Substituent effects on configurational stabilities at tetrahedral boron atoms in intramolecular borane-amine complexes: Structures, enantiomeric resolution, and rates of enantiomerization of [2-(dimethylaminomethyl)phenyl] phenylboranes

Article abstract of DOI:10.1246/bcsj.77.2081

The structures and stereochemistries of the five borane-amine complexes, (N-S)-{2-(Me₂NCH₂)C₆H₄}BPhX (X = Cl, F, Me, -OCOCF₃, and -OCOC₂F₅), were studied by some spectroscopic me

Full text of DOI:10.1246/bcsj.77.2081

Ó 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 2081–2088 (2004) 2081
Substituent Effects on Configurational Stabilities at Tetrahedral
Boron Atoms in Intramolecular Borane–Amine Complexes:
Structures, Enantiomeric Resolution, and Rates of Enantiomerization
of [2-(Dimethylaminomethyl)phenyl]phenylboranes#
ꢀ
Shinji Toyota, Fumiko Ito, Naoya Nitta, and Tomohiro Hakamata
Department of Chemistry, Faculty of Science, Okayama University of Science, 1-1 Ridaicho, Okayama 700-0005
Received June 1, 2004; E-mail: stoyo@chem.ous.ac.jp
The structures and stereochemistries of the five borane–amine complexes, (N–B)-{2-(Me2NCH2)C6H4}BPhX
(X = Cl, F, Me, –OCOCF3, and –OCOC2F5), were studied by some spectroscopic methods. The Cl complex prepared
by the standard method was converted to each of the other complexes by the treatment with an appropriate reagent
(MeLi or AgX). The enantiomers of the two perfluoroacyloxy complexes were resolved by chiral HPLC as a novel intra-
molecular borane–amine complex with a chiral center at the tetrahedral boron atom, and their chiroptical properties were
investigated by optical rotations and CD spectra. The classical kinetics or the saturation transfer measurements revealed
that the barriers to enantiomerization via the dissociation of the B–N coordination bond were enhanced from 94 to 116
kJ molꢁ1 in the following order of X: F < Me < –OCOCF3 ꢂ Cl < –OCOC2F5. The substituent effects on the config-
urational stability, which is closely related to the Lewis acidity of boron atoms, are discussed on the basis of the X-ray
structures of the three complexes.
We have studied the structures and the mechanism of dy-
namic behavior of intramolecular borane–amine complexes
1–3 with a 2-(dimethylaminomethyl)phenyl (DMP) group as
a bidentate C,N ligand (Chart 1).2–6 The spectroscopic analyses
revealed that the strength of the intramolecular B–N coordina-
tion bond was influenced by various factors, such as substitu-
ents and solvents. These results are valuable to estimate the
Lewis acidity of boron atoms and the Lewis basicity of nitro-
gen atoms. Moreover, a series of structural data led us to pro-
pose the ‘‘tetrahedral character’’ for the structural correlation
between the geometry of boron atoms and their Lewis acidity.7
In most of these complexes, the boron atoms are configuration-
ally liable in solution because of facile isomerization via the
B–N bond dissociation, even though the boron atom is bonded
to four different ligands including a DMP ligand.
Such boron compounds have been investigated from various
aspects,9 leading to several examples of isolable enantiopure
borates and coordinated boranes.10–16 In most cases, the con-
figuration at a boron atom is derived by other chiral elements
in an identical molecule with enantiopure moieties such as
amino acids and terpenes, some of which have been success-
fully utilized in the organic synthesis.11,12 Recently, enantio-
pure compounds with a chiral boron center as the sole stereo-
genic center have attracted interest, and two types of com-
plexes with phosphine and amine ligands (4 and 5) were re-
ported.17,18 These complexes carry one hydrogen ligand on
the boron atom, namely they are isoelectronic to secondary
carbons, and they provide direct evidence of the SN2 reaction
at the boron atom with inversion. These results tempted us to
explore chiral borane complexes by using the DMP ligand. Ac-
cordingly, we found that the title intramolecular borane–amine
complexes (6) were suitable for our purpose; here the boron
atom carried a phenyl group19 and a ligand X in addition to
the C,N-bidentate ligand (Chart 2). In the structural system,
Chiral compounds with a stereogenic center at the tetra-
hedral boron atom are intriguing targets in the stereochemical
studies in relation to the chemistry of tetrahedral carbons.8
NMe2
NMe2
Et
NMe2
O
NMe2
Ph
NMe2
X
B
B
B
Ph
Ph
O
B
B
Et
1
2
3
X
Ph
(S)-6
(R)-6
PR3
B
NMe3
B
a X = Cl
b X = Me
c X = F
d X = OCOCF3
e X = OCOC2F5
Br
NC
COOMe
R = cyclohexyl
CH2Ph
H
H
5
4
(R and S are vise verse for 6b.)
Chart 1.
Chart 2.
Published on the web November 11, 2004; DOI 10.1246/bcsj.77.2081

2082 Bull. Chem. Soc. Jpn., 77, No. 11 (2004)
Intramolecular Borane–Amine Complexes
Table 1. 11B NMR Chemical Shifts of 6 and Coordination-
we can introduce various substituents at the boron atom, facil-
itating the tuning of the structural and dynamic properties. For-
tunately, we could successfully resolve the enantiomers as
novel optically active intramolecular borane–amine complexes
when perfluoroacyloxy groups were introduced as X. We here-
in report the structure and configurational stability of a series
of (DMP)phenylboranes. The substituent effects on the Lewis
acidity of boron atoms are discussed on the basis of the kinetic
data and the X-ray structures.
Free Reference Compoundsa)
Compound (X) ꢀ/ppm
ꢀ/ppm (Compound)b)
61.0 (Ph2BCl)
70.6 (Ph2BMe)
47.4 (Ph2BF)
6a (Cl)
6b (Me)
10.8
6.8
6c (F)
6d (OCOCF3)
6e (OCOC2F5)
12.6
11.3
8.8
45.1 (Ph2BOEt), 47.8 (Et2BOAc)c)
a) Reference BF3 OEt2 at ꢀ 0. Solvent CDCl3. b) Refs. 20 and
ꢃ
21. c) 11B NMR data of acyloxydiphenylboranes (Ph2BO-
COR) are not available.
Results and Discussion
Synthesis and 11B NMR. Complexes 6 were prepared
according to Scheme 1. (DMP)lithium was prepared by o-lith-
iation of N,N-dimethylbenzylamine 7 with butyllithium in an
ordinary manner. This reagent was treated with dichlorophen-
ylborane to afford the corresponding chlorophenylborane 6a in
66% yield. The reaction of 6a with methyllithium or a silver
reagent gave the corresponding boranes 6b–e in good yields.
These compounds are colorless crystals that are stable under
ambient condition.
chemical shifts are shifted upfield by 35–65 ppm compared
with the reference compounds; such shifts are typical of tetra-
coordinated boron atoms.22
X-ray Analysis. Molecular structures of 6a, 6b, and 6e
were analyzed by X-ray crystallography. ORTEP drawings
are shown in Fig. 1. Selected structural parameters around
the boron atoms are listed in Table 2.
11B NMR spectra of 6 were measured in chloroform-d. The
chemical shift data are compiled in Table 1 together with those
of coordination free boranes as references.20,21 The signals of 6
were observed as broad bands in the range of ꢀ 6–13. These
In each complex, the boron atom is internally coordinated
by the amino-nitrogen atom to take a tetrahedral geometry.
ꢀ
The B–N bond distances are 1.67 A in 6a and 6e, and that
ꢀ
in 6b is longer by ca. 0.04 A than the others. The database
NMe2
B
MeLi
Ph
1) BuLi
2) PhBCl2
NMe2
Ph
6b
NMe2
Me
B
NMe2
Ph
Cl
7
6a
Ag+X–
B
6c-e
X
Scheme 1. Synthesis of complexes 6.
N
N
C(10)
B
C(1)
C(1)
B
C(10)
Cl
C(16)
6a
6b
N
C(1)
B
C(10)
O(1)
F
C(16)
O(2)
F
F
F
F
6e
Fig. 1. ORTEP drawings of 6a, 6b, and 6e.

S. Toyota et al.
Bull. Chem. Soc. Jpn., 77, No. 11 (2004) 2083
Table 2. Selected Structural Parameters in X-ray Structures of 6a)
6a
6b(A)b)
6b(B)b)
6e
ꢀ
Bond length/A
B–N
B–C(1)
B–C(10)
B–X0c)
1.667(2)
1.599(2)
1.597(3)
1.920(2)
1.707(4)
1.628(4)
1.618(4)
1.666(4)
1.704(4)
1.605(4)
1.635(4)
1.656(4)
1.668(4)
1.600(4)
1.604(4)
1.535(3)
Bond angle/degree
X0–B–C(1)
X0–B–C(10)
C(1)–B–C(10)
107.5(1)
109.7(1)
121.6(1)
110.0(2)
113.0(2)
117.1(2)
110.4(2)
112.6(2)
117.2(2)
110.4(2)
111.8(2)
120.2(2)
THC/%d)
67
63
63
56
a) Standard deviations in parentheses. b) Data for two independent molecules A and B in an
asymmetric unit. Figure 1 shows the structure of A. c) X0 stands for the atom directly at-
tached to the boron atom in group X. d) Tetrahedral character at the boron atom. Ref. 7.
analysis revealed that the distances of B–N coordination bonds
ꢀ
propanol 50:1 as eluent. The specific rotations of the first
22
were in the range of 1.67–1.76 A in ordinary borane–amine
and second fractions were ½ꢁꢅD þ180 and ꢁ180, respectively,
complexes.23 The observed distances of 6 are within the range,
suggesting that the coordinated form suffers from little strain
by the formation of the five-membered ring. The tetrahedral
character (THC) was calculated from bond angles at the boron
atom: this structural parameter was proposed for the correla-
tion between the structure and the strength of Lewis acidity
of boron compounds.7,24 The values increase from 56 to 67%
in the order of 6e < 6b < 6a. Generally, the THC value tends
to be large when the boron atom is bonded to electronegative
atoms. The large value of 6a is explained by the substitution of
an electronegative chlorine atom; the value is comparable to
that of a similar intramolecular chloroborane–amine complex,
in acetone. While the racemization of 6e was negligibly slow
in solutions at room temperature, it occurred rapidly at higher
temperatures.
CD spectra of enantiomers of 6e were measured in hexane
(Fig. 2(a)). The (þ)-isomer afforded a weak trough at 224
nm (ꢁ" ꢁ4:3) with a positive broad shoulder extending to
the shorter wavelength region; these features are mainly attrib-
ꢀ
uted to ꢂ–ꢂ transitions of benzene chromophors. The spec-
trum of the (ꢁ)-isomer was practically the mirror image of that
of the (þ)-isomer. The decrease in the CD intensity was ob-
served as an enantiopure sample underwent racemization by
heat: for example, the course of racemization of (ꢁ)-6e is in-
serted in Fig. 2(a). The partially resolved samples of 6d are al-
so CD active, and the easily eluted (þ)-isomer gave a similar
CD curve to that of (þ)-6e (Fig. 2(b)). This chiroptical similar-
ity suggests that the (þ)-isomers (or (ꢁ)-isomers) of 6d and 6e
must have the same stereochemistry, although their absolute
configuration cannot be assigned from the available data. The-
oretical studies or X-ray analysis will solve the question.
The above data are typical of enantiomers; their chiroptical
activity depends only on the chiral center at the boron atoms.
As for borane–Lewis base adducts, only two types of such op-
tically active organoboron compounds, 4 and 5, are known, in
which the tetrahedral boron atom is bonded to one hydrogen
atom and three nonhydrogen ligands.16,17 Accordingly, 6e is
the first example of an enantiopure borane complex with four
nonhydrogen ligands, to the best of our knowledge, the central
boron atom being isoelectronic to a tertiary carbon atom.
Barrier to Enantiomerization. The 1H NMR spectra of 6
afforded two singlets due to the N-Me groups and an AB quar-
tet due to the benzylic methylene protons at room temperature
and even at 130 ꢄC in 1,1,2,2-tetrachloroethane-d2 (TCE). This
signal pattern is consistent with the coordinated form, where
the methylene protons and the N-Me groups are diastereotopic
because of the chiral boron center. The absence of lineshape
changes suggests that the dynamic process takes place much
more slowly than the NMR time scale at the temperature, its
barrier being higher than 90 kJ molꢁ1. Because the complex
began to decompose at higher temperatures, we were not able
to adopt the lineshape analysis for the kinetic measurement.
ꢀ
(N–B)-[2-(Me2N)C6H4CH2]BClPh (B–N 1.685(4) A and THC
65%).25 The THC value of 6e is smaller than those of other
intramolecular diphenylborinate–amine adducts (74–80%).7,24
The correlation of these structural parameters with the rates
of the B–N bond dissociation will be discussed later.
In 6e, the torsion angles of N–B–O(1)–C(16) and B–O(1)–
C(16)–O(2) are 176.6(2) and 0.0(4)ꢄ, respectively. Namely,
the carbonyl carbon is anti relative to the amine ligand along
the B–O(1) bond, and the carbonyl oxygen is perfectly syn rel-
ative to the boron atom along the O(1)–C(16) bond.
Enantiomeric Resolution and Chiroptical Properties.
Resolution of the enantiomers of 6 was attempted by chiral
HPLC under various conditions. The chloride 6a completely
decomposed during the separation in an eluent system of hex-
ane–alcohol, even though its barrier to racemization was high
enough for the resolution at room temperature, as is mentioned
later. 6b and 6c were eluted as a single peak under all condi-
tions because of facile racemization or poor resolution.
In contrast, the use of a Daicel Chiralpak AD column gave
good results for the acyloxy compounds. The enantiomers of
6d were partially resolved at the retention times of 16.2 and
16.6 min (separation coefficient ꢁ ¼ 1:29) with hexane–2-
propanol 1:1 as eluent. However, further separation in a large
scale was difficult due to poor solubility and partial decompo-
sition. The partially resolved samples are dextro- and levorota-
tory in the order of elution. 6e was effectively resolved without
these problems. The enantiomers were eluted at 21.0 and 27.7
min with the base-line separation (ꢁ ¼ 1:49) with hexane–2-

2084 Bull. Chem. Soc. Jpn., 77, No. 11 (2004)
Intramolecular Borane–Amine Complexes
(a)
(b)
+40
+40
+20
0
(+)-6e
(+)-6d
(−)-6d
+20
0
ε
θ
−20
−20
−40
(−)-6e
−40
200
220
240
260
200
220
240
260
λ
/ nm
λ
/ nm
Fig. 2. CD spectra of 6d and 6e in hexane. (a) Enantiopure samples of (þ)- and (ꢁ)-6e. Spectral changes during the racemization of
(ꢁ)-6e upon heating for 0, 70, 300, 600, and 1080 min at 69 ꢄC are indicated by arrows. (b) Enantioenriched samples of (þ)- and
(ꢁ)-6d.
Table 3. Kinetic Data of Enantiomerization in Complexes 6 and Related Compounds
¼
Compound
Methoda)
Solvent
t/ꢄC
k/sꢁ1
ꢁGt /kJ molꢁ1
b)
6a
6b
6c
6c
6d
6d
6e
2c)
3d)
ST
ST
ST
ST
C2D2Cl4
C2D2Cl4
C2D2Cl4
135
140
120
120
135
73
83
120
120
4:4 ꢆ 10ꢁ2
112
103
94
0.75
2.6
C6D5NO2
C2D2Cl4
heptane
heptane
C6D5CD3
C2D2Cl4
2.1
0.12
9:6 ꢆ 10ꢁ5
6:4 ꢆ 10ꢁ5
1:2 ꢆ 103
15
95
ST
108
112
116
74
CK
CK
TLA
TLA
88
a) ST: saturation transfer. CK: classical kinetics. TLA: total lineshape analysis. b) 1,1,2,2-
Tetrachloroethane-d2 (TCE). c) Ref. 4. d) Ref. 5.
Thus, we carried out the saturation transfer (ST) experi-
height (112 kJ molꢁ1) is comparable to that determined by the
ST method.
ments;26,27 these allowed us to follow slower kinetic processes.
Actually, when one of the Me proton signals of 6c was irradi-
ated at 120 ꢄC in TCE, the intensity of the other Me proton sig-
nal became about a half of the original signal. A quantitative
analysis afforded the rate of site exchange between the two
Me groups at 2.6 sꢁ1, corresponding to 94 kJ molꢁ1 in free en-
ergy of activation. The magnetization transfers were small for
6a, 6b, and 6d, and negligible for 6e at 130–140 ꢄC, the high-
est limit of temperature for measurements without decomposi-
tion. The kinetic data are listed in Table 3.
Mechanism of Racemization. Before one compares the
data in Table 3, it is instructive to understand the kinetic rela-
tionship between the rate constants of possible dynamic proc-
esses. There are three elementary processes: the dissociation
of the B–N bond (rate constant: kdis), the rotation about the
B–C(1) bond (krot), and the inversion at the nitrogen atom
(kinv). These rate constants determine those of experimentally
28
observable processes, the enantiomerization (kena
)
and the
site exchange between the two N-Me signals (kMe). A possible
mechanism of enantiomerization and the related processes is
shown in Scheme 2.
The rates of enantiomerization were determined by the clas-
sical kinetics measurement for 6d and 6e. When an enantio-
pure sample of 6e was heated at 83 ꢄC in heptane, the racemi-
zation was almost completed in 4 h. The course of racemiza-
tion was monitored by chiral HPLC, and the kinetic analysis
In general, the nitrogen inversion takes place much faster
than the NMR time scale in acyclic amines: for example the
barrier to inversion is 45 kJ molꢁ1 for dialkylbenzylamines.29
As for the B–C bond rotation, the barriers are not so high in
triarylboranes and alkoxydiarylboranes (45–55 kJ molꢁ1) even
though aryl groups are sterically bulky such as the mesityl
group.30 These experimental facts strongly support that both
the N inversion and the B–C rotation take place rapidly after
the dissociation of the B–N bond under the conditions of the
kinetic measurements in 6, namely kdis ꢈ kinv, krot. Under
the situation, once the coordination bond dissociates, the N
afforded the rate constant (6:4 ꢆ 10ꢁ5
s
ꢁ1) and the free energy
of activation for enantiomerization (116 kJ molꢁ1). The rate
constants were also determined at five different temperatures
ꢄ
in the range of 73–93 C to give the following kinetic data:
¼
¼
ꢁH 146 ꢇ 10 kJ molꢁ1 and ꢁS 84 ꢇ 29 J molꢁ1 Kꢁ1. The
rate of 6d was determined with a partially resolved sample
ꢄ
in heptane at 73 C. The relative change in the enantiomeric
excess was monitored by the CD spectra. Thus obtained barrier

S. Toyota et al.
Bull. Chem. Soc. Jpn., 77, No. 11 (2004) 2085
MeA
MeB
MeB
MeA
MeA
MeB
MeB
MeA
N
N
N
N
dis
inv
dis
B
B
B
B
Ph
Ph
Ph
Ph
X
X
X
X
6
6
6'
rot
rot
MeA
MeA
MeB
MeB
MeA
MeB
MeA
MeB
N
N
N
N
dis
inv
dis
B
Ph
B
Ph
B
B
X
X
X
X
Ph
Ph
6'
Scheme 2. Circuit of dynamic processes in 6. dis: dissociation of B–N bond. inv: N inversion. rot: C–B bond rotation. Bars and
primes refer to the relations of enantiomers and topomers, respectively.
Me
Me
Me
Me
Me
Me
N
N
N
B+
B
B
Ph
X
Ph
X–
X
Ph
6
6
Scheme 3. Enantiomerization via ionic dissociation in 6.
atom recombines with the trigonal B atom from two prochiral
faces randomly as seen in the SN1 reaction at tertiary carbon
atoms (kena ¼ kdis=2). The site exchange between the N-Me
groups is also accompanied by the B–N dissociation. The fac-
ile N inversion in the dissociated form scrambles the magnetic
sites of the two Me groups regardless of enantiomerization.
Namely, a Me group comes back to the same site and the other
with the same probabilities via the dissociation (kMe ¼ kdis=2).
ular borane–amine system.
Substituent Effects on Lewis Acidity. Because the DMP
moiety is common in all complexes 6, the barrier to enantio-
merization is closely related to the Lewis acidity of boron
atoms: the higher the barrier, the more acidic. According
to this kinetic measure, the acidity is enhanced in the order
of 6c (F) < 6b (Me) < 6d (OCOCF3) ꢂ 6a (Cl) < 6e
(OCOC2F5). This order is not always consistent with the gen-
eral trend in BY3 type boranes: B(OR)3 < BR3 < BAr3 <
BF3 < BCl3 (R = alkyl, Ar = aryl).31,33 It is known that the
Lewis acidity of borane derivatives is influenced by various
factors such as electronic and steric effects. In the case of 6,
the effects at the coordinated and dissociated forms determine
the overall Lewis acidity. The weak Lewis acidity of 6c may
be explained by the fact that the stabilization of the dissociated
form (transition state) by ꢂ type B–F interactions works more
effectively in the system of 6 than in simple boranes. Rela-
tively little is known about the influences of acyloxy groups on
the Lewis acidity of attaching boron atoms. The above data un-
ambiguously show that the effect of a –OCOCF3 group is com-
parable to that of a Cl group, and a –OCOC2F5 group further
enhances the acidity. In general, oxygen substituents on B
atoms weaken the Lewis acidity by the resonance effect: for
example, weakly acidic boronic acids or alkyl borates. This ef-
fect is overwhelmed by a strongly electron-withdrawing moie-
ty in the perfluoroacyloxy compounds. The difference in barri-
ers to enantiomerization between 6d and 6e is attributed to the
inductive effect of extra fluorine atoms in the latter, which
slightly increases the Lewis acidity of the boron atom. This
finding is consistent with the pKa values of trifluoroacetic acid
(0.52) and pentafluoropropionic acid (ꢁ0:41).34 The use of a
–OCOC2F5 group not only enhances the barrier to enantiome-
rization but also increases the chemical stability and the solu-
bility. These advantages greatly facilitate the enantiomeric
resolution and the handling of enantiopure samples of 6e.
We can rationalize the kinetic relationship of kena ¼ kMe
¼
kdis=2 from the above discussion. It is a piece of supporting
evidence of this mechanism that the free energies of activation
for enantiomerization and the Me exchange determined by the
independent methods are nearly equal, namely kena ꢂ kMe at
the same temperature, for 6d.
Another possible pathway of racemization involves the dis-
sociation of the B–X bond instead of the B–N bond in 6. The
dissociation energies of the B–O, B–halogen, and B–C bonds
are so large that the homolytic dissociation is unlikely under
the conditions of the kinetic measurements.31 The ionic disso-
ciation mechanism shown in Scheme 3 is also eliminated for
the following reasons. Firstly, the rate of enantiomerization
was not affected so much for 6c even though nitrobenzene-
d5 was used as solvent (Table 3). If the B–F bond were to dis-
sociate, the strongly polar solvent should considerably acceler-
¼
ate the dissociation. Secondly, the entropy of activation (ꢁS )
of enantiomerization of 6e is large and positive; this tendency
is characteristic of the dissociation of the B–N coordination
bond rather than that of the B–O bond.32 While the former
process decreases the ionic character in the transition state
relative to the initial state, the situation should be vise versa
¼
in the latter process, to give a negative ꢁS value.
The above discussion ensures that the dissociation of the B–
N bond is rate-determining in the overall enantiomerization
process and that the kinetic data determined by the ST method
are equivalent to those of enantiomerization in the intramolec-

2086 Bull. Chem. Soc. Jpn., 77, No. 11 (2004)
Intramolecular Borane–Amine Complexes
¼
13:0 Hz), 7.20 (1H, d, J ¼ 7:3 Hz), 7.26–7.36 (5H, m), 7.57 (1H,
d, J ¼ 6:3 Hz), 7.66 (2H, dd, J ¼ 7:5, 1.7 Hz); 13C NMR (CDCl3)
ꢀ 46.4, 48.5, 67.8, 122.3, 127.2, 127.3, 127.4, 127.7, 130.8, 134.1,
138.8, 141.4 (br), 148.2 (br); 11B NMR (CDCl3) ꢀ 10.8 (half-band
width 151 Hz); Anal. Found: C, 69.67; H, 6.77; N, 5.33%. Calcd
for C15H17BClN: C, 69.95; H, 6.65; N, 5.44%.
The kinetic and structural data of 6 as well as of 1 (ꢁG 39
ꢄ
kJ molꢁ1 at 120 C,ꢄB–N 1.754 A, THC 51%) and of 2 (ꢁG
¼
ꢀ
74 kJ molꢁ1 at 120 C, B–N 1.746 A, THC 72%) indicate that
the barriers to enantiomerization are enhanced as the B–N
bonds become short. On the contrary, the barrier heights are
not always correlated with the THC in these compounds.
The THC value of 6e is smaller than those of 6a, 6b, and 2
regardless of the tight coordination bond. This finding means
that perfluoroacyloxy groups tend to enhance the Lewis acidity
of boron atoms with a small structural change from trigonal to
tetrahedral geometry. In the series of trialkyl(aryl)borane–
amine complexes, the barriers to dissociation are enhanced
in the order of 2 < 3 < 6b. The strong acidity of 6b is attrib-
uted to the inductive effect of a phenyl group, which is more
electronegative than alkyl groups. It is notable that the THC
value of 6b is smaller than that of 2 (72%) contrary to the tight
coordination in the former. These structural and kinetic fea-
tures of 2 are considerably influenced by the steric effect of
the bulky 9-BBN moiety, which destabilizes the coordinated
form by the F-strain. The above results show that the THC val-
ues are influenced not only by the strength of coordination
bond but also by various factors such as the electronegativity
of attached atoms and the steric effect.
ꢀ
[2-(Dimethylaminomethyl)phenyl]methylphenylborane (6b).
To a solution of 370 mg (1.42 mmol) of 6a in 10 mL of toluene
was added 1.3 mL (1.4 mmol) of a 1.1 mol Lꢁ1 methyllithium so-
lution in hexane under argon atmosphere. After the mixture was
stirred for 15 h at room temperature, the solvent was evaporated.
The residue was extracted with dichloromethane, and insoluble
materials were removed by filtration. The crude product was puri-
fied by recrystallization from hexane–dichloromethane to give 224
mg (66%) of the desired product as colorless crystals; mp 130–131
1
ꢄC; H NMR (CDCl3) ꢀ 0.25 (3H, s), 2.16 (3H, s), 2.63 (3H, s),
3.93 and 4.10 (2H, ABq, J ¼ 13:3 Hz), 7.16–7.27 (7H, m),
7.39–7.45 (2H, m); 13C NMR (CDCl3) ꢀ 4.7 (br), 46.6, 48.3,
67.8, 121.6, 125.1, 126.1, 127.0, 127.1, 130.5, 133.7, 138.5,
147.9 (br), 155.7 (br); 11B NMR (CDCl3) ꢀ 6.8 (half-band width
106 Hz); Anal. Found: C, 80.91; H, 8.55; N, 5.71%. Calcd for
C16H20BN: C, 81.03; H, 8.50; N, 5.91%.
[2-(Dimethylaminomethyl)phenyl]fluorophenylborane (6c).
To a solution of 105 mg (0.41 mmol) of 6a in 6 mL of acetonitrile
was added 89.4 mg (0.70 mmol) of silver fluoride under argon at-
mosphere. After the mixture was stirred for 4 h at room tempera-
ture, the solvent was evaporated. The residue was extracted with
chloroform, and insoluble materials were removed by filtration.
The evaporation afforded 73 mg (74%) of the practically pure
product as colorless amorphous material; mp 75.0–76.5 ꢄC;
1H NMR (CDCl3) ꢀ 2.23 (3H, s), 2.76 (3H, d, JHF ¼ 2:1 Hz),
3.94 and 4.34 (2H, ABq, J ¼ 13:6 Hz), 7.20 (1H, m), 7.21–7.35
In summary, a series of (DMP)phenylboranes with a tetrahe-
dral chiral boron center were synthesized to probe the substitu-
ent effects on the ease of enantiomerization or the Lewis acid-
ity. The stable enantiopure borane–amine complexes, especial-
ly the –OCOC2F5 complex, lead to a novel design of chiral tet-
rahedral boranes as enantioselective reagents in the organic
chemistry. The kinetic analysis of enantiomerization process
reveals the mechanism of the configurational liability at the
boron atom via the dissociation of B–N bond, which is isoelec-
tronically related to the SN1 reaction at a tertiary carbon atom.
(6H, m), 7.47–7.53 (2H, m); 13C NMR (CDCl3) ꢀ 44.7 (d, JCF
¼
8:1 Hz), 47.5, 67.0, 122.4, 127.4, 127.5, 131.6, 133.1, 133.2,
135.0, 140.2 (Two aromatic signals are missing because of com-
plicated couplings); 11B NMR (CDCl3) ꢀ 12.6 (half-band width
162 Hz); Anal. Found: C, 74.51; H, 7.18; N, 5.72%. Calcd for
C15H17BFN: C, 74.72; H, 7.11; N, 5.81%.
Experimental
General. Melting points are uncorrected. 1H and 13C NMR
spectra were measured on a Varian Gemini-300 spectrometer at
300 and 75 MHz, respectively. 11B and 19F NMR spectra were
measured on a JEOL Lambda-300 spectrometer at 96 and 282
MHz, respectively. Optical rotations were measured on a JASCO
DIP-1000 digital polarimeter with a 3.5 mm ꢃ ꢆ 100 mm cell. CD
spectra were measured on a JASCO J-810 polarimeter with a 1
mm cell.
Chloro[2-(dimethylaminomethyl)phenyl]phenylborane (6a).
To a solution of 2.0 mL (13 mmol) of N,N-dimethylbenzylamine
in 20 mL of dry hexane was added 9.2 mL (14 mmol) of a 1.0
mol Lꢁ1 butyllithium solution in hexane at ꢁ78 C under argon
[2-(Dimethylaminomethyl)phenyl]phenyl(trifluoroacetoxy)-
borane (6d). To a solution of 118 mg (0.46 mmol) of 6a in 3 mL
of dichloromethane was added 102 mg (0.46 mmol) of silver tri-
fluoroacetate under argon atmosphere. After the mixture was stir-
red for 1 h at room temperature, the solid was removed by filtra-
tion. The filtrate was evaporated, and the residue was recrystal-
lized from hexane–dichloromethane to give 121 mg (79%) of
ꢄ
1
the pure product as colorless crystals; mp 134–136 C; H NMR
(CDCl3) ꢀ 2.23 (3H, s), 2.90 (3H, s), 3.96 and 4.48 (2H, ABq, J ¼
13:5 Hz), 7.20 (1H, m), 7.31–7.44 (7H, m), 7.80 (1H, m);
13C NMR (CDCl3) ꢀ 44.3, 48.2, 67.9, 115.0 (q, JCF ¼ 286:3
Hz), 122.0, 127.3, 127.6, 127.7, 127.8, 132.7, 134.0, 139.7,
140.0 (br), 143.5 (br), 155.8 (q, JCF ¼ 39:6 Hz); 11B NMR
(CDCl3) ꢀ 11.3 (half-band width 81 Hz); 19F NMR (CDCl3) ꢀ
ꢁ72:6; IR (nujol) 1748 cmꢁ1 (C=O); Anal. Found: C, 60.57; H,
4.89; N, 4.42%. Calcd for C17H17BF3NO2: C, 60.93; H, 5.11;
N, 4.18%.
ꢄ
atmosphere. The reaction mixture was allowed to warm up to
room temperature, and then stirred for 24 h under reflux. The
white suspension thus prepared was slowly transferred with a can-
nula into a solution of dichlorophenylborane (1.8 mL or 13 mmol)
in 20 mL of dry hexane in an ice bath. After the mixture was stir-
red at room temperature for 4 h, the volatile materials were re-
moved by evaporation. The residue was extracted with dichloro-
methane. The organic solution was washed with aqueous sodium
hydrogencarbonate, dried over anhydrous magnesium sulfate, and
then evaporated. The crude product was purified by recrystalliza-
tion from hexane–dichloromethane to give 2.28 g (66%) of the
pure material as colorless needles; mp 154–156 ꢄC; 1H NMR
(CDCl3) ꢀ 2.26 (3H, s), 2.95 (3H, s), 3.94 and 4.54 (2H, ABq, J ¼
[2-(Dimethylaminomethyl)phenyl](pentafluoropropionyl-
oxy)phenylborane (6e). This compound was similarly prepared
from 6a and ꢄsilver pentafluoropropionate as above. Yield 66%;
1
mp 140–142 C; H NMR (CDCl3) ꢀ 2.22 (3H, s), 2.89 (3H, s),
3.95 and 4.46 (2H, ABq, J ¼ 13:2 Hz), 7.19 (1H, d, J ¼ 7:2
Hz), 7.28–7.32 (5H, m), 7.41 (2H, m), 7.78 (1H, d, J ¼ 7:2
Hz); 13C NMR (CDCl3) ꢀ 44.7, 48.2, 67.8, 105.9 (tq, JCF
¼

S. Toyota et al.
Bull. Chem. Soc. Jpn., 77, No. 11 (2004) 2087
261:1, 38.8 Hz), 118.1 (tq, JCF ¼ 34:7, 283.7 Hz), 122.1, 127.3,
127.7, 127.80, 127.83, 132.7, 133.9, 139.4 (br), 139.8, 143.7
(br), 156.5 (t, JCF ¼ 27:7 Hz); 11B NMR (CDCl3) ꢀ 8.8 (half-band
width 240 Hz); 19F NMR (CDCl3) ꢀ ꢁ79:1, ꢁ117:6; IR (nujol)
1742 cmꢁ1 (C=O); Found: C, 55.89; H, 4.78; N, 3.94%. Calcd
for C18H17BF5NO2: C, 56.13; H, 4.45; N, 3.64%.
to the following equation: k ¼ ½ðm=m0Þꢁ1 ꢁ 1ꢅ=T1eff, where m and
m0 are signal intensities under and without irradiation, respective-
ly. The data for each compound are as follows, where the solvent
is TCE unless otherwise mentioned. 6a: T1eff ¼ 2:5 s, m=m0 ¼
0:90 at 135 ꢄC. 6b: T1eff ¼ 2:0 s, m=m0 ¼ 0:40 at 140 ꢄC. 6c:
T1eff ¼ 0:36 s, m=m0 ¼ 0:52 at 120 ꢄC. 6c (nitrobenzene-d5):
T1eff ¼ 0:52 s, m=m0 ¼ 0:48 at 120 ꢄC. 6d: T1eff ¼ 0:71 s,
m=m0 ¼ 0:92 at 135 ꢄC. No magnetization transfer (m=m0 ꢂ 1)
Enantiomeric Resolution. The chiral HPLC was carried out
with a Daicel Chiralpak AD column (1 cm ꢃ ꢆ 25 cm). About 2
mg of a racemic sample was injected for each batch. For the reso-
lution of 6e, the enantiomers were eluted at 21.0 and 27.7 min
with the baseline separation (eluent: hexane–2-propanol = 50:1,
flow rate 2.0 mL minꢁ1). Easily eluted isomer (þ)-6e: mp 161–
ꢄ
was observed for 6e at 135 C.
Classical Kinetics. About 1 mg of an enantiopure sample of
6e was dissolved in 5 mL of heptane. The solution was heated at a
constant temperature. The enantiomeric ratio x (= [B]/[A], where
A is the starting enantiomer and B is the other enantiomer) was
monitored by chiral HPLC at appropriate intervals. The kinetics
was analyzed as reversible first-order equilibrium between two
enantiomers (A ꢀ B) by the equation of ln½ð1 þ xÞ=ð1 ꢁ xÞꢅ ¼
2kt, where k is the rate of enantiomerization. The rate constants
22
162 ꢄC, ½ꢁꢅD þ180 (c 0.10, acetone), CD (hexane) ꢄ (ꢁ")
224 (ꢁ4:2), 215 (sh, þ24). Less easily eluted isomer (ꢁ)-6e:
22
mp 161–162 ꢄC, ½ꢁꢅD ꢁ180 (c 0.10, acetone), CD (hexane) ꢄ
(ꢁ") 224 (þ3:8), 215 (sh, ꢁ23). For 6d, the enantiomers were
eluted at 16.2 and 16.6 min with partial separation (eluent: hex-
ane–2-propanol = 1:1, flow rate 1.0 mL minꢁ1). Chiroptical
measurements were carried out with partially resolved samples,
of which the enantiomeric excess was unknown. The first fraction:
ꢄ
k/sꢁ1 are as follows: ð1:4 ꢇ 0:2Þ ꢆ 10ꢁ5 at 73 C, ð3:1 ꢇ 0:3Þ ꢆ
10ꢁ5 at 78 ꢄC, ð6:4 ꢇ 0:2Þ ꢆ 10ꢁ5 at 8ꢄ3 ꢄC, ð1:1 ꢇ 0:2Þ ꢆ 10ꢁ4
ꢄ
at 88 C, and ð2:3 ꢇ 0:5Þ ꢆ 10ꢁ4 at 93 C. For 6d, the measure-
22
½ꢁꢅD þ17 (c 0.10, acetone), CD (hexane, 4:1 ꢆ 10ꢁ4 mol Lꢁ1) ꢄ
ment was carried out with a partially resolved sample in heptane
ꢄ
(ꢅ/mdeg) 224 (ꢁ3:0), 210 (sh, þ13:7). The second fraction:
at 73 C. The course of isomerization was followed by the angle
22
½ꢁꢅD ꢁ18 (c 0.12, acetone), CD (hexane, 4:4 ꢆ 10ꢁ4 mol Lꢁ1
)
of ellipticity ꢅ at 215 nm in the CD spectra. The rate constant
k was obtained by the least-square fitting of the equation:
lnðꢅt=ꢅ0Þ ¼ ꢁ2kt, where ꢅ0 and ꢅt are the ellipticity at time 0
ꢄ (ꢅ/mdeg) 224 (þ2:3), 210 (sh, ꢁ13:5). Complexes 6b and 6c
were eluted as single peaks without resolution.
Saturation Transfer Measurement. The measurements were
carried out on a JEOL Lambda-500 spectrometer at 500 MHz
(1H). The temperature was read from a thermocouple after the cal-
ibration with proton chemical shifts of 1,2-ethanediol. About 10
mg of sample was dissolved in ca. 0.7 mL of TCE or nitroben-
zene-d5, and the solution was degassed by bubbling argon gas.
The sample was kept at a constant temperature in the range of
120–140 ꢄC throughout the measurement. After the 90ꢄ pulse
width was determined in an ordinary manner, the effective spin
lattice relaxation time (T1eff/s) was measured by the inversion-re-
covery method. The intensity of one of the Me signals at the lower
magnetic field was monitored under the irradiation of the other Me
signal. The rate of site exchange (k/sꢁ1) was calculated according
and t, respectively, to give k ¼ ð9:6 ꢇ 0:3Þ ꢆ 10ꢁ5 sꢁ1
.
X-ray Analysis. Single crystals of 6a, 6b, and 6e were ob-
tained by crystallization from hexane–dichloromethane solutions.
The diffraction data were collected on a Rigaku RAXIS-IV imag-
ꢀ
ing plate diffractometer with Mo Kꢁ radiation (ꢄ ¼ 0:71070 A) to
a maximum 2ꢅ value of 55.1ꢄ. The reflection data were corrected
for the Lorentz-polarization effects and secondary extinction. The
structure was solved by the direct method (SIR 92) and refined by
the full-matrix least-squares method by using a teXsan program on
a comtec O2 workstation. The non-hydrogen atoms were refined
anisotropically. Some hydrogen atoms were refined isotropically,
and the rest were included in fixed positions. The additional crys-
tal and analysis data are listed in Table 4. Crystallographic data
Table 4. Crystal Data of Compounds 6a, 6b, and 6e
6a
6b
6e
Formula
F. W.
C15H17BClN
257.57
0:3 ꢆ 0:3 ꢆ 0:5
ꢁ50
2 C16H20BN
C18H17BF5NO2
385.14
ꢃ
237:15 ꢆ 2
0:4 ꢆ 0:4 ꢆ 0:5
ꢁ50
Crystal size/mm3
t/ꢄC
0:3 ꢆ 0:4 ꢆ 0:4
ꢁ180
Crystal system
Space group
monoclinic
P21=n (#14)
13.694(2)
6.595(1)
16.314(2)
109.17(2)
1391.7(4)
4
orthorhombic
Pca21 (#29)
12.924(1)
6.790(1)
31.292(2)
90
orthorhombic
P212121 (#19)
7.6462(5)
14.407(1)
16.385(1)
90
ꢀ
a/A
ꢀ
b/A
ꢀ
c/A
ꢆ/ꢄ
ꢀ 3
V/A
2745(1)
4
1.147
0.65
3025
0.140
0.048
0.161
1.02
1804.9(2)
4
1.417
1.25
2170
0.100
0.085
0.138
1.11
Z
Dc/g cmꢁ3
ꢇ/cmꢁ1
No. of data
p
1.229
2.55
2869
0.194
0.055
0.207
1.01
R1
wR2
GOF

2088 Bull. Chem. Soc. Jpn., 77, No. 11 (2004)
Intramolecular Borane–Amine Complexes
for the structures in this paper have been deposited at the Cam-
bridge Crystallographic Data Centre as supplementary publication
numbers CCDC 247164 (6a), 247165 (6b), and 227362 (6e).
Chuong, and C. Mioskowski, Chem. Commun., 2000, 2275.
19 Substitution of phenyl groups at boron atoms tends to
strengthen the B–N coordination bond compared with alkyl
groups in the (DMP)borane system. For instance, the tight coordi-
nation bond in {[1-(dimethylamino)ethyl]phenyl}diphenylborane
was reported: L. Horner, U. Kaps, and G. Simons, J. Organomet.
Chem., 287, 1 (1985).
20 H. Noth and B. Wrackmeyer, ‘‘Nuclear Magnetic Reso-
¨
nance Spectroscopy of Boron Compounds,’’ Sprinter Verlag, Ber-
lin (1978).
This work was partly supported by a grant from the Promo-
tion and Mutual Aid Corporation for Private Schools of Japan.
The authors thank Messrs. T. Shimasaki and S. Hirano for their
assistance of the X-ray analysis, and T. Yamamoto for his
technical assistance.
21 R. G. Kidd, ‘‘NMR of Newly Accessible Nuclei,’’ ed by
P. Laszlo, Academic Press, New York (1983), Vol. 2, Chap. 3.
22 11B NMR data of similar borane–amine complexes taken
from Ref. 20. Me3N!BEt2Cl (ꢀ 11.7), Me3N!BEt2F (ꢀ 9.5),
Me3N!BEt3 (ꢀ 4.3), (N–B)-Bu2B(CH2)3NEt2 (ꢀ 7.0), (N–B)-
PhB[(CH2)3]2NEt (ꢀ 9.4), (N–B)-Bu(AcO)B(CH2)3NMe2 (ꢀ
8.3), and (N–B)-Et2BOCOCH2NHMe (ꢀ 10.0).
References
# A part of this work was preliminary reported as communi-
cation. See Ref. 1.
1 S. Toyota, T. Hakamata, N. Nitta, and F. Ito, Chem. Lett.,
33, 206 (2004).
2
Applications of the DMP and related chelating ligands: a)
I. Omae, Coord. Chem. Rev., 83, 137 (1988). b) I. Omae, ‘‘Orga-
nometallic Intramolecular Coordination Compounds,’’ Elsevier,
Amsterdam (1986), p. 35.
23 The Cambridge Crystallographic Database (CSD ver. 5.25)
analysis was carried out for the substructures of C3N!BC2X
(X = C, O, or Cl). A total of 15 structures were found except
compounds with strongly electron-withdrawing groups or sterical-
ly strained structures. The ranges of the B–N bond distances are
¯
S. Toyota and M. Oki, Bull. Chem. Soc. Jpn., 63, 1168
3
(1990).
4
¯
ꢀ ꢀ
1.68–1.76 A (for 7 structures of X = C), 1.67–1.74 A (for 7 struc-
S. Toyota and M. Oki, Bull. Chem. Soc. Jpn., 64, 1554
ꢀ
tures of X = O), and 1.69 A (for 1 structure of X = Cl).
24 H. Hopfl, J. Organomet. Chem., 581, 129 (1999).
¨
25 H.-B. Tong, X.-H. Wei, S.-P. Huang, J.-F. Lia, and D.-S.
(1991).
5 a) S. Toyota, T. Futawaka, M. Asakura, H. Ikeda, and
M. Oki, Organometallics, 17, 4155 (1998). b) S. Toyota, T.
¯
¯
Futawaka, H. Ikeda, and M. Oki, J. Chem. Soc., Chem. Commun.,
Liua, Acta Crystallogr., Sect. E, 58, 1389 (2002).
´
26 a) S. Forsen and R. A. Hoffman, J. Chem. Phys., 39, 2892
´
(1963). b) R. A. Hoffman, B. Gestblom, and S. Forsen, J. Chem.
1995, 2499.
6
¯
a) S. Toyota and M. Oki, Bull. Chem. Soc. Jpn., 65, 2215
¯
´
Phys., 39, 486 (1963). c) S. Forsen and R. A. Hoffman, Acta
(1992). b) S. Toyota, M. Asakura, T. Futawaka, and M. Oki, Bull.
Chem. Soc. Jpn., 72, 1879 (1999).
7
(1992).
8
Chem. Scand., 17, 1787 (1963).
27 Reliability of kinetic data determined by the ST method
compared with those determined by other kinetic methods, see:
¯
M. Nakamura, H. Kihara, N. Nakamura, and M. Oki, Org. Magn.
Reson., 12, 702 (1979).
28 Enantiomerization means the conversion of one enantiom-
er into the other, and racemization means the conversion of an
enaniopure or enantioenriched material into a racemate. There-
fore, the rate of racemization (krac) is double of the rate of enan-
¯
S. Toyota and M. Oki, Bull. Chem. Soc. Jpn., 65, 1832
T. Murafuji, K. Kurotobi, N. Nakamura, and Y. Sugihara,
Curr. Org. Chem., 6, 1469 (2002), and references therein.
For example, a dynamic NMR study of configurational
stability in cyclic borates: S. Green, A. Nelson, S. Warriner, and
B. Whittaker, J. Chem. Soc., Perkin Trans. 1, 2000, 4403.
10 a) D. J. Owen, D. VenDerveer, and G. B. Schuster, J. Am.
Chem. Soc., 120, 1705 (1998). b) D. J. Owen and G. B. Schuster,
J. Am. Chem. Soc., 118, 259 (1996).
9
tiomerization, krac ¼ 2kena
.
29 a) C. H. Bushweller, ‘‘Acyclic Organonitrogen Stereody-
namics,’’ ed by J. B. Lambert and Y. Takeuchi, VCH, New York
(1992), Chap. 1. b) J. B. Lambert, Top. Stereochem., 6, 19 (1971).
30 a) P. Finocchiaro, D. Gust, and K. Mislow, J. Am. Chem.
Soc., 95, 7029 (1973). b) B. Meissner and H. A. Staab, Justus Lie-
bigs Ann. Chem., 753, 92 (1971). c) J. F. Blount, P. Finocchiaro,
D. Gust, and K. Mislow, J. Am. Chem. Soc., 95, 7019 (1973).
31 J. D. Odom, ‘‘Comprehensive Organometallic Chemistry,’’
ed by G. Wilkinson, F. G. A. Stone, and E. W. Abel, Pergamon
Press, Oxford (1982), Vol. 1, Chap. 5.1.
11 a) E. Vedejs, R. W. Chapman, M. Muller, and D. R.
¨
Powell, J. Am. Chem. Soc., 122, 3047 (2000). b) E. Vedejs, S.
C. Fields, R. Hayashi, S. R. Hitchcock, D. R. Powell, and M. R.
Schrimpf, J. Am. Chem. Soc., 121, 2460 (1999).
12 A. R. Rico, M. Tlahuextl, A. Flores-Parra, and R.
Contreras, J. Organomet. Chem., 581, 122 (1999).
13 O. N. Shevtsova and A. M. Yurkevich, Zh. Obshch. Khim.,
46, 348 (1976).
14 a) H. I. Beltran, L. S. Zamudio-Rivera, T. Mancilla,
R. Santillan, and N. Farfan, J. Organomet. Chem., 657, 194
(2002). b) T. Mancilla and R. Contreras, J. Organomet. Chem.,
321, 191 (1987).
¼
32 The entropies of activation (ꢁS ) of the dissociation of
,
B–N coordination bond in 1 and 2 are 56 and 64 J molꢁ1 Kꢁ1
respectively. See Refs. 3 and 4.
15 C. S. Shiner, C. M. Garner, and R. C. Haltiwanger, J. Am.
Chem. Soc., 107, 7167 (1985).
16 P. Vedrenne, V. Le Guen, L. Toupet, T. Le Gall, and C.
Mioskowski, J. Am. Chem. Soc., 121, 1090 (1999).
17 T. Imamoto and H. Morishita, J. Am. Chem. Soc., 122,
6329 (2000).
33 a) M. K. Murphy and J. L. Beauchamp, Inorg. Chem., 16,
2437 (1977). b) V. Jones, G. Frenking, and M. T. Reetz, J. Am.
Chem. Soc., 116, 8741 (1994).
34 ‘‘Ionisation Constants of Organic Acids in Aqueous Solu-
tions,’’ ed by E. P. Serjeant and B. Dempsey, Pergamon Press,
Oxford (1979).
18 L. Charoy, A. Valleix, L. Toupet, T. Le Gall, P. P. van