Synthesis and characterization of C- and N-bound isomers of transition metal α-cyanocarbanions [9]

Full text of DOI:10.1021/ja994387l

2960
J. Am. Chem. Soc. 2000, 122, 2960-2961
Synthesis and Characterization of C- and N-Bound
Isomers of Transition Metal r-Cyanocarbanions
Takeshi Naota,* Akio Tannna, and Shun-Ichi Murahashi*
Department of Chemistry
Graduate School of Engineering Science
Osaka UniVersity, Machikaneyama
Toyonaka, Osaka 560-8531, Japan
ReceiVed December 16, 1999
Transition metal R-cyanocarbanions are the subjects of con-
siderable attention in relation to the enolate chemistry of transition
metals,1 and also as a key intermediate for a family of catalytic
carbon-carbon bond forming reactions of nitriles with electro-
philes under neutral conditions.2,3 According to numerous studies
on the preparation of transition metal R-cyanocarbanions, two
types of structures, C-1,4 and N-bound ones,2b,5,6 are known as
stable forms. The C-bound complexes have been widely used as
R-cyanoalkylating reagents,1 while the N-bound ones, derived by
R-C-H activation of nitriles, have recently proven to be active
species for catalytic aldol and Michael reactions of nitriles.2b,5a
The basic studies on the dynamic behavior of C- and N-bound
complexes, e.g., reactivities toward electrophiles and intercon-
versions, are of particular importance since they will provide
significant information on the design and creation of novel C-C
bond forming process of nitriles via R-C-H activation.2,3
Synthesis of both exact isomers will lead us directly to the
systematic investigations; however, difficulties in obtaining the
isomers have long been preventing such a study.
Figure 1. Molecular structure of 1b. Thermal ellipsoids are shown at
the 30% probability level. Selected bond lengths (Å) and angles (deg):
Ru-P(1), 2.290(3); Ru-P(2), 2.289(3); Ru-C(1), 2.17(1); S-C(1),
1.80(1); C(1)-C(2), 1.45(2); C(2)-N, 1.17(1); S-O(1), 1.429(8);
S-O(2), 1.445(8); P(1)-Ru-P(2), 71.8(1); P(1)-Ru-C(1), 92.0(3);
P(2)-Ru-C(1), 101.7(3); Ru-C(1)-S, 112.4(5); Ru-C(1)-C(2), 113.1(8);
S-C(1)-C(2), 107.7(8).
resonance structure of enolate enables both isomers to be present
in a stable form. In this paper, we describe the first synthesis of
C- and N-bound isomers of transition metal R-cyanocarbanion,
RuCp[CH(CN)SO2Ph](PR3)2 (1) and Ru+Cp(NCCH-SO2Ph)(PR3)2
(2), and the first observation of their specific isomerizations.
The reaction of RuCpCl(PPh3)2 with the sodium salt of
(phenylsulfonyl)acetonitrile in EtOH/toluene (1:1) under argon
atmosphere at 25 °C gave the corresponding N-bound complex,
Ru+Cp(NCCH-SO2Ph)(PPh3)2 (2a), as the sole product in 89%
yield. When a similar reaction was carried out in EtOH/hexane
(1:1), the corresponding C-bound isomer, RuCp[CH(CN)SO2Ph]-
(PPh3)2 (1a), was formed as a 59:41 mixture of 1a and 2a.
Filtration of the precipitate under argon atmosphere gave pure
1a in 37% yield. Various N- and C-bound complexes of tertiary
phosphines can be prepared either by a similar treatment of
RuCpCl(PR3)2 or the ligand exchange of 1a or 2a with phosphines.
As a part of the program aiming at the development of novel
catalytic C-C bond forming reactions of nitriles,2 we are
investigating the structure and reactivity of transition metal
R-cyanocarbanion intermediates. We have designed a model
system for ruthenium phosphine complexes of R-cyanocarbanion
bearing an R-sulfonyl group, whose small contribution to the
1
These complexes are characterized by H, 13C NMR, IR, mass
spectra, and elemental analyses.
The almost complete R-metalated and zwitter ionic structures
of the C- and N-bound complexes have been unequivocally
established by X-ray analysis. Figure 1 shows the molecular
structure of RuCp[CH(CN)SO2Ph](dppm) (1b, dppm ) Ph2PCH2-
PPh2).7 The Ru-C(1) bond distance (2.12 Å) lies in the range of
a normal metal-carbon single bond of MCH2CN (2.1-2.2 Å).4d,f,g
The bond angles around the C(1) atom indicate sp3 configuration
of C(1). The C(2)-N bond distance (1.18 Å) is within the normal
range of the CN triple bond of free nitrile, and the C(1)-C(2)-N
bond angle is almost linear (178°). These observations show the
carbon-metal bond of 1b has complete σ-character and any
contribution to η2-coordination is negligible. The molecular
structure of N-bound complex 2a is illustrated in Figure 2.7 The
(1) (a) Stack, J. G.; Doney, J. J.; Bergman, R. G.; Heathcock, C. H.
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(6) Linear zwitter ion (M+-NC-C-R1R2) and bent azaallene (M-NdCd
CR1R2) structures have been an argument for the N-bound complex of transition
metals; however, the reported X-ray analyses indicate a strong contribution
of zwitter ionic structure.2b,5a-h
(7) Crystallographic data. 1b: orthorhombic, Pbca, a ) 42.400(1) Å, b )
19.648(7) Å, c ) 17.943(8) Å, V ) 14947(8) Å3, Z ) 16, R ) 0.055, Rw
)
0.069, GOF ) 1.74. 2a: monoclinic, P21/n, a ) 10.298(2) Å, b ) 22.889(4)
Å, c ) 17.333(1) Å, â ) 95.06(1)°, V ) 4069.5(10) Å3, Z ) 4, R ) 0.038,
Rw ) 0.039, GOF ) 1.33.
10.1021/ja994387l CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/10/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 12, 2000 2961
Scheme 1
standard. The reactions of inert C-bound complex 1c with
phosphines such as dppb (3d, dppb ) Ph2P(CH2)4PPh2), dppf (3e,
dppf ) 1,1′-bis(diphenylphosphino)ferrocene), and dcype (3f,
dcype ) 1,2-bis(dicyclohexylphosphino)ethane) afford the cor-
responding N-bound complexes 2d-f as a sole product in 79,
75, and 71% yields, respectively. On the contrary, similar
treatment with dppm (3b), dppe (3g, dppe ) Ph2PCH2CH2PPh2),
dmpe (3h, dmpe ) Me2PCH2CH2PMe2), and PMe2Ph (3i) gives
rise to only ligand exchange reactions to afford C-bound
complexes 1b,g-i. Almost the same preferences of C- and
N-bound structures have been observed in the N-to-C isomeriza-
tions. Thus, treatment of inert N-bound complex 2a with 3d-f
and P(i-Pr)3 (3j) gives the corresponding N-bound complexes 1d-
f,j, while addition of 3b,c,g-i affords the C-bound complexes
1b,c,g-i selectively in 51, 85, 72, 90, and 82% yields.
One plausible factor for determining the stable structures is
the steric circumstance around the metal center, where ligation
of bulky phosphines would favor the end-on coordination of the
N-bound complexes rather than the carbon-metal bond formation
of the C-bound ones. Indeed, the order of cone angles8,9 of tertiary
phosphines is well in accord with the tendency of relative thermal
stabilities of C- and N-bound complexes as shown in Scheme 1.
π-Basicity of ligands is not a major factor, as is clearly indicated
by the fact that two bidentate trialkyl phosphines, dmpe and dcype,
show different preferences. These isomerizations would proceed
intramolecularly via η2-coordination10 of nitriles, although the
intermolecular process including assembly of metals by bridging
R-cyanocarbanion moieties is also under consideration.
Figure 2. Molecular structure of 2a. Thermal ellipsoids are shown at
the 30% probability level. Selected bond lengths (Å) and angles (deg):
Ru-N, 2.050(4); Ru-P(1), 2.313(1); Ru-P(2), 2.333(1); N-C(2),
1.150(6); C(2)-C(1), 1.367(7); S-C(1), 1.662(6); S-O(1), 1.452(4);
S-O(2), 1.430(4); P(1)-Ru-P(2), 100.59(4); P(1)-Ru-N, 88.2(1);
P(2)-Ru-N, 91.1(1); Ru-N-C(2), 171.9(4); N-C(2)-C(1), 176.1(5);
C(2)-C(1)-S, 122.9(4).
linearity of the Ru-N-C(2)-C(1) atoms and the C(2)-C(1)-S
bond angle of 123° indicate the strong contribution of zwitter
ionic structure2b,5a-h including η1-coordination of R-cyanocarban-
ion, rather than the bent structure of azaalenyl metal.
The C-bound PPh3 complex 1a is less stable and readily
converted into the N-bound isomer 2a in solution. Upon heating
a 1.25 mM solution of 1a in benzene at 60 °C under argon
atmosphere, complex 1a is converted quantitatively into 2a after
3 h, while complex 2a is thermally stable in various boiling
solvents such as benzene, THF, and EtOH. Relative thermal
stabilities of the C- and N-bound complexes have proven to be
changed drastically depending on the choice of phosphine ligands.
In sharp contrast to the PPh3 complexes, C-bound dppm complex
1b is inert in various boiling solvents, while N-bound complex
2b undergoes quantitative N-to-C isomerization upon refluxing
in benzene for 24 h.
In conclusion, we have shown the first synthesis and charac-
terization of C- and N-bound isomers of R-cyanocarbanions.
Reactivity and catalysis of the C- and N-bound complexes toward
carbon electrophiles have been investigated.2 Catalytically active
species will be readily controlled by steric factors of external
ligands, overcoming their original stabilities mainly arising from
R-substituent of nitriles. These studies will serve as a new
approach for exploitation of novel and fundamental catalytic C-C
bond-forming reactions of nitriles fulfilling the future requirement
toward efficiency, selectivity, and atom economy.11 Efforts are
currently underway to provide the full scope of the reactions and
definitive mechanistic information and to apply the present
reactions to other systems.
To obtain a perspective view of the influence of phosphines
for thermal stabilities of C- and N-bound complexes, we
investigated the isomerization of 1 and 2 by the ligand exchange
reactions with various tertiary phosphines. A 1.25 mM solution
of thermally stable RuCp[CH(CN)SO2Ph](PMePh2)2 (1c) or 2a
in benzene was refluxed for 24 h in the presence of mono- and
diphosphines 3 (monophosphine, 4 equiv; diphosphine, 2 equiv)
under argon atmosphere. Yields of the products were determined
by 1H NMR analysis of the reaction mixtures using internal
Acknowledgment. This work was supported by Research for the
Future program, the Japan Society for the Promotion of Science, and a
Grant-in-Aid for Scientific Research, the Ministry of Education, Science,
Sports, and Culture, Japan.
(8) Tolman, C. A. Chem. ReV. 1977, 77, 313.
(9) The values for dppb and dppf were estimated from crystallographic
data of RuCl2(dppb)(PPh3) (MacFarlane, K. S.; Joshi, A. M.; Rettig, S. J.;
James, B. R. Inorg. Chem. 1996, 35, 7304) and RuCp(dppf)(CtCC5H4N-
{W(CO)4(PPh3)}) (Wu, I.-Y.; Lin, J. T.; Luo, J.; Sun, S.-S.; Li, C.-S.; Lin, K.
J.; Tsai, C.; Hsu, C.-C.; Lin, J.-L. Organometallics 1997, 16, 2038),
respectively.
Supporting Information Available: Experimental details for all
compounds and crystallographic data for 1b and 2a (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(10) (a) Andrews, M. A.; Knobler, C. B.; Kaesz, H. D. J. Am. Chem. Soc.
1979, 101, 7260. (b) Barrera, J.; Sabat, M.; Harman, W. D. J. Am. Chem.
Soc. 1991, 113, 8178.
(11) Murahashi, S.-I.; Naota, T. Bull. Chem. Soc. Jpn. 1996, 69, 1805.
JA994387L