Arenesulfonyl isocyanates 3+2 cycloaddition reactions

The reactivity of isocyanates in [2+2] cycloaddition reactions is as follows alkyl < aryl < nitroaryl << arenesulfonyl < halosulfonyl. Also, the reactivity of the substrate is determined by the substituents. For example, vinyl ethers and enamines are more reactive than olefins. Often the formation of the [2+2] cycloadducts involves polar linear intermediates, which can be intercepted by the isocyanate or the substrate to form six-membered ring [2+2+2] cycloadducts (see Section 3.3.1.4). Also, diynes react with isocyanates to give six-membered ring [2+2+2] cycloadducts. In the latter reactions catalysts play an important role. From Q, ty-diynes macrocyclic adducts are obtained. 

In contrast, reaction of diazomethane with phenyl carbonyl isocyanate affords an oxa-zolone derivative formed via a [4+2] cycloaddition sequence . Diazomethane reacts with arenesulfonyl isocyanates to give 1,2,3-oxathiazole derivatives. The reaction of phenyl isocyanate with photochemically generated diphenylcarbene gives 2,2-diphenylindoxyl. ... 

Across C=N bonds The cycloaddition reactions of isocyanates across C=N bonds usually afford [2-1-2] and [2-I-2-I-2] adducts but linear 1 1 adducts are also encountered. If the reaction is concerted, as in the dimerization of isocyanates, the four-membered ring [2-1-2] cycloadducts are obtained. However, when the reaction proceeds stepwise, the initial acyclic polar adduct can be intercepted by either the isocyanate or the substrate to give [2-I-2-I-2] cycloadducts. If the lifetime of the initial acyclic adduct is shorter than the time for rotation around the bond formed, the process is indistinguishable from the concerted reaction . An exchange reaction may proceed via the intramolecular [2-1-2] cycloaddition and the new set of double bonded substrates and heterocumulenes can participate in the reactions. An example is the cycloaddition reaction of arenesulfonyl isocyanates with carbodiimides. ... 

The polar cycloaddition reaction of arenesulfonyl isocyanates with carbodiimides gives rise to the formation of cyclic six-membered ring 2 1 adducts 56 and 57 . N-Sulfonylcarbodiimide is generated in an exchange reaction as shown in the following reaction scheme. 

---