Dimethyl acetylene dicarboxylate, insertion

Some aspects of the reactivity of the A-frames formed by Reaction 1 have been explored. Carbon monoxide and sulfur dioxide are readily lost from the respective adducts upon mild heating or exposure to vacuum. The insertions of isocyanides or sulfur have not been reversed. However the oxidation of Pd2(dpm)2(/Lt-S)Cl2 to Pd2(dpm)2-(/x-S02)Cl2 can be effected by using m-chloroperbenzoic acid as oxidant. Acetylene addition is photoreversible photolysis of Pd2(dpm)2-(/Lt-C2 C02CH3 2)C12 forms dimethylacetylene dicarboxylate and Pd2(dpm)2Cl2 (14). Pd2(dpm)2X2 is a catalyst for converting dimethyl-acetylene dicarboxylate into hexamethyl mellitate, and Pd2(dpm)2-(/it-C2 C02CH3 2)X2, which forms during the reaction, is presumed to be an intermediate. 

Similar cis insertion was observed for diphenylacetylene or dimethyl acetylene dicarboxylate (47). 

It is possible to compare the reactivity of some 1,3-dipoles inserted in cyclic structures with that of the same dipoles in open chains. This comparison, recalling that between cyclic and open-chain dienes (Section 4.1.3), is limited to two classes of 1,3-dipoles without a double bond , namely azomethine imines and oxides. For the former, there are data in Table 12 for the reaction with dimethyl acetylene dicarboxylate (iii) the cyclic azomethine imine (sydnone) is about 300 times less reactive than the open-chain cyanoazo-methine imine. In the case of nitrones, a typical comparison is. for reaction with ethyl crotonate in toluene at 100°C "... 

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