t-BuOOt-Bu is transferred to t-BuOꢀ and FeIV, which work as
distinct single electron oxidants for generation of aryl radicals
and oxidation of key carbon radical species, respectively.
This work has been supported financially in part by a
Grant-in-Aid for Scientific Research on Innovative Areas
‘‘Molecular Activation Directed toward Straightforward
Synthesis’’ (23105521 to E.S.) from MEXT. N.U. thanks the
JSPS for a Research Fellowship for Young Scientists. We are
grateful to Ms Yuki Yamamoto, Mr Mitsuru Harada, Dr
Kenji Kitayama, and Mr Ikuo Takahashi (Daicel Chemical
Industries, Ltd.) for ICP analysis.
13 E. Shirakawa, N. Uchiyama and T. Hayashi, J. Org. Chem., 2011,
76, 25.
14 Alkyl radicals are known to be generated by t-BuOꢀ from various
alkylboron compounds such as trialkylboranes, alkylboroxines, and
alkylboronic acid pinacol esters through homolytic substitution on
boron. For an early example, see: D. S. Matteson, J. Org. Chem.,
1964, 29, 3399. Generation of aryl radicals from triarylboranes by
t-BuOꢀ has also been reported. D. Griller, K. U. Ingold,
L. K. Patterson, J. C. Scaiano and R. D. Small, Jr, J. Am. Chem.
Soc., 1979, 101, 3780. Aryl radicals are assꢁumed to be generated by
ꢀ
the reaction of arylboronic acids with SO4 in the silver-catalyzed
oxidative coupling of arylboronic acids with pyridine derivatives: see
ref. 8a.
15 ICP-MS analysis of Fe(OTf)3 (Aldrich Co., product number
708801) used in the reactions showed that the contents of transition
metals (Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, Au) in Fe(OTf)3 are less
than 5 ppm (detection limit) except for Ni (26 ppm) and Cu
(29 ppm). The reaction of 1a with 2o under the conditions of entry
8 of Table 1 using Ni(OTf)2 or Cu(OTf)2 instead of Fe(OTf)3 gave
4ao in o1% or 7% yield, respectively. From these results, we
consider that Fe(OTf)3 is the catalyst that works in the reactions
disclosed here.
Notes and references
1 J. Hassan, M. Sevignon, C. Gozzi, E. Schulz and M. Lemaire,
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Chem. Rev., 2002, 102, 1359; J.-P. Corbet and G. Mignani, Chem.
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2 For recent reviews, see: D. Alberico, M. E. Scott and M. Lautens,
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3 Boronic Acids, ed. D. G. Hall, Wiley-VCH, Weinheim, 2005.
4 For an early example of the coupling of arylboronic acids or their
esters with arenes having a directing group, see: F. Kakiuchi,
S. Kan, K. Igi, N. Chatani and S. Murai, J. Am. Chem. Soc.,
2003, 125, 1698. For an early example of the coupling with
electron-rich arenes and/or heteroarenes, see: S.-D. Yang,
C.-L. Sun, Z. Fang, B.-J. Li, Y.-Z. Li and Z.-J. Shi, Angew. Chem.,
Int. Ed., 2008, 47, 1473.
16 Use of a reduced amount (1 equiv.) of 3 lowered the yield to 44%.
The reaction using H2O2 (in H2O, 30% w/w, 2 equiv.) or O2 (under
an oxygen atmosphere, 1 atm) instead of 3 gave 4ao only in 11% or
o1% yield.
17 The reaction of 1j at 80 1C gave 4jo in 49% yield, where a large
amount (0.43 to 4jo on GC integral) of anisole is formed. Raise of
the reaction temperature lowered the byproduction of anisole
(0.29 to 4jo) to increase the yield of 4jo to 65%. Such hydrolyzed
products were often observed as byproducts, especially in the
reaction of electron-rich arylboronic acids.
¨
5 A. S. Demir, O. Reis and M. Emrullahoglu, J. Org. Chem., 2003,
68, 578; A. S. Demir and H. Findik, Tetrahedron, 2008, 64, 6196;
A. S. Demir, H. Findik, N. Saygili and N. T. Subasi, Tetrahedron,
2010, 66, 1308; S. K. Guchhait, M. Kashyap and S. Saraf,
Synthesis, 2010, 1166.
18 The reaction of phenylboronic acids having a nitro, formyl, or
dimethylamino group at para-position gave the coupling products
only in o40% yields. No coupling product was obtained in the
reaction of butylboronic acid.
6 Reviews on HAS with aryl radicals: R. Bolton and G. H. Williams,
Chem. Soc. Rev., 1986, 15, 261; J. Fossey, D. Lefort and J. Sorba,
Free Radicals in Organic Chemistry, John Wiley and Sons,
Chichester, 1995, ch. 14, pp. 166–180; A. Studer and M. Bossart,
in Radicals in Organic Synthesis, ed. P. Renaud and M. P. Sibi,
Wiley-VCH, Weinheim, 2001, Vol. 2, ch. 1.4, pp. 62–80.
7 Non-radical mechanism is proposed. J. Wen, J. Zhang, S.-Y. Chen,
J. Li and X.-Q. Yu, Angew. Chem., Int. Ed., 2008, 47, 8897.
8 (a) I. B. Seiple, S. Su, R. A. Rodriguez, R. Gianatassio,
Y. Fujiwara, A. L. Sobel and P. S. Baran, J. Am. Chem. Soc.,
2010, 132, 13194. The same oxidation system (AgNO3 and K2S2O8)
has recently been applied to arylation of quinones; (b) Y. Fujiwara,
V. Domingo, I. B. Seiple, R. Gianatassio, M. D. Bel and
P. S. Baran, J. Am. Chem. Soc., 2011, 133, 3292.
19 Competition reaction between benzene (50 equiv.) and benzene-d6
(50 equiv.) with 1a showed KH/KD value of 1.12 (80% yield of 4ao
and 4ao-d5), which is within a standard range of HAS with aryl
radicals. For recent examples, see: E. Shirakawa, K. Itoh,
T. Higashino and T. Hayashi, J. Am. Chem. Soc., 2010,
132, 15537; W. Liu, H. Cao, H. Zhang, H. Zhang, K. H. Chung,
C. He, H. Wang, F. Y. Kwong and A. Lei, J. Am. Chem. Soc.,
2010, 132, 16737. See also ref. 10a.
20 Di-tert-butyl hyponitrite (5) is reported to undergo clean decomposition
into t-BuOꢀ and N2 with t1/2 of 29 min at 65 1C. H. Kiefer and
T. G. Traylor, Tetrahedron Lett., 1966, 7, 6163. For an example of use
of 5 as a source of t-BuOꢀ, which attacks alkylboron compounds to give
alkyl radicals see: A.-P. Schaffner and P. Renaud, Angew. Chem., Int.
Ed., 2003, 42, 2658.
9 FeC2O4ꢂ2H2O or FeCl3ꢂ6H2O as a catalyst in combination with air
as an oxidant is used, where non-radical mechanism is proposed.
J. Wen, S. Qin, L.-F. Ma, L. Dong, J. Zhang, S.-S. Liu, Y.-S. Duan,
S.-Y. Chen, C.-W. Hu and X.-Q. Yu, Org. Lett., 2010, 12, 2694.
10 Iron-catalyzed coupling of aryl halides with benzene derivatives
21 J. I. G. Cadogan, D. H. Hey and P. G. Hibbert, J. Chem. Soc.,
1965, 3939.
22 Direct oxidation by the O–O bond of peroxides is usual for FeII
complex, and used as a reliable method to obtain oxy radicals. For
reviews, see: A. Arnone, M. Cecere, R. Galli, F. Minisci,
M. Perchinunno, O. Porta and G. Gardini, Gazz. Chim. Ital.,
1973, 103, 13; F. Minisci, E. Vismara and F. Fontana, Heterocycles,
1989, 28, 489. For a dialkyl peroxide: D. H. R. Barton and
F. Launay, Tetrahedron, 1997, 53, 14565.
through HAS has been reported. (a) F. Vallee, J. J. Mousseau and
´
A. B. Charette, J. Am. Chem. Soc., 2010, 132, 1514; (b) W. Liu,
H. Cao and A. Lei, Angew. Chem., Int. Ed., 2010, 49, 2004. For
iron-catalyzed oxidative coupling of arylzinc compounds with
arenes having
a
directing group, see: (c) J. Norinder,
23 Upon treatment with Fe(OTf)3–L2 (1 equiv. each) in CHCl3 at
60 1C for 1 h, 73% of tert-butyl methyl ether was consumed, most
likely through cleavage of the t-Bu–OMe bond.
A. Matsumoto, N. Yoshikai and E. Nakamura, J. Am. Chem.
Soc., 2008, 130, 5858; (d) N. Yoshikai, A. Matsumoto, J. Norinder
and E. Nakamura, Synlett, 2010, 313. For iron-catalyzed
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X.-Y. Lu, S. Lin and Z.-J. Shi, Angew. Chem., Int. Ed., 2009,
48, 3817; (f) H. Egami, K. Matsumoto, T. Oguma, T. Kunisu and
T. Katsuki, J. Am. Chem. Soc., 2010, 132, 13633.
24 FeIII complexes having an Fe–O–Ot-Bu structure are known to
readily undergo homolysis at the O–O bond. For examples, see:
P. A. MacFaul, K. U. Ingold, D. D. M. Wayner and L. Que, Jr.,
J. Am. Chem. Soc., 1997, 119, 10594; J.-U. Rohde, S. Torelli,
X. Shan, M. H. Lim, E. J. Klinker, J. Kaizer, K. Chen, W. Nam
and L. Que, Jr., J. Am. Chem. Soc., 2004, 126, 16750.
11 For iron-catalyzed reactions of arylboronic acids with aldehydes,
see: T. Zou, S.-S. Pi and J.-H. Li, Org. Lett., 2009, 11, 453;
J. N. Rosa, R. S. Reddy, N. R. Candeias, P. M. S. D. Cal and
P. M. P. Gois, Org. Lett., 2010, 12, 2686.
12 For recent reviews on iron catalysis, see: C. Bolm, J. Legros, J. Le
Paih and L. Zani, Chem. Rev., 2004, 104, 6217; Iron Catalysis in
Organic Chemistry, ed. B. Plietker, Wiley-VCH, Weinheim, 2008;
C.-L. Sun, B.-J. Li and Z.-J. Shi, Chem. Rev., 2011, 111, 1293.
25 The reaction of 3 with Fe(OTf)3–L2 in the absence of 1a proceeded
in a rate (60 1C, 5 h, 97% conv.) comparable to that in its presence,
and addition of 1a to the reaction mixture after full consumption
of 3 (8 h) followed by further reaction for 5 h gave no 6. These
results are consistent with the mechanism shown in eqn (1), where
1a does not deal with generation of t-BuOꢀ but has to be present in
the reaction mixture to utilize short-lived t-BuOꢀ.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11671–11673 11673