Irradiation of imine-group VI carbene complexes in the presence of alkynes: A theoretical and experimental study

Article abstract of DOI:10.1021/jo0343073

The photoreaction between imine-substituted Fischer carbene complexes and alkynes is studied at both experimental and theoretical levels. 2H-Pyrrole derivatives are easily obtained as main products in moderate to good yields, with complete control of the regiochemistry. High-level theoretical calculations are carried out in order to explore and fully understand the reaction pathway. On the basis of the theoretical results, a mechanism that accounts for the experimental findings is proposed.

Full text of DOI:10.1021/jo0343073

Ir r a d ia tion of Im in e-Gr ou p VI Ca r ben e Com p lexes in th e
P r esen ce of Alk yn es: A Th eor etica l a n d Exp er im en ta l Stu d y
Pedro J . Campos, Diego Sampedro, and Miguel A. Rodr´ıguez*
Departamento de Quı´mica, Universidad de La Rioja, Grupo de S´ıntesis Quı´mica de La Rioja,
Unidad Asociada al C.S.I.C.; Madre de Dios, 51, E-26006 Logron˜o, Spain
miguelangel.rodriguez@dq.unirioja.es
Received March 8, 2003
The photoreaction between imine-substituted Fischer carbene complexes and alkynes is studied at
both experimental and theoretical levels. 2H-Pyrrole derivatives are easily obtained as main products
in moderate to good yields, with complete control of the regiochemistry. High-level theoretical
calculations are carried out in order to explore and fully understand the reaction pathway. On the
basis of the theoretical results, a mechanism that accounts for the experimental findings is proposed.
SCHEME 1. P h otor ea ctivity of Ch r om iu m
In tr od u ction
Im in e-Ca r ben e Com p lexes w ith Alk en es
The use in synthesis of complexes of the type (CO)5-
MdC(X)R (M ) group VI metal; X ) π-donor substituent,
usually with an O or N atom bonded to the metal; R )
alkyl, alkenyl, alkynyl or aryl group), although relatively
recent, has shown impressive potential.1 These com-
pounds, called Fischer carbene complexes after their
discoverer,2 have been shown to be extremely versatile
organometallic reagents with an extensive chemistry
including cycloadditions of almost any kind.3 More re-
cently, the photochemistry of group VI carbene complexes
has emerged as a new and very valuable route toward
different kinds of compounds such as cyclobutanones and
â-lactams.4 As part of our efforts to further develop the
synthetic utility of the photoreactivity of carbene com-
plexes, we reported in preceding papers the cycloaddition
of imine-substituted carbene complexes with alkenes and
alkynes5 and heteroatom-containing double bonds.6 Thor-
ough research7 of the reaction with olefins, including the
synthetic scope and the first photochemical study for this
kind of compounds, showed that this reaction actually
consists of two different, consecutive photochemical reac-
tions, the first one leading to a cyclopropane, which
subsequently rearranges7b to finally give the observed
1-pyrroline (Scheme 1).7 While it has been possible to
clarify the mechanism for the reaction of imine-carbene
complexes with alkenes, the corresponding cycloaddition
with alkynes still has some controversial points. In
contrast with the well-studied benzannulation procedures
of Fischer carbene complexes via both the experimental8
and theoretical9 approaches, there is no mechanistic
study of the cyclopentannulation of imine-group VI
carbene complexes with alkynes, thermal or photochemi-
cal. This reaction seems to be related to other cyclization
processes produced by Fischer carbene complexes, such
as the Do¨tz benzannulation. However, there are some
characteristics that make this reaction different. In
particular, the products obtained never include a CO from
the metal part. Therefore, the final products have five-
membered cycles, in contrast to the six-membered cycles
of the Do¨tz reaction, which are usually the main product.
(1) For reviews, see: (a) Barluenga, J .; Sua´rez-Sobrino, A. L.; Toma´s,
M.; Garc´ıa-Granda, S.; Santiago-Garc´ıa, R. J . Am. Chem. Soc. 2001,
123, 10494. (b) Davies, M. W.; J ohnson, C. N.; Harrity, J . P. A. J . Org.
Chem. 2001, 66, 3525. (c) Sierra, M. A. Chem. Rev. 2000, 100, 3591.
(d) Do¨rwald, F. Z. Carbenes in Organic Chemistry; Wiley-VCH:
Weinheim, 1999. (e) Do¨tz, K. H.; Tomuschat, P. Chem. Soc. Rev. 1999,
28 187. (f) Barluenga, J . Pure Appl. Chem. 1999, 71, 1385. (g) Aumann,
R.; Nienaber, H. Adv. Organomet. Chem. 1997, 41, 163. (h) Barluenga,
J . Pure Appl. Chem. 1996, 68, 543. (i) de Meijere, A. Pure Appl. Chem.
1996, 68, 61. (j) Harvey, D. F.; Sigano, D. M. Chem. Rev. 1996, 96,
271. (k) Hegedus, L. S. In Comprehensive Organometallic Chemistry
II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press:
Oxford, 1995; Vol. 12, p 549. (l) Wulff, W. D. In Comprehensive
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson,
G., Eds.; Pergamon Press: Oxford, 1995; Vol. 12, p 469. (m) Wulff, W.
D. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon Press: Oxford, 1991; Vol. 5, p 1065. (n) Do¨tz, K. H. Angew.
Chem. 1984, 96, 573; Angew. Chem., Int. Ed. Engl. 1984, 23, 587. (o)
Do¨tz, K. H.; Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schubert, U.;
Weiss, K. Transition Metal Carbene Complexes; VCH: Weinheim, 1983.
(2) Fischer, E. O.; Maasbo¨l, A. Angew. Chem. 1964, 76, 645; Angew.
Chem., Int. Ed. Engl. 1964, 3, 580.
(5) Campos, P. J .; Sampedro, D.; Rodr´ıguez, M. A. Organometallics
2000, 19, 3082.
(6) Campos, P. J .; Sampedro, D.; Rodrı´guez, M. A. Tetrahedron Lett.
2002, 43, 73.
(7) (a) Campos, P. J .; Sampedro, D.; Rodr´ıguez, M. A. Organome-
tallics 2002, 21, 4076. (b) Campos, P. J .; Soldevilla, A.; Sampedro, D.;
Rodr´ıguez, M. A. Org. Lett. 2001, 3, 4087.
(8) For a review, see: Wulff, W. D. In Comprehensive Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: New York,
1990; Vol. 5.
(3) (a) Barluenga, J .; Mart´ınez, S.; Sua´rez-Sobrino, A. L.; Toma´s,
M. J . Am. Chem. Soc. 2001, 123, 11113. (b) Barluenga, J .; Toma´s, M.;
Rubio, E.; Lo´pez-Pelegrin, J . A.; Garc´ıa-Granda, S.; Priede, M. P. J .
Am. Chem. Soc. 1999, 121, 3065. (c) Fru¨hauf, H. W. Chem. Rev. 1997,
97, 523. (d) Schmalz, H.-G. Angew. Chem. 1994, 106, 311; Angew.
Chem., Int. Ed. Engl. 1994, 33, 303.
(9) Gleichmann, M. M.; Do¨tz, K. H.; Hess, B. A. J . Am. Chem. Soc.
1996, 118, 10551.
(4) For a review, see: Hegedus, L. S. Tetrahedron 1997, 53, 4105.
10.1021/jo0343073 CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/09/2003
4674
J . Org. Chem. 2003, 68, 4674-4683

Irradiation of Imine-Group VI Carbene Complexes
SCHEME 2. P h otor ea ctivity of Ch r om iu m
Im in e-Ca r ben e Com p lexes w ith Alk yn es
At this point, one wonders whether the light effect is
only that of opening an empty coordination site by CO
dissociation or whether some of the subsequent steps
leading to the pyrrole derivative are also involved. Taking
into account that thermal reactions have been described
for some imine-carbene complexes,14 we carried out its
irradiation in the presence of alkynes. Since our results
seem analogous to those previously described, we assume
that after CO dissociation the reaction takes place on the
ground-state surface; that is, the light effect in this
reaction only consists of the activation of the carbene
complexes by creating an empty coordination site, which
can be filled by the alkyne. This suggests that all the
subsequent steps leading to the pyrrole formation should
take place on the ground state.
The value of this reaction as a synthetically useful way
to get 2H-pyrrole derivatives and as a Do¨tz-related
process prompted us to explore its mechanism. However,
due to the difficulties of experimental characterization
of the possible reaction intermediates as done in the case
of the reaction of imine-carbene complexes with al-
kenes,7 we decided to carry out the study of the cyclo-
pentannulation of imine-carbene complexes with alkynes
using theoretical tools.
Theoretical calculations of transition-metal complexes
are now a well-established tool for the study of organo-
metallic species.10 Especially important are density func-
tional theory (DFT) methods, due to their expediency,
which makes them viable for the study of large-size
molecules at a fraction of the time required for post-
Hartree-Fock (HF) calculations. An even more important
advantage is that in most cases the expectation values
derived from approximate DFT are in better agreement
with experiment than results obtained from HF calcula-
tions. In the particular case of group VI-carbene com-
plexes, DFT methods have shown remarkable agreement
with the experimental results.11 In particular, DFT
methods have achieved a number of successes over the
past few years in the field of group VI-carbene complex
chemistry, as they have been able to reproduce experi-
mental results with considerable accuracy.12 Encouraged
by these results, we tried to apply this methodology to
the study of the photochemical cyclopentannulation of
imine-carbene complexes with alkynes to make up for
the difficulties in the experimental characterization of
the intermediates involved.
Com p u ta tion Meth od s
The size of the imine-carbene complex under study
necessitated the use of (CO)5CrdCHNdCH2 and HCt
CH as models for the calculations. All calculations were
carried out using the Gaussian 98 program package.15
Since the DFT method combines the importance of
including electron correlation effects and the possibility
of dealing with large systems, ground-state molecular
geometries were optimized within the nonlocal density
approximation (NLDA), including Becke’s16 nonlocal
exchange corrections as well as Perdew’s17 inhomoge-
neous gradient corrections for correlation. For C, O, N,
and H, the standard split-valence 6-31G* basis set18 was
employed. For the chromium atom, we used the Hay-
Wadt effective core potential,19 with the minimal basis
set split to [341/2111/41].20 This basis set has proved its
ability to perform calculations on transition-metal com-
plexes with good accuracy.21 Geometry was fully opti-
(13) Details will be published elsewhere.
(14) (a) Dragisich, V.; Wulff, W. D.; Hoogsteen, K. Organometallics
1990, 9, 2867. (b) Dragisich, V.; Murray, C. K.; Warner, B. P.; Wulff,
W. D.; Yang, D. C. J . Am. Chem. Soc. 1990, 112, 1251. (c) Aumann,
R.; Heinen, H. J . Organomet. Chem. 1990, 389, C1. (d) Aumann, R.;
Heinen, H. J . Organomet. Chem. 1990, 391, C7.
The general accepted mechanism4 for the photochemi-
cal reactions of Fischer carbene complexes with unsatur-
ated systems involves CO insertion into the CrdC bond
in the first step to form a coordinated ketene followed
by reaction with the substrate. This ketene-complex
participation determines the formation of benzannulation
or cyclobutannulation products. In contrast with this
result, we have recently reported the first example of the
photochemically induced [3 + 2] cyclopentannulation of
imine-group VI carbene complexes with alkynes, which
gives 2H-pyrrole derivatives (Scheme 2). In these cases,
the absence of the CO ligand on the pyrrole structure
could indicate an initial CO loss instead of ketene-
complex formation after irradiation of carbene complexes.
To test this hypothesis, we checked that irradiation of
carbene complexes in the presence of Ph3P leads to the
substitution of one CO by the phosphine ligand.13
(15) Gaussian 98, Revision A.7: Frisch, M. J .; Trucks, G. W.;
Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J . R.;
Zakrzewski, V. G.; Montgomery, J . A., J r.; Stratmann, R. E.; Burant,
J . C.; Dapprich, S.; Millam, J . M.; Daniels, A. D.; Kudin, K. N.; Strain,
M. C.; Farkas, O.; Tomasi, J .; Barone, V.; Cossi, M.; Cammi, R.;
Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J .;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J . B.; Cioslowski, J .; Ortiz,
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P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J .; Keith, T.;
Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Chal-
lacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen, W.; Wong, M. W.;
Andres, J . L.; Gonzalez, M.; Head-Gordon, M.; Replogle, E. S.; Pople,
J . A. Gaussian, Inc., Pittsburgh, PA, 1998.
(16) Becke, A. D. Phys. Rev. 1988, A38, 3098.
(17) Perdew, J . P. Phys. Rev. 1986, B33, 8822; 1986, B34, 7406E.
(18) (a) Hariharan, P. C.; Pople, J . A. Theor. Chim. Acta 1973, 28,
213. (b) Hehre, W. J .; Ditchfield, R.; Pople, J . A. J . Chem. Phys. 1972,
56, 2257.
(19) Hay, P. J .; Wadt, W. R. J . Chem. Phys. 1985, 82, 299.
(20) Frenking, G.; Antes, I.; Bo¨hme, M.; Dapprich, S.; Ehlers, A. W.;
J onas, V.; Neuhaus, A.; Otto, M.; Stegmann, R.; Veldkamp, A.;
Vyboishchikov, S. F. In Reviews in Computational Chemistry; Lipko-
witz, K. B., Boyd, D. B., Eds.; VCH: New York, 1996; Vol. 8, p 63.
(21) (a) Ferna´ndez, E. J .; Lo´pez-de-Luzuriaga, J . M.; Monge, M.;
Rodr´ıguez, M. A.; Crespo, O.; Gimeno, M. C.; Laguna, A.; J ones, P. G.
Chem. Eur. J . 2000, 6, 636. (b) Barluenga, J .; Rodr´ıguez, F.; Vadecard,
J .; Bendix, M.; Fan˜ana´s, F. J .; Lo´pez-Ortiz, F.; Rodr´ıguez, M. A. J .
Am. Chem. Soc. 1999, 121, 8776. (c) Ferna´ndez, E. J .; Lo´pez-de-
Luzuriaga, J . M.; Monge, M.; Rodr´ıguez, M. A.; Crespo, O.; Gimeno,
M. C.; Laguna, A.; J ones, P. G. Inorg. Chem. 1998, 37, 6002.
(10) For a review see: Niu, S.; Hall, M. B. Chem. Rev. 2000, 100,
353.
(11) For a review, see: Ziegler, T. Chem. Rev. 1991, 91, 651.
(12) (a) Tafipolsky, M.; Scherer, W.; O¨ fele, K.; Artus, G.; Pedersen,
B.; Herrmann, W. A.; McGrady, G. S. J . Am. Chem. Soc. 2002, 124,
5865. (b) Torrent, M.; Duran, M.; Sola`, M. J . Chem. Soc., Chem.
Commun. 1998, 999. (c) Torrent, M.; Duran, M.; Sola`, M. Organome-
tallics 1998, 17, 1492. (d) Wang, C.-C.; Wang, Y.; Liu, H.-J .; Lin, K.-
J .; Chou, L.-K.; Chan, K.-S. J . Phys. Chem. A 1997, 101, 8887. (e)
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J . Org. Chem, Vol. 68, No. 12, 2003 4675

Campos et al.
TABLE 1. 2H-P yr r ole Der iva tive F or m a tion th r ou gh
P h otor ea ction of Im in e-Ca r ben e Com p lexes a n d
Alk yn es
TABLE 2. Absolu te a n d Rela tive En er gies of Ca lcu la ted
Str u ctu r es
total energy
(hartrees)
relative energy
(kcal/mol)
compd
11
TS-12
12
13
14
13-v
14-v
TS-15
15
TS-16
16
TS-17
17
TS-18
18
-749.866 147
-749.854 281
-749.898 810
-825.146 906
-825.124 392
-827.287 927
-827.308 506
-749.868 247
-749.885 259
-749.860 088
-749.875 899
-749.866 925
-749.903 637
-749.875 635
-749.964 735
0.0
+7.4
-20.5
0.0a
t
yield
+14.1a
0.0b
entry
M
complex R1
R2
Me Ph
Ph Ph
R3
Ph
Ph
Ph
H
(h) product (%)
1
2
3
4
5
6
7
Cr
Cr
Mo
Cr
Cr
Cr
W
1
2
3
1
1
1
4
5
5
7
5
5
6
5
7
8
9
9
51
49
+12.9b
-1.3
Me Ph
Me Ph
21
-12.0
+3.8
43a,b
23
Me CO2Me CO2Me 11
-6.1
-0.5
Me OEt
Me OEt
H
H
5
5
73a
63a
-23.5
-5.9
a
b
Only one regioisomer was found on the reaction crude. The
regiochemistry was wrongfully assigned in ref 5.
-61.9
a
b
Relative to 13. Relative to 13-v.
mized without any symmetry constraint employing Schle-
gel’s analytical gradient procedure,22 and the optimized
structures were characterized as minima or saddle points
by analytic frequency calculations, which also allowed the
obtaining of ZPE and thermal corrections.23 Searches for
transition states connecting minima were carried out by
quadratic synchronous transit approach (QST2).
Exp er im en ta l Resu lts
First, we prepared5 the imine-carbene complexes 1-4
(see Table 1) and carried out the direct irradiation in the
presence of alkynes using a series of different substitution
patterns. We tested the reaction in a variety of solvent
and reagent concentrations. The best results were ob-
tained when we irradiated, with a medium-pressure Hg
lamp, 0.25 mmol of imine-carbene complexes in hexane
together with 10 equiv of alkyne, through a Pyrex glass.
The use of THF, ether, or acetonitrile did not affect the
product distribution but increased the polymer formation
on the crude. A higher concentration on the starting
carbene complex or a lower concentration on the alkyne
caused the appearance of metathesis as a side reaction.5
This can be completely avoided by using the appropriate
concentrations. The results are shown in Table 1.
F IGURE 1. Geometrical parameters (bond lengths in Å and
angles in deg) of model carbene (CO)5CrdCHNdCH2 (10).
Supporting Information). In 7, the regiochemistry was
1
assigned on the basis of the analogy of their H and 13C
NMR spectra with that of 9.
The reaction proceeds in a synthetically useful way,
especially in those cases where the alkyne has electron-
donating groups. The three group VI metals were tested,
and the same qualitative results were found, as all three
are capable of producing the cycloaddition with alkynes.
However, from a practical point of view, the use of
chromium carbenes is recommended, due to the higher
yields obtained and the stability of the starting carbene.
The final products can be modified by changing the
carbenic carbon substituent and the alkyne substituents.
This way, the 2H-pyrrole obtained can bear different
substituents at the 3, 4, and 5 positions. Isomerization
to pyrrole is prevented by the two phenyl groups located
on the initial carbene’s iminic carbon.24 The regiochem-
istry of the reaction was determined by X-ray diffraction
of one of the 2H-pyrrole derivatives formed (9, see the
In an effort to work out the reaction mechanism, we
looked for reaction intermediates at shorter reaction
times. Unfortunately, we were unable to find any species
that could help to clarify the mechanism. At this point,
we decided to try studying the mechanism of the photo-
cycloaddition by means of theoretical calculations.
Th eor etica l Resu lts
The structures for all the minima and transition states
described in the paper are shown in the Supporting
Information, with the energies in Table 2. As the first
test for the accuracy of the calculations, we optimized
the geometry of a model compound of the carbene
complexes used. The most relevant geometrical param-
eters of the optimized structure of (CO)5CrdCHNdCH2
(10) are given in Figure 1. The CrdC(carbene) distance
is 2.076 Å, and the Cr-CO distances are between 1.874
and 1.897 Å, which reflect the weaker π-acceptor char-
acter of the carbene ligand as compared to CO. The
C(carbene)-N and N-C(imine) bond lengths (1.266 and
(22) Schlegel, H. B. J . Comput. Chem. 1982, 3, 214.
(23) Hehre, W. J .; Radom, L.; Schleyer, P. V. R.; Pople, J . A. Ab Initio
Molecular Orbital Theory; J . Wiley: New York, 1986.
(24) Details about the reaction of imine-carbene complexes bearing
different substituents on this position will be published elsewhere.
4676 J . Org. Chem., Vol. 68, No. 12, 2003

Irradiation of Imine-Group VI Carbene Complexes
SCHEME 3. F or m a tion of Meta lla cycle 12
1.267 Å, respectively) are those of an NdC double bond
(1.28 Å), and consistently, the C-N-C bond angle is
174.4°. The calculated structure is in good agreement
with the crystal structure of imine-carbene complex 1
(see the Supporting Information). In the molecular
structure of complex 1, distances of 2.113, 1.259, and
1.282 Å are found for the CrdC(carbene), C(carbene)-
N, and N-C(imine) bond lengths, respectively, together
with a C-N-C bond angle of 167.5°.
coordinated complex 11 to the insertion product 12 also
has a low energy barrier, in this case 7.4 kcal/mol. The
structures for 11, TS-12, and 12 are also shown in
Scheme 3. The alkyne moiety is completely symmetrical
in the complex 11, with both Cr-C(alkyne) distances
equal to 2.249 Å. The coordination with the metal center
clearly deforms the alkyne geometry, producing a C7-
C8 distance of 1.254 Å and a H-C7-C8 angle of 156.9°,
while the carbene ligand maintains its geometry. From
11, the main geometrical change needed to arrive at TS-
12 is the rotation of the alkyne ligand. In 11, the dihedral
angle between the planes of the two ligands is almost
90°, with distances between the carbenic carbon C2 and
the two alkyne carbons C7 or C8 of 2.923 Å. In TS-12,
the alkyne ligand is rotated to form an almost planar
substructure with the carbenic carbon and the metal
center. This rotation is needed to permit the interaction
between the two ligands, alkyne and carbenic carbon. The
bond distances give an idea about the progression of the
alkyne insertion. With regard to the alkyne ligand, one
Cr-C distance increases, while the other distance de-
creases. At the same time, the C7-C8 distance lengthens
from 1.254 to 1.290 Å. Clearly, the rotation of the alkyne
ligand causes a change in the bond order between it and
the chromium atom. The symmetry in compound 12 is
broken, and one alkyne carbon, C7, interacts strongly
with the metal while the other, C8, approaches the
carbene carbon C2. In addition, the carbene moiety is also
altered by the increasing interaction with the alkyne. It
must also rotate to permit the bond formation, and its
linearity decreases as the conjugation is broken due to
formation of the C8-C2 bond. The insertion process
progresses with the lengthening of the C2-Cr and C7-
C8 distances and the shortening of the two forming bonds,
C7-Cr and C8-C2.
Alk yn e In ser tion . Having checked the accuracy of the
method used for the calculations, at least with regard to
the structure of the carbene complexes, we aimed to
explore the reaction between the carbene complexes and
the alkynes. As discussed above, we studied the reaction
on the ground state and assume that the role of the light
on this process is restricted only to producing the empty
coordination position needed for the alkyne.25 Therefore,
the first step on the reaction path should be the substitu-
tion of one of the CO ligands by an alkyne, as shown by
the experiment of ligand photosubstitution with phos-
phine.13 This is in agreement with the two first steps on
the proposed mechanism for the Do¨tz benzannulation,
the loss of a CO ligand26 followed by the alkyne coordina-
tion.27 Taking this into account, we calculated the reac-
tion path from the alkyne-coordinated carbene complex
11 to the final 2H-pyrrole formation. The first step on
the calculated path is the insertion of the alkyne onto
the metal-carbon bond (Scheme 3). This step also
appears in the proposed mechanism for the benzannu-
lation and has been found by theoretical calculations to
be barrierless28 or with low energy barriers.29 As a
consequence, the alkyne complex 11 is taken to be a very
elusive intermediate that immediately rearranges to give
the insertion product 12. Our calculations show that the
transition state TS-12 that leads from the alkyne-
At this point, previous suggestions and studies on this
kind of reaction clearly differ from the present work, due
to the structure of the insertion product. The postulated
intermediate corresponds to a new carbene complex
formed by complete insertion of the alkyne into the
carbenic bond.29b,30 However, as shown in Scheme 3, the
structure for the intermediate which appears after the
alkyne coordination is that of a η3 vinylcarbene complex,
(25) Arrieta, A.; Coss´ıo, F. P.; Ferna´ndez, I.; Go´mez-Gallego, M.;
Lecea, B.; Manchen˜o, M. J .; Sierra, M. A. J . Am. Chem. Soc. 2000,
122, 11509.
(26) Fischer, H.; Mu¨hlemeier, J .; Ma¨rkl, R.; Do¨tz, K. H. Chem. Ber.
1982, 115, 1355.
(27) Do¨tz, K. H.; Siemoneit, S.; Hohmann, F.; Nieger, M. J . Orga-
nomet. Chem. 1997, 541, 285.
(28) Torrent, M.; Duran, M.; Sola`, M. Organometallics 1998, 17,
1492.
(29) (a) Gleichmann, M. M.; Do¨tz, K. H.; Hess, B. A. J . Am. Chem.
Soc. 1996, 118, 10551. (b) Hofmann, P.; Ha¨mmerle, M. Angew. Chem.,
Int. Ed. Engl. 1989, 28, 908.
(30) Do¨tz, K. H. Angew. Chem., Int. Ed. Engl. 1984, 23, 587.
J . Org. Chem, Vol. 68, No. 12, 2003 4677

Campos et al.
minimum on the potential energy surface for the reaction
of imine-carbene complexes with alkynes. The main
difference is the location of the N atom, which is directly
bonded to the carbenic carbon on structure 12 but via
the O atom in Sola`’s proposal. It may be that the
nitrogen’s lower electronegativity could permit an in-
creased sharing of its lone pair, in comparison with the
oxygen atom. The effect of the N atom substitution causes
a decrease in the Cr-C(carbene) distance from 2.630 Å,
on the structure calculated by Sola`, to 2.497 Å on 12.
Furthermore, the two distances between the three car-
bons bonded to the chromium atom are slightly shorter
for 12 (from 1.428 and 1.415 Å to 1.426 and 1.406 Å for
C7-C8 and C8-C2 in 12, respectively), suggesting a
higher bond order between the four atoms forming the
metallacyclobutene. These changes may account for the
increased stability of our complex, which therefore be-
comes a real minimum on the reaction course. As an
additional point, some years ago the Barluenga31 group
was able to isolate and fully characterize by X-ray
diffraction a carbene complex with similar structure, also
with a N atom next to one of the C atoms directly bonded
to the metal center. This result clearly shows that the N
atom causes a stabilization on this kind of intermediates,
which become stable enough to be trapped. On the other
hand, the absence of the N atom leads to a higher energy
intermediate, as Sola`’s calculations show, causing the
reaction path to diverge at this point from the one
proposed by the Do¨tz benzannulation.
Next, we were interested in the influence of the metal
on the reaction. It is not unusual to find a quite different
behavior between the complexes of these two metals in
the literature.1 Cases have been reported where the
product distribution differs greatly according to the use
of W or Cr as the metal center.33 However, the reaction
between imine-carbene complexes and alkynes shows
that the synthesis and reactivity of imine-carbene com-
plexes with tungsten or chromium are broadly similar,
so allowing a wider applicability range on a synthetic
way. The similarity could mean that the metal does not
exert a strong influence on the reaction path, that is, that
the key intermediates have similar structures. To test
this hypothesis we computed the analogue complex 12-
W, replacing the chromium atom with a tungsten atom.
The resulting structure is very similar, apart from the
slightly longer W-C distances due to the metal’s differing
characteristics (see the Supporting Information).
F IGURE 2. Geometrical parameters (bond lengths in Å) of
imine-carbene complex 12.
where alkyne insertion has not completed; that is, the
alkyne and carbenic carbons (C7, C8, and C2) are bonded
to the metal center. We propose that it is this, and not
the η1 carbene formed through a complete insertion, that
is the key intermediate of the 2H-pyrrole formation.
However, this kind of intermediate has not received much
attention despite the fact that it has been found both
experimentally31 and theoretically.32
The structure of the intermediate 12 shows that three
carbon atoms are coordinated to the metal, although the
degree of bond order varies greatly (Figure 2). The shorter
distance (1.874 Å) corresponds to one of the C atoms from
the alkyne (C7). This distance is within the range for a
CrdC bond, which suggests that the insertion of the
alkyne into the CrdC bond is almost completed. How-
ever, the other two Cr-C distances (2.281 and 2.497 Å
for C8 and C2, respectively) also indicate an interaction
between the atoms, thus suggesting that the insertion
has not proceeded completely. The angles around C2 and
C8 have values close to those of sp2 (125.2° for N9-C2-
C8 and 119.7° for the C7-C8-C2), although the three C
atoms clearly show pyramidalization to some extent. In
the same way, the distances between the C atoms (C2-
C8 1.406 Å and C7-C8 1.426 Å) show values between the
typical CdC and C-C bond distances. The carbene
moiety is twisted with a dihedral angle Cr-C7-C8-C2
of 52.8°, as against a twisting of the imine group through
a 48.4° dihedral angle on the C8-C2-N9-C17 subsystem,
thus making π-conjugation throughout the molecule
difficult. Consistently, the bond distances around the N9
(1.393 and 1.282 Å) clearly correspond to single and
double N-C bond distances, respectively.
In a recent paper,32 Sola` et al. found a minimum with
a similar structure as an intermediate of the Do¨tz
reaction. However, far from being the global minimum
in the reaction’s first steps, it is located 11.8 kcal/mol
higher in energy than the corresponding η1 complex, with
a 16.0 kcal/mol barrier to be passed. The reaction
mechanism proposed by Sola` consists of the formation
of the more stable η1 complex, which later rearranges to
the less stable η3 complex, finally progressing to yield the
benzannulation product. However it is worth pointing out
that this path supposes higher energy that the one the
authors proposed as the preferred one. In contrast with
Sola`’s results, structures such as 12 appear to be a
On the other hand, as discussed above, the reaction is
completely regiospecific (see Table 1, entries 4, 6, and
7). The alkyne moiety is completely symmetric in complex
11, while in the alkyne insertion product 12, this sym-
metry is broken. This step could therefore be responsible
for the reaction’s regiochemistry. Once the alkyne com-
plex has been formed, the presence of different groups
on the alkyne will lead to different insertion products,
causing the generation of two different products with
opposite regiochemistries. The experimental results sug-
gest that one of these products should be favored in some
way. To test this hypothesis, we calculated the two
insertion products produced from the model carbene
(31) Barluenga, J .; Aznar, F.; Mart´ın, A.; Garc´ıa-Granda, S.; Pe´rez-
Carren˜o, E. J . Am. Chem. Soc. 1994, 116, 11191.
(32) Torrent, M.; Duran, M.; Sola`, M. J . Am. Chem. Soc. 1999, 121,
1309.
(33) For some examples see: (a) Bertolini, T. M.; Nguyen, Q. H.;
Harvey, D. F. J . Org. Chem. 2002, 67, 8675. (b) Barluenga, J .; Aznar,
F.; Mart´ın, A.; Barluenga, S. Tetrahedron 1997, 53, 9323. (c) Harvey,
D. F.; Lund, K. P. J . Am. Chem. Soc. 1991, 113, 5066.
4678 J . Org. Chem., Vol. 68, No. 12, 2003

Irradiation of Imine-Group VI Carbene Complexes
F IGURE 3. Geometrical parameters (bond lengths in Å) and absolute (hartrees) and relative (in parentheses, kcal/mol) energies
of the two regioisomeric products of the alkyne HCtCOH insertion.
complex and H-CtC-OH. We then used this to repro-
duce the reaction between carbene complex 1 and ethynyl
ethyl ether (see Table 1, entry 6). We were able to find
the two structures (13 and 14) analogous to the compound
12, shown in Figure 3.
kcal/mol more stable than 14-v (see Table 2), and the
bond distances show values similar to those for 13 and
14.
Once the metallacyclobutene intermediate has been
reached, and given that the reaction yields a 2H-pyrrole
derivative as the only detectable product, a ring enlarge-
ment should occur between this point and the final
product formation. Also, a reductive elimination should
occur to generate the organic compound without the
metal part. Three different possibilities were considered,
depending on the chronological order of these steps: ring
enlargement, reductive elimination, and the regiochem-
istry of the ring enlargement. The three possible paths
are shown in Scheme 4. In routes B and C, the ring
enlargement takes place immediately after the metalla-
cyclobutene formation, generating a six-membered met-
allacycle, which, in a subsequent step, undergoes the
reductive elimination; there is a difference between the
two routes as regards the regiochemistry of the ring
enlargement. In route A, the ring enlargement takes
place after the reductive elimination. Taking the metal-
lacyclobutene 12 as a starting point, we calculated these
three paths, including the transition structures involved.
As can be seen in Scheme 4, although the three paths
have to go through transition structures of a comparable
height (19.2 kcal/mol for route A, 24.3 kcal/mol for route
B, and 20.0 kcal/mol for route C), the main difference
between them is the relative energies of the final
products. While the cyclopropene 15 generated through
route A is 8.5 kcal/mol higher in energy than the previous
intermediate 12, the final 2H-pyrrole 18 formed through
route B is 41.4 kcal/mol more stable. This difference
should be enough to lead to the formation of the pyrrole
derivatives through route B, producing a ring enlarge-
ment of compound 12 followed by a reductive elimination.
Finally, 17 is 37.4 kcal/mol less stable than 18.
The two intermediates have similar structures; how-
ever, they clearly differ in the bond distances of the
metallacyclobutene. The main difference seems to be the
degree of progression of the alkyne insertion, with
structure 13 showing almost complete insertion, with a
Cr-C2 distance of 2.583 Å. Also, the two bonds around
C8 have rather different distances (1.449 and 1.392 Å)
indicating a poor delocalization along the cycle. On the
other hand, 14 shows bond lengths more similar to the
compound 12, with shorter Cr-C2 distance (2.406 Å) and
similar bond distances around C8 (1.412 and 1.430 Å).
The effect of introducing an oxygen atom can be clearly
seen from the C-O distances. In 13, the C7-O14 is 1.334
Å, while the corresponding distance (C8-O14) in 14 is
1.384 Å. This variation illustrates the true difference
between these two systems; when the oxygen atom is
located next to the carbon atom close to the metal, this
contributes to a net stabilization of the compound. This
is a well-known feature of the Fischer carbene complexes,
where the placement of an atom with a lone pair (O, N,
S, etc.) onto the carbenic carbon allows the presence of a
resonance form which greatly increases the stability of
the compound.1 In this case, the presence of the O atom
causes a major difference in the stability of the two
insertion products. The difference of 14.1 kcal/mol should
be enough to lead to the formation of only one insertion
regioisomer. Thus, this result can account for the product
distribution found experimentally, where just one regio-
isomer was found in the case of the reaction with ethynyl
ethyl ether. Equally, substitution of the hydroxyl by a
vinyl group (HCtCCHdCH2 as alkyne model) affords the
same results. We calculated the equivalent products 13-v
and 14-v (see the Supporting Information), where we
tried to mimic the influence of the phenyl groups using
vinyl substituents. The same regiochemistry is also
expected for conjugated alkynes such as phenyl acetylene
used in entry 4, Table 1. Compound 13-v is around 13
Th e Cyclop r op en e Rou te. The transition state for
the route A, TS-15, leading to the cyclopropene 15
presents a structure related to that of the intermediate
12, but is placed 19.2 kcal/mol above. The bond distances
show the movements needed to complete the reductive
elimination. The C7-C8 distance is clearly shorter (1.346
Å) with a characteristic CdC value. This is caused by
J . Org. Chem, Vol. 68, No. 12, 2003 4679

Campos et al.
SCHEME 4. Th r ee Rou tes Sta r tin g fr om Com p ou n d 12a
a
Relative energies in kcal/mol.
the absence of conjugation resulting from the breakage
of the Cr-C2 bond at the same time as the C7-C2 bond
is forming. The shorter Cr-C2 distance is simply a
geometrical requirement which permits the formation of
the cyclopropene. The metallacycle breakage can also be
seen in the return to planarity of the three C atoms
forming the cycle and the metal, again a requirement for
cyclopropene formation.
The coordinative needs of the chromium atom in 15
are fulfilled by coordination of the nitrogen atom N9. The
last coordination site is partially taken up by the cyclo-
propene unit. However, the ring-closing geometry means
that the double bond C7-C8 does not have the right
orientation to allow the bonding with the metal. In this
situation, it is the newly formed C7-C2 bond which fills
this site. This can be clearly seen by comparing the two
C-C single bond distances of the cyclopropene moiety
(C2-C8 1.492 Å and C2-C7 1.547 Å). The second one is
too large for this kind of compound and is a consequence
of the coordination to the metal. Intermediate 15 could
evolve via two different routes: first, a direct ring
enlargement to give 18, or second, by analogy with the
reaction with alkenes (Scheme 1),7 so involving the loss
of the metal part followed by ring enlargement to afford
the 2H-pyrrole.34 The first possibility was discounted
since we were unable to find any transition structure
connecting 15 and 18. As an indirect proof for the second
alternative, and taking into account that no product with
the cyclopropene skeleton could be isolated or detected in
the reaction crude, we synthesized the iminecyclopropene
(34) The photorearrangement of vinylcyclopropene and related
compounds to yield five-membered rings is known. See, for example:
(a) Ziemmerman, H. E.; Wright, C. H. J . Am. Chem. Soc. 1992, 114,
6603. (b) Zimmerman, H. E.; Fleming, S. A. J . Org. Chem. 1985, 50,
2539. (c) Padwa, A.; Blacklock, T. J .; Getman, D.; Hatanaka, N. J . Am.
Chem. Soc. 1977, 99, 2344.
4680 J . Org. Chem., Vol. 68, No. 12, 2003

Irradiation of Imine-Group VI Carbene Complexes
SCHEME 5. Test for th e Cyclop r op en e-P yr r ole
Compound 16 has two similar Cr-C bond distances
(Cr-C7, 2.108 Å, and Cr-C2, 2.091 Å). This indicates a
similar bond order between the metal center and the two
carbon atoms. The situation is different in the case of
compound 17, where one of the Cr-C distances is clearly
longer than the other (Cr-C7, 1.983 Å, vs Cr-C17, 2.277
Å). These values suggest different natures for the two
complexes. While compound 16 has two single Cr-C
bonds, compound 17 presents a double CrdC7 bond and
a σ interaction with C17, which is slight but nonetheless
clear enough to be noted via the pyramidalization of the
C. At this point, the fate of these two compounds can be
postulated on the basis of the bond order found. The
compound 16 could yield the corresponding 2H-pyrrole
by reductive elimination of the metal center. The two σ
Cr-C bonds would have to be involved in forming the
new C2-C7 bond, which yields the final product. This
means that compound 16 would be connected with the
final product 18 by a transition state 0.2 kcal/mol higher
in energy. On the other hand, compound 17 could not
give this reductive elimination due to its different bond
distribution; thus the pyrrole formation path is blocked.
This can be understood by taking a closer look at the
structure. The reductive elimination needed to yield the
pyrrole derivative is prevented by the slight interaction
of the metal center with C17. Indeed, the bond order
between these two atoms is quite small due to the agostic
interaction between one hydrogen atom on C17 and the
metal (the Cr-H15 distance is 1.891 Å, which is a normal
value for this kind of interactions).35 This causes the C17-
H15 bond to lengthen considerably: 1.153 Å for the C17-H
distance with the hydrogen involved in the agostic
interaction vs 1.096 Å for the other C17-H distance in
the same carbon atom. By the way of confirmation, we
were unable to find any transition state connecting 17
with the final product 18. In summary, route C gives
metallacycle 17, whose further development is blocked.
Route B leads to metallacycle 16, which can evolve to
the most stable product 18. Thus, once 16 is formed, 18
is easily produced by reductive elimination of the metal
atom, through a transition state 14.6 kcal/mol higher in
energy than 12, that is, only 0.2 kcal/mol above 16. This
means that after the rearrangement from 12 to 16 takes
place, the 2H-pyrrole formation should be immediate. 18
is the most stable species on the reaction path, and its
formation should be the driving force for the reaction
completion. Looking at the energies of the different
transition structures, TS-12 is the highest one. Once the
first and highest transition state (TS-12) is passed, the
product distribution should be consistently under ther-
modynamic control. This causes the reaction to be
controlled by the relative energies of the final products,
that is, the reaction is directed toward the formation of
the more stable species, the 2H-pyrrole derivative with
the regiochemistry detected.
Rea r r a n gem en t
19 and we irradiated it under the same reaction condi-
tions (Scheme 5). After some time of irradiation we
obtained a complex crude, with tolane as the main
product, together with some other unidentified com-
pounds, none of them with pyrrole structure. This result
allows us to reject the hypothesis of formation of 2H-
pyrrole derivatives through the mechanism sketched on
route A, insofar as (a) 15 is clearly less favorable than
17 or 18; (b) we were unable to find any TS which could
explain the pyrrole formation from 15; (c) we were unable
to isolate or detect any compound with a cyclopropene
structure on the reaction crude; (d) the synthesized
imine-cyclopropene did not rearrange under the reaction
conditions. We see here a clear distinction between the
reaction of imine-carbene complexes with alkenes and
with alkynes. In the former, the five-membered cycle is
generated through two consecutive reactions and the
intermediate cyclopropane can be isolated in some cases.
In the latter, the pyrrole derivatives are formed directly,
and no intermediate can be trapped.
Th e Meta lla d ien e Rou tes. Routes B and C start from
compound 12 and give two different metallacycles through
two different ring enlargements (see Scheme 4). The
difference between these two possible pathways is the
direction of the ring enlargement. We computed both
routes from compound 12 to give the six-membered
metallacycle in two forms, 16 and 17. Scheme 4 shows
the two different cycles, together with the transition
structures leading to them. As can be seen, the two
possibilities clearly differ in the relative energies of both
minima and transition structures. The two isomeric
metallaazahexadienes differ as regards connectivity. In
compound 16, the ring enlargement takes place after the
approach of the imine moiety to the cycle from the
direction opposite to the metal center, with formation of
a C17-C8 bond. The corresponding transition state TS-
16 shows a structure intermediate between the two
minima 12 and 16. The bond distances around the N9
atom reflect these alterations: while the iminic bond
lengthens from a C17-N9 value of 1.282 Å in 12 to a C17-
N9 distance of 1.306 Å in TS-16 due to the bond formation
between the iminic carbon (C17) and C8 in the cycle, the
C2-N9 distance shortens from 1.393 to 1.266 Å, showing
the formation of a CdN double bond. At the same time,
C2, approaches the metal center, forming a σ-bond (Cr-
C2 2.497 Å in 12, 2.164 Å in TS-16, and 2.091 Å in 16).
However, formation of compound 17 requires a more
complex conformational change. The imine moiety has
to rotate in order to face the metal center. This causes a
minor modification in this part of the molecule in the TS
(distances around N9 of 1.282 Å in 12 vs 1.285 Å in TS-
17 and 1.393 Å in 12 vs 1.318 Å in TS-17), while the
more important change consists of the removal of one of
the C atoms in the cycle (C2) from the vicinity of the metal
center, needed to create a coordinative place for the
introduction of the approaching iminic carbon (C17) close
to the metal.
The energy profile for this reaction is shown in Scheme
6. As discussed before, only one regioisomer was found
on the reaction crude; the reaction path as calculated
explains this and the other experimental findings. Sev-
eral possibilities open up after the alkyne insertion;
according to our calculations, the reaction then proceeds
through the metallacycle route, with formation of 16,
(35) Hall, C.; Perutz, R. N. Chem. Rev. 1996, 96, 3125.
J . Org. Chem, Vol. 68, No. 12, 2003 4681

Campos et al.
SCHEME 6. En er gy P r ofile for th e Rea ction of Im in e-Ca r ben e Com p lexes w ith Alk yn esa
a
Relative energies in kcal/mol.
127.8, 128.0, 128.2, 128.3, 128.6, 128.7, 131.2, 132.4, 135.5,
136.7, 138.6, 140.8, 141.2, 147.4, 148.2, 170.1 (CdN); GC-MS
447 (M, 15), 370 (8), 292 (27), 252 (23), 165 (100), 139 (16),
103 (15), 77 (79), 51 (63); ES (+) 448 (M + 1); IR ν 3053 (m),
1712 (s), 1626 (m), 1596 (m), 1577 (m), 1492 (m) cm-1. Anal.
Calcd for C34H25N: C, 91.24; H, 5.63; N, 3.13. Found: C, 91.20;
H, 5.61; N, 3.19.
from which the most stable product 18 should be im-
mediately obtained.
Con clu sion s
Experimental studies and high level theoretical cal-
culations were done in order to explore the reactivity
between imine-carbene chromium complexes and alkynes.
Despite the existence of different reaction paths, the
relative energies of the intermediates and products
involved can account for the product distribution and the
regiochemistry experimentally obtained. This reaction
has been shown to be an easy and versatile doorway to
2H-pyrrole derivatives, as long as the regiochemistry of
the final products can be controlled. A different mecha-
nism from that previously proposed for the reaction with
alkenes is suggested. Despite the apparent similarities
between the reaction of imine-group VI carbene com-
plexes in the presence of alkynes and the Do¨tz reaction,
which include some of the intermediates in common, the
calculations show that the reaction mechanisms differ
immediately after the alkyne insertion. Once the light
opens up a coordination site by CO dissociation, the
reaction proceeds through the alkyne insertion, followed
by ring enlargement and reductive elimination. Once
again, Fischer carbene complexes show their complexity.
Related starting materials (alkenes and alkynes) give
related products (pyrrolines and 2H-pyrroles) through
quite different mechanisms.
2-Meth yl-3,5,5-tr ip h en yl-2H-p yr r ole (7): yellow solid;
1
yield 33 mg, 43%; H NMR δ 2.38 (s, 3H, CH3), 6.77 (s, 1H,
CH), 7.22-7.36 (m, 15H, arom); 13C NMR δ 18.9, 90.4, 126.2,
126.3, 126.9, 127.2, 127.5, 127.8, 127.9, 128.1, 128.3, 128.4,
128.6, 128.7, 133.5, 139.3, 170.7 (CdN); GC-MS 309 (M, 28),
265 (5), 230 (8), 191 (15), 165 (100), 139 (12), 102 (72), 77 (92),
51 (95); ES (+) 310 (M + 1); IR ν 3684 (w), 3058 (m), 1684 (w),
1619 (m), 1599 (m), 1492 (m). Anal. Calcd for C23H19N: C,
89.28; H, 6.19; N, 4.53. Found: C, 89.34; H, 6.15; N, 4.51.
2-Met h yl-3,4-d i(m et h oxyca r b on yl)-5,5-d ip h en yl-2H -
1
p yr r ole (8): yellow oil; yield 24 mg, 23%; H NMR δ 2.47 (s,
3H, NdCCH3), 3.69 (s, 3H, CO2CH3), 3.88 (s, 3H, CO2CH3),
7.1-7.4 (m, 10H, arom); 13C NMR δ 18.3 (NdCCH3), 52.4
(CO2CH3), 52.6 (CO2CH3), 92.3 (CPh2), 127.9, 128.0, 128.1,
128.1, 128.2, 128.3, 128.5, 134.3, 135.6, 138.2, 163.5 (CdO),
163.7 (CdO), 167.9 (CdN); GC-MS 349 (M, 43), 258 (35), 230
(29), 189 (28), 165 (10), 127 (56), 102 (15), 77 (39), 59 (100);
ES (+) 350 (M + 1); IR ν 3680 (w), 1737 (s), 1728 (s), 1639 (m)
cm-1
.
2-Meth yl-3-eth oxy-5, 5-d ip h en yl-2H-p yr r ole (9): yellow
solid; yield 13 mg, 73%; 1H NMR δ 1.38 (t, J ) 7 Hz, 3H,
CH2CH3), 2.26 (s, 3H, CH3CdN), 3.94 (c, J ) 7 Hz, 2H, CH2-
CH3), 6.40 (s, 1H, CH), 7.1-7.5 (m, 10H, arom); 13C NMR δ
14.4 (CH2CH3), 15.6 (CH3CdN), 66.2 (CH2CH3), 83.2 (CPh2),
119.8 (CdCO), 126.8, 127.4, 128.0, 142.9, 154.6 (CdCO), 169.1
(CdN); GC-MS 277 (M, 48), 248 (23), 207 (18), 179 (100), 152
(8), 77 (15); ES (+) 278 (M + 1). Anal. Calcd for C19H19NO: C,
82.28; H, 6.90; N, 5.05. Found: C, 82.38; H, 6.84; N: 5.00.
P r ep a r a t ion of Cyclop r op en e 19. Triphenylcyclopro-
penium tetrafluoroborate34b (1 mmol) was dissolved in 50 mL
of dry MeCN. One equivalent of the PhCHO was added. After
bubbling NH3, the mixture was stirred at room temperature
for 1 h. The solvent was removed with a rotary evaporator,
and the product was purified by flash column chromatography
(silica gel, hexane/Et2O). The irradiation procedure was analo-
gous to that previously described for N-cyclopropylimines.7b
3-(P h en ylm et h ylen ea m in o)-1,2,3-d ip h en ylcyclop r o-
Exp er im en ta l Section
Gen er a l P r oced u r e for Ir r a d ia tion . The carbene complex
(0.25 mmol) was dissolved in 50 mL of deoxygenated hexane.
Ten equivalents of alkyne was added, and the mixture was
irradiated at room temperature under an Ar atmosphere,
through a Pyrex glass with a 125 W medium-pressure mercury
lamp, until the carbene was consumed (TLC, hexane/CH2Cl2
1:1). The solvent was removed with a rotary evaporator, and
the crude product was filtered through Celite to remove
chromium residues. The product was purified by column
chromatography (silica gel, hexane/Et2O).
1
p en e (19): yellow oil; yield 13 mg, 53%; H NMR δ 7.1-7.5
(m, 12H, arom.), 7.6-7.7 (m, 6H, arom.), 7.8-7.9 (m, 2H,
arom.), 8.39 (s, 1H); 13C NMR δ 116.6, 126.8, 127.4, 128.0,
129.4, 130.0, 130.5, 130.9, 131.8, 132.5, 132.7, 135.7, 142.9,
154.6,156.1 (CdN); ES (+) 360 (M + 1).
Cr yst a llogr a p h ic Da t a for P en t a ca r b on yl[m et h yl-
(d ip h en ylm eth ylen ea m in o)m eth ylen e]ch r om iu m (1). An
orange prismatic crystal of 1 (C20H13CrNO5, MW) 399.32) was
transferred to the goniostat. The data were collected at room
temperature with an Enraf Nonius apparatus with κ geometry
and an electronic CCD detector. It was determined to have
monoclinic symmetry with a P2 space group. Unit cell param-
2-Meth yl-3,4,5,5-tetr aph en yl-2H-pyr r ole (5): yellow solid;
yield 49 mg, 51%; 1H NMR δ 2.30 (s, 3H, CH3), 6.8-7.5 (m, 20
H, arom); 13C NMR δ 18.4 (CH3), 90.1 (CPh2), 127.3, 127.6,
127.7, 127.8, 128.1, 128.3, 128.5, 128.6, 128.7, 129.3, 129.5,
130.1, 130.8, 134.0, 134.8, 139.5, 139.7, 164.8, 171.9 (CdN);
GC-MS 385 (M, 8), 384 (100), 344 (7), 305 (10), 265 (19), 153
(21); ES (+) 386 (M + 1). Anal. Calcd for C29H23N: C, 90.35;
H, 6.01; N, 3.63. Found: C, 90.55; H, 6.10; N, 3.35.
2,3,4,5,5-P en ta p h en yl-2H-p yr r ole (6): yellow solid; yield
1
55 mg, 49%; H NMR δ 6.95-7.88 (m, 25H, arom); 13C NMR
δ 92.2 (CPh2), 119.1, 123.5, 125.5, 126.4, 126.5, 127.5, 127.7,
4682 J . Org. Chem., Vol. 68, No. 12, 2003

Irradiation of Imine-Group VI Carbene Complexes
eters are a ) 12.268 Å, b ) 7.823 Å, c ) 19.884 Å, and â )
91.20°, and Dcalcd ) 1.309 g/cm3 for Z ) 4. A total of 4255
unique data were collected for 1.93° g 2θ g 27.44°. The
structure was solved by direct methods using SHELX97 with
a GOF ) 1.473. Final residuals are R(F) ) 0.055 and Rw(F) )
0.1545.
Cr ysta llogr a p h ic Da ta for 2-Meth yl-3-eth oxy-5, 5-d i-
p h en yl-2H-p yr r ole (9). A colorless prismatic crystal of 9
(C19H19NO, MW ) 277.37) was transferred to the goniostat.
The data were collected at room temperature with an Enraf
Nonius apparatus with κ geometry and an electronic CCD
detector. It was determined to have triclinic symmetry with a
P-1 space group. Unit cell parameters are a ) 8.856 Å, b )
8.856 Å, c ) 22.924 Å, R ) 89.43°, â ) 89.42°, and γ ) 62.97°,
and Dcalcd) 1.15 g/cm3 for Z ) 4. A total of 4521 unique data
were collected for 0.99° g 2θ g 21.04°. The structure was
solved by direct methods using SHELX97 with a GOF ) 1.344.
Final residuals are R(F) ) 0.082 and Rw(F) ) 0.2133.
Ack n ow led gm en t. This work was supported by
Spanish MCyT/FEDER (BQU2001-1625) and Univer-
sidad de La Rioja (API-01/B07). D.S. thanks the Spanish
Comunidad Auto´noma de La Rioja for a fellowship.
Su p p or tin g In for m a tion Ava ila ble: Cartesian coordi-
nates of all minima and transition structures 10-18, X-ray
data and ORTEP views of compounds 1 and 9, and general
experimental comments. This material is available free of
charge via the Internet at http://pubs.acs.org.
J O0343073
J . Org. Chem, Vol. 68, No. 12, 2003 4683