Cycloaddition with ketenes thioketenes

Cycloaddition may be varied by the use of fluorinated thioketenes in place of ketenes. Olefins and Schiff bases serve as the other component. Thus thioketene 56 cycloadds to a SchifT base to give product 57 in a 79% yield. In a further variation, reaction of dithiobenzoate esters with diphenylketene yields 61% of product 58. ... 

Thermal and photochemical cycloaddition reactions of 27r-electron species represent an important synthetic approach to four-membered rings. The reactions summarized in this section include 2 + 2 cycloaddition reactions of thioketones, thioketenes, isothiocyanates, sulfenes and iminosulfenes with alkenes, allenes, ketenes, ketenimines and alkynes. 

The cycloaddition reactions are subdivided into di-, tri- and oligomerization reactions, [2-1-1]-, [2-1-2]-, [3-1-2]- and [4- -2] cycloaddition reactions and other cycloaddition reactions. The insertion reactions into single bonds are also discussed. The cyclodimerization or cyclotrimerization reactions are special examples of the [2-1-2] and the [2-I-2-I-2] cycloaddition reactions, respectively. The cumulenes vary in their tendency to undergo these reactions. The highly reactive species, such as sulfines, sulfenes, thioketenes, carbon suboxide and some ketenes, are not stable in their monomeric form. Other cumulenes have an intermediate reactivity, i.e. they can be obtained in the monomeric state at room temperature and only heat or added catalysts cause di- or trimerization reactions. In this group, with decreasing order of reactivity, are allenes, phosphorus cumulenes, isocyanates, carbodiimides and isothiocyanates. 

As a general rule ketenes undergo non-catalyzed [2+2] cycloaddition reactions across their C=C bonds, with the exception of ketene itself. In contrast, disubstituted thioketenes undergo cyclodimerization across their C=S bonds. Mono substituted thioketenes undergo dimerization via a [3+2] cycloaddition reaction, also involving the C=S bonds. 

Carbon cumulenes undergo [2+2] cycloaddition reaction with numerous double or triple bonded substrates to give four-membered ring cycloadducts. Examples of cycloaddition to C=C, C=C, C=0, C=N, C=S, N=0, N=N. N=S. S=0, P=C, P=0, P=N and P=S bonds are known. When the two adjacent double bonds in the cumulenes are different, cycloaddition across either one of the double bonds occurs, and sometimes addition across both bonds is observed. However, more often the cycloaddition reactions follow only one pathway. As a general rule, in ketenes the non-catalyzed cycloaddition occurs preferentially across the C=C bond, whereas catalyzed cycloaddition reactions proceed across the C=0 bond. In thioketenes, isothiocyanates and sulfenes addition mainly occurs across the C=S bond. In isocyanates addition across the C=N bond is preferred. 

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