β-Boration of α,β-unsaturated carbonyl compounds in ethanol and methanol catalyzed by CCC-NHC pincer Rh complexes

Article abstract of DOI:10.1016/j.jorganchem.2015.11.010

Quantitative β-boration of α,β-unsaturated carbonyl compounds was achieved utilizing the eco-friendly solvent EtOH along with MeOH at room temperature in 1 h, by a CCC-NHC pincer Rh complex mixture. Substrates with β-substituents were successfully convert

Full text of DOI:10.1016/j.jorganchem.2015.11.010

Journal of Organometallic Chemistry 802 (2016) 32e38
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
b-Boration of
a,
b-unsaturated carbonyl compounds in ethanol and
methanol catalyzed by CCC-NHC pincer Rh complexes
Sean W. Reilly, Gopalakrishna Akurathi, Hannah K. Box, Henry U. Valle, T. Keith Hollis*,
Charles Edwin Webster
Department of Chemistry, Mississippi State University, Box 9573, Mississippi State, MS 39762, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Quantitative
solvent EtOH along with MeOH at room temperature in 1 h, by a CCC-NHC pincer Rh complex mixture.
Substrates with -substituents were successfully converted yielding challenging, quaternary CeB bonds.
The air- and water-stable pre-catalyst A, identified as a mixture of iodo and chloro CCC-NHC pincer Rh
amine complexes, was evaluated for catalytic activity. This report is the first example of a pincer Rh
complex demonstrating catalytic activity in a 1,4-addition at room temperature.
© 2015 Elsevier B.V. All rights reserved.
b-boration of a,b-unsaturated carbonyl compounds was achieved utilizing the eco-friendly
Received 16 October 2015
Received in revised form
12 November 2015
Accepted 14 November 2015
Available online 22 November 2015
b
Keywords:
Enones
Michael addition
N-Heterocyclic carbenes
Pincer ligands
Rhodium
1. Introduction
Cu systems have been the most commonly applied catalysts for
b-boration of a,b-unsaturated carbonyl compounds. Noteworthy
Organoborane compounds are highly sought after intermediates
in organic synthesis. The facile transformation of the CeB bond into
other functional groups such as alcohols, amines, and alkenes al-
lows direct access to a wide range of organic molecules [1e4]. In
examples have been reported using strictly protic solvents such as
MeOH [26] and H2O [36,37,58e61]. In 2009, Santos reported the
synthesis of an innovative sp2esp3 diboron reagent and demon-
strated its application in a Cu catalyzed
b-boration of acyclic sub-
1997, a Pt catalyst was utilized in the first report of
-unsaturated carbonyl compounds using a diboron reagent [5].
To date, the boration of -unsaturated carbonyl compounds has
b
-boration of
strates [32]. In 2008, the first asymmetric catalysis for b
acyclic compounds was reported using a chiral Cu phosphine
complex [15], opening the path for innovative work such as
-boration of
a,
b
a,b
been catalyzed with Pt [5e8], Pd [9,10], Rh [11e14], Cu [15e41], Ni
[10,42,43], Fe [44], metal-free phosphines [45e47], and N-hetero-
asymmetric
development of chiral
b
-borations conducted in water [37,58e60] and the
b
-tetrasubstituted carbon centers using b,b-
cyclic carbenes (NHCs) [48e51]. Subsequently,
unsaturated carbonyl compounds has become very useful due to
the additional carbonyl contained in the product
[6,11,17e22,42,44,52,53]. Oxidation of the product provides access
to -hydroxy compounds as an alternative to the aldol reaction or
reduction of -keto carbonyl compounds. -Hydroxy carbonyl
compounds are found widely in natural products and pharma-
ceuticals [54,55]. Additionally, -boro-carbonyl compounds have
b
-boration of
a
,
b
-
substituted substrates [62e64]. Subsequently, other examples of
asymmetric catalysis have been reported with Ni [10], Pd [10], Rh
[11,12,14], and Cu [18,21,22,27e30,33e35,39,40,61,65,66]. Metal
free-catalysis has also been developed, including asymmetric var-
iants [47,49], such as a chiral NHC salt reported by Hoveyda
[49,50,67].
b
b
b
Despite many reports utilizing various transition metals, base
additives or high temperatures have been required for efficient
b
been reported as being effective therapeutic agents in cancer
treatment [56,57].
rates, and limitations to acyclic a,b-unsaturated carbonyl substrates
have been noted. In addition, very few cyclic products with boron-
substituted quaternary carbon centers have been reported [48]. A
catalyst that provides cyclic b-boro-carbonyl compounds contain-
ing quaternary carbon centers and can operate at room tempera-
ture without the need for an exogenous base provides a straight-
* Corresponding author.
E-mail address: khollis@chemistry.msstate.edu (T.K. Hollis).
http://dx.doi.org/10.1016/j.jorganchem.2015.11.010
0022-328X/© 2015 Elsevier B.V. All rights reserved.

S.W. Reilly et al. / Journal of Organometallic Chemistry 802 (2016) 32e38
33
forward methodology in the formation of CeB bonds as well as b-
hydroxy carbonyl compound derived thereof.
Previously, we reported a mixture of the CCC-NHC pincer Rh
complexes 1/2 (Scheme 1) that efficiently catalyzed 1,4-addition of
aryl boronic acids to electron-deficient alkenes [68]. CCC-NHC
pincer Rh complexes 1/2 (Scheme 1), along with added NHMe2,
produced excellent yields (>90%) with various Michael acceptors
and arylboronic acids in protic solvents [68]. In looking toward the
preparation of chiral variants, further attempts to optimize the
synthesis of 1 led to a crude isolate (A) that was orange in color and
was obtained reproducibly. We report herein, the characterization
of mixture A, separate isolation and screening of the components
and material A in the b-boration of cyclic and acyclic, a,b-unsatu-
rated, electron-deficient alkenes. Furthermore, this report is among
the few examples [69,70] in the literature of a pincer complex
catalyzing 1,4-addition reactions at room temperature.
2. Results and discussion
2.1. Characterization of catalytically-active material A
Fig. 1. Two sets of diastereotopic methylene peaks indicating a mixture of CCC-NHC
pincer Rh complexes 1 and 1C in A. The signal at 4.75 ppm is overlapping from both
complexes.
The previously reported methodology for the synthesis of the
CCC-NHC pincer Rh complexes was modified by switching the
solvent to THF (Scheme 1) and using a careful work up procedure as
detailed in the experimental procedure. This sequence reproduc-
ibly yielded an orange microcrystalline to powdery solid A that was
highly catalytically active. Spectroscopic data indicated the orange
mixture A consisted of two CCC-NHC pincer Rh complexes evi-
denced by two sets of diastereotopic, overlapping methylene sig-
nals (Fig. 1 and ESI) in the 1H NMR spectrum and two carbene
carbon signals coupled to Rh in the 13C NMR spectra (Fig. 2 and ESI).
Two RheCaromatic and RheCcarbene doublets (Fig. 2) are observed in
the 13C NMR spectrum with identical RheCcarbene and RheCaromatic
doublet coupling values (1J ¼ 39 Hz and 1J ¼ 29 Hz, respectively)
observed for A. These data indicated that the complexes were
similar.
A dark orange solid was isolated from mixture A using silica gel
chromatography. Spectroscopic data analysis of the isolated mate-
rial lead to the assignment of axial amine adduct 1 as the structure
[71], which has NMR peaks coincident with the major component
of mixture A. Additionally, an orange X-ray quality crystal of 1 was
grown by vapor diffusion (toluene/DCM) from mixture A (Fig. 3)
[72]. The molecular structure was observed to be a distorted octa-
hedron with the amine ligand in an axial position. The metric data
(bond distances and angles) are typical for the CCC-NHC pincer
ligand system Rh complexes.
Fig. 2. Two RheCaromatic and RheCcarbene doublets are observed in the 13C NMR
spectrum of A.
variant (1C). Spectroscopic data from the 13C NMR also supported
the assignment and similarities of the two species. Complex 1C was
then prepared independently by starting with the chloro-
imidazolium salt (X ¼ Cl, Scheme 1). It was analyzed to deter-
mine if the 1H and 13C NMR signals of this complex corresponded to
Further analysis of the exact mass ESI-TOF spectrum of A con-
tained a peak at m/z 537.0942 ([M ꢀ 2H]þ, corresponding to a
theoretical mass for C22H30N5Cl2Rh 537.0928) indicating a chloro
Scheme 1. Equilibrium of amine adduct and iodo-bridged dimer.

34
S.W. Reilly et al. / Journal of Organometallic Chemistry 802 (2016) 32e38
independently prepared 2:1 ratio of 1 and 1C were each examine
with cyclohexenone and 2.5 equiv of B2Pin2 (Table 1). Only a 72%
conversion to the borated product was observed in 1 h (entry 1)
with 1, compared to the 57% conversion observed in 1 h using the
chloro adduct 1C (entry 2). These conversions trends are analogous
to our recent report on the halogen effects of CCC-NHC Zr and Hf
pincer complexes in hydroamination/cyclization rates (I > Br > Cl)
[73]. A 2:1 mixture of isolated 1 and 1C provided the lowest con-
version despite extended reaction times (entry 3). Despite the
lower conversions of 1 and 1C compared to A (Table 1, entry 4),
these results are consistent to the report of Santos and co-workers,
detailing the increased rate effect upon addition of amine base in
boration of -unsaturated carbonyl compounds [36]. While no
exogenous based was added when using A in -boration catalysis,
b-
a,b
b
excess NHMe2 in freshly prepared A undoubtedly assists in accel-
erating the reaction rate. This would explain the decrease in cata-
lytic efficiency observed when using “aged” A (i.e. was stored for
several months). Using a five month old sample of A for the bora-
tion of cyclohexenone resulted in 83% conversion in 1 h (entry 5),
compared to Table 1, entry 4, which provided quantitative conver-
sion in 1 h with freshly prepared A.
Fig. 3. Molecular structure 2-(1,3-bis(N-butylimidazol-2-ylidene)phenylene) (dime-
thylamine)bis(iodo) rhodium(III) (1). The hydrogens are omitted for clarity. Thermal
ellipsoids are shown at 50% probability. Selected bond lengths (Å) and angles (ꢁ):
Rh(1)-C(1), 1.948(4); Rh(1)-C(10), 2.074(4); Rh(1)-C(7), 2.083(4); Rh(1)-N(5), 2.136(3);
Rh(1)-I(2), 2.6610(4); Rh(1)-I(1), 2.8555(4); C(1)-Rh(1)-C(10), 78.41(15); C(1)-Rh(1)-
C(7), 78.48(14); C(10)-Rh(1)-C(7), 156.61(14).
Due to the superior performance of mixture A, it was further
analyzed for catalytic activity in b-boration of acyclic and cyclic
substrates due to the quantitative conversion obtained with
cyclohexenone. Because high yields were observed previously for
Michael addition products in a H2O/MeOH solvent mixture [68],
the second set of signals observed in the NMR spectra of A. Indeed,
the spectrum for both 1 and 1C match the 1H and 13C NMR signals
observed for A (see ESI). For example, an integration value of 2 was
set for the multiplet at 5.20 ppm (Fig. 2), a signal observed for
diastereotopic methylene peaks, yet an integration value of 3.30 is
observed for the other diastereotopic methylene signal at 4.75 ppm.
This integral intensity is due to overlapping diastereotopic meth-
ylene peaks of 1 and 1C at 4.75 ppm, while the other diastereotopic
peak corresponding to the minor complex (1C), is slightly down-
field at 4.95 ppm. Two doublets of doublets were observed at 1.59
pre-catalyst loading optimization of A in b-boration reactions was
conducted in MeOH with trans-3-nonen-2-one and B2pin2
(Table 2). Although the material A is a mixture, 2 mg (~4 mol %)
were used in a catalytic trial. The molecular weight of amine
adduct 1 (722.98 g/mol) was used as a surrogate for the entire
sample to calculate pre-catalyst loading (mol %) for comparative
purposes. Initially, 1.5 equiv of B2pin2 were used with trans-3-
nonen-2-one, and a 34% conversion to the borated product was
observed (entry 1). Previously, using 2.5 equiv of aryl boronic acids
3
and 1.64 ppm with identical coupling values (dd, JHeH ¼ 1.2 Hz,
3JRheH ¼ 6.3 Hz) in the 1H NMR spectrum due to the unique het-
with
a,b-unsaturated carbonyl compounds in 1,4-additions gave
eronuclear coupling of NMR active 103Rh to coordinated NHMe2.
higher conversion. Therefore, the B2pin2 concentration was
increased to 2.5 equiv, and as expected, conversion to the borated
trans-3-nonen-2-one of >99% was observed in 1.5 h (entry 2). A
sample from this crude reaction mixture was analyzed by exact
mass spectrometry in order to determine possible decomposition
of A (see SI). However, no evidence was observed to suggest
regeneration of the imidazolium salts, which strongly suggests that
2.2. Catalytic activity
The activity of these Rh complexes was investigated in b-bora-
tion catalytic trials under optimized reaction conditions from our
previous 1,4-addition report [68]. Isolated A, 1, 1C, and an
Table 1
b
-boration with isolated CCC-NHC Rh complexes.a
Table 2
Optimization of b
-boration reaction conditions.a
Entry
Solvent
Cat. (mol %)
B2pin2 (equiv)
Time
Conv %b
1
2
3
4
5
6
7
8
9
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
A (4)
A (4)
1.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
1 h
1 h
1 h
1 h
1 h
1.5 h
1 h
1 h
1 h
34
>99
0
Entry
Catalyst
Conv %b
e
1
2
1
1C
72 (1 h)
57 (1 h)
>99 (5 h)
17 (1 h)
54 (5 h)
>99 (1 h)
83 (1 h)
[Rh(COD)CI]2 (4)
NHMe2 (4)
A (2)
A (4)
A (4)
9
<3
>99
60c
93
3
2:1 mixture of 1 and 1C
4
5
A
H2O
A (4)
>99
A-5 months old
a
Trans-3-nonen-2-one (0.0735 mmol), B2pin2, A (2.00 mg, ~2.94
m
mol), and
a
2-Cyclohexenone (0.0735 mmol), bis(pinacolato)diboron (0.184 mmol), cat.
solvent (0.700 mL) were added to a 1 dram vial with a magnetic stir bar, sonicated
(2.94 mmol), and 0.700 mL of MeOH were added to a 1 dram vial with a magnetic stir
for 1 min and stirred for 1 h at 22 ꢁC.
b
bar, stirred at 22 ꢁC.
Conversion was determined by GCeMS analysis.
Reaction was conducted at 0 ꢁC.
b
c
Conversion was determined by GCeMS analysis.

S.W. Reilly et al. / Journal of Organometallic Chemistry 802 (2016) 32e38
35
Table 3
Scope of acyclic substrate reactivity in the
b
-boration with B2pin2.a
Entry
1
Product
MeOH conv %b (yield %)c
EtOH conv %b (yield %)c
>99 (86)
>99 (81)
2
3
4
5
6
>99 (86)
96 (82)
81 (66)
0d
>99 (84)
84 (61)
49 (22)
0d
0d
0d
7
8
>99 (90)
>99 (93)
>99 (94)
>99 (96)
9
0d
0d
10
>99 (84)
>99 (83)
a
Acyclic a,b-unsaturated compound (0.0735 mmol), bis(pinacolato)diboron (0.184 mmol), A (2.00 mg, ~2.94 mmol), and 0.700 mL of MeOH were added to a 1 dram
vial with a magnetic stir bar, and stirred for 1 h at 22 ꢁC.
b
Conversion was determined by GCeMS analysis.
Isolated yield.
After 24 h reaction time.
c
d
ꢀ
generation of a “free carbene” as the active catalyst is not occurring
with these complexes. A control experiment without added A
(entry 3) resulted in no reaction. Evaluating 4 mol % [Rh(COD)Cl]2
dimer as a potential catalyst (entry 4) provided only 1 turnover (8-
atom mol % Rh and 9% yield) in 1 h, thus eliminating extraneous
starting material as the active catalyst. To demonstrate that the
amine base was not the active catalyst, 4 mol % of NHMe2 only
produced <3% conversion (entry 5). The catalyst loading was low-
ered to examine the efficiency of A (entries 6 vs. 2). At 2 mol %
(1 mg) pre-catalyst loading (entry 6), >99% conversion was
observed in 1.5 h. The reaction was also conducted at 0 ꢁC to
determine the efficiency of the catalyst below rt (22 ꢁC) (entry 7),
which resulted in a respectable 60% conversion in only 1 h. High
conversion was also observed using eco-friendly solvents, H2O
(>99%) and EtOH (>93%) (entries 8 and 9). These results are
consistent with previous work of Yun and co-workers [26] and
Fernandez and co-workers [46], in which both demonstrated an
enhanced rate of -boration of -unsaturated carbonyl com-
b
a,b
pounds with the use of alcohol additives such as MeOH. Moreover,
Santos clearly demonstrated, through detailed mechanistic studies,
the ability of the amine base to activate a nucleophilic water
molecule generating the sp2esp3 diboryl adduct that participates in
transmetalation [36]. Protic solvents MeOH and EtOH were used
throughout the screening of activated acyclic and cyclic alkenes to
evaluate the effect on the rate of conversion. Because >99% con-
version was observed in 1 h with 4 mol % of A (entry 2, Table 3), the
scope and limitations studies were conducted under these opti-
mized conditions.
Utilizing the optimized reaction conditions, activated acyclic
alkenes were examined (Table 3). Excellent conversions (>99%) of
the aldehyde substrates (crotonaldehyde and cinnamaldehyde,
entries 1 and 2) were observed in both MeOH and EtOH. trans-4-

36
S.W. Reilly et al. / Journal of Organometallic Chemistry 802 (2016) 32e38
Table 4
cycloheptenone (75%) in good yields [13]. Thus, cyclic enones were
evaluated under the optimized conditions with 4 mol % (2.00 mg)
of A, and the results are presented in Table 4.
Scope of cyclic substrate reactivity in the b
-boration with B2pin2.a
Conversions of >99% in just 1 h were observed for all three non-
substituted cyclic substrates (entries 1e3) in MeOH and EtOH. This
rate of conversion of cyclohexenone is comparable to catalytic re-
ports with Cu (99%, 1 h) [36] and NHC (91%, 1 h) [48] catalysts. The
rate of product formation with cycloheptenone (entry 3, >93%), in
all solvents, is also comparable or faster than the best previous
reports [13,48]. Only modest conversions were observed for the a-
Entry Product
1
MeOH conv %b (yield %)c EtOH conv %b (yield %)c
substituted, 2-methylcyclopent-2-en-1-one (entry 4) in 1 h, but
upon extended reaction times improved conversions to the borated
product were obtained. No significant diastereoselectivity was
observed in the reactions with 2-methylcyclopent-2-en-1-one
(entry 4). Excellent conversions have been reported for sterically
>99 (90), 1 h
>99 (70) 1 h
hindered b-substituted cyclic enones in 1 h reaction times with Cu
or NHC catalysts [36,48,49]. However, extended reaction times
were required when pre-catalyst A was employed for both 2-
methyl cyclopentenone (entry 5) and 2-methyl cyclohexenone
(entry 6) in EtOH and MeOH, resulting in only fair to good con-
versions. After 1 h, moderate conversion (70%) was observed when
using MeOH as a solvent for 3-methyl-2-cyclopentenone (entry 5),
while less was observed in EtOH (57%). A drastic reduction in
conversion rate was observed for 3-methyl-2-cyclohexenone (entry
6). This observation is consistent with Nishiyama [11] and co-
workers' proposed catalytic cycle, which suggests that the steric
congestion due to the methyl group hinders the boryl-insertion at
2
>99 (88), 1 h
>99 (91), 1 h
>99 (84) 1 h
3
4
94 (90) 1 h
75, 1 h (dr ¼ 1.1:1)
70, 1 h (dr ¼ 1.7:1)
93 (71), 24 h (dr ¼ 1:1)
80 (51), 24 h (dr ¼ 1.6:1)
the b-position.
3. Conclusions
In conclusion, the CCC-NHC pincer Rh complex mixture A
5
6
a
70, 1 h
86 (51), 24 h
57, 1 h
63 (40), 24 h
catalyzed the
b-boration of a,b-unsaturated activated alkenes at
room temperature producing tertiary and quaternary carbon cen-
ters along with examples of quantitative conversion in just 1 h. The
reported method does not require the exclusion of air or H2O due to
the stability of pre-catalyst A. To our knowledge, this is the first
40, 1 h
51 (40), 24 h
26, 1 h
38 (25), 24 h
report of
b-boration using a pincer complex catalyst to produce
tertiary and quaternary
b-substituted carbon centers at room
temperature using MeOH or EtOH as the solvent. While complexes
1 and 1C have been found to be the identity of the components in
pre-catalyst A, 1, 1C and a mixture thereof was not as active a
catalyst as the initial isolate A. CCC-NHC pincer amine Rh adduct 1
was characterized by X-ray Crystallography. The higher activity of
initially isolated A is attributed to the presence of dimethyl amine.
Cyclic
a,b-unsaturated compound (0.0735 mmol), bis(pinacolato)diboron
(0.184 mmol), A (2.00 mg, ~2.94 mmol), and 0.700 mL of MeOH were added to a 1
dram vial with a magnetic stir bar, stirred for 1 or 24 h at 22 ꢁC.
b
Conversion was determined by GCeMS analysis.
c
Isolated yield.
4. Experimental section
Phenyl-3-buten-2-one (entry 3) and chalcone (entry 4) had the
least conversion in 1 h. Due to the lack of reported borated products
4.1. Preparation of pre-catalyst A
containing nitro functionalities, trans-b-nitro styrene (entry 5) and
The salt, 3-bis (1-butylimidazolium-3-yl) benzene iodide
(0.150 g, 0.260 mmol), and tetrakis(dimethylamido)zirconium
(0.173 g, 0.650 mmol) were weighed in under inert atmosphere and
dry THF (15 mL) was added into a 50 mL flask. The mixture was
stirred at room temperature for approximately 1 h or until a light
yellow homogenous solution was obtained. Once the reaction
became homogeneous, [Rh(COD)Cl]2 (0.128 g, 0.260 mmol) was
added to the flask and stirring continued at room temperature for
another 12 h. Deionized water (1 mL) was added, and it was stirred
vigorously for 10 min providing a white precipitate and a bright
orange supernatant. The orange supernatant was filtered and the
solvent was removed under reduced pressure yielding a dark or-
ange solid. The solid was washed with hexanes (25 mL) and ether
(25 mL) and dried to obtain dark orange/red solid (0.128 g, 68%).
Orange X-ray quality crystals were grown by vapor diffusion
1-(dimethylamino)-2-nitroethylene (entry 6) were both screened.
No product was observed for any of the nitro-containing substrates
despite extended reaction time of 24 h. Ethyl acrylate (entry 7)
exhibited excellent conversion (>99%) in both MeOH and EtOH.
Methyl vinyl ketone (entry 8) was another acyclic substrate to show
high conversion in MeOH and EtOH. Acrylonitrile (entry 9) was
evaluated but no product was observed. Boration of N,N-dimethy-
lacrylamide (entry 10) was successful (>99% conversion) in both
MeOH and EtOH.
Previously, little success has been reported in borating cyclic
enones with Rh catalysts. Reports include Rh(Phebox) [11] with 0%
conversion of cyclohexenone and Wilkinson's catalyst with
extended reactions times (12e13 h) and elevated temperature
(80 ꢁC) for the boration of cyclohexenone (78%) and

S.W. Reilly et al. / Journal of Organometallic Chemistry 802 (2016) 32e38
37
(toluene/DCM). Catalyst A mixture (two components: ratio ~2:1) 1H
NMR (600 MHz, CDCl3): 7.58e7.57 (m, 2.9H), 7.15e7.11 (m, 4.9H),
yellow homogenous solution was obtained. Once the reaction
became homogeneous, [Rh(COD)Cl]2 (0.031 g, 0.0632 mmol) was
added to the reaction mixture, and it was maintained at room
temperature for another 12 h. The reaction mixture was then
exposed to air for 10 min during which time a white precipitate
formed leaving a bright orange supernatant. The orange superna-
tant was filtered and the solvent was removed under reduced
pressure yielding a dark orange solid (0.028 g). A column was
packed with 230e400 mesh silica gel using CH2Cl2. The column
was initially eluted using CH2Cl2followed by 2:98 (MeOH: CH2Cl2)
to the fractions were combined and concentrated yielding a dark
orange solid (0.013 g, 38%), which had an Rf of 0.18. 1H NMR
d
7.10e7.04 (m, 3H), 5.25e5.21 (m, 2H), 4.97e4.94 (m, 1.1H),
4.79e4.74 (m, 3.1H), 3.72 (m, 1H), 3.69 (m, 0.6H), 2.05e1.99 (m,
5.9H), 1.65e1.63 (dd, 3J HeH ¼ 1.2 Hz, 3J RheH ¼ 6.3 Hz, 3.1H),
1.61e1.56 (dd, 3J HeH ¼ 1.2 Hz, 3J RheH ¼ 6.3 Hz, 5.4H), 1.53e1.50
(m, 6.3H), 0.99e0.96 (m, 9H). 13C {1H} (150 MHz, CDCl3):
d 180.6 (d,
J ¼ 39 Hz, RheCcarbene), 178.6 (d, J ¼ 39 Hz, RheCcarbene), 155.9 (d,
J ¼ 29 Hz, RheCaromatic), 154.1 (d, J ¼ 29 Hz, RheCaromatic), 145.8,
145.5, 124.0, 123.7, 121.6, 121.1, 115.8, 115.5, 108.6, 108.5, 51.7, 49.8,
44.6, 44.5, 33.6, 33.2, 20.1, 19.9, 14.1, 14.0. HRMS-ESI (m/z): [M ꢀ I]þ
calcd
for
C22H32IN5Rh,
596.0752;
found,
596.0740;
[M ꢀ I ꢀ NHMe2]þ calcd for C20H25IN4Rh, 551.0173; found,
(600 MHz, CDCl3):
d
7.58 (d, J ¼ 2.1 Hz, 2H), 7.19 (m, 1H), 7.10 (d,
551.0163;
calcd
for
C
20H25I2N4RhNa
J ¼ 7.7 Hz, 2H), 7.06 (d, J ¼ 2.1 Hz, 2H), 5.08e5.03 (m, 2H), 4.87e4.82
[M ꢀ NHMe2 þ Na]þ ¼ 700.9116, found 700.9110; calcd for
(m, 2H), 3.73 (s, br,1H, HNeH), 2.04e2.00 (m, 2H),1.98e1.94 (m, 2H),
3
3
C
22H30Cl2N5Rh [M ꢀ 2H]þ ¼ 537.0928, found 537.0942.
1.70e1.69 (dd, JHeH ¼ 1.2 Hz, JRheH ¼ 6.3 Hz, 6H), 1.58e1.54 (m,
4H), 1.03e1.01 (t, J ¼ 7.2 Hz, 6H). 13C {1H} (150 MHz, CDCl3):
d 181.1
4.2. Isolation of 2-((1,3-bis(N-butylimidazol-2-ylidene)phenylene)
(dimethylamido)bis(iodo)) rhodium(III) (1)
(d, J ¼ 39 Hz, CcarbeneeRh), 155.7 (d, J ¼ 29 Hz, CaromaticeRh), 146.1,
124.0, 121.0, 115.8, 108.8, 50.0, 45.7, 33.8, 20.2, 14.1. HRMS (ESI):
Calculated for C22H32ClN5Rh [M ꢀ Cl þ H]þ ¼ 504.1396, found
504.1380; Calculated for C22H33ClN5Rh [M-Cl þ Na]þ ¼ 528.1372,
found 528.1831.
A column was packed with 230e400 mesh silica gel using 1:1
(hexane: DCM). The column was initially eluted using 100% DCM
followed by 2:98 (ethyl acetate:DCM) to isolate a dark orange solid,
which had an Rf of 0.15. Isolated yield of the dark orange solid was
4.5. General procedure for catalyses
52 mg (26% yield). 1H NMR (600 MHz, CDCl3):
d
7.56 (d, J ¼ 2.1 Hz,
2H), 7.17 (m, 1H), 7.11 (d, J ¼ 7.7 Hz, 2H), 7.05 (d, J ¼ 2.1 Hz, 2H),
5.24e5.21 (m, 2H), 4.76e4.71 (m, 2H), 3.75 (m, 1H, HNeH),
A 1 dram vial containing a magnetic stir bar was charged with
a,
b-unsaturated compound (0.0735 mmol), bis(pinacolato)diboron
3
3
2.01e1.99 (m, 4H), 1.62e1.59 (dd, JHeH ¼ 1.2 Hz, JRheH ¼ 6.3 Hz,
(0.184 mmol), pre-catalyst A (2.00 mg, 2.94 mol), and solvent
m
6H), 1.58e1.55 (m, 4H), 0.99e0.97 (t, J ¼ 7.2 Hz, 6H). 13C {1H}
(0.700 mL). The vial was closed and stirred vigorously for 1 h at
22 ꢁC during which time a dark precipitate was observed to form.
(150 MHz, CDCl3):
d
178.9 (d, J ¼ 39 Hz, CcarbeneeRh), 156.2 (d,
J ¼ 29 Hz, CaromaticeRh), 145.7, 124.1, 121.7, 115.5, 108.7, 51.9, 44.6,
After 1 h, a 10 mL aliquot was passed through a short plug of celite
33.8, 20.0, 14.2. HRMS (ESI): Calculated for
C22H32IN5Rh
and a 0.45 um PTFE filter. It was injected into the GCeMS to
determine conversion. The crude reaction mixture was then filtered
through a plug of celite. Volatiles were then removed under
reduced pressure resulting in an oil. 1H NMR and HRMS was taken
of the crude product to confirm the borylated product. The crude
product was then purified by silica gel chromatography to deter-
mine an isolated yield.
[M ꢀ I þ H]þ ¼ 596.0752, found 596.0740; Calculated for
C
20H25IN4Rh [M ꢀ NHMe2, I]þ ¼ 551.0173, found 551.0163.
4.3. Structure solution and refinement of 2-((1,3-bis(N-
butylimidazol-2-ylidene)phenylene) (dimethylamido)bis(iodo))
rhodium(III) (1)
The structure was solved using ShelXS [74] and refined using
ShelXL [75] full-matrix least-squares refinement. Molecular draw-
ings and reports were generated using Olex2 [76]. The non-H atoms
were refined with anisotropic thermal parameters and all of the H
atoms were calculated in idealized positions and refined riding on
their parent atoms. In the final cycle of refinement 22,210 re-
4.6. Catalysis using a 2:1 mixture of 1 and 1C
A 2:1 ratio of 2-((1,3-bis(N-butylimidazol-2-ylidene)phenylene)
(dimethylamido)bis(iodo)) rhodium(III) (1), and 2-((1,3-bis(N-
butylimidazol-2-ylidene)phenylene) (dimethylamido)bis(chloro))
rhodium(III) (1C) (2.94
mmol total) were combined in a 1 dram vial
flections (of which 6172 are observed with I > 2s(I)) were used to
with -unsaturated compound (0.0735 mmol), bis(pinacolato)
a,b
refine 275 parameters and the resulting R1, wR2 and S (goodness of
fit) were 3.01%, 6.57%, 1.051, respectively. The refinement was car-
ried out by minimizing the wR2 function using F2 rather than F
values. R1 is calculated to provide a reference to the conventional R
value but its function is not minimized. All non-hydrogen atoms
were refined with anisotropic thermal displacement parameters.
Unless otherwise noted, hydrogen atoms were included in calcu-
lated positions. Thermal parameters for the hydrogens were tied to
the isotropic thermal parameter of the atom to which they are
bonded (1.5 ꢂ for methyl, 1.2 ꢂ for all others).
diboron (0.184 mmol), and solvent (0.700 mL). The general proce-
dure for catalyses was then followed.
Acknowledgments
Mississippi State University is gratefully acknowledged for
financial support. S.W.R. and H.K.B. also acknowledge the Depart-
ment of Education (GAANN-P200A120066) for fellowship support.
Appendix A. Supplementary data
4.4. Synthesis of 2-((1,3-bis(N-butylimidazol-2-ylidene)phenylene)
(dimethylamido)bis(chloro)) rhodium(III) (1C)
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2015.11.010.
The salt, 1,3-bis(1-butylimidazolium-3-yl)benzene chloride
(0.025 g, 0.0632 mmol), and tetrakis(dimethylamido)zirconium
(0.018 g, 0.0695 mmol) were weighed under an inert atmosphere
and combined with CD2Cl2 (0.8 mL) in a NMR tube. The mixture
was kept at room temperature for approximately 1 h until a light
Author contributions
SWR and GKA contributed equally to the experimental data
necessary for this work to be published.

38
S.W. Reilly et al. / Journal of Organometallic Chemistry 802 (2016) 32e38
ꢀ
[38] C. Sole, A. Bonet, A.H.M. de Vries, J.G. de Vries, L. Lefort, H. Gulyas,
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