K2CO3-promoted consecutive carbon-hydrogen and carbon-carbon bond activation of Cycloheptane with rhodium(III) porphyrin complexes: Formation of rhodium porphyrin cycloheptyl and benzyl

Article abstract of DOI:10.1021/om500313g

K₂CO₃-promoted carbon-hydrogen and carbon-carbon bond activations of cycloheptane are achieved with rhodium(III) tetrakis(4-tolyl) porphyrin chloride (Rh(ttp)Cl) at 120 °C to give Rh(ttp) cycloheptyl and benzyl complexes. On the basis of mechanistic studies, Rh(ttp)Cl first reacts by ligand substitution to give Rh(ttp)OH, which then undergoes reductive elimination to give Rh^II₂(ttp)₂. The metalloradical Rh^II(ttp), formed via dissociation of Rh ^II₂(ttp)₂, activates the CH bond of cycloheptane to form Rh(ttp)(cycloheptyl) and Rh(ttp)H. Rh(ttp)(cycloheptyl) slowly yields Rh(ttp)(cycloheptatrieneyl) by successive β-hydride elimination to olefins and Rh(ttp)H. K₂CO₃ promoted the dehydrogenation of Rh(ttp)H to give Rh^II₂(ttp)₂ and H₂. Both Rh(ttp)H and Rh^II₂(ttp) ₂ activate the cycloheptatriene to give Rh(ttp)(cycloheptatrienyl), which further undergoes a Rh^II(ttp)-catalyzed skeletal rearrangement to form Rh(ttp)Bn with rate enhancement much faster than that of the analogous organic isomerization of cycloheptatriene to toluene.

Full text of DOI:10.1021/om500313g

Article
pubs.acs.org/Organometallics
K2CO3‑Promoted Consecutive Carbon−Hydrogen and Carbon−
Carbon Bond Activation of Cycloheptane with Rhodium(III)
Porphyrin Complexes: Formation of Rhodium Porphyrin Cycloheptyl
and Benzyl
Kin Shing Chan* and Yun Wai Chan
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People’s Republic of China
S
* Supporting Information
ABSTRACT: K2CO3-promoted carbon−hydrogen and car-
bon−carbon bond activations of cycloheptane are achieved
with rhodium(III) tetrakis(4-tolyl)porphyrin chloride (Rh-
(ttp)Cl) at 120 °C to give Rh(ttp) cycloheptyl and benzyl
complexes. On the basis of mechanistic studies, Rh(ttp)Cl first reacts by ligand substitution to give Rh(ttp)OH, which then
undergoes reductive elimination to give RhII (ttp)2. The metalloradical RhII(ttp), formed via dissociation of RhII (ttp)2, activates
2
2
the CH bond of cycloheptane to form Rh(ttp)(cycloheptyl) and Rh(ttp)H. Rh(ttp)(cycloheptyl) slowly yields
Rh(ttp)(cycloheptatrieneyl) by successive β-hydride elimination to olefins and Rh(ttp)H. K2CO3 promoted the dehydrogenation
of Rh(ttp)H to give RhII (ttp)2 and H2. Both Rh(ttp)H and RhII (ttp)2 activate the cycloheptatriene to give
2
2
Rh(ttp)(cycloheptatrienyl), which further undergoes a RhII(ttp)-catalyzed skeletal rearrangement to form Rh(ttp)Bn with rate
enhancement much faster than that of the analogous organic isomerization of cycloheptatriene to toluene.
SEAr mechanism via a Rh(III) porphryin cation intermediate.7
INTRODUCTION
■
Aryl and alkyl aldehydes also undergo selective aldehydic CHA
with Rh(III) porphyrin chloride or methyl to give rhodium
porphyrin aryl and alkyl acyls.8 A σ bond metathesis mechanism
was proposed.8 Recently, we have reported the selective
benzylic CHA of toluenes promoted by base.9 Initial ligand
substitution of Rh(por)Cl with hydroxide gives Rh(por)OH,
which at high temperature by reductive elimination gives H2O2
and RhII(por).10 Then RhII(por) undergoes bimetalloradical
benzylic CHA on the basis of Wayland’s proposed mecha-
nism.11 The unique hydroxide reduction of easily handled
RhIII(por) halides to highly reactive RhII(por) metalloradical
provides a facile, alternative entry to RhII(por). Similar
hydroxide-promoted alkane CH activation of n-alkanes, cyclo-
pentane,12 cyclohexane,12 and cyclooctane13 was reported with
a Rh(II) porphyrin intermediate detected.9,12,13 To explore the
scope and gain further understanding of the base-promoted
bond activation chemistry with rhodium(III) porphyrin
complexes in alkane CHA, we have successfully discovered
the base-promoted aliphatic CHA of cycloheptane with
RhIII(ttp)Cl (ttp = 5,10,15,20-tetratolylporphyrinato dianion;
Figure 1) to give RhIII(ttp)(cycloheptyl) as a CHA product and
Rh(ttp)Bn as an unexpected carbon−carbon bond activation
(CCA)13 and CHA product. Mechanistic studies reveal that
metalloradical CHA is operating to initially produce Rh(ttp)H
and Rh(ttp)(cycloheptyl). Rh(ttp)(cycloheptyl) further under-
goes RhII(ttp)-catalyzed dehydrogenation to yield Rh(ttp)-
(cycloheptatrienyl), which further isomerizes into Rh(ttp)Bn
Carbon−hydrogen bond activation (CHA) of organic com-
pounds by transition-metal complexes, especially the late-
transition-metal complexes, is an important research area in
organometallic chemistry.1 The CHA of alkane is difficult due
to the low reactivity of nonpolar and strong CH bonds in
alkanes.2 Great efforts have been undertaken to achieve CHA.
Previous successful examples include low-valent transition-
metal complexes, while more recent systems involve high-valent
late-transition-metal complexes because of their added
advantages of broader functional group compatibility and
catalytic application.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, iridium(III)
pincer complexes were shown to catalyze the dehydrogenation
of cyclooctane.4c,d The alkyl CHA with bis-bidentate O-donor
iridium(III) complexes was also reported.4e,f
Bond activation chemistry of Rh(III) and Ir(III) are
commonly accepted to occur by either σ-bond metathesis or
heterolysis.4 However, recent reports provide evidence for the
oxidative addition mechanism in which a Rh(V) or Ir(V)
intermediate is formed and isolated.5 The isolation of oxidative
addition intermediates of strongly electron donating silyl Ir(V)
complexes provides further supporting evidence for such a
pathway;6 therefore, diverse mechanistic possibilities exist for
group 9 metal complexes undergoing CH activation.
We have reported examples of CHA by rhodium(III)
porphyrin (por) complexes. Rhodium(III) porphyrin chlorides
activate the meta C−H bond of benzonitrile to give m-
cyanophenyl rhodium porphyrin complexes selectively by an
Received: March 24, 2014
© XXXX American Chemical Society
A
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respectively, in less than 15 min (Scheme 2). Therefore, they
are intermediates for the CHA product 2 only.
Scheme 2. Reactions of Rh2(ttp)2 and Rh(ttp)H with
Cycloheptane
Figure 1. Structure of Rh(ttp)Cl.
via a RhII(ttp)-catalyzed skeletal rearrangement. We have
communicated that the metalloradical-catalyzed isomerization
is 1010 times faster than the analogous organic isomerization of
cycloheptatriene into toluene.14 We now report the full scope
of the K2CO3-promoted CHA and CCA13 of cycloheptane with
Rh(ttp)Cl.
Mechanism of CHA. As the mechanism of aliphatic CHA
has been proposed by Wayland via a bimetalloradical
mechanism11 and shown to operate analogously in our previous
reports,12,13 we therefore did not pursue its study in great detail
in this report. The Rh2(ttp)2 intermediate has also been
1
observed by H NMR (Table S1 and Figure S1 (Supporting
Information)).
RESULTS AND DISCUSSION
■
Conversion of Rh(ttp)(cycloheptyl) to Rh(ttp)Bn. We
reasoned that the CHA and CCA product Rh(ttp)Bn (3) is
formed from Rh(ttp)(cycloheptyl) (2). Rh(ttp)(cycloheptyl)
(2) was then heated under neutral and basic conditions
separately (eqs 2 and 3). Under neutral conditions, Rh(ttp)-
Initially, cycloheptane was found to react with Rh(ttp)Cl (1)
(Figure 1) to give Rh(ttp)(cycloheptyl) (2) in 18% yield as the
sole CHA product, while 70% of Rh(ttp)Cl was recovered
(Scheme 1). However, when K2CO3 (10 equiv) was
Scheme 1. Reaction of Rh(ttp)Cl with Cycloheptane
added,9,12,14,15 Rh(ttp)(cycloheptyl) (2), Rh(ttp)H (4), and
surprisingly the CCA product Rh(ttp)Bn (3)13 were formed in
30, 30, and 25% yields, respectively.
Since the thermal catalytic dehydrogenation of cycloheptane
to toluene in the presence of selenium takes place slowly at the
much higher temperature of 360 °C,16 Rh(ttp)Bn (3) most
likely comes from Rh(ttp)(cycloheptyl) (2) rather from the
benzylic CHA of isomerized toluene.
Reaction Profile. To gain further mechanistic under-
standing, the reaction of Rh(ttp)Cl with excess cycloheptane
at 120 °C in benzene-d6 in the presence of K2CO3 was
monitored by 1H NMR spectroscopy in a NMR tube (Table S1
and Figure S1 (Supporting Information)). After 2 h, only 15%
of Rh(ttp)Cl (1) remained while Rh(ttp)Bn (3), Rh(ttp)H (4),
and Rh2(ttp)2 (6) were formed in 13, 64, and 7% yields,
respectively. After 16 h, Rh(ttp)Cl (1) and Rh2(ttp)2 (6)
completely reacted; Rh(ttp)H (4) and Rh(ttp)Bn (3) still
remained unchanged while Rh(ttp)(cycloheptyl) (2) formed in
17% yield and cycloheptene (5) was observed in 42% yield (eq
1). Since the complete consumption of Rh2(ttp)2 (6)
(cycloheptyl) (2) yielded only Rh(ttp)H (4) and cycloheptene
(5) in 24 and 17% yields, respectively, after 6 days (eq 2 and
Table S2 and Figure S2 (Supporting Information)). Rh(ttp)H
and cycloheptene likely stem from the β-hydride elimination of
Rh(ttp)(cycloheptyl). In fact, the β-hydride elimination of
transition-metal alkyls17 and transition-metal metalloporphyrin
alkyls18 is well-known to give metal hydride and olefin. The
resultant metal hydrides and olefins can either undergo reverse
1,2-addition to give the metal alkyl or dissociation to produce
free alkene.
However, under basic conditions, Rh(ttp)Bn (3) was
observed in 6% yield over an extended time of 6 days, together
with a 28% yield of Rh(ttp)H and a 20% yield of cycloheptene
(eq 3). The conversion of Rh(ttp)(cycloheptyl) (2) to
Rh(ttp)Bn (3) occurs therefore only in the presence of
K2CO3 and also occurs slowly.
As the RhII (por)2 metal−metal-bonded dimer forms by the
2
thermal19,20 or much more rapid base-promoted9b,12 dehydro-
genative dimerization of RhIII(por)H, the possible catalytic role
of Rh2(ttp)2 in the conversion of Rh(ttp)(cycloheptyl) to
Rh(ttp)Bn was examined (Table S3 and Figure S3 (Supporting
Information)). To our delight, Rh(ttp)(cycloheptyl) added
with Rh2(ttp)2 (1 mol %) gave Rh(ttp)Bn in 6% yield in 22 h
(eq 4). Therefore, Rh2(ttp)2 catalyzes the conversion of 2 to 3.
significantly slowed down further conversion of Rh(ttp)-
(cycloheptyl) (2) to Rh(ttp)Bn (3), Rh2(ttp)2 (6) likely has
a promoting role in the transformation. Therefore, both
Rh2(ttp)2 and Rh(ttp)H are possible intermediates.
Rh2(ttp)2 and Rh(ttp)H were then reacted with cyclo-
heptane separately. Indeed, both Rh2(ttp)2 and Rh(ttp)H each
gave only Rh(ttp)(cycloheptyl) (2) in 76 and 73% yields,
B
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Proposed Mechanistic Pathways and Key Intermedi-
ates for 3. As Rh(ttp)(cycloheptyl) (2) was converted to
Rh(ttp)Bn (3) in the presence of K2CO3 or a catalytic amount
of Rh2(ttp)2, Scheme 3 shows three proposed mechanistic
outlines and their corresponding key intermediates.
((CH2)5(CHCH2)) (7) was then heated to 120 °C for 14
days, and no Rh(ttp)Bn (3) was observed (eq 6). In view of the
negative result, we did not conduct further RhII(ttp)-catalyzed
reactions. Pathway i is ruled out.
β-Alkyl Migration (Pathway ii). In pathway ii, Rh(ttp)-
(cycloheptyl) (2) undergoes β-alkyl migration to form
((cyclohexyl)methyl)Rh(ttp) (9), which then gives Rh(ttp)Bn
(3) upon further dehydrogenation, likely by a process discussed
in more detail in pathway iii below. This pathway has literature
precedence for a transition-metal-catalyzed process. The
conversion of cycloheptane to methylcyclohexane is catalyzed
by a Ta−H complex,21 and the WO2-catalyzed dehydrogen-
ation of methylcyclohexane is well-documented.23 Therefore,
((cyclohexyl)methyl)Rh(ttp) (9) was independently synthe-
sized by reductive alkylation with 8 (eq 7).22 However, when
((cyclohexyl)methyl)Rh(ttp) (9) was heated to 120 °C for 5
days in the presence of K2CO3, only a 5% yield of Rh(ttp)Bn
(3) and 60% yield of Rh(ttp)H (4) were observed (eq 8).
Therefore, pathway ii is at the most only a minor pathway on
the basis of its low yield and very slow rate.
Scheme 3. Proposed Key Intermediates for CCA Product 3
γ-H+ or H Elimination (Pathway i). Rh(ttp)(cycloheptyl)
(2) undergoes the ionic process (i) via a γ-H+ elimination with
base, followed by C(α)−C(β) cleavage to give a Rh(ttp)
olefinic α-carbon-centered carbanion and reprotonation to give
Rh(ttp)((CH2)5(CHCH2)) (7) or the analogous radical
process (2) via a γ-H abstraction with RhII(ttp), followed by
C(α)−C(β) cleavage to give a Rh(ttp) olefinic α-carbon-
centered radical and hydrogen atom abstraction to give 7. 7 can
undergo subsequent dehydrogenation and rearrangement,
leading to the formation of Rh(ttp)Bn (3) (Scheme 3). In
fact, a cycloheptyl tantalum complex has been proposed to
undergo γ-H elimination to generate Ta(CH2)5(CHCH2).21
Since Rh(ttp)(cycloheptyl) (2) did not undergo thermal
conversion to Rh(ttp)Bn, only the base- or RhII (ttp)2-
2
Conversion of Rh(ttp)(cycloheptyl) to Rh(ttp)-
(cycloheptatrienyl) via β-H Elimination (Pathway iii). As
Rh(ttp)(cycloheptyl) (2) gave Rh(ttp)H (4) and cycloheptene
via β-H elimination in the absence and presence of K2CO3 (eqs
2 and 3), we reasoned that cycloheptene can be further
dehydrogenated to give cycloheptatriene (CHT), which can
then produce Rh(ttp)Bn (3) by a RhII(ttp)-catalyzed process.
Scheme 4 illustrates the detailed proposed mechanism of the
Rh2(ttp)2-catalyzed dehydrogenation of Rh(ttp)(cycloheptyl)
(2) to give Rh(ttp)Bn (3). 2 initially undergoes reversible β-H
elimination18 to give Rh(ttp)H (4) and cycloheptene, which is
then rapidly trapped by Rh2(ttp)2 (6) to yield the 1,2-addition
product 10.20,24 Successive β-H elimination of 10 and a 1,4-
addition reaction of cyclohepta-1,3-diene with Rh2(ttp)2 (6)
catalyzed pathway is considered.
The suspected intermediate Rh(ttp)((CH2)5(CHCH2))
(7) was independently synthesized by the reductive alkylation
of Rh(ttp)Cl with 7-bromo-1-heptene (eq 5).22 Rh(ttp)-
Scheme 4. Proposed Mechanism for Rh(ttp)-Catalyzed CCA
C
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give 11. Repeated β-H elimination of 11 gives Rh(ttp)H and
CHT. Finally CHT reacts with Rh(ttp)H or Rh2(ttp)2 to give
Rh(ttp)(cycloheptatrienyl) (12).
Table 1. Additive Effect on the kobs Value on Isomerization
of 12 to 3
The conversion of 12 to 3 is a RhII(ttp)-catalyzed skeletal
rearrangement. Initially, RhII(ttp) dissociates from Rh2(ttp)2
through the weak Rh−Rh bond (16.5 kcal mol−1)25 and reacts
with 12 to give the carbon center radical 13, which then
isomerizes to give the cyclopropylmethyl radical 14 and
subsequently, upon ring opening and carbon−carbon bond
cleavage, produce 15 (Scheme 4).26 15 undergoes a further β-H
elimination to give the Rh-substituted benzyl radical 16.
Hydrogen atom transfer from Rh(ttp)H to 16 yields Rh(ttp)Bn
(3) and regenerates RhII(ttp). Thus, the conversion of 12 to 3
is catalyzed by RhII(ttp).
entry [12]0/103 M
additive
kobs/106 s−1 error/107 s−1
1
2
3
4
6.95
13.90
6.95
none
none
4.75
4.70
4.98
7.33
4.0
3.5
K2CO3 (10 equiv)
Rh2(ttp)2 (3.48 × 10−5
2.34
2.0
6.95
M)
5
6.95
Rh2(ttp)2 (3.48 × 10−4
21.53
12.7
M)
Supporting lines of evidence came from the following
experiments. Rh2(ttp)2 (6) reacted at room temperature with
cycloheptene rapidly to give the 1,2-addition product 10
quantitatively (eq 9). 10 at room temperature in 12 h
higher concentration of added Rh2(ttp)2 (Table 1, entries 4 and
5) as reported.14 A 10-fold increase of [Rh2(ttp)2] produces a
3-fold enhancement of the rate, supporting a half-order
dependence on Rh2(ttp)2 or first-order dependence on Rh(ttp),
thus validating the Rh(ttp)-catalyzed isomerization. This
supports the notion that K2CO3 only promotes the conversion
of Rh(ttp)H into Rh2(ttp)2 but not that from 12 into 3.9b,12 It
should be noted that this metalloradical-catalyzed isomer-
ization14 at 120 °C is much faster than the thermal
isomerization of CHT to toluene, which only occurs at 800−
1300 K at a sufficient rate.27
CONCLUSIONS
■
The mild conversion of cycloheptane to Rh(ttp)Bn via
sequential CHA and CCA with Rh(ttp)Cl promoted by
K2CO3 and RhII(ttp) has been observed. Rh(ttp)(cycloheptyl),
which formed from the base-promoted CHA of Rh(ttp)Cl and
cycloheptane, is the intermediate leading to the CCA product
of Rh(ttp)Bn. Rh(ttp)(cycloheptyl) undergoes multiple β-H
elimination to give Rh(ttp)H and CHT. CHT reacts with
Rh(ttp)H or Rh2(ttp)2 either generated from the base-
promoted dehydrogenative dimerization of Rh(ttp)H or
added separately to produce 12. The skeletal isomerization of
Rh(ttp)(cycloheptatrienyl) (12) to Rh(ttp)Bn (3) operates via
a metalloradical-catalyzed process.
decomposed partially to give Rh2(ttp)2 (6), cycloheptene,
and Rh(ttp)(cycloheptyl) (2) in 5, 2, and 42% yields,
respectively (eq 10). Upon heating at 120 °C for 21 h, 10
gave Rh(ttp)H (4), Rh(ttp)(cycloheptyl) (2), and Rh(ttp)Bn
(3) in 20, 58, and 19% yields, respectively (eq 11 and Table S1
and Figure S1 (Supporting Information)). Therefore, 10 is a
viable intermediate for the formation of Rh(ttp)Bn.
The proposed formation of Rh(ttp)(cycloheptatrienyl) (12)
from the CHA of CHT with Rh2(ttp)2 or Rh(ttp)H (Scheme
4) was confirmed separately. Both Rh2(ttp)2 and Rh(ttp)H
react with CHT quickly at room temperature to give 12
quantitatively (Scheme 5).
EXPERIMENTAL SECTION
■
General Procedures. All materials were obtained from
commercial suppliers and used without further purification unless
otherwise specified. Benzene was distilled from sodium under
nitrogen. Rhodium porphyrin complexes9,12,24 were prepared accord-
ing to the literature procedures, and they have been characterized. All
solutions used were degassed three times by freeze−thaw−pump
cycles and stored in a Teflon screwhead stoppered flask, which was
wrapped with aluminum foil to protect it from light.
Scheme 5. CHA Reaction of CHT
Yields from NMR sealed-tube experiments in benzene-d6 were
measured using the residual solvent proton as the internal standard.
NMR kinetic data were analyzed with OriginPro 7.5 software from
OriginLab Corp.
In order to validate the RhII(ttp)-catalyzed skeletal isomer-
ization of Rh(ttp)(c-heptatrienyl) (12) to Rh(ttp)Bn (3), the
rates of the quantitative conversion of 12 to 3 at 120 °C with
additives were monitored by 1H NMR spectroscopy (Table 1).
The disappearances of 12 followed a first-order decay, and the
kobs value does not change with the initial concentration of 12,
confirming the first-order kinetics (Table 1, entries 1 and 2, and
Figure S5 (Supporting Information)). Added K2CO3 did not
change the kobs value (Table 1, entry 3) and therefore does not
catalyze the reaction. However, the kobs value increases with
Experimental Procedures. Reaction of Cycloheptane with
Rh(ttp)Cl. Rh(ttp)Cl12 (1; 20.4 mg, 0.025 mmol) was added to
cycloheptane (3.0 mL). The red reaction mixture was degassed for
three freeze−thaw−pump cycles, purged with N2, and heated to 120
°C under N2 for 24 h. Excess cycloheptane was removed by vacuum
distillation. The residue was purified by column chromatography on
silica gel with a hexane/CH2Cl2 solvent mixture (1/1) as eluent to give
a red solid. Rh(ttp)(cycloheptyl) (2; 4.0 mg, 0.0046 mmol, 18%) was
collected and was further recrystallized from CH2Cl2/MeOH. Rf =
1
0.84 (hexane/CH2Cl2 1/1). H NMR (CDCl3, 300 MHz): δ −4.53
(m, 2), −4.05 (m, 3 H), −1.26 (m, 4 H), −0.22 (m, 2 H), −0.06 (m, 2
D
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H), 2.69 (s, 12 H, p-methyl), 7.54 (d, 8 H, J = 5.4 Hz, m-phenyl), 8.00
(d, 4 H, J = 7.5 Hz, o’-phenyl), 8.08 (d, 4 H, J = 7.8 Hz, o-phenyl), 8.69
(s, 8 H, pyrrole). 13C NMR (CDCl3, 75 MHz): δ 21.69, 22.09, 27.85,
added slowly to the suspension of Rh(ttp)Cl via a cannula. The
mixture was heated to 50 °C under N2 for 1 h. The solution was then
cooled to 0 °C under N2, and 7-heptenyl bromide (23 mg, 1.20 mmol)
was added. A reddish orange suspension was formed. After it was
stirred at room temperature for another 15 min under N2, the reaction
mixture was worked up by extraction with CH2Cl2/H2O. The
combined organic extract was dried (MgSO4), filtered, and rotary
evaporated. The reddish orange residue was purified by column
chromatography over silica gel (250−400 mesh) using a hexane/
CH2Cl2 solvent mixture (1/1) as the eluent. The major orange fraction
was collected and gave a reddish orange solid of Rh(ttp)-
((CH2)5(CHCH2)) (7; 96.0 mg, 0.11 mmol, 86%) as the product
1
33.24, 39.37 (d, JRh−C = 27.6 Hz), 122.87, 127.42, 127.55, 131.52,
133.63, 134.25, 137.23, 137.23, 139.53, 143.50. MS: calcd for
(C55H49N4Rh)+ m/z 868.3007, found m/z 868.3016.
Reaction of Cycloheptane and Rh(ttp)Cl with Potassium
Carbonate. Rh(ttp)Cl (20.4 mg, 0.025 mmol) and anhydrous
potassium carbonate (34.9 mg, 0.252 mmol) were added to
cycloheptane (3.0 mL). The red reaction mixture was degassed for
three freeze−thaw−pump cycles, purged with N2, and heated to 120
°C under N2 for 6 h. Excess cycloheptane 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 (1/1) as eluent. The
red solids Rh(ttp)(cycloheptyl) (2; 6.6 mg, 0.0076 mmol, 30%) and
Rh(ttp)Bn (3; 5.5 mg, 0.0064 mmol, 25%) were collected.
Sealed NMR Tube Experiment of Rh(ttp)Cl and Cycloheptane
with Potassium Carbonate in Benzene-d6. Rh(ttp)Cl (3.5 mg,
0.0043 mmol), cycloheptane (11 μL, 0.091 mmol), and potassium
carbonate (5.9 mg, 0.0427 mmol) were added to benzene-d6 (500 μL)
in a NMR tube. The red mixture was degassed for three freeze−thaw−
pump cycles and the NMR tube was flame-sealed under vacuum. It
was heated to 120 °C in the dark. The reaction was monitored with 1H
NMR spectroscopy.
Reaction of Cycloheptane with Rh2(ttp)2. Rh2(ttp)2 (9.5 mg,
0.0062 mmol) was added in cycloheptane (1.5 mL). The red reaction
mixture was degassed for three freeze−thaw−pump cycles, purged
with N2 and heated to 120 °C under N2 for 5 min. Excess
cycloheptane was removed by vacuum distillation. The residue was
purified by column chromatography on silica gel with a solvent
mixture of hexane/CH2Cl2 (1:1). Red solid, Rh(ttp)(cycloheptyl) (2)
(8.2 mg, 0.0093 mmol, 76%) was collected and was further
recrystallized from CH2Cl2/MeOH.
Reaction of Cycloheptane with Rh(ttp)H. Rh(ttp)H (9.5 mg, 0.012
mmol) was added in cycloheptane (1.5 mL). The red reaction mixture
was degassed for three freeze−thaw−pump cycles, purged with N2,
and heated to 120 °C under N2 for 15 min. Excess cycloheptane was
removed by vacuum distillation. The residue was purified by column
chromatography on silica gel with a hexane/CH2Cl2 solvent mixture
(1/1) as eluent. The red solid Rh(ttp)(cycloheptyl) (2; 7.8 mg, 0.090
mmol, 73%) was collected and was further recrystallized from
CH2Cl2/MeOH.
1
after rotary evaporation. Rf = 0.84 (hexane/CH2Cl2 1/1). H NMR
(CDCl3, 400 MHz): δ −4.96 (td, 2 H, J = 2.9, 9.0 Hz), −4.50 (qu, 2
H, J = 7.8 Hz), −1.56 (qu, 2 H, J = 7.4 Hz), −0.45 (qu, 2 H, J = 7.4
Hz), 0.80 (q, 2 H, J = 7.0 Hz), 2.69 (s, 12 H, p-methyl), 4.39 (dd, 1 H,
J = 1.6, 17.0 Hz), 4.50 (dd, 1 H, J = 1.1, 11.0 Hz), 5.06 (m, 1 H), 7.53
(t, 8 H, J = 7.7 Hz), 7.98 (d, 4 H, J = 7.6 Hz), 8.07 (d, 4 H, J = 7.6
Hz), 8.70 (s, 8 H, pyrrole). 13C NMR (CDCl3, 75 MHz): δ 21.69,
25.78, 26.84, 26.89, 32.54, 113.67, 122.50, 127.45, 127.52, 131.49,
133.79, 134.08, 137.25, 138.69, 139.44, 143.33. MS: calcd for
(C55H49N4Rh)+ m/z 868.3007, found m/z 868.3011.
Sealed NMR Tube Experiment of Rh(ttp)((CH2)5(CHCH2)) (7)
with Potassium Carbonate in Benzene-d6. Rh(ttp)((CH2)5(CH
CH2)) (7; 3.8 mg, 0.0044 mmol) and potassium carbonate (6.0 mg,
0.044 mmol) were added to benzene-d6 (500 μL) in a NMR tube. The
red solution was degassed for three freeze−thaw−pump cycles, and
the NMR tube was flame-sealed under vacuum and was heated to 120
°C in the dark. The reaction was monitored with 1H NMR
spectroscopy.
Independent Synthesis of Rh(ttp)(cyclohexylmethyl) (9).12
A
suspension of Rh(ttp)Cl (100 mg, 0.11 mmol) in EtOH (50 mL)
and a solution of NaBH4 (17 mg, 0.45 mmol) in aqueous NaOH (0.1
M, 2 mL) were purged with N2 for 15 min separately. The solution of
NaBH4 was added slowly to the suspension of Rh(ttp)Cl via a cannula.
The mixture was heated to 50 °C under N2 for 1 h. The solution was
then cooled to 0 °C under N2, and cyclohexylmethyl bromide (8; 23
mg, 1.20 mmol) was added. A reddish orange suspension was formed.
After it was stirred at room temperature for another 15 min under N2,
the reaction mixture was worked up by extraction with CH2Cl2/H2O.
The combined organic extract was dried (MgSO4), filtered, and rotary
evaporated. The reddish orange residue was purified by column
chromatography over silica gel (250−400 mesh) using a hexane/
CH2Cl2 solvent mixture (1/1) as the eluent. The major orange fraction
was collected and gave a reddish orange solid of Rh(ttp)CH2(c-C6H11)
(9; 92.5 mg, 0.11 mmol, 86%) as the product after rotary evaporation.
Sealed NMR Tube Experiment of Rh(ttp)(cycloheptyl) in Benzene-
d6. Rh(ttp)(cycloheptyl) (2; 3.8 mg, 0.0044 mmol) was added to
benzene-d6 (500 μL) in a NMR tube. The red solution was degassed
for three freeze−thaw−pump cycles, and the NMR tube was flame-
sealed under vacuum. The solution was heated to 120 °C in the dark,
1
Rf = 0.84 (hexane/CH2Cl2 1/1). H NMR (CDCl3, 400 MHz): δ
−5.47 (s, 1 H), −5.08 (dd, 2 H, J = 3.4, 5.0 Hz), −2.42 (d, 2 H, J =
12.2 Hz), −2.11 (qd, 2 H, J = 2.8, 12.6 Hz), −0.45 (qt, 2 H, J = 3.3,
12.7 Hz), −0.25 (qt, 1 H, J = 3.6, 12.8 Hz), 0.36 (td, 2 H, J = 2.8, 12.8
Hz), 0.64 (d, 1 H, J =12.8 Hz), 2.69 (s, 12 H), 7.53 (t, 8 H, J = 7.8
Hz), 7.96 (d, 4 H, J = 7.7 Hz), 8.09 (d, 4 H, J =7.7 Hz), 8.70 (s, 8 H,
pyrrole). 13C NMR (CDCl3, 75 MHz): δ 21.69, 25.21, 25.56, 30.64,
36.31, 122.56, 127.47, 127.52, 131.48, 133.70, 134.10, 137.24, 139.45,
143.43. MS: calcd for (C55H49N4Rh)+ m/z 868.3007, found m/z
868.3015.
1
and the reaction was monitored with H NMR spectroscopy.
Sealed NMR Tube Experiment of Rh(ttp)(cycloheptyl) (2) with
Potassium Carbonate in Benzene-d6. Rh(ttp)(cycloheptyl) (2; 3.8
mg, 0.0044 mmol) and potassium carbonate (6.0 mg, 0.044 mmol)
were added to benzene-d6 (500 μL) in an NMR tube. The red solution
was degassed for three freeze−thaw−pump cycles, and the NMR tube
was flame-sealed under vacuum. The solution was heated to 120 °C in
1
the dark. The reaction was monitored with H NMR spectroscopy at
Sealed NMR Tube Experiment of Rh(ttp)CH2(cycloC6H11) (9) with
Potassium Carbonate in Benzene-d6. Rh(ttp)CH2(c-C6H11) (9; 3.8
mg, 0.0044 mmol) and potassium carbonate (6.0 mg, 0.044 mmol)
were added to benzene-d6 (500 μL) in an NMR tube. The red solution
was degassed for three freeze−thaw−pump cycles, and the NMR tube
was flame-sealed under vacuum. The tube was heated to 120 °C in the
particular time intervals, and the NMR yields were taken.
Sealed NMR Tube Experiment of Rh(ttp)(cycloheptyl) with 1 mol
% of Rh2(ttp)2 in Benzene-d6. Rh(ttp)(cycloheptyl) (2; 3.8 mg,
0.0044 mmol) and Rh2(ttp)2 (0.034 mg, 4.4 × 10−5 mmol) which was
previously dissolved in 500 μL of degassed benzene-d6 were added
together in a NMR tube. The red solution was degassed for three
freeze−thaw−pump cycles, and the NMR tube was flame-sealed under
vacuum. The solution was heated to 120 °C in the dark. The reaction
1
dark, and the reaction was monitored with H NMR spectroscopy.
Sealed NMR Tube Experiment of Rh2(ttp)2 and Cycloheptene in
Benzene-d6. Cycloheptene (2 μL, 0.021 mmol) was added to a
solution of Rh2(ttp)2 prepared from a stock solution with subsequent
removal of solvent (3.4 mg, 0.0022 mmol) in 500 μL of degassed
benzene in an NMR tube. The solvent was removed by vacuum
distillation after 5 min. A 500 μL portion of degassed benzene-d6 was
then added. The red solution was degassed for three freeze−thaw−
1
was monitored with H NMR spectroscopy.
Independent Synthesis of Rh(ttp)(7-heptenyl).12 A suspension of
Rh(ttp)Cl (100 mg, 0.11 mmol) in EtOH (50 mL) and a solution of
NaBH4 (17 mg, 0.45 mmol) in aqueous NaOH (0.1 M, 2 mL) were
purged with N2 for 15 min separately. The solution of NaBH4 was
E
dx.doi.org/10.1021/om500313g | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
pump cycles, the NMR tube was flame-sealed under vacuum, and the
Rearrangement Reaction of Rh(ttp)(cycloheptatrienyl) (12) at
6.95 mM with 1 mol % of Rh2(ttp)2. Rh(ttp)(cycloheptatrienyl) (3.0
mg, 0.0035 mmol) and 1 mol % of Rh2(ttp)2 prepared from a stock
solution (0.027 mg, 1.7 × 10−5 mmol) were added to degassed
benzene-d6 (500 μL) in an NMR tube. The red reaction mixture was
degassed for three freeze−thaw−pump cycles, and the NMR tube was
flame-sealed under vacuum. The solution was heated to 120 °C in the
dark, the reaction was monitored with 1H NMR spectroscopy, and the
NMR yields were taken.
1
1
solution was immediately analyzed by H NMR. The H NMR signal
of Rh2(ttp)2 was not observed. Instead, a clean NMR spectrum of the
newly formed 1,2-addition product 10 appeared. 1H NMR (C6D6, 400
MHz): δ −4.24 (m, 2 H), −3.39 (m, 2 H), −2.70 (m, 2 H), −1.60 (m,
2 H), −1.20 (m, 2 H), −0.68 (m, 1 H), −0.18 (m, 1 H), 2.41 (s, 24 H,
p-methyl), 7.30 (d, 8 H, J = 6.4 Hz, m-phenyl), 7.33 (d, 8 H, J = 8.0
Hz, m′-phenyl), 8.13 (d, 8 H, J = 7.6 Hz, o′-phenyl), 8.19 (d, 8 H, J =
7.5 Hz, o-phenyl), 8.97 (s, 16 H, pyrrole). Compound 10 is too
unstable to allow a 13C NMR spectrum to be taken.
Rearrangement Reaction of Rh(ttp)(cycloheptatrienyl) (12) at
6.95 mM with 10 mol % of Rh2(ttp)2. Rh(ttp)(cycloheptatrienyl) (3.0
mg, 0.0035 mmol) and 10 mol % Rh2(ttp)2 (0.27 mg, 1.7 × 10−4
mmol) were added to degassed benzene-d6 (500 μL) in an NMR tube.
The red reaction mixture was degassed for three freeze−thaw−pump
cycles, and the NMR tube was flame-sealed under vacuum. The
solution was heated to 120 °C in the dark. The reaction was
Sealed NMR Tube Experiment of Rh2(ttp)2 and Cycloheptene in
Benzene-d6 at 120 °C. Cycloheptene (2 μL, 0.021 mmol) was added
to a solution of Rh2(ttp)2 prepared from a stock solution with
subsequent removal of solvent (3.4 mg, 0.0022 mmol) in 500 μL of
degassed benzene in an NMR tube. The solvent was removed by
vacuum distillation after 5 min. A 500 μL portion of degassed benzene-
d6 was then added. The red solution was degassed for three freeze−
thaw−pump cycles, and the NMR tube was flame-sealed under
1
monitored with H NMR spectroscopy, and the NMR yields were
taken.
1
vacuum. It was brought to H NMR analysis after it was sealed. The
ASSOCIATED CONTENT
1,2-addition product Rh(ttp) complex 10 was formed cleanly. The
reaction mixture was heated to 120 °C and was monitored with H
NMR spectroscopy.
■
1
S
* Supporting Information
Figures and tables giving the progress of reactions and
spectroscopic data for compounds 2, 7, 9, 10, and 12. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Sealed NMR Tube Experiment of Rh2(ttp)2 and Cycloheptatriene
in Benzene-d6. Rh2(ttp)2 (3.6 mg, 0.0047 mmol) prepared from a
stock solution with subsequent removal of solvent, cycloheptatriene (5
μL), and degassed benzene-d6 (500 μL) were placed in an NMR tube.
The red solution was degassed for three freeze−thaw−pump cycles,
and the NMR tube was flame-sealed under vacuum. The reaction
AUTHOR INFORMATION
■
1
mixture was kept at room temperature and monitored with H NMR
Corresponding Author
*E-mail for K.S.C.: ksc@cuhk.edu.hk.
spectroscopy at particular time intervals, and the NMR yields were
1
taken. 12 was obtained quantitatively. H NMR (C6D6, 400 MHz): δ
2.44 (s, 19), 7.32 (d, 8 H, J = 7.4 Hz, m-phenyl), 7.35 (d, 4 H, J = 7.8
Hz, o-phenyl), 8.13 (d, 4 H, J = 5.4 Hz, m′-phenyl), 8.14 (d, 4 H, J =
5.4 Hz, o′-phenyl), 8.93 (s, 8 H, pyrrole). 13C NMR (C6D6, 100
MHz): δ 21.50, 123.47, 131.82, 132.80, 134.22, 134.28, 134.92, 137.11,
140.43, 144.24. MS: calcd for (C55H43N4Rh)+ m/z 862.2537, found
m/z 862.2549.
Synthesis of Rh(ttp)(cycloheptatrienyl) (12). Rh(ttp)H (3.6 mg,
0.0047 mmol) and cycloheptatriene (5 μL) were added to degassed
benzene (500 μL). The red solution was degassed for three freeze−
thaw−pump cycles, purged with N2, and put into a water bath at 25 °C
for 15 min. Excess solvent was removed by vacuum distillation. A 500
μL portion of degassed benzene-d6 was then added. The product 12
was found to be air-sensitive and decomposed during column
chromatography. As the 1H NMR spectrum was clean, no further
purification step was required.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Research Grants Council of the HKSAR (No
400212) for financial support.
REFERENCES
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