benzyl radicals because of the lack of diastereoselectivity observed
in the product.
synthesized and characterized by UV-vis and ESR spectroscopy
as well as AFM. The B12-HBP showed good properties for use as
a homogenous catalyst; the covalently-immobilized B12 moieties
were efficiently solvated and the cobalt centers were accessible
for pyridine guests. The B12-HBP showed a high reactivity to 2-
phenethylbromide and increased selectivity for reductive dimer-
ization in the presence of TiO2 with UV light irradiation. These
results provide new insight into the effective organization of small
amounts of catalytically active metal complexes to explore new
catalytic reactions. The present system would be applicable to
radical reactions such as intramolecular coupling.
A proposed mechanism of the B12-HBP mediated reaction is
shown in Fig. 6 and Fig. 8. The reaction is initiated by the
photochemical reduction of Co(II) to the Co(I) state. The resulting
supernucleophile Co(I) species reacts with 2-phenethylbromide
to form the cobalt-phenethyl complex. The Co–C bond of the
complex then homolytically cleaves to form the corresponding
radical upon UV light irradiation.40 The resulting radical 9 and
rearrangement radical 10 can abstract hydrogen from the solvent
to form ethylbenzene. The benzyl radical 10 can also couple to
form 2,3-diphenylbutane. It is expected that this radical coupling
is close to a diffusion-controlled reaction (kd ~ 109 M-1 s-1 at
room temperature) and that the yield of 2,3-diphenylbutane is
largely dependent on the forming benzyl radical concentration
(i.e., a rate of 109 ¥ [10]2 for the formation of 2,3-diphenylbutane
from the benzyl radical).9 In the monomeric B12 system, hydrogen
abstraction can proceed prior to the radical coupling to give a
poor yield of 2,3-diphenylbutane. In contrast, in the B12-HBP
system, the benzyl radical coupling can be enhanced because
of the high local concentration of B12 units in 8 as described
above. Effective pre-organization of catalytic amounts of B12 on
the nano-sized scaffold could lead to production of the benzyl
radical in high concentration, promoting the intermolecular C–C
bond formation.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research
on Priority Areas (452 and 460), Global COE Program “Science
for Future Molecular System” from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT) of Japan, and
a Grant-in-Aid for Scientific Research (21.02310) and a Grant-in-
Aid for Scientific Research (A) (21245016) from the Japan Society
for the Promotion of Science (JSPS). Authors would like to thank
Prof. Kazuki Sada and Mr. Kota Sugikawa (Kyushu University)
for the AFM measurement.
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The present system is likely to show a positive correlation
between the cobalt density in the nano-sized catalyst and the
selectivity for the intermolecular coupling. Such effect dependent
on the density of metal centers in the nano-material is somewhat
similar to “dendritic effects” reported in the catalysis of metal-
lodendrimers. Indeed, the coupling reaction of organic radicals
has been reported to be involved in the atom-transfer radical
addition catalyzed by nickelated carbosilane dendrimers in the
pioneering work on dendritic effects.41,42 It has been demonstrated
that a proximity effect between the peripheral Ni active sites
can lead to the irreversible formation of inactive sites and the
coupling of two transient radicals. In the present work, we have
demonstrated a new synergetic nano-material that achieves the C–
C bond formation in the combination of a redox and coordination
rich metal complex and a randomly branched polymeric scaffold
without a decrease in catalytic activity on a substrate.
Conclusions
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In the present paper, a new hybrid catalyst composed of a
vitamin B12 derivative and a hyperbranched polymer (HBP) was
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 3035–3042 | 3041
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