Mechanism, metal catalyzed cyclotrimerization

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

The involvement of metalocyclopentadiene intermediates in the cyclotrimerization of alkynes (Scheme 6) has been established for some metal systems 162, 198, 201-204). However, there is no evidence to indicate participation of cobaltocyclopentadiene species in cyclotrimerization reactions involving carbonylcobalt complexes. It is worth noting, however, that 1,4-addition of an alkyne to such an intermediate would lead to the formation of 1,2,4- and 1,3,5-substituted benzenes. Moreover, the 1,2,4-derivative would be favored statistically. This line of reasoning has led to the incorporation of a cobaltocyclopentadiene intermediate in the mechanism proposed 120) (Scheme 7) for the Co4(CO)ia-catalyzed cyclotrimerization of PhC=CH. 

For the palladium-catalyzed cyclotrimerization of arynes, a mechanism similar to the accepted mechanism for [2+2+2] cycloaddition of alkynes may be proposed (Scheme 15). Though it has not been studied in depth, some experimental results support it. Firstly, aryne-forming conditions are necessary for the reaction to proceed (see Table 1, entry 8) no reaction of trifiate 55 takes place at room temperature in the presence of the catalyst if fluoride is absent, which rules out a mechanism initiated by the oxidative addition of the aryl trifiate to palladium. Data obtained in the closely related cocycloadditions of benzyne with alkynes, discussed below, hkewise point to benzyne as the reactive species. Secondly, the benzyne-palladium complex 67 is a plausible initial intermediate because the ability of group 10 metals to coordinate benzyne is well known (see Sect. 1.2.2), and although benzyne complexes of palladium have eluded isolation (apparently because of their instability) [7,26], they may well be able to exist as transient intermediates in a catalytic cycle such as that shown in Scheme 15. Thirdly, it is known that benzyne complex 34 can form metallacy-cles similar to 68, albeit with dcpe as hgand instead of PPhj [26]. 

A theoretical study of the mechanism of ruthenium-catalyzed formation of pyran-2-one and the corresponding sulfur and selenium analogues 8 from acetylene and CX2 (X = O, S, Se) has been reported (Equation 3) <2004NJC153>. This cyclotrimerization reaction has been experimentally carried out using carbon disulfide as a substrate <2002JA28>. The proposed mechanism involves formation of a bicyclic metal carbene intermediate. Formation of this intermediate seems to be particularly unfavorable energetically in the case of carbon diselenide. 

The cyclotrimerization of alkynes catalyzed by transition metals is a general method for building substituted benzenes from aliphatic precursors. Multiple bonds are formed in these reactions in a single operation. Although the reaction of thermal trimerization relates to allowed electro-cyclic processes, it is catalyzed by several transition metals, such as Co, Ni, Rh, Pd, Rh, and Ru [38]. Most recent publications show promise for the participation of transition metal complexes in [2+2+2] cycloaddition reactions based on zirconium, titanium, and indium [9]. This reaction has synthetic potential for using metallocyclopentadienes as intermediates in the cyclotrimerization of alkynes. The reaction mechanism is shown in Scheme 2.1 [3, 38]. Two alkyne molecules coordinated to the metal, that is, complex 2.1, couple to form cyclopentadiene 2.2. Next there is either addition of the alkyne to the metallocycle 2.3 to form the metallocycle... 

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