Dinuclear zinc hydride supported by an anionic bis(N-heterocyclic carbene) ligand

Article abstract of DOI:10.1002/asia.201301268

Methylene-linked bis(N,N′-di-tert-butylimidazol-2-ylidene) 1 reacted with diethylzinc to give dinuclear zinc ethyl compound 2, which contains a formally anionic bis(carbene) ligand as a result of deprotonation of the methylene bridge. The reaction of 2 with PhSiH₃ gave the phenylsilyl compound 3. The zinc hydride 4 was obtained by the reaction of 2 with LiAlH ₄ or Ph₃SiOH followed by treatment with PhSiH₃. X-ray diffraction studies show that compounds 2, 3, and 4 all have a similar dimeric structure with D_2h symmetry. The reaction of hydride 4 with carbon dioxide and N,N′-diisopropylcarbodiimide gave formato (5) and formamidinato (7) derivatives as a result of the insertion of the heterocumulene into both Zn-H bonds. Reaction with Ph₂CO gave the diphenylmethoxy compound 6. Hydride 4 shows catalytic activity in the hydrosilylation of 1,1-diphenylethylene and methanolysis of silanes. Copyright

Full text of DOI:10.1002/asia.201301268

FULL PAPER
DOI: 10.1002/asia.201301268
Dinuclear Zinc Hydride Supported by an Anionic Bis(N-Heterocyclic
Carbene) Ligand
Arnab Rit, Thomas P. Spaniol, and Jun Okuda*[a]
Abstract: Methylene-linked bis(N,N’-
di-tert-butylimidazol-2-ylidene) 1 react-
ed with diethylzinc to give dinuclear
zinc ethyl compound 2, which contains
or Ph3SiOH followed by treatment
with PhSiH3. X-ray diffraction studies
show that compounds 2, 3, and 4 all
have a similar dimeric structure with
D2h symmetry. The reaction of hydride
4 with carbon dioxide and N,N’-diiso-
propylcarbodiimide gave formato (5)
and formamidinato (7) derivatives as
a result of the insertion of the hetero-
À
a formally anionic bis
N
cumulene into both Zn H bonds. Re-
as a result of deprotonation of the
methylene bridge. The reaction of 2
with PhSiH3 gave the phenylsilyl com-
pound 3. The zinc hydride 4 was ob-
tained by the reaction of 2 with LiAlH4
action with Ph2CO gave the diphenyl-
methoxy compound 6. Hydride 4 shows
catalytic activity in the hydrosilylation
of 1,1-diphenylethylene and methanol-
ysis of silanes.
Keywords: carbon dioxide · homo-
geneous catalysis · hydrides · N-
heterocyclic carbenes · zinc
Introduction
NHC chelating ligand, methylidyne-3,3’-bis(N-tert-butylimi-
dazol-2-ylidene).
Zinc hydrides are used as reducing agents in organic synthe-
sis,[1] as models for zinc-containing enzymes,[2] and as molec-
ular precursors in materials science.[3] As a result of the low
thermal stability of polymeric (ZnH2)n,[4] more stable zinc
hydride derivatives of general formula RZnH were studied
and stabilized through chelation with monoanionic ligands[5]
or as Lewis base adducts, such as pyridine [(R)ZnH-
Results and Discussion
Dimeric Ethyl Compound 2
The bis-NHC ligand 1 was synthesized by following a litera-
ture report[12] and purified by crystallization from a saturated
solution in pentane at À358C. Treating a solution of 1 in
benzene with a 1m solution of ZnEt2 in hexane at ambient
ACHTUNGTRENNUNG
(pyridine)] (R=Et, Ph).[6] Most of these compounds contain
bridging hydrido ligands.[7] Well-defined zinc compounds
featuring a terminal hydride ligand remain still rare,[2,8] espe-
cially those containing both alkyl and hydrido ligands.[8e]
The extensive use of N-heterocyclic carbenes (NHCs)[9]
over the last two decades in homogeneous catalysis is based
on their ability to act as strong two-s-electron donors.
NHCs are well-established ligands for transition metals, but
used to a lesser extent for the Group 12 metal zinc.[10] We
have recently reported on molecular zinc dihydrides with
the composition [{ZnH2(L)}2] (L=1,3-bis(2,4,6-trimethyl-
phenyl/2,6-diisopropylphenyl)imidazol-2-ylidene) as adducts
with bulky NHC ligands.[11] Because the monodentate NHC
ligands are labile, we resorted to the use of a chelating
methylene-linked bis(N,N’-di-tert-butyl-imidazol-2-ylidene)
ligand.[12] Herein, we report the synthesis and reactivity of
a zinc hydride stabilized by a new type of monoanionic bis-
temperature gave the dimeric monoACTHNUTRGNEUNG(ethyl) compound 2 in
88% yield (Scheme 1).[10f] Compound 2 contains a monoa-
Scheme 1. Synthesis of compound 2.
nionic bisACHTNUTRGNE(UNG NHC) ligand of L2X type (L=neutral and X=
anionic ligand), which is formed by the unusual deprotona-
tion of the methylene proton of the ligand backbone by an
ethyl moiety under elimination of ethane. Clean formation
[a] Dr. A. Rit, Dr. T. P. Spaniol, Prof. Dr. J. Okuda
Institut fꢀr Anorganische Chemie
RWTH Aachen University
3
À
of 2 through activation of an sp -C H bond is remarkable
because reactions of similar types of ligand with other metal
alkyl reagents give either no activation or a mixture of prod-
ucts from which the activated products cannot be isolated in
a pure form.[13] Compound 2 is moderately soluble in tetra-
hydrofuran (THF), but not in aliphatic and aromatic hydro-
Landoltweg 1, 52056 Aachen (Germany)
Fax : (+49)241-80-92644
E-mail: jun.okuda@ac.rwth-aachen.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301268.
Chem. Asian J. 2014, 9, 612 – 619
612
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
J. Okuda et al.
carbons. It was characterized by NMR spectroscopy and
single-crystal X-ray diffraction.
1
The H NMR spectrum of 2 contains only one set of sig-
nals for both the NHC ligand and the ethyl group; this is
consistent with Ci symmetry in solution. In the 1H NMR
spectrum, deprotonation of the methylene group was evi-
dent from the integration ratio of the signal at d=5.82 ppm
relative to other resonances of the ligand and the ethyl
1
group. Ethane was detected by in situ H NMR spectrosco-
py. Coordination of the NHC ligand to the zinc center was
confirmed by a signal at d=186.4 ppm in the 13C{1H} NMR
spectrum; a value characteristic for a zinc(II)-bonded carbe-
ne.[10a–d]
Single crystals of the composition 2·2C4H8O were ob-
tained from a solution in THF/pentane at À358C. The mo-
lecular structure consists of a centrosymmetric dimer with
a core containing zinc atoms and methylidyne bridges of the
À
NHC ligands (Figure 1). As expected, the Zn CNHC bond
Scheme 2. Synthesis of compounds
3
and 4: a) PhSiH3, C6H6, 608C,
4 days; b) LiAlH4, THF, 8 h, 258C; c) i) Ph3SiOH, THF, 258C, 12 h;
ii) PhSiH3, THF, 258C, 2 h.
a result of the deprotonation of PhSiH3 by zinc ethyl in 2.
Compound 3 is highly soluble in THF and aromatic solvents,
but insoluble in aliphatic hydrocarbons. It was characterized
by multinuclear NMR spectroscopy along with single-crystal
X-ray diffraction.
1
The H NMR spectrum of 3 shows only one set of signals
for the NHC ligand and for the SiH2Ph group in a ratio of
1:1. The silyl groups give rise to a signal at d=4.0 ppm (1J-
1
N
at d=À57.34 ppm in the 29Si{1H} NMR spectrum in
[D8]THF.[15] The zinc-bound carbene signal was observed at
d=183.6 ppm in the 13C{1H} NMR spectrum. Molecular di-
hydrogen (1 bar) did not react with 3, even at 508C for
3 days.
Colorless crystals of the composition 3·2C4H8O suitable
for X-ray diffraction study were obtained from a solution in
THF/Et2O at À358C. The molecular structure consists of
a centrosymmetric dimer similar to 2 and with zinc coordi-
nated tetrahedrally by two NHC donors, one methylidyne
Figure 1. Molecular structure of 2. Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms and cocrystallized solvent mole-
cules of THF are omitted for clarity. Atoms labeled with an asterisk are
related by a symmetry operation (2Àx, Ày, 1Àz).
lengths[10b–d,11] (Zn1 C1 2.121(2) ꢂ, Zn1 C8 2.130(3) ꢂ) are
slightly longer than the bonds between zinc and the formally
À
À
À
À
À
anionic
ligands
(Zn1 C15*
2.108(2) ꢂ,
Zn1 C16
carbon atom and one SiH2Ph moiety (Figure 2). The Zn1
2.052(3) ꢂ). The zinc center coordinated by two NHC
donors, one methylidyne carbon and one ethyl group, is in
a distorted tetrahedral coordination environment. The dihe-
dral angle between two NHC rings of the same ligand is
130.3(1)8 with a bite angle of ligand 1 (C1-Zn1-C8) of
91.59(9)8. The Zn···Zn distance of 3.8367(6) ꢂ excludes
a bonding interaction between the zinc centers.
Si1 bond length of 2.4522(14) is comparable to that found in
related zinc silyl compounds.[15] The Zn···Zn distance
(3.692(2) ꢂ) is slightly shorter than that in 2.
Hydride Compound 4
The reaction of 2 with molecular dihydrogen (1 bar, 608C, 2
days) gave only unreacted starting material. When com-
pound 2 was treated with one equivalent of LiAlH4 at 258C
for 8 h in THF, hydride 4 was obtained (Scheme 2). This
compound was isolated as colorless crystals in 52% yield
after recrystallization from a solution in THF/pentane at
À358C. Compound 4 can also be synthesized by first react-
À
Phenylsilyl Compound 3
Metal alkyl complexes are reported to react with PhSiH3 to
give hydride complexes.[14] When compound 2 was treated
with PhSiH3, phenylsilyl compound 3 instead of a hydride
species was obtained (Scheme 2). Compound 3 was isolated
ing 2 with Ph3SiOH to yield [{ZnACHTNUTRGENN(GU OSiPh3)ACHTUGNTNER(NUGN 1 H)}2] (not iso-
1
as crystals in 39% yield. An in situ H NMR spectroscopy
lated) under ethane elimination, which readily reacts with
À
À
experiment showed the formation of ethane, probably as
PhSiH3 to give 4 in 71% yield through Zn OSiPh3/Si H
Chem. Asian J. 2014, 9, 612 – 619
613
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
J. Okuda et al.
Figure 2. Molecular structure of 3. Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms and solvent molecules are omit-
ted for clarity. Atoms labeled with an asterisk are related by a symmetry
operation (2Àx, Ày, 1Àz).
Figure 3. Molecular structures of 4. Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms, except that of the hydride coor-
dinated with zinc, are omitted for clarity. Atoms labeled with an asterisk
are related by a symmetry operation (1/2Àx, 1/2Ày, Àz).
bond metathesis. The reaction of 2 with other alcohols, such
as methanol and Me3SiOH, gave complicated product mix-
tures. Compound 4 is soluble in THF, but moderately solu-
ble in aromatic solvents, and is stable in solution at 258C for
days and can be stored for months at À358C.
Reactions of 4 with CO2, Ph2CO, and (iPrN)2C
Zinc catalysts are of potential interest for applications in the
copolymerization of CO2 with epoxides[16] and ring-opening
polymerization of lactide[17] to give polyesters. Zinc formates
are important intermediates in the Cu/ZnO-catalyzed syn-
thesis of methanol.[18] Compound 4 reacts readily with CO2
The hydride in 4 is characterized by an intense nZn vi-
À
H
brational absorption in the IR spectrum at n˜ =1548 cmÀ1.
This absorption shifts to a lower frequency of n˜ =1117 cmÀ1
(pACHTUGNTREN(UNNG CO2)=0.5 bar) at 258C to give formate species 5
À
for the corresponding deuteride isotopomer (nZn H/nZn
1.385). The H NMR spectrum in [D8]THF shows a broad
=
À
D
(Scheme 3) by double insertion into the Zn H bonds with
À
1
À
singlet at d=4.14 ppm for the Zn H resonance and a dou-
blet (3J
(H,H)=1.25 Hz) at d=5.48 ppm for the methylidyne
bridge proton because of the coupling with the hydrido frag-
1
ment, as evidenced by H–1H COSY NMR spectroscopy ex-
periments (see the Supporting Information). In the
13C{1H} NMR spectra, the zinc-bonded carbene signal is de-
tected at d=187.5 ppm.[10a–d]
Treating a solution of 4 in [D8]THF with D2 (1 bar) did
À
not result in any H D exchange, even at 608C over several
days. This type of inertness has also been observed for the
zinc dihydride [{ZnH2(L)}2] (L=1,3-bis(2,4,6-trimethylphe-
nyl)imidazol-2-ylidene (IMes), 1,3-bis(2,6-diisopropylpheny-
l)imidazol-2-ylidene, IPr).[11]
Crystals suitable for X-ray diffraction were obtained from
a solution in benzene/THF/pentane at 258C. The molecular
structure reveals that 4 is related to that of 2 and 3. The zinc
centers are tetrahedrally coordinated by two NHCs, one
alkyl donor and one terminal hydrido ligand (Figure 3). The
only reported compound that contains a terminal hydride
next to an alkyl moiety is [(Tptm=tris(2-pyridylthio)me-
thyl).[8e] The Zn H bond length of 1.61(3) ꢂ is 0.1 ꢂ longer
À
than those found in the four-coordinate compound [(k3-
Tptm)ZnH].[8e] The Zn CNHC bond lengths (Zn1 C1
À
À
À
2.105(2) ꢂ, Zn1 C9 2.095(2) ꢂ) are in the range of those
found for 2 and 3.
Scheme 3. Synthesis of compounds 5, 6, and 7: a) CO2 (0.5 bar), THF,
258C, 12 h; b) Ph2CO, THF, 258C, 2 h; c) CACTHNUTRGNE(UNG NiPr)2, THF, 258C, 2 h.
Chem. Asian J. 2014, 9, 612 – 619
614
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
J. Okuda et al.
retention of the dimeric structure. Compound 5 is insoluble
in common organic solvents, such as THF and aromatic hy-
drocarbons, but soluble in CH2Cl2 and dimethylsulfoxide
(DMSO).
compounds 6 and 7 were isolated as colorless crystals that
were highly soluble in common polar organic solvents.
In situ 1H NMR spectroscopic monitoring of both reac-
tions shows that the hydrido ligands in 4 have reacted within
1 h. The alkoxy unit in 6 was confirmed by the presence of
The formate unit in 5 is characterized by a singlet at d=
1
1
8.68 ppm in H NMR spectrum (CD2Cl2) and by a signal at
signals at d=6.17 and 83.0 ppm in the H and 13C{1H} NMR
d=167.9 ppm in the 13C{1H} NMR spectrum; these chemical
spectra, respectively. The formamidinato unit in 7 is charac-
terized by one singlet at d=6.74 ppm (1H NMR spectrum)
and by a signal at d=161.4 ppm (13C{1H} NMR spectrum).
These chemical shifts are similar to those reported for
shifts are in the range of those reported for [{Zn
(IMes)}2][11] and other related compounds.[19b] The IR spec-
trum of 5 shows two strong absorptions at n˜ =1621 (nasy(C=
(C=O)). The large separation be-
ACHTUGNTERN(NUGN O2CH)2
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
O)) and 1319 cmÀ1 (nsy
N
[Zn{CH
(iPrN)2}(Mesnacnac)]
(Mesnacnac=[HC{C(Me)-
tween these two absorptions (Dn=302 cmÀ1) indicates k1 co-
ordination of the formate unit,[19] which is in agreement with
the solid-state structure determination.
Colorless single crystals suitable for X-ray diffraction
were obtained from a solution in CH2Cl2/Et2O at 258C. The
The molecular structure of 7 is shown in Figure 5. The di-
meric structure contains a crystallographic inversion center
molecular structure shows that 5 is a dimer with two for-
1
À
mate groups in k -coordination mode (Figure 4). The Zn1
Figure 5. Molecular structure of 7. Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms, except for those of the formami-
dinato moieties and cocrystallized solvent molecules of THF, are omitted
for clarity. Atoms labeled with an asterisk are related by a symmetry op-
eration (2Àx, Ày, Àz).
À
within the Zn2C2 core. The Zn1 N5 bond length of
2.0181(16) ꢂ is in the range of other reported zinc formami-
Figure 4. Molecular structure of 5. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms, except for those of the formate moiet-
ies and cocrystallized solvent molecules of CH2Cl2, are omitted for clari-
ty. Atoms labeled with an asterisk are related by a symmetry operation
(1Àx, Ày, 1Àz).
dinato compounds.[19b] The N C bond lengths of the forma-
midinato group were found to be 1.343(3) and 1.293(3) ꢂ.
The non-coordinated N iPr fragments are directed away
from the central core to avoid steric crowding with the
À
À
À
O1 bond length (2.0204(17) ꢂ) is similar to that reported
bulky N tBu groups of the NHC ligands. The N5-C16-N6
bond angle (128.12(19)8) is similar to the observed O-C-O
bond angle (128.5(2)8) of the formate compound 5.
for the compound [(k3-Tptm)Zn
(O2CH)], with a terminal
zinc formate unit.[8e] The C O bond lengths within the for-
À
À
À
mate units (C1 O1 1.263(3) and C1 O2 1.220(3) ꢂ) indi-
cate partial delocalization of the double bond. The O1-C1-
O2 bond angle of 128.5(2)8 is in line with the sp2 character
of the formate carbon atom.
Hydrosilylation of DPE and Methanolysis of Silanes
Zinc hydride compounds have recently attracted attention
as an economical alternative to commonly used precious-
metal catalysts for catalytic silane coupling with protic sol-
vents, for example, alcohol or water, to give alkoxy or hy-
droxy silanes, respectively.[21] They are also reported to be
active in the catalytic hydrosilylation of aldehydes, ketones,
and CO2.[21a] The latter transformation allows the conversion
of cheap, abundant CO2 into renewable C1-feedstock chemi-
cals, such as ethyl formate and N,N-dimethylformamide.
When zinc hydride 4 was treated with benzophenone and
(iPrN)2C at ambient temperature, insertion of CO or CN
into both zinc hydride bonds resulted in the quantitative for-
mation of bisACHTUNGTRENNUNG(alkoxy) and bis(formamidinato) compounds 6
À
and 7, respectively. As with CO2, no insertion into the Zn
C(methylidyne bridge) was observed; this is in line with the
observation that dialkyl zinc only reacts with (iPrN)2C at
elevated temperature to give an insertion product.[20] Both
Chem. Asian J. 2014, 9, 612 – 619
615
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
J. Okuda et al.
Zinc hydride compounds have never been used in the cat-
alytic hydrosilylation of alkenes; an important reaction in
organic chemistry used to produce silicon-containing com-
pounds.[22] Therefore, we have tested 4 in the catalytic hy-
drosilylation of 1,1-diphenylethylene (DPE) and have moni-
Scheme 5. Catalytic methanolysis of silanes by using compound 4.
1
tored the conversion by H NMR spectroscopy. Preliminary
studies revealed that 4 is moderately active and only one re-
gioisomer (Markovnikov or 1,2-addition product) was selec-
tively obtained (Scheme 4). Hydrosilylation with PhSiH3 re-
gave the mono- and disubstituted products PhMeSiH
ACHTUNGTRENNUNG(OMe)
and PhMeSi(OMe)2, respectively, in a ratio of 93:7 within
AHCTUNGTRENNUNG
10 min. These results indicate the higher activity of 4 com-
pared with a related zinc hydride catalyst.[21b]
Conclusion
Scheme 4. Hydrosilylation of DPE catalyzed by 4.
Dimeric zinc ethyl compound 2, supported by an anionic
chelating bis-NHC ligand, was formed by the reaction of the
bis-NHC 1 with ZnEt2. The reaction of 2 with PhSiH3 gave
phenylsilyl compound 3 under ethane elimination. The zinc
hydride 4 was synthesized by reaction of 2 with LiAlH4 or
with Ph3SiOH followed by reaction with PhSiH3. X-ray crys-
tallography showed that compounds 2, 3, and 4 had similar
dimeric structures. The hydride 4 reacted with CO2 and
(iPrN)2C at ambient temperature to give formato (5) and
formamidinato (7) compounds, respectively. The formation
of compound 5 is of interest because of the relevance of
such species as intermediates in the Cu/ZnO-catalyzed syn-
thesis of methanol. Reaction with Ph2CO at ambient tem-
quires a higher reaction temperature than that required for
PhMeSiH2 when a stoichiometric amount of silane is used.
The tertiary silane PhMe2SiH did not give any hydrosilyla-
tion product (Table 1). When a slight excess of PhSiH3 with
Table 1. Hydrosilylation of DPE catalyzed by 4.
Silanes
T [8C]
t [h]
Conversion[a] [%]
25
18
3
15
53
PhSiH3
25[b]
60
89[c]
80[c]
88[c]
–
PhMeSiH2
PhMe2SiH
25
60(80)
12
6(4)
À
perature led to CO insertion into both Zn H bonds to give
[a] Conversion to [Ph2C(Me)ACHTNUGRTNEUNG
(SiRR1Ph)]; determined by in situ 1H NMR
diphenylmethoxy compound 6.
spectroscopy. [b] DPE/PhSiH3 =1:1.6 was used. [c] Remainder is
Ph2C(Me)H.
Compound 4 showed moderate activity in the hydrosilyla-
tion reaction of DPE to selectively give Markovnikov addi-
tion products. Compound 4 exhibited excellent activity in
the methanolysis of silanes to readily and selectively yield
respect to DPE (PhSiH3/DPE=1.6:1) was used, the reaction
rate was enhanced compared with that of the stoichiometric
reaction and 89% conversion was achieved within 3 h at
258C.
completely substituted poly
gen.
ACHTUNGTNER(NUNG methoxy)silanes and dihydro-
Apart from the hydrosilylation of DPE, the catalytic ac-
tivity of 4 in the methanolysis of silanes to give poly-
ACHTUNGTRENNUNG(methoxy)silanes and dihydrogen was investigated. This re-
Experimental Section
General Considerations
All manipulations were performed under an argon atmosphere by using
standard Schlenk or glove box techniques. Glassware was dried at 1308C
in an oven overnight. The solvents used for the synthesis and NMR spec-
troscopy experiments were dried, distilled, and degassed by standard
methods and stored over 4 ꢂ molecular sieves. NMR spectroscopy meas-
urements were performed on a Bruker Avance II 400 MHz spectrometer
À
action not only gives access to Si O bond containing com-
pounds for various applications in organosilicon[23] and relat-
ed materials chemistry,[24] but is also reported to be a potent
method for on-demand dihydrogen generation from liquid
storage materials.[21a]
1
at 238C. The chemical shifts (d, in ppm) of the H NMR spectra were ref-
1H NMR spectroscopic analysis revealed that all em-
ployed silanes (PhRSiH2, R=H, Me, and Ph) were selec-
tively converted with a requisite amount of methanol into
the corresponding substituted methyl siloxanes with a low
catalyst loading (0.5 mol%, 258C) within 10 min
(Scheme 5). No traces of partially substituted products were
obtained and crude reaction products contained only com-
pletely substituted methyl siloxanes, as determined by in
erenced to the residual proton signals of the deuterated solvents and re-
ported relative to tetramethylsilane. IR spectra were measured by using
an AVATAR 380 FTIR spectrometer. Abbreviations reported for the IR
spectra are as follows: w (weak), m (medium), and s (strong). Carbene
1 was prepared according to literature procedures[12] and PhSiD3 was pre-
pared by LiAlD4 reduction of PhSiCl3.[25] All other chemicals were used
as received without further purification. Elemental analyses were per-
formed in the Microanalytical Laboratory of this department.
1
situ H NMR spectroscopy. When one equivalent of metha-
nol was used under the same reaction conditions, PhMeSiH2
Chem. Asian J. 2014, 9, 612 – 619
616
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
J. Okuda et al.
Synthesis of 2
Synthesis of [D2]4
A 1m solution of Et2Zn in hexane (0.2 mL, 0.2 mmol) was slowly added
to a solution of 1 (0.052 g, 0.2 mmol) in benzene (2 mL). The reaction
mixture was stirred for 2 h at 258C. Pentane (5 mL) was added and the
precipitate was isolated by centrifugation, washed with pentane, and
dried under dynamic vacuum to give 2 (0.062 g, 0.088 mmol, 88%). Crys-
tals suitable for crystallography were obtained by slow diffusion of pen-
tane into a saturated solution of the compound in THF at À358C.
This compound was prepared from compound 2 (0.021 g, 0.03 mmol) and
Ph3SiOH (0.017 g, 0.06 mmol) by using PhSiD3 (0.007 g, 0.066 mmol) in-
stead of PhSiH3 in 82% yield (0.016 g, 0.0245 mmol). IR (KBr): n˜ =3122
(m), 3082 (w), 2968 (s), 2933 (w), 2872 (w), 1479 (w), 1470 (w), 1411 (m),
1391 (m), 1370 (s), 1314 (w), 1291 (m), 1247 (m), 1228 (m), 1209 (s), 1147
(w), 1117 (s, nZnÀD), 1089 (w), 1021 (w), 997 (w), 826 (w), 798 (w), 749
(m), 720 (s), 712 (s), 674 (m), 585 (s), 522 (s), 503 (w), 462 cmÀ1 (w).
1H NMR (400.1 MHz, [D8]THF): d=6.90 (d, 3J
ACHTUNGTRENNUNG
Synthesis of 5
H), 6.77 (d, 3J
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A solution of 4 (0.026 g, 0.04 mmol) in THF (2 mL) was pressurized with
0.5 bar of CO2 at room temperature for 10 min, resulting in the precipita-
tion of a white solid. After standing overnight, the precipitate was fil-
tered, washed with THF and pentane, and dried under reduced pressure
(0.025 g, 0.034 mmol, 85%). Colorless crystals suitable for X-ray diffrac-
tion analysis were grown from CH2Cl2/Et2O at 258C. 1H NMR
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG(CH3)3), 31.8 (CACHTUNGTRENNUNG(CH3)3), 16.4 (CH2CH3), 7.4 ppm (CH2CH3); elemental
analysis calcd (%) for C34H56N8Zn2 (707.63 gmolÀ1): C 57.71, H 7.98, N
15.84, Zn, 18.48; found: C 55.68, H 8.35, N 14.94, Zn 18.65.
(400.1 MHz, CD2Cl2, 298 K): d=8.68 (s, 2H; OCHO), 7.03 (d, 3J
1.76 Hz, 4H; Im-H), 6.85 (d, 3J
(H,H)=1.76 Hz, 4H; Im-H), 6.07 (s, 2H;
CH), 1.33 ppm (s, 36H; C
(CH3)3); 13C{1H} NMR (100.6 MHz, CD2Cl2):
d=180.4 (carbene-C), 167.9 (OCHO), 120.2 (Im-C), 115.7 (Im-C), 77.6
(CH), 57.4 (C(CH3)3), 31.1 ppm (C(CH3)3); IR (KBr): n˜ =3152 (w), 3121
(m), 3084 (w), 2975 (s), 2933 (w), 2875 (w), 2784 (m), 2699 (w), 1621 (s,
nasy(C=O)), 1478 (w), 1470 (w), 1412 (w), 1394 (m), 1371 (s), 1319 (s, nsy-
ACHTUNGTRENNUNG(H,H)=
Synthesis of 3
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
A solution of PhSiH3 (0.018 g, 0.168 mmol) in benzene (0.5 mL) was
added to a suspension of 2 (0.03 g, 0.042 mmol) in benzene (0.5 mL). The
reaction mixture was heated to 608C overnight. All solids dissolved to
give a clear solution. The reaction mixture was stirred for 4 days at this
temperature. During this time a crystalline solid precipitated. After the
reaction time, the reaction mixture was evaporated to dryness, dissolved
in benzene, and filtered over Celite. The filtrate was concentrated and
hexane was added. The obtained precipitate was washed with hexane and
dried in dynamic vacuum. Pure product was obtained after recrystalliza-
tion (0.014 g, 0.0162 mmol, 39%). Single crystals suitable for X-ray struc-
ture determination were obtained from THF/Et2O at À358C. 1H NMR
(400.1 MHz, [D8]THF): d=7.67–7.70 (m, 4H, oPh), 7.14–7.17 (m, 4H,
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGERTG(NNUN C=O)), 1289 (m), 1248 (m), 1232 (m), 1209 (s), 1151 (w), 1115 (s), 1082
(w), 825 (w), 756 (s), 742 (s), 719 (s), 678 (m), 585 (m), 505 cmÀ1 (m); ele-
mental analysis calcd (%) for C32H48N8Zn2O4 (739.544 gmolÀ1): C 51.97,
H 6.54, N 15.16; found: C 52.25, H 6.62, N 14.46.
Synthesis of 6
Ph2C(=O) (0.012 g, 0.066 mmol) in THF (1 mL) was slowly added to a so-
lution of 4 (0.020 g, 0.031 mmol) in THF (2 mL). After stirring the reac-
tion mixture for 2 h at 258C, all volatile compounds were removed under
dynamic vacuum. The remaining residue was washed with hexane and
dried in dynamic vacuum to yield a white solid (0.025 g, 0.0246 mmol,
79%). 1H NMR (400.1 MHz, [D8]THF, 298 K): d=7.60–7.62 (m, 8H;
mPh), 7.06 (tt, 3J
(H,H)=1.76 Hz, 4H; Im-H), 6.69 (d, J
(s, 2H; CH), 4.00 (1J
(Si,H)=139 Hz, 4H; SiH2Ph), 1.34 ppm (s, 36H; C-
(CH3)3); 13C{1H} NMR (100.6 MHz, [D8]THF): d=183.6 (carbene-C),
147.6 (iPh), 137.2 (oPh), 127.5 (mPh), 126.1 (pPh), 119.5 (Im-C), 115.5
(Im-C), 79.5 (CH), 57.6 (C(CH3)3), 32.1 ppm (C
(CH3)3); 29Si{1H} NMR
ACHTUNGTRENNUNG ACHTUNGTRENNUGN
(H,H)=7.4 Hz, 4J(H,H)=1.3 Hz, 2H; pPh), 6.86 (d, 3J-
3
G
ACHTUNGTNER(NUNG H,H)=1.76 Hz, 4H; Im-H), 5.51
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
oPh), 7.14–7.17 (m, 8H; mPh), 7.01 (tt, 3J
1.3 Hz, 4H; pPh), 6.79 (d, 3J(H,H)=1.8 Hz, 4H; Im-H), 6.42 (d, 3J-
(H,H)=1.5 Hz, 4H; Im-H), 6.17 (s, 2H; OCHPh2), 5.01 (s, 2H; CH),
1.35 ppm (s, 36H; C
(CH3)3); 13C{1H} NMR (100.6 MHz, [D8]THF): d=
183.0 (carbene-C), 155.6 (iPh), 128.6 (oPh), 128.0 (pPh), 125.5 (mPh),
119.2 (Im-C), 115.1 (Im-C), 83.0 (OCHPh2), 78.2 (CH), 57.5 (C(CH3)3),
ACHUTNGRENNUG ACHTUNGTRENNUNG(H,H)=
(H,H)=7.3 Hz, 4J
A
ACHTUNGTRENNUNG
(99.4 MHz, [D8]THF): d=À57.34 ppm; elemental analysis calcd (%) for
C42H60N8Si2Zn2 (863.92 gmolÀ1): C 58.39, H 7.00, N 12.97; found: C 56.96,
H 7.53, N 12.27.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Synthesis of 4
ACHTUNGTRENNUNG
31.7 ppm (CACTHUNGTRNEUG(N CH3)3); elemental analysis calcd (%) for C56H68N8Zn2O2
Method A: Compound 2 (0.057 g, 0.08 mmol) and Ph3SiOH (0.046 g,
0.164 mmol) were mixed in THF (3 mL) and stirred for 12 h at 258C.
After the reaction time, PhSiH3 (22 mL, 0.176 mmol) in THF (1 mL) was
added to the reaction mixture and stirred for 2 h at 258C. The reaction
mixture was filtered, evaporated to dryness, washed first with cold Et2O
(1 mL) and then with hexane/Et2O, and dried in vacuo to give a white
solid (0.037 g, 0.057 mmol, 71%).
(1015.944 gmolÀ1): C 66.20, H 6.75, N 11.03; found: C 65.45, H 7.02, N
10.27.
Synthesis of 7
N,N’-Diisopropylcarbodiimide (0.024 g, 0.092 mmol) in THF (1 mL) was
slowly added to a solution of 4 (0.028 g, 0.043 mmol) in THF (3 mL).
After stirring the reaction mixture for 2 h at 258C, all volatile compounds
were removed under dynamic vacuum. The remaining residue was
washed with cold pentane and dried in vacuo to give a white solid
(0.030 g, 0.0335 mmol, 78%). Crystals suitable for X-ray diffraction were
obtained from THF/Et2O at À358C. 1H NMR (400.1 MHz, [D8]THF,
Method B: A solution of LiAlH4 (0.005 g, 0.132 mmol) in THF (3 mL)
was added to a suspension of compound 2 (0.092 g, 0.13 mmol) in THF
(1 mL) over 5 min. The reaction mixture was stirred for 8 h at 258C. The
solution was filtered over Celite to give a clear, light-yellow filtrate. The
solution was concentrated to half of its volume and pentane was diffused
to precipitate a white solid. The solid was isolated and dried in vacuo
(0.044 g, 0.068 mmol, 52%). Crystals suitable for X-ray structure determi-
nation were obtained from a solution in benzene/THF/pentane at 258C.
298 K): d=7.01 (d, 3J(H,H)=1.8 Hz, 4H; Im-H), 6.98 (d, 3J
ACTHNUGTRENNUGN ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
1H NMR (400.1 MHz, [D8]THF): d=6.88 (d, 3J
H), 6.81 (d, 3J(H,H)=1.8 Hz, 4H; Im-H), 5.48 (d, 3J
pling with hydride, 2H; CH), 4.14 (s, 2H; Zn-H), 1.41 ppm (s, 36H; C-
(CH3)3); 13C{1H} NMR (100.6 MHz, [D8]THF): d=187.5 (carbene-C),
119.5 (Im-C), 114.4 (Im-C), 80.4 (CH), 57.6 (C(CH3)3), 31.6 ppm (C-
(CH3)3); IR (KBr): n˜ =3121 (w), 3079 (w), 2974 (s), 2936 (w), 2907 (w),
(H,H)=1.8 Hz, 4H; Im-
(H,H)=1.3 Hz, cou-
A
R
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
(CH
(CH3)2), 31.8 (C
E
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
2870 (w), 1548 (s, nZn H), 1478 (w), 1466 (w), 1407 (w), 1391 (m), 1367 (s),
À
1315 (w), 1283 (m), 1249 (m), 1229 (m), 1209 (s), 1147 (w), 1114 (s), 1081
(w), 1021 (w), 995 (w), 826 (w), 790 (w), 755 (m), 703 (s), 670 (m), 593
(s), 516 (s), 481 cmÀ1 (m); elemental analysis calcd (%) for C30H48N8Zn2
(651.52 gmolÀ1): C 55.30, H 7.43, N 17.20; found: C 55.72, H 7.84, N
16.26.
Catalytic Hydrosilylation of DPE with Silanes by Using 4
PhSiH3 (6.5 mg, 0.06 mmol), PhMeSiH2 (7.3 mg, 0.061 mmol), or
PhMe2SiH (8.2 mg, 0.06 mmol) and DPE (11 mg, 0.06 mmol) were added
in this order to a solution of 4 (2 mg, 3 mmol, 5.0 mol%) in [D8]THF
Chem. Asian J. 2014, 9, 612 – 619
617
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
J. Okuda et al.
(0.6 mL). The conversion was monitored by in situ 1H NMR spectrosco-
py.
Acknowledgements
We gratefully acknowledge financial support from the Cluster of Excel-
lence RWTH Aachen “Tailor-Made Fuels from Biomass” and the Deut-
sche Forschungsgemeinschaft through the International Research Train-
ing Group “Selectivity in Chemo- and Biocatalysis”.
Catalytic Methanolysis of Silanes with 4
PhMeSiH2 (41 mL, 0.3 mmol) and methanol (25 mL, 0.615 mmol) were
added in this order by means of a microliter syringe to a solution of 4
(1 mg, 3 mmol, 0.5 mol%) in [D8]THF (0.6 mL), resulting in the rapid
evolution of hydrogen gas and in the formation of the cross-dehydrocou-
pled product. The reaction mixture was monitored by 1H NMR spectros-
[1] a) Y. Gao, H. Urabe, F. Sato, J. Org. Chem. 1994, 59, 5521–5523;
b) Y. Gao, K. Harada, T. Hata, H. Urabe, F. Sato, J. Org. Chem.
1995, 60, 290–291; c) M. Uchiyama, F. Furumoto, M. Saiko, Y.
Kondo, T. Sakamato, J. Am. Chem. Soc. 1997, 119, 11425–11433.
[2] M. Rombach, H. Brombacher, H. Vahrenkamp, Eur. J. Inorg. Chem.
2002, 153–159.
copy, which showed complete conversion to PhMeSiACTHUNRGTNEUNG(OMe)2 within
10 min. In an analogous fashion, the reaction of PhSiH3 (37 mL,
0.3 mmol) with methanol (43 mL, 1.05 mmol) or Ph2SiH2 (56 mL,
0.3 mmol) with methanol (25 mL, 0.615 mmol) gave hydrogen and PhSi-
A
ACHTUNGTRENNUNG(OMe)2, respectively, within 10 min.
[3] W. Marciniak, K. Merz, M. Moreno, M. Driess, Organometallics
2006, 25, 4931–4933.
[4] a) A. E. Finholt, A. C. Bond, Jr., H. I. Schlesinger, J. Am. Chem.
Soc. 1947, 69, 1199–1203; b) J. J. Watkins, E. C. Ashby, Inorg. Chem.
1974, 13, 2350–2354; c) E. C. Ashby, J. J. Watkins, D. Greig, D. F.
Shriver, Inorg. Synth. 1977, 17, 6–8.
[5] For selected references, see: a) N. A. Bell, A. L. Kassyk, J. Organo-
met. Chem. 1988, 345, 245–251; b) A. Looney, R. Han, I. B. Gorrell,
M. Cornebise, K. Yoon, G. Parkin, A. L. Rheingold, Organometal-
lics 1995, 14, 274–288; c) H. Hao, C. Cui, H. W. Roesky, G. Bai, H.-
G. Schmidt, M. Noltemeyer, Chem. Commun. 2001, 1118–1119.
[6] A. J. De Koning, J. Boersma, G. J. M. Van Der Kerk, J. Organomet.
Chem. 1978, 155, C5–C7.
[7] a) M. P. Coles, S. M. El-Hamruni, J. D. Smith, P. B. Hitchcock,
Angew. Chem. 2008, 120, 10301–10304; Angew. Chem. Int. Ed. 2008,
47, 10147–10150; b) S. Schulz, T. Eisenmann, D. Schuchmann, M.
Bolte, M. Kirchener, R. Boese, J. Spielmann, S. Harder, Z. Natur-
forsch. B 2009, 64, 1397–1400; c) Z. Zhu, J. C. Fettinger, M. M.
Olmstead, P. P. Power, Organometallics 2009, 28, 2091–2095; d) Z.
Zhu, R. J. Wright, M. M. Olmstead, E. Rivard, M. Brynda, P. P.
X-ray crystallography data were collected on a Bruker CCD area-detec-
tor diffractometer with MoKa radiation (monolayer optics, l=0.71073 ꢂ)
by using w scans.[26a] The SMART program package was used for the
data collection and unit cell determination; processing of the raw frame
data was performed by using SAINT; absorption corrections were ap-
plied with SADABS.[26b] The structures were solved by direct methods
and refined against F2 by using all reflections with SHELXL-97, as imple-
mented in the WinGX program system.[26c–e] Non-hydrogen atoms were
refined anisotropically and hydrogen atoms were included with idealized
geometries. Only the hydrogen atoms of the CH unit between the imida-
zolidine rings (H15 in 2, H8 in 3 and 4, H9 in 5, and H8 in 7), the for-
mate (H1 in 5) and the formamidinate (H16 in 7), as well as the hydride
(H1 in 4) were refined in their position. Crystallographic and refinement
data for compounds 2, 3, 4, 5, and 7 is given in Table 2.
CCDC 930532 (2), 930533 (3), 930534 (4), 930535 (5), and 930536 (7)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from the Cambridge Crystallographic
Data Center via www.ccdc.cam.ac.uk/data_request/cif.
Table 2. Crystallographic and refinement data for compounds 2, 3, 4, 5, and 7.
2
3
4
5
7
formula
C34H56N8Zn2·2
851.52
G
R
C32H48N8O4Zn2·2
909.38
G
ACHTUNGTRENNUNG(C4H8O)
Mr [gmolÀ1
]
1008.11
1192.32
crystal size [mm]
crystal color and habit
crystal system
space group
a [ꢂ]
b [ꢂ]
c [ꢂ]
a [8]
b [8]
0.29ꢃ0.08ꢃ0.08
colorless rod
monoclinic
P21/c
11.3917(13)
10.3514(11)
18.589(2)
90.00
96.3733(19)
90.00
2178.4(4)
2
0.19ꢃ0.19ꢃ0.07
colorless plate
triclinic
0.20ꢃ0.15ꢃ0.11
colorless prism
monoclinic
C2/c
22.4773(19)
7.8926(7)
18.4466(16)
90.00
95.5660(16)
90.00
3257.1(5)
4
0.30ꢃ0.20ꢃ0.08
colorless block
monoclinic
P21/c
10.8353(9)
10.1314(8)
18.5847(15)
90.00
91.9958(13)
90.00
2038.9(3)
2
0.25ꢃ0.18ꢃ0.18
colorless plate
monoclinic
P21/c
10.7577(8)
17.8897(14)
16.8086(13)
90.00
99.6643(12)
90.00
3188.9(4)
2
¯
P1
9.504(7)
12.026(9)
12.774(10)
64.330(12)
86.365(14)
81.984(13)
1303.1(17)
1
g [8]
V [ꢂ3]
Z
1calcd [gcmÀ3
T [K]
]
1.299
100(2)
1.145
1.285
100(2)
1.012
1.329
100(2)
1.505
1.481
100(2)
1.485
1.242
100(2)
0.805
m
A
]
F
(000)
912
536
1376
944
1288
q range [8]
hkl indices
2.26–26.50
Æ14, Æ12, Æ23
1.77–28.59
Æ12, Æ16, Æ17
1.82–29.56
À31 to +29, Æ10,
Æ25
2.19–28.51
Æ14, Æ13, Æ24
1.68–26.46
Æ13, Æ22, Æ21
reflns collected
25696
18186
21770
27745
37312
reflns observed [I>2s(I)] 3719
independent reflns (Rint 4494 (0.0685)
data/restraints/parameters 4494/0/264
5254
3680
4371
5485
)
6530 (0.0620)
6530/0/303
0.0493, 0.1062
0.0629, 0.1135
1.043
4290 (0.0492)
4290/0/193
0.0429, 0.1062
0.0510, 1116
1.083
5162 (0.0509)
5162/0/247
0.0390, 0.1037
0.0486, 0.1109
1.040
6574 (0.0467)
6574/0/368
0.0351, 0.0793
0.0480, 0.0861
1.077
R1, wR2 [I>2 s(I)]
R1, wR2 (all data)
goodness of fit on F2
0.0385, 0.0856
0.0502, 0.929
1.062
D1max, min [eꢂÀ3
]
0.456, À0.390
0.754, À0.853
2.532, À0.310
0.548, À0.985
0.488, À0.355
Chem. Asian J. 2014, 9, 612 – 619
618
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
J. Okuda et al.
Power, Angew. Chem. 2006, 118, 5939–5942; Angew. Chem. Int. Ed.
2006, 45, 5807–5810; e) I. L. Fedushkin, O. V. Eremenko, A. A. Ska-
tova, A. V. Piskunov, G. K. Fukin, S. Y. Ketkov, E. Irran, H. Schu-
mann, Organometallics 2009, 28, 3863–3868.
E. B. Lobkovsky, G. W. Coates, J. Am. Chem. Soc. 2003, 125, 11911–
11924; d) M. Krçger, C. Folli, O. Walter, M. Dçring, Adv. Synth.
Catal. 2006, 348, 1908–1918; e) M. Cheng, N. A. Darling, E. B. Lob-
kovsky, G. W. Coates, Chem. Commun. 2000, 2007–2008; f) M.
Cheng, D. R. Moore, J. J. Reczek, B. M. Chamberlain, E. B. Lobkov-
sky, G. W. Coates, J. Am. Chem. Soc. 2001, 123, 8738–8749.
[17] a) M. Cheng, A. B. Attygalle, E. B. Lobkovsky, G. W. Coates, J. Am.
Chem. Soc. 1999, 121, 11583–11584; b) M. H. Chisholm, J. C. Huff-
man, K. Phomphrai, J. Chem. Soc. Dalton Trans. 2001, 222–224;
c) L. R. Rieth, D. R. Moore, E. B. Lobkovsky, G. W. Coates, J. Am.
Chem. Soc. 2002, 124, 15239–15248; d) for a review see: J. Wu, T.-L.
Yu, C.-T. Chen, C.-C. Lin, Coord. Chem. Rev. 2006, 250, 602–626.
[18] a) G. C. Chinchen, P. J. Denny, D. G. Parker, M. S. Spencer, D. A.
Whan, Appl. Catal. 1987, 30, 333–338; b) G. C. Chinchen, K. Mans-
field, M. S. Spencer, CHEMTECH 1990, 20, 692–699; e) K. C.
Waugh, Catal. Today 1992, 15, 51–75; c) M. S. Spencer, Top. Catal.
1999, 8, 259–266; d) M. Kurtz, J. Strunk, O. Hinrichsen, M. Muhler,
K. Fink, B. Meyer, C. Wçll, Angew. Chem. 2005, 117, 2850–2854;
Angew. Chem. Int. Ed. 2005, 44, 2790–2794.
[19] a) X.-M. Chen, Y.-X. Tong, T. C. W. Mak, Inorg. Chem. 1994, 33,
4586–4588; b) S. Schulz, T. Eisenmann, S. Schmidt, D. Blꢄser, U.
Westphal, R. Boese, Chem. Commun. 2010, 46, 7226–7228.
[20] a) M. Mꢀnch, U. Flçrke, M. Bolte, S. Schulz, D. Gudat, Angew.
Chem. 2008, 120, 1535–1537; Angew. Chem. Int. Ed. 2008, 47, 1512–
1514; b) S. Schulz, M. Mꢀnch, U. Flçrke, Z. Anorg. Allg. Chem.
2008, 634, 2221–2225; c) S. Schmidt, S. Gondzik, S. Schulz, D.
Blꢄser, R. Boese, Organometallics 2009, 28, 4371–4376.
[21] a) W. Sattler, G. Parkin, J. Am. Chem. Soc. 2012, 134, 17462–17465;
b) D. Mukherjee, R. R. Thompson, A. Ellern, A. D. Sadow, ACS
Catal. 2011, 1, 698–702.
[22] I. Ojima, Z. Li, J. Zhu, The Chemistry of Organosilicon Compounds,
Vol. 2 (Eds.: Z. Rappoport, Y. Apeloig), Wiley, New York, 1998,
p. 1687.
[23] a) T. W. Greene, P. G. Wuts, Protective Groups in Organic Synthesis,
2nd ed., Wiley, New York, 1991; b) F. Luo, C. Pan, J. Cheng, Curr.
Org. Chem. 2011, 15, 2816–2829.
[24] a) B. G. Trewyn, I. I. Slowing, S. Giri, H.-T. Chen, V. S.-Y. Lin, Acc.
Chem. Res. 2007, 40, 846–853; b) S. Huh, J. W. Wiench, B. G.
Trewyn, S. Song, M. Prusi, V. S.-Y. Lin, Chem. Commun. 2003,
2364–2365.
[8] a) R. Han, I. B. Gorrell, A. G. Looney, G. J. Parkin, J. Chem. Soc.
Chem. Commun. 1991, 717–719; b) W. Klꢄui, U. Schilde, M.
Schmidt, Inorg. Chem. 1997, 36, 1598–1601; c) C. Bergquist, L.
Koutcher, A. L. Vaught, G. Parkin, Inorg. Chem. 1999, 38, 422–423;
d) D. Mukherjee, A. Ellern, A. D. Sadow, J. Am. Chem. Soc. 2010,
132, 7582–7583; e) W. Sattler, G. Parkin, J. Am. Chem. Soc. 2011,
133, 9708–9711.
[9] For reviews on NHCs, see: a) F. E. Hahn, M. C. Jahnke, Angew.
Chem. 2008, 120, 3166–3216; Angew. Chem. Int. Ed. 2008, 47, 3122–
3172; b) M. Melaimi, M. Soleilhavoup, G. Bertrand, Angew. Chem.
2010, 122, 8992–9032; Angew. Chem. Int. Ed. 2010, 49, 8810–8849.
[10] a) A. J. Arduengo III, H. V. R. Dias, F. Davidson, R. L. Harlow, J.
Organomet. Chem. 1993, 462, 13–18; b) T. R. Jensen, L. E. Breyfo-
gle, M. A. Hillmyer, W. B. Tolman, Chem. Commun. 2004, 2504–
2505; c) T. R. Jensen, C. P. Schaller, M. A. Hillmyer, W. B. Tolman,
J. Organomet. Chem. 2005, 690, 5881–5891; d) D. Wang, K. Wurst,
M. R. Buchmeiser, J. Organomet. Chem. 2004, 689, 2123–2130;
e) For review on group 12 metal NHC complexes see: S. Budagum-
pi, S. Endud, Organometallics 2013, 32, 1537–1562; f) the reaction
of 1 with ZnMe2 first gave a neutral adduct, which underwent trans-
formation to give a compound analogous to 2, S. Dagorne, C. Flie-
del, private communication.
[11] A. Rit, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. 2013, 125,
4762–4765; Angew. Chem. Int. Ed. 2013, 52, 4664–4667.
[12] a) K. D. Wells, M. J. Ferguson, R. McDonald, M. Cowie, Organome-
tallics 2008, 27, 691–703; b) R. E. Douthwaite, D. Haꢀssinger,
M. L. H. Green, P. J. Silcock, Organometallics 1999, 18, 4584–4590;
c) S. Sauerbrey, P. K. Majhi, J. Daniels, G. Schnakenburg, G. M.
Brꢄndle, K. Scherer, R. Streubel, Inorg. Chem. 2011, 50, 793–799.
[13] K. A. Kreisel, G. P. A. Yap, K. H. Theopold, Organometallics 2006,
25, 4670–4679.
[14] For selected examples, see: a) S. Harder, J. Spielmann, J. Intemann,
H. Bandmann, Angew. Chem. 2011, 123, 4242–4246; Angew. Chem.
Int. Ed. 2011, 50, 4156–4160; b) A. Venugopal, W. Fegler, T. P.
Spaniol, L. Maron, J. Okuda, J. Am. Chem. Soc. 2011, 133, 17574–
17577; c) J. Zhou, J. Chu, Y. Zhang, G. Yang, X. Leng, Y. Chen,
Angew. Chem. 2013, 125, 4337–4340; Angew. Chem. Int. Ed. 2013,
52, 4243–4246; d) M. Arrowsmith, M. S. Hill, G. Kociok-Kçhn, D. J.
MacDougall, M. F. Mahon, Angew. Chem. 2012, 124, 2140–2142;
Angew. Chem. Int. Ed. 2012, 51, 2098–2100.
[25] J. P. Banovetz, H. Suzuki, R. M. Waymouth, Organometallics 1993,
12, 4700–4703.
[26] a) Bruker, SAINT-Plus, Bruker AXS Inc., Madison, WI, USA, 1999;
b) Bruker, SADABS, Bruker AXS Inc. Madison, WI, USA, 2004;
c) A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, J.
Appl. Crystallogr. 1993, 26, 343–350; d) G. M. Sheldrick, Acta Crys-
tallogr. Sect. A 2008, 64, 112–122; e) L. J. Farrugia, J. Appl. Crystal-
logr. 1999, 32, 837–838.
[15] N. Wiberg, W. Niedermayer, H.-W. Lerner, M. Bolte, Z. Anorg.
Allg. Chem. 2001, 627, 1043–1047.
[16] a) M. Cheng, E. B. Lobkovsky, G. W. Coates, J. Am. Chem. Soc.
1998, 120, 11018–11019; b) D. R. Moore, M. Cheng, E. B. Lobkov-
sky, G. W. Coates, Angew. Chem. 2002, 114, 2711–2714; Angew.
Chem. Int. Ed. 2002, 41, 2599–2602; c) D. R. Moore, M. Cheng,
Received: September 17, 2013
Published online: November 25, 2013
Chem. Asian J. 2014, 9, 612 – 619
619
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim