Mechanisms alkyne trimerization

Fig. 4. Generic mechanism for trimerization of alkynes to provide arenes by CpCoL2 catalysts (94).

Alternatively, with known metathesis catalysts such as (PCy3)2Cl2Ru=CHPh, a metal carbene mechanism of alkyne trimerization has been proposed.141... 

A characteristically different mechanism appears to operate in alkyne trimerization systems based on PdCb." Cationic metal complexes are involved in which initial halide transfer to an alkyne carbon is followed by sequential linear insertion of two more alkyne moieties. Metallacyclopentadiene intermediates are not involved in this sequence. Unique to this mechanism is the subsequent ring-closure to a cy-clopentadienylmethyl metal derivative, which, via halide transfer back to the metal, eventually leads to benzene via a bicyclo[3.1.0] system (Scheme 27). Support for this mechanism comes in part from the isolation of methylcyclopentadienyl-derived structures in several cases, including pentalene derivatives from further alkyne insertion followed by a second ring-closure." ... 

G. M. Whitesides and W. J. Ehmann, 7. Am. Chem. Soc., 1969, 91, 3800 see also G. A. Ville, K. P. C. Vollhardt and M. J. Winter, Organometallics, 1984, 3, 1177 for further studies along these lines, and J. R. Strickler, P. A. Wexler and D. E. Wigley, Organometallics, 1988, 7, 2067 for studies of relevant model systems associated with alkyne trimerizations catalyzed by early transition metals. Note that although cyclobutadiene-like structures are not involved in most alkyne trimerization mechanisms, metal-complexed cyclobutadienes are fairly common side products in alkyne trimerization reactions. As their yields are usually low, their formation does not present practical problems in arene synthesis. 

The major regiochemical consequence of this mechanism is that trimerization of unsymmetrically substituted alkynes gives rise mostly to benzenes in which the most sterically demanding substituents occupy positions 1, 2 and 4 around the ring. To a lesser extent products with the 1,3,5-substitution pattern... 

Metallacycles have been claimed to play pivotal roles in many transition metal-mediated multi-component coupling reactions [1]. For example, [2 -i- 2 -i- 2] alkyne cyclo-trimerization leading to benzenes - the Reppe reaction - has been considered to proceed via metallacyclopentadiene and elusive metallacycloheptatriene intermediates ("common mechanism ), while metallacyclopentenes have been proposed as intermediates for the [2 -i- 2 -i- 1] cyclo-coupling reactions of an alkyne, an alkene, and CO leading to a cyclopentenone (the Pauson-Khand reaction). A metallacyclic compound - which is defined here as a carbocyclic system with one atom replaced by a transition metal element - can be generally formed by oxidative cyclization of two unsaturated molecules with a low-valent transition metal fragment [2-4]. Alter-... 

These kinetic data support a reaction mechanism in which the normally trimeric hydride (R2AIH)3, must dissociate into its monomer (40), which attacks the alkyne in a syn fashion in the rate-determining step (Scheme 11). The importance of tricoordinate (40) in initiating the electrophilic attack on the alkyne is seen in the effect of donor solvents the rate of reaction in hexane is 10 times faster than that in THF. The latter solvent is known to form a strong 1 1 complex with (40). 

The trimerization reaction with benzene derivatives could sometimes follow a similar path but, since it also occurs with disubstituted acetylenes such as HOCH2C = CCH2OH, some other mechanism must also be operating. Schrauzer explained the cyclization of tolane with a catalyst based on bisacrylonitrilenickel and triphenylphosphine by the so-called V-complex, multicenter processes. With this process, three alkyne molecules would successively coordinate with nickel and then the ring closure would take place (See Figure 6). 

SCHEME3.29 Proposed mechanism for Ni-catalyzed 1 1 1 cross-trimerization of alkynes. 

A reasonable mechanism would involve formation of a bis-alkyne complex, followed by oxidative cy-clization to give a metallacyclopentadiene (Scheme 11.11). Coordination of a second alkyne, followed by insertion and reductive elimination from a metallacycloheptatriene, would yield the trimeric product. An analogous mechanism may be drawn for the formation of cyclooctatetraene, but with an additional alkyne coordination-insertion sequence prior to reductive elimination, a process that seems to be favoured by employing a nickel catalyst with fewer ligands. 

Gomplexes 36 and 37 (R = Bu ) are active catalysts for the insertion of R-G=G-Ph into biphenylene to form 9,10-disubstituted phenanthrenes (Scheme 4). Significantly, no cyclotrimerization of the alkyne was observed as long as biphenylene was present, but in its absence hexaphenylbenzene was produced from PhG=GPh at a slow rate (ca. 0.4 turnover/day). On the other hand, the GF3 derivative did not promote the formation of phenanthrenes, leading instead to the formation of cyclic trimers (Scheme 5). The proposed mechanism for the insertion of alkynes into biphenylene involves the dissociation of the Ni-NMe2 moiety to allow the coordination of the incoming biphenylene... 

Palladium-catalyzed trimerization of alkynes has been developed, " but simple terminal alkynes undergo dimerization to form enynes. A mechanism for the formation of head-o-tail enynes has been proposed that proceeds through palladium(iv) complexes 202 or 203. Probably, however, the acidic terminal alkyne will cleave the palladium-alkenyl bond to give the enyne product and an alkynylpalladium(ii) species that can enter a new catalytic cycle instead." ... 

Dahy ARA, Koga N. Trimerization of alkynes in the presence ofa hydrotris(pyrazolyl) borate iridium catalyst and the effect of substituent groups on the reaction mechanism a computational study. Organometallics. 2015 34 4965-4974. 

The precise mechanism of many oligomerization reactions remains open to discussion. We have already mentioned the question of whether cyclodimerization of butadiene, catalysed by nickel complexes, is a concerted or a stepwise process, of polymerization and then cyclization. The cyclo-trimerization of alkynes is often thought to proceed by co-ordination of three alkyne molecules to the metal catalyst, with a subsequent concerted cyclization step. Recent studies of the cyclotrimerization of hexafluorobut-2-yne in the presence of Ni(cod)j or of arsine complexes, however, suggest a stepwise process in which an intermediate (67) is formed, to which... 

In palladium-catalyzed [2 - - 2 - - 2] cycloaddition, arynes can be used as alkyne components. In 1994, Pena, Romero, and co-workers reported palladium-catalyzed homo-[2- -2- -2] cycloaddition of arynes (Scheme 6.9) [12]. The reactions of 2-silylaryl trifluoromethanesulfonates 26 with CsF generated the corresponding arynes 27, which were trimerized in the presence of a catalytic amount of Pd(PPh3)4 or Pd2(dba)3 to afford substituted triphenylenes 28. In the reactions of unsymmetric arynes, moderate to high regioselectivities were observed. A mechanism via pallada-cycle intermediates, generated through the oxidative cyclization of two molecules of arynes, was proposed. 

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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