Isolation and reaction of (indolin-3-ylidene)pentacarbonyltungsten generated from tungsten-containing azomethine ylide

Article abstract of DOI:10.1246/cl.2004.16

(Indolin-3-ylidene)pentacarbonyltungsten, a non-heteroatom-stabilized carbene complex of group 6 metal, was successfully isolated by the reaction of N-(o-alkynylphenyl)imine derivative, vinyl ether and tungstenhexacarbonyl under photoirradiation. This complex was moderately stable and fully characterized by spectroscopic analysis.

Full text of DOI:10.1246/cl.2004.16

16
Chemistry Letters Vol.33, No.1 (2004)
Isolation and Reaction of (Indolin-3-ylidene)pentacarbonyltungsten
Generated from Tungsten-containing Azomethine Ylide
Jun Takaya, Hiroyuki Kusama, and Nobuharu Iwasawaꢀ
Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551
(Received October 20, 2003; CL-030983)
(Indolin-3-ylidene)pentacarbonyltungsten,
a
non-hetero-
plex.
When Me-substituted alkyne 5 and t-butyl vinyl ether were
atom-stabilized carbene complex of group 6 metal, was success-
fully isolated by the reaction of N-(o-alkynylphenyl)imine
derivative, vinyl ether and tungstenhexacarbonyl under photoir-
radiation. This complex was moderately stable and fully charac-
terized by spectroscopic analysis.
treated with a stoichiometric amount of W(CO)6 in THF under
photoirradiation at room temperature, the reaction mixture im-
mediately became deep blue and the color remained even after
removal of the solvent. Further careful investigation revealed
that the deep blue oil could be separated in 12% yield by alumina
column chromatography with hexane-toluene (2:1) as an eluent
at ꢁ20 ꢂC without remarkable decomposition, along with the
Me-migrated tricyclic indole derivatives 6 (50% yield, cis:trans
= 55:45, Scheme 2).
Fischer-type carbene complexes of group 6 metal pentacar-
bonyl have long been employed as a useful reagent in synthetic
organic reactions. High utility of these complexes is partly due to
their stability caused by electron-donation from the heteroatom
on the carbene carbon.1 On the contrary, although non-hetero-
atom-stabilized carbene complexes of group 6 metal pentacar-
bonyl have appeared as a key intermediate in various reac-
tions,2,3 the isolation of such complexes has rarely been
achieved due to their high instability.4,5
We recently reported a novel method for the construction of
polycyclic indole skeletons by the tungstenpentacarbonyl cata-
lyzed reaction of N-(o-alkynylphenyl)imine derivatives (1) and
electron-rich alkenes (Scheme 1).6 It was postulated that novel
tungsten-containing azomethine ylide species were generated
by the reaction of 1 and tungstenpentacarbonyl, and they under-
went [3+2]-cycloaddition reaction with electron-rich alkenes to
afford non-heteroatom-stabilized tungsten carbene complexes
(3), from which 1,2-migration of the substituent R (R = H, alkyl,
aryl) occured to afford polycyclic indole derivatives. Such 1,2-
migration of alkyl or aryl group is quite rare in group 6 metal car-
bene complexes and isolation of the intermediate carbene com-
plex is highly desirable not only for the chemistry of non-hetero-
atom-stabilized carbene complexes but also for the
establishment of the mechanism of this reaction. Herein we wish
to report the successful isolation and characterization of the in-
termediate carbene complex 3 (R = Me, Scheme 1) and the
unique reactivity of this non-heteroatom-stabilized carbene com-
Ph
Alumina
h
ν
Column
100 mol%
W(CO)6
N
(at −20 °C)
+
O-t-Bu
(10 eq.)
Me
THF, rt
2 h
Ph
Ph
N
5
dropwise / 1 h
N
+
O-t-Bu
O-t-Bu
Me
Me
W(CO)5
7 : 12%
6 : 50% (cis:trans=55:45)
Scheme 2.
Extensive spectroscopic analysis (NMR (1H, 13C, DEPT,
HMBC, HMQC, NOE), IR, high resolution mass spectrometry
(FAB) and elemental analyses) revealed that this blue material
was the (indolin-3-ylidene)pentacarbonyltungsten 7.7 The ab-
sorption of the pentacarbonyl was observed at 2053 and
1909 cmꢁ1 in IR spectra and the carbene carbon was detected
at 310.9 ppm by 13C NMR, which was reasonable as compared
with the values of the reported non-heteroatom-stabilized car-
bene complexes.8 This carbene complex is rather stable and
can be stored for several weeks at ꢁ20 ꢂC under Ar without de-
composition. This unexpected stability of 7 can be explained by
considering the existence of a homologous conjugating effect
from the nitrogen through the aromatic ring. This is a quite rare
example of the isolation of non-heteroatom-stabilized group 6
metal pentacarbonyl carbene complex possessing alkyl substitu-
ent.9
Interestingly, although the 1,2-Me-migrated product 6 was
obtained as a mixture of cis- and trans-stereroisomers with
low diastereoselectivity (55:45), the isolated carbene complex
7 was obtained as a single stereoisomer whose stereochemistry
was determined by the measurement of the differential NOE
spectrum as shown in Scheme 2.10 We believe that the carbene
complex 7 is most stable among the possible four isomers as this
has the least steric repulsion between substituents (Me, Ph, and t-
BuO).
Ph
Ph
Ph
R
Ph
R
hν
W(CO)6
N
N
N
N
toluene
rt
1
(CO)5W
W(CO)5
R
R
W(CO)5
Ph
W-containing azomethine ylide 2
Ph
O-t-Bu
[3+2]-
cycloaddition
N
N
1,2-R-
migration
O-t-Bu
O-t-Bu
R
W(CO)5
R
4
Non-heteroatom-stabilized
carbene complex 3
Next, we investigated the reactivity of (indolin-3-ylidene)-
pentacarbonyltungsten 7. After several investigations, it was
Scheme 1.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.1 (2004)
17
found that 7 slowly decomposed in toluene at room temperature
to give the Me-migrated tricyclic indole derivative quantitatively
after 3 days. The product was obtained as a single stereoisomer,
which was confirmed to have the same trans relationship be-
tween the phenyl and t-BuO groups as shown in Scheme 3. This
result strongly supports that, in the W(CO)5 catalyzed prepara-
tion of polycyclic indole derivatives, a very rare 1,2-alkyl-migra-
tion is really occurring through the non-heteroatom stabilized
carbene intermediate 7 as proposed previously.11
Ph
N
CHO
Me
(38%-H)
excess D2O
+
7
+
cis-6 trans-6
THF-d8, rt
14 h
D
9
(20
:
24
:
56)
Scheme 5.
This work was supported in part by a Grants-in-Aid for Sci-
entific Research on Priority Areas (A) ‘‘Exploitation of Multi-
Element Cyclic Molecules’’ from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
trans-6
toluene-d8, rt, 3 d, quant.
7
cis-6
+
trans-6
THF-d8, rt, 1 d, quant.
(45 : 55)
References and Notes
a) F. Z. Dorwald, ‘‘Metal Carbenes in Organic Synthesis,’’ Wiley-
¨
1
Scheme 3.
VCH, Weinheim (1999). b) M. J. Winter, in ‘‘Comprehensive Organo-
metallic Chemistry II,’’ ed. by E. W. Abel, F. G. A. Stone, and G.
Wilkinson, Pergamon, New York (1995), Vol. 5, pp 172–192.
M. J. Winter, in ‘‘Comprehensive Organometallic Chemistry II,’’ ed.
by E. W. Abel, F. G. A. Stone, and G. Wilkinson, Pergamon, New York
(1995), Vol. 5, pp 168–172. See also ref 1a.
Examples for the catalytic process including a non-heteroatom-stabi-
lized carbene complex as the key intermediate, see; a) N. Iwasawa,
M. Shido, and H. Kusama, J. Am. Chem. Soc., 123, 5814 (2001). b)
K. Miki, F. Nishino, K. Ohe, and S. Uemura, J. Am. Chem. Soc.,
124, 5260 (2002).
Alkyl carbene derivatives, especially those having an ꢀ-hydrogen, are
highly reactive to give 1,2-hydrogen migration products. a) C. P.
Casey, L. D. Albin, and T. J. Burkhardt, J. Am. Chem. Soc., 99,
2533 (1977). b) J. Barluenga, A. Ballesteros, R. de la R. Bernardo, J.
Santamaria, E. Rubio, and M. Tomas, J. Am. Chem. Soc., 125, 1834
(2003).
Very interestingly, when 7 was left at rt in THF instead of
toluene, decomposition occurred within 1 day and the product
was obtained as a mixture of cis and trans 1,2-Me-migrated
products (cis-6:trans-6 = 45:55) quantitatively (Scheme 3). This
result indicates the existence of the equilibrium between the car-
bene complex 7 and a zwitterionic intermediate 8 through ring
opening and closing reactions (retro-aldol and aldol type reac-
tions) (Scheme 4). We propose that, in THF, recyclization from
8 proceeds in low stereoselectivity to afford up to four stereoiso-
meric carbene complexes, which undergo 1,2-Me-migration to
give the mixture of cis and trans-6 finally.12 Difference of the re-
activity in toluene and THF can be explained by considering the
stability of the zwitterionic intermediate 8, which is expected to
be stabilized in a more polar solvent like THF.
2
3
4
5
There are several reports on the synthesis and isolation of diarylcar-
bene complexes. a) C. P. Casey, T. J. Burkhardt, and C. A. Bunnell,
t-Bu
J. Am. Chem. Soc., 99, 2127 (1977). b) K. H. Dotz and J. Preiffer,
¨
O
Chem. Commun., 1996, 895. Other examples for the isolation of
non-heteroatom-stabilized carbene complexes, see; c) H. Fischer, S.
Zeuner, and K. Ackermann, J. Chem. Soc., Chem. Commun., 1984,
684. d) K. Miki, T. Yokoi, F. Nishino, K. Ohe, and S. Uemura, J.
Organomet. Chem., 645, 228 (2002).
H. Kusama, J. Takaya, and N. Iwasawa, J. Am. Chem. Soc., 124, 11592
(2002).
Spectroscopic data of 7; IR (neat) 2917, 2053, 1909, 1607 cmꢁ1 1H
;
Ph
N
Ph
N
Ph
N
H
Me
O-t-Bu
O-t-Bu
Me
Me
W(CO)5
W(CO)5
W(CO)5
6
7
1,2-Me-
Migration
1,2-Me-
Migration
Zwitterionic intermediate 8
NMR (500 MHz) ꢁ ¼ 1:16 (s, 9H), 1.78 (s, 3H), 2.67 (ddd, J ¼ 13:8,
10.7, 2.8 Hz, 1H), 3.12 (dd, J ¼ 13:8, 7.8 Hz, 1H), 4.73 (dd,
J ¼ 10:7, 7.8 Hz, 1H), 4.91 (d, J ¼ 2:8 Hz, 1H), 6.70 (brd,
J ¼ 8:5 Hz, 1H), 7.00 (ddd, J ¼ 8:5, 8.1, 1.0 Hz, 1H), 7.35–7.38 (m,
3H), 7.41–7.46 (m, 2H), 7.63 (ddd, J ¼ 8:5, 8.1, 1.2 Hz, 1H), 8.10
(brd, J ¼ 8:5 Hz, 1H); 13C NMR (125 MHz) ꢁ ¼ 24:1, 29.2, 46.2,
61.4, 74.5, 77.6, 103.1, 113.2, 121.9, 125.4, 127.4, 128.8, 136.2,
141.8, 142.1, 151.7, 160.1, 199.2, 207.0, 310.9; HRMS Calcd for
C27H25NO6W, M 643.1191. Found m=z 643.1164.; Anal. Calcd for
C27H25NO6W: C, 50.41; H, 3.92; N, 2.18. Found: C, 50.67; H, 4.12;
N, 2.13.
See Ref 1a, 4b, 5c.
There are few reports on the synthesis and isolation of alkylcarbene
complexes of group 6 metal pentacarbonyl; see for examples; a) J.
trans-6
cis-6
Scheme 4.
The existence of the zwitterionic intermediate 8 was further
confirmed by the experiment in the presence of D2O. By the
treatment of 7 in THF containing an excess amount of D2O, al-
dehyde 9, which is the hydrolyzed product of 8, was obtained
along with the 1,2-Me-migrated indoles (9:cis-6:trans-6 =
20:24:56, Scheme 5). Formation of the aldehyde moiety and in-
corporation of a deuterium at the 3-position of the indole skele-
ton also support the existence of the zwitterionic intermediate 8,
and thus, the existence of ring opening and closing equilibrium
by retro-aldol and aldol type reactions.
In summary, we have succeeded in isolating the non-hetero-
atom-stabilized tungsten carbene complex, (indolin-3-ylidene)-
pentacarbonyltungsten, possessing aryl and tert-alkyl substitu-
ents on the carbene carbon. This non-heteroatom-stabilized
tungsten carbene complex easily undergoes the 1,2-Me-migra-
tion to afford a tricyclic indole derivative and this result strongly
supports the proposed reaction mechanism of the tungsten-con-
taining azomethine ylides.
8
9
Preiffer and K. H. Dotz, Organometallics, 17, 1353 (1998). b) K. H.
¨
¨
Dotz, M. Papall, G. Muller, and K. Ackermann, J. Organomet. Chem.,
¨
383, 93 (1990).
10 NOEs were observed between the methyl group and o-proton of the
phenyl substituent and between the methyl group and the methin
proton on the t-BuO-substituted carbon.
11 1,2-Alkyl migration of group 6 metal carbene intermediates is reported
in a few, specific cases. See; a) M. Zora, J. W. Herndon, Y. Li, and J.
Rossi, Tetrahedron, 57, 5097 (2001). b) K. Nagao, M. Chiba, and
S.-W. Kim, Synthesis, 1983, 197.
12 No isomeric carbene complexes were observed during this NMR study.
Published on the web (Advance View) December 8, 2003; DOI 10.1246/cl.2004.16