[90054-58-3] · C12H28O4Si2 · 1,2-Diethoxy-1,2-bis(trimethylsilyloxy)ethylene · (MW 292.52)
(synthetic equivalent of the acyl anion -C(O)CO2Et; useful in ZnCl2-mediated nucleophilic additions to aldehydes and ketones,1 Michael additions to enones,1 and substitution reactions of SN1-active alkyl halides1 and carboxylic acid chlorides2)
Physical Data: bp 68-70 °C/5 mmHg.
Solubility: sol CH2Cl2, toluene, THF.
Preparative Method: can be prepared in one step from Diethyl Oxalate using the Rühlmann version2 of the acyloin condensation (eq 1).3 Although a yield of 64% of isolated product was reported,3 this could not be reproduced in a careful series of experiments.1,4 Rather, yields of 21-25% are consistently obtained (0.1-1.0 molar scale).1,4 The product consists of a (Z/E) isomeric mixture which is of no consequence for synthetic applications.
Handling, Storage, and Precautions: somewhat air and moisture sensitive and should therefore be handled and stored under an atmosphere of an inert gas such as N2 and in nonprotic solvents. Use in a fume hood.
The reagent described herein has the structural features of an O-silyl ketene acetal, and it therefore undergoes the typical reactions known for this class of compounds, especially Lewis acid-mediated C-C bond-forming processes.5 Since the reagent is extremely electron rich6 and therefore very reactive, catalytic amounts of mild Lewis acids such as ZnX2 suffice.1,4 The nucleophile which reacts with electrophilic organic substrates is a special case of an acyl anion equivalent (synthon) (1).
Group transfer-type7 of Mukaiyama aldol addition8 to aldehydes or ketones affords the isolable O-silylated aldol adducts, which can be hydrolyzed under acidic conditions to provide a-keto esters in high yield (eq 2).1 In the case of aldehydes the initial products are O-silyl-protected reductones.1
Group transfer-type of Michael additions to a,b-enones are just as smooth, with acidic workup affording a,d-diketo esters (eq 3).1,4
SN1-active alkyl halides require longer reaction times (typically 3-6 h),1,4 and the yields are lower. t-Butyl chloride and adamantyl bromide have been used successfully, but the generality of the procedure needs to be demonstrated (eq 4). In contrast, acylation reactions using a wide variety of carboxylic acid chlorides have been carried out (eq 5).9,10 The initial products are protected forms of a,b-diketo esters which can be isolated in yields of 72-93%. Deprotection is accomplished using MeOH/HCl.9,10 The products are vicinal tricarbonyl compounds having the usual characteristic chemical properties such as rapid formation of hydrated derivatives.11 The protected forms can be used directly in the synthesis of heterocycles.9,10
In summary, the title compound, readily available from diethyl oxalate, is a synthetically useful synthon in various C-C bond-forming reactions, requiring only catalytic amounts of mild Lewis acids such as Zinc Chloride. Alternatives to such an acyl anion equivalent include dithiane derivatives of glyoxylic acid esters, which have not been tested as widely and which require some synthetic effort in the final deprotection step.12
Manfred T. Reetz
Max-Planck-Institut für Kohlenforschung, Mulheim an der Ruhr, Germany