Metalloradical-Catalyzed Selective 1,2-Rh-H Insertion into the Aliphatic Carbon-Carbon Bond of Cyclooctane

Article abstract of DOI:10.1021/acs.organomet.5b00183

The selective aliphatic carbon-carbon activation of cyclo-octane (c-octane) was achieved via the Rh^II(ttp)-catalyzed 1,2-addition of Rh(ttp)H to give Rh(ttp)(n-octyl) (ttp = tetratolylporphyrinato dianion) in good yield under mild reaction conditions. This mechanism is further supported by DFT calculations. The reaction worked only with the sterically accessible Rh(ttp) porphyrin complex but not with the bulky Rh(tmp) system (tmp = tetrakismesitylporphyrinato dianion), thus showing the highly steric sensitivity of carbon-carbon bond activation by transition metal complexes. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00183

Article
pubs.acs.org/Organometallics
Metalloradical-Catalyzed Selective 1,2-Rh‑H Insertion into the
Aliphatic Carbon−Carbon Bond of Cyclooctane
Yun Wai Chan,† Bas de Bruin,*,‡ and Kin Shing Chan*,†
†Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People’s Republic of China
‡Van’t Hoff Institute for Molecular Sciences (HIMS), University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The
Netherlands
S
* Supporting Information
ABSTRACT: The selective aliphatic carbon−carbon activation of
cyclo-octane (c-octane) was achieved via the RhII(ttp)-catalyzed 1,2-
addition of Rh(ttp)H to give Rh(ttp)(n-octyl) (ttp = tetratolylpor-
phyrinato dianion) in good yield under mild reaction conditions. This
mechanism is further supported by DFT calculations. The reaction worked only with the sterically accessible Rh(ttp) porphyrin
complex but not with the bulky Rh(tmp) system (tmp = tetrakismesitylporphyrinato dianion), thus showing the highly steric
sensitivity of carbon−carbon bond activation by transition metal complexes.
dependence of the RhII(tmp) (tmp = tetrakismesitylporphyr-
INTRODUCTION
■
inato dianion) concentration in the rate laws, has been reported
Alkane functionalization in a homogeneous medium is an
by Wayland et al.8 The CCA mechanism has been shown to be
important and challenging process which involves either
dependent on RhII(tmp) concentration, with a first order rate
carbon−hydrogen bond activation (CHA)1 or carbon−carbon
dependence in both the reaction with nitroxide,10 and in the
bond activation (CCA)2 with organic, inorganic, and organo-
reaction with 2-methyl substituted nitrile11 and second order
metallics reagents followed by functionalization. Although
dependence in the reaction with cyclophane.12 We have
aliphatic C−C bonds are weaker than aliphatic C−H bonds,
previously communicated the CCA of c-octane via the 1,2-
CCA of alkanes by the attack of a transition metal complex is
addition of its carbon−carbon bond to Rh(ttp)H, catalyzed by
much less reported due to steric hindrance of the carbon atoms
the RhII(ttp) (ttp = tetratolylporphyrinato dianion) metal-
which are shielded by peripheral C−H bonds as well as for
statistical reasons associated with C−C bonds being typically
less abundant than C−H bonds in organic compounds.3
Cyclo-octane (c-octane) is a relatively unstrained cycloalkane
with a strain energy of 9.6 kcal/mol4 and therefore serves as a
commonly studied substrate in alkane functionalization, mostly
involving CHA. Some examples of CHA of c-octane are the
iridium(I) pincer dihydride-catalyzed dehydrogenation to c-
octene,5a the FeCl3-catalyzed aerobic oxidation to c-octanol and
c-octanone5b as well as the MnO2-catalyzed bromination to c-
octyl bromide.5c
loradical to give Rh(ttp) n-octyl selectively13 (Scheme 1) and
now report our full studies and theoretical calculations.
Scheme 1. Metalloradical Catalyzed CCA of c-Octane with
MH
Reports of CCA of c-octane are rare. CCA of c-octane in a
heterogeneous medium requires a very high reaction temper-
ature of 530 °C and consequently results in both CHA and
CCA.3a Oxidative CCA of c-octane catalyzed by N-hydroxyph-
thalides/Co(II)/Mn(II) at 100 °C in 14 h gives ω- dicarboxylic
acids in 2% yield only together with the major products being c-
octanol and c-octanone.3b
We have recently discovered the base-promoted CHA of
cyclic alkanes with Rh(III) porphyrins.6 In contrast to c-
pentane6 and c-hexane,6 c-heptane7 undergoes both CHA and
CCA to give Rh(III) porphyrin c-heptyl and benzyl.7 Both
CHA and CCA have been proposed to involve Rh(II)
porphyrin as a reagent or as a catalyst. These Rh(II) porphyrins
are unique metalloradicals and exhibit rich chemistry in bond
activations including CHA8,9 and CCA.10,11 The bimetallor-
adical CHA of methane and toluene, based on the second order
RESULTS AND DISCUSSION
■
Discovery and Optimization of CCA. Initially, c-octane
was found to react poorly with Rh(ttp)Cl 1 to give Rh(ttp)(c-
octyl) 2 and Rh(ttp)(n-octyl) 3 in 5% and 8% yields,
respectively (Table 1, entry 1). A 72% yield of Rh(ttp)Cl 1
was recovered, and a trace amount of Rh(ttp)H 4 was observed.
Both CHA and CCA products formed but the reaction was
inefficient. Upon addition of KOH (10 equiv) to the reaction
mixture, Rh(ttp)(c-octyl), Rh(ttp)(n-octyl), and Rh(ttp)H
were obtained in 6%, 25%, and 62% yields, respectively in 7.5
h (Table 1, entry 2). When K2CO3 (10 equiv) was added,9
Rh(ttp)Cl was consumed in 7.5 h, and Rh(ttp)(n-octyl) 3 and
Received: March 4, 2015
© XXXX American Chemical Society
A
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Table 1. Reaction of c-Octane with Rh(ttp)Cl 1
Table 3. Reaction Progress of Rh(ttp)(c-octyl) 2 with K2CO3
yield (%)
a
b
entry
additive
KOH
time
1
2
3
4
total
yield %
1
2
3
2 d
72
0
5
6
0
8
0
85
93
91
time (h)
2
3
4
5
total Rh pdt total org pdt
7.5 h
7.5 h
25
33
62
58
0
100
97
80
73
36
30
0
0
0
100
97
98
97
93
91
100
97
K2CO3
0
0.5
1.5
2.5
10
16
00
0
0
a
b
10 equiv. Recovery.
100
100
16
18
24
41
40
20
25
40
42
100
98
Rh(ttp)H 4 were obtained in 33% and 58% yields, respectively
(Table 1, entry 3). The CCA product 3 is the formal 1,2-
addition product of Rh(ttp)H 4 into the carbon−carbon bond
of c-octane. The c-octane sample was pure and found to be free
of n-octane and 1-octene by GC-MS analysis. Complexes 2 and
3 were independently synthesized by reductive alkylation with
NaBH4/R-Br (eq 1). Therefore, Rh(ttp)(n-octyl) 3 was
confirmed as the CCA product of c-octane with its X-ray
crystal structure communicated.13
92
21
93
reaction of Rh(ttp)Cl 1 with c-octane, the reaction was
monitored by H NMR spectroscopy in a sealed NMR tube
with excess c-octane and K2CO3 in benzene-d6 (Table 4).
1
Table 4. Reaction Progress of Rh(ttp)Cl with c-Octane with
K2CO3
yield %
a
Mechanistic Investigation: CHA as An Intermediate
for CCA? In some CCAs of hydrocarbons with transition metal
complexes, the CCA can be a parallel14a and/or consecutive
reaction channel14 with CHA. We thus investigated whether
the CHA product is an intermediate for CCA. Rh(ttp)(c-octyl)
2 was heated in benzene-d6 in both neutral and basic conditions
separately. Without K2CO3, Rh(ttp)(c-octyl) 2 gave Rh(ttp)(n-
octyl) 3, Rh(ttp)H 4, and c-octene 5 in 10%, 76%, and 36%
yields, respectively, after 21 h (Table 2). In the presence of
time (h)
1
3
4
5
6
total Rh
total org
0
100
100
100
82
0
0
0
0
0
100
100
100
95
0
0.5
1
0
0
0
0
0
0
0
0
0
0
4.5
17
62
76
240
0
0
0
13
62
0
0
0
21
54
56
53
18
50
52
48
83
18
79
78
74
0
29
26
26
83
0
0
82
0
0
79
a
6 = Rh2(ttp)2.
Table 2. Reaction Progress of the Thermal Reaction of
Rh(ttp)(c-octyl) 2
Initially, Rh(ttp)Cl was first converted to Rh2(ttp)2 6 in the
presence of K2CO3.15 At 4.5 h, 82% yield of Rh(ttp)Cl
remained, while Rh2(ttp)2 6 was formed in 13% yield. After 17
h, Rh(ttp)Cl was completely consumed. Rh2(ttp)2 6, Rh(ttp)H
4, and c-octene 5 were formed in 62%, 21%, and 18% yields,
respectively. After 62 h, Rh2(ttp)2 6 completely reacted. The
yields of Rh(ttp)H 4 and c-octene increased to 54% and 50%,
respectively, and only 29% yield of CCA product Rh(ttp)(n-
octyl) 3 was obtained. Finally, Rh(ttp)(n-octyl) 3 was
generated in prolonged heating of 62 h, and still, Rh(ttp)H
was consumed slowly and mostly remained unreacted even
after 10 days. Therefore, both Rh2(ttp)2 and Rh(ttp)H are
possible intermediates. The observed red gummy residue
yield %
time (h)
2
3
4
5
total Rh
total org
0
100
87
0
0
0
100
98
0
1
0
11
76
9
96
57
21
11
10
36
97
K2CO3 (10 equiv), Rh(ttp)(n-octyl) 3 was isolated in a higher
yield of 21% in 16 h with 2 recovered in 30% yield together
with 42% yield of c-octene (Table 3). However, both reactions
were low yielding and incomplete. We propose that the small
amount of 3 is formed from a multistep pathway. The slow β-H
elimination of 2 gives Rh(ttp)H 4 and octene. The slow
homolysis of 2 gives Rh(ttp) and the c-octyl radical which
undergoes disproportionation to give c-octane and c-octene. c-
Octane can then further react with Rh(ttp)H to Rh(ttp)-n-octyl
3 (see discussion below). Therefore, CHA product 2 is unlikely
a major intermediate leading to CCA product 3 and is more
reasonably a parallel reaction product.
1
showing H NMR upfield signals at δ = −5 to 1 ppm were
assigned to Rh(ttp)-incorporated c-octene oligomers (about
15% NMR yield), which indicate the occurrence of RhII(ttp)-
initiated oligomerization of c-octene.16,17 We did not pursue the
examination of the detailed structures of these oligomers. The
formation of c-octene, which forms from the β-H elimination of
Rh(ttp)(c-octyl) 2, indicates the occurrence of CHA of c-
octane. When c-octene accumulates, it serves as a trap for
Rh2(ttp)2 6 and therefore stops the CCA.
Rh(ttp)H as Intermediate in CCA? To investigate whether
Rh(ttp)H is the intermediate for CCA, Rh(ttp)H 4 was reacted
with c-octane. Rh(ttp)H indeed reacted with c-octane at 120 °C
Reaction Profile with Rh(ttp)Cl. To gain further
mechanistic understanding in order to enhance the CCA
B
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in 15 h to give Rh(ttp)(n-octyl) 3 selectively, though in only
21% yield, and was also recovered in 73% yield (eq 2).
Proposed Mechanism of RhII(ttp)-Catalyzed 1,2-In-
sertion of Rh(ttp)H into the Carbon−Carbon Bond of c-
Octane. We anticipated that the metalloradical RhII(ttp) 7/
Rh2(ttp)2 may act as a catalyst to facilitate the 1,2-addition of c-
octane with Rh(ttp). Indeed, Halpern reported in the elegant
mechanistic studies that RhII(oep) catalyzes (oep = octyle-
thylporphyrin dianion) the 1,2-addition of styrene to give
Rh(oep)CH2CH2Ph20 RhII(oep), which forms from homolysis
of the weak Rh−Rh bond in Rh2(oep)2,18 inserts into the C
C bond of styrene to give a Rh(oep)CH2C·HPh benzylic
carbon-centered radical, which is also a reversible reaction. The
Rh(oep)CH2CHPh radical then abstracts a hydrogen atom
from Rh(oep)H to yield the 1,2-addition product Rh(oep)-
CH2CH2Ph and regenerates RhII(oep). Thus, it is a RhII(ttp)-
catalyzed 1,2-insertion of Rh(ttp)H into the carbon−carbon
bond of styrene.
Prolonged heating of Rh(ttp)H in c-octane over 12 d yielded
Rh(ttp)(n-octyl) and Rh2(ttp)2 in 94% and 2% yields,
respectively (eq 3). As Rh(ttp)H underwent slow dehydrogen-
ative dimerization to give 6% yield of Rh2(ttp)2 at 120 °C in 1
day (eq 4), similar to the report by Wayland and co-workers,18
the small amount of Rh2(ttp)2 formed may facilitate the 1,2-
addition of Rh(ttp)H into c-octane (see the discussion below).
We thus recognized the similarity and anticipated that the
CCA of c-octane, being a 1,2-addition reaction, can also be
catalyzed by RhII(ttp) 7 (Scheme 3). Rh2(ttp)2 5 formed from
Scheme 3. Proposed Rh(ttp)-Catalyzed 1,2-Insertion of
Rh(ttp)H into the Carbon−Carbon Bond of c-Octane
It is intuitively tempting to suggest that Rh(ttp)H undergoes
a 1,2-insertion into the CC bond of c-octane by σ-bond
metathesis (Scheme 2).19 However, the requirement of a 12 d
Scheme 2. 4-Centered Transition State via σ-Bond
Metathesis Can Be Excluded
the thermolysis of Rh(ttp)H (eq 7, Scheme 3), initially
undergoes homolysis to give RhII(ttp) (eq 8 in ref ).18 RhII(ttp)
7 then reacts with c-octane in parallel CHA (eq 9, Scheme 3)
and CCA (eq 10, Scheme 3). For the CCA pathway, RhII(ttp)
can cleave the C−C bond of c-octane to generate the alkyl
radical 8 by a bimolecular homolytic substitution (SH2)-like
process22 (eq 10, Scheme 3) which can also reverse rapidly.20
Compound 8 can then abstract a hydrogen atom from the weak
(ttp)Rh-H bond (∼60 kcal mol−1),21 serving as an alkyl radical
trap to form a strong alkyl C−H bond21b to provide the driving
force of the reaction (eq 10, Scheme 3).
The branching of CHA and CCA is reasoned to be strongly
dependent on the concentration of Rh2(ttp)2. RhII(por) has
been shown to undergo CHA with alkane to give Rh(por)R
and Rh(por)H with a termolecular rate law (eq 9, Scheme 3).8
The proposed CCA should, however, follow first order kinetics
in RhII(ttp) or half order kinetics in Rh2(ttp)2, thus requiring
much lower RhII(ttp) concentrations. At the same time, a large
excess concentration of Rh(ttp)H facilitates trapping of the
carbon-centered radical 8 to give 3 and to regenerate RhII(ttp)
7. In this mechanism, Rh2(ttp)2 is the precatalyst or more
precisely RhII(ttp) is the catalyst, but the reaction requires the
presence of Rh(ttp)H in excess. To validate this mechanism,
and to develop a selective CCA process, we conducted a series
of experiments for the reaction with c-octane by increasing the
concentration ratio of Rh(ttp)H/Rh2(ttp)2 for more efficient
trapping.
reaction time rules out the fact that Rh(ttp)H is a direct
intermediate involved in the CC bond activation reaction. The
fairly crowded 4-centered transition state in the same size of Rh
porphyrin is geometrically not very attractive. Indeed, the
reaction mixture of 1 equiv of PPh3, Rh(ttp)H, and c-octane
upon heating at 120 °C for 1.5 d gave quantitative
Rh(ttp)H(PPh3) but no Rh(ttp) n-octyl 3 (eq 5). Rh(ttp)H-
(PPh3) did not undergo any dimerization, and therefore no
Ph3P was ligated to Rh2(ttp)2. No CCA of c-octane with
Rh(ttp)H alone occurs at a fast enough rate.
Rh2(ttp)2 as an Intermediate in CCA? We next examined
whether Rh2(ttp)2 6 is a possible intermediate. Rh2(ttp)2 6
reacted with c-octane at 120 °C for 15 h to give Rh(ttp)(c-
octyl) 2, Rh(ttp)(n-octyl) 3, and Rh(ttp)H 4 in 41%, 4%, and
46% yields, respectively (eq 6) with a very low yield of CCA
product 3 and is therefore not a major intermediate. Therefore,
both Rh(ttp)H 4 and Rh2(ttp)2 6 separately gave low yielding
reactions and are likely only minor reaction intermediates by
themselves. We therefore investigated the synergistic effect of
both Rh(ttp)H 4 and Rh2(ttp)2 6 to give Rh(ttp)(n-octyl) 3.
Synergetic Effect of Rh(ttp)H/Rh2(ttp)2 in CCA of c-
Octane. Indeed, mixtures of Rh(ttp)H and Rh2(ttp)2 were
much more efficient reagents, and the use of the combination
of both enhanced the total yields to up to 79% (Table 5, entries
2−4 vs 1). The selectivity toward CCA was further enhanced
C
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Table 5. RhII(ttp)-Catalyzed CCA of c-Octane with
Rh(ttp)H
resulting in a slight weakening of the Rh-c-octyl bond (Scheme
4).23 Indeed, the attempted synthesis of Rh(tmp)(c-octyl) 13
Scheme 4. Formation of Rh(tmp)H from CHA of RhII(tmp)
and c-Octane
a
entry Rh(ttp)H/Rh2(ttp)2 4:6 yield 2 (%) yield 3 (%) total yield (%)
b
1
2
3
4
01:0
02:1
05:1
10:1
00
60
53
00
21
18
26
73
21
78
79
73
a
b
The results are the average of at least duplicate measurements. 73%
Rh(ttp)H recovered.
by an increase of the Rh(ttp)H/Rh2(ttp)2 ratio. The CCA of c-
octane with the mixture of Rh(ttp)H/Rh2(ttp)2 in 2:1 ratio
gave Rh(ttp)(c-octyl) 2 and Rh(ttp)(n-octyl) 3 in 60% and
18% yields, respectively (Table 5, entry 2). When the
Rh(ttp)H/Rh2(ttp)2 ratio increased to 5:1, the yield of
Rh(ttp)(n-octyl) 3 increased to 26% yield but that of
Rh(ttp)(c-octyl) 2 decreased to 53% yield (entry 3). We
were glad to observe that Rh(ttp)(n-octyl) 3 was selectively
obtained in 73% yield from the reaction with the 10:1 ratio of
Rh(ttp)H/Rh2(ttp)2 (entry 4). The elimination of the CHA
channel (eq 9, Scheme 3) by a low concentration of Rh2(ttp)2 6
and efficient trapping of 8 by a higher concentration of Rh(ttp)
H were achieved. The selective aliphatic CCA of c-octane was
thus realized successfully, and the results confirm the proposed
RhII(ttp)-catalyzed 1,2-addition of Rh(ttp)H over the C−C
bond of c-octane.
Influence of Porphyrin Sterics on CCA. To find out the
steric sensitivity of CCA toward porphyrin structure, we further
examined the reaction of c-octane with the analogous but
sterically more hindered Rh(tmp) complexes (tmp =
5,10,15,20-tetramesitylporphyrinato dianion).8 Rh(tmp)Cl 10
reacted with c-octane (20 equiv) and K2CO3 in benzene-d6 at
120 °C in 3d to give Rh(tmp)H, Rh(tmp), and c-octene in
51%, 46%, and 28%, respectively without any CCA product (eq
11). When the mixture of Rh(tmp)H 11 and RhII(tmp) 12
by the reductive alkylation (NaBH4/c-octyl bromide) at a lower
temperature of 50 °C gave Rh(tmp)H 9 and c-octene in 89 and
77% yields, respectively, showing the highly labile nature of
Rh(tmp)-c-octyl 13 compared to Rh(ttp) c-octyl 2 (eq 1).
DFT Theoretical Calculations of the Proposed CCA
Step. To gain further understanding of the metalloradical CCA
step, DFT calculations were performed. Initial comparative
screening of different reaction pathways was performed at the
DFT-D3, b3-lyp, def2-TZVP levels using small atom Rh(por)
models of the actual Rh(tpp) species containing nonfunction-
alized porphyrin rings without meso-phenyl substituents.
All attempts to find a transition state corresponding to the σ-
bond metathesis 4-centered transition state described in
Scheme 2 were unsuccessful and led to the transition states
TSA or TSB corresponding to pathways A and B in Scheme 5.
These pathways correspond to hypothetical reactions in which
(por)Rh-octyl is formed directly from (por)Rh−H and the c-
octane reagent. In closed-shell singlet pathway A, simultaneous
heterolysis of the C−C bond of c-octane is accompanied by
proton transfer from the Rh-hydride moiety of (por)Rh−H to
the resulting negatively charged carbon atom formed from c-
octane in TSA. The transition state leads to subsequent proton
transfer from the beta-position of the “cationic” carbon to
rhodium, producing 1-octene with regeneration of (por)Rh−H.
Subsequent insertion of 1-octene into the Rh−H bond of
(por)Rh−H could produce (por)Rh-octyl, but the transition
state of this hypothetical step was not investigated in view of
the very high barrier of the preceding transition state TSA.
While interesting from a fundamental perspective, the high
transition state barrier of TSA makes pathway A rather
unrealistic. The same holds for pathway B, which proceeds at
the open-shell singlet (singlet biradical) surface employing the
broken-symmetry approach. This pathway involves C−C bond
homolysis of c-octane associated with simultaneous hydrogen-
atom transfer from (por)Rh−H to the (developing) ·C8H16·
diradical, thus producing (por)RhII and the 1-octyl radical ·
C8H17. Subsequent formation of (por)Rh-octyl is virtually
barrierless and only requires a proper orientation of the
RhII(por) and 1-octyl radical ·C8H17 reagents to allow Rh−C
bond formation. Again, pathway B is fundamentally interesting
but has a very high transition state barrier TSB, making this
pathway rather unrealistic in view of the experimental
observations.
(10:1) was reacted with c-octane at 120 °C for 15 h, no reaction
occurred, and 90% yield of Rh(tmp)H 11 was recovered (eq
12). RhII(tmp) 12 alone only underwent CHA with c-octane to
give Rh(tmp)H 11 and c-octene in 86% and 40% yields,
respectively (eq 13).
The formation of c-octene likely results from the
bimetalloradical CHA to give Rh(tmp)(c-octyl) 13, which
then rapidly undergoes facile β-hydride elimination to give c-
octene and Rh(tmp)H 7. The nearly 1:2 ratio of Rh(tmp)H to
c-octene further supports the occurrence of this process. The β-
hydride elimination of 13 is sterically enhanced by the
unfavorable steric interactions of the methyl groups at the 2,6
positions of mesityl groups in Rh(tmp) with the c-octyl group
In marked contrast to pathways A and B, the transition state
barrier TSC of pathway C involving a RhII(por) metalloradical
catalyzed reaction between (por)Rh−H and c-octane is much
lower, and hence, pathway C is a much more realistic scenario,
in good agreement with the experimental data (Scheme 5).
This pathway involves homolytic activation of the C−C bond
of c-octane by the RhII(por) metalloradical producing a
D
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Scheme 5. Comparison of DFT Calculated Free Energies (ΔG298 K in kcal mol−1) of Different Pathways Leading to the
Formation of (por)Rh-octyl from (por)Rh-H and c-Octane (DFT-D3, b3-lyp, and def2-TZVP)
terminal octyl-radical species (por)Rh−C8H16·, which sub-
sequently abstracts a hydrogen atom from (por)Rh−H to
regenerate the (por)RhII metalloradical “catalyst” with the
formation of the (por)Rh-octyl product.
The DFT calculated pathway (Scheme 6) is in qualitative
agreement with the proposed mechanism (Scheme 3). The first
steps from I to III are endothermic and endergonic. This agrees
with the experimental requirement to heat to drive the reaction.
The subsequent reaction between the Rh(tpp)octyl radical
complex III with Rh(tpp)(H) to form Rh(tpp)octyl and
RhII(tpp) is exothermic and exergonic, and provides the driving
force of the overall reaction. We were unable to find a transition
state for this last step, likely because the barrier is too low with
a flat reaction profile. The transition state of the first reaction
step (from II via TS to III) involves a metallo-radical induced
SH2-type homolysis of one of the C−C bonds of c-octane. In
this CCA pathway, the RhII metallo-radical traps one of the two
carbon radicals produced in this process in concert with C−C
bond homolysis (Figure 1).
Pathway C was further explored in more detail using the full
atom Rh(tpp) models. We argued that the full atom models
might lead to lower barriers in view of stronger (VdW)
dispersion interactions between c-octane and the Rh(tpp)
species. As such, we examined the hypothesized CCA reaction
between RhII(tpp) and c-octane, followed by reaction of the
Rh(tpp)octyl radical complex III with Rh(tpp)(H) to form
Rh(tpp)octyl and to regenerate RhII(tpp) (see Scheme 6). We
employed both the BP86 and the b3-lyp functional to examine
the details of pathway C. The def2-TZVP basis set and
Grimme’s D3-DFT dispersion corrections were used in both
cases. For the b3-lyp pathways, additional cosmo dielectric
solvent corrections (ε = 2.27 corresponding to benzene) were
included. The (free) energies associated with the reaction
sequence depicted in Scheme 6 are listed in Table 6.
Despite stronger attractive dispersion (VdW) forces between
Rh(tpp) and c-octane, the TSC transition barrier is still sizable
at the b3-lyp level (Table 1). However, it should be recalled at
this point that the experimental reaction requires prolonged
(15 h) heating to 120 °C to reach completion, and hence the
E
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def2-TZVP seems to be overestimated by several kcal mol−1
(which is not uncommon in DFT studies24).
Scheme 6. DFT Calculated Metallo-Radical Catalyzed
Reaction between c-Octane and (por)Rh-H (Pathway C)
a
Conclusions. We have discovered the RhII(ttp)-catalyzed
1,2-insertion of Rh(ttp)H into the carbon−carbon bond of c-
octane, which selectively produces Rh(ttp)-n-octyl in high yield.
The RhII(ttp) metallo-radical attacks the C−C bond of c-
octane, likely via an SH2 process. DFT theoretical calculations
support this mechanistic proposal. This represents a unique
metalloradical carbon−carbon activation process. Further
studies are being continued to develop catalytic C−C bond
functionalization.
Including Selected Bond Lengths
EXPERIMENTAL AND COMPUTATIONAL DETAILS
■
Reaction of c-Octane with Rh(ttp)Cl. Rh(ttp)Cl 1 (20.6 mg,
0.026 mmol) was added in c-octane (3.0 mL). The red reaction
mixture was degassed for three freeze−thaw−pump cycles, purged
with N2, and heated at 120 °C under N2 for 48 h. Excess c-octane was
removed by vacuum distillation. The residue was purified by column
chromatography on silica gel eluting with a solvent mixture of hexane/
CH2Cl2 (1:1). Red solid, Rh(ttp)(c-octyl) 2 (1.0 mg, 0.0011 mmol,
5%), and Rh(ttp)(n-octyl) 3 (1.9 mg, 0.0021 mmol, 8%) were
collected and further recrystallized from CH2Cl2/MeOH. The product
ratio was calculated by 1H NMR integration. Rh(ttp)Cl was recovered
(14.8 mg) after column chromatography. Characterization of Rh(ttp)-
a
DFT-D3, BP86, def2-TZVP, and DFT-D3, b3-lyp, def2-TZVP, and
cosmo ε = 2.27.
1
(c-octyl) 2: Rf = 0.84 (hexane/CH2Cl2 = 1:1). H NMR (C6D6, 400
MHz) δ −4.25 (m, 2), −3.66 (m, 3 H), −1.13 (m, 4 H), −0.32 (m, 2
H), 0.90 (m, 4 H), 2.41 (s, 12 H, p-methyl), 7.30 (d, 4 H, J = 7.3 Hz,
m-phenyl), 7.33 (d, 4 H, J = 7.2 Hz, m′-phenyl), 8.18 (d, 4 H, J = 7.7
Hz, o′-phenyl), 8.97 (s, 8 H, pyrrole). 13C NMR (CDCl3, 75 MHz) δ
Table 6. DFT Calculated Energies Associated with the
Computed Reaction Pathway Depicted in Scheme 6
a
ΔEZPE
ΔG°(298 K)
ΔH°
ΔS°
1
(kcal mol−1
)
(kcal mol−1
)
(kcal mol−1
)
(cal mol−1 K−1
BP86 b3-lyp
)
21.70, 22.54, 25.23, 25.85, 30.40, 40.62 (d, JRh−C = 26.4 Hz), 122.86,
BP86 b3-lyp
BP86 b3-lyp
BP86 b3-lyp
127.42, 127.54, 131.48, 133.62, 134.25, 137.22, 139.52, 143.52. HRMS
calcd. for (C56H51N4Rh+H)+: m/z 883.3242. Found: m/z 883.3214.
Characterization of Rh(ttp)(n-octyl) 2: Rf = 0.84 (hexane/CH2Cl2 =
I
= 0
= 0
= 0
= 0
II
−15.9
−10.2
+20.0
+34.5
+9.2
−8.3
−5.5
−15.7
−9.5
−24.8
−13.3
−28.0
−24.4
−9.7
1
1:1). H NMR (C6D6, 400 MHz) δ −4.55 (td, 2 H, J = 2.8, 8.7 Hz),
−4.11 (qu, 2 H, J = 8.2 Hz), −1.55 (qu, 2 H, J = 7.8 Hz), −0.50 (qu, 2
H, J = 8.0 Hz), 0.02 (qu, 2 H, J = 7.5 Hz), 0.44 (qu, 2 H, J = 7.4 Hz),
0.59 (t, 3 H, J = 7.2 Hz), 0.80 (qu, 2 H, J = 7.6 Hz), 2.41 (s, 12 H, p-
methyl), 7.27 (d, 4 H, J = 8.1 Hz, m-phenyl), 7.35 (d, 4 H, J = 6.4 Hz,
m′-phenyl), 8.12 (dd, 4 H, J = 1.7, 7.6 Hz, o-phenyl), 8.22 (dd, 4 H, J =
1.6, 7.6 Hz, o′-phenyl), 8.99 (s, 8 H, pyrrole). 13C NMR (CDCl3, 100
MHz) δ 14.00, 15.69 (d, 1JRh−C = 26.8 Hz), 21.68, 22.41, 26.28, 27.04,
27.52, 27.96, 31.31, 122.49, 127.43, 127.51, 131.47, 133.78, 134.09,
137.23, 139.48, 143.35. HRMS calcd. for (C56H53N4Rh)+: m/z
884.3320. Found: m/z 884.3336.
TS
III
IV
+28.5
+42.0
+13.7
+22.6
−17.8
−17.9
+20.1
+34.9
+10.8
+18.1
−17.0
−14.7
+17.0
−17.8
−16.0
−14.9
+2.6
+10.6
a
DFT-D3, BP86, def2-TZVP, and DFT-D3; b3-lyp, def2-TZVP, and
cosmo ε = 2.27.
Reaction of c-Octane and Rh(ttp)Cl with Potassium
Hydroxide. Rh(ttp)Cl (20.4 mg, 0.025 mmol) and potassium
hydroxide (14.2 mg, 0.254 mmol) was added in c-octane (3.0 mL).
The red reaction mixture was degassed for three freeze−thaw−pump
cycles, purged with N2 and heated at 120 °C under N2 for 7.5 h. Excess
c-octane was removed by vacuum distillation. The red residue was
added with benzene-d6 (500 μL) under N2 for 1H NMR spectroscopy,
and the NMR yield of Rh(ttp)H (62%) was estimated. The crude
mixture was then 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 eluting with a solvent
mixture of hexane/CH2Cl2 (1:1). Red solids, Rh(ttp)(c-octyl) 2 (1.3
mg, 0.0015 mmol, 6%), and Rh(ttp)(n-octyl) 3 (5.6 mg, 0.0063 mmol,
25%) were collected.
Reaction of c-Octane 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 in c-octane
(3.0 mL). The red reaction mixture was degassed for three freeze−
thaw−pump cycles, purged with N2, and heated at 120 °C under N2
for 7.5 h. Excess c-octane was removed by vacuum distillation. The red
Figure 1. Structure of transition state TS representing RhII-mediated
SH2-type CCA of c-octane (b3-lyp, def2-TZVP).
experimental barrier is also high (estimated at ∼30−35 kcal
mol−1). The barrier (from II to TSC) calculated at the BP86,
def2-TZVP level (ΔG⧧ = 36.8 kcal mol−1; ΔH⧧ = 35.8 kcal
mol−1) is lower and in reasonable agreement with the
experimental estimate. The barrier calculated at the b3-lyp,
1
residue was added with benzene-d6 (500 μL) under N2 for H NMR
spectroscopy, and the NMR yield of Rh(ttp)H (58%) was estimated.
The crude mixture was then extracted with CH2Cl2/H2O. The organic
layer was collected, dried, and evaporated to dryness, and the residue
F
DOI: 10.1021/acs.organomet.5b00183
Organometallics XXXX, XXX, XXX−XXX

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Article
was purified by column chromatography on silica gel eluting with a
solvent mixture of hexane/CH2Cl2 (1:1). Red solids, Rh(ttp)(n-octyl)
3 (7.5 mg, 0.0085 mmol, 33%), were collected.
Reaction of c-Octane with Rh(ttp)H and PPh3. Rh(ttp)H (9.6
mg, 0.012 mmol) and PPh3 (3.2 mg, 0.012 mmol) were added in c-
octane (1.5 mL). The red reaction mixture was degassed for three
freeze−thaw−pump cycles, purged with N2, and heated at 120 °C
under N2 for 15 h. Excess c-octane was removed by vacuum
distillation. The residue was added with benzene-d6 (500 μL) under
N2 protection for 1H NMR spectroscopy. Rh(ttp)H(PPh3) was
Independent Synthesis of Rh(ttp)(c-octyl) 2.6 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 aq. 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 at 50 °C under N2 for 1 h. The solution was then cooled to 30
°C under N2 and c-octyl bromide (23 mg, 1.20 mmol) was added. A
reddish orange suspension was formed. After stirring 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 rotatory evaporated. The
reddish orange residue was purified by column chromatography over
silica gel (250−400 mesh) using a solvent mixture of hexane/CH2Cl2
(1:1) as the eluent. The major orange fraction was collected and gave a
reddish orange solid of Rh(ttp)(c-octyl) 2 (94.1 mg, 0.11 mmol, 86%)
as the product after rotary evaporation.
1
obtained in quantitative NMR yield. H NMR (C6D6, 400 MHz) δ
−33.42 (b, 1 H), 2.39 (s, 12 H, p-methyl), 4.16 (t, 2 H, J = 9.2 Hz),
6.28 (td, 2 H, J = 2.4, 7.6 Hz, m-phenyl), 6.52 (t, 1 H, J = 6.8 Hz), 7.35
(d, 4 H, J = 7.2 Hz, m′-phenyl), 7.84 (d, 4 H, J = 7.6 Hz, o-phenyl),
7.89 (d, 4 H, J = 7.6 Hz, o′-phenyl), 8.98 (s, 8 H, pyrrole).
Thermal Dehydrogenative Dimerization of Rh(ttp)H. Rh-
(ttp)H (3.2 mg, 0.0041 mmol) was added in benzene-d6 (500 μL).
The red reaction mixture was degassed for three freeze−thaw−pump
cycles, and the NMR tube was flame-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 of
Rh2(ttp)2 6 were taken. The H2 concentration in solution was too low
to be detected.
Independent Synthesis of Rh(ttp)(n-octyl) 3.6 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 aq. 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 at 50 °C under N2 for 1 h. The solution was then cooled to 30
°C under N2, and n-octyl bromide (23 mg, 1.20 mmol) was added. A
reddish orange suspension was formed. After stirring 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 rotatory evaporated. The
reddish orange residue was purified by column chromatography over
silica gel (250−400 mesh) using a solvent mixture of hexane/CH2Cl2
(1:1) as the eluent. The major orange fraction was collected and gave a
reddish orange solid of Rh(ttp)(n-octyl) 3 (96.5 mg, 0.11 mmol, 88%)
as the product after rotary evaporation.
Reaction of c-Octane with Rh2(ttp)2 6. Rh2(ttp)2 (9.6 mg, 0.012
mmol) was added in c-octane (1.5 mL). The red reaction mixture was
degassed for three freeze−thaw−pump cycles, purged with N2, and
heated at 120 °C under N2 for 15 h. Excess c-octane was removed by
vacuum distillation. The red residue was added with benzene-d6 (500
1
μL) under N2 protection for H NMR spectroscopy, and the yield of
Rh(ttp)H (46%) was estimated. The residue was purified by column
chromatography on silica gel eluting with a solvent mixture of hexane/
CH2Cl2 (1:1). Red solids, Rh(ttp)(c-octyl) 2 (4.5 mg, 0.0051 mmol,
41%) and Rh(ttp)(n-octyl) 3 (0.4 mg, 0.00045 mmol, 4%), were
collected, and the product ratio was calculated by 1H NMR
integration.
Reaction of c-Octane with a 2:1 Mixture of Rh(ttp)H and
Rh2(ttp)2. Rh(ttp)H (9.6 mg, 0.012 mmol) and Rh2(ttp)2 (4.8 mg,
0.0031 mmol) were added to c-octane (1.5 mL). The red reaction
mixture was degassed for three freeze−thaw−pump cycles, purged
with N2, and heated at 120 °C under N2 for 15 h. Excess c-octane was
removed by vacuum distillation. The residue was purified by column
chromatography on silica gel eluting with a solvent mixture of hexane/
CH2Cl2 (1:1). Red solids, Rh(ttp)(c-octyl) 2 (9.8 mg, 0.011 mmol,
60%) and Rh(ttp)(n-octyl) 3 (3.0 mg, 0.0034 mmol, 18%), were
collected, and the product ratio was calculated by 1H NMR
integration.
Reaction of c-Octane with a 5:1 Mixture of Rh(ttp)H and
Rh2(ttp)2. Rh(ttp)H (9.6 mg, 0.012 mmol) and Rh2(ttp)2 (1.9 mg,
0.0012 mmol) were added to c-octane (1.5 mL). The red reaction
mixture was degassed for three freeze−thaw−pump cycles, purged
with N2, and heated at 120 °C under N2 for 15 h. Excess c-octane was
removed by vacuum distillation. The residue was purified by column
chromatography on silica gel eluting with a solvent mixture of hexane/
CH2Cl2 (1:1). Red solids, Rh(ttp)(c-octyl) 2 (6.9 mg, 0.0078 mmol,
53%) and Rh(ttp)(n-octyl) 3 (3.4 mg, 0.0038 mmol, 26%), were
collected, and the product ratio was calculated by 1H NMR
integration.
Reaction of c-Octane with a 10:1 Mixture of Rh(ttp)H and
Rh2(ttp)2. Rh(ttp)H (9.6 mg, 0.012 mmol) and Rh2(ttp)2 (1.0 mg,
0.00065 mmol) were added to c-octane (1.5 mL). The red reaction
mixture was degassed for three freeze−thaw−pump cycles, purged
with N2, and heated at 120 °C under N2 for 15 h. Excess c-octane was
removed by vacuum distillation. The residue was purified by column
chromatography on silica gel eluting with a solvent mixture of hexane/
CH2Cl2 (1:1). Red solid, Rh(ttp)(n-octyl) 3 (8.9 mg, 0.010 mmol,
73%) was collected and was further recrystallized from CH2Cl2/
MeOH.
Thermal Stability of Rh(ttp)(c-octyl) in Benzene-d6. Rh(ttp)(c-
octyl) 2 (3.9 mg, 0.0044 mmol) was added into 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. It
1
was heated at 120 °C in the dark. It was monitored with H NMR
spectroscopy at particular time intervals, and the NMR yields were
taken.
Stability of Rh(ttp)(c-octyl) with Potassium Carbonate in
Benzene-d6. Rh(ttp)(c-octyl) 2 (3.9 mg, 0.0044 mmol) and
potassium carbonate (6.0 mg, 0.044 mmol) were added into
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. 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 taken.
Reaction of Rh(ttp)Cl and c-Octane with Potassium
Carbonate in Benzene-d6. Rh(ttp)Cl (3.5 mg, 0.0043 mmol), c-
octane (11 μL, 0.087 mmol), and potassium carbonate (5.9 mg, 0.0427
mmol) were added into 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 at 120 °C in
1
the dark. It was monitored with H NMR spectroscopy at particular
time intervals, and the NMR yields were taken.
Reaction of c-Octane with Rh(ttp)H. Rh(ttp)H (9.6 mg, 0.012
mmol) was added in c-octane (1.5 mL). The red reaction mixture was
degassed for three freeze−thaw−pump cycles, purged with N2, and
heated at 120 °C under N2 for 15 h. Excess c-octane was removed by
vacuum distillation. The residue was added with benzene-d6 (500 μL)
1
under N2 protection for H NMR spectroscopy, and the recovered
Reaction of Rh(tmp)Cl 10 and c-Octane with Potassium
Carbonate in Benzene-d6. Rh(tmp)Cl9,10 (4.0 mg, 0.0043 mmol),
c-octane (11 μL, 0.087 mmol), and potassium carbonate (5.8 mg,
0.0420 mmol) were added into 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 at
yield of Rh(ttp)H (73%) was estimated. The residue was purified by
column chromatography on silica gel eluting with a solvent mixture of
hexane/CH2Cl2 (1:1). Red solid, Rh(ttp)(n-octyl) 3 (2.3 mg, 0.0026
mmol, 21%), was collected and was further recrystallized from
CH2Cl2/MeOH.
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DOI: 10.1021/acs.organomet.5b00183
Organometallics XXXX, XXX, XXX−XXX

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Article
1
120 °C in the dark. It was monitored with H NMR spectroscopy at
particular time intervals, and the NMR yields were taken. Rh(tmp)H,
RhII(tmp), and c-octene were in 51%, 46%, and 28% yields,
respectively at 120 °C in 3 d.
the full atom TPP models optimized at the b3-lyp level, additional
dielectric constant corrections (cosmo35) were taken into account
based on single point calculations, using the dielectric constant of
benzene (ε = 2.27).
Reaction of c-Octane with a 10:1 Mixture of Rh(tmp)H 11
and RhII(tmp) 12. Rh(tmp)H8 (10.6 mg, 0.012 mmol) and
RhII(tmp)8 (1.1 mg, 0.0012 mmol) were added to c-octane (1.5
mL). The red reaction mixture was degassed for three freeze−thaw−
pump cycles, purged with N2, and heated at 120 °C under N2 for 24 h.
Excess c-octane was removed by vacuum distillation. The colorless
ASSOCIATED CONTENT
■
S
* Supporting Information
Reaction progress and computational data. The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.organomet.5b00183.
1
organic distillate was added with benzene-d6 for H NMR spectros-
copy, and c-octene was not observed. Degassed benzene-d6 was added
1
to N2 for H NMR spectroscopy. A red solution of Rh(tmp)H (90%
AUTHOR INFORMATION
1
■
yield, estimated by H NMR spectroscopy) was obtained.
Reaction of c-octane with RhII(tmp) 12. RhII(tmp) (10.6 mg,
0.0012 mmol) was added to c-octane (1.5 mL). The red reaction
mixture was degassed for three freeze−thaw−pump cycles, purged
with N2, and heated at 120 °C under N2 for 24 h. Excess c-octane was
removed by vacuum distillation. The colorless organic distillate was
added with benzene-d6 for 1H NMR spectroscopy, and c-octene (40%
yield, estimated by 1H NMR) was observed. Degassed benzene-d6 was
Corresponding Authors
*(B.D.B.) E-mail: bdebruin@science.uva.nl.
*(K.S.C.) E-mail: ksc@cuhk.edu.hk.
Notes
The authors declare no competing financial interest.
1
added to N2 for H NMR spectroscopy. A red solution of Rh(tmp)H
ACKNOWLEDGMENTS
■
1
(86% yield, estimated by H NMR spectroscopy) was obtained.
We thank the General Research Fund of the Research Grants
Council of Hong Kong (No: 400212), The Netherlands
Organization for Scientific Research, Chemical Sciences
(NWO-CW VICI project 016.122.613), and the NWO/RGC
Joint Research Scheme sponsored by the Research Grants
Council of Hong Kong and The Netherlands Organization for
Scientific Research (D-HK002/11T) for financial support.
Attempted Synthesis of Rh(tmp)(c-octyl) 13. A suspension of
Rh(tmp)Cl (20.0 mg, 0.022 mmol) in EtOH (5 mL) and a solution of
NaBH4 (3 mg, 0.087 mmol) in aq. NaOH (0.1 M, 0.4 mL) were
purged with N2 for 15 min separately. The solution of NaBH4 was
added slowly to the suspension of Rh(tmp)Cl via a cannula. The
mixture was heated at 50 °C under N2 for 1 h. The solution was then
cooled to 30 °C under N2, and c-octyl bromide was added. A reddish
orange suspension was formed. After stirring at room temperature for
another 15 min under N2, the reaction mixture was vacuum-distilled,
and the distillate went through 1H NMR spectroscopy after extraction
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1
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Computational Details. Geometry optimizations were carried out
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Several methods have been proposed for corrections of gas phase to
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entropy correction term (SCP = SGP + Rln{1/24.5}) for all species,
affecting relative free energies (298 K) of all associative steps of −2.5
kcal mol−1.32 Larger correction terms of −6.0 kcal mol−1 have been
suggested based on solid arguments.33,34 While it remains a bit
debatable as to which entropy correction term is best to translate gas
phase DFT data into free energies relevant for reactions in solution, in
this article we adapted the suggested correction term of −6.0 kcal
mol−1.33,34 For the reactions described in Scheme 6 and Table 6, using
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DOI: 10.1021/acs.organomet.5b00183
Organometallics XXXX, XXX, XXX−XXX