Rare-earth-catalyzed C-H bond addition of pyridines to olefins

Article abstract of DOI:10.1021/ja208129t

An efficient and general protocol for the ortho-alkylation of pyridines via C-H addition to olefins has been developed, using cationic half-sandwich rare-earth catalysts, which provides an atom-economical method for the synthesis of alkylated pyridine der

Full text of DOI:10.1021/ja208129t

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pubs.acs.org/JACS
Rare-Earth-Catalyzed CꢀH Bond Addition of Pyridines to Olefins
Bing-Tao Guan and Zhaomin Hou*
Organometallic Chemistry Laboratory and Advanced Catalyst Research Team, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako,
Saitama 351-0198, Japan
S Supporting Information
b
and copolymerization of a variety of olefins.12,13 However, the
ABSTRACT: An efficient and general protocol for the
use of the cationic rare-earth alkyls in the CꢀH addition of
pyridines to olefins has remained unexplored to date.14 Herein,
ortho-alkylation of pyridines via CꢀH addition to olefins has
been developed, using cationic half-sandwich rare-earth
we report the first highly efficient, ortho-selective CꢀH addition
catalysts, which provides an atom-economical method for
the synthesis of alkylated pyridine derivatives. A wide range
of pyridine and olefin substrates including α-olefins, sty-
renes, and conjugated dienes are compatible with the
catalysts.
of pyridines to olefins catalyzed by cationic half-sandwich rare-
earth alkyl complexes. The selectivity and functional group toler-
ance of the present rare-earth catalysts are complementary to
those of late transition metal catalysts. A wide range of pyridine
and olefin substrates, including α-olefins, styrenes, and conju-
gated dienes, are applicable to afford a new family of function-
alized pyridine derivatives.
On the basis of catalyst screening for the reaction of α-picoline
with norbornene (see Supporting Information, Table S1), we
first chose thecombinationofthehalf-sandwichscandiumbis(benzyl)
yridine moieties are among the most important heterocyclic
P
structural motifs and exist widely in a large number of natural
products, pharmaceuticals, ligands, and functional materials.1
Therefore, the development of efficient, atom-economical pro-
cesses for the synthesis of pyridine-containing compounds through
direct CꢀH functionalization of pyridine has received intensive
attention.2ꢀ6 Among possible approaches to the functionaliza-
tion of pyridines, catalytic CꢀH bond addition to olefins is the
most atom-economical way for the synthesis of alkylated pyridine
derivatives. However, direct CꢀH alkylation of pyridines has met
with limited success to date, partly because of the low activity of
pyridyl species and easy β-H elimination of transition metal alkyl
species.
Pioneering work reported in 1989 by Jordan and co-workers
showed that cationic zirconium metallocenes, which are active
olefin polymerization catalysts, could effect the catalytic CꢀH
addition of α-picoline to propylene in the presence of H2.7
Recently, Bergman and Ellman reported that Rh(I)ꢀphosphine
complexes could catalyze the ortho-alkylation of pyridines at high
temperatures (165 °C).8 Nakao and Hiyama found that, in the
presence of Lewis acids, N-heterocyclic carbene nickel com-
plexes catalyzed the alkylation of pyridines with unprecedented
C-4-selectivity.9 In most of these reactions, aliphatic alkenes were
applicable, but styrenes and conjugated dienes seemed not suitable
for the catalytic transformation.10
Various rare-earth alkyl complexes have been reported pre-
viously to undergo ortho-metalation of pyridine through CꢀH
bond activation.11 However, reports on the insertion of olefins
into a rare-earth pyridyl bond remain scarce.11d,f Teuben and co-
workers reported in 1994 that yttrium metallocene complexes
catalyzed the ethylation of pyridine under high temperature
(110 °C) and high ethylene pressure (40 bar).11d This is the
only precedent of rare-earth-catalyzed alkylation of pyridine.
Unfortunately, this catalyst was active only for ethylene, whereas
the insertion of a higher olefin was much slower and could not be
achieved catalytically.
13e
complex (C5Me5)Sc(CH2C6H4NMe2-o)2 and B(C6F5)3 as
catalyst to examine the ethylation of various pyridine derivatives.
Some representative results are summarized in Table 1. The
o-CꢀH ethylation of α-picoline took place easily under 3 atm of
ethylene at 70 °C to give 2-methyl-6-ethylpyridine (3aa) almost
quantitatively (entry 1). Ethylene polymerization did not take
place until α-picoline was completely consumed. Ethylation of
2-ethylpyridine and 2-isopropylpyridine also took place selec-
tively (entries 2 and 3), while reaction of the bulkier 2-tert-butyl-
pyridine with ethylene yielded a mixture of oligomers under the
same conditions, possibly owing to the steric hindrance of the
t-Bu group, which could retard the abstraction of hydrogen from
2-t-Bu-pyridine by a 2-t-Bu-pyridylethylꢀSc species and thus lead
to continuous insertion of ethylene into the pyridylethylꢀSc bond.
However, when the yttrium complex (C5Me5)Y(CH2C6H4NMe2-o)2
was used in place of the Sc analogue, the ethylation reaction of
2-t-Bu-pyridine took place selectively and quantitatively (entry 4),
probably because the larger Y ion in the pyridylethyl yttrium
intermediate could promote more efficiently the coordination
and subsequent deprotonation of 2-t-Bu-pyridine rather than the
insertion of another molecule of ethylene.
Tetrahydroquinoline (1e), 2,3-cyclopentenopyridine (1f),
quinoline (1g), and 2-phenylpyridine (1h) could also be o-ethy-
lated selectively by the scandium catalyst (Table 1, entries 5ꢀ8).
It is noteworthy that the ethylation of 2-phenylpyridine took
place selectively at the ortho position of the pyridine unit (rather
than the phenyl group) (entry 8), in contrast with late transition
metal-catalyzed reactions, in which the pyridine unit usually
serves as a directing group to lead the CꢀH activation taking
place at the phenyl ring.15 Remarkably, 2-bromopyridine (1i)
and 2-iodopyridine (1j) could also undergo selective CꢀH
ethylation to give the corresponding Br- and I-containing
We recently demonstrated that cationic half-sandwich rare-earth
alkyl complexes can serve as excellent catalysts for the polymerization
Received: August 29, 2011
Published: October 14, 2011
r
2011 American Chemical Society
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Table 1. Catalytic CꢀH Addition of Pyridine Derivatives to
Table 2. Catalytic CꢀH Addition of 2-Ethylpyridine to
Various Olefinsa
Ethylenea
a Reactions were carried out with 1 mmol of pyridine in 3 mL of toluene
under 3 atm of ethylene, unless otherwise noted. b Isolated yield. c Yield
obtained by converting the product to its hydrochloride salt. d C5Me5Y-
(CH2C6H4NMe2-o)2 was used. e 0.5 mmol of pyridine 1f, 4% catalyst.
f 0.5 mmol of quinoline 1g, 8% catalyst.
ethylpyridine derivatives (entries 9, 10); no dehalogenation was
observed.
To examine the scope of olefin substrates, 2-ethylpyridine was
then used to react with various olefins (Table 2). The o-CꢀH
addition of 2-ethylpyridine to norbornene took place smoothly
in the presence of 2 mol % of the Sc catalyst to afford the
corresponding alkylation product 3bb quantitatively (entry 1). In
the case of dicyclopentadiene (2c), the reaction took place
selectively at the norbornene unit, leaving the cyclopentene unit
unchanged.16 Two regioisomer products (3bc and 3bc0) due to
the asymmetry of the norbornene unit in dicyclopentadiene were
obtained in a ca. 2:1 ratio (entry 2). In the alkylation with
α-olefins such as 1-hexene and 1-octene, branched isomer
products were obtained exclusively (entries 3 and 4), in contrast
with late transition metal-catalyzed reactions, which always gave
the linear isomers as a predominant product. 8a,9
The scandium catalyst showed rather poor activity for the
reaction of 2-ethylpyridine with styrene, although it was ex-
tremely active for the polymerization of styrene in the absence of
a pyridine compound.13k However, the larger yttrium catalyst,
though less active for the polymerization of styrene,13a showed
excellent catalytic activity for o-alkylation of 2-ethylpyridine with
styrene to give selectively the corresponding linear product (Table 2,
entry 5). This reaction is also in contrast with the Ni-catalyzed
a Reactions were carried out with 0.5 mmol of pyridine and 1 mmol of
an olefin in 2 mL of toluene, unless otherwise noted. b Isolated yield.
c 1 mmol pyridine, 1.5 mmol olefin and 2% catalyst. d 1 mL toluene and
1 mL olefin. e 3bc/3bc0 = 2:1, determined by 1H NMR. f 3bm/3bm0 =
1.2:1, determined by 1H NMR. A small amount of 3,4-isoprene insertion
products was also observed.
alkylation of pyridine with styrene, in which the branched isomer
was formed as a major product and CꢀC bond formation took
place at the C-4 position of pyridine.9 Alkyl-, fluoro-, chloro-, and
methoxy-substituted styrenes are also compatible with the yttrium
catalyst to afford selectively the corresponding linear alkylation
products (entries 6ꢀ10). To the best of our knowledge, this is
the first example of catalytic o-selective alkylation of a pyridine
compound with styrenes.10
The yttrium catalyst is also suitable for CꢀH addition of
2-ethylpyridine to 1,3-cyclohexadiene to afford the correspond-
ing allylated pyridine product 3bl (entry 11). In the case of
isoprene, both mono- and double-insertion products were ob-
tained, even when an equimolar amount of isoprene was used
(entry 12). These reactions represent the first example of catalytic
CꢀH addition of a pyridine compound to a conjugated diene.10
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Scheme 1. Mono- and Dialkylation of Pyridine with
Norbornene
Scheme 3. A Possible Mechanism for Catalytic CꢀH Addi-
tion of Pyridines to Olefins
Scheme 2. CꢀH vs CꢀD Addition of Pyridine to Norbor-
nene: Kinetic Isotope Effect
pyridines to a variety of olefins such as 1-alkenes, styrenes, and 1,3-
conjugated dienes to afford straightforwardly a series of alkylated
pyridine derivatives in an atom-economical way. The present rare-
earth catalysts are complementary to late transition metal catalysts in
terms of selectivity, functional group tolerance, and substrate scope.
Further studies on rare-earth-catalyzed CꢀH functionalizations
with other substrates are in progress.
’ ASSOCIATED CONTENT
Similar to Zr catalysts,7a no alkylation was observed in the
reaction of pyridine with ethylene and styrene, probably because
of the formation of a relatively stable pyridine-coordinated metal
species. However, the reaction between pyridine and norbornene
took place smoothly in the presence of scandium catalyst
(Scheme 1). Double o-alkylation of pyridine could also be
achieved after most of the pyridine was consumed, but no dialkyla-
tion product was observed at the early stage. If the reaction was
quenched after the first 27 h, the monoalkylation product 3kb
was isolated in 79% yield. Further alkylation of the monoalkyla-
tion product took place rapidly to afford the dialkylation product
3kbb in 99% isolated yield in ca. 3 h (Scheme 1).
S
Supporting Information. Detailed experimental proce-
b
dures and characterization data for all new compounds. This
material is available free of charge via the Internet at http://pubs.
acs.org.
’ AUTHOR INFORMATION
Corresponding Author
houz@riken.jp
’ ACKNOWLEDGMENT
The reaction of a 1:1 mixture of pyridine and pyridine-d5 with
norbornene and the reaction of 2-D-pyridine with norbornene
showed significant kinetic isotope effects (kH/kD = 4.9 and 4.0,
respectively, Scheme 2), suggesting that CꢀH bond activation
(deprotonation) could be involved in the rate-determining step
of the present catalytic reaction.
This work was partly supported by a Grant-in-Aid for Scien-
tific Research (S) (21225004) from JSPS and the Key Project of
International Cooperation of NSFC (20920102030). Dr. Jia-Rui
Wang and Dr. Hao Zhang are gratefully appreciated for con-
ducting initial studies of the related work.
On the basis of the observations described above and reported
previously,7,11d,f,13a,h,i,m a possible mechanism for the present
catalytic alkylation of pyridines can be proposed as shown in
Scheme 3. Coordination of a pyridine compound 1 to the metal
center of the cationic alkyl species A, generated from its neutral
dialkyl precursor and an activator such as B(C6F5)3, would
promote o-CꢀH activation (deprotonation) of 1 to give pyridyl
species B.17 The 2,1-insertion of a 1-alkene into the me-
talꢀpyridyl bond in B would be sterically favored to afford C,
which on subsequent deprotonation of another molecule of
pyridine 1 should yield the branched alkylation product 3 and
regenerate B. In the case of styrene, 1,2-insertion would be
preferred because of possible formation of a stable benzallylic
species such as D, which after protonation with a pyridine
compound should give the linear alkylation product 30.
’ REFERENCES
(1) (a) Michael, J. P. Nat. Prod. Rep. 2005, 22, 627–646. (b) Henry,
G. D. Tetrahedron 2004, 60, 6043–6061. (c) Bagley, M. C.; Glover, C.;
Merritt, E. A. Synlett 2007, 2459–2482. (d) Arena, C. G.; Arico, G. Curr.
Org. Chem. 2010, 14, 546–580. (e) Bianchini, C.; Giambastiani, G.;
Luconi, L.; Meli, A. Coord. Chem. Rev. 2010, 254, 431–455. (f) Holliday,
B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 2022–2043. (g)
Kurth, D. G.; Higuchi, M. Soft Matter 2006, 2, 915–927.
(2) A recent review: Nakao, Y. Synthesis 2011, 20, 3209–3219.
(3) For examples of borylation, see: (a) Cho, J.; Iverson, C. N.;
Smith, M. R. J. Am. Chem. Soc. 2000, 122, 12868–12869. (b) Takagi, J.;
Sato, K.; Hartwig, J. F.; Ishiyama, T.; Miyaura, N. Tetrahedron Lett. 2002,
43, 5649–5651. (c) Mkhalid, I. A. I.; Coventry, D. N.; Albesa-Jove, D.;
Batsanov, A. S.; Howard, J. A. K.; Perutz, R. N.; Marder, T. B. Angew.
Chem., Int. Ed. 2006, 45, 489–491. (d) Murphy, J. M.; Liao, X.; Hartwig,
J. F. J. Am. Chem. Soc. 2007, 129, 15434–15435. (e) Fischer, D. F.;
Sarpong, R. J. Am. Chem. Soc. 2010, 132, 5926–5927. (f) Hurst, T. E.;
Macklin, T. K.; Becker, M.; Hartmann, E.; Gel, W. K.; Salle, J. P.;
Batsanov, A. S.; Marder, T. B.; Snieckus, V. Chem.—Eur. J. 2010,
16, 8155–8161.
In summary, we have demonstrated that half-sandwich rare-
earth dialkyl complexes such as (C5Me5)Ln(CH2C6H4NMe2-o)2
(Ln = Sc, Y) in combination with an activator such as B(C6F5)3 can
serve as an excellent catalyst for ortho-selective CꢀH addition of
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(4) For examples of arylation, see: (a) Godula, K.; Sezen, B.; Sames,
D. J. Am. Chem. Soc. 2005, 127, 3648–3649. (b) Berman, A. M.; Lewis,
J. C.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2008, 130,
14926–14927. (c) Tobisu, M.; Hyodo, I.; Chatani, N. J. Am. Chem. Soc.
2009, 131, 12070–12071. (d) Yanagisawa, S.; Ueda, K.; Taniguchi, T.;
Itami, K. Org. Lett. 2008, 10, 4673–4676. (e) Kobayashi, O.; Uraguchi,
D.; Yamakawa, T. Org. Lett. 2009, 11, 2679–2682. (f) Li, M.; Hua, R.
Tetrahedron Lett. 2009, 50, 1478–1481. (g) Seiple, I. B.; Su, S.;
Rodriguez, R. A.; Gianatassio, R.; Fujiwara, Y.; Sobel, A. L.; Baran,
P. S. J. Am. Chem. Soc. 2010, 132, 13194–13196.
(5) For examples of olefination, see: (a) Murakami, M.; Hori, S.
J. Am. Chem. Soc. 2003, 125, 4720–4721. (b) Nakao, Y.; Kanyiva, K. S.;
Hiyama, T. J. Am. Chem. Soc. 2008, 130, 2448–2449. (c) Tsai, C.; Shih,
W.; Fang, C.; Li, C.; Ong, T.; Yap, G. P. A. J. Am. Chem. Soc. 2010,
132, 11887–11889. (d) Ye, M.; Gao, G.; Yu, J. J. Am. Chem. Soc. 2011,
133, 6964–6967.
(6) For some other examples, see: (a) Moore, E. J.; Pretzer, W. R.;
O’Connell, T. J.; Harris, J.; LaBounty, L.; Chou, L.; Grimmer, S. S. J. Am.
Chem. Soc. 1992, 114, 5888–5890. (b) Kawashima, T.; Takao, T.;
Suzuki, H. J. Am. Chem. Soc. 2007, 129, 11006–11007. (c) Beveridge,
R. E.; Arndtsen, B. A. Synthesis 2010, 1000–1008. (d) Tran, L. D.;
Daugulis, O. Org. Lett. 2010, 12, 4277–4279. (e) Li, B.; Shi, Z. Chem. Sci.
2011, 2, 488–493. (f) Correia, C. A.; Yang, L.; Li, C. Org. Lett. 2011,
13, 4581–4583.
(7) (a) Jordan, R. F.; Taylor, D. F. J. Am. Chem. Soc. 1989,
111, 778–779. (b) Jordan, R. F.; Taylor, D. F.; Baenziger, N. C.
Organometallics 1990, 9, 1546–1557. (c) Rodewald, S.; Jordan, R. F.
J. Am. Chem. Soc. 1994, 116, 4491–4492. (d) Guram, A. S.; Jordan, R. F.
Organometallics 1991, 10, 3470–3479.
F.; Nishiura, M.; Koshino, H.; Hou, Z. Macromolecules 2011, 44,
6335–6344. (l) Guo, F.; Nishiura, M.; Koshino, H.; Hou, Z. Macro-
molecules 2011, 44, 2400–2403. (m) Luo, Y.; Luo, Y.; Qu, J.; Hou, Z.
Organometallics 2011, 30, 2908–2919. (n) Pan, L.; Zhang, K.; Nishiura,
M.; Hou, Z. Angew. Chem., Int. Ed. 2011 in press. DOI: 10.1002/
anie.201104011.
(14) For stoichiometric o-metalation of pyridine by cationic rare-
earth alkyls, see refs 11iꢀ11k. For examples of the use of cationic rare-
earth catalysts in organic synthesis, see: (a) Lauterwasser, F.; Hayes,
P. G.; Br€ase, S.; Piers, W. E.; Schafer, L. L. Organometallics 2004,
23, 2234–2237. (b) Tazelaar, C. G. J.; Bambirra, S.; van Leusen, D.;
Meetsma, A.; Hessen, B.; Teuben, J. H. Organometallics 2004, 23,
936–939. (c) Ge, S.; Meetsma, A.; Hessen, B. Organometallics 2009,
28, 719–726. (d) Takimoto, M.; Usami, S.; Hou, Z. J. Am. Chem. Soc.
2009, 131, 18266–18268.
(15) Similar regioselectivity was observed in the Zr-mediated stoi-
chiometric reaction of 2-phenylpyridine with propylene; see ref 7c. For
examples of the use of a pyridine unit as a directing group in late
transition metal-catalyzed aromatic CꢀH activation, see: (a) Lim, Y.;
Kim, Y. H.; Kang, J. J. Chem. Soc., Chem. Commun. 1994, 2267–2268. (b)
Hull, K. L.; Anani, W. Q.; Sanford, M. S. J. Am. Chem. Soc. 2006,
128, 7134–7135. (c) Tsai, A. S.; Tauchert, M. E.; Bergman, R. G.;
Ellman, J. A. J. Am. Chem. Soc. 2011, 133, 1248–1250.
(16) No reaction between 2-ethylpyridine and cyclopentene or
cyclohexene was observed under the same conditions. For the Rh-
catalyzed alkylation of quinoline with cyclohexene, see ref 8a. For
stoichiometric addition of Zr-pyridyl species to 2-butene, see ref 7c.
(17) Alkylation of 2-Me-6-D-pyridine with ethylene or styrene in the
presence of (C5Me5)Ln(CH2C6H4NMe2-o)2 (Ln = Sc or Y)/B(C6F5)3
in toluene at 70 °C gave the corresponding o-alkylation products with
partial deuteration at both the o-methyl group and the β-carbon atom. In
the absence of an olefin, H/D exchange between the methyl CꢀH and
the pyridyl CꢀD units was observed. This is under further investigation
and will be reported in due course.
(8) (a) Lewis, J. C.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc.
2007, 129, 5332–5333. (b) Yotphan, S.; Bergman, R. G.; Ellman, J. A.
Org. Lett. 2010, 12, 2978–2981.
(9) Nakao, Y.; Yamada, Y.; Kashihara, N.; Hiyama, T. J. Am. Chem.
Soc. 2010, 132, 13666–13668.
(10) Stoichiometric addition of zirconiumꢀpyridyl species to sty-
rene and conjugated dienes was reported; see refs 7c and 7d. Ni-catalyzed
C-4-alkylation of pyridine with styrene was reported recently; see ref 9.
(11) Selected examples: (a) Watson, P. L. J. Chem. Soc., Chem.
Commun. 1983, 276–277. (b) 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. (c) Den Haan, K. H.;
Wielstra, Y.; Teuben, J. H. Organometallics 1987, 6, 2053–2060. (d)
Deelman, B.; Stevels, W. M.; Teuben, J. H.; Lakin, M. T.; Spek, A. L.
Organometallics 1994, 13, 3881–3891. (e) Duchateau, R.; van Wee,
C. T.; Teuben, J. H. Organometallics 1996, 15, 2291–2302. (f) Duchateau,
R.; Brussee, E. A. C.; Meetsma, A.; Teuben, J. H. Organometallics 1997,
16, 5506–5516. (g) Jantunen, K. C.; Scott, B. L.; Gordon, J. C.;
Kiplinger, J. L. Organometallics 2007, 26, 2777–2781. (h) Carver,
C. T.; Diaconescu, P. L. J. Am. Chem. Soc. 2008, 130, 7558–7559.
(i) Arndt, S.; Elvidge, B. R.; Zeimentz, P. M.; Spaniol, T. P.; Okuda, J.
Organometallics 2006, 25, 793–795. (j) Yahia, A.; Kramer, M. U.; Okuda,
J.; Maron, L. J. Organomet. Chem. 2010, 695, 2789–2793. (k) Kramer,
M. U.; Yahia, A.; Maron, L.; Okuda, J. C. R. Chim. 2010, 13, 626–632.
(12) Recent reviews: (a) Hou, Z. M.; Luo, Y. J.; Li, X. F. J. Organomet.
Chem. 2006, 691, 3114–3121. (b) Nishiura, M.; Hou, Z. M. Nature
Chem. 2010, 2, 257–268.
(13) For examples, see: (a) Luo, Y.; Baldamus, J.; Hou, Z. J. Am.
Chem. Soc. 2004, 126, 13910–13911. (b) Zhang, L.; Luo, Y.; Hou, Z.
J. Am. Chem. Soc. 2005, 127, 14562–14563. (c) Li, X.; Hou, Z. Macro-
molecules 2005, 38, 6767–6769. (d) Li, X. F.; Baldamus, J.; Nishiura, M.;
Tardif, O.; Hou, Z. M. Angew. Chem., Int. Ed. 2006, 45, 8184–8188. (e)
Li, X. F.; Nishiura, M.; Mori, K.; Mashiko, T.; Hou, Z. M. Chem.
Commun. 2007, 4137–4139. (f) Yu, N.; Nishiura, M.; Li, X. F.; Xi, Z. F.;
Hou, Z. M. Chem.—Asian J. 2008, 3, 1406–1414. (g) Zhang, H.; Luo, Y.;
Hou, Z. Macromolecules 2008, 41, 1064–1066. (h) Li, X.; Nishiura, M.;
Hu, L.; Mori, K.; Hou, Z. J. Am. Chem. Soc. 2009, 131, 13870–13882. (i)
Li, X.; Hou, Z. Macromolecules 2010, 43, 8904–8909. (j) Pan, L.; Zhang,
K.; Nishiura, M.; Hou, Z. Macromolecules 2010, 43, 9591–9593. (k) Guo,
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