General Papers ARKIVOC 2006 (i) 119-127
HBr in an alcoholic medium and other organic solvent.21 The pH of the neat acetic acid recorded was 0.8 which drop to a value of -0.9 on addition of TBATB under the identical reaction condition. The HBr with pKa (-9) is sufficiently acidic as compared to protonated carboxylic acid pKa (-7) and protonated alcohol pKa (-2).23 Thus, alcohol would preferentially protonated over carboxylic acid. The nucleophilic attack of carboxylate on the oxonium species will yield acylated product as shown in Scheme 1.
Bu4NBr3 + CH3COOH
HBr
R O H R R O H H R Br R'COOH R O R R' O H R O R R' O + HBr + -H+ + Scheme 1. Mechanism of acylation
By employing this reagent a wide variety of aliphatic, aromatic primary, secondary benzylic alcohols containing electron releasing and electron withdrawing groups could be acetylated to produce the corresponding esters in good to excellent yields (Table 1). Under the present optimized reaction condition, aliphatic alcohols (Table 1, entries 1-3) were transformed to corresponding acetates in excellent yields. Benzylic alcohols with electron donating substituent (Table 1, entry 4) and electron withdrawing substituent (Table 1, entry 5) could also be acetylated in shorter time. Acetylation of hindered primary alcohols (Table 1, entries 7-8) could be achieved in good yields. However, symmetrical diols (Table 1, entries 9-11) were diacetylated completely under the present reaction condition. Hindered secondary alcohols (Table 1, entries 12-15) were converted to their acetates in moderate yield. Small percentage of brominated product (< 5%) was observed as side product for substrate containing double bond (Table 1, entries 16-17). More over, acid sensitive groups such as OMe (Table 1, entry 4); allyloxy group (Table 1, entry 17) as well as base sensitive benzoate group (Table 1, entries 1819) remained intact under the described reaction condition revealing the functional group compatibility of this method.
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