Amidines from ketenimines

Another example in which a Grignard base generates a reactive intermediate by elimination is depicted in Eq. (32) [39]. a-Cyanoeneamines are deprotonated by MeMgl, giving the reactive trialkyl ketenimine. The ketenimine subsequently reacts with primary amines, yielding amidines. The critical step is cyanide elimination from the deprotonated eneamine. The keteneimines could be isolated either by careful distillation or could be directly reacted with amines. 

The reaction of thioketens and amino-azirines proceeds by C=N cleavage to give the dipolar species (293) or ketenimines (294), depending upon substitution hydrolysis of the latter leads to 2-thiazolin-5-ones (295). There are few general routes to 1,3,4-thiadiazolines and a new method, discovered accidentally, provides the derivatives (296) which, since they have amidine characteristics, will probably prove to be useful synthons. Thiadiazolidine diones are readily available from metallated iV-(trimethylsilyl) alkylamines. 

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