Synthesis, structure and isomerization of arylphosphoranes with anti-apicophilic bonding modes using a novel bidentate ligand with two C 2F5 groups

Article abstract of DOI:10.1039/b802947d

A series of anti-apicophilic pentacoordinate phosphoranes (with one chelating substituent in an O-equatorial, C-apical bonding mode at pentacoordinated phosphorus atom) bearing a para-substituted aryl group (-C ₆H₄(p-X); X = H, CF₃, F, OMe) or a mesityl (2,4,6-trimethylphenyl) group were isolated using a novel bulky bidentate ligand with two C₂F₅ groups. These phosphoranes were stable to isomerization at room temperature, and quantitatively converted into the corresponding more stable isomers (O-apical) at elevated temperatures in solution. On the basis of a kinetic study, the free energy of activation (ΔG^?) of the stereomutation of the O-equatorial mesitylphosphorane to its O-apical isomer was higher than that of the CF ₃ derivative by 2.6 kcal mol^-1, giving rise to a further example of the steric effect of the C₂F₅ group to freeze the isomerization of the pentacoordinate phosphorus compounds. Kinetic measurements of the isomerization of the O-equatorial ortho-unsubstituted derivatives (-C₆H₄(p-X)) to the corresponding O-apical isomers suggested that the O-equatorial isomers were stabilized by the π → σ*_P-O interaction in the ground state.

Full text of DOI:10.1039/b802947d

An international journal of inorganic chemistry
www.rsc.org/dalton
Number 28 | 28 July 2008 | Pages 3621–3760
ISSN 1477-9226
PAPER
Yamamoto et al.
HOT PAPER
Synthesis, structure and isomerization of Linti and Seifert
arylphosphoranes with anti-apicophilic A decagallane(6) cluster
bonding modes using a novel bidentate Ga10[Si(CMe3)3]6 with an
ligand with two C2F5 groups
unprecedented structure

PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis, structure and isomerization of arylphosphoranes with
anti-apicophilic bonding modes using a novel bidentate
ligand with two C2F5 groups†
Xin-Dong Jiang, Shiro Matsukawa and Yohsuke Yamamoto*
Received 20th February 2008, Accepted 12th May 2008
First published as an Advance Article on the web 13th June 2008
DOI: 10.1039/b802947d
A series of anti-apicophilic pentacoordinate phosphoranes (with one chelating substituent in an
O-equatorial, C-apical bonding mode at pentacoordinated phosphorus atom) bearing a
para-substituted aryl group (–C6H4(p-X); X = H, CF3, F, OMe) or a mesityl (2,4,6-trimethylphenyl)
group were isolated using a novel bulky bidentate ligand with two C2F5 groups. These phosphoranes
were stable to isomerization at room temperature, and quantitatively converted into the corresponding
more stable isomers (O-apical) at elevated temperatures in solution. On the basis of a kinetic study, the
free energy of activation (DG‡) of the stereomutation of the O-equatorial mesitylphosphorane to its
O-apical isomer was higher than that of the CF3 derivative by 2.6 kcal mol−1, giving rise to a further
example of the steric effect of the C2F5 group to freeze the isomerization of the pentacoordinate
phosphorus compounds. Kinetic measurements of the isomerization of the O-equatorial
ortho-unsubstituted derivatives (–C6H4(p-X)) to the corresponding O-apical isomers suggested that the
O-equatorial isomers were stabilized by the p → r*P–O interaction in the ground state.
by the Berry pseudorotation (BPR).9,10 The barrier to the BPR is
Introduction
very low (calculated to be about 2–3 kcal mol−1 for PH5)11 without
any steric restrictions.
Hypervalent1 10-P-52 phosphorus compounds (phosphoranes) are
regarded as intermediates (or transition states) in the formation or
hydrolysis of biologically relevant phosphorus compounds3 such
as DNA or RNA. Furthermore, they also play important roles as
intermediates in the Wittig reaction.4 Phosphorane chemistry is
crucially important to understand the mechanisms of the above-
described reactions, thus, a number of studies on phosphoranes
have been carried out to establish many important properties.5
However, phosphorane chemistry is still complicated due to their
kinetic and thermodynamic properties. Phosphoranes generally
prefer a trigonal-bipyramid (TBP) structure which has two distinct
bonds, i.e., an apical bond and equatorial bond. The apical
bond is described as a three-center–four-electron (hypervalent)
bond whereas the equatorial bond is described as an sp2 bond.
The former is more polar than the latter, and the apical site
is sterically more congested than the equatorial site,6 therefore,
the electronegative and sterically less bulky groups prefer the
apical site. The relative preference of substituents occupying
the apical site is specifically known as apicophilicity.7,8 From a
kinetic standpoint, on the other hand, a pentacoordinate molecule
causes rapid stereomutations, giving rise to an exchange between
the apical and equatorial ligand, therefore, such a molecule
generally exists as an equilibrium mixture containing several (up
to 20) stereoisomers in the solution state. This nondissociative
intramolecular site exchange of substituents is usually advocated
Using the rigid bidentate ligand called the Martin ligand
(A),12 our group succeeded in synthesizing the anti-apicophilic
phosphoranes (with one chelating substituent in an O-equatorial,
C-apical bonding mode at pentacoordinated phosphorus atom)
having an alkyl monodentate ligand (O-equatorial 1) (Fig. 1).13
Unlike the case of other anti-apicophilic phosphoranes,14 these
O-equatorial phosphoranes are kinetically stabilized products,
and can still be converted into the corresponding more stable
stereoisomers (O-apical 2). Although the Martin ligand is capable
of stabilizing various types of hypervalent compounds both
thermodynamically and kinetically, isolation of the O-equatorial
phosphoranes bearing a small or electronegative monodentate
ligand has still been troublesome because such compounds are
isomerized too fast to be isolated. For example, the O-equatorial
1a (R = Me) could not be isolated in the pure form because the
Department of Chemistry, Graduate School of Science, Hiroshima Uni-
versity, 1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8526, Japan. E-mail:
yyama@sci.hiroshima-u.ac.jp; Fax: +81-82-424-0723
† CCDC reference numbers 672351 (12a), 672352 (12b), 672353 (12c),
672354 (12d), 672355 (12e), 672356 (13d) and 672357 (13e). For crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b802947d
Fig. 1 Phosphoranes bearing bidentate ligands for freezing BPR.
3678 | Dalton Trans., 2008, 3678–3687
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The Royal Society of Chemistry 2008
©

irreversible isomerization of 1a to 2a was relatively fast. Thus,
we developed a bulky bidentate ligand with two C2F5 groups
(B) in place of the CF3 groups of the Martin ligand.15 This
allowed us to isolate the O-equatorial methylphosphorane 3a
in the pure form, and the free energy of activation (DG‡333) of
the isomerization of 3a to 4a was found to be greater than that
of 1a to 2a by 3.6 kcal mol−1. Furthermore, ligand B was also
effective for freezing the isomerization of the “anti-apicophilic”
10-As-5 arsoranes (O-equatorial, C-apical) to the more stable
stereoisomers (O-apical, C-equatorial).16
As a more electronegative monodentate ligand than the alkyl
group, we chose aryl groups as the next targets. Although several
attempts were made to isolate the O-equatorial arylphosphoranes
5 using the Martin ligand (A), it was found that these phospho-
ranes, except for the TIP (2,4,6-triisopropylphenyl) derivative 5c,
rapidly isomerized to the O-apical counterparts (Scheme 1).17 For
example, the isomerization of the phenyl derivative (5a to 6a) was
completed within 30 min at r.t., which was in contrast to the
O-equatorial alkyl derivatives (1). We assumed that the stereo-
mutation of the O-equatorial to O-apical isomer passes through
a high-energy intermediate (Int-1) which bears the monodentate
ligand (R) at the apical site, therefore, the more apicophilic R
could accelerate the isomerization (Scheme 2). Thus, the rapid
isomerization was mainly due to the much higher apicophilicity of
the aryl group than the alkyl group.7b,c,i,j This required employing
a very bulky TIP group to isolate the O-equatorial isomer.
of the nN → r*P–C interaction based upon the crystal structure
of 7 (Fig. 2).7a By use of a pair of O-equatorial and O-apical
benzylphosphoranes, we subsequently demonstrated that the O-
equatorial a-carbanion 8 was less basic than the corresponding
O-apical isomer 9.18 This result was supported by the density
functional calculations showing that the nC → r*P–O interaction of
the O-equatorial a-anion 8 was much greater than the nC → r*P–C
interaction of the corresponding O-apical isomer 9.18 Moreover,
based on the kinetic study of the isomerization of the O-equatorial
aminophosphorane 10 to the O-apical isomer 11, we succeeded
in determining the energy of the nN → r*P–O interaction to be
4.4 kcal mol−1, which was in good agreement with the calculated
value (4.0 kcal mol−1).19
Fig. 2 The n → r* interaction of the a-anions and aminophosphoranes.
Consequently, the interaction between the p-orbital of the
monodentate aromatic ligand and the r* orbital in the equatorial
plane has been of significant interest. However, in the Martin
ligand system (A), we isolated only the O-equatorial isomer with
a TIP group (5c) as described above. Therefore, no insights into
the p → r* interaction were established in the study. Thus, if
the O-equatorial isomer (12) bearing an ortho-unsubstituted aryl
monodentate ligand could be prepared (Fig. 3), and if the para-
substituents (X) of the monodentate ligand affect the rate of the
isomerization of the O-equatorial (12) to O-apical (13) isomer, the
p → r*P–O interaction in the O-equatorial isomer (12) could be
estimated.
Scheme 1 Synthesis of O-equatorial arylphosphoranes using the Martin
ligand.
Scheme 2 High energy intermediate formed during the isomerization of
the O-equatorial to O-apical isomer.
Theoretical studies have suggested that the p-donating sub-
stituents prefer to remain at the equatorial site of the TBP structure
because such substituents are capable of interacting with the anti-
bonding r* orbitals.8 Our group experimentally demonstrated that
a dimethylamino group is less apicophilic than expected on the
basis of its electronegativity, and this could be due to the existence
Fig. 3 Possible p → r* interaction of arylphosphoranes.
Dalton Trans., 2008, 3678–3687 | 3679
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©

In this context, we determined to employ the bulky bidentate
ligand B for stabilizing the anti-apicophilic O-equatorial arylphos-
phoranes (12). We now report the isolation of O-equatorial
arylphosphoranes 12 using ligand B. These O-equatorial phos-
phoranes, even the ortho-unsubstituted derivatives (12a–d), were
found to be stable at r.t., and could still be converted into the
O-apical isomer 13 at elevated temperatures. The structures of
the O-equatorial phosphoranes as well as the O-apical isomers
were unambiguously determined by an X-ray crystallographic
analysis. The kinetic study furnished the activation parameters
for the stereomutation of the O-equatorial arylphosphoranes 12
to the O-apical isomers 13. Furthermore, the effect of the solvent
and the para-substituent on the rate of the isomerization were
also investigated in order to provide an insight into the p → r*P–O
interaction in the O-equatorial arylphosphoranes. Full details are
given in the following sections.
Scheme 3 Synthesis of the O-equatorial (12) and O-apical (13) arylphos-
phoranes. Reagents and conditions: (a) ArLi, Et2O, r.t., 1 h, then I2, −78 ◦C
to 0 ◦C, 1 h, 12a: 85%, 12b: 58%, 12c: 76%, 12d: 48%, 12e: 83%; (b) C6D6,
80 ◦C, 8 h, 13a: 98%, 13b: 96%, 13c: 96%, 13d: 97%, 13e: 95%.
steric effect of the C2F5 groups for freezing the isomerization is
also remarkable in the present case.
Results and discussion
The structures of the O-equatorial (12a–e) and O-apical (13d
and 13e) phosphoranes were confirmed by an X-ray crystallo-
graphic analysis (Fig. 4). All compounds have a distorted trigonal-
bipyramidal (TBP) structure, in which the aryl monodentate
ligand occupies one of the equatorial sites. Two independent
molecules were found in the unit cell for each of the 12a, 12c and
12e. The angles and distances involving the phosphorus atom of
12 and 13 (Table 1) were similar to those of the corresponding
alkylphosphoranes 3 and 4, respectively. For the O-equatorial
isomers, the equatorial O2–P–C1 angle of 12a–d (12a: 120.5◦,
124.0◦; 12b: 122.5◦; 12c: 121.7◦, 124.1◦; 12d: 123.6◦) is only slightly
larger than those of the alkylphosphoranes (3a: 119.5◦; 3b: 119.7◦◦;
3c: 118.0◦),15a although those of the mesityl derivative 12e (112.8 ,
Synthesis and structure of the O-equatorial and O-apical
arylphosphoranes
Using the new bidentate ligand B, the O-equatorial arylphos-
phoranes (12) were successfully synthesized by the reaction of
the P–H spirophosphorane 1415a with excess ArLi, followed by
treatment with I2 (Scheme 3).17 All of the O-equatorial 12 was
quantitatively converted into the corresponding O-apical isomers
13 by heating in an organic solvent. Considering the fact that the
O-equatorial arylphosphoranes 5 except for 5c (Ar = TIP) have
not been isolated,17 it is noted that even the ortho-unsubstituted
derivatives (12a–d) are isolable using ligand B, indicating that the
Fig. 4 ORTEP diagrams of arylphosphoranes with the thermal ellipsoids shown at the 30% probability level. For 12a, 12c and 12e, one of the two
independent molecules is shown. All hydrogen atoms are omitted for clarity.
3680 | Dalton Trans., 2008, 3678–3687
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The Royal Society of Chemistry 2008
©

◦
˚
Table 1 Selected bond lengths (A) and angles ( ) for the O-equatorial (12) and O-apical (13) phosphoranes
Compound
12aa
12b
12ca
12d
12ea
13d
13e
P–O1
P–O2
P–C1
P–C2
P–C3
1.775(4), 1.807(4)
1.666(5), 1.655(4)
1.824(7), 1.823(7)
1.889(6), 1.900(6)
1.825(6), 1.820(7)
1.788(5)
1.643(5)
1.813(7)
1.878(7)
1.841(7)
1.7870(12), 1.7800(12)
1.6501(12), 1.6564(12)
1.8215(17), 1.8229(17)
1.8805(17), 1.8763(18)
1.8245(17), 1.8191(17)
1.7742(14)
1.6654(13)
1.8264(17)
1.8810(19)
1.8125(18)
1.7781(19), 1.7845(18)
1.6636(18), 1.6722(18)
1.828(3), 1.817(3)
1.890(3), 1.887(3)
1.853(3), 1.850(2)
1.7435(13)
1.7526(13)
1.8335(18)
1.8294(17)
1.8045(18)
1.7663(10)
1.7663(10)
1.8298(15)
1.8282(14)
1.8422(15)
O1–P–O2
O1–P–C1
O1–P–C2
O1–P–C3
O2–P–C1
O2–P–C2
O2–P–C3
C1–P–C2
C1–P–C3
C2–P–C3
83.7(2), 81.32(19)
86.7(2), 86.6(2)
171.5(3), 168.6(2)
87.7(2), 89.5(2)
120.5(2), 124.0(2)
87.9(3), 87.3(2)
116.3(3), 116.9(3)
98.9(3), 99.4(3)
121.8(3), 117.4(3)
94.7(2), 96.2(3)
83.0(2)
86.4(3)
171.3(3)
88.7(3)
122.5(3)
88.4(3)
116.2(3)
99.6(3)
119.9(3)
93.7(3)
82.26(6), 82.76(6)
87.06(7), 87.12(7)
170.13(7), 170.22(7)
89.17(7), 88.69(7)
121.70(7), 124.12(7)
87.97(7), 87.53(7)
118.47(7), 119.00(7)
99.32(8), 99.29(8)
118.48(8), 115.50(8)
94.32(7), 95.12(8)
82.34(6)
86.95(7)
169.59(8)
89.20(7)
123.63(7)
87.25(8)
117.75(7)
99.12(8)
117.24(8)
95.43(8)
83.75(9), 82.60(9)
167.89(7)
87.16(8)
88.47(7)
95.94(7)
88.89(8)
87.12(7)
96.17(7)
139.54(8)
110.17(8)
110.29(8)
174.02(5)
86.85(6)
90.68(6)
93.52(6)
90.28(6)
86.72(6)
92.46(6)
125.52(7)
118.85(6)
115.63(7)
86.76(11), 87.12(10)
170.73(11), 169.61(10)
87.89(10), 88.06(10)
112.78(11), 115.18(10)
87.23(10), 87.14(10)
121.26(11), 125.08(11)
98.73(12), 98.81(11)
124.64(12), 118.23(11)
95.00(11), 96.52(11)
a Bond parameters for the two independent molecules are shown.
115.2◦) are apparently smaller than those of 12a–d and comparable
to that of the O-equatorial TIP derivative 5c (114.0◦).17 Similarly,
for the O-apical phosphoranes, the equatorial C1–P–C2 angle of
13e (125.5◦) is smaller than that of 13d (139.5◦). This is mainly
due to the steric repulsions between the bulky mesityl group and
the aromatic rings of the bidentate ligand.
The degree of the tilt of the monodentate aryl ring can be
represented by the tilt angle h, which is the dihedral angle between
the root mean plane consisting of P and the equatorial atoms (C1,
O2 and C3 for 12, C1, C2 and C3 for 13) and that consisting of the
six atoms of the monodentate aromatic ring (Fig. 5 and Table 2).
For the ortho-unsubtituted O-equatorial derivatives (12a, 12b, 12c
and 12d), h ranges from 14◦ to 33◦, and the degree of the tilt is
independent of the electronic property of the para-substituent. In
addition, the two independent molecules for 12c have different
tilt angles (27.7◦ and 14.1◦). This implies that the p → r*
interactions in these arylphosphoranes are not strong enough to
dominate the tilt angle of the crystal structure. Comparing the
O-equatorial and O-apical isomers, the former has a smaller h
than the latter probably due to steric repulsions originating from
the apical aromatic ring of the bidentate ligand. Overall, the crystal
structures are mainly governed by steric reasons.
Kinetic study of the isomerization of O-equatorial 12 to
O-apical 13
The rates of the isomerization of 12 to 13 were measured by
monitoring the change in the integrals of the 19F NMR signals
of the trifluoromethyl group, and the rates satisfied first-order
kinetics. At first, in order to investigate the solvent effect for
the isomerization, measurements of the stereomutation of 12a to
◦
13a were carried out at 50 C using DMSO-d6, CD3CN, MeOH
N
20
and C6D6. The rates and solvent parameters (er, l and ET
)
are summarized in Table 3. The rates were found to be slightly
affected by the polarity of the solvents, in which the polar solvent
decelerated the isomerization.
In order to investigate the p-substituent effect, the rates of the
isomerization for 12a–d were then measured in C6D6 at 50 ◦C
(Table 4). Although the rates of 12a–d ((2.77, 3.77, 2.57 and 2.02) ×
10−4 s−1 for 12a, 12b, 12c and 12d, respectively) were of the same
order of magnitude, it was found that the stereomutation was
decelerated by the electron-donating OMe group and accelerated
by the electron-withdrawing CF3 group. Table 4 shows the rate
constants along with the substituent parameters (rp, rR and
rI).21 Although the isomerization rates (k) of 12 to 13 do not
nicely correlate with each substituent parameter, the resonance
Fig. 5
Table 2 Tilt angles h of phosphoranes
Table 3 The rates for the stereomutation from O-equatorial 12a to O-
apical 13a in various solvents at 323 K
Compound
Ar
Y
h/◦
era
1030la/C m
ET
kb/s−1
Na
12a
12b
12c
12d
12e
13d
13e
Ph
O2
O2
O2
O2
O2
C2
C2
32.9, 29.8a
24.4
Solvent
C6H4(p-CF3)
C6H4(p-F)
C6H4(p-OMe)
Mes
C6H4(p-OMe)
Mes
27.7, 14.1a
18.8
DMSO-d6
CD3CN
MeOH
C6D6
46.45
35.94
32.66
2.27
13.5
13.0
9.6
0.444
0.460
0.762
0.111
(0.84 0.05) × 10−4
(1.51 0.03) × 10−4
(1.90 0.01) × 10−4
(2.77 0.03) × 10−4
24.8, 20.4a
42.6
34.0
0.0
a Values of undeuterated solvents are shown. b Error is given as the standard
deviation.
a Tilt angles for the two independent molecules are shown.
This journal is
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Dalton Trans., 2008, 3678–3687 | 3681
©

Table 4 Effect of p-substituents on the isomerization rate in C6D6 at
323 K
Process
X
rp
0.00
0.51
0.06
rR
0.00
0.11
−0.48
−0.58
rI
104k/s−1
12a → 13a
12b → 13b
12c → 13c
12d → 13d
H
CF3
F
0.00
0.40
0.54
0.30
2.77 0.03
3.77 0.03
2.57 0.03
2.02 0.02
OMe
−0.28
substituent parameter (rR) seemed to be a greater contribution
than the inductive constant (rI). Thus, we carried out a multi-
regression analysis using rR and rI as variants, and obtained the
following equation with a very good fit of R2 = 0.996.
k = (2.31rR + 1.72rI + 2.80) × 10−4
This equation shows that the contribution of the resonance
effect on the isomerization rate is 1.3 times greater than that of
the inductive effect. Since the r*P–O orbital of the O-equatorial
phosphorane has found to be significantly lower in energy than
the r*P–C orbital of the O-apical isomer,18 the ground state (i.e.,
TBP structure) of the O-equatorial ortho-unsubstituted deriva-
tives (12a–d) should be somewhat stabilized by the p → r*P–O
interaction, whereas such an interaction can be negligible for the
O-apical isomers (Fig. 3). In any case, this result agrees with the
fact that the p-donating nature reduces the apparent apicophilicity
of the substituent although the degree of the interaction is not
significant.
Scheme 4 Frozen one-step pseudorotation and P–C(ipso) rotation in the
O-equatorial mesitylphophorane (12e)
between RP* and SP*. These observations indicate that the P–
C(ipso) rotation in the mesityl derivative (12e) is slow and that in
the ortho-unsubstituted derivatives (12a–d) is fast on the NMR
timescale. Therefore, the degree of freedom in the P–C(ipso) bond
rotation is higher for 12a–d than for 12e in the solution state. Thus,
the ortho-unsubstituted aryl group of 12a–d should be capable of
being nearly perpendicular to the equatorial plane, whereas the
mesityl group of 12e is most likely fixed as the crystal structure
even in solution. Therefore, the p → r*P–O interaction should work
well for the O-equatorial ortho-unsubstituted derivative (12a–d),
giving rise to stabilization of the ground states of 12a–d.
In order to investigate the difference between the ortho-
unsubstituted aryl and mesityl groups, the rates of the isomer-
ization of the phenyl (12a to 13a) and the mesityl (12e to 13e)
derivatives were measured in the temperature range of 313–333 K
(Table 5), and obtained the activation parameters from the Eyring
plot (Fig. 6). The free energy of activation of the mesityl derivative
The reason for the rather small interaction could be due to the
small tilt angle described above (Fig. 5). Therefore, we investigated
the structure in solution based on the 31P and 19F NMR. All the
isolated phosphoranes showed singlets in 31P NMR (CDCl3), and
the chemical shifts for the O-equatorial phosphoranes (d = −10.6
(12a), −11.4 (12b), −11.1 (12c), −10.7 (12d) and −2.6 (12e) ppm)
were shifted downfield compared to those of the O-apical isomers
(d = −26.9 (13a), −28.2 (13b), −27.7 (13c) and −27.1 (13d) and
−23.6 (13e) ppm) similar to our previous results.13,15,17–19 The 19F
NMR spectra of the O-apical phosphoranes (13) showed only
two CF3 signals due to their C2 symmetrical structure. For the
O-equatorial isomers, four CF3 signals were observed for 12e (d =
−78.1, −78.4, −79.1 and −79.2 ppm) at room temperature or even
at 60 ◦C, whereas there were only two CF3 signals for 12a–d. As we
have already reported, for the O-equatorial phosphoranes bearing
a bulky monodentate ligand (5b and 5c), the four chemically
nonequivalent CF3 groups have been observed distinctly in the 19
F
NMR. If a fast exchange between the RP* and SP* isomers takes
place,22 the two endo-CF3 groups (a-endo-CF3 and b-endo-CF3)
coalesce into one, and the same occurs for the exo-CF3 groups (a-
exo-CF3 and b-exo-CF3). Therefore, this observation implies that
the one-step stereomutation between the RP* and SP* enantiomers
of 12e is slow on the NMR timescale (Scheme 4).
1
In the H NMR, two kinds of ortho-methyl protons (d = 2.65
(s, 3H) and 2.27 ppm (s, 3H)) as well as meta-protons (d = 6.81
4
4
(d, JH, P = 4.4 Hz, 1H), 6.73 ppm (d, JH, P = 8.3 Hz, 1H)) were
clearly observed for 12e, whereas only a set of ortho- and meta-
aromatic protons was observed for 12a–d. That is, sites a and
b of 12e are not equivalent, whereas sites c and d of 12a–d are
1
equivalent in the H NMR (Scheme 4). If the conformation of
the monodentate ligand is fixed, sites a (or c) and b (or d) are
not exchanged with each other upon the one-step pseudorotation
Fig. 6 Eyring plot for the isomerization in C6D6. Squares: 12a → 13a;
circles: 12e → 13e.
3682 | Dalton Trans., 2008, 3678–3687
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a
Table 5 Activation parameters of the stereomutation in C6D6
Process
T/K
k/s−1
DH‡/kcal mol−1
DS‡/cal K−1 mol−1
−0.6 0.6
DG‡298/kcal mol−1
24.1
12a → 13a (Ar = Ph)
313
318
323
328
333
(0.81 0.01) × 10−4
(1.56 0.01) × 10−4
(2.77 0.03) × 10−4
(5.00 0.04) × 10−4
(8.96 0.10) × 10−4
23.9 0.2
12e → 13e (Ar = Mes)
313
318
323
328
333
(2.08 0.03) × 10−5
(3.56 0.02) × 10−5
(6.49 0.06) × 10−5
(11.3 0.03) × 10−5
(17.1 0.20) × 10−5
21.6 0.6
−10.8 1.9
24.8
a Error is given as the standard deviation.
(DG‡ = 24.8 kcal mol−1) is slightly higher than that of the
298
phenyl derivative (24.1 kcal mol−1). When compared to the CF3
derivative, the free energy of activation for 12e to 13e (DG‡
24.8 kcal mol−1) was higher than that of 5b to 6b (DG‡
=
298
=
298
22.2 kcal mol−1) by 2.6 kcal mol−1, and was comparable to that
of the TIP derivative 5c to 6c (DG‡ = 24.1 kcal mol−1).17b This
298
again proves the steric hindrance of the C2F5 group being more
effective for freezing the isomerization than that of the CF3 group.
As for the activation enthalpy (DH‡) of the phenyl derivative
(12a → 13a) and of the mesityl derivative (12e → 13e), the former
is greater than the latter by 2.3 kcal mol−1 in clear contrast to
the free energy of activation. When considering the high-energy
intermediates in the three-step BPR from the O-equatorial 12
to the O-apical 13,13 the mesityl derivative (Int-2e) could be less
stable than the phenyl derivative (Int-2a) because the former has a
bulky mesityl group at the sterically congested apical site (Fig. 7).
Therefore, the difference in enthalpy has to be explained by some
factors in the ground state because the activation enthalpy is lower
in the mesityl derivative (12e → 13e) than in the phenyl derivative
(12a → 13a). The p → r*P–O stabilizing interaction in 12a can be
one of the reasons for the higher activation enthalpy for 12a →
13a. The activation entropy (DS‡) for the mesityl derivative (12e →
13e) is lower than that for the phenyl derivative (12a → 13a). The
steric repulsion between the bulky mesityl group and the bidentate
ligands even in Int-2e could be the reason.
Fig. 7 Energy diagram for the isomerization of the O-equatorial phos-
phoranes to the O-apical isomers.
Conclusions
The new bidentate ligand with two C2F5 groups was used to isolate
the O-equatorial arylphosphoranes 12 (Ar = Ph, C6H4(p-CF3),
C6H4(p-F), C6H4(p-OMe) and Mes), which could be quantitatively
converted into the corresponding O-apical isomers 13 by heating
at elevated temperatures. The structures were confirmed by an
X-ray analysis, showing that the tilt angle of the monodentate
ligand was found to be independent of the para-substituents.
However, the kinetic study showed a small para-substituent effect
on the stereomutations. The multi-regression analysis for the para-
substituents revealed that the contribution of the resonance effect
(rR) on the rate constant was superior to the inductive effect (rI).
This indicates that the p → r*P–O stabilizing interaction in the
O-equatorial isomer plays some role in the isomerization. This
result shows a general insight into the stereoelectronic effect of the
pentacoordinated molecules.
Experimental
General
Melting points were measured using a Yanaco micro melting point
1
apparatus. The H NMR (400 MHz), 19F NMR (376 MHz), 31P
NMR (162 MHz) were recorded using JEOL EX-400 or JEOL
AL-400 spectrometers. The 1H NMR chemical shifts (d) are given
in ppm downfield from Me4Si, determined by residual chloroform
(d = 7.26 ppm). The 19F NMR chemical shifts are given in ppm
downfield from the external CFCl3. The 31P NMR chemical shifts
are given in ppm downfield from the external 85% H3PO4. The
elemental analyses were performed using a Perkin-Elmer 2400
CHN elemental analyzer. All reactions were carried out under N2.
Diethyl ether (Et2O) was freshly distilled from Na/benzophenone,
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Dalton Trans., 2008, 3678–3687 | 3683
©

2
and the other solvents were distilled from CaH2. Merck silica gel
60GF254 was used for the preparative thin-layer chromatography.
Ar-F), −114.9 (d, JF, F = 292.0 Hz, 2 F, CF2CF3), −116.1 (d,
2JF, F = 292.0 Hz, 4 F, CF2CF3), −118.5 (dm, JF, F = 292.0 Hz,
2
2 F, CF2CF3) ppm. 31P NMR (162 MHz, CDCl3): d −11.1 ppm.
Anal. Found: C, 41.48; H, 1.18. Calc. for C28H12F21O2P: C, 41.50;
H, 1.49%.
General procedure for the synthesis of O-equatorial
arylphosphoranes
Synthesis of 12d. Following the above-described procedure
using p-bromoanisole (d = 1.494 g mL−1, 0.060 mL, 0.48 mmol)
and 14 (62.5 mg, 0.0872 mmol), 12d (34.7 mg, 0.0421 mmol,
48%) was isolated as a white solid. Colorless crystals of 12d
suitable for the X-ray analysis were obtained by recrystallization
from n-hexane–CH2Cl2. Mp 124.0–125.0 ◦C (decomp.). 1H NMR
(400 MHz, CDCl3): d 7.96–8.03 (m, 2 H, Ar–H), 7.76 (br s, 2
H, Ar–H), 7.58–7.61 (m, 4 H, Ar–H), 7.50 (dd, 3JH, P = 15.0 Hz,
3JH, H = 8.8 Hz, 2 H, Ar–H), 6.78 (d, 4JH, P = 7.8 Hz, 3JH, H = 8.8 Hz,
2 H, Ar–H), 3.74 (s, 3 H, Ar-OCH3) ppm. 19F NMR (376 MHz,
Synthesis of 12a. PhLi (1.14 M cyclohexane–Et2O solution,
0.19 mL, 0.216 mmol) was added to a solution of P–H phospho-
rane 14 (49.9 mg, 0.0696 mmol) in Et2O (3.0 mL) at −78 ◦C. The
mixture was then stirred for 1 h at room temperature. I2 (52.0 mg,
0.204 mmol) was added to the mixture at −78 ◦C, and the mixture
was stirred for 1 h at 0 ◦C. The reaction was then quenched
with aqueous Na2S2O3 (30 mL). The mixture was extracted with
Et2O (2 × 40 mL), and the organic layer was washed with brine
(2 × 40 mL) and dried over anhydrous MgSO4. After removing
the solvents by evaporation, the resulting crude product was
separated by preparative TLC (n-hexane–CH2Cl2 = 7 : 1) to afford
12a (47.0 mg, 0.0593 mmol, 85%) as a white solid. Colorless
crystals of 12a suitable for the X-ray analysis were obtained
by recrystallization from n-hexane–CH2Cl2. Mp 115.0–116.0 ◦C
(decomp.). 1H NMR (400 MHz, CDCl3): d = 7.95–7.99 (m, 2 H,
Ar–H), 7.74 (br s, 2 H, Ar–H), 7.59–7.62 (m, 4 H, Ar–H), 7.49–
7.55 (m, 2 H, Ar–H), 7.24–7.28 (m, 3 H, Ar–H) ppm. 19F NMR
3
CDCl3): d −78.8 (s, 6 F, CF2CF3), −79.4 (dd, JF, F = 9.8 Hz,
2
5JF, F = 9.8 Hz, 6 F, CF2CF3), −115.0 (d, JF, F = 292.0 Hz, 2 F,
2
CF2CF3), −116.2 (br d, JF, F = 292.0 Hz, 4 F, CF2CF3), −118.5
(dm, 2JF, F = 292.0 Hz, 2 F, CF2CF3) ppm. 31P NMR (162 MHz,
CDCl3): d −10.7 ppm. Anal. Found: C, 42.05; H, 1.60. Calc. for
C29H15F20O3P: C, 42.35; H, 1.84%.
Synthesis of 12e. Following the above-described procedure
using 2-bromomesitylene (d = 1.301 g mL−1, 0.10 mL, 0.65 mmol)
and 14 (71.3 mg, 0.0995 mmol), 12e (69.2 mg, 0.0829 mmol, 83%)
was isolated as a white solid. Colorless crystals of 12e suitable
for the X-ray analysis were obtained by recrystallization from
(376 MHz, CDCl3): d −78.8 (s, 6 F, CF2CF3), −79.4 (dd, 3JF, F
=
9.8 Hz, 5JF, F = 9.8 Hz, 6 F, CF2CF3), −114.8 (d, 2JF, F = 295.6 Hz,
2 F, CF2CF3), −116.1 (d, 2JF, F = 295.6 Hz, 4 F, CF2CF3), −118.4
(dm, 2JF, F = 295.6 Hz, 2 F, CF2CF3) ppm. 31P NMR (162 MHz,
CDCl3): d −10.6 ppm. Anal. Found: C, 42.43; H, 1.26. Calc. for
C28H13F20O2P: C, 42.44; H, 1.65%.
◦
1
CH3CN. Mp 113.1–113.9 C. H NMR (400 MHz, CDCl3): d
7.95–7.98 (m, 1 H, Ar–H), 7.81 (br d, 3JH, H = 8.0 Hz, 1 H, Ar–H),
3
4
7.69 (td, JH, H = 8.0 Hz, JH, H = 1.2 Hz, 1 H, Ar–H), 7.61 (br
Synthesis of 12b. Following the above-described procedure
using p-bromobenzotrifluoride (d = 1.607 g mL−1, 0.040 mL,
0.29 mmol) and 14 (51.8 mg, 0.0723 mmol), 12b (36.0 mg,
0.0418 mmol, 58%) was isolated as a white solid. Colorless
crystals of 12b suitable for the X-ray analysis were obtained
by recrystallization from n-hexane–CH2Cl2. Mp 106.0–107.0 ◦C
3
d, JH, H = 8.0 Hz, 1 H, Ar–H), 7.44–7.53 (m, 4 H, Ar–H), 6.81
4
4
(d, JH, P = 4.4 Hz, 1 H, Ar–H), 6.73 (d, JH, P = 8.3 Hz, 1 H,
Ar–H), 2.65 (s, 3 H, Ar–CH3), 2.27 (s, 3 H, Ar–CH3), 2.18 (s, 3
H, Ar–CH3) ppm. 19F NMR (376 MHz, CDCl3): d −78.1 (s, 3 F,
CF2CF3), −78.4 (s, 3 F, CF2CF3), −79.1 (s, 3 F, CF2CF3), −79.2 (s,
3 F, CF2CF3), −112.9 (d, 2JF, F = 292.0 Hz, 1 F, CF2CF3), −113.8
(d, 2JF, F = 292.0 Hz, 1 F, CF2CF3), −114.1 (d, 2JF, F = 292.0 Hz,
2 F, CF2CF3), −114.9 (d, 2JF, F = 292.0 Hz, 1 F, CF2CF3), −115.3
(d, 2JF, F = 292.0 Hz, 1 F, CF2CF3), −115.7 (d, 2JF, F = 292.0 Hz, 1
1
(decomp.). H NMR (400 MHz, CDCl3): d 7.92–7.96 (m, 2 H,
Ar–H), 7.76 (br s, 2 H, Ar–H), 7.62–7.66 (m, 4 H, Ar–H), 7.61
(dd, 3JH, P = 15.0 Hz, 3JH, H = 8.3 Hz, 2 H, Ar–H), 7.53 (d, 4JH, P
=
3
7.8 Hz, JH, H = 8.3 Hz, 2 H, Ar–H) ppm. 19F NMR (376 MHz,
F, CF2CF3), −117.8 (d, 2JF, F = 292.0 Hz, 1 F, CF2CF3) ppm. 31
P
CDCl3): d −63.5 (s, 3 F, Ar–CF3), −78.8 (s, 6 F, CF2CF3), −79.5
NMR (162 MHz, CDCl3): d −2.7 ppm. Anal. Found: C, 44.54; H,
(dd, 3JF, F = 9.8 Hz, 5JF, F = 9.8 Hz, 6 F, CF2CF3), −114.8 (d, 2JF, F
=
2.07. Calc. for C31H19F20O2P: C, 44.62; H, 2.30%.
2
292.0 Hz, 2 F, CF2CF3), −116.2 (br dm, JF, F = 292.0 Hz, 4 F,
2
CF2CF3), −118.4 (dm, JF, F = 292.0 Hz, 2 F, CF2CF3) ppm. 31P
Synthesis of 13a. A solution of 12a (12.2 mg, 0.0153 mmol) in
C6D6 (1.0 mL) was heated at 80 ◦C for 8 h. After concentration in
vacuo, 13a was obtained (12.0 mg, 0.0151 mmol, 98%) as a white
solid. Mp 149.1–150.0 ◦C. 1H NMR (400 MHz, CDCl3): d 8.70–
NMR (162 MHz, CDCl3): d −11.4 ppm. Anal. Found: C, 40.19;
H, 1.01. Calc. for C29H12F23O2P: C, 40.49; H, 1.41%.
Synthesis of 12c. Following the above-described procedure
using p-bromofluorobenzene (d = 1.593 g mL−1, 0.040 mL,
0.36 mmol) and 14 (47.7 mg, 0.0665 mmol), 12c (41.2 mg,
0.0508 mmol, 76%) was isolated as a white solid. Colorless
crystals of 12c suitable for the X-ray analysis were obtained
by recrystallization from n-hexane–CH2Cl2. Mp 112.5–113.5 ◦C
8.78 (m, 2 H, Ar–H), 7.69–7.82 (m, 6 H, Ar–H), 7.41 (dd, 3JH, P
=
3
15.8 Hz, JH, H = 8.0 Hz, 2 H, Ar–H), 7.18–7.27 (m, 3 H, Ar–
H) ppm. 19F NMR (376 MHz, CDCl3): d −78.3 (s, 6 F, CF2CF3),
−79.8 (dd, 3JF, F = 19.7 Hz, 5JF, F = 6.2 Hz, 6 F, CF2CF3), −116.7
2
(s, 2 F, CF2CF3), −116.8 (s, 2 F, CF2CF3), −116.8 (dq, JF, F
=
=
3
2
285.8 Hz, JF, F = 19.7 Hz, 2 F, CF2CF3), −121.9 (dq, JF, F
1
(decomp.). H NMR (400 MHz, CDCl3): d 7.93–7.98 (m, 2 H,
285.8 Hz, 5JF, F = 6.2 Hz, 2 F, CF2CF3) ppm. 31P NMR (162 MHz,
CDCl3): d −26.9 ppm. Anal. Found: C, 42.30; H, 1.64. Calc. for
C28H13F20O2P: C, 42.44; H, 1.65%.
3
Ar–H), 7.76 (br d, JH, H = 8.0 Hz, 2 H, Ar–H), 7.61–7.64 (m, 4
3
3
4
H, Ar–H), 7.54 (ddd, JH, P = 15.0 Hz, JH, H = 9.0 Hz, JH, F
=
3
3
6.0 Hz, 2 H, Ar–H), 6.96 (ddd, JH, F = 9.0 Hz, JH, H = 9.0 Hz,
4JH, P = 3.6 Hz, 2 H, Ar–H) ppm. 19F NMR (376 MHz, CDCl3): d
−78.8 (s, 6 F, CF2CF3), −79.5 (dd, 3JF, F = 9.8 Hz, 5JF, F = 9.8 Hz,
Synthesis of 13b. A solution of 12b (14.0 mg, 0.0162 mmol) in
◦
C6D6 (1.0 mL) was heated at 80 C for 8 h. After concentration
3
4
6 F, CF2CF3), −111.0 (tt, JH, F = 9.0 Hz, JH, F = 6.0 Hz, 1 F,
3684 | Dalton Trans., 2008, 3678–3687
in vacuo, 13b was obtained (13.5 mg, 0.0157 mmol, 96%) as a
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©

white solid. Mp 115.0–116.0 ◦C. 1H NMR (400 MHz, CDCl3): d
8.69–8.74 (m, 2 H, Ar–H), 7.71–7.84 (m, 6 H, Ar–H), 7.51 (dd,
3
4
3JH, P = 15.4 Hz, JH, H = 8.0 Hz, 2 H, Ar–H), 7.45 (dd, JH, P
=
7.6 Hz, JH, H = 8.0 Hz, 2 H, Ar–H) ppm. 19F NMR (376 MHz,
3
CDCl3): d −63.5 (s, 3 F, Ar–CF3), −78.2 (s, 6 F, CF2CF3), −79.9
3
5
(dd, JF, F = 19.7 Hz, JF, F = 6.2 Hz, 6 F, CF2CF3), −116.7 (s,
2
2 F, CF2CF3), −116.8 (s, 2 F, CF2CF3), −116.8 (dq, JF, F
=
=
3
2
285.8 Hz, JF, F = 19.7 Hz, 2 F, CF2CF3), −121.8 (dq, JF, F
285.8 Hz, 5JF, F = 6.2 Hz, 2 F, CF2CF3) ppm. 31P NMR (162 MHz,
CDCl3): d −28.2 ppm. Anal. Found: C, 40.13; H, 1.07. Calc. for
C29H12F23O2P: C, 40.49; H, 1.41%.
Synthesis of 13c. A solution of 12c (11.8 mg, 0.0145 mmol) in
C6D6 (1.0 mL) was heated at 80 ◦C for 8 h. After concentration in
vacuo, 13c was obtained (11.4 mg, 0.0140 mmol, 96%) as a white
solid. Mp 120.0–121.0 ◦C. 1H NMR (400 MHz, CDCl3): d 8.67–
8.72 (m, 2 H, Ar–H), 7.71–7.83 (m, 6 H, Ar–H), 7.37–7.45 (m, 2
H, Ar–H), 6.84–6.91 (m, 2 H, Ar–H) ppm. 19F NMR (376 MHz,
3
CDCl3): d −78.3 (s, 6 F, CF2CF3), −79.8 (dd, JF, F = 19.7 Hz,
5JF, F = 6.2 Hz, 6 F, CF2CF3), −110.2 to −110.3 (m, 1 F, Ar–F),
−116.7 (s, 2 F, CF2CF3), −116.8 (s, 2 F, CF2CF3), −116.8 (dq,
3
2JF, F = 285.8 Hz, JF, F = 19.7 Hz, 2 F, CF2CF3), −121.8 (dq,
5
2JF, F = 285.8 Hz, JF, F = 6.2 Hz, 2 F, CF2CF3) ppm. 31P NMR
(162 MHz, CDCl3): d −27.7 ppm. Anal. Found: C, 41.37; H, 1.33.
Calc. for C28H12F21O2P: C, 41.50; H, 1.49%.
Synthesis of 13d. A solution of 12d (12.1 mg, 0.0147 mmol) in
C6D6 (1.0 mL) was heated at 80 ◦C for 8 h. After concentration in
vacuo, 13d was obtained (11.8 mg, 0.0143 mmol, 97%) as a white
solid. Colorless crystals of 13d suitable for the X-ray analysis were
obtained by recrystallization from n-hexane–CH2Cl2. Mp 117.0–
117.8 ◦C. 1H NMR (400 MHz, CDCl3): d 8.68–8.72 (m, 2 H, Ar–
H), 7.67–7.78 (m, 6 H, Ar–H), 7.38 (dd, 3JH, P = 15.2 Hz, 3JH, H
=
4
3
8.8 Hz, 2 H, Ar–H), 6.69 (dd, JH, P = 3.4 Hz, JH, H = 8.8 Hz, 2
H, Ar–H), 3.73 (s, 3 H, Ar–OCH3) ppm. 19F NMR (376 MHz,
3
CDCl3): d −78.3 (s, 6 F, CF2CF3), −79.7 (dd, JF, F = 19.7 Hz,
5JF, F = 6.2 Hz, 6 F, CF2CF3), −116.7 (s, 2 F, CF2CF3), −116.8
3
(s, 2 F, CF2CF3), −116.8 (dq, 2JF, F = 285.8 Hz, JF, F = 19.7 Hz,
2 F, CF2CF3), −121.8 (dq, 2JF, F = 285.8 Hz, 5JF, F = 6.2 Hz, 2 F,
CF2CF3) ppm. 31P NMR (162 MHz, CDCl3): d −27.1 ppm. Anal.
Found: C, 42.27; H, 1.46. Calc. for C29H15F20O3P: C, 42.35; H,
1.84%.
Synthesis of 13e. A solution of 12e (21.6 mg, 0.0258 mmol) in
◦
C6D6 (2.0 mL) was heated at 80 C for 8 h. After concentration
in vacuo, 13e was obtained (20.5 mg, 0.0245 mmol, 95%) as a
white solid. Colorless crystals of 13e suitable for the X-ray analysis
were o◦btained by recrystallization from n-hexane/CH2Cl2. 190.0–
1
191.0 C. H NMR (400 MHz, CDCl3): d 8.56–8.61 (m, 2 H,
Ar–H), 7.73 (br s, 2 H, Ar–H), 7.66–7.71 (m, 4 H, Ar–H), 6.58
4
(d, JH, P = 6.2 Hz, 2 H, Ar–H), 2.15 (s, 3 H, Ar-CH3), 2.00 (s,
6 H, Ar–CH3) ppm. 19F NMR (376 MHz, CDCl3): d −77.0 (s,
3
5
6 F, CF2CF3), −79.7 (dd, JF, F = 16.0 Hz, JF, F = 8.6 Hz, 6 F,
2
3
CF2CF3), −114.6 (dq, JF, F = 284.6 Hz, JF, F = 16.0 Hz, 2 F,
2
4
CF2CF3), −115.9 (dd, JF, F = 284.6 Hz, JF, F = 33.2 Hz, 2 F,
2
CF2CF3), −116.94 (d, JF, F = 284.6 Hz, 1 F, CF2CF3), −116.97
(d, 2JF, F = 284.6 Hz, 1 F, CF2CF3), −118.1 (ddq, 2JF, F = 284.6 Hz,
5
4JF, F = 33.2 Hz, JF, F = 8.6 Hz, 2 F, CF2CF3) ppm. 31P NMR
(162 MHz, CDCl3): d −23.6 ppm. Anal. Found: C, 44.47; H, 1.96.
Calc. for C31H19F20O2P: C, 44.62; H, 2.30%.
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Dalton Trans., 2008, 3678–3687 | 3685
©

Single-crystal X-ray analyses of 12a, 12b, 12c, 12d, 12e,
13d and 13e
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Crystals suitable for the X-ray structural determination were
mounted on a Mac Science DIP2030 imaging plate diffractometer
and irradiated with graphite monochromated Mo-Ka radiation
˚
(k = 0.71073 A) for the data collection. The unit cell parameters
were determined by separately autoindexing several images in
each data set using the DENZO program (MAC Science).23 For
each data set, the rotation images were collected in 3◦ increments
with a total rotation of 180◦ about the φ axis. The data were
processed using SCALEPACK. The structure was solved by
a direct method using the SHELXS-97 program.24 Refinement
on F2 was carried out using a full-matrix least-squares by the
SHELXL-97 program.24 All non-hydrogen atoms were refined
using the anisotropic thermal parameters. The hydrogen atoms
were included in the refinement along with isotropic thermal
parameters. The crystallographic data are summarized in Table 6.
CCDC reference numbers 672351 (12a), 672352 (12b), 672353
(12c), 672354 (12d), 672355 (12e), 672356 (13d) and 672357 (13e).
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b802947d
Kinetic measurements of the isomerization of 12 to 13
Samples (ca. 5 mg) in C6D6 (0.6 mL) were sealed in an NMR
tube under N2. Kinetic measurements of the pseudorotation
process were carried out using a JEOL EX-400 spectrometer
by monitoring the 19F NMR signals in a variable temperature
mode, and the specified temperatures were maintained throughout
each set of measurements (error within
1
◦C). The observed
temperatures were calibrated with the 19F NMR chemical shift
difference in the signals of neat 1,3-propanediol (high temperature
region) and MeOH (low temperature region). The data were
analyzed on the basis of the first-order kinetics using the following
equation: ln (C0/C12) = kT, in which C0 = ratio of 12 at t = 0, C12
=
ratio of 12 at arbitrary intervals. Here C0 = C12 + C13, C0/C12
=
8 (a) R. Hoffmann, J. M. Howell and E. L. Muetterties, J. Am. Chem.
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(C12 + C13)/C12 = 1 + C13/C12. The C13/C12 ratio was monitored
by the integration of the 19F NMR signals of the trifluoromethyl
group at 40–60 ◦C. The rate constants and activation parameters
for the stereomutation from 12 to 13 are shown in Tables 3–5.
Acknowledgements
The authors are grateful to Central Glass Co., Ltd., for the
generous gift of the p-bromobenzotrifluoride. This work was
supported by two Grants-in-Aid for Scientific Research on Priority
Areas (Nos. 14340199, 17350021) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
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