Cyclotrimerization of Triynes or Dienynes

Transition metal-catalyzed intermolecular [2 + 2 + 2] cyclotrimerization of alkynes to benzene derivatives has been extensively studied. In this section, the focus is on the cyclo-trimerizations of the substrates bearing three independent unsaturated bond components. The key issue with this type of process usually involves the challenge of controlling regioselectivity [1—1]. However, 1,3,5-trisubstituted benzene 44 can be obtained as the sole product in good yield when 3-butyn-2-one 43 is used as the substrate for the cyclotrimerization catalyzed by Rh2(pfb)4 (pfb=perfluorobutyrate) in the presence of EtsSiH under a CO atmosphere (Eq. 11) [30]. 

A cationic rhodium complex-catalyzed codimerization of 1,3-dienes with alkynes gives the corresponding cyclohexadienes in good yields with high regioselectively, as exemplified in the reaction of 2-methyl-l,3-butadiene with phenylacetylene (Eq. 12) [31]. 

A bimetaUic rhodium complex has also been employed for the cyclotrimerization of (trifluoromethyl)acetylene 45 at room temperature to afford 1,3,5-trifluoromethyl-benzene 46 in good yield (Eq. 13) [32]. 

It is possible to carry out the [2+2+2] cyclotrimerization reaction in a regioselective manner by using a partially or completely intramolecular approach. Rhodium-catalyzed intramolecular cyclotrimerization of 1,6,11-triynes, which construct fused 5-6-5 ring-systems, has been studied extensively [33-36]. Cyclization of 1,6,11-triyne 47 catalyzed by RhCl(PPh3)3, gives the tricyclic benzene 48 in good yield (Eq. 14) [33a]. 

An interesting appHcation of the rhodium-catalyzed cyclotrimerization of diynes with alkynes to the synthesis of C-acyl glycosides has been reported, wherein ethynylglycal 52 (Eq. 15), as well as diethynylglucose derivative 54 (Eq. 16), are employed to give the corresponding adducts 53 and 55, respectively [38]. 

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