Aerobic oxidative esterification of benzyl alcohols with catalytic tetrabromomethane under visible light irradiation

Tetrahedron Letters 53 (2012) 5306–5308

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Aerobic oxidative esterification of benzyl alcohols with catalytic tetrabromomethane under visible light irradiation Tomoya Nobuta, Akitoshi Fujiya, Shin-ichi Hirashima, Norihiro Tada, Tsuyoshi Miura, Akichika Itoh ⇑ Gifu Pharmaceutical University 1-25-4, Daigaku-nishi, Gifu 501-1196, Japan

a r t i c l e

i n f o

Article history: Received 28 May 2012 Revised 12 July 2012 Accepted 20 July 2012 Available online 27 July 2012

a b s t r a c t We report a useful method for the facile synthesis of aromatic esters from benzyl alcohols with molecular oxygen and catalytic tetrabromomethane in alcohol under visible light irradiation with a fluorescent lamp. This is the first metal-free reaction using molecular oxygen as the terminal oxidant. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Esterification Tetrabromomethane Visible light Aerobic Benzyl alcohol

Aromatic esters have always attracted a great deal of interest in organic synthesis as intermediates such as liquid crystal polymers, cosmetics, pharmaceuticals, agrochemicals, and food additives because of their versatility. In general, aromatic esters have been synthesized for the esterification of carboxylic acid, which is obtained by the oxidation of alcohols with heavy metals such as Cr or Mn.1 Recently, the direct oxidative esterification of alcohols has attracted a great deal of attention from the viewpoint of reducing energy consumption, labor, and solvents. These reactions involve the use of stoichiometric amounts of oxidants such as Ca(OCl)2,2 isocyanuric chloride,3 I2,4 KI/t-BuOOH,5 NaIO4/LiBr,6 PhI(OAc)2/I2,7 and PhIO/KBr8 or transition metal catalysts such as Mo,9 Mn,10 Pd,11 Ir,12 and Ru.13 On the other hand, the aerobic catalytic oxidative esterification of alcohols has been developed in recent years. Molecular oxygen is photosynthesized by plants and is a more efficient oxidant than other oxidants such as toxic heavy metals or complex organic reagents, theoretically producing only water as the end product of oxidation. However, these reactions require transition metal reagents such as Au14 or Pd.15 Thus, more environmental friendly and economical methods of oxidative esterification of alcohols are needed. We have developed various aerobic photooxidation reactions under oxygen atmosphere and light irradiation.16 Recently, we reported the aerobic photooxidative synthesis of aromatic methyl esters from methyl aromatics in the presence of catalytic CBr4.17 This facile and efficient method is of interest in green chemistry

because it does not use heavy metals, whereas it makes use of molecular oxygen and inexpensive reagents. However, this reaction has limited substrate scope. When electron-poor substrates were used, a xenon lamp was required, which resulted in low yields. Furthermore, only methyl carboxylates were obtained by this reaction, because other alcohols, such as ethanol or propanol that is used as solvent, are more easily oxidized than methanol. Therefore, we studied the oxidative esterification of benzyl alcohols under visible light irradiation from general purpose fluorescent lamp. In this Letter, we report the details of the aerobic photooxidative synthesis of aromatic esters from benzyl alcohols (Scheme 1). Table 1 shows the reaction conditions for the aerobic photooxidative synthesis of 4-tert-butyl methylbenzoate (2a) from 4-tertbutyl benzylalcohol (1a) as the test substrate using methanol as solvent under an O2 atmosphere irradiated with four 22 W fluorescent lamps. Among the bromine sources examined, 0.3 equiv of CBr4 were found to afford the desired product 2a most efficiently (entries 1–10). The excellent yield of 2a was obtained by prolonging the reaction time to 20 h (entry 11). It is noted that using of two fluorescent lamps resulted in low yield (entry 12). The fact that 2a was not obtained without CBr4, irradiation, and molecular oxygen shows that this reaction needs them (entries 13–15). It is noted

O Ar

⇑ Corresponding author. Tel./fax: +81 58 230 8108. E-mail address: [email protected] (A. Itoh). 0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2012.07.091

OH

O2, hν, CBr4 ROH Scheme 1.

Ar

OR

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T. Nobuta et al. / Tetrahedron Letters 53 (2012) 5306–5308 Table 1 Study of reaction conditions for aerobic photooxidation

O2, hν (VIS) bromine source MeOH, rt

OH t

Bu 1a (0.3 mmol)

O2, hν (VIS) CBr4 (0.3 equiv)

CHO

5b (0.3 mmol) t

2b (71%)

Bu 2a

OMe

Bromine source (equiv)

Time (h)

Yielda (%)

1 2 3 4 5 6 7 8 9b 10c 11 12d 13 14e 15f 16g

LiBr (0.3) NaBr (0.3) KBr (0.3) MgBr2 (0.3) CaBr2 (0.3) Aq HBr (0.3) Br2 (0.3) CBr4 (0.3) CBr4 (0.2) CBr4 (0.1) CBr4 (0.3) CBr4 (0.3) — CBr4 (0.3) CBr4 (0.3) CBr4 (0.3)

10 10 10 10 10 10 10 10 10 10 20 20 20 20 20 20

0 0 0 0 0 0 34 74 0 0 94 10 0 0 3 74

OMe

O2, hν (VIS) CBr4 (0.3 equiv)

c d e f g

2b (87%) O2, hν (VIS) CBr4 (0.3 equiv)

CO2H

2b (37%)

Scheme 2. Study of reaction mechanism.

hν

1) CBr4

hν

Br Br

Br2

HBr

OH

O2

CO2Me MeO

Ar

2i (41%)b

Bu

2a (94%)

CO2Me F

2d (82%)

CO2Me

MeO2C

2e (85%) CO2Me

2g (66%)

CO2Me

CO2Me

CO2Me

10 c

2l (5%) 2k (93%) 2j (84%)b CO2Pr

CO2Et t

a b c

Bu

t

3a (76%)

Bu

Br2

OH

H2O

OH Ar

Ar CHO

OH

ROH

Ar

Br

HBr

OR

OR Ar

OR

RO

OH

Ar

OR

O2

RO

OO

Ar

OR

HBr

Br

6 RO

OOH

Ar

OR

HBr

Br2

ROH Ar CO2R

CO2Me

2h (93%)

2f (92%)

HBr

Scheme 3. Plausible path for aerobic photooxidation of benzyl alcohol to aromatic esters.

CO2Me O2N

OH

2b (73%)

Cl

Cl

CO2Me

Ar

Br

5

CO2Me Cl

2c (61%)

OH

HBr

2

CO2Me t

Ar

OO

8

OOH

product

ROH, rt, 20 h

(3)

MeOH, rt, 10 h

1

Table 2 Aerobic photooxidative synthesis of aromatic esters from benzyl alcoholsa

substrate (0.3 mmol)

CO2Me

7b (0.3 mmol)

OR

O2, hν (VIS) CBr4 (0.3 equiv)

(2)

MeOH, rt, 10 h

2) Ar

recovered in 86% yield. recovered in 98% yield. out using two fluorescent lamps. out in the dark. out under an N2 atmosphere. out under an air atmosphere.

CO2Me

6b (0.3 mmol)

a 1

H NMR analysis. Starting material 1a was Starting material 1a was The reaction was carried The reaction was carried The reaction was carried The reaction was carried

(1)

MeOH, rt, 10 h

CO2Me

Entry

b

CO2Me

4a (63%)b

Isolated yields. The reaction was carried out for 36 h. Determined by 1H NMR.

that air instead of molecular oxygen can be also used in this oxidative esterification (entry 16). Table 2 presents the scope and limitation of the aerobic photooxidative synthesis of aromatic esters from benzyl alcohols under the optimized reaction conditions.18 In general, the corresponding aromatic esters were obtained in good to high yields regardless of

the electron-donating or electron-withdrawing group in the benzene ring (2a–2h). Methyl 4-methoxybenzoate (2i) was obtained in moderate yield in spite of prolonging the reaction time. Both 1-naphthalenemethanol (1j) and 2-naphthalenemethanol (1k) were suitable substrates for this reaction affording the corresponding esters 2j and 2k in high yields, respectively. Unfortunately, aliphatic alcohol was a poor substrate and resulted in low yield (2l). Interestingly, ethanol and propanol can be used under these oxidative esterification conditions giving the aromatic esters 3a and 4a in good yields, respectively. In order to examine the intermediates of this reaction, benzaldehyde (5b) and benzaldehyde dimethyl acetal (6b) were subjected to the same aerobic photooxidation conditions for 10 h, and methyl benzoate (2b) was obtained in good yields (Scheme 2, Eqs. 1 and 2). In addition, the esterification of carboxylic acid (7b) was slow under these conditions, and methyl ester (2b) was obtained in 37% yield (Scheme 2, Eq. 3). Therefore, these results suggest that the reaction proceeds with aldehyde (5b) and demethyl acetal (6b) as intermediates. Scheme 3 shows a plausible path of this oxidation, which is postulated by considering all the above mentioned results and the necessity of molecular oxygen and continuous irradiation in this reaction. The first step involves the abstraction of the hydrogen radical from the benzyl position of benzyl alcohols 1 with bromine radical, generated from CBr4 under light irradiation, to produce the radical species 8. This is followed by oxygenation to afford alde-

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T. Nobuta et al. / Tetrahedron Letters 53 (2012) 5306–5308

hyde 5, which is transformed to acetal 6 under the reaction conditions in alcohol. Acetal 6 is oxidized to aromatic ester 2 under aerobic photooxidative conditions. In conclusion, we have developed an aerobic photooxidative synthesis method of aromatic esters in the presence of a catalytic amount of CBr4. This method is of great value from the viewpoint of green chemistry and organic synthesis because it uses inexpensive bromine source, harmless visible light irradiated from a general purpose fluorescent lamp, and molecular oxygen as the terminal oxidant. Further application of this photooxidation reaction to other reactions is now in progress in our laboratory.

14.

15.

16.

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