Cyclotrimerization and Cyclotetramerization of Alkynes

Benzene and cyclooctatetraene (COT) derivatives are formed by [2+2+2] and [2+2+2+2] cycloadditions of alkynes. At first the metallacyclopropene 107 and metallacyclopentadiene 108 are formed. Benzene and COT (106) are formed by reductive elimination of the metallacycloheptatriene 109 and the metallacyclononate-traene 110. Formation of benzene by the [2+2+2] cycloaddition of acetylene is catalysed by several transition metals. Synthesis of benzene derivatives from 

Although some degree of regioselectivity is observed in the formation of cobaltacyclopentadienes 112 from unsymmetrically substituted alkynes, the reaction has little synthetic interest because the cyclization of terminal alkynes and unsymmetric internal alkynes gives a mixture of all possible regioisomers of 

As one partial solution, the selective cross-cyclization is possible by stepwise reaction of Cp2Zr (113), generated from zirconocene dichloride and BuLi, with three different symmetric internal alkynes in one pot. The method is based on the conversion of zirconacyclopentadiene 114 to the benzene derivative 115 by the treatment with dimethyl acetylenedicarboxylate and CuCl [52], 

Control of the regioselectivity in Co-catalysed cross-cyclization has been solved in an ingenious way utilizing 1,5-hexadiyne (116) as one component and bis(trimethyl-silyl)acetylene (118) as the other. Although bulky bis(trimethylsilyl)acetylene (118) itself cannot cyclotrimerize to hexasilylbenzene due to steric hindrance, it reacts with the cobaltacyclopentadiene 117 formed from the less bulky diyne 116 to produce the 

The phyllocladane skeleton 131 was constructed efficiently by stereoselective formations of six carbon-carbon bonds and four rings via a one-pot sequence of cyclizations the ene type, [2+2+2], and [4+2] cycloadditions. In this synthesis, the Conia ene reaction of 127 takes place under mild conditions to generate 128, and the cyclotrimerization of its diyne with 118 gives 129. These two reactions are catalysed by CpCo(CO)2. Finally, ring-opening to give 130 and intramolecular Diels-Alder reaction in the presence of DPPE produced the phyllocladane skeleton 131 in a total yield of 42% [55]. 

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Whereas the transition metal catalyzed cyclotrimerization and cyclotetramerization of alkynes leading to benzene or cyclooctatetraene and their derivatives is a rather common reaction, there exist only a few examples of cooligomerizations between alkynes and alkenes or 1,3-butadienes leading to 1,3- or 1,4-cyclohexadiene derivatives20S). It is therefore surprising that the [3+2]-cycloaddition between methylenecyclopropanes and alkynes, catalyzed by triarylphosphite modified Ni(0) compound, is a rather convenient method to synthesize 4-methylenecyclopentenes 206). A wide range of methylenecyclopropanes and alkynes, in the latter case mainly 1,2-disubstituted ones, can be used for these reactions (Eqs. 98-100, see p. 127-128). 

Cyclotrimerization and cyclotetramerization of alkynes to form benzene and cyclooctatetraene derivatives was first achieved with various nickel catalysts26. Several other metal systems are also active and numerous applications have been reported3 5. Various concerted or stepwise reaction paths have been suggested3,5,2 -29. 

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