changed, and only trace benzaldehyde (<1%,
no turnovers) had formed.
Control Reaction of (dipic)VIV(O)(DMSO)2 (7) with 2-Phen-
oxyethanol in DMSO-d6. In a resealable Teflon-capped NMR
tube under an atmosphere of Ar, 2-phenoxyethanol (11.1 mg,
0.080 mmol) and 7 (8.8 mg, 0.023 mmol) were dissolved in
DMSO-d6 containing 1,3,5-tri-tert-butylbenzene (2.5 mM) as an
internal standard. An initial 1H NMR spectrum was recorded,
and then the solution was heated for 1 week at 100 °C.
Examination of the 1H NMR spectrum showed no reaction of
2-phenoxyethanol. No change in the green color of the solution
occurred.
Control Reaction of (dipic)VIV(O)(DMSO)2 (7) with 1-Phen-
yl-2-phenoxyethanol in DMSO-d6. Under an atmosphere of Ar
in a resealable Teflon-capped NMR tube, 1-phenyl-2-phenox-
yethanol (10.4 mg, 0.049 mmol) and 7 (11.1 mg, 0.029 mmol)
were dissolved in DMSO-d6 (0.6 mL) containing 1,3,5-tri-tert-
A similar reaction in a resealable Teflon-capped NMR tube
involving thermolysis of 2 at 100 °C for 6 h showed formation of
6, acetone, 2-methoxypropene, and pinacol monomethyl ether.
After initially turning green, the solution color did not change
upon heating for a total of 6 days at 100 °C, nor were any
changes in the composition of the organic products observed,
indicating no conversion to the previously reported V(III) μ-oxo
1
dimer,35 which is dark purple (λ = 518 nm ε = 5000 M-1 cm-1
λ = 598 nm, ε = 4600 M-1 cm-1).
,
Thermolysis of (dipic)V(O)(OPE). In an NMR tube, complex
3 (6.6 mg, 0.018 mmol) was dissolved in pyr-d5 (0.6 mL)
containing p-xylene (3.4 mM) as an internal standard. A H
1
NMR spectrum was immediately recorded, and then the solu-
tion heated at 100 °C for 10 min. After cooling to room
temperature, examination of the H NMR spectrum revealed
1
complete disappearance of 3 and formation of 2-phenoxyace-
taldehyde (78%) and 2-phenoxyethanol (98%). Yields were
determined by the average of 4 runs of this type. The UV-vis
spectrum of the reaction mixture was consistent with the vana-
dium product (dipic)V(O)(pyr)2 (6). Spiking the reaction mix-
ture with 2-phenoxyacetaldehyde prepared by a published
procedure (Swern oxidation)36 confirmed the identity of this
product.
1
butylbenzene (2.5 mM) as an internal standard. An initial H
Thermolysis of 4 in pyr-d5. Complex 4 (3.4 mg, 0.0076 mmol)
was dissolved in pyr-d5 (0.6 mL) containing p-xylene as an
internal standard (19.6 mmol). Within 10 min, the solution
changed color from yellow to green. Examination of the 1H
NMR spectrum showed complete consumption of the vana-
dium(V) and a 1:1 mixture of 2-phenoxyacetophenone and
1-phenyl-2-phenoxyethanol. The identity of the 2-phenoxyace-
tophenone was confirmed by spiking the reaction mixture with
the authentic compound prepared according to a published
procedure.37 The UV-vis spectrum of the reaction mixture
was consistent with the vanadium(IV) product 6.
NMR spectrum was recorded, then the solution was heated at
100 °C for 1 week. Integration against the internal standard
revealed that no reaction of 1-phenyl-2-phenoxyethanol oc-
curred. The solution color did not change from its initial green.
Catalytic Oxidation of 1-Phenyl-2-phenoxyethanol in DMSO-
d6. 1-Phenyl-2-phenoxyethanol (33.3 mg, 0.156 mmol) and
10 mol % (dipic)V(O)OiPr (5.4 mg, 0.015 mmol) were dissolved
in DMSO-d6 (1 mL) containing 1,3,5-tri-tert-butylbenzene (2.4
mM) as an internal standard. An initial 1H NMR spectrum was
recorded, then the solution was transferred under air to a thick-
walled 100 mL Schlenk tube equipped with a Teflon stopcock.
The reaction was heated at 100 °C with stirring and monitored
periodically by 1H NMR. After 2 days, approximately 50% of
the starting material had been consumed, with the products
consisting of 2-phenoxyacetophenone (35%), benzoic acid
(15%), formic acid (10%), and phenol (13%). After 1 week,
95% of the starting material had been consumed, with the
products consisting of formic acid (46%), benzoic acid (81%),
phenol (77%), and 2-phenoxyacetophenone (9%). Yields are
expressed as a percentage of the theoretical maximum based on
the initial amount of substrate, and were determined by an
average of two runs of this type. A 51V NMR spectrum recorded
at the end of the reaction revealed a single peak at -518 ppm,
corresponding to the cis-dioxo anion [(dipic)V(O)2]-. The 13C
NMR spectrum showed a heptet at 41.35 ppm (J = 20 Hz)
corresponding to dimethylsulfone-d6. A GC-MS trace of the
reaction mixture also showed dimethylsulfone-d6 M/Z = 100.
A control reaction using an analogous procedure was con-
ducted where 1-phenyl-2-phenoxyethanol (16.8 mg, 0.079
mmol) was heated in DMSO-d6 under air at 100 °C (no
vanadium). After 7 days, no oxidation products were detected
by 1H NMR, and integration against the internal standard
( p-xylene) revealed that none of the substrate had reacted.
Attempted Oxidation of Formic Acid with (dipic)V(O)OiPr. In
an NMR tube, formic acid (0.043 g, 0.930 mmol), and 10 mol %
(dipic)V(O)OiPr (32.4 mg, 0.092 mmol) were dissolved in
DMSO-d6 containing 1,3,5-tri-tert-butylbenzene (2.5 mM) as
an internal standard. An initial 1H NMR spectrum was re-
corded. The reaction mixture was transferred to a thick-walled
Thermolysis of 5 in pyr-d5. Complex 5 (6.1 mg, 0.013 mmol)
was dissolved in pyr-d5 (0.6 mL) containing p-xylene as an
internal standard (6.3 mM). Over approximately 5 min at room
temperature the solution color changed from yellow to green.
Examination of the 1H NMR spectrum after 10 min revealed a
1:1 ratio of benzoin methyl ether/1,2-diphenyl-2-methoxyetha-
nol (u diastereomer). The UV-vis spectrum of the solution was
consistent with the vanadium product (dipic)V(O)(pyr)2 (6).
Thermolysis of 5 in DMSO-d6. Under argon, complex 5(4.5mg,
0.010 mmol) was dissolved in DMSO-d6 (0.6 mL, used as
received from Cambridge Isotope Laboratory, not dried) con-
taining 1,3,5-tri-tert-butylbenzene (2.5 mM) as an internal
standard. An initial 1H NMR spectrum was recorded, and the
solution immediately heated at 100 °C for 30 min, during which
time the color changed from the initial yellow to green. Exam-
ination of the 1H NMR spectrum revealed formation of copro-
ducts methanol and benzaldehyde (66%), benzoin methyl ether
(33%), and 1,2-diphenyl-2-methoxyethanol (u diastereomer)
(100%). The UV-vis spectrum of the solution was consistent
with formation of (dipic)VIV(O)(DMSO)2 (7).
Preparation of (dipic)VIV(O)(DMSO)2 (7). In a small thick-
walled glass vessel equipped with
a Teflon stopcock,
(dipic)VV(O)(Hpin) (0.119 g, 0.341 mmol) was suspended in
DMSO (1 mL). The suspension was heated at 100 °C for 30 min,
during which time all the material dissolved, forming a dark
green solution. Slow diffusion of diethyl ether (2 mL) at
room temperature gave large green crystals. The crystals were
washed with diethyl ether (3 ꢀ 1.5 mL) and dried under vacuum.

5618 Inorganic Chemistry, Vol. 49, No. 12, 2010
Hanson et al.
100 mL Schlenk tube equipped with a Teflon stopcock and
heated under air with stirring at 100 °C. After 3 days, examina-
tion of the 1H NMR spectrum revealed that less than 5% of the
formic acid had been consumed.
theoretical maximum based on the initial amount of substrate).
Benzil and benzoin could also be detected (less than 2% each)
in the 1H NMR spectrum. After 6 days at 100 °C, the starting
material had been completely consumed. The products con-
sisted of benzoic acid (85%), methyl benzoate (84%), benzoin
methyl ether (9%), benzaldehyde (9%), methanol (6%), benzil
(less than 5%), and benzoin (less than 5%).
A control reaction with 1,2-diphenyl-2-methoxyethanol only
(no vanadium) in pyr-d5 under air showed no reaction when
heated at 100 °C for 1 week.
Catalytic Oxidation of 1,2-Diphenyl-2-methoxyethanol in
DMSO-d6. 1,2-Diphenyl-2-methoxyethanol (31.8 mg, 0.140
mmol, an 85:15 mixture of u:l diastereomers) and 5 mol %
(dipic)V(O)OiPr (15.5 μL of a 0.45 M stock solution in DMSO-
d6, 0.007 mmol) were dissolved in DMSO-d6 (1 mL) containing
1,3,5-tri-tert-butylbenzene (0.91 mM) as an internal standard.
1
An initial H NMR spectrum was recorded, and under air the
Attempted Catalytic Oxidation of 2-Phenoxyethanol. 2-Phe-
noxyethanol (30.8 mg, 0.223 mmol) and 10 mol % (dipic)V-
(O)OiPr (7.8 mg, 0.022 mmol) were dissolved in DMSO-d6
(1 mL) containing 1,3,5-tri-tert-butylbenzene (3.3 mM) as an
internal standard. An initial 1H NMR spectrum was recorded.
The solution was transferred to a 100 mL Schlenk tube equipped
with a Teflon stopcock and heated at 100 °C under air. After
1 week at 100 °C, integration of the 1H NMR spectrum against
the internal standard revealed that only approximately 20% of
the starting material had been consumed. The organic products
consisted of phenol (18%), formic acid (6%), as well as several
other unidentified products. The 51V NMR spectrum showed
a single peak at -518 ppm, corresponding to the cis-dioxo
anion [(dipic)V(O)2]-. Dimethylsulfone-d6 was detected in both
the 13C NMR spectrum and the GC-MS trace of the reaction
mixture. A control reaction with 2-phenoxyethanol only (no
vanadium) in DMSO-d6 showed no reaction after 1 week under
air at 100 °C.
reaction mixture was transferred to a 100 mL glass vessel
equipped with a Teflon stopcock and stirbar. The vessel was
sealed and heated with stirring at 100 °C. It was periodically
opened to air and monitored by 1H NMR spectroscopy (by
transferring the solution to an NMR tube). After 20 h at 100 °C,
integration of the 1H NMR spectrum revealed that 94% of the
starting material had been consumed (19 turnovers). The pro-
ducts consisted of benzaldehyde (73%), methanol (69%), ben-
zoin methyl ether (13%), and methyl benzoate and benzoic acid
(approximately 5% or less each) (yields expressed in terms of
theoretical maximum per initial amount of substrate). A 51V
NMR spectrum recorded at this time showed a single peak
at -518 ppm, corresponding to the cis-dioxo anion [(dipic)-
V(O)2]-. Both the 13C NMR spectrum and the GC-MS trace of
the reaction mixture indicated that dimethylsulfone-d6 was
present at the end of the catalytic run. No dimethylsulfide-d6
was detected.
An analogous procedure was conducted using 1,2-diphenyl-
2-methoxyethanol (34.6 mg, 0.152 mmol, an 85:15 mixture of u:l
diastereomers) and 2.5 mol % (dipic)V(O)OiPr (8.4 μL of a
0.45 M stock solution in DMSO-d6, 0.004 mmol) in DMSO-d6
(1 mL) containing 1,3,5-tri-tert-butylbenzene (3.7 mM) as an
Reaction of 6 with 1-Phenyl-2-phenoxyethanol. Complex 6 (4.4
mg, 0.011 mmol) and 1-phenyl-2-phenoxyethanol (3.8 mg, 0.018
mmol) were dissolved in pyr-d5 containing p-xylene (4.7 mM) as
an internal standard. An initial 1H NMR spectrum was re-
corded, then the solution heated at 100 °C for 1 week. No color
change was observed, nor any reaction of the 1-phenyl-2-
phenoxyethanol detected by 1H NMR.
1
internal standard. After 40 h at 100 °C, integration of the H
NMR spectrum revealed that 93% of the starting material
had been consumed (37 turnovers). The products consisted of
benzaldehyde (72%), methanol (73%), benzoin methyl ether
(11%), and methyl benzoate and benzoic acid (5% or less each).
A control reaction with 1,2-diphenyl-2-methoxyethanol only
(no vanadium) in DMSO-d6 under air showed no reaction when
heated at 100 °C for 1 week.
Reaction of 7 with Air. In an NMR tube, DMSO vanadium-
(IV) complex 7 (10.2 mg, 0.026 mmol) was dissolved in DMSO-
d6 (0.4 mL). The solution was opened to air, and then the NMR
tube was capped and heated at 100 °C for 18 h. During this time,
the solution changed color from green to a very pale blue. After
cooling to room temperature, the cis-dioxo anion [(dipic)-
V(O)2]- was detected by both 1H and 51V NMR.
Catalytic Oxidation of 1,2-Diphenyl-2-methoxyethanol in pyr-
d5. 1,2-Diphenyl-2-methoxyethanol (29.8 mg, 0.131 mmol, an
85:15 mixture of u:l diastereomers) and (dipic)V(O)OiPr (4.8
mg, 0.013 mmol) were dissolved in pyr-d5 (0.8 mL) containing
p-xylene (5 μL, 0.041 mmol) as an internal standard. An initial
1H NMR spectrum was recorded, and under air the reaction
mixture was transferred to a 100 mL glass vessel equipped with a
Teflon stopcock and stirbar. The mixture was heated at 100 °C
with stirring and monitored periodically by 1H NMR spectro-
scopy. After 3 days, 56% of the starting material had been
consumed. The products consisted of benzoin methyl ether
(45%), benzaldehyde (5%), methanol (3%), benzoic acid
(4%), and methyl benzoate (5%) (yields expressed in terms of
Acknowledgment. This work was supported by Los Alamos
National Laboratory LDRD (Director’s PD Fellowship to S.K.
H.) and NSF via the Center for Enabling New Technologies
through Catalysis (CENTC). We thank L. A. (Pete) Silks
(LANL), R. Wu (LANL), and Professors W. T. Borden
(UNT), S. L. Scott (UCSB), and P. C. Ford (UCSB) for helpful
discussions.
Supporting Information Available: CIF files for 2, 3, 5, and 7,
and X-ray crystallographic data for 2, 3, 5, and 7. This material
is available free of charge via the Internet at http://pubs.acs.org.