METALATED KETENIMINES - DEPROTONATION OF N-ISOPROPYL-DIPHENYLKETENIMINE AND SUBSEQUENT TRAPPING REACTIONS WITH ELECTROPHILES - A THEORETICAL AND EXPERIMENTAL-STUDY

Deprotonation of N-isopropyl-diphenylketenimine 1 may be achieved by surplus strong organic base (2,2,6,6-tetramethylpiperidine, n-butyllithium, potassium tert.butylate in tert.butylmethyl ether). The resulting organometallic suspension is investigated by means of trapping reactions using various electrophiles. From the structures of the trapping products obtained the intermediacy of a substituted 1-methylene-2-aza-allyl anion 9 and of a 3-azapentadienyl anion 10 is deduced. Thus, proton sources yield a mixture of the 2-azabutadiene 2 together with two dimeric products, the 3-azahexatriene 3 and the heterocyclus 4, Methyl iodide leads to the formation of a cross-conjugated dimeric product 5. The 2-azabutadiene derivatives 6 and 7 are the trapping products using trimethylacetyl chloride or dimethyl disulfide, resp., as electrophiles, exhibiting multiple addition to intermediate substituted 3-azapentadienyl anion systems. Quantum chemical calculations (MNDO, PM3, ab initio-methods) are used for the prediction of the gas phase acidities of N-iso-propylformimine and N-isopropylketenimine as model systems; the structures obtained by complete geometry optimizations of the postulated monomeric anions and lithium compounds are discussed.