Six- and Eight-Membered Carbocycles

The stability of the / -Cp-cobalt core even allows elemental sulfur to be incorporated in the alkyne co-cyclization. In the presence of excess alkynes the -Cp-cobalt half-sandwich complexes catalyze the formation of thiophenes (eq. (22)) [54]. Kajitani [55] has expanded this work to rhodium complexes and included selenium as an additional heteroatom. 

In practice the alkyne cyclotrimerization to benzene and its derivatives may be performed using both homogeneous and heterogeneous catalysts. Most catalysts reported in the survey of the catalysts (cf. Section 3.3.8.2) may be applied, giving good yields, to the cyclotrimerization of unsymmetrically substituted terminal and also internal alkynes. As mentioned above, in the case of terminal alkynes 

The regiochemical product distribution of the co-cyclization of two or three different alkynes occurs statistically. In some cases carefully controlled reaction conditions allow isolation of a main product from mixed cyclotrimerizations. For example, l,2,3,4-tetraphenyl-5,6-diethylbenzene can be obtained from cobalt-catalyzed reaction of tolane and 3-hexyne in good yield [62]. The first example of an intermolecular, regiospecific cross-benzannulation reaction catalyzed by Pd(PPh3)4 was reported by Yamamoto [63]. The reaction of 2-alky 1-but-l-ene-3-yne with disubstituted diynes leads exclusively in high yields to 

4- dialkyl-2-ethynylbenzene. No other isomers are formed. The selective synthesis of radiolabeled toluene and p-xylene via co-eyclotrimerization was obtained using a heterogeneous chromium catalyst described in [57, 64]. 

Instead of a second or third alkyne, an alkene C=C double bond may be incorporated into the cyclotrimerization reaction. Iron [65], rhodium [66], nickel [67], palladium [68], or cobalt [69] catalysts have been used to form cyclohexa-dienes. However, the preparative use of this catalytic co-cyclization is disturbed by consecutive side reactions of the resulting dienes such as cycloaddition or dehydrogenation. Itoh, Ibers and co-workers [70] have reported the straight palladium-catalyzed co-cyclization reaction of C2(C02Me)2 and norbomene (eq. (24)). 

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This section focuses on the transition metal-catalyzed formation of ring systems using alkynes in the homogeneous phase. Because of the great number of possible products, this survey is restricted to five- and six-membered heterocycles, and six-and eight-membered carbocycles. 

A number of metal complexes catalyses specific alkyne polymerizations, giving rise to four-, six- or eight-membered carbocyclic rings. The first work in this area was the nickel-catalysed formation of cyclooctatetraene (40) from acetylene by the group of Reppe ", but since then formation of cyclic systems from acetylenes has been found to be also catalysed by molybdenum, cobalt, iridium and tantalum. ... 

Taxol, the anti-cancer compound from yew trees14 (Taxol spp.) has two six- and one eight-membered carbocyclic rings 58. Nicolaou15 saw that two C-C disconnections would give two simple six-membered ring precursors 57 and 59. 

The ring junction stereochemistry in the fused lactones was found to be cis in all cases (Bryan and Chan, 1996). A two-atom ring expansion based on the indium-mediated Barbier-type allylation was recently reported by Li et al. (1996). This novel carbocycle enlargement was applied to five-, six-, seven-, eight- and twelve-membered ring compounds ... 

Similar to cyclodimerizations, transition metal catalyzed cyclooligomerizations and cocyclooligomerizations involve dicarborative addition steps. The most important examples of this type are represented by cyclotrimerization and cyclotetramerization of alkenes. dienes, alkynes and systems with hetero multiple bonds, leading to carbocyclic and heterocyclic six-, eight- or twelve-membered rings1 ... 

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