Metalloradical-catalyzed aliphatic carbon-carbon activation of cyclooctane

Article abstract of DOI:10.1021/ja101586w

The aliphatic carbon-carbon activation of c-octane was achieved via the addition of Rh(ttp)H to give Rh(ttp)( n-octyl) in good yield under mild reaction conditions. The aliphatic carbon-carbon activation was Rh^II(ttp)- catalyzed and was very se

Full text of DOI:10.1021/ja101586w

Published on Web 05/04/2010
Metalloradical-Catalyzed Aliphatic Carbon-Carbon Activation of Cyclooctane
Yun Wai Chan and Kin Shing Chan*
Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, New Territories,
Hong Kong SAR, The People’s Republic of China
Received February 23, 2010; E-mail: ksc@cuhk.edu.hk
Alkane functionalization in a homogeneous medium is an
important and challenging process which involves either carbon-
hydrogen activation (CHA)1 or carbon-carbon activation (CCA)2
with organic, inorganic and organometallics reagents. Although
aliphatic C-C bonds are weaker than aliphatic C-H bonds, CCA
of alkanes is much less reported due to the steric hindrance of the
C-C bond by the attack of a transition metal complex.3
Cyclooctane (c-octane) is a relatively unstrained cycloalkane and
therefore serves as a commonly investigated substrate in alkane
functionalization, mostly involving CHA. Some examples of CHA
of c-octane are the iridium(I) pincer dihydride-catalyzed dehydro-
genation to c-octene,4a the FeCl3-catalyzed aerobic oxidation to
c-octanol and c-octanone,4b and the MnO2-catalyzed bromination
to c-octyl bromide.4c Examples of CCA of c-octane are rarely
Figure 1. ORTEP presentation of Rh(ttp)(n-octyl) 2 (30% probability
reported. A CCA of c-octane in a heterogeneous medium requires
a very high reaction temperature of 530 °C and consequently results
in both CHA and CCA.3a An oxidative CCA of c-octane catalyzed
by N-hydroxyphthalides/Co(II)/Mn(II) at 100 °C in 14 h gives R,ω-
dicarboxylic acids in 2% yield only.3b
We have recently discovered the base-promoted CHA of alkane
with Rh(III) porphyrin5 as well as the aliphatic CCA of nitroxides
by Rh(II) porphyrin.6 We now report the selective aliphatic CCA
of c-octane by rhodium(III) porphyrin hydride to give a high yield
of rhodium porphyrin n-octyl under mild reaction conditions and
the mechanistic studies identifying the unique catalytic role of Rh(II)
porphyrin (Scheme 1).
displacement ellipsoids). Rh-C ) 2.03 Å, R ) 0.0522.
and basic conditions separately. Without K2CO3, Rh(ttp)(c-octyl)
1 gave Rh(ttp)(n-octyl) 2, Rh(ttp)H 3, and c-octene 4 in 10%, 76%,
and 36% yields, respectively after 21 h (eq 2, Figure S1, Table
S1). In the presence of K2CO3 (10 equiv), Rh(ttp)(n-octyl) 2 was
isolated in a higher yield of 21% in 16 h (eq 3, Figure S2, Table
S2). However, both reactions were low yielding and incomplete.
Therefore, the CHA product is not a major intermediate leading to
the CCA product.
Scheme 1. CCA of c-Octane with MH
Initially, c-octane was found to react poorly with Rh(ttp)Cl (ttp
)5,10,15,20-tetratolylporphyrinato dianion) to give Rh(ttp)(c-octyl)
1 and Rh(ttp)(n-octyl) 2 in 5% and 8% yields, respectively (eq 1).
A 72% yield of Rh(ttp)Cl was recovered, and a trace amount of
Rh(ttp)H 3 was observed. Both CHA and CCA products formed,
but the reaction was inefficient. When K2CO3 (10 equiv) was
added,7 Rh(ttp)Cl was consumed in 7.5 h and Rh(ttp)(n-octyl) 2
and Rh(ttp)H 3 were obtained in 33% and 58% yields, respectively.
The CCA product 2 is the formal 1,2-addition product of Rh(ttp)H
into c-octane. The structures of 1 and 2 were confirmed by
independent syntheses.8 2 was further characterized by X-ray
crystallography (Figure 1).
To enhance the CCA reaction of Rh(ttp)Cl with c-octane based
on mechanistic understandings, the reaction was monitored by H
1
NMR spectroscopy in a sealed NMR tube (eq 4, Figure S3, Table
S3). Initially, Rh(ttp)Cl was first converted to Rh2(ttp)2 5.5 5 then
slowly and completely reacted with the gradual formation of
Rh(ttp)H 3. Finally, Rh(ttp)(n-octyl) was generated in prolonged
heating and still, Rh(ttp)H was consumed slowly and mostly
remained unreacted. Therefore, both Rh2(ttp)2 and Rh(ttp)H are
possible intermediates. The observed 1H NMR upfield signals at δ
) -5 to 1 ppm (Figure S4) 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.10
To investigate the intermediacy of Rh(ttp)H and Rh2(ttp)2,
Rh(ttp)H 3 and Rh2(ttp)2 5 were then separately reacted with
c-octane. Rh(ttp)H 3 indeed reacted with c-octane at 120 °C in
To investigate whether the CHA product is an intermediate for
CCA,9 Rh(ttp)(c-octyl) 1 was heated in benzene-d6 in both neutral
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C O M M U N I C A T I O N S
Scheme 3. Formation of Rh(tmp)H from CHA of RhII(tmp) and
c-Octane
15 h to give Rh(ttp)(n-octyl) 2 selectively, though in only 21%
yield, and was also recovered in 73% yield (eq 5). As Rh(ttp)H
underwent slow dehydrogenative dimerization to give 6% yield of
Rh2(ttp)2 at 120 °C in 1 day, similar to the report by Wayland and
co-workers (eq 6),11 the small amount of Rh2(ttp)2 formed in eq 5
likely facilitates the 1,2-addition of Rh(ttp)H into c-octane.12 The
other possible intermediate Rh2(ttp)2 5 was also reacted with
c-octane. Rh(ttp)(c-octyl) 1, Rh(ttp)(n-octyl) 2, and Rh(ttp)H 3 were
formed in 41%, 4%, and 46% yields, respectively (eq 7) with a
very low yield of CCA product. Therefore, both Rh(ttp)H 3 and
Rh2(ttp)2 5 gave low yielding reactions and are likely only minor
reaction intermediates by themselves.
2-4 vs 1). The selectivity toward CCA was further enhanced 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 a 2:1 ratio gave Rh(ttp)(c-
octyl) and Rh(ttp)(n-octyl) in 60% and 18% yields, respectively
(Table 1, entry 2). When the Rh(ttp)H/Rh2(ttp)2 ratio increased to
5:1, the yield of Rh(ttp)(n-octyl) increased to 26% yield but that
of Rh(ttp)(c-octyl) decreased to 53% yield (entry 3). Rh(ttp)(n-
octyl) was selectively obtained in 73% yield from the reaction with
the 10:1 ratio of Rh(ttp)H/Rh2(ttp)2 (entry 4). The aliphatic CCA
of c-octane was thus achieved successfully with the RhII-catalyzed
1,2-addition of Rh(ttp)H.
Based on the mechanism of the RhII-catalyzed insertion of
Rh(oep)H (oep ) octylethylporphyrin dianion) into styrene reported
by Halpern et al.,12 we proposed that the CCA, being a 1,2-addition
reaction, is catalyzed by RhII (Scheme 2). Rh2(ttp)2 5 formed from
thermolysis of Rh(ttp)H initially undergoes homolysis to give
RhII(ttp) (eqs 8 and 9).11 RhII(ttp) then reacts with c-octane in
parallel CHA (pathway iii, eq 10) and CCA (pathway iv, eq 11).
RhII(por) (por ) porphyrinato dianion) has been shown to undergo
CHA with alkane to give Rh(por)R and Rh(por)H.5,13 For the CCA
pathway, RhII(ttp) can cleave the C-C bond of c-octane to generate
the alkyl radical 6 (pathway iv, eq 11) which can also reverse back
rapidly.14 6 can then abstract a hydrogen atom from the weak
(ttp)Rh-H bond15a to form a strong alkyl C-H bond,15b providing
the driving force of the reaction (pathway v). The proposed
mechanism can be validated qualitatively by increasing the ratio
of Rh(ttp)H/Rh2(ttp)2 for more efficient trapping of 6 to 2 (Table
1, eq 12).
The sterically more hindered Rh(tmp) was not effective for CCA
(tmp ) 5,10,15,20-tetramesitylporphyrinato dianion). When the
mixture of Rh(tmp)H and RhII(tmp) (10:1) was reacted with
c-octane at 120 °C for 15 h, no reaction occurred and 90% yield of
Rh(tmp)H was recovered (eq 13). RhII(tmp) only underwent CHA
with c-octane to give Rh(tmp)H and c-octene in 86% and 40%
yields, respectively (eq 14). The formation of c-octene likely results
from the CHA product Rh(tmp)(c-octyl) which rapidly undergoes
facile ꢀ-hydride elimination to give c-octene and Rh(tmp)H
(Scheme 3). Indeed, the attempted synthesis of Rh(tmp)(c-octyl)
by reductive alkylation (NaBH4/c-octyl bromide) gave Rh(tmp)H
and c-octene in 89% and 77% yields, respectively.
In conclusion, we have discovered the mild, selective RhII-
catalyzed 1,2-addition of Rh(ttp)H to c-octane via an aliphatic CCA.
Further studies are ongoing.
Indeed, mixtures of Rh(ttp)H and Rh2(ttp)2 were more efficient
reagents and enhanced the total yields up to 79% (Table 1, entries
Acknowledgment. We thank the Research Grants Council of
the HKSAR, the People’s Republic of China (CUHK 400307) for
financial support. We thank reviewers for helpful suggestions and
Mr. Kwong Shing Choi for helpful discussions and assistance.
Scheme 2. Proposed Mechanism of RhII-Catalyzed 1,2-Addition of
c-Octane with RhH
Supporting Information Available: Detailed experimental section,
1H and 13C NMR data of 1 and 2, reaction time profiles of eqs 2-4,
and details of crystallographic studies of 2 (including CIF file). This
material is available free of charge via the Internet at http://pubs.acs.org.
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