Oxidation of benzyl alcohols, benzyl halides, and alkylbenzenes with oxone

Tetrahedron 68 (2012) 9763e9768

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Oxidation of benzyl alcohols, benzyl halides, and alkylbenzenes with oxone Keshaba Nanda Parida, Samik Jhulki, Susovan Mandal, Jarugu Narasimha Moorthy * Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 May 2012 Received in revised form 3 September 2012 Accepted 5 September 2012 Available online 11 September 2012

Oxidation of benzyl alcohols, benzyl halides, and alkylbenzenes to their corresponding oxidation products has been shown to be accomplished directly with oxone. The methodology that involves mere stirring/heating of the reactants and oxone in acetonitrile/water (1:1, v/v) is simple and practical, but is limited to substrates that do not contain sensitive functionalities and heteroaromatic rings. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Oxidation of primary alcohols to corresponding aldehydes/carboxylic acids and secondary alcohols to ketones is a much visited chemistry, and can be accomplished with a number of organic and transition metal-based oxidation reagents.1 In our laboratories, we have recently shown that alcohols can be conveniently oxidized with modified IBXs2a,b as well as with precursor iodo-acids catalytically in the presence of oxone as a terminal oxidant.2c We surmised that benzyl halidesdlike those of benzyl alcoholsdshould be possible to be converted to the carbonyl compounds using iodoacid as a catalyst and oxone as a co-oxidant; indeed, we showed some time ago that primary and secondary halides can be converted to aldehydes and ketones using IBX in DMSO at high temperatures.3 In our attempts to obviate the use of DMSO as a solvent and to develop a catalytic protocol, we discovered that oxone itself is effective in some of the oxidations. Oxone is a mild, inexpensive, easy-to-handle, and environmentally benign oxidation reagent that has found extensive application in organic synthesis.4 It has largely been employed as a co-oxidant in the presence of a catalyst for a variety of oxidative transformations.2c,5 There are only a few reports in which oxone has been directly employed for oxidations.6e12 Given that oxone is largely employed as a cooxidant, exploration of its true potential in oxidations directly was deemed necessary. It is ironical that IBX has emerged as a fantastic oxidation reagent than its triacetate, namely DesseMartin periodinane. Now, we seem to realize that oxone itself is great, at least for some oxidations, than IBX, which is produced by oxidation of o-iodobenzoic acid with oxone. Herein, we report the oxidation of benzyl alcohols, benzyl halides, and aromatic

* Corresponding author. Tel.: þ91 512 2597438; fax: þ91 512 2597436; e-mail address: [email protected] (J.N. Moorthy). 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2012.09.028

hydrocarbons using oxone and point out limitations with the use of oxone. 2. Results In our initial attempts to optimize the reaction conditions for oxidation of p-bromobenzyl bromide to the corresponding carboxylic acid using o-iodobenzoic acid and oxone in acetonitrile/ water, we discovered from a control experiment that oxone itself performs the oxidation at reflux conditions. From solvent screening studies, CH3CN/H2O (1:1) mixture was reckoned to be the best combination to examine the oxidation of a range of substrates. Thus, both benzyl chlorides and bromides were found to be converted to their corresponding carboxylic acids when heated at reflux with 3e5 equiv of oxone in acetonitrile/water (entries 1e7, Table 1). TLC monitoring of the reactions indicated that the reactions in all cases proceeded through the intermediacy of aldehydes, which subsequently underwent oxidation to the carboxylic acids with oxone, vide infra. It should be noted that the oxidation of aldehydes to acids with oxone in DMF is a well-established synthetic protocol.13 A similar attempt with the oxidation of secondary benzyl bromide, namely 1-bromo-4-(1-bromoethyl)benzene, in refluxing acetonitrile/water revealed that the oxidation completed within 10 min; product analysis showed that the corresponding a-bromoketone formed as a side product along with the major expected ketone in 85% yield (Eq. 1). When the reaction was conducted at room temperature, the reaction was found to be clean affording the ketone, i.e., 4-bromoacetophenone, primarily. Thus, secondary benzyl halides containing both electron-donating as well as electron-withdrawing groups were found to be oxidized; the substrate with electron-withdrawing group was found to undergo completion over a longer period and at reflux conditions, cf. Table 1 (entries 8e11).

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Table 1 Oxidation of primarya and secondaryb benzyl halides using oxone Entry

Substrate

Equiv/time (h)

Product

Isolated yield (%)

3/5

98

3/15 5/24 4/12 3/9

98 74c 73d 96

6

4/12

85

7

5/24

80e

1

X=H 2 3 4 5

Br NO2 CN Cl

8

0.6/1.8

85

0.6/1 0.6/1.8 1.2/5.5

92 97 93f

X=H 9 10 11

Br Cl NO2

a

Reactions were run at reflux in 1:1 (v/v) acetonitrile/water mixture. Unless mentioned otherwise, the reactions were run at rt in 1:1 (v/v) acetonitrile/water mixture. c Starting material (14%) was recovered. d Formation of unidentified nonpolar compound was also observed, which seemed to account for loss of the starting material. e Starting material (3%) was recovered. f The reaction was performed at reflux conditions. p-Nitrobenzoic acid was isolated as a side product in 5% yield. b

Buoyed by the facile oxidation of both primary and secondary benzyl halides with oxone, we investigated the oxidation of primary and secondary benzyl alcohols using oxone in CH3CN/H2O at reflux. As shown in Table 2, a variety of benzyl alcohols were converted to the corresponding carbonyl compoundseketones in the case of secondary alcohols (entries 10e14, Table 2) and acids (entries 1e6) in the case of primary alcohols; yields of the isolated products are quantitative for alcohols with the exception of electron-poor substrates (entries 3 and 6). Benzyl alcohol with a-carboxy group and benzil were found to undergo oxidation to benzoic acid in high isolated yields (entries 15 and 16). The oxidation of aliphatic alcohols led to surprising results. While the oxidation of dodecyl alcohol and 2-octyloxyethanol led to an intractable mixture, that of the secondary cyclooctanol gave rise to octanedicarboxylic acid in 60% isolated yield (entries 7, 9, and 18). In contrast, cholestanol was almost recovered intact after 36 h (entry 17). Oxidation of cyclohexyl methanol yielded adipic acid in 57% isolated yield (entry 8). We believe that the products in these cases

may arise from radical-mediated reactions. In principle, oxone may undergo dissociation upon thermolysisdlike H2O2dto produce hydroxyl radicals that lead to complicated reaction mixtures; alternative pathway involving the formation of dioxirane and subsequent reactions with it is ruled out, since dioxirane is produced in basic pH conditions.14 In our experiments with oxidation of alcohols, we observed that the reaction of 3-phenylpropanol with oxone led to a complex mixture of products as revealed by TLC, which made us wonder if oxone performs the benzylic oxidation along the lines of KMnO4 as well as the oxidation of aromatics; replacement of transition metal-based oxidation chemistry for alkylbenzenes with a green reagent such as oxone was deemed impressive. Thus, when substituted toluenes were heated at reflux in the presence of 3e6 equiv of oxone for 36 h, the corresponding acids were indeed isolated, cf. Table 3. While the yield of the acid was remarkable in the case of p-bromotoluene (entry 1), the reactions of all other electron-poor arenes were found to be rather sluggish (entries 2e6); the isolated yields of the acids were 48e64%. Oxidation of naphthalene led to trans-o-carboxycinnamic acid in 56% isolated yield together with 1,4-naphthoquinone in 34% yield (entry 7). The formation of o-carboxycinnamic acid may occur via initial epoxidation of the C1eC2 double bond followed by ring opening to afford 2-hydroxynapthalene, which has been reported to yield ocarboxycinnamic acid upon treatment with peracetic acid.15 Oxidation of anthracene afforded bianthronyl16 in 55% isolated yield; of course, 9,10-anthraquinone was also isolated in 23% yield (entry 8). While fluorene underwent a very sluggish reaction to afford fluorenone, its acyclic analog, i.e., diphenylmethane, was found to react better yielding benzophenone in good isolated yields (entries 9 and 10).

3. Mechanism of oxidation To gain insights into the mechanism of oxidation of benzyl halides, progress of the reaction was monitored by 500 MHz 1H NMR spectroscopy with an attempt to observe the intermediate en route to the oxidation product. The 1H NMR spectral profiles for the reaction of p-bromobenzyl bromide, a representative case, in 80:20 mixture of acetonitrile-d3/D2O in the presence of 3 equiv of oxone revealed the formation of aldehyde in small amounts (Fig. 1), which converted rapidly to the acid. At higher conversions after 24 h, the 1 H NMR spectra revealed the formation of a small amount of benzyl alcohol, which isn’t observed in the initial stages. Thus, the primary benzyl halide seemingly undergoes oxidation rather slowly to the aldehyde, which is in turn oxidized rapidly to the acid. The reason as to why the alcohol shows up only after a longer reaction time should be explicable from the following considerations: it is well established that oxone decomposes with time in the acidic medium.17 Thus, the unreacted benzyl halide, after a few

K.N. Parida et al. / Tetrahedron 68 (2012) 9763e9768 Table 2 Oxidation of primary and secondary alcohols using oxonea Entry

Substrate

Equiv/time (h)

Product

3/9.5

1

Table 3 Oxidation of aromatic hydrocarbons using oxonea Yieldb (%)

93

Entry

1

Br NO2 CN Cl

6

Substrate

Me

X

Equiv/time (h)

Product

3/5.5

Yield (%)

96

X = p-Br

X=H 2 3 4 5

9765

3/12 5/24 3/13 4/20

94 92c 99 99

5/24

74d

2 3 4 5 6

p-NO2 p-CN o-NO2 m-CO2H p-CO2H

5/24 5/36 6/24 5/36 5/36

X

COOH

61b 50c 48d 64e 56f

7

5/36

56g

8

5/36

55h

9

5/36

18i

10

5/36

72j

de

7

5/36

8

5/36

9

5/36

10

2/1

62f

2/1.2 2/2 4/20

86 95 93

14

5/22.5

95

15

6/30

85

16

8/48

97

57

de

X=H 11 12 13

Cl Br NO2

a

17

18

Cholestanol

5/36

5/36

dg

60

a

All reactions were run in at reflux in 1:1 (v/v) acetonitrile-water mixture. Isolated yields, unless mentioned otherwise. Starting material (4%) was recovered. d Starting material (14%) was recovered. e An intractable product mixture was observed. f Starting material (21%) was recovered. g No reaction was observed with starting compound recovered almost quantitatively. b c

hours, may slowly undergo hydrolysis to the benzyl alcohol in the absence of active oxone to account for the formation of small amounts of the alcohol. Based on the medium employed, namely acetonitrile/water (50:50), we believe that the oxidation of benzyl halides proceeds via SN1 mechanism, Scheme 1. The sluggish reaction with electron-poor substrates and relatively shorter reaction times for secondary benzyl bromides are consistent with the rate-determining ionization followed by attack of the oxone to yield the ketone. It is noteworthy that one observes a singlet at ca. d 6.9. We presumed that it possibly corresponds to the addition product between the aldehyde and oxone, cf. Scheme 1. However, independent 1H NMR monitoring of the reaction of 4bromobenzaldehyde did not yield a similar signal (Fig. S2,

Oxidations were run in acetonitrile/water mixture (1:1, v/v) at reflux. Starting material (24%) was recovered. c Starting material (43%) was recovered. d Starting material (34%) was recovered. e Starting material (26%) was recovered. f Starting material (38%) was recovered. g Starting material (5%) was recovered and 34% 1,4-naphthoquinone was obtained. h Starting material (3%) was recovered and 9,10-anthraquinone was obtained in 23% isolated yield. i Starting material (71%) was recovered. j Starting material (17%) was recovered. b

Supplementary data). It is not yet clear to us as to what this signal indeed corresponds to. Insofar as the mechanism of oxidation of alcohols is concerned, we believe that the strongly acidic conditions noted above may facilitate the SN1 ionization via protonation, cf. Scheme 1, such that the subsequent steps are similar to those of benzyl halides. 500 MHz 1H NMR monitoring of the reaction reveals formation of the aldehyde in small amounts with rapid formation of the acids, cf. Fig. 1. The ionization appears to be ratedetermining as revealed from rather slow oxidation, for example, of p-nitrobenzyl alcohol (entry 3, Table 2). In this respect, it is worth mentioning that Wu et al. reported the oxidation of various primary and secondary alcohols by oxone in the presence of AlCl3 as an additive in water.18 Quite explicably, the role of AlCl3 in this instance must be to generate HCl, which facilitates ionization of the alcohols. 4. Limitations with oxone as a direct oxidation reagent Given the cheap cost and environmentally benign attributes, oxone should in principle constitute a sought-after oxidation reagent. For benzyl halides, activated benzyl alcohols and some of the alkylarenes as those shown in Tables 1e3, oxone is indisputably

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Fig. 1. 1H NMR monitoring of oxidation of p-bromobenzyl alcohol (left) and p-bromobenzyl bromide (right) in CD3CN/D2O (4:1) with oxone (3 equiv) at reflux. (a) Before addition of oxone, (b) after 1 h of shaking at rt for alcohol and heating at reflux for benzyl bromide, (c) after 2.5 h of heating, (d) after 4.5 h of heating at reflux, and (e) after 24 h of heating at reflux for benzyl bromide.

were found to be quite messy. It has been shown that oxone reacts with pyridines and sulfides to give N-oxides9a and sulfoxides/sulfones,6 respectively; presumably, the double bonds in furans react with oxone.13

5. Conclusions In conclusion, we have found that oxone can be employed directly for oxidation of a number of benzyl halides. In an analogous manner, activated primary and secondary alcohols are shown to be converted to the corresponding acids and ketones, respectively. Based on the medium employed for the reaction and different rates of oxidations observed for differently-substituted substrates, SN1 mechanism has been proposed for the oxidation of benzyl alcohols and benzyl halides. Aromatics and alkylaromatics are also found to undergo oxidation to yield products in respectable yields. Although the methodology reported herein is simple and practical, it suffers from certain limitations. Scheme 1. Mechanism of oxidation of benzyl halides and benzyl alcohols.

6. Experimental section 6.1. General

a good reagent. However, we note several limitations with the use of oxone, which are enumerated below:  Oxone is known to decompose at low pH.17 The saturated solution of oxone is strongly acidic (pH ca. 3.0), and the amount of oxone that is decomposed increases with duration of the reaction. Thus, for substrates that react poorly, the reactions tend to be too sluggish as is evident for the oxidation of p-nitrobenzyl alcohol (entry 3, Table 2).  For unreactive aliphatic alcohols, the reactions are likely to proceed via radical pathways leading to intractable mixtures of products.  Substituted arenesdunless very reactive to oxone oxidationdyield side chain oxidation products competitively.  The reactions of p-methoxy-substituted benzene derivatives, i.e., p-methoxytoluene, p-methoxybenzyl alcohol, p-methoxybenzyl halides, etc., with oxone led to competitive reactions yielding intractable mixtures of products. We believe that these substrates initially afford p-quinonemethides that subsequently react under the employed conditions of the reaction to yield mixtures.19 Further, the reactions of alkylated heteroaromatic compounds such as pyridines, thiophenes, furans, etc.

Solvents were distilled prior to use and double distilled water was used for the reaction. All the reactions were carried out in an open atmosphere without any precaution. The products were isolated by column chromatography with silica gel of 100e200 mm particle size. NMR spectra were recorded with 400 and 500 MHz spectrometers. 6.2. General procedure for oxidation of benzyl halides, alcohols, and aromatic hydrocarbons To 16 mL of acetonitrile/water (1:1 v/v) mixture was added 0.5e1.2 mmol of the starting compound. The contents were heated at reflux with introduction of oxone (cf. entries for each case) incrementally over the entire duration of the reaction. For secondary benzyl halides, the reactions were run at room temperature. The progress of the reaction in each case was monitored by TLC analysis. After completion of the reaction, the reaction mixture was cooled to room temperature and the organic matter was extracted with ethyl acetate. The combined organic extract was dried over anhyd Na2SO4 and concentrated in vacuo. Short pad silica gel column chromatography of the residue led to isolation of the pure product.

K.N. Parida et al. / Tetrahedron 68 (2012) 9763e9768

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6.2.1. o-Nitrobenzoic acid.20 Colorless solid; Rf (10% CH3OH/EtOAc) 0.31; 1H NMR (CDCl3, 400 MHz) d 7.73e7.68 (m, 2H), 7.92e7.86 (m, 2H).

137.6, 145.1, 168.1, 168.7. ESI-MSþ m/z calcd for C10H8O4Na 215.0320 [MþNaþ], found 215.0327.

6.2.2. p-Bromobenzoic acid.2c Colorless solid; Rf (50% EtOAc/pet. ether) 0.48; 1H NMR (CDCl3, 500 MHz) d 7.62 (d, J¼8.55 Hz, 2H), 7.96 (d, J¼8.55 Hz, 2H).

6.2.17. Naphthalene-1,4-dione.30 Yellow solid; Rf (5% EtOAc/pet. ether) 0.5; 1H NMR (CDCl3, 400 MHz) d 6.98 (s, 2H), 7.74e7.77 (m, 2H), 8.07e8.09 (m, 2H).

6.2.3. o-Bromobenzoic acid.21 Colorless solid; Rf (50% EtOAc/pet. ether) 0.49; 1H NMR (CDCl3, 500 MHz) d 7.38e7.43 (m, 2H), 7.72(d, J¼8.2 Hz, 1H), 8.02 (dd, J1¼7.3 Hz, J2¼2.1 Hz, 1H).

6.2.18. Bianthronyl.16 Yellow solid; Rf (17.5% EtOAc/pet. ether) 0.57; mp 265e268  C (lit. 256e258  C); IR (KBr, cm1) 2923, 2852, 1661, 1597, 1463, 1319; 1H NMR (CDCl3, 400 MHz) d 4.78 (s, 1H), 6.86e6.88 (m, 2H), 7.41e7.43 (m, 4H), 7.92e7.95 (m, 2H). ESI-MSþ m/z calcd for C28H19O2Na 387.1385 [MþNaþ], found 387.1385.

6.2.4. p-Cyanobenzoic acid.22 Colorless solid; Rf (50% EtOAc/pet. ether) 0.19; 1H NMR (DMSO-d6, 500 MHz) d 7.98 (d, J¼8.6 Hz, 2H), 8.07 (d, J¼8.6 Hz, 2H). 6.2.5. p-Nitrobenzoic acid.2c Colorless solid; Rf (10% CH3OH/EtOAc) 0.32; 1H NMR (CDCl3, 400 MHz) d 8.27 (d, J¼8.8 Hz, 2H), 8.33 (d, J¼8.8 Hz, 2H). 6.2.6. p-Fluorobenzoic acid.23 Colorless solid; Rf (50% EtOAc/pet. ether) 0.46; 1H NMR (CDCl3, 500 MHz) d 7.15 (t, J¼8.8 Hz, 2H), 8.12e8.15 (m, 2H). 6.2.7. p-Bromoacetophenone.2c Colorless solid; Rf (10% EtOAc/pet. ether) 0.43; 1H NMR (CDCl3, 500 MHz) d 2.58 (s, 3H), 7.60 (d, J¼8. 5 Hz, 2H), 7.82 (d, J¼8.5 Hz, 2H). 24

6.2.8. 2-Bromo-1-(4-bromophenyl)ethanone. Colorless solid; Rf (10% EtOAc/pet. ether) 0.6; 1H NMR (CDCl3, 400 MHz) d 4.4 (s, 2H), 7.64 (d, J¼8.5 Hz, 2H), 7.85 (d, J¼8.5 Hz, 2H). 6.2.9. Acetophenone.2c Colorless liquid; Rf (10% EtOAc/pet. ether) 0.59; 1H NMR (CDCl3, 400 MHz) d 2.61 (s, 3H), 7.46 (t, J¼7.3 Hz, 2H), 7.54e7.59 (m, 1H), 7.96 (dd, J1¼8.5 Hz, J2¼1.4 Hz, 2H). 6.2.10. Benzoic acid.2c Colorless solid; Rf (25% EtOAc/pet. ether) 0.37; 1H NMR (CDCl3, 400 MHz) d 7.49 (t, J¼7.8, 2H), 7.62 (t, J¼7.8 Hz, 1H), 8.12 (d, J¼7.3 Hz, 2H). 6.2.11. Benzophenone.25 Colorless solid; Rf (10% EtOAc/pet. ether) 0.45; 1H NMR (CDCl3, 400 MHz) d 7.48 (t, J¼7.8 Hz, 4H), 7.59 (t, J¼7.3 Hz, 2H), 7.8 (d, J¼8.2 Hz, 4H). 6.2.12. p-Nitroacetophenone.2c Straw-yellow solid; Rf (17.5% EtOAc/ pet. ether) 0.45; 1H NMR (CDCl3, 400 MHz) d 2.68 (s, 3H), 8.11 (d, J¼8.7 Hz, 2H), 8.31 (d, J¼8.7 Hz, 2H). 6.2.13. p-Chloroacetophenone.26 Colorless solid; Rf (10% EtOAc/pet. ether) 0.53; 1H NMR (CDCl3, 400 MHz) d 2.59 (s, 3H), 7.43 (d, J¼8.3 Hz, 2H), 7.89 (d, J¼8.3 Hz, 2H). 6.2.14. p-Chlorobenzoic acid.27 Colorless solid; Rf (25% EtOAc/pet. ether) 0.39; 1H NMR (CDCl3, 400 MHz) d 7.45 (d, J¼8.3 Hz, 2H), 8.04 (d, J¼8.3 Hz, 2H). 6.2.15. 9-Fluorenone.28 Yellow solid; Rf (10% EtOAc/pet. ether) 0.61. H NMR (CDCl3, 400 MHz) d 7.29 (t, J¼7.3 Hz, 2H), 7.47e7.54 (m, 4H), 7.66 (d, J¼7.3 Hz, 2H).

1

6.2.16. o-Carboxycinnamic acid. Colorless solid; Rf (10% MeOH/ EtOAc) 0.2; mp 160e164  C (lit. 170.5e173  C);29 IR (KBr, cm1) 2968, 1759, 1680, 1423, 1278; 1H NMR (acetone-d6, 400 MHz), d 6.39 (d, J¼15.8 Hz, 1H), 7.52 (t, J¼7.6 Hz, 1H), 7.62 (t, J¼7.3 Hz, 1H), 7.81 (d, J¼7.6 Hz, 1H), 8.01 (d, J¼7.8 Hz, 1H), 8.55(d, J¼16 Hz, 1H); 13C NMR (acetone-d6, 125 MHz) d 122.1, 129.3, 130.9, 131.7, 132.2, 133.8,

6.2.19. Anthracene-9,10-dione.31 Yellow solid; Rf (5% EtOAc/pet. ether) 0.54; 1H NMR (CDCl3, 400 MHz) d 7.80e7.82 (m, 4H), 8.31e8.33 (m, 4H). 6.2.20. Isophthalic acid.32 Colorless solid; Rf (10% CH3OH/EtOAc) 0.23; 1H NMR (DMSO-d6, 500 MHz) d 7.63 (t, J¼7.6 Hz, 1H), 8.16 (dd, J1¼7.8 Hz, J2¼1.6 Hz, 2H), 8.48 (s, 1H). 6.2.21. Octane-1,8-dicarboxylic acid.33 Colorless solid; Rf (10% CH3OH/EtOAc) 0.23; mp 128e130  C (lit. 133e135  C); 1H NMR (acetone-d6, 500 MHz) d 1.36e1.38 (m, 4H), 1.59e1.62 (m, 4H), 2.29 (t, J¼7.45 Hz, 4H). 6.2.22. Adipic acid.34a Colorless solid; Rf (10% MeOH/EtOAc) 0.24; mp 146e148  C (lit. mp 147.5e149  C);34b 1H NMR (acetone-d6, 500 MHz) d 1.63e1.67 (m, 4H), 2.30e2.35 (m, 4H). 6.2.23. Benzene-1,4-dicarboxylic acid.32 Colorless solid; Rf (10% CH3OH/EtOAc) 0.2; 1H NMR (DMSO-d6, 500 MHz) d 8.03 (s, 4H), 12.7 (br s, 2H).

Acknowledgements J.N.M. is thankful to the Department of Science and Technology, India for generous financial support. K.N.P., S.M., and S.J. thank CSIR for their research fellowships. Supplementary data 1

H and 13C NMR spectral reproductions for the products of oxidations and 1H NMR monitoring of the oxidations. Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.tet.2012.09.028. References and notes 1. Tojo, G.; Fernandez, M. In Oxidation of Alcohols to Aldehydes and Ketones; Tojo, G., Ed.; Springer ScienceþBusiness Media: New York, NY, 2006. 2. (a) Moorthy, J. N.; Senapati, K.; Singhal, N. Tetrahedron Lett. 2009, 50, 2493; (b) Moorthy, J. N.; Senapati, K.; Parida, K. N. J. Org. Chem. 2010, 75, 8416; (c) Moorthy, J. N.; Senapati, K.; Parida, K. N.; Jhulki, S.; Sooraj, K.; Nair, N. N. J. Org. Chem. 2011, 76, 9593. 3. Moorthy, J. N.; Singhal, N.; Senapati, K. Tetrahedron Lett. 2006, 47, 1757. 4. He, W. Synlett 2006, 3548. 5. (a) Bolm, C.; Magnus, A. S.; Hildebrand, J. P. Org. Lett. 2000, 2, 1173; (b) Travis, B. R.; Narayan, R. S.; Borhan, B. J. Am. Chem. Soc. 2002, 124, 3824; (c) Yang, D. Acc. Chem. Res. 2004, 37, 497; (d) Thottumkara, A. P.; Bowsher, M. S.; Vinod, T. K. Org. Lett. 2005, 7, 2933; (e) Schulze, A.; Giannis, A. Synthesis 2006, 257; (f) Yusubov, M. S.; Zagulyaeva, A. A.; Zhdankin, V. V. Chem.dEur. J. 2009, 15, 11091; (g) Uyanik, M.; Akakura, M.; Ishihara, K. J. Am. Chem. Soc. 2009, 131, 251. 6. Trost, B. M.; Curran, D. P. Tetrahedron Lett. 1981, 22, 1287. 7. Ceccherelli, p.; Curini, M.; Epifano, F.; Marcotullio, M. C.; Rosati, O. J. Org. Chem. 1995, 60, 8412. 8. (a) Molander, G. A.; Cavalcanti, L. N. J. Org. Chem. 2011, 76, 623; (b) Webb, K. S.; Levy, D. Tetrahedron Lett. 1995, 36, 5117. } ck9. (a) Fields, J. D.; Kropp, P. J. J. Org. Chem. 2000, 65, 5937; (b) Priewisch, B.; Ru Braun, K. J. Org. Chem. 2005, 70, 2350. 10. Frigerio, M.; Santagostino, M.; Sputore, S. J. Org. Chem. 1999, 64, 4537.

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