Aliphatic carbon-carbon bond activation of nitriles by rhodium(II) porphyrin

Article abstract of DOI:10.1021/om060828f

The aliphatic C(α)-C(β) bonds of a series of nitriles were activated by Rh(tmp) using Ph₃P as the promoting ligand from 100 to 130°C. Kinetic studies at 130°C revealed the rate law, rate = k′K₁[Rh(tmp)][Ph₃P]ⁿ + k ₃K₂(K₁[Ph₃P])/ (1 + K ₁[Ph₃P])[Rh(tmp)][^tBuCN] and suggested the carbon-carbon bond activation occurred at the coordinated ^tBuCN with Rh(tmp) in a 1:1 ratio in the transition state.

Full text of DOI:10.1021/om060828f

20
Organometallics 2007, 26, 20-21
Aliphatic Carbon-Carbon Bond Activation of Nitriles by
Rhodium(II) Porphyrin
Kin Shing Chan,* Xin Zhu Li, Chun Wah Fung, and Lirong Zhang
Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, New Territories, Hong Kong,
People’s Republic of China
ReceiVed September 10, 2006
Table 1. Optimization of CCA Conditions
Summary: The aliphatic C(R)-C(â) bonds of a series of nitriles
were actiVated by Rh(tmp) using Ph3P as the promoting ligand
from 100 to 130 °C. Kinetic studies at 130 °C reVealed the
rate law, rate ) k′K1[Rh(tmp)][Ph3P]n + k3K2(K1[Ph3P])/
(1 + K1[Ph3P])[Rh(tmp)][tBuCN] and suggested the carbon-
amt of PPh3
(equiv)
reaction
time (h)
entry
temp (°C)
product (yielda (%))
1
2
3
4
0
1
1
1
130
110
130
150
120
60
60
Rh(tmp)Me (trace)
Rh(tmp)Me (20)
Rh(tmp)Me (52)
Rh(tmp)Me (30)
Rh(tmp)Et (16)
t
carbon bond actiVation occurred at the coordinated BuCN
48
with Rh(tmp) in a 1:1 ratio in the transition state.
Carbon-carbon bond activation (CCA) by a transition-metal
complex is an active and challenging research area.1 Selective
aliphatic CCA is difficult and is often associated with competi-
tive carbon-hydrogen bond activation (CHA).1 The activation
of the C(R)-CN bond in alkyl or aryl nitriles is of great interest,
because the alkyl or aryl metal cyanides formed have been
proposed as intermediates in a number of important catalytic
processes such as cyanation of aryl halides,2 utilization of aryl
cyanides for cross-coupling,3 arylcyanation of alkynes,4 and
methylation using acetonitrile.5 Most reports of CCA of nitriles
by transition-metal complexes show that it occurs at the C(R)-
CN bonds6 rather than at the aliphatic C(R)-C(â) bonds. The
more facile C(R)-CN activation may likely be steric in origin,
a Average yield of at least two runs.
CCA, we have investigated the reactions of Rh(tmp) with alkyl
nitriles. Herein, we report the discovery of the selective and
ligand-enhanced aliphatic CCA at the C(R)-C(â) positions of
nitriles as well as results of mechanistic studies.
When the prototypical substrate of tert-butyl cyanide was
reacted with Rh(tmp) in benzene at 70 °C for 2 days, no reaction
was observed. When the reaction temperature was increased to
130 °C, a trace of Rh(tmp)Me was detected (Table 1, entry 1
and eq 1). Further optimization of CCA was achieved by the
C6H6, N2
tBuCN
5 equiv
Rh(tmp)Me
2 (trace to 52%)
Rh(tmp) +
8
(1)
since the bond energies of C(R)-CN (103-125 kcal mol-1
)
1
are higher than those of C(R)-C(â) bonds (60-83 kcal mol-1).7
The formation of a strong metal cyanide may also contribute
to the driving force of the activation.
We have reported that rhodium(II) meso-tetramesitylporphyrin
(1; Rh(tmp)8) activates the aliphatic carbon-carbon bonds in
nitroxides9 and ketones.10 In expanding the scope of the aliphatic
addition of Ph3P to form a more electron-rich and reactive
Rh(tmp)Ph3P.10,11 At 110 °C, when 1 equiv of Ph3P was used,
20% of Rh(tmp)Me was obtained in 120 h. At 130 °C, a higher
yield (52%) of Rh(tmp)Me was isolated in 60 h. However, at
150 °C, the reaction gave a mixture of Rh(tmp)Me (30%) and
Rh(tmp)Et (16%). The formation of Rh(tmp)Et was rationalized
from the reaction of Rh(tmp)Me with Rh(tmp) to give Rh(tmp)-
CH2, which further reacted with Rh(tmp)Me to give Rh(tmp)-
Et. Indeed, Rh(tmp) reacted with Rh(tmp)Me at 150 °C in 60
h to give 26% Rh(tmp)Et and recovered Rh(tmp)Me in 43%.
Therefore, the CCA of tert-butyl cyanide appears to have a
narrow temperature range from about 110 to 130 °C.
* To whom correspondence should be addressed. E-mail: ksc@
cuhk.edu.hk.
(1) Crabtree, R. H. Chem. ReV. 1985, 85, 245-269. (b) Murakami, M.;
Ito, Y. In Topics in Organometallic Chemistry; Murai, S., Ed.; Springer:
Berlin, 1999; Vol. 3, pp 97-129. (c) Rybtchinski, B.; Milstein, D. Angew.
Chem., Int. Ed. 1999, 38, 870-883. (d) Jun, C.-H. Chem. Soc. ReV. 2004,
33, 610-618.
(2) For a recent review: Sundermeier, M.; Zapf, A.; Beller, M. Eur. J.
Inorg. Chem. 2003, 3513-3526.
(3) (a) Miller, J. A. Tetrahedron Lett. 2001, 42, 6991-6993. (b) Miller,
J. A.; Dankwardt, J. W. Tetrahedron Lett. 2003, 44, 1907-1910. (c) Miller,
J. A.; Dankwardt, J. W.; Penney, J. M. Synthesis 2003, 1643-1648. (d)
Penney, J. M.; Miller, J. A. Tetrahedron Lett. 2004, 45, 4989-4992.
(4) Nakao, Y.; Oda, S.; Hiyama, T. J. Am. Chem. Soc. 2004, 126, 13904-
13905.
The optimized reaction conditions were then applied to a
series of nitriles (eq 2), and Table 2 gives the results. Except in
PPh3, C6H6, N2
Rh(tmp) +
130 °C, 24-60 h8 Rh(tmp)Me
(2)
R-CN
5 equiv
(5) Nakazawa, H.; Kamdata, K.; Itazaki, M. Chem. Commun. 2005,
4004-4006.
CH3CN, which underwent C(R)-CN activation to give a low
yield of Rh(tmp)Me (Table 2, entry 1), all of the other nitriles
showed selective aliphatic C(R)-C(â) CCA. The yields and
rates of reactions increased with decreasing C(R)-C(â-Me)
(6) (a) Brunkan, N. M.; Brestensky, D. M.; Jones, W. D. J. Am. Chem.
Soc. 2004, 126, 3627-3641. (b) Garc´ıa, J. J.; Are´valo, A.; Brunkan, N.
M.; Jones, W. D. Organometallics 2004, 23, 3997-4002. (c) Taw, F. L.;
Mueller, A. H.; Bergman, R. G.; Brookhart, M. J. Am. Chem. Soc. 2003,
125, 9808-9813. (d) Lu, T.; Zhuang, X.; Li, Y.; Chen, S. J. Am. Chem.
Soc. 2004, 126, 4760-4761.
(9) Zhang, L.; Chan, K. S. J. Organomet. Chem. 2006, 691, 3782-3787.
(10) Mak, K. W.; Yeung, S. K.; Chan, K. S. Organometallics 2002, 21,
2362-2364.
(11) Formation of a 1:1 Rh(tmp)-ligand complex has been reported:
(a) Wayland, B. B.; Sherry, A. E. Bunn, A. G. J. Am. Chem. Soc. 1993,
115, 7675-7684. (b) Collman, J. P.; Boulvtov, R. J. Am. Chem. Soc. 2000,
122, 11812-11821.
(7) (a) McMillen, D. F.; Golden, D. M. Annu. ReV. Phys. Chem. 1982,
33, 493-532. (b) Luo, Y. R. Handbook of Bond Dissociation Energies in
Organic Compounds, 1st ed.; CRC Press: Boca Raton, FL, 2003.
(8) (a) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991,
113, 5305-5311. (b) Tse, M. K.; Chan, K. S. Dalton Trans. 2001, 510-
511.
10.1021/om060828f CCC: $37.00 © 2007 American Chemical Society
Publication on Web 11/29/2006

Communications
Table 2. CCA of Nitriles with Rh(tmp)
Organometallics, Vol. 26, No. 1, 2007 21
tion). Since Rh(tmp) was stable in benzene at 130 °C for 2 days
t
C(R)-C(â) BDE7
time
(h)
yielda
(%)
(Figure S6, Supporting Information), the BuCN-independent
entry
nitriles
(kcal mol-1
)
reaction was found to be dependent on Ph3P, as evidenced by
the spectral change of Ph3Rh(tmp) at 130 °C (Figure S7,
Supporting Information). We rationalized that Ph3PRh(tmp)
undergoes disproportionation (k′) to RhI(tmp)- and (Ph3P)2RhIII-
1
2
3
4
5
6
7
8
9
MeCN
EtCN
Me2CHCN
Me3CCN
Me2C(CN)2
PhMeCHCN
PhMe2CCN
Ph2MeCCN
4-MeC6H4CMe2CN
4-CF3C6H4CMe2CN
60
48
48
24
48
36
24
36
24
24
10
38
41
52
47
48
61
38
57
65
83.2
79.5
74.7
76
63
59.9
t
(tmp)+ at high temperature.11,17,18 For the BuCN-dependent
process relevant to aliphatic CCA, the rate was found to be first
t
order in both Rh(tmp) and BuCN and exhibited saturation
kinetics in excess Ph3P.14
A proposed mechanism for CCA conforming to the kinetic
evidence is depicted in Scheme 1. Initially, facile coordination
of Ph3P with Rh(tmp) occurs to give Ph3PRh(tmp) (k1), which
10
a Average isolated yield of at least two runs.
t
is then complexed by BuCN to give Ph3PRh(tmp)tBuCN (k2).
bond dissociation energy (BDE).7 The absence of Rh(tmp)Ph
suggests that the reaction goes through a radical12 rather than a
concerted oxidative addition process via an elusive Rh(IV).13,14
The slight decrease in reactivity in Ph2MeCCN as compared to
that in PhMe2CCN (Table 2, entry 8 vs 7) can be accounted for
by an increase in large steric hindrance caused by the phenyl
group. The electron-withdrawing CF3 appeared to increase the
yield of Rh(tmp)Me slightly to 65% in comparison to the more
electron rich Me-substituted species (Table 2, entries 9 and 10
vs 7).
Competitive carbon-hydrogen bond activation at the benzylic
or aliphatic hydrogens did not likely occur to give Rh(tmp)H.15
If there were any Rh(tmp)H formed, PhMe2CCHdNH[Rh(tmp)]
would have been observed, as Rh(tmp)H was found to react
with PhMe2CCN to give PhMe2CCHdNH[Rh(tmp)] in 27%
yield at 130 °C in 24 h. The absence of other tertiary Rh alkyls
is likely due to the instability of the tertiary rhodium macrocycle
caused by steric hindrance.16
The mode of nitrile coordination is likely N-bound rather than
side-bound, to minimize steric hindrance. Subsequently, Rh
t
abstracts the methyl group of loosely bound BuCN to yield
Ph3PRh(tmp)Me in the transition state in the rate-determining
step (k3), which is in contrast with the termolecular transition
state in the activation of methane by Rh(tmp).8,15 The fate of
the nitrile-containing fragment, presumably a carbon-centered
radical, remains unclear. We were not able to detect any
coupling product of (NCMe2C)2 in the reaction mixture of
Rh(tmp) with tBuCN by GC-MS. Presumably, NCMe2C readily
abstracts a hydrogen atom rather than coupling with either itself
or Rh(tmp).19,20
Scheme 1
Rh(tmp) + Ph3P yk1z Ph3PRh(tmp)
(4)
k-1
Ph3PRh(tmp) + Me3CCN yk2z Ph3PRh(tmp)NCCMe3 (5)
k-2
Kinetic studies were carried out to gain further mechanistic
insight into the CCA. First, the stoichiometry and binding
constants of Ph3P with Rh(tmp) were measured spectrally at
522 nm from 20 to 50 °C (Table S1 and Figure S1, Supporting
Information) and analyses of the data confirmed the formation
of 1:1 complex and yielded K1,130 °C(extrapolated) ) 10.7 M-1
()k1/k-1), ∆H1 ) -12 ( 1 kcal mol-1, and ∆S1 ) -26 ( 2
cal mol-1 K-1 (Figure S2, Supporting Information).
k3
Ph3PRh(tmp)NCCMe3
98 Ph3PRh(tmp)Me + Me2CCN (6)
rds
Ph3PRh(tmp)Me 8 Rh(tmp)Me + Ph3P
(7)
fast
In summary, the selective aliphatic C(R)-C(â) bonds of a
series of nitriles have been activated by Rh(tmp) using PPh3 as
the promoting ligand. The CCA was affected by bond energy,
steric hindrance, and the small electronic effect of aryl aceto-
nitriles. No C(aryl)-C(R) bond nor CH activation was observed.
Further studies of bond activation are ongoing.
Kinetic measurements of the reaction of Rh(tmp) and tBuCN
with excess Ph3P carried out spectrally at 522 nm under the
conditions 130 °C and initial concentrations (5.55-16.65) ×
t
10-5 M Rh(tmp), (2.97-8.90) × 10-3 M BuCN, and 1.10 ×
10-3 M Ph3P for at least 4 half-lives yielded the rate law shown
in eq 3.
Acknowledgment. We thank the Research Grants Council
of Hong Kong of the SAR of China for financial support (No.
400203).
-d[Rh(tmp)]/dt ) k′K1[Rh(tmp)]0[Ph3P]n + k3K2(K1[Ph3P])/
(1 + K1[Ph3P])[Rh(tmp)]0[tBuCN] (3)
Supporting Information Available: Text, tables, and figures
giving experimental details and kinetic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
The rate is composed of two parallel processes, with the first
t
t
process being BuCN independent and the second one BuCN
OM060828F
dependent (Figures S3-5 and Table S2, Supporting Informa-
(17) Wayland, B. B.; Balkus, K. J., Jr.; Franos, M. D. Organometallics
1989, 8, 950-955.
(12) (a) Ingold, K. U.; Roberts, B. P. Free Radical Substitution Reactions;
Wiley: New York, 1971. (b) Davis, A. G.; Roberts, B. P. In Free Radicals;
Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. 2, pp 547-589.
(13) Pestovsky, O.; Bakac, A. Inorg. Chem. 2002, 15, 3975-3982.
(14) Chan, K. S.; Zhang, L.; Fung, C. W. Organometallics 2004, 23,
6097-6098.
(18) The kinetic order of Rh(tmp) was preliminarily estimated from the
first-order fit of the absorbance change. We did not investigate the kinetic
order of Ph3P: i.e., n in eq 3.
(19) No tBuRh(tmp) was observed in the reaction of tBuNC with
Rh(tmp).11a The formation of Rh(tmp)CMe2CN is likely sterically hindered.
(20) Reaction of BrCMe2CN with Rh(tmp) gave Rh(tmp)Br but no
Rh(tmp)CMe2CN. However, the sterically less hindered BrCH2CN reacted
with Rh(tmp) at room temperature in 1 h to give Rh(tmp)Br and Rh(tmp)-
CH2CN in 38 and 47%, yields, respectively. These reactions demonstrated
the importance of steric effects in coupling.
(15) (a) Cui, W.; Zhang, X. P.; Wayland, B. B. J. Am. Chem. Soc. 2003,
125, 4994-4995. (b) Cui, W.; Wayland, B. B. J. Am. Chem. Soc. 2004,
126, 8266-8274.
(16) Giese, B.; Hartung, J.; Kesselheim, C.; Lindner, H. J.; Svoboda, I.
Chem. Ber. 1993, 126, 1193-1200.