Chiral copper(II)-catalyzed enantioselective boron conjugate additions to α,β-unsaturated carbonyl compounds in water

Article abstract of DOI:10.1002/anie.201207343

Copper pins on the boron: The enantioselective 1,4-addition of diboron to α,β-unsaturated compounds proceeds smoothly in the presence of catalytic amounts of Cu(OH)₂ and chiral 2,2′-bipyridine ligand in water. A wide substrate scope of α,β-unsaturated carbonyl compounds, including acyclic, cyclic, and β,β-disubstituted enones, α,β-unsaturated esters, amides, and a nitrile, has been shown. Copyright

Full text of DOI:10.1002/anie.201207343

Angewandte
Chemie
DOI: 10.1002/anie.201207343
Asymmetric Catalysis
Chiral Copper(II)-Catalyzed Enantioselective Boron Conjugate
Additions to a,b-Unsaturated Carbonyl Compounds in Water**
Shu¯ Kobayashi,* Pengyu Xu, Toshimitsu Endo, Masaharu Ueno, and Taku Kitanosono
Organic reactions are usually carried out in organic solvents
in modern organic chemistry, and it is very rare to use water as
the reaction medium despite water being safe, benign,
environmentally friendly, and inexpensive compared with
organic solvents.[1] In organic reactions in aqueous media,
there are two major obstacles to be surmounted. First, many
reactive substrates, reagents, and catalysts are decomposed or
deactivated by water. Second, most organic substances are
insoluble in water. On the other hand, we have investigated
organic reactions in water from the standpoint that the most
ideal reactions, enzymatic reactions in vivo, are carried out in
water, and have found unique reactivity and selectivity in
aqueous media, which are not observed in organic solvents
where water plays key roles.[2]
were shown to be suitable and stable catalysts in aqueous
media, and that exchange processes from boron to the second
elements were crucial to promote the reactions. Based on
a hint obtained from this work, we assumed a similar
exchange process from the B atom of bis(pinacolato)diboron
(B2(pin)2) to a second element. Herein, we report enantiose-
lective boron conjugate additions to various a,b-unsaturated
carbonyl compounds and nitrile in water catalyzed by
Cu(OH)2 with a chiral ligand. Very rare chiral CuII catalysis
in water is also described.[11,12]
First, we conducted the reaction of B2(pin)2 with chalcone
in water in the presence of several promising metal hydrox-
ides that were expected to work well in water. It was found
that some metal hydroxides worked well with ligands. For
example, when Cu(OH)2 was combined with dibenzylamine
(DBA), the desired 1,4-addition product 1a was obtained in
88% yield (Table 1, entry 1). Zn(OH)2 showed inferior
reactivity (entry 2). We then investigated several chiral
ligands in an attempt to produce enantioselective reactions.
Asymmetric catalysis in water is extremely difficult to achieve
because many chiral catalysts decompose rapidly in the
presence of water.[13] We have already investigated water-
compatible Lewis acidic metals (cations), in which CuII and
ZnII are involved.[14] We have also demonstrated that combi-
nations of CuII and ZnII with chiral 2,2’-bipyridine ligand L1[15]
were effective for enantioselective ring-opening reactions of
meso-epoxides[12] and allylation reactions of aldehydes,[10b,d]
respectively, in aqueous media. It was found that Cu(OH)2-L1
showed very promising results; namely, the desired addition
product was obtained in 83% yield with a 90.5:9.5 enantio-
meric ratio (e.r.) in the presence of Cu(OH)2 (10 mol%) with
L1 (12 mol%) at RT in water for 12 h (entry 3). Whereas
Zn(OH)2-L1 showed lower yield and enantioselectivity
(entry 4), combinations of Cu(OH)2 with chiral ligands L2
and L3 gave high yields with moderate enantioselectivities
(entries 5 and 6). The use of water as a solvent is essential for
reactivity and selectivity; the reactions did not proceed at all
or proceeded very sluggishly in typical organic solvents such
as THF, toluene, CH2Cl2, DMF, DMSO, MeOH, and EtOH
(entries 7–13). With 5 mol% catalyst loading, the enantiose-
lectivity decreased slightly (entry 14). We then examined
several reaction conditions to further improve the yield and
the enantioselectivity, and found that some additives were
effective for that purpose. Among them, the use of acetic acid
(entry 16) or trifluoroacetic acid (entry 17) were found to be
the most effective to improve both yield and enantioselectiv-
ity. Boronic acid was also effective as an additive (entry 19).
Finally, the desired product was obtained in 95% yield with
99.5:0.5 e.r. when the reaction was conducted at 58C
(entry 21). It was also confirmed that the product was isolated
a-Chiral boron derivatives are an important class of
À
compounds, because the C B linkage can be transferred into
À
À
À
C O, C N, and C C bonds through 1,2-migration of inter-
mediary ate complexes with appropriate nucleophiles, with
retention of stereogenic centers.[3] Enantioselective boron
conjugate addition to a,b-unsaturated carbonyl and related
compounds provides one of the most efficient routes to a-
chiral boron compounds. Based on seminal reports of CuI-
catalyzed conjugate borylation,[4] Yun et al. reported an
enantioselective method for CuI-catalyzed conjugate boryla-
tion.[5] After that, several catalytic asymmetric borylations
using CuI with chiral ligands were reported.[6] Furthermore,
other metal-catalyzed[7] and metal-free[8] enantioselective
boron conjugate additions to a,b-unsaturated carbonyl and
related compounds have also been reported. In all of those
cases, the reactions were carried out in organic solvents, and
to the best of our knowledge, there has been no report of this
conjugate addition in water.[9]
In previous papers, we have shown that addition reactions
of allylboronates and allenylboronates to aldehydes and
acylhydrazones proceeded smoothly in the presence of
metal hydroxides such as Zn(OH)2, Cu(OH)2, Bi(OH)3, and
others in aqueous media.[10] In these reactions, metal hydrox-
ides that had not been used as catalysts in organic synthesis
[*] Prof. Dr. S. Kobayashi, P. Xu, T. Endo, Dr. M. Ueno
Department of Chemistry, School of Science
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: shu_kobayshi@chem.s.u-tokyo.ac.jp
[**] This work was partially supported by a Grant-in-Aid for Science
Research from the Japan Society for the Promotion of Science
(JSPS), the Global COE Program, the University of Tokyo, MEXT
(Japan), and NEDO.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207343.
Angew. Chem. Int. Ed. 2012, 51, 12763 –12766
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12763

.
Angewandte
Communications
Table 1: Optimization of reaction conditions.[a]
Table 2: Substrate scope.
Entry
M
Solvent
Additive
Ligand
Yield [%]
e.r.
Entry
R1
R2
R3
Yield [%]
e.r.
1
2
3
4
5
6
7
8
Cu
Zn
Cu
Zn
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
H2O
H2O
H2O
H2O
H2O
–
–
–
–
–
–
–
–
–
–
–
–
–
–
DBA
DBA
L1
L1
L2
L3
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
88
64
83
17
79
–
–
1
2
3
4
5
6
7
8
Ph
4-FC6H4
4-FC6H4
4-MeOC6H4
Ph
Ph
Ph
Ph
Ph
Me
Ph
Ph
2-ClC6H4
Ph
4-ClC6H4
4-MeOC6H4
H
H
H
H
H
H
H
H
H
H
95
91
90
90
90
95
98
91
87
88
quant.
87
91
85
73
80
76
80
85
99.5:0.5
94.5:5.5
98:2
96:4
94:6
94.5:5.5
91:9
94:6
95.5:4.5
90:10
91:9
98.5:1.5
96.5:3.5
91:9
90.5:9.5
73:27
68:32
68.5:31.5
–
H2O
THF
80
NR
NR
NR
trace
NR
17
1
84
72
93
Me
iPr
tBu
Ph
À
toluene
CH2Cl2
DMF
DMSO
MeOH
EtOH
H2O
H2O
H2O
H2O
H2O
–
–
–
–
9
9
10
11
12
13
14
15
16
17
18
19
20
21[b]
10
11
12[a]
13[a]
14[b]
15[b]
16
17
18
19
À
(CH2)3
Ph
64.5:35.5
–
90:10
85:15
94.5:5.5
93:7
90.5:9.5
93.5:6.5
90.5:9.5
99.5:0.5
Ph
Me
Me
Me
H
H
H
Ph
Ph
Ph
Me
Ph
MeO
EtO
EtO
NMe2
NEt2
pyridine
AcOH
TFA
PhCO2H
H3BO3
AcOK
AcOH
91:9
93:7
95.5:4.5
94.5:5.5
90.5:9.5
93
86
94
90
H
H
Ph
H2O
H2O
H2O
=
NCCH CHPh
95
[a] B2(pin)2 (1.6 equiv) was used. [b] Without AcOH.
[a] For entries 1–13: M(OH)2 (10 mol%); for entries 14–21 M(OH)2
(5 mol%). [b] The reaction was conducted at 58C. Ac=acetyl, DBA=
dibenzylamine, DMF=dimethylformamide, DMSO=dimethylsulfoxide,
NR=no reaction, Pin=pinacol, TFA=trifluoroacetic acid.
(entries 11–13). It should be noted that optically active
tertiary alcohols were synthesized with excellent ee values
in the latter two cases (entries 12 and 13). a,b-Unsaturated
esters (entries 14–16) and amides (entries 17 and 18) are also
good substrates, and the desired addition products were
obtained in high yields with high enantioselectivities. More-
over, cinnamyl nitrile reacted with B2(pin)2 under these
conditions to afford the corresponding boron product in high
yield and with high enantioselectivity (entry 19). It should be
noted that a single chiral CuII catalyst can be applicable to
a wide variety of a,b-unsaturated carbonyl compounds and
nitrile in this asymmetric boron conjugate addition reaction;
to the best of our knowledge, this is the first example of such
a case. Furthermore, the experimental procedure is very
simple: substrates are combined in the presence of inex-
pensive and readily available Cu(OH)2 and chiral bipyridine
ligand L1 (and sometimes acetic acid) in water.
as an a-chiral boron derivative 1a, which was quantitatively
converted to b-hydroxy ketone 2a by treatment with NaBO3
(Scheme 1). It was found to be more convenient for deter-
mining the enantioselectivity of 2a by HPLC analysis on
a chiral stationary phase.
In addition to the use of water as a solvent, CuII catalysis is
very rare in the boron conjugate addition to a,b-unsaturated
carbonyl compounds.[9,16] Indeed, 1,4-addition of diboron to
a,b-unsaturated carbonyl compounds has been catalyzed by
CuI in organic solvents in many cases.[7,8] On the other hand,
whereas Cu(OH)2 was found to be very effective in water, the
activity of CuI is not clear, because CuI species are known to
be unstable in water.[17] Based on the experimental results, we
propose the catalytic cycle of the chiral Cu(OH)2-catalyzed
asymmetric boron conjugate addition to a,b-unsaturated
carbonyl compounds in water shown in Scheme 2. The
Cu(OH)2-L1 complex 3 reacts with B2(pin)2 to form key
CuII intermediate 4. We assume that 4 is sufficiently Lewis
acidic to form complex 5, in which a,b-unsaturated carbonyl
compounds are activated by 4, and boron addition might
occur via six-membered cyclic transition states to afford 6.
Scheme 1. Conversion of an a,b-unsaturated ketone into a b-hydroxyl
carbonyl compound.
The substrate scope of a,b-unsaturated carbonyl com-
pounds was surveyed using Cu(OH)2 (5 mol%) with chiral
ligand L1 (6 mol%) at 58C in water for 12 h (Table 2). Several
acyclic a,b-unsaturated ketones reacted smoothly with
B2(pin)2 under the conditions to afford the desired 1,4-
addition products in high yields with high enantioselectivities
(entries 1–10). Remarkably, a cyclic a,b-unsaturated ketone
and b,b-disubstitued enones also reacted with B2(pin)2
smoothly with the same catalyst to afford the corresponding
addition products in high yields with high enantioselectivities
12764 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 12763 –12766

Angewandte
Chemie
Scheme 4. Turnover frequency test.
In summary, we have developed enantioselective 1,4-
additions of diboron to a,b-unsaturated carbonyl compounds
and a nitrile catalyzed by Cu(OH)2 and a chiral bipyridine
ligand in water. The desired products were obtained in high
yields and with high enantioselectivities. Previously, similar
types of reactions have been carried out predominantly by
using CuI with a chiral ligand in organic solvents. In contrast,
this report has shown the first example of a chiral CuII-
catalyzed asymmetric 1,4-addition of diboron in water. Under
our conditions, a wide substrate scope of a,b-unsaturated
carbonyl compounds, including acyclic, cyclic, and b,b-disub-
stituted enones, a,b-unsaturated esters, a,b-unsaturated
amides, and an a,b-unsaturated nitrile, has been shown. This
is also the first case where a single catalyst reacts with all of
these substrates to give products with high yields and high
enantioselectivities. Furthermore, several synthetic advan-
tages of this method have been demonstrated in this reaction:
1) The reactions proceeded smoothly using inexpensive and
readily available Cu(OH)2 in water; no dry solvents or
anhydrous conditions are needed. 2) The experimental pro-
cedure is very simple; the substrates, Cu(OH)2, and chiral
bipyridine ligand L1 (and sometimes AcOH) are mixed in
water. 3) A high TOF value was attained. Further investiga-
tions to elucidate the reaction mechanism, especially the
detailed roles of CuII and water, are now in progress.
Scheme 2. Proposed catalytic cycle.
CuII enolate 6 is then immediately protonated by H2O to give
desired compound 1, along with regeneration of 3. Chiral
induction might occur via the aforementioned six-membered
cyclic transition states; these relatively rigid transition states
would be formed with excellent chiral environments created
by bipyridine ligand L1 to realize high enantioselectivities
with a wide substrate scope. As for the additive, acetic acid
(AcOH) may react with Cu(OH)2; however, a 1:1 mixture of
Cu(OH)2/AcOH gave the best results after testing ratios of
Cu(OH)2 and AcOH. Therefore, the exclusive formation of
Cu(OAc)2 is impossible under the conditions. When
Cu(OAc)2 (10 mol%) was used independently as the catalyst
at RT for 12 h, product 2a was obtained in 90% yield with
92.5:7.5 e.r.
As an extension of this work, we conducted the reaction of
a,b,g,d-unsaturated ketone 7 with B2(pin)2 under the standard
conditions (Scheme 3). In this experiment, the 1,4-addition
product 8 was predominantly obtained (79% yield, 1,4-
Received: September 11, 2012
Published online: November 19, 2012
Keywords: asymmetric catalysis · boron · b-borylation · copper ·
.
synthetic methods
[1] a) C. J. Li, Chem. Rev. 2005, 105, 3095; b) Organic Reactions in
Water (Ed.: U. M. Lindstroem), Wiley-Blackwell, Oxford, 2007;
c) Water in Organic Synthesis (Science of Synthesis) (Ed.: S.
Kobayashi), Thieme, Stuttgart, 2012.
[2] a) D. C. Rideout, R. Breslow, J. Am. Chem. Soc. 1980, 102, 7816;
b) H. Yorimitsu, T. Nakamura, H. Shinokubo, K. Oshima, K.
Omoto, H. Fujimoto, J. Am. Chem. Soc. 2000, 122, 11041; c) S.
Narayan, J. Muldoon, M. G. Finn, V. V. Fokin, H. C. Kolb, K. B.
Sharpless, Angew. Chem. 2005, 117, 3339; Angew. Chem. Int. Ed.
2005, 44, 3275; d) U. Schneider, M. Ueno, S. Kobayashi, J. Am.
Chem. Soc. 2008, 130, 13824; e) L. Ackermann, J. Pospech, Org.
Lett. 2011, 13, 4153; f) T. Kitanosono, M. Sakai, M. Ueno, S.
Kobayashi, Org. Biomol. Chem. 2012, 10, 7134.
[3] a) M. Tortosa, Angew. Chem. 2011, 123, 4036; Angew. Chem. Int.
Ed. 2011, 50, 3950; b) S. Nave, R. P. Sonawane, T. G. Elford,
V. K. Aggarwal, J. Am. Chem. Soc. 2010, 132, 17096; c) J. Chang,
H. Lee, D. G. Hall, J. Am. Chem. Soc. 2010, 132, 5544; d) D.
Imao, B. W. Glasspoole, V. S. Laberge, C. M. Crudden, J. Am.
Chem. Soc. 2009, 131, 5024; e) J. L. Stymiest, V. Bagutski, R. M.
French, V. K. Aggarwal, Nature 2008, 456, 778; f) D. G. Hall,
Boronic Acids. Preparation, Applications in Organic Synthesis
Scheme 3. The reaction of a,b,g,d-unsaturated ketone 7 with
B2(pin)2.
adduct/1,6-adduct = 98:2) in 94.5:5.5 e.r., which might support
the cyclic transition state 5 of this asymmetric transformation.
Finally, the turnover frequency (TOF) of the chiral CuII
catalyst in water was confirmed to be very high. In the
presence of Cu(OH)2 (0.005 mol%), L1 (0.006 mol%), and
AcOH (0.006 mol%), chalcone reacted with B2(pin)2 at RT in
water for 15 min to afford 2a in 22% yield with 92.5:7.5 e.r.
The TOF reached 17,600 hÀ1, which is the highest value ever
reported in an asymmetric boron conjugate addition reaction
(Scheme 4).[18]
Angew. Chem. Int. Ed. 2012, 51, 12763 –12766
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 12765

.
Angewandte
Communications
and Medicine, Wiley-VCH, Weinheim, 2005; g) E. Hupe, P.
Marek, I. Knochel, Org. Lett. 2002, 4, 2861; h) Y. Yamamoto, N.
Asao, Chem. Rev. 1993, 93, 2207.
[11] a) S. Kobayashi, S. Nagayama, T. Busujima, Chem. Lett. 1999, 71;
b) S. Kobayashi, S. Nagayama, T. Busujima, Tetrahedron 1999,
55, 8739; c) S. Kobayashi, Y. Mori, S. Nagayama, K. Manabe,
Green Chem. 1999, 1, 175; d) S. Otto, J. B. F. N. Engberts, J. C. T.
Kwak, J. Am. Chem. Soc. 1998, 120, 9517; e) B. Y. Yang, X. M.
Chen, G. J. Deng, Y. L. Zhang, Q. H. Fan, Tetrahedron Lett.
2003, 44, 3535.
[4] a) T. Ishiyama, N. Miyaura, K. Takahashi, J. Organomet. Chem.
2001, 625, 47; b) T. Ishiyama, N. Miyaura, K. Takahashi, Chem.
Lett. 2000, 982; c) A. Hosomi, H. Ito, J. Tateiwa, H. Yamanaka,
Tetrahedron Lett. 2000, 41, 6821.
[5] a) X. Feng, J. Yun, Chem. Eur. J. 2010, 16, 13609; b) H. Chea,
H. S. Sim, J. Yun, Adv. Synth. Catal. 2009, 351, 855; c) H. S. Sim,
X. Feng, J. Yun, Chem. Eur. J. 2009, 15, 1939; d) J. E. Lee, J. Yun,
Angew. Chem. 2008, 120, 151; Angew. Chem. Int. Ed. 2008, 47,
145; e) S. Mun, J. E. Lee, J. Yun, Org. Lett. 2006, 8, 4887.
[6] a) J. C. H. Lee, R. McDonald, D. G. Hall, Nat. Chem. 2011, 3,
894; b) A. L. Moure, R. G. Arrayꢀs, J. C. Carretero, Chem.
Commun. 2011, 47, 6701; c) G. A. Molander, S. A. McKee, Org.
Lett. 2011, 13, 4684; d) J. K. Park, H. H. Lackey, M. D. Rexford,
K. Kovnir, M. Shatruk, D. T. McQuade, Org. Lett. 2010, 12, 5008;
e) M. Kanai, N. Kato, E. Ichikawa, M. Shibasaki, Synlett 2005,
1491; f) J. A. Ma, D. Cahard, Angew. Chem. 2004, 116, 4666;
Angew. Chem. Int. Ed. 2004, 43, 4566.
[7] For rhodium, see: T. Shiomi, T. Adachi, K. Toribatake, L. Zhou,
H. Nishiyama, Chem. Commun. 2009, 5987; for nickel and
palladium, see: E. Fernꢀndez, M. J. Geier, V. Lillo, S. A.
Westcott, Org. Biomol. Chem. 2009, 7, 4674.
[8] a) H. Wu, S. Radomkit, J. M. OꢁBrien, A. H. Hoveyda, J. Am.
Chem. Soc. 2012, 134, 8277; b) J. M. OꢁBrien, K. S. Lee, A. M.
Hoveyda, J. Am. Chem. Soc. 2010, 132, 10630; c) A. Bonet, E.
Fernꢀndez, H. Gulyꢀs, Angew. Chem. 2010, 122, 5256; Angew.
Chem. Int. Ed. 2010, 49, 5130; d) J. A. Schiffner, K. Mꢂther, M.
Oestreich, Angew. Chem. 2010, 122, 1214; Angew. Chem. Int. Ed.
2010, 49, 1194; e) K. S. Lee, A. R. Zhugralin, A. H. Hoveyda, J.
Am. Chem. Soc. 2009, 131, 7253.
[12] a) M. Kokubo, T. Naito, S. Kobayashi, Tetrahedron 2010, 66,
1111; b) C. Ogawa, M. Kokubo, S. Kobayashi, J. Synth. Org.
Chem. Jpn. 2010, 68, 718; c) M. Kokubo, T. Naito, S. Kobayashi,
Chem. Lett. 2009, 38, 904.
[13] a) S. Kobayashi, K. Manabe, Pure. Appl. Chem. 2000, 72, 1373;
b) D. Sinou, Adv. Synth. Catal. 2002, 344, 221; c) K. Manabe, S.
Kobayashi, Chem. Eur. J. 2002, 8, 4094; d) K. Manabe, S.
Kobayashi, Acc. Chem. Res. 2002, 35, 209; e) S. Kobayashi, C.
Ogawa in Asymmetric Synthesis-The Essentials (Eds.: M.
Christmann, S. Brꢃse), Wiley-VCH, Weinheim, 2007, p. 110;
f) “Acid Catalysis in Water”: C. Ogawa, S. Kobayashi in Organic
Reactions in Water (Ed.: U. M. Lindstroem), Wiley-Blackwell,
Oxford, 2007, p. 60; g) S. Kobayashi, Pure. Appl. Chem. 2007, 79,
235; h) “Toward Truly Efficient Organic Reactions in Water”: C.
Ogawa, S. Kobayashi in Process Chemistry in the Pharmaceutical
Industry, Vol. 2 (Eds.: K. Gadamasetti, T. Braish), CRC, Boca
Raton, 2008, pp. 249 – 265; i) C. Pan, Z. Wang, Coord. Chem.
Rev. 2008, 252, 736.
[14] a) S. Kobayashi, S. Nagayama, T. Busujima, J. Am. Chem. Soc.
1998, 120, 8287; b) S. Kobayashi, C. Ogawa, Chem. Eur. J. 2006,
12, 5954.
[15] a) C. Bolm, M. Zehnder, D. Bur, Angew. Chem. 1990, 102, 206;
Angew. Chem. Int. Ed. Engl. 1990, 29, 205; b) C. Bolm, M.
Ewald, M. Felder, G. Schlingloff, Chem. Ber. 1992, 125, 1169;
c) S. Ishikawa, T. Hamada, K. Manabe, S. Kobayashi, Synthesis
2005, 2176.
[9] a) During this research, achiral copper(II) catalysis in water was
reported; see: S. B. Thorpe, J. A. Calderone, W. L. Santos, Org.
Lett. 2012, 14, 1918.
[16] I. Ibrahem, P. Breistein, A. Cꢄrdova, Angew. Chem. 2011, 123,
12242; Angew. Chem. Int. Ed. 2011, 50, 12036.
[10] a) S. Kobayashi, M. Ueno, T. Kitanosono, Top. Curr. Chem. 2012,
311, 1; b) S. Kobayashi, T. Endo, M. Ueno, Angew. Chem. 2011,
123, 12470; Angew. Chem. Int. Ed. 2011, 50, 12262; c) S.
Kobayashi, T. Kitanosono, M. Ueno, Synlett 2010, 2033; d) S.
Kobayashi, T. Endo, U. Schneider, M. Ueno, Chem. Commun.
2010, 46, 1260; e) M. Fujita, T. Nagano, U. Schneider, T.
Hamada, C. Ogawa, S. Kobayashi, J. Am. Chem. Soc. 2008,
130, 2914.
[17] The immediate disproportionation of CuI to form Cu0 and CuII is
well known; see: a) P. A. Korzhavyi, I. L. Soroka, E. I. Isaev, C.
Lilja, B. Johansson, Proc. Natl. Acad. Sci. USA 2012, 109, 686;
´
ˇ ´
´
´
b) M. Plavsic, D. Krznaric, B. Cosovic, Electroanalysis 1994, 6,
469; c) J. L. Proust, Ann. Chim. 1799, 1, 32.
[18] In previous reports,[5–8,16] the TOF was less than 50 hÀ1, except for
one case where a 5580 hÀ1 TOF was attained using a chiral
carbene CuI complex in toluene, see Ref. [6d].
12766 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 12763 –12766