Understanding the formation of N-H tautomers from α-substituted pyridines: Tautomerization of 2-ethylpyridine promoted by osmium

Article abstract of DOI:10.1021/ja073673r

Reaction of [OsH(η²-H₂){η²-C,C-κ¹-N-(CH₂CH-o-C₅H₄N)}(PⁱPr₃)₂]BF₄ with benzophenone affords [OsH₂{κ²-C,O-C₆H₄C(O)Ph}{κ-C-(HNC₅H₃Et)}(PⁱPr₃)₂]BF₄ as a result of the orthometalation of the ketone, the hydrogenation of the vinyl substituent of the pyridine, and its subsequent tautomerization. In acetonitrile, the ketone is released and complex [OsH{κ-C-(HNC₅H₃Et)}(NCCH₃)(PⁱPr₃)₂]BF₄ is formed. Copyright

Full text of DOI:10.1021/ja073673r

Published on Web 08/16/2007
Understanding the Formation of N-H Tautomers from r-Substituted
Pyridines: Tautomerization of 2-Ethylpyridine Promoted by Osmium
Mar´ıa L. Buil, Miguel A. Esteruelas,* Karin Garce´s, Montserrat Oliva´n, and Enrique On˜ate
Departamento de Qu´ımica Inorga´nica, Instituto de Ciencia de Materiales de Arago´n,
UniVersidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
Received May 22, 2007; E-mail: maester@unizar.es
Scheme 1
N-H tautomers of R-substituted pyridines, postulated 70 years
ago1 and experimentally proved in the gas phase for pyridine,2 have
been recently synthesized by means of an iridium mediated process
that involves formally a 1,2-H shift from carbon to nitrogen.3 At
the same time, we showed the stabilization of N-H tautomers of
quinoline and 8-methylquinoline by coordination to osmium and
ruthenium and an additional Cl‚‚‚HN interaction between the NH
hydrogen and a chlorine of the metal fragment.4 A few months
later, Whittlesey5 et al. reported ruthenium isomers resulting from
the N- or C-bond tautomers of isopropyl-4,5-dimethylimidazol, in
agreement with the paper of Crabtree, Eisenstein, and Sini about
the possibility that C-bound imidazoles could have some existence
in metalloprotein chemistry.6
This type of tautomerization is important not only in biological
processes, where the energetically less stable tautomer is often an
active intermediate that dictates the mechanism and the formed
product, but also in some relevant catalytic reactions,7 in particular
those involving N-aromatic compounds.8 Thus, Bergman, Ellman
et al.9 have proposed that the C,N-1,2-H rearrangement is the key
step for the Rh(I) catalyzed ortho alkylation of pyridines and
quinolines. An exciting question arises now: Are such tautomer-
izations more frequent than previously thought?
as a result of the transfer of the coordinated hydrogen molecule
A process related to the ortho alkylation of pyridines and
quinolines is the Murai’s reaction for the insertion of olefins in an
ortho-CH bond of aromatic ketones. Like for the nitrogen hetero-
cycles, the ortho-CH addition of the aromatic ring to the ruthenium
catalyst is a fundamental step.10 In agreement with the trend of
osmium to provide stable models of reactive intermediates proposed
in transformations with ruthenium counterparts,11 osmium-poly-
hydride compounds have proven to activate an ortho-CH bond of
aromatic ketones to give orthometalated derivatives, which are
reminiscent species of the intermediates proposed for the Murai’s
reaction.12
from the metal center to the C-C double bond; (ii) ortho-CH bond
activation of the ketone by the resulting monohydride, in agreement
with the trend shown by the osmium-hydride complexes to add
aromatic ketones; and (iii) C,N-1,2-H rearrangement of 2-ethylpy-
ridine.
The tautomerization appears to be favored by the steric hindrance
experienced between the ethyl substituent and the orthometalated
ketone. In agreement with this, the reaction of 1 with methyl vinyl
ketone does not lead to a N-H tautomer, the species [OsH{CHd
Complex OsH6(PiPr3)2 also activates a C(sp2)-H bond of the CH2
CHC(O)CH3}2(PiPr3)2]BF4 is isolated instead. The presence of the
substituent in R-position is certainly determinant for the tautomer-
ization of the heterocycle. Recently, we have also observed that
2-methylpyridine reacts with MH2Cl2(PiPr3)2 (M ) Ru, Os) to give
N-H tautomers related to those isolated with quinoline and
8-methylquinoline, while under the same conditions pyridine affords
the usual species coordinated by the nitrogen atom.
2-Ethylpyridine is the third pyridine undergoing C,N-tautomer-
ization, after 2-methylpyridine and 2-phenylpyridine,3 and the
tautomerization is the first one of a pyridine promoted by a metal
of the eighth group.14 A 2,6-lutidinium ylide bound to osmium
through the carbon atom at 4-position has been produced by
isomerization of an η2-C,C-2,6-lutidine species.15 A few pyridine-
or N-alkylpyridine-2-ylidene and -4-ylidine complexes of other
metals than osmium have been also prepared by indirect methods
from suitable precursors.16
group of the substituent of 2-vinylpyridine to yield OsH3(NC5H4-
o-CHdCH)(PiPr3)2, which reacts with HBF4 to give 1 as a result
of the regeneration of the 2-vinylpyridine molecule and the trans-
formation of the Os-trihydride unit into Os-hydride-dihydrogen.13
Now, we have observed that the treatment at 50 °C of the white
complex 1 with 5.0 equiv of benzophenone in the absence of solvent
gives rise after 24h to a red melt that, by stirring in diethyl ether at
room temperature, affords 2 (Scheme 1) as a red solid in 55% yield.
The addition of some drops of acetonitrile or acetone to the dis-
carded diethyl ether solution produces the precipitation of the side
complexes [Os{C6H4C(O)C6H5}(η2-H2)L(PiPr3)2]BF4 (L ) CH3CN,
(CH3)2CO), which were characterized by X-ray diffraction analysis.
The formation of 2 is a one-pot tandem process of three
reactions: (i) hydrogenation of the vinyl substituent of the pyridine
9
10998
J. AM. CHEM. SOC. 2007, 129, 10998-10999
10.1021/ja073673r CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
The X-ray structure of 2 proves the orthometalation of the ketone
and the stabilization of the N-H tautomer of the pyridine, which
coordinates through C(32). Furthermore it suggests that, like in the
previous quinoline complexes,4 a hydrogen bond involving the NH
group plays an important role in the stabilization of 2. Thus, the
NH-hydrogen points out the oxygen atom of the ketone, the
separation between them being (2.05(5) Å) shorter than the sum
of the van der Waals radii of hydrogen and oxygen.12b,17
one should expect a rapid growth of the number of transition-metal
complexes containing N-H tautomers of R-substituted pyridines
in the near future.
Acknowledgment. Financial support from the Spanish MEC
(Projects CTQ2005-00656 and Consolider Ingenio 2010 CSD2007-
00006) and Diputacio´n General de Arago´n (E35) are acknowledged.
K.G. thanks the Spanish MEC for her grant.
The coordination geometry around the osmium atom can be
rationalized as a distorted pentagonal bipyramid with axial phos-
phines (P(1)-Os-P(2) ) 163.79(4)°) and the hydride ligands lying
in the equatorial plane between the metalated carbon atom of the
ketone and the heterocycle, which are transoid disposed (C(1)-
Os-C(32) ) 158.11(17)°). In the 1H NMR spectrum in CD2Cl2 at
room temperature, they give rise to a broad signal at -9.8 ppm.
This resonance is temperature dependent. At 263 K decoalescence
occurs, and at temperatures lower than 253 K an ABX2 spin system
is observed. The value of the H-H coupling constant (55.7 Hz)
suggests the operation of a weak quantum exchange coupling18
between the hydride ligands, in addition to the thermally activated
site exchange process. The ∆Hq (13.6 ( 0.5 kcal mol-1) and ∆Sq
(4.7 ( 1.2 eu) values compare well with those found for the other
Os-H2 compounds with the hydride ligands separated by less than
1.6 Å.17,19 In agreement with this, at 300 MHz, a T1(min) value of
59 ms was obtained, which corresponds to a H-H distance of 1.40
Å (1.59(6) Å by X-ray) assuming slow spinning.20
The Os-C(32) bond length (2.110(5) Å) is similar to the
distances reported for the scarce Os-imidazolylidene complexes
characterized by X-ray diffraction analysis.21 Although the Os-
C(1) distance (2.084(4) Å) is lightly shorter than the Os-C(32)
bond length, and despite the chelate nature of the orthometalated
ligand, the elimination of the ketone from 2 is favored with regard
to the retrotautomerization of the heterocycle22 or the elimination
of 2-ethylpyridinium. Thus, at room temperature in acetonitrile, 2
evolves to 3 (90% yield).
Supporting Information Available: Experimental details for the
synthesis, characterization and crystallographic data for 2, 3, and [Os-
{C6H4C(O)C6H5}(η2-H2)L(PiPr3)2]BF4 (L ) CH3CN, (CH3)2CO). This
material is available free of charge via the Internet at http://pubs.acs.org.
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160.98(5)°) and the N-H tautomer of the heterocycle (Os-C(5)
) 1.993(6) Å) cis disposed to the hydride ligand (H(01)). The NH-
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the H-H distance in rings of the type LH‚‚‚HN with an electrostatic
hydrogen-hydrogen interaction.23 The hydrogen bond maintains
in solution. In CD2Cl2, at 300 MHz, the T1(min) value of the hydride
resonance (δ ) -16.22; JH-P ) 19.5 Hz) is lower than that of
[OsH(CH3CN)3(PiPr3)2]BF4 (198 ms versus 257 ms). Assuming that
the excess relaxation rate (1.16 s-1) is due to the proximity of the
NH hydrogen atom, and that the H(01)‚‚‚H(1) vector rotates with
the molecule as a whole, one can calculate a H(1)‚‚‚H(01) separation
of 2.2 Å, in good agreement with that obtained by X-ray diffraction
analysis.
In conclusion, the steric hindrance experienced by the substituent
and the co-ligands of the complex is a strong determining factor
for the promotion of the metal mediated rearrangement of R-sub-
stituted pyridines into N-H tautomers. The retrotautomerization
and release of the heterocycle from the metal center is disfavored
with regard to the elimination of strongly coordinating ligands,
including orthometalated aromatic ketones.24 Like in the previously
reported Os- and Ru-quinoline derivatives,4 hydrogen bonds involv-
ing the NH group appear to play an important role in the
stabilization of this type of compounds. In the light of these results,
Note that complex [Os{C6H4C(O)C6H5}(η2-H2)(CH3CN)(PiPr3)2]BF4 for-
mally resulting from the replacement of the heterocycle by acetonitrile in
2 is stable and has been characterized by X-ray diffraction analysis (see
Supporting Information).
(22)
(23) (a) Buil. M. L.; Esteruelas, M. A.; On˜ate, E.; Ruiz, N. Organometallics
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M.; On˜ate, E.; Tolosa, J. I. Organometallics 2000, 19, 275.
Complexes [OsH2{C(Ph)dCHC(O)R}{κ-C-[HNC5H3Et]}(PiPr3)2]+ (R )
Me, Ph) react in the same manner as 2 to give 3.
(24)
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