Phosphonium ester salts and Cs-phosphonate esters

Remarkable advances have been made in the past years in the synthesis of isoprenoid Cs-ester units by using 2-methyIbut-3-enenitrile (106) as a starting material. The nitrile 106 is a byproduct or waste product in adiponitrile production and results from direct hydro-cyanation of butadiene. 

A first route [39] starts from 2-methylbut-3-enoic acid ethyl ester (107), which is accessible from 106 by a Pinner reaction. The addition of bromine to 107 is achieved at -20°C in the absence of solvents and the subsequent dehydrohalogenation of 108 to 4-bromo-2-methylbut-2-enoic acid ethyl ester (109) can be effected with NaOH in ethanol. The bromide 109 serves as precursor to the Horner reagent 100 as well as to the phosphonium bromide 99. 

Recently, a novel process has been developed [40] for the manufacture of the phosphonium chloride 98 and the corresponding ethyl sulphate 110. The process comprises a-chlorination of 2-methylbut-3-enenitrile (106) by means of an alkali-metal hypochlorite, after which the 2-chloro-2-methylbut-3-enenitrile (111) formed is subjected either first to an ethanolysis to give the corresponding 2-chloro-2-methylbut-3-enoic acid ethyl ester (112) and then reaction of 112 with triphenylphosphine to give the desired phosphonium chloride 98, or first the reaction of 111 with triphenylphosphine to obtain (3-cyanobut-2-enyl)triphenyl-phosphonium chloride (113) and then the ethanolysis of 113 with concentrated sulphuric acid to give the corresponding ethyl sulphate 110. 

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