Silyl-substituted diazoacetic esters in superacid media—a stable ion and solvolysis study

Abstract

For methyl diazo(triisopropylsilyl)acetate (1) and methyl diazo(pentamethyldisilanyl)acetate (2), the protonation and subsequent chemistry in superacids, as well as the solvolysis reactions with non-aqueous acids (FSO₃H, CF₃SO₃H, HF) have been studied. Upon solvolysis of 1 by FSO₃H or CF₃SO₃H, protonation, dediazoniation and gegenion addition occur without concomitant desilylation, whereas with HF, partial desilylation takes place. Upon solvolysis of 2, only desilylated esters are obtained. When 1 is allowed to react with the superacid FSO₃H·SbF₅(1 : 1)/SO₂ at ⩽–75 °C, five major ions are formed, among them the silyl-substituted enoldiazonium ions (Z)- and (E)-9 corresponding to O-protonated 1 and the protonated (fluorosulfonyloxy)acetate 4-H⁺. In the lower-acidity superacid FSO₃H/SO₂ the O-protonated silyldiazoesters [(Z)-9, (E)-9] and the C,O-diprotonated (14-H⁺) silyldiazoesters are present. Ion 14-H⁺ is slowly transformed into 3-H⁺ by N₂loss and gegenion addition. In the reaction mixture from 2 and FSO₃H·SbF₅(1:1)/SO₂, the O-protonated silyldiazoester [enoldiazonium ion (Z)-15] as well as the desilylated ions 4-H⁺ and 10(or 11) can be identified in relative yields that are temperature dependent (–75 or –85 °C). The silyl-substituted ions 9 and 15 are the first enoldiazonium ions derived from a diazoacetic ester that have been observed, and 16 is the first alkanediazonium ion of this type.