The fluorine-NHC gauche effect: A structural and computational study

Article abstract of DOI:10.1016/j.tet.2013.02.071

Herein, we report the synthesis and X-ray structural analysis of a collection of fluorinated metal N-heterocyclic carbenes (Ag, Au, Pd, Rh, Ir) and their precursor salts. The common structural feature of these species is a flanking fluoroethyl group, which is either freely rotating or embedded within a bicyclic framework. Solid state analysis confirmed a gauche conformational preference in all cases with the fluorine adopting a syn clinal arrangement (φ_[NCCF]～60°) with respect to the triazolium nitrogen at the vicinal position of the NHC. A density functional theory analysis was employed to quantify these effects and evaluate the influence of electronic modulation of the carbenic carbon [(CN⁺); neutral carbene (C:); metal-bound carbene (CM)], on the relative gauche/anti preference, thus highlighting the potential of this conformational phenomenon as a useful molecular design strategy for controlling the topology of organometallic complexes.

Full text of DOI:10.1016/j.tet.2013.02.071

Tetrahedron xxx (2013) 1e13
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
The fluorine-NHC gauche effect: a structural and computational
study
,
Susann Paul a, W. Bernd Schweizer a, Graham Rugg b, Hans Martin Senn b
*
,
,
Ryan Gilmour c
*
a
€
€
ETH Zurich, Laboratory for Organic Chemistry, Wolfgang-Pauli-Strasse 10, 8093 Zurich, Switzerland
b University of Glasgow, WestCHEM and School of Chemistry, Glasgow G12 8QQ, Scotland, UK
c
€
€
€
€
Westfalische Wilhelms-Universitat Munster, Organic Chemistry Institute, Corrensstrasse 40, 48149 Munster, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein, we report the synthesis and X-ray structural analysis of a collection of fluorinated metal
N-heterocyclic carbenes (Ag, Au, Pd, Rh, Ir) and their precursor salts. The common structural feature of
these species is a flanking fluoroethyl group, which is either freely rotating or embedded within a bicyclic
framework. Solid state analysis confirmed a gauche conformational preference in all cases with the
fluorine adopting a syn clinal arrangement (f[NCCF]w60ꢀ) with respect to the triazolium nitrogen at the
vicinal position of the NHC. A density functional theory analysis was employed to quantify these effects
and evaluate the influence of electronic modulation of the carbenic carbon [(C]Nþ); neutral carbene
(C:); metal-bound carbene (C]M)], on the relative gauche/anti preference, thus highlighting the po-
tential of this conformational phenomenon as a useful molecular design strategy for controlling the
topology of organometallic complexes.
Received 14 January 2013
Received in revised form 18 February 2013
Accepted 21 February 2013
Available online xxx
This article is dedicated, with respect and
admiration, to Professor Melanie Sanford on
her receipt of the Tetrahedron Young In-
vestigator Award 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Computational chemistry
Conformational analysis
Fluorine
Gauche effect
Molecular design
1. Introduction
Fluorinated N-heterocyclic carbenes are emerging as valuable
organocatalysts and powerful ancillary ligands for transition-
metal catalysed processes.1 Pertinent examples include the tri-
azolium Stetter catalyst reported by Rovis et al. (1) where the
introduction of fluorine improves efficiency,2 and the ruthenium
metathesis catalyst disclosed by Grubbs et al. (2)3 in which a flu-
orineemetal interaction leads to a rate acceleration. In both cases,
the fluorine atom that is inextricably linked to catalyst perfor-
mance is vicinal to one of the flanking nitrogens of the NHC
(Fig. 1). In these examples, the fluorine atom influences the overall
conformation of the catalysts by virtue of stereoelectronic re-
finement4 or remote metalefluorine interactions,5 whilst the in-
ductive effect is likely diminished relative to an aliphatic
Fig. 1. Examples of fluorinated N-heterocyclic carbenes.
Our interest in N-heterocyclic carbene (NHC) complexes con-
taining a single, aliphatic fluorine substituent positioned vicinal to
one of the flanking nitrogens stems from our efforts to exploit the
fluorine gauche effect6 as an approach to acyclic conformational
control in catalysis.7
b
-fluoroamine motif (þIp vs ꢁIs).
This phenomenon, which is principally associated with fluo-
rinated organic molecules, is
conjugative interactions between the low lying
orbital of the CeF bond and adjacent -orbitals or non-bonding
a
manifestation of hyper-
s
* antibonding
* Corresponding authors. E-mail address: ryan.gilmour@uni-muenster.de
(R. Gilmour).
s
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.02.071
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2
S. Paul et al. / Tetrahedron xxx (2013) 1e13
electron pairs.8 Consequently, it is possible to access conformer
populations, whilst keeping the steric footprint of the steering
group to a minimum, in a manner that is complementary to
steric conformational control. As a general rule, fluoro sub-
stituents prefer to be gauche to vicinal electron-withdrawing
groups (FdꢁeCH2eCH2eEWG; f[FeCeCeEWG]z60ꢀ) rendering this
phenomenon potentially useful for molecular design (Scheme 1;
top). However, in the case of freely rotatable scaffolds, such as
1,2-difluoroethane, it is important to note that two possible
gauche conformers can be populated with equal probability
N
NPh
τ2
θ
N-heterocyclic carbene precursors
τ1
N
bicyclic to monocyclic
F
N
NPh
X
N
NPh
N
+
+
+
+
N
N
N
NPh
_
N
NPh
_
_
_
Ph
Ph
X
X
X
3
4
5
6
F
F
F
F
(
f[FeCeCeEWG]zꢂ60ꢀ).
3o to 1o fluoride
triazolium to imidazolium
metal N-heterocyclic carbene complexes
N
NPh
N
NPh
N
NPh
N
N
N
NPh
N
Ph
Ph
[M]Ln
7
[M]Ln
8
[M]Ln
10
[M]Ln
9
F
F
F
F
M = Ag, Au
M = Pd, Rh, Ir
L = ligand
M = Ag, Au
M = Rh, Ir
L = ligand
M = Ag
M = Pd
L = ligand
M = Ag
M = Pd
L = ligand
Fig. 2. Synthetic targets for this study.
Scheme 1. Pre-organisation of metal N-heterocyclic carbene complexes by a fluorine-
NHC gauche effect. X¼C or N; M¼coinage or transition metal; L¼ligand.
esterification of 11 to form the methyl ester 12 (81%). Addition of
phenylmagnesium bromide furnished the expected carbinol 13 in
a
respectable 70% yield. Deoxyfluorination using dieth-
Herein, we disclose the synthesis, structural analysis and DFT
study of various fluorinated NHCs and explore this conformational
effect within the framework of organometallic complexes. The
possibility of discriminating between two gauche scenarios (gau-
che1 and gauche2) by relying on remote metalefluorine interactions
is also considered (Scheme 1; bottom). Moreover, a computational
study to evaluate the influence of electronic modulation of the
carbenic carbon [(C]Nþ); neutral carbene (C:); metal-bound car-
bene (C]M)], on the relative gauche/anti preference of these li-
gands is disclosed.
ylaminosulfur trifluoride (DAST) successfully installed the tertiary
fluorine centre in 64% yield. Subsequent treatment with the
Meerwein’s reagent, phenylhydrazine and trimethylorthoformate
furnished the desired triazolium tetrafluoroborate in a three step,
one pot sequence (61% overall). NHC precursor 19 was also pre-
pared from the methyl ester 12 by reduction and subsequent
protection as the TBS ether.
Construction of the triazolium ring followed the same three
step, one pot procedure as previously described to deliver the TBS
precursor 18 in 17% yield (three steps). Finally, treatment of this
species with an excess of DAST effected an in situ deprotection/
deoxyfluorination sequence to deliver the desired product in up to
98% yield. Unfortunately, this last step proved capricious and yields
were variable.
Previously, this laboratory has reported the synthesis and
conformational analysis of a representative Au(I) NHC. It was
noted that by inserting a CeF bond in a vicinal relationship to
the triazolium nitrogen atom at the ring junction of the ligand,
the relative molecular topology of
a
flanking diphenyl-
An alternate route to access compound 19 was explored in-
volving the conversion of alcohol 16 to the corresponding bromide
by treatment with CBr4 and PPh3.11 Halogen exchange proved
facile by simple treatment of 20 with AgF in a Finkelstein reaction
to furnish the primary fluoride 21 in a respectable 54% yield. The
three step alkylation/phenylhydrazine addition/cyclisation se-
quence furnished the target structure 19 in 78% yield over three
steps.
fluoromethyl group could be controlled (f[NCCF]¼ꢁ61.9ꢀ and
ꢁ48.6ꢀ) by the aforementioned effect.9 To establish the gener-
ality of this gauche conformational preference in NHC scaffolds
bearing a pendant fluoroethyl motif, three ligand substructures
were conceived (Fig. 2). Scaffolds 3 and 7 would partially embed
the fluoroethyl motif in a bicyclic triazolium framework thus
restricting rotation around one of the two bonds of the fluo-
roethyl group (
s
1 and
s
2). Similarly, structures 4 and 8 would also
Gratifyingly, it was possible to grow crystals of the triazolium
tetrafluoroborate salt 19 that were suitable for X-ray crystallo-
graphic analysis (Fig. 3). The compound crystallised in the or-
thorhombic space group P212121 with four molecules in the unit
cell.10 A completely different packing arrangement shows the
fluorodiphenylmethyl tetrafluoroborate salt 15, which crystal-
lised in the monoclinic space group C2.9 Importantly, measure-
reduce torsional freedom but offer the possibility of syn clinal-
endo versus syn clinal-exo arrangements due to the removal of
the bulky phenyl substituents from the fluorine-bearing carbon
atom.
Finally, the non-constrained species 5/9 and 6/10 may allow
for the detection of remote metalefluorine interactions by
relaxing the CeCeN bond angle (
q
). In all of these ligand classes,
ment of the NCCF torsion angle (f) confirmed the expected
Plenio’s criteria of having the metal and fluorine atom separated
by a minimum of four bonds is satisfied.5 A solid state analysis of
complexes 7e10, containing a group 11 coinage metal (Ag, Au) or
a group 9 or 10 transition metal (Pd, Rh, Ir) allows for comparison
of the metal on the conformational behaviour. The synthesis of
bicyclic triazolium salts of type 3 and 4 proved facile from com-
mercially available (S)-pyroglutamic acid 11 (Scheme 2). Prepa-
ration of the diphenylfluoromethyl derivative 15 began by
gauche conformation (f[NCCF]¼67.9ꢀ). Intriguingly, whereas in
the fluorodiphenylmethyl species a syn clinal-endo conforma-
tion was found, possibly due to the steric hindrance of the two
phenyl groups, a syn clinal-exo arrangement is present in 19. It is
also noteworthy that the plane of the N-phenyl substituent and
the plane of the triazolium ring system of an adjacent molecule
are pointing away from each other in contrast to 15 where the
planes are almost parallel with an angle of 5.3ꢀ.
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S. Paul et al. / Tetrahedron xxx (2013) 1e13
synthesis of the N-heterocyclic carbene precursors
3
-
i) Me3O+BF4 , CH2Cl2
PhMgBr
THF
Et2N-SF3
CH2Cl2
O
O
O
ii) PhNHNH2, rt, 24 h
NH
NH
NH
0oC - rt
16 h, 70%
0oC - rt
20 h, 64%
iii) HC(OMe)3, MeOH,
80oC, 24 h
61% (3 steps)
CO2R
F
OH
Ph
Ph
Ph
13
Ph
14
SOCl2, MeOH
-18oC - rt, 81%
11 R = H
12 R = Me
-
i) Me3O+BF4 , CH2Cl2
ii) PhNHNH2, rt, 24 h
iii) HC(OMe)3, MeOH,
80oC, 24 h
NaBH4
EtOH
0oC - rt
19 h
+
+
+
Et2N-SF3
CH2Cl2
O
N
N
N
N
N
NPh
NPh
NPh
_
_
_
N
NH
OR
BF4
BF4
BF4
80%
0oC - rt
20 h
up to 98%
17% (3 steps)
F
F
OTBS
Ph
Ph
15
16 R = H
TBSCl, imidazole,
18
19
17 R = OTBS
CH2Cl2, rt, 4 h, 66%
i) Me3O+BF4 , CH2Cl2
ii) PhNHNH2, rt, 15 h
iii) HC(OMe)3, MeOH,
80oC, 8 h, 78%
-
PPh3, CBr4
O
O
MeCN, 0oC - rt
AgF, MeCN
rt, 14 h, 54%
16
NH
NH
(3 steps)
15 h, 70%
20
21
Br
F
Scheme 2. Synthesis of the bicyclic N-heterocyclic carbene precursor salts 15 and 19.
space group P21/c with four molecules in the unit cell. Short fluo-
rineefluorine contacts between the fluorine of the pendant arm of
the imidazole and the triflate anion were observed with a distance
ꢀ
of dCeF/FeC¼2.72 A, which is less than the sum of the van der Waals
16
ꢀ
radii of 2.94 A (Fig. 5). This is certainly the effect of the disorder
and not the correct distance in the crystal. The nature of these in-
teractions is the subject of much conjecture.17
Fig. 3. X-ray crystal structure of the triazolium tetrafluoroborate salt 19 (f[NCCF]¼67.9ꢀ;
syn clinal-exo). Thermal ellipsoids set at the 50% probability level.10
Inspired by a recent study by Kysilka et al. describing the al-
kylation of imidazoles with fluoroalkyltriflates,12 the imidazolium
and triazolium triflates 23 and 25 were prepared by alkylation of 22
and 24 with fluoroethyltriflate in toluene at reflux (95% and 94%,
respectively, Scheme 3). Phenylimidazole 22 was synthesised via
the copper-catalysed amination of phenylbromide with imidaz-
ole.13 Similarly, the 1,2,4-triazole 24 was obtained by copper-
catalysed coupling of triazole with phenyl iodide.
Fig. 4. X-ray crystal structure of the imidazolium triflate 23 showing the fluorine
gauche effect
(
f[NCCF]¼ꢂ71.1ꢀ). Reliability of the F/F contacts indicated in red
ꢀ
(d¼2.72 A) suffer from the disordered CF3 group. Thermal ellipsoids set at the 50%
probability level.14
Scheme 3. Synthesis of the imidazolium and triazolium triflates 23 and 25.
It was possible to grow crystals of the imidazolium and tri-
azolium salts 23 and 25 that were suitable for single crystal X-ray
analysis.14,15 The solid state structure of 23 revealed a clear gauche
relationship of the fluorine relative to the nitrogen centre
Fig. 5. F/F contacts of type II in 23. Thermal ellipsoids set at the 50% probability
level.14
(
fNCCFꢂ71.1ꢀ, Fig. 4). The molecule crystallised in the monoclinic
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4
S. Paul et al. / Tetrahedron xxx (2013) 1e13
Murray-Rust et al. have classified such halogenehalogen in-
teractions into two classes (Fig. 5): type I in which the CeXeX
angles are equal (q1 q2) and the less common type II in which one
agents.18,20e22 Initially, the palladium bis-NHC complex 26 was
prepared from the tetrafluoroborate salt 15 by a two step pro-
cedure involving transmetallation from the silver complex with
PdCl2, to furnish the desired species in a moderate 33% yield
(Scheme 5).
¼
angle (q1) is approximately 180ꢀ and the other (q2) around 90ꢀ. The
fluorineefluorine interaction found in the crystal structure of 23 is
reminiscent of type II with q1¼157.1ꢀ and q2¼103.2ꢀ17a The gauche
conformational preference was also observed in the X-ray structure
of 25 (Fig. 6).15
Scheme 5. Synthesis of the PdCl2(NHC)2 complex 26.
Single crystal X-ray analysis of the PdCl2(NHC)2 complex 26
(Fig. 7)23 confirmed the expected gauche orientation of the
fluorine with respect to the nitrogen of the triazolium ring
system (f[NCCF]¼60.4ꢀ). Interestingly, the syn clinal-exo confor-
mation is observed, in contrast to the Ag(I) and Au(I) complexes
with the identical NHC ligand where the syn clinal-endo con-
former is present.9 This is possibly due to the bis structure of
compound 26. It is interesting to note that the NHC ligands are
tilted at an angle of 81.1ꢀ relative to each other. Furthermore,
the N-phenyl group is rotated by 42.7ꢀ with respect to the tri-
ꢀ
azolium ring plane (Fig. 7). The interatomic distance of 2.30 A is
Fig. 6. X-ray crystal structure of the triazolium triflate 25. The fluorine gauche effect
(
considerably smaller than the sum of the Van der Waals radii of
fluorine and hydrogen (2.67 A). O’Hagan et al. have disclosed the
fNCCF¼ꢂ65.8ꢀ and ꢂ69.6ꢀ) isindicated. Thermal ellipsoidsset at the50%probabilitylevel.15
ꢀ
results of a Cambridge Structural Database (CSD) search for
short F/H contacts in fluorine-containing organic compounds,
which revealed that CeF/HeC interactions are the most com-
mon contacts.24 The authors also mention that the CeF/HeC
interactions have practically no influence on packing
arrangements.
2. Pd N-heterocyclic carbene complexes (type 7)
In order to investigate the potential application of the
fluorine-NHC gauche effect for controlling the molecular topology
of transition metal complexes and thus complement our recent
study of Ag(I) and Au(I) systems,9 a synthesis and structural
analysis of palladium complexes was initiated. In contrast to Ag(I)
and Au(I) species in which the coordination geometry is pre-
dominantly linear, Pd(II) complexes would allow this conforma-
tional analysis to be extended to square-planar arrangements.
Moreover, a study of transition metal complexes would allow the
notion of further conformational refinement by remote metal-
efluorine interactions to be investigated (Scheme 4). Given
the importance of Pd(II) species in catalysis, we envisaged that
new approaches to control the ligand topology may find further
applications.
Scheme 4. Possible gauche effect/remote metalefluorine interactions in transition
metal complexes.
Fig. 7. X-ray structure of the PdCl2(NHC)2 complex 26; f[NCCF]¼60.4ꢀ, dH-F¼2.296 A,
ꢀ
23
ꢀ
dPd-F¼3.24 A. Thermal ellipsoids set at the 50% probability level.
Regarding the synthesis of these Pd, and other transition
metal complexes described herein, modified literature pro-
cedures were followed.18,19 Generally, transmetallation from
the corresponding silvereNHC complexes proved to be the
method of choice. This process has attracted broad attention
due to the capability of such species to act as carbene transfer
In this example, the distance between fluorine and palladium
was measured to be 3.24 A, which is greater than the threshold
value of 3.0 A. The characteristic square-planar coordination ge-
ometry of palladium(II) complexes is satisfied in this particular
ꢀ
5
ꢀ
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S. Paul et al. / Tetrahedron xxx (2013) 1e13
5
case, and thus the fluorine atom is not necessary to complete the
coordination sphere. Therefore, the preparation of a palladium
complex bearing a single NHCeligand was attempted. To that end,
the Pd(II)(allyl)(NHC) complex 28 was prepared by trans-
metallation of the silver NHC complex 27 with the allyl palladiu-
m(II)chloride dimer (52%, Scheme 6).
similar to the precursor salt 15. Furthermore, the X-ray analysis also
revealed a distorted square-planar coordination around the Pd(II)
and the expected h
3 coordination mode of the allyl-palladium frag-
ment. The distances between the palladium and the allylic carbon
ꢀ
atoms were found to be similar (2.097, 2.137 and 2.183 A), with the
longest bond being to the carbon trans to the NHC ligand. Solid state
analysis of the complex revealed that the fluorine atom is pointing
ꢀ
away from the metal centre (d¼4.95 A), thus no remote fluo-
rineepalladium interactionwas detected. Unfortunately, attempts to
generate the corresponding cationic palladium complex by abstrac-
tion of the chloride ligand by treatment with AgPF6 yielded a com-
plex mixture of decomposition products.
3. Rh and Ir N-heterocyclic carbene complexes (type 7)
Scheme 6. Synthesis of the Pd(allyl)(NHC)2 complex 28.
X-ray analysis revealed that the molecule crystallised in the same
tetragonal space group P41212 as the PdCl2(NHC)2 complex 26 with
eight molecules in the unit cell (Fig. 8).25 However, in contrast to 26,
the fluorine atom adopts a syn clinal-endo conformationwith respect
to the nitrogen of the triazolium ring system (f[NCCF]¼ꢁ51.7ꢀ) com-
parable to the conformations observed in the corresponding Au and
Ag species. In addition, the plane of the N-phenyl substituent is ro-
tated by 29.5ꢀ with respect to the triazolium ring plane, which is very
Like palladium(II) complexes, the coordination behaviour of
rhodium(I) and iridium(I) complexes is dominated by the forma-
tion of square-planar complexes due to their d8 configuration. To
extend this study of metal NHC systems of type 7 (Fig. 2), the
synthesis of complexes 29 (M¼Rh) and 30 (M¼Ir) was attempted.
This proved facile by treating the common Ag(I) transfer agent
27 with either the chloro(1,5-cyclooctadiene)-rhodium(I) or -iri-
dium(I) dimer at 40 ꢀC in CH2Cl2 (Scheme 7). The desired complexes
29 and 30 were isolated in 70% and 83% yield, respectively. It is
important to note that a one pot procedure can also be employed to
deliver the desired target molecule directly from the starting NHC
precursor 15. This process involves suspending the chloro(1,5-
cyclooctadiene)-rhodium(I) or -iridium(I) dimer complexes in
ethanol followed by the addition of a solution of sodium ethoxide in
ethanol. The triazolium precursor 15 can then be added in one
portion to generate the Rh(COD)(NHC)Cl complex 29 and
Ir(COD)(NHC)Cl complex 30. Despite the reduction in yield (Rh,
41%; Ir, 61%) the advantage of this route is that the preparation of
the Ag(I) NHC precursor 27 is avoided.26
Scheme 7. Synthesis of the Rh and Ir complex 29 and 30.
However, repeated attempts to crystallise these complexes
proved fruitless, with the complexes rapidly decomposing. Fortu-
nately, some structural insights could be gleaned from spectro-
scopic studies. In the 19F NMR spectra of both complexes 29 and 30,
two signals were observed in a ratio of 1:0.07 and 1:0.16, re-
spectively, indicating the presence of two discrete species. The
signals of the minor isomer could also be detected in the 1H NMR
spectra of both complexes but could not be assigned to the in-
dividual protons. This observation can be rationalised by consid-
ering two isomers that differ in the orientation of the MeCl bond
with respect to the NHC ligand. This is consistent with a report from
Ganter et al. describing two sets of signals in the 1H NMR spectrum
of a Rh(COD)(NHC)Cl complex.27 In the absence of crystallographic
1
data, it was envisaged that determination of the JCF coupling
constants might give insights as to the presence or absence of re-
mote metalefluorine interactions, since the coupling constant
would likely decrease upon coordination with the metal centre.28,29
However, by comparing the coupling constants of the NHC pre-
cursor salt 15 (J¼185.0 Hz) with those of the Rh complex 29
(J¼185.0 Hz) and the Ir complex 30 (J¼186.4 Hz), no significant
deviation was observed.
Fig. 8. X-ray structure of the Pd(allyl)(NHC)2 complex 28; f[NCCF]¼ꢁ51.7ꢀ, dHeF¼2.46 A.
ꢀ
Thermal ellipsoids set at the 50% probability level (top). CeF/HeC contacts high-
lighted with dotted lines (bottom).25
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6
S. Paul et al. / Tetrahedron xxx (2013) 1e13
4. Ag and Au N-heterocyclic carbene complexes (type 8)
To complement the aforementioned structural studies of the
fluorodiphenylmethyl NHCs, selected NHC metal complexes of the
analogous fluoromethyl species were synthesised. Deletion of the
phenyl groups leads to a sterically less congested scaffold and may
conceivably give a clearer picture of the interactions of interest. The
investigation began with the synthesis of the silver and gold
complexes 32 and 33 (Scheme 8).
Scheme 8. Synthesis of type 8 Ag and Au complexes 32 and 33.
The desired Ag(NHC)Cl complex 33 was prepared from the tet-
rafluoroborate salt 19 and Ag2O in presence of NMe4Cl in 86% yield
(Scheme 8, right).30 This complex was subsequently processed to
the analogous Au(I) NHC 32 by treatment with AuCl(SMe2) (76%
from 19). Intriguingly, treatment of 19 with Ag2O under basic phase
transfer conditions (Scheme 8, left)18 furnished the bis-NHC 31 in
a modest 51% yield.
It was possible to grow crystals of the silver mono- and bis-
NHC complexes 33 and 31 that were suitable for X-ray analysis
(Figs. 9 and 10). The complex 33 crystallised in the same ortho-
rhombic space group P212121 as the NHC precursor salt 19 (Fig. 3)
with four molecules in the unit cell.31
Fig. 10. X-ray structure of the Ag(I) bis-NHC complex 31; f[NCCF] (Ag1)¼57.4ꢀ and
63.0ꢀ; f[NCCF] (Ag2)¼68.5 and ꢁ65.5ꢀ; two symmetry independent molecules in the
asymmetric unit (upper); enlarged view of the crystal lattice with intermolecular
contacts (dotted lines, bottom). Thermal ellipsoids set at the 50% probability level.36
The measurement of the NCCF torsion angle revealed a syn clinal-
endo conformation (f[NCCF]¼ꢁ61ꢀ), whereas the corresponding tet-
rafluoroborate salt 19 showed a syn clinal-exo orientation of the
fluorine with respect to the nitrogen of the triazolium ring system.
Furthermore, it is noteworthy that the silver atom is not in the ex-
pected linear coordination environment as in the case of the fluo-
rodiphenyleNHC complex 27, but instead has a CeAgeCl bond angle
of 132.5ꢀ (dAgeCl¼2.60 A). The silver atom is tetrahedrally coordinated
ꢀ
by one carbenic carbon atom and three chloride atoms. This ar-
rangement results in two parallel zigzag chains in which the Ag(NHC)
Cl units are connected in a head-to-tail manner via the bridging
chlorine atoms with AgeCl distances of 2.07 A and 2.76 A (Fig. 9). This
alignment is consistent with previous reports from Lin, Zhang, Chen
Fig. 9. X-ray structure (top) and intermolecular connections (bottom) of the Ag(mono-
ꢀ
ꢀ
NHC)Cl complex 33; f[NCCF]¼ꢁ61.0ꢀ; dAgeCl¼2.07 and 2.76 A. Thermal ellipsoids set at
ꢀ
the 50% probability level.31
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S. Paul et al. / Tetrahedron xxx (2013) 1e13
7
and Lee who have also observed this staircase type arrangement of
silver imidazolylidene and triazolylidene halides.32e35 The distance
were obtained from the chloro(1,5-cyclooctadiene)erhodium(I) or
iridium(I) dimer complexes that were initially suspended in etha-
nol and treated with a solution of sodium ethoxide in ethanol
(Scheme 9). Addition of the NHC precursor salt 19 furnished the
desired Rh(COD)(NHC)Cl complex 34 and Ir(COD)(NHC)Cl complex
35 in 47% and 36% yield, respectively (over two steps).
ꢀ
between the fluorine atom and the silver atom in 33 is 4.29 A, which is
ꢀ
considerablylongerthanthethresholdofabout3A thatisindicativeof
a remote metalefluorine interaction.5 The silver carbene bond length
ꢀ
is marginally longer than in the fluorodiphenyl complex 27 (2.09 A),
which might be a consequence of the differences in the coordination
environments of the silver atom. The distance between two adjacent
silver atoms that are connected via two bridging chlorine atoms is
ꢀ
about3.62A,whichislongerthanthesumoftheVanderWaalsradiiof
3.44 A,16 indicating that there are no argentophilic interactions in this
ꢀ
structure. A distinct feature is the almost planar alignment of the tri-
azoleringandtheN-phenylsubstituent, whicharerotatedbyonly4.3ꢀ
to each other. The NHC ligands of one zigzag chain are stacked with
ꢀ
long ring to ring distances of 4.48 A and the triazole planes of both
zigzag chains are tilted by 80.7ꢀ relative to each other. This rotation
might be a result of a weak CeF/HeC contact between the fluorine
atom and an ortho hydrogen atom of the N-phenyl groupof a molecule
Scheme 9. Synthesis of the Rh and Ir complex 34 and 35.
Pleasingly, single crystals of the rhodium complex 34 were ob-
24
ꢀ
tained that were suitable for X-ray crystal analysis (Fig. 11).38 The
of the neighbouring zigzag chain with a distance of 2.33 A.
Single crystal X-ray analysis of the Ag(I) bis-NHC complex 31
(Fig. 10) revealed two symmetry independent molecules Ag1 and
Ag2 in the asymmetric unit inwhich the silver atoms are in an almost
linear environment with CeAgeC bond angles of 171.5ꢀ and 175.2ꢀ
(Fig.10, upper).36 The CeAg bond lengths are similar with 2.09 A and
ꢀ
ꢀ
ꢀ
ꢀ
2.08 A for molecule Ag1 and 2.11 A and 2.12 A in molecule Ag2. In
molecule Ag1, both fluorine atoms adopt a syn clinal-exo confor-
mation with respect to the triazolium nitrogens with f[NCCF]¼57.4ꢀ
and 63.0ꢀ. In molecule Ag2 an NCCF torsion angle of f[NCCF]¼68.5ꢀ
(syn clinal-exo) was measured for one NHC ligand, whereas in the
other ligand a syn clinal-endo orientation of the fluorine atom to the
triazolium nitrogen was found (f[NCCF]¼ꢁ65.5ꢀ). Curiously, one
fluorine atom orients towards the silver atom with a short FeAg
ꢀ
ꢀ
contact of 2.93 A, whilst the other points away (d¼4.91 A).
Both fluorine atoms in Ag2 form short CeF/HeC contacts with
the para hydrogen atom of the N-phenyl group of the adjacent Ag2
ꢀ
ꢀ
molecule (d¼2.40 A and 2.54 A). Similar intermolecular contacts are
present in molecule Ag1 in which a short CeF/HeC contact is
formed between the fluorine atom and the meta hydrogen atom of
the adjacent Ag1 molecule with a distance of 2.13 A. The shortest
FeAg distance was measured to be 3.15 A, which creates a T-shaped
ꢀ
ꢀ
coordination at the silver atom. However, 13C NMR analysis revealed
similar 1JCF coupling constants of 175.4 Hz and 173.9 Hz for the silver
mono- and bis-NHC complexes 33 and 31. These values are close to
the 1JCF coupling constant of the precursor salt 19 (173.2 Hz). In this
case, no remote metalefluorine interactions could be identified. The
tetrafluoroborate anions form several F/H contacts in the range of
ꢀ
2.30e2.64 A with the hydrogen atoms of the phenyl groups and of
the NHC backbones. Thus, the tetrafluoroborate is bridging the
strands of Ag1 and Ag2 molecules (Fig. 10, bottom).
Unfortunately, it was not possible to grow single crystals of the
corresponding gold complex 32 that were suitable for X-ray anal-
ysis. 13C NMR analysis revealed a 1JCF coupling constant of 177.2 Hz,
which is similar to those of the silver complexes 31 and 33, and the
precursor salt 19, thus indicating that no interaction between the
fluorine atom and the gold centre occurs.
5. Rh and Ir N-heterocyclic carbene complexes (type 8)
In order to investigate possible remote metalefluorine in-
teractions involving aliphatic fluorine centres, rhodium and iridium
complexes were synthesised. Recently, Togni et al. disclosed an
elegant study describing the synthesis and structural analysis of
various iridium complexes displaying the aforementioned in-
teraction.37 Consequently, Rh and Ir complexes 34 and 35 were
synthesised to complement the previously described structural
study of fluorodiphenyl NHC structures 31 and 33. Both complexes
Fig. 11. X-ray structure of the Rh complex 34; f[NCCF]¼ꢁ59.9ꢀ; dRheF¼4.67 A (top) and
ꢀ
visualisation of the square-planar coordination geometry (bottom). Thermal ellipsoids
set at the 50% probability level.38
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8
S. Paul et al. / Tetrahedron xxx (2013) 1e13
compound crystallised in the orthorhombic space group P212121
with four molecules in the unit cell. The fluorine atom adopts
a clear syn clinal-endo orientation with respect to the triazolium
nitrogen with f[NCCF]¼ꢁ59.9ꢀ. Importantly, no interaction between
Ag2O furnished the silver bis-imidazolyl complex 36 in 82% yield
(vide infra).
It was possible to grow single crystals of the silver triazolyl
complex 37 that were suitable for X-ray crystal analysis
(Fig. 12).41 Measurement of the NCCF torsion angles revealed the
expected gauche arrangement with f[NCCF]¼ꢂ61.8ꢀ and ꢂ67.9ꢀ.
The compound crystallised in the orthorhombic space group
Pbca with eight molecules in the unit cell. A marginal distortion
of the linear environment of the silver centre was found with
a CeAgeC bond angle of 170ꢀ. This is possibly the consequence
of a weak coordination between the silver atom and an oxygen
ꢀ
the fluorine and rhodium centres was evident (dRheF¼4.67 A). The
short contacts between the fluorine atom and an olefinic proton of
ꢀ
the cyclooctadiene ligand of an adjacent molecule (d¼2.56 A), and
the weak RheCl/HeC(phenyl) interactions might be due to crystal
packing forces. A slightly distorted square-planar coordination ge-
ometry around the rhodium atom was found with bond angles of
carbeneerhodiumecentroid(c2)¼176.3ꢀ and chlorideerhodiume
centroid(c1)¼177.7ꢀ (see Fig. 11, bottom). Measurement of the
distances between the rhodium atom and the centroids c1 and c2
ꢀ
atom of the triflate anion (d¼2.93 A) resulting in an overall
T-shape geometry at the metal centre (Fig. 12, bottom). Both
ꢀ
of the cyclooctadiene double bonds revealed a 0.1 A longer dis-
fluorine atoms orient towards the metal with FeAg distances of
ꢀ
ꢀ
tance to the double bond located trans to the NHC ligand with
3.64 A and 3.69 A.
ꢀ
dRhec2¼2.096 A. As was observed for the corresponding Rh and Ir
fluorodiphenyl complexes 29 and 30, two discrete species were
found by 19F NMR spectroscopy occurring in a ratio of approxi-
mately 1:0.15. The chemical shifts of the NHC ligand are very
similar for both complexes. For example, the protons at carbon
atom 6 appear at
species and at
¼6.19 ppm and 4.83 ppm for the iridium com-
pound. The 13C chemical shift was found to be almost identical
¼82.7 ppm and 82.6 ppm). The largest shifts were observed for
d
¼6.53 ppm and 4.87 ppm for the rhodium
d
(
d
the carbon and hydrogen atoms of the cyclooctadiene ligand.
These are shifted upfield by 12e17 ppm in the 13C NMR spectrum
and about 0.3 ppm in the 1H NMR from the rhodium to the
iridium complex. These slight shift differences are consistent
with literature reports of isostructural Rh and Ir(NHC) complexes
reported by Messerle and Hazari who compared the molecular
structures of such compounds by X-ray crystallographic
analysis.39,40
6. Ag and Au N-heterocyclic carbene complexes (type 9 and 10)
In order to investigate the possibility of molecular pre-organisa-
tion by a fluorine gauche effect, as well as by a remote metalefluorine
interaction, structurally simplified metal triazolium and imidazo-
lium complexes were synthesised bearing a pendant fluoroethyl
moiety to allow greater degrees of rotational freedom (Scheme 10).
Naturally, two gauche conformations can be populated with
equal probability in these structures, either with the fluorine ori-
ented towards the metal centre, thus allowing a possible remote
metalefluorine interaction, or the fluorine atom is oriented away
from the metal centre.
Fig. 12. X-ray structure of the Ag triazolyl complex 37; f[NCCF]¼ꢂ61.8ꢀ and ꢂ67.9ꢀ;
ꢀ
ꢀ
dAgeO¼2.93 A; dAgeF¼3.69 and 3.64 A (top) and visualisation of the T-shape structure
(bottom). Thermal ellipsoids set at the 50% probability level.41
Scheme 10. Synthesis of the Ag complexes 36 and 37.
7. Cationic Pd type 9 and 10 NHC complexes
With the silver complexes 36 and 37 in hand, attention was
focussed on preparing a representative palladium(II) complex of
this monocyclic type for comparison. Upon treatment of the silver
NHC complexes 36 and 37 with dimeric palladium allyl chloride
To that end, the silver triazolyl complex 37 was obtained in 58%
yield upon treatment of the triazolium salt 25 with Ag2O in pres-
ence of NMe4Cl. Analogously, reaction of imidazolium salt 23 with
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S. Paul et al. / Tetrahedron xxx (2013) 1e13
9
[Pd(allyl)Cl]2, the structurally novel palladium bis-NHC complexes
38 and 39 were obtained in moderate yields (58% and 78%, re-
spectively, Scheme 11).
an NCCF torsion angle f[NCCF]¼ꢂ67.7ꢀ (dPdeF¼4.13 A). In contrast,
the second gauche conformation orients the fluorine away from the
metal with an NCCF torsion angle of f[NCCF]¼ꢂ72.9ꢀ and a PdeF
ꢀ
ꢀ
distance of d¼5.65 A.
In this synthesis and structural survey of a small collection of
bicyclic fluorinated NHC complexes, we have observed that in the
solid state, a syn clinal-endo arrangement dominates the confor-
mational behaviour of the bicyclic species 27, 28, 33 and 34, irre-
spective of the steric demand of the fluorine-bearing carbon
(Fig. 14). However, in the case of the bis-NHC palladium complex
26, both a syn clinal-exo arrangement predominates with the
fluorine atoms pointing towards the metal centre. Intriguingly,
both gauche arrangements can be observed in one of the two solid
state species of complex 31(I). Notably, in the monocyclic NHCs
evaluated in this study, both gauche arrangements (gauche1 and
gauche2) are present in the palladium complexes 38 and 39, yet the
silver species 37 displays the gauche2 arrangement (Fig. 14, right).
Scheme 11. Synthesis of the Pd complexes 38 and 39.
Single crystal X-ray analysis of the palladium complexes 3842
and 3943 revealed the expected
h
3-coordination of the allyl frag-
ment and the distorted square-planar coordination sphere of the
palladium centres (Fig. 13). The two NHC ligands attached to the
palladium atom are almost perpendicular to each other with
CePdeC bond angles of 97.5ꢀ in complex 39 and 96.0ꢀ in the Pd
imidazoyl complex 38.
8. Computational conformational studies
Although, strictly speaking, we have been unable to detect
remote-fluorine interactions in the complexes studied, a number of
intriguing candidates have been identified which clearly adopt
conformations in which the fluorine atoms point towards the metal
centre (26, 31, 37e39). Common to all of these studies is the pos-
tulated gauche preference. Therefore to quantify this phenomenon,
a theoretical study at the DFT level was instigated. We selected
the imidazolium cation 23, the neutral NHC derived by
deprotonation and the cationic bis(NHC) allyl palladium complex
39 (where NHC is the triazolyl carbene derived from 25).
For each system, we calculated the NCCF rotational profile and
optimised the three rotamers; results are summarised in Table 1
and Fig. 15. The optimised structures of 23 and 39 agree very well
with the experiment (vide supra Figs. 4 and 13). In 23, the largest
deviation is for fNCCF (expt 71ꢀ, calcd 62ꢀ), which is caused by the
F/F interactions in the solid state (see Fig. 4). In the palladium
complex 39, the NCCF torsions are in good agreement (expt 65ꢀ and
66ꢀ, calcd both 63ꢀ).
However, the torsion angles between the phenyl and triazolyl
rings deviate somewhat (expt ꢁ39ꢀ and ꢁ53ꢀ, calcd ꢁ44ꢀ and
ꢁ42ꢀ), which is again due to intermolecular interactions in the solid
state.
In all three systems, the relative energies of the two gauche
rotamers are within 2 kJ molꢁ1, i.e., practically degenerate. Because
the 2-fluoroethyl group is oriented almost perpendicularly to the
NHC ring system (the CcarbNCC torsion angle is between 90ꢀ and
105ꢀ), there are no differential interactions between the fluorine
and the nitrogen lone pair. This is unlike the situation in (2-
fluoroethyl)imines,44 where one gauche rotamer is destabilised by
the repulsion between the fluorine and imine lone pairs, to the
extent that it becomes even less stable than the anti rotamer.
For the imidazolium cation 23, the preferred þgauche rotamer is
more stable than the anti by 11 kJ molꢁ1. The magnitude of this
stabilisation is characteristic of an electrostatic gauche effect, which
orients the partially negatively charged fluorine preferentially to-
wards the cationic moiety, similarly to the situation seen in N-(2-
fluoroethyl)iminium,7a -pyridinium,45 or -ammonium6,46,47 sys-
tems. The Hirshfeld partial charges for 23 (which are practically
independent of the torsion angle) are ꢁ0.13e (F), 0.00 (both N) and
þ0.24e (CcarbH). Considering only the chargeecharge interactions
between fluorine and the atoms of the [N]CHeN]þ unit, the
þgauche rotamer is stabilised by 5 kJ molꢁ1 relative to the anti
rotamer. In the neutral NHC, however, there is no such electrostatic
stabilisation. While the charge on fluorine does not change much
(ꢁ0.15e), the formally neutral [NeC:eN] unit now has a charge of
ꢁ0.27e (of which ꢁ0.18e on Ccarb). The preferred ꢁgauche rotamer
Fig. 13. X-ray structures of 38 and 39 (upper, lower).42,43 Thermal ellipsoids shown at
the 50% probability level. Hydrogen atoms and the counter ions have been omitted for
clarity.
The triazolyl complex 39 crystallised in the monoclinic space
group P21/n. The fluorine atoms adopt a clear gauche conformation
with respect to the triazolium ring nitrogen with NCCF torsion
angles of f[NCCF]¼ꢂ65.3ꢀ and ꢂ66.1ꢀ. Although one of the pendant
fluorine substituents adopts a gauche conformation that might al-
low for a remote metalefluorine interaction, the PdeF distance was
ꢀ
measured to be 4.05 A. Furthermore, the second fluorine atom is
ꢀ
oriented away from the Pd centre (dPdeF¼5.43 A). A similar obser-
vation was found for the imidazoyl complex 38, which crystallised
in the triclinic space group P1 with two molecules in the unit cell.
One fluorine atom adopts a gauche conformation with respect to
the imidazolium nitrogen orienting towards the metal centre with
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10
S. Paul et al. / Tetrahedron xxx (2013) 1e13
Fig. 14. An overview of X-ray structural studies.
is therefore electrostatically destabilised relative to the anti by
3 kJ molꢁ1. Consequently, the net preference for the ꢁgauche
rotamer reduces to 6 kJ molꢁ1. This value is typical of the stabili-
Table 1
Calculateda energies and torsion angles of rotamers of the imidazolium ion 23, the
neutral NHC derived from 23 and the Pd(NHC) complex 39. Energies are relative to
the lowest-energy rotamer for each compound
sation imparted by antiperiplanar hyperconjugative sCeH
and sCeH interactions, in preference to sCeN
sCeF/s*CeN. Similar stereoelectronic gauche effects in NCCF sys-
/s*
CeF
Compound
NHCeHþ (23)
Rotamer
D
E [kJ molꢁ1
]
f(NCCF) [ꢀ]
/s
*
CeN
/s
*
and
CeF
ꢁgauche
þgauche
anti
ꢁgauche
þgauche
anti
ꢁgauche
þgauche
anti
1
0
11
0
2
6
0
0
7
ꢁ63
62
ꢁ178
ꢁ65
64
ꢁ179
ꢁ63
63
ꢁ173
tems have previously been described for imines,44 amines6,45e47
and amides.6,48 Free NHCs and their precursor cations therefore
follow the pattern found in other NCCF systems as far as the gauche
preference is concerned.
NHC
[Pd(C3H5)(NHC)2]þ (39)
In the complex 39, the þgauche rotamer is preferred to the anti
by 7 kJ molꢁ1, only slightly more than in the free NHC. The analysis
of Hirshfeld partial charges reveals that the main difference be-
tween the free and the Pd-bound NHC is the decreased amount of
charge on Ccarb (þ0.01e) and also the adjacent nitrogens. This is
consistent with the NHC donating electron density towards the
metal centre. Overall, the [NeC:eN] unit is almost neutral (ꢁ0.01e).
The chargeecharge electrostatic interaction between fluorine and
[NeC:eN] is therefore small in magnitude and differs in-
significantly (<1 kJ molꢁ1) between the rotamers. The absence of
the destabilising electrostatic component in the complexed NHC
compared to the free NHC thus accounts for the slightly stronger
gauche preference in the latter. However, the electron-withdrawing
effect of the metal on the [NeC:eN] unit is not nearly as strong as
the effect of protonation in the cation. A Natural Bond Orbital
analysis confirmed that the torsion-dependent hyperconjugative
interactions in the NCCF unit are practically the same for free and
complexed NHC. The metal-bound NHC thus essentially behaves as
a free NHC with respect to gauche preference.
a
Results obtained with M06-L/def2-TZVP in continuum CH2Cl2.
9. Experimental section
9.1. General
All reactions using air or moisture sensitive compounds were
carried out in flame-dried and evacuated glassware under an at-
mosphere of argon. Solvents were dried according to standard
Fig. 15. Rotational profiles, calculated at M06-L/def2-TZVP/PCM(CH2Cl2) level, for 23,
its neutral NHC, and the bis(NHC) Pd complex 39. In 39, the NCCF torsion was scanned
only in one NHC, while the other was freely optimised.
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S. Paul et al. / Tetrahedron xxx (2013) 1e13
11
procedures or were taken from the solvent drying system. All
chemicals were reagent grade and used as supplied. Solvents
for extractions and chromatography were technical grade and dis-
tilled prior to use. Reactions were monitored by thin layer chroma-
tography, using pre-coated Merck silica gel 60 F254 glass plates
(0.25 mm). Visualisation was accomplished using ultraviolet light
J¼28.2, 8.2 Hz, 1H, H5*), 4.90e4.80 (m, 1H, CHeallyl), 4.42e4.32 (m,
1H, CHeallyl*), 4.06 (dd, J¼7.4,1.5 Hz,1H, CH2eallyl), 3.98 (dd, J¼7.4,
1.7 Hz,1H, CH2eallyl*), 3.23e3.14 (m,1H, H3), 3.08e2.95 (m,1H, H4;
m, 2ꢄH4*, 3H, CH2eallyl*), 2.91e2.82 (m, 1H, H3a; m, 1H, H3*),
2.66e2.57 (m, 2H, H4, CH2eallyl; m, 2H, H3*, CH2eallyl*), 2.16 (d,
J¼6.5 Hz, 1H, CH2eallyl), 1.34e1.26 (m, 1H, CH2eallyl*); 1.01 (d,
J¼12.0 Hz, 1H, CH2eallyl); not all quaternary carbons of the minor
(l¼254 nm) or by dipping in cerium ammonium molybdate (CAM)
stain [(NH4)6Mo7O24$4H2O (50 g), Ce(SO4)2 (10 g) and H2SO4
(100 ml) in water (900 ml)] or in ninhydrin stain (prepared by dis-
solving 0.3 g of ninhydrin in 100 ml of n-BuOH containing 1 ml of
acetic acid), followed by heating. Flash column chromatography was
carried out using Fluka silica gel 60 (230e400 mesh). Concentration
in vacuo was performed at w8 mbar and 30e40 ꢀC, drying at the
high vacuum at w10ꢁ2 mbar and rt. 1H NMR, 19F NMR and 13C NMR
spectra were recorded on Varian Mercury-vx 300 MHz, Bruker
AVANCE 300 MHz, Bruker DRX 400 MHz or Bruker AV 400 MHz
rotamer could be assigned: 13C NMR (100 MHz, CDCl3)
d 179.9,163.0,
162.7*, 141.9, 140.9*, 140.7, 140.4*, 129.1, 129.0, 128.8, 128.78, 128.7,
128.5, 128.4, 128.3, 127.9, 127.5, 126.6, 126.55, 126.4, 126.0, 125.5,
125.4, 125.1, 125.0, 124.9, 116.0*, 115.1, 101.8, 71.4, 70.9*, 62.6, 52.5,
50.8*, 34.2*, 32.2, 22.5*, 21.6; 19F NMR (376 MHz, CDCl3)
ꢁ167.8; IR (neat) 3060,1671,1598,1543,1493,1449,1391,1343,1247,
1203, 762, 752, 694; mass spectrum HRMS (ESIþ) m/z: (MþꢁCl),
found: 516.1082. C27H25PdFN3 requires 516.1072.
d
ꢁ161.6*,
spectrometers. Chemical shifts
relative to the residual solvent. Melting points were measured on
d
are reported in parts per million
9.1.3. 1,5-Cyclooctadienyl-(S)-(5-(fluorodiphenylmethyl)-2-phenyl-
6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]triazol-3(5H)-ylidene)rhodium(I)
chloride (29). A freshly prepared solution of NaH (9.6 mg,
0.24 mmol, 60% dispersion in mineral oil) in EtOH (1.2 ml) was
added slowly to a suspension of [Rh(COD)Cl]2 (29.6 mg, 0.06 mmol)
in EtOH (0.8 ml). After stirring at rt for 45 min, 15 (55 mg,
0.12 mmol) was then added and the mixture was stirred for a fur-
ther 24 h at rt. The solvent was evaporated and the crude product
was purified by column chromatography (hexane/ethyl acetate 1/
0/3/5) to yield the title compound as a pale brown solid (30 mg,
41%). Mp: 133 ꢀC (decomposition); Rf 0.15 (hexane/ethyl acetate 3/
€
a Buchi B540 melting point apparatus and are uncorrected. IR
spectra were measured on a PerkineElmer Spectrum 100 FTIR
spectrometer and reported in cmꢁ1. Optical rotations were obtained
using a JASCO DIP-370 polarimeter. High-resolution mass spectra
were performed by the MS service at the Laboratory for Organic
€
Chemistry (ETH Zurich). Elemental analysis was performed by the
micro-laboratory at the Laboratory for Organic Chemistry (ETH
€
Zurich).
9.1.1. Bis[(S)-5-(fluorodiphenylmethyl)-2-phenyl-6,7-dihydro-2H-pyr-
rolo[2,1-c][1,2,4]triazol-3(5H) ylidene]palladium(II) chloride (26). A
mixture of (S)-5-(fluorodiphenylmethyl)-2-phenyl-6,7-dihydro-5H-
pyrrolo[2,1-c][1,2,4]triazol-2-ium tetrafluoroborate 15 (93 mg,
0.2 mmol), Ag2O (25 mg, 0.12 mmol) and NMe4Cl (22 mg, 0.2 mmol)
in anhydrous CH2Cl2 (6.8 ml) was stirred for 24 h at rt. The mixture
was filtered through an Acrodisc (Chromafil PET 20/25), which was
washed with CH2Cl2 (1 ml). The filtrate was concentrated and the
residue re-dissolved in CH2Cl2 (5 ml). PdCl2 (18 mg, 0.1 mmol) was
added and the mixture stirred for an additional 24 h. The solvent was
removed in vacuo and the crude product was purified by column
chromatography (cyclohexane/ethyl acetate 1/0/1/1) to the title
compound as a pale brown solid (30 mg, 33%); mp: 150 ꢀC (de-
1);½a 2D0
ꢃ
ꢁ155.0 (c 0.10, CH2Cl2); 1H NMR (400 MHz, CDCl3)
d 8.45 (d,
J¼7.5 Hz, 2H, Ph), 7.59e7.49 (m, 8H, Ph), 7.46e7.29 (m, 3H, Ph), 7.01
(d, J¼7.5 Hz, 2H, Ph), 6.28 (dd, J¼8.3, 5.6 Hz, 1H, H5), 5.10 [s, br, 1H,
CH(COD)], 4.86 [s, br, 1H, CH(COD)], 3.76 [s, br, 1H, CH(COD)],
3.04e2.99 (m, 1H, H4), 2.92 [s, br, 1H, CH(COD)], 2.51e2.38 (m, 2H,
H3, H4), 2.28e2.20 [m, 3H, CH2(COD)], 1.81e1.69 [m, 3H,
CH2(COD)], 1.63e1.54 [m, 1H, CH2(COD)], 1.47e1.38 (m, 1H, H3); 13
C
NMR (100 MHz, CDCl3) d 185.6, 162.1, 141.1, 139.9, 139.0,129.5,129.1,
128.8, 128.7, 128.5, 128.3, 128.2, 128.1, 127.5, 125.6, 98.6, 97.3, 96.8,
71.2, 70.6, 62.5, 32.6, 32.0, 31.7, 29.2, 28.3, 20.2; 19F NMR
(376.6 MHz, CDCl3)
d
ꢁ120.2* (minor), ꢁ127.7; IR (neat) 2924, 2872,
1598, 1497, 1447, 1393, 1330, 1251, 1076, 1032, 966, 910, 861, 808,
761, 723, 692, 635, 618; mass spectrum HRMS (MALDIþ) m/z:
(MþꢁCl), found 580.1634. C32H32FN3Rh requires 580.1630.
composition); Rf 0.28 (cyclohexane/ethyl acetate 1/1); ½a D20
ꢁ197.7 (c
ꢃ
0.09, CH2Cl2); 1H NMR (400 MHz, CD2Cl2)
d 8.22e8.21 (m, 4H, Ph),
7.54e7.02 (m, 26H, Ph), 6.10e6.07 (m, 2H, H5), 3.10e2.99 (m, 2H,
H4), 2.63e2.50 (m, 2H, H4), 2.46e2.39 (m, 2H, H3),1.52e1.40 (m, 2H,
9.1.4. 1,5-Cyclooctadienyl-(S)-(5-(fluorodiphenylmethyl)-2-phenyl-
6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]triazol-3(5H)-ylidene)iridium(I)
chloride (30). A freshly prepared solution of NaH (4.8 mg,
0.12 mmol, 60% dispersion in mineral oil) in EtOH (0.6 ml) was
added slowly to a suspension of [Ir(COD)Cl]2 (20.0 mg, 0.03 mmol)
in EtOH (0.4 ml). After stirring at rt for 45 min, 15 (48 mg,
0.105 mmol) was added and the mixture was stirred for further 24 h
at rt. The solvent was removed and the crude product was purified
by column chromatography (cyclohexane/ethyl acetate 3/1/1/1)
to yield the product as a pale brown solid (26 mg, 61%). Mp: 115 ꢀC
H3); 13C NMR (150 MHz, CD2Cl2)
d
170.7, 161.5, 140.8, 139.9 & 138.8,
130.5e128.5, 125.5, 98.8, 63.3, 31.3, 20.5; 19F NMR (376 MHz,
CD2Cl2)
d
ꢁ114.7, ꢁ119.2; IR (neat) 3060, 2925, 1733, 1683, 1602,
1499, 1449, 1407, 1353, 1252, 1037, 973, 910, 860, 761, 724, 692, 635;
mass spectrum HRMS (MALDIþ) found 879.2016 (MþꢁCl).
C
48H40ClF2N6Pd requires 879.2014.
9.1.2. (S)-(5-(Fluorodiphenylmethyl)-2-phenyl-6,7-dihydro-5Hpyrrolo
[2,1-c][1,2,4]triazol-3(5H)ylidene)(allyl)palladium(II) chloride (28). A
flask was charged with (S)-(5-(fluorodiphenylmethyl)-2-phenyl-
6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]-triazol-3(5H)ylidene) silver(I)
chloride 27 (76.9 mg, 0.15 mmol) and [Pd(allyl)Cl]2 (27.5 mg,
0.075 mmol) under an atmosphere of argon. CH2Cl2 (5.5 ml) was
added and the reaction was stirred at rt for 20 h. The mixture was
then filtered through a pad of Celite. Evaporation of the solvent and
recrystallisation of the resulting solid from CH2Cl2/pentane yielded
the product as a pale brown solid (43 mg, 52%). Mp: decomposition
(decomposition); Rf 0.29 (cyclohexane/ethyl acetate 2/1); ½a D20
ꢃ
ꢁ112.5 (c 0.12, CH2Cl2); 1H NMR (400 MHz, CDCl3)
d 8.31e8.29 (m,
2H, Ph), 7.50e7.28 (m, 11H, Ph), 7.04e7.02 (m, 2H, Ph), 6.19 (dd,
J¼7.9, 5.5 Hz, 1H, H5), 4.82 [s, br, 1H, CH(COD)], 4.43 [s, br, 1H,
CH(COD)], 3.37 [s, br, 1H, CH(COD)], 3.05e3.00 (m, 1H, H4),
2.53e2.42 (m, 3H, H3, H4), 2.11e1.95 [m, 3H, CH2(COD)], 1.62e1.50
[m, 3H, CH2(COD)], 1.46e1.38 [m, 1H, CH2(COD)], 1.30e1.24 [m, 1H,
CH2(COD)]; 13C NMR (100 MHz, CDCl3)
d
182.3, 161.8, 141.0, 139.8,
139.1,129.4,129.1,128.8,128.5,128.4,128.1,128.0,127.6,127.5,125.7,
98.3, 84.2, 84.16, 62.3, 54.5, 54.4, 33.4, 32.7, 31.8, 29.5, 28.9, 20.4; 19
NMR (376.6 MHz, CDCl3)
>136 ꢀC; ½a 2D0
ꢃ
ꢁ96.3 (c 0.11, CHCl3); two rotamers were identified
F
with a ratio of 1/0.2 (* denotes the minor rotamer): 1H NMR
d
ꢁ121.4* (minor), ꢁ128.2; IR (neat) 2929,
(400 MHz, CDCl3)
d
7.84e7.82 (m, 2H, Ph*), 7.77e7.75 (m, 2H, Ph),
2879, 1596, 1493, 1447, 1329, 1251, 1075, 1032, 971, 907, 808, 760,
724, 693, 635; mass spectrum HRMS (MALDIþ) m/z: (MþꢁCl), found
670.2202. C32H32FIrN3 requires 670.2205.
7.67e7.57 (m, 2H, Ph; m, 2H, Ph*), 7.49e7.17 (m, 9H, Ph; m,10H, Ph*),
7.10e7.07 (m, 1H, Pha), 6.77 (dd, J¼30.7, 8.5 Hz, 1H, H5a), 6.47 (dd,
Please cite this article in press as: Paul, S.; et al., Tetrahedron (2013), http://dx.doi.org/10.1016/j.tet.2013.02.071

12
S. Paul et al. / Tetrahedron xxx (2013) 1e13
9.1.5. 1,5-Cyclooctadienyl-(S)-(5-(fluoromethyl)-2-phenyl-6,7-
dihydro-2H-pyrrolo[2,1-c][1,2,4]-triazol-3(5H)-ylidene)rhodium(I)
chloride (34). A freshly prepared solution of NaH (9.6 mg,
0.24 mmol, 60% dispersion in mineral oil) in EtOH (1.2 ml) was
added slowly to a suspension of [Rh(COD)Cl]2 (29.6 mg, 0.06 mmol)
in EtOH (0.8 ml). After stirring at rt for 45 min, 19 (36.6 mg,
0.12 mmol) was added and the mixture was stirred for a further
24 h at rt. The solvent was removed in vacuo and the crude product
was purified by column chromatography (cyclohexane/ethyl ace-
tate 1/0/1/2) to yield the product as a pale brown solid (26 mg,
47%) [Found: C, 51.91; H, 5.30; N, 8.92; F, 3.96; Cl, 7.81.
d
177.2, 140.2, 129.9, 129.3, 125.0, 123.7, 122.5, 119.1, 82.3, 61.3, 51.1;
19F NMR (376.5 MHz, CDCl3)
d
ꢁ78.2 (s, 3F, OTf), ꢁ222.2 (tt, J¼47.3,
27.9 Hz, 2F, CH2F); IR (neat) 3097, 2920, 2851, 1598, 1497, 1415,1363,
1261, 1221, 1141, 1101, 1030, 1013, 909, 863, 768, 741, 694, 635; mass
spectrum HRMS (ESIþ) m/z: (Mþ) found, 527.1241. C25H27F2N4Pd
requires 527.1243.
9.1.8. Allyl-bis(4-(2-fluoroethyl)-1-phenyl-1H-1,2,4-triazol-5(4H)-
ylidene)palladium(II) triflate (39). Bis(4-(2-fluoroethyl)-1-phenyl-
1H-1,2,4-triazol-5(4H)ylidene) silver (I) triflate 37 (23 mg,
0.036 mmol) and [Pd(allyl)Cl]2 (12.6 mg, 0.035 mmol) were dis-
solved in CH2Cl2 (0.7 ml) and the mixture was stirred at rt for 24 h.
The resulting suspension was filtered through a short pad of Celite
and the filtrate concentrated to give a crude product, which was
then purified by column chromatography (CH2Cl2/MeOH 50/
0/20/5) to furnish the product as a white solid (19.1 mg, 78%). Mp:
115 ꢀC (decomposition); Rf 0.31 (CH2Cl2/MeOH 9/1); 1H NMR
C
20H24N3FClRh requires C, 51.80; H, 5.22; N, 9.06; F, 4.10; Cl, 7.64].
Mp: 198 ꢀC (decomposition); Rf 0.49 (cyclohexane/ethyl acetate 1/
1); ½a 2D0
ꢃ
ꢁ84.0 (c 0.1, CH2Cl2); 1H NMR (400 MHz, CDCl3)
d 8.43 (d,
J¼7.6 Hz, 2H, H9, H13), 7.56e7.52 (m, 2H, H10, H12), 7.47e7.43 (m,
1H, H11), 6.53 (dd, J¼48.4, 10.1 Hz, 1H, H6), 5.21e5.17 [m, 1H,
CH(COD)], 5.10e4.97 [m, 2.5H, H6, H5, CH(COD)], 4.87 (d, J¼10.2 Hz,
0.5H, H6), 3.35 [s, br, 1H, CH(COD)], 3.07e2.94 (m, 2H, H3),
2.87e2.73 [m, 3H, H4, CH(COD)], 2.37e2.26 [m, 3H, CH2(COD)],
1.94e1.77 [m, 4H, CH2(COD)], 1.67e1.59 [m, 1H, CH2(COD)]; 13C
(400 MHz, CD2Cl2)
d
8.28 (d, J¼1.1 Hz, 2H, H3), 7.52e7.48 (m, 6H,
H5, H6, H7), 7.39e7.36 (m, 2H, H5, H6), 5.56 (tt, J¼13.2, 7.3 Hz, 1H,
H10), 4.63e4.54 (m, 2H, H1), 4.52e4.42 (m, 2H, H1), 4.18 (d,
J¼7.4 Hz, 2H, H9 or H11), 3.94e3.91 (m, 2H, H2), 3.87e3.84 (m, 2H,
H2), 2.97 (d, J¼13.2 Hz, 2H, H9 or H11); 13C NMR (100 MHz, CD2Cl2)
NMR (100 MHz, CDCl3)
99.15, 99.06, 82.7, 71.1, 69.6, 58.0, 33.1, 32.0, 29.2, 28.7, 28.3, 21.2; 19
(376.6 MHz, CDCl3)
d 181.4, 160.8, 140.6, 128.8, 128.5, 124.0,
F
d
ꢁ236.8; IR (neat) 2939, 2229,1967,1596,1493,
d 180.9, 144.4, 139.9,130.4,130.3,124.7, 120.9, 82.3,121.5, 63.9, 49.7;
1449, 1331, 1069, 973, 910, 865, 812, 760, 725, 693, 639; mass
19F NMR (376.5 MHz, CD2Cl2)
d
ꢁ78.9 (s, 3F, OTf), ꢁ222.4 (tt, J¼47.2,
spectrum HRMS (ESIþ) m/z: (MþꢁCl), found 428.0999.
28.0 Hz, 2F, CH2F); IR (neat) 3119, 2924, 1597, 1539, 1499, 1463,
1409, 1374, 1357, 1263, 1220, 1151, 1076, 1029, 961, 911, 858, 766,
718, 696, 634; mass spectrum HRMS (ESIþ) m/z: (Mþ) found,
529.1133. C23H25F2N6Pd requires 529.1147.
Full experimental details are provided in the Supplementary
data.
C
20H24FN3Rh requires 428.1004.
9.1.6. 1,5-Cyclooctadienyl-(S)-(5-(fluoromethyl)-2-phenyl-6,7-
dihydro-2H-pyrrolo[2,1-c][1,2,4]-triazol-3(5H)-ylidene)iridium(I)
chloride (35). A freshly prepared solution of NaH (2.4 mg,
0.06 mmol, 60% dispersion in mineral oil) in EtOH (0.3 ml) was
added slowly to a suspension of [Ir(COD)Cl]2 (10.1 mg, 0.015 mmol)
in EtOH (0.2 ml). After stirring at rt for 45 min, 19 (18.3 mg,
0.06 mmol) was added and the mixture was stirred for a further
24 h at rt. The solvent was evaporated and the crude product was
purified by column chromatography (cyclohexane/ethyl acetate 2/
1) to yield the product as a pale brown solid (6 mg, 36%). Mp: 175 ꢀC
Acknowledgements
We gratefully acknowledge generous financial support from the
€
Alfred Werner Foundation, the ETH Zurich, the University of Glas-
gow and the Westfalische Wilhelms-Universitat Munster.
€
€
€
(decomposition); Rf 0.74 (CH2Cl2/MeOH 20/1); ½a D20
ꢁ6.6 (c 0.06,
ꢃ
Supplementary data
CH2Cl2); 1H NMR (400 MHz, CDCl3)
d 8.30e8.29 (m, 2H, H9, H13),
7.50e7.43 (m, 3H, H10, H11, H12), 6.19 (dd, J¼47.8, 9.7 Hz, 1H, H6),
5.01e4.65 [m, 4H, H5, H6, 2ꢄ CH(COD)], 3.08e3.02 (m, 2H, H3),
2.89e2.76 [m, 3H, H4, CH(COD)], 2.45e2.40 [m, 1H, CH(COD)],
2.21e2.11 [m, 3H, CH2(COD)], 1.84e1.76 [m, 1H, CH2(COD)],
1.72e1.64 [m, 2H, CH2(COD)], 1.58e1.48 [m, 1H, CH2(COD)],
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.tet.2013.02.071.
References and notes
1.44e1.37 [m, 1H, CH2(COD)]; 13C NMR (100 MHz, CDCl3)
d 179.0,
1. (a) Zimmer, L. E.; Sparr, C.; Gilmour, R. Angew. Chem. 2011, 123, 12062e12074;
Angew. Chem., Int. Ed. 2011, 50, 11860e11871; (b) Hunter, L. Beilstein J. Org. Chem.
2010, 6, http://dx.doi.org/10.3762/bjoc.6.38
2. DiRocco, D. A.; Oberg, K. M.; Dalton, D. M.; Rovis, T. J. Am. Chem. Soc. 2009, 131,
10872e10874.
3. Ritter, T.; Day, M. W.; Grubbs, R. J. Am. Chem. Soc. 2006, 128, 11768e11769.
4. DiRocco, D. A.; Noey, E. L.; Houk, K. N.; Rovis, T. Angew. Chem. 2012, 124,
2441e2444; Angew. Chem., Int. Ed. 2012, 51, 2391e2394.
5. For an excellent review see Plenio, H. Chem. Rev. 1997, 97, 3363e3384.
6. For a recent theoretical treatment of the gauche effect in substituted fluoro-
ethanes see: Buissonneaud, D. Y.; Van Mourik, T.; O’Hagan, D. Tetrahedron 2010,
66, 2169e2202 and references cited therein.
160.5, 140.4, 128.7, 128.3, 124.1, 86.4, 86.35, 82.6, 57.8, 54.8, 53.3,
33.3, 33.2, 29.9, 29.2, 28.4, 21.3; 19F NMR (376.6 MHz, CDCl3)
d
ꢁ236.9; IR (neat) 2921, 1969, 1729, 1595, 1498, 1448, 1384, 1353,
1287, 1155, 1066, 1018, 981, 934, 817, 765, 692; mass spectrum
HRMS (MALDIþ) m/z: (MþꢁCl) found: 518.1578. C20H24FIrN3 re-
quires 518.1579.
9.1.7. Allyl-bis(1-(2-fluoroethyl)-3-phenyl-1H-imidazol-2(3H)-yli-
dene)palladium(II) triflate (38). Bis(1-(2-fluoroethyl)-3-phenyl-1H-
imidazol-2(3H)-ylidene) silver(I) triflate 36 (31 mg, 0.0486 mmol)
and [Pd(allyl)Cl]2 (17 mg, 0.0486 mmol) were dissolved in CH2Cl2
(1 ml) and the mixture was stirred at rt for 19 h. The resulting
suspension was filtered through a short pad of Celite and the filtrate
was concentrated to give a yellow oil, which was purified by col-
umn chromatography (CH2Cl2/MeOH 50/1) to yield the product as
a white solid (19.1 mg, 58%). Mp: 134.3e136.2 ꢀC; Rf 0.47 (CH2Cl2/
7. For examples from this laboratory see: (a) Sparr, C.; Schweizer, W. B.; Senn, H.
M.; Gilmour, R. Angew. Chem. 2009, 121, 3111e3114; Angew. Chem., Int. Ed. 2009,
48, 3065e3068; (b) Sparr, C.; Tanzer, E.-M.; Bachmann, J.; Gilmour, R. Synthesis
ꢁ
2010, 1394e1397; (c) Seebach, D.; Gilmour, R.; Groselj, U.; Deniau, G.; Sparr, C.;
ꢁ
ꢁ
Ebert, M.-O.; Beck, A. K.; McCusker, L. B.; Sisak, D.; Uchimaru, T. Helv. Chim. Acta
2010, 93, 603e634; (d) Bucher, C.; Mondelli, C.; Baiker, A.; Gilmour, R. J. Mol.
Catal. A: Chem. 2010, 327, 87e91; (e) Sparr, C.; Gilmour, R. Angew. Chem. 2010,
122, 6670e6673; Angew. Chem., Int. Ed. 2010, 49, 6520e6523; (f) Tanzer, E.-M.;
Schweizer, W. B.; Ebert, M.-O.; Gilmour, R. Chem.dEur. J. 2012, 18, 2006e2013;
(g) Tanzer, E.-M.; Zimmer, L. E.; Schweizer, W. B.; Gilmour, R. Chem.dEur. J.
2012, 18, 11334e11342.
MeOH 9/1); 1H NMR (400 MHz, CDCl3)
d 7.47e7.44 (m, 6H, H6, H7,
8. Irwin, J. J.; Ha, T.-K.; Dunitz, J. D. Helv. Chim. Acta 1990, 73, 1805e1817.
9. Paul, S.; Schweizer, W. B.; Ebert, M.-O.; Gilmour, R. Organometallics 2010, 29,
4424e4427; www.ccdc.cam.ac.uk/data_request/cif Selected crystallographic
data for the type 7 Au(NHC)Cl complex: Orthorhombic; Space group P212121;
H8), 7.25e7.24 (m, 2H, H3), 7.15e7.14 (m, 2H, H4), 7.09e7.07 (m, 4H,
H6, H7, H8), 5.28 (tt, J¼13.2, 7.3 Hz, 1H, H11), 4.52 (dm, J¼47.3 Hz,
4H, H1), 3.88 (d, J¼7.3 Hz, 2H, H10 or H12), 3.68 (dm, J¼27.7 Hz, 2H,
H2), 2.59 (d, J¼13.2 Hz, 2H, H10 or H12); 13C NMR (100 MHz, CDCl3)
a¼9.8352 A; b¼9.8352 A; c¼27.0406 A; d[AueAu]¼3.31 A; f[NCCF]¼ꢁ61.9ꢀ/ꢁ48.
ꢀ
ꢀ
ꢀ
ꢀ
Please cite this article in press as: Paul, S.; et al., Tetrahedron (2013), http://dx.doi.org/10.1016/j.tet.2013.02.071

S. Paul et al. / Tetrahedron xxx (2013) 1e13
13
6ꢀ; CCDC 782698 contains the supplementary crystallographic data. These data
can be obtained free of charge from The Cambridge Crystallographic Data
Centre via.
30. Herrmann, W. A.; Schneider, S. K.; Ofele, K.; Sakamoto, M.; Herdtweck, E. J.
Organomet. Chem. 2004, 689, 2441e2449.
€
31. Selected crystallographic data for 33. Orthorhombic, P212121, z¼4, a¼4.
ꢀ
ꢀ
ꢀ
10. Selected crystallographic data for 19. Orthorhombic, P212121, z¼4, a¼6.
4753(4) A, b¼14.464(2) A, c¼18.633(2) A, R¼0.036 for 163 parameters and 2466
ꢀ
ꢀ
ꢀ
8397(7) A, b¼11.8537(11) A, c¼16.459(2) A, R¼0.076 for 190 parameters and
I>2s(I), CCDC 919185 contains the supplementary crystallographic data. These
1933 I>2
s
(I), CCDC 919179 contains the supplementary crystallographic data.
data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
These data can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
32. Lee, K. M.; Wang, H. M. J.; Lin, I. J. B. J. Chem. Soc., Dalton Trans. 2002,
2852e2856.
33. Liu, Q.-X.; Xu, F.-B.; Li, Q.-S.; Zeng, X.-S.; Leng, X.-B.; Chou, Y. L.; Zhang, Z.-Z.
11. Silverman, R. B.; Levy, M. A. J. Org. Chem. 1980, 45, 815e818.
ꢂ
ꢂ
12. Kysilka, O.; Rybackova, M.; Skalicky, M.; Kvıcalova, M.; Cvacka, J.; Kvıcala, J. J.
Fluorine Chem. 2009, 130, 629e639.
Organometallics 2003, 22, 309e314.
13. Meyer, D.; Strassner, T. J. Org. Chem. 2011, 76, 305e308.
34. Chen, W.; Liu, F. J. Organomet. Chem. 2003, 673, 5e12.
35. Liao, C.-Y.; Chan, K.-T.; Chiu, P.-L.; Chen, C.-Y.; Lee, H. M. Inorg. Chim. Acta 2008,
361, 2973e2978.
ꢀ
14. Selected crystallographic data for 23. Monoclinic, P21/c, z¼4, a¼15.2434(13) A,
ꢀ
ꢀ
b¼9.2224(7) A, c¼10.0075(8) A,
b
¼91.290(4)ꢀ, R¼0.063 for 247 parameters and
ꢀ
1666 I>2
s
(I), CCDC 919180 contains the supplementary crystallographic data.
36. Selected crystallographic data for 31. Monoclinic, P21, z¼4, a¼12.1211(5) A,
¼92.168(1)ꢀ, R¼0.043 for 683 parame-
ꢀ
ꢀ
b
These data can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
b¼7.5474(3) A, c¼26.4352(11) A,
ters and 9378 I>2s(I), CCDC 919186 contains the supplementary crystal-
15. Selected crystallographic data for 25. Monoclinic, P21/n, z¼8, a¼16.
lographic data. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
¼108.611(3)ꢀ, R¼0.055 for 485
ꢀ
ꢀ
ꢀ
1386(19) A, b¼9.5708(11) A, c¼18.719(2) A,
b
parameters and 3769 I>2s(I), CCDC 919181 contains the supplementary
€
crystallographic data. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
37. Stanek, K.; Czarniecki, B.; Aardoom, R.; Ruegger, H.; Togni, A. Organometallics
2010, 29, 2540e2546.
38. Selected crystallographic data for 34. Orthorhombic, P212121, z¼4, a¼9.
ꢀ
ꢀ
ꢀ
16. Bondi, A. J. Phys. Chem. 1964, 68, 441e451.
5952(8) A, b¼10.7997(9) A, c¼17.834(2) A, R¼0.015 for 330 parameters and
17. For selected examples see: (a) Ramasubbu, N.; Parthasarathy, R.; Murray-Rust,
P. J. Am. Chem. Soc. 1986, 108, 4308e4314; (b) Schwarzer, A.; Seichter, W.;
Weber, E.; Stoeckli-Evans, H.; Losada, M.; Hulliger, J. CrystEngComm 2004, 6,
567e572; (c) Barcelo-Oliver, M.; Estarellas, C.; Garcia-Raso, A.; Terron, A.;
Frontera, A.; Quinonero, D.; Mata, I.; Molins, E.; Deya, P. M. CrystEngComm 2010,
12, 3758e3767; (d) Granifo, J.; Toledo, D.; Garland, M. T.; Baggio, R. J. Fluorine
Chem. 2010, 131, 510e516; (e) Schwarzer, A.; Bombicz, P.; Weber, E. J. Fluorine
Chem. 2010, 131, 345e356; (f) Chopra, D.; Row, T. N. G. CrystEngComm 2011, 13,
2175e2186; (g) Berger, R.; Resnati, G.; Metrangolo, P.; Weber, E.; Hulliger, J.
Chem. Soc. Rev. 2011, 40, 3496e3508.
4151 I>2s(I), CCDC 919187 contains the supplementary crystallographic data.
These data can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
39. Field, L. D.; Messerle, B. A.; Vuong, K. Q.; Turner, P. Organometallics 2005, 24,
4241e4250.
40. Ashley, J. M.; Farnaby, J. H.; Hazari, N.; Kim, K. E.; Luzik, E. D., Jr.; Meehan, R. E.;
Meyer, E. B.; Schley, N. D.; Schmeier, T. J.; Tailor, A. N. Inorg. Chim. Acta 2012,
380, 399e410.
ꢀ
41. Selected crystallographic data for 37. Orthorhombic, Pbca, z¼8, a¼9.221(2) A,
ꢀ
ꢀ
b¼19.977(3) A, c¼25.889(4) A, R¼0.044 for 409 parameters and 3148 I>2
s(I),
18. Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972e975.
CCDC 919188 contains the supplementary crystallographic data. These data can
be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
ꢂ
19. Fremont, P.; Scott, N. M.; Stevens, E. D.; Nolan, S. P. Organometallics 2005, 24,
2411e2418.
ꢀ
20. Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105, 3978e4008.
21. Lin, I. J. B.; Vasam, C. S. Coord. Chem. Rev. 2007, 251, 642e670.
22. Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B.
Chem. Rev. 2009, 109, 3561e3598.
42. Selected crystallographic data for 38. Triclinic, Pꢁ1, z¼2, a¼9.7693(3) A, b¼12.
ꢀ
ꢀ
1509(4) A, c¼12.6102(4) A,
a
¼105.997(1)ꢀ,
b
¼95.575(2)ꢀ,
g
¼107.310(1)ꢀ, R¼0.
026 for 482 parameters and 5337 I>2s(I), CCDC 919189 contains the supple-
mentary crystallographic data. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
ꢀ
23. Selected crystallographic data for 26. Tetragonal, P41212, z¼4, a¼12.3737(2) A,
ꢀ
c¼26.1445(5) A, R¼0.048 for 268 parameters and 5419 I>2
s(I), CCDC 919183
ꢀ
contains the supplementary crystallographic data. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
43. Selected crystallographic data for 39. Monoclinic, P21/n, z¼4, a¼12.3511(5) A,
ꢀ
ꢀ
b¼15.3862(6) A, c¼15.0217(6) A,
b
¼111.834(1)ꢀ, R¼0.027 for 496 parameters
and 5285 I>2s(I), CCDC 919190 contains the supplementary crystallographic
24. Howard, J. A. K.; Hoy, V. J.; O’Hagan, D.; Smith, G. T. Tetrahedron 1996, 52,
12613e12622.
data. These data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
44. Sparr, C.; Salamanova, E.; Schweizer, W. B.; Senn, H. M.; Gilmour, R. Chem.dEur.
J. 2011, 17, 8850e8857.
45. Gooseman, N. E. J.; O’Hagan, D.; Peach, M. J. G.; Slawin, A. M. Z.; Tozer, D. J.;
Young, R. J. Angew. Chem., Int. Ed. 2007, 46, 5904e5908.
46. Briggs, C. R. S.; Allen, M. J.; O’Hagan, D.; Tozer, D. J.; Slawin, A. M. Z.; Goeta, A. E.;
Howard, J. A. K. Org. Biomol. Chem. 2004, 2, 732e740.
ꢀ
25. Selected crystallographic data for 28. Tetragonal, P41212, z¼8, a¼9.5501(1) A,
ꢀ
c¼52.3946(9) A, R¼0.050 for 303 parameters and 6109 I>2
s(I), CCDC 919184
contains the supplementary crystallographic data. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
€
26. Kocher, C.; Herrmann, W. A. J. Organomet. Chem. 1997, 532, 261e265.
27. Hildebrandt, B.; Frank, W.; Ganter, C. Organometallics 2011, 30, 3483e3486.
28. Takemura, H.; Nakashima, S.; Kon, N.; Yasutake, M.; Shinmyozu, T.; Inazu, T. J.
Am. Chem. Soc. 2001, 123, 9293e9298.
47. Hyla-Kryspin, I.; Grimme, S.; Hruschka, S.; Haufe, G. Org. Biomol. Chem. 2008, 6,
4167e4175.
48. O’Hagan, D.; Bilton, C.; Howard, J. A. K.; Knight, L.; Tozer, D. J. J. Chem. Soc.,
Perkin Trans. 2 2000, 605e607.
29. Plenio, H. ChemBioChem 2004, 5, 650e655.
Please cite this article in press as: Paul, S.; et al., Tetrahedron (2013), http://dx.doi.org/10.1016/j.tet.2013.02.071