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Transition metal-free oxidative esterification of benzylic alcohols in

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aqueous medium Supravat Samanta,a Venkatanarayana Pappula,a Milan Dindaa and Subbarayappa Adimurthy* a

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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Oxidative esterification of benzylic alcohols with catalytic amount of HBr-H2O2 in aqueous medium under mild conditions is reported with wide range of substrate scope for both benzylic and aliphatic alcohols. The conditions are also suitable for selective monoesterification of ethylene glycol and glycerol. With catalytic amount of HBr (20 mol%) and H2O2, the generation of reactive intermediate species BrOH has been ascertained by UV-visible spectra.

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Introduction

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Esters are the most common and fundamental building blocks used in variety of organic transformations.1 Particularly, aromatic carboxylic esters received much attention due to their diverse interest in synthesis of organic intermediates such as polymers, cosmetics, pharmaceuticals, agrochemicals, food additives and advanced materials.2 As a result, remarkable progress has been achieved by various groups over the past few years on the synthesis of ester derivatives. Traditionally, the reaction between carboxylic acids and alcohols under the assistance of activating reagents or water abstracting reagents are well established procedure for ester synthesis. In literature, stoichiometric amounts of heavy-metal oxidants (KMnO4, CrO3, ozone, oxone, N-iodosuccinimide) and transition-metal catalysts including precious metals such as silver, palladium, ruthenium, rhodium, gold, etc. were usually employed for the success of these transformations.3 Metal-free oxidative esterification of alcohols are rarely reported in literature,4 also separation of metal catalyst from products is of particular importance. Moreover, transition metal-catalyzed reactions also generate hazardous waste which is environmentally problematic and hence, should to be avoided wherever possible. Furthermore, it is also highly desirable to develop environmentally benign oxidative esterification processes without requirement of any metal catalyst. In continuation of our efforts on the development of green and sustainable methods for the oxidation of organic substrates,5

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Scheme 1.

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To the best of our knowledge, no similar HBr-catalyzed reactions are known to date. Therefore, the development of an efficient, and environmentally benign catalytic system for oxidative esterification with benzylic alcohols remains challenging.

Results and discussion

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Academy of Scientific & Innovative Research CSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar –364 002, India E-mail: [email protected] †Electronic Supplementary Information (ESI) available: [General procedure; copies of NMR spectra and mass data for selective compounds.] See DOI: 10.1039/b000000x/

herein, we wish to report a general catalytic oxidative crossesterifications of benzylic and aliphatic alcohols (Scheme 1), which proceeds with high selectivity under mild conditions.

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We initiated our studies with benzyl alcohol 1a as substrate and subjected oxidative esterification with catalytic amount (20 mol %) of commercially available aqueous hydrobromicacid (HBr) and hydrogen peroxide (6.0 eqv. w.r.t 1a) in methanol at room temperature. After 16 h the desired product methyl benzoate 3a was obtained in 31% isolated yield (Table 1, entry 1). When the reaction temperature was increased to 40 °C, the yield of the product was also increased to 47% (Table 1, entry 2). However, at 60 °C the product was obtained in 87% isolated yield (Table 1, entry 3). When the reaction was performed with reducing the amount of HBr to 10 and 15 mol%, the yield was decreased to 76% and 81% respectively (Table 1, entries 4 and 5). Further no improvements were observed either by decreasing the oxidant or in absence of both oxidant and HBr catalyst (Table 1, entries 6 10). By decreasing the reaction time from 16 h, the yield of the product was dropped (Table 1, entries 11-13). Then we focused on the effect of other solvents on the oxidative esterification of 1a [journal], [year], [vol], 00–00 | 1

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Table 2. Substrate scope of esterificationa OH

R1

R2

HBr (0.2 mmol)

R2OH

R1

H2O2 (6.0 mmol ), 60°C 2

1

OMe

OMe Cl

3a;87%

OMe

OMe

OMe

3f;73%

Cl

3g; 72%

OEt

3e; 83%

OMe

OEt

Br 3i; 82%

3h;79%

OEt

OEt

Br

OMe O2N

3d;71%

3c; 92

NO2

F

OMe

Br

3b; 85%

OMe

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3

3j; 84%

OnBu

OnBu

Cl 3k; 89%

3l; 69%

OnBu Cl

3n; 80%b

3m; 81%

OnBu

OnOct

Cl 3q; 85%b

OnOct Br 3u; 80%

3r; 76%b

O S

b

OnOct

OnOct

Br 3p; 79%b

3o; 68%b

3v; 65%

OMe

3s; 65%b

OMe

3t;77%b

OMe

N 3w; 0%a,b

3x; 0%a,b

a Reaction conditions:1.0 mmol of benzyl alcohol 1, 6.0 mmol of H2O2, 0.2 mmol HBr (46% aqueous solution), in 2.0 mL of 2 at 60 °C for 16 h; isolated yield. bReactions carried out at 70 -75 °C. 35

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Reaction conditions:1.0 mmol of benzyl alcohol 1a, 6.0 mmol of H2O2, 0.2 mmol HBr (46% aqueous solution), in 2.0 mL of MeOH at 60 °C for 16 h. bIsolated yield. c0.1 mmol of HBr was used. d0.15 mmol HBr was used. eAbsence of HBr. f5.0 mmol of MeOH was added. (DCE: Dichloro ethane)

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with methanol by fixing temperature (60 °C), time (16 h), catalyst (20 mol%) and oxidant (6.0 equivalents). The reaction was not favorable in various solvents (CH3CN, DMF, DMSO, CHCl3, CH2Cl2, DCE, CH2Br2, toluene, water, and 1,4-dioxane) screened out for the present transformation (Table 1, entries 14-23).

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Having identified the optimized reaction conditions, we turned our attention to the examination of substrate scope and limitation of this catalytic oxidation system (Table 2). As evident from Table 2, the oxidative esterification of a variety of benzyl alcohols with methanol were smoothly transformed into the corresponding products in moderate to excellent yield. The presence of a variety of electron-donating/-withdrawing groups (CH3, Cl, Br, F, and NO2) at either (para/meta) position of benzyl alcohol could be tolerated and afford the corresponding methyl ester in 71-92% yields (3a–3i). With ethanol similar yields of corresponding ethyl ester were obtained (3j–3m). The variety of benzyl alcohols with aliphatic alcohols such as n-butanol and noctanol were also afforded the corresponding butyl and octyl esters (3n–3u). However, for long chain aliphatic alcohols it required 70-75 °C for efficient conversion. Electronic effects associated with electron- donating/withdrawing substituents did not affect the efficiency of the process. Then we extended the

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strategy for thiophen-2-ylmethanol with methanol, the corresponding methyl ester 3v was also obtained in 65% yield. Unfortunately, the present catalytic system is not applicable for pyridin-2-ylmethanol and unsaturated aromatics. Finally, to explore the potential applications of this protocol, we investigated the selective mono esterification of polyalcohols with benzylic alcohols under the optimized conditions (Table 3). To our delight, we obtained good yields of mono esters of ethylene glycol and naturally abundant glycerol with representative benzylic alcohols 5a-5f. Table 3. Monoesterification of polyalcoholsa

a Reaction conditions:1.0 mmol of benzyl alcohol 1, 6.0 mmol of H2O2, 0.2 mmol HBr (46% aqueous solution), in 2.0 mL of 4 at 70 -75 °C for 16 h; Isolated yield.

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Table 1. Optimization of reaction conditions for 3aa

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To understand the reactive species involved in the present transformation, we recorded the UV-absorption spectra of mixture of HBr-H2O2 system, in presence and absence of organic substrate (Fig 1). The absorption spectra were recorded at different interval (1, 3, 5 and 7 h) of time. As evident from the

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Conclusion

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In conclusion, we have developed transition metal-free oxidative esterification of benzylic alcohols with catalytic HBr in aqueous medium. The method has no influence on both electrondonating/withdrawing substituents and also applicable for selective mono-esterification of ethylene glycol and glycerol under mild conditions. We have also demonstrated the in-situ generation of the reactive species BrOH with catalytic HBr-H2O2 using UV-visible spectroscopic evidence and these species are accountable for the oxidative esterification.

Fig 1. UV-Absorption spectra of reaction mixture (a) in presence of benzyl alcohol, (b) in absence of benzyl alcohol.

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plot of the absorbance v/s wavelength, the aqueous bromine generated in-situ from HBr- H2O2 will disproportionate to BrOH and HBr (Scheme 3). The absorption bands at 220 nm, 266 nm and 390 nm correspond to HBr, BrOH and Br2 respectively.6 Figure 1, indicates the different species formed during the reaction (fig 1a) in presence of alcohol, the peak at 220 nm (HBr) decreases initially and then slowly increases with the progress of the reaction.7 Initially, the peak at 266 nm (BrOH) reaches maximum and then it completely disappears as reaction proceeds. This clearly indicates that reactive species BrOH will oxidize the alcohol to aldehyde and bromide atom ends up as HBr after completion of the reaction. Whereas in Fig 1b, in absence of benzyl alcohol, the peak at 266 nm (BrOH) increases as there was no organic substrate available for oxidation by these species in the system. In this manner the catalytic HBr generates the reactive species in-situ to accelerate the oxidative esterification of alcohols to methyl esters and makes the process environmentally friendly reagent dispensing the use of precious/heavy metals. Further, to confirm the radical reaction pathway,4 we performed a

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Scheme 3. Proposed reaction mechanism

Acknowledgment.

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Scheme 2. Mechanistic study 70

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reaction under our optimized conditions using TEMPO as radical scavenger, no desired product 3a was observed (Scheme 2). This study supports that, the reaction proceeds through radical mechanism. Hence, based on the above observations, a plausible reaction mechanism has been proposed in Scheme 3. Firstly, the generated bromide radical reacted with benzyl alcohol to give the corresponding radical species I. Then the reaction of radical species I with bromine generated benzaldehyde and bromide radical; subsequently followed by proton abstraction to provide the benzoyl radical II. The formed benzoyl radical II was further reacted with bromide radical to give the desired benzoyl bromide. The formed benzoyl bromide was oxidized to give the desired methyl ester after reacting with methanol. This journal is © The Royal Society of Chemistry [year]

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(CSIR-CSMCRI- Communication No. 109/2014). S. S, M. D. and P.V. are thankful to CSIR and UGC New Delhi, India for their fellowships. We are thankful “Analytical Discipline and Centralized Instrumental Facility” division of CSMCRI for providing instrumentation facilities. We also thank, CSIR (CS0123 INDUS MAGIC) and DST, Govt. India (SR/S1/OC13/2011) for financial support.

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Transition metal-free oxidative esterification of benzylic alcohols in aqueous medium.

Oxidative esterification of benzylic alcohols with a catalytic amount of HBr-H2O2 in aqueous medium under mild conditions is reported with a wide rang...
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