Kinetic resolution of secondary alcohols by NHC-catalyzed oxidative esterification

Article abstract of DOI:10.1055/s-0030-1260030

Kinetic resolution of racemic secondary alcohols by oxidative esterification using carbene catalysis is described. Good to moderate selectivity has been achieved. N-Heterocyclic carbenes (NHCs) were systematically varied and several aldehydes were included in this study as acyl donors. As an oxidant, a readily available bisquinone-type system was used. Georg Thieme Verlag Stuttgart - New York.

Full text of DOI:10.1055/s-0030-1260030

1974
PAPER
Kinetic Resolution of Secondary Alcohols by NHC-Catalyzed Oxidative
Esterification
K
inetic Resolu
u
tionof Seco
mndaryAlcohols by
O
a
xidative
E
n
sterification De Sarkar, Anup Biswas, Chie Hoon Song, Armido Studer*
Institute of Organic Chemistry and NRW Graduate School of Chemistry, University of Münster, Corrensstr. 40, 48149 Münster,
Germany
Fax +49(251)8336523; E-mail: studer@uni-muenster.de
Received 18 March 2011
Dedicated to Prof. Dieter Hoppe on the occasion of his 70th birthday
acyl transfer
chiral NHC
O
Abstract: Kinetic resolution of racemic secondary alcohols by ox-
idative esterification using carbene catalysis is described. Good to
moderate selectivity has been achieved. N-Heterocyclic carbenes
(NHCs) were systematically varied and several aldehydes were in-
cluded in this study as acyl donors. As an oxidant, a readily avail-
able bisquinone-type system was used.
R1
O
R*
1
O
N
Suzuki and Maruoka
R1
X
N
O
R*
chiral NHC
oxidation
Key words: carbene catalysis, kinetic resolution, oxidative esterifi-
cation, secondary alcohols, acylazolium ions
R1
H
3
2
chiral acyl-
azolium ion
this work
OH
O
In recent years, a large number of catalysts based on small
organic molecules have been prepared and successfully
applied in a variety of transformations rendering the field
‘organocatalysis’1,2 one of the hottest areas in asymmetric
catalysis. Along this line, N-heterocyclic carbenes
(NHCs)3 have become prominent candidates which allow
catalyzation of a broad range of useful asymmetric trans-
formations including the benzoin condensation,4 the
Stetter reaction,5 conjugate umpolung or homoenolate re-
actions,6 redox processes7,8 and acyl-transfer reactions.9
R2
R3
R1
O
OH
4
racemic alcohol
+
R2
R3
R2
R3
6
5
kinetic resolution
enantioenriched
alcohol
enantioenriched
ester
Scheme 1 Catalytic generation and use of acylazolium ions as
asymmetric acylating reagents
ondary alcohols (Scheme 1). To our knowledge, kinetic
resolution by the acylation of racemic alcohols using an
oxidative esterification approach has not been reported.15
The development of nonenzymatic catalysts for the kinet-
ic resolution of racemic alcohols has been intensively in-
vestigated over the last decade;10 however, the use of
chiral NHCs in this context has not been well explored.
Suzuki and co-workers reported the first example of a
chiral NHC-catalyzed kinetic resolution of various 1-
arylethanols.11a Their strategy is based on the generation
of a chiral acylazolium ion12 3 by acyl transfer from a vi-
nyl acetate 1 to a chiral NHC, and subsequent stereoselec-
tive acylation of the racemic alcohol 4 (Scheme 1).13 The
same approach was later used by Maruoka and co-workers
who found that high selectivities can be achieved by using
bulky vinyl esters.11b
For optimization studies we chose the resolution of 1-(1-
naphthyl)ethanol (4a) using cinnamaldehyde (2a) in com-
bination with various chiral carbene precursors 7–13 as a
test system (Figure 1, Table 1). The aldehyde 2a (1 equiv)
was reacted with an excess of racemic alcohol 4a (3
equiv) in the presence of the triazolium salt (5 mol%), 1,8-
diazabicyclo[5.4.0]undec-7-ene (10 mol%) and bisqui-
none oxidant 1416 in tetrahydrofuran as a solvent (Table 1).
In contrast to most studies on kinetic resolutions where re-
actions using 1 equivalent of alcohol are stopped at lower
conversions, esterifications were allowed to go to maxi-
mum conversion at room temperature (2 to 12 hours de-
pending on the activity of the NHC). Ester 15a was
readily isolated and the enantiomeric excess (ee) was de-
termined by chiral HPLC. This ee value cannot be used
for calculation of the stereoselectivity factor (S-value)
since reactions were not conducted under pseudo-first-
order conditions in alcohol; however, the ee measured can
be taken as a good estimate for the evaluation of the effi-
ciency of the given NHC for the kinetic resolution of al-
cohol 4a. For the most efficient system, we also ran
reactions under pseudo-first-order conditions (large ex-
cess of alcohol, see below).
We have recently explored the biomimetic oxidation of al-
dehydes by oxidative NHC catalysis applying external or-
ganic oxidants. Our efforts culminated in novel mild
esterification, amidation and azidation protocols.14 Based
on these results, we envisioned the use of chiral acylazo-
lium ions of type 3, generated from the corresponding al-
dehydes 2 by oxidative NHC catalysis, as asymmetric
acylating reagents for the kinetic resolution of chiral sec-
SYNTHESIS 2011, No. 12, pp 1974–1983
x
x.
x
x
.
2
0
1
1
Advanced online publication: 06.05.2011
DOI: 10.1055/s-0030-1260030; Art ID: C30511SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
Kinetic Resolution of Secondary Alcohols by Oxidative Esterification
1975
With catalyst 7,17 the ester 15a was isolated in 84% yield
and 14% ee within 2 hours (Table 1, entry 1). We found
that the selectivity increased when switching from the
tert-butyldimethylsilyl to the bulkier tert-butyldiphenyl-
silyl ether 8 (36% ee, 88% yield; Table 1, entry 2). The S-
enantiomer was formed in excess.18 Selectivity with bi-
functional catalysts (i.e., 9, 1019 and 1119) was unsatisfac-
tory (Table 1, entries 3–5) indicating that the additional
functional group on the carbene does not influence selec-
tivity to a large extent. Chiral amino alcohol derived tria-
zolium salt 1220 provided a moderate selectivity (30% ee;
Table 1, entry 6), whereas electron-deficient catalyst 1321
was found to be inefficient in the oxidative esterification
under the applied conditions (Table 1, entry 7). Further
optimization studies were conducted using catalyst 8. We
found that by lowering reaction temperature, and by using
a stoichiometric amount of base, both ee and yield in-
creased (Table 1, entries 8 and 9).
N
N
Ph
Ph
N
N
O
N
N
N
N
N
O
BF4
BF4
NH
Ph
OTBS
OTBDPS
Ph
BF4
7
8
9
N
N
N
N
N
N
O
S
NH
NH
Ph
Ph
BF4
BF4
11
NH
NH
Ph
Ph
10
O
O
N
N
N
N
N
N
C6F5
Mes
Cl
BF4
12
13
Figure 1 Chiral carbene precursors used for kinetic resolution
Table 1 Kinetic Resolution of 1-(1-Naphthyl)ethanol (4a) Using
Various Carbene Precursors
We next studied the effect of the structure of the aldehyde
on the kinetic resolution of alcohol 4a using triazolium
salt 8 as a precatalyst under the optimized reaction condi-
tions to give esters 15b–i and 16a (Table 2). Electron-rich
cinnamaldehyde derivative 2b was found to be a good
acyl donor at –20 °C and ester 15b was isolated in 71%
yield with 59% ee (Table 2, entry 1). Aliphatic enal 2c de-
livered ester 15c with moderate ee (Table 2, entry 2). Ox-
idative esterifications with benzaldehyde derivatives were
performed at room temperature since they reacted very
slowly at –20 °C (Table 2, entry 3). para-Substituted elec-
tron-poor aldehydes 2d and 2e showed moderate to low
selectivity in the kinetic resolution of alcohol 4a (Table 2,
entries 4 and 5). With 2-bromobenzaldehyde (2f), the
ester 15f was isolated with a good selectivity (60% ee;
Table 2, entry 6), whereas 2-methylbenzaldehyde (2g)
provided a moderate result (37% ee; Table 2, entry 7).
3-Chlorobenzaldehyde (2h) and 2-naphthaldehyde (2i)
showed similar activity and the products were isolated
with moderate ee (Table 2, entries 8 and 9). As a summary
of these screenings, we can state that a,b-unsaturated al-
dehydes are more reactive than the aromatic aldehydes for
steric reasons, and best selectivity at room temperature
was achieved with 2-bromobenzaldehyde.
O
HO
O
t-Bu
t-Bu
t-Bu
H
+
+
4a (3 equiv)
2a (1 equiv)
t-Bu
O
14 (1 equiv)
O
7–13 (5 mol%)
O
DBU (10 mol%)
THF (0.1 M)
r.t., 2–12 h
15a
Entry
Catalyst
Time (h)
Yielda (%) of eeb (%) of
ester 15a
ester 15a
1
7
8
2
2
84
14
2
88
36
3
9
6
72
7
4
10
11
12
13
8
6
85
24
5
6
80
14
6
6
78
30c
n.d.d
50
To study the effect of the alcohol structure on the reaction,
four different alcohols were reacted by oxidative esterifi-
cation with cinnamaldehyde using catalyst 8 (Table 3).
With 1-(2-naphthyl)ethanol (4b) at –20 °C, the ester 17b
was isolated in 73% yield with 43% ee (Table 3, entry 1).
This selectivity is slightly lower than the selectivity ob-
tained with 1-(1-naphthyl)ethanol (4a) (cf. Table 1, entry
8). The bulky aromatic naphthyl substituent is important
for enantiotopic differentiation since 1-phenylethanol (4c)
provided the ester 17c with significantly lower selectivity
(Table 3, entry 2). As expected, changing the electronic
environment of the alcohol moiety by adding an electron-
donating substituent at the ortho position did not lead to
7
12
6
<5
8e
9e,f
81
8
6
98
52
a Isolated yield.
b Enantiomeric excess of the isolated product was determined using
chiral HPLC. The absolute configuration of the major enantiomer was
determined by comparing the HPLC retention time with that of an au-
thentic sample.
c The other enantiomer was obtained as the major isomer.
d n.d. = not determined.
e Reaction performed at –20 °C.
f Using 1.1 equivalents of DBU.
Synthesis 2011, No. 12, 1974–1983 © Thieme Stuttgart · New York

1976
S. De Sarkar et al.
PAPER
Table 2 Kinetic Resolution of 1-(1-Naphthyl)ethanol (4a) Using
Various Aldehydes
resolution in combination with catalyst 8 (see Table 2, en-
try 10), showed higher selectivity in combination with
carbene precursor 13 in the kinetic resolution of 1-(1-
naphthyl)ethanol (57% ee; Table 4, entry 1). For this par-
ticular system, the ee was further increased when toluene
was used as a solvent (63% ee; Table 4, entry 2). We
therefore decided to test the resolution of other alcohols
using aldehyde 2j and catalyst 13 (Table 4); however, as
for the aldehyde 2a/catalyst 8 couple, for 1-(2-methoxy-
phenyl)ethanol (4d), and also for 1-phenylpropan-1-ol
(4f), significantly lower selectivity was obtained (Table 4,
entries 3 and 4). By increasing the steric bulk in the alco-
hol moiety, a slightly better resolution was achieved
(Table 4, entry 5). The difficult kinetic resolution of di-
arylmethanols occurred with low selectivity (Table 4, en-
tries 6–8).
HO
8 (5 mol%)
DBU (1.1 equiv)
O
+
RCHO
R
O
14 (1 equiv)
THF (0.1 M)
2–12 h
2b–j
4a (3 equiv)
15b–i, 16a
Entry
R
Conditions
Ester Yielda eeb
(%)
71
61
<5
51
67
65
59
67
68
92
(%)
59
35
n.d.c
38
14
60
37
42
41
0
1
2
4-MeOC6H4CH=CH –20 °C, 12 h
15b
15c
15d
15d
15e
15f
i-PrCH=CH
4-MeO2CC6H4
4-MeO2CC6H4
4-O2NC6H4
2-BrC6H4
–20 °C, 12 h
–20 °C, 12 h
r.t., 2 h
3
4
5
r.t., 2 h
Table 4 Kinetic Resolution of Various Alcohols Using Pyridine-2-
carbaldehyde (2j) as the Acyl Source
6
r.t., 12 h
r.t., 12 h
r.t., 12 h
r.t., 12 h
r.t., 2 h
O
R2
13 (5 mol%)
DBU (1.1 equiv)
7
2-MeC6H4
3-ClC6H4
15g
15h
15i
OH
R2
N
CHO
N
O
R1
+
R1
4 (3 equiv)
14 (1 equiv)
toluene (0.05 M)
r.t., 1–6 h
8
9
2-naphthyl
2-pyridyl
2j
16
10
16a
Entry R2
R3
Time Ester
Yielda eeb
(h)
(%)
76
80
78
83
40
87
77
50
(%)
57
63
15
20
30
35
11
7
a Isolated yield.
b Enantiomeric excess of the isolated product was determined using
chiral HPLC. The absolute configuration of the major isomer was not
assigned.
1c
2
3
4
5
6
7
8
1-naphthyl
1-naphthyl
2-MeOC6H4
Ph
Me
Me
Me
Et
1
16a
16a
16d
16f
16g
16h
16i
3
c n.d. = not determined.
4
an increase in the selectivity (Table 3, entry 3), and also
an increase in the size of the alkyl substituent of the sec-
ondary alcohol [see cyclohexyl(phenyl)methanol (4e);
Table 3, entry 4] did not influence the reaction outcome to
a large extent.
4
Ph
t-Bu
Ph
4
2-BrC6H4
2-MeC6H4
1-naphthyl
2
Ph
3
Interestingly, pyridine-2-carbaldehyde (2j), which turned
out to deliver an inefficient acylazolium ion for kinetic
Ph
6
16j
a Isolated yield.
Table 3 Kinetic Resolution of Various Alcohols by NHC-Cata-
lyzed Cinnamoylation
b Enantiomeric excess of the isolated product was determined using
chiral HPLC. For 16a, the absolute configuration of the major isomer
is S and was determined by comparing the HPLC retention time with
that of an authentic sample. For the other products, the absolute con-
figuration was not assigned.
8 (5 mol%)
O
R2
OH
R2
DBU (1.1 equiv)
CHO
+
Ph
Ph
O
R1
R1
14 (1 equiv)
THF (0.1 M)
c Reaction conducted in THF.
2a
4b–e (3 equiv)
17b–e
Entry R2
R3
Conditions
Ester Yielda eeb
The mechanism of the acylation of a secondary alcohol
with a chiral acylazolium ion 3 is a two-step process
(Scheme 2). The alcohol first reacts with ion 3 to give ad-
duct A. This step is likely reversible.22 Moreover, we have
shown by DFT calculations that free carbenes activate al-
cohols by strong hydrogen-bonding interactions.14b
Hence, a second carbene is probably involved in adduct
formation. The rate-determining step might be the frag-
mentation of A to liberate the carbene and the product es-
ter.22 Considering the effect of the free carbene on adduct
formation and the pre-equilibrium, the equation
(Equation 1) suggested by Kagan and Fiaud that is usually
(%)
73
56
59
50
(%)
43
10
10
19
1
2
3
4
2-naphthyl
Ph
Me
Me
–20 °C, 12 h 17b
r.t., 6 h
17c
17d
2-MeOC6H4 Me
Ph Cy
r.t., 12 h
–20 °C, 12 h 17e
a Isolated yield.
b Enantiomeric excess of the isolated product was determined using
chiral HPLC. The absolute configuration was assigned by analogy to
15a.
Synthesis 2011, No. 12, 1974–1983 © Thieme Stuttgart · New York

PAPER
Kinetic Resolution of Secondary Alcohols by Oxidative Esterification
1977
ment at 300 K. For flash column chromatography, Merck or Fluka
silica gel 60 (40–63 mm) was used at approximately 0.4 bar. Infrared
spectra were recorded on a Varian FT-IR 3100 Excalibur or a
Shimadzu FTIR 8400S spectrophotometer. Mass spectra were re-
corded on a Bruker Daltonics MicroTOF (ESI) instrument. A
Hewlett Packard Binary Pump System with a Hewlett Packard Se-
ries 1100 ChemStation for LC was used for HPLC analysis. The
column, eluent and retention times used for the determination of
enantiomer ratios are given with the details of the relevant experi-
ment. Full characterization is provided for previously unknown
compounds.
applied for calculation of the S-value should not be valid
(C = conversion, ee = enantiomeric excess of the prod-
uct).23
*
*
OR2
R
R
O
HO
R1
R2OH
O
N
N
R1
R1
OR2
– NHC
– H+
N
N
R
X
X
*
*
R
A
3
Scheme 2 Suggested mechanism
Preparation of the Chiral Triazolium Salts 8 and 9
(S)-5-[(tert-Butyldiphenylsilyloxy)methyl]-2-phenyl-6,7-dihy-
dro-5H-pyrrolo[2,1-c][1,2,4]triazol-2-ium Tetrafluoroborate
(8)
ln[1 – C(1 + ee)]
S =
ln[1 – C(1 – ee)]
Trimethyloxonium tetrafluoroborate (320 mg, 2.2 mmol) was add-
ed to a soln of (S)-5-[(tert-butyldiphenylsilyloxy)methyl]pyrroli-
din-2-one28 (0.7 g, 2.0 mmol) in CH2Cl2 (8 mL) at r.t. and the
mixture was stirred overnight. Then, phenylhydrazine (238 mg, 2.2
mmol) was added and the mixture was allowed to stir at r.t. for 12
h. The solvent was removed under reduced pressure; the product
was used without further purification. Trimethyl orthoformate (20
mL) was added and the mixture was refluxed for an additional 12 h.
The solvent was removed under reduced pressure and the crude
product was purified by column chromatography (CH2Cl2–MeOH,
50:1) to afford 8 as a yellow solid; yield: 904 mg (83%).
Equation 1
Indeed, running the reaction of aldehyde 2b with 1-(1-
naphthyl)ethanol (4a, 1 equiv) at –20 °C until 20% con-
version provided ester 15b with 48% ee. According to
equation 1 this leads to an S-value of 3.2 which is signifi-
cantly lower than expected based on the experiment using
3 equivalents of alcohol (not pseudo first order) described
in Table 2 (entry 1, 59% ee at 71% conversion; assuming
pseudo first order in alcohol, this would lead to an S-value
of 3.9). We therefore repeated experiments with an even
larger excess of racemic alcohol 4a while keeping the al-
dehyde concentration constant. When a large excess of the
alcohol is used, reaction occurs under pseudo-first-order
conditions with respect to the alcohol and the ee of the
product ester, irrespective of the conversion, reflects the
S-value. With 10 equivalents of rac-4a at 92% conver-
sion, the ester was isolated with 68% ee (S-value = 5.3);
with 15 equivalents the ee further increased to 73% (S-
value = 6.4), and with 25 equivalents the ee was raised to
76% (S-value = 7.3). No further increase in selectivity
was observed when the amount of alcohol was increased
FTIR (neat): 3132, 3073, 2957, 2934, 2889, 2859, 2360, 1684,
1590, 1470, 1429, 1391, 1106, 1053, 1026, 857, 824, 762, 745, 703,
688 cm–1.
1H NMR (300 MHz, CDCl3): d = 9.43 (s, 1 H), 7.66–7.58 (m, 2 H),
7.50–7.43 (m, 3 H), 7.43–7.34 (m, 3 H), 7.30–7.22 (m, 3 H), 7.22–
7.09 (m, 4 H), 5.08–5.04 (m, 1 H), 4.35–4.26 (m, 1 H), 3.97–3.88
(m, 1 H), 3.26–3.05 (m, 2 H), 3.01–2.83 (m, 1 H), 2.77–2.63 (m, 1
H), 0.92 (s, 9 H).
13C NMR (75 MHz, CDCl3): d = 163.2, 136.4, 135.6, 135.5, 135.4,
132.3, 132.0, 130.8, 130.4, 130.4, 130.2, 128.2, 128.1, 120.8, 64.8,
62.2, 29.7, 27.0, 22.4, 19.2.
HRMS (ESI): m/z [M]+ calcd for C28H32N3OSi: 454.2309; found:
454.2303.
to 50 equivalents. Hence, in these systems the selectivity (S)-(5-Oxopyrrolidin-2-yl)methyl Phenylcarbamate
Phenyl isocyanate (655 mg, 5.5 mmol) and a catalytic amount of py-
ridine were added to a soln of (S)-5-(hydroxymethyl)pyrrolidin-2-
one (575 mg, 5.0 mmol) in THF (20 mL) at r.t. The resulting mix-
ture was refluxed for 24 h. Solvent was removed under reduced
factor S varies as a function of the initial alcohol concen-
tration and the best selectivities are obtained by using a
large excess of the alcohol under pseudo-first-order con-
ditions.
pressure and the crude product was purified by column chromatog-
raphy (MTBE–MeOH, 20:1) to afford the title compound as a white
solid; yield: 602 mg (52%).
FTIR (neat): 3300, 2225, 1714, 1655, 1236, 727 cm–1.
1H NMR (300 MHz, CDCl3): d = 8.02 (s, 1 H), 7.57 (s, 1 H), 7.44–
7.28 (m, 2 H), 7.25–7.13 (m, 2 H), 7.01–6.89 (m, 1 H), 4.14–4.09
(m, 1 H), 3.95–3.75 (m, 2 H), 2.37–1.99 (m, 3 H), 1.76–1.58 (m, 1
H).
13C NMR (75 MHz, CDCl3): d = 179.0, 153.5, 138.2, 129.0, 123.3,
118.8, 67.5, 53.5, 30.0, 22.8.
HRMS (ESI): m/z [M + Na]+ calcd for C12H14N2O3Na: 257.0897;
In conclusion, we have presented an application of oxida-
tive esterification for the kinetic resolution of secondary
alcohols employing various chiral carbenes. The key in-
termediate chiral acylazolium ion is generated oxidatively
by using an aldehyde as the acyl source. Although the se-
lectivities are currently moderate, we expect that further
modification of the carbene structure will lead to the de-
velopment of an efficient catalyst for the kinetic resolu-
tion using our oxidative esterification protocol.
found: 257.0892.
All reactions were carried out in heat-gun-dried glassware under an
argon atmosphere. THF was freshly distilled from potassium under
argon. All other solvents and reagents, including alcohols 4e,24 4h25
and 4j,26 were purified according to standard procedures or were
used as received from Aldrich, Acros, ABCR or Fluka. 1,3-Dimeth-
yltriazolium iodide27 was used to prepare racemic samples. 1H and
13C NMR spectroscopy was conducted on a Bruker DPX 300 instru-
(S)-2-Phenyl-5-[(phenylcarbamoyloxy)methyl]-6,7-dihydro-
5H-pyrrolo[2,1-c][1,2,4]triazol-2-ium Tetrafluoroborate (9)
A flame-dried 25-mL round-bottom Schlenk flask was charged with
(S)-(5-oxopyrrolidin-2-yl)methyl phenylcarbamate (234 mg, 1.0
mmol) and CH2Cl2 (8 mL). Trimethyloxonium tetrafluoroborate
Synthesis 2011, No. 12, 1974–1983 © Thieme Stuttgart · New York

1978
S. De Sarkar et al.
PAPER
(163 mg, 1.1 mmol) was added and the mixture was stirred at r.t.
overnight. Then, phenylhydrazine (119 mg, 1.1 mmol) was added
and the mixture was allowed to stir at r.t. for 24 h. The solvent was
removed under reduced pressure; the product was used without fur-
ther purification. MeOH (1 mL) and trimethyl orthoformate (10
mL) were added and the mixture was refluxed for an additional 24
h. The solvent was removed under reduced pressure. Crystallization
from hot MeOH afforded 9 as a colorless crystalline solid; yield:
200 mg (47%).
The enantiomeric excess (52%) was determined by chiral HPLC;
column: Chiralcel OD-H; solvent: cyclohexane–i-PrOH (99.5:0.5);
flow: 1.0 mL/min; major enantiomer tR = 17.6 min, minor enantio-
mer tR = 33.6 min. The absolute configuration of the major enantio-
mer was determined by comparing the HPLC retention time with
that of an authentic sample.
1-(1-Naphthyl)ethyl (E)-3-(4-Methoxyphenyl)acrylate (15b)
According to GP I with (E)-3-(4-methoxyphenyl)acrylaldehyde
(2b; 41 mg, 0.25 mmol), DBU (42 mg, 0.28 mmol), catalyst 8 (6.8
mg, 12.5 mmol), 1-(1-naphthyl)ethanol (4a; 129 mg, 750 mmol) and
3,3¢,5,5¢-tetra-tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in
THF (2.5 mL) at –20 °C for 12 h and silica gel chromatography
(pentane–Et2O, 9:1) to afford 15b as a light yellow solid; yield: 59
mg (71%).
FTIR (neat): 3382, 3152, 2361, 1738, 1595, 1540, 1214, 1074,
1022, 761 cm–1.
1H NMR (300 MHz, CD3CN): d = 9.80 (s, 1 H), 8.03 (s, 1 H), 7.88–
7.77 (m, 2 H), 7.74–7.59 (m, 3 H), 7.52–7.40 (m, 2 H), 7.38–7.25
(m, 2 H), 7.14–7.03 (m, 1 H), 5.15–4.99 (m, 1 H), 4.68 (dd, J = 11.8,
3.4 Hz, 1 H), 4.22 (dd, J = 11.8, 8.3 Hz, 1 H), 3.41–3.16 (m, 2 H),
3.10–2.95 (m, 1 H), 2.71–2.57 (m, 1 H).
13C NMR (75 MHz, CD3CN): d = 164.1, 153.6, 139.4, 138.5, 136.7,
132.1, 131.3, 130.1, 124.4, 122.4, 119.6, 65.2, 60.9, 30.1, 22.3.
FTIR (neat): 2977, 1703, 1600, 1509, 1249, 1157, 1032, 780 cm–1.
1H NMR (300 MHz, CDCl3): d = 8.17 (d, J = 8.3 Hz, 1 H), 7.93–
7.86 (m, 1 H), 7.82 (d, J = 8.2 Hz, 1 H), 7.75–7.67 (m, 2 H), 7.60–
7.45 (m, 5 H), 6.90 (d, J = 8.8 Hz, 2 H), 6.80 (q, J = 6.5 Hz, 1 H),
6.41 (d, J = 15.9 Hz, 1 H), 3.83 (s, 3 H), 1.80 (d, J = 6.6 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 166.5, 161.4, 144.7, 137.6, 133.9,
130.4, 129.7, 128.9, 128.4, 127.2, 126.3, 125.6, 125.4, 123.3, 123.2,
115.8, 114.3, 69.3, 55.3, 21.7.
HRMS (ESI): m/z [M]+ calcd for C19H19N4O2: 335.1503; found:
335.1512.
Procedure for the Catalyst Screening Experiments (Table 1)
DBU (10 mol%) was added to a soln of a chiral triazolium salt 7–13
(5 mol%) in THF (0.1 M) under argon atmosphere. The mixture was
stirred for 5 min, then 1-(1-naphthyl)ethanol (4a, 3.0 equiv) and
3,3¢,5,5¢-tetra-tert-butyldiphenoquinone (14, 1.0 equiv) were added.
After the mixture was stirred for 5 min, (E)-cinnamaldehyde (2a, 1
equiv) was added. The resulting solution was stirred at r.t. for 2–6
h. The solvent was removed under reduced pressure and the residue
was purified by flash silica gel column chromatography (pentane–
Et2O, 50:1 to 20:1).
HRMS (ESI): m/z [M + Na]+ calcd for C22H20O3Na: 355.1305;
found: 355.1308.
The enantiomeric excess (59%) was determined by chiral HPLC;
column: Chiralpak AD-H; solvent: cyclohexane–i-PrOH (99.0:1.0);
flow: 1.0 mL/min; major enantiomer tR = 45.7 min, minor enantio-
mer tR = 26.2 min. The absolute configuration of the major enantio-
mer was not assigned.
1-(1-Naphthyl)ethyl (E)-4-Methylpent-2-enoate (15c)
According to GP I with (E)-4-methylpent-2-enal (2c; 25 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 8 (6.8 mg, 12.5 mmol),
1-(1-naphthyl)ethanol (4a; 129 mg, 750 mmol) and 3,3¢,5,5¢-tetra-
tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in THF (2.5 mL)
at –20 °C for 12 h and silica gel chromatography (pentane–Et2O,
100:1 to 20:1) to afford 15c as a light yellow oil; yield: 41 mg
(61%).
General Procedure (GP I) for the Kinetic Resolution of Second-
ary Alcohols Using Catalyst 8 (Tables 2 and 3)
DBU (10 mol% or 1.1 equiv) was added to a soln of triazolium salt
8 (5 mol%) in THF (0.1 M) under argon atmosphere. The mixture
was stirred for 5 min, then an alcohol 4a–e (3.0 equiv) and 3,3¢,5,5¢-
tetra-tert-butyldiphenoquinone (14, 1.0 equiv) were added. After
the mixture was stirred for 5 min, an aldehyde 2a–j (1 equiv) was
added at r.t. or –20 °C. The resulting solution was stirred for 2–12 h
at that temperature. The solvent was removed under reduced pres-
sure and the residue was purified by flash silica gel column chroma-
tography (pentane–Et2O).
FTIR (neat): 2963, 1715, 1650, 1455, 1263, 1166, 1065, 980, 781
cm–1.
1H NMR (300 MHz, CDCl3): d = 8.13 (d, J = 8.3 Hz, 1 H), 7.91–
7.84 (m, 1 H), 7.81 (d, J = 8.2 Hz, 1 H), 7.64 (d, J = 7.0 Hz, 1 H),
7.59– 7.43 (m, 3 H), 7.02 (dd, J = 15.7, 6.5 Hz, 1 H), 6.73 (q, J =
6.6 Hz, 1 H), 5.86 (dd, J = 15.7, 1.4 Hz, 1 H), 2.47 (dqd, J = 13.7,
6.8, 1.4 Hz, 1 H), 1.74 (d, J = 6.6 Hz, 3 H), 1.07 (d, J = 6.8 Hz, 6 H).
13C NMR (75 MHz, CDCl3): d = 166.3, 155.9, 137.6, 133.8, 130.3,
128.8, 128.4, 126.2, 125.6, 125.3, 123.3, 123.2, 118.7, 69.2, 30.9,
21.7, 21.2.
1-(1-Naphthyl)ethyl (E)-Cinnamate (15a)
According to GP I with (E)-cinnamaldehyde (2a; 66 mg, 0.5 mmol),
DBU (83 mg, 0.55 mmol), catalyst 8 (13.5 mg, 25 mmol), 1-(1-
naphthyl)ethanol (4a; 258 mg, 1.5 mmol) and 3,3¢,5,5¢-tetra-tert-
butyldiphenoquinone (14; 205 mg, 500 mmol) in THF (5.0 mL) at
–20 °C for 6 h and silica gel chromatography (pentane–Et2O, 50:1
to 20:1) to afford 15a as a light yellow solid; yield: 148 mg (98%).
HRMS (ESI): m/z [M + Na]+ calcd for C18H20O2Na: 291.1356;
found: 291.1363.
FTIR (neat): 3415, 3055, 2980, 2935, 2361, 2340, 1714, 1639,
1338, 1317, 1167, 1074, 1005, 798, 768 cm–1.
The enantiomeric excess (35%) was determined by chiral HPLC;
column: Chiralpak AD-H; solvent: cyclohexane–i-PrOH (99.5:0.5);
flow: 1.0 mL/min; major enantiomer tR = 10.7 min, minor enantio-
mer tR = 7.2 min. The absolute configuration of the major enantio-
mer was not assigned.
1H NMR (300 MHz, CDCl3): d = 8.17 (d, J = 8.5 Hz, 1 H), 7.90 (d,
J = 9.5 Hz, 1 H), 7.83 (d, J = 8.2 Hz, 1 H), 7.76 (d, J = 16.0 Hz, 1
H), 7.69 (d, J = 6.8 Hz, 1 H), 7.62–7.46 (m, 5 H), 7.44–7.34 (m, 3
H), 6.81 (q, J = 6.6 Hz, 1 H), 6.54 (d, J = 16.0 Hz, 1 H), 1.81 (d,
J = 6.6 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 166.4, 145.2, 137.6, 134.6, 134.0,
130.5, 130.4, 129.0, 129.0, 128.6, 128.2, 126.5, 125.8, 125.5, 123.4,
118.5, 69.7, 30.5, 21.9.
HRMS (ESI): m/z [M + Na]+ calcd for C21H18O2Na: 325.1199;
found: 325.1195.
Methyl 1-(1-Naphthyl)ethyl Terephthalate (15d)
According to GP I with methyl 4-formylbenzoate (2d; 82 mg, 0.5
mmol), DBU (83 mg, 0.55 mmol), catalyst 8 (13.5 mg, 25 mmol), 1-
(1-naphthyl)ethanol (4a; 258 mg, 1.5 mmol) and 3,3¢,5,5¢-tetra-tert-
butyldiphenoquinone (14; 205 mg, 500 mmol) in THF (5.0 mL) at
Synthesis 2011, No. 12, 1974–1983 © Thieme Stuttgart · New York

PAPER
Kinetic Resolution of Secondary Alcohols by Oxidative Esterification
1979
r.t. for 2 h and silica gel chromatography (pentane–Et2O, 50:1 to
mer tR = 7.1 min. The absolute configuration of the major enantio-
20:1) to afford 15d as a light yellow solid; yield: 85 mg (51%).
mer was not assigned.
FTIR (neat): 3422, 2954, 2937, 2392, 2294, 1716, 1597, 1577,
1442, 1275, 1113, 1099, 1070, 800, 778, 730 cm–1.
1H NMR (300 MHz, CDCl3): d = 8.30–8.05 (m, 5 H), 7.95–7.78 (m,
2 H), 7.70 (d, J = 6.9 Hz, 1 H), 7.61–7.42 (m, 3 H), 6.90 (q, J = 6.6
Hz, 1 H), 3.95 (s, 3 H), 1.86 (d, J = 6.6 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 166.4, 165.2, 137.3, 134.4, 134.1,
134.0, 130.4, 129.8, 129.7, 129.1, 128.8, 126.6, 125.9, 125.5, 123.5,
123.3, 71.0, 52.5, 22.0.
1-(1-Naphthyl)ethyl 2-Methylbenzoate (15g)
According to GP I with 2-methylbenzaldehyde (2g; 30 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 8 (6.8 mg, 12.5 mmol),
1-(1-naphthyl)ethanol (4a; 129 mg, 750 mmol) and 3,3¢,5,5¢-tetra-
tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in THF (2.5 mL)
at r.t. for 12 h and silica gel chromatography (pentane–Et2O, 100:1
to 50:1) to afford 15g as a yellow oil; yield: 43 mg (59%).
FTIR (neat): 2977, 1702, 1598, 1448, 1248, 1071, 862, 740 cm–1.
HRMS (ESI): m/z [M + Na]+ calcd for C21H18O4Na: 357.1097;
found: 357.1090.
1H NMR (300 MHz, CDCl3): d = 8.18 (d, J = 8.4 Hz, 1 H), 7.99 (d,
J = 7.6 Hz, 1 H), 7.87 (d, J = 7.9 Hz, 1 H), 7.80 (d, J = 8.2 Hz, 1 H),
7.69 (d, J = 7.1 Hz, 1 H), 7.59–7.43 (m, 3 H), 7.38 (t, J = 7.4 Hz, 1
H), 7.24 (t, J = 7.2 Hz, 2 H), 6.89 (q, J = 6.6 Hz, 1 H), 2.59 (s, 3 H),
1.83 (d, J = 6.6 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 166.8, 140.3, 137.6, 133.9, 131.9,
131.7, 130.6, 130.3, 129.9, 128.9, 128.4, 126.3, 125.7, 125.7, 125.4,
123.2, 123.2, 69.9, 21.9, 21.7.
The enantiomeric excess (38%) was determined by chiral HPLC;
column: Chiralpak AD-H; solvent: cyclohexane–i-PrOH (99.5:0.5);
flow: 1.0 mL/min; major enantiomer tR = 17.9 min, minor enantio-
mer tR = 19.6 min. The absolute configuration of the major enantio-
mer was not assigned.
1-(1-Naphthyl)ethyl 4-Nitrobenzoate (15e)
HRMS (ESI): m/z [M + Na]+ calcd for C20H18O2Na: 313.1199;
found: 313.1202.
According to GP I with 4-nitrobenzaldehyde (2e; 76 mg, 0.5 mmol),
DBU (83 mg, 0.55 mmol), catalyst 8 (13.5 mg, 25 mmol), 1-(1-
naphthyl)ethanol (4a; 258 mg, 1.5 mmol) and 3,3¢,5,5¢-tetra-tert-
butyldiphenoquinone (14; 205 mg, 500 mmol) in THF (5.0 mL) at
r.t. for 2 h and silica gel chromatography (pentane–Et2O, 50:1 to
20:1) to afford 15e as a yellow oil; yield: 108 mg (67%).
The enantiomeric excess (37%) was determined by chiral HPLC;
column: Chiralpak AD-H; solvent: cyclohexane–i-PrOH (99.5:0.5);
flow: 1.0 mL/min; major enantiomer tR = 7.0 min, minor enantio-
mer tR = 5.4 min. The absolute configuration of the major enantio-
mer was not assigned.
FTIR (neat): 3054, 2985, 2389, 2349, 2280, 1720, 1604, 1525,
1268, 1101, 777, 718 cm–1.
1H NMR (300 MHz, CDCl3): d = 8.31–8.22 (m, 4 H), 8.18 (d,
J = 8.4 Hz, 1 H), 7.94–7.81 (m, 2 H), 7.70 (d, J = 6.9 Hz, 1 H),
7.62–7.47 (m, 3 H), 6.93 (q, J = 6.6 Hz, 1 H), 1.90 (d, J = 6.6 Hz, 3
H).
13C NMR (75 MHz, CDCl3): d = 164.1, 150.7, 136.9, 136.0, 134.0,
131.0, 129.2, 129.0, 126.7, 126.0, 125.5, 123.7, 123.5, 123.1, 71.6,
21.9.
1-(1-Naphthyl)ethyl 3-Chlorobenzoate (15h)
According to GP I with 3-chlorobenzaldehyde (2h; 35 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 8 (6.8 mg, 12.5 mmol),
1-(1-naphthyl)ethanol (4a; 129 mg, 750 mmol) and 3,3¢,5,5¢-tetra-
tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in THF (2.5 mL)
at r.t. for 12 h and silica gel chromatography (pentane–Et2O, 100:1
to 50:1) to afford 15h as a yellow oil; yield: 52 mg (67%).
FTIR (neat): 3064, 1719, 1576, 1426, 1254, 1125, 1072, 902, 743,
630 cm–1.
HRMS (ESI): m/z [M + Na]+ calcd for C19H15NO4Na: 344.0893;
1H NMR (300 MHz, CDCl3): d = 8.18 (d, J = 8.3 Hz, 1 H), 8.09 (s,
1 H), 8.00 (d, J = 7.8 Hz, 1 H), 7.90 (d, J = 7.9 Hz, 1 H), 7.84 (d,
J = 8.2 Hz, 1 H), 7.70 (d, J = 7.1 Hz, 1 H), 7.62–7.46 (m, 4 H), 7.38
(t, J = 7.9 Hz, 1 H), 6.91 (q, J = 6.6 Hz, 1 H), 1.86 (d, J = 6.6 Hz, 3
H).
13C NMR (75 MHz, CDCl3): d = 164.6, 137.2, 134.5, 133.9, 133.0,
132.2, 130.3, 129.7, 129.7, 129.0, 128.6, 127.8, 126.4, 125.7, 125.4,
123.3, 123.1, 70.7, 21.8.
found: 344.0889.
The enantiomeric excess (14%) was determined by chiral HPLC;
column: Chiralpak AD-H; solvent: cyclohexane–i-PrOH (99.5:0.5);
flow: 1.0 mL/min; major enantiomer tR = 16.4 min, minor enantio-
mer tR = 14.5 min. The absolute configuration of the major enantio-
mer was not assigned.
1-(1-Naphthyl)ethyl 2-Bromobenzoate (15f)
According to GP I with 2-bromobenzaldehyde (2f; 46.3 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 8 (6.8 mg, 12.5 mmol),
1-(1-naphthyl)ethanol (4a; 129 mg, 750 mmol) and 3,3¢,5,5¢-tetra-
tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in THF (2.5 mL)
at r.t. for 12 h and silica gel chromatography (pentane–Et2O, 100:1
to 50:1) to afford 15f as a viscous yellow oil; yield: 58 mg (65%).
HRMS (ESI): m/z [M + Na]+ calcd for C19H15ClO2Na: 333.0653;
found: 333.0654.
The enantiomeric excess (42%) was determined by chiral HPLC;
column: Chiralpak AD-H; solvent: cyclohexane–i-PrOH (99.5:0.5);
flow: 1.0 mL/min; major enantiomer tR = 8.3 min, minor enantio-
mer tR = 7.1 min. The absolute configuration of the major enantio-
mer was not assigned.
FTIR (neat): 3057, 1725, 1591, 1437, 1250, 1112, 1034, 746 cm–1.
1H NMR (300 MHz, CDCl3): d = 8.12 (d, J = 8.4 Hz, 1 H), 7.82 (d,
J = 8.3 Hz, 1 H), 7.79–7.71 (m, 2 H), 7.66 (d, J = 7.1 Hz, 1 H),
7.63–7.57 (m, 1 H), 7.55–7.37 (m, 3 H), 7.32–7.17 (m, 2 H), 6.87
(q, J = 6.6 Hz, 1 H), 1.81 (d, J = 6.6 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 165.5, 137.0, 134.3, 133.8, 132.5,
132.4, 131.3, 130.3, 128.9, 128.6, 127.1, 126.4, 125.7, 125.4, 123.5,
123.2, 121.7, 71.1, 21.7.
1-(1-Naphthyl)ethyl 2-Naphthoate (15i)
According to GP I with 2-naphthaldehyde (2i; 39 mg, 0.25 mmol),
DBU (42 mg, 0.28 mmol), catalyst 8 (6.8 mg, 12.5 mmol), 1-(1-
naphthyl)ethanol (4a; 129 mg, 750 mmol) and 3,3¢,5,5¢-tetra-tert-
butyldiphenoquinone (14; 102 mg, 250 mmol) in THF (2.5 mL) at
r.t. for 12 h and silica gel chromatography (pentane–Et2O, 50:1 to
20:1) to afford 15i as a light yellow solid; yield: 55 mg (68%).
HRMS (ESI): m/z [M + Na]+ calcd for C19H15BrO2Na: 377.0148;
found: 377.0153.
FTIR (neat): 3050, 1712, 1628, 1507, 1445, 1355, 1271, 1222,
1086, 977, 766 cm–1.
1H NMR (300 MHz, CDCl3): d = 8.69 (s, 1 H), 8.25 (d, J = 8.4 Hz,
1 H), 8.14 (dd, J = 8.6, 1.5 Hz, 1 H), 7.97 (d, J = 7.7 Hz, 1 H), 7.93–
The enantiomeric excess (60%) was determined by chiral HPLC;
column: Chiralpak AD-H; solvent: cyclohexane–i-PrOH (99.5:0.5);
flow: 1.0 mL/min; major enantiomer tR = 10.3 min, minor enantio-
Synthesis 2011, No. 12, 1974–1983 © Thieme Stuttgart · New York

1980
S. De Sarkar et al.
PAPER
7.86 (m, 3 H), 7.84 (d, J = 8.3 Hz, 1 H), 7.78 (d, J = 7.0 Hz, 1 H),
7.63–7.47 (m, 5 H), 6.97 (q, J = 6.5 Hz, 1 H), 1.91 (d, J = 6.6 Hz, 3
H), 6.79 (d, J = 8.2 Hz, 1 H), 6.42 (d, J = 16.0 Hz, 1 H), 6.30 (q,
J = 6.5 Hz, 1 H), 3.76 (s, 3 H), 1.48 (d, J = 6.5 Hz, 3 H).
H).
13C NMR (75 MHz, CDCl3): d = 166.2, 156.2, 144.7, 134.7, 130.6,
130.3, 129.0, 128.7, 128.2, 126.0, 120.8, 118.8, 110.7, 67.5, 55.6,
21.35.
HRMS (ESI): m/z [M + Na]+ calcd for C18H18O3Na: 305.1148;
found: 305.1148.
13C NMR (75 MHz, CDCl3): d = 166.0, 137.6, 135.6, 133.9, 132.5,
131.2, 130.4, 129.4, 128.9, 128.5, 128.2, 128.2, 127.8, 126.6, 126.4,
125.7, 125.4, 125.4, 123.3, 123.3, 70.4, 21.9.
HRMS (ESI): m/z [M + Na]+ calcd for C23H18O2Na: 349.1199;
found: 349.1200.
The enantiomeric excess (10%) was determined by chiral HPLC;
column: Chiralcel OD-H; solvent: cyclohexane–i-PrOH (99.5:0.5);
flow: 1.0 mL/min; major enantiomer tR = 18.2 min, minor enantio-
mer tR = 22.7 min. The absolute configuration of the major enantio-
mer was determined by analogy to 15a.
The enantiomeric excess (41%) was determined by chiral HPLC;
column: Chiralcel OD-H; solvent: cyclohexane–i-PrOH (99.5:0.5);
flow: 1.0 mL/min; major enantiomer tR = 11.7 min, minor enantio-
mer tR = 15.6 min. The absolute configuration of the major enantio-
mer was not assigned.
Cyclohexyl(phenyl)methyl (E)-Cinnamate (17e)
1-(2-Naphthyl)ethyl (E)-Cinnamate (17b)
According to GP I with (E)-cinnamaldehyde (2a; 33 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 8 (6.8 mg, 12.5 mmol),
cyclohexyl(phenyl)methanol (4e; 143 mg, 750 mmol) and 3,3¢,5,5¢-
tetra-tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in THF (2.5
mL) at –20 °C for 12 h and silica gel chromatography (pentane–
Et2O, 100:1 to 20:1) to afford 17e as a yellow oil; yield: 40 mg
(50%).
According to GP I with (E)-cinnamaldehyde (2a; 33 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 8 (6.8 mg, 12.5 mmol),
1-(2-naphthyl)ethanol (4b; 129 mg, 750 mmol) and 3,3¢,5,5¢-tetra-
tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in THF (2.5 mL)
at –20 °C for 12 h and silica gel chromatography (pentane–Et2O,
100:1 to 20:1) to afford 17b as a yellow oil; yield: 55 mg (73%).
FTIR (neat): 3056, 2981, 1710, 1636, 1500, 1449, 1307, 1268,
1168, 1062, 984, 861 cm–1.
FTIR (neat): 2929, 2855, 1712, 1638, 1450, 1310, 1273, 1077, 990,
863, 762 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.91–7.82 (m, 4 H), 7.78–7.70 (m,
1 H), 7.59–7.46 (m, 5 H), 7.42–7.36 (m, 3 H), 6.53 (d, J = 16.0 Hz,
1 H), 6.21 (q, J = 6.6 Hz, 1 H), 1.72 (d, J = 6.6 Hz, 3 H).
1H NMR (300 MHz, CDCl3): d = 7.68 (d, J = 16.0 Hz, 1 H), 7.55–
7.27 (m, 10 H), 6.49 (d, J = 16.0 Hz, 1 H), 5.63 (d, J = 7.7 Hz, 1 H),
1.98–1.59 (m, 5 H), 1.51–1.36 (m, 1 H), 1.32–0.85 (m, 5 H).
13C NMR (75 MHz, CDCl3): d = 166.2, 144.9, 139.1, 134.5, 133.2,
133.1, 130.3, 128.9, 128.4, 128.1, 128.0, 127.7, 126.2, 126.0, 125.0,
124.1, 118.4, 72.5, 22.2.
13C NMR (75 MHz, CDCl3): d = 166.3, 144.7, 139.8, 134.5, 131.6,
130.2, 129.4, 128.8, 128.2, 128.1, 127.7, 127.1, 118.5, 80.4, 43.1,
35.2, 29.1, 29.0, 26.3, 25.9, 25.9.
HRMS (ESI): m/z [M + Na]+ calcd for C21H18O2Na: 325.1199;
HRMS (ESI): m/z [M + Na]+ calcd for C22H24O2Na: 343.1699;
found: 325.1198.
found: 343.1694.
The enantiomeric excess (43%) was determined by chiral HPLC;
column: Chiralpak AD-H; solvent: cyclohexane–i-PrOH (99.5:0.5);
flow: 1.0 mL/min; major enantiomer tR = 39.2 min, minor enantio-
mer tR = 17.2 min. The absolute configuration of the major enantio-
mer was determined by analogy to 15a.
The enantiomeric excess (19%) was determined by chiral HPLC;
column: Chiralcel OD-H; solvent: cyclohexane–i-PrOH (99.5:0.5);
flow: 1.0 mL/min; major enantiomer tR = 20.2 min, minor enantio-
mer tR = 11.7 min. The absolute configuration of the major enantio-
mer was determined by analogy to 15a.
1-Phenylethyl (E)-Cinnamate (17c)
General Procedure (GP II) for the Kinetic Resolution of Sec-
ondary Alcohols Using Catalyst 13 (Table 4)
According to GP I with (E)-cinnamaldehyde (2a; 66 mg, 0.5 mmol),
DBU (83 mg, 0.55 mmol), catalyst 8 (13.5 mg, 25 mmol), 1-phe-
nylethanol (4c; 183 mg, 1.5 mmol) and 3,3¢,5,5¢-tetra-tert-butyl-
diphenoquinone (14; 205 mg, 500 mmol) in THF (5.0 mL) at r.t. for
6 h and silica gel chromatography (pentane–Et2O, 50:1 to 20:1) to
afford 17c as a yellow oil; yield: 71 mg (56%).
DBU (1.1 equiv) was added to a soln of triazolium salt 13 (5 mol%)
in toluene (0.05 M) under argon atmosphere. The mixture was
stirred for 5 min, then an alcohol 4 (3.0 equiv) and 3,3¢,5,5¢-tetra-
tert-butyldiphenoquinone (14, 1.0 equiv) were added. After the
mixture was stirred for 5 min, aldehyde 2j (1 equiv) was added at
r.t. The resulting solution was stirred for 1–6 h at that temperature.
The solvent was removed under reduced pressure and the residue
was purified by flash silica gel column chromatography [pentane–
methyl tert-butyl ether (MTBE)].
The physical data are in agreement with those reported in the liter-
ature.29
The enantiomeric excess (10%) was determined by chiral HPLC;
column: Chiralcel OD-H; solvent: cyclohexane–i-PrOH (99.5:0.5);
flow: 1.0 mL/min; major enantiomer tR = 9.7 min, minor enantio-
mer tR = 27.9 min. The absolute configuration of the major enantio-
mer was determined by analogy to 15a.
1-(1-Naphthyl)ethyl Picolinate (16a)
According to GP II with pyridine-2-carbaldehyde (2j; 27 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 13 (5.8 mg, 12.5 mmol),
1-(1-naphthyl)ethanol (4a; 129 mg, 750 mmol) and 3,3¢,5,5¢-tetra-
tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in toluene (5.0
mL) at r.t. for 3 h and silica gel chromatography (pentane–MTBE,
2:1) to afford 16a as a pale yellow oil; yield: 56 mg (80%).
1-(2-Methoxyphenyl)ethyl (E)-Cinnamate (17d)
According to GP I with (E)-cinnamaldehyde (2a; 66 mg, 0.5 mmol),
DBU (83 mg, 0.55 mmol), catalyst 8 (13.5 mg, 25 mmol), 1-(2-
methoxyphenyl)ethanol (4d; 228 mg, 1.5 mmol) and 3,3¢,5,5¢-tetra-
tert-butyldiphenoquinone (14; 205 mg, 500 mmol) in THF (5.0 mL)
at r.t. for 12 h and silica gel chromatography (pentane–Et2O, 50:1 to
20:1) to afford 17d as a yellow oil; yield: 83 mg (59%).
FTIR (neat): 3056, 1720, 1583, 1512, 1441, 1298, 1241, 1129,
1075, 998, 781, 740 cm–1.
1H NMR (300 MHz, CDCl3): d = 8.85–8.75 (m, 1 H), 8.20 (d,
J = 8.4 Hz, 1 H), 8.13 (d, J = 7.8 Hz, 1 H), 7.92–7.72 (m, 4 H),
7.60–7.41 (m, 4 H), 6.98 (q, J = 6.6 Hz, 1 H), 1.90 (d, J = 6.6 Hz, 3
H).
FTIR (neat): 3061, 3031, 2982, 2937, 2838, 2252, 1709, 1638,
1494, 1451, 1245, 1172, 1065, 755, 733, 632 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.63 (d, J = 16.0 Hz, 1 H), 7.50–
7.39 (m, 2 H), 7.38–7.25 (m, 4 H), 7.23–7.13 (m, 1 H), 6.89 (m, 1
Synthesis 2011, No. 12, 1974–1983 © Thieme Stuttgart · New York

PAPER
Kinetic Resolution of Secondary Alcohols by Oxidative Esterification
1981
13C NMR (75 MHz, CDCl3): d = 164.4, 150.0, 148.3, 137.1, 136.9,
133.8, 130.3, 128.9, 128.6, 126.8, 126.4, 125.7, 125.4, 125.2, 123.4,
123.1, 71.1, 21.7.
3,3¢,5,5¢-tetra-tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in
toluene (5.0 mL) at r.t. for 4 h and silica gel chromatography (pen-
tane–MTBE, 2:1) to afford 16g as a white solid; yield: 27 mg (40%).
HRMS (ESI): m/z [M + Na]+ calcd for C18H15NO2Na: 300.0995;
found: 300.0987.
FTIR (neat): 2962, 1727, 1580, 1443, 1286, 1239, 1128, 1037, 975,
739, 698 cm–1.
The enantiomeric excess (63%) was determined by chiral HPLC;
column: Chiralcel OD-H; solvent: cyclohexane–i-PrOH (98:2);
flow: 1.0 mL/min; major enantiomer tR = 18.7 min, minor enantio-
mer tR = 14.5 min. The absolute configuration of the major enantio-
mer was determined by comparing the HPLC retention time with
that of an authentic sample.
1H NMR (300 MHz, CDCl3): d = 8.77 (d, J = 4.3 Hz, 1 H), 8.13 (d,
J = 7.8 Hz, 1 H), 7.83 (td, J = 7.7, 1.5 Hz, 1 H), 7.44 (dd, J = 6.9,
5.0 Hz, 1 H), 7.41–7.33 (m, 2 H), 7.33–7.20 (m, 3 H), 5.82 (s, 1 H),
1.02 (s, 9 H).
13C NMR (75 MHz, CDCl3): d = 164.0, 150.1, 148.5, 138.1, 136.9,
127.8, 127.6, 126.6, 124.9, 84.1, 35.5, 26.2.
HRMS (ESI): m/z [M + Na]+ calcd for C17H19NO2Na: 292.1308;
found: 292.1302.
1-(2-Methoxyphenyl)ethyl Picolinate (16d)
According to GP II with pyridine-2-carbaldehyde (2j; 27 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 13 (5.8 mg, 12.5 mmol),
1-(2-methoxyphenyl)ethanol (4d; 114 mg, 750 mmol) and 3,3¢,5,5¢-
tetra-tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in toluene
(5.0 mL) at r.t. for 4 h and silica gel chromatography (pentane–
MTBE, 2:1) to afford 16d as a colorless oil; yield: 50 mg (78%).
The enantiomeric excess (30%) was determined by chiral HPLC;
column: Chiralcel OD-H; solvent: cyclohexane–i-PrOH (98:2);
flow: 1.0 mL/min; major enantiomer tR = 5.9 min, minor enantio-
mer tR = 9.2 min. The absolute configuration of the major enantio-
mer was not assigned.
FTIR (neat): 2982, 1722, 1590, 1453, 1293, 1244, 1134, 1059, 866,
752 cm–1.
(2-Bromophenyl)(phenyl)methyl Picolinate (16h)
According to GP II with pyridine-2-carbaldehyde (2j; 27 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 13 (5.8 mg, 12.5 mmol),
(2-bromophenyl)(phenyl)methanol (4h; 200 mg, 750 mmol) and
3,3¢,5,5¢-tetra-tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in
toluene (5.0 mL) at r.t. for 2 h and silica gel chromatography (pen-
tane–MTBE, 2:1) to afford 16h as a colorless oil; yield: 80 mg
(87%).
1H NMR (300 MHz, CDCl3): d = 8.80 (d, J = 4.7 Hz, 1 H), 8.14 (d,
J = 7.8 Hz, 1 H), 7.83 (td, J = 7.7, 1.3 Hz, 1 H), 7.52 (d, J = 7.5 Hz,
1 H), 7.47 (dd, J = 7.5, 4.9 Hz, 1 H), 7.30–7.22 (m, 1 H), 6.96 (t,
J = 7.5 Hz, 1 H), 6.89 (d, J = 8.2 Hz, 1 H), 6.57 (q, J = 6.5 Hz, 1 H),
3.86 (s, 3 H), 1.68 (d, J = 6.5 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 164.2, 156.1, 149.9, 148.5, 136.9,
130.1, 128.7, 126.7, 126.0, 125.1, 120.7, 110.6, 68.9, 55.5, 21.2.
FTIR (neat): 3060, 1722, 1580, 1439, 1289, 1240, 1118, 1033, 968,
749, 701 cm–1.
HRMS (ESI): m/z [M + Na]+ calcd for C15H15NO3Na: 280.0944;
found: 280.0955.
1H NMR (300 MHz, CDCl3): d = 8.79 (d, J = 5.3 Hz, 1 H), 8.17 (d,
J = 7.8 Hz, 1 H), 7.84 (t, J = 7.7 Hz, 1 H), 7.65 (d, J = 7.8 Hz, 1 H),
7.59 (d, J = 8.0 Hz, 1 H), 7.53–7.41 (m, 4 H), 7.41–7.27 (m, 4 H),
7.18 (t, J = 7.7 Hz, 1 H).
13C NMR (75 MHz, CDCl3): d = 163.9, 150.0, 148.0, 139.1, 138.3,
136.9, 133.1, 129.5, 128.8, 128.5, 128.2, 127.8, 127.7, 126.9,
125.34, 123.2, 77.1, 27.0.
The enantiomeric excess (15%) was determined by chiral HPLC;
column: Chiralcel OD-H; solvent: cyclohexane–i-PrOH (98:2);
flow: 1.0 mL/min; major enantiomer tR = 21.0 min, minor enantio-
mer tR = 13.3 min. The absolute configuration of the major enantio-
mer was not assigned.
1-Phenylpropyl Picolinate (16f)
HRMS (ESI): m/z [M + Na]+ calcd for C19H14BrNO2Na: 390.0100;
found: 390.0099.
According to GP II with pyridine-2-carbaldehyde (2j; 27 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 13 (5.8 mg, 12.5 mmol),
1-phenylpropan-1-ol (4f; 102 mg, 750 mmol) and 3,3¢,5,5¢-tetra-
tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in toluene (5.0
mL) at r.t. for 4 h and silica gel chromatography (pentane–MTBE,
2:1) to afford 16f as a yellow oil; yield: 50 mg (83%).
The enantiomeric excess (35%) was determined by chiral HPLC;
column: Chiralcel OD-H; solvent: cyclohexane–i-PrOH (98:2);
flow: 1.0 mL/min; major enantiomer tR = 12.6 min, minor enantio-
mer tR = 9.3 min. The absolute configuration of the major enantio-
mer was not assigned.
FTIR (neat): 2969, 1722, 1582, 1444, 1304, 1132, 1083, 896, 751,
702 cm–1.
1H NMR (300 MHz, CDCl3): d = 8.79–8.74 (m, 1 H), 8.10 (dd,
J = 7.8, 0.9 Hz, 1 H), 7.79 (td, J = 7.7, 1.7 Hz, 1 H), 7.48–7.40 (m,
3 H), 7.37–7.23 (m, 3 H), 5.97 (t, J = 7.0 Hz, 1 H), 2.22–2.07 (m, 1
H), 2.06–1.91 (m, 1 H), 0.95 (t, J = 7.4 Hz, 3 H).
Phenyl(2-tolyl)methyl Picolinate (16i)
According to GP II with pyridine-2-carbaldehyde (2j; 27 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 13 (5.8 mg, 12.5 mmol),
phenyl(2-tolyl)methanol (4i; 149 mg, 750 mmol) and 3,3¢,5,5¢-tetra-
tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in toluene (5.0
mL) at r.t. for 3 h and silica gel chromatography (pentane–MTBE,
2:1) to afford 16i as a colorless oil; yield: 58 mg (77%).
13C NMR (75 MHz, CDCl3): d = 164.5, 149.9, 148.4, 140.1, 136.9,
128.4, 127.9, 126.7, 126.7, 125.1, 78.9, 29.3, 10.1.
HRMS (ESI): m/z [M + Na]+ calcd for C15H15NO2Na: 264.0995;
found: 264.0991.
FTIR (neat): 3058, 1722, 1582, 1450, 1298, 1243, 1125, 964, 748
cm–1.
The enantiomeric excess (20%) was determined by chiral HPLC;
column: Chiralcel OD-H; solvent: cyclohexane–i-PrOH (98:2);
flow: 1.0 mL/min; major enantiomer tR = 9.4 min, minor enantio-
mer tR = 12.8 min. The absolute configuration of the major enantio-
mer was not assigned.
1H NMR (300 MHz, CDCl3): d = 8.73–8.70 (m, 1 H), 8.09 (d,
J = 7.8 Hz, 1 H), 7.74 (td, J = 7.8, 1.7 Hz, 1 H), 7.48 (dd, J = 5.1,
3.9 Hz, 1 H), 7.41–7.08 (m, 10 H), 2.30 (s, 3 H).
13C NMR (75 MHz, CDCl3): d = 164.1, 150.1, 148.2, 139.0, 137.7,
136.8, 135.9, 130.6, 128.5, 128.0, 128.0, 127.7, 127.2, 126.8, 126.1,
125.2, 75.6, 19.5.
2,2-Dimethyl-1-phenylpropyl Picolinate (16g)
HRMS (ESI): m/z [M + Na]+ calcd for C20H17NO2Na: 326.1151;
found: 326.1158.
According to GP II with pyridine-2-carbaldehyde (2j; 27 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 13 (5.8 mg, 12.5 mmol),
2,2-dimethyl-1-phenylpropan-1-ol (4g; 123 mg, 750 mmol) and
Synthesis 2011, No. 12, 1974–1983 © Thieme Stuttgart · New York

1982
S. De Sarkar et al.
PAPER
(7) For selected examples of NHC-catalyzed esterifications
The enantiomeric excess (11%) was determined by chiral HPLC;
column: Chiralcel OD-H; solvent: cyclohexane–i-PrOH (98:2);
flow: 1.0 mL/min; major enantiomer tR = 10.5 min, minor enantio-
mer tR = 8.7 min. The absolute configuration of the major enantio-
mer was not assigned.
using internal redox reactions, see: (a) Zeitler, K.; Rose, C.
A. J. Org. Chem. 2009, 74, 1759. (b) Chow, K. Y.-K.;
Bode, J. W. J. Am. Chem. Soc. 2004, 126, 8126.
(c) Reynolds, N. T.; Read de Alaniz, J.; Rovis, T. J. Am.
Chem. Soc. 2004, 126, 9518. (d) Grasa, G. A.; Güveli, T.;
Singh, R.; Nolan, S. P. J. Org. Chem. 2003, 68, 2812.
(8) For NHC-catalyzed oxidative esterifications using an
external oxidant, see: (a) Maki, B. E.; Chan, A.; Phillips, E.
M.; Scheidt, K. A. Tetrahedron 2009, 65, 3102.
1-Naphthyl(phenyl)methyl Picolinate (16j)
According to GP II with pyridine-2-carbaldehyde (2j; 27 mg, 0.25
mmol), DBU (42 mg, 0.28 mmol), catalyst 13 (5.8 mg, 12.5 mmol),
1-naphthyl(phenyl)methanol (4j; 176 mg, 750 mmol) and 3,3¢,5,5¢-
tetra-tert-butyldiphenoquinone (14; 102 mg, 250 mmol) in toluene
(5.0 mL) at r.t. for 6 h and silica gel chromatography (pentane–MT-
BE, 2:1) to afford 16j as an orange solid; yield: 43 mg (50%).
(b) Noonan, C.; Baragwanath, L.; Connon, S. J. Tetrahedron
Lett. 2008, 49, 4003. (c) Miyashita, A.; Suzuki, Y.;
Nagasaki, I.; Ishiguro, C.; Iwamoto, K.-I.; Higashino, T.
Chem. Pharm. Bull. 1997, 45, 1254. (d) Tam, S. W.;
Jimenez, L.; Diederich, F. J. Am. Chem. Soc. 1992, 114,
1503.
FTIR (neat): 3054, 1722, 1586, 1514, 1443, 1293, 1239, 1119, 956,
743, 699 cm–1.
1H NMR (300 MHz, CDCl3): d = 8.72 (m, 1 H), 8.10 (d, J = 7.8 Hz,
1 H), 8.03 (dd, J = 6.2, 3.5 Hz, 1 H), 7.88 (s, 1 H), 7.84–7.64 (m, 4
H), 7.47–7.34 (m, 6 H), 7.31–7.16 (m, 3 H).
13C NMR (75 MHz, CDCl3): d = 164.2, 150.1, 148.1, 139.4, 136.9,
134.9, 133.9, 130.8, 129.1, 128.8, 128.5, 128.1, 127.6, 126.9, 126.4,
125.9, 125.7, 125.3, 125.2, 124.0, 75.8.
(9) (a) Campbell, C. D.; Duguet, N.; Gallagher, K. A.;
Thomson, J. E.; Lindsay, A. G.; O’Donoghue, A. C.; Smith,
A. D. Chem. Commun. 2008, 3528. (b) Thomson, J. E.; Rix,
K.; Smith, A. D. Org. Lett. 2006, 8, 3785.
(10) For selected examples of the nonenzymatic kinetic
resolution of alcohols, see: (a) Hu, B.; Meng, M.; Wang, Z.;
Du, W.; Fossey, J. S.; Hu, X.; Deng, W.-P. J. Am. Chem. Soc.
2010, 132, 17041. (b) Rendler, S.; Plefka, O.; Karatas, B.;
Auer, G.; Fröhlich, R.; Mück-Lichtenfeld, C.; Grimme, S.;
Oestreich, M. Chem. Eur. J. 2008, 14, 11512. (c) Vedejs,
E.; Jure, M. Angew. Chem. Int. Ed. 2005, 44, 3974.
(d) Miller, S. J. Acc. Chem. Res. 2004, 37, 601.
HRMS (ESI): m/z [M + Na]+ calcd for C23H17NO2Na: 362.1151;
found: 362.1152.
The enantiomeric excess (7%) was determined by chiral HPLC; col-
umn: Chiralpak AD-H; solvent: cyclohexane–i-PrOH (98:2); flow:
1.0 mL/min; major enantiomer tR = 16.3 min, minor enantiomer
tR = 27.0 min. The absolute configuration of the major enantiomer
was not assigned.
(11) (a) Suzuki, Y.; Yamauchi, K.; Muramatsu, K.; Sato, M.
Chem Commun. 2004, 2770. (b) Kano, T.; Sasaki, K.;
Maruoka, K. Org. Lett. 2005, 7, 1347.
(12) For recent reports involving acylazolium ions, see:
(a) De Sarkar, S.; Studer, A. Angew. Chem. Int. Ed. 2010, 49,
9266; Angew. Chem. 2010, 122, 9452. (b) Zhu, Z. Q.; Xiao,
J. C. Adv. Synth. Catal. 2010, 352, 2455. (c) Kaeobamrung,
J.; Mahatthananchai, J.; Zheng, P.; Bode, J. W. J. Am. Chem.
Soc. 2010, 132, 8810. (d) Ryan, S. J.; Candish, L.; Lupton,
D. W. J. Am. Chem. Soc. 2009, 131, 14176.
Acknowledgment
We thank the DFG for funding this project within the priority pro-
gram ‘organocatalysis’ and the NRW Graduate School of Chemi-
stry for providing a scholarship to A.B.
(13) For NHC-catalyzed transesterifications, see: (a) Grasa, G.
A.; Güveli, T.; Singh, R.; Nolan, S. P. J. Org. Chem. 2003,
68, 2812. (b) Nyce, G. W.; Lamboy, J. A.; Connor, E. F.;
Waymouth, R. M.; Hedrick, J. L. Org. Lett. 2002, 4, 3587.
(14) (a) De Sarkar, S.; Studer, A. Org. Lett. 2010, 12, 1992.
(b) De Sarkar, S.; Grimme, S.; Studer, A. J. Am. Chem. Soc.
2010, 132, 1190. (c) Guin, J.; De Sarkar, S.; Grimme, S.;
Studer, A. Angew. Chem. Int. Ed. 2008, 47, 8727; Angew.
Chem. 2008, 120, 8855.
(15) Scheidt and co-workers reported the desymmetrization of
meso-diols using oxidative carbene catalysis; see ref. 8a.
(16) Bisquinone 14 is readily prepared by treatment of 2,6-di-
tert-butylphenol (Aldrich US$37.50/1 kg) with O2; see:
Kharasch, M. S.; Joshi, B. S. J. Org. Chem. 1957, 22, 1439.
(17) Enders, D.; Niemeier, O.; Balensiefer, T. Angew. Chem. Int.
Ed. 2006, 45, 1463.
(18) The absolute configuration of the major enantiomer of 15a
formed using catalyst 8 was determined by comparing the
HPLC retention time with that of an authentic sample.
(19) Brand, J. P.; Siles, J. I. O.; Waser, J. Synlett 2010, 881.
(20) Struble, J. R.; Bode, J. W. Org. Synth. 2010, 87, 362.
(21) Vora, H. U.; Lathrop, S. P.; Reynolds, N. T.; Kerr, M. S.;
deAlaniz, J. R.; Rovis, T. Org. Synth. 2010, 87, 350.
(22) Mahatthananchai, J.; Zheng, P.; Bode, J. W. Angew. Chem.
Int. Ed. 2011, 50, 1673.
References
(1) (a) Berkessel, A.; Gröger, H. Asymmetric Organocatalysis:
From Biomimetic Concepts to Applications in Asymmetric
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Enantioselective Organocatalysis: Reactions and
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Kinetic Resolution of Secondary Alcohols by Oxidative Esterification
1983
(25) Luo, Y.; Herndon, J. W.; Cervantes-Lee, F. J. Am. Chem.
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(26) Ryu, E.-H.; Cho, H. K.; Zhao, Y. Org. Lett. 2007, 9, 5147.
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Synthesis 2011, No. 12, 1974–1983 © Thieme Stuttgart · New York