Cyclotrimerization of alkynes

For the cyclotrimerization of alkynes, several mechanisms have been proposed. The most plausible ones are a concerted fusion of three ir-bonded alkyne molecules, and stepwise processes involving a cyclobutadiene complex or a five-membered metallocyclic intermediate (98). In the case of the cyclotrimerization of a-alkynes it is possible to discriminate between a reaction pathway via a cyclobutadiene complex and the other reaction pathways, by analysis of the products. If cyclotrimerization proceeds via a cyclobutadiene complex and if steric factors do not affect the reaction,... 

Recently, it has been demonstrated that coordination vacancies on the surface metal cations are relevant to the unique redox reactivity of oxide surfaces]2]. Oxidation of fonnaldehyde and methyl formate to adsorbed formate intermediates on ZnO(OOOl) and reductive C-C coupling of aliphatic and aromatic aldehydes and cyclic ketones on 1102(001) surfaces reduced by Ar bombardment are observed in temperature-prognunmed desorption(TPD). The thermally reduced 1102(110) surface which is a less heavily damaged surface than that obtained by bombardment and contains Ti cations in the -t-3 and +4 states, still shows activity for the reductive coupling of formaldehyde to form ethene]13]. Interestingly, the catalytic cyclotrimerization of alkynes on TiO2(100) is also traced in UHV conditions, where cation coordination and oxidation states appear to be closely linked to activity and selectivity. The nonpolar Cu20( 111) surface shows a... 

Catalytic Formation of Carbon-Carbon Bonds in Ultrahigh Vacuum Cyclotrimerization of Alkynes on Reduced Ti02 Surfaces... 

The cyclotrimerization of alkynes to aromatic compounds, observed to occur efficiently on reduced Ti02 (001) surfaces during TPD experiments, can also be carried out as a genuinely catals tic reaction at low pressures. A kinetic model of the cyclotrimerization reaction describing the pressure and temperature dependence of the behavior observed was constructed (Scheme 1). 

Scheme 1. Proposed kinetic model for the cyclotrimerization of alkynes to aromatic compoimds on reduced Ti02 (001) surfaces. Acetylene is used as the representative alkyne for clarity.

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]. 

Intermolecular cyclotrimerization of alkynes using Wilkinson s catalyst has also afforded substituted carbazole derivatives. This reaction was extended to an intramolecular version by offering the possibility for a six-membered ring annotation that cannot be achieved easily in the corresponding intermolecular version. Intramolecular cyclotrimerization is completely regioselective due to an additional tether. 

While the DA reaction is a very elegant way to generate cyclohexene and cyclohexa-diene derivatives, the cyclotrimerization of alkynes is an atom-economic and efficient... 

Okamoto and coworkers recently described the iron-catalyzed cyclotrimerization of alkynes utilizing a low-valent iron-diimine complex that was generated in situ upon reduction with zinc dust (Scheme 9.34) [92]. 

An analogous cleavage of a nickeladisilacyclopentene with alkyne is shown in entry 116. It has been pointed out (297) that these reactions show certain similarities to metal-catalyzed cyclotrimerization of alkynes or to cycloaddition of alkynes and substituted disilanes, postulated to involve Si-metal intermediates. 

Selective cyclotrimerization of alkynes with nitriles produced pentasubstituted pyridines (1) with little formation of benzenoid products (Scheme 1) <94CB(127)2535>. 

Vollhardf s group adopted the cobalt-catalyzed photochemical cyclotrimerization of alkynes for the rapid construction of a novel class of helicenes, helical [n]phe-... 

Co-Cyclotrimerizations of Alkynes with Carbon-Heteroatom Multiple Bonds... 

Numerous transition metal-mediated [2 + 2+2] cycloadditions have been utilized in the synthesis of pyridines . Selective cyclotrimerization of alkynes with nitriles leads to pentasubstituted pyridines 310 with minimal formation of benzenoid byproducts (Scheme 157) <20000L3131>. Different alkynes can be utilized in the same strategy if a sequential approach is used (Scheme 158) <2000JA4994>. 

The Pauson-Khand reaction (PKR) is among the most powerful transformations in terms of molecular complexity increment [1]. Only a few of other reactions like the Diels-Alder, or the cyclotrimerization of alkynes can compete with the PKR, which consists formally of a [2 + 2 + 1] cycloaddition in which a triple bond, a double bond and carbon monoxide form a cy-clopentenone [2-12], This constitutes one of the best ways to construct cyclopentenones, which upon further transformations can be converted into structures present in numerous natural products (Scheme 1). 

Cyclotrimerization of alkynes mediated by the cationic complex [(77-Cp)Ru(acetonitrile)3](PF6) was shown by the DFT methods to proceed via the ruthenacyclopentadiene intermediates in accord with experimental findings <2003JOM(682)204>. One illustration for the transformation of such an intermediate into the final product is illustrated... 

Iridacyclopentadienes tend to be the intermediates of [2-F2-P1] cyclotrimerization of alkynes leading finally to iridabenzenes <2004CEJ4518>. Iridacyclopentadienes 81 (R = COOMe) <2004JA1610> and 81 (R = H)... 

Rhodacyclopentadienes are often postulated as intermediates in the catalytic cyclotrimerization of alkynes <20000M4289, 2002CC2984, 20020L745, 2003IC7701, 2003JA784, 2003JMO(204)333, 2003JOM(678)10,... 

Metallacyclopentadienes are often postulated as intermediates in the alkyne-based synthesis of various organic compounds like cyclotrimerization of alkynes to yield derivatives of benzene or in the synthesis of quinines and tropones <1994MI211, 1995ICA(230)215, 1995COMC-II(12)741, 1996JOM(511)263, 1999JOM(573)139,... 

The formation of organic compounds from reactions of alkynes and carbonylcobalt compounds has been mentioned briefly in preceding sections. Several reaction sequences that lead to the formation of organic products are summarized in Scheme 4. The most extensively studied reaction of this type is undoubtedly the cyclotrimerization of alkynes. 

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. 

Reactions between a metallaborane and alkyne are not always predictable. For example, itio-l,2-(Cp Rh)2B3H7 catalyzes the cyclotrimerization of alkynes rather than undergoing insertion of the alkyne into the cluster cage. ... 

Reactions of [MoCl3(thf)3] with cyclic trithioethers C-S3 afforded the Mo complexes, [MoCl3(c-S3)] and [MoCl3(ttob)], and with neat tetrahydrothiophene (tht), mer-[MoCl3(tht)3] was formed. This latter complex was an active catalyst for the selective polymerization and cyclotrimerization of alkynes. 

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