Selective activation of benzylic carbon-hydrogen bonds of toluenes with rhodium(III) porphyrin methyl: Scope and mechanism

Article abstract of DOI:10.1021/om9009119

Toluenes underwent selective benzylic carbon-hydrogen bond activation (BnCHA) with rhodium(III) porphyrin methyl. The ortho-, meta-, and para-substituted toluenes yielded the corresponding rhodium porphyrin benzyls in high yields in solvent-free conditions as well as in benzene solvent. Mechanistically, Rh(ttp)Me likely undergoes a σ-bond metathesis pathway. The small value of the kinetic isotope effect (2.7) indicates a bent transition state. The negative slope (-1.1) of the linear free energy relationship Hammett plot supports that the benzylic carbon builds up a positive charge in the transition state.

Full text of DOI:10.1021/om9009119

624 Organometallics 2010, 29, 624–629
DOI: 10.1021/om9009119
Selective Activation of Benzylic Carbon-Hydrogen Bonds of Toluenes
with Rhodium(III) Porphyrin Methyl: Scope and Mechanism
Kwong Shing Choi, Peng Fai Chiu, and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories,
Hong Kong, People’s Republic of China
Received October 16, 2009
Toluenes underwent selective benzylic carbon-hydrogen bond activation (BnCHA) with rhodium(III)
porphyrin methyl. The ortho-, meta-, and para-substituted toluenes yielded the corresponding rhodium
porphyrin benzyls in high yields in solvent-free conditions as well as in benzene solvent. Mechanistically,
Rh(ttp)Me likely undergoes a σ-bond metathesis pathway. The small value of the kinetic isotope effect
(2.7) indicates a bent transition state. The negative slope (-1.1) of the linear free energy relationship
Hammett plot supports that the benzylic carbon builds up a positive charge in the transition state.
Introduction
We have been interested in the chemistries of alkyl,6 aryl,7
benzylic,8 and aldehydic9 carbon-hydrogen bond activations
and silicon-hydrogen activation of silanes10 by both rhodium-
(III) and iridium(III) porphyrin chlorides.11 These types of
metalloporphyrins are unique due to the apparent steric diffi-
culties of cis-coordination of a substrate to the five-coordinate
metalloporphyrin complex in the course of bond activation and
the relative electronic inaccessibility of an oxidative addition
intermediate that requires a formal M(V) oxidation state. Pre-
viously, we have also discovered the base-promoted selective
benzylic (Bn) CHA of toluenes by rhodium(III) tetrakis-
(4-tolylporphyrin) chloride (RhIII(ttp)Cl).8 To gain further
mechanistic understanding of the BnCHA of toluenes by me-
talloporphyrins, studies have been extended to RhIII(ttp)Me
bearing a nonionizable Rh-Me group. We now report the
synthetic results and mechanistic investigation revealing a bent
transition state via SBM.
The carbon-hydrogen bond activation (CHA) using transi-
tion metal complexes plays an important role in converting
saturated hydrocarbons to functionalized organic compounds.1
Late transition metal complexes are more appealing than their
early transition metal counterparts due to the broader functional
group compatibilities.2 Therefore, both exploratory and mecha-
nistic studies would aid in discovering the rich chemistry of high-
valent late transition metal complexes in bond activations.
For CHA by high-valent late transition metal complex, a few
mechanistic possibilities exist, such as oxidative addition (OA),3
σ-bond metathesis (SBM),4 and (base-promoted) heterolysis.5
However, the OA and SBM are often not easily distin-
guished for rhodium(III) and iridium(III) complexes. The
successful isolation of high-valent metal(V) intermediates in
the Si-H activation has firmly established the OA pathway in
some cases.3
Results and Discussion
*Corresponding author. E-mail: ksc@cuhk.edu.hk.
(1) (a) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970–1973.
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Yung, C. M.; Arndtsen, B. A.; Adamson, D. R.; Bergman, R. G. J. Am.
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2464. (d) Davies, D. L.; Donald, S. M. A.; Macgregor, S. A. J. Am. Chem.
Soc. 2005, 127, 13754–13755.
Rh(ttp)Me reacted with toluene in solvent-free conditions
successfully and selectively in BnCHA at 150 °C in 1 day to
give Rh(ttp)Bn in 91% yield (eq 1, Table 1, entry 2), while no
reaction occurred at 120 °C in 2 days (Table 1, entry 1). In
contrast with the nonselective reaction with Rh(ttp)Cl, no
aryl CHA product was observed.8 The BnCHA also pro-
ceeded smoothly in benzene solvent using 10 equiv of toluene
but with slightly lower efficiency. A longer reaction time of
4 days was required, and slightly lower product yield of 69%
was obtained (Table 1, entry 3). The reaction was faster at
200 °C and gave a high product yield of 91% (Table 1, entry 4).
Thus Rh(ttp)Me is less reactive but more selective than
Rh(ttp)Cl. Methane was also identified and formed in 76%
(8) Chan, K. S.; Chiu, P. F.; Choi, K. S. Organometallics 2007, 26,
1117–1119.
(9) (a) Chan, K. S.; Lau, C. M. Organometallics 2006, 25, 260–265.
(b) Chan, K. S.; Lau, C. M; Yeung, S. K.; Lai, T. H. Organometallics 2007,
26, 1981–1985.
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(b) Cheung, C. W.; Chan, K. S. Organometallics 2008, 27, 3043–3055. (c)
Li, B.; Chan, K. S. Organometallics 2008, 27, 4034–4042.
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Chim. Acta 1998, 270, 551–554. (b) Zhou, X.; Wang, R.-J.; Xue, F.; Mak, T.
C. W.; Chan, K. S. J. Organomet. Chem. 1999, 580, 22–25. (c) Zhou, X.;
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2000, 598, 80–86.
r
pubs.acs.org/Organometallics
Published on Web 01/12/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 3, 2010 625
Table 1. Temperature Effect on CHA of Toluene with Rh(ttp)Me
PhCH3
RhðttpÞMe
RhðttpÞBn
ð1Þ
f
temp,time,N2
entry
temp/oC
time
yield/%
1
2
3
4
120
150
150
200
2 d
1 d
4 d
2 h
no rxna
91
69b
91
a Rh(ttp)Me recovered in 92%. b 10 equiv of toluene in benzene, CH4
formed in 76%.
Table 2. Base Effect on CHA of Toluene
RhðttpÞMe þ PhCH3 10 equiv of base RhðttpÞBn ð2Þ
Figure 1. ORTEP drawing of Rh(ttp)Bn(4-tBu) (1b) (30%
probability displacement ellipsoids).
f
150°C,6 h,N2
recovered
yield of
Rh(ttp)Me/%
yield
Rh(ttp)Bn/%
total
yield/%
entry
base
1
2
3
4
none
80
33
14
0
12
55
72
50b
92
88
86
50b
K2CO3
K3PO4
Ph3Pa
a 3 equiv was used. b (Ph3P)Rh(ttp)Me was obtained.
Table 3. Scope of Substrate
RhðttpÞMe þ FG-BnH
RhðttpÞBn-FG ð3Þ
f
150°C,N2
Figure 2. ORTEP drawing of Rh(ttp)Bn(4-F) (1d) (30% prob-
ability displacement ellipsoids).
entry
FG
time
yield (%)
˚
Table 4. Selected Bond Lengths (A) and Max. Deviation of
Rh(ttp)Bn(4-tBu) and Rh(ttp)Bn(4-F)
1
2
3
4
5
6
7
8
9
4-Me
4-tBu
4-H
4-F
4-Cl
4-CF3
4-CN
3-Me
2-Me
7 h
4 h
1 d
15 d
1 d
4 d
17 d
3 d
4 d
1a (76)
1b (82)
1c (91)
1d (48a)
1e (69)
1f (83)
1g (71)
1h (77)
1i (79)
max. deviation
from the
least-squares
Rh-CR
length/A
˚
˚
˚
entry
FG
Rh-Nav/A
plane/A
1
2
4-tBu, 1b
4-F, 1d
2.085(5)
2.064(3)
2.019
2.024
0.162(5)
0.132(4)
a Rh(ttp)Me dissolved poorly in 4-fluorotoluene and was recovered in
24%.
Ph3P ligand did not produce any BnCHA product but yielded
only (Ph3P)Rh(ttp)Me in 50% yield (Table 2, entry 4) and
shut down the CHA reaction completely.
when the reaction was carried out in a sealed NMR tube in
benzene-d6 (Table 1, entry 3; see also SI).
Scope of Substrate. To examine the substrate scope of
toluene, the optimized reaction conditions were applied to
various substituted toluenes. Generally, high yields of rho-
dium porphyrin benzyls were obtained (Table 3, eq 3).
Electron-rich toluenes gave faster reaction rates than elec-
tron-poor toluenes (Table 3, entries 1-7). More steri-
cally hindered ortho- and meta-xylene were also applicable,
but the reactions required longer times (Table 3, entries 8
and 9).
Base Effect. With the reported base-promoted selective
BnCHA by Rh(ttp)Cl at 120 °C,8 we thus explored the effect
of added bases. The control experiment was carried out in 6 h.
Rh(ttp)Me did not react completely and was recovered in
12% yield together with Rh(ttp)Bn formed in 80% yield (eq 2,
Table 2, entry 1). When 10 equiv of K2CO3 was added, only a
33% yield of Rh(ttp)Bn was isolated and a 55% yield of
Rh(ttp)Me recovered (Table 2, entry 2). Similarly, the addi-
tion of 10equiv of K3PO4 gavea 14% yieldofRh(ttp)Bnanda
72% yield of recovered Rh(ttp)Me (Table 2, entry 3). There-
fore, Rh(ttp)Bn was formed in lower yields and more slowly in
the presence of bases. It is likely that a base can coordinate to
Rh(ttp)Me to retard the BnCHA reaction, as the Rh center is
also Lewis acidic. Indeed, the addition of the coordinating
X-ray Structures. Figures 1 and 2 show the X-ray struc-
tures of Rh(ttp)Bn(4-tBu) and Rh(ttp)Bn(4-F). Table 4 lists
some selected bond lengths and maximum deviations of the
core atoms and rhodium from the 24-atom least-squares
planes of Rh(ttp)Bn(4-tBu) and Rh(ttp)Bn(4-F). The Rh-
CR bond lengths of Rh(ttp)Bn(4-tBu) and Rh(ttp)Bn(4-F)
˚
are similar and are 2.085(5) and 2.064(3) A, respectively

626 Organometallics, Vol. 29, No. 3, 2010
Table 5. Temperature-Dependent Kinetic Isotope Effect of BnCHA
Choi et al.
RhðttpÞMe þ C6H5CH3=C6D5CD3
1 : 1
RhðttpÞBn=RhðttpÞBn-d7
f
temp,N2
ð4Þ
a
entry
temp/oC
time
(kH/kD)obs
1
2
150
200
1 d
2.5 h
2.7 ( 0.1
2.8 ( 0.1
a Determined by 1H NMR.
Table 6. Competition Experiments of BnCHA
yield of
Rh(ttp)Bn-FG/%
a
entry
p-FG
σp
yield of Rh(ttp)Bn/%
log k(HFG/HH)
1
2
3
4
5
Me
tBu
F
Cl
CF3
-0.14
-0.15
0.15
0.34
0.53
59
65
49
32
25
21
23
35
50
47
0.45
0.45
0.16
-0.19
-0.28
aσp: para-substituent constant.
(Table 4, entries 1 and 2). The average Rh-N bond lengths in
relationship was observed from the Hammett plot using
the substituent constant σp.13 As the para substituent
becomes more electron-rich, the rate of reaction increases.
The plot of log k(HFG/HH) against σp values yields the
F value of -1.14. Therefore, in the bent transition state,
a positive charge buildup occurs on the benzylic carbon.14
In addition, the linear free energy relationship in the
Hammett plot suggests that any prior coordination of the
benzylic C-H to the rhodium center unlikely occurs to a
significant extent with a change of substituent to affect the
rate.9b
Proposed Mechanism. Rh(ttp)Me can in principle un-
dergo homolysis to give RhII(ttp) and methyl radicals or
heterolysis into Rh(ttp)þ/Me- or Rh(ttp)-/Meþ. The rate of
formation of RhII(ttp) and Me radical by homolysis is too
slow to account for the reaction rate, due to the strong
Rh-CH3 bond (about 57 kcal/mol).6,15 Likewise, heterolysis
is also unlikely, especially in a nonpolar solvent.6 The
remaining possibility of classical oxidative addition or
SBM reported in many CHA reactions3,4 is experimen-
tally very difficult to distinguish, unless a stable intermediate
is isolated. In this porphyrin system, an OA demands a
Rh(V) intermediate complex, which is less common.
Furthermore the sterics of a Rh porphyrin complex with
three cis substituents are highly unfavorable to further argue
against the OA. We therefore favor that the BnCHA of
toluene with Rh(ttp)Me undergoes SBM to give Rh(ttp)Bn
(Scheme 1).
Rh(ttp)Bn(4-tBu) and Rh(ttp)Bn(4-F) do not have any
˚
significant difference, which are 2.019 and 2.024 A, respec-
tively (Table 4, entries 1 and 2). On the other hand, the
substituted benzyls do not cause a large distortion from
planarity of the rhodium porphyrin complexes (Table 4,
entries 1 and 2).
Kinetic Isotope Effect. The observed kinetic isotope
effects (kH/kD)obs for the BnCHA of toluene with Rh(ttp)Me
were measured by competition experiments with an equimo-
lar mixture of solvent toluene and toluene-d8. (kH/kD)obs
were measured to be 2.7 and 2.8 at 150 and 200 °C,
respectively (eq 4, Table 5, entries 1 and 2). Since Rh(ttp)Bn
did not exchange with toluene-d8 under the same reaction
conditions, the measured values are truly kinetic ones.
Therefore, the Bn-H bond cleavage occurs at the rate-
limiting step. The nearly identical KIEs suggest little change
in Arrhenius behavior12 and the same reaction mechanism
at both temperatures. The small values further indicate
a bent transition state in the benzylic C-H cleavage rate-
limiting step.
Hammett Studies. To gain some mechanistic understand-
ings of the electronic effect of BnCHA, the Hammett
plot was constructed from a series of competition experi-
ments using an equimolar mixture of 4-substituted (FG)
toluene and toluene in the reaction with Rh(ttp)Me at
150 °C in 3 days in benzene (eq 5, Table 6). Since Rh(ttp)Bn
did not undergo any exchange with 4-FG-BnH, the ratios
are kinetic ones. Figure 3 shows that the linear free energy
(14) (a) Ess, D. H.; Nielsen, R. J.; Goddard, W. A.III; Periana, R. A.
J. Am. Chem. Soc. 2009, 131, 11686–11688. (b) Hull, K. L.; Sanford, M. S.
J. Am. Chem. Soc. 2009, 131, 9651–9653.
(15) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991,
113, 5305–5311.
(12) Kwart, H. Acc. Chem. Res. 1982, 15, 401–408.
(13) Isaacs, N. S. Physical Organic Chemistry; ELBS, Longman: Avon,
U.K., 1987.

Article
Organometallics, Vol. 29, No. 3, 2010 627
Figure 3. Hammett plot of BnCHA of toluene with Rh(ttp)Me.
Scheme 1. BnCHA of Toluene with Rh(ttp)Me
In this BnCHA system with Rh(ttp)Me, the polarity
reverse of Rh-Me likely occurs in the transition state since
a Rh-C is commonly viewed to be a polar covalent bond with
the rhodium center bearing the positive charge.14a,20,21 The
nature and direction of the overall charge transfer in carbon-
hydrogen bond activation have been reported by Ess et al.14a
They also computed the Hammett plots of BnCHA in selected
metal complexes, which indicate that the benzylic hydrogens can
transfer in a hydric, protic, or hydrogen atom like manner
depending on the metal complexes. The nucleophilic susbstitu-
tion reactions of Rh(ttp)Me with K2CO322 and the intramole-
cular ether formation of Rh porphyrin hydroxylalkyl added
with base23 support the electrophilic nature of the carbon
R to Rh in the transition states. Likewise, the measured F value
of -1.14 from the Hammett studies supports that the benzylic
C-H bond cleaves as a hydric-like hydrogen in the transition
state with the corresponding Rh-Me carbon as an electrophilic
center. This demonstrated electronic effect in carbon-hydrogen
bond activation would hopefully aid the choice of reagent and
design of porphyrin ligand to facilitate the functionalization of
Rh-alkyl bonds.
Many transition metal complexes have been exploited to
break C-H bonds by electrophilic activation.16 For exam-
ple, (H2O)Pt(II)Cl2 and (HSO4)(SO4)Au(III) reacted with
methane, and the Pt and Au centers are both also highly
electrophilic.16d,17 While the reaction between the cationic
[Cp*(PMe3)IrMe]þ complex with methane is presumed to be
promoted by electrophilic C-H activation,18 Ess et al. have
reported recently that the C-H bond cleavage in the
[Cp(PH3)Ir(Me)-CH4]þ system is not so electrophilic by
theoretical calculation, in contrast to the ground-state ca-
tionic nature of the complex.14a A change of polarity in the
ground state and the transition state of the metal complexes
can occur.14a,19 Experimental studies of the polarity and
extent of polarity of bond activation with transition metal
complexes will be fruitful for both fundamental understand-
ings and enhancement of reactivity.
Conclusion
In summary, we have discovered a selective and functional
group compatible benzylic CHA of toluenes with Rh-
(ttp)Me. Rh(ttp)Me undergoes a polar SBM pathway with
a positive charge buildup on the benzylic carbon occurring in
the bent transition state.
(16) (a) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh,
T.; Fujii, H. Science 1998, 280, 560–564. (b) Periana, R. A.; Taube, D. J.;
Evitt, E. R.; Loffler, D. G.; Wentrcek, P. R.; Voss, G.; Masuda, T. Science
1993, 259, 340–343. (c) Periana, R. A.; Mironov, O.; Taube, D.; Bhalla, G.;
Jones, C. J. Science 2003, 301, 814–818. (d) Jones, C. J.; Taube, D.;
Ziatdinov, V. R.; Periana, R. A.; Nielsen, R. J.; Oxgaard, J.; Goddard, W.
A.III. Angew. Chem., Int. Ed. 2004, 43, 4626–4629. (e) Gretz, E.; Oliver, T.;
Sen, A. J. Am. Chem. Soc. 1987, 109, 8109–8111. (f) Kao, L.-C.; Hutson, A.
C.; Sen, A. J. Am. Chem. Soc. 1991, 113, 700–701. (g) Labinger, J. A.;
Herring, A. M.; Bercaw, J. E. J. Am. Chem. Soc. 1990, 112, 5628–5629.
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5961–5976.
Experimental Section
Unless otherwise noted, all reagents were purchased from
commercial suppliers and directly used without further purifica-
tion. Hexane was distilled from anhydrous calcium chloride.
Benzene and toluene were distilled from sodium. Thin-layer
chromatography was performed on precoated silica gel 60 F254
(17) (a) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97, 2879–2932.
(b) Zhu, H.; Ziegler, T. J. Organomet. Chem. 2006, 691, 4486–4497.
(18) (a) Tellers, D. M.; Yung, C. M.; Arndtsen, B. A.; Adamson, D.
R.; Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 1400–1410. (b) Tellers,
D. M.; Bergman, R. G. Organometallics 2001, 20, 4819–4832.
ꢀ
ꢀ
(19) (a) Kristjansdottir, S. S.; Norton, J. R. Organometallics 1991, 10,
2357–2361.
(21) (a) Uddin, J.; Morales, C. M.; Maynard, J. H.; Landis, C. R.
ꢀ
(20) The 1,2-rearrangement rate of rhodium porphyrin alkyls with its
rate-limiting β-hydride elimination step is enhanced by an electron-
deficient porphyrin and is due to the resulting weaker Rh-C bond. (a)
Mak, K. W.; Chan, K. S. J. Am. Chem. Soc. 1998, 120, 9686–9687. (b)
Mak, K. W.; Xue, F.; Mak, T. C. W.; Chan, K. S. J. Chem. Soc., Dalton
Trans. 1999, 3333–3334.
Organometallics 2006, 25, 5566–5581. (b) Clot, E.; Megret, C.; Eisenstein,
O.; Perutz, R. N. J. Am. Chem. Soc. 2009, 131, 7817–7827.
(22) Fung, H. S.; Chan, Y. W.; Cheung, C. W.; Choi, K. S.; Lee, S. Y.;
Qian, Y. Y.; Chan, K. S. Organometallics 2009, 28, 3981–3989.
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588–590.

628 Organometallics, Vol. 29, No. 3, 2010
Choi et al.
plates. Silica gel (Merck, 70-230 mesh) was used for column
chromatography.
Rh(ttp)Bn 1c (3 mg, 0.0035 mmol, 14%) and recovered Rh-
(ttp)Me (14 mg, 0.018 mmol, 72%).
1H NMR and 13C NMR spectra were recorded on a Bruker
DPX-300 at 300 and 75 MHz, respectively. Chemical shifts were
referenced with the residual solvent protons in C6D6 (δ = 7.15
ppm), CDCl3 (δ = 7.26 ppm), or tetramethylsilane (δ = 0.00
D. Addition of 3 equiv of Ph3P. Rh(ttp)Me(20mg, 0.025mmol),
Ph3P (20 mg, 3 equiv), and toluene (1.5 mL) were degassed in a
Teflon-stoppered tube covered by aluminum foil and heated at
150 °C under N2 for 6 h. The solvent was then removed under
vacuum, and the red crude mixture was isolated by column chro-
matography on silica gel eluting with CH2Cl2 to give (Ph3P)Rh-
(ttp)Me (14 mg, 0.013 mmol, 50%). 1H NMR (CDCl3, 300 MHz):
-6.95 (d, 3H, J=2.4 Hz), 4.90 (br s, 6 H), 6.73 (t, 6 H, J=7.2 Hz),
6.98 (t, 3 H, J=7.2 Hz), 7.48 (d, 8 H, J=8.2 Hz), 7.73 (d, 4 H, J=
8.5 Hz), 7.94 (d, 4 H, J = 8.4 Hz), 8.59 (s, 8 H).
ppm) in 1H NMR spectra and CDCl3 (δ = 77.16 ppm) in 13
C
NMR spectra as the internal standards. Chemical shifts (δ) were
reported as part per million (ppm) in δ scale downfield from
TMS. Coupling constants (J) were reported in hertz (Hz).
High-resolution mass spectra (HRMS) were recorded on a Thermo-
Finnigan MAT 95 XL mass spectrometer. Fast atom bombard-
ment spectra were performed with 3-nitrobenzyl alcohol (NBA)
as the matrix. All samples for combustion analyses were recrys-
tallized from CH2Cl2/MeOH and vacuum-dried at room tem-
perature for at least 2 days before submission.
General Procedure for Reactions of Rh(ttp)Me with Various
Substituted Toluenes. Reaction of Rh(ttp)Me with p-Xylene.
Rh(ttp)Me (20 mg, 0.025 mmol) and p-xylene (1.5 mL) were de-
gassed for three freeze-thaw-pump cycles and heated at 150 °C
under N2 for 7 h with the reaction tube covered by aluminum
foil. The solvent was then removed under vacuum, and the red
crude mixture was isolated by column chromatography on silica
gel eluting with a solvent mixture of hexane/CH2Cl2 (1:1) to give
4-CH3C6H4CH2Rh(ttp) 1a (17 mg, 0.019 mmol, 76%) as a red
solid. Rf = 0.65 (n-hexane/CH2Cl2, 1:1). 1H NMR (CDCl3, 300
MHz): -3.77 (d, 2 H, J = 3.6 Hz), 1.68 (s, 3 H), 2.70 (s, 12 H),
2.87 (d, 2 H, J = 7.8 Hz), 5.66 (d, 2 H, J = 7.8 Hz), 7.54 (dd, 8 H,
J = 2.1, 6.8 Hz), 7.96 (dd, 4 H, J = 2.1, 7.1 Hz), 8.07 (dd, 4 H,
J = 2.1, 7.7 Hz), 8.67 (s, 8 H). Calcd for (C56H45N4Rh)þ: m/z
876.2694; found m/z 876.2667. Anal. Calcd for C56H45N4Rh: C,
76.70; H, 5.17; N, 6.39. Found: C, 76.64; H, 5.28; N, 6.06.
Reaction of Rh(ttp)Me with 4-tert-Butyltoluene. Rh(ttp)Me
(20 mg, 0.025 mmol) and 4-tert-butyltoluene (1.5 mL) were
degassed for three freeze-thaw-pump cycles and heated at 150 °C
under N2 for 4 h to give 4-tBuC6H4CH2Rh(ttp) 1b (19 mg,
0.021 mmol, 82%) as a red solid. Rf = 0.75 (n-hexane/CH2Cl2,
1:1). 1H NMR (CDCl3, 300 MHz): -3.79 (d, 2 H, J = 3.6 Hz),
0.96 (s, 9 H), 2.70 (s, 12 H), 2.93 (d, 2 H, J = 8.1 Hz), 5.89 (d, 2 H,
J = 8.4 Hz), 7.54 (t, 8 H, J = 6.0 Hz), 8.04 (t, 8 H, J = 7.9 Hz),
The synthesis of Rh(ttp)Me followed a literature method.20
General Procedure for Reactions of Rh(ttp)Me with Toluene at
Various Temperature and Benzene. At 120 °C. Rh(ttp)Me (20
mg, 0.025 mmol) and toluene (1.5 mL) were degassed in a
Teflon-stoppered tube covered by aluminum foil and heated at
120 °C under N2 for 2 days. No reaction occurred.
At 150 °C. Rh(ttp)Me (20 mg, 0.025 mmol) and toluene
(1.5 mL) were degassed in a Teflon-stoppered tube covered by
aluminum foil and heated at 150 °C under N2 for 1 day. The
solvent was then removed under vacuum, and the red crude
mixture was isolated by column chromatography on silica gel
eluting with a solvent mixture of hexane/CH2Cl2 (1:1) to give
Rh(ttp)Bn9a 1c (20 mg, 0.023 mmol, 91%).
At 150 °C with Benzene. Rh(ttp)Me (20 mg, 0.025 mmol) and
toluene (27 μL) with benzene (1.5 mL) were degassed in a Teflon
screw-capped tube covered by aluminum foil and heated at 150 °C
under N2 for 4 days. The solvent was then removed under
vacuum, and the red crude mixture was isolated by column
chromatography on silica gel eluting with a solvent mixture of
hexane/CH2Cl2 (1:1) to give Rh(ttp)Bn (15 mg, 0.017 mmol,
69%).
8.65 (s, 8 H). 13C NMR (CDCl3, 75 MHz): 13.00 (d, 1JRh-C
=
26.6 Hz), 21.91, 31.01, 34.48, 122.75, 113.30, 124.47, 127.60,
131.68, 134.13, 134.23, 137.41, 139.76, 143.49, 146.20. Calcd for
(C59H51N4Rh)þ: m/z 918.3163; found m/z 918.3139. Anal.
Calcd for C55H43N4Rh: C, 77.11; H, 5.59; N, 6.09. Found: C,
76.66; H, 5.65; N, 5.91.
At 200 °C. Rh(ttp)Me (20 mg, 0.025 mmol) and toluene (1.5
mL) were degassed in a Teflon-stoppered tube covered by
aluminum foil and heated at 200 °C under N2 for 2 h. The
solvent was then removed under vacuum, and the red crude
mixture was isolated by column chromatography on silica gel
eluting with a solvent mixture of hexane/CH2Cl2 (1:1) to give
Rh(ttp)Bn 1c (20 mg, 0.023 mmol, 91%).
General Procedure for Reactions of Rh(ttp)Me with Toluene
and Various Bases. A. Without Base. Rh(ttp)Me (20 mg, 0.025
mmol) and toluene (1.5 mL) were degassed for three freeze-
thaw-pump cycles and heated at 150 °C under N2 for 6 h with
the reaction tube covered by aluminum foil. The solvent was
then removed under vacuum, and the red crude mixture was
isolated by column chromatography on silica gel eluting with a
solvent mixture of hexane/CH2Cl2 (1:1) to give Rh(ttp)Bn 1c
(17 mg, 0.020 mmol, 80%) and recovered Rh(ttp)Me (2.3 mg,
0.003 mmol, 12%).
B. Addition of K2CO3. Rh(ttp)Me (20 mg, 0.025 mmol), 10
equiv of K2CO3 (35 mg, 0.25 mmol), and toluene (1.5 mL) were
degassed for three freeze-thaw-pump cycles and heated at 150 °C
under N2 for 6 h with the reaction tube covered by aluminum
foil. The solvent was then removed under vacuum, and the red
crude mixture was isolated by column chromatography on silica
gel eluting with a solvent mixture of hexane/CH2Cl2 (1:1) to give
Rh(ttp)Bn 1c (7 mg, 0.0083 mmol, 33%) and recovered Rh-
(ttp)Me (11 mg, 0.014 mmol, 55%).
C. Addition of K3PO4. Rh(ttp)Me (20 mg, 0.025 mmol), 10
equiv of K3PO3 (53 mg, 0.25 mmol), and toluene (1.5 mL) were
degassed for three freeze-thaw-pump cycles and heated at 150 °C
under N2 for 6 h with the reaction tube covered by aluminum
foil. The solvent was then removed under vacuum, and the red
crude mixture was isolated by column chromatography on silica
gel eluting with a solvent mixture of hexane/CH2Cl2 (1:1) to give
Reaction of Rh(ttp)Me with 4-Fluorotoluene. Rh(ttp)Me (20
mg, 0.025 mmol) and 4-fluorotoluene (1.5 mL) were degassed
for three freeze-thaw-pump cycles and heated at 150 °C under
N2 for 15 days to give 4-FC6H4CH2Rh(ttp) 1d (11 mg, 0.012
mmol, 48%) as a red solid. Rf = 0.60 (n-hexane/CH2Cl2, 1:1).
1H NMR (CDCl3, 300 MHz): -3.83 (d, 2 H, J = 3.9 Hz), 2.70 (s,
12 H), 2.89 (dd, 2 H, J = 2.7, 5.7 Hz,), 5.55 (t, 2 H, J = 8.7 Hz),
7.55 (t, 8 H, J = 6.6 Hz), 7.98 (dd, 4 H, J = 2.1, 8.0 Hz), 8.06 (d, 4
H, J = 1.2, 8.0 Hz), 8.68 (s, 8 H). 13C NMR (CDCl3, 75 MHz):
11.43 (d, 1JRh-C = 27.3 Hz), 21.89, 112.96 (d, 1JC-F = 20.9 Hz),
122.83, 126.08, 126.18, 127.78, 131.83, 134.16, 134.21, 137.52,
139.61, 143.45. Calcd for (C55H43N4Rh)þ: m/z 880.2443; found
m/z 880.2426. Anal. Calcd for C55H43N4Rh: C, 74.61; H, 5.07;
N, 6.39. Found: C, 74.99; H, 4.81; N, 6.36.
Reaction of Rh(ttp)Me with 4-Chlorotoluene. Rh(ttp)Me (20
mg, 0.025 mmol) and 4-chlorotoluene (1.5 mL) were degassed
for three freeze-thaw-pump cycles and heated at 150 °C under
N2 for 4 days to give 4-ClC6H4CH2Rh(ttp) 1e (15 mg, 0.017
mmol, 69%) as a red solid. Rf = 0.60 (n-hexane/CH2Cl2, 1:1).
1H NMR (CDCl3, 300 MHz): -3.84 (d, 2 H, J = 3.9 Hz), 2.70 (s,
12 H), 2.89 (d, 2 H, J = 8.4 Hz), 5.58 (d, 2 H, J = 8.4 Hz), 7.55 (t,
8 H, J=7.2 Hz), 7.96 (dd, 4 H, J=1.5, 7.1 Hz), 8.06 (dd, 4 H, J=
1.5, 7.1 Hz), 8.69 (s, 8 H). 13C NMR (CDCl3, 75 MHz): 10.67 (d,
1JRh-C=27.4 Hz), 21.89, 122.87, 126.23, 126.25, 127.79, 131.90,
134.22, 137.54, 139.56, 140.12, 143.45. Calcd for (C55H42N4-
ClRh)þ: m/z 896.2148; found m/z 896.2138. Anal. Calcd for
C55H42N4ClRh: C, 73.62; H, 4.70; N, 6.24. Found: C, 73.21; H,
5.12; N, 6.22.

Article
Reaction of Rh(ttp)Me with 4-Methylbenzotrifluoride. Rh-
Organometallics, Vol. 29, No. 3, 2010 629
m/z 876.2703. Anal. Calcd for C56H45N4Rh: C, 76.70; H, 5.17; N,
6.39. Found: C, 76.60; H, 5.11; N, 6.33.
(ttp)Me (20 mg, 0.025 mmol) and 4-methylbenzotrifluoride (1.5
mL) were degassed for three freeze-thaw-pump cycles and
heated at 150 °C for 4 days. The red crude mixture was isolated
by column chromatography on silica gel eluting with a solvent
mixture of hexane/CH2Cl2 (1:1) to give 4-F3CC6H4CH2Rh(ttp) 1f
(19 mg, 0.020 mmol, 83%) as a red solid. Rf = 0.60 (n-hexane/
CH2Cl2, 1:1). 1H NMR (CDCl3, 400 MHz): -3.81 (d, 2 H, J = 3.9
Hz), 2.70 (s, 12 H), 2.96 (d, 2 H, J = 8.0 Hz), 6.08 (d, 2 H, J = 8.1
Hz), 7.54 (d, 4 H, J = 7.8 Hz), 7.55 (d, 4 H, J = 7.6 Hz), 7.94 (d, 4
H, J = 8.0 Hz), 8.05 (d, 4 H, J = 8.0 Hz), 8.70 (s, 8 H). 13C NMR
Temperature-Dependent Isotope Effect with Rh(ttp)Me. Kinetic
Isotope Effect at 150 °C. Rh(ttp)Me (20 mg, 0.025 mmol) and a
premixed equimolar solvent mixture of toluene/toluene-d8 (1.5 mL)
were degassed for three freeze-thaw-pump cycles in the
rota-flo tube. The tube was covered by aluminum foil and
heated to 150 °C under N2 for 1 d. The isotope ratio of
Rh(ttp)Bn to Rh(ttp)Bn-d7 was determined to be 2.7 by integra-
1
tion of H NMR. (The integration of benzylic protons (1.470)
was used to calculate the ratio with the integration of pyrrole
signal set as 8.000). The results were the average of at least two
runs.
1
(CDCl3, 100 MHz): 9.49 (d, JRh-C = 27.5 Hz), 21.68, 122.73,
122.78, 125.20 (q, JC-F = 31.2 Hz), 125.70, 127.60, 131.77,
1
132.28, 133.70, 137.41, 139.25, 143.24, 145.78. Calcd for
(C56H42N4F3Rh)þ: m/z 930.2411; found m/z 930.2391.
Kinetic Isotope Effect at 200 °C. Rh(ttp)Me (20 mg, 0.025
mmol), a premixed equimolar solvent mixture of toluene/
toluene-d8 (1.5 mL) were degassed for three freeze-thaw-pump
cycles in a RotaFlo tube. The tube was covered by aluminum foil
and heated to 200 °C under N2 for 2.5 h. The isotope ratio of
Rh(ttp)Bn to Rh(ttp)Bn-d7 was determined to be 2.8 by integra-
Reaction of Rh(ttp)Me with 4-Methylbenzonitrile. Rh(ttp)Me
(20 mg, 0.025 mmol) and 4-methylbenzonitrile (1.5 mL) were
degassed for three freeze-thaw-pump cycles and heated at 150 °C
for 17 days. The red crude mixture was isolated by column
chromatography on silica gel eluting with a solvent mixture of
hexane/CH2Cl2 (1:1) to give 4-NCC6H4CH2Rh(ttp) 1g (16 mg,
0.018 mmol, 71%) as a red solid. Rf = 0.30 (n-hexane/CH2Cl2,
1:1). 1H NMR (CDCl3, 300 MHz) -3.87 (d, 2 H, J = 3.9 Hz), 2.71
(s, 12 H), 2.90 (d, 2 H, J = 8.4 Hz), 6.11 (d, 2 H, J = 8.1 Hz), 7.57
(t, 8 H, J = 8.4 Hz), 7.93 (dd, 4 H, J = 1.5, 8.3 Hz), 8.03 (dd, 4 H,
J=1.2, 7.7Hz), 8.90(s, 8H). 13CNMR(CDCl3, 75MHz) 9.27(d,
1JRh-C = 28.0 Hz), 21.89, 106.25, 120.01, 122.98, 124.92, 127.89,
129.69, 132.04, 134.12, 134.29, 137.32, 139.32, 143.39. Calcd for
(C56H42N5Rh)þ: m/z 887.2490; found m/z 887.2487.
Reaction of Rh(ttp)Me with m-Xylene. Rh(ttp)Me (20 mg,
0.025 mmol) and m-xylene (1.5 mL) were degassed for three
freeze-thaw-pump cycles and heated at 150 °C under N2 for 3
days with the reaction tube covered by aluminum foil. The
solvent was then removed under vacuum, and the red crude
mixture was isolated by column chromatography on silica gel
eluting with a solvent mixture of hexane/CH2Cl2 (1:1) to give 3-
CH3C6H4CH2Rh(ttp) 1h (17 mg, 0.019 mmol, 77%) as a red
solid. Rf = 0.65 (n-hexane/CH2Cl2, 1:1). 1H NMR (CDCl3, 300
MHz): -3.79 (d, J = 3.9 Hz, 2 H), 1.58 (s, 3 H), 2.63 (s, 1 H), 2.64
(s, 1 H), 2.70 (s, 12 H), 2.80 (d, J = 8.1 Hz, 1 H), 5.77 (t, J = 7.7
Hz, 1 H), 6.32 (d, J = 7.8 Hz, 1 H), 7.53 (d, J = 5.7 Hz, 4 H), 7.55
(d, J = 5.7 Hz, 4 H), 8.00 (dd, J = 2.6, 5.9 Hz, 4 H), 8.06 (dd, J =
2.4, 5.1 Hz, 4 H), 8.67 (s, 8 H). Calcd for (C56H45N4Rh)þ: m/z
876.2694; found m/z 876.2682. Anal. Calcd for C56H45N4Rh: C,
76.70; H, 5.17; N, 6.39. Found: C, 76.42; H, 5.09; N, 6.37.
Reaction of Rh(ttp)Me with o-Xylene. Rh(ttp)Me (20 mg,
0.025 mmol) and o-xylene (1.5 mL) were degassed for three
freeze-thaw-pump cycles and heated at 150 °C under N2 for 4
days with the reaction tube covered by aluminum foil. The solvent
was then removed under vacuum, and the red crude mixture was
isolated by column chromatography on silica gel eluting with a
solvent mixture of hexane/CH2Cl2 (1:1) to give 2-CH3C6H4-
CH2Rh(ttp) 1i (18 mg, 0.020 mmol, 79%) as a red solid. Rf =
0.65 (n-hexane/CH2Cl2, 1:1). 1H NMR (CDCl3, 300 MHz): -3.57
(d, J = 3.6 Hz, 2 H), -0.91 (s, 3 H), 2.56 (d, J = 7.5 Hz, 1 H), 2.70
(s, 12 H), 5.66 (t, J = 7.5 Hz, 1 H), 5.72 (d, J = 7.8 Hz, 1 H), 6.31
(t, J = 7.2 Hz, 1 H), 7.53 (d, J = 3.3 Hz, 4 H), 7.54 (d, J = 1.8 Hz,
4 H), 7.96 (dd, J = 1.7, 7.7 Hz, 4 H), 8.05 (dd, J = 2.1, 7.4 Hz, 4
H), 8.67 (s, 8 H). Calcd for (C56H45N4Rh)þ: m/z 876.2694; found
1
tion of H NMR. (The integration of benzylic protons (1.475)
was used to calculate the ratio with the integration of the pyrrole
signal set as 8.000.) The results were the average of at least two
runs.
Attempted H/D Exchange. Rh(ttp)Bn (10 mg, 0.012 mmol)
and toluene-d8 (1.5 mL) were degassed for three free-
ze-thaw-pump cycles and then heated at 200 °C under N2
for 2.5 h to give only recovered Rh(ttp)Bn (9.2 mg, 0.0107 mmol,
92%).
Competition Experiments of 4-Substituted Toluene. Rh-
(ttp)Me (10 mg, 0.013 mmol), a premixed equimolar solvent
mixture of toluene (13.5 μL, 0.13 mmol), and p-xylene (16 μL,
0.13 mmol) in benzene (1.5 mL) as the solvent were degassed for
three freeze-thaw-pump cycles and heated at 150 °C under N2
for 3 days in a Teflon-stoppered tube. After the reaction was
complete, the 1H NMR spectrum of the crude reaction mixture
was taken. The ratio of k(HFG/HH) was calculated from the
ratio of the benzylic protons with reference to the pyrrole proton
1
in the H NMR spectrum from at least two runs. A series of
experiments was carried out using various functionalized to-
luenes, i.e., 4-tert-butyltoluene (22 μL, 0.13 mmol), 4-fluoroto-
luene (14 μL, 0.13 mmol), 4-chlorotoluene (15 μL, 0.13 mmol),
and 4-methylbenzotrifluoride (18 μL, 0.13 mmol). The ratios of
k(HFG/HH) were calculated from the integration of the benzylic
protons in the 1H NMR spectra. The results were the average of
at least two runs of experiments.
Attempted Exchange. Rh(ttp)Bn (10.0 mg, 0.012 mmol) and 4-
tert-butyltoluene (1.5 mL) were degassed for three freeze-
thaw-pump cycles and then heated at 150 °C under N2 for
1 day to give only Rh(ttp)Bn (9.0 mg, 0.0104 mmol, 90%).
Acknowledgment. We thank the Research Grants
Council of the HKSAR, China (CUHK 400307), for
financial support.
Supporting Information Available: Text, tables, and figures of
crystallographic data for complexes 1b and 1d (CIF and PDF),
1H and 13C NMR spectra for 1f and 1g. This material is available
free of charge via the Internet at http://pubs.acs.org.