Reactions cyclotrimerization

CHEMO- AND REGIOSELECTIVITY ENHANCEMENT IN SOLID-SUPPORTED REACTIONS 

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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). 

Among the most exciting frontiers in boratabenzene chemistry is the development of transition metal-boratabenzene complexes as catalysts. As early as 1984, it had been demonstrated that these adducts can accelerate useful reactions— specifically, Bonnemann established that (C5H5B-Ph)Co(cod) serves as a catalyst for pyridine-forming cyclotrimerization reactions of alkynes and nitriles.39... 

Saaby S, Baxendale IR, Ley SV (2005) Non-metal-catalysed intramolecular alkyne cyclotrimerization reactions promoted by focused microwave heating in batch and flow modes. Org Biomol Chem 3 3365-3368... 

NbBrs, and NbCls-Pl Sn evidently proceeds via cyclotrimerization of diynes, which most probably involve cyclic carbometallation, details are not very clear.246 2463 Related reactions of Ta and Mo complexes were also investigated in this study. Formation of tantallacyclopropenes by complexation of alkynes with Ta complexes has also been reported247 (Scheme 51). In addition to the Ta-catalyzed polymerization of diynes mentioned above, Ta-catalyzed or -promoted cyclotrimerization reactions of alkynes to produce benzene derivatives, a Ta-promoted ethylene... 

It is possible to carry out the [2+2+2] cyclotrimerization reaction in a regioselective manner by using a partially or completely intramolecular approach. Rhodium-catalyzed intramolecular cyclotrimerization of 1,6,11-triynes, which construct fused 5-6-5 ring-systems, has been studied extensively [33-36]. Cyclization of 1,6,11-triyne 47 catalyzed by RhCl(PPh3)3, gives the tricyclic benzene 48 in good yield (Eq. 14) [33a]. 

Witulski and Alayrac reported the synthesis of clausine C (clauszoline-L) (101) by a rhodium-catalyzed alkyne cyclotrimerization of diyne 1014 and propiolic ester 635 (561). Analogous to the hyellazole (245) synthesis (see Scheme 5.75), the diyne precursor 1014 required for this key cyclotrimerization reaction was obtained starting from readily available 2-iodo-5-methoxyaniline. Using Wilkinson s catalyst, [RhClfPPhsls], crossed-alkyne cyclotrimerization of 1014 and 635 led to N-tosylclausine C (1015) in 78% yield in an isomeric ratio of 3.8 1. Finally, deprotection of the tosyl group with TBAF in refluxing TFIF afforded clausine C (clauszoline-L) (101) (561) (Scheme 5.147). 

The cyclotrimerization of isocyanates is initiated by anionic type of catalysts and proceeds via propagation, transfer and termination steps ( ). It was found that in the case where the cyclotrimerization reaction proceeds with a long kinetic chain length, the kinetics of the reaction followed second order with respect to the isocyanate as measured by the disappearance of the isocyanate groups and was first order with respect to the initial concentration of the catalyst ( ,5). ... 

A fascinating series of cyclodimerization or cyclotrimerization reactions was first observed in the labs of Wilke to produce 1,5-cydooctadiene... 

These cyclodimerization and cyclotrimerization reactions are catalyzed by low valent Ziegler-type Ni catalysts (139—144). Large ligands, such as tris-o-biphenylyl phosphite on nickel tend to favor cydooctadiene (COD) formation while smaller ligands favor the linear dimer, 1,3,7-octatriene. The dimer yield at 80°C and 101.3 kPa (1 atm) is 96%. The nickel catalyst can also be placed on a support so that it can be recycled (145). Many other type catalysts have been reported for this reaction (146). The linear 1,3,7-octatriene and its 1,3,6 isomer are also obtained by a Pd catalyzed dimerization (147—151). The kinetics of thermally induced dimerization to COD has also been studied (152). 

Cyclotrimerization of nitriles is the best-known route to 1,3,5-triazines (for a detailed discussion see CHEC 2.18). The reaction is of value for preparing the symmetrical derivatives only. Nevertheless, many important triazines, such as cyanuric chloride, are made in this way. Other cyclotrimerization reactions are useful thus, an easy route to 1,3,5-triazine involves heating ammonium acetate with ethyl orthoformate. 

An example of cyclotrimerization reactions is the homopolymerization of aromatic cyanate esters, forming triazine rings (Fig. 3.17). As these particular stepwise polymerizations cannot be described by the equations developed so far, we will consider them in this section with the usual assumptions for an ideal polymerization. 

Figure 3.18 Different states of reaction of a monomer in a cyclotrimerization reaction.

The cyclotrimerization reaction described above can be converted into a cyclodimerization reaction by blocking one of the vacant coordination positions around the nickel atom (77, 66, 83). Phosphines and phosphites in a ligand-to-nickel ratio of 1 1 have been found to be particularly effective. 

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. 

Again, with the help of spectral analyses, the hyperbranched structures of the poly(aroylarylene)s were confirmed. Thanks to the 1,3,5-regioselectivity of the cyclotrimerization reaction, the NMR spectra of the hb-PAAs were... 

Dibromopyrrole 1084 (R = Br) and benzyne generated from anthranilic acid and isoamyl nitrile gave dibromo-derivative 1086, the metallation of which led to product 1087 (Scheme 211) <20050L1003>, which was later successfully used in a cyclotrimerization reaction. 

Co2(CO)8 and Hg[Co(CO)j2 (41), i7T-CMCo CO)2 191), Co CO)aNO 192), Co2(CO)8(RCaR ) (193), and Co4(CO)io(RC2R ) 48). The trimeri-zation reactions with symmetrically disubstituted alkynes, RC=CR, lead inevitably to hexasubstituted benzenes. Unsymmetrical alkynes RC=CR are trimerized generally to 1,2,4- rather than 1,3,5-trisubstituted benzenes. These cyclotrimerization reactions can be achieved readily by... 

Carbonylcobalt complexes do not appear to be effective catalysts for the cyclotrimerization of certain alkynes. Thus, the alkynes PhC= CSiMea, MegSiC CSiMea, o-BrCeH CsCCeHiBr-o, tcrt-BuC=CH, tert-hnC=CR i-tert 41, 190), PhC=CBu-tert (59), -PrC=CPr-t 194, 195), and CH2=C(Me)C=CC(Me)=CH2 196) do not participate readily in catalytic cyclotrimerization reactions. These alkynes all have bulky substituents and it seems logical to conclude that the cyclotrimerization reactions are sterically inhibited. 

Many individual experiments have given some information about the mechanism of cyclotrimerization reactions involving alkynes and carbo-nylcobalt complexes, but details of the overall mechanism remain obscure. 

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. 

The possibility that Coa(CO)4(RCaR )3 complexes are intermediates in the catalytic cyclotrimerization reactions has been alluded to in a previous section. Certainly, these complexes have been used as stoichio-... 

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