Metalloradical activations of aliphatic carbon-carbon bonds of nitriles: Scope and mechanism

Article abstract of DOI:10.1021/om070064j

The C(sp³)-C(sp³) bonds of a series of α-alkylacetonitriles, 2-silylacetonitriles and 2-alkylbenzonitriles, have been activated by Rh(tmp) using Ph₃P as the optimized promoter ligand at 130°C. Selective aliphatic-aliphatic carbon-carbon bond activation (CCA) occurred for α-alkylacetonitriles and 2-alkylbenzonitriles without aromatic-aliphatic or aromatic-cyanide bond activation. Competitive activations of C-Si and C-C bonds were observed for 2-silylacetonitriles. The yields of Rh(tmp) alkyls were affected by bond energy and steric hindrance of the nitriles. Kinetic studies for the carbon-carbon bond activation (CCA) of ^tBuCN at 130°C revealed the rate law: rate = k′K ₁[Rh(tmp)]^m[Ph₃P]ⁿ + k ₃K₂(K₁[Ph₃P])/(1 + K ₁[Ph₃P])[Rh(tmp)][^tBuCN]. The CCA is proposed to occur at the coordinated ^tBuCN with Rh(tmp) in a 1:1 ratio in the transition state.

Full text of DOI:10.1021/om070064j

Organometallics 2007, 26, 2679-2687
2679
Metalloradical Activations of Aliphatic Carbon-Carbon Bonds of
Nitriles: Scope and Mechanism
Kin Shing Chan,* Xin Zhu Li, Lirong Zhang, and Chun Wah Fung
Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, New Territories, Hong Kong,
People’s Republic of China
ReceiVed January 22, 2007
The C(sp3)-C(sp3) bonds of a series of R-alkylacetonitriles, 2-silylacetonitriles and 2-alkylbenzonitriles,
have been activated by Rh(tmp) using Ph3P as the optimized promoter ligand at 130 °C. Selective
aliphatic-aliphatic carbon-carbon bond activation (CCA) occurred for R-alkylacetonitriles and 2-alky-
lbenzonitriles without aromatic-aliphatic or aromatic-cyanide bond activation. Competitive activations
of C-Si and C-C bonds were observed for 2-silylacetonitriles. The yields of Rh(tmp) alkyls were affected
by bond energy and steric hindrance of the nitriles. Kinetic studies for the carbon-carbon bond activation
(CCA) of tBuCN at 130 °C revealed the rate law: rate ) k′K1[Rh(tmp)]m[Ph3P]n + k3K2(K1[Ph3P])/(1 +
t
K1[Ph3P])[Rh(tmp)][tBuCN]. The CCA is proposed to occur at the coordinated BuCN with Rh(tmp) in
a 1:1 ratio in the transition state.
of acetonitrile was reported by Nakazawa.2g Silylation of aryl
nitrile catalyzed by Rh was also documented.7
Introduction
Carbon-carbon bond activation (CCA) by a transition metal
complex remains a great challenge in the field of organometallic
chemistry.1 The activation of the C(R)-CN bond in alkyl or
aryl nitriles is an important process in fundamental studies and
organic syntheses.2
Most reports of the activation of nitriles by transition metal
complexes occur at the C(R)-CN3,8 bond rather than the
aliphatic C(R)-C(â) bonds, although the C(R)-CN bond
energies (BDE ) 103-125 kcal mol-1) are higher than those
of C(R)-C(â) bonds (BDE ) 60-83 kcal mol-1).9 The
formation of a strong metal-cyanide bond may account for the
driving force of the activation.
Rhodium(II) tetramesitylporphyrin, Rh(tmp) 1 (Figure 1), is
a metalloradical and monomeric10,11 in solution. It exhibits rich
chemistry, especially in bond activations.12 We have reported
that Rh(tmp)12h,13 1 activates the aliphatic carbon-carbon bonds
in nitroxides12h and ketones12i and the silicon-nitrogen bond
in silyl and alkyl isonitriles.14 In expanding the scope of the
Some stoichiometric activations of nitriles have been docu-
mented. Bergman and Brookhart reported CCA of nitriles at
room temperature using a cationic silyl Rh(III) complex in
excellent yields.3 The C-CN bond of acetonitrile can be
activated by [Me2Si(C5Me4)2]MoH24 under photochemical con-
ditions and the dinuclear copper(II) cryptate.5
Catalytic nitrile-group transfer from alkyl nitriles as solvents
to aryl halides by Ni, Pd, or Pt in the presence of Zn has been
utilized in organic syntheses.6 The nickel-catalyzed cross-
couplings of alkyl and alkenyl Grignard reagents with aryl
nitriles afford good yields of the aryl alkanes or aryl alkenes
via the activation of aryl C-CN bonds.2b,c Photochemical CCA
(7) Tobisu, M.; Kita, Y.; Chatani, N. J. Am. Chem. Soc. 2006, 128, 8152-
8153.
(8) (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.
* Corresponding author. 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:
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Chem., Int. Ed. 1999, 38, 870-883. (d) Jun, C.-H. Chem. Soc. ReV. 2004,
33, 610-618.
(2) (a) For a recent review: Sundermeier, M.; Zapf, A.; Beller, M. Eur.
J. Inorg. Chem. 2003, 3513-3526. (b) Miller, J. A. Tetrahedron Lett. 2001,
42, 6991-6993. (c) Miller, J. A.; Dankwardt, J. W. Tetrahedron Lett. 2003,
44, 1907-1910. (d) Miller, J. A.; Dankwardt, J. W.; Penney, J. M. Synthesis
2003, 1643-1648. (e) Penney, J. M.; Miller, J. A. Tetrahedron Lett. 2004,
45, 4989-4992. (f) Nakao, Y.; Oda, S.; Hiyama, T. J. Am. Chem. Soc.
2004, 126, 13904-13905. (g) Nakazawa, H.; Kamdata, K.; Itazaki, M.
Chem. Commun. 2005, 4004-4006. (h) Nakazawa, H.; Kawasaki, T.;
Miyoshi, K.; Suresh, C. H.; Koga, N. Organometallics 2004, 23, 117-
126.
(3) (a) Taw, F. L.; White, P. S.; Bergman, R. G.; Brookhart, M. J. Am.
Chem. Soc. 2002, 124, 4192-4193. (b) Taw, F. L.; Mueller, A. H.; Bergman,
R. G.; Brookhart, M. J. Am. Chem. Soc. 2003, 125, 9808-9813.
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M.; Parkin, G. Organometallics 1999, 18, 2403-2406.
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4760-4761.
(6) Luo, F.-H.; Chu, C.-I.; Cheng, C.-H. Organometallics 1998, 17,
1025-1030.
(9) (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.
(10) (a) Wayland, B. B. Polyhedron 1988, 7, 1545-1555. (b) Wayland,
B. B.; Sherry, A. E. Inorg. Chem. 1992, 31, 148-150.
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27, 2745-2747.
(12) (a) Sherry, A. E.; Wayland, B. B. J. Am. Chem. Soc. 1990, 112,
1259-1261. (b) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc.
1991, 113, 5305-5311. (c) Del Rossi, K. J.; Zhang, X. X.; Wayland, B. B.
J. Organomet. Chem. 1995, 504, 47-56. (d) Del, Rossi, K. J.; Wayland,
B. B. J. Chem. Soc., Chem. Commun. 1986, 1653-1655. (e) Wayland, B.
B.; Poszmik, G.; Fryd, M. Organometallics 1992, 11, 3534-3542. (f)
Anderson, J. E.; Yao, C.-L.; Kadish, K. M. Inorg. Chem. 1986, 25, 718-
719. (g) Poszmik, G.; Carroll, P. J.; Wayland, B. B. Organometallics 1993,
12, 3410-3417. (h) Carbon-carbon bond activation occurred only at 70
°C in 4 h, but not at room temperature. See: Tse, M. K.; Chan, K. S. J.
Chem. Soc., Dalton Trans. 2001, 510-511. (i) Zhang, L.; Chan, K. S. J.
Organomet. Chem. 2006, 691, 3782-3787.
(13) Mak, K. W.; Yeung, S. K.; Chan, K. S. Organometallics 2002, 21,
2362-2364.
(14) (a) Chan, K. S.; Zhang, L.; Fung, C. W. Organometallics 2004, 23,
6097-6098. (b) Zhang, L.; Fung, C. W.; Chan, K. S. Organometallics 2006,
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10.1021/om070064j CCC: $37.00 © 2007 American Chemical Society
Publication on Web 04/10/2007

2680 Organometallics, Vol. 26, No. 10, 2007
Chan et al.
Scheme 1. Proposed Mechanism for Forming
Rh(tmp)CH2CH3
electron-rich aryl phosphine (4-MeOPh)3P yielded Rh(tmp)Me
(Table 2, entry 4), presumably due to Me-O bond cleavage.
Finally, Ph3P was found not to yield any Rh(tmp)Ph (Table 2,
entry 5) and appeared to be stable. Therefore, Ph3P was chosen
to be the promoter ligand.
Table 1 lists the results of the optimization of CCA with the
addition of Ph3P to form a more electron-rich and reactive Rh-
(tmp)Ph3P.13,16 In the presence of Ph3P, a higher, 52% yield of
Rh(tmp)Me was isolated in 24 h at 130 °C than that at 110 °C
(Table 1, entries 3 and 4). However, at 150 °C, the reaction
gave a mixture of Rh(tmp)Me 2a (30%) and Rh(tmp)Et 2b
(16%) (Table 1, entry 5). 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 (Scheme 1). Indeed, an independent reaction
of Rh(tmp) with Rh(tmp)Me, prepared in about 1:1 ratio from
the incomplete photolyis of Rh(tmp)Me,10,11 produced Rh(tmp)-
Et and recovered Rh(tmp)Me in 26% and 43% yields, respec-
tively (eq 3). Therefore, the CCA of tert-butyl cyanide appears
to have a narrow temperature range of 110-130 °C.
Figure 1. Structure of Rh(tmp).
Table 1. Optimization of CCA Conditions
entry
PPh3/equiv
temp/°C
time/h
product and yielda/%
1
2
3
4
5
0
0
1
1
1
70
130
110
130
150
48
120
60
24
48
no reaction
Rh(tmp)Me 2a (trace)
Rh(tmp)Me 2a (20)
Rh(tmp)Me 2a (52)
Rh(tmp)Me 2a (30)
Rh(tmp)Et 2b (16)
a Average yield of at least two runs.
Table 2. Effect of Phosphine Ligand
entry
phosphine
time/days
product yields/%
1
2
3
4
5
Ph2PMe
Ph2PEt
Ph2PBu
(4-MeOPh)3P
Ph3P
2
3
4
1
7
Rh(tmp)Me 2a (21)
Rh(tmp)Et 2b (23)
Rh(tmp)Bu 2c (26)
Rh(tmp)Me 2a (21)
no Rh(tmp) alkyls
aliphatic CCA, we have found that Rh(tmp) underwent selective
aliphatic C(R)-C(â) activation of alkyl nitriles.15 Herein, we
disclose the full results of CCA of nitriles with Rh(tmp) and
mechanistic studies.
CCA of Rh(tmp) with Alkyl Nitriles and R-Alkylphenyl-
acetonitriles. A series of nitriles were then examined to
investigate the scope of CCA using the optimized conditions
(eq 4). Table 3 lists the results of successful aliphatic CCA.
CH3CN underwent C(R)-CN activation to give a low yield of
Rh(tmp)Me (Table 3, entry 1). Although the C(R)-CΝ bond
is strong (BDE ) 124.7 kcalmol-1),9b this successful CCA is
likely due to an unhindered methyl group adjacent to a CN group
coordinated to Rh(tmp). For nitriles 3c-3j, the C(R)-CΝ bonds
are much stronger than the C(R)-C(â) bonds.9b Consequently,
C(R)-C(â) bonds of the nitriles (Table 3, entries 2-8) were
preferentially cleaved. In entries 6-8, no C(phenyl)-C(alkyl)
Results and Discussion
Optimization of Reaction Conditions. tert-Butyl cyanide,
without any R-acidic proton,12i was used as the protocol substrate
to examine the feasibility of CCA.12i When tBuCN was reacted
with Rh(tmp) in benzene at 70 °C for 2 days, no reaction was
observed (eq 1, Table 1, entry 1). Upon increasing the reaction
temperature to 130 °C, a trace amount of Rh(tmp)Me was
detected (Table 1, entry 2).
bond activation was observed, likely due to the fact that Caryl
-
Calkyl bonds (BDE(Ph-Me) ) 102 kcal mol-1) are stronger than
C(alkyl)-C(alkyl) bonds and kinetically more hindered. The
yields and rates of reactions increased with decreasing C(R)-
C(â-Me) bond dissociation energy (BDE)19 except that of
2-methylphenylacetonitrile (Table 3, entry 6), in which the
increased steric hindrance of the additional phenyl group likely
disfavors the coordination with Rh(tmp). In 2,2-dimethylphe-
nylacetonitrile (entry 7), the steric hindrance also increased. The
stronger effect of the decreased bond energy of the C(R)-C(â)
bond, however, dominates the effect of increased steric hin-
drance to give a higher CCA yield. The absence of Rh(tmp)Ph
suggests that the reaction goes through a radical17 rather than
concerted oxidative addition process via an elusive Rh(IV).14,18
The absence of any CCA product in the reaction with PhCN at
130 °C for 7 days further attests to this proposal. Electron-
withdrawing CF3 appeared to increase the yield of Rh(tmp)Me
Since ligands can coordinate to Rh(tmp) to form five-
coordinate complexes16 with enhanced reactivity, electron-rich
phosphines were then examined. However, the control reactions
of phosphines with Rh(tmp) (eq 2, Table 2, entries 1-3) at
130 °C yielded carbon-phosphorus bond activation products
of Rh(tmp) alkyls in low yields. Unfortunately, even the
(15) Chan, K. S.; Li, X. -Z.; Fung, C. W.; Zhang, L. Organometallics
2007, 26, 20-21.
(16) Formation of the Rh(tmp)-ligand 1:1 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.
(17) (a) Ingold, K. U.; Roberts, B. P. Free Radical Substitution Reactions;
Wiley: New York, 1971. (b) Davis, A. G.; Roberts, B. P. Free Radicals;
Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. 2, pp 547-589.
(18) Pestovsky O.; Bakac, A. Inorg. Chem. 2002, 15, 3975-3982.
(19) Kametani, T.; Yukawa, H.; Suzuki, K.; Honda, T. J. Chem. Soc.,
Perkin Trans. 1 1985, 2151-2154.
(20) Culkin, D. A.; Hatwig, J. F. J. Am. Chem. Soc. 2002, 124, 9330-
9331.

Metalloradical ActiVations of Aliphatic C-C Bonds of Nitriles
Table 3. CCA of Nitriles
Organometallics, Vol. 26, No. 10, 2007 2681
BDE7 CR-CN/
BDE7 C(R)-C(â)/
entry
nitrilea
MeCN 3b
kcal mol-1
kcal mol-1
time/h
% yieldb
1
2
3
4
5
6
124.7
121.1
120.4
117.2
60
48
48
24
48
36
10
38
41
52
47
48
EtCN 3c
83.2
79.5
74.7
76
Me2CHCN 3d
Me3CCN 3a
Me2C(CN)2 3e
PhMeCHCN 3f
104.6
102.8
63 (CMe-CCH)
92.7 (CPh-CCH
)
7
PhMe2CCN19 3g
59.9 (CMe-CC)
90.6 (CPh-CC)
24
61
8
9
10
Ph2MeCCN 3h
36
24
24
38
57
65
4-MeC6H4CMe2CN20 3i
4-CF3C6H4CMe2CN20 3j
b
a 5 equiv of nitriles. Average isolated yield of at least two runs.
Scheme 2. Proposed Mechanism of CCA
t
Scheme 3. Transition State of the Reaction of Rh(tmp) with BuCN
slightly to 65% when compared to the more electron-rich Me-
substituted one (Table 3, entries 9 and 10 vs 7).
alkyl radicals were possibly generated in CCA but could not
couple with Rh(tmp) due to steric hindrance.
Coproducts. The fate of the nitrile-containing fragment after
carbon-carbon cleavage remained unidentified (Table 3).
Presumably, R-cyano alkyl radical intermediates had formed.
For sterically unhinderd substrates (Table 3, entries 1-3), the
reaction yields might be too low to allow accurate determination
of coproduct. For higher yielding and more hindered nitriles
(Table 3, entries 4-10), we proposed that the steric hindrance
of the nitrile-containing fragment is too large to couple with
Rh(tmp) to give R-cyano alkyl Rh(tmp). In order to validate
this proposal, several R-bromoacetonitriles were prepared and
reacted with Rh(tmp). The bromine atom abstraction would
likely generate the R-cyano alkyl radicals for examining their
reactivities toward Rh(tmp).21
The R-bromoacetonitriles 3k-3n22 were reacted with Rh-
(tmp) at room temperature to give Rh(tmp)Br 2d as the only
isolated rhodium porphyrin complex (eq 5). No cyanoalkyl
rhodium porphyrin was observed. However, the sterically less
hindered R-bromoacetonitrile 3o gave Rh(tmp)Br23 2d and
rhodium porphyrin cyanomethyl 2e in 38% and 47% yield,
respectively (eq 6). Therefore, the sterically hindered R-cyano
Competitive carbon-hydrogen bond activation at the benzylic
or aliphatic hydrogens did not likely occur at least significantly,
as Rh(tmp)H would have been formed.24,2525 If there were any
Rh(tmp)H 2f formed, PhMe2C-C(NdH)Rh(tmp) 2g would
have been observed even in small amount, as Rh(tmp)H was
separately found to react PhMe2CCN to give PhMe2C-C(Nd
H)Rh(tmp) 2g in 27% yield at 130 °C in 24 h (eq 7). The
(24) (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.
(25) (a) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D. J.
Am. Chem. Soc. 1996, 118, 12406-12415. (b) Sundermann, A.; Uzan, O.;
Milstein, D.; Martin, J. M. L. J. Am. Chem. Soc. 2000, 19, 3338-3346. (c)
Brunkan, N. M.; Brestensky, D. M.; Jones, W. D. J. Am. Chem. Soc. 2004,
126, 3627-3641.
(21) Paonessa, R. S.; Thomas, N. C.; Halpern, J. J. Am. Chem. Soc. 1985,
107, 4333-4335.
(22) (a) Couvreur, P.; Bruylants, A. J. Org. Chem. 1953, 18, 501-506.
(b) Molina, P.; Leonardo, C. L.; Botia, J. L. Foces, C. F.; Castano, C. F.
Tetrahedron 1996, 52, 9629-9642.
(23) Ogoshi, H.; Setsune, J.; Omura, T.; Yoshida, Z. J. Am. Chem. Soc.
1975, 97, 6461-6465.

2682 Organometallics, Vol. 26, No. 10, 2007
Table 4. CCA of r-Alkylphenylacetonitriles
Chan et al.
substratea
product
yield/%
total yield/%
53
PhCH(Et)CN 3p
Rh(tmp)Me 2a
Rh(tmp)Et 2b
Rh(tmp)Me 2a
Rh(tmp)Et 2b
Rh(tmp)Pr 2h
Rh(tmp)Me 2a
30
23
27
16
9
PhCH(Pr)CN 3q
52
39
PhMeC(Bn)CN 3r
39
a 5 equiv of nitriles.
absence of other tertiary Rh alkyls is likely due to the unstable
tertiary rhodium porphyrin and related macrocycle by steric
hindrance.26
Figure 2. Binding studies of Rh(tmp) with PPh3 at 20 °C (inset:
spectral changes upon titration with Ph3P). UV titration and analysis
curves of Rh(tmp) with PPh3 at 20 °C.
Regioselectivity of CCA. The selectivity of CCA in alkyl
nitriles19 was examined with Et, Pr, and benzyl R-substituted
phenylacetonitriles 3p, 3q, and 3r (eq 8, Table 4). There were
two CCA products produced for R-ethylphenylacetonitrile 3p,
Rh(tmp)Et 2b and Rh(tmp)Me 2a; three CCA products formed
for R-propylphenylacetonitrile 3q, Rh(tmp)Pr 2h, Rh(tmp)Et 2b,
and Rh(tmp)Me 2a; and one CCA product for 3r, Rh(tmp)Me
2a. The regioselectivity of alkyl CCA was not high, with only
slight preference for terminal methyl group activation (BDEMe-Bn
) 76.4 kcalmol-1) for 3p and 3q.9b For 3r, only Rh(tmp)Me
was observed without any Rh(tmp)Bn formed. As Rh(tmp)Bn
was thermally stable at 130 °C in 48 h in benzene, if it formed,
it would have been observed. Therefore, the selective formation
of Rh(tmp)Me from 3r reflected the steric preference of less
hindered methyl cleavage. Even the much weaker benzyl-
benzyl bond (BDEBn-Bn ) 62.6 kcalmol-1 9b) was not competi-
tive.
Figure 3. van’t Hoff plot of the binding of Rh(tmp) with PPh3.
Co-N(py) (pyCo(oep)CH3) ) 1.983 Å).27 Pyridine, presumably
with a stronger binding constant (at 30 °C, 1.87 × 104 ( 1.10
M-1, see binding constant of Ph3P below)14 and shorter Rh-
N(py) distance than Rh-P, exerts a pronounced effect.27 Thus
the proximal and less hindered CH2-CN bond underwent
activation to give pyRh(tmp)CN.
t
Reactions of R-Silylacetonitrile. In order to expand the
substrate scope, the reactions of R-silylactonitrile were examined
(eq 9, Table 5). In the presence of pyridine, Me3SiCH2CN 3s
reacted with Rh(tmp) to give pyRh(tmp)CN in 21% yield via
C(alkyl)-CN cleavage (Table 5, entry 1). With Ph3P added, in
Me3SiCH2CN 3s, the weaker Si-C(R) and Si-CH3 bonds
The sterically more hindered BuMe2SiCH2CN 3t behaved
differently. Addition of pyridine enhanced the total reaction
yields much higher than that of Ph3P (Table 5, entries 3-5). A
very low yield of Rh(tmp)Me, 8%, was observed with Ph3P
added (Table 5, entry 5). Rh(tmp)Me could form via either CCA
or CSiA. With pyridine added, 3t gave over 40% of Rh(tmp)-
Me at both 110 and 130 °C together with lower yields of pyRh-
(tmp)CN 2j (Table 5, entries 3 and 4). We do not understand
the effect of the ligand on the difference in reactivities of 3s
and 3t.
(BDEMe3Si-CH3 ) 94 kcal mol-1 9b
)
were cleaved rather than the
stronger C(R)-CN bond (BDECH3-CN ) 121 kcal mol-1 9b
)
(Table 5, entry 2). Three types of Si-C bond activation
products, Rh(tmp)SiMe3 2i, Rh(tmp)CH2CN 2e, and Rh(tmp)-
Me 2a, were produced. A much higher yield of Rh(tmp)CH2-
CN in 63% yield was obtained. Its stronger Rh-CH2CN bond
formed due to the electron-withdrawing CN group.9a The direct
comparison of Rh-C and Rh-Si bond strengths is lacking
literature data to allow the comparison of the stability with 2i.
The formation of pyRh(tmp)CN 2j could be rationalized from
steric preference, since pyridine in a five-coordinate metal-
loporphyrin could pull the rhodium atom down to the porphyrin
plane from the four-coordinate metalloporphyrin (analogous
CCA of Rh(tmp) and 2-Alkyl Benzonitriles. 2-Alkylben-
zonitriles 3u-w28 were examined. Rh(tmp) with PPh3 added
successfully also activated the benzylic-methyl carbon-carbon
(27) Summers, J. S.; Petersen, J. L.; Stolzenberg, A. M. J. Am. Chem.
Soc. 1994, 116, 7189-7195.
(28) Windhorst, A. D.; Timmerman, H; Worthington, E. A.; Bijloo, G.
J.; Nederkoorn, P. H. J.; Menge, W. M. P. B.; Leurs, R.; Herscheid, J. D.
M. J. Med. Chem. 2000, 43, 1754-1761.
(26) Giese, B.; Hartung, J.; Kesselheim, C.; Lindner, H. J.; Svoboda, I.
Chem. Ber. 1993, 126, 1193-1200.

Metalloradical ActiVations of Aliphatic C-C Bonds of Nitriles
Table 5. Reactions of r-Silylactonitriles
temp/°C
Organometallics, Vol. 26, No. 10, 2007 2683
entry
substratea
ligand
product (yield/%)
1
2
3
4
5
Me3SiCH2CN 3s
pyb
110
110
130
110
110
pyRh(tmp)CN 2j (21)
Me3SiCH2CN 3s
Ph3Pc
pyb
Rh(tmp)SiMe3 2i (17)
Rh(tmp)Me 2a (47)
Rh(tmp)Me 2a (42)
Rh(tmp)Me 2a (8)
Rh(tmp)CH2CN 2e (63)
pyRh(tmp)CN 2j (30)
pyRh(tmp)CN 2j (15)
Rh(tmp)Me 2a (7)
tBuMe2SiCH2CN 3t
tBuMe2SiCH2CN 3t
tBuMe2SiCH2CN 3t
pyb
Ph3Pc
b
c
a 10 equiv. 2 equiv. 1 equiv.
Table 6. CCA Results of 2-Alkylbenzonitriles
entry
nitrile
C-C (BDE/kcal mol-1
)
time/h
Rh(tmp)Me/%a
1
2
3
2-ethylbenzonitrile 3u
2-isopropylbenzonitrile 3v
2-tert-butylbenzonitrile 3w
CNPhCH2-Me (76.4)
CNPhCH-Me2 (74.6)
CNPhC-Me3 (72.5)
60
48
48
9
13
29
a Average yield of at least two runs.
t
Table 7. Rate Constants of CCA of Rh(tmp) and BuCN with Ph3P Addition at 130 °C
entry
[Rh(tmp)] × 105 (M)
[PPh3] × 102 (M)
[tBuCN] × 103 (M)
kobs′ × 104 (s-1
)
kobs× 102 (M-1 s-1
)
1
2
3
4
5
6
7
5.55
11.10
16.65
5.55
5.55
5.55
1.10
1.10
1.10
1.10
1.10
1.10
1.10
5.93
5.93
5.93
2.97
4.45
7.42
8.90
1.19 ( 0.03
1.13 ( 0.03
1.04 ( 0.02
0.74 ( 0.02
0.94 ( 0.01
1.39 ( 0.06
1.59 ( 0.06
2.00
2.31
1.91
2.49
2.11
1.88
1.79
5.55
bonds selectively at 130 °C to give only Rh(tmp)Me (eq 10,
Table 7). 3w was more reactive and higher-yielding than 3u
and 3v likely due to the weaker carbon-carbon bond in 3w
(Table 6). The yields were in general lower than that of
2-alkylacetonitriles (Table 3) due to larger steric hindrance of
the adjacent aryl ring and stronger carbon-carbon bond of
3u-w.
× 10-3 M Ph3P with at least 4 half-lives. A linear pseudo-first-
order plot is obtained as shown in Figure 4. The pseudo-first-
order rate constants kobs′ (kobs′ ) kobs[tBuCN]), derived from
the slopes, were plotted against the [tBuCN] to yield linear plots
(Figure 5) with a nonzero intercept. The observed rate law is
shown in eq 13.
The term k in eq 13 suggested that some background reaction
occurred. Rh(tmp) has been reported by Wayland to be stable
in C6H6 over a period of months at 353 K.12b We also observed
the same stability at 130 °C by UV-vis spectroscopy as
evidenced by no spectral change. However, upon addition of
Ph3P, the resultant Ph3PRh(tmp) showed spectral change with
increasing time at 130 °C (Figure 6). Therefore, the term k
comes from the decomposition of Ph3PRh(tmp), a pathway
UV Titration of [Rh(tmp)] and Ph3P. The stoichiometry
and binding constants of Rh(tmp) and Ph3P were measured
spectrophotometrically at 522 nm from 20 to 50 °C (eq 11).
The titration curves of Rh(tmp) with Ph3P at 20.0 °C are shown
in Figure 2 (for definition of terms and details, see Experimental
Section). Analyses of the data confirmed a 1:1 adduct and
yielded the binding constant at 30.0 °C; K1,30°C (k1/k-1) ) 1.42
× 103 ( 1.05 M-1, K1,130°C(extrapolated) ) 10.7 M-1 (see
Experimental Section). The enthalpy and entropy of the Ph3P
coordination were estimated by plotting ln K1 against 1/T (Figure
3) to be ∆H1 ) -12 ( 1 kcal mol-1 and ∆S1 ) -26 ( 3 cal
mol-1 K-1, respectively. The negative entropy was due to the
loss of rotational and translational freedom of the ligand and
the rhodium porphyrin upon coordination.
t
independent of BuCN. The kinetic order of Rh(tmp) in this
pathway was preliminary estimated to be first order (m ) 1 in
eq 14) by the linear relationship of the first-order fit of the
absorbance change (Figure 6, inset). The details were not further
pursued since it is not directly relevant to the CCA.
The overall rate law is expressed in eq 14 with reference to
Scheme 2 as well where [Rh(tmp)]0 is equal to [Rh(tmp)]total
.
The rate is composed of two parallel processes with the first
t
t
one being BuCN independent and the second one BuCN
dependent (Scheme 2). We rationalize that Ph3PRh(tmp)
undergoes disproportionation to Rh(I)(tmp)- and (Ph3P)2Rh-
(III)(tmp)+ at high temperature (eq 16).16,29,3030 For the tBuCN-
dependent processes relevant to aliphatic CCA, the rate was
t
Kinetic Studies of CCA of Rh(tmp) with BuCN. Kinetic
studies were carried out to gain further mechanistic insight into
the CCA (eq 12). Kinetic measurements of the reaction of Rh-
(tmp) and tBuCN with excess Ph3P were carried out spectrally
at 522 nm at 130 °C, with initial concentrations (5.55-16.65)
(29) Wayland, B. B.; Balkus, K. J., Jr.; Franos, M. D. Organometallics
1989, 8, 950-955.
(30) The kinetic order of Rh(tmp) was preliminarily estimated from the
first-order fit of the absorbance change. We did not pursue investigating
the kinetic order of Ph3P, i.e., n in eq 13.
t
× 10-5 M Rh(tmp), (2.97-8.90) × 10-3 M BuCN, and 1.10

2684 Organometallics, Vol. 26, No. 10, 2007
Chan et al.
Figure 4. Pseudo-first-order rate plot (inset: spectral changes).
Figure 6. Decomposition of Ph3PRh(tmp) (inset: first-order
fitting).
Figure 5. Plot of the kobs′ vs [tBuCN].
t
found to be first order on Rh(tmp) and BuCN and exhibited
saturation kinetics in excess Ph3P (Table 8, Figure 7).14
Figure 7. Saturation kinetics on Ph3P.
(SH2) is very difficult.33 It further accounts for the selective
coordination-promoted CCA without any significant CHA.
The fate of the nitrile containing fragment, presumably a
carbon-centered radical remains unclear. (NCMe2C)2 could not
be detected in the reaction mixture of Rh(tmp) with tBuCN by
GC-MS analysis. Presumably, NCMe2C readily abstracts a
hydrogen atom rather couples with either itself or Rh(tmp).32
Reaction of BrCMe2CN with Rh(tmp) gave only Rh(tmp)Br but
no Rh(tmp)CMe2CN (eq 5). 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 (eq 6). These reactions demonstrated the
importance of steric effect in coupling with the fairly crowded
Rh(tmp) conferred by the 2,6-dimethyl groups at the mesityl
rings. Rh(tmp)CMe2CN did not form likely due to steric
hindrance.
A proposed mechanism for CCA conforming to the kinetic
evidence is depicted in eqs 15-19. Initially, facile coordination
of Ph3P with Rh(tmp) occurs to give Ph3PRh(tmp) (k1), which
t
is then complexed by BuCN to give Ph3PRh(tmp)tBuCN (k2).
The mode of nitrile coordination is likely N- rather than side
bound to minimize steric hindrance, even though it is kinetically
indistinguishable (Scheme 3). The precoordination of Rh(tmp)
with Ph3P to form Ph3PRh(tmp) is supported by the facile
coordination of the ligand to Rh(tmp),16 the pyridine-promoted
bond activation of Me3SiCN with Rh(tmp),14 and the saturation
kinetics with respect to [Ph3P] (Figure 7). 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, Scheme 3), which is in contrast with the termolecular
Another possible mechanism involving an electron-transfer
process either intermolecularly or intramolecularly via nitrile
coordination is listed in eqs 20-23. Initially, the Rh(III) cation
and nitrile radical anion intermediates are formed from the
reaction of Ph3PRh(tmp) with tBuCN as shown in eq 20. Methyl
radical is subsequently generated (eq 21) and rapidly reacts with
transition state in the activation of methane by Rh(tmp).12h,31
A
precoordination step (eq 17) is more reasonable to mediate the
homolytic cleavage of the carbon-carbon bond, as the unas-
sisted homolytic bimolecular substitution at a carbon center
(31) (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.
(33) (a) Ingold, K. U.; Roberts, B. P. Free Radical Substitution Reactions;
Wiley: New York, 1971. (b) Davis, A. G.; Roberts, B. P. Free Radicals;
Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. 2, pp 547-589. (c) Fossey,
J.; Lefort, D.; Sorba, J. Free Radicals in Organic Chemistry; Wiley: New
York, 1995; pp 125-137.
t
t
(32) No BuRh(tmp) was observed in the reaction of BuNC with Rh-
(tmp): ref 11a. The formation of Rh(tmp)CMe2CN is likely sterically
hindered. See also: Hetterscheid, D. G. H.; Bens, M.; de Bruin, B. Dalton
Trans. 2005, 979-984.

Metalloradical ActiVations of Aliphatic C-C Bonds of Nitriles
Table 8. Saturation Kinetics on Ph3P at 130 °C
Organometallics, Vol. 26, No. 10, 2007 2685
spectra were performed on a Hitachi U-3300 spectrophotometer
equipped with a Neslab RTE-110 temperature circulator for
temperature control with a mixture of ethylene glycol and water
(v/v ) 1:1) used as the circulating liquid. The temperature was
measured by a Fluka thermometer connected to a K-type thermo-
couple wire placed in an adjacent dummy UV cell filled with
benzene inside the sample compartment.
[Rh(tmp)]
[PPh3]
[tBuCN]
kobs
′
kobs
entry × 105 (M) × 103 (M) × 103 (M) × 104 (s-1
)
× 102 (M-1 s-1
)
1
2
3
5.55
5.55
5.55
11.0
1.10
0.22
8.90
8.90
8.90
1.59 ( 0.06
1.79
1.62
0.65
1.44 ( 0.03
0.58 ( 0.02
CCA Reaction of Rh(tmp) and Nitriles with Ligand. Typical
procedure: A benzene solution of PPh3 (10 µL, 1.1 M in C6H6)
was added to the benzene solution of Rh(tmp) (0.088 mmol,
4.0 mL) under N2 with stirring. Then nitrile (0.055 mmol) was
added to the mixture under N2. The mixture was heated at 130 °C
under N2 in the absence of light. After reaction, the crude product
was purified by chromatography on silica gel with hexane/CH2Cl2
(10:1) to hexane/CH2Cl2 (7:1) as the eluent to give the product.
Rh(tmp) to furnish Rh(tmp)Me. This mechanism conforms with
the rate law but remains less likely. The formation of stable
Ph3PRh(tmp)+ limits the maximum yield of Rh(tmp)Me to less
than 50%, which is contrary to the observed results.
Preparation of (5,10,15,20-Tetramesitylporphyrinato)bromo
Rhodium(III) [Rh(tmp)Br] (2d).22 Magnesium powder (4.1 mg,
0.168 mmol) was added to a Teflon screw-head stoppered flask.
Then it was heated under vacuum for 1 h. After cooling down to
room temperature, the reactor was filled with N2. Dried Et2O
(10 mL) was added to the flask with a syringe. EtBr (13 µL, 0.168
mmol) was dropped into the flask with a syringe under N2. Then
the mixture was refluxed for 3 h. The prepared Grignard reagent
EtMgI in dry ether (10 mL) was directly added to a solution of
Rh(tmp)Cl (103 mg, 0.112 mmol) in dry 1,2-dimethoxyethane
(100 mL) under N2. The mixture was stirred for 24 h at room
temperature and then poured into aqueous NH4Cl solution. The
extract obtained with dichloromethane was washed with water, dried
over anhydrous Na2SO4, and evaporated to dryness. The crude
product was purified by chromatography on silica gel, eluting with
a solvent mixture of hexane/CH2Cl2 (1:1) to give a red solid. It
was recrystallized from hexane-CH2Cl2 to give a red crystal (43.2
mg, 0.045 mmol, 40%). Rf ) 0.28 (CH2Cl2/hexane ) 1:1): 1H
NMR (C6D6, 300 MHz) δ 1.67 (s, 12 H), 2.33 (s, 12 H), 2.44 (s,
12 H), 7.02 (s, 4 H), 7.25 (s, 4 H), 8.90 (s, 8 H); 13C NMR (CDCl3,
75 MHz) δ 21.65, 21.78, 21.97, 119.80, 127.58, 128.06, 131.87,
137.73, 138.41, 140.29, 143.24; HRMS (FAB) calcd for (C56H52N4-
BrRh) [M]+ m/z 962.2425, found m/z 962.2458.
We propose that in the transition state (tBuCN)Rh(tmp)Ph3P
dissociates to form a side-bound nitrile-Rh complex (Scheme
3). The rhodium radical then abstracts the methyl group via
possibly a backside attack to form Ph3PRh(tmp)Me, which then
dissociates rapidly to give Rh(tmp) and Ph3P. This is a type of
coordination-mediated bimolecular homolytic substitution.33 The
methyl-abstraction step appears not to be very sensitive to the
size of substituents, as seen in the poor selectivity of R-substi-
tuted phenylacetonitriles 3p and 3q. Only in 3r was the much
smaller methyl selectively activated rather than the benzyl group.
Both C-C activation of tBuCN and Si-CN activation of Me3-
SiCN14 undergo a bimolecular transition state. Precoordinating
t
with nitrile (or isonitrile) occurs rapidly. For BuCN, Rh(tmp)
abstracts the methyl group in the rate-determining step, while
for Me3SiCN, cyanide group transfer to Rh(tmp) occurs.
Conclusion
The aliphatic C(R)-C(â) bonds of a series of alkyl nitriles,
R-alkylphenylacetonitriles, R-silylacetonitriles, and 2-alkylben-
zonitriles, have been activated selectively by Rh(tmp) using PPh3
as the promoting ligand. The CCA yields were affected by bond
energy and steric hindrance and the small electronic effect of
aryl acetonitriles. The CCA was not regioselective for small
differences in the size of alkyl groups. R-Silylacetonitriles were
activated by Rh(tmp) in the presence of PPh3 or pyridine to
give rhodium porphyrin alkyls and cyanide complexes in
satisfactory yields via C-Si or C-C bond activation.
Reaction between Rh(tmp) and R-Bromoacetonitriles. De-
gassed R-bomoacetonitrile (0.055 mmol) was added to the benzene
solution of Rh(tmp) (0.088 mmol, 4.0 mL) under N2 with stirring.
The mixture was stirred at room temperature under N2 in the
absence of light. The crude product was purified by chromatography
on silica gel using a solvent mixture of hexane/CH2Cl2 (1:1) to
give an orange solid. The solid was dried under high vacuum
overnight at 70 °C. Rh(tmp)Br was obtained with a 1H NMR
spectrum identical to that of an authentic sample.
The mechanism of selective C(R)-C(â) bond activation of
tBuCN was investigated by kinetic studies. Two parallel
processes occurred: the disproportionation of Ph3PRh(tmp) and
CCA Reaction of Rh(tmp) with R-Ethylphenylacetonitrile
with Ph3P. Degassed R-ethylphenylacetonitrile (0.055 mmol, 8.0
mg) was added to the benzene solution of Rh(tmp) (0.088 mmol,
4.0 mL) under N2 with stirring. The crude product was purified by
chromatography on silica gel using hexane/CH2Cl2 (10:1) to hexane/
CH2Cl2 (5:1) as eluent to give an orange solid. Rh(tmp)Me (0.0026
mmol, 2.4 mg, 30%) and Rh(tmp)Et (0.0020 mmol, 1.8 mg, 23%)
were obtained.
t
the CCA of BuCN. A bimolecular substitution of the methyl
group of coordinated tBuCN by Rh(tmp) to give Rh(tmp)Me is
proposed.
Experimental Section
Unless otherwise noted, all chemicals were obtained from
commercial suppliers and used before purification. Triphenylphos-
phine was recrystalized in hexane and dried under high vacuum.
Hexane for chromatography was distilled from anhydrous calcium
chloride. Thin-layer chromatography was performed on precoated
silica gel 60 F254 plates.
1H NMR spectra were recorded on a Bruker DPX-300 (300
MHz). Chemical shifts are reported with reference to the residual
solvent protons in CDCl3 (δ 7.24 ppm) or with tetramethylsilane
(δ 0.00 ppm) as the internal standard. Coupling constants (J) are
reported in hertz (Hz). Mass spectra were recorded on a Finnigan
MAT 95XS mass spectrometer (FAB-MS and ESI-MS). UV-vis
CCA Reaction of Rh(tmp) with R-Propylphenylacetonitrile
with Ph3P. Degassed R-propylphenylacetonitrile (0.055 mmol, 8.8
mg) was added to the benzene solution of Rh(tmp) (0.088 mmol,
4.0 mL) under N2 with stirring. The crude product was purified by
chromatography on silica gel using hexane/CH2Cl2 (10:1) to hexane/
CH2Cl2 (5:1) as eluent to give an orange solid. Rh(tmp)Me: 2.4
mmol, 2.1 mg, 27%. Rh(tmp)Et: 1.3 mmol, 1.4 mg, 16%; Rf )
1
0.44 (CH2Cl2/hexane, 1:5); H NMR (C6D6, 300 MHz) δ -4.31
(dq, 2 H, 2JRhH ) 3.0 Hz, 3JHH ) 7.5 Hz), -3.83 (dt, 3 H, 3JRhH
)
1.5 Hz, 3JHH ) 7.5 Hz), 1.88 (s, 12 Hz), 2.12 (s, 12 H), 2.44 (s, 12
H), 6.93 (s, 4H), 7.19 (s, 4 H), 8.75 (s, 12 H); 13C NMR (C6D6, 75
MHz) δ 21.4, 22.1, 22.5, 22.7, 23.6, 30.8, 32.5, 35.5, 120.8, 131.5,

2686 Organometallics, Vol. 26, No. 10, 2007
Chan et al.
µmol, 4%): 1H NMR (C6D6, 300 MHz) δ -5.26 (d, 3 H, 2JRh-H
138.2, 139.6, 139.9, 149.7; HRMS (FAB) calcd for (C58H54N4Rh+)
m/z 912.3631, found m/z 912.3715. Rh(tmp)Pr (0.8 mmol, 0.7 mg,
9%): Rf ) 0.45 (CH2Cl2/hexane, 1:5); 1H NMR (C6D6, 300 MHz)
)
2.7 Hz), 1.72 (s, 12 H), 2.25 (s, 12 H), 2.43 (s, 12 H), 7.07 (s, 4
H), 7.20 (s, 4 H), 8.75 (s, 8 H). A deep red solid of Rh(tmp)CH2-
CN (4.4 mg, 4.8 µmol, 55%): Rf ) 0.47 (hexane/CH2Cl2, 1:1); 1H
NMR (CDCl3, 300 MHz) δ -5.09 (d, 2 H, J ) 4.2 Hz), 1.81 (s,
12 H), 1.90 (s, 12 H), 2.62 (s, 12 H), 7.27 (s, 8 H), 8.69 (s, 8 H);
13C NMR (CDCl3, 100 MHz) 0.29, 21.76, 21.99, 128.01, 128.22,
131.69, 138.02, 139.11, 139.51, 142.90; FAB calcd for (C58H54N5-
Rh+) m/z 958.2933, found m/z 958.3001.
Reaction of [Rh(tmp)] and Trimethylsilylacetonitrile with
Pyridine Added. Distilled and degassed pyridine (2 µL, 0.02 mmol)
was added to the solution of [Rh(tmp)] (8.8 µmol) at rt. Trimeth-
ylsilylacetonitrile (0.01 mL, 0.08 mmol) was added, and the solution
was heated at 110 °C for 1 day under N2 in the absence of light.
The crude product was purified by chromatography on silica gel
to give a deep red solid of pyRh(tmp)CN14 (1.7 mg, 1.7 µmol,
19%): 1H NMR (CDCl3, 300 MHz) δ 1.26 (d, 2 H, J ) 6.0 Hz),
1.59 (s, 12 H), 1.95 (s, 12 H), 2.60 (s, 12 H), 4.97 (t, 2 H, J ) 6.9
Hz), 6.00 (t, 1 H, J ) 7.8 Hz), 7.19 (s, 4 H), 7.28 (s, 4 H), 8.60 (s,
8 H).
Reaction of [Rh(tmp)] and tert-Butyldimethylacetonitrile with
PPh3 Added. Triphenylphosphine solution (0.1 mL, 0.01 mmol,
0.1 M in C6H6) was added to the solution of [Rh(tmp)] (8.8 µmol)
at rt. Degassed tert-butyldimethylacetonitrile solution (0.10 mL,
0.08 mmol, 0.88 M in benzene) was added to the adduct solution,
and the mixture was heated at 110 °C for 1 day under N2 in the
absence of light. The crude product was purified by chromatography
on silica gel to give a red solid of Rh(tmp)CH3 (0.6 mg, 0.7 µmol,
8%).
Reaction of [Rh(tmp)] and tert-Butyldimethylacetonitrile with
Pyridine Added. Distilled and degassed pyridine (2 µL, 0.02 mmol)
was added to the solution of [Rh(tmp)] (8.8 µmol) at rt. Degassed
tert-butyldimethylacetonitrile solution (0.10 mL, 0.08 mmol, 0.88
M in benzene) was added to the adduct solution, and the mixture
was heated at 110 °C for 1 day under N2 in the absence of light.
The crude product was purified by chromatography on silica gel
to give a red solid of Rh(tmp)CH3 (3.3 mg, 3.7 µmol, 42%). pyRh-
(tmp)CN (1.3 mg, 1.3 µmol, 15%) was also obtained.
2
3
δ -4.44 (dt, 2 H, JRhH ) 3.0 Hz, JHH ) 7.5 Hz), -3.88 (sextet,
2 H, J ) 7.5 Hz), -1.42 (t, 3 H, J ) 7.4 Hz), 1.90 (s, 12 H),
2.18(s, 12 H), 2.44 (s, 12 H), 7.11 (s, 4H), 7.20 (s, 4 H), 8.76 (s,
12 H); 13C NMR (C6D6, 75 MHz) δ 11.41, 17.06, 21.55, 21.83,
22.13, 30.23, 120.27, 131.02, 137.72, 139.03, 139.31, 143.18;
HRMS (FAB) calcd for (C59H59N4Rh+) m/z 926.3789, found m/z
926.3777. Anal. Calcd for C44H48NO: C, 76.44; H, 6.41; N, 6.04.
Found: C, 76.31; H, 6.56; N, 6.14.
CCA Reaction of Rh(tmp) with R-Benzyl-R-methylphenyl-
acetonitrile with Ph3P. Degassed R-benzyl-R-methylphenylaceto-
nitrile (0.055 mmol, 12.2 mg) was added to a benzene solution of
Rh(tmp) (0.088 mmol, 4.0 mL) under N2 with stirring. The crude
product was purified by chromatography on silica gel using hexane/
CH2Cl2 (10:1) to hexane/CH2Cl2 (5:1) as eluent to give an orange
solid: Rh(tmp)Me (0.0034 mmol, 3.1 mg, 39%).
Preparation of (5,10,15,20-Tetramesitylporphyrinato)hy-
dridorhodium(III) [Rh(tmp)H].34 A red suspension of Rh(tmp)I
(50 mg, 0.049 mmol) in EtOH (40 mL) and a solution of NaBH4
(7.5 mg, 0.20 mmol) in aqueous NaOH (0.5 M, 2 mL) were purged
with N2 separately for about 15 min. The solution of NaBH4 was
added slowly to the suspension of Rh(tmp)I via a cannula. The
reaction mixture was heated at 55 °C for 3 h, and the color changed
to deep brown. The reaction mixture was then cooled to 0 °C under
N2, and HCl (40 mL, 0.1 M) was added via a syringe. An orange
suspension formed immediately and was stirred for 15 min at
0 °C. The workup was carried out under N2. The reaction mixture
was worked up by addition with degassed benzene/H2O. The crude
product was extracted with degassed benzene (50 mL), washed with
H2O (10 mL × 3), dried over MgSO4, and filtered under N2 using
a cannula with the tip wrapped with a filter paper. Then the solvent
was removed under high vacuum. A red solid (41.6 mg, 0.047
mmol, 96%) was obtained: 1H NMR (C6D6, 300 MHz) δ -40.07
(d, 1 H, 1JRhH ) 45 Hz), 1.80 (s, 12 H), 2.14 (s, 12 H), 2.44 (s, 12
H), 7.10 (s, 4 H), 7.19 (s, 4 H), 8.77 (s, 8 H).
Reaction between Rh(tmp)H and R,R-Dimethylphenylaceto-
nitrile. R,R-Dimethylphenylacetonitrile (76.7 mg, 0.528 mmol) was
added to a solution of Rh(tmp)H (156 mg, 0.176 mmol) in dried
and degassed benzene (50 mL) under N2. Then the mixture was
heated at 130 °C with stirring for 24 h under N2 in the absence of
light. After cooling, solvent was removed under reduced pressure.
The residue was purified by chromatography on silica gel eluting
with a solvent mixture of CH2Cl2 (100%) to ethyl acetate/CH2Cl2
(5:95) to give the red solid (5,10,15,20-tetramesitylporphyrinato)-
1-imino-2-methyl-2-phenylpropylrhodium(III), 2g (49.4 mg, 0.048
mmol, 27%): Rf ) 0.19 (100% CH2Cl2). 1H NMR (C6D6, 300 MHz)
δ -0.70 (s, 6 H), 1.94 (s, 24 H), 2.44 (s, 12 H), 4.75 (d, 2 H, J )
7.8 Hz), 6.47 (t, 2 H, J ) 7.6 Hz), 6.57 (t, 1 H, J ) 7.2 Hz), 7.09
(s, 4 H), 8.96 (s, 8 H); 13C NMR (CDCl3, 75.5 MHz) δ 21.46,
21.65, 22.18, 26.84, 29.86, 35.95, 118.36, 119.46, 122.90, 127.70,
127.96, 128.62, 131.15, 137.46, 137.76, 138.79, 140.26, 142.12;
HRMS (FAB or ESI) only peak of [Rh(tmp)]+ (m/z 884) was
observed; IR (CH2Cl2, cm-1) ν(CdN) 1611 (s), ν(N-H) 3440 (s).
Reaction of [Rh(tmp)] and Trimethylsilylacetonitrile with
PPh3 Added. Triphenylphosphine in benzene (0.1 mL, 0.01 mmol,
0.1 M in C6H6) was added to a solution of [Rh(tmp)] (8.8 µmol) at
rt. Trimethylsilylacetonitrile (0.01 mL, 0.08 mmol) was added to
the adduct solution, and the mixture was heated at 110 °C for 1
day under N2 in the absence of light. The crude product was purified
by chromatography on silica gel to give an orange solid of Rh-
(tmp)SiMe314 (1.1 mg, 1.1 µmol, 13%): 1H NMR (C6D6, 300 Hz)
δ -3.07 (s, 9 H), 1.63 (s, 12 H), 2.41 (s, 24 H), 6.84 (s, 4 H), 7.41
(s, 4 H) 8.69 (s, 8 H). A red solid of Rh(tmp)CH3 (0.3 mg, 0.3
UV Titration of Coordination between [Rh(tmp)] and PPh3.
The UV titrations were carried out on a UV-vis spectrometer
equipped with a temperature controller. The titrations were carried
out at temperatures of 20.0, 25.0, 30.0, 35.0, 40.0, 45.0, and 50.0
((0.2) °C. The temperatures of the solutions were measured by a
thermocouple wire placed in a UV cell, which was placed inside
the sample compartment. Benzene was freshly distilled over sodium
under N2, and pyridine was distilled over NaOH under N2. Stock
solutions of Rh(tmp) in benzene (∼5.45 × 10-6 M) and PPh3 in
benzene (∼3.0 × 10-1 M) were prepared. The solution of Rh(tmp)
(3.00 mL) was transferred to a Teflon-stoppered Schlenk UV cell
with a gastight syringe under N2. PPh3 solution was then titrated
into the Rh(tmp) solutions via a gastight microsyringe at 2.0 µL
steps up to a total of 20.0 µL and then at 4.0 µL steps up to a total
of 60.0 µL PPh3 solution added. Finally 100 µL of PPh3 was added
to the sample solution to obtain the estimated absorbance for the
Rh(tmp)-PPh3 complex. The experimental absorbance was mea-
sured at 522 nm. The binding constants and the number of PPh3
ligands coordinated to each Rh(tmp) complex were calculated by
the equation35
Ae - A
An - Ae
log Kn ) log
m - n log[L]
where n ) no. of Ph3P ligands coordinated to each Rh(tmp), Kn )
binding constant, [L] ) conc of pyridine in the UV sample, Ae )
experimental absorbance measured (volume correction made), Am
(34) Wayland, B. B.; VanVoorhees, S. L.; Walker, C. Inorg. Chem. 1986,
25, 4039-4042.
(35) Perkampus, H. H. UV-VIS Spectroscopy and Its Applications;
Springer-Verlag: Berlin, Heidelberg, 1992.

Metalloradical ActiVations of Aliphatic C-C Bonds of Nitriles
Organometallics, Vol. 26, No. 10, 2007 2687
) absorbance of the Rh(tmp) without pyridine, An ) theoretical
absorbance obtained for the Rh(tmp)-Ph3P complex (obtained from
the absorbance measured for the infinite point with volume
correction).
stock solution of Rh(tmp), Ph3P, and benzene were added to a
cuvette Schlenck UV cell and thermally equilibrated at 80 °C for
t
30 min. Then the BuCN solution in benzene was added to the
mixture quickly, and the cell was heated in an oil bath at 130.0 (
3.0 °C. The sample was monitored at an interval of 15 or 20 min
at 80.0 ( 0.2 °C in a thermostated UV spectrometer for 4-5 half-
lives. The reactions monitored at room temperature produced less
precise absorbance.
The measured Ae values were corrected for volume change due
to the addition of ligand solution and the thermal expansion/
contraction by Vt ) V0(1 + ât) and â ) 0.00372 (for benzene).36
t
Kinetic Studies on Reaction of [Rh(tmp)] and BuCN with
Ph3P Added. Kinetic studies were carried out at the temperature
range 130 ( 3.0 °C in an oil bath. Rh(tmp)CH3 (2.5 mg, 0.00225
mmol) was dissolved in benzene in a 2.00 mL volumetric flask,
then was transferred to a Rotaflo flask and degassed. After
photolysis for 10 h at 6-10 °C, the stock solution of Rh(tmp) (1.11
Acknowledgment. We thank the Research Grants Council
of Hong Kong of the SAR of China for financial support (No.
400203).
t
× 10-3 M) was produced. Ph3P (0.2199 M) and substrate BuCN
Supporting Information Available: NMR spectra. This mate-
rial is available free of charge via the Internet at http://pubs.acs.org.
(0.445 M) were prepared and degassed. Measured amounts of the
(36) Atkins. P. Physical Chemistry, 6th ed.; Oxford, 1998.
OM070064J