Light-induced aminocarbene to imine dyotropic rearrangement in a chromium(0) center: An unprecedented reaction pathway

Article abstract of DOI:10.1021/ja035614t

1,2-Dyotropic rearrangement can be induced by irradiation of properly functionalized Fischer carbenes. This novel reaction takes place by a stepwise mechanism and with double inversion of configuration at the static scaffold. Good yields are obtained with both cyclic and acyclic structures, thus suggesting that this unprecedented transformation can be extended to other organometallic systems. Copyright

Full text of DOI:10.1021/ja035614t

Published on Web 07/22/2003
Light-Induced Aminocarbene to Imine Dyotropic Rearrangement in a
Chromium(0) Center: An Unprecedented Reaction Pathway
Miguel A. Sierra,*,† Israel Ferna´ndez,† Mar´ıa J. Manchen˜o,† Mar Go´mez-Gallego,†
M. Rosario Torres,‡ Fernando P. Coss´ıo,*,§ Ana Arrieta,§ Begon˜a Lecea,| Ana Poveda, and
Jesu´s Jime´nez-Barbero#
Departamento de Qu´ımica Orga´nica and Laboratorio de Difraccio´n de Rayos X, Facultad de Qu´ımica,
UniVersidad Complutense, 28040-Madrid, Spain, Kimika Fakultatea, Euskal Herriko Unibertsitatea, P.K. 1072,
28080 San Sebastia´n-Donostia, Spain, Farmazi Fakultatea, P.K. 450, 01080 Vitoria-Gasteiz, Spain,
SIDI-UniVersidad Auto´noma, Cantoblanco, 28053-Madrid, Spain, and Centro de InVestigaciones Biolo´gicas (CSIC),
Vela´zquez 144, 28006-Madrid, Spain
Received April 14, 2003; E-mail: sierraor@quim.ucm.es; qopcomof@sq.ehu.es
Photochemistry of chromium(0)carbene complexes1 is one of the
fundamental pillars on which the synthetic versatility2 of these
compounds is based. The usefulness of the light-induced reactions
of these complexes stems from their ability to reversibly generate
ketene-like intermediates,3 initially postulated in 1988 by D’Andrea
and Hegedus. Since then, the different efforts directed toward the
detection of these elusive intermediates have been fruitless.4,5
Recently, we reported a theoretical-experimental study on the
mechanism of the photoreaction of group 6 alkoxychromium(0)-
carbene complexes with imines.6 Further work by our group resulted
in the light-induced carbene transfer from group 6 carbene
complexes to olefins.7,8 We have now studied the photochemistry
of complexes 1 having the carbene heteroatom substituent tethered
to the metal nucleus by a phosphine ligand and a carbon chain of
variable length and rigidity. Reported herein is the discovery of a
new photochemical aminocarbene to imine dyotropic rearrangement.
Complexes 1 were prepared by intramolecular ligand displace-
ment on aminocarbene complexes 2 (boiling hexanes/benzene, 1:1
mixtures).9 These latter compounds were obtained by aminolysis
of pentacarbonyl[methoxymethylcarbene]chromium(0), 3, with the
corresponding aminophosphines 4 (Figure 1).
Figure 1.
plex) being the static scaffold. Computational studies11-13 on the
transformation of model chromium(0)carbene 1d into coordinated
imine 5d showed that the 1d f 5d reaction is exothermic by 14.7
kcal/mol at the ground state S0. Because in the experimentally stud-
ied imines 5a-c the methyl group is always syn to the chromium
atom (R2 ) Me, Scheme 1), we have calculated the structures and
energies of both syn- and anti-5e. Our calculations indicate that the
syn stereoisomer is 0.8 kcal/mol more stable than its anti analogue.9
Therefore, both the 1,2-dyotropic transformations and the double
inversion stereochemistry are thermodynamically favored processes.
Scheme 1
Irradiation (450 W Hg-medium-pressure lamp, Pyrex filter, and
Pyrex well) of complexes 1 in MeCN/MeOH solutions resulted in
their smooth conversion to new organometallic compounds that
lacked the carbene moiety (Scheme 1). A combination of 1D- and
2D-homonuclear (COSY and NOESY) and heteronuclear (1H-13C
HSQC, 1H-13C HMBC, 1H-31P HMBC, and 1H-15N HSQC)
experiments was performed to assign most of the 1H, 13C, and 31
P
signals of compounds 5b and 5c. The structure of the reaction pro-
ducts was finally resolved by X-ray diffraction analysis9 carried out
in a single monocrystal of 5c. Thus, instead of the photocarbonyl-
ation process, a new carbene to imine rearrangement has occurred.
The reaction occurs with tethers of different lengths (4a,b) and
rigidities (4c), showing that the rearrangement process is general.
Experimental results above indicate that chromium(0)carbenes
1a-c are transformed upon irradiation into N-metalated imines
5a-c instead of the usually proposed ketenes.1 This transformation
is formally a 1,2-dyotropic rearrangement of type 1 according to the
definition proposed by Reetz.10 Both C-Cr and N-H bonds migrate,
and the Cr and H atoms interchange their positions, the CdN moiety
(structure II contributes significantly to the description of the com-
We have found14 that the lowest energy excited state of 1d is a
triplet, with a vertical excitation energy of 2.66 eV (465.5 nm).
The first excited singlet state S1 lays 0.80 eV above the triplet. It
can be assumed that the dyotropic reaction starts with a short-lived
triplet state T1, formed after initial photoexcitation of the singlet
ground state So to the excited singlet state S1.15,16 The triplet state
of 1d has a C-Cr distance ca. 0.2 Å larger than that found at the
So state. In addition, the spin densities of both centers are 0.582
and 1.675 au, respectively, thus indicating that in this excited state
there is a significant contribution of the biradical structure depicted
in Scheme 2.
† Universidad Complutense.
‡ Difraccio´n de Rayos X, Universidad Complutense.
§ Kimika Fakultatea, Euskal Herriko Unibertsitatea.
| Farmazi Fakultatea, Euskal Herriko Unibertsitatea.
Universidad Auto´noma.
# CSIC.
9
9572
J. AM. CHEM. SOC. 2003, 125, 9572-9573
10.1021/ja035614t CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
work (CIF and PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
References
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We have not been able so far to locate and characterize a con-
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pathway is kinetically favored. Therefore, this pathway is in full
agreement with the experimental data. The rigidity imposed by the
cyclic structure of complexes 1 is not responsible for this new photo-
chemical reaction because the irradiation of complex 8 produced
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(9) See the Supporting Information for a full experimental procedure,
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calculations reported in this work.
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Scheme 3
(13) The LANL2 DZ effective core potential and basis set were used for chro-
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In conclusion, an unprecedented photochemical pathway alterna-
tive to the now classical Hegedus’s photocarbonylation has been
disclosed. Both theoretical and experimental results are in full agree-
ment with this new dyotropic rearrangement18 of the carbene ligand
to an imine ligand in complexes 1. Efforts directed to fully under-
stand the general mechanism of the photochemistry of group 6
metal-carbene complexes are now deeply active in our laboratories.
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(16) It has been reported experimentally that in photochemical reactions of
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(18) Another very interesting experimental and theoretical study of a stepwise
dyotropic rearrangement has been reported while this paper was under
revision. See: Zhang, X.; Houk, K. N.; Lin, S.; Danishefsky, S. J. J. Am.
Chem. Soc. 2003, 125, 5111.
Acknowledgment. Support for this work from grant BQU2001-
1283 (Madrid-group), grant BQU2001-0904 (San Sebastia´n-Donos-
tia group) from the Ministerio de Ciencia y Tecnolog´ıa (Spain),
and the Euskal Herriko Unibertsitatea (9/UPV 00040.215-13548/
2001) (San Sebastia´n-Donostia group) is gratefully acknowledged.
I.F. thanks the Ministerio de Educacio´n y Cultura (Spain) for a
predoctoral (FPU) grant.
Supporting Information Available: Full experimental details for
the preparation of all new compounds, X-ray characterization of
compound 5c, and full computational data for the calculations in this
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