Cyclotrimerization under

Alkyl cyanides with electron-withdrawing substituents (e.g., halogen atoms) are more reactive and cyclotrimerize under acidic and basic conditions, as well as at high pressure. In the presence of hydrogen chloride, the formation of an intermediate adduct of two nitrile molecules, which cyclizes upon the addition of a third nitrile, has been suggested145 (cf. Houben-Weyl, Vol. E5,... 

Compound 105 has been prepared by an independent protocol. Phosphaalkyne la undergoes quantitative cyclotrimerization under the influence of a Lewis acid such as AICI3 to furnish the spirocyclic diphosphirenium betaine 106 [91 ]. The Lewis acid can be removed by treatment with the weak Lewis base DMSO. The spirocycUc system 107 cannot be isolated since it rearranges rapidly to the 1,3,5-triphospha-Dewar-phosphinine 19 which - again - cannot be isolated under these reaction conditions but is, in turn, trapped by added phosphaalkyne in a homo-Diels-Alder reaction to afford the phosphaalkyne tetramer 105 [92]. 

Lanthanide(IlI)-catalyzed cyclotrimerization under mild conditions... 

In addition to 6, different ruthenium complexes have been investigated as catalysts for alkyne cyclotrimerizations and cyclocotrimerization of alkynes with nitriles. Among them, Grubbs-type carbene complexes are of particular importance since they catalyze inter- and intramolecular metathesis-cascade cyclotrimerizations under mild conditions. In addition, they have good tolerance toward a variety of functional groups. 

In 1988, Linstrumelle and Huynh used an all-palladium route to construct PAM 4 [21]. Reaction of 1,2-dibromobenzene with 2-methyl-3-butyn-2-ol in triethylamine at 60 °C afforded the monosubstituted product in 63 % yield along with 3% of the disubstituted material (Scheme 6). Alcohol 15 was then treated with aqueous sodium hydroxide and tetrakis(triphenylphosphine)palladium-copper(I) iodide catalysts under phase-transfer conditions, generating the terminal phenylacetylene in situ, which cyclotrimerized in 36% yield. Although there was no mention of the formation of higher cyclooligomers, it is likely that this reaction did produce these larger species, as is typically seen in Stephens-Castro coupling reactions [22]. 

Steady-state molecular beam studies of the reaction of methylacetylene on reduced Ti02 (001) surfaces were undertaken to determine whether this reaction could be performed catalytically under UHV conditions. A representative experiment is presented in Figure 1. Prior to each experiment, the surface was sputtered and annealed to a temperature between 400 K and 550 K surfaces prepared in this manner have the highest fraction of Ti(+2) sites (ca. 30% of all surface cations) of any surface we have been able to create by initial sputtering [3]. Thus these are the surfaces most active for cyclotrimerization in TPD experiments [1]. Steady-state production of trimethylbenzene (as indicated by the m/e 105 signal detected by the mass spectrometer) was characterized by behavior typical of more traditional catalysts a jump in activity upon initial exposure of the crystal to the molecular beam, followed by a decay to a lower, constant level of activity over a longer time scale. Experiments of up to 6 hours in duration showed... 

Under supercritical C02 at 102 °G, the [2 + 2 + 2]-cyclotrimerization of 3-hexyne with C02 was achieved by use of Ni(cod)2/dppb as a catalyst system (Scheme 27).39,40 In the case of diynes, the intramolecular cyclization/carboxyla-tion took place under high pressure and high temperature (Scheme 28).41,41a... 

A Ni(dppe)Br2-Zn system effectively catalyzes co-cydotrimerization of an allene with a propiolate. The reaction is highly regio- and chemoselective to afford a poly-substituted benzene derivative in good yield. (Scheme 16.82) [92], From the observation that no desired [2 + 2 + 2] product is obtained for the reaction of 1-hexyne and phenylacetylene with w-butylallene under similar conditions, the presence of an electron-withdrawing C02Me group in the alkyne moiety is essential for the success of the present [2 + 2 + 2]-co-cyclotrimerization. 

Comparison of the different types of cobalt catalysts shows that the in situ system [Eq.(2)] is most accessible while the Rep-, R(ind)-, and bori-ninato ligands having electron-withdrawing substitutents are the most active. The difference between the 14e" and the 12e core complexes makes itself apparent in the chemoselectivity of the reaction. Catalysts containing a 14-electron core favor pyridine formation, whereas those containing a 12-electron core (i.e., the rj -allylcobalt systems) favor the formation of benzene derivatives by cyclotrimerization of the alkynes. For example, in the reaction of propyne and propionitrile at 140°C in the presence of a 12-electron system (5), a 2 1 ratio of benzene to pyridine product is formed, whereas a catalyst containing the cpCo moiety (a 14-electron system) leads (under identical conditions) to the predominant formation of pyridine derivatives (84HCA1616). 

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]. 

The [2+2+2] cyclotrimerization of triynes has been reported using rhodium catalysts <2003JA12143> under biphasic conditions. Thus, the cyclization of triyne 174 proceeds rapidly in a biphasic system to produce the tricyclic compound in good yield using an in t/r -generated water-soluble rhodium catalyst (Equation 108) <2003JA7784>. 

Electron-deficient acetylenes 96 and 98 form 97 and 99, respectively, in poor yields with the assistance of Rh2(pfb)4 (Equation (19)). When Rh4(CO)i2 is used as the catalyst, cyclotrimerization of these acetylenes proceeds even in the presence of Me2PhSiH under GO pressure. ... 

The volatility of difunctional isocyanates (such as tolylene diisocyanates, hexamethylene diisocyanate, etc.) creates many environmental problems in the urethane industry. These difficulties can be overcome by preparation of NCO-terminated oligomers with low vapor pressure. One approach is the preparation of NCO-ter-minated oligomers by partial cyclotrimerization of difunctional isocyanates. Usually this is achieved by a multi-step process which includes also deactivation of the catalyst at a certain conversion. During our work on cyclotrimerization of isocyanates we found that cyclic sulfonium zwitterions are very active cyclotrimerization catalysts (2). Recently we found that cyclic sulfonium zwitterions under certain reaction conditions act as anionic initiators. This behavior of cyclic sulfonium zwitterions permits preparation of isocyanate oligomers containing isocyanurate rings by a one-step procedure, eliminating the deactivation step. 

The kinetics of catalysis of cyclotrimerization was studied on the model system phenyl isocyanate/ace-tonitrile (solvent). Acetonitrile (AN, 99.64%, from Vinstron Corp.) was purified by refluxing with phosphorus pentoxide (5 g/1), then with calcium hydride (2 g/1) followed by distillation under nitrogen. Phenyl isocyanate was obtained from the Upjohn Company with a purity of 99.5%, and was purified by distillation. Tolylene diisocyanates (2,4 and 80/20 2,4/2,6 isomers) were obtained from the Mobay Chemical Co., and were purified by distillation. Cyclic sulfonium zwitterions (SZ) were obtained from the Dow Chemical Co. 

Alkyl cyanides with electron withdrawing substituents are more reactive and cyclotrimer-ize under both acidic and basic conditions as well as at high pressure (Table 14). Complexes of Lewis acids and hydrogen halides are particularly valuable catalysts. Two mechanisms for the acid-catalyzed cyclotrimerization have been postulated. Grundmann et al. suggest... 

Cyclotrimerization of nitriles with heteroatomic substituents are also facile and important reactions. Melamine is formed on heating cyanamide above its melting point (equation 74) (59HC(l3)l, p. 309). Substituted cyanamides react to give either the expected 1,3,5-triazine or the isomer (142). The 1,3,5-triazine is the preferred product at high temperatures, under acid catalysis, and for cyanamides with bulky substituents, whilst the isomer is favoured by basic conditions and low temperatures (Scheme 80). The mechanism of formation of (142) has been proposed (Scheme 81) (78RCR975). 

The phyllocladane skeleton 131 was constructed efficiently by stereoselective formations of six carbon-carbon bonds and four rings via a one-pot sequence of cyclizations the ene type, [2+2+2], and [4+2] cycloadditions. In this synthesis, the Conia ene reaction of 127 takes place under mild conditions to generate 128, and the cyclotrimerization of its diyne with 118 gives 129. These two reactions are catalysed by CpCo(CO)2. Finally, ring-opening to give 130 and intramolecular Diels-Alder reaction in the presence of DPPE produced the phyllocladane skeleton 131 in a total yield of 42% [55]. 

A highly electron-deficient carbon-oxygen double bond can also participate in the co-cyclotrimerization with alkynes under the ruthenium catalysis. The cycloaddition of commercially available diethyl ketomalonate with the diynes 21 proceeded at 90 °C in the presence of 5-10 mol % Cp RuCl(cod). The expected fused 2ff-pyrans 27, however, underwent thermal electrocyclic ringopening to produce cyclopentene derivatives 28 (Eq. 14) [23]. 

The spectroscopic data for 9 support the proposed structure. In particular, the wSi NMR chemical shift of 5 43.19 as a triplet (Jsi-p(C/ ) = 40.11 Hz) resembles the literature value reported for the m-NiSijPC complex. 3 Compound 9 was found to be a good reactive intermediate for the double silylation reaction. The reaction of 1 with 1-phenylprop-l-yne (1 equiv) in the presence of a catalytic amount of 9 (0.03 equiv) for 6 h afforded the double-silylated producted 10 in 94 % (GC) yield. The reaction was quite sensitive to the reaction conditions. When the same reaction was carried out at higher temperature (70-75 °C), the major component was identified as the acetylene cyclotrimerization product 11 (Scheme 2), which has been characterized by spectroscopic techniques. When hex-l-yne is employed as a terminal alkyne in the reaction with 1 under the same condition, the five-membered disilyl ring compound 12 is isolated as a colorless liquid in 71 % yield. 

An examination of the photodecoloration of bis-[4-(dimethylamino)dithio-benzyl]nickel has shown it to proceed in two stages, the first of which increases with increasing concentration of dissolved oxygen.Rate enhancements of thirty- to forty-fold are reported for the cyclotrimerization of acetylene under u.v. irradiation in the presence of a Ni"-Si02 catalyst pretreated with... 

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