Copper-NHC-Mediated Semihydrogenation and Hydroboration of Alkynes: Enhanced Catalytic Activity Using Ring-Expanded Carbenes

Article abstract of DOI:10.1021/acs.organomet.8b00467

A series of two-coordinate copper tert-butoxide complexes bearing five-, six-, and seven-membered ring N-heterocyclic carbenes, prepared by protonolysis of (NHC)CuMes with ^tBuOH, have been used as catalytic precursors in the semihydrogenation of alkynes with silanes/^tBuOH and the hydroboration of alkynes with HBPin. Both processes proceed with high regioselectivity and show enhancements with six- and seven-membered ring carbenes.

Full text of DOI:10.1021/acs.organomet.8b00467

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Copper-NHC-Mediated Semihydrogenation and Hydroboration of
Alkynes: Enhanced Catalytic Activity Using Ring-Expanded Carbenes
Jonathan W. Hall, Darcy M. L. Unson, Paul Brunel, Lee R. Collins, Mateusz K. Cybulski, Mary F. Mahon,
and Michael K. Whittlesey*
Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
S
* Supporting Information
ABSTRACT: A series of two-coordinate copper tert-butoxide complexes
bearing five-, six-, and seven-membered ring N-heterocyclic carbenes, prepared
t
by protonolysis of (NHC)CuMes with BuOH, have been used as catalytic
precursors in the semihydrogenation of alkynes with silanes/tBuOH and the
hydroboration of alkynes with HBPin. Both processes proceed with high
regioselectivity and show enhancements with six- and seven-membered ring
carbenes.
t
of PhCCMe (1) and BuOH at room temperature led to
INTRODUCTION
■
semihydrogenation of the alkyne to give just the Z-isomer of
PhCHCHMe (2, Scheme 1). A likely pathway for the
Manipulation of the stereoelectronic properties of a ligand to
enhance activity within a metal coordination sphere is the
name of the game in homogeneous catalysis. In the case of
Scheme 1. Semihydrogenation of PhCCMe (1) by (6-
heterocyclic carbene ligands, this has driven the development
Mes)CuOtBu (I)
of alternatives to the original1 Arduengo-type unsaturated/
saturated five-membered ring N-heterocyclic carbenes
(NHCs),2,3 in particular, cyclic alkyl amino carbenes
(CAACs),4 abnormal/mesoionic carbenes,5 and bisoxazoline-
derived NHCs.6 An additional class of carbene ligands that has
received some attention in recent years is one based on ring
sizes of >5.7 Often referred to as ring-expanded carbenes or
RE-NHCs, these offer enhanced donor capabilities and an
increased steric profile in comparison to that of their five-
membered ring counterparts,8 features that should help to
facilitate key processes in catalytic cycles including oxidative
addition (electronic enhancement) and reductive elimination
(steric enhancement). Moreover, the wide N−C−N angle that
is a particular feature of RE-NHCs results in the N-substituted
wingtips being pushed down toward the metal, stabilizing low
coordination numbers and/or unusual oxidation states.9 As a
result, there may be further enhancement of a desired catalytic
pathway or, alternatively, the advent of new processes.
The use of metal RE-NHC complexes in catalysis is still
relatively limited and has for the most part focused on Pd,10
together with a smaller number of reports employing Ru,11
Rh,12 Ir,13 Ni,14 Pt,15 Cu,16 and Au.17 The paucity of work is
somewhat surprising given that, in the vast majority of direct
comparisons between RE-NHCs (most commonly, six- and
seven-membered ring systems) and imidazolidin-2-ylidenes, a
beneficial effect is observed.
reaction involves syn-hydrocupration of the alkyne to afford a
Cu-alkenyl species, followed by protonolysis by the alcohol.
Circumstantial support for this mechanism was provided by
the isolation and structural characterization of alkenyl complex
II from a stoichiometric reaction of I, Et3SiH, and 1.
Recent years have witnessed a number of reports on
homogeneous alkyne semihydrogenation catalysts, with the
aim being to develop systems that alleviate some of the issues
of selectivity and over-reduction associated with the well-
known commercially used heterogeneous Lindlar catalyst.21
Much of the focus has been on cheap and sustainable metals,
including Mn,22 Fe,23 Co,24 and Ni.25 Copper complexes26
have proved very versatile in preventing the over-reduction of
typically challenging substrates such as aryl- and diaryl-
substituted alkynes, especially ones containing functional
groups. Encouraged by our initial findings above, as well as
the reports by Tsuji and Lalic with the five-membered ring
NHC precursor (IPr)CuOtBu,26a,b we now describe a catalytic
investigation with a series of (NHC)CuOtBu complexes that
We have been interested in the reactivity of RE-NHCs in
conjunction with the first-row metals Ni14a,b,d,9a,18 and Cu.19
During efforts to prepare a hitherto unknown mononuclear
copper hydride complex, (6-Mes)CuH,19a,20 through reaction
of the tert-butoxide complex (6-Mes)CuOtBu (I) with silanes,
we found that the combination of I and Et3SiH in the presence
Received: July 5, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00467
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Article
differ both in ring size (five-, six-, and seven-membered ring
carbenes) and N-substituents. In the course of investigating the
influence of other variables (reductant, substrate, and alcohol)
on catalytic activity, we have established that this same range of
(NHC)CuOtBu complexes catalyze the α-hydroboration of
internal alkynes. In both catalytic processes, the presence of
RE-NHC ligands leads to activity higher than that found with
the five-membered ring analogues.
Cu−O distances and the N−C−N bond angle across the series
of tert-butoxide complexes shown in Scheme 2. A further
comparison of the different NHCs was provided by
calculations of the percentage buried volume (%Vbur) using
the SambVca program.28 There is a fairly steady increase in
steric profile of the six-membered NHC ligands on moving
from o-tolyl (IV, entry 3) to mesityl (I, entry 1) to Dipp (VI,
entry 4) substituents, whereas the increase in ring size upon
moving from 6-Mes to 7-Mes (entries 1 and 2) brings about
only a small increase.
RESULTS AND DISCUSSION
The preliminary catalytic study shown in Scheme 1 employed
5 mol % of I, 1.1 equiv of Et3SiH, and 1.1 equiv of BuOH.
■
The activity of I versus that of III−VIII for the
semireduction of 2 with PHMS is summarized in Table 3.
Entries 1, 2, and 6 reveal a clear trend in decreasing product
yield as a function of ring size in the order 6 > 5 > 7 for N-
mesityl-substituted carbenes. These differences are highlighted
more clearly at a lower Cu loading (entries 8−10). The results
also reveal a dependency on the N-substituent, with product
yield decreasing in the order N-mesityl > N-xylyl ≫ N-o-tolyl
≫ N-Dipp (entries 1 and 3−5). This suggests there is a
“Goldilocks” effect in which substituents are either too small
(tolyl) or too large (Dipp) to be effective, whereas the 2,6-
dimethylphenyl substitution pattern present in both I and V is
“just right” to be effective. Interestingly, studies with the 6-
Dipp complex VI revealed that this effectively inactive complex
19a
t
Under these conditions, the conversion of PhCCMe (1) to
(Z)-PhCHCHMe (2) was very slow, requiring almost 2
weeks to go to completion at room temperature. The reaction
time could be cut to 17 h by increasing the temperature to 60
°C, although there was now a slight loss of selectivity (96:4 Z/
E). No evidence for over-reduction to alkane was found.
In a number of instances, Et3SiH has been shown to be a
relatively unreactive silane for semireduction reactions.23b,26a,b
Indeed, upon trialling a range of other trialkyl and mixed aryl/
alkyl silanes for the semireduction of 1 (Table 1, entries 1−5),
Table 1. Catalytic Semihydrogenation of PhCCMe by 1 as
t
a
could be “activated” to some extent by changing from BuOH
a Function of Silane
i
to EtOH or PrOH, which afforded product yields of 52 and
31%, respectively, after 2 h (2 mol % loading).29 Activity
increased further with iPrOH over a longer reaction time (73%
after 48 h) but was unchanged with EtOH. Such variations
could reflect the relative stabilities of the alkoxide complexes
(6-Dipp)CuOiPr and (6-Dipp)CuOEt in their own right,30
their susceptibility to react with silane to yield [(6-Dipp)-
CuH]2, as well as the ease of protonolysis of the alkenyl
intermediate. The tert-butoxide precursors were subsequently
tested in the semihydrogenation of PhCCPh (3), as well as
the terminal alkynes 1-hexyne (4), and PhCCH (5). For 3,
there was a change in order of activity (Scheme 3), with the
SIMes complex VII now the most active. I and III were the
least effective of the copper precursors, at least in part as a
result of the limited solubility of the alkenyl complexes (6/7-
Mes)CuC(Ph)CH(Ph), which were deposited as fine yellow
powders during the course of catalytic runs.31 The 6-Mes
derivative (6-Mes)CuC(Ph)CH(Ph) (IX) was isolated and
structurally characterized (Supporting Information). With the
terminal alkynes 4 and 5, no precipitation took place, and as a
result, the performance of I and III improved (Scheme 3).
Similar activity was found for all of the copper complexes, with
the exception of IV, suggesting again that the N-ortho-tolyl
group is simply too small to be effective.
Further studies (Scheme 4) with a low loading of I (0.5 mol
%) were probed to discriminate the reactivity of different
internal alkynes. Replacement of the Me group in 1 by a
trimethylsilyl group (10) shut down reactivity, whereas a butyl
group afforded good product yields with both electron-
donating and -withdrawing substituents on the aryl ring
(11−14). Dialkyl-substituted alkynes (16, 17) proved reactive
in the short (3 h) term but underwent no further conversion
over 24 h. The same behavior was observed upon
incorporation of functionalized substituents onto an aryl ring
(15).32
b
entry
silane
I (mol %)
yield of 2 (%)
1
2
3
4
5
EtMe2SiH
Me2PhSiH
MePh2SiH
Ph3SiH
Ph3SiH
PHMS
1
1
1
1
5
1
trace
5
34
2
57
85
c
6
a
Conditions: 1 (0.22 mmol), I (1 or 5 mol %), 1.1 equiv of silane, 1.1
equiv of BuOH, C6D6 (0.5 mL), 25 °C, 2 h. Yield of 2 determined
by H NMR spectroscopy using 1,3,5-(MeO)3C6H3 as an internal
b
t
1
c
standard. Reaction time of 1 h.
we observed improved activity in all cases, as evidenced by
product formation within 2 h, even at a lowered catalyst
loading of 1 mol %. Polymethylhydrosiloxane (PHMS) proved
to be particularly active (entry 6), generating an 85% yield of
product within only 1 h.
Investigation of the reaction as a function of NHC ligand led
us to prepare a number of new (RE-NHC)CuOtBu complexes
using the method employed previously for I, namely,
incorporation of free RE-NHC into polynuclear [Cu(Mes)]n
to give (RE-NHC)CuMes (Supporting Information), followed
t
by protonolysis with BuOH. This afforded the new 7-Mes, 6-
o-Tol, and 6-xylyl derivatives III−V as highly air-sensitive,
cream-colored solids in yields of 17−93%. To broaden the
scope of the (NHC)CuOtBu complexes for investigation, the
same synthetic approach was used to make the known 6-Dipp
analogue VI,20d as well as the known five-membered ring
SIMes and IPr derivatives VII27 and VIII.20a
The X-ray structures of III and IV are shown in Figure 1. As
VI and VII were only previously characterized by NMR
spectroscopy,18,27 they too were isolated for structural analysis
Catalytic Hydroboration of Alkynes. Given the central
role of [(NHC)CuH]2 in the catalytic semihydrogenation, the
activity of I was tested with other E−H substrates besides
(Figure 1). Table 2 shows a comparison of the Cu−CNHC
/
B
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Article
Figure 1. Molecular structures of III, IV, VI, and VII, with ellipsoids shown at the 30% probability level. In all cases, hydrogen atoms have been
omitted for clarity. Also omitted for IV is the minor disordered component of C11. For VI, only one of the two molecules present in the
asymmetric unit is shown.
Table 2. Structural Comparison of (NHC)CuOtBu Complexes
a
entry
(NHC)CuOtBu
r(Cu−CNHC) (Å)
r(Cu−O) (Å)
N−C−N (deg)
%Vbur
1
2
3
4
5
6
I
III
IV
VI
VII
VIII20a
1.874(2)
1.8818(14)
1.881(5)
1.885(3)
1.844(3)
1.8641(18)
1.8016(15)
1.8032(10)
1.809(3)
1.808(2)
1.788(2)
117.65(18)
119.51(13)
117.3(4)
116.8(3)
107.8(3)
44.0 (38.6)
45.4 (40.2)
38.1 (33.5)
52.0 (46.4)
38.2 (32.9)
44.6 (39.1)
1.8104(13)
103.42(15)
a
Value calculated at M−CNHC distance of 2.0 Å and, in parentheses, 2.28 Å. Parameters used: Bondi radii scaled by 1.17, a 3.5 Å sphere radius, 0.1
exhaustiveness and excluding hydrogens.
Indeed, a low-temperature 1H NMR spectrum following
addition of HBPin to I confirmed generation of [(6-
Mes)CuH]2.20d,36 As with semihydrogenation, the use of Cu
catalysts for hydroboration has received considerable interest.
Although many of the studies (including some with NHC
ligands) have focused on terminal alkynes,37 there are far fewer
examples of Cu-catalyzed hydroboration of more challenging
internal alkynes.38
Scheme 2. Synthesis of (NHC)CuOtBu Complexes
Using 1 once again as a test substrate, I was shown to retain
both high activity and high regioselectivity even at catalyst
loadings as low as 0.2 mol % (Table 4, entries 1−3). As
expected, given the proposed hydrocupration pathway, the 6-
Mes alkenyl complex II exhibited similarly high activity
compared to that of I (entry 4). In comparison to I, both IV
and VI again gave poor yields. In addition, both of the five-
membered ring NHC complexes VII and VIII now also
showed poor activity, whereas the 7-Mes complex III proved
to be the most active (entry 5).
silanes (Scheme 1). Although NH3·BH3 gave very little
conversion of 1,22 HBPin proved to be reactive, although to
bring about quantitative hydroboration rather than semi-
hydrogenation (Table 4).33 The reaction was highly
regioselective, giving a 96:4 ratio of α- and β-vinylboronate
products 27 and 28.34,35 The preferential formation of the α-
product 27 is consistent with a hydrocupration pathway.33
C
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Article
Table 3. Catalytic Semihydrogenation of PhCCMe by
Scheme 4. Substrate Scope for Semihydrogenation with I,
Showing Conversions after 3 h and (in Parentheses) 24 h
a
a
(NHC)CuOtBu
b
entry
(NHC)CuOtBu
yield of 2 (%)
1
2
3
4
5
6
7
8
9
10
I
94
82
22
87
trace
85
33
86
66
76
III
IV
V
VI
VII
VIII
I
c
c
III
VII
c
a
Conditions: 1 (0.22 mmol), (NHC)CuOtBu (5 mol %), 1.1 equiv of
b
t
PHMS, 1.1 equiv of BuOH, C6D6 (0.5 mL), 25 °C, 2 h. Yield of 2
determined by 1H NMR spectroscopy using 1,3,5-(MeO)3C6H3 as an
c
internal standard. 2 mol % (NHC)CuOtBu.
a
Conditions: RCCR′ (0.22 mmol), I (0.5 mol %), 2 equiv of
t
Scheme 3. Semihydrogenation Activity of (NHC)CuOtBu as
PHMS, 2 equiv of BuOH, C6D6 (0.5 mL), 25 °C. Product yields
determined by 1H NMR spectroscopy using 1,3,5-(MeO)3C6H3 as an
internal standard.
a
a Function of Alkyne
Table 4. (NHC)CuOtBu-Catalyzed Hydroboration of
a
PhCCMe (1) with HBPin
(NHC)CuOtBu
(mol %)
yield of 27 + 28
27:28
b
b
entry
t (h)
(%)
ratio
1
2
3
I (1)
I (0.5)
I (0.2)
II (0.2)
III (0.2)
IV (0.2)
VI (0.2)
VII (0.2)
VIII (0.2)
1
2
3
3
3
3
3
3
3
97
64
59
77
89
2
44
13
21
96:4
96:4
96:4
96:4
96:4
a
c
4
Conditions as given in Table 3.
5
6
7
8
9
A preliminary probe of the activity of I toward a series of
internal alkynes with different substituents (Scheme 5)
revealed similarities to the results of semihydrogenation, such
as reduced activity of ArCCSiMe3 relative to ArCCMe
and ArCC(butyl).
63:37
96:4
78:22
a
Conditions: (NHC)CuOtBu, 1.1 equiv of HBPin, C6H6 (0.5 mL),
room temperature. Values are the average of two runs. Determined
by GC using 1,3,5-(MeO)3C6H3 as an internal standard. II = (6-
Mes)CuC(Ph)CH(Me).
b
A recent DFT study of (NHC)Cu-catalyzed alkyne
hydroboration39 showed that the rate-determining step in the
formation of α-hydroboration products was the initial
interaction of the alkyne with (NHC)CuH. When HBPin
was added to (6-Mes)CuC(Ph)CH(Me) (II) at 196 K and
c
Scheme 5. Yield and α (29−32) and β (33−36) Ratio (in
Parentheses) of Hydroboration Products Catalyzed by I
1
the reaction monitored at 222 K by H NMR spectroscopy,
[(6-Mes)CuH]2 had already formed, consistent with the high
reactivity of the intermediate formed upon interaction of the
borane with the alkenyl complex that makes it too short-lived
to detect even at low temperature.
SUMMARY AND CONCLUSIONS
■
A series of (NHC)CuOtBu complexes bearing five-, six-, and
seven-membered ring N-heterocyclic carbene ligands have
been prepared and investigated as precursors for the
semihydrogenation and hydroboration of alkynes with silanes
and pinacolborane, respectively. Both reactions take place with
D
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was recrystallized from THF/hexane to yield V as an off-white solid.
Yield 122 mg (23%). 1H NMR (500 MHz, C6D6): δ 6.99 (t, JHH = 7.7
Hz, 2H, Ar), 6.92 (d, JHH = 7.7 Hz, 4H, Ar), 2.41 (m, 4H, NCH2),
2.13 (s, 12H, CH3), 1.28 (m, 2H, NCH2CH2), 1.21 (s, 9H,
C(CH3)3). 13C{1H} NMR (126 MHz, C6D6): δ 202.7 (s, NCN),
145.2 (s), 135.1 (s), 129.4 (s), 68.4 (s), 43.6 (s), 37.0 (s), 20.7 (s),
18.1 (s). Anal. Calcd for C24H33N2OCu: C, 67.18; H, 7.75; N, 6.53;
Found: C, 66.78; H, 7.75; N, 6.55.
high selectivity to afford (Z)-alkenes and α-hydroboration
products, respectively, through a common hydrocupration
pathway.
The enhancement of catalysis by six- and seven-membered
ring NHCs compared to their five-membered ring counterparts
is the major finding of these studies. Moreover, we have also
observed that 2,6-dimethylphenyl-derived N-substituents on
the NHC are optimal for catalysis. Additional studies of
(NHC)Cu-catalyzed reactions are underway in our group to
probe the generality of ring-expanded NHC enhancements in
catalysis.
(6-Dipp)CuOtBu (VI). Compound VI was prepared as for III by
reaction of 6-Dipp (prepared in situ from [6-Dipp·H]Br (300 mg,
0.62 mmol) and KHMDS (136 mg, 0.68 mmol)) with Cu(Mes)n
t
(113 mg, 0.62 mmol) followed by reaction with BuOH to afford VI
as an off-white solid. Crystalline material was obtained from benzene/
hexane. Yield 110 mg (33%). NMR data matched those in the
literature.20d
EXPERIMENTAL SECTION
■
(SIMes)CuOtBu (VII). Compound VII was prepared as for III by
reaction of SIMes (made in situ from [SIMes·H]Cl (563 mg, 1.64
mmol) and KHMDS (330 mg, 1.6 mmol)) with Cu(Mes)n (300 mg,
All manipulations were carried out using standard Schlenk, high-
vacuum and glovebox techniques. Solvents were purified using an
MBraun SPS solvent system (hexane, pentane, Et2O) or under a
nitrogen atmosphere from sodium benzophenone ketyl (benzene,
THF). C6D6 and THF-d8 were vacuum transferred from potassium.
NMR spectra were recorded at 298 K (unless otherwise stated) on
Bruker Avance 400/500 MHz and Agilent 500 MHz NMR
spectrometers and referenced to solvent signals as follows: benzene
(1H, δ 7.16; 13C{1H}, δ 128.0), THF (1H, δ 3.58; 13C{1H}, δ 67.2;
19F, externally to CFCl3 (δ 0.0). High-resolution mass spectrometry
was conducted using a MaXis HD quadrupole APCI time-of-flight
(APCI-QTOF) mass spectrometer (Bruker) with analyses performed
in APCI positive mode. GC−MS analysis was performed on an
Agilent 7890A gas chromatograph. Elemental analyses were
performed by Elemental Microanalysis Ltd., Okehampton, Devon,
UK. Literature methods were used for the preparation of Cu(Mes)n,40
(6-Mes)CuOtBu (I),19a 6-o-Tol,7 6-Xylyl,7 6-Dipp,7 and 7-Mes.7
(7-Mes)CuOtBu (III). 7-Mes (141 mg, 0.42 mmol) and Cu(Mes)n
(77 mg, 0.42 mmol) were combined in benzene (5 mL) in a J Young
resealable ampule and stirred for 1 h. The resulting solution was
concentrated, and pentane was added to yield a near colorless
precipitate of (7-Mes)CuMes. The solid was isolated by cannula
filtration and dried under vacuum. It was then dissolved in benzene (5
t
1.64 mmol) followed by reaction with BuOH to give VII as an off-
white solid. Crystalline material was obtained from toluene/pentane
at −30 °C. Yield 520 mg (72%). 1H NMR data matched those in the
literature.27 13C{1H} NMR (126 MHz, C6D6): δ 204.9 (s, NCN),
138.3 (s), 136.1 (s), 135.8 (s), 129.9 (s), 68.0 (s), 50.4 (s), 34.4 (s),
21.0 (s), 18.0 (s). The extreme air sensitivity of VII precluded all
attempts to measure CHN microanalysis data.
(IPr)CuOtBu (VIII). Compound VIII was prepared as for III by
reaction of IPr (400 mg, 1.03 mmol) with Cu(Mes)n (188 mg, 1.03
mmol) in C6H6 (5 mL), followed by reaction with tBuOH (1 mL) in
C6H6 to give VIII as an off-white solid. Yield 268 mg (49%). 1H NMR
data matched those in the literature.41
Typical Catalytic Procedures. Alkyne Semihydrogenation: To
a flame-dried J. Youngs NMR tube were added 1-phenylpropyne
(0.22 mmol), PMHS (0.24 mmol), 1,3,5-(MeO)3C6H3 (0.022 mmol,
internal standard), and 500 μL of 2.2 × 10−3 M C6D6 stock solution
t
of I. Catalysis was initiated by addition of BuOH (0.24 mmol) and
reactions followed by 1H NMR spectroscopy. After the desired
reaction time, solutions were filtered through a silica plug (hexane as
eluent), and the filtrate was reduced to dryness using a flow of N2.
The residue was dissolved in minimal CHCl3 and washed through a
silica plug with hexane to afford alkene products.
t
mL), and BuOH (1 mL) was added. After being stirred for 2 h at
room temperature, the solution was reduced to dryness, and the
residue was redissolved in a minimum amount of C6H6 and then
reprecipitated with hexane. This process was repeated twice more,
after which the resulting precipitate was isolated as a colorless solid.
Crystalline material was obtained from benzene/hexane. Yield 163 mg
(93%). 1H NMR (500 MHz, C6D6): δ 6.75 (s, 4H, Ar), 3.05 (m, 4H,
NCH2CH2), 2.24 (s, 12H, CH3), 2.10 (s, 6H, CH3), 1.49 (quint, 3JHH
= 3.0 Hz, 4H, NCH2CH2), 1.18 (s, 9H, C(CH3)3). 13C{1H} NMR
(126 MHz, C6D6): δ 213.3 (s, NCN), 145.3 (s), 137.4 (s), 134.3 (s),
130.3 (s), 68.3 (s), 52.0 (s), 37.0 (s), 25.5 (s), 21.0 (s), 18.7 (s). Anal.
Calcd for C27H39N2OCu: C, 68.83; H, 8.34; N, 5.95. Found: C,
68.96; H, 8.34; N, 5.84.
Alkyne Hydroboration: To a flame-dried J. Youngs resealable
NMR tube were added 1-phenylpropyne (66.5 μL, 0.51 mmol), 1,3,5-
(MeO)3C6H3 (100 μL from a 0.2 M stock solution in C6D6, internal
standard), and I (400 μL from a 0.025 × 10−3 M stock solution in
C6D6). HBPin (81.2 μL, 0.560 mmol, 1.1 equiv) was added to initiate
the reaction. The solution was shaken at room temperature for 3 h at
which point the clear yellow solution was flushed through a silica plug
with CH2Cl2. The resulting solution was reduced to dryness and
analyzed by GC.
X-ray Crystallography. Data for III, IV, and VI were collected
using a Rigaku SuperNova instrument and a Cu Kα source, and those
for VIII, (7-Mes)CuMes (Supporting Information), (6-Xylyl)CuMes
(Supporting Information), (6-Dipp)CuMes (Supporting Informa-
tion), and IX (Supporting Information) were obtained using a Rigaku
Xcalibur instrument equipped with Mo Kα radiation. All experiments
were conducted at 150 K, with the exception of that for III, which was
implemented at 210 K. Structures were solved using Olex-242 and
refined using SHELXL.43 Refinements were generally straightforward
with a few notable exceptions which follow herein.
The data pertaining to IV are representative of a small single crystal
obtained after cardinal efforts to extract a suitable sample from a batch
of very twinned material. The result is an unequivocal testimony to an
excellent modern diffractometer and area detector. The methyl group
based on C11, in the compound, was seen to be disordered in an
80:20 ratio. C−C distances involving the two components of this
atom were restrained to being similar in order to assist convergence.
The motif in VI was seen to consist of two molecules, and the
proximity of a hydrogen atom attached to the apical NHC carbon
(C3) in one moiety is suggestive of a C−H···O interaction with the
tBu oxygen (O2) in the other molecule [H3A−O2, 2.29 Å; O2−C3,
(6-o-Tol)CuOtBu (IV). Compound IV was prepared as for III by
reaction of 6-o-Tol (prepared in situ from [6-o-Tol·H]BF4 (500 mg,
1.42 mmol) and KHMDS (312 mg, 1.56 mmol)) with Cu(Mes)n
(259 mg 1.42 mmol) followed by reaction with tBuOH to yield IV as
an off-white solid. Yield 95 mg (17%). Crystalline material was
obtained from benzene/hexane. 1H NMR (500 MHz, C6D6): δ 7.16−
6.94 (m, 8H, Ar), 2.62−2.46 (m, 4H, NCH2),‡* 2.13 (s, 6H, CH3),
1.28 (m, 2H, NCH2CH2), 1.26 (s, 9H, C(CH3)3). 13C{1H} NMR
(126 MHz, C6D6): δ 201.9 (s, NCN), 146.9 (s), 134.7 (s)*, 134.6
(s)‡, 131.8 (s)‡, 131.7 (s)*, 128.6 (s), 128.4 (s), 128.3 (s), 128.2 (s),
127.7 (s), 68.6 (s), 45.4 (s)*, 45.3 (s)‡, 37.1 (s), 21.1 (s)*, 21.0 (s)‡,
18.0 (s)‡, 17.9 (s)*. More than one set of signal was apparent
(Supporting Information), which we assign to major (‡) and minor
(*) conformers.18a Anal. Calcd for C22H29N2OCu: C, 65.89; H, 7.29;
N, 6.99. Found: C, 65.85; H, 7.27; N, 6.68.
(6-Xylyl)CuOtBu (V). Compound V was prepared as for III by
reaction of 6-Xylyl (prepared in situ from [6-Xylyl·H]BF4 (500 mg,
1.22 mmol) and KHMDS (269 mg, 1.34 mmol)) with Cu(Mes)n
(223 mg, 1.22 mmol), followed by reaction with tBuOH. The product
E
DOI: 10.1021/acs.organomet.8b00467
Organometallics XXXX, XXX, XXX−XXX

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Article
3.075(4) Å; C3−H3A−O2; 138.0°]. The asymmetric unit in VII was
seen to contain half of a benzene molecule, proximate to a
crystallographic inversion center, in addition to one molecule of the
copper complex. Data integration and subsequent refinement of the
model take account of the fact that the crystal was a twinned by a
180° rotation, in direct space, about 1 0 0.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00467.
Multinuclear NMR spectra of (RE-NHC)CuMes, I−
VIII, alkynes, semihydrogenation, and hydroboration
products; X-ray structures of (RE-NHC)CuMes and IX
(PDF)
Accession Codes
CCDC 1845505−1845512 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(6) (a) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F.
Sterically Demanding, Bioxazoline-Derived N-Heterocyclic Carbene
Ligands with Restricted Flexibility for Catalysis. J. Am. Chem. Soc.
2004, 126, 15195−15201. (b) Wurtz, S.; Glorius, F. Surveying
Sterically Demanding N-Heterocyclic Carbene Ligands with Re-
stricted Flexibility for Palladium-Catalyzed Cross-Coupling Reactions.
Acc. Chem. Res. 2008, 41, 1523−1533.
(7) Iglesias, M.; Beetstra, D. J.; Knight, J. C.; Ooi, L.-L.; Stasch, A.;
Coles, S.; Male, L.; Hursthouse, M. B.; Cavell, K. J.; Dervisi, A.; Fallis,
I. A. Novel Expanded Ring N-Heterocyclic Carbenes: Free Carbenes,
Silver Complexes, And Structures. Organometallics 2008, 27, 3279−
3289.
AUTHOR INFORMATION
Corresponding Author
*E-mail: m.k.whittlesey@bath.ac.uk. Tel.: 44 1225 383748.
ORCID
■
(8) Li, J.; Shen, W. X.; Li, X. R. Recent Developments of Expanded
Ring N-Heterocyclic Carbenes. Curr. Org. Chem. 2012, 16, 2879−
2891.
Michael K. Whittlesey: 0000-0002-5082-3203
Notes
The authors declare no competing financial interest.
(9) (a) Poulten, R. C.; Page, M. J.; Algarra, A. G.; Le Roy, J. J.;
́
Lopez, I.; Carter, E.; Llobet, A.; Macgregor, S. A.; Mahon, M. F.;
ACKNOWLEDGMENTS
Murphy, D. M.; Murugesu, M.; Whittlesey, M. K. Synthesis,
Electronic Structure, and Magnetism of [Ni(6-Mes)2]+: A Two-
Coordinate Nickel(I) Complex Stabilized by Bulky N-Heterocyclic
Carbenes. J. Am. Chem. Soc. 2013, 135, 13640−13643. (b) Phillips,
N.; Dodson, T.; Tirfoin, R.; Bates, J. I.; Aldridge, S. Expanded-Ring
N-Heterocyclic Carbenes for the Stabilization of Highly Electrophilic
Gold(I) Cations. Chem. - Eur. J. 2014, 20, 16721−16731. (c) Ellul, C.
E.; Lowe, J. P.; Mahon, M. F.; Raithby, P. R.; Whittlesey, M. K.
[Ru3(6-NHC)(CO)10]: Synthesis, Characterization and Reactivity of
Rare 46-Electron Tri-Ruthenium Clusters. Dalton Trans. 2018, 47,
4518−4523.
(10) (a) Mayr, M.; Wurst, K.; Ongania, K.-H.; Buchmeiser, M. R.
1,3-Dialkyl- and 1,3-Diaryl-3,4,5,6-tetrahydropyrimidin-2-ylidene
Rhodium(I) and Palladium(II) Complexes: Synthesis, Structure,
and Reactivity. Chem. - Eur. J. 2004, 10, 1256−1266. (b) Rogers,
M. M.; Wendlandt, J. E.; Guzei, I. A.; Stahl, S. S. Aerobic
Intramolecular Oxidative Amination of Alkenes Catalyzed by NHC-
Coordinated Palladium Complexes. Org. Lett. 2006, 8, 2257−2260.
(c) Dunsford, J. J.; Cavell, K. J. Expanded Ring N-Heterocyclic
Carbenes: A Comparative Study of Ring Size in Palladium(0)
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9135. (d) Spallek, M. J.; Riedel, D.; Rominger, F.; Hashmi, A. S. K.;
Trapp, O. Six-Membered, Chiral NHCs Derived from Camphor:
Structure-Reactivity Relationship in Asymmetric Oxindole Synthesis.
■
We acknowledge financial support from the EPSRC Centre for
Doctoral Training in Catalysis (EP/L016443/1) for J.H. and
EPSRC Doctoral Training Awards to L.R.C. and M.K.C. P.B.
́
thanks the Universite de Toulouse (Programme ATUPS 2016)
and Agence Nationale de la Recherche (ANR CE6-CYCLO-
OP) for supporting his stay in Bath.
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