Carbon-carbon bond activation of R-CN (R = ME, Ar, iPr, tBu) using a cationic Rh(III) complex

Article abstract of DOI:10.1021/ja0255094

Addition of 1.0 equiv of Ph₃SiH to [Cp*(PMe₃)Rh(Me)(CH₂Cl₂)]⁺BAr'₄- (1) resulted in release of methane and quantitative formation of [Cp*(PMe₃)Rh(SiPh₃)(CH₂

Full text of DOI:10.1021/ja0255094

Published on Web 03/28/2002
i
t
Carbon-Carbon Bond Activation of R-CN (R ) Me, Ar, Pr, Bu) Using a
Cationic Rh(III) Complex
Felicia L. Taw, Peter S. White, Robert G. Bergman,*,1 and Maurice Brookhart*
Department of Chemistry, UniVersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290
Received January 2, 2002
Scheme 1
Activation of carbon-carbon bonds by metal complexes in
homogeneous media remains a challenge in the field of organo-
metallic chemistry. Success has primarily been limited to systems
in which strain relief or aromatization is a driving force, or where
the C-C bond activation is promoted by directing or activating
groups.2 We report here the C-C bond activation of R-CN (R )
i
t
Me, Ph, (4-CF3)Ph, (4-MeO)Ph, Pr, Bu) using a cationic Rh(III)
complex.
Addition of 1.0 equiv of Ph3SiH to the previously reported
-
complex [Cp*(PMe3)Rh(Me)(CH2Cl2)]+BAr′4 (1, Ar′ ) 3,5-
C6H3(CF3)2) resulted in release of methane and quantitative
-
formation of [Cp*(PMe3)Rh(SiPh3)(CH2Cl2)]+BAr′4 (2, Scheme
1).3 At room temperature, this reaction was complete within seconds
and is analogous to the Si-H activation reaction reported by
Bergman for the corresponding Ir(III) system.4 Addition of 1.0 equiv
of MeCN to 2 caused immediate displacement of dichloromethane
Scheme 2
-
to form the η1-nitrile adduct [Cp*(PMe3)Rh(SiPh3)(NCMe)]+BAr′4
(3). Alternately, addition of MeCN to 1 resulted in formation of
[Cp*(PMe3)Rh(Me)(NCMe)]+BAr′4 (4). Subsequent addition of
-
Ph3SiH to 4 resulted in Si-H activation and release of methane to
form 3 (Scheme 1).
Complex 2 is difficult to isolate as decomposition occurred upon
removal of solvent even at -40 °C. However, the η1-nitrile complex
(3) can be isolated as a thermally sensitive, orange solid. In solution,
3 is stable below -20 °C for prolonged periods of time. However,
upon standing in solution at room temperature overnight, complex
3 converted quantitatively to another product which we have
characterized as the C-C activation product, [Cp*(PMe3)Rh-
-
(Me)(CNSiPh3)]+BAr′4 (5, eq 1).
2 was not isolable, it was generated in-situ by adding 1.0 equiv of
Ph3SiH to a dichloromethane solution of 1 (as in Scheme 1). Adding
1.0 equiv of PhCN to a solution of 2 resulted in quantitative
Complex 5 exhibited in the 1H NMR spectrum resonances attribut-
able to Cp*, PMe3, SiPh3, and Me moieties. Assignment of these
resonances was supported by 13C{1H} NMR data.5 Of note is the
appearance of a signal at δ 173.4 ppm (dd, 1JRh-C ) 23.1 Hz, 2JP-C
) 65.7 Hz),which is diagnostic of the isonitrile carbon. An alternate
structure for 5 is the Rh(V) species [Cp*(PMe3)Rh(SiPh3)(Me)-
(CN)]+BAr′4- resulting from direct oxidative addition of Me-CN.
However, this possibility was ruled out by the 29Si NMR spectrum
of 5 which revealed a singlet at -17.95 ppm, indicating that SiPh3
is not directly bound to the Rh center. Use of CD3CN instead of
-
formation of [Cp*(PMe3)Rh(Ph)(CNSiPh3)]+BAr′4 (6) within 1
h at room temperature (Scheme 2). Similarly, adding 1.0 equiv of
4-trifluoromethylbenzonitrile or 4-methoxybenzonitrile to 2 resulted
in the corresponding C-C activation products (7 and 8, Scheme
2). Complete conversion to complex 7 occurred within 15 min,
and complete conversion to 8 occurred within 4 h.
i
Addition of 1.0 equiv of PrCN to a solution of 2 also resulted
in formation of the C-C activation product, [Cp*(PMe3)Rh(iPr)-
-
(CNSiPh3)]+BAr′4 (9, Scheme 2). This reaction was complete
CH3CN resulted in exclusive formation of [Cp*(PMe3)Rh(CD3)-
within 2.5 days at room temperature. We were able to grow yellow-
orange crystals of 9 in 75% isolated yield (quantitative yield by
NMR). The X-ray crystal structure of 9 is shown in Figure 1.6 This
structure confirmed the Rh-C-N-Si bond linkage and that
-
(CNSiPh3)]+BAr′4
.
In light of these results, we examined other nitrile substrates to
determine the scope of this C-C activation reaction. Since complex
i
* To whom correspondence should be addressed. E-mail: mbrookhart@unc.edu.
cleavage of the Pr-CN bond occurred.
9
4192
J. AM. CHEM. SOC. 2002, 124, 4192-4193
10.1021/ja0255094 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
acquiring a 29Si NMR spectrum allowed characterization of the
intermediate. A singlet corresponding to the intermediate was
observed at δ -21.76 ppm, indicating that the Si atom in the
intermediate is not directly bound to the Rh center. Thus, 12c is
the only plausible intermediate. Precedent for this type of η2-
iminoacyl complex exists in the literature.7 However, none of the
examples exhibit reactivity similar to the system described here.
Parkin has shown that photolysis of an ansa molybdenocene,
[Me2Si(C5Me4)2]MoH2, in the presence of MeCN results in the loss
of H2 and oxidative addition of the C-C bond of MeCN to form
[Me2Si(C5Me4)2]Mo(Me)(CN).8 Examples of C-C cleavage of aryl
cyanides are more common.9 A recent example from Jones showed
that reaction of [(dippe]NiH]2 with PhCN leads to initial formation
of an η2-nitrile complex which then undergoes oxidative addition
to form (dippe)Ni(Ph)(CN).10
Figure 1. ORTEP diagram of [Cp*(PMe3)Rh(iPr)(CNSiPh3)]+ (9).
In this work we have shown that a cationic Rh(III) complex will
C-C activate the bonds of a wide range of nitriles, including cases
involving cleavage of a secondary or tertiary carbon center. With
the exception of tBuCN, facile cleavage of the C-CN bond occurred
quantitatively at 25 °C. Studies are currently underway to extend
the scope and establish full mechanistic details of this reaction.
Figure 2. Possible structures for intermediate.
Acknowledgment. Research performed at UNC was supported
by the NSF (CHE-0107810). Research performed at UCB was
supported by the Director, Office of Energy Research, Office of
Basic Energy Sciences, Chemical Sciences Division, U.S. Depart-
ment of Energy, under Contract No. DE-AC03-76SF00098. We
thank Professor Joseph L. Templeton, Dr. Peter J. Alaimo, and Dr.
Olafs Daugulis for helpful discussions and Dr. Marc ter Horst for
NMR technical assistance.
t
Addition of 1.0 equiv of BuCN to a solution of 2 resulted in
immediate formation of the η1-nitrile adduct [Cp*(PMe3)Rh(SiPh3)-
(NCtBu)]+BAr′4- (10). Complex 10 is relatively stable, and small
amounts of the C-C activation product were observed only after
several days at room temperature. Heating a solution of 10 to 50
°C for 3 days resulted in approximately 50% conversion to
[Cp*(PMe3)Rh(tBu)(CNSiPh3)]+BAr′4- (11, Scheme 2). However,
conversion to 11 was incomplete and only a mixture of decomposi-
tion products were formed after prolonged heating.
Supporting Information Available: Synthesis and characterization
of new compounds, including all crystallographic data for complex 9
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
In the course of our studies on the C-C activation reactions
discussed above, we observed by H and 31P{1H} NMR spectros-
1
copy the appearance of variable amounts (depending on the nitrile
substrate used) of a transient intermediate which grew in as the
reaction progressed and disappeared upon quantitative formation
of product. Possible structures for this intermediate are shown in
Figure 2. The first possibility, 12a, is a Rh(V) species formed by
oxidative addition of R-CN. Migration of the silyl group to
nitrogen would result in the C-C activation product. Complex 12b
is a Rh(III) η2-nitrile complex which can then undergo oxidative
addition of R-CN with subsequent or concerted silyl migration to
form the product. The last possibility, 12c, is a Rh(III) η2-iminoacyl
complex which can form the final product by migration of the R
group to the Rh center.
Reactions involving aryl cyanides exhibited significant build-
up of the transient intermediate species before complete conversion
to product. Thus, we could generate the intermediate at low
temperatures and completely characterize it by NMR spectroscopy.
For example, addition of 4-methoxybenzonitrile to 2 at -40 °C
led to exclusive formation of the η1-nitrile complex [Cp*(PMe3)-
Rh(SiPh3)(NC(4-OMe)Ph)]+BAr′4- (13). A 29Si NMR spectrum of
References
(1) Department of Chemistry, University of California and Division of
Chemical Sciences, Lawrence Berkeley National Laboratory, Berkeley,
CA 94720-1460.
(2) For general reviews, see: (a) Rybtchinski, B.; Milstein, D. Angew. Chem.,
Int. Ed. 1999, 38, 870. (b) Murakami, M.; Ito, Y. In Topics in
Organometallic Chemistry; Murai, S., Ed.; Springer-Verlag: New York,
1999. (c) Crabtree, R. H. Chem. ReV. 1985, 85, 245.
(3) Taw, F. L.; Mellows, H.; White, P. S.; Hollander, F. J.; Bergman, R. G.;
Brookhart, M.; Heinekey, D. M. J. Am. Chem. Soc. 2002. In press.
(4) Klei, S. R.; Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2000, 122,
1816.
(5) See Supporting Information for full spectroscopic details.
(6) See Supporting Information for bond lengths and bond angles.
(7) For a few representative examples, see: (a) Shin, J. H.; Savage, W.;
Murphy, V. J.; Bonanno, J. B.; Churchill, D. G.; Parkin, G. J. Chem.
Soc., Dalton Trans. 2001, 1732. (b) Sa´nchez-Nieves, J.; Royo, P.;
Pellinghelli, M. A.; Tiripicchio, A. Organometallics 2000, 3161. (c) Wu,
Z.; Diminnie, J. B.; Xue, Z. Organometallics 1999, 1002. (d) Daff, P. J.;
Monge, A.; Palma, P.; Poveda, M. L.; Ruiz, C.; Valerga, P.; Carmona, E.
Organometallics 1997, 2263. (e) Bochmann, M.; Wilson, L. M.; Hurst-
house, M. B.; Motevalli, M. Organometallics 1988, 1148. (f) Chetcuti,
P. A.; Knobler, C. B.; Hawthorne, M. F. Organometallics 1988, 650. (g)
Campion, B. K.; Falk, J.; Tilley, T. D. J. Am. Chem. Soc. 1987, 2049.
13 (at -20 °C) revealed a resonance at δ 17.52 ppm (dd, JRh-Si
)
JP-Si ) 20 Hz), corresponding to the silyl group. Since the Si atom
is bound directly to the Rh center, coupling to both 103Rh and 31P
nuclei was observed. A spectrum of the product (8) revealed a
resonance at δ -19.54 ppm (s) corresponding to the silyl group.
Since the Si atom is three bonds away from Rh, no coupling to
either 103Rh or 31P was observed. If a solution of the η1-nitrile
complex is allowed to warm to 15 °C for 20 min, a mixture of the
η1-nitrile complex (13), the intermediate (14), and the product (8)
is observed in an approximate ratio of 5:90:5 (by NMR). Cooling
this reaction to -20 °C to prevent further product formation and
(8) Churchill, D.; Shin, J. H.; Hascall, T.; Hahn, J. M.; Bridgewater, B. M.;
Parkin, G. Organometallics 1999, 18, 2403.
(9) For examples, see: (a) Miller, J. A. Tetrahedron Lett. 2001, 6991. (b)
Abla, M.; Yamamoto, T. J. Organomet. Chem. 1997, 532, 267. (c) Favero,
G.; Morvillo, A.; Turco, A. J. Organomet. Chem. 1983, 241, 251. (d)
Parshall, G. W. J. Am. Chem. Soc. 1974, 2360. (e) Gerlach, D. H.; Kane,
A. R.; Parshall, G. W.; Jesson, J. P.; Muetterties, E. L. J. Am. Chem. Soc.
1971, 3543.
(10) Garcia, J. J.; Jones, W. D. Organometallics 2000, 19, 5544.
JA0255094
9
J. AM. CHEM. SOC. VOL. 124, NO. 16, 2002 4193