Amidines ketenimines

The substituent R determines the reactivity of the isocyanate. Aromatic isocyanates react faster than aliphatic isocyanates, and carbonyl and sulfonyl isocyanates are considerably more reactive than the former. Isocyanate groups attached to oxygen or nitrogen are not stable in their monomeric forms. In cycloaddition reactions, isocyanates react preferentially across their C=N bonds, but additions across the C=0 bonds are also encountered. In this respect, isocyanates resemble ketenes (see Chapter 4, Section 4.1.). Suitable substrates for cycloaddition reactions are carbon multiple bonds (acetylenes, olefins, ketenes, etc.), C=N bond-containing compounds (imines, amidines, ketenimines, azines, carbodiimldes, etc.), C=0 bonds and C=S bond-containing substrates and phosphorus multiple-bond-containing substrates. Cycloaddition reactions of isocyanates across multiple metal bonds are also known. 

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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