Stereochemical stability differences between axially chiral 6-Aryl-substituted picolinic esters and their benzoic ester derivatives: Sp2 N: Vs. sp2 CH in CH3, C6 H5, and CH3 O ortho-substitution effect

Article abstract of DOI:10.1246/bcsj.20150262

An axially chiral naphthyl-substituted picolinic acid (R-Naph-PyCOOH) or its allyl ester (R-Naph-PyCOOAll) acts as an excellent ligand for catalytic asymmetric dehydrative intramolecular allylation. Towards the future development of a high performance cat

Full text of DOI:10.1246/bcsj.20150262

Stereochemical Stability Differences between Axially Chiral 6-Aryl-
Substituted Picolinic Esters and Their Benzoic Ester Derivatives:
sp2N: vs. sp2CH in CH3, C6H5, and CH3O ortho-Substitution Effect
Shinji Tanaka,3 Yusuke Suzuki,1 Masaharu Matsushita,2 and Masato Kitamura*1
1Graduate School of Pharmaceutical Sciences, Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8601
2Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8601
3Research Center for Materials Science, Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8601
E-mail: kitamura@os.rcms.nagoya-u.ac.jp
Received: July 30, 2015; Accepted: September 4, 2015; Web Released: December 15, 2015
Masato Kitamura
Masato Kitamura received his Doctor of Agriculture at Nagoya University (NU) (Professors T. Goto and M.
Isobe) in 1983. After postdoctoral work with Professor G. Stork (Columbia University), he joined the Noyori
group at Faculty of Science in NU as Assistant Professor (1983), and was promoted to Associate Professor
(1990). In 1998, he moved to Research Center for Materials Science in NU as Full Professor. Currently, he is
studying at Graduate School of Pharmaceutical Sciences in NU from 2012. He is a recipient of The CSJ
Award for Young Chemists (1989) and Synthetic Organic Chemistry Award, Japan (2012).
Abstract
since the atropisomerism of 6,6¤-dinitro-2,2¤-diphenic acid
was first detected in 1922.4 The stereochemical stability of
individual enantiomers is determined mainly by the steric
factor of the ortho-substituents of the biphenyls (Ph-Ph), L, S,
L¤, and S¤, where L and L¤ are larger in size than S and S¤,
respectively (Figure 1a), and is generally discussed by use
of the concept of the effective radius (ER), as defined by
Sternhell.5 Replacement of sp2C-S with sp2N reduces the
rotational energy barrier ¦G‡ value because the S substituent
then is replaced by the lone pair on N (Figure 1b). In 2009,
Clayden quantitatively investigated the substituent effect in the
rotation through the sp2C-sp2C bond joining two aryl rings in
6-phenylpyridine derivatives (Ph-Py) by use of a dynamic
NMR technique (DNMR).6 Two years later, Schlosser esti-
mated the degree of space-filling of a pyridine lone pair in
An axially chiral naphthyl-substituted picolinic acid (R-
Naph-PyCOOH) or its allyl ester (R-Naph-PyCOOAll) acts
as an excellent ligand for catalytic asymmetric dehydrative
intramolecular allylation. Towards the future development of a
high performance catalysis using R-Naph-PyCOOH or its
carba-analogue (R-Naph-PhCOOH), the stereochemical stabil-
ity of R-Naph-PyCOOAll and R-Naph-PhCOOAll has been
investigated by a systematic change in the ortho-substituent
R on the naphthalene ring from CH3 to C6H5 and CH3O to
provide a range of effective radius (ER) and Hammett standard
· constant (·p). Enantiomerically pure R-Naph-PyCOOAll and
R-Naph-PhCOOAll were prepared, and the rotational energy
barriers ¦G‡(sp2N) and ¦G‡(sp2CH) as well as the racemiza-
tion half-life (t1/2) were determined by time-interval chiral
HPLC analysis of the enantiomer ratio (er). These experiments
revealed have been i) that replacement of sp2CH with sp2N
lowers ¦G‡ at least by 14 kJ mol¹1 (3.3 kcal mol¹1), and ii) that
the degree of the ¦G‡ lowering (¦¦G‡) is not proportional to
ER, but has a high linear correlation to ·p with a negative µ
value. In this particular system, a synergistic effect of the steric
and electronic factors stabilizes TSº180 with a 180° dihedral
angle of CH3OC(2¤)=C(1¤)-C(6)=N(1) over TSº0, decreasing
the t1/2 of CH3O-Naph-PyCOOAll to 7 days from 1600 years
of CH3-Naph-PyCOOAll.
a
Ph-Ph
C
C
L
S
L'
L
S
S
R
G‡
S'
S'
L'
b
Ph-Py
N
N
L
R
S'
L
S
G‡
S'
L'
L'
Axially chiral biaryls have constituted one of the core
structural motifs in chiral ligands,1 molecular machines,2 and
both natural and unnatural biologically active compounds3
Figure 1. Atropisomerism of ortho-substituted biaryl com-
pounds Ph-Ph and Ph-Py (L > S; L¤ > S¤). The upper aryl
ring is fixed for simplicity.
1726 | Bull. Chem. Soc. Jpn. 2015, 88, 1726–1734 | doi:10.1246/bcsj.20150262
© 2015 The Chemical Society of Japan

comparison to the corresponding C-H analogue by DNMR and
a DFT calculation of systematically designed Ph-Py and Ph-Ph
compounds, and revealed that the ¦G‡ values of Ph-Pys are
Results and Discussion
System Design. Figure 3 represents the general energy
diagram for the racemization of ortho-disubstituted biaryls
(X = sp2N and sp2CH). As defined in Figure 1, L and L¤ are
larger substituents than S, which is either H or a lone pair in
this particular case, and S¤, respectively. The conformation in
the ground state is expected to be orthogonal because both aryl
rings have ortho-substituted skeletons. Clockwise rotation of
the lower aryl ring through the sp2C-sp2C single bond gives a
planar transition state TSº0. Here, s-cis S¤-C=C-C=X with the
dihedral angle of 0° generates a relatively less crowded cove
region, while the two large ortho-substituents, L and L¤, on the
same side creates an over crowded fjord region.9 Anticlockwise
rotation gives TSº180, in which small sp2N: or sp2CH and large
L¤ are located in the cove region and L and S¤ are in the fjord
region. The S/L¤ (H or lone pair/L¤) and L/S¤ combination
is preferred over the S/S¤ (H or lone pair/S¤) and L/L¤
combination, and therefore the enantiomer more easily inverts
the stereochemistry via TSº180. At first glance, the ER size of L
and S¤ might be thought to be the decisive factor determining
the ¦G‡ value of TSº180, but the actual situation is not so
simple. The TS could be stabilized or destabilized by electronic
factors involving a conjugation in S¤-C=C-C=X or L¤-C=
C-C=X and an electronic repulsion between S¤ or L¤ and sp2N.
In order to avoid complications in analysis of the steric and
electronic effects on the stereochemical stability of future
axially chiral ligands, the basic skeletons are fixed to be allyl
5-methyl-6-(2-R-naphthyl)picolinate (2, R-Naph-PyCOOAll)
and the benzoate analogues (3, R-Naph-PhCOOAll) (Chart 1),
in which three R groups, CH3 (ER, 1.80 ¡; ·p, ¹0.14), C6H5
(1.62, 0.02), and CH3O (1.52, ¹0.28), are adopted. The
carboxylic acid was protected as the allyl (All) ester to avoid
protonation of the sp2N atom.
¹1
up to 17.6 kJ mol (4.2 kcal mol¹1) smaller than those of the
carba-analogous Ph-Phs.7 Thanks to these pioneering studies,
the torsional barriers of Ph-Pys can now be discussed as an
extension of those of Ph-Ph-derivatives, enabling the rational
design and synthesis of various axially chiral Ph-Py-type
ligands in asymmetric catalysts.1c,1d
We have recently found that axially chiral (S)- or (R)-Cl-
Naph-PyCOOAll ((S)- or (R)-allyl 6-(2-chloronaphthalen-1-yl)-
5-methylpyridine-2-carboxylic ester) (1) works as an excellent
ligand in combination with [RuCp(CH3CN)3]PF6 for catalytic
asymmetric dehydrative cyclization of various allylic alcohols
having protonic nucleophiles, as shown in Figure 2.8 Their use
enables efficient enantioselective construction of hetero- and
carbo-cycles, which are key chiral starting materials for the
synthesis of pharmaceuticals and natural products. This first
example of the high utility of chiral picolinic acid derivatives,
as well as the first successful example of a paradigm shift from
traditional salt-liberation-type Tsuji-Trost allylation to the
H2O-liberation version, attracted a great deal of attention from
the organic synthetic chemist community.8e,8f Here, we would
like to report the results of a quantitative analysis of the stereo-
chemical stability of a series of new axially chiral ligands as a
step towards the design of chiral picolinic acid derivatives and
carba-analogues with a much higher performance in asymmet-
ric catalysis.
0.05–1 mol%
cat*
C
NuH
R2
C
Nu
*
–H2O
OH
C'
C'
R2
R1
R1
quant
up to >99:1 er
cat* = [RuCp(CH3CN)3]PF6/
Cl-Naph-PyCOOR (1)
COOR
COOR
Synthesis of («)-R-Naph-PyCOOAll and («)-R-Naph-
PhCOOAll. For the purposes of this quantitative study of
rotation around the C(6)-C(1¤) bond of the sp2N-type chiral 2
N
N
Cl
Cl
COOAll
COOAll
5
4
N
(S)-Cl-Naph-PyCOOR
(S)-1
(R)-Cl-Naph-PyCOOR
(R)-1
6
3
1'
1'
R
R
2'
2'
Figure 2. Asymmetric dehydrative cyclization of ω-NuH-
substituted allylic alcohols (NuH: OH, NHCOR, NHSO2R,
COOH) catalyzed by a CpRu(II) complex with a new
axially chiral picolinic acid-type ligand. R = H or
CH2CH=CH2 (All).
2
3
a: CH3, b: C6H5, c: OCH3
Chart 1.
TS
0
TS
L/L'
S/S'
180
X
L
L'
fjord cove
L/S'
S/L'
S'
X
L
fjord cove
S'
L'
X
X
X
L
L
L
c
loc
S'
L'
L'
S'
L'
S'
kwise
e
G‡
ockwis
icl
t
an
G‡
= 0°
180°
Figure 3. Schematic energy diagram in the racemization process of ortho-disubstituted biaryls (X = sp2N and sp2CH). L > X (S),
L¤ > S¤ > X (S). The upper aryl ring is fixed for simplicity.
Bull. Chem. Soc. Jpn. 2015, 88, 1726–1734 | doi:10.1246/bcsj.20150262
© 2015 The Chemical Society of Japan | 1727

COOAll
Figure 5a(1) shows the change in the er of 2a over time. The
er decreases from >99:1 to 85:15 (11 h), 77:23 (20 h), 66:34
(38 h), and 54:46 (81 h). Logarithmic plots of the enantiomeric
excess (ee = «R ¹ S«/«R + S«), converted from the er values,
versus time is shown in Figure 5a(2). The linear relationship,
N
N
6
(S)- or (R)-2
(
-
1'
chiral
R
R
HPLC
4
( )-2
ln(ee) = ¹8.64 © 10¹6t + 4.61, with a correlation coefficient
R = CH3, C6H5, CH3O: i), ii), iii), iv), v)
¹1
(R2) of 0.998 determined the krac value to be 8.64 © 10¹6 s
.
COOR'
The energy barrier ¦G‡ for the R/S stereoinversion can be
empirically related to 8.314T ln(2.084 © 1010 T/krac) by use of
the Eyring equation,11 therefore ¦G‡ at 120 °C was calcu-
COOAll
5
X
3
(S)- or (R)-3
(
-
¹1
lated to be 135 kJ mol (33 kcal mol¹1). The half-life (t1/2) of
1'
R
chiral
HPLC
Y
R
(R)-2a was deduced from ln 2/krac to be 1600 years at 25 °C.
Figures 5b-5f shows the HPLC profiles as well as the expo-
nential decay curves for 2b, 2c, 3a, 3b, and 3c. Since the decline
in the er of 2c was too fast to measure under the standard
conditions,6i the rate of racemization was measured at 50 °C.
Table 1 summarizes the results together with the effective radius
(ER) and Hammett standard ·p constant for R as indices for
the steric effect and the electronic effect, respectively.
( )-3
6
R = CH3; R' = CH3, X = B(OH)2, Y = Br: vi), vii), iv), v)
R = C6H5; R' = H, X = Br, Y = B(OH)2: vi), iv), v)
R = CH3O; R' = CH3, X = Br, Y = B(OH)2: vi), vii), iv), v)
Figure 4. Systematic synthesis of («)-R-Naph-PyCOOAll
((«)-2) and («)-R-Naph-PhCOOAll ((«)-3). i) mCPBA. ii)
ClCON(CH3)2, (CH3)3SiCN. iii) aq HCl. iv) SOCl2. v)
AllOH. vi) [Pd(P(C6H5)3)4], Na2CO3 or K2CO3. vii) aq
NaOH. Chiral HPLC separation: CHIRALCEL OD-H for
2a, 2b, and 3c; CHIRALPAK AD-H for 2c, 3a, and 3b.
Effect of Substituent R on Stereochemical Stability. As
expected from Schlosser’s investigation,7 the rotation energy
barrier ¦G‡(CH) values of sp2CH-type biaryls 3 are higher than
the ¦G‡(N) values of sp2N-type biaryl 2 in all cases with
R = CH3, C6H5, and CH3O (Table 1). Among the six com-
pounds 2a-2c and 3a-3c, 3a is stereochemically the most
and C(3)-C(1¤) bond for the sp2CH-type chiral 3, six racemic
compounds were initially prepared in a systematic way as
shown in Figure 4. The reaction conditions were not optimized.
All of the precursors, 4a (R = CH3),8a 4b (R = C6H5),8a and 4c
(R = CH3O),10 for the 2 series are known compounds. These
were converted in 32-61% overall yields in 5 steps via i) N-
oxidation, ii) Reissert-Henze cyanation, iii) acid-promoted
hydrolysis, iv) acid chloride formation, and v) alcoholysis by
AllOH. The corresponding carba-analogues 3 were also
similarly synthesized in 36-78% overall yields from benzoic
acid derivatives 5 (X = B(OH)2, R¤ = CH3 for 3a; X = Br,
R¤ = H for 3b; X = Br, R¤ = CH3 for 3c) via i) Suzuki-
Miyaura coupling with 6 (Y = Br for 3a, Y = B(OH)2 for 3b
and 3c), ii) acid chloride formation, and iii) alcoholysis by
AllOH. With 5 (R¤ = CH3), a hydrolysis step is required after
the C(3)-C(1¤) coupling.
¹1
stable with a barrier of 153 kJ mol (36.3 kcal mol¹1) and
1500000 years half life, while 2c is the most unstable, where
¹1
the ¦G‡ and t1/2 values decrease to 107 kJ mol
(25.6
kcal mol¹1) and 7 days, respectively. The significant difference
in the stereochemical stability is apparently ascribable to the
following two steric factors: i) sp2N (lone pair) vs. sp2CH (H)
in the upper aryl ring and ii) CH3 (1.80 ER) and CH3O (1.52
ER) at C(2¤) of the lower naphthalene ring. The smaller
substituent combination in 2c remarkably facilitates the C(6)-
C(1¤) bond rotation, while the larger substituent combination
in 3a impedes the C(3)-C(1¤) bond rotation. Interestingly,
however, the energy difference ¦¦G‡ between ¦G‡(CH) and
¹1
¦G‡(N) is not proportional to the ER value: 18 kJ mol
¹1
(4.3 kcal mol¹1) for R = CH3 (1.80 ER); 14 kJ mol
(3.3
(5.5
¹1
¹1
kcal mol
) for R = C6H5 (1.62 ER); 23 kJ mol
Enantiomer Separation.
The R and S enantiomers of
kcal mol¹1) for R = CH3O (1.52 ER). A correlation can be
seen rather between ¦¦G‡ and the Hammett standard · con-
stant, ·p, for R. As the ·p value decreases from 0.02 to ¹0.14
and ¹0.28, the ¦¦G‡ value increases from 14 to 18 and then
23, implying that there is an electronic effect in the sp2N-type
R-Naph-PyCOOAll that does not exist in the sp2CH ana-
logue R-Naph-PhCOOAll. As shown in Figure 6, the Hammett
plot of ¦¦G‡ versus ·p exhibited a linear relationship with a
correlation coefficient of 0.99. Although there are only three
data points, the high linear correlation with a negative slope
(µ = ¹29.9) clearly indicates that the degree of decrease in the
TS energy level produced by the replacement of sp2CH with
sp2N increases as the electron donor character of R increases.
In the present biaryls, R-Naph-PyCOOAll and R-Naph-
PhCOOAll, the sp2C-sp2C bond rotation is thought to proceed
via TSº180, as in the general discussion in Figure 3. With
R = CH3O, however, the s-trans conformation is preferred over
the corresponding s-cis conformation of TSº0 for both steric
and electronic reasons (Figure 7a). The L/S¤ combination (L:
(«)-2 and («)-3 were separated by use of a preparative HPLC
packed with a chiral stationary phase (2a, 2b, and 3c:
CHIRALCEL OD-H; 2c, 3a, and 3b: CHIRALPAK AD-H)
by 10 injections, each being 2 mg (Figure 4). The absolute con-
figurations of the longer retention time 2a and 2b compounds
were determined to be R and S, respectively, by the X-ray
crystallographic analyses of (1R,2S,5R)-menthyl ester of CH3-
Naph-PyCOOH and (R)-1-phenylethylamide of C6H5-Naph-
PyCOOH. The details are described in the previous report.8a
As there was no correlation between the HPLC elution order
and the configuration in the above two cases, the absolute
configurations of 2c, 3a, 3b, and 3c were not assigned.
Racemization. The enantiomerically pure 2a-2c and 3a-3c
obtained above were used for measurement of the racemization
rate constants krac. The standard conditions were set to [2 or 3] =
10 mM, DMA, and 120 °C (393 K). At appropriate time inter-
vals, an aliquot of the mixture was sampled and subjected to
chiral HPLC analysis to determine the enantiomer ratio (er).
1728 | Bull. Chem. Soc. Jpn. 2015, 88, 1726–1734 | doi:10.1246/bcsj.20150262
© 2015 The Chemical Society of Japan

a
(1)
(2)
5
4
3
2
ln(ee) = –8.64 x 10–6 t + 4.61
R2 = 0.998
COOAll
N
time, h
er
1 : >99
15 : 85
23 : 77
34 : 66
46 : 54
0
11
20
38
81
CH3
(x 105)
2a
0
1.08
1.80
t, s
2.88
6
8
10
12
14
t, min
(2)
b
(1)
5
4
3
2
ln(ee) = –7.78 x 10–5 t + 4.59
R2 = 0.999
COOAll
time, h
er
N
0
1
1 : >99
14 : 86
C6H5
5
7
38 : 62
43 : 57
6
8
(x 103)
4
10
12 14 16
0
7.2
14.4
21.6
28.8
2b
t, s
t, min
c
(2)
(1)
5
4
3
2
ln(ee) = –2.95 x 10–5 t + 4.58
R2 = 0.994
time, h
er
COOAll
0
1 : 99
8 : 92
1.5
3.0
4.5
6.2
7.8
N
14 : 86
21 : 79
CH3O
24 : 76
29 : 71
(x 103)
0
7.2
14.4
21.6
28.8
8
10
12
14
16
t, s
2c
t, min
d
4.7
(1)
(2)
ln(ee) = –4.01 x 10–8 t + 4.60
4.6
4.5
4.4
4.3
4.2
R2 = 0.980
COOAll
time, h
0
er
0.1 : >99.9
60
0.5 : 99.5
1.0 : 99.0
CH3
132
0
14.4
28.8
t, s
43.2
(x 105)
6
8
10
12
14
3a
t, min
e
(1)
(2)
4.7
4.6
4.5
4.4
4.3
4.2
ln(ee) = –9.87 x 10–7 t + 4.61
R2 = 0.998
COOAll
time, h
0
er
1 : >99
2 : 98
4 : 96
5 : 95
C6H5
10
20
32
3b
0
3.6
7.2
10.8
14.4 (x 104)
8
10
12
14
16
t, s
t, min
f
(1)
(2)
5
4
3
2
COOAll
ln(ee) = –3.82 x 10–5 t + 4.61
R2 = 0.999
time, h
er
0
2
3
4
5
6
7
>99 : 1
89 : 11
83 : 17
79 : 21
75 : 25
71 : 29
69 : 31
CH3O
18
4
54 : 46
3c
7.2 (x 104)
6
8
10
12
0
1.8
3.6
5.4
t, s
t, min
Figure 5. Determination of the racemization rate krac, the rotational energy barrier ¦G‡, and the half-life t1/2. (1): HPLC charts of
2 and 3 sampled at appropriate time intervals. (2): the logarithmic plots of the ees over time.
Bull. Chem. Soc. Jpn. 2015, 88, 1726–1734 | doi:10.1246/bcsj.20150262
© 2015 The Chemical Society of Japan | 1729

Table 1. Relation between effective radius (ER) and Hammett standard · constant (·p) for R and the rotational energy barrier ¦G‡ for the
enantiomerically pure R-Naph-PyCOOAll and R-Naph-PhCOOAll (R = CH3, C6H5, and CH3O)a)
2
3
¹1 c)
Entry
R
¦¦G‡
ER/¡
·
p
¦G‡393K/kJ mol
t1/2¢298K
¦G‡393K/kJ mol
t1/2¢298K
¹1 c)
1
2
3
CH3
C6H5
CH3O
135
128
107b)
1600 y
100 y
7 d
153
142
130
1500000 y
27000 y
200 y
18
14
23
1.80
1.62
1.52
¹0.14
0.02
¹0.28
a) Conditions: [2 or 3] = 10 mM; DMA; 120 °C. b) 50 °C (323 K). c) Calculated by the Eyring equation.
G‡
R =
a
CH3O
TS
TS
180
0
CH3
COOAll
COOAll
COOAll
20
C6H5
10
anti-
clockwise
L
L
L
S
S
S
N
fjord cove
N
N
clockwise
5
= –29.9
(R2 = 0.99)
fjord cove
8'
OCH3
CH3O
CH3O
L'
L'
L'
S'
S'
S'
= 0°
= 180°
2c
sterically
p
–0.3
–0.2
–0.1
0
disfavor
disfavor
favor
favor
electronically
Figure 6. Hammett plots of ¦¦G‡ as a function of ·p
values.
b
TS
G‡ = 61.5 kJ mol–1
TS
0
180
C(5)CH3, S¤: CH3O) in the fjord region is sterically less dis-
favored than the L/L¤ combination (L: C(5)CH3, L¤: C(8¤)H).
The degree of conjugation in the CH3O-C=C-C=N moiety
is enhanced by the involvement of an n-orbital on the O of
the CH3O substituent, and the dipole moment generated by the
anti-located C=N and CH3O-C decreases the molecular polar-
ity. This synergistic effect would be a reason for the easier
racemization of 2c in comparison to 2a and 2b. On the con-
trary, the situation is completely reversed by alteration of the
2-naphthalenepyridine systems 2 to 2-phenylpyridine systems
7 and 8 (Figure 7b). The rotation barrier of 7 (R = CH3O)
3
L
L
S
S anti-
clockwise
L
S
N
N
N
clockwise
cove cove
6'
fjord bay
OCH3
L'
CH3O
CH3O
S'
S'
L'
S'
L'
= 0°
favor
7
= 180°
sterically
disfavor
disfavor
electronically
favor
G‡ = 58.0 kJ mol–1
TS
TS
0
180
3
L
L
S
S anti-
clockwise
L
S
N
N
N
¹1
is 3.5 kJ mol (0.8 kcal mol¹1) higher than that of 8 (R =
clockwise
cove cove
6'
fjord bay
¹1
¹1
CH3) (61.5 kJ mol (14.7 kcal mol¹1) vs. 58.0 kJ mol (13.9
kcal mol¹1)) as reported by Clayden.6a The tendency is con-
sistent neither with the ER-based view, nor with our observa-
tions. As shown in Figure 3, the anticlockwise/clockwise route
to TSº180 or TSº0 is determined by the relative ER-size differ-
ence between the ortho-substituents on the two aryl rings. The
S/L¤-L/S¤ combination is sterically advantageous in compar-
ison to the S/S¤-L/L¤ combination. In the case of 7 (R =
CH3O), as shown in Figure 7b, the clockwise rotation gives a
sterically favored, but electronically disfavored, TSº0 (S/L¤-
L/S¤: sp2N:/CH3O-C(3)Et/C(6¤)H), and the anticlockwise
rotation has sterically disfavored, but electronically favored,
TSº180 (S/S¤-L/L¤: sp2N:/C(6¤)H-C(3)Et/CH3O). The higher
¦G‡ value of 7 (R = CH3O, 1.52 ER) than that of 8 (R = CH3,
1.80 ER) might indicate a racemization process that proceeds
via TSº0. Electronic repulsion caused between the sp2N lone
pair and CH3O lone pairs would raise the rotational energy
more than expected simply from their ER values.
CH3
L'
CH3
L'
CH3
L'
S'
S'
S'
= 0°
8
= 180°
favor
favor
sterically
electronically
disfavor
disfavor
Figure 7. Supposed transient planar structures of CH3O-
Naph-PyCOOAll (2c) and the related aryl-pyridine com-
pounds 7 and 8.
advantage of our groundbreaking finding of the utility of a
chiral picolinic acid-type ligand in a Ru-catalyzed asymmetric
dehydrative allylation,8 the stereochemical stability of enantio-
merically pure R-Naph-PyCOOAll (2) and the carba-analogue
R-Naph-PhCOOAll (3) have been investigated in a systematic
way by fixing R at C(2¤) of the naphthalene ring to CH3 (1.80
ER; ¹0.14 ·p), C6H5 (1.62 ER; 0.02 ·p), and CH3O (1.52 ER;
¹0.28 ·p), and by setting the upper aryl group to allyl 5-
methylpicolinate or allyl 4-methylbenzoate. The steric and
electronic effect of R on the rotational energy barrier ¦G‡ as
well as the racemization half-life t1/2 was quantitatively ana-
lyzed to reveal i) that the replacement of sp2C(2)H in 3 with
Conclusion
Information on the stereochemical stability of axially chiral
compounds serves as a platform for designing chiral functional
materials, particularly chiral ligands that can contribute to
the development of catalytic asymmetric reactions. Taking
sp2N lowers ¦G‡ at least by 14 kJ mol (3.3 kcal mol¹1), and
¹1
1730 | Bull. Chem. Soc. Jpn. 2015, 88, 1726–1734 | doi:10.1246/bcsj.20150262
© 2015 The Chemical Society of Japan

ii) that the degree of the ¦G‡ lowering (¦¦G‡) is not
proportional to ER, but has a high linear correlation to ·p with
a negative µ value in the Hammett plot. The phenomenon
could be understood by a synergistic effect of the steric and
electronic factors. With the particular biaryl 2, TSº180 with the
s-trans-CH3O-C(2¤)=C(1¤)-C(6)=N(1) is both sterically and
electronically preferred over TSº0, decreasing the t1/2 of 2c
(R = CH3O) to 7 days from the 1600 years of the methyl
derivative 2a (R = CH3). In contrast, R-Ph-Py-type biaryls 7
and 8 show a reversed pattern, in which the ¦G‡ value of 7
(R = CH3O) is 6% higher than that of 8 (R = CH3). A mis-
match between the steric and electronic factors for the stabi-
lization of the TS causes the reversed effect. One tends to
recognize the sp2N only as a smaller substituent for sp2CH, but
these results show that care must be taken when designing a
pyridine-containing axially chiral biaryl ligand, particularly in
an electronically conjugatable system.
lnaphthalen-1-yl)boronic acid. Kishida chemical: allyl alcohol
(AllOH), sodium bicarbonate (NaHCO3), sodium hydroxide
(NaOH), and sodium sulfonate (Na2SO4). Nacalai Tesque:
distilled water (distilled H2O), 12 M hydrochloric acid H2O
solution (12 M aqueous HCl), potassium carbonate (K2CO3),
sodium carbonate (Na2CO3), thionyl chloride (SOCl2), and
magnesium (Mg). Tokyo Chemical Industry: 3-bromo-4-meth-
ylbenzoic acid, 1-bromo-2-methylnaphthalene, triethyl borate
((EtO)3B), methyl 3-bromo-4-methylbenzoate, N,N-dimethyl-
carbamoyl chloride (ClCONMe2), trimethylsilyl cyanide
(TMSCN),
and
tetrakis(triphenylphosphine)palladium(0)
([Pd(PPh3)4]). Wako: 69-75% m-chloroperoxybenzoic acid
(mCPBA) and sodium chloride (NaCl). The following com-
pounds for the synthesis of 2c, 3a, 3b, and 3c were prepared in
accordance with reported procedures: 2-(2-methoxynaphthalen-
1-yl)-3-methylpyridine,9 [5-(methoxycarbonyl)-2-methylphen-
yl]boronic acid,12 1-bromo-2-phenylnaphthalene.13
General Manipulations. A Teflon-coated magnetic bar
was used for stirring a reaction mixture. Reactions at 0 °C and
at ¹78 °C were carried out by use of an ice bath and of a dry
ice/CH3OH bath, respectively. Room temperature (rt) was in
the range of 25 to 28 °C. Remaining solvents after a general
workup process were removed by means of a rotary evaporator.
Concentration of a reaction mixture in a Schlenk flask was
performed by connecting to a vacuum-Ar line via a cold trap
cooled by liquid N2. Organic extracts obtained by a general
partition-based workup were dried over anhydrous Na2SO4
for ca. 30 min. “Aqueous” and “saturated” are abbreviated as
“aq” and “sat,” respectively. Brine means sat aq NaCl. All of
organometallic reactions were carried out under an Ar atmos-
phere using a general Schlenk technique unless otherwise
specified. A Schlenk flask with a Teflon J. Young valve is
specified as a “Young-type Schlenk.” Schlenk flasks were dried
before use at ca. 250 °C by use of a heat gun under a reduced
pressure, and silicon grease was used for connecting to a cold
finger and a glass stopper. Liquid reagents were introduced
by use of a syringe via a septum rubber. After introduction,
the septum was replaced with a glass stopper or with a Young
valve. Heating in a closed system was carried out after reducing
the pressure of the whole system or after raising the tem-
perature, followed by closing the system. A cold finger was
used for reflux processes in the Schlenk system. Solvents and
liquid chemicals were transferred by use of a gas-tight syringe
or cannulation method. One freeze-thaw cycle consists of i)
freezing a liquid mixture, ii) evacuation of the system at the
freezing stage, iii) closing the system, iv) thawing the frozen
liquid, and v) releasing the negative pressure to atmospheric
pressure by filling with Ar gas. The reaction conditions for the
synthesis of ligands were not optimized.
Experimentals
Instruments. Nuclear magnetic resonance (NMR) spectra
1
were recorded on a JEOL JNM-ECA-600 (600 MHz for H,
and 152 MHz for 13C), and the chemical shifts are expressed in
parts per million (ppm) downfield from Si(CH3)4 or in ppm
1
relative to CHCl3 (¤ 7.26) in H NMR and to CDCl3 (¤ 77.0)
in 13C NMR. The H signal coupling patterns are indicated as
1
follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet;
and br, broad signal. The resolutions of the H- and 13C NMR
1
spectra were 0.689 and 1.44 Hz, respectively. All of the NMR
spectra were measured at 25 °C. High-resolution mass spec-
tra (HRMS) were measured by ESI on a Bruker Daltonics
micrOTOF-QII system. A Shimadzu 10AD with 6A pump was
used for high performance liquid chromatography (HPLC)
analyses as well as preparative separation of enantiomers.
Materials. Gas: Argon (Ar) gas was purified by being
passed through a column of BASF R3-11 catalyst at 80 °C and
then through a column of granular CaSO4.
Solvents: Solvents for preparation of ligands and for the
racemization experiments were dried and degassed at the reflux
temperature in the presence of appropriate drying agents (2.5
g L¹1) under an Ar stream for 6 h and then distilled into the
storage part of the distillation system: tetrahydrofuran (THF)
and 1,2-dimethoxyethane (DME) from sodium benzophenone
ketyl; dichloromethane (CH2Cl2) and N,N-dimethylacetaminde
(DMA) from CaH2. The CH2Cl2 used for the distillation was
first distilled over P2O5, and kept in the presence of molecular
sieves 4A (MS 4A). Chloroform-d (CDCl3) was purchased
from Cambridge Isotope Laboratories, and passed through
ICN-Alumina-B-Super I (5 g/100 mL) before use. HPLC grade
ethyl acetate (EtOAc), hexane, and 2-propanol (i-PrOH) were
used for analysis of enantiomer ratios (er) and for separa-
tion of enantiomers using the chiral HPLC column. First
grade CH2Cl2, diethyl ether (Et2O), and hexane were used for
extraction and flash silica-gel column chromatography (SiO2-
chromatography) without purification: Distilled water (H2O)
was used for reactions, and tap water for extraction.
Synthesis of («)-R-Naph-PyCOOAll and («)-R-Naph-
PhCOOAll.
(«)-CH3-Naph-PyCOOAll ((«)-2a) and («)-
C6H5-Naph-PyCOOAll ((«)-2b) were prepared in accordance
with reported procedures.8a
(«)-CH3O-Naph-PyCOOAll ((«)-2c).
The procedures
essentially followed those reported in ref 8a.
Reagents and Chemicals. All reagents were purchased
from companies and used without further purification. These
are listed below in the alphabetical order neglecting the number
suffix. Aldrich: 1,2-dibromoethane ((BrCH2)2) and (2-methoxy-
Process I:
A 50-mL Schlenk flask was charged with
2-(2-methoxynaphthalen-1-yl)-3-methylpyridine (1.17 g, 4.68
mmol)9 and CH2Cl2 (10 mL). The resulting colorless solution
was cooled to 0 °C, and three portions of mCPBA (69-75%,
Bull. Chem. Soc. Jpn. 2015, 88, 1726–1734 | doi:10.1246/bcsj.20150262
© 2015 The Chemical Society of Japan | 1731

1.62 g) were added at 10-min intervals. The temperature was
gradually raised to rt, and the colorless solution was stirred for
30 min. After cooling again to 0 °C, 1 M aq NaOH (10 mL) was
slowly added. The organic layer was washed with 1 M aq
NaOH (10 mL) and brine (10 mL), and then dried over Na2SO4
(5 g). Filtration/evaporation gave 2-(2-methoxynaphthalen-1-
yl)-3-methylpyridine-1-oxide as a brownish oil (986 mg),
which was immediately used for Process ii without further
purification.
Process II: A 100-mL Schlenk flask was charged with the
above obtained crude N-oxide compound (986 mg) and CH2Cl2
(30.0 mL). To the brown solution was added ClCONMe2 (560
mg, 5.20 mmol) dissolved in CH2Cl2 (10.0 mL). After 30 min at
rt, TMSCN (738 mg, 7.44 mmol) dissolved in CH2Cl2 (10.0
mL) was added. The Schlenk line was equipped with a cold
finger. The whole mixture was stirred at rt for 12 h, and then
heated at 50 °C for 3 h. After cooling down to rt, the mix-
ture was washed with sat aq NaHCO3 (20 mL) and H2O (20
mL), and the organic layer was separated and concentrated to
give 6-(2-methoxynaphthalen-1-yl)-5-methylpicolinonitrile as
a brown oil (1.5 g), which was immediately used for Process iii
without further purification.
CH2Cl2 (10 mL © 3), and the organic extracts were dried over
Na2SO4 (ca. 5 g), filtered, and concentrated to give the crude
product (0.3 g) as a yellow oil. This was used for the next
reaction without further purification.
Process II: To the yellow oil compound (0.3 g) charged
into a 50 mL round-bottom flask was added dioxane (5.00 mL),
H2O (5.00 mL), and NaOH (120 mg, 3.00 mmol). After being
stirred at rt for 18 h, the reaction mixture was acidified by
addition of 1 M aq HCl (3 mL), and then the aq layer was
extracted with EtOAc (10 mL © 4). The combined organic
layers were washed with brine (30 mL), and dried over Na2SO4
(ca. 10 g). Filtration/evaporation afforded a white solid (249
mg), which was used for the next reaction without further
purification.
Process III: To the white solid product (249 mg) placed in
a 20-mL Schlenk flask was added SOCl2 (3.00 mL), and the
mixture was stirred at 60 °C for 2 h. After removal of the
volatiles in vacuo, AllOH (3.00 mL) was added at rt. The mix-
ture was stirred at rt for 6 h, and concentrated in vacuo to
give a yellow oil (ca. 0.5 g). The residue was purified by SiO2-
chromatography (50 g, 1:20 EtOAc-hexane eluent) to give («)-
allyl 4-methyl-3-(2-methylnaphthalen-1-yl)benzoate ((«)-3a)
1
Process III: The crude nitrile compound (1.5 g) and 6 M
aq HCl (10.0 mL) were placed in a 50-mL round-bottomed
flask equipped with a reflux condenser. The whole mixture
was heated at 100 °C for 12 h, cooled to rt and concentrated
in vacuo. To this crude residue containing 6-(2-methoxy-
naphthalen-1-yl)-5-methylpicolinic acid was added SOCl2
(5.00 mL), and the mixture was stirred at rt for 2 h. Removal
of all of the volatiles in vacuo gave a crude acid chloride as a
brown oil, which was dissolved in CH2Cl2 (4.00 mL) and
AllOH (1.50 mL). After 2 h at rt, the mixture was concentrated
in vacuo. The residue was dissolved in CH2Cl2 (20 mL). This
was washed with sat aq NaHCO3 (10 mL © 2), and dried over
Na2SO4 (ca. 5 g). Filtration/evaporation afforded a brown
solid, which was purified by SiO2-chromatography (50 g, 1:8
EtOAc-hexane eluent) to give («)-2a as a white solid (500 mg,
as a white solid (254 mg, 78% total yield): H NMR (CDCl3):
¤ 1.98 (s, 3H, CH3), 2.15 (s, 3H, CH3), 4.80 (d, J = 5.50 Hz,
2H, COOCH2CHCH2), 5.25 (d, J = 11.0 Hz, 1H, COOCH2-
CHCHH), 5.38 (d, J = 17.2 Hz, 1H, COOCH2CHCHH), 6.01
(m, 1H, COOCH2CHCH2), 7.16 (d, J = 8.25 Hz, ar), 7.31 (dd,
J = 7.56, 7.56 Hz, 1H, ar), 7.40-7.45 (m, 3H, ar), 7.81 (d,
J = 8.25 Hz, 1H, ar), 7.84 (s, 1H, ar), 7.85 (d, J = 9.62 Hz, 1H,
ar), 8.06 (dd, J = 9.62, 1.37 Hz, 1H, ar); 13C NMR (CDCl3): ¤
19.8, 20.3, 65.5, 118.3, 124.9, 125.4, 126.1, 127.5, 127.9,
128.2, 128.6, 128.8, 130.2, 131.3, 132.0, 132.3, 133.2, 136.3,
139.5, 142.8, 166.4; HRMS m/z (M + Na+) obsd 339.1357,
calcd for C22H20NaO2 339.1356.
(«)-C6H5-Naph-PhCOOAll ((«)-3b). Process I: A 50-
mL three-necked round-bottomed flask equipped with a drop-
ping funnel, a reflux condenser, and a three-way stopcock was
charged with Mg (45.7 mg, 1.88 mmol), THF (10.0 mL), and
two drops of (BrCH2)2. A 10-mL THF solution of 1-bromo-2-
phenylnaphthalene (443 mg, 1.56 mmol)13 was introduced for
1 h from a dropping funnel so as to keep a gentle reflux. After
another 1-h stirring followed by cooling to ¹78 °C, (EtO)3B
(0.530 mL, 3.13 mmol) was added, and the temperature was
raised to rt. The mixture was stirred at rt for 5 h, and partitioned
between 2 M aq HCl (10 mL) and Et2O (15 mL). The aq layer
was extracted with Et2O (15 mL © 3), and the combined
extracts were washed with brine (15 mL). Drying over Na2SO4
(ca. 10 g)/filtration/evaporation afforded a crude product
containing ca. 50% of (2-phenylnaphtalene-1-yl)boronic acid,
which was used for the next process without further purification.
Process II: The procedure was the same as Process I in the
synthesis of («)-3a. Conditions: the above obtained crude (2-
phenylnaphthalene-1-yl)boronic acid (424 mg, 0.782 mmol as
the pure form); 3-bromo-4-methylbenzoic acid (368 mg, 1.71
mmol), 2 M aq K2CO3 (2.28 mL); DME (4.60 mL); [Pd(PPh3)4]
(93.0 mg, 80.5 ¯mol); 100 °C; 12 h. The crude product (1.16 g)
was used for Process iii without purification.
1
32% total yield): H NMR (CDCl3): ¤ 2.09 (s, 3H, CH3), 3.83
(s, 3H, OCH3), 4.85-4.92 (m, 2H, COOCH2CHCH2), 5.26 (dd,
J = 10.33, 1.38 Hz, 1H, COOCH2CHCHH), 5.39 (dd, J =
17.21, 1.38 Hz, 1H, COOCH2CHCHH), 6.01-6.07 (m, 1H,
COOCH2CHCH2), 7.09 (dd, J = 8.26, 1.38 Hz, 1H, aromatic
proton (ar)), 7.29-7.35 (m, 3H, ar), 7.78 (d, J = 7.57 Hz, 1H,
ar), 7.81 (dd, J = 7.23, 2.07 Hz, 1H, ar), 7.92 (d, J = 8.95
Hz, 1H, ar); 13C NMR (CDCl3): ¤ 19.2, 56.7, 66.3, 113.5,
118.7, 122.7, 123.8, 124.3, 124.5, 126.9, 128.1, 129.4, 130.4,
132.4, 133.2, 138.2, 138.5, 146.0, 154.3, 156.7, 165.5; HRMS
m/z (M + Na+) obsd 356.1277, calcd for C21H19NNaO3
356.1257.
(«)-CH3-Naph-PhCOOAll ((«)-3a). Process I: A 20-
mL Schlenk flask was charged with [5-(methoxycarbonyl)-2-
methylphenyl]boronic acid (200 mg, 1.03 mmol),12 1-bromo-2-
methylnaphthalene (80.0 ¯L, 521 ¯mol), 2.00 M aq Na2CO3
(1.00 mL, 2.00 mmol), and DME (4.00 mL). After degassing
the whole mixture by three freeze-thaw cycles, to the sus-
pension was added [Pd(PPh3)4] (60.0 mg, 51.9 ¯mol). The
mixture was heated at 100 °C for 18 h. The reaction mixture
was cooled to rt, and then the diphase solution was acidified
by addition of 1 M aq HCl (10 mL). This was extracted with
Process III: The procedure was the same as Process II in
the synthesis of («)-3a. Conditions: the above obtained crude
1732 | Bull. Chem. Soc. Jpn. 2015, 88, 1726–1734 | doi:10.1246/bcsj.20150262
© 2015 The Chemical Society of Japan

4-methyl-3-(2-phenylnaphthalen-1-yl)benzoic acid (1.16 g);
SOCl2 (5.00 mL); AllOH (5.00 mL). Purification: 2-times
SiO2-chromatography (1st, 100 g, 1:6 EtOAc-hexane eluent;
2nd, 50 g, 1:6 EtOAc-hexane eluent). («)-Allyl 4-methyl-3-
(2-phenylnaphthalen-1-yl)benzoate ((«)-3b) (90.0 mg, 36%
total yield): 1H NMR (CDCl3): ¤ 1.89 (s, 3H, CH3), 4.76-
4.79 (m, 2H, COOCH2CHCH2), 5.25 (dd, J = 11.02, 1.38 Hz,
1H, COOCH2CHCHH), 5.35 (dd, J = 17.21, 1.38 Hz, 1H,
COOCH2CHCHH), 5.70-6.03 (m, 1H, COOCH2CHCH2),
7.15-7.18 (m, 5H, ar), 7.28 (d, J = 8.26 Hz, 1H, ar), 7.34 (d,
J = 8.26 Hz, 1H, ar), 7.39 (ddd, J = 8.66, 8.57, 1.38 Hz, 1H,
ar), 7.50 (ddd, J = 7.57, 7.57, 1.38 Hz, 1H, ar), 7.59 (d, J =
8.26 Hz, 1H, ar), 7.90-7.97 (m, 4H, ar); 13C NMR (CDCl3): ¤
19.8, 20.3, 65.5, 118.3, 124.9, 125.4, 126.1, 127.5, 127.9,
128.2, 128.6, 128.8, 130.2, 131.3, 132.0, 132.3, 133.2, 136.3,
139.5, 142.8, 166.4; HRMS m/z (M + Na+) obsd 401.1523,
calcd for C27H22NaO2 401.1512.
(«)-C6H5-Naph-PhCOOAll ((«)-3b):
Column,
CHIRALPAK AD-H (2 cm º © 25 cm); eluent, 10:1 hexane-
i-PrOH; flow rate, 10 mL min¹1; detection, 254-nm light; tR,
9.07 and 22.08 min; sample loading, 2 mg/injection © 10.
(«)-CH3O-Naph-PhCOOAll ((«)-3c):
Column,
CHIRALCEL OD-H (2 cm º © 25 cm); eluent, 5:1 hexane-
i-PrOH; flow rate, 10 mL min¹1; detection, 254-nm light; tR,
10.55 and 16.62 min; sample loading, 2 mg/injection © 10.
Determination of the Rotational Energy Barrier ¦G‡ and
t1/2
.
A typical procedure is represented by that for the
enantiomerically pure CH3-Naph-PyCOOAll (2a). A 20-mL
Young-type Schlenk flask was charged with 2a (3.30 mg,
10.0 ¯mol) and DMA (1.00 mL). After degassing the system by
three-freeze/thaw cycles, the mixture was heated at 120 °C by
use of a Riko MH-5D oil bath, which was adjusted so that the
measurement temperature was kept the same throughout the
whole process. After 11 h, the Schlenk flask was moved to an
ice bath, and an aliquot of the mixture (0.1 mL) was sampled
under Ar. Immediately after sampling (5 min), the Schlenk
flask was inserted into the 120 °C oil bath. The sample was
partitioned between Et2O (1 mL) and H2O (1 mL). The organic
layer was washed with H2O (1 mL © 2), and concentrated
in vacuo. This was subjected to er analysis by HPLC (column,
CHIRALCEL OD-H (0.46 cm º © 25 cm); eluent, 5:1 hexane-
i-PrOH; flow rate, 1 mL min¹1; detection, 254-nm light; tR, 7.22
and 11.43 min), determining the er to be 85.0:15.0. In the
same way, the ers were measured at 20 h, 38 h, and 81 h to be
77.0:23.0, 66.25:33.75, and 54.0:46.0, respectively. The loga-
rithmic plot of ln(ee) versus time determined the racemization
rate krac to be 8.64 © 10¹6 s¹1. By use of the Eyring equation
(¦G‡ = 8.314T ln(2.084 © 1010 T/krac)), the rotational energy
(«)-CH3O-Naph-PhCOOAll ((«)-3c).
The procedures
were the same as Processes I-III in the synthesis of («)-3a.
Process I: Conditions: (2-methoxynaphthalen-1-yl)boronic
acid (300 mg, 1.49 mmol); methyl 3-bromo-4-methylbenzoate
(136 ¯L, 873 ¯mol); 2 M aq Na2CO3 (1.75 mL, 3.50 mmol);
DME (5.00 mL); [Pd(PPh3)4] (101 mg, 87.4 ¯mol); 100 °C;
18 h. The crude product (0.5 g) was directly used for the next
reaction.
Process II: Conditions: dioxane (5.00 mL); H2O (5.00 mL);
NaOH (86.2 mg, 2.15 mmol); rt, 16 h. The crude product (0.2 g)
was used for the next allylation.
Process III: Conditions: the above obtained crude 3-(2-
methoxynaphthalen-1-yl)-4-methylbenzoic acid (0.2 g); SOCl2
(3.00 mL); 60 °C; 2 h; AllOH (3.00 mL). Purification: SiO2-
chromatography (50 g, 1:15 EtOAc-hexane eluent). («)-Allyl
3-(2-methoxynaphthalen-1-yl)-4-methylbenzoate (198 mg, 68%
barrier ¦G‡ was found to be 135.2 kJ mol at 393 K. The
HPLC charts and ln(ee)/time relation is shown in Figures 5a(1)
¹1
1
total yield): H NMR (CDCl3): ¤ 2.06 (s, 3H, CH3), 3.84 (s,
and 5a(2), respectively. The HPLC conditions, ee change, krac,
3H, CH3O), 4.80 (d, J = 5.50 Hz, 2H, COOCH2CHCH2), 5.25
(dd, J = 10.3, 1.37 Hz, 1H, COOCH2CHCHH), 5.38 (dd, J =
17.9, 1.37 Hz, 1H, COOCH2CHCHH), 5.98-6.04 (m, 1H,
COOCH2CHCH2), 7.20 (d, J = 8.25 Hz, ar), 7.31-7.36 (m, 1H,
ar), 7.38 (d, J = 9.12 Hz, 1H, ar), 7.44 (d, J = 8.25 Hz, 1H, ar),
7.84 (dd, J = 8.26, 1.38 Hz, 1H, ar), 7.90-7.93 (m, 2H, ar), 8.05
(dd, J = 9.28, 2.11 Hz, 1H, ar); 13C NMR (CDCl3): ¤ 20.0, 56.5,
65.4, 113.3, 118.1, 123.2, 123.6, 124.7, 126.6, 127.8, 128.0,
128.8, 129.0, 129.4, 130.0, 132.2, 132.4, 133.2, 136.5, 143.7,
153.7, 166.4; HRMS m/z (M + Na+) obsd 339.1357, calcd for
C22H20NaO2 339.1356.
Enantiomer Separation. Enantiomers of («)-CH3-Naph-
PyCOOAll ((«)-2a) and («)-C6H5-Naph-PyCOOAll ((«)-2b)
were obtained in accordance with the procedures reported
earlier.8a All other enantiomers were separated by chiral HPLC
column under the following conditions, and were immediately
subjected to the er decay experiments.
and ¦G‡ values obtained for 2b, 2c, 3a, 3b, and 3c are listed
below. The er decay of 2c was measured at 50 °C because of
the stereochemical lability.
C6H5-Naph-PyCOOAll (2b): HPLC conditions: column,
CHIRALCEL OD-H (0.46 cm º © 25 cm); eluent, 85:15
hexane-i-PrOH; flow rate, 1 mL min¹1; detection, 254-nm
light; tR, 6.60 and 12.39 min. Ee change: 0 h (>99.9% ee),
1.0 h (72.6% ee), 5.0 h (25.1% ee), 7.0 h (13.6% ee). krac
=
¹1
7.78 © 10¹5 s¹1. ¦G‡393K = 128.1 kJ mol (Figure 5b).
CH3O-Naph-PyCOOAll (2c): HPLC conditions: column,
CHIRALPAK AD-H (0.46 cm º © 25 cm); eluent, 9:1 hexane-
i-PrOH; flow rate, 1 mL min¹1; detection, 254-nm light; tR,
10.50 and 12.98 min. The origin of the peak at 9.6 min is
unidentified. Ee change: 0 h (97.4% ee), 1.5 h (84.4% ee), 3.0 h
(72.0% ee), 4.5 h (58.5% ee), 6.2 h (52.8% ee), 7.8 h (42.0% ee)
at 50 °C. krac = 2.95 © 10¹5 s¹1. ¦G‡323K = 107.3 kJ mol
(Figure 5c).
¹1
(«)-CH3O-Naph-PyCOOAll ((«)-2c):
Column,
CH3-Naph-PhCOOAll (3a): HPLC conditions: column,
CHIRALPAK AD-H (0.46 cm º © 25 cm); eluent, 10:1 hex-
ane-i-PrOH; flow rate, 1 mL min¹1; detection, 254-nm light; tR,
7.71 and 10.41 min. Ee change: 0 h (>99.9% ee), 60 h (98.9%
ee), 132 h (98.1% ee). krac = 4.01 © 10¹8 s¹1. ¦G‡393K = 152.8
kJ mol (Figure 5d).
C6H5-Naph-PhCOOAll (3b): HPLC conditions: column,
CHIRALPAK AD-H (0.46 cm º © 25 cm); eluent, 10:1 hex-
CHIRALPAK AD-H (2 cm º © 25 cm); eluent, 9:1 hexane-
i-PrOH; flow rate, 10 mL min¹1; detection, 254-nm light; tR,
15.9 and 20.0 min; sample loading, 2 mg/injection © 10.
(«)-CH3-Naph-PhCOOAll ((«)-3a):
Column,
¹1
CHIRALPAK AD-H (2 cm º © 25 cm); eluent, 50:1 hexane-
i-PrOH; flow rate, 10 mL min¹1; detection, 254-nm light; tR,
15.45 and 18.67 min; sample loading, 2 mg/injection © 10.
Bull. Chem. Soc. Jpn. 2015, 88, 1726–1734 | doi:10.1246/bcsj.20150262
© 2015 The Chemical Society of Japan | 1733

ane-i-PrOH; flow rate, 1 mL min¹1; detection, 254-nm light; tR,
4
614.
5
G. H. Christie, J. Kenner, J. Chem. Soc., Trans. 1922, 121,
G. Bott, L. D. Field, S. Sternhell, J. Am. Chem. Soc. 1980,
7.82 and 14.57 min. Ee change: 0 h (>99.9% ee), 10 h (96.4%
¹1
ee), 20 h (92.7% ee), 32 h (89.3% ee). krac = 9.87 © 10¹7 s
.
¹1
¦G‡393K = 142.3 kJ mol (Figure 5e).
102, 5618.
6
a) J. Clayden, S. P. Fletcher, J. J. W. McDouall, S. J. M.
CH3O-Naph-PhCOOAll (3c): HPLC conditions: column,
CHIRALPAK AD-H (0.46 cm º © 25 cm); eluent, 10:1 hex-
ane-i-PrOH; flow rate, 1 mL min¹1; detection, 254-nm light; tR,
6.35 and 9.71 min. Ee change: 0 h (>99.9% ee), 2.0 h (78.6%
ee), 3.0 h (66.9% ee), 4.0 h (58.1% ee), 5.0 h (50.0% ee), 6.0 h
(42.3% ee), 7.0 h (37.6% ee), 18 h (8.68% ee). krac = 3.82 ©
Rowbottom, J. Am. Chem. Soc. 2009, 131, 5331. For other reports
on determination of stereochemical stability of 2-aryl-substituted
pyridine, see: b) J. R. Pedersen, Acta Chem. Scand. 1972, 26, 929.
c) S. C. Tucker, J. M. Brown, J. Oakes, D. Thornthwaite,
Tetrahedron 2001, 57, 2545. d) F. Y. Kwong, Q. Yang, T. C. W.
Mak, A. S. C. Chan, K. S. Chan, J. Org. Chem. 2002, 67, 2769.
e) S. B. Cortright, R. A. Yoder, J. N. Johnston, Heterocycles 2004,
62, 223. f) R. W. Baker, S. O. Rea, M. V. Sargent, E. M. C.
Schenkelaars, T. S. Tjahjandarie, A. Totaro, Tetrahedron 2005, 61,
3733. g) B. A. Sweetman, H. Müller-Bunz, P. J. Guiry,
Tetrahedron Lett. 2005, 46, 4643. h) R. W. Baker, S. O. Rea,
M. V. Sargent, E. M. C. Schenkelaars, T. S. Tjahjandarie, A.
Totaro, Tetrahedron 2005, 61, 3733. i) M. Hapke, K. Kral, C.
Fischer, A. Spannenberg, A. Gutnov, D. Redkin, B. Heller, J. Org.
Chem. 2010, 75, 3993.
¹1
10¹5 s¹1. ¦G‡393K = 130.4 kJ mol (Figure 5f).
This work was aided by a Grant-in-Aid for Scientific
Research (No. 25E07B212) and (Nos. 22750088, 24510112,
and 24106713) from the Ministry of Education, Culture,
Sports, Science and Technology (Japan), and an Advanced
Catalytic Transformation Program for Carbon Utilization
(ACT-C) from Japan Science and Technology Agency (JST).
Supporting Information
7
A. Mazzanti, L. Lunazzi, S. Lepri, R. Ruzziconi, M.
Schlosser, Eur. J. Org. Chem. 2011, 6725.
a) S. Tanaka, T. Seki, M. Kitamura, Angew. Chem., Int. Ed.
NMR spectra of 2c and 3a-3c and HPLC charts of 2c and
3a-3c for enantiomer separation. This material is available
electronically on J-STAGE.
8
2009, 48, 8948. b) T. Seki, S. Tanaka, M. Kitamura, Org. Lett.
2012, 14, 608. c) Y. Suzuki, T. Seki, S. Tanaka, M. Kitamura,
J. Am. Chem. Soc. 2015, 137, 9539. d) K. Miyata, M. Kitamura,
Synthesis 2012, 44, 2138. e) M. Kitamura, K. Miyata, T. Seki, N.
Vatmurge, S. Tanaka, Pure Appl. Chem. 2013, 85, 1121. f) M.
Kitamura, S. Tanaka, M. Yoshimura, J. Synth. Org. Chem., Jpn.
2015, 73, 690.
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1734 | Bull. Chem. Soc. Jpn. 2015, 88, 1726–1734 | doi:10.1246/bcsj.20150262
© 2015 The Chemical Society of Japan