Base-promoted carbon-hydrogen bond activation of alkanes with rhodium(III) porphyrin complexes

Article abstract of DOI:10.1021/om800397p

Base-promoted carbon-hydrogen bond activation of alkanes was achieved in the reactions of alkanes with rhodium(III) porphyrin chlorides (Rh(por)Cl) at 120°C to give rhodium porphyrin alkyls in moderate yields. This carbon-hydrogen activation (CHA) of alkane provided a facile synthesis of Rh(por)R. Mechanistic investigation of CHA suggested that Rh(por)H and [Rh(por)]₂ were key intermediates for the CHA step.

Full text of DOI:10.1021/om800397p

Organometallics 2008, 27, 4625–4635
4625
Base-Promoted Carbon-Hydrogen Bond Activation of Alkanes with
Rhodium(III) Porphyrin Complexes
Yun Wai Chan and Kin Shing Chan*
Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, New Territories, Hong Kong,
People’s Republic of China
ReceiVed May 4, 2008
Base-promoted carbon-hydrogen bond activation of alkanes was achieved in the reactions of alkanes
with rhodium(III) porphyrin chlorides (Rh(por)Cl) at 120 °C to give rhodium porphyrin alkyls in moderate
yields. This carbon-hydrogen activation (CHA) of alkane provided a facile synthesis of Rh(por)R.
Mechanistic investigation of CHA suggested that Rh(por)H and [Rh(por)]2 were key intermediates for
the CHA step.
Table 1. Effect of Temperature on CHA of Cyclohexane
Introduction
Carbon-hydrogen bond activation (CHA) of organic com-
pounds by transition-metal complexes is an important area of
research in organometallic chemistry.1 The CHA of alkane is
challenging, due to the lack of reactivity of alkanes.2 Previous
examples include low-valent transition-metal complexes, while
more recent systems involved high-valent late-transition-metal
complexes because of their added advantages of broader
functional group compatibility.3 Examples of high-valent late-
transition-metal Rh(III) and Ir(III) complexes undergoing CHA
are well-known.4 Schwartz and co-workers demonstrated the
Rh(III)-catalyzed chlorination of methane.4a,b Recently, irid-
ium(III) pincer complexes catalyzed the dehydrogenation of
cylcooctane.4c,d Periana and co-workers worked on the alkyl
CHA with bis-bidentate O-donor iridium(III) complexes.4e,f
entry
temp (°C)
yield (%)
1
2
3
4
5
80
100
120
150
200
trace
3
31
16
18
porphyrin complexes selectively via an SEAr mechanism.6 Aryl
and alkyl aldehydes also undergo selective aldehydic CHA to
give rhodium porphyrin aryl and alkyl acyls.7 Recently, we have
reported the selective benzylic CHA of toluenes promoted by
base.8 These types of CHA are mechanistically puzzling, though
we have suggested a heterolytic or σ-bond metathesis pathway.
To explore the scope and gain further understanding of the bond
activation chemistry of rhodium(III) porphyrin complexes, we
have successfully discovered the base-promoted aliphatic CHA
of alkanes and identified some key features of the reaction
mechanism and herein report our results.
The bond activation chemistry of Rh(III) and Ir(III) is
commonly accepted to occur by either σ-bond metathesis or
heterolysis. However, recent reports have provided evidence for
oxidative addition in which a Rh(V) or Ir(V) intermediate is
formed.5 Therefore, diverse mechanistic possibilities exist.
We have been interested in CHA by rhodium(III) porphyrin
complexes. Rhodium porphyrin chloride can activate the meta
C-H bond of benzonitrile to give (m-cyanophenyl)rhodium
Results and Discussion
Initially, rhodium(III) tetrakis(4-tolylporphyrin) chloride (Rh(t-
tp)Cl; 1a) reacted with cyclohexane at 80 and 100 °C for 24 h
to give a trace amount of Rh(ttp)(c-hexyl) (eq 1, Table 1, entries
1 and 2). At 120 °C for 24 h, successful CHA of cyclohexane
occurred and Rh(ttp)(c-hexyl) (2a) was obtained in 31% yield
(Table 1, entry 3). When the temperature was further increased
to 150 or 200 °C, Rh(ttp)(c-hexyl) (2a) was obtained in lower
yields of 16% and 18%, respectively (Table 1, entries 4 and 5).
Likely, Rh(ttp)(c-hexyl) (2a) is thermally unstable. Indeed, when
Rh(ttp)(c-hexyl) (2a) was heated in cyclohexane at 120 and 150
°C for 1 day, the recovery yields were 80 and 41%, respectively.
Therefore, the optimal reaction temperature was selected to be
120 °C.
* To whom correspondence should be addressed. E-mail: ksc@
cuhk.edu.hk.
(1) (a) Crabtree, R. H. Dalton Trans. 2001, 17, 2437–2450. (b) Lersch,
M.; Tilset, M. Chem. ReV. 2005, 105, 2471–2526.
(2) (a) Arndtsen, R. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H.
Acc. Chem. Res. 1995, 28, 154–162. (b) Sen, A. Acc. Chem. Res. 1998, 31,
550–557.
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J. -S.; Roesky, P. W.; Blechert, S. Angew. Chem., Int. Ed. 2005, 44, 7794–
7798.
(4) (a) Ward, M. D.; Schwartz, J. Organometallics 1982, 1, 1030–1032.
(b) Kitajima, N.; Schwartz, J. J. Am. Chem. Soc. 1984, 106, 2220–2222.
(c) Gottker-Schnetmann, I.; Brookhart, M. J. Am. Chem. Soc. 2004, 126,
9330–9338. (d) Gottker-Schnetmann, I.; White, P. S.; Brookhart, M.
Organometallics 2004, 23, 1766–1776. (e) Wong-Foy, A. G.; Bhalla, G.;
Liu, X. Y.; Periana, R. A. J. Am. Chem. Soc. 2003, 125, 14292–14293. (f)
Bhalla, G.; Periana, R. A. Angew. Chem., Int. Ed. 2005, 44, 1540–1543.
(g) Aoki, T.; Crabtree, R. H. Organometallics 1993, 12, 294–298.
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Commun. 1983, 15, 825–826. (b) Monti, D.; Frachey, G.; Bassetti, M.;
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Chim. Acta 1995, 240, 485–493. (c) Cho, J.-Y.; Tse, M. K.; Holmes, D.;
Maleczka, R. E., Jr.; Smith, M. R. Science 2002, 295, 305–308.
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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.; Tse,
M. K.; Wu, D.-D.; Mak, T. C. W.; Chan, K. S. J. Organomet. Chem. 2000,
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1117–1119.
10.1021/om800397p CCC: $40.75
2008 American Chemical Society
Publication on Web 08/21/2008

4626 Organometallics, Vol. 27, No. 18, 2008
Chan and Chan
Table 2. Base Effect in CHA
Table 5. Activation of Alkanes with Rh(ttp)Cl
entry
base
none
PPh3
time (h)
yield (%)
entry
substrate
time (h)
product (yield (%))
1
2
3
4
5
6
7
8
9
10
24
24
48
24
24
6
6
6
6
24
31
0a
1
2
3
4
5
cyclopentane
cyclohexane
n-pentane
n-hexane
n-heptane
6
6
24
24
24
2b (76)
2a (59)
2c (29)
2d (40)
2e (58)
2,2′-bpyb
2,6-dbpyc
2,6-dppyd
2,6-dppyd
NaOH
50
50
58
23
47
51
59
40
NaOAc
K2CO3
K2CO3
Scheme
1.
Proposed
Decomposition
Pathway
of
Rh(ttp)(cyclopentyl)
a Rh(ttp)Cl(PPh3) (2f) was obtained in 83% yield. b 2,2′-bpy
)
)
2,2′-bipyridine. c 2,6-dbpy
)
2,6-di-tert-butylpyridine. d 2,6-dppy
2,6-diphenylpyridine.
Table 3. Effect of K2CO3 Loading in CHA of Cyclohexane
but the reaction rate was not faster (Table 3, entry 2 vs 1). A
higher loading of 10 equiv of K2CO3 increased both the reaction
yield and the rate (Table 3, entry 3). However, a further increase
to 20 equiv of K2CO3 did not result in any further enhancement
in yield (Table 3, entry 4). Therefore, the optimized reaction
conditions were found to require 10 equiv of K2CO3 at 120 °C.
Visual inspection of the reaction mixture showed that even 5
equiv of K2CO3 did not dissolve completely at 120 °C; therefore,
the reaction mixture was heterogeneous.
entry
amt of K2CO3 (equiv)
time (h)
yield (%)
1
2
3
4
0
5
10
20
24
24
6
31
35
59
56
6
Table 4. CHA of Cyclohexane with Rh(por)Cl
The structures of porphyrins in the rhodium porphyrin
chlorides affect the rates and yields of the CHA of cyclohexane.
The electronic effects of CHA were examined by three Rh-
(por)Cl species, including Rh(bocp)Cl (1c; bocp ) 2,3,7,8,12,13,
17,18-octachloro-5,10,15,20-tetrakis(p-tert-butylphenyl)porphy-
rinato dianion), Rh(tpp)Cl (1b; tpp ) 5,10,15,20-tetraphenylpor-
phyrinato dianion), and Rh(ttp)Cl (1a) (Table 4, eq 4). The
reaction rates followed the order of electron-deficient Rh(por)Cl:
Rh(bocp)Cl > Rh(tpp)Cl > Rh(ttp)Cl (Table 4, entries 1-3).
The optimized K2CO3-promoted reaction conditions were
successfully applied to other alkanes. Cyclopentane and cyclo-
hexane gave the cyclopentyl and cyclohexyl complexes, in 76
and 59% yields, respectively in 6 h (eq 5, Table 5, entries 1
and 2). The straight-chain alkanes reacted with Rh(ttp)Cl (1a)
more slowly than cyclohexane (Table 5, entries 3-5 vs 2). A
longer time of 24 h was required. The yields of Rh(ttp) alkyls
increased with the chain length, presumably due to the observed
increasing solubility of Rh(ttp)Cl (1a) in longer chain hydro-
carbons. Selective terminal CHA took place to give only the
primary Rh(ttp) alkyls. While the thermal isomerization of
Rh(ttp)CH2CH2CH3 into Rh(ttp)CH(CH3)2 has been reported,
the time to establish equilibrium requires 10 days.10 Therefore,
the isomerization of these Rh(ttp) alkyls did not occur in 24 h.
We were not able to detect any cyclohexanol, cyclohexyl
chloride, cyclohexene, or cyclohexanone by GC-MS analysis
of the crude reaction mixture. It may be that the concentration
of these species, if formed, was low in the presence of 1000
times more cyclohexane.
entry
por
ttp
tpp
time (h)
product (yield (%))
1
2
3
6
5
1
2a (59)
4a (52)
4b (61)
bocp
With the reported base-promoted benzylic CHA of toluenes
by RhIII(ttp)Cl8 and other examples of base-promoted CHA by
transition-metal complexes,9 we sought to examine the promot-
ing effect of various bases. Table 2 and eq 2 give the results of
the screenings. The ligand, PPh3, only gave coordination
complex 2f, without any CHA product (Table 2, entry 2). On
the other hand, non-coordinating bases of 2,2′-bipyridine, 2,6-
di-tert-butylpyridine, and 2,6-diphenylpyridine gave higher
yields of over 50% of Rh(ttp)(c-hexyl) in 1 day (eq 2, Table 2,
entries 3-5). However, a shorter reaction time of 6 h resulted
in the much lower yield of 23% (Table 2, entry 6).
To our delight, nucleophilic inorganic bases were found to
promote both the yields and rates of CHA (Table 2, entries 7-9).
When NaOH was added, the reaction only took 6 h and the
yield of 2a was 47%. NaOAc gave a slightly higher yield of
51%, and K2CO3 gave the highest yield of 59%. These
nucleophilic bases required just 6 h for the reaction to be
complete. Prolonged heating to 1 day with K2CO3 resulted in a
lower yield of 40% (Table 2, entry 10). From these results, the
optimal reaction conditions were found to require K2CO3 in 6 h.
The loading of base was further optimized (eq 3, Table 3).
Five equivalents of K2CO3 increased the reaction yield slightly,
In order to understand the lower yield of Rh(ttp) alkyl with
longer reaction time (Table 2, entries 9 and 10), the thermal
stability of Rh(ttp)(c-pentyl) in the presence of K2CO3 at 120
°C was examined and monitored by 1H NMR spectroscopy (eq
(9) (a) Liang, L. C.; Lin, J. M.; Lee, W. Y. Chem. Commun. 2005, 19,
2462–2464. (b) Harkins, S. B.; Peters, J. C. Organometallics 2002, 21, 1753–
1755. (c) Wang, C.; Ziller, J. W.; Flood, T. C. J. Am. Chem. Soc. 1995,
117, 1647–1648.
(10) Mak, K. W.; Xue, F.; Mak, T. C. W.; Chan, K. S. J. Chem. Soc.,
Dalton Trans. 1999, 19, 3333–3334.

C-H Bond ActiVation of Alkanes with Rh Porphyrins
Organometallics, Vol. 27, No. 18, 2008 4627
Table 6. Selected Bond Lengths (Å) and Angles (deg) for Rh(por)R
dihedral angle between Ph group
and mean porphyrin plane (deg)
max deviation from
least-squares plane (Å)
entry
Rh(ttp)R
Rh-C length (Å)
Rh-Nav (Å)
1
2
3
4
Rh(ttp)(cyclohexyl) (2a)
Rh(ttp)(cyclopentyl) (2b)
Rh(ttp)(n-heptyl) (2e)
2.126(7)
2.073(7)
2.048(3)
1.970(4)
80.79 (7)
80.98 (3)
82.25(5)
0.465(4)
0.452(7)
0.552(3)
2.019
2.017
2.019
2.027
12
Rh(oep)CH3
6). After the reaction mixture was heated for 30 min, the pyrrole
signal of Rh(ttp)(cyclopentyl) shifted up from δ 8.99 to 8.59
ppm, assigned to be that of Rh(ttp)- (34% yield), and Rh(ttp)H
was formed in 8% yield. Then after 13 h, a small amount of a
cyclopentadienyl anion 1H NMR signal appeared (δ 5.69
ppm).11 After 5 days, its yield increased to 6%. When dilute
aqueous HCl was added into the reaction mixture, Rh(ttp)H (1d)
(ꢀ-H at δ 9.03 ppm) was observed, further supporting the
formation of Rh(ttp)-.
97%.15 This synthetic route can access a variety of rhodium
porphyrin alkyls directly from alkanes in one step.
X-ray Structure Determination. Table 6 gives selected bond
lengths and angles for complexes 2a,b,e. Figures 1-3 show
the molecular structures of 2a,b,e, respectively (30% thermal
ellipsoids). The Rh-C bond lengths of 2a,b,e range from 2.07
to 2.13 Å (Table 6, entries 1-3) and are similar to the reported
Rh-C bond lengths of Rh(oep)Me (oep ) 2,3,7,8,12,13,17,18-
octaethylporphyrinato dianion) (1.97 Å)16 (Table 6, entry 4).
The Rh-Nav bond lengths do not vary significantly (2.017-2.019
Å). The Rh-C bond lengths appear to follow the steric size of
alkyls in the order cyclohexyl > cyclopentyl > n-heptyl (Table
6, entries 1-3). The various alkyls do not cause large distortion
of the mean porphyrin plane from planarity in complexes 2a
(0.465 Å), 2b (0.452 Å), and 2e (0.552 Å).
Proposed Mechanism for CHA. Scheme 2 shows two
possible pathways for the proposed mechanism of alkyl CHA
with Rh(ttp)Cl (1a).
We rationalize that K2CO3 abstracts the ꢀ-alkyl proton of
Rh(ttp)(cyclopentyl) by an E2 elimination to give Rh(ttp)- and
cyclopentene (Scheme 1). As Rh(ttp)H is a moderately strong
acid with a pKa value of about 11,12 Rh(ttp)- is therefore a
good leaving group. Similar base-induced ꢀ-elimination has also
been reported.13 Then cyclopentene can further undergo CHA
at the allylic position and subsequent ꢀ-proton elimination to
give cyclopentadiene, which mostly either dimerizes or poly-
merizes but still yields a small amount of cyclopentadienyl anion
upon reaction with base.
Pathway Ia. In the absence of base, Rh(ttp)Cl (1a) initially
undergoes heterolysis to form Rh(ttp) cation and chloride anion,
most likely as an ion pair.6-8 An alkane then coordinates to
the Rh metal center to form a C-H-Rh complex.17 Finally,
the alkyl C-H bond is cleaved to give Rh(ttp)H. The coordina-
tion of alkane to the Rh metal center is supported by the
inhibition of CHA by PPh3 (Table 2, entry 2). Upon addition
of PPh3, Rh(ttp)Cl (1a) did not react with cyclohexane at 120
°C in 24 h to give any Rh(ttp)(cyclohexyl) (2a).
Monitoring the reaction of Rh(ttp)Cl (1a) with cyclohexane
(50 equiv) in benzene-d6, a poorer hydrogen donor than
cyclohexane, in a sealed NMR tube at 120 °C over the course
of 12 h by 1H NMR spectroscopy did not reveal any intermedi-
ate, only the formation of Rh(ttp)H and Rh(ttp)(cyclohexyl) in
68 and 4% yields, respectively. No stable, long-lived intermedi-
ate likely formed, suggesting the Rh(por)Cl ion pair or
C-H-Rh complex is highly reactive. Furthermore, the major
product is Rh(ttp)H in benzene solvent rather than Rh(ttp)(cy-
clohexyl) in cyclohexane solvent. Probably, the reaction of
Rh(ttp)H with a slight excess of cyclohexane is slow.
Rh(ttp)(cyclohexyl) (2a) was more stable than Rh(ttp)(cy-
clopentyl) (2b) under the same basic, thermal conditions (eq
7). After 5 days, 27% remained of Rh(ttp)(cyclohexyl) together
with Rh(ttp)H formed in 44% yield, presumably from the
protonation of the Rh(ttp)- intermediate with some residual
water present in the solvent or K2CO3.
Pathway Ib. In the base-promoted reaction, Rh(ttp)Cl initially
undergoes ligand substitution with MX to give Rh(ttp)X (MX
) NaOH, NaOAc, K2CO3). Recently, iridium(III) hydroxide,18a
The higher stability of Rh(ttp)(cyclohexyl) is likely due to
the smaller dihedral angle of Rh-CR-Cꢀ-Hꢀ, disfavoring the
antiperiplanar transition state of an E2 elimination14 (dihedral
angles of Rh-CR-Cꢀ-Hꢀ of Rh(ttp)(cyclopentyl) (2b) and
Rh(ttp)(cyclohexyl) (2a) are 131 and 122°, respectively; see the
Supporting Information for X-ray data).
(15) (a) Brothers, P. J.; Collman, J. P. Acc. Chem. Res. 1986, 19, 209–
215. (b) Lexa, D.; Mispelter, J.; Saveant, J. M. J. Am. Chem. Soc. 1981,
103, 6806–6812. (c) Perree-Fauvent, M.; Gaudemer, A.; Boucly, P.;
Devynck, J. J. Organomet. Chem. 1976, 120, 439–451. (d) Chan, K. S.;
Mak, K. W.; Tse, M. K.; Yeung, S. K.; Li, B. Z.; Chan, Y. W. J. Organomet.
Chem. 2008, 693, 399–407.
These CHA reactions further provide a facile, convenient
synthesis of Rh(por) alkyls. For comparison, a previous synthesis
of rhodium porphyrin alkyl was achieved by a two-step process
via reductive alkylation (NaBH4/RBr) with yields from 48 to
(16) Whang, D. M.; Kim, K. M. Acta Crystallogr., Sect. C 1991, 47,
2547–2550.
(17) Bengali, A. A.; Schultz, H.; Moore, C. B.; Bergman, R. G. J. Am.
Chem. Soc. 1994, 116, 9369–9377.
(11) Wayland, B. B.; van Voorhees, S. L.; Wilker, C. Inorg. Chem. 1986,
25, 4039–4042.
(12) Nelson, A. P.; Dimagno, S. G. J. Am. Chem. Soc. 2000, 122, 8569–
8570.
(13) Sanford, M. S.; Groves, J. T. Angew. Chem., Int. Ed. 2004, 43,
588–590.
(18) (a) Tenn, W. J.; Young, K. J. H.; Oxgaard, J.; Nielsen, R. J.;
Goddard, W. A.; Periana, R. A. Organometallics 2006, 25, 5173–5175. (b)
Oxgaard, J.; Tenn, W. J.; Nielson, R. J.; Periana, R. A.; Goddard, W. A.
Organometallics 2007, 26, 1565–1567. (c) Fu, X. F.; Wayland, B. B. J. Am.
Chem. Soc. 2004, 126, 2623–2631. (d) Feng, Y.; Lail, M.; Barakat, K. A.;
Cundari, T. R.; Gunnoe, T. B.; Peterson, J. L. J. Am. Chem. Soc. 2005,
127, 14174–14175. (e) Collman, J. P.; Boulatov, R. Inorg. Chem. 2001,
40, 560–563.
(14) March, J., AdVanced Organic Chemistry: Reactions, Mechanisms
and Structure, 3rd ed.; Wiley: New York, 1985.

4628 Organometallics, Vol. 27, No. 18, 2008
Chan and Chan
Figure 1. ORTEP presentation of the molecular structure with numbering scheme for complex 2a (30% probability displacement ellipsoids).
Figure 2. ORTEP presentation of the molecular structure with numbering scheme for complex 2b (30% probability displacement ellipsoids).
iridium(III) alkoxide,18b rhodium(III) hydroxide,18c ruthenium(II)
hydroxide,18d and rhodium(III) alkoxide18e have been reported
and these ruthenium and iridium complexes can undergo C-H
activation.18a,b,d Rh(ttp)X is then rapidly reduced by an alkane
to form Rh(ttp)H (1d), which then yields the metal-metal-
bonded [Rh(ttp)]2 (1e)19 quickly in basic media.
composed, and its recovery yield was 31%. The decomposed
Rh(ttp) complexes possibly precipitated and were not detected
by H NMR.
1
When the reaction mixture with K2CO3 in benzene-d6 was
followed by 1H NMR spectroscopy (eq 8, Figure 4), no Rh(ttp)H
but only [Rh(ttp)]2 was observed after 30 min. Likely, Rh(ttp)H
was converted rapidly to [Rh(ttp)]2 in a basic medium. After
3 h, Rh(ttp)(cyclohexyl) slowly grew in to about 15% yield and
Rh(ttp)H formed quickly in up to 50% yield. The composition
of the reaction mixture remained more or less the same up to
12 h with a major amount of Rh(ttp)H and small amounts of
[Rh(ttp)]2 and Rh(ttp)(cyclohexyl) present. It is likely that with
a slight excess of cyclohexane in benzene instead of cyclohexane
as the solvent, the base-promoted E2 elimination of Rh(ttp)(cy-
clohexyl) becomes competitive to re-form Rh(ttp)- or Rh(ttp)H
after protonation from the small amount of water present as
well as forming the coproduct cyclohexene. Cyclohexene may
also further undergo multiple CHA and E2 elimination to give
benzene, analogous to the case for cyclopentene. Indeed,
Independent experiments showed that [Rh(ttp)]2 (1e) and
presumably H2 were formed quickly over 15 min at room
temperature from Rh(ttp)H (1d) added with base (eq 9).
However, Rh(ttp)H was thermally stable in the absence of base,
even upon heating at 120 °C for 6 days with 90% recovery (eq
10). Therefore, both [Rh(ttp)]2 and Rh(ttp)H can activate RH
to give Rh(ttp)R in pathways IIa-c (Scheme 2).
1
benzene was observed by H NMR spectroscopy when cyclo-
hexane-d12 was used as the solvent. After 5 days of heating,
1
the H NMR signal of benzene was observed (δ 7.21 ppm) in
To further understand why Rh(ttp)H did not react completely
with a slight excess of cyclohexane in benzene solvent, the
cyclohexane-d12 with 8% yield. Rh(ttp)(cyclohexyl) (2a) de-

C-H Bond ActiVation of Alkanes with Rh Porphyrins
Organometallics, Vol. 27, No. 18, 2008 4629
Figure 3. ORTEP presentation of the molecular structure with numbering scheme for complex 2e (30% probability displacement ellipsoids).
Scheme 2. Proposed Mechanism of Alkyl CHA with
Rh(ttp)Cl
Table 7. KIE Values for Reactions of Rh(ttp)X with Cyclohexane
time
yield
entry Rh(ttp)X base (amt (equiv)) (h) KIE by 1H NMR (%)
1
2
3
4
5
Rh(ttp)Cl
Rh(ttp)H
Rh(ttp)H
[Rh(ttp)]2
[Rh(ttp)]2
K2CO3 (10)
none
K2CO3 (10)
none
6
3
3
6
6
9.1 ( 0.3a
8.9 ( 0.3
8.5 ( 0.4
8.7 ( 0.3
9.0 ( 0.3
32
54
59
38
42
K2CO3 (10)
thermal CHA reaction of Rh(ttp)H with cyclohexane at 120 °C
in 3 h was carried out. Indeed, Rh(ttp)(cyclohexyl) was formed
in 36% yield and supported the intermediancy of Rh(ttp)H (eq
11). The low yield of Rh(ttp)(cyclohexyl) in a slight excess of
cyclohexane is probably due to the unfavorable equilibrium with
limited cyclohexane as well as the base-promoted or thermal
ꢀ-H elimination of Rh(ttp)(cyclohexyl).
a The KIE determined by MS was 9.7 ( 0.2.
To gain further information about the nature of the transition
state and the possible Rh species involved (pathways Ic and
IIa-c), the kinetic isotope effects of the CHA reaction were
measured by a series of competition experiments. Rh(ttp)Cl (1a)
was reacted with an equimolar mixture of cyclohexane and
cyclohexane-d12 in the presence of 10 equiv of K2CO3 at 120
°C over 6 h. The ratio of Rh(ttp)(cyclohexyl) (2a) to Rh(ttp-
1
)(cyclohexyl)-d11 was determined to be 9.1:1.0 by H NMR
spectroscopy. The large kinetic isotope effect (KIE) supported
the notion that the C-H cleavage step to give Rh(ttp)(cyclo-
hexyl) is involved in the rate-limiting step. The observed KIE
is truly a kinetic value, as Rh(ttp)(cyclohexyl) did not exchange
with cyclohexane-d12 under the same conditions. Likewise, the
KIEs of the CHA with Rh(ttp)H and [Rh(ttp)]2 with or without
K2CO3 were measured and are about 9.0 (eq 12, Table 7, entries
2 and 3 vs 4 and 5). Rh(ttp)H was also found to be more reactive
than [Rh(ttp)]2.
To gain further support that [Rh(ttp)]2 is an intermediate, the
reaction of [Rh(ttp)]2 (1e) with cyclohexane in benzene-d6 in a
sealed NMR tube was monitored by 1H NMR spectroscopy (eq
13). After 8 h, both Rh(ttp)(cyclohexyl) (2a) and Rh(ttp)H (1d)
were obtained in 12 and 82% yields, respectively (eq 13). The
(19) (a) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991,
113, 5305–5311. (b) Thibblin, A.; Sidhu, H. J. Am. Chem. Soc. 1992, 114,
7403–7407.
Figure 4. Time profile of Rh(ttp)Cl and cyclohexane with K2-
CO3.

4630 Organometallics, Vol. 27, No. 18, 2008
Chan and Chan
metathesis processes,22 but they are similar to the KIE values
of other CHA reactions involving methane-eliminating σ-bond
metathetical events (8.7-9.1).23a,b As Rh(ttp)H (1d) reacts with
cyclohexane to give Rh(ttp)(cyclohexyl) (2a), it may undergo
σ-bond metathesis with H2 elimination. Such σ-bond metathesis
with H2 elimination was also proposed in the alkane metathesis
catalyzed by silica-supported tantalum hydride.23c
Scheme 3. Proposed Pathways of Alkyl CHA with Rh(ttp)H
Conclusions
much higher yield of Rh(ttp)H suggests that, even under thermal
conditions, Rh(ttp)(cyclohexyl) still can generate Rh(ttp)H by
ꢀ-H elimination, as ascertained by the independent thermal
experiment shown in eq 14. Therefore, [Rh(ttp)]2 is confirmed
to be a viable intermediate of the CHA reaction under both
neutral and basic conditions. The expected coproduct, cyclo-
hexene, was not observed, which was rationalized by further
dehydrogenation of cyclohexene to yield benzene.
Base-promoted aliphatic CHA of alkanes was achieved with
rhodium(III) porphyrin complexes to give rhodium(III) porphy-
rin alkyls. Mechanistic investigations suggested that both
Rh(ttp)H (1d) and [Rh(ttp)]2 are key intermediates for the
parallel CHA step. The roles of base are (i) to facilitate the
formation of Rh(ttp)X, (ii) to enhance the CHA rate with alkane
and generate Rh(ttp)H by a Rh(ttp)X species more reactive than
Rh(ttp)Cl, and (iii) to provide a parallel CHA pathway by
[Rh(ttp)]2.
Experimental Section
Unless otherwise noted, all reagents were purchased from
commercial suppliers and used before purification. Hexane for
chromatography was distilled from anhydrous calcium chloride.
Rh(ttp)Cl (1a), Rh(tpp)Cl (1b), and Rh(bocp)Cl (1c) were prepared
according to literature procedures. Thin-layer chromatography was
performed on precoated silica gel 60 F254 plates. Silica gel (Merck,
70-230 and 230-400 mesh) was used for column chromatography
in air.
Since the large values of KIEs of the CHA of CH4 (8.6 at 25
°C and 5.1 at 80 °C) and of toluene (6.5 at 80 °C) with RhII(tmp)
(tmp ) 5,10,15,20-tetramesitylporphyrinato dianion) are sup-
portive of a bimetalloradical linear, termolecular transition
state,19a the similar magnitude of KIEs in the reaction of
cyclohexane with Rh(ttp) complexes suggest that RhII(ttp) also
undergo similar bimetalloradical activation with cyclohexane.
However, the conversion of Rh(ttp)H to [Rh(ttp)]2 in the absence
of base is very slow; therefore, we tend to favor that both
Rh(ttp)H and [Rh(ttp)]2 are parallel reacting species under both
neutral and basic conditions. However, we could not fully
understand the high KIE value of 8.9 measured for the thermal
reaction with Rh(ttp)H (Table 7, entry 2). A possible reason is
the large KIE observed for a branching reaction from a common
intermediate:19b in this case, the branching reaction of Rh2(ttp)2
with cyclohexane from the common intermediate of Rh(ttp)H.
1H NMR spectra were recorded on a Bruker DPX 300 (300 MHz)
spectrometer. Spectra were referenced internally to the residual
proton resonance in C6D6 (δ 7.15 ppm) or CDCl3 (δ 7.26 ppm) or
with tetramethylsilane (TMS, δ 0.00 ppm) as the internal standard.
Chemical shifts (δ) are reported in parts per million (ppm). 13C
NMR spectra were recorded on a Bruker DPX 300 (75 MHz)
spectrometer and referenced to CDCl3 (δ 77.10 ppm) spectra.
Coupling constants (J) are reported in hertz (Hz). Mass spectra
(HRMS) were performed on a Thermofinnigan MAT 95 XL
instrument (FABMS).
The CHA reactions were carried out in N2 in the dark with
Teflon-stoppered reaction tubes covered with aluminum foil. Unless
otherwise stated, the reactions were duplicated and the yields are
the average yields. The reactions were traced by TLC. Unless
otherwise stated, all the reactions were stopped once the starting
materials (Rh(por)Cl) were consumed.
As Rh(ttp)H was shown to be a viable intermediate of the
CHA reaction (eq 11), two detailed mechanistic possibilities,
oxidative addition and σ-bond metathesis, could exist (Scheme
3). For the oxidative addition, a seven-coordinated Rh(V)
complex (A) formed at first with the three ligands R, H, and H
on the same face of the porphyrin, which then underwent
reductive elimination to give Rh(ttp)R and H2. Though Rh(V)
organometallic complexes are uncommon, they can be stabilized
by strongly σ-donating ligands such as silyl20 and hydride. The
two cis dihydrides, e.g. [(η5-C5Me5)Rh(H)2(SiMe3)2] and [(η5-
C5Me5)Rh(H)2(SiEt3)2],20 are not sterically demanding enough
to rule out the oxidative addition pathway. Alternatively, a
concerted σ-bond metathesis or its variants such as σ-complex
(B) assisted metathesis21 could be a viable pathway.
Reaction of Alkanes with Rh(ttp)Cl (1a).6a,c,7,24,25 (5,10,
15,20-Tetratolylporphyrinato)(cyclohexyl)rhodium(III), [Rh-
(ttp)(cyclohexyl)] (2a). Method A. Rh(ttp)Cl (1a; 20.1 mg, 0.025
mmol) was added into cyclohexane (3.0 mL). The red suspension
was degassed for three freeze-thaw-pump cycles and was then
heated at 120 °C under N2 in the dark for 24 h. After 24 h, the
mixture turned dark red. Excess cyclohexane was removed by
vacuum distillation. The dark red residue was then purified by
column chromatography on silica gel with a hexane/CH2Cl2 solvent
(22) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan,
M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem.
Soc. 1987, 109, 203–219.
(23) (a) Zhang, S.; Piers, W. E.; Gao, X.; Parvez, M. J. J. Am. Chem.
Soc. 2000, 122, 5499–5509. (b) Conroy, K. D.; Hayes, P. G.; Piers, W. E.;
Parvez, M. Organometallics 2007, 26, 4464–4470. (c) Basset, J. M.; Coperet,
C.; Lefort, L.; Maunders, B. M.; Maury, O.; Le Roux, E.; Saggio, S.;
Soulivong, D.; Sunley, G. J.; Taoufik, M.; Thivolle-Cazat, J. J. Am. Chem.
Soc. 2005, 127, 8604–8605.
The large KIE values of cyclohexane CHA with Rh(ttp)Cl
(9.1) and with Rh(ttp)H (8.9) are not typical of “normal” σ-bond
(20) Ruiz, J.; Mann, B. E.; Spencer, C. M.; Taylor, B. F.; Maitlis, P. M.
J. Chem. Soc., Dalton Trans. 1987, 8, 1963–1966.
(21) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46,
2578–2592.
(24) Mak, K. W.; Yeung, S. K.; Chan, K. S. Organometallics 2002, 21,
2362–2364.
(25) Ogoshi, H.; Setsune, J.; Omura, T.; Yoshida, Z. J. Am. Chem. Soc.
1975, 97, 6461–6466.

C-H Bond ActiVation of Alkanes with Rh Porphyrins
Organometallics, Vol. 27, No. 18, 2008 4631
mixture (4/1) as eluent to give Rh(ttp)(cyclohexyl) (2a) as a red
solid (6.6 mg, 0.007 mmol, 31%), which was further recrystallized
from CH2Cl2/MeOH. Rf ) 0.84 (hexane/CH2Cl2 ) 1/1). 1H NMR
for three freeze-thaw-pump cycles and was heated at 150 °C under
N2 in the dark for 24 h. Excess cyclohexane was removed by
vacuum distillation. The dark red residue was purified by column
chromatography on silica gel with a solvent mixture of hexane/
CH2Cl2 (4:1) as eluent. Rh(ttp)(cyclohexyl) (2a) was obtained as a
red solid (4.3 mg, 0.005 mmol, 41%).
Method B. Rh(ttp)Cl (1a; 20.4 mg, 0.025 mmol) and anhydrous
potassium carbonate (34.9 mg, 0.252 mmol) were added into
cyclohexane (3.0 mL). The red reaction mixture was degassed for
three freeze-thaw-pump cycles and was heated at 120 °C under
N2 for 6 h. Excess cyclohexane was removed by vacuum distillation,
and the dark red crude product was extracted with CH2Cl2/H2O.
The organic layer was collected, dried, and evaporated to dryness,
and the residue was purified by column chromatography on silica
gel with a hexane/CH2Cl2 solvent mixture (4:1) as eluent. Rh(ttp)-
(cyclohexyl) (2a; 12.7 mg, 0.015 mmol, 59%) was collected as a
red solid.
(CDCl3, 300 MHz): δ -4.25 (m, 5 H, Ha, Hb, and Hb′), -1.23 (q,
2 H, J ) 12.3 Hz, Hc′), -0.95 (tq, 1 H, J ) 3.3, 12.9 Hz, Hd),
-0.58 (d, 2 H, J ) 12.6 Hz, Hc), -0.08 (d, 1 H, J ) 12.6 Hz,
Hd′), 2.69 (s, 12 H, p-methyl), 7.52 (d, 8 H, J ) 7.5 Hz, m-phenyl),
8.01 (d, 4 H, J ) 8.4 Hz, o′-phenyl), 8.06 (d, 4 H, J ) 7.8 Hz,
o-phenyl), 8.68 (s, 8 H, pyrrole). 13C NMR (CDCl3, 75 MHz): δ
1
21.69, 25.17, 26.95, 33.36, 39.37 (d, JRh-C ) 27.6 Hz), 122.81,
127.40, 127.55, 130.88, 131.52, 131.71, 134.25, 137.23, 139.53,
143.40. HRMS: calcd m/z for (C54H46N4Rh)+ 854.2850, found m/z
854.2859. Anal. Calcd for C54H46N4Rh: C, 75.87; H, 5.54; N, 6.55.
Found: C, 75.41; H, 5.57; N, 6.50. Single crystals for X-ray
diffraction analysis were grown from CH2Cl2/methanol.
Method A. Rh(ttp)Cl (1a; 20.2 mg, 0.025 mmol) was added
into cyclohexane (3.0 mL). The red suspension was degassed for
three freeze-thaw-pump cycles and was then heated at 80 °C
under N2 in the dark for 24 h. After 24 h, the mixture turned dark
red. Excess cyclohexane was removed by vacuum distillation. The
dark red residue was then purified by column chromatography on
silica gel with a hexane/CH2Cl2 solvent mixture (4/1) as eluent to
give Rh(ttp)(cyclohexyl) (2a) as a red solid (0.2 mg, 0.000 23 mmol,
1%).
The cyclohexane fraction removed by vacuum distillation was
extracted with water (3.0 mL). The colorless organic layer was
diluted by dichloromethane (3.0 mL) and then was injected for GC-
MS analysis.
Method C. Rh(ttp)Cl (1a; 20.4 mg, 0.025 mmol) and sodium
hydroxide (10.1 mg, 0.252 mmol) were added into cyclohexane
(3.0 mL). The red reaction mixture was degassed for three
freeze-thaw-pump cycles and was heated at 120 °C under N2 for
6 h. Excess cyclohexane was removed by vacuum distillation, and
the dark red crude product was extracted with CH2Cl2/H2O. The
organic layer was collected, dried, and evaporated to dryness, and
the residue was purified by column chromatography on silica gel
with a hexane/CH2Cl2 solvent mixture (4:1) as eluent. Rh(ttp)(cy-
clohexyl) (2a) as a red solid was collected as the only product (10.2
mg, 0.012 mmol, 47%). The cyclohexane fraction removed by
vacuum distillation was extracted with water (3.0 mL). The colorless
organic layer was diluted by dichloromethane (3.0 mL) and then
was injected for GC-MS analysis.
Method D. Rh(ttp)Cl (1a; 20.1 mg, 0.025 mmol) and sodium
acetate (20.4 mg, 0.249 mmol) were added into cyclohexane (3.0
mL). The red reaction mixture was degassed for three freeze-thaw-
pump cycles and was heated at 120 °C under N2 for 6 h. Excess
cyclohexane was removed by vacuum distillation, and the dark red
crude product was extracted with CH2Cl2/H2O. The organic layer
was collected, dried, and evaporated to dryness, and the residue
was purified by column chromatography on silica gel with a hexane/
CH2Cl2 solvent mixture (4/1) as eluent to give Rh(ttp)(cyclohexyl)
(2a) as a red solid (10.9 mg, 0.013 mmol, 51%).
Method E. Rh(ttp)Cl (1a; 20.6 mg, 0.026 mmol) and 2,2′-
bipyridine (39.9 mg, 0.255 mmol) were added into cyclohexane
(3.0 mL). The red reaction mixture was degassed for three
freeze-thaw-pump cycles and was heated at 120 °C under N2 for
48 h. Excess cyclohexane was removed by vacuum distillation, and
the residue was purified by column chromatography on silica gel
with a hexane/CH2Cl2 solvent mixture (4/1) as eluent to give the
Rh(ttp)(cyclohexyl) (2a) as a red solid (11.0 mg, 0.013 mmol, 50%).
Method F. Rh(ttp)Cl (1a; 19.8 mg, 0.025 mmol) and 2,6-
diphenylpyridine (57.2 mg, 0.245 mmol) were added into cyclo-
hexane (3.0 mL). The red reaction mixture was degassed for three
freeze-thaw-pump cycles and was heated at 120 °C under N2 for
24 h. Excess cyclohexane was removed by vacuum distillation, and
the residue was purified by column chromatography on silica gel
with a hexane/CH2Cl2 solvent mixture (4/1) as eluent to give the
Rh(ttp)(cyclohexyl) (2a) as a red solid (12.2 mg, 0.014 mmol, 58%).
Method G. Rh(ttp)Cl (1a; 19.7 mg, 0.024 mmol) and 2,6-di-
tert-butylpyridine (46.4 mg, 54.5 µL, 0.243 mmol) were added into
cyclohexane (3.0 mL). The red reaction mixture was degassed for
three freeze-thaw-pump cycles and was heated at 120 °C under
N2 for 24 h after excess cyclohexane was removed by vacuum
distillation; the residue was then purified by column chromatography
Method A. Rh(ttp)Cl (1a; 19.9 mg, 0.025 mmol) was added
into cyclohexane (3.0 mL). The red suspension was degassed for
three freeze-thaw-pump cycles and was then heated at 100 °C
under N2 in the dark for 24 h. After 24 h, the mixture turned dark
red. Excess cyclohexane was removed by vacuum distillation. The
dark red residue was then purified by column chromatography on
silica gel with a hexane/CH2Cl2 solvent mixture (4/1) as eluent to
give Rh(ttp)(cyclohexyl) (2a) as a red solid (0.6 mg, 0.000 70 mmol,
3%).
Method A. Rh(ttp)Cl (1a; 20.0 mg, 0.025 mmol) was added
into cyclohexane (3.0 mL). The red suspension was degassed for
three freeze-thaw-pump cycles and was then heated at 150 °C
under N2 in the dark for 24 h. After 24 h, the mixture turned dark
red. Excess cyclohexane was removed by vacuum distillation. The
dark red residue was then purified by column chromatography on
silica gel with a hexane/CH2Cl2 solvent mixture (4/1) to give
Rh(ttp)(cyclohexyl) (2a) as a red solid (3.4 mg, 0.0040 mmol, 16%).
Method A. Rh(ttp)Cl (1a; 20.0 mg, 0.025 mmol) was added
into cyclohexane (3.0 mL). The red suspension was degassed for
three freeze-thaw-pump cycles and was then heated at 200 °C
under N2 in the dark for 24 h. After 24 h, the mixture turned dark
red. Excess cyclohexane was removed by vacuum distillation. The
dark red residue was then purified by column chromatography on
silica gel with a hexane/CH2Cl2 solvent mixture (4/1) as eluent to
give Rh(ttp)(cyclohexyl) (2a) as a red solid (3.8 mg, 0.0045 mmol,
18%).
Thermal Stability of Rh(ttp)(cyclohexyl) (2a) at 120 °C.
Rh(ttp)(cyclohexyl) (2a; 10.9 mg, 0.013 mmol) was added into
cyclohexane (3.0 mL). The red solution was degassed for three
freeze-thaw-pump cycles and was heated at 120 °C under N2 in
the dark for 24 h. Excess cyclohexane was removed by vacuum
distillation. The dark red residue was purified by column chroma-
tography on silica gel with a hexane/CH2Cl2 solvent mixture (4/1)
as eluent. Rh(ttp)(cyclohexyl) (2a) was obtained as a red solid (8.7
mg, 0.010 mmol, 80%).
Thermal Stability of Rh(ttp)(cyclohexyl) (2a) at 150 °C.
Method A. Rh(ttp)(cyclohexyl) (2a; 10.4 mg, 0.012 mmol) was
added into cyclohexane (3.0 mL). The red solution was degassed

4632 Organometallics, Vol. 27, No. 18, 2008
Chan and Chan
on silica gel with a hexane/CH2Cl2 solvent mixture (4/1) as eluent
to give Rh(ttp)(cyclohexyl) (2a) as a red solid (10.3 mg, 0.012
mmol, 50%).
(5,10,15,20-Tetraphenylporphyrinato)(cyclohexyl)rhodium-
(III), [Rh(tpp)(cyclohexyl)] (4a). Method B. Rh(tpp)Cl (1b)25,26
(20.2 mg, 0.027 mmol) and anhydrous potassium carbonate (37.2
mg, 0.269 mmol) were added into cyclohexane (3.0 mL) and to
form a bright red reaction mixture. This reaction mixture was
degassed for three freeze-thaw-pump cycles and was heated at
120 °C under N2 in the dark for 5 h. After 5 h, the mixture turned
dark red. Excess cyclohexane was removed by vacuum distillation.
The dark red residue was extracted with CH2Cl2/H2O. The organic
layer was collected, dried, and evaporated to dryness and then
purified by column chromatography on silica gel with a hexane/
CH2Cl2 solvent mixture (4/1) as eluent. Rh(tpp)(cyclohexyl) (4a)
was collected as a red solid (11.2 mg, 0.014 mmol, 52%), Rf )
Method B. Rh(ttp)Cl (1a; 20.1 mg, 0.025 mmol) and anhydrous
potassium carbonate (34.7 mg, 0.252 mmol) were added into
cyclohexane (3.0 mL). The red reaction mixture was degassed for
three freeze-thaw-pump cycles and was heated at 120 °C under
N2 for 24 h. Excess cyclohexane was removed by vacuum
distillation, and the dark red crude product was extracted with
CH2Cl2/H2O. The organic layer was collected, dried, and evaporated
to dryness, and the residue was purified by column chromatography
on silica gel with a hexane/CH2Cl2 solvent mixture (4/1) as eluent
to give Rh(ttp)(cyclohexyl) (2a) as a red solid (8.6 mg, 0.010 mmol,
40%).
1
0.83 (hexane/CH2Cl2 1/1). H NMR (CDCl3, 300 MHz): δ -4.26
Method F. Rh(ttp)Cl (1a; 20.3 mg, 0.025 mmol) and 2,6-
diphenylpyridine (57.0 mg, 0.245 mmol) were added into cyclo-
hexane (3.0 mL). The red reaction mixture was degassed for three
freeze-thaw-pump cycles and was heated at 120 °C under N2 for
6 h. Excess cyclohexane was removed by vacuum distillation, and
the residue was purified by column chromatography on silica gel
with a hexane/CH2Cl2 solvent mixture (4/1) as eluent to give the
Rh(ttp)(cyclohexyl) (2a) as a red solid (4.9 mg, 0.0057 mmol, 23%).
(m, 5 H, Ha, Hb, and Hb′), -1.21 (q, 2 H, J ) 12.3 Hz, Hc′), -0.94
(tq, 1 H, J ) 3.3, 12.9 Hz, Hd), -0.56 (d, 2 H, J ) 11.4 Hz, Hc),
-0.07 (d, 1 H, J ) 12.9 Hz, Hd′), 7.50 (m, 8 H, m-phenyl), 8.13
(d, 4 H, J ) 4.5 Hz, o′-phenyl), 8.19 (d, 4 H, J ) 6.6 Hz, o-phenyl),
8.67 (s, 8 H, pyrrole). 13C NMR (CDCl3, 75 MHz): δ 25.14, 26.95,
33.39, 39.65 (d, 1JRh-C ) 29.5 Hz), 122.87, 126.70, 126.84, 127.69,
131.61, 133.78, 134.34, 142.39, 143.30. HRMS: calcd m/z for
(C50H39N4Rh)+ 798.2224, found m/z 798.2171. Anal. Calcd for
C50H39N4Rh: C, 75.18; H, 4.92; N, 7.01; Found: C, 74.68; H, 4.89;
N, 6.70.
[2,3,7,8,12,13,17,18-Octachloro-5,10,15,20-tetrakis(p-tert-bu-
tylphenyl)porphyrinato](cyclohexyl)rhodium(III), Rh(bocp)(cy-
clohexyl) (4b). Method B. Rh(bocp)Cl (1c)25,26 (21.7 mg, 0.017
mmol) and anhydrous potassium carbonate (24.0 mg, 0.174 mmol)
were added into cyclohexane (3.0 mL) to form a bright red reaction
mixture. This reaction mixture was degassed for three freeze-thaw-
pump cycles and was heated at 120 °C under N2 in the dark for
1 h. After 1 h, the mixture turned dark red. Excess cyclohexane
was removed by vacuum distillation. The dark red residue was
extracted with CH2Cl2/H2O. The organic layer was collected, dried,
and evaporated to dryness and then purified by column chroma-
tography on silica gel with a hexane/CH2Cl2 solvent mixture (4/1)
as eluent to give Rh(bocp)(cyclohexyl) (4b) as a red solid (13.8
mg, 0.011 mmol, 61%). Rf ) 0.86 (hexane/CH2Cl2 1/1). 1H NMR
Chloro(5,10,15,20-Tetratolylporphyrinato)(triphenylphos-
phine)rhodium(III), [Rh(ttp)Cl(PPh3)] (2f). Rh(ttp)Cl (1a; 19.9
mg, 0.025 mmol) and triphenylphosphine (64.7 mg, 0.247 mmol)
were added into cyclohexane (3.0 mL). The red reaction mixture
was degassed for three freeze-thaw-pump cycles and was heated
at 120 °C under N2 for 24 h. Excess cyclohexane was removed by
vacuum distillation, and the residue was purified by column
chromatography on silica gel with ethyl acetate as eluent to give
Rh(ttp)Cl(PPh3) (2f; 21.9 mg, 0.021 mmol, 83%). Rf ) 0.65 (ethyl
acetate). 1H NMR (CDCl3, 300 MHz): δ 2.68 (s, 12 H, p-methyl),
3.84 (dd, 6 H, J ) 7.6, 10.7 Hz, m-phenyl of PPh3), 6.51 (td, 6 H,
J ) 2.2, 7.8 Hz, o-phenyl of PPh3), 6.90 (td, 3 H, J ) 5.3, 11.2
Hz, p-phenyl of PPh3), 7.49 (d, 8 H, J ) 8.0 Hz, m-phenyl), 7.66
(d, 4 H, J ) 7.6 Hz, o′-phenyl), 8.04 (d, 4 H, J ) 7.49 Hz,
o-phenyl), 8.72 (s, 8 H, pyrrole). 13C NMR (CDCl3, 75 MHz): δ
21.63, 121.63, 126.79, 126.92, 127.00, 127.66, 129.36, 131.02,
131.14, 132.15, 132.65, 134.16, 135.05, 137.05, 139.31, 142.75.
HRMS: calcd m/z for (C66H51N4PRh)+ 1033.2901, found m/z
1033.2885.
Investigation of Base Loading in CHA of Cyclohexane.
Method B. Rh(ttp)Cl (1a; 20.3 mg, 0.025 mmol) and anhydrous
potassium carbonate (34.9 mg, 0.126 mmol) were added into
cyclohexane (3.0 mL). The red reaction mixture was degassed for
three freeze-thaw-pump cycles and was heated at 120 °C under
N2 for 24 h. Excess cyclohexane was removed by vacuum
distillation, and the dark red crude product was extracted with
CH2Cl2/H2O. The organic layer was collected, dried, and evaporated
to dryness, and the residue was purified by column chromatography
on silica gel with a hexane/CH2Cl2 solvent mixture (4/1) as eluent.
Rh(ttp)(cyclohexyl) (2a) as a red solid (7.5 mg, 0.0088 mmol, 35%)
was collected.
Method B. Rh(ttp)Cl (1a; 20.1 mg, 0.025 mmol) and anhydrous
potassium carbonate (68.8 mg, 0.498 mmol) were added into
cyclohexane (3.0 mL). The red reaction mixture was degassed for
three freeze-thaw-pump cycles and was heated at 120 °C under
N2 for 6 h. Excess cyclohexane was removed by vacuum distillation,
and the dark red crude product was extracted with CH2Cl2/H2O.
The organic layer was collected, dried, and evaporated to dryness,
and the residue was purified by column chromatography on silica
gel with a hexane/CH2Cl2 solvent mixture (4/1) as eluent. Rh(ttp)-
(cyclohexyl) (2a) as a red solid (11.9 mg, 0.014 mmol, 56%) was
collected.
(CDCl3, 300 MHz): δ -3.63 (q, 2 H, J ) 9.6 Hz, Hb), -3.46 (q,
2 H, J ) 9.3 Hz, Hb′), -3.17 (td, 1 H, J ) 3.0, 9.0 Hz, Ha), -0.85
(q, 2 H, J ) 12.3 Hz, Hc′), -0.71 (t, 1 H, J ) 12.9 Hz, Hd), -0.25
(d, 2 H, J ) 12.0 Hz, Hc), 1.36 (d, 1 H, J ) 12.0 Hz, Hd′), 1.55 (s,
36 H, tBu), 7.70 (d, 8 H, J ) 8.7 Hz, m-phenyl), 8.54 (d, 4 H, J )
7.2 Hz, o′-phenyl), 7.95 (d, 4 H, J ) 7.2 Hz, o-phenyl). 13C NMR
(CDCl3, 75 MHz): δ 25.18, 27.61, 29.86, 31.85, 35.11, 35.78,
122.56, 124.71, 133.02, 134.05, 134.73, 138.11, 152.76. HRMS:
m/z calcd for (C66H63N4Cl8Rh)+ 1298.1551, found m/z 1298.1519.
Anal. Calcd for C66H63N4Cl8Rh: C, 61.04; H, 4.89; N, 4.31; Found:
C, 61.11; H, 5.05; N, 4.19.
(5,10,15,20-Tetratolylporphyrinato)(cyclopentyl)rhodium(III),
[Rh(ttp)(cyclopentyl)] (2b). Method B. Rh(ttp)Cl (1a; 20.0 mg,
0.025 mmol) and anhydrous potassium carbonate (34.2 mg, 0.248
(26) (a) Buchler, J. W.; Dreher, C.; Kunzel, F. M. In Metal Complexes
with Tetrapyrrole Ligands III; Buchler, J. W., Ed.; Springer-Verlag, Berlin,
Heidelberg, 1995; Structure and Bonding 84. (b) Mak, K. W.; Chan, K. S.
J. Am. Chem. Soc. 1998, 120, 9689–9687.

C-H Bond ActiVation of Alkanes with Rh Porphyrins
Organometallics, Vol. 27, No. 18, 2008 4633
mmol) were added into cyclopentane (3.0 mL). The red reaction
mixture was degassed for three freeze-thaw-pump cycles and was
heated at 120 °C under N2 in the dark for 6 h. Excess cyclopentane
was removed by vacuum distillation. The dark red crude product
was extracted with CH2Cl2/H2O. The organic layer was collected,
dried, and evaporated to dryness, and the residue was purified by
column chromatography on silica gel with a hexane/CH2Cl2 solvent
mixture (4/1) as eluent. Rh(ttp)(cyclopentyl) (2b) was collected as
a red solid (16.0 mg, 0.017 mmol, 75%). Rf ) 0.85 (hexane/CH2Cl2
300 MHz): δ -4.96 (td, 2 H, J ) 3.0, 9.0 Hz, Ha), -4.51 (qu, 2
H, J ) 7.8 Hz, Hb), -1.58 (qu, 2 H, J ) 7.5 Hz, Hc), -0.57 (qu,
2 H, J ) 7.5 Hz, Hd), 0.10 (m, 2 H, He), 0.17 (t, 3 H, J ) 7.2 Hz,
Hf), 2.68 (s, 12 H, p-methyl), 7.52 (d, 4 H, J ) 5.7 Hz, m′-phenyl),
7.54 (d, 4 H, J ) 5.7 Hz, m-phenyl), 7.97 (d, 4 H, J ) 7.2 Hz,
o′-phenyl), 8.06 (d, 4 H, J ) 8.3 Hz, o-phenyl), 8.69 (s, 8 H,
1
1/1). H NMR (CDCl3, 300 MHz): δ -4.90 (m, 2 H, Hb), -4.37
pyrrole). 13C NMR (CDCl3, 75 MHz): δ 13.53, 15.69 (d, 1JRh-C
)
27.2 Hz), 21.40, 21.69, 26.06, 27.13, 29.84, 122.53, 127.47, 130.89,
131.51, 131.85, 133.83, 134.11, 137.25, 139.52, 143.39 HRMS:
calcd m/z for (C54H49N4Rh)+ 856.3007, found m/z 856.3017. Anal.
Calcd for C54H49N4Rh: C, 75.69; H, 5.76; N, 6.54. Found: C, 75.61;
H, 5.72; N, 6.53.
(m, 1 H, Ha), -3.46 (m, 2 H, Hb′), -1.03 (m, 4 H, Hc), 2.69 (s, 12
H, p-methyl), 7.52 (t, 8 H, J ) 6.2 Hz, m-phenyl), 8.00 (d, 4 H, J
) 8.5 Hz, o′-phenyl), 8.09 (d, 4 H, J ) 8.6 Hz, o-phenyl), 8.69 (s,
8 H, pyrrole). 13C NMR (CDCl3, 75 MHz): δ 18.87, 22.14, 29.52,
34.53, 123.20, 127.86, 127.92, 131.91, 134.19, 134.61, 137.69,
139.94, 143.93. HRMS: m/z calcd for (C53H45N4Rh)+ 840.2694,
found m/z 840.2694. Anal. Calcd for C53H45N4Rh: C, 75.71; H,
5.39; N, 6.66. Found: C, 75.29; H, 5.37; N, 6.53. Single crystals
for X-ray diffraction analysis were grown from CH2Cl2/ethanol.
(5,10,15,20-Tetratolylporphyrinato)(n-pentyl)rhodium(III),
[Rh(ttp)(n-pentyl)] (2c). Method B. Rh(ttp)Cl (1a; 20.4 mg,
0.025 mmol) and anhydrous potassium carbonate (34.9 mg, 0.252
mmol) were added into n-pentane (3.0 mL) to form a bright red
reaction mixture. This reaction mixture was degassed for three
freeze-thaw-pump cycles and was heated at 120 °C under N2 in
the dark for 24 h. After 24 h, the mixture turned dark red. Excess
n-pentane was removed by vacuum distillation. The dark red residue
was extracted with CH2Cl2/H2O. The organic layer was collected,
dried, and evaporated to dryness and then purified by column
chromatography on silica gel with a hexane/CH2Cl2 solvent mixture
(4/1) as eluent. Rh(ttp)(n-pentyl) (2c). Rh(ttp)(n-pentyl) (2c) as a
red solid was collected as the major product (6.5 mg, 0.008 mmol,
31%). Rf ) 0.78 (hexane/CH2Cl2 1/1). 1H NMR (CDCl3, 300 MHz):
(5,10,15,20-Tetratolylporphyrinato)(n-heptyl)rhodium(III),
[Rh(ttp)(n-heptyl)] (2e). Method B. Rh(ttp)Cl (1a; 20.9 mg,
0.026 mmol) and anhydrous potassium carbonate (35.3 mg, 0.255
mmol) were added into n-heptane (3.0 mL) to form a bright red
reaction mixture. This reaction mixture was degassed for three
freeze-thaw-pump cycles and was heated at 120 °C under N2 in
the dark for 24 h. After 24 h, the mixture turned dark red. Excess
n-heptane was removed by vacuum distillation. The dark red residue
was extracted with CH2Cl2/H2O. The organic layer was collected,
dried, and evaporated to dryness and then purified by column
chromatography on silica gel with a hexane/CH2Cl2 solvent mixture
(4/1) as eluent. Rh(ttp)(n-heptyl) (2e) was collected as the major
product as a red solid (13.2 mg, 0.015 mmol, 59%). Rf ) 0.83
(hexane/CH2Cl2 1/1). 1H NMR (CDCl3, 300 MHz): δ -4.97 (td, 2
H, J ) 3.0, 8.7 Hz, Ha), -4.53 (qu, 2 H, J ) 7.2 Hz, Hb), -1.63
(qu, 2 H, J ) 7.2 Hz, Hc), -0.55 (qu, 2 H, J ) 7.2 Hz, Hd), -0.04
(d, 2 H, J ) 9.0 Hz, He), 0.45 (q, 3 H, J ) 6.0 Hz, Hg), 0.52 (qu,
2 H, J ) 7.2 Hz, Hf), 2.69 (s, 12 H, p-methyl), 7.51 (d, 4 H, J )
6.6 Hz, m′-phenyl), 7.53 (d, 4 H, J ) 6.3 Hz, m-phenyl), 7.98 (d,
4 H, J ) 6.9 Hz, o′-phenyl), 8.08 (d, 4 H, J ) 7.2 Hz, o-phenyl),
8.70 (s, 8 H, pyrrole). 13C NMR (CDCl3, 75 MHz): δ 13.89, 15.71
1
δ -4.97 (td, 2 H, J ) 2.4, 7.5 Hz, Ha), -4.50 (qu, 2 H, J ) 7.2
Hz, Hb), -1.59 (qu, 2 H, J ) 2.1 Hz, Hc), -0.49 (q, 2 H, J ) 6.9
Hz, Hd), -0.25 (t, 3 H, J ) 7.2 Hz, He), 2.69 (s, 12 H, p-methyl),
7.53 (t, 8 H, J ) 6.0 Hz, m-phenyl), 7.98 (d, 4 H, J ) 8.6 Hz,
o′-phenyl), 8.08 (d, 4 H, J ) 8.3 Hz, o-phenyl), 8.70 (s, 8 H,
pyrrole) (qu ) quintet). 13C NMR (CDCl3, 75 MHz): δ 12.81, 15.66
(d, 1JRh-C ) 27.2 Hz), 20.72, 21.69, 26.86, 28.50, 122.51, 127.47,
127.52, 131.49, 133.81, 134.10, 137.25, 139.50, 143.37 HRMS:
calcd m/z for (C53H47N4Rh)+ 840.2694, found m/z 840.2682. Anal.
Calcd for C53H47N4Rh: C, 75.52; H, 5.62; N, 6.64. Found: C, 75.43;
H, 5.67; N, 6.36.
(d, JRh-C ) 27.5 Hz), 21.69, 22.10, 26.25, 27.06, 27.24, 30.57,
122.52, 126.22, 127.45, 127.52, 131.49, 133.80, 134.11, 137.23,
139.51, 143.38 HRMS: calcd m/z for (C55H51N4Rh)+ 870.3163,
found m/z 870.3167. Anal. Calcd for C55H51N4Rh: C, 75.85; H,
5.90; N, 6.43. Found: C, 75.77; H, 5.96; N, 6.31. Single crystals
for X-ray diffraction analysis were grown from CH2Cl2/methanol.
Decomposition of Rh(ttp)(cyclopentyl) (2b) with K2CO3 in
Benzene-d6. Rh(ttp)(cyclopentyl) (6.7 mg, 0.0080 mmol) and
potassium carbonate (11.0 mg, 0.080 mmol) were added into
benzene-d6 (520 µL) in an NMR tube. The red reaction mixture
was degassed for three freeze-thaw-pump cycles, and the NMR
tube was sealed under vacuum. It was heated to 120 °C, the reaction
mixture was monitored with 1H NMR spectroscopy, and NMR
yields were determined.
(5,10,15,20-Tetratolylporphyrinato)(n-hexyl)rhodium(III),
[Rh(ttp)(n-hexyl)] (2d). Method B. Rh(ttp)Cl (1a; 20.6 mg, 0.026
mmol) and anhydrous potassium carbonate (35.3 mg, 0.255 mmol)
were added into n-hexane (3.0 mL) to form a bright red reaction
mixture. This reaction mixture was degassed for three freeze-thaw-
pump cycles and was heated at 120 °C under N2 in the dark for
24 h. After 24 h, the mixture turned dark red. Excess n-hexane
was removed by vacuum distillation. The dark red residue was
extracted with CH2Cl2/H2O. The organic layer was collected, dried,
and evaporated to dryness and then purified by column chroma-
tography on silica gel with a hexane/CH2Cl2 solvent mixture (4/1)
as eluent. Rh(ttp)(n-hexyl) (2d) was collected as the major product.
Rh(ttp)(n-hexyl) (2d) was collected as a red solid (10.6 mg, 0.012
For the observation of Rh(ttp)H, a dilute HCl solution was added
into the NMR tube with shaking. A 1H NMR spectrum was
measured immediately after the addition of dilute HCl.
Decomposition of Rh(ttp)(cyclohexyl) (2a) with K2CO3 in
Benzene-d6. Rh(ttp)(cyclohexyl) (6.9 mg, 0.0081 mmol) and
potassium carbonate (11.0 mg, 0.080 mmol) were added into
benzene-d6 (520 µL) in an NMR tube. The red reaction mixture
was degassed for three freeze-thaw-pump cycles, and the NMR
tube was sealed under vacuum. It was heated to 120 °C, the reaction
mixture was monitored with 1H NMR spectroscopy, and NMR
yields were determined.
1
mmol, 49%). Rf ) 0.84 (hexane/CH2Cl2 1/1). H NMR (CDCl3,

4634 Organometallics, Vol. 27, No. 18, 2008
Chan and Chan
Sealed NMR Tube Experiment of Rh(ttp)Cl and Cyclohex-
ane in Benzene-d6. Rh(ttp)Cl (1a; 5.1 mg, 0.0066 mmol) and
cyclohexane (34 µL, 0.316 mmol) were added into benzene-d6 (520
µL) in an NMR tube. The red mixture was degassed for three
freeze-thaw-pump cycles, and the NMR tube was sealed under
vacuum. It was heated at 120 °C in the dark. It was monitored
with 1H NMR spectroscopy at particular time intervals, and the
NMR yields were determined.
deuterium incorpation (4.680). kH/kD is equal to the integration of
alkyl proton (δ 4.680) over the integration of alkyl deuterium (δ
5.193-4.680). The calculated kH/kD is 9.1 ( 0.3.
(ii) MS Method. The product ratio is calculated to be the
molecular peak intensity of Rh(ttp)(cyclohexyl) (observed to be
60.0) over the molecular peak intensity of Rh(ttp)(cyclohexyl)-d11
(observed to be 6.1). The product ratio is calculated to be 9.7 (
0.2.
Alkyl Exchange of Rh(ttp)(cyclohexyl) (2a) with Cyclohex-
ane-d12 at 120 °C. Rh(ttp)(cyclohexyl) (2a; 5.4 mg, 0.0063 mmol)
was added into cyclohexane-d12 (0.5 mL) in a Teflon screw capped
NMRtube.Theredsolutionwasdegassedforthreefreeze-thaw-pump
cycles. The NMR tube was sealed under vacuum. It was heated at
Sealed NMR Tube Experiment of Rh(ttp)Cl and Cyclohex-
ane with K2CO3 in Benzene-d6. Rh(ttp)Cl (1a; 5.1 mg, 0.0066
mmol), cyclohexane (34 µL, 0.316 mmol), and K2CO3 (8.7 mg,
0.063 mmol) were added into benzene-d6 (520 µL) in an NMR
tube. The red mixture was degassed for three freeze-thaw-pump
cycles, and the NMR tube was sealed under vacuum. It was heated
at 120 °C in the dark. It was monitored with 1H NMR spectroscopy
at particular time intervals, and the NMR yields were determined.
Reaction of Rh(ttp)H with KOH.11 Rh(ttp)H (1d; 8.0 mg, 0.010
mmol) and potassium hydroxide (14.3 mg, 0.104 mmol) were added
into degassed benzene-d6 (520 µL) in an NMR tube. The reaction
mixture was degassed for three freeze-thaw-pump cycles, and
the NMR tube was sealed under vacuum. The reaction mixture was
then warmed to 23 °C in a water bath for 15 min. It was monitored
by 1H NMR spectroscopy. [Rh(ttp)]2 was obtained (55% NMR
yield).
Thermal Stability of Rh(ttp)H. Rh(ttp)H (1d; 7.9 mg, 0.010
mmol) was added into degassed benzene-d6 (520 µL) in an NMR
tube. The reaction mixture was degassed for three freeze-thaw-
pump cycles, and the NMR tube was sealed under vacuum. The
reaction mixture was then heated to 120 °C for 6 days. It was
monitored by 1H NMR spectroscopy. The NMR yield of Rh(ttp)H
was found to be 90%.
1
120 °C in the dark, and the reaction was monitored by H NMR.
The product ratio was calculated as follows.
For the case of a 3 h reaction, the integration of the alkyl proton
at δ -4.171 (observed δ 5.370) was used to calculate the ratio,
with the integration of the pyrrole signal of 2a (δ 8.63) being taken
as 8. Let the integration of that alkyl deuterium be Y. Y is equal to
the integration of that alkyl proton without deuterium incorporation
(δ 5.370) minus the observed integration of that alkyl proton with
deuterium incorpation (δ 5.144). kH/kD is equal to the integration
of alkyl proton over the integration of alkyl deuterium. kH/kD is
calculated to be 22.8.
For the case of a 42 h reaction, kH/kD is equal to the integration
of the alkyl proton (δ 5.191) over the integration of alkyl deuterium
(δ 5.370-5.191). The calculated kH/kD is 22.9.
Competition Reaction of Cyclohexane and Cyclohexane-d12
with Rh(ttp)H (1d). Method A. Rh(ttp)H (1d; 10.1 mg, 0.013
mmol) was added into a mixture of cyclohexane (1.50 mL, 14.000
mmol) and cyclohexane-d12 (1.49 mL, 14.000 mmol). The red
mixture was degassed for three freeze-thaw-pump cycles and was
heated at 120 °C under N2 in the dark for 3 h. Then the excess
cyclohexane/cyclohexane-d12 mixture was removed by vacuum
distillation. The dark red residue was examined by 1H NMR
spectroscopy. It was then purified by column chromatography on
silica gel with a hexane/CH2Cl2 solvent mixture (4/1) as eluent.
The red fraction was collected. After rotary evaporation, the product
mixture of Rh(ttp)(cyclohexyl) (2a) and Rh(ttp)(cyclohexyl)-d11 was
obtained as a red solid (6.0 mg, 0.0070 mmol, 54%). Integration
of the alkyl proton of the Rh(ttp)(cyclohexyl)/Rh(ttp)(cyclohexyl)-
d11 mixture (δ -4.26) was found to be 4.668. The calculated kH/kD
was 8.9 ( 0.3.
Competition Reaction of Cyclohexane and Cyclohexane-d12
with Rh(ttp)H (1d) and K2CO3. Method B. Rh(ttp)H (1d; 10.0
mg, 0.013 mmol) and potassium carbonate (17.9 mg, 0.130 mmol)
were added into a mixture of cyclohexane (1.50 mL, 14.000 mmol)
and cyclohexane-d12 (1.49 mL, 14.000 mmol). The red mixture was
degassed for three freeze-thaw-pump cycles and was heated at
120 °C under N2 in the dark for 3 h. Then the excess cyclohexane/
cyclohexane-d12 mixture was removed by vacuum distillation. The
dark red residue was examined by 1H NMR spectroscopy to
determine the product ratio. It was then extracted with CH2Cl2/
H2O and purified by column chromatography on silica gel with a
hexane/CH2Cl2 solvent mixture (4/1) as eluent. The red fraction
was collected. After rotary evaporation, the product mixture of
Rh(ttp)(cyclohexyl) (2a) and Rh(ttp)(cyclohexyl)-d11 was obtained
as a red solid (6.2 mg, 0.0073 mmol, 56%). Integration of the alkyl
proton of the Rh(ttp)(cyclohexyl)/Rh(ttp)(cyclohexyl)-d11 mixture
(δ -4.26) was found to be 4.647. The calculated kH/kD was 8.5 (
0.4.
Reaction of Alkanes with Rh(ttp)H (1d).7,25 Method A.
Rh(ttp)H (1d; 10.0 mg, 0.013 mmol) was added into cyclohexane
(3.0 mL). The red suspension was degassed for three freeze-thaw-
pump cycles and was then heated at 120 °C under N2 in the dark
for 3 h. After 3 h, the mixture turned red. Excess cyclohexane was
removed by vacuum distillation. The dark red residue was then
purified by column chromatography on silica gel with a hexane/
CH2Cl2 solvent mixture (4/1) as eluent to give Rh(ttp)(cyclohexyl)
(2a) as a red solid (4.0 mg, 0.0047 mmol, 36%).
Competition Reaction of Cyclohexane and Cyclohexane-d12
with Rh(ttp)Cl (1a) and K2CO3. Method B. Rh(ttp)Cl (1a; 19.6
mg, 0.021 mmol) and potassium carbonate (33.6 mg, 0.243 mmol)
were added into a mixture of cyclohexane (1.50 mL, 14.000 mmol)
and cyclohexane-d12 (1.49 mL, 14.000 mmol). The red mixture was
degassed for three freeze-thaw-pump cycles and was heated at
120 °C under N2 in the dark for 6 h. Then the excess cycloxane/
cyclohexane-d12 mixture was removed by vacuum distillation. Part
of the red residue (0.2 mg) was examined by mass spectroscopy to
obtain the ratio of the two products. The remaining crude product
was monitored by 1H NMR spectroscopy to determine the product
ratio. It was then extracted with CH2Cl2/H2O and purified by column
chromatography on silica gel with a hexane/CH2Cl2 solvent mixture
(4/1) as eluent. The red fraction was collected. After rotary
evaporation, the product mixture of Rh(ttp)(cyclohexyl) (2a) and
Rh(ttp)(cyclohexyl)-d11 was obtained as a red solid (6.7 mg, 0.0078
mmol, 32%).
The product ratio was calculated as follows.
(i) 1H NMR Method. Integration of the alkyl protons of
Rh(ttp)(cyclohexyl) at δ -4.26 (observed δ 5.193) and integration
of the alkyl protons of Rh(ttp)(cyclohexyl)/Rh(ttp)(cyclohexyl)-d11
at δ -4.26 (observed δ 4.680) were used to calculate the ratio with
the integration of the pyrrole signal of 2a (δ 8.63) being taken as
8. Let the integration of that alkyl deuterium be Y. Y is equal to the
integration of that alkyl proton without deuterium incorporation
(5.193) minus the observed integration of that alkyl proton with
Competition Reaction of Cyclohexane and Cyclohexane-d12
with [Rh(ttp)]2 (1e).11 Method A. [Rh(ttp)]2 (1e; 10.3 mg, 0.0067
mmol) was added into a mixture of cyclohexane (1.50 mL, 14.000
mmol) and cyclohexane-d12 (1.49 mL, 14.000 mmol). The red
mixture was degassed for three freeze-thaw-pump cycles and was
heated at 120 °C under N2 in the dark for 6 h. Then the excess

C-H Bond ActiVation of Alkanes with Rh Porphyrins
Organometallics, Vol. 27, No. 18, 2008 4635
cyclohexane/cyclohexane-d12 mixture was removed by vacuum
distillation. The dark red residue was examined by 1H NMR
spectroscopy. It was then purified by column chromatography on
silica gel with a hexane/CH2Cl2 solvent mixture (4/1) as eluent.
The red fraction was collected. After rotary evaporation, the product
mixture of Rh(ttp)(cyclohexyl) (2a) and Rh(ttp)(cyclohexyl)-d11 was
obtained as a red solid (4.3 mg, 0.0050 mmol, 38%). Integration
of the alkyl proton of the Rh(ttp)(cyclohexyl)/Rh(ttp)(cyclohexyl)-
d11 mixture (δ -4.26) was found to be 4.659. The calculated kH/kD
was 8.7 ( 0.3.
Sealed NMR Tube Experiment with [Rh(ttp)]2 (1e) and
Cyclohexane in Benzene-d6.11 [Rh(ttp)]2 (1e; 5.1 mg, 0.0033
mmol) and cyclohexane (34 µL, 0.316 mmol) were added into
benzene-d6 (520 µL) in an NMR tube. The red mixture was
degassed for three freeze-thaw-pump cycles, and the NMR tube
was sealed under vacuum. It was heated at 120 °C in the dark. It
was monitored with 1H NMR spectroscopy at particular time
intervals, and the NMR yields were determined.
ꢀ-Hydride Elimination of Rh(ttp)(cyclohexyl) (2a). Rh(ttp)-
(cyclohexyl) (6.8 mg, 0.0081 mmol) was added into benzene-d6
(520 µL) in an NMR tube. The red reaction mixture was degassed
for three freeze-thaw-pump cycles, and the NMR tube was sealed
under vacuum. It was heated to 120 °C, the reaction mixture was
monitored with 1H NMR spectroscopy, and NMR yields were
detemined.
Competition Reaction of Cyclohexane and Cyclohexane-d12
with [Rh(ttp)]2 (1e) and K2CO3.11 Method A. [Rh(ttp)]2 (1e; 10.1
mg, 0.0065 mmol) and potassium carbonate (9.0 mg, 0.065 mmol)
were added into a mixture of cyclohexane (1.50 mL, 14.000 mmol)
and cyclohexane-d12 (1.49 mL, 14.000 mmol). The red mixture was
degassed for three freeze-thaw-pump cycles and was heated at
120 °C under N2 in the dark for 6 h. Then the excess cyclohexane/
cyclohexane-d12 mixture was removed by vacuum distillation. The
Acknowledgment. We thank Ms. C. M. Lau for prelimi-
nary investigation, Ms. H. S. Chan for the X-ray crystal-
lographic determinations, and the Research Grants Council
of the HKSAR, People’s Republic of China (CUHK 400105),
for financial support.
1
dark red residue was examined by H NMR spectroscopy. It was
then purified by column chromatography on silica gel with a hexane/
CH2Cl2 solvent mixture (4/1) as eluent. The red fraction was
collected. After rotary evaporation, the product mixture of Rh(t-
tp)(cyclohexyl) (2a) and Rh(ttp)(cyclohexyl)-d11 was obtained as
a red solid (4.7 mg, 0.0055 mmol, 42%). Integration of the alkyl
proton of the Rh(ttp)(cyclohexyl)/Rh(ttp)(cyclohexyl)-d11 mixture
(δ -4.26) was found to be 4.673. The calculated kH/kD was 9.0 (
0.3.
Supporting Information Available: Figures giving spectro-
scopic data for compounds 2a-f and 4a,b and text, tables, figures,
and CIF files giving crystallographic data for complexes 2a,b,e.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM800397P