Copper-catalyzed oxidative transformation of secondary alcohols to 1,5-disubstituted tetrazoles

Article abstract of DOI:10.1002/adsc.201300863

A mild and convenient oxidative transformation of secondary alcohols to 1,5-disubstituted tetrazoles is uncovered by employing trimethylsilyl azide (TMSN₃) as a nitrogen source in the presence of a catalytic amount of copper(II) perchlorate hexahydrate [Cu(ClO₄)₂ ^.6 H₂O] (5 mol%) and 2,3-dichloro-5,6-dicyano-para- benzoquinone (DDQ) (1.2 equiv.) as an oxidant. This reaction is performed under ambient conditions and proceeds through C-C bond cleavage.

Full text of DOI:10.1002/adsc.201300863

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DOI: 10.1002/adsc.201300863
Copper-Catalyzed Oxidative Transformation of Secondary
Alcohols to 1,5-Disubstituted Tetrazoles
Balaji V. Rokade,a Karthik Gadde,a and Kandikere Ramaiah Prabhua,*
a
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, Karnataka, India
Fax : (+91)-80-2360-0529; e-mail: prabhu@orgchem.iisc.ernet.in
Received: September 24, 2013; Revised: December 13, 2013; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300863.
Abstract: A mild and convenient oxidative transfor-
mation of secondary alcohols to 1,5-disubstituted
tetrazoles is uncovered by employing trimethylsilyl
azide (TMSN3) as a nitrogen source in the presence
TMSN3 (5.5 equiv.), harsh reaction conditions (808C),
and requires additives and long reaction times.
In continuation of our investigations on the utility
À
of azides in C N bond forming reactions, especially
of a catalytic amount of copper(II) perchlorate
leading to the formation of nitriles,[13] and in the light
of Jiaoꢀs work,[11] we anticipated that secondary alco-
hols in the presence of a Cu salt, an excess of trime-
thylsilyl azide (TMSN3) and 2,3-dichloro-5,6-dicyano-
1,4-benzoquinone (DDQ), may form the correspond-
ing azide in situ and then undergo a rearrangement to
form the nitrilium ion, which may be trapped in
a [2+3]cycloaddition fashion by the excess azide pres-
ent in the reaction medium (Scheme 1). Herein, we
present our detailed investigation for converting sec-
ondary alcohols directly to 1,5-disubstituted tetrazoles
in a one-pot fashion under ambient conditions.
.
hexa
hydrate [Cu
N
dichloro-5,6-dicyano-para-benzoquinone
(DDQ)
(1.2 equiv.) as an oxidant. This reaction is per-
formed under ambient conditions and proceeds
À
through C C bond cleavage.
Keywords: alcohols; azides; copper; regioselectivi-
ty; tetrazoles
We began our study by investigating the Cu-cata-
Metal-mediated transformations are useful and indis- lyzed reaction of (E)-1,3-diphenylprop-2-en-1-ol 1a
À
À
pensable strategies for C C and C heteroatom bond with TMSN3 as a nitrogen source and DDQ as an oxi-
forming reactions in organic synthesis.[1] In addition, dant (Table 1). Preliminary studies were carried out
À
C N bond forming strategies are not only challenging using Cu(I) and Cu(II) salts such as CuCl, CuBr, CuI,
but have a great impetus as they provide avenues for Cu
A
G
ACHTUNGTRENNUNG(OTf)2.
the synthesis of biologically active and therapeutically Among these copper reagents, Cu
ACHTUNGTRENNUNG
useful heterocycles.[2] Among the heterocyles, tetra- Cu
AHCTUNGTRENNUNG
containing molecules[3] due to their well-known bio- ic amount of Cu
ACHTUNRGTENN(GU ClO4)2·6H2O or CuACHTUNGTRENN(GUN OTf)2 furnished
logical activities[4] as well as vast applications in phar- the expected product (E)-1-phenyl-5-styryl-1H-tetra-
maceuticals[5a] and material sciences.[5c] Traditionally, zole 2a in near quantitative yields (entries 5 and 6,
1,5-disubstituted tetrazoles are prepared by (i) the re-
action of nitriles with alkyl or aromatic azides,[6] (ii)
the reaction of ketones[7] or oximes[8] with sodium
azide or hydrazoic acid and (iii) the reaction of
amides[9] with sodium azide in the presence of PCl5 or
triflic anhydride, etc. Gold-catalyzed synthesis of tet-
razoles using alkynes has been recently reported by
Echavarren and Gaydou.[10] Recently, Jiao and co-
workers developed a method to synthesize 1,5-disub-
stituted tetrazoles using 1,3-diphenylpropene,[11] which
requires precursors that are relatively difficult to
access and require multistep synthetic sequences.[12]
Additionally, this protocol uses a large excess of Scheme 1.
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Balaji V. Rokade et al.
Table 1. Optimization studies.[a]
Table 2. Substrate scope for cinnamyl alcohols.[a]
Entry
Yield
Substrate (1)
Product (2) Yield [%][b]
Entry
Cu Catalyst
Oxidant
(equiv.)
G
[%][b]
1
R=4-MeC6H4
R=4-MeOC6H4 1c
R=3-BnOC6H4
R=1-naphthyl
R=2-FC6H4
1b 2b
80
69
76
70
69
72
85
74
40
2
3
4
5
6
7
8
9
2c
1
2
3
4
5
6
7
8
CuCl
CuBr
CuI
DDQ (2.2)
DDQ (2.2)
DDQ (2.2)
DDQ (2.2)
DDQ (2.2)
DDQ (2.2)
DDQ (2.2)
no oxidant
DDQ (2.2)
DDQ (1.5)
DDQ (1.2)
DDQ (1.2)
nd
nd
nd
nd
99
99
nd
1d 2d
1e
1f
1g
2e
2f
2g
Cu
Cu(ClO4)2·6H2O
Cu(OTf)2
no catalyst
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
(ClO4)2·6H2O
(ClO4)2·6H2O
(ClO4)2·6H2O
(ClO4)2·6H2O
(ClO4)2·6H2O
(ClO4)2·6H2O
(ClO4)2·6H2O
(ClO4)2·6H2O
(ClO4)2·6H2O
A
R=4-FC6H4
N
R
R=4-ClC6H4
R=4-BrC6H4
R=2-thiophene
1h 2h
1i
1j
2i
2j
N
G
nd
9
nd[c]
99
[a]
Reaction conditions: 1 (0.5 mmol), TMSN3 (1.1 mmol),
Cu(ClO4)2·6H2O (0.025 mmol), DDQ (0.6 mmol),
CH2Cl2 (2 mL) at room temperature for 1 h.
Isolated yield.
10
11
12
13
14
15
16
17
18
19
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
N
AHCTUNGTRENNUNG
99
78[d]
99[e]
nd
[b]
DDQ (1.2)
benzoquinone (1.2)
chloranil (1.2)
NaOCl (1.2)
TBHP in decane (1.2)
TBHP in water (1.2)
DDQ (2)
nd
nd
nd
CH2Cl2 (2 mL) at room temperature was chosen as
the optimal conditions to investigate the scope of the
reaction.
nd
The optimized reaction conditions were found to be
applicable to a wide range of substrates (Table 2 and
Table 3). As depicted in Table 2, a variety of cinnamyl
alcohols (substituted with the same aryl groups at the
1 and 3-positions) were converted into their corre-
sponding tetrazoles with good to excellent yields. Sub-
stituents with electron-releasing and electron-with-
drawing natures on the phenyl ring had no effect on
the outcome of the reaction, as it afforded the corre-
sponding tetrazoles 2b, 2c, 2d,[14] 2e, 2f, 2g, 2h and 2i
in good to excellent yields (entries 1–8, Table 2). As
CuI
nd[f]
[a]
Reaction conditions: 1a (0.5 mmol), TMSN3 (1.1 mmol),
catalyst (0.025 mmol), oxidant, dichloroethane (2 mL) at
room temperature for 1 h. nd=not detected.
Isolated yield.
[b]
[c]
[d]
[e]
[f]
NaN3 used instead of TMSN3.
CH3CN used as a solvent.
CH2Cl2 used as a solvent.
Reaction conditions similar to those of ref.[11]
Table 1). The formation of the product tetrazole was can be seen, the halo-substituted tetrazoles 2h and 2i,
not observed when the reaction was carried out either which are potential precursors for further transforma-
in the absence of CuACHTUNGRTNEUNG(ClO4)2·6H2O or DDQ (entries 7 tions, were obtained in good yields (entries 7 and 8,
and 8, Table 1). Further studies pointed out that NaN3 Table 2). Although, heterocyclic substrate such as
is not a suitable nitrogen source for this transforma- (E)-1,3-di(thiophen-2-yl)prop-2-en-1-ol 1j reacted
tion as the reaction of alcohol 1a with NaN3 and well under the optimal conditions, the reaction fur-
DDQ did not furnish the product 2a (entry 9, nished the corresponding tetrazole 2j in lower yield
Table 1). Solvent screening studies revealed that sol- (40%).
vents such as CH3CN and CH2Cl2 are most compati-
After studying the scope of allylic alcohols that are
ble solvents for the reaction (entries 12 and 13, substituted with the same aryl groups at the 1 and 3
Table 1). Other oxidants such as benzoquinone, chlor- positions, the applicability of the reaction was investi-
anil, NaOCl and TBHP were found to be incompati- gated on a spectrum of unsymmetrically substituted
ble as these reactions did not yield the expected tetra- allyl alcohols (Table 3). Reactions of alcohols contain-
zole 2a (entries 14–18, Table 1).
ing a methyl group on the phenyl ring at the 2 and 4
The reaction of 1a under Jiaoꢀs conditions (which positions (1k, 1l and 1m) led to the formation of the
uses 1,3-diphenylpropene) did not furnish the tetra- corresponding tetrazoles 2k, 2l and 2m in good yields
zole 2a, and resulted in the formation of the corre- with different ratios of regiomers (entries 1–3,
sponding chalcone
(entry 19, Table 1).[11] From these screening studies,
the reaction of 1a (0.5 mmol), TMSN3 (1.1 mmol), on phenyl ring such as 1n and 1o were regioselectively
–
1,3-diphenyl-2-propen-1-one Table 3).
Alcohols containing electron-donating substituents
CuACHTUNGTRENNUNG(ClO4)2·6H2O (0.025 mmol), DDQ (0.6 mmol), in transformed into their tetrazoles 2n (12:88) and 2o
2
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Copper-Catalyzed Oxidative Transformation of Secondary Alcohols
Table 3. Substrate scope for cinnamyl alcohols.[a]
Entry
Substrate (1)
Product (2:2’)
2k:2k’(55:45)
2l:2l’ (37:63)
2m:2m’ (64:36)
2n:2n’ (12:88)
Yield [%][b]
1
2
3
4
R1 =Ph, R2 =2-MeC6H4
R1 =Ph, R2 =4-MeC6H4
R1 =4-MeC6H4, R2 =Ph
R1 =Ph, R2 =4-MeOC6H4
1k
1l
1m
1n
G
80
73
71
62
5
R1 =Ph,
1o
2o:2o’ (25:75)
60
6
7
8
9
10
11
12
13
14
15
16
17
18
R1 =Ph, R2 =1-napthyl
R1 =Ph, R2 =2-napthyl
R1 =Ph, R2 =4-NO2C6H4
R1 =Ph, R2 =2-ClC6H4
R1 =Ph, R2 =3-ClC6H4
R1 =Ph, R2 =4-ClC6H4
R1 =Ph, R2 =2,4-ClC6H3
R1 =Ph, R2 =2-FC6H4
R1 =Ph, R2 =4-FC6H4
R1 =Ph, R2 =2-BrC6H4
R1 =Ph, R2 =3-BrC6H4
R1 =4-MeC6H4, R2 =2-napthyl
R1 =Me, R2 =Ph
1p
1q
1r
1s
1t
1u
1v
1w
1x
1y
1z
2p:2p’ (56:44)
2q:2q’ (51:49)
2r:2r’ (100:0)
2s:2s’ (39:61)
2t:2t’ (64:36)
2u:2u’ (47:53)
2v:2v’ (45:55)
2w:2w’ (39:61)
2x:2x’ (41:59)
2y:2y’ (50:50)
2z:2z’ (64:36)
2aa:2aa’ (49:51)
2ab:2ab’ (100:0)
73
65
79
71
83
88
71
80
73
75
62
64
54
1aa
1ab
[a]
Reaction conditions: 1 (0.5 mmol), TMSN3 (1.1 mmol), Cu
at room temperature for 1 h.
Isolated yield.
ACHTUGNRTNE(NGNU ClO4)2·6H2O (0.025 mmol), DDQ (0.6 mmol), CH2Cl2 (2 mL)
[b]
(25:75) in good yields (entries 4 and 5, Table 3). Alco- regioselective formation of the corresponding tetra-
hols such as 1p and 1q gave rise to the formation of zole 2ab in moderate yield (entry 18, Table 3).
tetrazoles 2p and 2q in moderate yields in almost the
In addition, disubstituted benzylic alcohols are also
same regioisomeric ratios (entries 6-7, Table 3). An al- served as good substrates under the optimized reac-
cohol with an electron-withdrawing aryl moiety, such tion conditions as alcohols such as 3a, 3b, 3c, and 3d
as (E)-3-(4-nitrophenyl)-1-phenylprop-2-en-1-ol 1r, furnished the corresponding tetrazoles in moderate
resulted in regioselective formation (100:0) of the cor- yields (entries 1–4, Table 4).
responding tetrazole, e.g., 2r in 79% yield (entry 8,
To expand the substrate scope and to examine the
Table 3). The reaction of alcohols containing chloro functional group tolerance for this reaction, the fol-
as a substituent at different positions on the phenyl lowing experiments were performed. Under the opti-
ring such as 1u (4-Cl) and 1v (2,4-Cl) furnished their mal reaction conditions, a mixture of alcohol 1a and
corresponding tetrazoles 2u and 2v with poor regiose- ketone (5a) or ester (5b) or nitrile (5c) or acid (5d) or
lectivity (entries 11 and 12, Table 3). In contrast, alco- amide (5e) was subjected to this oxidative transforma-
hols 1s and 1t with chloro substitution at the 2- and 3- tion. As expected, the alcohol reacted under the con-
position of the phenyl ring respectively furnished cor- ditions to produce the corresponding tetrazole 2a in
responding tetrazoles 2s and 2t with the opposite re- almost quantitative yields and the ketone (5a), ester
[14]
gioselectivity (entries 9 and 10, Table 3). Alcohols (5b), nitrile (5c),
acid (5d) or amide (5e) remained
containing fluoro and bromo substituents, such as 1w, intact (Table 5).[15]
1x, 1y and 1z produced the corresponding tetrazoles
The suitability of this methodology for scaling up
2w, 2x, 2y and 2z in close to 50:50 ratios in good to was demonstrated by carrying out the reaction on
moderate yields (entries 13–16, Table 3). Unsymmetri- a gram scale. As can be seen, reaction of 1a (1.05 g,
cal alcohol 1aa was also converted into its tetrazole 5 mmol) under the optimal conditions furnished the
2aa in 64% yield in a 49:51 ratio (entry 17, Table 3). product 2a in 71% yield (Scheme 2).
It was interesting to observe that the reaction of alco-
Several experiments were performed using a variety
hol 1ab, containing styryl substitution, resulted in the of substituted alcohols (see Scheme-2, Supporting In-
formation) to understand the origin of the regioselec-
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Balaji V. Rokade et al.
Table 4. Substrate scope for benzyl alcohol.[a]
rich phenyl ring over the electron-poor phenyl ring.
This phenomenon can be attributed to the stabiliza-
tion of the carbocation by the electron-rich phenyl
ring. Furthermore, the azide gets oxidized to the cor-
responding nitrilium ion, where the electron-rich
group migrates to the nitrogen atom and this is em-
phasized by comparing the ratio of regioisomers
formed in the azide stage and the tetrazole stage.
Based on these observations, the overall observed mi-
gratory aptitude of a variety of substituents is as fol-
lows; aryl group (eÀ-rich> eÀ-poor)>alkyl group>
vinyl group (see Supporting Information, Scheme-5).
It is also noteworthy that only highly Lewis acidic
Entry
Substrate
Product Yield
(3)
(4)
[%][b]
1
2
3
4
R1 =4-MeOC6H4, R2 =Me 3a 4a
50
48c
69
48
R1 =4-PhC6H4, R2 =Me
R1 =R2 =4-MeOC6H4
R1 =R2 =4-Me2NC6H4
3b 4b
3c 4c
3d 4d
[a]
copper salts such as CuACTHNUTRGENNUG(ClO4)2·6H2O and CuACHTUNGTRENNUNG(OTf)2
Reaction conditions: 3 (0.5 mmol), TMSN3 (1.1 mmol),
Cu(ClO4)2·6H2O (0.025 mmol), DDQ (0.6 mmol),
ACHTUNGTRENNUNG
are effective for this conversion, while the other
copper salts lead to the formation of the ketone as
the sole product. Also, we observed that the order of
addition of reagents is crucial for the success of the
reaction. If DDQ is added prior to the addition of the
copper salt, the alcohol gets oxidized to the corre-
sponding ketone and the reaction ceases.
CH2Cl2 (2 mL) at room temperature for 1 h.
Isolated yield.
8 h.
[b]
[c]
Table 5. Selective formation of tetrazoles.[a]
Based on these observations (Supporting Informa-
tion, Scheme-3 and Scheme-4), and in light of recent
reports by Echavarren,[10] and Jiao[11,16] we believe
that the reaction proceeds through the corresponding
azide, which was further oxidized to the tetrazole. A
tentative mechanism is presented in Scheme 3.
To summarize, we have found a mild and conven-
ient method to synthesize 1,5-disubstituted tetrazoles
using easily accessible secondary alcohols by employ-
ing TMSN3 as a nitrogen source. This one-pot reac-
tion is performed in the presence of a catalytic
Entry
Substrate (5)
Product [%][b]
(2a)
(5)
1
2
3
4
5a: R1 =-C(O)CH3; R2 =-OCH3
5b: R1 =-C(O)OCH3; R2 =-H
5c: R1 =-CN; R2 =-OBn
42
48
42
52
58
52
52
48
5d: R1 =-COOH; R2 =-OCH3
amount of CuACHTUNGTRNEG(UN ClO4)2·6H2O using DDQ as an oxidant
under ambient conditions, and tolerates a variety of
functional groups such as ketone, ester, nitrile, acid
and amides.
5
5e: R1 =H,
47
53
[a]
Reaction conditions: 1a (0.5 mmol), TMSN3 (1.1 mmol),
Cu(ClO4)2·6H2O (0.025 mmol), DDQ (0.6 mmol),
ACHTUNGTRENNUNG
CH2Cl2 (2 mL) at room temperature for 1 h.
Ratio based on H NMR data.
[b]
1
tivity. Based on these experiments, we believe that
the electronic nature of substituent on the aromatic
ring determines the regioselectivity of the reaction,
which is in agreement with the observation of Jiao
and co-workers.[11] We observed that the regioselec-
tive formation of azide is the determining factor, as
unsymmetrically substituted alcohols preferentially
furnished the azide that was benzylic to the electron-
Scheme 2. Scaling up of the reaction.
Scheme 3. Plausible mechanism.
4
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Copper-Catalyzed Oxidative Transformation of Secondary Alcohols
2003, 125, 9983–9987; e) F. Couty, F. Durrat, D. Prim,
Experimental Section
Tetrahedron Lett. 2004, 45, 3725–3728; f) P. B. Palde,
T. F. Jamison, Angew. Chem. 2011, 123, 3587–3590;
Angew. Chem. Int. Ed. 2011, 50, 3525–3528; g) D. Can-
tillo, B. Gutmann, C. O. Kappe, J. Am. Chem. Soc.
2011, 133, 4465–4475.
Note: Proper safety precautions should be followed when
using azides.[17]
Typical Experimental Procedure
[7] a) H. Suzuki, Y. S. Hwang, C. Nakaya, Y. Matano, Syn-
thesis 1993, 1218–1220; b) A. S. El-Ahl, S. S. Elmorsy,
H. Soliman, F. A. Amer, Tetrahedron Lett. 1995, 36,
7337–7340; c) A. G. Schultz, A. Wang, C. Alva, A. Se-
bastian, S. D. Glick, D. C. Deecher, J. M. Bidlack, J.
Med. Chem. 1996, 39, 1956–1966.
Trimethylsilyl azide (1.1 mmol) was added dropwise to
a
well-stirred mixture of alcohol (0.5 mmol), Cu-
ACHTUNGTRENNUNG(ClO4)2·6H2O (0.025 mmol) in CH2Cl2 (2 mL), after 15 min
DDQ was added (0.6 mmol) and the reaction mixture was
stirred at room temperature for 1 h. Then the reaction mix-
ture was dissolved in small amount of EtOAc (2 mL),
passed through alumina, and purified on a silica gel column
using EtOAc/hexane as eluent.
[8] R. N. Butler, D. A. O. Donoghue, J. Chem. Res. Synop.
1983, 18.
[9] a) J. Zabrocki, J. B. Dunbar Jr, K. W. Marshall, M. V.
Toth, G. R. Marshall, J. Org. Chem. 1992, 57, 202–209;
b) E. W. Thomas, Synthesis 1993, 767–768; c) A. LeTir-
an, J. P. Stables, H. Kohn, Bioorg. Med. Chem. 2001, 9,
2693–2708; d) J. Xiao, X. Zhang, D. Wang, C. Yuan, J.
Fluorine Chem. 1999, 99, 83–85; e) A. F. Hegarty, N. M.
Tynan, S. Fergus, J. Chem. Soc. Perkin Trans. 2 2002,
1328–1334.
[10] During the revision of this manuscript, Echavarran and
co-workers reported a gold-catalyzed synthesis of tetra-
zoles. See: M. Gaydou, A. M. Echavarren, Angew.
Chem. 2013, 125, 13710–13713; Angew. Chem. Int. Ed.
2013, 52, 13468–13471.
[11] F. Chen, C. Qin, Y. Cui, N. Jiao, Angew. Chem. 2011,
123, 11689–11693; Angew. Chem. Int. Ed. 2011, 50,
11487–11491. The reaction of 1a under Jiaoꢀs conditions
did not furnish the tetrazole 2a, and resulted in the for-
mation of the corresponding chalcone (1,3-diphenyl-2-
propen-1-one) (see entry 19, Table 1).
[12] a) J. Wang, W. Huang, Z. Zhang, X. Xiang, R. Liu, X.
Zhou, J. Org. Chem. 2009, 74, 3299–3304; b) Y. Liu, B.
Yao, C. L. Deng, R. Y. Tang, X. G. Zhang, J. H. Li,
Org. Lett. 2011, 13, 1126–1129; c) M. Mayer, W. M.
Czaplik, A. Jacobi von Wangelin, Adv. Synth. Catal.
2010, 352, 2147–2152.
[13] a) M. Lamani, K. R. Prabhu, Angew. Chem. 2010, 122,
6772–6775; Angew. Chem. Int. Ed. 2010, 49, 6622–6625;
b) M. Lamani, P. Devadig, K. R. Prabhu, Org. Biomol.
Chem. 2012, 10, 2753–2759; c) B. V. Rokade, S. K. Mal-
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2012, 77, 5364–5370.
[14] Unlike the reactions of benzyl ethers with DDQ, these
reactions are selective as benzyl group is tolerated
under the reaction conditions.
[15] See the Supporting Information for relevant spectra.
[16] While we were preparing this manuscript Jiao and co-
workers reported a silver-catalyzed nitrogenation of al-
kynes to access tetrazoles. See: T. Shen, T. Wang, C.
quin, N. Jiao, Angew. Chem. 2013, 125, 6809–6812;
Angew. Chem. Int. Ed. 2013, 52, 6677–6680.
Acknowledgements
KRP acknowledges IISc, CSIR (No. 01/2415/10-EMR-II) and
RL Fine Chem for financial support. We thank to Dr. A. R.
Ramesha for useful discussion. BVR thanks CSIR for
a senior research fellowship.
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[17] S. Brꢂse, K. Banert, (Eds.), Organic azides: Synthesis
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pp 1–28.
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