Mechanism of the interconversions between C- and N-bound transition metal α-cyanocarbanions

Article abstract of DOI:10.1021/ja017868p

The first presentation of the intra- and intermolecular mechanisms of the C-N interconversions of transition metal -cyanocarbanions is described. A pair of N- and C-bound isomers of isonitrile complex Ru⁺Cp(NCCH-SO₂Ph)(PPh₃)(CN-t-Bu) (1) and RuCp[CH(CN)SO₂Ph](PPh₃)(CN-t-Bu) (2) was synthesized for the mechanistic studies on the N-to-C isomerizations. Structural characterization by X-ray diffractions of 1 and 2 indicated their typical zwitterionic and α-metalated structures. The kinetic studies on the irreversible isomerization of 1 to 2 in benzene-d₆ at 333-348 K were carried out using ¹H NMR spectroscopy, affording the first-order rate constants k₁ and the activation parameters ΔH = 107 ± 2 kJ-mol^-1 and ΔS = -22 ± 5 J-K^-1-mol^-1. The almost identical values of k₁ were obtained upon similar treatment of 1 with 4 equiv of external ligands such as PPh₃, CH₃CN, and t-BuNC at 333 K, indicating that the N-to-C isomerization proceeds in an intramolecular manner without dissociation of a ligand. As a model system for the C-to-N isomerization, the irreversible transformation of RuCp[CH(CN)SO₂Ph](PPh₃)₂ (3) to Ru⁺Cp(NCCH-SO₂Ph)(PPh₃)₂ (4) was investigated under various reaction conditions. The reaction of 3 at room temperature in THF affords the coordination dimers (R_Ru *, S_C *, R_Ru *, S_C *)-{RuCp[CH(CN)SO₂Ph](PPh₃)}₂ (5) stereoselectively, and its distorted μ²-C,N-bound structure was determined by X-ray analysis. The reaction profiles for the isomerization of 3 includes the generation- and temperature-dependent decays of dimeric species 5 and its diastereomer 6, which strongly suggests that the intra- and intermolecular pathways are included in the C-to-N isomerization. The intramolecular process of the C-to-N isomerization of 3 has been confirmed by the kinetic studies on the isomerization of 3 with excess amount of PPh₃ in benzene-d₆ at 333-348 K which afford the first-order kinetics with the activation parameters of ΔH = 121 ± KJ.mol ^-1 and ΔS= 42 ± 4 J-K^-1-mol^-1. Treatment of 5 with PPh₃ in boiling benzene gives rise to the quantitative formation of N-bound complex 4. The controlled kinetic experiments on the cleavage of 5 with PPh₃ have concluded that the cleavage of 5 with PPh₃ proceeds via simultaneous C-Ru and N-Ru bond scissions, indicating the temperature-dependent participation of intermolecular process in the C-to-N isomerization of 3. Copyright

Full text of DOI:10.1021/ja017868p

Published on Web 05/25/2002
Mechanism of the Interconversions between C- and N-Bound Transition Metal
r-Cyanocarbanions
Takeshi Naota,*,†,‡ Akio Tannna,† Shigeaki Kamuro,† and Shun-Ichi Murahashi*,†,§
Department of Chemistry, Graduate School of Engineering Science, Osaka UniVersity, PRESTO, Japan Science and
Technology Corporation (JST), Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Received December 22, 2001
Compared to the marked progress in the chemistry of transition
metal enolates,1 much less has been made on the studies on the
structure and reactivity of transition metal R-cyanocarbanions.2,3
Recently, we reported the first interconversion between C- and
N-bound transition metal R-cyanocarbanions after success in
obtaining their exact isomers.4 The elucidation of the mechanism
of the C-N interconversion is a subject of great urgency, since it
could be a crucial step for a new family of catalytic C-C bond
formations via R-C-H activation of nitriles.5 The movement of
the metal fragments in the C-N interconversion is also of particular
interest in view of linkage isomerism,6 because of ambiguities
arising from the exceptionally long distance between each binding
site on the R-cyanocarbanion moieties. In this communication we
describe the first mechanistic rationale on the C-to-N and N-to-C
isomerizations of transition metal R-cyanocarbanions, including the
intramolecular processes with the unprecedented participation of
intermolecular process via self-assembly of metals as shown in
Scheme 1.
2.89(2) × 10-5 s-1 (343 K), and 4.853(3) × 10-5 s-1 (348 K),
respectively.8 The rate data correlate well (R2 ) 0.999) with Eyring
relationship of ln(k1/T) versus 1/T, where the activation parameters
∆Hq and ∆Sq were determined to be 107 ( 2 kJ‚mol-1 and -22 (
5 J‚K-1‚mol-1, respectively. The first-order kinetics clearly indicates
that the reaction proceeds in an intramolecular manner. To gain
insight into the intramolecular mechanism, kinetics on the reactions
of 1 with an excess amount of external ligands such as PPh3, CH3-
CN, and t-BuNC were carried out at 333 K in benzene-d6 using
similar NMR technique ([1]0 ) 2.00 × 10-2 M, [ligand]0 ) 8.00
× 10-2 M). All reactions afforded exclusive formation of 2 with
the clean first-order kinetics, where the observed first-order rate
constants k1 were almost same as that obtained in the absence of
PPh3 (PPh3: 8.91(5) × 10-6 s-1; CH3CN: 8.67(8) × 10-6 s-1
;
t-BuNC: 8.86(9) × 10-6 s-1). The reaction in CDCl3 also afforded
a similar k1 value of 8.99(3) × 10-6 s-1. These results exclude the
possibility of the formation and rebound of a 16-electron ion pair
intermediate [RuCp(PPh3)(CN-t-Bu)]+(NCCH-SO2Ph), concluding
that the N-to-C isomerization proceeds intramolecularly without
dissociation of any ligand.
Scheme 1
The irreversible transformation of RuCp[CH(CN)SO2Ph](PPh3)2
(3) into Ru+Cp(NCCH-SO2Ph)(PPh3)2 (4)4 has been investigated
as a representative C-to-N isomerization. The reaction courses for
the isomerization of 3 in benzene-d6 at 313-373 K were monitored
by the similar 1H NMR analysis ([3]0 ) 1.00-3.00 × 10-2 M). In
all cases most of complex 3 was consumed at the early stage of
the reactions, accompanying with the formation of product 4,
coordination dimer (RRu*,SC*,RRu*,SC*)-{RuCp[CH(CN)SO2Ph]-
(PPh3)}2 (5), and its minor diastereomer 6.9,10 Prolonged reactions
at 348 and 373 K afforded faster decays of [5] and [6] with the
corresponding increment of [4] after complete consumption of 3,
while the reaction profile at 313 K showed only a slight decline of
these dimer species, suggesting temperature-dependent participation
of the intermolecular process in the C- to-N isomerizations.
As a suitable model system for the kinetic studies on the N-to-C
isomerization, we designed and prepared a pair of the C- and
N-bound isomers of isonitrile complexes Ru+Cp(NCCH-SO2Ph)-
(PPh3)(CN-t-Bu) (1) and RuCp[CH(CN)SO2Ph](PPh3)(CN-t-Bu)
(2).7 Due to their strong π-acidity isonitrile ligands have proven to
show a remarkable acceleration effect for the N-to-C isomerizations,
and the reaction of 1 to 2 proceeds irreversibly under milder
conditions than those of any other N-bound bis-phosphine complex.4
Kinetic studies on the isomerization of 1 in benzene-d6 were carried
out by means of 1H NMR analysis using internal standard
(bibenzyl). The consumption rate of 1 exhibited clean first-order
dependence on the concentration of 1 at 333-348 K ([1]0 ) 2.00
× 10-2 M), and the first-order rate constants k1 were determined
to be 8.96(2) × 10-6 s-1 (333 K), 1.574(7) × 10-5 s-1 (338 K),
* To whom correspondence should be addressed. E-mail: naota@
chem.es.osaka-u.ac.jp.
† Osaka University.
‡ PRESTO, JST.
§ Present address: Department of Applied Chemistry, Faculty of Engineering,
Okayama University of Science, Ridaicho, Okayama 700-0005, Japan.
9
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10.1021/ja017868p CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
tions driven by the formation and cleavage of the molecular
assemblies.
In conclusion, we have shown the first presentation of the intra-
and intermolecular mechanism of C-N interconversions of transi-
tion metal R-cyanocarbanions. The results will provide significant
information on the controlled generation of N-bound R-cyano-
carbanion active species3c-e,5b for catalytic C-C bond forming
reactions of nitriles. Further studies are currently in progress.
Supporting Information Available: Experimental details for all
reactions, representative kinetic data for the isomerizations of 1 and 3,
crystallographic data for 1, (RRu*,SC*)-2 and 5 (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 1. Molecular structure of 5. Thermal ellipsoids are shown at the
30% probability level. Selected bond distances (Å) and angles (deg.):
Ru(1)-C(1), 2.220(5); C(1)-C(2)*, 1.449(8); C(2)-N(1), 1.145(7); N(1)-
Ru(1), 2.073(4); Ru(1)-C(1)-C(2)*, 103.8(3); C(1)-C(2)*-N(1)*, 168.8(5);
C(2)-N(1)-Ru(1), 156.7(4); N(1)-Ru(1)-C(1), 80.5(2).
References
(1) (a) Slough, G. A.; Bergman R. G.; Heathcock, C. H. J. Am. Chem. Soc.
1989, 111, 938. (b) Stack, J. G.; Doney, J. J.; Bergman, R. G.; Heathcock,
C. H. Organometallics 1990, 9, 453. (c) Hartwig, J. F.; Andersen, R. A.;
Bergman, R. G. J. Am. Chem. Soc. 1990, 112, 5670. (d) Stack, J. G.;
Simpson, R. D.; Hollander, F. J.; Bergman, R. G.; Heathcock, C. H. J.
Am. Chem. Soc. 1990, 112, 2716.
(2) C-Bound complexes: (a) Ittel, S. D.; Tolman, C. A.; English, A. D.; Jesson,
J. P. J. Am. Chem. Soc. 1978, 100, 7577. (b) Crocco, G. L.; Lee, K. E.;
Gladysz, J. A. Organometallics 1990, 9, 2819. (c) Falvello, L. R.;
Ferna´ndez, S.; Navarro, R.; Urriolabeitia, E. P. Inorg. Chem. 1997, 36,
1136. (d) Reference 1b. (e) Kujime, M.; Hikichi, S.; Akita, M. Organo-
metallics 2001, 20, 4049.
(3) N-Bound complexes: (a) Ricci, J. S.; Ibers, J. A. J. Am. Chem. Soc. 1971,
93, 2391. (b) Tanabe, Y.; Seino, H.; Ishii, Y.; Hidai, M. J. Am. Chem.
Soc. 2000, 122, 1690. (c) Hirano, M.; Takenaka, A.; Mizuho, Y.; Hiraoka,
M.; Komiya, S. J. Chem. Soc., Dalton Trans. 1999, 3209. (d) Murahashi,
S.-I.; Take, K.; Naota, T.; Takaya, H. Synlett 2000, 7, 1016. (e) Naota,
T.; Tannna, A.; Murahashi, S.-I. Chem. Commun. 2001, 63. (f) Reference
2e.
To verify the presence of the intramolecular process, kinetic
studies on the reaction of 3 with an excess amount of PPh3 in
1
benzene-d6 (eq 2) were carried out by H NMR analysis ([3]0 )
2.00 × 10-2 M, [PPh3]0 ) 4.00 × 10-1 M). The formation of the
coordination dimers was retarded to negligible amounts (<1%),
and the consumption rate of 3 showed clean first-order dependence
on [3] at 333-348 K. From the obtained first-order rate constants
k2: 1.31(1) × 10-4 s-1 (333 K); 2.52(2) × 10-4 s-1 (338 K); 4.70(7)
× 10-4 s-1 (343 K); 7.50(20) × 10-4 s-1 (348 K), the ∆Hq and
∆Sq values were estimated to be 121 ( 1 kJ‚mol-1 and 42 ( 4
J‚K-1‚mol-1 by the satisfactory Eyring relationship (R2 ) 1.000),
indicating the intramolecular process in the C-to-N isomerization
of 3.
(4) Naota, T.; Tannna, A.; Murahashi, S.-I. J. Am. Chem. Soc. 2000, 122,
2960.
(5) (a) Naota, T.; Taki, H.; Mizuno, M.; Murahashi, S.-I. J. Am. Chem. Soc.
1989, 111, 5954. (b) Murahashi, S.-I.; Naota, T.; Taki, H.; Mizuno, M.;
Takaya, H.; Komiya, S.; Mizuho, Y.; Oyasato, N.; Hiraoka, M.; Hirano,
M.; Fukuoka, A. J. Am. Chem. Soc. 1995, 117, 12436. (c) Takaya, H.;
Naota, T.; Murahashi, S.-I. J. Am. Chem. Soc. 1998, 120, 4244.
(6) The linkage isomerization of transition metal enolates: (a) Fish, R. H. J.
Am. Chem. Soc. 1974, 96, 6664. (b) Baba, S.; Ogura, T.; Kawaguchi, S.
Bull. Chem. Soc. Jpn. 1974, 47, 665. (c) Komiya, S.; Kochi, J. K. J. Am.
Chem. Soc. 1977, 99, 3695. (d) References 1a-c.
Cleavage of dimer species 5 (see Figure 1 for the structure of 5)
with PPh3 was examined to clarify the suggested intermolecular
pathway. When a 1.25 mM solution of 5 in benzene was heated at
373 K in the presence of PPh3 (4 equiv), the dimeric structure was
collapsed to afford N-bound complex 4 in a quantitative yield (eq
3). The reaction course of the cleavage of 5 with an excess amount
of PPh3 (40 equiv) at 313 K in benzene-d6 showed that the
concentrations of 3 and 4 increase with good linear time-dependence
until conversion of 5 reaches up to 20%. The observed rate constants
for the initial formation of 3 and 4 (kobs ) d[3]/dt, d([4]/dt) were
estimated to be 5.44(11) × 10-8 M‚s-1 (R2 ) 0.996) and 1.03(2)
× 10-8 M‚s-1 (R2 ) 0.998), the latter of which is 1.5 times larger
than that for the initial formation of 4 obtained in the C-to-N
(7) Zwitterionic and R-metalated structures of 1 and (RRu*,SC*)-2 (major
diastereomer) have been confirmed by X-ray diffraction. 1: orthorhombic,
P212121 (No. 19), a ) 17.95 (1) Å, b ) 20.169 (6) Å, c ) 10.467 (5) Å,
V ) 3790(2) Å3, Z ) 4, R ) 0.040, wR2 ) 0.114, GOF ) 1.01.
(RRu*,SC*)-2: triclinic, P-1 (No. 2), a ) 12.77 (1) Å, b ) 13.09 (6) Å,
c ) 11.292 (6) Å, a ) 98.67 (6), b ) 91.78 (7), g ) 61.87 (4), V ) 1643
(2) Å3, Z ) 2, R ) 0.031, wR2 ) 0.067, GOF ) 1.01.
(8) 1H NMR monitoring showed that the concentration of 1 declines with
constant and exclusive formation of 59:41 mixture of (RRu*,SC*)- and
(RRu*,RC*)-2, which indicates that the consumption rate of 1 can be
regarded as a sum of the rate of two independent isomerizations; 1 to
(RRu*,SC*)- and (RRu*,RC*)-2. These diastereomers were also confirmed
to be inert under the kinetic conditions.
isomerization of 3 under similar conditions (6.67(12) × 10-9 M‚s-1
,
R2 ) 0.994).11 These results clearly indicate the presence of both
C-Ru and N-Ru scissions in the cleavage of 5, showing the
inclusion of the intermolecular process in the C-to-N isomerization
of 3.
(9) The reaction of 3 in benzene at room temperature gives a 73:27 mixture
of 5 and 6, while similar treatment in THF affords 5 (22%) as a sole
diastereomer (eq 3). Selected characterization data: 5: IR (KBr) 2195
(CN) cm-1 1H NMR (270 MHz, C6D6) δ 2.95 (d, J ) 5.0 Hz, 2 H,
;
CHCN), 4.75 (s, 10 H, CpH); FABMS m/z 1218 [M]+. 6: 1H NMR (270
MHz, C6D6) δ 4.18 (d J ) 1.5 Hz, 1 H, CHCN), 4.22 (d, J ) 4.2 Hz, 1
H, CHCN), 4.63 (s, 10 H, CpH); FABMS m/z 1218 [M]+. Considering
from unsymmetrical H(1) NMR signals of 6, the stereochemistry of 6
was assigned to be (RRu*,RC*,RRu*,SC*) or (RRu*,RC*,SRu*,RC*).
The schematic representation of intramolecular pathways for
the C-to-N and N-to-C isomerizations are shown in Scheme 1a,
where the metal fragments would undergo slippage on the
C-C-N surfaces and η1-η2 conversion on the coordinated nitriles.
The wide-angle rotation of the η1-η2 conversion can be rationalized
by assuming the bent azaallenyl intermediates (M-NdCdC).
Acceleration with isonitriles could be arising from the ligand-
assisted enhancement of this initiating step. The intermolecular
pathway participates only in the C-to-N isomerization as shown in
Scheme 1b. Due to requirement for the severe conditions in the
cleavage of the coordination dimers, the intermolecular process is
accompanied with the intramolecular one with high temperature
dependence. This is a quite rare example of molecular transforma-
(10) µ2-C,N-Bound structure of the dimer species has been unequivocally
established by X-ray analysis of 5 as shown in Figure 1, although it has
been lying on the level of speculation in the literatures: (a) Cummins,
D.; Higson, B. M.; McKenzie, E. D. J. Chem. Soc., Dalton 1973, 414.
(b) Ros, R.; Renaud, J.; Roulet, R. HelV. Chim. Acta 1975, 58, 133. (c)
Oehme, G.; Ro¨ber, K.-C.; Pracejus, H. J. Organomet. Chem. 1976, 105,
127. (d) References 1d, 2c. Crystallographic data for 5: monoclinic, C2/
c, a ) 17.430 (3) Å, b ) 16.621 (5) Å, c ) 21.903 (3) Å, b ) 96.88 (1)°,
V ) 6299 (2) Å3, Z ) 8, R ) 0.054, wR2 ) 0.136, GOF ) 1.00.
(11) The kinetic data were obtained by the similar 1H NMR analysis of the
isomerization of 3 in the presence of excess PPh3 in benzene-d6 at 313 K
until 10% conversion ([3]0 ) 1.71 × 10-3 M, [PPh3]0 ) 6.83 × 10-2 M).
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