Novel ruthenium-mediated conversion of aldimines and aminals to aminocarbene complexes

Article abstract of DOI:10.1021/om020664c

The reaction of [RuCp(L)(CH₃CN)₂]PF₆(L = CH₃CN,PMe₃) with (E)-N-(phenylmethylene)-2-pyridiamine (py-N=CHPh) resulted in the cyclic aminocarbene complexes [RuCp(L)(=CPhnh-Py]PF₆. It was found that with L = PPh₃, CO the reaction stopped already at the stage of the imine complex. This reaction did not proceed via a direct 1,2-hydrrogen shift but it involved hydrido iminiacyl intermediates.

Full text of DOI:10.1021/om020664c

© Copyright 2002
Volume 21, Number 23, November 11, 2002
American Chemical Society
Communications
Novel Ru th en iu m -Med ia ted Con ver sion of Ald im in es a n d
Am in a ls to Am in oca r ben e Com p lexes
Christina M. Standfest-Hauser,† Kurt Mereiter,‡ Roland Schmid,† and
Karl Kirchner*,†
Institute of Applied Synthetic Chemistry and Institute of Chemical Technologies and Analytics,
Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria
Received August 14, 2002
Summary: The reaction of [RuCp(L)(CH3CN)2]PF6 (L )
CH3CN, PMe3) with (E)-N-(phenylmethylene)-2-pyridin-
amine (py-NdCHPh) affords the cyclic aminocarbene
complexes [RuCp(L)(dCPhNH-py)]PF6, whereas with
L ) PPh3, CO the reaction stops already at the stage of
the imine complex [RuCp(L)(κ2N,N-py-NdCHPh)]PF6.
This reaction arguably does not proceed via a direct 1,2-
hydrogen shift but seems to involve hydrido iminoacyl
intermediates as a result of C-H bond activation and
deprotonation steps.
available from simple organic precursors in a one-pot
procedure. This has been achieved recently by convert-
ing amines,4 ethers,5 and activated olefins (see Scheme
1)6 into the respective carbene complexes.
All these reactions are thermodynamically favorable,
primarily due to the presence of a strong π-donor group.
In fact, for nonactivated olefins (X ) H in Scheme 1),
the reaction is strongly endothermic and carbenes are
often converted to the respective olefins; i.e., the reverse
process takes place.7 The analogous conversion of aldi-
mines to aminocarbenes (Scheme 2), which is to the best
of our knowledge not known, would be related to this
reaction. We now report for the first time a facile
rearrangement of aldimines and also aminals to yield
cyclic aminocarbenes.
Heteroatom-stabilized carbene complexes play a cen-
tral role in organometallic chemistry.1 They have found
widespread application as reactive intermediates in
organic synthesis, initiating a wide range of C-C and
C-heteroatom bond-forming reactions.2 Moreover, het-
eroatom-stabilized carbenes, in particular N-heterocyclic
ones, have been recognized as extraordinarily useful
spectator ligands instead of or in addition to phosphine
ligands.3 It is thus of great interest to find new synthetic
routes to afford carbene complexes, especially if they are
Accordingly, treatment of [RuCp(CH3CN)3]PF6 (1) or
[RuCp(PMe3)(CH3CN)2]PF6 (2a ) with 1 equiv of (E)-N-
(phenylmethylene)-2-pyridinamine (py-NdCHPh)8 af-
(3) (a) Hermann, W. A. Angew. Chem. 2002, 114, 1342. (b) Arduengo,
A. J . Acc. Chem. Res. 1999, 32, 913.
(4) Lee, D.-H.; Chen, J .; Faller, J . W.; Crabtree, R. H. Chem.
Commun. 2001, 213.
(5) Slugovc, C.; Mereiter, K.; Trofimenko, S.; Carmona, E. Angew.
Chem., Int. Ed. 2000, 39, 2158.
(6) (a) Coalter, J . N.; Spivak, G. J .; Gerard, H.; Clot, E.; Davidson,
E. R.; Eisenstein, O.; Caulton, K. G. J . Am. Chem. Soc. 1998, 120, 9388.
(b) Coalter, J . N.; Bollinger, J . C.; Huffman, J . C.; Zwanziger, U. W.;
Caulton, K. G.; Davidson, E. R.; Gerard, H.; Clot, E.; Eisenstein, O.
New J . Chem. 2000, 24, 9.
(7) Becker, E.; Ru¨ba, E.; Mereiter, K.; Schmid, R.; Kirchner, K.
Organometallics 2001, 20, 3851 and references therein.
(8) For related reactions of this ligand see: (a) J un, C.-H.; Moon, C.
W.; Lee, D.-Y. Chem. Eur. J . 2002, 8, 2422. (b) Suggs, J . W. J . Am.
Chem. Soc. 1979, 101, 489.
* To whom correspondence should be addressed. E-mail: kkirch@
mail.zserv.tuwien.ac.at.
† Institute of Applied Synthetic Chemistry.
‡ Institute of Chemical Technologies and Analytics.
(1) (a) Schrock, R. R. J . Chem. Soc., Dalton Trans. 2001, 2541.
(2) (a) Sierra, M. A. Chem. Rev. 2000, 100, 3591. (b) Zaragoza-
Do¨rwald, F. Metal Carbenes in Organic Synthesis; Wiley-VCH: Wein-
heim, Germany, 1999. (c) Aumann, R.; Nienaber, H. Adv. Organomet.
Chem. 1997, 41, 163. (d) Harvey, D. F.; Sigano, D. M. Chem. Rev. 1996,
96, 271. (e) Hegedus, L. S. Acc. Chem. Res. 1995, 28, 299. (f) Wul, W.
D. Compr. Organomet. Chem. 1995, 12, 470. (g) Do¨tz, K. H. In
Reactions of Coordinated Ligands; Braterman, P. R., Ed.; Plenum:
New York, 1986, Chapter 4, p 285.
10.1021/om020664c CCC: $22.00 © 2002 American Chemical Society
Publication on Web 10/15/2002

4892 Organometallics, Vol. 21, No. 23, 2002
Communications
Sch em e 1
structure of 8 (in the form of 8‚(CH3)2CO) has been
determined by single-crystal X-ray diffraction (Scheme 3).
Characteristic 13C{1H} NMR spectroscopic features of
7 and 8 comprise marked low-field resonances at 266.5
and 264.9 ppm, respectively, assignable to the carbene
carbon atom of the dCPhNH-py moiety. Complex 8
adopts a typical three-legged piano-stool conformation.
The Ru-C(14) bond distance is 1.959(1) Å, comparable
to that in other heteroatom-stabilized ruthenium car-
bene complexes.
Interestingly, if [RuCp(PPh3)(CH3CN)2]PF6 (2b) and
[RuCp(CO)(CH3CN)2]PF6 (3) are reacted with py-Nd
CHPh, the reaction does not proceed to give an amino-
carbene but affords the complexes [RuCp(PPh3)(κ2N,N-
py-NdCHPh)]PF6 (5) and [RuCp(CO)(κ2N,N-py-Nd
CHPh)]PF6 (6), featuring a κ2N,N-coordinated py-Nd
CHPh ligand (Scheme 3), even if kept at elevated
temperature for 24 h. Complexes 5 and 6 have been fully
characterized by NMR spectroscopy and elemental
analyses. The structure of 5 has been confirmed also
by X-ray crystallography.
Sch em e 2
The reaction mechanism shown in Scheme 3 repre-
sents our initial working hypothesis for providing the
amino carbene complexes, and the four key intermedi-
ates A-D are proposed. The formulations of these
intermediates are also supported by preliminary DFT
(B3LYP) calculations using py-NdCH2 and HCN as
model ligands (Figure 1).9 The overall reaction is
exothermic by 8.0 kcal/mol. For the first step, the
initially formed κ2N,N-py-NdCHPh complex converts
into A, where the imine moiety is side-on coordinated.
Subsequently, C-H bond activation occurs via the
intermediacy of B to give eventually the hydrido imino-
acyl intermediate C.10 Such intermediates have been
also suggested recently in the reaction of [RuHCl(P-i-
Pr3)2]2 with imines, eventually yielding, however, isoni-
triles rather than aminocarbenes.11 Moreover, the ob-
servation that neither 2b nor 3 undergoes a C-H
activation/oxidative addition reaction may also support
our proposal. While in the first case steric restrictions
may account for the lack of reactivity, in the latter case
the strongly π-accepting CO ligand may prevent an
oxidative addition step. Although we have as yet no
conclusive evidence for this pathway, we believe that
C undergoes a facile deprotonation (e.g., assisted by
adventitious water, counterion, or solvent) to give the
strongly basic neutral iminoacyl complex D, which
readily uptakes a proton to afford the final products. A
Sch em e 3
(9) All calculations were performed using the Gaussian98 software
package: 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, J . V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J . A. Gaussian 98, revision A.5; Gaussian, Inc.: Pittsburgh, PA,
1998.
(10) For hydrido acyl-hydroxycarbene rearrangements see: (a)
Casey, C. P.; Czerwinski, C. J .; Fusie, K. A.; Hayashi, R. J . Am. Chem.
Soc. 1997, 119, 3971. (b) Steinborn, D.; Gerisch, M.; Bruhn, C.; Davies,
J . A. Inorg. Chem. 1999, 38, 680.
(11) Coalter, J . N.; Huffman, J . C.; Caulton, K. G. Organometallics
2000, 19, 3569.
forded the aminocarbene complexes [RuCp(CH3CN)-
(dCPhNH-py)]PF6 (7) and [RuCp(PMe3)(dCPhNH-
py)]PF6 (8) in high yields (Scheme 3). This process is
not restricted to RuCp complexes; RuTp(COD)Cl (4) also
readily reacts with py-NdCHPh at elevated tempera-
tures to give the aminocarbene complex RuTp(dCPhNH-
py)Cl (9). In addition to full spectroscopic and analytical
characterizations of all the products, the solid-state

Communications
Organometallics, Vol. 21, No. 23, 2002 4893
F igu r e 1. Optimized B3LYP geometries and relative energies (kcal/mol) for the conversion of [RuCp(κ2N,N-py-NdCH2)-
(HCN)]+ to the aminocarbene [RuCp(HCN)(dCHNH-py)]+ (Ru sdd; C, H, N (6-31g**).
Sch em e 4
direct 1,2-hydrogen shift process converting B directly
into aminocarbene complexes seems to be unlikely. In
fact, DFT calculations suggest such a process to be as
high as 53.7 kcal (Figure 1).
In summary, we have shown for the first time that
the reaction of aldimines and aminals with both RuCp
and RuTp complexes furnishes a simple preparation for
a class of carbene complexes that does not rely on the
conventional routes to carbenes utilizing, for example,
diazoalkanes. Our efforts are currently directed toward
extending the scope of these reactions to other transi-
tion-metal complexes as well as other aldimines and
aminals.
Finally, we have investigated whether aminals which
are readily derived from aldimines and amines can be
converted into aminocarbenes. Indeed, the reaction of
1 and 2a with the aminal N,N′-di-2-pyridinyl-1-propyl-
diamine (py-NHCH(Et)CHNH-py) affords the respective
cyclic aminocarbene complexes [RuCp(py-NH2)(dC(NH-
py)Et)]PF6 (10) and [RuCp(PMe3)(dC(NH-py)Et]PF6
(11) in high yields (Scheme 4). With 2b the reaction does
not stop at the aminocarbene stage but proceeds further
to give the olefin complex 12. All these reactions involve
both C-H and C-N activation steps. It is interesting
to note, however, that 2b does not react with py-Nd
CHPh to give an aminocarbene complex or a rearrange-
ment product thereof, suggesting that in the case of
aminals a different mechanism may be operative.
Ack n ow led gm en t. Financial support by the “Fonds
zur Fo¨rderung der wissenschaftlichen Forschung” (Project
No. P14681-CHE) is gratefully acknowledged. Support
from the EC through the COST D17 Action is also
acknowledged.
Su p p or tin g In for m a tion Ava ila ble: X-ray crystallo-
graphic data for 5 and 8‚(CH3)2CO and spectroscopic data for
4-12. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM020664C