Cyclization hydrocarbons, metal-catalyzed

Yasuo Yamazaki and Tadashi Kawai Metal-Catalyzed Cyclization Reactions of Hydrocarbons Zoultan Paal... 

Catalytic cyclization is a very important reaction, both commercially and theoretically. Here we review metal-catalyzed cyclization of alkylaromatic hydrocarbons, and we present reaction mechanisms that govern these reactions. 

Cyclization can be catalyzed by acids and by metals. Whereas the mech-ansim of acid-catalyzed cyclization is well understood, many questions remain about the nature of metal-catalyzed cyclization. It is, however, generally observed that the presence of an aromatic ring enhances the rate of cyclization alkylaromatics cyclize at a much higher rate than aliphatic hydrocarbons. 

Transition metals catalyze a very wide variety of hydrocarbon reactions ranging from hydrogenation of olefins and exchange of paraffins with deuterium at lower temperatures to skeletal rearrangement, cyclization, hydrogenolysis, cracking, and carbiding under more severe conditions. Because of this flexibility in... 

A common feature of any cyclization reaction is that a new intramolecular C—C bond is produced that would not have been formed in the absence of the catalyst. Those reactions in which one ring closure step is sufficient to explain the formation of a given cyclic product will be called simple cyclization processes, although their mechanism is, as a rule, complex. We shall distinguish those cases in which any additional skeletal rearrangement step(s) is (are) required to explain the process. Some specific varieties of hydrocarbon ring closure processes are not included. A recent excellent review deals with the formation of a second ring in an alkyl-substituted aromatic compound (12). Dehydrocyclodimerization reactions have also to be omitted—all the more since it is doubtful whether a metallic function itself is able to catalyze this process (13). 

Numerous examples of an organometallic based methodology for the formation of selenophenes have been discussed. In these cases, a selenolate anion is formed in situ followed by cyclization. The application to selenophenes is due to the Brandsma group <83TL2203>. Indeed, when hydrocarbon (49) is first metallated and then treated with selenium, a mixture of (50) (46%) and (51) (42%) is formed after hydrolysis (Equation (11)). The base catalyzed isomerization of (50) to (51) is quantitative. 

---