Isothiocyanates cyclotrimerization reactions

Kinetic and thermodynamic evaluation of the NHC - isothiocyanate coupling reaction yielding (141) has disclosed the reversible nature of this reaction it therefore proved possible to use (141) as a latent catalyst and thereby to achieve efficient 2 -I- 2 -b 2-cyclotrimerization of phenylisocyanate leading to (142). 

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. 

The cyclotrimerization of carbon cumulenes is usually initiated by heat or catalysis. Especially, the use of a catalyst assures that trimerization can be accomplished in quantitative yields. The base catalyzed cyclotrimerization reaction seems to be limited to ketenes, isocyanates, isothiocyanates and carbodiimides. In the trialkylphosphine catalyzed trimerization of methyl isocyanate an asymmetric trimer is obtained. 

Cyclotrimerization of nitriles, isocyanates, isothiocyanates, imidates, and carbodiimides all lead to symmetrical 1,3,5-triazines. New catalysts have been introduced in some cases to carry out these transformations with increased yields and under relatively milder reaction conditions. The cyclotrimerization of monocyanamides has been reviewed <89RCR879>. 

Cyclopentadienylcobalt complexes are also good for co-cyclotrimerization of alkynes with other unsaturated compounds containing the carbon-heteroatom double bonds, especially when they are part of the cumulene system such as isocyanates, diimides, and carbon dioxide. The reaction conditions are essentially the same as in the previously mentioned processes. However, the biggest problem remains the selectivity for the formation of heterocycles, because of the strong competition for the formation of benzene derivatives. Whereas co-cyclotrimerization of diimides and isocyanates results in the formation of reasonable yields of the corresponding heterocycles 170 and 171 (Scheme 75), in the case of carbon dioxide the yields are generally low [108, 109]. Recently, it has been shown that the ruthenium complex 106 is capable of efficient catalysis of co-cyclotrimerization of diynes and isocyanates [110] and isothiocyanates [111] under mild reaction conditions. 

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