Cyclotrimerization study

Other Methods. A variety of other methods have been studied, including phenol hydroxylation by N2O with HZSM-5 as catalyst (69), selective access to resorcinol from 5-methyloxohexanoate in the presence of Pd/C (70), cyclotrimerization of carbon monoxide and ethylene to form hydroquinone in the presence of rhodium catalysts (71), the electrochemical oxidation of benzene to hydroquinone and -benzoquinone (72), the air oxidation of phenol to catechol in the presence of a stoichiometric CuCl and Cu(0) catalyst (73), and the isomerization of dihydroxybenzenes on HZSM-5 catalysts (74). 

Moulijn et al. (33) studied the reactions of some linear alkynes over a W08-Si02 catalyst in a fixed-bed flow reactor. Besides metathesis, cyclotrimerization to benzene derivatives occurred. Thus, propyne yielded, in addition to metathesis products, a mixture of trimethylbenzenes. From this an indication of the mechanism of the metathesis of alkynes can be obtained. 

Since this scheme regenerates the original coordinatively unsaturated Ti+2 centers upon desorption of the aromatic, it could, in principle, represent a catalytic cycle for heterogeneous alkyne cyclization. The present study reports a test of that h3T>othesis—the feasibility of catal5hic cyclotrimerization—on a reduced Ti02 surface in UHV. 

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... 

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... 

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]. 

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]. 

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. 

It appears difficult to propose a unified mechanism to explain all experimental observations of the cyclotrimerization of acetylene. The most common pathway, studied mainly with cobalt complexes,72 73 involves a metallacyclopentadiene intermediate ... 

Cyclotrimerization of acetylenes to form benzene derivatives is studied intensively mainly because of the challenge of chemo- and regioselectivities. A regiose-lective and highly chemoselective method for preparing substituted benzenes was described via PdCl2-catalyzed cyclotrimerization in the presence of CuCl2 537... 

A survey of the studies of metal-catalyzed acetylene cyclotrimerization on single-crystal, supported, and bimetallic catalysts is available.539 A new study with size-selected Pdn clusters found that clusters as small as Pd7 are able to induce benzene formation at 157°C.540... 

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). 

That sulfides coordinate to palladium(II) more strongly than the corresponding sulfoxides has received an interesting confirmation in a study of the complexes (19)-(21) (R = 4-MeQjHt).149 The complex (19) is the best catalyst both for cyclotrimerization of diphenylethyne and for isomerization of allyl ethanoates. In both processes a vacant metal coordination site is essential and the thioether sulfur is too strongly bound (in 20 and 21) for facile dissociation. 

PhCH2=Ru(PCy3)2Cl2 catalyses the cyclooligomerization of trienes (143) to benzene derivatives (144) via a cascade of four metathesis reactions (Scheme 54).273 Isocyanate cyclotrimerization catalysed by dimethylbenzylamme-phenyl glycidyl ether-phenol has been studied by IR and PMR spectroscopy.274... 

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 intramolecular version was studied to control the selectivity of the insertion 268.433 Cyclotrimerization of bis(boryl) acetylene provided a novel hexa(boryl)benzene 269 (cat represents catecholato group).434,435... 

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. 

A systematic study using a wide variety of reaction conditions was completed to determine the conditions which favor the selective formation of CTV or CTTV [75], CTV was determined to be the kinetic cyclization product and CTTV was determined to be the thermodynamic product, Therefore, it was possible to determine reaction conditions which favored predominant formation of the desired product. CTV was formed preferentially by two methods. The treatment of 3,4-bis(methyloxy)benzyl chloride with stoichiometric amounts of AgBF4 in methylene chloride resulted in the formation of CTV in a 91 9 ratio to other products. In this case 9% of the product was CTTV and higher molecular weight products. The cyclotrimerization of 3,4-bis(methyloxy)benzyl alcohol with superacids... 

Because preparative cyclotrimerization reactions are usually conducted at high concentration, the initial, faster rates in this study were considered more important. For each run, the rates of disappearance of the substituted aryl acetylene and phenylacetylene, along with the reaction ratio, are listed in Table I. 

Arene complexes are usually prepared by the following methods (i) metal vapor synthesis (see Metal Vapor Synthesis of Transition Metal Compounds).(ii) AEAIX3 reduction of a metal halide in the presence of the arene and (hi) alkyne cyclotrimerization (see Cyclodimerization -tri-merization Reactions). Synthetic procedmes to obtain r] -arene derivatives have been reviewed by Pampaloni and Calderazzo. Over the past ten years, studies in this... 

The transition metal-catalyzed [2 -i- 2 -i- 2] cyclocotrimerization of two molecules of an alkyne with an alkene has studied to a lesser degree compared to the parent alkyne cyclotrimerization [9], although the resultant cyclohexadiene is a valuable synthetic intermediate (e.g., a diene component for the Diels-Alder reaction). This is because a 2 1 coupling of an alkyne and an alkene is generally difficult to compete with the more facile alkyne cyclotrimerization. The success of the selective coupling depends on the electronic balance between the employed alkyne and alkene components the combinations of an electron-deficient alkene with a neutral alkyne [35] or an electron-deficient alkyne with a neutral alkene [36] were successful in the previous... 

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