【反应机理】亚磷酸三苯酯和碘甲烷体系碘代反应是什么机理？

亚磷酸三苯酯和碘甲烷体系可以将醇转化为相应的碘代烃。亚磷酸三苯酯先和碘甲烷反应得到甲基三苯氧基碘化鏻【碘化甲基（三苯氧基）鏻】，其能和醇进行酯交换，发生苯酚的取代。鏻衍生物是一个好的离去基，能形成膦酸盐。因此，碘化物进攻磷酸盐发生构型转换。

反应机理

此反应机理可以参考和，如下图

反应中亚磷酸酯是甲酯或乙酯，而此反应利用苯酯，因此不能进行上述反应，但是亚磷酸三苯酯和碘甲烷反应生成的甲基三苯氧基碘化鏻中间体，可以迅速和底物醇发生酯交换得到类似A的中间体, 然后发生类似的历程发生构型反转，碘离子发生SN2取代得到碘代烃产物。

反应实例

A 500-ml., two-necked, round-bottomed flask fittedwith a reflux condenser equipped with a calcium chloride drying tube is chargedwith 136 g. (115 ml., 0.439mole) of triphenyl phosphite, 35.2 g. (0.400 mole) of neopentylalcohol, and 85 g.(37 ml., 0.60 mole) of methyl iodide.A thermometer of sufficient length extends into the liquid contents of the flask. The mixture is heated under gentle reflux with an electric heatingmantle until the temperature of the refluxing liquid rises from its initialvalue of 75–80° to about 130°, and the mixture darkens and begins to fume. Thetime required is about 24 hours. It is necessary to adjust the heat input fromthe mantle from time to time as the reaction proceeds and the reflux ratediminishes. The reaction mixture is distilled under reduced pressure through a13-cm. Vigreux column. The fraction boiling below 65° (50 mm.) is collected and washed with 50 ml. of water,then with 50-ml. portions of cold 1 N sodiumhydroxide until the washings no longer contain phenol.The product is washed again with 50 ml. of water, dried over calciumchloride and redistilled, yielding 51–60 g. (64–75%) of neopentyl iodide, b.p. 54–55° (55 mm.), nD21 1.4882.

【 OrganicSyntheses, Coll. Vol. 6, p.830; Vol. 51, p.44】

A 500-ml., two-necked, round-bottomed flask fittedwith a reflux condenser equipped with a calcium chloride drying tube is chargedwith 124 g. (107 ml., 0.400mole) of triphenyl phosphite and 85 g.(37 ml., 0.60 mole) of methyl iodide.A thermometer of sufficient length extends into the liquid contents of theflask. The mixture is heated under gentle reflux with a heating mantle untilthe internal temperature has risen to about 120°. At this point the mixture isdark and viscous. The flask is cooled, and 40 g.(0.40 mole) of cyclohexanol is added to the oily methyltriphenoxyphosphoniumiodide. The mixture is shaken gently until homogeneous and allowedto stand overnight at room temperature. The mixture is distilled through a13-cm. Vigreux column, yielding 62.5–63 g.(74–75%) of iodocyclohexane, b.p. 66–68° (12 mm.), nD22 1.5475.

【Organic Syntheses, Coll. Vol. 6, p.830; Vol. 51, p.44】

To a solution of methyltriphenoxyphosphoniumiodide (2.3 mmol) in anhydrous dimethylformamide was added alcohol(2 mmol) at room temperature. Afterstirred for 18 h, the solvent was removed in vacuo and the residue wasdissolved in chloroform, extracted with 5% sodium thiosulfate, washed withwater and dried. Direct crystallizationfrom chloroform-hexane gave the iodide in 77% yield as needles.

【 J. A. C. S. 1964, 2093-2095】

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