Addition of oxime derivatives to alkynyl fischer carbene complexes

Article abstract of DOI:10.1021/om300570q

The addition of oxime derivatives to alkynyl Fischer carbene complexes has been explored through a combined experimental and computational approach. Up to four different heterocycles with diverse regiochemistry and ring size were found, depending on the starting ketone. The reaction mechanism was studied by DFT methods.

Full text of DOI:10.1021/om300570q

Article
pubs.acs.org/Organometallics
Addition of Oxime Derivatives to Alkynyl Fischer Carbene Complexes
́
Hector F. Gonzalez, Marina Blanco-Lomas, Laura Rivado-Casas, Miguel A. Rodrıguez, Pedro J. Campos,
́
́
and Diego Sampedro*
́
́
́
Departamento de Quımica, Centro de Investigacion
́
en Sıntesis Quımica, CISQ, Universidad de La Rioja, Madre de Dios, 51,
E-26006 Logrono, Spain
̃
S
* Supporting Information
ABSTRACT: The addition of oxime derivatives to alkynyl
Fischer carbene complexes has been explored through a
combined experimental and computational approach. Up to
four different heterocycles with diverse regiochemistry and
ring size were found, depending on the starting ketone. The
reaction mechanism was studied by DFT methods.
Scheme 1. Photochemical Addition of Acyloximes to Alkynyl
Fischer Carbene Complexes
INTRODUCTION
■
In the last few decades Fischer carbene complexes have become
a powerful synthetic tool for organic chemists.1−4 In particular,
these compounds have proven to be extremely useful for the
synthesis of carbocycles of different sizes through [3 + 2 + 1]
(Dotz benzannulation),5 [2 + 1] (cyclopropanation),6 [2 + 2],7
̈
[3 + 2],8 [3 + 3],9 [4 + 2],10 and [4 + 3]11,12 cyclizations. These
organometallic species have also been used in the construction
of an increasing number of heterocycles.13−15 Due to the
extremely versatile behavior of Fischer carbene complexes, they
have been labeled as “chemical multitalents”.16−18 Different
reactivity levels are found when Fischer carbene complexes are
used in combination with unsaturated nitrogen compounds.
The most important work was carried out by Hegedus, who
developed a useful photochemical synthesis of β-lactams from
carbene complexes and imines.19 Other reports include the
1,2-20 and 1,4-addition21 of imines to α,β-unsaturated carbene
complexes and the cycloaddition of azadienes to unsaturated
Fischer carbene complexes.11,22−24 We recently described25 the
use of acyloximes in the photochemical synthesis of isoquino-
line derivatives through a two-step, one-pot reaction sequence.
First, the iminyl radical is easily generated by a nitrogen−
oxygen bond cleavage. The subsequent addition of the newly
formed radical to unsaturated systems yields the heterocyclic
compounds with ease and in good yields. This reaction
mechanism has been further explored by means of theoretical
calculations26 and EPR studies.27 Both aryl and alkynyl groups
can be used in either inter- or intramolecular reactions. We
extended the scope of this photochemical behavior of
acyloximes when confronted with Fischer alkynyl carbene
complexes (Scheme 1).14 We showed how nitrogen-centered
radicals generated by the action of UV light are capable of
reacting with alkynyl Fischer carbene complexes in two ways,
i.e. 1,2- and 1,4-addition, and our results constitute the first
reported example of a photochemically driven reaction of this
kind.
After a comprehensive mechanistic exploration, it was
demonstrated that 3 and 4 were formed through two different
pathways (Schemes 2 and 3).
The photochemical nature of the reaction was explored by
performing a test at room temperature in the dark. After the
Scheme 2. Proposed Mechanism for the Formation of 3
Received: June 21, 2012
Published: September 6, 2012
© 2012 American Chemical Society
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Scheme 3. Proposed Mechanism for the Formation of 4
Table 1. Addition of Acetyloximes to 2a
product (yield, %)
entry
ketone
1
2
3
4
benzophenone
fluorenone
6a (31)
6d (19)
7a (5)
8a (4)
8b (24)
indanone
9c (15)
acetophenone
Scheme 5. Thermal Addition of Indanone Oxime Derivatives
to Alkynyl Fischer Carbene Complexes
reaction mixture was stirred for 3 h, only traces of 3a were
obtained, together with a large amount of unreacted starting
compounds 1a and 2a.14 However, this incipient result
together with experimental difficulties associated with the
photochemical reactivity of oxime derivatives with alkynyl
Fischer carbene complexesprompted us to explore the
thermal version of this reaction.
product structure differs in the regiochemistry or the size of the
cycle (products 6−9). Only on using benzophenone (entry 1)
and acetophenone (entry 4) was the reaction outcome similar.
The absence of the minor products is presumably due to the
lower reaction yields obtained.
RESULTS AND DISCUSSION
■
The thermal reaction was carried out under several sets of
reaction conditions by varying solvents, temperature, and
reaction time. The most relevant results were obtained on
heating a mixture of 1a and 2a (1.2 equiv) in toluene at 90 °C
for 18 h. Three different products were isolated (Scheme 4).
As can be seen, compound 6a was the main product. We
have previously described the formation of this compound
through a thermal pathway.14 Two minor compounds were
isolated by HPLC along with the main product. Compound 7a
(related to 4 in Scheme 3 and in previous work14) has a seven-
membered ring (in contrast to 6a), while 8a also has a five-
membered ring with a different regiochemistry relative to 6a.
The general applicability of the reaction was assessed. With
this aim in mind, several ketones were employed to generate
the acetyloximes and these were subsequently reacted with
alkynyl carbene 2a. The results are summarized in Table 1.
It can be seen from the results that this reaction is extremely
sensitive to the oxime derivative employed. As explained above,
the compound with the five-membered ring (6a) was the main
product for the benzophenone acetyloxime. However, when the
fluorenone acetyloxime was tested, the five-membered ring with
the opposite regiochemistry (8b) was the only isolated product.
In the case of indanone acetyloxime, a new structure with a six-
membered ring, 9c, was formed (Scheme 5). Finally,
compound 6d was isolated in the case of the acetophenone
acetyloxime. In all four cases shown in Table 1 products from
the addition of the oxime derivative and the carbene complex
were obtained, but these results are, at first glance, far from
being easily optimized. In the three first entries the main
The structures of 6a, 7a, 8a,b, and 9c28 were confirmed by X-
ray diffraction studies (Figure 1), whereas the formation of 6d
was determined by comparison of its spectroscopic data with
those of 6a. In each case, the acetyl moiety is lost during the
course of the reaction and we therefore checked the effect of
changing this unit on the reaction outcome. The equivalent
reaction was carried out under the same conditions but with the
acetyloximes replaced by oximes. In every case, the change to
oximes did not lead to substantial changes in either the product
ratio or yield. For instance, replacement of benzophenone
acetyloxime 1a by the equivalent oxime gave a similar crude
product. The main product was 6a, and this was obtained in
40% yield (in comparison with 31% using the acetyloxime;
Table 1, entry 1). In the case of acetophenone derivatives, the
yield of 6d changed from 19% (acetyloxime; Table 1, entry 4)
to 13% (oxime). Similarly, the use of chromium carbene
complexes did not alter the reactivity, but in any case slightly
lower yields were found. This difference is believed to be
caused by the lower stability of the chromium analogues.
Due to the very different reaction outcome, even when slight
modifications were introduced, we decided to perform a
theoretical study of the thermal reaction in order to fully
understand the reaction mechanism. This step is a prerequisite
before the reaction can be turned into a synthetically useful
process.
Computational Details. All calculations were carried out
using the Gaussian 03 program package.29 Since DFT methods
combine the importance of including electron correlation
effects and the possibility of dealing with large systems, ground
Scheme 4. Thermal Addition of Benzophenone Oxime Derivatives to Alkynyl Fischer Carbene Complexes
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Figure 1. X-ray diffraction structures for 6a, 7a, 8a,b, and 9c.
state molecular geometries were optimized within the nonlocal
density approximation (NLDA), including Becke’s30 nonlocal
exchange corrections as well as Perdew’s31 inhomogeneous
gradient corrections for correlation (BP86). For C, O, N, and
H, the standard split-valence 6-31G* basis set32 was employed.
For the tungsten atom, the Hay−Wadt effective core
potential33 was used with the minimal basis set split to [441/
2111/21] (LanL2DZ). The geometry was fully optimized
without any symmetry constraint for all model compounds.
Optimized structures were characterized as minima or saddle
points by frequency calculations, which also allowed the ZPE
and thermal corrections to be obtained. The Fischer carbene
complex was calculated without any simplification. Although
both chromium and tungsten carbene complexes have proven
utility in these reactions, we used the tungsten complex despite
the greater computational cost, as better experimental results
were obtained. Oximes were used in the calculation, as they
yield the same experimental compounds as acetyloximes, but at
a smaller computational cost.
Theoretical Calculations. We started the theoretical study
by modeling the reaction between the benzophenone oxime
and alkynyl carbene complex 2a. As explained above, this
reaction led to the formation of 6a−8a. According to the
literature,1 two different reactive pathways could emerge when
confronting a nucleophile with alkynyl carbene complexes.
These two possibilities are due to the presence of two reactive
points: i.e., the carbene carbon atom and the β alkyne carbon
atom. Thus, we computed these two paths and the results are
shown in Figure 2.
In the first route, transition structure B-TS is formed through
nucleophilic attack of the nitrogen oxime atom at the β alkyne
carbon atom. The geometry of the carbene complex is modified
accordingly by decreasing the alkyne C−C−C angle and
lengthening the C−C triple-bond distance (1.240 Å in A, 1.277
Å in B-TS). Modifications in the W−C bond length and
Ccarbene−Cakyne distance also respond to this attack. The second
route involves the attack on the carbene carbon atom, and this
leads to an increase in the W−C bond length (2.170 Å in A,
2.322 Å in C-TS) while the alkyne moiety is barely altered. A
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Figure 2. Nucleophilic attack. Distances are given in angstroms and free energies in kcal/mol relative to A.
Figure 3. Metal fragment migration in the formation of 6a.
comparison of the two transition structures reveals only minor
differences in the CN distances, a finding consistent with the
nucleophilic attack by the nitrogen atom lone pair, but large
differences in the N−C bond distances that are being formed.
In B-TS the N−Calkyne distance is 1.919 Å, whereas in C-TS the
N−Ccarbene distance is 1.795 Å. This situation should be
reflected in the relative stabilities of the two structures, as in B-
TS the negative charge is mainly located on the metal fragment.
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Figure 4. Cyclization step in the formation of 6a.
These two transition structures lead to two different products,
for which the addition of the oxime has taken place on the
alkyne triple bond (B) or the WC bond (C). In the case of B
the structure of the Fischer carbene complex is maintained,
with a WC bond of 2.275 Å, and the relative energy reflects
this fact. In the case of C, the addition to the WC bond leads
to a longer W−C distance (2.379 Å) and a less stable structure.
B is clearly more stable than C, and it is formed through a
pathway of lower energy (B-TS vs C-TS). Thus, at this point
there will be a clear preference for the formation of B.
The energy profile for the formation of E is shown in Figure
5. As can be seen, the steps with the highest energy barriers are
The next step in the reaction mechanism consists of a [1,2]-
W(CO)5 migration (Figure 3). In D-TS the metal fragment
moves from the carbenic carbon to the adjacent atom with W−
C distances of 2.540 and 2.754 Å. This geometrical change is
associated with a considerable increase in energy, and this
causes intermediate D to be quite unstable. Thus, D will rapidly
progress along the reaction path or will return to the previous,
more stable minimum B. This migration was initially proposed
by Dotz et al.34 to explain cyclodimerization of Fischer alkynyl
̈
Figure 5. Energetic profile for the formation of 6a.
carbene complexes, and the intermediate is stable enough to be
identified spectroscopically.35 In addition, this migration was
also found in related reactions of Fischer carbene complexes.11
In these examples, [1,2]-migration promotes the subsequent
cyclization to yield the reaction products, as happens in the case
described here.
the initial attack of the oxime nitrogen atom at the β alkyne
carbon atom to yield intermediate B (18.2 kcal/mol) together
with the formation of D (also 18.2 kcal/mol), although the
formation of E-TS corresponds with the maximum in energy
along the reaction path. On the other hand, a global minimum
in the reaction path is reached with the formation of E, which is
3.4 kcal/mol more stable than the reactants.
Experimentally 6a was found to be the main reaction
product, but another two products were also isolated from the
reaction mixture, 7a and 8a. Thus, we also explored by
computational means the fate of C, which is formed after
nucleophilic attack at the carbenic atom. The next step in the
reaction mechanism for C is shown in Figure 6. The new, more
stable minimum F was found through a [1,3]-W(CO)5
migration. The driving force for this step is the generation of
F, a more stable structure. In the transition structure F-TS, the
metal fragment migrates through a cyclic structure. The new
intermediate F has extended conjugation, and this accounts for
its higher stability. This step has been suggested previously22
Once D is formed, it will quickly progress through a
cyclization step. The electron-rich carbon atom located next to
the metal fragment can attack the iminic carbon to yield a five-
membered cycle (Figure 4). As can be seen, in E-TS this
cyclization step takes place, as the C−C distance is already
2.626 Å. Accordingly, the CN distance lengthens from 1.355
Å in D to 1.370 Å in E-TS and the C−C bond distance changes
from 1.269 Å in D to 1.314 Å in E-TS. Cyclization is completed
in E to yield a precursor of 6a. Due to the high energy of D, the
cyclization step has to surmount an energy barrier of only 8.6
kcal/mol. Formation of 6a from E involves the loss of the metal
fragment and the removal of the OH of the oxime functionality.
This kind of transformation has already been found in related
reactions of alkynyl Fischer carbene complexes and 1-hydroxy-
1-azabutadiene derivatives.22
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Figure 6. Metal fragment migration step in the formation of 7a.
Figure 7. Cyclization step in the formation of 7a.
and was later found12 in cyclization reactions of Fischer carbene
complexesin the latter case charge stabilization was invoked
to justify the formation of analogues of F.
The next step in the reaction mechanism corresponds to a
cyclization step (Figure 7). The molecule is now in the right
disposition to allow the interaction between its two extremes.
Electron density located on the metal fragment can promote a
nucleophilic attack on the closest aromatic ring. At the same
time, the metal fragment moves to the neighboring carbon
atom in a complex geometrical change that finally leads to the
transition structure G-TS. Although both changes have an
influence on the geometry, the biggest deformation comes from
the movement of the metal fragment and the nucleophilic
attack on the aromatic ring is less influential. Due to the
complex atomic movement involved, we performed IRC
calculations to connect the three stationary points in order to
ensure that G-TS is really the transition structure that connects
these two minima (Figure 8).
Figure 8. IRC connecting F, G-TS, and G.
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Figure 9. Hydrogen atom migration step in the formation of 7a.
Figure 10. Energetic profile for the formation of 7a.
Figure 11. Cyclization step in the formation of 8a.
Once cyclization has taken place, the final structure that is
similar to 7a is formed through a hydrogen migration (Figure
9). The driving force for this step comes from the high
stabilization of H with respect to G due to the recovery of
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aromaticity in the phenyl ring. Thus, H is 0.3 kcal/mol more
stable than the reactants and 15.3 kcal/mol more stable than G.
In the transition structure H-TS the hydrogen atom is located
between the initial and final positions (C−H distances of 1.355
and 1.445 Å) and the imaginary frequency is 1083.0i, a
characteristic value for a hydrogen migration.
and the formation of 7a and 8a. B-TS is ca. 5 kcal/mol more
stable than C-TS, a finding that is consistent with experimental
results. Similar calculations were carried out on a fluorenone
oxime (Figure 13).
The final step corresponds to a reductive elimination with
loss of the metal fragment to yield 7a. This step has been
previously found in related transformations.22 The energetic
profile for this reaction is shown in Figure 10. For the
formation of G-TS and H-TS it is necessary to surmount
energetic barriers of considerable height. In addition, the final
computed product is only slightly more stable than the
reactants. These facts help to explain the low yield observed
for the formation of 7a.
As shown in Scheme 4, another product was detected in the
reaction of alkynyl carbene complex 2a with benzophenone
acetyl oxime 1a. A product with a five-membered ring was
found in low yield. Compound 8a has a regiochemistry
different from that of 6a. The formation of 8a can be explained
by starting from intermediate C, which is obtained by
nucleophilic attack of the oxime derivative nitrogen atom at
the carbenic carbon. Once C is formed, a direct cyclization step
leads to Ithe precursor of 8a (Figure 11). The formation of
the final product 8a involves reductive elimination as described
above.
The energetic profile for this process is shown in Figure 12.
As can be seen, the first step is common in the two routes
Figure 13. Energetic profile for the reaction of alkynyl carbene
complex 2a with fluorenone oxime.
It can be seen that the formation of K-TS (equivalent to B-
TS, attack at the β alkyne C atom) has to surmount the same
energy barrier (18.2 kcal/mol), whereas L-TS (equivalent to C-
TS, attack at the carbenic C atom) is more stable (15.0 kcal/
mol). This situation is due to the planar disposition of the
fluorenone moiety compared to the benzophenone, which
eliminates the steric hindrance with the pentacarbonylmetal
fragment. In this case, L-TS will be preferred and the formation
of 8b will eventually be achieved, as confirmed by experimental
data.
The mechanism for the reaction should change when
different oxime derivatives are used, as reflected by the data
shown in Table 1. Specifically, in the case of indanone (entry 3)
a different product was obtained (Scheme 5). The first steps in
the reaction mechanism should be very similar to those
discussed above. However, at a certain point, the acidic α
protons could provide a competitive pathway. Fischer carbene
complexes and related compounds are known to have acid/base
reactivity.1 A detailed mechanistic exploration of this step is
currently underway using both computational and experimental
tools, and the results will be published in due course.
As explained above, three different products (6a−8a) were
obtained from the reaction of 1a and 2a (Scheme 4), with 6a
obtained as the major product. However, as inferred from the
computational data presented in Figures 6, 10, and 12, the
formation of 7a and 8a should be preferred, as the points with
highest energy along the path have values of 31−32 kcal/mol,
while in the case of the formation of 6a the value is 38.2 kcal/
mol. These data are in clear disagreement with experimental
results. It therefore seems clear that an alternative path must be
operating. Considering that the formation of 6a requires the
removal of the OH of the oxime functionality at some point,
this could take place in any of the steps prior to surmounting
Figure 12. Energetic profile for the formation of 8a.
leading to the formation of 7a and 8a. Furthermore, the rate-
determining step in both cases has a similar energy barrier (G-
TS and I-TS). Thus, 7a and 8a should be formed in similar
yieldsa situation that was confirmed experimentally.
As explained above, a very different reaction outcome was
found when diverse oxime derivatives were used. The
formation of 6a−8a was detected on using benzophenone
acetyloxime 1a. However, only 8b was isolated in the case of
the reaction with fluorenone acetyloxime and only 9c was
obtained for the indanone derivative. Given that the oxime
derivatives share some common features, it seems reasonable to
expect a general reaction mechanism as outlined for 1a. The
variation in product ratios is probably due to small differences
in the energy for the relevant transition structures for the path
leading to each type of product. Thus, in order to explain the
different reactivity, we carried out selected calculations on the
key structures for the oxime derivatives. As shown in Figure 2,
two reaction paths are available for the reaction of oxime
derivatives with alkynyl Fischer carbene complexes: i.e.,
nucleophilic attack on the carbenic C atom or on the β alkyne
C atom. In the case of 2a, attack at the β alkyne C atom leads to
B-TS and subsequently to the formation of 6a, the main
product, while attack onat the carbenic C atom leads to C-TS
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the high energy barrier leading to E by any negatively charged
metal fragment. Metal species with a certain electron density
are ubiquitous in the chemistry of Fischer carbene complexes,
but the exact nature of these structures and the more favorable
step for the loss of OH are still unknown. However, indirect
evidence for this alternative mechanism comes from the excess
complex required in this and other examples14,22 for the
reaction to reach completion. This step has also been invoked
previously for similar reactions.22 Furthermore, bearing in mind
that reaction of 3a in refluxing acetonitrile leads to the
formation of 6a in 76% yield (Scheme 6),14 it seems reasonable
to believe that the loss of the OH fragment could take place in
the initial steps, before D is formed, through metal fragment
cyclization.
previously, as the species computed have different numbers of
atoms. To properly compare the different paths, the exact
formation of 3a should be assessed. As stated above, this
information is currently unavailable and a discussion of the
formation of this compound is beyond the scope of this paper.
However, as 3a constitutes a common intermediate for these
and other relevant transformations, the mechanism leading to
its formation is currently under investigation.
CONCLUSIONS
■
We have presented here a new type of reactivity that allows the
generation of up to four different types of nitrogenated
heterocycles by using oxime derivatives and alkynyl Fischer
carbene complexes. Both the regiochemistry and the size of the
cycle can be modified by changing the starting ketone used to
synthesize the oxime derivative. Thus, the mechanism of ring
formation was found to be very dependent on the ketone
moiety. On the other hand, chromium and tungsten can be
used in the carbene moiety and both oxime and acetyloximes
yield similar products. Consequently, both the metal and the
oxime derivative were found to affect the mechanism to minor
extent. The mechanism of the reaction was explored in an effort
to understand this transformation before any subsequent
modification could be devised to improve the reaction
outcome. However, with the data currently available, the
formation of some products found in the crude reaction
mixture cannot be completely explained and further studies will
be needed. Although in the present form this reaction provides
low yields, it represents a new type of reactivity that would
allow the synthesis of complex heterocycles with ease and in
very few steps. Further efforts are required to turn this reaction
into a synthetically useful tool and to expand the synthetic
scope.
Scheme 6. Thermal Formation of 6a from 3a
Unfortunately, without further experimental information on
the species involved in the process of OH loss, the
computational study would be a complex task and is well
beyond the scope of this paper. However, some evidence could
be obtained from the formation reaction shown in Scheme 5 by
computational means (Figure 14). As can be seen from the IRC
calculation, the formation of M (equivalent to 6a) takes place in
only one step from 3a after surmounting a barrier of 24.5 kcal/
mol. Furthermore, M is much more stable (34.3 kcal/mol) than
3a. Thus, the formation of M could drive the reaction to
completion. It should be noted that the data shown in Figure
14 cannot be compared with the energy values discussed
Figure 14. IRC calculation for the formation of M from 3a.
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(21) Funke, F.; Duetsch, M.; Stein, F.; Noltemeyer, M.; Demeijere,
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(22) Barluenga, J.; Tomas, M.; Ballesteros, A.; Santamaria, J.;
EXPERIMENTAL SECTION
■
General Procedure. To 1 equiv of oxime acetate dissolved in
toluene was added 1.2 equiv of the corresponding alkynyl Fischer
carbene, and the mixture was heated for ca. 18 h at 90 °C. The
reaction was monitored by TLC. Once the reaction was complete, the
solution was cooled to room temperature. The solvent was removed
and the crude product was purified by silica gel column
chromatography (hexane/EtOAc 90/10).
Carbajo, R. J.; Lop
Eur. J. 1996, 2, 88.
́
ez-Ortiz, F.; Garcia-Granda, S.; Pertierra, P. Chem.
(23) Barluenga, J.; Tomas, M.; Ballesteros, A.; Santamaria, J.; Corzo-
Suarez, R.; Garcia-Granda, S. New J. Chem. 2001, 25, 8.
(24) Barluenga, J.; Tomas, M.; Ballesteros, A.; Santamaria, J.; Lop
́
ez-
Ortiz, F. J. Chem. Soc., Chem. Commun. 1994, 321.
́
(25) Alonso, R.; Campos, P. J.; García, B.; Rodrıguez, M. A. Org. Lett.
ASSOCIATED CONTENT
* Supporting Information
Text, figures, and tables giving experimental procedures,
characterization data, X-ray diffraction data, and Cartesian
coordinates for the geometries discussed in the text. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
2006, 8, 3521.
S
(26) Alonso, R.; Campos, P. J.; Rodriguez, M. A.; Sampedro, D. J.
Org. Chem. 2008, 73, 2234.
(27) Portela-Cubillo, F.; Alonso-Ruiz, R.; Sampedro, D.; Walton, J.
C. J. Phys. Chem. A 2009, 113, 10005.
(28) In the case of 9c, the low quality of the different crystals used for
the data collection did not allow us to obtain a better data set.
However, despite the fact that the quality of the structure is not good
enough, the molecular connectivity was confirmed.
(29) Gaussian 03; Revision C.02 ed.; Gaussian Inc., Pittsburgh PA,
2003.
AUTHOR INFORMATION
Corresponding Author
*E-mail: diego.sampedro@unirioja.es.
■
(30) Becke, A. D. Phys. Rev. 1988, A38, 3098.
(31) Perdew, J. P. Phys. Rev. 1986, B34.
(32) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(33) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
Notes
The authors declare no competing financial interest.
(34) Dotz, K. H.; Christoffers, C.; Knochel, P. J. Organomet. Chem.
1995, 489, C84.
(35) Iwasawa, N.; Ochiai, T.; Maeyama, K. J. Org. Chem. 1998, 63,
3164.
̈
ACKNOWLEDGMENTS
■
Financial support from the MICINN (CTQ2007-64197 and
CTQ2011-24800) is gratefully acknowledged. M.B.-L. thanks
the Spanish MEC for her grant. H.F.G. and L.R.-C. thank the
́
Comunidad Autonoma de La Rioja for their grants. We thank
́
Fernando Rodriguez, Reza Kia and Alegrıa Caballero for their
assistance in solving the X-ray diffraction structures.
REFERENCES
■
(1) Dorwald, F. Z. Carbenes in Organic Chemistry; Wiley-VCH:
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