Zincocene and dizincocene N-heterocyclic carbene complexes and catalytic hydrogenation of imines and ketones

Article abstract of DOI:10.1002/chem.201402875

The N-heterocyclic carbene (NHC) adducts Zn(Cp^R) ₂(NHC)] (Cp^R=C₅HMe₄, C ₅H₄SiMe₃; NHC=ItBu, IDipp (Dipp=2,6- diisopropylphenyl), IMes (Mes=mesityl), SIMes) were prepared and shown to be active catalysts for the hydrogenation of imines, whereas decamethylzincocene [ZnCp*₂] is highly active for the hydrogenation of ketones in the presence of noncoordinating NHCs. The abnormal carbene complex [Zn(OCHPh₂)₂(aItBu)]₂ was formed from spontaneous rearrangement of the ItBu ligand during incomplete hydrogenation of benzophenone. Two isolated Zn^I adducts [Zn₂Cp* ₂(NHC)] (NHC=ItBu, SIMes) are presented and characterized as weak adducts on the basis of ¹³C NMR spectroscopic and X-ray diffraction experiments. A mechanistic proposal for the reduction of [ZnCp* ₂] with H₂ to give [Zn₂Cp*₂] is discussed.

Full text of DOI:10.1002/chem.201402875

DOI: 10.1002/chem.201402875
Full Paper
&
Homogeneous Catalysis
Zincocene and Dizincocene N-Heterocyclic Carbene Complexes
and Catalytic Hydrogenation of Imines and Ketones
Phillip Jochmann[a] and Douglas W. Stephan*[a, b]
Abstract: The N-heterocyclic carbene (NHC) adducts Zn-
(CpR)2(NHC)] (CpR =C5HMe4, C5H4SiMe3; NHC=ItBu, IDipp
(Dipp=2,6-diisopropylphenyl), IMes (Mes=mesityl), SIMes)
were prepared and shown to be active catalysts for the hy-
drogenation of imines, whereas decamethylzincocene
[ZnCp*2] is highly active for the hydrogenation of ketones in
the presence of noncoordinating NHCs. The abnormal car-
bene complex [Zn(OCHPh2)2(aItBu)]2 was formed from spon-
taneous rearrangement of the ItBu ligand during incomplete
hydrogenation of benzophenone. Two isolated ZnI adducts
[Zn2Cp*2(NHC)] (NHC=ItBu, SIMes) are presented and char-
acterized as weak adducts on the basis of 13C NMR spectro-
scopic and X-ray diffraction experiments. A mechanistic pro-
posal for the reduction of [ZnCp*2] with H2 to give [Zn2Cp*2]
is discussed.
engo et al., reporting the first zincocene NHC adduct,[14] only
a limited number of related complexes has been isolated.
NHC-supported zinc alkoxides have been employed for the
polymerization of d,l-lactide,[7a,b] and rac-lactide.[7c] NHC ad-
ducts of zinc bromide, chlorides, and acetates have also been
used to effect cycloaddition of CO2 to epoxides and the co-
polymerization of CO2 with cyclohexene oxide.[15,7d] Rapid func-
tionalization of CO2 by insertion into ZnÀC bonds and ring-
opening polymerization of epoxides are additional examples of
applications of zinc–NHC complexes.[16,17]
Introduction
The activation of dihydrogen and the catalytic reduction of un-
saturated organic molecules is a fundamentally important pro-
cess.[1] Indeed, the hydrogenation of imines and ketones con-
stitutes an atom economic way for the production of amines
and alcohols.[2] Platinum-group metals for catalytic hydrogena-
tion are increasingly undesirable due to their rising cost and
toxicity. As a result, there has been increasing interest in earth-
abundant element-based catalyst systems in recent years.[3] On
the main group side, we and others have described efficient
metal-free hydrogenation utilizing frustrated Lewis pairs
(FLPs).[4] In terms of earth-abundant metals, a number of
groups have focused on s-block metals,[5] as well as Fe-, Ni-,
and Co-based catalysts.[6] In this vein, zinc-based catalysts have
been developed for important transformations including (de)-
polymerization,[7] hydrosilylation,[8] dehydrocoupling,[8b,9] and
hydroamination reactions.[10,11] In the case of hydrogenation,
while Beller and co-workers had previously exploited Zn(OTf)2
to generate an in situ catalyst for the hydrogenation of imines,
Even though decamethylzincocene [ZnCp*2] (1; Cp*=C5Me5)
and related zincocenes have been known for decades,[18]
recent attention[19] has focused on the ZnI compound [Zn2Cp*2]
(2), which contains a ZnÀZn bond.[20] In extending our earlier
communication, we report a series of zincocene- and dizinco-
cene–NHC adducts. The former complexes were evaluated in
the catalytic hydrogenation of imines and ketones. The reac-
tion of 1 with H2 to form the latter ZnI compounds is discussed
from a mechanistic point of view.
we have reported the first example of
a well-defined
organozinc(hydride) precatalyst, [Zn(Cp*)(H)(SIMes)] (SIMes= Results and Discussion
1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene).[12]
Zincocene complexes
This catalyst, derived from the combination of an organozinc
reagent with N-heterocyclic carbenes (NHCs) is a rare example
of a (Cp*)Zn–NHC complex.[13] After pioneering work by Ardu-
Because the above-described previous results demonstrated
that NHCs can stabilize cyclopentadienyl–zinc hydrides, we
sought to probe related systems. To that end, the parent octa-
methyl- and bis(trimethylsilyl)zincocenes, [Zn(CpMe4)2] and [Zn-
(CpTMS)2], which are known to be coordination polymers in the
solid state, were prepared and converted with NHCs
(Scheme 1).[21] Although the reported syntheses used
[LiC5H4SiMe3] and [Mg(C5Me4H)2] as metathesis reagents, the
present approach is similar to that reported for analogous
ZnCp*2 derivatives.[18] Thus, the pro-ligands CpRH were treated
with 1.2 equivalent of NaH and subsequently reacted with
[a] Dr. P. Jochmann, Prof. Dr. D. W. Stephan
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, Ontario, M5S 3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
[b] Prof. Dr. D. W. Stephan
Chemistry Department, Faculty of Science
King Abdulaziz University (KAU), Jeddah 21589 (Saudi Arabia)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402875.
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Scheme 1. Synthesis of compounds 3–10.
ZnCl2. The isolated yields were 65% for [Zn(CpMe4)2] and 30%
for [Zn(CpTMS)2]. The particularly low latter yield was attributed
to contamination of the crude product with residual chloride
species. This dictated a minimum of three extractions with
pentane and subsequent drying in vacuo to give [Zn(CpTMS)2]
free of chloride impurities. Adduct formations of [Zn(CpMe4)2]
and [Zn(CpTMS)2] were achieved by simply mixing equimolar
amounts of zinc species and free carbene in benzene or tolu-
ene for one hour. In this fashion, the compounds [Zn(CpMe4)2-
(NHC)] (Scheme 1, 3–6,) and [Zn(CpTMS)2(NHC)] (7–10) (NHC=
1,3-di-tert-butyl-imidazol-2-ylidene : tBu, 1,3-bis(2,6-diisopropyl-
phenyl)imidazol-2-ylidene: IDipp, 1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene: IMes, 1,3-Bis(2,4,6-trimethylphenyl)-4,5-dihy-
droimidazol-2-ylidene: SIMes) were prepared, isolated, and
characterized by H and 13C NMR analysis. Solutions of adducts
1
Figure 1. Pov-ray depiction of [Zn(CpMe4)2(ItBu)](C6H6) (3; top) and [Zn-
(CpMe4)2(SIMes)](THF)2 (6; bottom). Co-crystallized solvent molecules are
omitted for clarity. Only C5Me4H protons are shown for clarity.
1
3–10 in aromatic solvents showed H NMR coupling patterns
indicative of h5-coordinated CpR ligands.
1H NMR spectra of [Zn(CpMe4)2(NHC)] (NHC=ItBu, IDipp,
IMes, SIMes) (3–6) showed a strong dependence of the chemi-
cal shift of the CH proton of the CpMe4 ligand on the nature of
the coordinated NHC (3: d=4.30, 4: 3.32, 5: 3.68 and 6:
3.63 ppm). Due to the very low solubility of 5 and 6 in aliphatic
and aromatic solvents (<2 mgmLÀ1), no reliable 13C NMR data
could be obtained. The more soluble adducts 3 and 4 showed
13C NMR resonances for the metal bound CNHC atoms at d=
173.5 and 189.4 ppm, respectively. Single crystals of [Zn-
(CpMe4)2(ItBu)] (3) and [Zn(CpMe4)2(SIMes)] (6) were obtained by
cooling concentrated solutions in benzene and THF, respective-
ly. Even though this method allowed recrystallization of the
other adducts (i.e., 4 and 5), the obtained materials were fre-
quently found to be microcrystalline or of low quality and thus
not suitable for X-ray structure determination. The molecular
structures of 3 and 6 in the solid state feature a trigonal planar
coordination around the metal center with the sum of the
angles about the metal totaling 3608 (Figure 1). The ZnÀC dis-
tances for the carbene ligands were found to be 2.062(2) and
2.074(3) ꢁ. A ring slip of the CpMe4 rings leads to comparatively
short ZnÀC bond lengths of 2.099(2) and 2.111(2) ꢁ for 3 and
2.037(3) and 2.126(3) ꢁ for 6. The cyclopentadienyl hydrogen
atoms of the CpMe4 ligands gave rise to ZnÀH distances of
1.886 and 1.833 ꢁ in 3 and 2.567 and 2.745 ꢁ in 6. For com-
pound 3, these values suggest the possibility of an agostic in-
teraction. Although the C5HMe4 ring in 3 is coplanar with the
remaining proton in the C5-plane, this proton is markedly out-
of-plane (above) in 6. In the former case, the coordination can
be described as bis(h1-p)-coordination, reflecting sp2 hybridiza-
tion of the zinc-bound carbon. In contrast, in compound 6, the
zinc-bound carbon atom is sp3 hybridized, and thus the coordi-
nation is best described as bis(h1-s). Steric interactions of the
CpMe4 fragment with the carbene substituents in 6 may ac-
count for the twisting of the CpMe4 ligand away from the car-
bene. The orientation of the CpMe4 ligand towards the carbene
in 3 may be stabilized by this agostic interaction; however, the
dynamic nature of the bis(h5)-Cp* binding in solution preclud-
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ed confirmation of such interactions in
spectroscopy.
3
by NMR
Table 1. Catalytic hydrogenation of imines.[a,c]
The 1H NMR shifts of [Zn(CpTMS)2(NHC)] (NHC=ItBu, IDipp,
IMes, SIMes) (7–10) appeared to be insensitive to the nature of
the NHC. Only [Zn(CpTMS)2(ItBu)] (7) displayed slightly upfield
CpTMS resonances (d=6.90 and 6.11 ppm), whereas the other
three adducts 8–10 showed resonances in the narrow range of
6.19 to 5.66 ppm, and the TMS groups gave resonances be-
tween 0.39 and 0.37 ppm for 7–10. The zinc-bound CNHC
carbon atoms showed 13C resonances at d=186.6 (7), 178.0
(8), 176.3 (9), and 201.3 ppm (10). Single crystals of [Zn(CpTMS)2-
(ItBu)] (7) were obtained by cooling a concentrated solution in
toluene and featured slipped CpTMS rings, a three-coordinated
metal center with an angle sum about Zn of 3608 and a ZnÀ
CNHC distance of 2.048(4) ꢁ (Figure 2). The zinc–carbon bond
No.
Cat.
Substrate[b]
Yield [%]
1
3
tBuN=CHPh
0
99
0
0
0
2
3
PhCH2N=CHPh
3
4
5
6
7
8
9
3
3
3
4[b]
4[b]
4[b]
4[b]
4[b]
7
7
7
7
7
8
8
8
8
8
9
9
9
9
9
10
10
10
10
10
tBuN=CH-p-tBu-C6H4
tBuN=CH-p-Br-C6H4
tBuN=CH-m-Br-C6H4
tBuN=CHPh
3
PhCH2N=CHPh
100
trace
4
tBuN=CH-p-tBu-C6H4
tBuN=CH-p-Br-C6H4
tBuN=CH-m-Br-C6H4
tBuN=CHPh
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
9
0
PhCH2N=CHPh
25
0
trace
0
11
100
19
25
22
7
tBuN=CH-p-tBu-C6H4
tBuN=CH-p-Br-C6H4
tBuN=CH-m-Br-C6H4
tBuN=CHPh
PhCH2N=CHPh
tBuN=CH-p-tBu-C6H4
tBuN=CH-p-Br-C6H4
tBuN=CH-m-Br-C6H4
tBuN=CHPh
PhCH2N=CHPh
100
9
tBuN=CH-p-tBu-C6H4
tBuN=CH-p-Br-C6H4
tBuN=CH-m-Br-C6H4
tBuN=CHPh
10
11
trace
15
0
PhCH2N=CHPh
tBuN=CH-p-tBu-C6H4
tBuN=CH-p-Br-C6H4
tBuN=CH-m-Br-C6H4
0
0
[a] H2 (100 bar), 72 h, 508C, [Zn(CpR)2(NHC)] (10 mol%, conc. ca. 4 mm in
C6D6). [b] This experiment was performed at a catalyst loading of 7 mol%,
1
due to limited solubility. [c] Yields are based on H NMR data.
ligand and Zn0 or a colorless precipitate after two weeks. Even
though not investigated in detail, the latter precipitate is be-
lieved to be ZnH2 or a derivative thereof. In addition, 3–4 and
7–9 were evaluated as catalysts for the hydrogenation of C=N
double bonds (Table 1). In general, these species were not
highly effective, giving only moderate reduction at best. The
one exception was the substrate PhCH2N=CHPh, which was ef-
fectively reduced by using 3–4 and 7–9 as the catalyst
precursors.
Figure 2. Pov-ray depiction of [Zn(CpTMS)2(ItBu)] (7). Hydrogen atoms are
omitted for clarity.
lengths for the CpTMS ligands of 2.126(4) and 2.140(4) ꢁ are in-
dicative of two h1-CpTMS ligands. However, the zinc-bound CH
moieties are not fully pyramidalized, what speaks for a certain
degree of charge delocalization in the slipped CpTMS ligands.
Notably, the zinc center binds to the carbon atoms in g posi-
tion of the CpTMS ligands. This is likely a consequence of steric
effects, and a similar binding was observed for the parent com-
pound [Zn(CpTMS)2]n.[21] The overall observed complex geometry
is reminiscent of the paddle-wheel nature of 6.
In a similar fashion, catalytic hydrogenation of a series of ke-
tones by 1/NHC (NHC=ItBu, IDipp, SIMes) was assessed using
10 mol% catalyst, and 100 bar H2 at 258C typically for 72 h
(Table 2). No significant reduction was observed for 2-methyl-
pentan-3-one, benzophenone, and diisopropylphenyl methyl
ketone (Table 2, entries 5–7, 12–14, 19–21). In contrast, moder-
ate to quantitative yields of the corresponding alcohols were
obtained for the hydrogenation of iPr2CO (entries 1, 8, 15) and
tBu2CO (entries 2, 9, 16). In these cases, 1/SIMes is the most ef-
ficient catalyst. It is noteworthy that SIMes did not form an
adduct with 1 due to steric crowding, and thus this situation
resembles frustrated Lewis pair (FLP) behavior. The poorer cat-
alytic ability of 1/NHC to reduce ketone versus imine sub-
strates is consistent with the greater bond strength of ZnÀO
over ZnÀN.[12] This is thought to slow product release and thus
Catalytic hydrogenations
Efforts to react 3, 4, and 7–10 with H2 (4 bar) were monitored
in situ by NMR spectroscopy. Compounds 5 and 6 were found
to be unsuitable, due to their low solubility. Although 3 and 8
showed no apparent reaction, 4, 7, 9, and 10 reacted slowly
with H2 leading to hydrogenolysis of the cyclopentadienyl
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Table 2. Catalytic hydrogenation of ketones with 1/NHC.
No.[a]
Cat.
Substrate
Yield [%][b]
1
2
3
4
5
6
7
8
3
3
3
3
3
3
3
4
4
4
4
4
4
4
6
6
6
6
6
6
6
iPr2C=O
tBu2C=O
90
57
22
23
trace
0
iPr(Me)C=O
Cy(Me)C=O
iPr(Et)C=O
Ph2C=O
Dipp(Me)C=O
iPr2C=O
0
54
32
40
26
trace
0
9
tBu2C=O
10
11
12
13
14
15
16
17
18
19
20
21
iPr(Me)C=O
Cy(Me)C=O
iPr(Et)C=O
Ph2C=O
Dipp(Me)C=O
iPr2C=O
0
100
62
26
14
trace
0
tBu2C=O
iPr(Me)C=O
Cy(Me)C=O
iPr(Et)C=O
Ph2C=O
Figure 3. Pov-ray depiction of 11. Solvent molecules and hydrogen atoms
are omitted for clarity.
Dipp(Me)C=O
0
[a] H2 (100 bar), 72 h, 258C, [ZnCp*2]/NHC (10 mol%, conc. ca. 4 mm in
1
C6D6). [b] Yields are based on H NMR data. Cy=cyclohexyl.
tures an oxygen-bridged dimer. The bridging ligands exhibited
slightly elongated ZnÀO bond lengths of 2.016(1) ꢁ, whereas
the nonbridging alkoxy ligands displayed ZnÀO distances of
1.927(1) ꢁ. The terminal aItBu ligands showed longer bond
lengths of ZnÀC 2.027(2) ꢁ, which is slightly shorter compared
to previously reported NHC adducts of zinc.[23a]
slow hydrogenation. In agreement with this hypothesis, single
crystals of zinc alkoxide products were repeatedly obtained in
hydrogenation reactions (see below).
Crystals of a product 11 were obtained from the attempted
hydrogenation of benzophenone with [ZnCp*2] (1) in the pres-
ence of ItBu (Scheme 2). This product was independently syn-
thesized by initial reaction of 1 with Ph2CHOH and subsequent
The corresponding reactions with IDipp and Ph2CO or
tBu2CO gave crystals of [Zn(OCHPh2)2(IDipp)] (12) and [Zn-
(OCHtBu2)2(IDipp)] (13). The latter species was isolated in 50%
yield from the hydrogenation of the ketone, whereas the
former species 12 was more conveniently prepared from the
reaction of [ZnCp*2] with benzhydrol in the presence of IDipp
(Scheme 3). The nature of 12 and 13 was confirmed crystallo-
graphically (Figure 4). These species exhibit trigonal planar ge-
Scheme 2. Reaction of 1 to the abnormal adduct 11.
Scheme 3. Formation of alkoxides 12 and 13.
treatment of the intermediate alkoxide product with ItBu.[22]
This product proved to be insoluble in aromatic solvents and
unstable in CD2Cl2. However, when a freshly prepared solution
of 11 in CD2Cl2 was kept in liquid N2 and warmed prior to ac-
ometries at the Zn center with angle sums of >3598. The ZnÀ
O bond lengths are 1.835(4) to 1.846(4) ꢁ for 12 and 1.859(1)
and 1.866(1) ꢁ for 13. The IDipp ligands gave rise to ZnÀC dis-
tances of 1.981(6) and 1.994(6) ꢁ in 12 and 2.052(2) ꢁ in 13.
The closely related [Zn(OCH2Ph)2(IMes)]2 and [ZnCl(OCH2Ph)-
(IDipp)]2 were structurally characterized by Tolman and co-
workers and shown to act as catalysts for the polymerization
of lactide.[7a,b] It is noteworthy that these species were ob-
served to exist as dimers in the solid state, but appear to be
monomers in solution.
1
quisition of NMR data, H resonances at d=7.56 and 6.86 ppm
and a 13C resonance at 155.3 ppm were consistent with the
presence of an abnormal carbene.[23] Two sets of resonances in-
ferred two alkoxide (OCHPh2)À fragments leading to specula-
tion of a dimeric structure in solution. The corresponding reac-
tion of 1 with benzophenone under D2 effected the analogous
conversion, prompting the formulation of the product as [Zn-
(OCDPh2)2(aItBu)] ([D1]11). Crystallographic data confirmed this
formulation of 11 (Figure 3). The molecular structure of 11 fea-
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resonances at 205.8 and 208.4 ppm for the coordinating
carbon atoms of the NHCs, respectively.[12b] These values indi-
cate a low sensitivity of the Cp* resonances to carbene coordi-
nation. The carbene carbon 13C resonances are significantly
shifted downfield, compared to NHC adducts of ZnII cyclopen-
tadienyls.[12b,14,16] The molecular solid-state structures of 14 and
15 were established and revealed the Cp* rings on the Zn
atom to which the carbene binds to adopt slipped binding
modes (Figure 5). The ZnÀZn bond lengths are 2.3392(4) and
2.3682(6) ꢁ for 14 and 15, respectively, and are between the
corresponding distances observed for the parent compound 2
(2.305(3) ꢁ)[20a] and [Zn2Cp*2(dmap)2] (dmap=4-dimethylami-
nopyridine 2.418(1) ꢁ)[24] and within the general range for
Figure 4. Pov-ray depiction of (12; top) and (13; bottom); one iPr group per
IDipp ligand was disordered. In the case of 12, there are two independent
molecules per unit cell, one is shown. All hydrogen atoms are omitted for
clarity.
Dizincocenes
We reported earlier the in situ observation of dizincocene ad-
ducts [Zn2Cp*2(ItBu)] (14) and [Zn2Cp*2(SIMes)] (15) during the
conversion of 1 to 2 in the presence of H2 and NHCs.[12b] These
adducts were independently synthesized by the combination
of equimolar amounts of 2 and NHCs in toluene (Scheme 4).
1
Compounds 14 and 15 showed characteristic H NMR reso-
nances at d=2.07 and 2.11 ppm for the h5-Cp* ligands and 13C
Figure 5. Pov-ray depiction of (14; top) and (15; bottom). For compound
14, one of two disordered slipped Cp* positions is shown. All hydrogen
atoms are omitted for clarity.
Scheme 4. Synthesis of compounds 14 and 15.
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Zn2R2 compounds.[19c] Although one ring remains coordinated intermediacy of [Cp*ZnH]1À2 was further supported by our
in a slightly slipped h5-fashion (ZnÀC bond lengths: 2.269(4) to recent report of [Cp*ZnH(NHC)] (NHC=SIMes), which can be
2.384(4) ꢁ for 14 and 2.323(5)–2.414(4) ꢁ for 15), the other ring regarded as the NHC-stabilized intermediate. Similarly, Rit et al.
is best described as h2 (ZnÀC 2.209(15)–2.472(18) ꢁ) for 14 and have described [ZnH2(NHC)]2 (NHC=IDipp, IMes).[9b] Interest-
h3 (ZnÀC 2.126(4) to 2.548(4) ꢁ) for 15. This slippage is consis- ingly, reduction of [ZnCp*2] or [ZnH2(NHC)]2 with H2 at elevated
tent with a minimal pyramidalization around the zinc-bound temperatures gave metallic Zn0.[9b,12] This implies that soluble
carbon atoms, because the methyl groups are slightly above zinc hydrides can undergo reductive elimination under reduc-
the C5 plane (0.41 and 0.47 ꢁ for 14 and 15, respectively). The ing conditions.
ZnÀNHC bond distances are 2.101(3) (14) and 2.147(4) ꢁ (15),
Because dimeric zinc hydrides are key intermediates en
slightly elongated compared to known ItBu and SIMes adducts route to 2, NHCs react with the transient Zn hydride account-
of [Cp*ZnR] (R=C6F5, H). This is in agreement with a diminished ing for the observed formation of NHC-derived aminals. Trans-
2+
Lewis acidity of the Zn2 unit.
fer of a hydride to the NHC and subsequent hydrogenolysis is
The original in situ observation of dizincocene adducts 14 thought to release the aminal and regenerate the zinc hydride.
and 15 was made during the conversion of 1 and the corre- Indeed, independent reactions showed that this oxidative addi-
sponding NHC with H2 to cleanly give 2.[12a,b] This facile and tion of H2 to the NHC is catalytic in zinc. Such hydride-transfer
high-yielding route gave 2 in the presence or absence of reactions were previously observed for beryllium, silicon and
NHCs. However, in the presence of NHCs, the formation of boron reagents and calculated to constitute the first step in
[Cp*ZnH(NHC)] and NHC-derived aminals were observed as in- the ring expansion of NHCs.[28]
termediates and/or byproducts. The formation of 2 is com-
monly believed to proceed by dimerization of [ZnCp*]· radicals.
Conclusion
This view was supported by the detailed work of Carmona and
co-workers,[20d] reactivity studies of the hydride complexes
The present results expand and document straightforward syn-
[ArMH]2 (M=Zn, Cd; Ar=C6H3-2,6-(C6H2-2,4,6-iPr3)2, C6H2-2,6-
(C6H2-2,4,6-iPr3)2-4-SiMe3) reported by Power and co-workers[25]
and theoretical calculations.[26]
thetic routes to NHC adducts of zincocenes and dizincocenes.
2+
Although the Zn2 unit was characterized as a weak Lewis
acid, our findings are in line with H2 elimination from a hy-
dride-bridged dimer en route to dizincocenes. The NHC–zinco-
cenes showed moderate to high activity in catalytic hydroge-
nation, and the first example of organozinc-catalyzed hydroge-
nation of ketones to give alcohols is reported. In several cases,
the incomplete hydrogenation of ketones also allowed the iso-
lation of monomeric NHC coordinated zinc alkoxides. The un-
precedented spontaneous rearrangement of an NHC in the co-
ordination sphere of zinc led to the formation of an abnormal
alkoxy complex. The utility and reactivity of these Zn–NHC
complexes continues to be the subject of study in our labora-
tories.
In the case of the reaction of 1 with H2, the stability of 1 pre-
cludes a pathway involving the homolytic dissociation into
[ZnCp*]· and Cp*· radicals in a pre-equilibrium. This view was
further supported by calculations by Hepperle and Wang that
showed this dissociation is uphill by 32.3 kcalmolÀ1 [26]
al-
,
though 6.7 kcalmolÀ1 less costly than the analogous dissocia-
tion for [ZnCp2]. A more likely pathway involves heterolytic
cleavage of H2 to eliminate Cp*H and generate [Cp*ZnH]. Di-
merization and reductive elimination of H2 would lead to the
formation of 2 (Scheme 5). This mechanistic scenario was inti-
mated by Power and co-workers[25a] and was demonstrated for
Experimental Section
General considerations
All manipulations were performed under an atmosphere of dry,
oxygen-free N2 by means of standard Schlenk or glovebox tech-
niques (MBraun glovebox equipped with a À408C freezer), unless
otherwise noted. C6D6 and [D8]toluene were dried over Na/benzo-
phenone ketyl, CD2Cl2 was dried over CaH2 and vacuum transferred
before use. Other solvents were purified by using an Innovative
Technologies solvent-purification system. [ZnCp*2],[18] [Zn2Cp*2],[20c,
d,12]
and SIMes[29] were prepared according to published protocols.
Scheme 5. Proposed reaction pathway for 1 with H2 to yield 2 and NHC-de-
[Zn(CpMe4)2] and [Zn(CpTMS)2] were prepared according to the proto-
col for [ZnCp*2].[18] Carbenes ItBu (TCI), IDipp (Aldrich), and IMes
(Aldrich) were used as received. NMR spectra were recorded on
Bruker Avance 400 MHz, an Agilent DD2 500, or an Agilent DD2
600 Hz spectrometers. CNHC shifts were determined by heteronu-
clear multiple-bond coherence (HMBC) experiments, if not ob-
served by 1D 13C NMR spectroscopy. Chemical shifts were refer-
enced internally by using the residual solvent resonances and re-
ported relative to tetramethylsilane. X-ray crystallography (Bruker
rived aminal.
the cadmium homologue.[25d] In support of this view, a sample
of 1 pressurized with HD gave 2, H2, D2, Cp*H, and Cp*D. A
similar mechanistic proposal was recently reported by Fischer
and co-workers, in which the formation of 2 from 1 and ZnH2
was explained by in situ formation of [Cp*ZnH] and reductive
elimination upon reaction with another equivalent of 1.[27] The Kappa Apex II) and elemental analysis (PerkinElmer CHN Analyzer)
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Synthesis of [Zn(CpTMS)2(NHC)] (NHC=ItBu) 7, IDipp 8, IMes
9, SIMes 10)
were performed in house. We note that satisfactory carbon con-
tents were obtained only for few of the new compounds, even
after repeated elemental analysis. These species are highly air and
oxygen sensitive and appear to be pure by all spectroscopic meth-
ods. A similar phenomenon has been described earlier.[22a]
These compounds were prepared in a similar fashion, and thus
only one preparation is detailed. A vial was charged with [Zn-
(CpTMS)2] (12 mg, 0.035 mmol) and ItBu (6 mg, 0.033 mmol). The
solids were dissolved in toluene (1.5 mL) and stirred overnight. All
volatiles were removed under reduced pressure, and the resulting
solid was washed with n-pentane. After drying in vacuo, the prod-
uct was obtained.
Synthesis of [Zn(CpMe4)2(NHC)] (NHC=ItBu 3, IDipp 4, IMes 5,
SIMes 6)
These compounds were prepared in a similar fashion, and thus
only one preparation is detailed. A solution of ItBu (12 mg,
0.067 mmol) in toluene (2 mL) was added to a solution of [Zn-
(CpMe4)2] (20 mg, 0.065 mmol) in toluene (2 mL). All volatiles of the
colorless solution were removed, and the resulting powder was re-
crystallized from toluene at À358C to give colorless crystals that
were dried in vacuo (31 mg, 0.064 mmol, 96%).
Data for 7
Pale orange solid (14 mg, 0.027 mmol, 82%). 1H NMR (400 MHz,
C6D6): d=6.90 (m, 4H, CHCp), 6.17 (s, 2H, CHItBu), 6.11 (m, 4H, CHCp),
1.12 (s, 18H, MetBu), 0.39 ppm (s, 18H, SiMe3); 13C NMR (151 MHz,
C6D6): d=186.6 (CItBu), 127.1 (CCp), 123.3 (CHCp), 119.2 (CHItBu), 101.8
(CHCp), 58.3 (CtBu), 31.2 (MetBu), 1.3 ppm (SiMe3); elemental analysis
calcd (%) for C27H46N2Si2Zn (520.23): C 62.34, H 8.91, N 5.38; found:
C 61.80, H 9.71, N 5.55.
Data for 3
1H NMR (400 MHz, C6D6): d=6.23 (s, 2H, CHItBu), 4.30 (br hept,
4J(H,H)=1 Hz, 2H, CHCp), 2.24 (s, 12H, MeCp), 2.15 (s, 12H, MeCp),
0.93 ppm (s, 18H, tBu); 13C NMR (125 MHz, C6D6): d=173.5 (CItBu),
131.5 (CCp), 125.5 (CCp), 119.0 (CHItBu), 65.3 (CHCp), 57.6 (CtBu), 29.4
(MetBu), 15.6 (MeCp), 12.4 ppm (MeCp); elemental analysis calcd (%)
for C29H46N2Zn (488.08): C 71.36, H 9.50, N 5.74; found: C 70.46, H
9.46, N 5.52.
Data for 8
Off-white powder (18 mg, 0.025 mmol, 71%). 1H NMR (400 MHz,
C6D6): d=7.27 (t, 3J(H,H)=7.8 Hz, 2H, p-CHIDipp), 7.12 (d, 3J(H,H)=
7.7 Hz, 4H, m-CHIDipp), 6.34 (s, 2H, CHIDipp), 6.19 (br, 4H, CHCp), 5.66
(br, 4H, CHCp), 2.68 (pent, 3J(H,H)=6.8 Hz, 4H, CHiPr), 1.38 (d,
3J(H,H)=6.9 Hz, 12H, MeiPr), 0.89 (d, 3J(H,H)=6.9 Hz, 12H, MeiPr),
0.37 ppm (s, 18H, SiMe3); 13C NMR (151 MHz, C6D6): d=178 (CIDipp),
145.9 (o-CIDipp), 135.3 (i-CIDipp), 131.7 (p-CHIDipp), 125.2 (CHIDipp), 124.9
(m-CHIDipp), 119.0 (CHCp), 103.8 (CHCp), 29.4 (CHiPr), 26.4 (MeiPr), 23.3
(MeiPr), 1.6 ppm (SiMe3); elemental analysis calcd (%) for
C43H62N2Si2Zn (728.52): C 70.89, H 8.58, N 3.85; found: C 70.34, H
8.63, N 4.10.
Data for 4
Recrystallized from toluene at À358C gave yellow crystals that
1
were dried in vacuo (36 mg, 0.052 mmol, 81%). H NMR (400 MHz,
C6D6): d=7.22 (t, 3J(H,H)=7.8 Hz, 2H, p-CHIDipp), 7.10 (d, 3J(H,H)=
7.8 Hz, 4H, m-CHIDipp), 6.48 (s, 2H, CHIDipp), 3.32 (br s, 2H, CHCp), 2.85
Data for 9
3
(hept, J(H,H)=6.8 Hz, 4H, CHiPr), 2.07 (s, 12H, MeCp), 1.93 (s, 12H,
Orange powder (12 mg, 0.019 mmol, 66%). 1H NMR (400 MHz,
MeCp), 1.29 (d, 3J(H,H)=6.8 Hz, 12H, MeiPr), 0.98 ppm (d, 3J(H,H)=
6.8 Hz, 12H, MeiPr); 13C NMR (151 MHz, C6D6): d=189.4 (CIDipp),
146.2 (o-CIDipp), 137.7 (i-CIDipp), 130.3 (CCp), 128.7 (p-CHIDipp), 126.7
(CCp), 124.6 (m-CHIDipp), 123.5 (CHIDipp), 90.2 (CHCp), 29.3 (CHiPr), 25.4
(MeiPr), 24.1 (MeiPr), 15.2 (MeCp), 12.5 ppm (MeCp); elemental analysis
calcd (%) for C45H62N2Zn (696.37): C 77.61, H 8.97, N 4.02; found: C
75.18, H 8.78, N 4.35.
C6D6): d=6.78 (s, 4H, m-CHIMes), 6.16 (t, J(H,H)=1.9 Hz, 4H, CHCp),
3
3
5.82 (t, J(H,H)=1.9 Hz, 4H, CHCp), 5.78 (s, 2H, CHIMes), 2.17 (s, 6H,
MeIMes), 1.92 (s, 12H, MeIMes), 0.37 ppm (s, 18H, SiMe3); 13C NMR
(125 MHz, C6D6): d=176.3 (CIMes), 140.6 (p-CIMes), 135.4 (i-CIMes), 135.1
(o-CIMes), 130.3 (m-CHIMes), 128.3 (CCp), 123.3 (CHIMes), 116.4 (CHCp),
105.6 (CHCp), 21.4 (MeIMes), 18.6 (MeIMes), 1.5 ppm (SiMe3); elemental
analysis calcd (%) for C37H50N2Si2Zn (644.37): C 68.97, H 7.82, N
4.35; found: C 67.71, H 8.50, N 5.23.
Data for 5
Data for 10
Colorless powder (10 mg, 0.016 mmol, 70%). 1H NMR (400 MHz,
C6D6): d=6.79 (s, 4H, m-CHIMes), 5.79 (s, 2H, CHIMes), 3.68 (br, 2H,
CHCp), 2.21 (s, 12H, MeCp), 2.11 (s, 6H, MeIMes), 1.90 (s, 12H, MeIMes),
1.86 ppm (s, 12H, MeCp); no 13C NMR data were available due to
low solubility; elemental analysis calcd (%) for C39H50N2Zn (612.22):
C 76.51, H 8.23, N 4.58; found: C 75.81, H 8.87, N 4.01.
Off-white powder (10 mg, 0.015 mmol, 52%). 1H NMR (400 MHz,
C6D6): d=6.80 (s, 4H, CHSIMes), 6.18 (t, 3J(H,H)=2.1 Hz, 4H, CHCp),
3
5.76 (t, J(H,H)=2.1 Hz, 4H, CHCp), 2.81 (s, 4H, CH2SIMes), 2.16 (s, 6H,
MeSIMes), 2.11 (s, 12H, MeSIMes), 0.37 ppm (s, 18H, SiMe3); 13C NMR
(125 MHz, C6D6): d=201.3 (CSIMes), 139.9 (i-CSIMes), 136.3 (o-CSIMes),
134.8 (p-CSIMes), 130.1 (m-CHSIMes), 128.6 (CCp), 117.6 (CHCp), 104.6
(CHCp), 51.11 (CH2SIMes), 21.4 (MeSIMes), 18.8 (MeSIMes), 1.4 (SiMe3); ele-
mental analysis calcd (%) for C37H52N2Si2Zn (646.38): C 68.75, H 8.11,
N 4.33; found: C 65.15, H 8.10, N 4.09.
Data for 6
Colorless powder (9 mg, 0.015 mmol, 65%). 1H NMR (400 MHz,
C6D6): d=6.80 (s, 4H, m-CHSIMes), 3.63 (br, 2H, CHCp), 2.87 (s, 4H,
CH2SIMes), 2.21 (s, 12H, MeCp), 2.11 (s, 6H, MeSIMes), 2.09 (s, 12H,
MeSIMes), 1.81 ppm (s, 12H, MeCp); no 13C NMR data were available
due to low solubility, elemental analysis calcd (%) for C39H52N2Zn
(614.23): C 76.26, H 8.53, N 4.56; found: C 77.57, H 9.04, N 4.14.
Synthesis of [Zn(OCHPh2)2(NHC)]2 (NHC=aItBu 11. IDipp 12)
These compounds were prepared in a similar fashion, and thus
only one preparation is detailed. A solution of in ItBu (6 mg,
0.033 mmol) in benzene (0.2 mL) was added to a solution of [Zn-
(OCHPh2)2] (15 mg, 0.031 mmol) in benzene (0.4 mL). After stirring
Chem. Eur. J. 2014, 20, 1 – 10
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overnight, the colorless precipitate was separated by decantation
and washed with toluene and n-pentane. After drying in vacuo,
the product was obtained.
Data for 14
Colorless crystals (9 mg, 0.015 mmol, 60%). 1H NMR (500 MHz,
C6D6): d=6.67 (s, 2H, CHItBu), 2.07 (s, 30H, MeCp*), 1.43 ppm (s, 18H,
tBu); 13C NMR (125 MHz, C6D6): d=205.8 (CItBu), 116.0 (CHItBu), 110.1
(CCp*), 56.6 (CtBu), 31.6 (MetBu), 11.2 ppm (MeCp*); elemental analysis
calcd (%) for C31H50N2Zn2 (581.52): C 64.03, H 8.67, N 4.82; found: C
62.02, H 9.13, N 4.66.
Data for 11
Colorless solid (16 mg, 0.014 mmol, 90%). 1H NMR (400 MHz,
3
4
CD2Cl2): d=7.74 (d, J(H,H)=7.1 Hz, 4H, o-CHPh1), 7.56 (d, J(H,H)=
1.6 Hz, 2H, 2-CHaItBu), 7.26 (t, J(H,H)=7.2 Hz, 4H, m-CHPh1), 7.19 (m,
3
Data for 15
3
4
4H, o-CHPh2), 7.10 (t, J(H,H)=7.3 Hz, 2H, p-CHPh1), 6.86 (d, J(H,H)=
1.7 Hz, 2H, 5-CHaItBu), 6.76 (m, 4H, m-CHPh2), 6.65 (m, 2H, p-CHPh2),
6.37 (s, 2H, OCH1), 6.02 (s, 2H, OCH), 1.40 (s, 18H, tBu), 1.24 ppm
(s, 18H, tBu); 13C NMR (125 MHz, CD2Cl2): d=155.3 (CaItBu), 154.1 (i-
C), 151.4 (i-C), 128.0 (CHPh), 127.9 (CHPh), 127.7 (CHPh), 127.7 (CHPh),
127.3 (5-CHaItBu), 126.2 (2-CHaItBu), 125.3 (CHPh), 125.2 (CHPh), 80.6
(OCH), 80.2 (OCH), 58.8 (CtBu), 56.8 (CtBu), 30.6 (MetBu), 30.5 ppm
(MetBu); elemental analysis calcd (%) for C74H84N4O4Zn2 (1224.26): C
72.60, H 6.92, N 4.58; found: C 72.63, H 6.68, N 3.58.
1
Yellow crystals (12 mg, 0.017 mmol, 68%). H NMR (500 MHz, C6D6):
d=6.79 (s, 4H, m-CHSIMes), 2.95 (s, 4H, CH2SIMes), 2.25 (s, 12H,
MeSIMes), 2.11 (s, 30H, MeCp*), 2.07 ppm (s, 6H, MeSIMes); 13C NMR
(125 MHz, C6D6): d=208.4 (CSIMes), 139.1 (i-CSIMes), 136.1 (o-CSIMes
)
135.3 (p-CSIMes), 130.2 (m-CHSIMes), 110.8 (CCp*), 51.2 (CH2SIMes), 21.3
(MeSIMes), 19.3 (MeSIMes), 12.0 ppm (MeCp*); elemental analysis calcd
(%) for C41H56N2Zn2 (707.67): C 69.59, H 7.98, N 3.96; found: C
68.82, H 9.02, N 4.07.
CCDC-994020 (3), CCDC-994021 (6), CCDC-994022 (7), CCDC-
994023 (11), CCDC-994024 (12), CCDC-994025 (13), CCDC-994026
(14), and CCDC-994027 (15) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Data for 12
Colorless crystalline solid (12 mg, 0.015 mmol, 83%). 1H NMR
(400 MHz, C6D6): d=7.35 (t, 3J(H,H)=7.8 Hz, 2H, p-CHIDipp), 7.20–
7.07 (br m, 14H, m-CHIDipp and o-, m-, p-CHPh), 6.36 (s, 2H, CHIDipp),
5.65 (s, 2H, OCH), 2.61 (hept, 3J(H,H)=6.9 Hz, 4H, CHiPr), 1.31 (d,
3J(H,H)=6.9 Hz, 12H, MeiPr), 1.03 ppm (d, 3J(H,H)=6.9 Hz, 12H,
MeiPr); 13C NMR (125 MHz, C6D6): d=178.6 (CIDipp), 152.5 (i-CPh),
146.4 (o-CIDipp), 134.7 (i-CIDipp), 131.3 (p-CHIDipp), 128.0, 127.5 and
125.8 (o-, m-, p-CHPh), 125.0 (m-CHIDipp), 124.1 (CHIDipp), 80.1 (OCH),
29.4 (CHiPr), 25.1 (MeiPr), 24.4 ppm (MeiPr); elemental analysis calcd
(%) for C53H58N2O2Zn (820.43): C 77.59, H 7.13, N 3.41; found: C
75.86, H 7.01, N 3.39.
Acknowledgements
P.J. is grateful to the Alexander von Humboldt Foundation for
a Feodor Lynen Research Fellowship and wants to thank Conor
Pranckevicius for his help with X-ray structure determination.
D.W.S. gratefully acknowledges the financial support of the
NSERC of Canada and the award of a Canada Research Chair.
Keywords: carbene ligands
·
homogeneous catalysis
·
Synthesis of [Zn(OCHtBu)2(IDipp)] (13)
hydrogenation · reaction mechanisms · zinc
tBu2CO (9 mg, 0.063 mmol) was added to a solution of 1 (10 mg,
0.030 mmol) and IDipp (12 mg, 0.031 mmol) in toluene (0.5 mL).
The reaction mixture was exposed to H2 (4 bar) for 4 d, after which
all volatiles were removed. The crude product was washed with n-
pentane and dried in vacuo to give an off-white powder (11 mg,
0.015 mmol, 50%). 1H NMR (400 MHz, C6D6): d=7.22 (t, 3J(H,H)=
7.7 Hz, 2H, p-CHIDipp), 7.11 (d, 3J(H,H)=7.7 Hz, 2H, m-CHIDipp), 6.33 (s,
2H, CHIDipp), 3.58 (s, 2H, OCH), 2.72 (hept, 3J(H,H)=6.9 Hz, 4H,
CHiPr), 1.51 (d, 3J(H,H)=6.9 Hz, 4H, MeiPr), 1.10 (s, 36H, MetBu),
[1] a) J. G. de Vries, C. J. Elsevier, The Handbook of Homogeneous Hydroge-
nation, Wiley-VCH, Weinheim, 2007; b) C. Jꢂkel, R. Paciello, Chem. Rev.
2006, 106, 2912–2942.
[2] a) D. J. Ager, A. H. M. de Vries, J. G. de Vries, Chem. Soc. Rev. 2012, 41,
3340–3380; b) H.-U. Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner, M.
Studer, Adv. Synth. Catal. 2003, 345, 103–151.
[3] a) L. J. Hounjet, D. W. Stephan, Org. Proc. Res. Dev. 2014, 18, 385–391;
b) Z.-X. Wang, L. Zhao, G. Lu, H. Li, F. Huang in Frustrated Lewis Pairs I,
Vol. 332 (Eds.: G. Erker, D. W. Stephan), Springer, Berlin Heidelberg,
2013, pp. 231–266; c) J. Paradies, Angew. Chem. 2014, 126, 3624–3629;
Angew. Chem. Int. Ed. 2014, 53, 3552–3557; d) J. Paradies, Angew. Chem.
Int. Ed. 2014, 53, 3552–3557.
[4] a) D. W. Stephan, S. Greenberg, T. W. Graham, P. Chase, J. J. Hastie, S. J.
Geier, J. M. Farrell, C. C. Brown, Z. M. Heiden, G. C. Welch, M. Ullrich,
Inorg. Chem. 2011, 50, 12338–12348; b) D. W. Stephan, G. Erker, Angew.
Chem. 2010, 122, 50–81; Angew. Chem. Int. Ed. 2010, 49, 46–76;
c) D. W. Stephan, G. Erker, Angew. Chem. Int. Ed. 2010, 49, 46–76;
d) D. W. Stephan, Org. Biomol. Chem. 2012, 10, 5740–5746; e) Q.-A.
Chen, Z.-S. Ye, Y. Duan, Y.-G. Zhou, Chem. Soc. Rev. 2013, 42, 497–511;
f) D. Chen, Y. Wang, J. Klankermayer, Angew. Chem. 2010, 122, 9665–
9668; Angew. Chem. Int. Ed. 2010, 49, 9475–9478; g) D. Chen, V. Leich,
F. Pan, J. Klankermayer, Chem. Eur. J. 2012, 18, 5184–5187; h) D. Chen,
J. Klankermayer, Chem. Commun. 2008, 2130–2131; i) J. M. Farrell, Z. M.
Heiden, D. W. Stephan, Organometallics 2011, 30, 4497–4500; j) J. M.
Farrell, J. A. Hatnean, D. W. Stephan, J. Am. Chem. Soc. 2012, 134,
15728–15731; k) T. Mahdi, Z. M. Heiden, S. Grimme, D. W. Stephan, J.
Am. Chem. Soc. 2012, 134, 4088–4091.
3
1.01 ppm (d, J(H,H)=6.9 Hz, 4H, MeiPr); 13C NMR (125 MHz, C6D6):
d=179.4 (CIDipp), 145.9 (o-CIDipp), 135.7 (o-CIDipp), 131.3 (p-CHIDipp),
125.1 (m-CHIDipp), 124.6 (CHIDipp), 90.0 (OCH), 38.9 (CtBu), 30.6 (MetBu),
29.5 (CHiPr), 24.9 (MeiPr), 24.5 ppm (CHiPr); elemental analysis calcd
(%) for C45H74N2O2Zn (740.47): C 72.99, H 10.07, N 3.78; found: C
72.82, H 9,73, N 3.89.
Synthesis of [Zn2Cp*2(NHC)] (NHC=ItBu 14, SIMes 15)
These compounds were prepared in a similar fashion, and thus
only one preparation is detailed.
A solution of ItBu (5 mg,
0.028 mmol) in toluene (0.5 mL) was added to a solution of 2
(10 mg, 0.025 mmol) in toluene (0.5 mL). All volatiles of the color-
less solution were removed, and the resulting powder was recrys-
tallized from toluene at À358C to give colorless crystals that were
dried in vacuo.
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[5] a) J. Spielmann, F. Buch, S. Harder, Angew. Chem. 2008, 120, 9576–9580;
Angew. Chem. Int. Ed. 2008, 47, 9434–9438; b) S. Harder, Chem. Rev.
2010, 110, 3852–3876; c) G. Zeng, S. Li, Inorg. Chem. 2010, 49, 3361–
3369.
[15] X. Liu, C. Cao, Y. Li, P. Guan, L. Yang, Y. Shi, Synlett 2012, 23, 1343.
[16] P. Jochmann, D. W. Stephan, Organometallics 2013, 32, 7503–7508.
[17] N. Merle, K. W. Tçrnroos, V. R. Jensen, E. Le Roux, J. Organomet. Chem.
2011, 696, 1691–1697.
[6] a) C. Federsel, C. Ziebart, R. Jackstell, W. Baumann, M. Beller, Chem. Eur.
J. 2012, 18, 72–75; b) W. H. Harman, J. C. Peters, J. Am. Chem. Soc.
2012, 134, 5080–5082; c) G. Zhang, B. L. Scott, S. K. Hanson, Angew.
Chem. 2012, 124, 12268–12272; Angew. Chem. Int. Ed. 2012, 51, 12102–
12106; d) J. Y. Wu, B. N. Stanzl, T. Ritter, J. Am. Chem. Soc. 2010, 132,
13214–13216; e) K. T. Sylvester, P. J. Chirik, J. Am. Chem. Soc. 2009, 131,
8772–8774; f) S. Monfette, Z. R. Turner, S. P. Semproni, P. J. Chirik, J. Am.
Chem. Soc. 2012, 134, 4561–4564; g) P. O. Lagaditis, P. E. Sues, J. F. Son-
nenberg, K. Y. Wan, A. J. Lough, R. H. Morris, J. Am. Chem. Soc. 2014,
136, 1367–1380.
[7] a) T. R. Jensen, L. E. Breyfogle, M. A. Hillmyer, W. B. Tolman, Chem.
Commun. 2004, 2504–2505; b) T. R. Jensen, C. P. Schaller, M. A. Hillmyer,
W. B. Tolman, J. Organomet. Chem. 2005, 690, 5881–5891; c) P. L.
Arnold, I. J. Casely, Z. R. Turner, R. Bellabarba, R. B. Tooze, Dalton Trans.
2009, 7236–7247; d) D. Wang, K. Wurst, M. R. Buchmeiser, J. Organomet.
Chem. 2004, 689, 2123–2130; e) G. Schnee, C. Fliedel, T. Avilꢃs, S. Dag-
orne, Eur. J. Inorg. Chem. 2013, 2013, 3699–3709; f) E. Piedra-Arroni, C.
Ladaviꢄre, A. Amgoune, D. Bourissou, J. Am. Chem. Soc. 2013, 135,
13306–13309.
[8] a) O. Jacquet, X. Frogneux, C. Das Neves Gomes, T. Cantat, Chem. Sci.
2013, 4, 2127–2131; b) W. Sattler, G. Parkin, J. Am. Chem. Soc. 2012,
134, 17462–17465; c) V. Bette, A. Mortreux, D. Savoia, J.-F. Carpentier,
Tetrahedron 2004, 60, 2837–2842; d) N. A. Marinos, S. Enthaler, M.
Driess, ChemCatChem 2010, 2, 846–853; e) S. Enthaler, K. Schrçder, S.
Inoue, B. Eckhardt, K. Junge, M. Beller, M. Drieß, Eur. J. Org. Chem. 2010,
2010, 4893–4901; f) S. Enthaler, B. Eckhardt, S. Inoue, E. Irran, M. Driess,
Chem. Asian J. 2010, 5, 2027–2035; g) C. Boone, I. Korobkov, G. I. Niko-
nov, ACS Catal. 2013, 3, 2336–2340.
[9] a) A. Rit, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. 2013, 125,
4762–4765; Angew. Chem. Int. Ed. 2013, 52, 4664–4667; b) A. Rit, T. P.
Spaniol, L. Maron, J. Okuda, Angew. Chem. Int. Ed. 2013, 52, 4664–4667.
[10] a) S. Enthaler, ACS Catal. 2013, 3, 150–158; b) Y. Yin, W. Ma, Z. Chai, G.
Zhao, J. Org. Chem. 2007, 72, 5731–5736; c) K. Okuma, J.-i. Seto, K.-i. Sa-
kaguchi, S. Ozaki, N. Nagahora, K. Shioji, Tetrahedron Lett. 2009, 50,
2943–2945; d) N. Meyer, K. Lçhnwitz, A. Zulys, P. W. Roesky, M. Doch-
nahl, S. Blechert, Organometallics 2006, 25, 3730–3734; e) K. Alex, A.
Tillack, N. Schwarz, M. Beller, ChemSusChem 2008, 1, 333–338; f) J.
Jenter, A. Lꢅhl, P. W. Roesky, S. Blechert, J. Organomet. Chem. 2011, 696,
406–418; g) P. Horrillo-Martꢆnez, K. C. Hultzsch, Tetrahedron Lett. 2009,
50, 2054–2056.
[18] R. Blom, J. Boersma, P. H. M. Budzelaar, B. Fischer, A. Haaland, H. V.
Volden, J. Weidlein, Acta Chem. Scand. 1986, 40a, 113–120.
[19] a) S. Mebs, M. A. Chilleck, S. Grabowsky, T. Braun, Chem. Eur. J. 2012, 18,
11647–11661; b) M. A. Chilleck, T. Braun, B. Braun, Chem. Eur. J. 2011,
17, 12902–12905; c) T. Li, S. Schulz, P. W. Roesky, Chem. Soc. Rev. 2012,
41, 3759–3771; d) S. Schulz, S. Schmidt, D. Blꢂser, C. Wçlper, Eur. J.
Inorg. Chem. 2011, 2011, 4157–4160.
[20] a) I. Resa, E. Carmona, E. Gutierrez-Puebla, A. Monge, Science 2004, 305,
1136–1138; b) D. del Rꢆo, A. Galindo, I. Resa, E. Carmona, Angew. Chem.
2005, 117, 1270–1273; Angew. Chem. Int. Ed. 2005, 44, 1244–1247; c) D.
del Rꢆo, A. Galindo, I. Resa, E. Carmona, Angew. Chem. 2005, 117, 1270–
1273; Angew. Chem. Int. Ed. 2005, 44, 1244–1247; d) A. Grirrane, I. Resa,
A. Rodriguez, E. Carmona, E. Alvarez, E. Gutierrez-Puebla, A. Monge, A.
Galindo, D. del Rꢆo, R. A. Andersen, J. Am. Chem. Soc. 2006, 128, 693–
703.
[21] R. Fernꢇndez, A. Grirrane, I. Resa, A. Rodrꢆguez, E. Carmona, E. ꢈlvarez,
E. Gutiꢃrrez-Puebla, ꢈ. Monge, J. M. Lꢉpez del Amo, H.-H. Limbach, A.
Lledꢉs, F. Maseras, D. del Rꢆo, Chem. Eur. J. 2009, 15, 924–935.
[22] a) M. H. Chisholm, J. C. Gallucci, H. Yin, H. Zhen, Inorg. Chem. 2005, 44,
4777–4785; b) M. H. Chisholm, N. W. Eilerts, J. C. Huffman, S. S. Iyer, M.
Pacold, K. Phomphrai, J. Am. Chem. Soc. 2000, 122, 11845–11854;
c) G. E. Coates, D. Ridley, J. Chem. Soc. A 1966, 1064–1069.
[23] a) Y. Wang, Y. Xie, M. Y. Abraham, R. J. Gilliard, P. Wei, C. F. Campana,
H. F. Schaefer, P. v. R. Schleyer, G. H. Robinson, Angew. Chem. 2012, 124,
10320–10323; Angew. Chem. Int. Ed. 2012, 51, 10173–10176; b) T. K.
Sen, S. C. Sau, A. Mukherjee, P. K. Hota, S. K. Mandal, B. Maity, D. Koley,
Dalton Trans. 2013, 42, 14253–14260.
[24] D. Schuchmann, U. Westphal, S. Schulz, U. Flçrke, D. Blꢂser, R. Boese,
Angew. Chem. 2009, 121, 821–824; Angew. Chem. Int. Ed. 2009, 48,
807–810.
[25] a) Z. Zhu, J. C. Fettinger, M. M. Olmstead, P. P. Power, Organometallics
2009, 28, 2091–2095; b) Z. Zhu, R. J. Wright, M. M. Olmstead, E. Rivard,
M. Brynda, P. P. Power, Angew. Chem. 2006, 118, 5939–5942; Angew.
Chem. Int. Ed. 2006, 45, 5807–5810; c) Z. Zhu, R. C. Fischer, J. C. Fetting-
er, E. Rivard, M. Brynda, P. P. Power, J. Am. Chem. Soc. 2006, 128, 15068–
15069.
[26] S. S. Hepperle, Y. A. Wang, J. Phys. Chem. 2008, 112, 9619–9622.
[27] K. Freitag, H. Banh, C. Ganesamoorthy, C. Gemel, R. W. Seidel, R. A.
Fischer, Dalton Trans. 2013, 42, 10540–10544.
[28] a) K. J. Iversen, D. J. D. Wilson, J. L. Dutton, Organometallics 2013, 32,
6209–6217; b) M. R. Momeni, E. Rivard, A. Brown, Organometallics 2013,
32, 6201–6208.
[11] A. Lennartson, Nat. Chem. 2014, 6, 166–166.
[12] a) P. Jochmann, D. W. Stephan, Angew. Chem. 2013, 125, 10014–10018;
Angew. Chem. Int. Ed. 2013, 52, 9831–9835; b) S. Werkmeister, S.
Fleischer, K. Junge, M. Beller, Chem. Asian J. 2012, 7, 2562–2568; c) S.
Werkmeister, S. Fleischer, S. Zhou, K. Junge, M. Beller, ChemSusChem
2012, 5, 777–782.
[29] A. J. Arduengo, J. R. Goerlich, W. J. Marshall, J. Am. Chem. Soc. 1995, 117,
11027–11028.
[13] S. Budagumpi, S. Endud, Organometallics 2013, 32, 1537–1562.
[14] A. J. Arduengo III, F. Davidson, R. Krafczyk, W. J. Marshall, M. Tamm, Or-
ganometallics 1998, 17, 3375–3382.
Received: March 31, 2014
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
9
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
Reaction mechanisms: A series of N-
heterocyclic carbene (NHC) adducts of
zincocenes [Zn(CpR)2(NHC)]
&
Homogeneous Catalysis
P. Jochmann, D. W. Stephan*
(CpR =C5HMe4, C5H4SiMe3; NHC=ItBu,
IDipp, IMes, SIMes) was isolated and
tested in the catalytic hydrogenation of
imines. Catalytic hydrogenation of ke-
tones was achieved with [ZnCp*2]
(Cp*=C5Me5)/NHC. Adducts of zinc alk-
oxides and [Zn2Cp*2] were isolated, and
their mechanistic implications are dis-
cussed (see scheme).
&& – &&
Zincocene and Dizincocene N-
Heterocyclic Carbene Complexes and
Catalytic Hydrogenation of Imines and
Ketones
&
&
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
10
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!