Chemoselective oxidation of polyols with chiral palladium catalysts

Article abstract of DOI:10.1021/om4001549

Chiral palladium-based catalysts derived from pyridinyl oxazoline (pyOx) ligands catalyze the oxidation of alcohols, including 1,2-diols, triols, and tetraols, with high regio- and chemoselectivity. Screening of various chiral oxazoline-derived ligands for the oxidation of a model diol, 1,2-propanediol (1,2-PD), revealed that the nature of the ligand had a significant influence on the activity and chemoselectivity for oxidation of vicinal diols. The PyOx ligands containing an α-methyl substituent were the most active for the oxidation of 1,2-PD using benzoquinone as the terminal oxidant. Oxidation of vicinal diols and polyols occurs selectively at the secondary alcohol to afford α-hydroxy ketones in isolated yields of 62-87%. Chemoselective oxidation of meso-erythritol with the chiral [(S)-(α-Me(tert-Bu)PyOx)Pd(OAc)] ₂[OTf]₂ afforded (S)-erthyrulose in 62% yield and 24% ee.

Full text of DOI:10.1021/om4001549

Article
pubs.acs.org/Organometallics
Chemoselective Oxidation of Polyols with Chiral Palladium Catalysts
Antonio G. De Crisci,† Kevin Chung,† Allen G. Oliver,‡ Diego Solis-Ibarra,† and Robert M. Waymouth*,†
†Department of Chemistry, Stanford University, Stanford, California 94305, United States
‡Molecular Structure Facility, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556,
United States
S
* Supporting Information
ABSTRACT: Chiral palladium-based catalysts derived from pyridinyl
oxazoline (pyOx) ligands catalyze the oxidation of alcohols, including
1,2-diols, triols, and tetraols, with high regio- and chemoselectivity.
Screening of various chiral oxazoline-derived ligands for the oxidation
of a model diol, 1,2-propanediol (1,2-PD), revealed that the nature of
the ligand had a significant influence on the activity and chemo-
selectivity for oxidation of vicinal diols. The PyOx ligands containing
an α-methyl substituent were the most active for the oxidation of 1,2-
PD using benzoquinone as the terminal oxidant. Oxidation of vicinal
diols and polyols occurs selectively at the secondary alcohol to afford
α-hydroxy ketones in isolated yields of 62−87%. Chemoselective oxidation of meso-erythritol with the chiral [(S)-(α-Me(tert-
Bu)PyOx)Pd(OAc)]2[OTf]2 afforded (S)-erthyrulose in 62% yield and 24% ee.
the catalytic activity and chemoselectivity for diol and polyol
oxidation.
INTRODUCTION
■
The oxidation of alcohols to carbonyl compounds is one of the
most widely used synthetic transformations.1−6 Selective
catalytic oxidations are environmentally attractive alternatives
to those utilizing stoichiometric heavy-metal oxidants.6−11
Palladium-catalyzed oxidations have proven attractive due to
the mild conditions, high chemo-12−14 and stereoselectiv-
ities,3,15−17 and the use of air or oxygen as terminal
oxidants.18−30
RESULTS AND DISCUSSION
■
Synthesis. To expand the scope of catalysts for chemo- and
stereoselective alcohol oxidation, we targeted cationic Pd
complexes ligated by chiral oxazoline ligands,40−45 including
the chiral bis(oxazoline) tert-BuBOX (L1),40,46 the bi-
(oxazolines) L2a,b,41,47,48 and the pyridinyl oxazolines
(pyOx) L3a−d (Figure 1).44,49−55
We recently reported that cationic Pd complexes12,31−33
ligated by neocuproine (2,9-dimethylphenanthroline)7,18 li-
gands catalyze the chemoselective oxidation of vicinal diols and
triols to α-hydroxy ketones at room temperature.12 α-Hydroxy
ketones are versatile synthetic intermediates and a common
functional group in biologically active natural products;34−37
the chemoselective oxidation of vicinal diols to hydroxy ketones
provides a mild and selective strategy to these intermediates.
Cationic Pd acetate complexes ligated by phenanthrolines
substituted in the 2- or 2,9-positions are active catalysts for the
oxidation of primary or secondary alcohols31−33 and vicinal
diols12 at room temperature. Cationic monoacetate complexes
were more active than either the dicationic complexes31 or the
diacetates;38,39 we have reasoned that the presence of both an
acetate ligand and exchangeable cationic coordination site
facilitates ligand exchange and deprotonation of the alcohol to
generate cationic Pd alkoxides.31−33 The presence of 2,9-
substituents on the phenanthroline ligands is critical; Pd
complexes ligated by unsubstituted phenanthrolines are inactive
at room temperature.31
Figure 1. Oxazoline ligands.
The chiral bis(oxazoline) tert-BuBox (L1) is commercially
available; the syntheses of the bi(oxazolines) L2a,b were carried
out following literature protocols.46 The syntheses of the pyOx
ligands L3a−d were carried out using slightly modified
literature procedures50,56,57 from commercially available β-
amino alcohols and substituted 2-pyridinecarbonitriles.
To assess the suitability of the chiral ligands for low-
temperature alcohol oxidation, screening studies were carried
Herein, we describe efforts to develop chiral Pd complexes
for chemo- and enantioselective polyol oxidation reactions and
describe features of the ligand geometry which influence both
Received: February 25, 2013
Published: March 21, 2013
© 2013 American Chemical Society
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out with cationic Pd complexes either isolated or generated in
situ31 with ligands L1−L3. For these studies, the Pd acetate
complex (L1)PdOAc2 and dicationic complex [(L1)Pd-
(MeCN)2][BF4]2 were generated by reaction of a dichloro-
methane solution of the dichloropalladium complex (L1)-
Reasoning that the conformational flexibility of the bis-
(oxazoline) ligand L160 might have contributed to the
undesired ligand carbometalation, we investigated the related
bi(oxazoline) ligands (L2a,b) which adopt more planar five-
membered chelates with Pd.61,62 Nevertheless, when (L2a)Pd-
(OAc)2 and [(L2a)Pd(CH3CN)2](OTf)2 were combined in
CH3CN and left to react for 1 h at room temperature, analysis
of the resulting mixture by MS (ESI) revealed ions at m/z
370.12 and 329.14, consistent with the cyclopalladated
metallacycles [(L2a∧C)Pd(MeCN)]+ and [(L2a∧C)Pd]+, re-
spectively.
58
PdCl2 with AgOAc or AgOTf/CH3CN, respectively.
Attempted oxidation of 1,2-propanediol in DMSO-d6 with 2
equiv of benzoquinone (25 °C) with the product resulting from
combining31,33 (L1)PdOAc2 and [(L1)Pd(MeCN)2][BF4]2
was unsuccessful. Subsequent analysis of the reaction product
1
by H NMR and electrospray ionization mass spectrometry
Attempts to generate a cationic Pd acetate in situ32 by
treating (L2a)Pd(OAc)2 with 1 equiv of HOTf (to Pd, as a
0.33 M MeCN solution) was similarly unsuccessful; this
mixture was inactive for oxidation of 1,2-propanediol (2 equiv
of benzoquinone) after 16 h at 25 °C in DMSO-d6.
(ESI-MS) revealed that the reaction of (L1)PdOAc2 and
[(L1)Pd(MeCN)2][BF4]2 generated the palladacycle [(L1∧C)-
Pd(MeCN)]+ resulting from carbometalation of one of the tert-
butyl groups of L1. This was established by the observation of
characteristic ions at m/z 443.1695 ([(L1∧C)Pd(CD3CN)]+)
and m/z 399.1236 ([(L1∧C)Pd]+) in the HR-MS desorption
ESI (DESI; see the Supporting Information) and by
diasterotopic CH2 peaks at 2.13 ppm (qd, J = 9.0, 1.2 Hz)
and the tert-butyl group derived diastereotopic methyl groups at
0.95 (d, J = 1.2 Hz) and 0.93 ppm in CD3CN solution. The
cyclopalladation of t-Bu-substituted oxazolines has been
observed previously.59 DFT computations (M06-L) of
[(L1∧C)Pd(MeCN)]+ (Figure 2; see the Supporting Informa-
More promising reactivity was observed with the tert-butyl-
substituted ligand L2b when an external base was added. When
[(L2b)Pd(CH3CN)2](BF4)2 (prepared by reaction with L2b
and ([(MeCN)4Pd][BF4]2) was combined with 0.5 equiv of
Cs2CO3 or CsOAc (relative to Pd), 1,2-propanediol, and
benzoquinone in DMSO-d6, hydroxyacetone was observed after
heating at 40−60 °C for 2 days, although under these
conditions, the yields were modest (≤23%) and Pd black also
formed. Similarly, oxidation of 1,2-propanediol with [(L2b)-
Pd(CH3CN)2](BF4)2 activated with 1 equiv of aqueous
NaOH63 afforded a 19% yield of hydroaxyacetone after 16 h
at 60 °C in DMSO-d6.
In light of the modest activities observed with ligands L1 and
L2, we investigated the activity of Pd complexes of the pyOx
ligands L3. Complexes 6a−d were prepared as shown in
Scheme 1.
Figure 2. Observed C(sp3)−H activation of L1 upon comproportio-
nation, characterized by 1H NMR, 13C NMR including DEPT135, and
HR-MS (DESI). See the Supporting Information.
Initial screening of complex 6a by 1H NMR revealed it to be
inactive at room temperature for the oxidation of 1,2-
propanediol with 10 mol % Pd and 1.15 equiv (relative to
diol) of benzoquinone in DMSO-d6 or 9/1 CD3CN/D2O
mixtures. Higher conversions (29% at 2 h) was observed at 60
°C to afford a 12% yield of hydroxyacetone. As the reaction
progressed to 17 h, conversion increased to approximately 60%
tion) indicate that the palladacycle adopts a puckered five-
membered ring with the C(Me)2 moiety out of plane. The
nitrogen trans to the Pd−C bond is longer by 0.21 Å at 2.22 Å
in comparison to the Pd−N trans to the acetonitrile.
Scheme 1. Representative Synthesis of [(RPyOX)Pd(OAc)]22+(OTf−)2 with Conditions and Yields Reported for 6d (Dimer
a
Shown)
a
Legend: (i) NaOMe, MeOH, 4 h, room temperature; (ii) (S)-2-amino-3,3-dimethyl-1-butanol (L-tert-leucinol), neat, 50 °C, Ar purge; (iii)
(PhCN)2PdCl2, dichloromethane, 18h, room temperature; (iv) AgOAc, dichloromethane (dark), 45 min, room temperature; (v) MeCN/HOTf
solution, 40 min, room temperature.
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but the yield of hydroxyacetone dropped to 9% accompanied
by the formation of Pd black.
As previous studies with phenanthroline-type ligands had
indicated that α-methyl substitution was beneficial,12,31−33 we
prepared the methylpyridine complexes 6b−d. 1H NMR
screening experiments of complexes 6b−d revealed these
complexes to be more active and selective than those derived
from 6a; oxidation of 1,2-propanediol with 6b afforded 37% of
hydroxyacetone and 7% of lactaldehyde (NMR vs internal
standard and 2 equiv of BQ in DMSO-d6) after 8 h at room
temperature. Similarly, oxidation of 1,2-propanediol with 6c
afforded 23% of hydroxyacetone after 13 h at room temperature
with 2 equiv of BQ. Complex 6d gave the most promising rates
and selectivity: oxidation of 1,2-propanediol in CD3CN/D2O
(9/1) with 10 mol % Pd and 2 equiv of benzoquinone at 55 °C
led to complete conversion of diol and formation of 1-
hydroxyacetone with 99% selectivity in less than 45 min. These
screening studies revealed that, for oxidation of diols with the
pyOx ligands, the presence of both an α-methyl substituent on
the pyridine and a t-Bu substituent on the oxazoline were
optimal for high rates and selectivities. In light of these
promising results, preparative oxidation reactions were carried
out with complex 6d. In addition, complexes 4c,d and 6a,d
were characterized by X-ray crystallography.
Figure 3. Single-crystal X-ray structure of 4c. Ellipsoids are drawn at
the 50% probability level. The solvent molecule is omitted for clarity.
Relevant bond distances (Å) and bond angles (°): Pd1−N1, 2.003(2):
Pd1−N2, 2.131(2); Pd1−Cl1, 2.280(1); Pd1−Cl2, 2.307(2); N2−
C11, 1.349(3); C11−C12, 1.488(3); N1−Pd1−N2, 79.7(1); N1−
Pd1−Cl1, 90.35(5); N2−Pd1−Cl1, 169.8(1); N1−Pd1−Cl2,
177.27(5); N2−Pd1−Cl2, 102.5(1); Cl1−Pd1−Cl2, 87.39(2); N1−
C16−C2, 111.0(1); N1−C16−C8, 101.0(2); C11−N2−Pd1
132.0(1); C10−N2−Pd1, 111.1(1).
The synthesis of complexes 4−6 is summarized in Scheme 1;
the synthesis of 6d is described in detail below. Palladation of
ligand L3d with Cl2Pd(PhCN)2 in dichloromethane afforded
the dichloride 4d in 95% yield. Treatment of 4d with AgOAc
afforded the diacetate, which was subsequently converted to 6d
by treatment with 1 equiv of triflic acid.32 Attempts to generate
5d directly from ligand L3d and Pd(OAc)2 in refluxing toluene
or benzene led to mixtures which were not readily purified. It is
known that pyBox ligands are susceptible to nucleophilic ring
opening from acetate attack64 and may be responsible for the
failure to generate 5d directly from Pd(OAc)2. Complex 5d has
limited stability and was converted to 6d on the same day it was
made to prevent decomposition. A similar observation was
reported by Yoo with pyOx Pd acetates.52 Complex 6d is
obtained in 86% yield as its bridging dimer after precipitation
with diethyl ether to give an air-stable orange precipitate. We
note that 6d is stable for at least several days (more stable than
5d) at room temperature when protected from light with
aluminum foil (taken as a precaution).
The single-crystal X-ray structure of 4c is shown in Figure 3.
The complex crystallizes as dark orange blocklike crystals from
slow evaporation in a concentrated chloroform-d solution at
room temperature. The Pd is coordinated in a distorted four-
coordinate square-planar geometry by two chlorine atoms and
the two nitrogens of the R enantiomer of the pyOx ligand L3c.
The palladium−nitrogen distances are Pd1−N1 = 2.003(2)
and Pd1−N2 = 2.131(2) Å. The pyridyl Pd−N distance is
0.104 Å longer in 4c than in the nonmethylated analogue
reported by Jones et al.,61 likely as a consequence of the steric
demands of the methyl group. The presence of an α-methyl
group is also evident in the Cl−Pd−N angles, N2−Pd1−Cl1 =
169.8° and N1−Pd1−Cl2 = 177.27° for 4c, in comparison to
the nonmethylated version, 173.07 and 173.02°,61 revealing a
larger deviation along the angle defined at the pyridine nitrogen
of 4c.
Figure 4. Single-crystal X-ray structure of 4d. Ellipsoids are drawn at
the 50% probability level. The dichloromethane solvent molecule is
omitted for clarity. Relevant bond distances (Å) and bond angles and
torsion angles (°): Pd1−N1, 2.001(2); Pd1−N2, 2.112(2); Pd1−Cl1,
2.2786(6); Pd1−Cl2, 2.2900(6); N1−Pd1−N2, 80.08(9); N1−Pd1−
Cl1, 91.29(7); N2−Pd1−Cl1, 169.05(6); N1−Pd1−Cl2, 177.64(6);
N2−Pd1−Cl2, 99.98(6); Cl1−Pd1−Cl2, 88.37(2); N2−Pd1−N1−
C3, 16.13(19); Cl1−Pd1−N1−C3, −157.09(19); Cl2−Pd1−N1−C3,
−75.6(17); N2−Pd1−N1−C1, −169.5(3); Cl1−Pd1−N1−C1,
17.3(3); Cl2−Pd1−N1−C1, 98.8(16).
coordinate square-planar geometry by two chlorine atoms and
the two nitrogens of the pyOx ligand. The distortion evident in
the N2−Pd1−Cl1 angle of 169.05(6)° is likely due to the steric
demands of the tert-butyl substituent of the oxazolinyl ligand.
Similar distortions have been noted previously;27 Yoo and co-
workers52 reported the X-ray structure for a compound
analogous to 4d, lacking an α-methyl substituent on the
pyridine. Introduction of the α-methyl substituent causes a
slight lengthening of the Pd1−N2 bond of 4d (2.11 vs 2.05 Å
for the complex52 lacking this substituent).
Single crystals of 6a were obtained as yellow needlelike
blocks from an acetonitrile/diethyl ether solution at room
temperature. The dimeric dication crystallizes in space group
C2221 and is shown in Figure 5. The Pd−N pyridyl bond
lengths are approximately 2.01 Å, while those of the oxazolyl
A single-crystal X-ray structure of 4d was also obtained and is
shown in Figure 4. The complex crystallizes as yellow blocklike
crystals from a red dichloromethane/hexane solution at room
temperature. The Pd is coordinated in a distorted four-
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Figure 6. X-ray structure of the bridging acetate dimer 6d. The two
triflate counterions are omitted for clarity. Ellipsoids are drawn at the
50% probability level. Selected bond distances (Å) and bond angles
and torsion angles (deg): Pd1−Pd2, 2.9094(4); Pd1−N1, 1.992(3);
Pd1−O3, 2.006(3); Pd1−O5, 2.024(3); Pd1−N2, 2.096(3); Pd2−N3,
1.979(3); Pd2−O6, 2.000(3); Pd2−O4, 2.021(3); Pd2−N4, 2.078(3);
N1−Pd1−O5, 174.1(2); O3−Pd1−N2, 170.0(1); O6−Pd2−N4,
170.0(1); N1−Pd1−N2, 80.5(1); N3−Pd2−N4, 80.8(1); O6−Pd2−
O4, 86.2(1); O3−Pd1−O5, 86.8(1); N1−Pd1−Pd2−N3, 100.1(1);
O3−Pd1−Pd2−N3, −171.49(13); O5−Pd1−Pd2−N3, −83.0(1);
N2−Pd1−Pd2−N3, 18.4(1); N1−Pd1−Pd2−O6, −169.1(1); O3−
Pd1−Pd2−O6, −80.7(1); O5−Pd1−Pd2−O6, 7.8(1); N2−Pd1−
Pd2−O6, 109.21(1).
Figure 5. X-ray structure of the bridging acetate dimer 6a. The two
triflate counterions are omitted for clarity. Ellipsoids are drawn at the
50% probability level. Selected bond distances (Å) and bond angles
(°): Pd1−Pd2, 3.0742(9); Pd1−O6, 1.986(7); Pd1−N2, 1.987(8);
Pd1−O4, 2.011(6); Pd1−N1, 2.012(8); Pd2−N4, 1.997(9); Pd2−O3,
2.001(7); Pd2−O5, 2.019(7); Pd2−N3, 2.022(9); O4−Pd1−N,
194.1(3); O6−Pd1−Pd2, 79.8(2); N2−Pd1−Pd2, 113.7(2); O4−
Pd1−Pd2, 77.4(2); N1−Pd1−Pd2, 106.3(2); N4−Pd2−O3, 168.3(3);
N4−Pd2−O5, 95.0(3); O3−Pd2−O5, 91.3(3); N4−Pd2−N3,
80.5(4); O3−Pd2−N3, 92.9(3); O5−Pd2−N3, 175.3(3); N4−Pd2−
Pd1, 112.8(2); O3−Pd2−Pd1, 78.3(2); O5−Pd2−Pd1, 76.2(2); N3−
Pd2−Pd1, 106.8(2); O6−Pd1−N2, 95.3(3); O6−Pd1−O4, 89.1(3);
N2−Pd1−O4, 168.6(3); O6−Pd1−N1, 173.7(3); N2−Pd1−N1,
80.6(3).
5 mM in in CD3CN, the complex exists primarily in its
monomeric form, [(L6d)Pd(OAc)(CD3CN)]OTf (Scheme 2),
1
as revealed by the concentration-dependent H and 13C NMR
Pd−N are approximately 1.99 Å. The ligands adopt an eclipsed
orientation (when viewed down the Pd−Pd axis) where both i-
Pr groups are on the same side of the molecule, with one i-Pr
substituent directed toward the interior of the dimeric
sandwich. The Pd atoms are linked through bridging acetates,
giving rise to a Pd1−Pd2 distance of 3.0742(9) Å. The 1H and
13C NMR spectra of this complex are complex in CD3CN,
likely as a consequence of the diastereotopic isomeric dimers
that exist in solution. Complex 5a, however, has very well-
defined NMR spectra, as do all other palladium bis-acetates,
5b−d.
spectra.
Scheme 2. Concentration-Dependent Dimer-to-Monomer
Equilibration
Single crystals of 6d were obtained as orange-yellow blocks
from an acetonitrile/diethyl ether solution at −20 °C. This
dimeric complex crystallizes in the space group P1; shown in
Figure 6 is the dicationic dimer (two triflate anions also exist in
the unit cell but are not within bonding distance to the dimer),
which adopts a structure analogous to that of [(neocuproine)-
Pd(μ-OAc)]2OTf2.31 The orientation of the ligand and bridging
acetates is such that the pyOx ligands are situated in a partially
eclipsed fashion and there is a close contact of 2.9094(4) Å
from Pd1 to Pd2. The Pd−N(pyOx) bond distances (1.992(3)
and 1.979(3) Å) are shorter than the Pd−N(pyridine)
distances (2.096(3) and 2.078(3) Å). The Pd−O distances
are all similar. The bond angles about the Pd atoms (excluding
the Pd−Pd interaction) reflect a square-planar geometry with
angles of 90°. In solution, complex 6d exhibits a dynamic
equilibrium with its monomer, as reported previously for the
neocuproine Pd dimers.31 At a concentration of approximately
Chemoselective Oxidation of Polyols to Hydroxy
Ketones. As our screening experiments had revealed that
complex 6d was both the most active and most chemoselective
of the chiral complexes investigated for the oxidation of 1,2-
propanediol, we investigated several other alcohols to assess
both the chemo- and stereoselectivity. Summarized in Table 1
are results with representative diols, triols, and tetraols. At 60
°C, oxidation of 1,2-propanediol with benzoquinone afforded
hydroxyacetone with 99% selectivity after 45 min (Table 1,
entry 1). Oxidation of glycerol afforded dihydroxyacetone in
87% isolated yield; by 1H NMR, we could not detect any of the
aldehyde resulting from oxidation of the primary alcohol. This
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f
Table 1. Chemo-, Regio-, and Enantioselective Oxidation of Polyols with 6d
a
b
1
Determined by H NMR with p-xylene as internal standard. Entry 1 is NMR scale yield with internal standard, entries 2−5 are isolated yields
c
d
e
including both enantiomers where applicable. Determined from HPLC analysis. Determined by NMR. Determined from derivatized tris-
f
acetylated compound (see the Experimental Section). All reactions were performed in 9/1 CH3CN/H2O, except for entry 5, where a 2.6/1 ratio was
used; deuterated solvents were used for NMR scale reactions. Benzoquinone65 was used as a co-oxidant at approximately 1.15 equiv with respect to
alcohol.
Scheme 3. Proposed Mechanism for the Oxidation of Diols with Catalyst 6d
high chemoselectivity for oxidation of the secondary alcohol is
analogous to that observed with the achiral [(neocuproine)-
Pd(μ-OAc)]2OTf2.12 Oxidation of 1,2,4-butanetriol (entry 3,
Table 1) gave 1,4-dihydroxybutanone in 85% yield after 24 h.
Oxidation of meso-erythritol afforded a 62% isolated yield of
erythrulose; NMR experiments reveal that the chemoselectivity
in the oxidation of this tetraol is approximately 71% in favor of
the α-hydroxy ketone, illustrating the high chemoselectivity of
these oxidation reactions for polyols.
Complex 6d is also efficient at oxidizing mono alcohols such
as 2-propanol and 1-propanol to their corresponding ketone or
aldehyde. Oxidation of 2-propanol with 6d (10 mol % Pd) and
benzoquinone gave a 56% yield of acetone after 1 h at 65 °C in
CD3CN/D2O. Under comparable conditions, oxidation of 1-
propanol provided a 78% yield of propanal after 1 h, suggesting
that the primary alcohol is oxidized faster than the secondary
alcohol.39 This was confirmed by oxidation of a 1/1 mixture of
2-propanol and 1-propanol under similar conditions: after 1 h
at 65 °C, 1-propanol was oxidized more rapidly than 2-
propanol to give a combined 58% yield of propanal and acetone
in a ratio of 1.8. These data indicate that the chemoselectivity
for oxidation of primary and secondary alcohols with Pd
catalysts derived from 6d is lower and opposite from that of
1,2-propanediol: 1,2-propanediol is oxidized selectively at the
secondary alcohol to afford the hydroxy ketone, whereas
mixtures of primary and secondary alcohols exhibit lower
selectivities in favor of the primary alcohol.
While the chemoselectivities observed with the chiral
complex 6d are high, the enantioselectivities for the oxidation
of meso-polyols were modest. Oxidation of meso-hydrobenzoin
with complex 6d led to the clean conversion of hydrobenzoin
to benzoin with no evidence of the diketone under these
conditions (55 °C, approximately 1 h). Analysis of the hydroxy
ketone by chiral HPLC revealed the formation of (R)-benzoin
in 11% ee. To assess if the low enantioselectivities were a
consequence of racemization of the resulting hydroxyketones,
both (R,R)-hydrobenzoin and (S,S)-hydrobenzoin were inves-
tigated as substrates. NMR scale oxidations of (R,R)-hydro-
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benzoin or (S,S)-hydrobenzoin at 60 °C with 6d afforded (R)-
benzoin or (S)-benzoin with >90% ee after less than 2 h,
implicating that racemization of the hydroxy ketones is minimal
under these conditions. Higher enantioselectivities (≤76% ee)
were reported for the oxidation of meso-hydrobenzoin with
Cu(OTf)2 in the presence of phenyl box ligands of type L1.66
Oxidation of meso-erythritol (entry 5, Table 1) with 6d
cleanly afforded (S)-erythrulose in 62% isolated yield and 24%
ee after column purification. In contrast to meso-hydrobenzoin,
the R alcohol of meso-erythritol was oxidized preferentially,
albeit with low stereoselectivity.
α-hydroxy ketones with high chemoselectivity in isolated yields
of 62−87%. While the chemoselectivities are high, the
enantioselectivities observed for the oxidation of meso-hydro-
benzoin or meso-erythritol with the chiral complex 6d were
modest at 11−24% ee.
EXPERIMENTAL SECTION
■
1
General Information. All H and 13C{1H} NMR spectra were
obtained at room temperature on Varian 400 MHz, Unity/Inova
Varian 500 MHz, or Unity/Inova Varian 600 MHz spectrometers and
taken at room temperature. Enantiomeric excess was determined using
chiral HPLC (see below for details). For 1,2-propanediol oxidation
monitored by 1H NMR spectra were acquired with d1=60 s, nt=4 or 8,
gain=20 and autoshimmed on a Varian 400 MHz instrument. The
chemical shifts are reported in parts per million (δ), and the
multiplicities are reported as s (singlet), d (doublet), (triplet), q
(quartet), m (multiplet), sept (septet), app (apparent), and br
(broad). All peaks were monitored with respect to the p-xylene peak
(7.00 ppm in CD3CN/D2O solvent mixture and added via microliter
syringe). Residual proton and carbon peaks were used as solvent
references: 1H (δ (ppm): chloroform-d, 7.26; deuterium oxide-d2,
4.78; acetonitrile-d3, 1.95; dimethyl sulfoxide-d6, 2.49); 13C (δ (ppm):
chloroform-d, 77.3; acetonitrile, 1.13, 117.1; dimethyl sulfoxide-d6,
39.51). 19F NMR spectra were referenced to the external standard
0.05% α,α,α-CF3C6H5 in C6D6 (δ (ppm): C6D6; −63.72). LCMS data
was obtained from a Waters 2795 HPLC system with a dual-
wavelength UV detector and ZQ single quadrupole MS with
electrospray ionization (ESI) source and 0.1% formic acid acetonitrile
solvent with direct loop injection. High-resolution MS measurements
were obtained from an Orbitrap ESI via direct injection or by
desorption ESI (DESI; see the Supporting Information). An HP 7890/
5975 GC-MS, single quadrupole MS instrument with electron impact
ionization source was also used to obtain GC data. Elemental and trace
silver analysis was performed at Robertson Micrlolit Laboratories Inc.,
Ledgewood, NJ.
Proposed in Scheme 3 is a mechanism12,31 for the oxidation
of vicinal diols by complex 6d. We propose that complexation
of the diol to the monomeric solvate 6dm is followed by
intramolecular deprotonation67 of the coordinated alcohol with
loss of HOAc to generate a Pd alkoxide alcohol chelate. β-
Hydride elimination would generate the hydroxy ketone adduct
of the palladium hydride. Displacement of the hydroxy ketone,
subsequent oxidation of the Pd−H by benzoquinone, and
substitution by another diol would regenerate the Pd diolate.
The high chemoselectivities exhibited by 6d for the oxidation
of vicinal diols to hydroxy ketones are analogous to those
observed for the achiral neocuproine catalysts.12 On the basis of
kinetic and theoretical studies to be reported elsewhere,68 we
propose that the high chemoselectivity for oxidation of 1,2-
propanediol derives from product-determining β-H elimination
from the secondary Pd alkoxide to yield the hydroxy ketone.
The lower chemoselectivities observed for primary and
secondary alcohols is proposed to arise from product-
determining equilibration of the Pd alkoxides, favoring the
primary alkoxide.
The modest enantioselectivities observed preclude an
interpretation of the origin of the enantioselectivity (at 55
°C, an ee of 24% corresponds to a ΔΔG⧧ value of <0.5 kcal/
mol). If β-H elimination from the solvated diolate S (Scheme
3) is the stereodifferentiating step, there are at least two
diasteromeric Pd diolates that can form from meso-hydro-
benzoin, due the lack of C2 symmetry for ligands 6a−d. For
meso-erythritol, the situation is even more complicated, as
diolates could form from either the 1,2-position or the 2,2′-
position. Crystal structures of neutral Pd diolate complexes of
erythritol69,70 suggest that the diolate complexes can coordinate
at the 1,2-diol position or the 2,2′-position, depending on the
coligands. These considerations highlight the challenges for the
chemo- and enantioselective oxidation of unprotected polyols.
Nevertheless, the high chemoselectivities observed with Pd
complexes derived from the chiral ligand L3d and the achiral
neocuproine,12 and the impressive enantioselectivities observed
with sparteine ligands,3,6,15,16 provide promising leads for the
further development of alcohol oxidation catalysts capable of
both high high chemo- and enantioselectivities.
X-ray Crystallography. Single crystals were mounted on either a
Bruker Kappa X8 or a Bruker D8 Venture diffractometer equipped
with an Apex II CCD or Photon 100 CMOS detector, respectively.
Frames were collected using ω and ψ scans and integrated with
SAINT. Multiscan absorption correction (SADABS) was applied.71
The structures were solved by direct methods (SHELXS) and refined
using full-matrix least squares on F2 with SHELXL-9772 using
established procedures.73 Weighted R factors, Rw, and all goodness
of fit indicators are based on F2. All non-hydrogen atoms were refined
anisotropically. The hydrogen atoms of the C−H bonds were placed in
idealized positions. Further crystallographic details and the corre-
sponding CIF files can be found in the Supporting Information.
Chemicals. Sodium methoxide (97%), 6-methyl-2-pyridinecarbo-
nitrile (97%), 2-pyridinecarbonitrile (99%), anhydrous toluene
(99.8%), glacial acetic acid, L-tert-leucinol ((S)-2-Amino-3,3-dimeth-
yl-1-butanol) (98%), L-valinol ((S)-2-amino-3-methyl-1-butanol
(96%)), (R)-(−)-2-phenylglycinol (or (R)-2-amino-2-phenylethanol)
(98%, 99% ee), (S,S)-2,2′-isopropylidene-bis(4-tert-butyl-2-oxazoline)
(L1; 99%), silver acetate (99%), silver trifluoromethanesulfonate
(≥99.5%), silver tetrafluoroborate (98%), trifluoromethanesulfonic
acid (99%), 1,2-propanediol (≥99.5%), benzoquinone (≥99.5%),
meso-erythritol (99%), meso-hydrobenzoin (99%), (R,R)-hydrobenzoin
(99%, 99% ee), (S,S)-hydrobenzoin (99%, 99% ee), DL-benzoin (98%),
1,2,4-butanetriol (98%), and europium(III) trifluoromethanesulfonate
(98%) were purchased from Sigma Aldrich. With the exception of
benzoquinone (recrystallized in petroleum ether 40−60 °C), all
reagents were used as received from Sigma Aldrich. Anhydrous
methanol (99.8%) was purchased from Arcos Organics and used as
received. Dichloromethane, hexanes, acetonitrile, diethyl ether, ethyl
acetate, and ethanol (all ACS grade), p-xylene (99.4%), glycerol
(99.8%), HPLC grade isopropyl alcohol, and heptane were purchased
from Fischer Scientific. rac-Erythulose was prepared independently
using a previously reported achiral palladium catalyst12,68 and meso-
erythritol and used as the racemic standard for this study.
SUMMARY
■
Cationic palladium complexes ligated by chiral pyOx ligands are
efficient catalysts for the chemoselective oxidation of vicinal
polyols to afford α-hydroxy ketones. The activity and selectivity
of the Pd catalysts for alcohol oxidation depend sensitively on
the ligand environment; complex 6d, derived from the tert-Bu
pyOx ligand containing an α-methyl substituent, is more active
than pyOx complexes lacking the α-methyl substituent or those
derived from bis(oxazoline) or bi(oxazoline) ligands. The
oxidation of the vicinal diols 1,2-propanediol and hydro-
benzoin, the triol glycerol, and the tetraol erythritol yields the
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Dichlorobis(benzonitrile)palladium(II) (99%) and tetrakis-
(acetonitrile)palladium(II) tetrafluoroborate (99%) were purchased
from Strem Chemicals and used as received. CDCl3, CD3CN, and
D2O were obtained from Cambridge Isotopes Laboratories and used
as received.
with 50 °C heating. Yield: 7.89 g, 92% (with a half-molecule of
dichloromethane in molecular weight). Attempts to remove the solvate
with prolonged heating (50−90 °C) under vacuum or redissolving in
other solvents (MeCN, ether) led to decomposition or new solvent
1
incorporation, respectively. H NMR (300 MHz, CDCl3): 7.96 (t, J =
Ligand Synthesis. Ligand L1, (S,S)-2,2′-isopropylidene-bis(4-tert-
butyl-2-oxazoline), was purchased from Sigma-Aldrich and used as
received.
7.76 Hz, 1H), 7.67 (dd, J = 7.51 Hz, 1H), 7.53 (dd, J = 8.01 Hz, 1H),
5.28 (s, 1H, 1/2 dichloromethane), 4.86 (dd, J = 9.32 Hz, 1H), 4.74 (t,
J = 9.27 Hz, 1H), 4.41 (dd, J = 9.18 Hz, 1H), 3.14 (s, 3H), 1.05 (s,
9H). 13C NMR (125 MHz, CDCl3): 169.07, 167.86, 144.70, 139.54,
131.72, 123.01, 73.97, 69.54, 53.7 (DCM), 35.46, 27.61, 26.04. Anal.
Calcd for [(L3d)PdCl2)]·1/4CH2Cl2 (C13H18Cl2N2OPd·1/4CH2Cl2):
C, 38.18; H, 4.47; N, 6.72. Found: C, 38.31; H, 4.21; N, 6.63. MS
(ESI): m/z found 400.1 (calcd for [(L3d)Pd(MeCN)Cl]+ 400.04). X-
ray-quality crystals were grown by slow evaporation of a dichloro-
methane solution at room temperature, to reveal large red crystals (see
Figure 4 and the Supporting Information).
Bi(oxazoline) ligands L2a,b were prepared by literature methods46
and characterized by 1H NMR and GC-MS (see the Supporting
Information).
Ligands L3a−d were prepared according to Scheme 1 from 2-
pyridinecarbonitrile or 6-methylpicolinonitrile. A detailed procedure is
given below for L3d; ligands L3a−c were prepared following a similar
procedure56 (see the Supporting Information). Compounds 2a,b were
synthesized according to the literature.42
Synthesis of 2b.42 NaOMe (4.15 g, 0.0768 mol) was placed in a
50 mL pear-shaped round-bottomed flask in a nitrogen glovebox.
Under a stream of argon, anhydrous MeOH (18 mL) was added and
stirred, dissolving the NaOMe. During this time the solution became
warm. 6-Methyl-2-pyridinecarbonitrile (1; 4.0 g, 0.0338 mol) was
dissolved in 20 mL of anhydrous toluene. The toluene solution was
added to the stirred MeOH solution and left to react for at least 4 h at
room temperature. HOAc (4.61 g, 0.0767 mol) was added to the
solution, during which the solution became gelatinous. The solution
was then dried under vacuum to reveal a white powder in an oily
liquid. The residue was then dissolved in 25 mL of dichloromethane
and the insoluble residue was filtered by gravity. The brown solution
was dried under vacuum to reveal a brown oil. The brown oil was
vacuum-distilled at 60 °C at 60 mTorr to give a colorless oil and used
the same day. Yield: 4.1 g (81%). 1H NMR (CDCl3, 400 MHz): 7.58−
7.68 (m, 2H), 7.18 (ddd, J = 6.87, 0.41 Hz, 1H), 3.97 (s, 3H), 2.55 (s,
3H). 13C NMR (125 MHz, CDCl3): 167.34, 158.45, 146.99, 137.57,
125.31, 118.23, 54.08, 24.69. GC-MS (EI): calculated for C8H10N2O
150.08, found m/z (EI) M+ 150.1.
Synthesis of (S)-4-(tert-Butyl)-2-(6-methylpyridin-2-yl)-4,5-
dihydrooxazole (L3d). Compound 2b (4.12 g, 0.0274 mol) was
placed in a 50 mL round-bottomed flask. L-tert-Leucinol ((S)-2-amino-
3,3-dimethyl-1-butanol; 3.22 g, 0.0274 mol) was added. The flask was
heated to 65 °C and the mixture stirred under a gentle stream of argon
(to aid in evaporation of volatiles) for 16 h. During this time, the
slightly greenish solution solidified. The reaction mixture was cooled
to room temperature, and the residue was recrystallized from
petroleum ether (36−60 °C) and dried under vacuum to reveal a
white, needlelike powder. Yield: 5.05 g (84%). Alternatively, the ligand
can be purified by flash silica gel chromatography, with 5/1 hexanes/
ethyl acetate as eluent. 1H NMR (CDCl3, 400 MHz): 7.94 (d, J = 7.60
Hz, 1H), 7.65 (t, J = 7.74 Hz, 1H), 7.24 (d, J = 7.17 Hz, 1H,
overlapping with residual solvent peak), 4.45 (dd, J = 10.18 Hz, 1H),
4.31 (t, J = 8.50 Hz, 1H), 4.10 (dd, J = 10.24 Hz, 1H), 2.63 (s, 3H),
0.96 (s, 9H). 13C NMR (125 MHz, CDCl3): 162.86, 158.82, 146.66,
136.90, 125.46, 121.50, 76.56, 69.58, 34.23, 26.17, 24.92. GC-MS (EI):
calculated for C13H18N2O 218.14, found m/z (EI) M+ 218.1.
Pd Complexes. Screening of palladium complexes with ligands L1
and L2 and their syntheses are described in the Supporting
Information. The syntheses of compounds 4d−6d are described in
detail below; complexes 4a−c, 5a−c, and 6a−c were prepared
analogously (see the Supporting Information).
Synthesis of Diacetatopalladium (S)-4-(tert-Butyl)-2-(6-methyl-
pyridin-2-yl)-4,5-dihydrooxazole-6-methylpicolinonitrile (5d). Com-
1
pound 4d (7.33 g, 0.0176 mol, with /2 MeCN solvate as dry weight
1
determined by H NMR, solvent corrected mol wt of 416.14 g/mol)
was placed in a 250 mL round-bottomed flask equipped with a stir bar
and wrapped in aluminum foil. AgOAc (6.28 g, 0.0376 mol) was added
to the flask under a stream of argon. A 50 mL portion of
dichloromethane was added, and the mixture was stirred in the dark
for 45 min. Under a stream of argon, the solution was filtered through
a plug of Celite and evaporated to dryness for several hours to reveal
an orange precipitate. Yield: 8.48 g (89% with 1/2 dichloromethane as
1
judged by H NMR). The precipitate was used immediately for the
1
next step. H NMR (300 MHz, CDCl3): 7.96 (t, J = 7.47 Hz, 1H),
7.57 (d, J = 7.44 Hz, 1H), 7.43 (d, J = 7.47 Hz, 1H), 4.90−4.51 (m,
2H), 3.87 (dd, J = 8.54 Hz, 1H), 2.66 (s, 3H), 2.00 (s, 3H), 1.96 (s,
3H), 0.97 (s, 9H). 13C NMR (125 MHz, CDCl3): 178.75, 178.356,
169.966, 166.354, 144.822, 139.719, 131.462, 122.822, 73.602, 72.209,
34.860, 25.930, 23.961, 23.086, 23.018 (br sh). MS (ESI): m/z found
[M − OAc]+ 383.1 (100%) (calcd 383.06, 100%).
Synthesis of [(pyOx)Pd(OAc)]2[OTf]2, (6d). A 0.33 M HOTf
solution was prepared freshly from anhydrous acetonitrile. A 53 mL
portion (0.0179 mol) of this solution was added directly to a powder
1
containing compound 5d (8.48 g, 0.0174 mol, with
/ dichloro-
2
methane solvate as dry weight determined by 1H NMR, solvent
corrected mol wt of 485.26 g/mol), and the mixture was stirred for 45
min. A deep red solution formed. A 650 mL portion of anhydrous
diethyl ether was added to the solution, and an orange precipitate
formed. The solution was placed in a −20 °C freezer for 1 h. The
supernatant was carefully decanted, and the residue was washed with 3
× 100 mL of ether or until the washings became clear. The orange
precipitate was dried overnight under vacuum at 40 °C to give 7.20 g
of orange product. Another 0.61 g was recovered from the washing
solutions following the same procedure as above. Overall yield: 7.80 g,
86% (78% from (PhCN)2PdCl2). 1H NMR of [(pyOx)Pd(OAc)-
(CH3CN)][OTf] (6dm) (600 MHz, CD3CN, 4.8 mM solution,
approximately 5% dimer present): 8.13 (t, J = 7.82 Hz, 1H), 7.74 (dd,
J = 7.59 Hz, 1H), 7.67 (d, J = 7.89 Hz, 1H), 4.99 (dd, J = 9.66 Hz,
1H), 4.82 (t, J = 9.53 Hz, 1H), 3.88 (v br, 1H), 2.69 (s, 3H), 1.93 (br
1
s, 3H), 1.00 (s, 9H). H NMR of [(pyOx)Pd(OAc)]2[OTf]2 (6dd)
(600 MHz, CD3CN, 448 mM solution, approximately 35% 6dm): 8.29
(t, J = 7.81 Hz, 1H), 7.89 (d, J = 7.90 Hz, 1H), 7.68 (d, J = 7.44 Hz,
1H, overlapping with 6dm), 4.80 (dd, J = 10.42 Hz, 1H, overlapping
with 6dm), 4.04 (t, J = 9.89 Hz, 1H), 3.51 (dd, J = 9.29 Hz, 1H), 2.72
(s, 3H), 2.17 (s, 3H), 0.99 (s, 9H, overlapping with 6dm). 13C NMR of
[(pyOx)Pd(OAc)]2[OTf]2 (6dd) (125 MHz, CD3CN, 448 mM
solution): 188.151, 170.386, 167.385, 142.909, 133.812, 126.752,
Synthesis of Dichloropalladium (S)-4-(tert-Butyl)-2-(6-methylpyr-
idin-2-yl)-4,5-dihydrooxazole-6-methylpicolinonitrile (4d). Com-
pound L3d (4.25 g, 0.0194 mol) was placed in a 200 mL round-
bottomed flask equipped with a stir bar. (PhCN)2PdCl2 (7.305g,
0.01904 mmol) was added to the flask. A 50 mL portion of
dichloromethane was added, and a deep red solution formed. The
solution was stirred for 18 h at room temperature. The volume was
reduced to one-fourth of the original solvent under vacuum. Hexanes
was added to precipitate out an orange-brown product. The
supernatant was carefully decanted, and the precipitate was washed
and triturated with 3 × 100 mL of hexanes or until the supernatant
became clear. The precipitate was dried under vacuum (60 mTorr)
−
74.370, 71.010, 35.550, 24.906, 24.563, 24.068, 23.272. CF3SO3 not
observed. 19F NMR (376 MHz, CD3CN): −80.41. HR-MS ESI
Orbitap (neat CD3CN): found m/z (relative intensity) 383.0581
−
(100%) [0.5M − CF3SO3 ] (calcd 383.0582 (100%) agrees within 0.2
ppm for C15H21N2O3Pd+, i.e. Pd monomer (singly cationic) species)
complete isotope pattern 383.0581 (100%), 385.0579 (88.78%),
382.0596 (80.00%), 387.0590 (38.64%), 381.0584 (35.69%), 384.0610
(13.90%), 386.0611 (13.32%), 388.0621 (5.40%), 379.0600 (2.61%).
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Anal. Calcd for [(L3d)Pd(OAc)][OTf] (C16H21F3N2O6PdS): C,
36.07; H, 3.97; N, 5.26. Found: C, 35.82; H, 3.71; N, 5.15. X-ray-
quality single crystals were grown from diethyl either diffusion into a
concentrated acetonitrile solution at approximately −20 °C (see
Figure 6 and the Supporting Information). We also performed ICP
trace metal analysis for silver and obtained 2650 ppm (0.26%) of silver
in this sample. This sample was used for all preparatory scale oxidation
catalyses described in this paper. Control experiments reveal that
Ag(L3d)+ is inactive for alcohol oxidation (see the Supporting
Information).
Alcohol Oxidation: Screening Experiments. A series of
screening experiments (NMR and preparatory scale) were conducted
in order to determine which ligand set had the most promising activity
for 1,2-propanediol oxidation (NMR scale). Screening experiments
were done on systems containing ligands L1, L2a, L2b, and L3a−L3d,
where the latter was found to be superior and studied in detail.
Oxidation of 1,2-Propanediol with [(L2b)Pd(MeCN)2][BF4]2/
Cs2CO3. [(L2b)Pd(MeCN)2][BF4]2 (5 mg, 0.00813 mmol) was
added to a 0.7 mL DMSO-d6 solution containing 1,2-propanediol
(6.2 mg, 5.9 uL, 0.0815 mmol), Cs2CO3 (1.3 mg, 0.00398 mmol),
benzoquinone (17.6 mg, 0.162 mmol), and p-xylene (8.6 mg, 10 uL,
0.081 mmol). At room temperature, less than 7% conversion was
observed over the course of 2 days; however, heating the same
solution to 60 °C for 1 h led to a 23% yield of hydroxyacetone, as
determined by integration vs the internal standard p-xylene (with
relaxed 1H NMR spectra, 400 MHz) with significant Pd black
formation. Other attempts to generate the active catalyst in situ
involved addition of HOTf to the palladium bis-acetate complexes;32
addition of 1 equiv of HOTf (29 μL, 0.33 M HOTf in a MeCN
solution) to a 0.7 mL CD3CN stock solution containing (L2a)PdOAc2
(4.3 mg, 0.01 mmol), benzoquinone (21.g mg, 0.200 mmol), and 1,2-
propandiol (7.5 mg, 0.100 mmol) revealed no evidence of
hydroxyacetone formation after 18 h at room temperature.
was performed. Evaporation of the relevant fractions gave 88 mg (0.85
mmol, 85% yield) of a light brownish oil. 1H NMR (500 MHz, D2O):
2.63 (t, J = 5.86 Hz, 2 H) 3.78 (t, J = 5.86 Hz, 2 H) 4.30 (s, 2 H). 13C
NMR (126 MHz, D2O): 40.92 (s, 1 C) 57.02 (s, 1 C) 68.30 (s, 1 C)
212.82 (s, 1 C).
Oxidation of meso-Hydrobenzoin. meso-Hydrobenzoin (250 mg,
1.16 mmol) and recrystallized benzoquinone (150 mg, 1.37 mmol)
were dissolved in 18.0 mL of a 9/1 CH3CN/H2O mixture in a 200 mL
pear-shaped round-bottomed flask. The solution was stirred until the
reactants dissolved. Pd catalyst 6d (62.1 mg, 0.116 mmol using
monomer molecular weight of 532.01 g/mol) was added to the vial.
The flask was capped and placed in a 55 °C oil bath and the reaction
monitored by 1H NMR spectroscopy. Previous NMR experiments
revealed a maximum yield of 100% for this reaction under the same
conditions. After 1.2 h, the solution was evaporated to dryness with 35
°C heating to reveal a dark black-green mixture. The crude product
was dissolved in a minimum amount of EtOAc solution and triturated
with slight heating and sonication and then loaded onto a column
using 15/1 hexanes/EtOAc eluent. The silica column had dimensions
of 18 cm × 2.5 cm in a 35 cm length column. The product has an Rf
value of 0.3 and comes out immediately after the BQ-HQ fraction.
Some coelution is observed with the BQ-HQ fractions, which is
responsible for lower isolated yields. The product -containing fractions
were combined and evaporated to dryness to reveal a white precipitate
1
in 71% yield (176 mg). H NMR (500 MHz, CDCl3): 7.94 (dd, J =
8.44 Hz, 2H), 7.55 (t, J = 7.44 Hz, 1H), 7.42 (t, J = 7.77 Hz, 2H),
7.39−7.27 (m, 5H), 5.98 (d, J = 6.10 Hz, 1H), 4.59 (d, J = 6.09 Hz,
1H). 13C NMR (125 MHz, CDCl3): 199.168, 139.229, 134.195,
133.671, 129.410, 129.400 (sh), 128.952, 128.850, 128.027, 76.460.
GC-MS (EI): m/z 212. Chiral HPLC was carried out by a modified
literature preparation:66 Chiralcel OJ-H, iPrOH/heptane 10/90, flow
rate of 0.8 mL/min and 210 nm detection with manual injection; (R)-
benzoin, 18.9 min; (S)-benzoin, 22.8 min; 5−11% ee was observed
over multiple runs in favor of the R enantiomer (major). The sample
was compared to the commercial DL-benzoin.
Oxidation of meso-Erythritol. meso-Erythritol (250 mg, 2.05
mmol) and recrystallized benzoquinone (234 mg, 2.16 mmol) were
dissolved in a 30.0 mL solution of a 2.6/1 CH3CN/H2O mixture in a
200 mL pear-shaped round-bottom flask. The solution was stirred until
the reactants dissolved. Pd catalyst 6d (108.9 mg, 0.204 mmol) was
added to the flask. The solution was sealed and placed in a 60 °C
heated oil bath, and the conversion was monitored by 1H NMR
spectroscopy. After 1.2 h, the solution was evaporated to dryness at 35
°C to reveal a dark black-green syrupy mixture. The crude product was
dissolved in a minimal amount of a 20/1 EtOAc/EtOH mixture,
triturated, heated slightly, and sonicated. The crude mixture was
loaded onto a silica column (18 cm × 2.5 cm in a 35 cm length
column) and eluted with a 20/1 EtOAc/EtOH mixture. The product
has an Rf value of 0.4 and comes out soon after the BQ-HQ fraction.
The product-containing fractions were combined and evaporated to
dryness to reveal a syrupy colorless oil in 61% yield (140 mg).
Previous NMR experiments reveal a maximum yield of 71% for this
reaction under the same conditions. 1H NMR (400 MHz, D2O):
4.61−4.44 (m, 2H), 4.42 (t, J = 4.08 Hz, 1H), 3.83 (q, J = −1.5 Hz,
2H). 13C NMR (125 MHz, D2O with MeOH internal standard, 49.13
ppm): 212.546, 76.149, 66.107, 63.138.
NMR Scale Oxidation Reactions. Oxidation of 1,2-Propanediol.
1,2-Propanediol (3.57 mg, 0.0469 mmol), benzoquinone (5.6 mg,
0.0518 mmol), and p-xylene (5.5 mg, 0.0519 mmol) was dissolved in
0.7 mL of a 8/1 CD3CN/D2O solution. To this solution was added 2.5
mg of 6d (0.00468 mmol), and the NMR tube was placed in a heated
1
oil bath at 60 °C. After 45 min, another H NMR measurement was
taken to reveal the formation of hydroxyacetone. 1H NMR (9/1
CD3CN/D2O, 400 MHz): 4.12 (s, 2H), 2.04 (s, 3H). See the
1
Supporting Information for H NMR.
Preparative Scale Oxidation Reactions. Oxidation of Glycerol.
Glycerol (100 mg, 1.09 mmol) and benzoquinone (130 mg, 1.2 mmol)
were dissolved in 10 mL of a 9/1 acetonitrile/water mixture. Catalyst
6d (26.6 mg 0.05 mmol) was added, and over a few minutes, the
reaction mixture darkened. The mixture was stirred for 24 h at 55 °C.
Upon completion of the reaction, a palladium mirror was observed on
the walls of the vial. After completion, the mixture was rotary
evaporated at 40 °C. This mixture was rotary evaporated, and the
residue was dissolved in 15/1 ethyl acetate/ethanol and then purified
through silica gel chromatography, using a gradient elution of neat
ethyl acetate to 7/1 ethyl acetate/ethanol. Evaporation of the relevant
fractions gave 85 mg (0.944 mmol, 87% yield) of off-white flakes. In
D2O, dihydroxyacetone exists in equilibrium between the free and
hydrated forms. 1H NMR (500 MHz, D2O): 4.34 (s, 1 H) (free). 13
NMR (126 MHz, D2O): 65.48, 212.61 (free). H NMR (500 MHz,
D2O): ppm 3.50 (s, 1 H) (hydrated). 13C NMR (126 MHz, D2O):
64.17, 95.71 (hydrated). See the Supporting Information.
C
1
Derivatization of Erythrulose to Erythrulose Triacetate for
Chiral HPLC Analysis. This procedure was performed by a modified
literature procedure.74 When Eu(OTf)3 was used rather than
Er(OTf)3, we found that the isolated yields doubled. Erythrulose
(137 mg, 1.14 mmol) and Eu(OTf)3 (8.0 mg, 0.0135 mmol) was
placed in a 100 mL pear-shaped round-bottomed flask equipped with a
stir bar. The flask was filled with argon. Simultaneously, 7 mL of
anhydrous MeCN and Ac2O (0.40 mL, 0.432 g, 4.23 mmol) were
added to a septum-sealed flask with dynamic argon. The solution was
stirred at room temperature for 17 h under static argon. After
approximately 2 h of stirring, all of the erythrulose had completely
dissolved to give a slightly green solution. The reaction was monitored
Oxidation of 1,2,4-Butanetriol. 1,2,4-Butanetriol (106 mg,1 mmol)
and benzoquinone (130 mg, 1.2 mmol) were dissolved in 10 mL of a
9/1 acetonitrile/water mixture. Catalyst 6d (26.6 mg, 0.05 mmol) was
added, and over a few minutes, the reaction mixture darkened. The
mixture was stirred for 24 h at 55 °C. The reaction progress was
monitored via TLC, using 20/1 ethyl acetate/ethanol as the eluent.
After completion of the reaction, the mixture was rotary evaporated at
40 °C. The residue was dissolved in a 2 mL portion of acetonitrile and
then twice purified through silica gel chromatography. The first time
20/1 ethyl acetate/ethanol was used as an eluent, and the second time
a gradient elution of neat ethyl acetate to 15/1 ethyl acetate/ethanol
1
by H NMR for completion. After 17 h, 40 mL of ethyl acetate was
added to the mixture and it was extracted with a 50 mL saturated
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NaHCO3 solution. The organic phase was dried under vacuum and
purified by flash silica chromatography using 1/1 EtOAc/hexanes as
eluent to give a colorless oil with a slight green tinge to it. The product
had an Rf factor of approximately 0.6. Yield,: 236 mg (62%). 1H NMR
(400 MHz, CDCl3): 5.40 (dd, J = 5.00 Hz, 1H), 4.91−4.80 (dd, 2H),
4.50 (dd, J = 12.26 Hz, 1H), 4.38 (dd, J = 12.26 Hz, 1H), 2.18 (s, 3H),
2.17 (s, 3H), 2.08 (s, 3H). 13C NMR (100 MHz, CDCl3): 198.621,
170.602, 170.216, 170.041, 74.902, 66.866, 62.586, 20.842, 20.750,
20.591. Chiral HPLC analysis: Chiralcel AD-H, iPrOH/heptane 5/95,
flow rate 0.8 mL/min and 210 nm detection with manual injection;
(R)-erythrulose trisacetate, 22.0 min; (S)-erythrulose trisacetate, 25.4
min; 24% ee was observed (38% (R)-erythrulose trisacetate, 62% (S)-
erythrulose trisacetate). The sample was compared with rac-
erythrulose trisacetate synthesized in our laboratory (see Chemicals
above).
DFT Computations. Computations were performed using the
Gaussian 0975 software package in a Linux multiprocessor environ-
ment. Geometry optimizations and vibration analysis were performed
using the M06-L DFT functional as described by Truhlar and Zhao76
with the double-ζ polarized 6-31G(d,p)77−86 basis set for light atoms
and SDD87 ecp for Pd and added f coefficients88−90 and default
convergence criteria in the gas phase. All calculations were performed
with the a cationic charge and singlet ground state multiplicities in the
gas phase. The GEDIIS algorithm was used for geometry
optimizations. All stationary point structures were confirmed to be
local minima as determined by vibration analysis, where no imaginary
or negative frequencies were present for the intermediates. SCF
energies are not ZPE corrected. See the Supporting Information for
atomic coordinates and SCF energies.
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ASSOCIATED CONTENT
* Supporting Information
■
S
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Soc. 2004, 126, 6554−6555.
Text, figures, tables, and CIF files giving NMR spectra, GC and
LS-MS data, chiral HPLC traces, X-ray crystallography data,
and DFT computed atomic coordinates. This material is
available free of charge via the Internet at http://pubs.acs.org.
The crystal structures have been deposited at the Cambridge
Crystallographic Data Centre (CCDC) and allocated with the
deposition numbers CCDC 928778−928781.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for R.M.W.: waymouth@stanford.edu.
■
(26) Ferreira, E. M.; Stoltz, B. M. J. Am. Chem. Soc. 2001, 123, 7725−
7726.
(27) Trend, R. M.; Stoltz, B. M. J. Am. Chem. Soc. 2008, 130, 15957−
15966.
Notes
(28) Jensen, D. R.; Pugsley, J. S.; Sigman, M. S. J. Am. Chem. Soc.
2001, 7475−7476.
The authors declare no competing financial interest.
(29) Melero, C.; Shishilov, O. N.; Alvarez, E.; Palma, P.; Campora, J.
Dalton Trans. 2012, 41, 14087−14100.
ACKNOWLEDGMENTS
■
This research was supported by Department of Energy (DE-
FG02-10ER16198). A.G.D.C. acknowledges Stanford Univer-
sity for a Center for Molecular Analysis and Design (CMAD)
postdoctoral fellowship. K.C. is grateful for a Stanford Graduate
Fellowship. We thank Autumn Maruniak in the Trost
Laboratory for assistance with chiral HPLC. We thank Dr.
Stephen R. Lynch for assistance with NMR measurements, Dr.
Richard Perry for Orbitrap HRMS of complex 6d and Andrew
Ingram for 4c and the CH-activated Pd complex of L1. We
thank GCEP, Professor T. Daniel P. Stack, and Eric Stenehjem
for access and maintenance of the Redox computer cluster
(Stanford University) to perform the DFT computations.
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