N-heterocyclic carbene-amide rhodium(I) complexes: Structures, dynamics, and catalysis

Article abstract of DOI:10.1021/om200631w

The amide-functionalized imidazolium salts [BocNHCH₂CH ₂ImR]X (R = Me, X = I, 1a; R = benzyl, X = Br, 1b; R = trityl, X = Cl, 1c) bearing increasingly bulky N-alkyl substituents were prepared in high yields by direct alkylation of the (2-imidazol-1-yl-ethyl)carbamic acid tert-butyl ester; 1c is a crystalline solid also characterized by X-ray diffraction. These salts are precursors for the synthesis of rhodium(I) complexes [Rh(NBD)X(NHC)] (NHC = 1-(2-NHBoc-ethyl)-3-R-imidazolin-2-ylidene; X = Cl, R = Me (3a), R = benzyl (3b), R = trityl (3c); X = I, R = Me (4a)). All the complexes display restricted rotation about the metal-carbene bond; however, while the rotation barriers calculated for 3a,b and 4a matched the experimental values, unexpectedly this was not true in the case of 3c, where the experimental value was equal to that obtained for compound 3b (58.6 kJ mol^-1) and much smaller with respect to the calculated one (100.0 kJ mol^-1). The catalytic activity of the neutral rhodium(I) complexes 3a-c in the hydrosilylation of terminal alkynes with HSiMe₂Ph has been investigated with PhC≡CH, TolC≡CH, ⁿBuC≡CH, Et ₃SiC≡CH, and (CPh₂OH)C≡CH as substrates. The steric hindrance on the N-heterocyclic ligand and on the alkyne substrates affects conversion and selectivity: for the former the best results were achieved employing the less encumbered 3a catalyst with TolC≡CH, whereas by employing hindered alkynes such as Et₃SiC≡CH or (CPh ₂OH)C≡CH the hydrosilylation leads only to the formation of the β-(E)-vinylsilane and α-bis(silyl)alkene isomers. The complexes 3a,b have also been employed in the addition of arylaldehydes with phenylboronic acid, and like in the hydrosylylation case, the best results were obtained using 3a in the presence of aldehydes bearing electron-withdrawing groups, such as 4-cyanobenzaldehyde and 4-acetylbenzaldehyde as substrates.

Full text of DOI:10.1021/om200631w

ARTICLE
pubs.acs.org/Organometallics
N-Heterocyclic Carbene-Amide Rhodium(I) Complexes: Structures,
Dynamics, and Catalysis
Luigi Busetto,† M. Cristina Cassani,*,† Cristina Femoni,† Michele Mancinelli,‡ Andrea Mazzanti,‡
Rita Mazzoni,*,† and Gavino Solinas†
†Dipartimento di Chimica Fisica ed Inorganica, Universitꢀa degli Studi di Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy
‡Dipartimento di Chimica Organica “A. Mangini”, Universitꢀa degli Studi di Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy
S Supporting Information
b
ABSTRACT: The amide-functionalized imidazolium salts
[BocNHCH2CH2ImR]X (R = Me, X = I, 1a; R = benzyl, X =
Br, 1b; R = trityl, X = Cl, 1c) bearing increasingly bulky N-alkyl
substituents were prepared in high yields by direct alkylation of
the (2-imidazol-1-yl-ethyl)carbamic acid tert-butyl ester; 1c is a
crystalline solid also characterized by X-ray diffraction. These
salts are precursors for the synthesis of rhodium(I) complexes
[Rh(NBD)X(NHC)] (NHC = 1-(2-NHBoc-ethyl)-3-R-imida-
zolin-2-ylidene; X = Cl, R = Me (3a), R = benzyl (3b), R = trityl
(3c); X = I, R = Me (4a)). All the complexes display restricted
rotation about the metalꢀcarbene bond; however, while the
rotation barriers calculated for 3a,b and 4a matched the experimental values, unexpectedly this was not true in the case of 3c, where
the experimental value was equal to that obtained for compound 3b (58.6 kJ molꢀ1) and much smaller with respect to the calculated
one (100.0 kJ molꢀ1). The catalytic activity of the neutral rhodium(I) complexes 3aꢀc in the hydrosilylation of terminal alkynes
with HSiMe2Ph has been investigated with PhCtCH, TolCtCH, nBuCtCH, Et3SiCtCH, and (CPh2OH)CtCH as substrates.
The steric hindrance on the N-heterocyclic ligand and on the alkyne substrates affects conversion and selectivity: for the former the
best results were achieved employing the less encumbered 3a catalyst with TolCtCH, whereas by employing hindered alkynes such
as Et3SiCtCH or (CPh2OH)CtCH the hydrosilylation leads only to the formation of the β-(E)-vinylsilane and α-bis(silyl)alkene
isomers. The complexes 3a,b have also been employed in the addition of arylaldehydes with phenylboronic acid, and like in the
hydrosylylation case, the best results were obtained using 3a in the presence of aldehydes bearing electron-withdrawing groups, such
as 4-cyanobenzaldehyde and 4-acetylbenzaldehyde as substrates.
selectivities were reported by Oro3d when using rhodium(I)
1. INTRODUCTION
complexes with alkylamino-functionalized NHC ligands for the
N-Heterocyclic carbenes (NHCs) have attracted considerable
hydrosilylation of terminal alkynes. The addition of aryl boronic
attention as a new class of ligands over the past decade.1 The
derivatives to carbonyl compounds represents another active
heterocyclic carbene moiety offers great possibilities for fine-
tuning the ligand structure and, thereby, the catalytic properties,
through the introduction of appropriate substituents at the
nitrogen atoms or at the five-membered-ring carbon atoms. The
use of heteroatom-functionalized NHC carbenes in which a
donor group is attached to a strongly bonded imidazolyl ring
in the design of homogeneous catalysts has received increasing
field of research for [(NHC)Rh] catalysts:1b Furstner first,
followed by Buchmeiser, demonstrated that “common” dialkyl-
or diaryl-NHC rhodium complexes are efficient at +80 °C with
1 mol % catalyst loading for different substituted benzaldehyde,
€
while Ozdemir and co-workers have synthesized and tested a
large number of different NHC architecture scaffolds,5 including
amino-functionalized benzimidazolylidene systems.6
attention, as it can provide coordination versatility and enhanced
In this research field we recently described the selective,
catalytic activity.2 In this context, amino/amide-functionalized
high-purity synthesis of tert-butyloxylcarbonyl (Boc)-protected
NHC ligands are found to be important building blocks of active
1-(2-aminoethyl)-3-methylimidazolium [R1NHCH2CH2ImR]X
transition metal catalysts that are used for cross-coupling and
(1a, Im = imidazole, R1 = Boc, R = Me, X = I, Chart 1) and
hydrosilylation reactions.3 With regard to the latter class of
deprotected [NH2CH2CH2ImMe]+. We had found that when
reaction, most of the recent efforts concern the design of efficient
the amino group is protected, the formation of stable silver
catalysts for the hydrosilylation of alkynes aimed at the selec-
tive formation of (Z) and (E)-alkenylsilanes.4 Surprisingly the
role played in such reactions by rhodium(I) NHC complexes
Received: July 13, 2011
Published: September 09, 2011
has been little investigated,1b and good yields and fair to good
r
2011 American Chemical Society
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Chart 1
complexes with different bonding motifs in the solid state
occurred, while with the deprotected ligand a silver complex of
the type [(NHC-NH2)AgI] is likely to form, but it is unstable in
solution.7 The studies relative to the coordinating properties of
the Boc-protected amino-functionalized imidazolium cation
[NH(Boc)CH2CH2ImMe]+ also revealed that the presence
of the Boc group significantly reduces the nucleophilic power
of the nitrogen atom on the side arm that never coordinates to
the metal center. Successively [NH2CH2CH2ImMe]+ has been
employed in the preparation of a gold(III)-aminoethyl imidazo-
lium aurate salt, [Cl3AuNH2(CH2)2ImMe)][AuCl4],8 used as a
well-defined ionic liquid-stabilized AuNPs precursor.9,10
Figure 1. ORTEP diagram of 1c depicted with displacement ellipsoids
at the 30% probability level. The counterion and water molecules have
been omitted for clarity.
Following our initial report,7 in this work we describe the
synthesis and characterization of novel functionalized NHC
ligand precursors of the type [NH(Boc)CH2CH2ImR]+ (1b, R =
benzyl; 1c, R = trityl) and their silver(I) and rhodium(I)
complexes. The dynamic behavior of the rhodium systems has
been investigated by means of VT NMR studies and DFT
calculations, and the application of these complexes in the
hydrosilylation of alkynes as well as the addition of arylaldehydes
to phenylboronic acid is also described.
ether (1:4). The molecular structure is shown in Figure 1; crystal
data and experimental details are presented in Table 2, while for
the full list of bond lengths and angles see the Supporting
Information. The single crystals selected for X-ray analysis
crystallize in the centrosymmetric P21/c space group (Z = 4),
and the unit cell also contains four water molecules. The bond
distances and angles are in the expected range compared to those
of known imidazolium salts,12 and also the N(1)ꢀC(19) dis-
tance for the NꢀC(trityl) bond of 1.5016(16) Å is in keeping
with literature data.13 In the latter group two phenyl rings
(C(7)ꢀC(12) and C(13)ꢀC(18)) are linked in a helical shape,
whereas the third phenyl (C(1)ꢀC(6)) generates a second
helical system with imidazole.14 The NH group is not involved
in hydrogen bonds with the carbenic hydrogen, but, on the
contrary, it acts as donor with one of the chloride counteranions
’ RESULTS AND DISCUSSION
Synthesis of Imidazolium Ligand Precursors and NHC-Silver
Complexes. The novel imidazolium salts [BocNHCH2CH2ImR]X
(1b, R = benzyl, X = Br; 1c, R = trityl, X = Cl) bearing increasingly
bulky N-alkyl substituents were prepared in high yields by alkylation
of the (2-imidazol-1-yl-ethyl)carbamic acid tert-butyl ester respectively
with benzyl bromide and trityl chloride in dichloromethane at room
temperature (Chart 1).
(N(3) Cl(1) = 3.262 Å). Each of the four water molecules
3 3 3
(see Figure S1 in the Supporting Information) also has hydrogen
bonds with two chloride anions (3.182 and 3.233 Å) and with the
carbenic hydrogen (3.034 Å).
Like 1a, the salt 1b was isolated as a colorless to pale yellow
viscous liquid, whereas 1c is a crystalline solid; they are soluble in
chlorinated solvents and were fully characterized by elemental
analysis, NMR spectroscopy, electrospray ionization mass spec-
trometry (ESI-MS), and in case of 1c also X-ray diffraction. The
NMR resonances of the imidazolium fragment were observed at
chemical shifts typical for imidazolium salts, and it is worth
noting that, to the best of our knowledge, substitution at the 1,3-
positions of the imidazole unit with the trityl group has been
reported only in one case regarding rhodium-catalyzed hydro-
silylation/cyclization reactions.11
Silver(I) halide complexes of NHC have been shown to be
useful as NHC ligand transfer agents to metals,15 and we have
previously described that upon treatment of 1a with Ag2O in a
2:1 stoichiometric ratio in dichloromethane, the biscarbene salt
[(NHC)2Ag][AgI2] (NHC = 1-(2-NHBoc-ethyl)-3-R-imidazo-
lin-2-ylidene, R = Me, (2a)) is readily obtained.7 Likewise, 1b
and 1c have been successively reacted with Ag2O to give the
silver complexes 2b,c, although in the latter case longer reaction
times (48 h instead of 2 h) are required due to the steric
hindrance of the trityl group. The formation of 2b,c was
unambiguously confirmed by NMR spectroscopy and electro-
spray ionization mass spectrometry (ESI-MS); however, unlike
what was previously done for 2a,7 the silver complexes were not
Crystals of 1c suitable for single-crystal X-ray diffraction were
grown from a double layer of dichloromethane and petroleum
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Scheme 1. Synthesis of Rhodium(I) Complexesa
Figure 2. 1H NMR of 3b at various temperatures (600 MHz in CDCl3).
a Reactions and conditions for 3aꢀc: (i) Ag2O, CH2Cl2, RT; (ii)
[Rh(NBD)Cl]2, CH2Cl2, RT, 2 h. Reaction and conditions for 4a:
(iii) [Rh(NBD)(OtBu)]2, THF, RT, 3 h.
stable, and NMR monitoring for over 10 days of a sample
prepared in air with not anhydrous chloroform did not present
any significant decomposition.
They have been fully characterized by elemental analysis,
ESI-MS mass spectrometry, and 1H and 13C NMR using gCOSY,
gHSQC, and gHMBC experiments for full resonance assignments.
The 1H NMR (CDCl3) spectra for 3a show a broad signal at
6.74 ppm assigned to the dCH backbone protons, while in the
13C{1H} NMR spectra the coordination of the carbene to the
rhodium center becomes evident as a doublet resonance at 184.2
ppm (d, JC,Rh = 57.1 Hz). These chemical shifts and coupling
constant values lie in the usual range for related Rh(I)-NHC
complexes.3d,17 The olefinic protons of the norbornadiene ligand
feature three 1:1:1 resonances (2 protons each) at 4.86, 3.82, and
3.49 ppm, respectively, and three doublets in the 13C{1H} NMR
spectra at 63.6 (JC,Rh = 4.8 Hz), 51.0 (JC,Rh = 4.8 Hz), and 48.1
(JC,Rh = 12.3 Hz). This observation can be explained by the lack
of an effective symmetry plane in the molecules as the result of
the hindered rotation about the carbeneꢀrhodium bond. Due to
the steric hindrance exerted by the two side arms, the norborna-
diene moiety and the chlorine atom are displaced in an out-of-
plane disposition with respect to the imidazole ring. If the two
side arms are different, the molecule has C1 symmetry; therefore
a pair of conformational enantiomers is generated.
isolated but straightforwardly used in situ. The silver complexes
2b,c showed 1H and 13C NMR in CDCl3 features very similar to
those previously described for 2a: disappearance of the imida-
zolium NCHN proton resonance coupled with appearance of a
singlet at ca. 180 ppm for the AgꢀCcarbene carbons. The electro-
spray ionization mass spectrometry analysis in methanol indi-
cated the presence in solution of the cations [(NHC)2Ag]+ with
the observed isotopic distributions in perfect agreement with
the calculated ones.
Synthesis of Rhodium(I) Complexes and Solution NMR
Studies. The rhodium complexes [Rh(NBD)Cl(NHC)] (NBD =
2,5-norbornadiene; 3a, R = Me; 3b, R = benzyl; 3c, R = trityl)
were synthesized by transmetalation from the NHC-Ag com-
plexes 2aꢀc in dichloromethane (Scheme 1, method A). The
iodide analogue [Rh(NBD)I(NHC)] (4a, R = Me) was instead
prepared either by reaction with the in situ generated carbene in
THF (method B, reaction conditions iii) or by ion exchange
methatesis from 3a with an excess of NaI.15
We found that whereas the transmetalation reaction with
[Rh(NBD)Cl]2 always gave the yellow neutral mononuclear
metal carbene complexes 3 in a very efficient and selective way
(70ꢀ90% yield), with the synthetic procedure B the iodide
complex 4a was always isolated in lower yields (30ꢀ40%). This
is probably due to the erratic oxidation of tetrahydrofuran
to γ-butyrolactone catalyzed by Rh(I) (either the starting
[Rh(NBD)Cl]2 or the metal carbene itself) in the presence of
trace amounts of water.16 With the transmetalation method A,
after filtering off any unreacted Ag2O or any formed AgX
followed by gradient column chromatography under argon on
anhydrous silica gel, complexes 3 were isolated as yellow micro-
crystalline solids. They are completely soluble in chlorinated
solvents and acetonitrile, partially soluble in diethyl ether, and
completely insoluble in petroleum ether. Complexes 3a,b and 4a
are air sensitive in the solid state and in solution but not moisture
sensitive: continuous monitoring of the NMR samples in CDCl3
showed that decomposition to unidentified products rapidly
occurs after exposure to air (ca. 1 h) but not after controlled
additions of degassed water at either room temperature (over
two weeks) or 55 °C (30 min.). On the contrary 3c is perfectly air
The 1H spectra of compound 3b, bearing a benzyl group, are
displayed in Figure 2. The spectrum recorded at room tempera-
ture (middle trace) shows very broad signals, and, in particular,
the signal of the benzylic CH2 is close to the coalescence point.
When the temperature is raised to +55 °C, all the signals sharpen,
whereas on lowering the temperature, the benzylic CH2 decoa-
lesces into an AB system (5.58 and 6.02 ppm) and the methylenic
protons of the amide side arm splits into diastereotopic signals
too. The carbonyl stretching frequency (νCO) of the carbamic
group ꢀNHC(O)Oꢀ does not give any information on the kind
of coordination and is similar to that of the starting imidazolium
precursor always appearing at about 1710 cmꢀ1 for the rhodium
complexes, and although the complexes are neutral, positive
ESI-MS analyses for the complexes 3aꢀc and 4a always showed a
major m/z peak corresponding to the [M ꢀ X]+ fragment.
All attempts to facilitate a kN coordination of the amine
nitrogen by removing the halide ligand as AgX resulted in decom-
position products even when the reactions were performed in
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Scheme 2. (Left) Experimental X-ray Structures of 3a and 4a;
(Right) Calculated Structures
Figure 3
observed barrier should be due to steric effects only. When the
steric hindrance on one side arm is reduced, as in compound 3a,
the rotational barrier is lower (55.3 kJ molꢀ1), and the enantio-
merization process could be monitored by observing the splitting
of a CH signal of norbornadiene (Figure S6). On the contrary,
when the chlorine atom of 3a is exchanged with iodine (as in
compound 4a), the rotational barrier is increased, and the signals
of norbornadiene are split at room temperature. On raising the
temperature, the coalescence point is reached at +57 °C, and a
single broad signal was observed at +90 °C (spectra at higher
temperatures cannot be recorded due to decomposition). The
rotational barrier was derived to be 72.4 kJ molꢀ1 (Figure S7).
These data agree very well with those of Enders and Gielen,
confirming a steric origin of the rotation barrier.20
An analogous behavior is observed for 3c (Figures S2ꢀS5);
moreover the presence of the three phenyl rings of the trityl
group causes a ring current effectthatleadstoasignificant downfield
shift of one of the NCH2ꢀ wingtip diasterotopic protons that is
found at 6.02 ppm, whereas on the opposite, a proton of the
norbornadiene is high-field shifted to an unusually low 2.1 ppm.
The variable-temperature spectra taken in the case of com-
pound 3c (Figure S8) showed that the rotational barrier
(58.8 kJ molꢀ1) was identical, within error, to that obtained in
the case of the benzyl-substituted compound 3b; however, the
spectra simulations show a noticeable increase of the free energy
of activation on raising the temperature, therefore indicating a
negative entropic activation.
DFT calculations of compounds 3aꢀc and 4a were performed
at the B3LYP/LANL2DZ level. The geometry of the calculated
ground states of 3a and 4a were almost identical to that observed
in the solid state (Scheme 2). In particular the calculations
correctly reproduced the RhꢀC, RhꢀCl, and RhꢀI bond
lengths. In addition, the torsion angles that generate the two
conformational enantiomers observed in solution were also
correctly reproduced (Cl(2)ꢀRh(1)ꢀC(8)ꢀN(2) = 89.15°;
Figure 4. Variable-temperature spectra of 3b showing the evolution of
the benzylic CH2 signal (1H NMR at 400 MHz in CDCl3). On the right
the simulations with the corresponding rate constants are reported.
coordinating solvents (acetone or acetonitrile) and/or in the
presence of coordinating anions (triflate).
Stereodynamics. The stereodynamics and the rotation bar-
riers about the Rhꢀcarbene bond have been determined by
making use of variable-temperature NMR spectroscopy,19 on
increasing the steric hindrance of the N-alkyl substituent, and by
varying the halogen atom (Figure 3).
In the case of 3b, the signal of the benzylic CH2 shows the
typical shape of the coalescence at +25 °C. As shown in Figure 4,
the signal sharpens at high temperature, whereas it decoalesces
into an AB system at low temperature. Line shape simulation at
various temperatures yielded the rate constants for the enantio-
merization, from which an activation energy of 58.6 kJ molꢀ1
was derived by the Eyring equation. The activation energy was
found to be constant with respect to the temperature, indicating a
negligible value of the activation entropy. This implies that the
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Scheme 3. Enantiomerization Pathway for Compounds
3aꢀc and 4a
the amide-ethyl group (TS-1), with the simultaneous cros-
sing of the norbornadiene on the second side arm (R1). The
calculations also suggested that the energy barrier is strongly
related to the steric hindrance of R1, and for this reason the
barrier calculated for compound 3c is predicted to be very high
(100.0 kJ molꢀ1).
While the rotation barriers calculated for 3a and 3b matched
the experimental values, this was not true in the case of 3c, where
the experimental value was equal to that obtained for compound
3b and much smaller with respect to the calculated one. In
addition, the energy barrier derived for 3c from line shape
simulation showed a strong dependence on the temperature,
while the barriers measured for 3a,b did not show this effect. In
particular, the large negative activation entropy derived from
simulations (ꢀ40 ( 10 eu) indicates that a strongly organized
transition state or a different interconversion pathway could take
place in the case of compound 3c. The inversion process may
proceed either via a cleavage of the RhꢀCl bond before or during
the rotation21 or by partial dissociation of the trityl group into a
contact ionic pair, followed by the rotation of the RhꢀCl moiety
and subsequent re-formation of nitrogenꢀtrityl carbon bond.22
The experimental evidence that the replacement of chlorine with
iodine raises the rotational barrier in the case of 3a and 4a (as
correctly indicated also by the calculations) suggests that the
latter hypothesis should be considered more likely. In addition,
the possible coordination of a phenyl ring of the trityl moiety to
rhodium23 in the transition state might be responsible for the
partial dissociation of the trityl group and for lowering the energy
of the transition state. In this framework the value obtained for
compound 3c is a threshold value for the true steric rotational
barrier.
Crystal Structure Determination for 3a, 4a, and 3c. The
molecular structures of the rhodium(I) complexes 3a, 4a, and 3c
were determined via X-ray diffraction and are reported in
Figures 5, 6, and 7, respectively. The crystal data and experi-
mental details together with the RhꢀCcarbene and RꢀX distances
are reported in Table 2, whereas the full list of bond lengths and
angles are reported in the Supporting Information. The structural
analysis confirmed that the Boc protection removes most of the
nucleophilicity of the amide nitrogen that is never involved in any
coordinative bond. Crystals of 3a and 4a suitable for diffraction
were grown by slow evaporation of a mixture of CH2Cl2 and
petroleum ether.
The structure of 3a, which crystallized in a centric space group
(P21/c), consists of separated C18H27ClN3O2Rh molecules ar-
ranged in eclipsed, alternating columns of enantiomers. There
are, in fact, two possible orientations of the BocNHCH2CH2ꢀ
and ꢀCH3 substituents on the functionalized imidazolium ring,
and both are generated by the cell symmetry operators from the
unique Rh complex in the asymmetric unit. Rhodium shows a
classic square-planar coordination and is bonded to the Cl(2)
atom, the carbenic C(8) atom, and the bidentate norbornadiene
fragment. The stronger trans effect that the carbene exercises
compared with the chlorine ligand is inferable from the distances
between Rh and C atoms on the norbornadiene unit: the
RhꢀC(1) and RhꢀC(2) contacts trans to the carbene are
2.204(2) and 2.206(2) Å, respectively, and the RhꢀC(3) and
RhꢀC(4) bond lengths trans to Cl atom are remarkably shorter,
both 2.083(2) Å. Despite the presence of donor chlorides anions,
there are not significant intra- or intermolecular hydrogen bonds
in the solid state, with the exception of a very weak intermole-
cular interaction between Cl(2) and N(3) (3.366 Å). In addition
Table 1. Calculated and Experimental Energy Barriers for the
Enantiomerization of 3aꢀc and 4a (energies in kJ molꢀ1
,
calculations at the B3LYP/LANL2DZ level)
compd
TS-1
TS-2
exptl (kJ molꢀ1
)
3a
3b
3c
4a
58.8
67.9
67.8
77.7
55.3a
58.6a
58.8b
72.4a
100.0
126.4
79.2
83.3
a ΔGq invariant with the temperature. b ΔGq at ꢀ1.5 °C. ΔHq = 46.1 kJ
molꢀ1; ΔSq = ꢀ43 J Kꢀ1 molꢀ1
.
I(1)ꢀRh(1)ꢀC(8)ꢀN(9) = 96.72°). These results actually
show that the theoretical level employed in the calculations is
suitable to tackle the conformational analysis of these complexes.
The two enantiomeric ground states GS and GS0 can inter-
convert into each other by two possible transition states due to
the rotation around the carbeneꢀrhodium bond (Scheme 3).
The first one (TS-1) is reached by a 90° clockwise rotation
starting from GS and corresponds to the crossing of the halogen
atom on the CH2 of the amide-ethyl moiety (denoted as R2),
whereas the second transition state (TS-2) takes place when a
counterclockwise rotation forces the halogen to cross the second
alkyl side arm group (R1) on the imidazole. The steric hindrance
of the halogen and R1 obviously influence the activation energy of
the rotational barrier. DFT calculations of the two possible
transition states (see Table 1) suggested that the threshold path-
way (i.e., that with the lowest transition-state energy) corre-
sponded in all the cases to the passage of the halogen atom on
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Figure 5. ORTEP diagram of 3a depicted with displacement ellipsoids at the 30% probability level.
Figure 6. ORTEP diagram of 4a depicted with displacement ellipsoids at the 30% probability level.
Figure 7. ORTEP diagram of 3c depicted with displacement ellipsoids at the 30% probability level.
there is no evidence of a positional disorder in the structure even
at room temperature, and this is an index of the very high efficient
solid packing of the molecules, which is mainly stabilized by steric
effects.
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Table 2. Crystal Data and Data Collection and Refinement Parameters for 1c, 3a, 4a, and 3c
1c H2O
3a
4a
3c 2CH3CN
3
3
formula
fw
C
29H34ClN3O3
C18H27ClN3O2Rh
455.79
C18H27IN3O2Rh
547.24
C40H45ClN5O2Rh
766.17
508.04
T, K
296(2)
298(2)
296(2)
296(2)
λ, Å
0.71073
0.71073
0.71073
0.71073
cryst syst
space group
a, Å
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/n
8.5286(9)
17.2903(18)
18.910(2)
90
11.9725(7)
18.1627(10)
9.9190(5)
90
12.0829(13)
15.0838(16)
12.2761(13)
90
13.1294(13)
9.7921(10)
29.874(3)
90
b, Å
c, Å
α, deg
β, deg
93.2710(10)
90
112.3730(10)
90
108.2450(10)
90
93.5440(10)
90
γ, deg
cell volume, Å3
2751.8(5)
4
1994.55(19)
4
2124.7(4)
4
3833.4(7)
4
Z
Dc, g cmꢀ3
μ, mmꢀ1
F(000)
cryst size, mm
θ limits, deg
index ranges
1.226
1.518
1.711
1.328
0.173
1.006
2.272
0.555
1080
936
1080
1592
0.80 ꢁ 0.70 ꢁ 0.60
1.61 to 28.71
ꢀ10 e h e 11,
ꢀ22 e k e 21,
ꢀ24 e l e 25
31 214
0.30 ꢁ 0.25 ꢁ 0.20
1.84 to 25.00
ꢀ14 e h e 14,
ꢀ21 e k e 21,
ꢀ11 e l e 11
18 769
0.20 ꢁ 0.15 ꢁ 0.10
1.77 to 25.00
ꢀ14 e h e 14,
ꢀ17 e k e 17,
ꢀ14 e l e 14
20 041
0.60 ꢁ 0.40 ꢁ 0.20
1.37 to 28.72
ꢀ17 e h e 17,
ꢀ13 e k e 13,
ꢀ40 e l e 39
43 389
reflns collected
indep reflns
6711 [R(int) = 0.0219]
100.0%
3513 [R(int) = 0.0209]
100.0%
3740 [R(int) = 0.0449]
100.0%
3740 [R(int) = 0.0276]
100.0%
completeness to θ = 25.00°
data/restraints/params
goodness on fit on F2
R1 (I > 2σ(I))
6711/0/348
0.903
3513/0/226
1.030
3740/24/255
1.009
9350/0/459
0.998
0.0419
0.0217
0.0303 and 0.0623
0.0503 and 0.0717
0.639 and ꢀ0.634
2.020(4)
0.0319
wR2 (all data)
0.1139
0.0617
0.0759
largest diff peak and hole, e Åꢀ3
RhꢀC bond length (Å)
RhꢀX bond length (Å)
0.235 and ꢀ0.289
0.388 and ꢀ0.283
2.026(2)
0.519 and ꢀ0.277
2.0549(18)
2.4283(6)
2.3794(6)
2.6597(5)
The structure of 4a (Figure 6), which crystallized in the same
centric space group as its chloride congener, does not signifi-
cantly differ from the latter in terms of solid-state packing and
atomic coordination, the only exception being a slight positional
disorder found in the terminal tert-butyl group of the ligand.
Again the complex crystallized in a racemic mixture with both
enantiomers present in a 1:1 ratio. The same strong trans effect
of the carbenic atom C(8) is also observed, as revealed by the
RhꢀC bond lengths: the RhꢀC(1) and RhꢀC(2) distances, in
trans position with respect to C(8), are both 2.207(4) Å, while
the RhꢀC(4) and RhꢀC(5) bonds, trans to the I atom, are
2.103(4) and 2.105(4) Å in that order.
hindrance exerted by the large trityl group. The dihedral angle
Cl(2)ꢀRh(1)ꢀC(18)ꢀN(2) that yields the two enantiomers
whose barrier was observed in solution is 111°, to be compared
with that of 92.47° observed for Cl(2)ꢀRh(1)ꢀC(8)ꢀN(1) in 3a.
Catalysis. The potential of the Rh(I) complexes 3aꢀc as
catalysts as well as the influence of the carbene substituents on
the catalytic activity was examined in the hydrosilylation of
1-alkynes and in the addition of aryladehydes to phenylboronic acid.
Hydrosilylation of 1-Alkynes. The complexes were found to be
active catalyst precursors for the hydrosilylation of terminal
alkynes. The catalytic reactions were carried out in CDCl3 at
+25 °C using a slight excess of HSiMe2Ph and were routinely
1
Crystals of 3c suitable for X-ray diffraction were obtained from
a concentrated acetonitrile solution at ꢀ20 °C.
monitored by H NMR spectroscopy. The influence of the
1-alkyne has been studied using phenylacetylene, tolylacetylene,
1-hexyne, (triethylsilyl)acetylene, and 1,1-diphenyl-2-propyn-1-
ol as substrates (Table 3).24 Catalyst loadings as low as 0.5 and
0.1 mol % of 3a also provided high activities at room temperature
(Table 3, entries 12ꢀ15), although in the latter case slower
kinetics were found.
As generally reported for transition metal catalyzed hydro-
silylation of 1-alkynes,4b and in particular for neutral and cationic
Rh(I)-NHC hydrosilylation catalysts,3d the reaction is unselective,
and complexes 3aꢀc convert phenylacetylene and tolylacetylene to a
Compound 3c crystallizes in the centrosymmetric P21/n
group, and both enantiomers are generated by the center of
symmetry, as in the previous cases. At variance with 3a and 4a,
the unit cell contains also eight molecules of acetonitrile, which
makes the crystal package less efficient. As in the case of 3a and
4a, the rhodium has a square-planar coordination, with the
RhꢀCl and RhꢀCcarbene bond distances slightly longer with
respect to 3a (2.055 Å for Rh(1)ꢀC(18) and 2.428 Å for
Rh(1)ꢀCl(2); see Table 2). This could be ascribed to the steric
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mixture of the three possible isomeric vinylsilane derivatives: β-(Z)-
or β-(E)-1-silyl-1-alkenes from the anti-Markovnikov addition and
α-2-silyl-1-alkene from the Markovnikov addition. Furthermore the
formation of the corresponding alkene has been observed from these
substrates (Scheme 4, Figure 8).25
PhCtCH and dimethylphenylsilane catalyzed by 3a. The graph
shows that β-(Z) and β-(E) vinylsilane profiles cross when the
conversion is complete, and the reaction goes further until β-(Z)
completely converts to β-(E). We can also observe that the
formation of an α isomer (10ꢀ12%) and styrene (14ꢀ16%)
(alkene in Scheme 4 and Figures 8 and 10) affects the selectivity
of the reaction.
Data for the other catalysts and substrates reported in Table 3
and Figures 11 and 12 (see the Supporting Information for
further graphs and details) show that the α isomer is always
identified in variable amounts, 11ꢀ49%, while the alkene for-
mation only affected the phenyl and tolylacetylene substrates
(Figures 8 and 11). The best results in terms of selectivity have
been obtained employing 3a as catalyst and tolylacetylene as the
substrate: this reaction leads to the formation of β-(E) (85%) and
α isomers (15%) within 6 h, whereas no alkene formation has
been observed in this case.
Although 3aꢀc completely converted all the substrates, the
less encumbered 3a showed the best reaction rates; for instance
in the phenylacetylene case turnover frequencies (TOF) of 48 hꢀ1
were found for 3a, 40 hꢀ1 for 3b, and 13 hꢀ1 for 3c (see also the
profile of conversion of PhCtCH with 3aꢀc in Figure 9; further
details on substrate profiles of conversion and selectivity vs time are
available in the Supporting Information, Figures S9ꢀS21).
Contrary to what was reported by Oro et al. employing similar
amino-alkyl-functionalized rhodium complexes,3d in the hydro-
silylation of phenylacetylene we did not observe the formation of
polyphenylacetylene. The β-(Z) vinylsilane is the major product
until the substrate completely disappears. However, once com-
plete conversion is reached, β-(Z) isomerizes to β-(E) vinylsi-
lane, which, at the end of the reaction, is always the major product
(bar diagram in Figure 8). In Figure 10 we report an example of
conversion and selectivity profiles vs time for the reaction of
When nBuCtCH is employed as substrate (Table 3,
Figure 12), the reaction rate is generally slower, showing with
3a a TOF of 24 hꢀ1 (cf. with phenylacetylene TOF = 48 hꢀ1 and
tolylacetylene TOF = 50 hꢀ1). Furthermore once the conversion
is complete, the β-(Z) vinylsilane isomerizes into the hex-2-
enyldimethylphenylsilane (allyl in Figure 12) (32ꢀ46%) instead
of the β-(E) vinylsilane (Scheme 5). This behavior is in line
with what was previously reported by Crabtree et al. in the
hydrosilylation of 1-hexyne with HSiMePh2 catalyzed by
[Rh(PPh3)3Cl].25d
It is worth noting that, as above stated, the hydrosilylation of
1-hexyne requires a longer initiation time (cf. Figure S10 with
Figure 9 for phenylacetylene and Figure S9 for tolylacetylene). In
view of the fact that the alkyne's steric encumbrance negatively
affects the reaction rate (vide infra) and by comparison with
literature data on the hydrosilylation of 1-hexyne,3d,26 this
behavior has to be ascribed to an electronic effect. In particular
a less activated metalꢀalkyne bond could inhibit the catalyst
turnover, slowing the insertion step.27
Opposite of what was observed with the previously discussed
substrates (phenyl, tolyl, and n-butylacetylene) and in agreement
with what was previously reported,3d the reaction of dimethyl-
phenylsilane with (triethylsilyl)acetylene, Et3SiCtCH, in the
presence of 3a leads to the formation of only the two isomers
β-(E) and α. This behavior has been attributed to both the steric
hindrance and the electronic characteristics of the precursor and
can be confirmed catalyzing the reaction between 1,1-diphenyl-2-
propyn-1-ol, (CPh2OH)CtCH, and dimethylphenylsilane with
3a: also in this case only the formation of the β-(E) and α isomers
occurred, with a better selectivity in the former (Figure 11,
Scheme 6).
Table 3. Hydrosilylation of Terminal Alkynes
conv
%
%
%
entry
alkyne
cat.a (% NMR) β-(E) β-(Z) % α alkene
1
PhCtCH
PhCtCH
PhCtCH
TolCtCH
TolCtCH
TolCtCH
nBuCtCH
nBuCtCH
nBuCtCH
3a
3b
3c
3a
3b
3c
3a
3b
3cb
95
79
21
24
32
54
27
43
33
42
0
63
50
19
33
48
27
40
58
0
16
15
21
13
13
31
27
2
11
28
3
26
4
>99
76
5
12
6
26
7
48
8
32
9
0
0
49
33
10
11
Et3SiCtCH 3a
>99
47
51
67
(CPh2OH)
CtCH
3a
12
13
14
15
PhCtCH
TolCtCH
PhCtCH
TolCtCH
3ac
3ac
3ad
3ad
94
>99
72
24
46
31
30
55
36
26
48
16
13
22
18
5
3
20
68
4
a Reaction time = 2 h; catalyst loading (1 mol %) unless otherwise stated.
b In this case the substrate conversion presented a longer induction time:
after 24 h the conversion achieved 26% (selectivities: % β-(E) = 37%; %
c
β-(Z) = 42%; % α = 21%). Catalyst loading (0.5 mol %). d Catalyst
loading (0.1 mol %).
Scheme 4
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Figure 10. Reaction profile of conversion and selectivities vs time for
the hydrosilylation of PhCtCH with 3a.
Figure 8. Selectivity vs conversion and time for the hydrosilylation of
phenylacetylene (top) and tolylacetylene (bottom) catalyzed by 3a, 3b,
and 3c.
Figure 11. Selectivity vs conversion and time for the hydrosilylation of
all the substrates under study catalyzed by 3a.
Figure 9. Reaction profile of conversion vs time for the hydrosilylation
of PhCtCH with complexes 3a, 3b, and 3c.
Figure 12. Selectivity vs conversion and time for the hydrosilylation of
butylacetylene catalyzed by 3a, 3b, and 3c.
By comparing the alkyne conversions when employing the less
encumbered catalyst 3a, we can observe that the steric encum-
brance of the alkyne also affects the reaction rate. While all the
other substrates completely convert within 6 h, the tertiary
propargyl alcohol (CPh2OH)CtCH reaches complete conver-
sion only after 144 h (Figure 11).
Generally while the reaction rate is affected by the steric
encumbrance of both the complexes 3aꢀc and the alkynes, the
substituents on the N-heterocyclic ring of the catalysts 3aꢀc do
not seriously affect the selectivity, which is otherwise influenced
by the steric hindrance and electronics of the alkyne. This behavior is
in accordance with the mechanism proposed for neutral and
cationic amino rhodium NHC complexes.3d
Addition of Arylaldehydes to Phenylboronic Acid. The com-
plexes 3a and 3b were also found to be active catalyst precursors
for the addition of aryladehydes to phenylboronic acid.28 The
catalytic reactions were carried out in dimethoxyethane/water
(3:1) in the presence of the aldehyde, an equimolar amount of
KOtBu, and an excess of phenylboronic acid, heating at +80 °C in
the presence of rhodium carbene catalyst (1% mol) (Scheme 7).
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Scheme 5
Scheme 6
Scheme 7
Table 4. Addition of Arylaldehydes with Phenylboronic Acid
(entries 9ꢀ12). When lowering the electron deficiency of the
aldehydes, the conversions and reaction rates contextually de-
crease (entries 1ꢀ8). This behavior is in line with what was
reported by Miyaura for rhodium-catalyzed addition of arylboro-
nic acid to aldehydes, giving secondary alcohols, which is in
general facilitated by the presence of an electron-withdrawing
group on the aldehydes,30 and with the results obtained by
Imlinger and Buchmeiser for rhodium 1,3-R2-tetrahydropyrimi-
din-2-ylidenes, indicating the need for a highly nucleophilic
metal center.5d
entry
aldehyde
catalyst time (h) conv (%) yield (%)
1
2
3
4
5
6
7
8
9
4-Cl-PhCHO
3a
3b
3a
3b
3a
3b
3a
3b
3a
3b
3a
3b
5
5
8
8
8
8
8
8
1
1
2
2
99
83
42
32
66
45
66
49
99
96
99
99
78
80
38
29
63
42
43
39
87
75
82
80
4-Cl-PhCHO
4-OMe-PhCHO
4-OMe-PhCHO
3,4,5-(tri-OMe)-PhCHO
3,4,5-(tri-OMe)-PhCHO
4-tBu-PhCHO
4-tBu-PhCHO
4-CN-PhCHO
’ CONCLUSIONS
10 4-CN-PhCHO
We have described the synthesis of novel imidazolium salts
that are precursors for rhodium(I) complexes [Rh(NBD)X-
(NHC)] (NHC = 1-(2-NHBoc-ethyl)-3-R-imidazolin-2-yli-
dene; X = Cl, R = Me (3a), R = benzyl (3b), R = trityl (3c);
X = I, R = Me (4a)), in which the NHC ligands bear an amide
functional group on one nitrogen and increasing bulky N-alkyl
substituents (Me, benzyl, trityl) on the other. All the complexes
display restricted rotation about the metalꢀcarbene bond;
however while the rotation barriers calculated for the complexes
in which R = Me, benzyl (3a,b and 4a) matched the experimen-
tal values, this was not true in the trityl case 3c, where the ex-
perimental value was equal to that obtained for compound 3b
and much smaller with respect to the calculated one. In addi-
tion, the energy barrier derived for 3c from line shape simula-
tion showed a strong dependence on the temperature, while
the barriers measured for 3a,b did not show this effect. In
particular, the large negative activation entropy derived from
simulations (ꢀ40 ( 10 eu) indicates that a strongly organized
11 4-C(O)Me-PhCHO
12 4-C(O)Me-PhCHO
Conversions and isolated yields were monitored by 1H NMR
spectroscopy.29 The influence of the aldehyde has been studied
using 4-chlorobenzaldehyde, 4-methoxybenzaldehyde, 3,4,5-tri-
methoxybenzaldehyde, 4-tert-butylbenzaldehyde, 4-cianobenzal-
dehyde, and 4-acetylbenzaldehyde. The results are summarized
in Table 4.
Catalysts 3a,b convert all the tested aldehydes in moderate to
good yields. Better results have been obtained by employing the
less encumbered 3a catalyst, which was shown to be faster in all
cases (e.g., TOFs for 3a and 3b with 4-Cl-PhCHO as substrate
are 20 and 16 hꢀ1, respectively), and employing electron-with-
drawing ꢀCN- and ꢀC(O)Me-functionalized aldehydes (e.g.,
with 4-CN-PhCHO the TOF for 3a is 99 hꢀ1), which quanti-
tatively convert (NMR) within 2 h with high isolated yields
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transition state and a different interconversion pathway takes
place in the case of compound 3c. We believe that these
observations may bring important implications in further re-
search in NHCꢀM formation, and studies in this direction are in
progress.
Celite was dried in an oven at 150 °C. Melting points were taken with a
Stuart Scientific SMP3 melting point apparatus and were uncorrected.
Synthesis of Triphenylchloromethane (Trityl Chloride). This product
was synthesized using a modified literature method.34 A three-neck, dry,
round-bottom flask was charged with triphenylmethanol (4.0 g, 15.4
mmol) and dry toluene (10 mL) under argon. A condenser was attached,
and the mixture was heated to 80 °C. Acetyl chloride (1.1 mL, 15.4
mmol) was added with a dropping funnel while stirring vigorously with a
magnetic stirrer. Once the solid had dissolved, additional acetyl chloride
(1.90 mL, 27.8 mmol) was added over the course of 10 min. The
solution was heated for 30 min and then cooled in an ice bath, and
petroleum ether (80 mL) was added. This mixture was left in the ice bath
for 2 h, at which point a dark brown product separated out and was
discarded. Volatiles were removed from the filtrate under vacuum to give
a pale yellow solid, which after washing with petroleum ether (3 ꢁ 5 mL)
and drying afforded the title compound as an off-white powder (3.34 g,
78%) that must be stored under argon. 1H NMR (300.1 MHz, CDCl3):
δ 7.34ꢀ7.25 (m, Ph). 13C{1H} NMR (75.50 MHz, CDCl3): δ 145.25
(C5), 129.67 (C4), 127.76 (C2), 127.71 (C3), 81.34 (C1).
The neutral rhodium(I) complexes 3aꢀc are efficient catalyst
precursors for the hydrosilylation of terminal alkynes. The steric
hindrance on the N-heterocyclic ligand and on the alkyne
substrates affects conversion and selectivity: for the former the
best results have been obtained employing the less encumbered
3a catalyst with tolylacetylene and dimethylphenylsilane as
substrates, whereas by employing hindered alkynes such as
Et3SiCtCH or (CPh2OH)CtCH the hydrosilylation leads
only to the formation of β-(E) and α isomers. The complexes
3a,b have also been employed in the addition of arylaldehydes
with phenylboronic acid, and like in the hydrosilylation case, the
best results were obtained using 3a in the presence of aldehydes
bearing electron-withdrawing groups, such as 4-cyanobenzalde-
hyde and 4-acetylbenzaldehyde as substrates. Studies concerning
the use of the deprotected imidazolium salt [NH2CH2CH2ImMe]+
as precursor for a primary-amino-functionalized NHC ligand are
under way.
’ EXPERIMENTAL SECTION
Materials and Procedures. All reactions were carried out under
argon using standard Schlenk techniques. Solvents were dried and
distilled under nitrogen prior to use; the deuterated solvents used after
being appropriately dried and degassed were stored in ampules under
ꢁ
Synthesis of 1-(2-BocNH-ethyl-)-3-benzylimidazolium Bromide, 1b.
In a 100 mL round-bottom flask to a solution of BocNHCH2CH2Im
(1.02 g, 4.80 mmol) in CH2Cl2 (10 mL) was added an excess of benzyl
bromide (7.20 mmol). After stirring for 12 h at room temperature, the
solvent was removed under vacuum, and the resulting pale yellow,
viscous oil was thoroughly washed with diethyl ether (3 ꢁ 10 mL). After
separation from the washings the oil was kept under vacuum at 40 °C for
argon on 4 Å molecular sieves. The prepared derivatives were character-
ized by elemental analysis and spectroscopic methods. The IR spectra
were recorded with a FT-IR Perkin-Elmer Spectrum 2000 spectrometer.
The NMR spectra were recorded using Varian Inova 300 (1H, 300.1;
13C, 75.5 MHz), Varian MercuryPlus VX 400 (1H, 399.9; 13C, 100.6
MHz), Varian Inova 600 (1H, 599.7; 13C, 150.8 MHz) instruments. The
spectra were referenced internally to residual solvent resonances, and
unless otherwise stated, they were recorded at 298 K for characterization
1
several hours to yield 1.87 g (100%) of 1b. H NMR (399.9 MHz,
1
CDCl3): δ 10.20 (s, 1H, NCHN), 7.45 (s, 1H, CHim), 7.35ꢀ7.27 (m,
5H, Ph), 7.22 (s, 1H, CHim), 6.12 (br s, 1H, NH), 5.52 (s, 2H,
PhCH2N), 4.56 (br t, 2H, NCH2), 3.69 (br t, 2H, CH2NH), 1.36 (s,
9H, CH3). 13C{1H} NMR (100.6 MHz, CDCl3): δ 156.1 (CdO),
137.9 (CH, NCHN), 132.7 (Ph, C5), 128.9 (Ph, 2C4), 128.8 (Ph, 2C3),
purposes; full H and 13C NMR assignments were done, when neces-
sary, by gCOSY, gHSQC, gHMBC, and NOESY NMR experiments
using standard Varian pulse sequences; J.Young valve NMR tubes
(Wilmad) were used to carry out NMR experiments under inert
conditions. ESI-MS analyses were performed by direct injection of
methanol solutions of the metal complexes using a Waters ZQ 4000
mass spectrometer. Elemental analyses were performed on a Thermo-
Quest Flash 1112 Series EA instrument. The chemicals 2-bromoethy-
lamine hydrobromide, imidazole (ImH), benzyl bromide, and Ag2O
were used as purchased from Aldrich; [Rh(NBD)Cl]2 was purchased
from Strem and used as received; the starting building blocks—2-t-Boc-
aminoethyl bromide (carbamic acid 2-bromoethyl-tert-butyl ester),31
(2-imidazol-1-yl-ethyl)carbamic acid tert-butyl ester (BocNHCH2CH2Im),7,32
1-(2-BocNH-ethyl-)-3-methylimidazolium iodide (1a), 1-(2-BocNH-ethyl)-
3-methylimidazolin-2-ylidenesilver iodide (2a)7—were prepared according to
literature procedures. Phenylacetylene, 1-hexyne, 4-ethynyltoluene, triethylsi-
lilacetylene, 1,1-diphenyl-2-propyn-1-ol, dimethyphenylsilane, 4-chlorobenzal-
dehyde, 4-methoxybenzaldehyde, 3,4,5-trimethoxybenzaldehyde, 4-tert-butyl-
benzaldehyde, 4-cianobenzaldehyde, 4-acetylbenzaldehyde, and phenylboro-
nic acid were used as purchased from Sigma Aldrich. Petroleum ether (Etp)
refers to a fraction of bp 60ꢀ80 °C. The reactions were monitored by
thin-layer chromatography (TLC) on highly purified silica gel on polyester
(w/UV indicator) and visualized using UV light (254 nm). Column chroma-
tography was carried out under argon on silica gel previously heated at about
200 °C while a slow stream of a dry nitrogen was passed through it;33
t
128.3 (Ph, C2), 123.3 (CHim), 121.4 (CHim), 79.4 (Cq, Bu), 53.3
(PhCH2), 49.5 (NCH2), 40.2 (CH2NH), 28.6 (CH3, tBu). IR
(CH2Cl2, cmꢀ1): 1706 (vs, νCO); (neat, cmꢀ1): 3399 (vs, νNH),
3136, 3068, 2977, 1701 (vs, νCO), 1560, 1509, 1457, 1366, 1255,
1164. ESI-MS (MeOH, m/z): 302 (100) [M]+; 79 (97), 81 (100)
[Br]ꢀ. Anal. Calcd (%) for C17H24BrN3O2: C, 53.41; H, 6.33; N, 10.99.
Found: C, 53.80; H, 6.62; N, 11.33.
Synthesis of 1-(2-BocNH-ethyl-)-3-tritylimidazolium Chloride, 1c. In
a 100 mL round-bottom flask to a solution of BocNHCH2CH2Im (0.93
g, 4.41 mmol) dissolved in CH2Cl2 (20 mL) was added trityl chloride
(1.30 g, 4.66 mmol). After stirring for 16 h at room temperature, the
solvent was removed under vacuum and the resulting solid was
thoroughly washed with diethyl ether (3 ꢁ 10 mL) to yield 2.05 g
(95%) of a white solid identified as 1c. 1H NMR (399.9 MHz, CDCl3): δ
9.12 (s, 1H, NCHN), 8.01 (s, 1H, CHim), 7.39 (m, 9H, Ph), 7.13 (m, 6H,
Ph), 6.96 (s, 1H, CHim), 6.61 (br s, 1H, NH), 4.78 (t, 2H, JH,H = 5.5 Hz,
NCH2), 3.61 (m, 2H, JH,H = 5.4 Hz, CH2NH), 1.31 (s, 9H, CH3).
13C{1H} NMR (100.6 MHz, CDCl3): δ 156.6 (CdO), 139.5 (C5),
137.3 (NCHN), 129.5 (C4), 129.3 (C2), 128.9 (C3), 123.5 (CHim),
123.3 (CHim), 79.5 (Cq, tBu), 79.2 (C1), 49.9 (NCH2), 40.5 (CH2NH),
t
28.3 (CH3, Bu). IR (CH2Cl2, cmꢀ1): 1707 (vs, νCO); (KBr, cmꢀ1):
5268
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3368 (vs, νNH), 3166, 3144, 3061, 2977, 1702 (vs, νCO), 1493, 1446,
1365, 1252, 1168. ESI-MS (MeOH, m/z): 454 (100) [M]+. Anal. Calcd
(%) for C29H32ClN3O2: C, 71.09; H, 6.58; N, 8.58. Found: C, 71.16; H,
6.50; N, 8.65. Mp = 68 °C. Crystals of 1c suitable for single-crystal X-ray
diffraction were grown from a double layer of dichloromethane and
petroleum ether (1:4) at room temperature.
C7H2, NBD), 54.5 (CH2, CH2Ph), 51.1 (d, JC,Rh = 15.7 Hz, CH, NBD),
49.4 (d, JCꢀRh = 12.1 Hz, CH, NBD), 49.2 (NCH2), 48.6 (d, JCꢀRh
=
11.5 Hz, CH, NBD), 41.8 (CH2NH), 28.3 (CH3, tBu). IR
(CH2Cl2, cmꢀ1): 1710 (vs, νCO). IR (KBr, cmꢀ1): 3340 (s, νNH),
3167, 3123, 3091, 3060, 3032, 2978, 2928, 1708 cmꢀ1 (vs, νCO), 1509,
1365, 1250, 1173. ESI-MS (MeOH, m/z): 554 (12) [M + Na]+, 496
(100) [M ꢀ Cl]+; 530 (100) [M ꢀ H]ꢀ. Anal. Calcd (%) for
C24H31ClN3O2Rh: C, 54.19; H, 5.87; N, 7.90. Found: C, 54.21; H,
5.89; N, 7.87. Mp = 97 °C.
Preparation of [Rh(NBD)Cl{1-(2-NHBoc-ethyl)-3-R-imidazolin-2-
ylidene}], R = Me (3a), Bnz (3b), Trit (3c). Synthesis of 3a. To a
solution of 1a (0.258 g 0.73 mmol) in CH2Cl2 (ca. 30 mL) was added
Ag2O (0.107 g, 0.37 mmol). The suspension was stirred for 2 h
and filtered on Celite, and the filtrate was added to a solution of
[Rh(NBD)Cl]2 (0.168 g 0.37 mmol) in CH2Cl2. After stirring for 3 h
the yellow AgI was filtered off, and the solvent was removed under
vacuum. The crude material was purified by column chromatography on
silica gel using first CH2Cl2 and then CH2Cl2/MeOH [100:3 (v/v)] as
eluent to afford 0.23 g (71%) of 3a as a yellow solid. Rf: 0.60 (CH2Cl2/
Synthesis of 3c. To a solution of 1c (0.200 g, 0.41 mmol) in CH2Cl2
(20 mL) was added Ag2O (0.050 g, 0.21 mmol). The suspension was
stirred for 48 h, then solid [Rh(NBD)Cl]2 (0.092 g, 0.20 mmol) was
directly added to the reaction mixture. After stirring for a further 2 h the
crude material was filtered on a Celite pad, the insoluble material was
thoroughly washed with CH2Cl2, and the solvent was removed under
vacuum. The residue was purified by column chromatography on silica
gel treated with 5% v/v triethylamine in Et2O, using Et2O/CH2Cl2, 1:3
(v/v), to afford 0.22 g (78%) of 3c as a yellow solid. Rf: 0.71 (CH2Cl2/
MeOH, 100:5); Rf: 0.22 (Et2O). Suitable crystals for an X-ray diffraction
analysis were obtained from a concentrated solution of 3c in CH3CN
at ꢀ20 °C.
1
MeOH, 100:5). H NMR (300.1 MHz, CDCl3): δ 6.76 (br s, 2H,
CHim), 5.34 (br s, 1H, NH), 4.86 (m, 2H, NBD), 4.49 (m, 2H, NCH2),
4.10 (s, 3H, NCH3), 3.92 (m, 2H, CH2NH), 3.82 (m, 2H, NBD), 3.49
t
(m, 2H, NBD), 1.41 (s, 9H, Bu), 1.36 (t, 2H, JH,H = 1.4 Hz, C7H2
NBD). 13C{1H} NMR (75 MHz, CDCl3): δ 184.2 (d, JC,Rh = 57.1 Hz),
156.3 (CdO), 121.8 (CHim), 121.6 (CHim), 79.4 (d, JC,Rh = 4.6 Hz,
NBD), 79.2 (Cq, tBu), 63.6 (d, JC,Rh = 4.8 Hz; NBD), 51.0 (d, JC,Rh = 2.4
Hz, NBD), 49.6 (NCH2), 48.1 (d, JC,Rh = 12.3 Hz, NBD), 41.3
(CH2NH), 37.9 (NCH3), 28.4 (CH3, tBu). IR (CH2Cl2, cmꢀ1): 1712
(vs, νCO). IR (KBr, cmꢀ1): 3386 (s, νNH), 3156, 3120, 3104, 3056, 3041,
2981, 2922, 1700 cmꢀ1 (vs, νCO), 1500, 1364, 1254, 1173. ESI-MS
(MeOH, m/z): 478 (11) [M + Na]+, 420 (100) [M ꢀ Cl]+; 490 (100)
[M + Cl]ꢀ. Anal. Calcd (%) for C18H27ClN3O2Rh: C, 47.43; H, 5.97; N,
9.22. Found: C, 47.10; H, 6.04; N, 9.23. Mp = 136 °C (dec). Suitable
crystals of 3a for X-ray diffraction have been obtained by slow evapora-
tion from a mixture of CH2Cl2 and petroleum ether.
The silver intermediate 2c was not isolated, but a 1H NMR and ESI-
MS analysis was carried out on the crude material to confirm its
1
formation. H NMR of 2c (399.9 MHz, CDCl3): δ 7.36ꢀ7.21 (m,
15H, Ph), 7.29 (s, 1H, CHim), 7.03 (s, 1H, CHim), 4.24 (t, 2H, JH,H = 6.0
Hz, NCH2), 3.51 (t, 2H, JH,H = 6.0 Hz, CH2NH), 1.43 (s, 9H, tBu), the
NH resonance was not observed. 13C{1H} NMR (100.6 MHz, CDCl3):
δ 183.9 (s, C-Ag), 156.8 (CdO), 142.0 (C5), 130.0 (C4), 128.4 (C2,
t
C3), 123.5 (CHim), 119.5 (CHim), 80.0 (C1), 79.9 (Cq, Bu), 52.6
t
(NCH2), 41.4 (CH2NH), 28.3 (CH3, Bu). ESI-MS (MeOH, m/z):
1015 (50) [C58H64AgN6O4]+, 243 (100) [Ph3C]+.
1H NMR of 3c (599.7 MHz, CDCl3, 243.2 K): δ 7.38ꢀ7.17 (m, 15H,
Ph), 6.98 (s, 1H, CHim), 6.46 (s, 1H, CHim), 6.02 (br s, 1H, NCH2), 6.14
(t, JH,H = 5.1 Hz, 1H, NH), 4.60 (br s, 1H, NCH2), 4.53 (s, 1H, NBD),
3.88 (br s, 1H, CH2NH), 3.72 (br s, 1H, CH2NH), 3.62 (s, 1H, CH,
NBD), 3.57 (s, 1H, CH, NBD), 3.23 (s, 1H, CH, NBD), 2.82 (s, 1H,
Synthesis of 3b. To a solution of 1b (0.500 g, 1.31 mmol) in CH2Cl2
(30 mL) was added Ag2O (0.154 g, 0.66 mmol). After stirring for 2 h at
room temperature, the suspension was filtered on Celite and the filtrate
added to a solution of [Rh(NBD)Cl]2 (0.301 g, 0.65 mmol) in CH2Cl2.
After 3 h, AgBr was filtered off and the solvent was removed under
vacuum. The crude material was purified by column chromatography on
silica gel using first CH2Cl2 and then CH2Cl2/MeOH, 100:1 (v/v), as
eluent to afford 0.67 g (78%) of 3b as a yellow solid. Rf: 0.65 (CH2Cl2/
MeOH, 100:5).
t
CH, NBD), 2.1 (s, 1H, CH, NBD), 1.41 (s, 9H, Bu), 0.98 (m, 2H,
NBD). 13C{1H} NMR (150.8 MHz, CDCl3, 243.2 K): δ 185.4 (d,
JC,Rh = 56.1 Hz), 156.6 (CdO), 142.1 (C5), 128.7 (C4), 128.3 (C2),
127.4 (C3), 124.8 (CHim), 119.7 (CHim), 79.1 (Cq, tBu), 77.3 (C1), 73.5
(CH, NBD), 64.3 (CH, NBD), 61.9 (C7H2, JC,Rh = 5.1 Hz, NBD), 51.2
(NCH2), 49.3 (CH, NBD), 48.7 (CH, NBD), 45.1 (CH, NBD), 44.7
The silver intermediate of 1-(2-BocNH-ethyl)-3-benzylimidazolin-2-
ylidene, silver bromide 2b, was not isolated, but a NMR and ESI-MS
analysis was carried out on the crude material to confirm its formation.
1H NMR of 2b (399.9 MHz, CDCl3): δ 7.21ꢀ6.98 (m, 5H, Ph), 7.02 (s,
t
(CH, NBD), 40.8 (CH2NH), 28.8 (CH3, Bu). IR (CH2Cl2, cmꢀ1):
1702 (vs, νCO). IR (KBr, cmꢀ1): 3319 (s, νNH), 3141, 3089, 3058, 3032,
2975, 2930, 1707 (vs, νCO), 1492, 1445, 1250, 1170. ESI-MS (MeOH,
m/z): 648 (100) [M ꢀ Cl]+; 682 [M ꢀ H]ꢀ. Anal. Calcd (%) for
C36H39ClN3O2Rh: C, 63.21; H, 5.75; N, 6.14. Found: C, 63.51; H, 5.98;
N, 6.04. Mp = 108 °C.
1
1H, CHim), 6.81 (s, 1H, CHim), 5.74 (br s, H, NH), 5.14 (s, 2H,
PhCH2N), 4.16 (br t, 2H, NCH2), 3.39 (br t, 2H, CH2NH), 1.28 (s, 9H,
CH3). 13C{1H} NMR (100.6 MHz, CDCl3): δ 181.5 (s, C-Ag), 155.9
(CdO), 135.7 (Ph, C5); 128.7 (Ph, C4); 128.1 (Ph, C3); 127.3 (Ph, C2),
122.0 (CHim), 120.9 (CHim), 78.9 (Cq, tBu), 55.2 (CH2, PhCH2), 51.1
(CH2, NCH2), 41.2 (CH2, CH2NH), 28.1 (CH3, tBu). IR
(CH2Cl2, cmꢀ1): 1707 (vs, νCO). ESI-MS (MeOH, m/z): 709 (100)
[C34H46AgN6O4]+; 79 (100), 81 (97) [Br]ꢀ.
Synthesis of [Rh(NBD)I{1-(2-NHBoc-ethyl)-3-methylimi-
dazolin-2-ylidene}], 4a, by Method B (iii). [Rh(NBD)Cl]2
(0.050 g, 0.11 mmol) and KOtBu (0.024 g, 0.22 mmol) were reacted
in THF (10 mL) for 3 h. Successively the imidazolium salt 1a (0.076 g,
0.212 mmol) was added, and after stirring for a further 3 h the solid was
removed by filtration and the resulting orange solution was evaporated
to dryness. After washing with Et2O 0.049 g (41%) of 4a was obtained,
and suitable crystals of 4a for X-ray diffraction have been obtained by
slow evaporation from a mixture of CH2Cl2 and petroleum ether.
Synthesis of 4a by Ion Exchange. Alternatively to a solution of
3a (0.050 mg, 0.11 mmol) in degassed methanol (2 mL) was added an
excess of NaI (0.030 g, 0.2 mmol). After stirring for 20 h the suspension
was filtered, and after removal of the volatiles in vacuo the product was
used without further workup. 1H NMR (399.9 MHz, CDCl3): δ 6.78 (s,
2H, CHim), 5.25 (br s, 1H, NH), 5.09 (m, 1H, CH, NBD), 4.98 (m, 1H,
CH, NBD), 4.64 (m, 1H, NCH2), 4.26 (m, 1H, NCH2), 3.98 (s, 3H,
1H NMR of 3b (599.7 MHz, CDCl3, 228.18 K): δ 7.35 (m, 5H, Ph),
6.79 (d, 1H, JH,H = 1.8 Hz, CHim), 6.68 (d, 1H, JH,H = 1.8 Hz, CHim),
6.01 (d, 1H, JH,H = 15.2 Hz, CH2Ph), 5.57 (d, 1H, JH,H = 15.2 Hz,
CH2Ph), 5.45 (t, 1H, JH,H = 5.6 Hz, NH), 4.83 (m, 2H, NBD), 4.70 (m,
1H, NCH2); 4.31 (m, 1H, NCH2), 4.18 (m, 1H, CH2NH), 3.81 (m, 1H,
NBD), 3.76 (m, 1H, CH2NH), 3.67 (m, 1H, NBD), 3.51 (m, 1H, NBD),
3.23 (m, 1H, NBD), 1.39 (s, 9H, CH3), 1.31 (d, 1H, JH,H = 8.1 Hz, C7H2,
NBD), 1.26 (d, 1H, JH,H = 8.1 Hz, C7H2, NBD). 13C{1H} NMR (150.8
MHz, CDCl3, 228.2 K): δ 185.2 (d, JC,Rh = 55.2), 156.3 (CdO), 136.8
(Ph, C5); 128.9 (Ph, C4); 128.6 (Ph, C3); 128.1 (Ph, C2), 122.3 (CHIm),
120.5 (CHIm), 79.3 (Cq, tBu), 79.2 (CH, NBD), 63.6 (d, JC,Rh = 4.8 Hz,
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Organometallics
ARTICLE
CH3), 3.89 (m, 1H, CH2NH), 3.82 (m, 2H, CH, NBD), 3.72 (m, 1H,
CH2NH), 3.63 (m, 2H, CH, NBD), 1.42 (s, 9H, tBu), 1.26 (m, 2H, CH2,
NBD). 13C{1H} NMR: δ 183.7 (d, JC,Rh = 56.2 Hz), 122.0 (CHim),
121.9 (CHim), 78.9 (Cq, tBu), 76.8 (CH, NBD), 76.4 (CH, NBD), 64.9
(7CH2, NBD), 49.2 (NCH2), 37.7 (CH3), 40.3 (CH2NH), 51.2 (CH,
NBD), 50.6 (CH, NBD), 28.0 (tBu). IR (THF, cmꢀ1): ν(CdO): 1715.
IR (KBr, cmꢀ1): 3354 (s, νNH), 3165, 3094, 3052 (m, CHim), 2975,
2924 (br vs, CH), 1700 (vs, νCO). ESI-MS(+) (MeOH, m/z): 420 (100)
[M ꢀ I]+; 127 (100) [Iꢀ]. Anal. Calcd (%) for C18H27IN3O2Rh: C, 39.51;
H, 4.97; N, 7.68. Found: C, 40.00; H, 5.14; N, 7.61. Mp = 148 °C (dec).
X-ray Crystal Structure Determination for 1c, 3a, 4a, and
3c. Crystal data were collected at room temperature on a Bruker APEX
II diffractometer equipped with a CCD detector operating at 50 kV and
30 mA, using graphite-monochromated Mo Kα radiation (λ = 0.71073
Å). An empirical absorption correction was applied on both structures
by using SADABS.35 They were solved by direct methods and refined by
full-matrix least-squares based on all data using F2 with SHELXL97.36 All
non-hydrogen atoms were refined anisotropically, with the exception of
the hydrogen atoms, which were set geometrically and given fixed
isotropic thermal parameters. There is a positional disorder in the
structure of C18H27IN3O2Rh (4a), more specifically in the tert-butyl
group, so the involved carbon atoms were split in two positions, and
anisotropic displacement parameter restraints were applied. Crystal data
are collected in Table 2.
optimized ground state the frequency analysis showed the absence of
imaginary frequencies, whereas each transition state showed a single
imaginary frequency. Visual inspection of the corresponding normal
mode was used to confirm that the correct transition state had been
found. All the reported energy values represent total electronic energies.
In general, these give the best fit with experimental DNMR data.42
Therefore, the computed numbers have not been corrected for zero-
point energy contributions or other thermodynamic parameters. This
avoids artifacts that might result from the ambiguous choice of the
adequate reference temperature, from the empirical scaling,43 and from
the treatment of low-frequency vibration as harmonic oscillators.44
Catalysis. General Procedure for the Hydrosilylation of 1-Alkynes
with HSiMe2Ph. A J. Young valve NMR tube was charged under argon
with the catalyst precursor (3aꢀc) (7.7 ꢁ 10ꢀ4 mmol), CDCl3
(0.6 mL), the corresponding alkyne (PhCtCH, TolCtCH,
nBuCtCH, Et3SiCtCH, or (CPh2OH)CtCH) (0.077 mmol), and
a slight excess of HSiMe2Ph (0.085 mmol). The solution was kept at
T = 25 °C and monitored by 1H NMR spectroscopy. The new products
were characterized by 1H NMR and by comparison with similar compounds
reported in the literature.25
1
(E)-2-(Dimethyl(phenyl)silyl)-1-tolylethene. H NMR (300.1 MHz,
CDCl3): δ 7.60ꢀ7.16 (m, 9H), 6.96 (d, 1H, JH,H = 19.2 Hz), 6.45
(d, 1H, JH,H = 19.2 Hz,), 2.36 (s, 3H), 0.49 (s, 6H) ppm.
1-(Dimethyl(phenyl)silyl)-1-tolylethene. 1H NMR (300,1 MHz,
CDCl3): δ 7.6ꢀ7.3 (m, 4H), 5.93 (d, 1H, JH,H = 2.9 Hz), 5.66
(d, 1H, JH,H = 2.9 Hz), 2.35 (s, 3H), 0.30 (s, 6H) ppm.
Variable-Temperature NMR. NMR spectra were recorded using
spectrometers operating at fields of 9.6 and 14.4 T (400 and 600 MHz
(E)-2-(dimethylphenylsilyl)-1-(CPh2OH)ethene. 1H NMR (300,1
MHz, CDCl3): δ 7.77ꢀ7.24 (m, 15H), 6.74 (d, 1H, JH,H = 18.8 Hz),
6.16 (d, 1H, JH,H = 18.8), 0.35 (s, 6H).
1
for H). The NMR tubes containing the compounds were prepared
under an argon atmosphere using a vacuum line. Low-temperature 1H
spectra were acquired without spinning using a 5 mm dual direct probe
with a 6000 Hz (at 400 MHz) or 9000 Hz (at 600 MHz) sweep width,
40° tip angle pulse width, 3 s acquisition time, and 1 s delay time. A
shifted sine bell weighting function equal to the acquisition time (i.e., 3
s) was applied before the Fourier transformation. Usually 32 to 64 scans
were collected. When operating the NMR apparatus at low temperature,
a flow of dry nitrogen was first passed through a precooling unit adjusted
to ꢀ50 °C. Then the gas entered into an inox steel heat-exchanger
immersed in liquid nitrogen and connected to the NMR probe head by a
vacuum-insulated transfer line. Gas flows of 10 to 20 L minꢀ1 were
required to descend to the desired temperature. Temperature calibra-
tions were performed before the experiments, using a digital thermo-
meter and a Cu/Ni thermocouple placed in an NMR tube filled with
isopentane for the low-temperature range and with tetrachloroethane
for the high-temperature range. The conditions were kept as identical as
possible with all subsequent work. In particular, the sample was not spun
and the gas flow was the same as that used during the acquisition of the
spectra. The uncertainty in temperature measurements can be estimated
from the calibration curve as (2 °C.
1-(Dimethylphenylsilyl)-1-(CPh2OH)ethene. 1H NMR (300,1 MHz,
CDCl3): δ 7.77ꢀ7.24 (m, 15H), 5.72 (d, 1H, JH,H = 1.8), 5.28 (d, 1H,
JH,H = 1.8), 0.39 (s, 6H).
General Procedure for the Addition of Arylaldehydes with Phenyl-
boronic Acid. Phenylboronic acid (0.600 g, 4.9 mmol), KOtBu (2.45
mmol), the substituted aldehyde (4-chlorobenzaldehyde, 4-methoxy-
benzaldehyde, 3,4,5-trimethoxybenzaldehyde, 4-tert-butylbenzaldehyde,
4-cianobenzaldehyde, 4-acetylbenzaldehyde) (2.45 mmol), the rhodium
catalyst 3a or 3b (1% mol), and dimethoxyethane (7.5 mL) were
introduced in a Schlenck tube, and then degassed H2O (2.5 mL) was
added. The resulting mixture was heated for 1ꢀ8 h at 80 °C, cooled to
ambient temperature, and extracted with ethyl acetate (20 mL). After
drying over Na2SO4 the organic phase was evaporated, and the residue
was purified by flash chromatography. The isolated yield was checked by
1H NMR spectroscopy. The reaction products were identified by NMR
comparison with literature reported data.28
’ ASSOCIATED CONTENT
Line shape simulations were performed using a PC version of the QCPE
DNMR6 program.37 Electronic superimposition of the original spectrum and
of the simulated one enabled the determination of the most reliable rate
constant. The rate constants, thus obtained at various temperatures, afforded
the free energy of activation ΔGq by applying the Eyring equation.38 ΔHq and
ΔSq were evaluated by linear regression of the ΔGq value vs the temperature.
Except for 3c, in all cases investigated, the activation energy ΔGq was found to
be virtually invariant in the given temperature range, thus implying a very
small or negligible activation entropy ΔSq.19,39
S
Supporting Information. Computational details; struc-
b
tural data for 1c, 3a, 4a, 3c; 1H, 13C, and HSQC NMR spectra for
3c; VT NMR for 3a, 4a, 3c; details and full characterization data
for the catalysis. X-ray data in the form of CIF files. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: maria.cassani@unibo.it (M.C.C.), rita.mazzoni@unibo.it
(R.M.). Fax: +39 0512093694. Phone: +39 051 2093700.
DFT Calculations. Conformational searches were performed by
Molecular Mechanics (MMFF force field as implemented in Titan 1.0.5,
Wavefunction Inc.). Final geometry optimizations were carried out at
the B3LYP/LanL2DZ level40 by means of the Gaussian 09 series of
programs41 (see the Supporting Information). The standard Berny
algorithm in redundant internal coordinates and default criteria of
convergence were employed in all the calculations. Harmonic vibrational
frequencies were calculated for all the stationary points. For each
’ ACKNOWLEDGMENT
The authors thank the University of Bologna and the Ministero
dell’Universitꢀa e della Ricerca (MUR) (project: “New strategies for
5270
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Organometallics
ARTICLE
the control of reactions: interactions of molecular fragments with
metallic sites in unconventional species”, PRIN 2007) for financial
support.
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(16) Herrmann, W. A.; Gooβen, L. J.; Spiegler, M. J. Organomet.
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