Solvent-controlled copper-catalyzed oxidation of benzylic alcohols to aldehydes and esters

Article abstract of DOI:10.1002/ejoc.201300668

A procedure for the copper-catalyzed selective oxidation of primary alcohols to esters and aldehydes was developed. Under solvent-free conditions, self-oxidative esterification and cross-esterification of benzyl alcohols with various aliphatic alcohols proceed smoothly to give the corresponding esters in good yields. If DMF was used as a solvent, the benzyl alcohols were selectively converted into the corresponding aldehydes in excellent yields. Depend on solvent! Under solvent-free conditions, Cu-catalyzed oxidative self-esterification and cross-esterification of benzylic alcohols with various aliphatic alcohols proceed smoothly to give the corresponding esters in good yields. If DMF is used as a solvent, the benzylic alcohols are selectively converted into the corresponding aldehydes in excellent yields. DTBP = di-tert-butyl peroxide. Copyright

Full text of DOI:10.1002/ejoc.201300668

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300668
Solvent-Controlled Copper-Catalyzed Oxidation of Benzylic Alcohols to
Aldehydes and Esters
Yefeng Zhu[a] and Yunyang Wei*[a]
Keywords: Synthetic methods / Oxidation / Esterification / Alcohols / Aldehydes / Copper
A procedure for the copper-catalyzed selective oxidation of
primary alcohols to esters and aldehydes was developed. Un-
der solvent-free conditions, self-oxidative esterification and
cross-esterification of benzyl alcohols with various aliphatic
alcohols proceed smoothly to give the corresponding esters
in good yields. If DMF was used as a solvent, the benzyl
alcohols were selectively converted into the corresponding
aldehydes in excellent yields.
such as gold,[8] palladium,[9] iridium,[10] or ruthenium[11] as
catalysts. The use of earth-abundant first-row transition
Introduction
Esters are important intermediates that are widely applied metals instead of catalysts based on noble metals is interest-
in the synthesis of bulk chemicals, drugs, synthetic materi- ing and important from academic, industrial, and sus-
als, and polymers.[1] Traditionally, esters are prepared by tainability aspects. Recently, a general and efficient zinc-cat-
the reaction of activated carboxylic acid derivatives (acyl alyzed cross-esterification of benzylic alcohols with meth-
chlorides and anhydrides) with alcohols.[2] Recent develop- anol was reported by Wu.[12] In this method, more than
ments in palladium-catalyzed three-component esterifica- one equivalent of CF3COOH was used as an additive and
tion reactions of alcohols, aryl halides, and CO provide a methanol was required as a solvent to achieve full conver-
more direct approach for ester synthesis.[3,4] The oxidative sion of the benzylic alcohols, which makes the reaction less
esterification of aldehydes with alcohols is also reported for applicable and less practical.
the synthesis of esters.[5] As the required aldehydes are nor-
Copper is one of the most abundant metals on the earth,
mally synthesized by selective oxidation of alcohols,[6,7] the and it is one of the most inexpensive and environmentally
direct conversion of alcohols into esters in the presence of friendly ones. Despite the fact that great attention has been
catalysts is certainly a step forward towards green, econ- paid to the copper-catalyzed selective oxidation of alcohols
omic, and sustainable processes.[8–11] These one-pot pro- to aldehydes in the past decades,[7] reports on the copper-
cedures reported usually require the use of noble metals catalyzed oxidative esterification of alcohols are rare. Dur-
Scheme 1. Copper-catalyzed selective oxidation of alcohols to esters or aldehydes.
ing the course of our continued interests in investigating
[a] School of Chemical Engineering, Nanjing University of Science
copper-catalyzed coupling reactions and the selective oxi-
dation of alcohols,[13] herein we wish to report the solvent-
controlled copper-catalyzed selective oxidation of alcohols
to esters or aldehydes under neutral conditions (Scheme 1).
To the best of our knowledge, no similar copper-catalyzed
reactions are known to date.
and Technology,
Nanjing 210094, China
Fax: +86-25-84317078
E-mail: ywei@mail.njust.edu.cn
Homepage: http://sce.njust.edu.cn/inc/ArticleRead.aspx?
ArticleID=786
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300668.
Eur. J. Org. Chem. 2013, 4503–4508
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Y. Z h u , Y. We i
SHORT COMMUNICATION
pendently proved to be ineffective as a ligand in this trans-
formation. Encouraged by these results, 1,3-diketimine
(nacac) ligand L9[14] was synthesis from L7 and 2,6-dimeth-
ylbenzenamine. To our delight, the desired benzyl benzoate
product was obtained in 92% yield if L9 was used as the
ligand (Table 1, entry 10). The electron-rich character of the
ligand is probably beneficial to the oxidative addition of
DTBP to the catalyst. However, the steric hindrance of the
ligand may effectively prevent a competitive reaction of the
substrate with the ligand.
With L9 as the ligand, we then examined the effective-
ness of various copper sources and solvents for the reaction
(Table 2). As shown in Table 2, in the absence of any copper
source, no esterification product was obtained (Table 2, en-
try 1). If CuI, CuBr, or CuCl was used as the catalyst, 2a
was obtained in 92, 82, or 75% yield, respectively (Table 2,
entries 2–4). The use of CuCl2, Cu(OAc)2, and Cu(acac)2
(acac = acetylacetonate) as catalysts resulted in lower yields
of 2a and higher yields of benzaldehyde (3a; Table 2, en-
tries 5–7). Notably, the selectivity between aldehyde and es-
ter could be modified by carrying out the reaction in dif-
ferent solvents (Table 2, entries 8–13). Generally, nonpolar
solvents favored ester formation, whereas polar solvents fa-
vored aldehyde formation. If the reaction was carried out
in DMF or dimethylacetamide (DMA) instead of the sol-
vent-free conditions, aldehyde 3a was found to be the only
product (Table 2, entries 8 and 9). Other peroxides such as
Results and Discussion
We started our research with benzyl alcohol as the sub-
strate and commercially available di-tert-butyl peroxide
(DTPB) was employed as the oxidant under solvent-free
conditions. As shown in Table 1, in the presence of CuI
(10 mol-%) without any ligand, only a trace amount of
benzyl benzoate (2a) was observed (Ͻ5% by GC–MS). The
major product was benzaldehyde (42% by GC–MS;
Table 1, entry 1) along with unreacted starting material. To
improve the activity of the catalyst system, the influence of
various ligands was investigated. Ligands such as tetra-
methylethylenediamine (L1), 2-picolinic acid (L2), 2,2Ј-bi-
pyridyl (L3), 1,10-phenanthroline (L4), and l-proline (L5)
were not efficient in the reaction (Table 1, entries 2–6). With
the use of CuI and Mannich base L6, which has been
proved to be an efficient ligand in copper-catalyzed C–N
cross-coupling reactions by us,[13b,13c] 2a was obtained in
only 8% yield (Table 1, entry 7). However, when the β-di-
ketone acetylacetone (L7) was used as the ligand, benzyl
benzoate was obtained in 21% yield. We noted that a major
byproduct was enol ester L8. Enol ester L8 prepared inde-
Table 1. Survey of ligands for the copper-catalyzed oxidative esteri-
fication of benzyl alcohol.[a]
Table 2. Optimization of the reaction conditions for the selective
oxidation of benzyl alcohol.[a]
Entry
[Cu]
Oxidant
Solvent
Yield[b] [%]
2a 3a
1
2
3
4
5
6
7
8
–
CuI
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
TBHP
BPO
–
–
–
–
–
–
–
0
92
82
75
12
8
38
0
0
28
Ͻ1
Ͻ5
Ͻ5
72
65
32
91
69
52
71
9
CuBr
CuCl
CuCl2
Cu(OAc)2
Cu(acac)2
CuI
Entry
Ligand
Yield[b] [%]
2a
3a
DMF
DMA
tBuOH
THF
benzene 85
n-hexane 71
1
2
3
4
5
6
7
8
9
–
Ͻ5
Ͻ5
Ͻ5
Ͻ5
Ͻ5
Ͻ5
8
21
13
92
42
58
46
50
53
52
61
49
41
Ͻ1
9
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
L1
L2
L3
L4
L5
L6
L7
L8
L9
10
11
12
13
14[c]
15
16[d]
17[e]
47
Ͻ5
8
–
–
–
–
Ͻ5
Ͻ5
58
51
48
8
DTBP
DTBP
CuI
75
17
10
[a] Reaction conditions: A mixture of benzyl alcohol (1a, 1 mmol),
[Cu] (10 mol-%), L9 (10 mol-%), and oxidant (4 mmol) was stirred
at 90 °C for 6 h under an atmosphere of N2. [b] Determined by
GC–MS. [c] 70% tBuOOH in water. [d] Temperature was 70 °C.
[e] DTBP (3 mmol) was used.
[a] Reaction conditions: A mixture of benzyl alcohol (1a, 1 mmol),
CuI (10 mol-%), ligand (10 mol-%), and DTBP (4 mmol) was
stirred at 90 °C for 6 h under an atmosphere of N2. [b] Determined
by GC–MS.
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Cu-Catalyzed Oxidation of Benzylic Alcohols
tert-butyl hydroperoxide (TBHP) or benzoyl peroxide selectivity of the reaction decreased markedly. For example,
(BPO) were less effective in the reaction (Table 2, entries 14 if 4-methoxybenzyl alcohol was used as the substrate, the
and 15), and their use led to only a trace amount of 2a. A ratio of ester to aldehyde was 90:10 (Table 3, entry 4). Het-
decrease in the amount of peroxide from 4 to 3 equiv. and eroaromatic alcohols such as 2-furanmethanol or 2-thenoyl-
a decrease in the reaction temperature from 90 to 70 °C led methanol also gave the corresponding esters in good yields
to a decrease in the yield of the product and a decrease in of 81 and 83%, respectively (Table 3, entries 12 and 13).
the selectivity (Table 2, entries 16 and 17).
Notably, steric hindrance had a great impact on this trans-
The oxidative self-esterification of different benzylic and formation, as was evidenced by the fact that no correspond-
heterobenzylic alcohols was then examined with CuI/L9 as ing ester was obtained if ortho-substituted benzylic alcohols
the catalyst under solvent-free conditions. On the whole, were used as substrates (Table 3, entries 10 and 11). The
electron-withdrawing groups and electron-donating groups only product detected was benzaldehyde.
were well tolerated (Table 3, entries 1–8). Benzylic alcohols
Encouraged by the success of the oxidative self-esterifica-
bearing electron-withdrawing groups (Table 3, entries 5–8) tion of benzylic alcohols, we tried to extend the protocol to
or weak electron-donating groups (Table 3, entries 2 and 3) the oxidative cross-esterification of benzylic alcohols with
reacted smoothly under the optimized conditions to form aliphatic alcohols. 4-Bromobenzyl alcohol and n-butyl
the corresponding esters in high yields with trace amounts alcohol were first used in this cross-esterification reaction.
of aldehydes. With strong electron-donating groups, the The desired product, butyl 4-bromobenzoate, was obtained
in 88% yield in the presence of CuI (20 mol-%) and L9
(20 mol-%) under solvent-free conditions (Table 4, entry 1).
Table 3. Copper-catalyzed oxidative self-esterification of benzylic
alcohols.[a]
Then, various benzylic alcohols were used to couple with
n-butyl alcohol. As shown in Table 4, both substituted
benzylic alcohols and heteroarylmethanols gave the corre-
sponding n-butyl esters in good yields (Table 4, entries 2–6).
Other aliphatic alcohols such as methanol, ethanol, 1,1,1-
trifluoroethanol, n-pentyl alcohol, n-heptyl alcohol, and n-
octyl alcohol were also tested and all gave the desired prod-
ucts (Table 4, entries 7–12). Notably, for all the reactions in
Table 4, the ratio of benzylic alcohol to aliphatic alcohol
was 1:1.5 and only a trace amount of the self-oxidative es-
terification product was observed.
Finally, the scope of the copper-catalyzed selective oxi-
dation of benzylic alcohols to the corresponding aldehydes
was examined by using DMF as a solvent. As shown in
Scheme 2, all of the benzylic alcohols tested afforded the
corresponding aldehydes in excellent yields. The electronic
and steric character of the substrates showed little effect on
the performance of the catalyst. To our delight, heteroaryl-
methanols such as 2-furanmethanol and 2-thenoylmethanol
could also be transformed into the corresponding aldehydes
in good yields. In all cases, no oxidative self-esterification
product was detected.
Although the detailed mechanism for the present trans-
formation is not clear, a possible mechanism is proposed
according to the above results and related work.[14] As
shown in Scheme 3, oxidative addition of the DTBP oxi-
dant to the CuILn complex formed between CuI and L9
generates intermediate LnCuII–OtBu (I1).[14c] Alkoxide li-
gand exchange with an alcohol produces alkoxy copper in-
termediate I2.[15] Direct β-hydride elimination affords the
aldehyde and copper hydride intermediate I3, which then
undergoes oxidative dehydrogenation to regenerate the
LnCuI species (Scheme 3, path a). This will result in the se-
lective formation of the aldehyde. Further reaction of this
resulting aldehyde with I2 will generate hemiacetal copper
intermediate I5, from which β-hydride elimination will re-
lease the ester and copper hydride I3. Overall, this will re-
sult in the oxidative esterification of primary alcohols
(Scheme 3, path b).
[a] Reaction conditions: A mixture of benzylic alcohol (1 mmol),
CuI (10 mol-%), L9 (10 mol-%), and DTBP (4 mmol) was stirred
at 90 °C for 6 h under an atmosphere of N2. [b] Isolated yield.
[c] Based on GC–MS.
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Y. Z h u , Y. We i
SHORT COMMUNICATION
Table 4. Copper-catalyzed oxidative cross-esterification of benzylic
alcohols.[a]
Scheme 2. Copper-catalyzed selective oxidation of benzylic
alcohols to aldehydes. Reaction conditions: CuI (10 mol-%), L9
(10 mol-%), benzylic alcohol (1.0 mmol), DTBP (4 mmol), DMF
(2 mL), 90 °C, 6 h, yield determined by GC–MS.
Scheme 3. The proposed reaction mechanism.
reactivity of the carbonyl group of DMF, intermediate I6 is
prone to β-hydride elimination rather than transformation
into I7 through carbonyl insertion. Hence, the aldehyde is
selectively formed.
Scheme 4. The proposed reaction mechanism in DMF.
If the reaction is carried out under solvent-free condi-
tions, the aldehyde formed by β-hydride elimination may
coordinate with copper to form complex I4, which is trans-
formed into I5 by carbonyl insertion. If an aliphatic RЈOH
alcohol is added to the catalytic system, more electron-rich
[a] Reaction conditions: A mixture of benzylic alcohol (0.5 mmol),
RЈOH (0.75 mmol), CuI (20 mol-%), L9 (20 mol-%), and DTBP
(4 mmol) was stirred at 90 °C for 6 h under an atmosphere of N2.
[b] Isolated yield.
Alkoxy copper complex I2 is a key intermediate in this complex I4 is formed, and the subsequent carbonyl inser-
transformation. If the reaction is conducted in DMF, the tion would be favored. A competitive experiment performed
abundance of solvent may easily coordinate to copper com- in Scheme 5 showed that the reaction activity order was
plex I2 to form complex I6 (Scheme 4). Owing to the low nBuOH Ͼ PhCH2OH Ͼ tBuOH.
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Cu-Catalyzed Oxidation of Benzylic Alcohols
to room temperature, dichloromethane (20 mL) was added to the
mixture. The resulting mixture was washed with water (3ϫ 10 mL).
The organic layer was separated and dried with sodium sulfate. The
organic layer was used for GC–MS analysis.
Supporting Information (see footnote on the first page of this arti-
cle): General considerations, preparation of Mannich base L6 and
imine L9, general procedure for the oxidation of benzylic alcohols,
characterization data of the esters, and 1H NMR and 13C NMR
spectra.
Acknowledgments
Scheme 5. Competitive experiment. Reaction conditions: CuI
(20 mol-%), L9 (20 mol-%), 4-bromobenzaldehyde (1.0 mmol),
benzyl alcohol (1.5 mmol), n-butyl alcohol (1.5 mmol), DTBP
(4 mmol), 90 °C, 6 h, yield determined by GC–MS.
The authors are grateful to a project funded by the Priority Aca-
demic Program Development of Jiangsu Higher Education Insti-
tutions (PAPD).
[1] a) K. Ishihara, Tetrahedron 2009, 65, 1085; b) J. Otera, Esterifi-
cation: Methods, Reactions, and Applications, Wiley-VCH,
Weinheim, Germany, 2003; c) T. W. Greene, P. G. M. Wuts,
Protective Groups in Organic Synthesis Wiley, New York, 1991.
[2] a) J. McNulty, A. Capretta, V. Laritchev, J. Dyck, A. J. Robert-
son, Angew. Chem. 2003, 115, 4185; Angew. Chem. Int. Ed.
2003, 42, 4051; b) T. A. Houston, B. L. Wilkinson, J. T. Blanch-
field, Org. Lett. 2004, 6, 679; c) P. Gopinath, S. Nilaya, K. M.
Muraleedharan, Org. Lett. 2011, 13, 1932; d) K. C. K. Swamy,
N. N. B. Kumar, E. Balaraman, K. V. P. P. Kumar, Chem. Rev.
2009, 109, 2551.
[3] a) A. Schoenberg, R. F. Heck, J. Org. Chem. 1974, 39, 3327; b)
A. Brennführer, H. Neumann, M. Beller, Angew. Chem. 2009,
121, 4176; Angew. Chem. Int. Ed. 2009, 48, 4114; c) C. M.
Kormos, N. E. Leadbeater, Org. Biomol. Chem. 2007, 5, 65; d)
Y. Yamamoto, Adv. Synth. Catal. 2010, 352, 478; e) Z. Xin,
T. M. Gogsig, A. T. Lindhardt, T. Skrydstrup, Org. Lett. 2012,
14, 284.
Conclusions
In summary, a procedure for the solvent-controlled cop-
per-catalyzed selective oxidation of primary alcohols to al-
dehydes or esters was developed. Depending on the catalyst
system, the highly selective formation of the corresponding
aldehydes or esters is observed. Both oxidative homocou-
pling reactions as well as cross-esterifications of benzylic
alcohols with various aliphatic alcohols proceed under mild
conditions to give the corresponding esters in good yields.
Studies into the mechanistic aspects and synthetic applica-
tions of this reaction are currently underway.
Experimental Section
[4] Recently, the transition-metal-free alkoxycarbonylation of aryl
halides was reported: H. Zhang, R. Shi, A. Ding, L. Lu, B.
Chen, A. Lei, Angew. Chem. 2012, 124, 12710; Angew. Chem.
Int. Ed. 2012, 51, 12542.
General Procedure for the Self-Oxidative Esterification of Benzyl
Alcohols: Under an atmosphere of N2, a mixture of benzyl alcohol
1 (1 mmol), DTBP (4 mmol), CuI (19.1 mg, 0.1 mmol, 10 mol-%),
and imine L9 (30.6 mg, 0.1 mmol, 10 mol-%) was sealed in a tube
with a screw cap containing a Teflon septum. The mixture was
stirred in an oil bath at 90 °C for 6 h. After cooling to room tem-
perature, dichloromethane (20 mL) was added to the mixture. The
mixture was then filtered, and the filtrate was evaporated in vacuo.
The residue was purified by flash column chromatography (silica
gel, ethyl acetate/petroleum ether = 1:100) to afford desired esters
2.
[5] a) K. Ekoue-Kovi, C. Wolf, Chem. Eur. J. 2008, 14, 6302; b) B.
Xu, X. Liu, J. Haubrich, C. M. Friend, Nat. Chem. 2010, 2,
61; c) X. F. Wu, C. Darcel, Eur. J. Org. Chem. 2009, 1144; d)
R. Gopinath, B. Barkakaty, B. Talukdar, B. K. Patel, J. Org.
Chem. 2003, 68, 2944; e) A. S. K. Hashmi, C. Lothschuetz, M.
Ackermann, R. Doepp, S. Anantharaman, B. Marchetti, H.
Bertagnolli, F. Rominger, Chem. Eur. J. 2010, 16, 8012; f) N.
Mori, H. Togo, Tetrahedron 2005, 61, 5915; g) N. N. Karade,
V. H. Budhewar, A. N. Katkar, G. B. Tiwari, ARKIVOC 2006,
11, 162; h) B. R. Travis, M. Sivakumar, G. O. Hollist, B. Bor-
han, Org. Lett. 2003, 5, 1034; i) R. Gopinath, B. K. Patel, Org.
Lett. 2000, 2, 577; j) C. Marsden, E. Taarning, D. Hansen, L.
Johansen, S. K. Klitgaard, K. Egeblad, C. H. Christensen,
Green Chem. 2008, 10, 168; k) G. A. Hero-poulos, C. Villa-
longa-Barber, Tetrahedron Lett. 2011, 52, 5319; l) S. Yamada,
D. Morizono, K. Yamamoto, Tetrahedron Lett. 1992, 33, 4329;
m) C. Liu, S. Tang, L. Zheng, D. Liu, H. Zhang, A. Lei, Angew.
Chem. 2012, 124, 5760; Angew. Chem. Int. Ed. 2012, 51, 5662;
n) W. J. Yoo, C. J. Li, J. Org. Chem. 2006, 71, 6266; o) W. J.
Yoo, C. J. Li, Tetrahedron Lett. 2007, 48, 1033; p) C. Liu, S.
Tang, L. Zheng, D. Liu, H. Zhang, A. Lei, Angew. Chem. 2012,
124, 5760; Angew. Chem. Int. Ed. 2012, 51, 5662.
General Procedure for the Cross-Oxidative Esterification of Benzyl
Alcohols: Under an atmosphere of N2, a mixture of benzyl alcohol
1 (0.5 mmol), aliphatic alcohol (0.75 mmol), DTBP (4 mmol), CuI
(19.1 mg, 0.1 mmol, 20 mol-%), and imine L9 (30.6 mg, 0.2 mmol,
10 mol-%) was sealed in a tube with a screw cap containing a Tef-
lon septum. The mixture was stirred in an oil bath at 90 °C for 6 h.
After cooling to room temperature, dichloromethane (20 mL) was
added to the mixture. The mixture was then filtered, and the filtrate
was evaporated in vacuo. The residue was purified by flash column
chromatography (silica gel, ethyl acetate/petroleum ether = 1:100)
to afford desired esters 4–10.
[6] a) D. I. Enache, J. K. Edwards, P. Landon, B. Solsona-Espriu,
A. F. Carley, A. A. Herzing, M. Watanabe, C. J. Kiely, D. W.
Knight, G. J. Hutchings, Science 2006, 311, 362; b) R. A. Shel-
don, I. W. C. E. Arends, G.-J. ten Brink, A. Dijksman, Acc.
Chem. Res. 2002, 35, 774; c) M. S. Sigman, D. R. Jensen, Acc.
Chem. Res. 2006, 39, 221; d) R. Liu, C. Dong, X. Liang, X.
Wang, X. Hu, J. Org. Chem. 2005, 70, 729; e) R. Liu, X. Liang,
General Procedure for the Oxidation of Benzyl Alcohols to Alde-
hydes: Under an atmosphere of N2, a mixture of benzyl alcohol 1
(1 mmol), DTBP (4 mmol), CuI (19.1 mg, 0.1 mmol, 10 mol-%),
and imine L9 (30.6 mg, 0.1 mmol, 10 mol-%) in DMF (2 mL) was
sealed in a tube with a screw cap containing a Teflon septum. The
mixture was stirred in an oil bath at 90 °C for 6 h. After cooling
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Y. Z h u , Y. We i
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C. Dong, X. Hu, J. Am. Chem. Soc. 2004, 126, 4112; f) T.
Mallat, A. Baiker, Chem. Rev. 2004, 104, 3037; g) M. J. Schultz,
M. S. Sigman, Tetrahedron 2006, 62, 8227.
2795; e) S. Arita, T. Koike, Y. Kayaki, T. Ikariya, Chem. Asian
J. 2008, 3, 1479.
[11] a) J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, J. Am. Chem.
Soc. 2005, 127, 10840; b) J. Zhang, M. Gandelman, L. J. W.
Shimon, D. Milstein, Dalton Trans. 2007, 107; c) C. Gunana-
than, L. J. W. Shimon, D. Milstein, J. Am. Chem. Soc. 2009,
131, 3146; d) N. A. Owston, A. J. Parker, J. M. J. Williams,
Chem. Commun. 2008, 624; e) N. A. Owston, T. D. Nixon, A. J.
Parker, M. K. Whittlesey, J. M. J. Williams, Synthesis 2009,
1578; f) T. Zweifel, J.-V. Naubron, H. Grútzmacher, Angew.
Chem. 2009, 121, 567; Angew. Chem. Int. Ed. 2009, 48, 559.
[12] X. F. Wu, Chem. Eur. J. 2012, 18, 8912.
[13] a) Y. Zhu, H. Zhao, Y. Wei, Synthesis 2013, 45, 952; b) Y. Zhu,
Y. Shi, Y. Wei, Monatsh. Chem. 2010, 141, 1009; c) Y. Zhu, Y.
Wei, Can. J. Chem. 2011, 89, 645; d) L. Liu, L. Ji, Y. Wei,
Catal. Commun. 2008, 9, 1379; e) L. Liu, J. Ma, L. Ji, Y. Wei,
J. Mol. Catal. A 2008, 291, 1; f) C. Zhu, Y. Wei, Adv. Synth.
Catal. 2012, 354, 313.
[7]
a) M. F. Semmelhack, C. R. Schmid, D. A. Cortés, C. S. Chou,
J. Am. Chem. Soc. 1984, 106, 3374; b) I. E. Markó, P. R. Giles,
M. Tsukazaki, S. M. Brown, C. J. Urch, Science 1996, 274,
2044; c) I. A. Ansari, R. Gree, Org. Lett. 2002, 4, 1507; d) P.
Gamez, I. W. C. E. Arends, J. Reedijk, R. A. Sheldon, Chem.
Commun. 2003, 2414; e) I. E. Markó, A. Gautier, R. Dumeun-
ier, K. Doda, F. Philippart, S. M. Brown, C. J. Urch, Angew.
Chem. 2004, 116, 1614; Angew. Chem. Int. Ed. 2004, 43, 1588;
f) N. Jiang, A. J. Ragauskas, J. Org. Chem. 2006, 71, 7087; g) O.
Onomura, H. Arimoto, Y. Matsumura, Y. Demizu, Tetrahedron
Lett. 2007, 48, 8668; h) C. Liu, J. Han, J. Wang, Synlett 2007,
643; i) P. J. Figiel, M. N. Kopylovich, J. Lasri, M. F. C. Gu-
edes da Silva, J. J. R. Fraústo da Silva, A. J. L. Pombeiro,
Chem. Commun. 2010, 46, 2766; j) J. M. Hoover, S. S. Stahl, J.
Am. Chem. Soc. 2011, 133, 16901; k) C. Han, M. Yu, W. Sun,
X. Yao, Synlett 2011, 2363; l) S. G. Babu, P. A. Priyadarsini,
R. Karvembu, Appl. Catal. A 2011, 392, 218.
[14] Recently, Warren and co-workers showed that the β-diketimin-
ato copper(I) complex [{(Cl2NN)Cu}2(m-benzene)], in: con-
3
junction with the mild oxidant tBuOOtBu enables Csp –H am-
[8]
a) B. Xu, X. Liu, J. Haubrich, R. J. Madix, C. M. Friend, An-
gew. Chem. 2009, 121, 4270; Angew. Chem. Int. Ed. 2009, 48,
4206; b) R. L. Oliveira, P. K. Kiyohara, L. M. Rossi, Green
Chem. 2009, 11, 1366; c) H. Miyamura, T. Yasu-kawa, S. Ko-
bayashi, Green Chem. 2010, 12, 776.
ination: a) S. Wiese, Y. M. Badiei, R. T. Gephart, S. Mossin,
M. S. Varonka, M. M. Melzer, K. Meyer, T. R. Cundari, T. H.
Warren, Angew. Chem. 2010, 122, 9034; Angew. Chem. Int. Ed.
2010, 49, 8850; b) R. T. Gephart III, D. L. Huang, M. J. B. Ag-
uila, G. Schmidt, A. Shahu, T. H. Warren, Angew. Chem. 2012,
124, 6594; Angew. Chem. Int. Ed. 2012, 51, 6488; c) R. T. Ge-
phart III, C. L. McMullin, N. G. Sapiezynski, E. S. Jang,
M. J. B. Aguila, T. R. Cundari, T. H. Warren, J. Am. Chem.
Soc. 2012, 134, 17350.
[9] a) S. Gowrisankar, H. Neumann, M. Beller, Angew. Chem.
2011, 123, 5245; Angew. Chem. Int. Ed. 2011, 50, 5139; b) C.
Liu, J. Wang, L. Meng, Y. Deng, Y. Li, A. Lei, Angew. Chem.
2011, 123, 5250; Angew. Chem. Int. Ed. 2011, 50, 5144; c) C.
Liu, S. Tang, A. Lei, Chem. Commun. 2013, 49, 1324.
[10] a) T. Suzuki, K. Morita, M. Tsuchida, K. Hiroi, J. Org. Chem.
2003, 68, 1601; b) T. Suzuki, K. Morita, Y. Matsuo, L. Hiroi,
Tetrahedron Lett. 2003, 44, 2003; c) A. Izumi, Y. Obora, S.
Sakaguchi, Y. Ishii, Tetrahedron Lett. 2006, 47, 9199; d) S. Ar-
ita, T. Koike, Y. Kayaki, T. Ikariya, Organometallics 2008, 27,
[15] a) Y. J. Kim, K. Osakada, A. Takenaka, A. Yamamoto, J. Am.
Chem. Soc. 1990, 112, 1096; b) J. W. Tye, Z. Weng, R. Giri,
J. F. Hartwig, Angew. Chem. 2010, 122, 2231; Angew. Chem.
Int. Ed. 2010, 49, 2185.
Received: May 7, 2013
Published Online: June 12, 2013
4508
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Eur. J. Org. Chem. 2013, 4503–4508