The photochemical reactivity of the "photo-inert" tungsten (Fischer) carbene complexes

Article abstract of DOI:10.1002/anie.200501590

(Chemical Equation Presented) The light fantastic: Tungsten(0) Fischer carbene complexes, previously considered to be photo-inert, produce, by adequately choosing the ligands, three classes of photoproducts (see scheme). Thus, irradiation of tetracarbonyl

Full text of DOI:10.1002/anie.200501590

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to yield b-lactams began the synthetically useful photochem-
istry of group 6 (Fischer)carbene complexes. [2,3] Both alkoxy-
and aminocarbene complexes of chromium(0)are active in
these reactions.[4] The reversible formation of chromium-
bound ketenes by irradiation of chromium(0)carbene com-
plexes with visible light was further postulated based on
indirect evidence.[5] In contrast, tungsten(0)alkoxycarbene
complexes do not carbonylate, and all the previous exper-
imental data collected about the photochemical behavior of
these compounds have led them to be considered as photo-
inert.[6] Recently, we reported[7] a new photochemical dyo-
tropic rearrangement from aminocarbenes 1 to imines 2
(Scheme 1), which unambiguously demonstrates the possibil-
Scheme 1. Dyotropic rearrangement from aminocarbene complexes 1
to imine complexes 2.
Carbene Complex Chemistry
ity of other reaction pathways in these organometallic
complexes that do not necessarily involve a photocarbonyla-
tion step. According to our calculations, the first triplet state
(T1)in these compounds has a biradical character rather than
the metallacyclopropanone structure proposed for the T1 of
alkoxycarbene complexes.[5,8] This short-lived species evolves
stepwise to products 2 on the T1 hypersurface.
The discovery of these new photochemical reactions in
chromium(0)aminocarbene complexes led us to propose that
tungsten(0)aminocarbene complexes may, in fact, experience
reactions analogous to their chromium counterparts. Should
this hypothesis be correct, the photo-inert tungsten(0)car-
bene complexes could be made photo-active by adequately
modifying their structures. Herein we present the experimen-
tal confirmation of this hypothesis and show not only that
tungsten(0)carbene complexes can be photo-active, but also
that they undergo photocarbonylation processes.
The tungsten(0)carbene complexes 3a–c were prepared
following our previously reported procedure.[7] Thus, com-
plexes 4a–c, obtained by aminolysis of pentacarbonyl[ethoxy-
(methyl)carbene]tungsten(0)with the corresponding amino-
phosphanes, were heated in toluene to afford the desired
products 3 in good yield (Scheme 2).
Irradiation of complexes 3a–c in MeCN/MeOH yielded
the syn-metalated imine complexes 5a–c in acceptable yields
(Scheme 3). The structures of the syn-metalated cyclic imines
5 were assigned by comparison of their spectroscopic data
with those of their chromium analogues.[7] To the best of our
knowledge, the 1,2-dyotropic rearrangement represented in
Scheme 3 is the first photoreaction reported to date of the
presumed photochemically unreactive tungsten(0)carbene
complexes.[6]
DOI: 10.1002/anie.200501590
The Photochemical Reactivity of the “Photo-
Inert” Tungsten (Fischer) Carbene Complexes**
Israel Fernµndez, Miguel A. Sierra,* Mar Gómez-Gal-
lego, María J. Mancheæo, and Fernando P. Cossío*
The seminal discovery in 1982[1] of the sunlight-promoted
reaction between chromium(0)carbene complexes and imines
[*] I. Fernµndez, Prof. M. A. Sierra, Prof. M. Gómez-Gallego,
Dr. M. J. Mancheæo
Departamento de Química Orgµnica
Facultad de Química
Universidad Complutense
28040 Madrid (Spain)
Fax: (+34)91-394-4310
E-mail: sierraor@quim.ucm.es
Prof. F. P. Cossío
Kimika Fakultatea
Euskal Herriko Unibertsitatea, P.K. 1072
20080 San Sebastiµn–Donostia (Spain)
Fax: (+34)943015270
E-mail: fp.cossio@ehu.es
[**] Support for this work under grants CTQ2004-06250-C02-01/BQU
(Madrid group) and CTQ2004-0681/BQU (San Sebastiµn–Donostia
group) from the Ministerio de Ciencia y Tecnología (Spain), and the
Euskal Herriko Unibertsitatea (9/UPV 00040.215-13548/2001) (San
Sebastian–Donostia group) is gratefully acknowledged. I.F. thanks
the Ministerio de Educación y Cultura (Spain) for a predoctoral
(FPU) grant.
It may be possible that the dyotropic reaction represented
in Scheme 3 is a particular process restricted to complexes 3.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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In fact, two different classes of compounds were obtained,
either as mixtures (for complex 6c)or as single products (for
complexes 6a and 6b). The structure of a cyclic tetracarbo-
nyl(phosphanylamino)tungsten complex was assigned to
compounds 7a and 7b (derived from complexes 6a and 6b)
on the basis of their spectroscopic data. Thus, complex 7a
shows a set of four 13C NMR signals in the range d = 202.9–
210.8 ppm as doublets (due to P–C coupling)corresponding
to the [W(CO)4] fragment, while the signal for the N-Me
moiety appears at d = 56.1 ppm. It should be pointed out that
complexes 7 lack the Me group and the carbene carbon atom
of the starting complexes 6. The product isolated from 6c
lacks the metal moiety and has an amino ester structure.[9] The
13C NMR spectrum of this compound contains a signal at d =
174.0 ppm attributable to the ester carbonyl group. This
feature shows unambiguously that carbonylation has occur-
red. The oxidation of the phosphorus atom was confirmed
from the 31P NMR spectrum (d ꢀ 34 ppm)and additionally by
ESI mass spectrometry (m/z 407), thus confirming the
presence of the phosphane oxide moiety (Scheme 4).
Scheme 2. Synthesis of cyclic tungsten(0) carbene complexes 3. F or
details, see Experimental Section.
The UV-visible photochemistry of tungsten(0)amino-
carbene complexes is therefore quite sensitive to the sub-
stitution at the nitrogen atom and can lead to up to three
different photochemical reactions. This is remarkable for a
class of compounds that had previously been considered as
photo-inert. To confirm that this reactivity could also be
observed in chromium(0)aminocarbene complexes, com-
pound 6d was prepared and irradiated under similar con-
ditions. Now, in a very efficient process, both the N-
methylamino ester 9 and the (aminophosphanyl)tetracarbo-
nylchromium complex 10 were obtained (Scheme 5).
Scheme 3. Photochemical metalladyotropic type I rearrangement of
complexes 3. For details, see Experimental Section.
Therefore, to extend our knowledge of this reaction and to
check the generality of tungsten(0)–carbene photochemistry,
complexes 6 were prepared in almost quantitative yields by
alkylation of complexes 3 with MeI/Cs2CO3, and irradiated in
MeCN/MeOH (10:1; complexes 6a and 6b)or MeCN/THF/
MeOH (5:5:1; complex 6c; Scheme 4). Complexes 6 did not
lead to the expected products analogous to 5 derived from a
type I 1,2-dyotropic rearrangement.
Scheme 5. Competitive a-fragmentation and carbonylation reactions of
chromium(0) carbene 6d.
To rationalize the results obtained above we propose
that the irradiation of complex 11 produces the biradical
species 11* (Scheme 6). This excited species evolves into
the final N-metalated imine 13 via carbene 12. This reaction
pathway is analogous to that observed for chromium(0)
carbene complexes.[7] In contrast, the biradical 14* derived
from the irradiation of 14, which has a methyl substituent at
À =
nitrogen, evolves by fragmentation of the a-N C [W]
bond to form a new biradical 15. This process is similar to
the a fragmentation of amides,[10] which are the isolobal
analogues of aminocarbene complexes.[11] Capture of two
hydrogen atoms from the solvent forms the species 16. The
evolution of this kind of complex into a metalated amine
product like 17 has been previously observed by us and
others.[12,13]
Scheme 4. Photochemical a-fragmentation and carbonylation reactions of N-methy-
lated complexes 6. For details, see Experimental Section.
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which may be of future interest both theoretically and
synthetically. Thus, we have demonstrated experimentally
that the excitation of tetracarbonyltungsten(0)(phosphanyla-
mino)carbene complexes produces species having either a
biradical structure or a tungstacyclopropanone structure.
These species coexist and produce three different classes of
products depending mainly on the substitution pattern on the
nitrogen atom: N-unsubstituted complexes produce cyclic
syn-metalated imine complexes through a type I metalladyo-
tropic process, while N-methylated complexes produce pen-
tacarbonyl(phosphanylamino)tungsten(0)complexes and a-
amino esters. Further work in this emerging area of tungsten
photochemistry as well as the study of other related processes
in chromium(0)and molybdenum( 0)carbene complexes is
currently in progress.
Experimental Section
General procedure for the synthesis of complexes 4: A solution of
stoichiometric amounts of pentacarbonyl[ethoxy(methyl)carbene]-
tungsten(0)and the corresponding aminophosphane, in dry CH Cl2,
2
was stirred at room temperature until the disappearance of the
starting material (checked by TLC). Then, the solvent was removed
and the product was purified by flash column chromatography to
yield pure compounds.
Scheme 6. Proposed reaction pathways for the photoreaction of tung-
sten(0) aminocarbene complexes to yield syn-metalated imines 13
(path a) or a-fragmentation products 17 (path b).
1
4a: yellow oil (58%). H NMR (300 MHz, CDCl3): d = 1.75 (m,
2H; CH2P), 2.56 (s, 3H; CH3), 3.95 (m, 2H; CH2N), 7.32–7.72 (m,
10H; HAr), 8.46 ppm (br.s, 1H; NH). 13C NMR (50 MHz, CDCl3): d =
28.1 (d, JC,P = 15.3 Hz; CH2P), 47.1 (CH3), 52.7 (d, JC,P = 16.5 Hz;
The formation of a-amino ester 8 from tungsten(0)
complex 6c and the mixture of N-metalated amine 10 and
the carbonylation product a-amino ester 9 from chromium(0)
carbene 6d indicate the co-existence of different activated
species that are formed upon irradiation of these complexes
(Scheme 7). It is reasonable to assume that compound 10
CH2N), 128.9 (d, JC,P = 6.4 Hz; CAr), 129.4 (CAr), 132.7 (d, JC,P
=
19.1 Hz; CAr), 136.5 (d, JC,P = 11.4 Hz; CAr), 198.2 (CO), 203.2 (CO),
255.6 ppm (W C). IR (CCl4): n˜ = 2062, 1965, 1927 cmÀ1
.
=
C21H18NO5PW: calcd. C 43.55, H 3.13, N 2.42; found C 43.69, H
3.00, N 2.28.
General Procedure for the synthesis of complexes 3: A solution of
the (phosphanylamino)carbene complex was heated at reflux in
toluene until the disappearance of the starting material (checked by
TLC). The solvent was then removed under reduced pressure to yield
pure compounds (unless otherwise specified).
3a: yellow solid (91%). 1H NMR (300 MHz, CDCl3): d = 2.36 (m,
2H; CH2P), 2.77 (s, 3H; CH3), 3.85 (m, 1H; CH2N), 3.92 (m, 1H;
CH2N), 7.30–7.52 (m, 10H; HAr), 8.92 ppm (s, 1H; NH). 13C NMR
(75.5 MHz, CDCl3): d = 25.2 (d, JC,P = 23.6 Hz; CH2P), 46.6 (d, JC,P
=
4.1 Hz; CH3), 48.9 (d, JC,P = 6.1 Hz; CH2N), 128.5 (d, JC,P = 9.3 Hz;
CAr), 129.8 (d, JC,P = 1.4 Hz; CAr), 131.9 (d, JC,P = 12.0 Hz; CAr), 137.0
(d, JC,P = 38.3 Hz; CAr), 203.2 (d, JC,P = 7.0 Hz; CO), 209.2 (d, JC,P
=
26.8 Hz; CO), 213.4 (d, JC,P = 7.3 Hz; CO), 261.4 ppm (d, JC,P
=
.
9.6 Hz; W C). IR (KBr): n˜ = 3304, 2002, 1875, 1838 cmÀ1
=
C20H18NO4PW: calcd. C 43.58, H 3.29, N 2.54; found C 43.74, H
3.51, N 2.70.
General Procedure for the synthesis of complexes 6: A solution of
complex 3 in degassed acetone (0.025m)was treated with two
equivalents of MeI, three equivalents of Cs2CO3, and water (drops)at
room temperature overnight. The crude reaction mixture was filtered
through a short pad of celite and the solvent was removed under
reduced pressure to yield pure compounds (unless otherwise speci-
fied).
Scheme 7. Proposed reaction pathways for the competitive processes
observed in the irradiation of complexes 6c and 6d.
1
6a: yellow solid (99%). H NMR (300 MHz, CDCl3): d = 2.31
(m, 2H; CH2P), 2.67 (s, 3H; CH3C), 3.26 (s, 3H; CH3N); 4.05 (m, 1H;
would arise from biradical intermediate 18, while compounds
8 and 9 are formed from coordinated-ketene intermediates
19.[13]
In conclusion, tungsten(0)carbene complexes, which were
previously considered to be photo-inert, show a rich and
complex photoreactivity when they are properly substituted,
CH2N), 4.14 (m, 1H; CH2N), 7.26–7.46 ppm (m, 10H; HAr). 13C NMR
(75.5 MHz, CDCl3): d = 24.8 (d, JC,P = 23.6 Hz; CH2P), 42.0 (d, JC,P
37.2 Hz; CH2N), 53.4 (CH3C), 62.5 (d, JC,P = 7.1 Hz; CH3N), 128.5 (d,
C,P = 9.3 Hz; CAr), 129.7 (d, JC,P = 1.6 Hz; CAr), 131.9 (d, JC,P
=
J
=
12.0 Hz; CAr), 137.3 (d, JC,P = 38.2 Hz; CAr), 203.9 (d, JC,P = 6.9 Hz;
CO), 210.1 (d, JC,P = 28.1 Hz; CO), 213.0 (d, JC,P = 6.7 Hz; CO),
Angew. Chem. Int. Ed. 2006, 45, 125 –128
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127

Communications
=
259.7 ppm (d, JC,P = 10.1 Hz; W C). IR (KBr): n˜ = 2000, 1902, 1857,
1842 cmÀ1. C21H20NO4PW: calcd. C 44.63, H 3.57, N 2.48; found C
44.44, H 3.40, N 2.55.
Pergamon, Oxford, 1995, p. 549; c)M. A. Schwindt, J. R. Miller,
L. S. Hegedus, J. Organomet. Chem. 1991, 413, 143.
[3] For reviews on different aspects of metal–carbene chemistry,
see: a)K. H. Dötz, H. Fischer, P. Hofmann, R. Kreissel, U.
Schubert, K. Weiss, Transition Metal Carbene Complexes, Verlag
Chemie, Deerfield Beach, FL, 1983; b)K. H. Dötz, Angew.
Chem. 1984, 96, 573; Angew. Chem. Int. Ed. Engl. 1984, 23, 587;
c)W. D. Wulff in Comprehensive Organic Synthesis, Vol. 5 (Eds.:
B. M. Trost, I. Fleming), Pergamon, Oxford, 1991, p. 1065; d)H.
Rudler, M. Audouin, E. Chelain, B. Denise, R. Goumont, A.
Massoud, A. Parlier, A. Pacreau, M. Rudler, R. Yefsah, C.
Alvarez, F. Delgado-Reyes, Chem. Soc. Rev. 1991, 20, 503;
e)D. B. Grötjahn, K. H. Dötz, Synlett 1991, 381; f)W. D. Wulff
in Comprehensive Organometallic Chemistry II, Vol. 12 (Eds.:
E. W. Abel, F. G. A. Stone, G. Wilkinson), Pergamon, Oxford,
1995, p. 470; g)D. F. Harvey, D. M. Sigano, Chem. Rev. 1996, 96,
271; h)R. Aumann, H. Nienaber, Adv. Organomet. Chem. 1997,
41, 163; i)B. Alcaide, L. Casarrubios, G. Domínguez, M. A.
Sierra, Curr. Org. Chem. 1998, 2, 551; j)M. A. Sierra, Chem. Rev.
2000, 100, 3591; k)A. de Meijere, H. Schirmer, M. Duetsch,
Angew. Chem. 2000, 112, 4124; Angew. Chem. Int. Ed. 2000, 39,
3964; l)J. Barluenga, J. Flórez, F. J. Faæanµs, J. Organomet.
Chem. 2001, 624, 5.
General Procedure for the photochemical reactions of complexes 3
and 6: Photochemical reactions were conducted with a 450-W
medium-pressure Hg lamp through a Pyrex filter in dry, degassed
MeCN containing MeOH (10:1 ratio)or in MeCN, THF, and MeOH
(5:5:1 ratio)in a sealed Pyrex tube filled with argon. In a typical
experiment, after irradiation for 10 h the solution (0.015m)was
filtered through a short pad of celite, the solvents were removed
under reduced pressure, and the crude product was submitted to flash
chromatography to give pure complexes, unless otherwise specified.
5a: yellow solid (71%). 1H NMR (300 MHz, CDCl3): d = 2.28 (d,
J = 5.2 Hz, 3H; CH3), 2.46 (m, 2H; CH2P), 3.85 (m, 1H; CH2N), 3.93
(m, 1H; CH2N), 7.34–7.59 (m, 10H; HAr), 7.86 ppm (q, J = 5.2 Hz,
1H; CH). 13C NMR (75.5 MHz, CDCl3): d = 26.0 (CH3), 30.7 (d, JC,P
21.2 Hz; CH2P), 69.6 (d, JC,P = 10.1 Hz; CH2N), 128.8 (d, JC,P = 9.9 Hz;
Ar), 130.2 (d, JC,P = 1.7 Hz; CAr), 131.9 (d, JC,P = 12.2 Hz; CAr), 135.5
=
C
(d, JC,P = 40.4 Hz; CAr), 172.9 (CH), 203.0 (d, JC,P = 7.0 Hz; CO), 210.1
(d, JC,P = 32.0 Hz; CO), 211.6 ppm (d, JC,P = 5.1 Hz; CO). IR (KBr):
n˜ = 2008, 1867, 1838 cmÀ1. C20H18NO4PW: calcd. C 43.58, H 3.29, N
2.54; found C 43.39, H 3.33, N 2.41.
7a: yellow solid (46%). 1H NMR (300 MHz, CDCl3): d = 2.21 (m,
1H; CH2P), 2.48 (m, 1H; CH2P), 2.76 (m, 1H; CH2N), 2.89 (d, J =
5.9 Hz, 3H; CH3), 3.20 (m, 1H; CH2N), 7.32–7.62 ppm (m, 10H; HAr).
13C NMR (75.5 MHz, CDCl3): d = 30.0 (d, JC,P = 21.6 Hz; CH2P), 48.1
(CH3), 56.1 (d, JC,P = 10.2 Hz; CH2N), 128.7 (d, JC,P = 3.9 Hz; CAr),
128.9 (d, JC,P = 3.6 Hz; CAr), 130.3 (CAr), 131.7 (d, JC,P = 12.7 Hz; CAr),
132.1 (d, JC,P = 12.6 Hz; CAr), 134.8 (d, JC,P = 19.9 Hz; CAr), 135.3 (d,
[4] A. Hafner, L. S. Hegedus, G. de Weck, B. Hawkins, K. H. Dötz,
J. Am. Chem. Soc. 1988, 110, 8413.
[5] L. S. Hegedus, G. de Weck, S. DꢀAndrea, J. Am. Chem. Soc. 1988,
110, 2122.
[6] Strictly speaking tungsten(0)Fischer carbene complexes may
experience loss of CO ligands as well as syn–anti isomerization
upon irradiation. Nevertheless, to the best of our knowledge
these complexes are photo-inert toward nucleophiles. The
irradiation of chromium(0)carbene complexes with UV light
also produces CO extrusion and syn–anti isomerization of the
group tethered to the ligand. See: K. O. Doyle, M. L. Gallagher,
M. T. Pryce, A. D. Rooney, J. Organomet. Chem. 2001, 617, 269.
For the purposes of this work we will always refer to visible light
photochemistry, using medium-pressure Hg lamps, a Pyrex filter,
and a Pyrex well. Under these conditions medium-pressure Hg
lamps radiate predominantly at 365 – 366 nm as well as signifi-
cant amounts in the visible region at 404 – 408, 436, 546, and
577 – 579 nm. Under these experimental conditions, to the best
of our knowledge, only photocarbonylation and carbene-transfer
processes have been reported for chromium(0)and molybde-
num(0)carbene complexes. See references [1] and [2].
JC,P = 22.9 Hz; CAr), 202.9 (d, JC,P = 6.8 Hz; CO), 204.0 (d, JC,P
=
7.8 Hz; CO), 210.2 (d, JC,P = 31.9 Hz; CO), 210.8 ppm (d, JC,P
=
4.1 Hz; CO). IR (CCl4): n˜ = 2012, 1954, 1888 cmÀ1. C19H18NO4PW:
calcd. C 42.33, H 3.37, N 2.60; found C 42.28, H 3.50, N 2.74.
8: pale-yellow oil (11%). The solvent was removed in vacuo and
the residue was dissolved in a mixture of hexane and EtOAc (1:1)and
exposed to direct sunlight until a clear solution was obtained. The
solution was then filtered through a short pad of celite, the solvent
eliminated, and the residue was purified by flash column chromatog-
raphy to yield pure compound. 1H NMR (300 MHz, CDCl3): d = 0.86
(d, J = 6.9 Hz, 3H; CH3C), 1.91 (s, 3H; CH3N), 3.31 (q, J = 6.9 Hz,
1H; CHCH3), 3.54 (s, 3H; CH3O), 3.92 (d, J = 10.6 Hz, 2H; AB
system, CH2), 6.95–7.59 ppm (m, 14H; HAr). 13C NMR (125 MHz,
CDCl3): d = 14.0 (CH3C), 36.5 (CH3N), 51.1 (CH3O), 56.9 (CH), 60.7
(CH2), 126.1 (d, JC,P = 13.2 Hz; CAr), 128.4 (d, JC,P = 9.1 Hz; CAr),
130.1, 131.2 (d, JC,P = 9.4 Hz; CAr), 131.6 (CAr), 131.8 (d, JC,P = 8.6 Hz;
[7] M. A. Sierra, I. Fernµndez, M. J. Mancheæo, M. Gómez-Gallego,
M. R. Torres, F. P. Cossío, A. Arrieta, B. Lecea, A. Poveda, J.
JimØnez-Barbero, J. Am. Chem. Soc. 2003, 125, 9572.
C
J
Ar), 132.0 (CAr), 133.2 (CAr), 133.6 (d, JC,P = 12.0 Hz; CAr), 145.5 (d,
C,P = 9.4 Hz; CAr), 174.0 ppm (CO). 31P NMR (internal H3PO4
reference): d = 34.2 ppm. IR (CCl4): n˜ = 1736, 1437, 1194, 1159,
1119 cmÀ1. C24H20NO3P: calcd. C 70.75, H 6.43, N 3.44; found C
70.60, H 6.61, N 3.59.
[8] I. Fernµndez, M. A. Sierra, M. Gómez-Gallego, M. J. Mancheæo,
F. P. Cossío, Chem. Eur. J. 2005, 20, 5988.
[9] The 1H NMR spectrum of the reaction mixture shows the
presence of the cyclic phosphane–amine analogue to 7 (signals at
d = 2.73 (d)and 3.62 ppm (m)as the main reaction product.
However, all attempts to isolate this compound were fruitless.
The crude reaction mixture was oxidized and chromatographed
to isolate compound 8 (see Scheme 4).
Received: May 10, 2005
Revised: September 23, 2005
Published online: November 22, 2005
[10] a)J. D. Coyle, Chem. Rev. 1978, 78, 97; b)A. G. Gilbert, J.
Baggott, Essentials of Molecular Photochemistry, Blackwell
Science, Oxford, 1991, pp. 328 – 329.
[11] a)R. Hoffmann, Science 1981, 211, 995; b)R. Hoffmann, Angew.
Chem. 1982, 94, 711; Angew. Chem. Int. Ed. Engl. 1982, 21, 711.
[12] a)C. K. Murray, B. P. Warner, V. Dragisich, W. D. Wulff, R. D.
Rogers, Organometallics 1990, 9, 3142; b)I. Fernµndez, M. J.
Mancheæo, M. Gómez-Gallego, M. A. Sierra, T. Lejon, L. K.
Hansen, Organometallics 2004, 23, 1851.
Keywords: carbene ligands · carbonylation · photochemistry ·
rearrangement · tungsten
.
[1] a)M. A. McGuire, L. S. Hegedus, J. Am. Chem. Soc. 1982, 104,
5538; b)L. S. Hegedus, M. A. McGuire, L. M. Schultze, C. Yijun,
O. P. Anderson, J. Am. Chem. Soc. 1984, 106, 2680.
[2] For reviews on the photochemistry of Group 6 metal–carbene
complexes, see: a)L. S. Hegedus, Tetrahedron 1997, 53, 4105;
b)L. S. Hegedus in Comprehensive Organometallic Chemistry
II, Vol. 12 (Eds.: E. W. Abel, F. G. A. Stone, G. Wilkinson),
[13] Preliminary DFT calculations carried out at the uB3LYP/6-
31 g(d)&LANL2DZ + DZPVE level of theory are consistent
with these mechanistic proposals.
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