Dynamic behavior of N-heterocyclic carbene boranes: Boron-carbene bonds in B, B -disubstituted N, N -dimethylimidazol-2-ylidene boranes have substantial rotation barriers

Article abstract of DOI:10.1021/acs.joc.5b00324

Dynamic NMR spectroscopy has been used to measure rotation barriers in five B,B-disubstituted 1,3-dimethylimidazol-2-ylidene boranes. The barriers are attributed to the sp²-sp³ bond between C(1) of the N-heterocyclic carbene ring and the boron atom. Bonds to boron atoms bearing a thexyl (1,1,2-trimethylpropyl) group show especially high barriers, ranging from 75-86 kJ mol^-1. 2-Isopropyl-1,3,5-trimethylbenzene is used as a comparable to help understand the nature and magnitude of the barriers.

Full text of DOI:10.1021/acs.joc.5b00324

Article
pubs.acs.org/joc
Dynamic Behavior of N‑Heterocyclic Carbene Boranes: Boron−
Carbene Bonds in B,B‑Disubstituted N,N‑Dimethylimidazol-2-ylidene
Boranes Have Substantial Rotation Barriers
Krishnan Damodaran, Xiben Li, Xiangcheng Pan, and Dennis P. Curran*
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
S
* Supporting Information
ABSTRACT: Dynamic NMR spectroscopy has been used to
measure rotation barriers in five B,B-disubstituted 1,3-dimethyli-
midazol-2-ylidene boranes. The barriers are attributed to the sp2−
sp3 bond between C(1) of the N-heterocyclic carbene ring and the
boron atom. Bonds to boron atoms bearing a thexyl (1,1,2-
trimethylpropyl) group show especially high barriers, ranging from
75−86 kJ mol−1. 2-Isopropyl-1,3,5-trimethylbenzene is used as a
comparable to help understand the nature and magnitude of the barriers.
INTRODUCTION
■
N-Heterocyclic carbene boranes have garnered interest in diverse
areas such as main group chemistry,1 organic synthesis,2 and
polymer chemistry3 because they blend chemical stability with an
interesting reactivity profile.4 This profile does not include
hydroboration,5 the most common reaction of B−H bonds in
boranes, because free boranes are not easily released. With
hydroboration excluded, a wealth of other interesting reactions of
B−H bonds materialize.
The static structures of many NHC-boranes are understood
thanks to the complementary techniques of NMR spectroscopy,
X-ray crystallography, and calculations.3 6 However, little is
a,
known about the dynamic behavior of NHC-boranes.7 NMR
spectra of N,N-dialkyl NHC boranes 1 with stereogenic boron
atoms (X ≠ Y) typically show chemical shift equivalence of
groups on either side of the imidazolylidene ring (for example,
the groups RA in Figure 1a exhibit one resonance). This means
that rotation of the bond between the NHC ring and boron is fast
on the NMR time scale, as expected for such an sp2−sp3 bond.
1,3-(2,6-Diisopropylphenyl)imidazol-2-ylidene boranes 2 ex-
hibit slow rotation around the N−Ar bonds, a feature shared by
many N-aryl substituted NHC complexes of both metals and
main group elements.8 For example, achiral molecules such as
dipp-Imd-BH2Cl9 3 and dipp-Imd-BHCl210 4 shown in Figure 1b
exhibit two doublets for the isopropyl methyl groups in their 1H
NMR spectra. One resonance represents the four “A” Me groups,
the other, the four “B” Me groups. This slow N−Ar rotation is a
variant of standard biaryl rotation that involves Csp2−Nsp2
Figure 1. Dynamic aspects of representative NHC-boranes (formal
bonds.11
charges omitted).
The chemical shift equivalence is reduced for molecules like
dipp-Imd-BHClOTf 5,10 which now has a stereogenic boron
To summarize, N,N-dialkylimidazolylidene boranes typically
atom. The 1H NMR spectrum of this compound exhibits four Me
exhibit simple NMR spectra consistent with rapid rotation of all
bonds. 1H NMR spectra of dipp and related N,N-diary-
doublets, each representing pairs of Me groups (A−D). The
bond between the boron atom and C2 of the imidazolylidene
ring must be rotating rapidly on the NMR time scale; otherwise,
there would be eight Me doublets.
Received: February 11, 2015
Published: April 3, 2015
© 2015 American Chemical Society
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Article
limidazolylidene boranes commonly exhibit features of slow
rotation that can be attributed to the N−Ar bonds.
corresponding enthalpy ΔH⧧, 38.0 kJ mol−l, and the entropy
ΔS⧧, −67.7 J K−1.
Within this backdrop, we were initially surprised when we
observed evidence of dynamic behavior in the 1H NMR
spectrum of a chiral B,B-disubstituted N,N-dimethylimidazol-2-
ylidene borane. Here, we report the results of experiments
conducted to understand and quantify this phenomenon. We
have measured the barriers for dynamic processes of five
representative B,B-disubstituted NHC-boranes and can con-
fidently attribute the barriers to slow rotation of the Csp2−Bsp3
bond rather than dissociation or epimerization at boron. The
barriers are substantial, 56.4 kJ mol−l (13.5 kcal mol−1) and up.
These data cannot be understood either by dissociation of the
boron from the NHC ring (decomplexation) or by epimerization
at boron or carbon because all three processes would result in
interconversion of the two diastereomers 7a,b, not coalescence
of resonances in one of the diastereomers. Further, chiral NHC-
boranes with boron stereocenters have been resolved and shown
to be thermally stable well above room temperature.2b Thus, the
rotation of the bond between the boron atom and the
imidazolylidene ring is responsible for the 56.4 kJ mol−l
activation barrier.
The difference between 7b and other B,B-disubstituted NHC-
boranes 1 that do not show dynamic NMR behavior is that one of
the carbon atoms bonded to boron is secondary in 7b. To further
increase the rotation barrier of the B−C bond, we prepared four
new B,B-disubstituted NHC-boranes with one of the boron
substituents being a tertiary carbon. The precursor for these
substrates, B-thexylborane 8 (thexyl is 1,1,2-trimethylpropyl), is
itself readily available by borenium-catalyzed hydroboration of
tetramethylethylene (2,3-dimethyl-2-butene).5
RESULTS AND DISCUSSION
■
We encountered slow rotation in one of the products of Rh-
catalyzed insertions of diazoesters into the B−H bonds of
substituted NHC-boranes.2b Reaction of α-NHC-boryl ester 6
with methyl 2-diazo-2-phenylacetate and 1% Rh2(esp)2,12 as
shown in Figure 2a, provided a 56/44 mixture of two separable
Rh-catalyzed insertion reactions of 8 with ethyl diazoacetate
and trimethylsilyl diazomethane provided products 9 and 10 in
36% and 15% yields, respectively, after purification by flash
chromatography. The ester substituent on the diazo compound
increased the yield of 9 relative to 10. This is sensible given that
NHC-boranes are relatively nucleophilic13 and prefer to insert
into transient metal carbenes that are derived from electrophilic
diazo precursors.
Two compounds with B−S bonds were also made by reaction
of 8 with the corresponding benzothiazole 2-thiol or N-
phenyltetrazole disulfide.14 Heating of 8 and benzothiazole 2-
thiol (BtSH) in benzene at 80 °C, followed by evaporation and
automated flash chromatography, gave boryl sulfide 11 in 58%
yield. In the reaction between 8 and N-phenyltetrazole disulfide
(PtSSPt), the corresponding boryl sulfide 12 formed at room
temperature in 59% isolated yield. The four new compound 9−
12 were fully characterized (Scheme 1).
Figure 2. Synthesis and dynamic features of a B,B-disubstituted NHC-
borane. (a) esp is 1,3-benzenedipropanoate, C6H4-1,3-(CH2CH2CO2)2.
All four compounds 9−12 exhibited doubled resonances in
their 1H NMR spectra for either (or both) the N-methyl groups
or the pair of protons on the imidazolylidene ring at room
temperature. This doubling (as opposed to broadening) already
diastereomers of 7a,b in 70% yield. The diastereomers arise
because both the boron atom and one of its substituents are
stereocenters. Neither of the isomers is crystalline, and NMR
spectroscopy did not provide a clear indication of relative
configuration. Interestingly, while the 1H NMR spectrum of the
major diastereomer 7a was standard, the 1H NMR spectrum of
the minor diastereomer 7b lacked the usual six-proton singlet for
the two N-methyl groups of the imidazolylidene ring. In its place
was a very broad hump at about 3.7 ppm that integrated for about
six protons. This suggests that slow rotation is interconverting
the two N-methyl groups of 7b, as shown in Figure 2b.
Apparently, the rt spectrum happens to be very close to the
coalescence point at 400 MHz.
Scheme 1. Synthesis of Four B-Thexyl NHC-boranes
1
A series of H NMR spectra were then recorded at five
temperatures ranging from 260 to 301 K. As projected, the broad
6 H resonance at 298 K (room temperature) resolved into two
broad 3 H signals at 260 K. These signals then gradually
coalesced on rewarming. The data were processed by line shape
analysis in the usual way to provide rate constants at the various
temperatures (see the Supporting Information). Further treat-
ment including an Eyring plot of this data provided the activation
free energy ΔG⧧, 56.4 kJ mol−l (13.5 kcal mol−1), and the
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Table 1. Activation Parameters for B-Thexyl NHC-boranes 9−12 As Measured by Temperature- and Time-Dependent 1D or 2D
EXSY Experiments
a
entry
cmd
resonance
ΔG⧧, kJ mol−1 (kcal mol−1
)
ΔH⧧, kJ mol−1
ΔS⧧, J K−1
1
2
3
4
9
10
11
12
Me
Me
H
81.8 (19.6)
86.4 (20.7)
74.9 (17.9)
77.7 (18.6)
71.5
81.3
60.9
62.6
−34.6
−17.0
−46.9
−50.5
H
a
The doubled resonances used for the line shape analysis; Me for the N-methyl groups, H for the imidazolylidene CH’s.
suggests higher rotation barriers for 9−12 compared to 7b.
Further, the doubling of the NHC ring protons is consistent with
the slow rotation of the NHC−B bond. Because the processes
were too slow for coalescence experiments, 1D and 2D EXSY
experiments with variable mixing times were used to collect data
sets for the barrier measurements.
Sets of spectra were recorded at appropriate temperatures in
the range of 290−330 K, and the data were processed to give rate
constants and energies of activation (see the Supporting
Information). In the spectra of 9 and 10, the degree of exchange
between the two N-methyl singlets was assessed in 2D EXSY
experiments to provide the data in Table 1. In the spectra of 11
and 12, the degree of exchange between the two imidazolylidene
doublets was assessed in 1D EXSY experiments.
These four compounds have significantly higher rotation
barriers than 7b. The two compounds with B−S bonds, 11 and
12, have barriers of 74.9 kJ mol−1 (17.9 kcal mol−1) and 77.7 kJ
mol−1 (18.6 kcal mol−1) (Table 1, entries 3 and 4). The two
compounds with B−CH2 bonds, 9 and 10, have even higher
barriers of 81.8 kJ mol−1 (19.6 kcal mol−1) and 86.4 kJ mol−1
(20.6 kcal mol−1) (Table 1, entries 1 and 2). The larger barriers in
9 and 10 compared to 11 and 12 presumably arise because B−C
bonds are shorter than B−S bonds.
All the compounds so far have stereocenters at boron, but the
local plane of symmetry in the NHC ring renders the two
rotamers identical. If the symmetry of the NHC ring is broken,
then the slow rotation should engender diastereomers. The
highest barriers observed in this work, 82 and 86 kJ mol−1 with 9
and 10, are just below the level needed for chromatographic
separation of diastereomers (atropisomers) at room temper-
ature.
Both 1H and 13C NMR analysis showed that 13 was about a 70/
30 mixture of rotamers; however, the sample could not be
resolved into its component atropisomers at room temperature
by flash chromatography. On the other hand, HPLC injection
resulted in partial separation, with the minor rotamer eluting
before the major rotamer. The chromatogram exhibited features
of atropisomers that partially equilibrate during the time scale of
the experiment (see the Supporting Information).
Thus, unsymmetrical NHC-borane 13 probably has a rotation
barrier close to that of the related symmetric sample 9. Its
rotamers can only be partially resolved at room temperature, and
they then reequilibrate rather quickly. Nonetheless, the
observation of the diastereomeric rotamers and their equilibrium
on the time scale of some minutes at rt is consistent with the
picture of NHC-B bond rotation that emerged from the data in
Table 1.
As far as we know, these are the first measurements of rotation
barriers for bonds between N-heterocyclic carbenes and a main
group element.15 Touchstones in other areas of NHC chemistry
include a number of interesting metal complexes that exhibit
slow rotation of the NHC−metal bond that have been prepared
by Enders and others.16 Chiral complexes of this type have been
used in catalysis.17 However, most such complexes have a square-
planar metal atom and are, therefore, geometrically different
from the tetrahedral boron atom of 7 and 9−12.
Simple touchstones in carbon chemistry include 2-isopropyl-
1,3,5-trimethylbenzene 14 and related molecules with a
secondary alkyl group bonded to a benzene ring and flanked
by two ortho substituents (Figure 4).18 The rotation barrier of
the Csp2−Csp3 bond between the benzene ring and the
isopropyl group of 14 is 53.6 kJ mol−1 (12.8 kcal mol−1). In
this simple analogy, the 1,3-dimethyl-substituted benzene ring of
14 maps onto the N,N-dimethyl imidazolylidene ring of NHC-
boranes like 15, while the central isopropyl carbon atom of 14
maps onto the boron atom of 15. The first of the NHC-boranes
To test the prediction that diastereomers would result, NHC-
borane 13 (Figure 3) was made starting from 1-isopropyl-1-
methylimadozol-2-ylidene borane by a two-step sequence of
borenium-catalyzed hydroboration of tetramethylethylene,
followed by Rh-catalyzed B−H insertion with ethyl diazoacetate.
Figure 3. Rotational isomers of 13.
Figure 4. 2-Isopropyl-1,3,5-trimethylbenzene 13 as a touchstone.
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0.012, JAB = 2.0 Hz), 3.94 (s, 3H), 3.84 (q, J = 6.8 Hz, 2H) 3.79 (s, 3H),
1.67−1.76 (m, 2H), 1.36 (sep, J = 6.8 Hz, 1H), 0.98 (t, J = 6.8 Hz, 3H),
0.86 (d, J = 6.8 Hz, 3H), 0.79 (d, J = 6.8 Hz, 3H), 0.74 (s, 3H), 0.55 (s,
3H); 13C NMR (100 MHz, CDCl3) δ 181.4, 121.1, 121.0, 58.2, 37.8,
37.4, 37.0, 26.6, 24.9, 18.9, 18.1, 14.4; 11B NMR (128 MHz, CDCl3) δ
−16.3 (d, JBH = 87 Hz); IR (film) 3121, 2951, 2857, 2353, 1694, 1472
cm−1; HRMS (ESI) m/z (M+) calcd for C15H28BN2O2 279.2238, found
279.2229.
(1,3-Dimethyl-1H-imidazol-3-ium-2-yl)(2,3-dimethylbutan-
2-yl)((trimethylsilyl)methyl)hydroborate 10. The (1,3-dimethyl-
1H-imidazol-3-ium-2-yl)(2,3-dimethylbutan-2-yl)dihydroborate 8 (62
mg, 0.32 mmol, 1.0 equiv) and Rh2(esp)2 (2.4 mg, 0.0032 mmol) were
dissolved in dry DCM (2 mL) under argon. The solution was dark green.
The reaction mixture was heated to reflux. A solution of
trimethylsilyldiazomethane (0.24 mL, 0.47 mmol, 1.5 equiv) in dry
DCM (2 mL) was added via syringe pump over a period of 4 h. The
color of the solution turned orange. After 4 h, the solvent was removed
and the crude 1H NMR and 11B NMR were taken. The conversion was
estimated by crude 11B NMR as 18%. The mixture was concentrated and
purified by flash chromatography to give the product (13 mg, 0.046
mmol, 15%) as a colorless oil: 1H NMR (400 MHz, CDCl3) δ 6.80 (d,
1H, J = 2.0 Hz), 6.72 (d, 1H, J = 2.0 Hz), 3.84 (s, 3H), 3.79 (s, 3H), 1.46
(sep, J = 6.8 Hz, 1H), 0.86 (t, J = 6.4 Hz, 3H), 0.82 (d, J = 6.8 Hz, 3H),
0.70 (s, 3H), 0.49 (s, 3H), −0.36 (s, 9H); 13C NMR (100 MHz, CDCl3)
δ 121.0, 120.5, 37.5, 37.3, 36.5, 26.8, 24.3, 19.1, 17.9, 0.25; 11B NMR
(128 MHz, CDCl3) δ −18.7 (d, JBH = 83 Hz); IR (film) 2926, 2854,
1718, 1556, 1468, 1379 cm−1; HRMS (ESI) m/z (M+) calcd for
C15H32BN2Si 279.2422, found 279.2421.
(Benzo[d]thiazol-2-ylthio)(1,3-dimethyl-1H-imidazol-3-ium-
2-yl)(2,3-dimethylbutan-2-yl)hydroborate 11. Benzothiazole-2-
thiol (17.6 mg, 0.10 mmol) was added to a benzene (0.5 mL) solution
of diMe-Imd-BH2thexyl 8 (19.4 mg, 0.10 mmol). The colorless solution
was charged to a sealed tube and heated in an oil bath at 80 °C for 12 h.
The mixture was cooled to room temperature; then, the solvent was
evaporated and the title product (21.0 mg, 58%) was obtained as an oil
after flash chromatographic purification. 1H NMR (400 MHz, CDCl3) δ
7.64 (dd, J = 8.0, 0.4 Hz, 1H), 7.57 (dd, J = 7.2, 0.4 Hz, 1H), 7.30−7.23
(m, 1H), 7.12 (td, J = 7.2, 0.8 Hz, 1H), 6.91 (d, J = 2.0 Hz, 1H), 6.78 (d, J
= 2.0 Hz, 1H), 4.05 (s, 3H), 4.04 (s, 3H), 3.70−3.30 (m, 1H), 1.71 (sep,
J = 6.8 Hz, 1H), 0.98 (d, J = 6.8 Hz, 3H), 0.95 (s, 3H), 0.88 (d, J = 6.8 Hz,
3H), 0.69 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 175.2, 154.2, 135.7,
125.2, 122.4, 122.0, 121.5, 120.4, 120.1, 38.0, 37.5, 36.4, 27.0, 23.6, 19.2,
17.7; 11B NMR (128.4 MHz, C6D6) δ −11.0 (d, J = 84.7 Hz); FTIR
(thin film, CH2Cl2) ν = 2952, 1578, 1454, 1416, 1234, 1115, 998, 756,
727 cm−1; HRMS (ESI) m/z (M+ + H) calculated for C18H27BN3S2
360.1739, found 360.1743.
7b has the closest structure to 14, and indeed the rotation
barriers of 7b and 14 are roughly comparable.
According to this analogy, larger N-alkyl groups or larger
boron substituents should result in higher rotation barriers. Axial
chirality results if the two, the imidazolyl nitrogen or carbon
substituents, are different, and stable atropisomers are within
reach based on the barriers reported herein. Suitably substituted
NHC-boranes with three boron substituents should surely show
features of slow rotation in their NMR spectra. Indeed,
Braunschweig, Stephan, and co-workers have recently observed
doubling of resonances in NMR spectra of a trisubstituted NHC-
borane7a and a hindered disubstituted NHC-borane.7b Both
groups assigned the doubling to slow rotation of the NHC−B
bond. These observations mesh nicely with the barriers reported
here.
In summary, substantial barriers have been measured for five
different B,B-disubstituted N-heterocyclic carbene boranes that
bear one primary and either one secondary or one tertiary
substituent on the boron atom. The barriers are attributed to
slow rotation around the bond between the NHC ring and the
boron atom and range for 56 kJ mol−1 (secondary boron
substituent) up to 75−86 kJ mol−1 (tertiary boron substituents).
EXPERIMENTAL SECTION
■
(1,3-Dimethyl-1H-imidazol-3-ium-2-yl)(2-ethoxy-2-
oxoethyl)(2-methoxy-2-oxo-1-phenylethyl)hydroborate (7a,b).
(1,3-Dimethyl-1H-imidazol-3-ium-2-yl)(2-ethoxy-2-oxoethyl)dihydro-
borate 6 (38.0 mg, 0.19 mmol, 1.0 equiv) and the Rh2(esp)2 (1.4 mg,
0.0019 mmol, 0.01 equiv) were dissolved in dry DCM (2 mL) under
argon. The solution was dark green. The reaction mixture was heated to
reflux. A solution of methyl 2-diazo-2-phenylacetate (66.7 mg, 0.38
mmol, 2.0 equiv) in dry DCM (2 mL) was added via syringe pump over
a period of 4 h. The color of the solution turned orange. After 4 h, the
solvent was removed and the crude 1H NMR and 11B NMR were taken.
The mixture was concentrated and purified by flash chromatography
(Hex:EA = 1:2) to give two diastereomers as a colorless oil.
1
Diastereomer 7a (25.6 mg, 0.075 mmol, 39%): H NMR (400 MHz,
CDCl3) δ 7.18−7.20 (m, 2H), 7.06−7.10 (m, 2H), 6.90−6.99 (m, 1H),
6.58 (s, 2H), 3.87 (qd, J1 = 7.2 Hz, J2 = 2.0 Hz, 2H), 3.62 (s, 3H), 3.47 (s,
6H), 3.22 (br, 1H), 1.75 (br, 2H), 1.03 (t, J = 7.2 Hz); 13C NMR (100
MHz, CDCl3) δ 179.4, 178.6, 144.0, 127.5, 127.5, 124.2, 121.0, 58.7,
50.8, 49.0 (br), 36.4, 28.0 (br), 14.4; 11B NMR (128 MHz, CDCl3) δ
−16.4 (d, JBH = 96 Hz); IR (film) 2950, 2377, 1703, 1482, 1453, 1434,
1363 cm−1; HRMS (ESI) m/z (M+ + H) calcd for C18H26BN2O4
345.1980, found 345.1974. Diastereomer 7b (20.1 mg, 0.059 mmol,
31%): 1H NMR (400 MHz, CDCl3) δ 7.25−7.27 (m, 2H), 7.11−7.19
(m, 2H), 6.99−7.02 (m, 1H), 6.73 (s, 2H), 3.74−3.85 (m, 2H), ∼3.7
(very broad, 6H), 3.40 (s, 3H), 3.24 (br, 1H), 1.53 (br, 2H), 0.97 (t, J =
7.2 Hz); 13C NMR (100 MHz, CDCl3) δ 179.5, 178.9, 143.3, 128.5,
127.6, 124.4, 121.2, 58.6, 50.6, 49.0 (br), 36.7, 26.5 (br), 14.3; 11B NMR
(128 MHz, CDCl3) δ −16.5 (d, J = 95 Hz); IR (film) 3134, 2950, 2376,
1703, 1598, 1577, 1481, 1453, 1432 cm−1; HRMS (ESI) m/z (M+ + H)
calcd for C18H26BN2O4 345.1980, found 345.1990. The minor
diastereomer shows slow rotation.
(1,3-Dimethyl-1H-imidazol-3-ium-2-yl)(2,3-dimethylbutan-
2-yl)((1-phenyl-1H-tetrazol-5-yl)thio)hydroborate 12. 1-Phenyl-
tetrazoldisulfide (35.4 mg, 0.10 mmol) was added to a solution of diMe-
Imd-BH2thexyl 8 (19.4 mg, 0.10 mmol) in benzene (0.5 mL). The
colorless solution was charged to a sealed tube. The sealed tube was
stirred at rt for 1 day. After all the NHC-borane was consumed, the
solvent was evaporated, and the title product (22.2 mg, 59%) was
1
obtained as an oil after flash chromatographic purification. H NMR
(400 MHz, CDCl3) δ 7.75 (d, J = 7.8 Hz, 2H), 7.53 (t, J = 7.6 Hz, 2H),
7.47 (d, J = 7.2 Hz, 1H), 6.92 (d, J = 1.6 Hz, 1H), 6.78 (d, J = 1.6 Hz,
1H), 3.97 (s, 3H), 3.95 (s, 3H), 1.65 (br s, 1H), 1.55 (sep, J = 6.8 Hz,
1H), 0.93−0.91 (m, 6H), 0.85 (d, J = 6.8 Hz, 3H), 0.70 (s, 3H); 13C
NMR (125 MHz, CDCl3) δ 159.0, 135.2, 129.1, 128.9, 124.2, 121.9,
121.3, 37.9, 37.7, 36.9, 27.0, 24.1, 19.1, 17.9; 11B NMR (128.4 MHz,
CDCl3) δ −10.9 (br s); FTIR (thin film, CH2Cl2) ν = 2954, 1597, 1499,
1477, 1375, 1232, 1090, 1005, 760, 694 cm−1; HRMS (ESI) m/z (M+ +
H) calculated for C18H28BN6S 371.2189, found 371.2192.
(1,3-Dimethyl-1H-imidazol-3-ium-2-yl)(2,3-dimethylbutan-
2-yl)(2-ethoxy-2-oxoethyl)hydroborate 9. (1,3-Dimethyl-1H-imi-
dazol-3-ium-2-yl)(2,3-dimethylbutan-2-yl)dihydroborate 8 (58.2 mg,
0.30 mmol, 1.0 equiv) and Rh2(esp)2 (2.3 mg, 0.0030 mmol) were
dissolved in dry DCM (2 mL) under argon. The solution was dark green.
The reaction mixture was heated to reflux. A solution of ethyl 2-
diazoacetate (0.089 mL, 0.60 mmol, 2.0 equiv) in dry DCM (2 mL) was
added via syringe pump over a period of 4 h. The color of the solution
(2,3-Dimethylbutan-2-yl)(3-isopropyl-1-methyl-1H-imida-
zol-3-ium-2-yl)dihydroborate. Triflimide (56.6 mg, 0.19 mmol, 0.2
equiv) was added to a solution of (3-isopropyl-1-methyl-1H-imidazol-3-
ium-2-yl)trihydroborate (0.134 g, 0.96 mmol, 1.1 equiv) and 2,3-
dimethylbut-2-ene (0.13 mL, 1.05 mmol, 1.1 equiv) in DCM (2 mL).
The reaction mixture was refluxed overnight. Then, the solvent was
1
turned orange. After 4 h, the solvent was removed and the crude H
NMR and 11B NMR were taken. The conversion was estimated by crude
11B NMR as 55%. The mixture was concentrated and purified by flash
chromatography to give the product (30.3 mg, 0.11 mmol, 36%) as a
1
1
colorless oil: H NMR (400 MHz, CDCl3) δ 6.79 (ABq, 2H, ΔδAB
=
removed and the crude H NMR and 11B NMR were taken. The
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The Journal of Organic Chemistry
Article
conversion was estimated by crude 11B NMR as 65%. The mixture was
(3) Lalevee
́
, J.; Telitel, S.; Tehfe, M. A.; Fouassier, J. P.; Curran, D. P.;
concentrated and purified by flash chromatography to give the product
Lacote, E. Angew. Chem., Int. Ed. 2012, 51, 5958−5961.
̂
1
(113 mg, 0.50 mmol, 53%) as a colorless oil: H NMR (400 MHz,
(4) Curran, D. P.; Solovyev, A.; Makhlouf Brahmi, M.; Fensterbank, L.;
CDCl3) δ 6.89 (ABq, 2H, ΔδAB = 0.085, JAB = 1.6 Hz), 5.22 (sept, J = 6.8
Hz, 1H), 3.80 (s, 3H), 1.38 (d, J = 6.8 Hz, 6H), 0.91 (d, J = 6.8 Hz, 6H),
0.67 (s, 6H); 13C NMR (100 MHz, CDCl3) δ 120.8, 114.8, 49.6, 39.9,
36.4, 28.3, 23.0, 18.9; 11B NMR (128 MHz, CDCl3) δ −23.9 (t, JBH = 83
Hz); IR (film) 3172, 3111, 2938, 2282, 1573, 1428, 1408 cm−1; HRMS
(ESI) m/z (M+) calcd for C13H26BN2 221.2184, found 221.2181.
(2,3-Dimethylbutan-2-yl)(2-ethoxy-2-oxoethyl)(3-isopropyl-
1-methyl-1H-imidazol-3-ium-2-yl)hydroborate 13. (1,3-Dimeth-
yl-1H-imidazol-3-ium-2-yl)(2,3-dimethylbutan-2-yl)dihydroborate (49
mg, 0.22 mmol, 1.0 equiv) and Rh2(esp)2 (1.6 mg, 0.0022 mmol) were
dissolved in dry DCM (2 mL) under argon. The solution was dark green.
The reaction mixture was heated to reflux. A solution of ethyl 2-
diazoacetate (0.27 mL, 2.2 mmol, 10 equiv) in dry DCM (2 mL) was
added via syringe pump over a period of 10 h. The color of the solution
Malacria, M.; Laco
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135, 14433−14437. (b) Prokofjevs, A.; Boussonnier
H.; Lacote, E.; Curran, D. P.; Vedejs, E. J. Am. Chem. Soc. 2012, 134,
12281−12288.
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te, E. Angew. Chem., Int. Ed. 2011, 50, 10294−10317.
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2010, 16, 6861−6865. (b) Telitel, S.; Vallet, A.-L.; Schweizer, S.;
Delpech, B.; Blanchard, N.; Morlet-Savary, F.; Graff, B.; Curran, D. P.;
Robert, M.; Laco
16947.
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te, E.; Lalevee, J. J. Am. Chem. Soc. 2013, 135, 16938−
(7) (a) Farrell, J. M.; Posaratnanathan, R. T.; Stephan, D. W. Chem. Sci.
2015, 6, 2010−2015. (b) Braunschweig, H.; Claes, C.; Damme, A.; Dei;
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1630.
1
turned orange. After 10 h, the solvent was removed and the crude H
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NMR and 11B NMR were taken. The conversion was estimated by crude
11B NMR as 61%. The mixture was concentrated and purified by flash
chromatography to give two inseparable diastereomers (with a ratio of
73/27, 23 mg, 0.074 mmol, 34%) as a colorless oil: 1H NMR (400 MHz,
CDCl3) δ 6.82−6.94 (m, 2H), 5.45 (sept, J = 6.8 Hz, 0.73H), 4.97 (sept,
J = 6.8 Hz, 0.27H), 3.94 (s, 2.2H), 3.87−3.72 (m, 2H), 3.75 (s, 0.8H),
1.86 (br, 2H), 1.51 (d, J = 6.8 Hz, 0.81H), 1.47 (d, J = 6.8 Hz, 0.81H),
1.41 (d, J = 6.8 Hz, 2.19H), 1.23 (d, J = 6.8 Hz, 2.19H), 1.40−1.25 (m,
1H), 1.02−1.00 (m, 3H), 0.86−0.78 (m, 6H), 0,73 (s, 3H), 0.54 (s, 3H);
13C NMR (100 MHz, CDCl3) δ 181.4, 121.8, 121.8, 115.3, 115.0, 58.3,
49.9, 49.3, 38.2, 37.7, 37.4, 36.9, 26.6, 25.4, 24.8, 24.1, 23.6, 23.4, 22.0,
19.0, 19.0, 18.5, 18.0, 14.4, 14.0; 11B NMR (128 MHz, CDCl3) δ −16.3
(d, JBH = 86 Hz); IR (film) 3119, 2936, 2856, 2355, 1740, 1696, 1559,
1453 cm−1; HRMS (ESI) m/z (M+) calcd for C17H32BN2O2 307.2551,
found 307.2544.
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ASSOCIATED CONTENT
■
(14) Pan, X.; Vallet, A.-L.; Schweizer, S.; Dahbi, K.; Delpech, B.;
S
* Supporting Information
Blanchard, N.; Graff, B.; Geib, S. J.; Curran, D. P.; Lalevee
́ ̂
, J.; Lacote, E.
Contains general information on the synthesis and NMR
J. Am. Chem. Soc. 2013, 135, 10484−10491.
1
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11B, and 13C NMR spectra of the new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: curran@pitt.edu (D.P.C.).
Notes
The authors declare no competing financial interest.
(17) Wang, F.; Liu, L.-J.; Wang, W.; Li, S.; Shi, M. Coord. Chem. Rev.
2012, 256, 804−853.
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ACKNOWLEDGMENTS
■
We thank the National Science Foundation for support. We also
thank Dr. Emmanuel Laco
helpful discussions. This paper is dedicated to Professor Iwao
Ojima on the occasion of his 70th birthday.
̂
te (CNRS and University of Lyon) for
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