Apparent 1,2-silyl migrations in aromatic carbenes occur by intermolecular silyl exchanges [16]

Full text of DOI:10.1021/ja980797i

9100
J. Am. Chem. Soc. 1998, 120, 9100-9101
Apparent 1,2-Silyl Migrations in Aromatic Carbenes
Occur by Intermolecular Silyl Exchanges
Ste´phane Sole´, Heinz Gornitzka, Olivier Guerret, and
Guy Bertrand*
Laboratoire de Chimie de Coordination du CNRS
205 route de Narbonne
F-31077 Toulouse Cedex, France
ReceiVed March 10, 1998
ReVised Manuscript ReceiVed July 7, 1998
Figure 1. (a) Correlation diagram of the in-plane 1,2-H-migration for
aromatic carbenes of type 2. (b) Energy diagram of the calculated out of
plane 1,2-H-migration for 2.
In the past few years, carbene chemistry has undergone a
profound revolution with the appearance of stable singlet carbenes
1-61-6 in the literature. However, the true carbenic nature of
these species remains a highly debatable topic.7
Scheme 1
Heinemann et al. demonstrated that even though the reaction
would be exothermic (-26.1 kcal/mol at the RHF/MP2DSQ
level11a and -29 kcal/mol at the DFT/B3LYP level11b), the carbene
2 (R ) H) should be kinetically stable toward 1,2-shifts, since
the activation energy of this rearrangement is high (+46.8 kcal/
mol,11a +39.8 kcal/mol11b). Recently, Ma¨ıer et al. obtained similar
results for the thiazolylidene system 3 (R ) H) (∆H ) - 34.0
kcal/mol, Eq ) + 42.3 kcal/mol).11c
It is now well established that the 1,2-migration is a funda-
mental reaction for singlet carbenes and occurs via a unimolecular
concerted mechanism.8 Alternative intermolecular pathways such
as those involving carbene-olefin π-complexes9 have recently been
ruled out.10 1,2-Hydrogen migrations in aromatic carbenes cannot
proceed through an intramolecular process in the plane of the
ring, since this mechanism would impose the crossing of two
orbitals with the same symmetry as shown in the correlation
diagram (Figure 1a). An alternative possibility, which has been
studied theoretically,11 involves the interaction of the N-H bond
with the out of plane pπ orbital of the carbene. This process
induces a deformation of the ring and thus the loss of the
electronic delocalization of the nitrogen lone pairs (Figure 1b).
Here, we report that 1,2 migrations can occur for aromatic
carbenes of type 1, but via intermolecular processes.
Since the ability of silyl groups to migrate is well established,
we chose to prepare 1H-4-silyl-1,2,4-triazolium salts (7a-d)12
as precursors of the corresponding heterocyclic carbenes 1a-d
(Scheme 1). Deprotonation of salts 7a-d, with various bases,
readily occurred at 0 °C, as shown by the disappearance of the
corresponding 1H NMR signal. Interestingly, no 13C NMR signal
in the range expected for carbene centers (∼200 ppm)13 was
observed; instead, the signals for the quaternary carbon atoms
appeared around 155 ppm. Moreover, the 29Si NMR signals were
shifted to high field (∆δ ≈ 35 ppm) compared to those observed
for the triazolium precursors 7a-d. After workup, the products
8a-d were isolated in 42-81% yields and were fully character-
ized;12 a single-crystal X-ray diffraction study for compound 8a
was performed. 1H-5-Silyl-1,2,4-triazoles 8a-d formally result
from the 1,2-migration of the silyl group from nitrogen to the
carbene center of the transient derivatives 1a-d.
(1) Enders, D.; Breuer, K.; Raabe, G.; Runsink, J.; Teles, J. H.; Melder, J.
P.; Ebel, K.; Brode, S. Angew. Chem., Int. Ed. Engl. 1995, 34, 1021.
(2) (a) Arduengo, A. J., III.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361. (b) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am.
Chem. Soc. 1992, 114, 5530. (c) Arduengo, A. J., III; Davidson, F.; Dias, H.
V. R.; Goerlich, J. R.; Khasnis, D.; Marshall, W. J.; Prakasha, T. K. J. Am.
Chem. Soc. 1997, 119, 12742.
(3) Arduengo, A. J., III; Goerlich, J. R.; Marshall, W. J. Liebigs Ann. 1997,
365.
(4) Arduengo, A. J., III; Goerlich, J. R.; Marshall, W. J. J. Am. Chem.
Soc. 1996, 118, 11027.
(5) Alder, R. W.; Allen, P. R.; Murray, M.; Orpen, G. Angew. Chem., Int.
Ed. Engl. 1996, 35, 1121.
(6) (a) Igau, A.; Gru¨tzmacher, H.; Baceiredo, A.; Bertrand, G. J. Am. Chem.
Soc. 1988, 110, 6463. (b) Igau, A.; Baceiredo, A.; Trinquier, G.; Bertrand, G.
Angew. Chem., Int. Ed. Engl. 1989, 28, 621. (c) Gilette, G.; Baceiredo, A.;
Bertrand, G. Angew. Chem., Int. Ed. Engl. 1990, 29, 1429. (d) Soleilhavoup,
M.; Baceiredo, A.; Treutler, O.; Ahlrichs, R.; Nieger, M.; Bertrand, G. J. Am.
Chem. Soc. 1992, 114, 10959. (e) Dyer, P.; Baceiredo, A.; Bertrand, G. Inorg.
Chem. 1996, 35, 46.
(12) Physical and spectroscopic data for selected compounds. For 7a: 1H
3
NMR (CD3CN) δ 1.25 (d, J(H-H) ) 7.2 Hz, 18 H, CH3CH), 1.77 (sept.,
(7) (a) Dagani, R. Chem. Eng. News 1991, 69 (4), 19. (b) Regitz, M. Angew.
Chem., Int. Ed. Engl. 1991, 30, 674. (c) Dagani, R. Chem. Eng. News 1994,
72 (18), 20. (d) Regitz, M. Angew. Chem., Int. Ed. Engl. 1996, 35, 725. (d)
Heinemann, C.; Mu¨ller, T.; Apeloig, Y.; Schwarz, H. J. Am. Chem. Soc. 1996,
118, 2023. (e) Boehme, C.; Frenking, G. J. Am. Chem. Soc. 1996, 118, 2039.
(8) (a) Hoffmann, R.; Zeiss, J. D.; Van Dine, G. W. J. Am. Chem. Soc.
1968, 90, 1485. (b) Nickon, A. Acc. Chem. Res. 1993, 26, 84. (c) Storer, J.
W.; Hook, K. N. J. Am. Chem. Soc. 1993, 115, 10426. (d) Sander, W.; Bucher,
G.; Wierlacher, S. Chem. ReV. 1993, 93, 1583. (e) Jackson, J. E.; Platz, M. S.
AdVances in Carbene Chemistry; Brinker, U.; Ed.; JAI: Greenwich, CT, 1994;
Vol. 1.
(9) (a) Liu, M. T. H. Acc. Chem. Res. 1994, 27, 287. (b) Bonneau, R.; Liu,
M. T. H.; Kim, K. C.; Goodman, J. L. J. Am. Chem. Soc. 1997, 119, 3829.
(10) Keating, A. E.; Garcia-Garibay, M. A.; Houk, K. N. J. Am. Chem.
Soc. 1997, 119, 10805.
(11) (a) Heinemann, C.; Thiel, W. Phys. Chem. Lett. 1994, 217, 11. (b)
McGibbon, G. A.; Heinemann, C.; Lavorato, D. J.; Schwarz, H. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 1478. (c) Maier, G.; Endres, J.; Reisenauer, H. P.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1709.
3J(H-H) ) 7.2 Hz, 3 H, CHSi), 4.23 (s, 3 H, CH3N), 8.72 (s, 1 H, CH), 9.49
(s, 1 H, CH); 13C NMR (CD3CN) δ 10.8 (s, CHSi), 16.9 (s, CH3CH), 38.9 (s,
1
CH3N), 120.4 (q, J(C-F) ) 320.1 Hz, CF3), 144.7 (s, CH), 146.5 (s, CH);
29Si NMR (CD3CN) δ +41.4; mp 77 °C. For 8a: 1H NMR (C6D6) δ 1.04 (d,
3J(H-H) ) 7.1 Hz, 18 H, CH3CH), 1.29 (sept., 3J(H-H) ) 7.1 Hz, 3 H,
CHSi), 3.45 (s, 3 H, CH3N), 8.13 (s, 1 H, CH); 13C NMR (C6D6) δ 12.4 (s,
CHSi), 18.6 (s, CH3CH), 37.4 (s, CH3N), 152.0 (s, CH), 155.3 (s, CSi); 29Si
NMR (C6D6) δ +1.2; MS (DCI, NH3) m/z ) 240 (M + 1); mp 40-42 °C.
For 9a: 1H NMR (CDCl3) δ 0.99 (d, 3J(H-H) ) 3.9 Hz, 18 H, CH3CH),
3
1.17 (sept., J(H-H) ) 3.9 Hz, 3 H, CHSi), 3.69 (s, 3 H, CH3N), 6.30 (s, 1
H, CHOSi) 7.21-7.42 (m., 5 H, C6H5), 7.73 (s, 1 H, CH); 13C NMR (CDCl3)
δ 12.0 (s, CHSi), 17.9 (s, CH3CH), 36.1 (s, CH3N), 69.9 (s, CHOSi), 125.1,
126.8, 127.7, 128.4 (C6H5), 149.7 (s, CH), 155.9 (s, C-CHOSi); 29Si NMR
(C6D6) δ +19.0; MS (DCI, NH3) m/z ) 346 (M + 1). For 12′: 1H NMR
(CD3CN) δ 0.05 (s, 6 H, CH3Si), 1.12 (s, 9 H, CH3C), 7.60-8.02 (m, 15 H,
C6H5); 13C NMR (CD3CN) δ -4.6 (s, CH3Si), 18.4 (s, CH3C), 26.7 (s, CH3C),
1
120.2 (q, J(C-F) ) 320.3 Hz, CF3), 123-141 (m, C6H5), 154.3 (s, CSi).
(13) Herrmann, W. A.; Ko¨cher, C. Angew. Chem., Int. Ed. Engl. 1997, 36,
2162.
S0002-7863(98)00797-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/22/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 35, 1998 9101
Scheme 2
Scheme 4
Scheme 3
along with triazole 13 (Scheme 4). Compounds 12 and 13 could
then react together via a nucleophilic attack of the nitrogen of 13
at the N-silyl group of the cation 12, affording the rearrangement
product 8 and regenerating the starting salt 7. The first step is
supported by the reaction of the stable carbene 1e with triazolium
salt 7b at 0 °C, which led to the C-silylated triazolium salt 12′
and triazole 13a.12 The second part of the postulated mechanism
has been modelized by reacting the triazole 13b with the
N-silylated triazolium 7b. A large excess of 13b was necessary
to displace the equilibrium toward the formation of 7d and 13a,
and the reaction only occurred at room temperature. Therefore,
it could be surprising that derivatives 12 and 13 were not detected
by monitoring the reaction of the cations 7 with KH at low
temperature; however, it is quite clear that the sterically congested
triazolium salt 12 is much more reactive than the model compound
7b (Scheme 4).
All attempts to spectroscopically characterize carbenes 1a-d
failed, even when monitoring the deprotonation reaction at -78
°C. To prove that the 1,2-migration occurred from the expected
transient carbenes, 1a,b were generated in the presence of a large
excess of benzaldehyde. The resulting products 9a,b (66-72%
yields) and 10 (19-15%) have been fully characterized;12 a single-
crystal X-ray diffraction study for 10 was performed. Hetero-
cycles 9a,b clearly result from the attack of the nucleophilic
carbenes 1a,b at the carbonyl group leading to the zwitterion
11a,b, followed by migration of the silyl group. On the other
hand, 10 is the product of the coupling of two benzaldehyde
molecules, which is reminiscent of the Breslow benzoin conden-
sation catalyzed by thiazolylidene, the first experimental evidence
for the transient formation of aromatic carbenes14 (Scheme 2).
The next question was the intra- or intermolecular nature of
the rearrangement of carbenes 1a-d into triazoles 8a-d. Depro-
tonation of a 1/1 mixture of triazolium salts 7a and 7d, bearing
different substituents at both nitrogen atoms, led to a mixture of
the four rearrangement products 8a-d, which were identified by
1H NMR and GC/MS of the bulk reaction mixture. Starting from
a 1/1 mixture of triazolium salts 7b,c, the same products 8a-d
were obtained (Scheme 3). We then checked that in solution no
exchange reactions of the silyl groups occurred between the
triazolium salts 7a,d, and between 7b,c. Similarly, no silyl group
exchanges occurred between the triazoles 8a-d. These results
as a whole prove that carbenes 1a-d were formed by deproto-
nation of triazolium salts 7a-d and that the formation of triazoles
8a-d resulted from an intermolecular process.
In conclusion, 1,2-migrations can occur for aromatic carbenes
such as 1a-d, but via intermolecular processes in contrast to all
known transient carbenes.10 This rearrangement implies the
carbene and an electrophilic partner. This is reminiscent of the
specific behavior of the stable carbenes 1-5 toward dimerization,
for which the protonated form of the carbene is also involved.15,16
Acknowledgment. We are grateful to the Alexander von Humboldt
Foundation for a grant to H.G. and to the CNRS for financial support of
this work.
Supporting Information Available: Experimental procedures, physi-
cal and spectroscopic data for all new compounds; X-ray structures, tables
of crystal and intensity collection data, position and thermal parameters,
and interatomic distances for derivatives 8a and 10 (21 pages, print/PDF).
See any current masthead page for ordering information and Web access
instructions.
It was then of interest to gain greater insight into the mechanism
of this intermolecular rearrangement. Aromatic carbenes are very
good nucleophiles.7d Therefore, a nucleophilic attack of a carbene
of type 1 on the silyl group of a starting cation 7 could result in
the formation of the C- and N-silyl-substituted triazolium salt 12,
JA980797I
(15) (a) Trinquier, G.; Malrieu, J. P. J. Am. Chem. Soc. 1987, 109, 5303.
(b) Driess, M.; Gru¨tzmacher, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 828.
(16) (a) Alder, R. W.; Blake, M. E. J. Chem. Soc., Chem. Commun. 1997,
1513. (b) Denk, M. K.; Thadani, A.; Hatano, K.; Lough, A. J. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2607. See also ref 3.
(14) (a) Breslow, R. J. Am. Chem. Soc. 1958, 80, 3719. (b) Teles, J. H.;
Melder, J. P.; Ebel, K.; Schneider, R.; Gehrer, E.; Harder, W.; Brode, S. HelV.
Chim. Acta 1996, 79, 61.