Zinc triflate catalyzed aerobic cross-dehydrogenative coupling (CDC) of alkynes with nitrones: A new entry to isoxazoles

Article abstract of DOI:10.1021/ol200849k

A mild, operationally simple cross-dehydrogenative coupling between nitrones and terminal alkynes is described by using cheap, readily available 3,3′,5,5′-tetra-tert-butyldipheno-quinone and dioxygen as oxidants. These cross-couplings can be performed on

Full text of DOI:10.1021/ol200849k

ORGANIC
LETTERS
2011
Vol. 13, No. 10
2746–2749
Zinc Triflate Catalyzed Aerobic Cross-
Dehydrogenative Coupling (CDC) of
Alkynes with Nitrones: A New Entry to
Isoxazoles
Sandip Murarka and Armido Studer*
€
Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat, Corrensstrasse 40,
€
48149 Mu€nster, Germany
studer@uni-muenster.de
Received April 1, 2011
ABSTRACT
A mild, operationally simple cross-dehydrogenative coupling between nitrones and terminal alkynes is described by using cheap, readily
available 3,30,5,50-tetra-tert-butyldipheno-quinone and dioxygen as oxidants. These cross-couplings can be performed on various nitrones and
alkynes with good to excellent yields. Product nitrones are readily transformed to pharmaceutically important 3,5-disubstituted isoxazoles.
Development of selective and efficient CꢀC bond form-
ing reactions is of paramount importance for organic
chemistry. Transition-metal-catalyzed cross-coupling re-
actions using two prefunctionalized substrates belong to
the most powerful processes for CꢀC bond formation.1
Recently, transition-metal-catalyzed CꢀH bond activa-
tion with subsequent CꢀC bond formation has gained
great attention.2 However, in these cases one coupling
partner still has to be prefunctionalized. Cross-dehydro-
genative coupling (CDC) has emerged as a more atom
economic approach for the construction of CꢀC bonds
under oxidative conditions starting without prefunctiona-
lized substrates.3 Formally, two different CꢀH bonds are
directly connected to the corresponding CꢀC bond under
liberation of H2. In recent years, some excellent pioneering
studies have been published along this line.3ꢀ6
(4) (a) sp3 CꢀH and sp3 CꢀH: (i) Li, Z.; Li, C.-J. J. Am. Chem. Soc.
2005, 127, 3672.(ii)Li, Z.;Li,C.-J.J. Am. Chem. Soc. 2005, 127, 6968. (b) sp3
CꢀH and sp CꢀH: (i) Li, Z.; Li, C.-J. J. Am. Chem. Soc. 2004, 126, 11810.
(ii) Murahashi, S.-I.; Komiya, N.; Terai, H.; Nakae, T. J. Am. Chem. Soc.
2003, 125, 15312. (c) sp2 CꢀH and sp2 CꢀH: (i) Hatamoto, Y.; Sakaguchi,
S.; Ishii, Y. Org. Lett. 2004, 6, 4623. (ii) Jia, C.; Kitamura, T.; Fujiwara, Y.
Acc. Chem. Res. 2001, 34, 633. (iii) Tsuji, J.; Nagashima, H. Tetrahedron
1984, 40, 2699. (d) sp2 CꢀH and sp3 CꢀH: DeBoef, B.; Pastine, S. J.; Sames,
D. J. Am. Chem. Soc. 2004, 126, 6556. (e) sp CꢀH and sp CꢀH: Nicolaou,
K. C.; Zipkin, R. E.; Petasis, N. A. J. Am. Chem. Soc. 1982, 104, 5558.
(5) (a) Zhao, L.; Basle, O.; Li, C.-J. Proc. Natl. Acad. Sci. U.S.A.
2009, 106, 4106. (b) Guo, X.; Deng, G.; Li, C.-J. Adv. Synth. Catal. 2009,
351, 2071. (c) Deng, G.; Li, C.-J. Org. Lett. 2009, 11, 1171. (d) Correia,
C. A.; Li, C.-J. Adv. Synth. Catal. 2010, 352, 1446. (e) Cai, G.; Fu, Y.; Li,
Y.; Wan, X.; Shi, Z.-J. J. Am. Chem. Soc. 2007, 129, 7666. (f) Li, B.-J.;
Tian, S.-L.; Fang, Z.; Shi, Z.-J. Angew. Chem., Int. Ed. 2008, 47, 1115. (g)
Hull, K. L.; Sanford, M. S. J. Am. Chem. Soc. 2007, 129, 11904. (h)
Dwight, T. A.; Rue, N. R.; Charyk, D.; Josselyn, R.; DeBoef, B. Org.
Lett. 2007, 9, 3137. (i) Li, Z.; Guo, X.; Yu, R.; Li, H. J. Am. Chem. Soc.
2009, 131, 17387. (j) Dohi, T.; Ito, M.; Morimoto, K.; Iwata, M.; Kita,
Y. Angew. Chem., Int. Ed. 2008, 47, 1301. (k) Liegault, B.; Fagnou, K.
Organometallics 2008, 27, 4841. (l) Stuart, D. R.; Fagnou, K. Science
2007, 316, 1172. (m) Stuart, D. R.; Villemure, E.; Fagnou, K. J. Am.
Chem. Soc. 2007, 129, 12072. (n) Xia, J.-B.; You, S.-L. Organometallics
(1) (a) Diederich, F.; Stang, P. J., Eds. Metal-Catalyzed Cross-
Coupling Reactions; Wiley-VCH: New York, 1998. (b) Li, Z.-P.; Bohle,
D.-S.; Li, C.-J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 8928.
(2) (a) Dyker, G. Handbook of C-H Transformations; Wiley-VCH:
Weinheim, 2005. (b) Herrerias, C.; Yao, X.; Li, Z.; Li, C.-J. Chem. Rev. 2007,
107, 2546. (d) Alberico, D.; Scott, M. E.; Lautens, M. Chem. Rev. 2007,
107, 174. (e) Chen, X.; Engle, K.-M.; Wang, D.-H.; Yu, J.-Q. Angew.
Chem., Int. Ed. 2009, 48, 5094.
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2007, 26, 4869. (o) Pinter, A.; Sud, A.; Sureshkumar, D.; Klussmann, M.
Angew. Chem., Int. Ed. 2010, 49, 5004. (p) Xie, J.; Huang, Z.-Z. Angew.
Chem., Int. Ed. 2010, 49, 10181. (q) Guo, C.; Song, J.; Luo, S.-W.; Gong,
L.-Z. Angew. Chem., Int. Ed. 2010, 49, 5558.
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V. M. Chem. Rev. 2011, 111, 1215.
r
10.1021/ol200849k
Published on Web 04/19/2011
2011 American Chemical Society

There are several reports on CDC processes comp-
rising sp2(CꢀH)/sp3(CꢀH),4d,5c,5i,5k sp2(CꢀH)/sp2-
(CꢀH),4c,5b,5g,5h,5lꢀ5n and sp(CꢀH)/sp(CꢀH)4e couplings
(Scheme 1). However, there are very few reports on
successful sp2(CꢀH)/sp(CꢀH) CDC reactions.6 Herein, we
disclose the first examples of zinc triflate catalyzed CDC
reactions between sp2(CꢀH) of nitrones and sp(CꢀH) of
terminal alkynes in the presence of 3,30,5,50-tetra-tert-
butyldipheno-quinone 37 and dioxygen as oxidants for the
preparation of alkynylated nitrones 5 (Scheme 1). Moreover,
we will show that such nitrones are readily transformed to the
corresponding isoxazoles.
followed by mild oxidation of A to the cross-dehydrogena-
tively coupled R-alkynylated nitrone 5.
Scheme 2. Working Model for the CDC between N-tBu Ni-
trones and Terminal Alkynes
Scheme 1. CꢀC Bond Formation via Cross-Dehydrogenative
Coupling
The reason behind choosing the tBu group as an N-
substituent inthe starting nitrones is3-fold: (a) preliminary
studies revealed that oxidation is fast for N-tBu-substi-
tuted hydroxylamines, (b) oxidation occurs regioselec-
tively,13 and (c) the tBu group is an established acid-
sensitive protecting group. We started our studies by
reacting isopropyl N-tBu nitrone 1a with alkyne 2a
(3 equiv) in the presence of Zn(OTf)2 (20 mol %) and
NEt3 (50 mol %) in DCM at 45 °C. Various organic
oxidants were employed for the initial screenings. No reac-
tion was observed using DDQ (1 equiv), but the 2,2,6,6-
tetramethylpiperidine-N-oxyl radical14 (TEMPO, 2 equiv)
delivered the desired 5a in 74% yield (Table 1, entries 1, 2).
The yield was improved to 84% with the readily available
diquinone 3 as an oxidant (entry 3). Importantly, 3 can be
regenerated from byproduct 4.7 Replacing iPr2NEt with
Et3N gave a slightly lower yield (77%), which was improved
to 83% by stirring the reaction mixture at rt, albeit at the
expense of a longer reaction time (entries 4, 5). Hence, the
optimal oxidant and base in terms of yield and reaction time
were found to be diquinone 3 and iPr2NEt.
CꢀC bond formation via reaction of a metalated term-
inal alkyne8 with an electrophile is a classical approach for
alkynylation. Products obtained are amenable to further
synthetic transformations.9 Transition-metal-catalyzed
addition of alkynes to CdN bonds has been intensively
studied.10,11 Carreira’s Zn(II)-catalyzed alkynylation of
nitrones to propargyl N-hydroxylamines has caught our
attention.11a We envisioned oxidizing propargyl N-
hydroxylamine12 intermediates of type A to the R-alkyny-
lated nitrones 5, while avoiding oxidative homocoupling of
the metalated alkyne (Scheme 2). Hence, the oxidant must
be compatible with the organozinc species. Our proposed
cascade comprises a Zn(OTf)2/NR3 catalyzed nucleophilic
addition of a terminal alkyne to an N-tBu nitrone to A,
We then tested the scope of our CDC under the opti-
mized conditions. The (6-methoxy-2-naphthyl)alkynyl Zn
compound underwent smoothtransformationto5einhigh
(6) (a) Wei, Y.; Zhao, H. Q.; Kan, J.; Su, W. P.; Hong, M. C. J. Am.
Chem. Soc. 2010, 132, 2522. (b) Haro, T. de.; Nevado, C. J. Am. Chem.
Soc. 2010, 132, 1512.
(7) Kharasch, M. S.; Joshi, B. S. J. Org. Chem. 1957, 22, 1439. 3 is
readily prepared by treatment of o,o0-di-tert-butylphenol with O2.(a) De
Sarkar, S.; Grimme, S.; Studer, A. J. Am. Chem. Soc. 2010, 132, 1190. (b)
De Sarkar, S.; Studer, A. Org. Lett. 2010, 12, 1992. (c) De Sarkar, S.;
Studer, A. Angew. Chem., Int. Ed. 2010, 49, 9266.
(13) Under identical conditions using 2.2 equiv of TEMPO isopropyl
N-Bn nitrone gave the desired product in 76% yield along with the
undesired regioisomer formed via oxidation at the benzylic position in
9% yield.
(8) Negishi, E.-I.; Anastasia, L. Chem. Rev. 2003, 103, 1979.
(9) Trost, B. M.; Weiss, A. H. Adv. Synth. Catal. 2009, 351, 963.
(10) (a) Li, C.-J. Acc. Chem. Res. 2010, 43, 581. (b) Carreira, E. M.;
(14) Review: Vogler, T.; Studer, A. Synthesis 2008, 845. Other
applications of TEMPO as a mild oxidant in catalysis:Vogler, T; Studer,
A. Org. Lett. 2008, 10, 129. Kirchberg, S.; Vogler, T.; Studer, A. Synlett
2008, 18, 2841. Vogler, T.; Studer, A. Adv. Synth. Catal. 2008, 350, 1963.
Guin, J.; De Sarkar, S.; Grimme, S.; Studer, A. Angew. Chem., Int. Ed.
2008, 47, 8727. Maji, M. S.; Pfeifer, T.; Studer, A. Angew. Chem., Int. Ed.
€
Frantz, D. E.; Fassler, R.; Tomooka, C. S. Acc. Chem. Res. 2000, 33,
373. (c) Chen, L.; Li, C. J. Adv. Synth. Catal. 2006, 348, 1459. (e) Zani, L.;
Bolm, C. Chem. Commun. 2006, 4263.
€
(11) (a) Frantz, D. E.; Fassler, R.; Carreira, E. M. J. Am. Chem. Soc.
€
1999, 121, 11245. (b) Fassler, R.; Frantz, D. E.; Oetiker, J.; Carreira,
€
2008, 47, 9547. Kirchberg, S.; Frohlich, R.; Studer, A. Angew. Chem.,
E. M. Angew. Chem., Int. Ed. 2002, 41, 3054. (c) Topic, D.; Aschwanden,
Int. Ed. 2009, 48, 4235. Maji, M. S.; Studer, A. Synthesis 2009, 2467.
Pouliot, M.; Renaud, P.; Schenk, K.; Studer, A.; Vogler, A. Angew.
Chem., Int. Ed. 2009, 48, 6037. Maji, S. M.; Pfeifer, T.; Studer, A.
€
P.; Fassler, R.; Carreira, E. M. Org. Lett. 2005, 7, 5329. (d) Miura, M.;
Enna, M.; Okuro, K.; Nomura, M. J. Org. Chem. 1995, 60, 4999.
(12) (a) Hamer, J.; Macaluso, A. Chem. Rev. 1964, 64, 473. (b) Cicchi,
S.; Marradi, M.; Goti, A.; Brandi, A. Tetrahedron Lett. 2001, 42, 6503.
(c) Qureshi, A. K.; Sklarz, B. J. Chem. Soc. C 1966, 412. (d) Cicchi, S.;
Corsi, M.; Goti, A. J. Org. Chem. 1999, 64, 7243. (e) Cardona, F.;
Gorini, L.; Goti, A. Lett. Org. Chem. 2006, 3, 118.
€
Chem.;Eur. J. 2010, 16, 5872. Kirchberg, S.; Frohlich, R.; Studer, A.
Angew. Chem., Int. Ed. 2010, 49, 6877. Maji, S. M.; Murarka, S.; Studer,
A. Org. Lett. 2010, 12, 3878. Kirchberg, S.; Tani, S.; Ueda, K.;
Yamaguchi, Y.; Studer, A.; Itami, K. Angew. Chem., Int. Ed. 2011, 50,
2387.
Org. Lett., Vol. 13, No. 10, 2011
2747

yield (entry 4). Terminal alkynes bearing sterically de-
manding aliphatic substituents (Me3Si and tBu) reacted
with 1a to give 5i, j in high yields (entries 5,6). The nitrone
substituent R1 could also be varied (see 5k, 5l, entries 7, 8).
However, the presence of 3 clearly leads to faster and
cleaner reactions with higher yields. Decreasing the Zn-
(OTf)2 loading led to worse results (entries 12ꢀ14), and
switching to Et3N as a base further improved the yield
(76%, entry 13). The optimal loading of cooxidant 3 in
terms of economy, yield, and reaction time was found to be
15 mol % (entries 8, 11, 15ꢀ17).
Table 1. CDC between Various Nitrones 1 and Alkynes 2 Using
a Stoichiometric Amount of Oxidant 3
Table 2. CDC between Nitrone 1a and Alkyne 2a under Dif-
ferent Conditions Using Dioxygen and 3 as a Cooxidant
[Zn]
iPr2NEt
3
yield
(%)b
entry
(mol %)
(mol %)
(mol %)
solv/t (°C)
1
20
20
20
20
20
20
20
20
20
20
20
10
15
15
20
20
20
50
50
50
50
50
50
50
50
50
50
50
25
40
40
50a
50a
50a
20
20
20
20
20
20
15
10
30
40
ꢀ
DCM/40
THF/66
DCE/84
BTF/100
BTF/75
BTF/60
BTF/60
BTF/60
BTF/60
BTF/60
BTF/60
BTF/60
BTF/60
DCE/60
BTF/60
BTF/60
BTF/60
28c
46c
59d
45d
68d
72e
71
70f
74g
80h
55i
12j
66
2
3
4
5
entry
1 (R1)
2 (R2)
product
yield (%)a
6
7
1
2
Me2CH
Me2CH
Me2CH
Me2CH
Me2CH
Me2CH
Me2CH
Me2CH
c-C6H11
c-C5H9
Ph
Ph
Ph
Ph
Ph
5a
5a
5a
5a
5a
5e
5i
n.r.b
74%c
84
77d
83e
87
8
9
3
10
11
12
13
14
15
16
17
4
5
20
20
10
15
10
ꢀ
6
6-MeO-2-naphthyl
46c,j
76
74f
54i
7
Me3Si
tBu
Ph
85
8
5j
80
9
5k
5l
87
10
Ph
91
a Yield of isolated product. b 1.1 equiv of DDQ was used as an
oxidant. c 2.2 equiv of TEMPO were used as an oxidant. d NEt3 used as a
base. e At room temperature for 24 h using NEt3 as a base.
a With NEt3 as a base. b Yield of isolated product; reactions were
carried out on a 0.5 mmol scale for 24 h. c Oxidation did not go to
completion. d Decomposition was observed. e Reaction was finished in
20 h. f Reaction time extended to 30 h. g Reaction was finished in 18 h.
h Reaction was finished in 16 h. i Reaction was carried out in absence of
oxidant. j Addition did not go to completion.
We next envisioned running our CDC reaction catalytic
in oxidant 3 in the presence of O2 and began our studies by
reacting 1a with 2a in the presence of 20 mol % Zn(OTf)2,
Under optimized conditions reaction scope was ex-
plored (Table 3). Arylacetylenes bearing electron-donating
and -withdrawing substituents at the aryl moiety reacted
with 1a to give 5bꢀe with very good yields (entries 2ꢀ5).
The bisalkyne homocoupling products were not identified.
Enynes were observed to be good substrates (entry 6), and
aliphatic terminal alkynes afforded 5g,h,j in acceptable
yields (entries 7, 8, and 10). Silyl protected alkyne under-
went smooth transformation to 5i (entry 9).
A wide range of nitrones 1 were then tested using
phenylacetylene as a reaction partner. Aliphatic nitrones
derived from cyclic aldehydes provided 5k,l,n in good
yields (entries 11, 12, and 14). Nitrones derived from
acyclic aldehydes also worked well (entries 13, 15). Aro-
matic nitrones either worked sluggishly or did not react at
all (results not shown).11a
€
50 mol % Hunig’s base, 20 mol % of oxidant 3, and
dioxygen (1 atm, balloon, Table 2). Several solvents in-
cluding DCM, THF, and DCE were screened. As com-
pared to the stoichiometric protocol, 5a was isolated in
lower yields using DCM or THF, due to incomplete
oxidation (entries 1, 2). An increase of the temperature
by switching to DCE improved the yield to 59%; however,
the product partly decomposed (entry 3). As fluorinated
solvents do better dissolve dioxygen, we tested R,R,R-
trifluorotoluene (BTF)15and the yield was improved to
72% at 60 °C (entry 6). Lower yields due to product
decomposition were obtained at higher temperatures
(entries 4, 5). Changing the loading of 3 to 15, 10, and 30
mol % did not affect the yield to a large extent (entries
7ꢀ9), and the highest yield was obtained with 40 mol % of
3 (80%, entry 10). CDC with O2 in the absence of 3 gave 5a
in 55% yield (entry 11). These results indicate that back
ground aerobic oxidation is occurring at a reasonable rate
and regeneration of 3 from 4 with O2 is not very efficient.
Isoxazoles belong to an important class of five-mem-
bered nitrogen heterocyclesthatare embedded ina number
of pharmaceutically important compounds.16 Various
methods for their preparation have been reported; how-
ever, only a few of them are general, regioselective, and
(15) (a) Ogawa, A.; Curran, D. P. J. Org. Chem. 1997, 62, 450. (b)
Deschamps, J.; Costa Gomes, F. M.; Padua, A. H. J. Fluorine Chem.
2004, 125, 409.
(16) Sperry, J.; Wright, D. Curr. Opin. Drug Discovery Dev. 2005, 8, 723.
Org. Lett., Vol. 13, No. 10, 2011
2748

isoxazole 7a. Unfortunately, most of them either failed to
react (scandium tiflate, ytterbium triflate, copper triflate,
tin triflate) or provided multiple products (conc. HCl, HCl
in dioxane, HCl in methanol, HCl in isopropanol, triflic
acid, trifluoroacetic acid). In some cases 7a was obtained in
a poor yield along with 5a (BF3/Et2O, BBr3). However,
BCl3 (30 mol %) in dichloroethane at 90 °C provided 7a in
a good yield (71%). We also found that 5a undergoes
quantitative cleavage of the tBu group to give oxime 6a
with TiCl4 in CH2Cl2 (sealed tube at 50 °C for 24 h). Gold-
catalyzed cycloisomerization18,21 of 6a eventually afforded
7a in 85% overall yield. The two-step protocol was also
successfully applied to the preparation of 7b and 7c.
Table 3. CDC between Various Nitrones 1 and Alkynes 2 Using
Oxidant 3 (15 mol %) and Dioxygen As Oxidants
entry
1 (R1)
Me2CH
2 (R2)
product yield (%)a
1
Ph
5a
5b
5c
5d
5e
5f
76
2
Me2CH
Me2CH
Me2CH
Me2CH
Me2CH
Me2CH
Me2CH
Me2CH
Me2CH
c-C6H11
c-C5H9
3-Me-C6H4
4-MeO-C6H4
4-CF3ꢀC6H4
6-MeO-2-naphthyl
1-Cyclohex-1-enyl
nPr
82
3
72
4
77
5
80
6
65
7
5g
5h
5i
67
Scheme 3. Transformation of Alkynylnitrones 5 to 3,5-Disub-
stituted Isoxazoles 7
8
c-C6H11
52
9
Me3Si
68
10
11
12
13
14
15
tBu
5j
70
Ph
5k
5l
82
Ph
74
1-Phenylethyl Ph
c-C3H5 Ph
C6H5CH2CH2 Ph
5m
5n
5o
65 (76)b
64
62
a Yield of isolated product and reactions were carried out on a 0.5
mmol scale. b Yield in the bracket obtained with 30 mol % Zn(OTf)2 and
60 mol % of NEt3.
high yielding.17ꢀ19 The [3 þ 2] cycloaddition of alkynes
with nitrile oxides, which probably belongs to the most
direct route to isoxazoles, shows some drawbacks such as
competing dimerization of the nitrile oxides, low yields,
and occasional lack of regioselectivity.20 We thought that
selective removal of the N-tBu group of the CDC products
5 should deliver oximes 6, which should undergo cycliza-
tion to give 3,5-disubstituted isoxazoles 7 (Scheme 3).
We first tested various Brønsted and Lewis acids under
different conditions for the direct transformation of 5a to
In conclusion, we have documented an efficient zinc
triflate catalyzed CDC between N-tBu nitrones and term-
inal alkynes using 3 and O2 as oxidants. Reactions can be
performed on a wide array of alkynes and nitrones.
Importantly, alkynylated nitrones were successfully trans-
formed to regioisomerically pure 3,5-disubstituted isoxa-
zoles. To our knowledge, this sequence represents a novel
approach for the preparation of isoxazoles.
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Acknowledgment. We thank the SFB 858 for support-
ing our work (stipend to S.M.).
Supporting Information Available. Experimental de-
tails and characterization data for the products. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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2749