這是最為經(jīng)典和簡(jiǎn)單的甲基芳基醚水解去甲基的方法。一般使用過(guò)量的濃HI酸回流下、或者大大過(guò)量的48% HBr或37% HCl在HOAc或Ac2O中回流，反應(yīng)完后，濃縮去除過(guò)量的酸，加入水，用有機(jī)溶劑萃取，然后進(jìn)一步純化即可。特點(diǎn)是操作和后處理簡(jiǎn)單、方便。但要求底物對(duì)強(qiáng)酸穩(wěn)定，對(duì)于有些底物，易形成鹵代苯副產(chǎn)物。
在48%HBr中加入相轉(zhuǎn)移催化劑是對(duì)僅使用強(qiáng)質(zhì)子酸如48%HBr的改進(jìn)。加入相轉(zhuǎn)移催化劑后，能提高去甲基化的效率，大大縮短反應(yīng)的時(shí)間。此反應(yīng)是在一個(gè)多相體系（水和有機(jī)）中進(jìn)行，可使用的相轉(zhuǎn)移催化劑有四丁基溴化銨，十六碳烷基三丁基溴化磷,四辛基溴化銨，三辛基甲基溴化銨等。對(duì)于甲基芳基醚，一般使用5 mol量的HBr，對(duì)于甲基烴基醚，一般使用10 mol量的HBr；值得注意的是，反應(yīng)的收率和時(shí)間不是依賴(lài)于使用那一種相轉(zhuǎn)移催化劑，而是依賴(lài)于相轉(zhuǎn)移催化劑的濃度和其在有機(jī)相的溶解度。
反應(yīng)實(shí)例
Typical Pocedure
The ether (300 mg) was dissolved in 48% HBr (50 mL) and heated for 1 hrunder reflux. The reaction mixture was evaporated to dryness and extracted withether. The ethereal extract was washed with sodium carbonate solution andwater, dried and evaporated. The residue was recrystallized from chloroform togive the product (120 mg, 40%).
【 I. Kawasaki, K. Matsuda, and T. Kaneko, Bull. Chem. Soc. Jpn., 1971, 44, 1986.】
Typical Pocedure
Cleavage of Di-n-octyl Ether. Di-n-octyl ether (12.1 g, 0.05 mol), 47% aqueous hydrobromic acid (56mL,0.5 mol), and hexadecyltributylphosphonium bromide (2.5 g, 0.005 mol) are mixed in a flask equipped with amagnetic stirrer and reflux condenser, and heated at 115°C (inner temperature) with stirring for 5 hrs.After this time GLC analysis (SE 30, 3% over chromosorb sulfate, and distilledto give pure 1-bromooctane. The organic layer is separated, dried with sodiumsulfate, and distilled to give pure 1-bromooctane; yield: 17.5 g (91%), bp 88°/torr. By treating thedistillation residue with hexane, 2.3 g(92%) of pure phosphonium bromide are recovered, mp 54-56°C. In the case of aryl alkyl ethers, aqueousalkaline extraction of the organic phase affords the corresponding phenol.
【 D. Landini, F. Montanari and F. Rolla, Synthesis, 1978,771.】
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