Kinetics Phenylacetylene

Thermodynamics and kinetics need not go hand in hand. Consider all possible products resulting from addition of one equivalent of bromine to phenylacetylene (phenylacetylene+Br2) and to styrene (styrene+Br2). Calculate the heat of reaction for each addition. (The energy of Br2 is given at right.) Is addition to the alkyne or to the alkene more favorable ... 

The presence of soluble Rh nanoparticles after catalysis is demonstrated by TEM. The kinetic of the catalytic reaction was found to be zero-order in respect to the substrate and first order with respect to hydrogen and catalyst. Curiously, under the same conditions (60 °C, 7 bar H2), ethylcyclohexane is not detected at the end of phenylacetylene hydrogenation and the formation of methylcyclohexane from toluene was only obtained under drastic conditions 40 bar H2 and 80 °C. 

Independent studies of the reduction of C=C and C=C bonds indicate that the latter is kinetically favored. Thus, in the absence of phenylacetylene, the rate of hydrogenation of styrene to ethylbenzene is about one order of magnitude faster than those for C=C bond reduction, indicating that the origin of the selectivity cannot be kinetic. The styryl compound represents a thermodynamic sink that causes virtually all the osmium present in solution to be tied up in this form, and therefore the kinetically unfavorable pathway becomes essentially the only one available in the presence of alkyne.31... 

The complex OsHCl(CO)(P Pr3)2 is also an effective catalyst precursor for the selective hydrogenation of benzylideneacetone to 4-phenylbutan-2-one.97 In contrast to the hydrogenation of phenylacetylene, kinetic studies on the hydrogenation of benzylideneacetone by OsHCl(CO)(P Pr3)2 show that the reaction is independent of the pressure of hydrogen and first order with respect to the concentration... 

Catalytic studies and kinetic investigations of rhodium nanoparticles embedded in PVP in the hydrogenation of phenylacetylene were performed by Choukroun and Chaudret [90]. Nanoparticles of rhodium were used as heterogeneous catalysts (solventless conditions) at 60 °C under a hydrogen pressure of 7 bar with a [catalyst]/[substrate] ratio of 3800. Total hydrogenation to ethylbenzene was observed after 6 h of reaction, giving rise to a TOF of 630 h 1. The kinetics of the hydrogenation was found to be zero-order with respect to the al-kyne compound, while the reduction of styrene to ethylbenzene depended on the concentration of phenylacetylene still present in solution. Additional experi-... 

It is reasonable to assume that 5 and styrene generate the diradical 527, which collapses to give 528 at 30 °C under kinetic control. At temperatures of 75 °C and above, the last step is reversible and an equilibrium is established in that 528 and the methylenecyclobutane derivative 529 maintain a ratio of 1 6 [215]. The [2 + 2]-cydoadduct 530 of 5 to phenylacetylene should be formed analogously via a diradical of the type 527 and is closely related in its structure to the cephalosporin derivatives 437 and 438 (R= Ph, Scheme 6.89). In addition to 530, 2-phenylindene was obtained, which has to be considered as the product of the thermal rearrangement of 530 [216]. Akin to such a process, 526 [194] and 529 [215] were converted into indane derivatives on heating. 

Yashima et al. showed an example where the polymer helicity was controlled by enzymatic enantioselective acylation of the monomers [109]. Optically active phenylacetylenes containing hydroxyl or ester groups were obtained by the kinetic resolution of the corresponding racemic hydroxy-functional phenylacetylene (see Scheme 16). Polymerization of the phenylacetylenes afforded an optically active poly(phenylacetylene) with a high molecular weight (Mn = 89kDa PDI = 2.0) and... 

The Diels-Alder reaction between l,3-diarylbenzo[c]furans and simple alkenes and alkynes has been much investigated kinetically. Glass and Smith found a remarkable enhancement of dienophilicity by the trifluoromethane sulfonyl group in a series of substituted phenylacetylenes of type 205 with 205 (R = SiMcj) no reaction occurred. 

Other laboratories have used FTIR spectroscopy to determine the kinetics of reactions on different polymer supports39 and to enumerate factors regulating site interactions in different types of supports (see Chapter 7, p. 219).40 A major weak point of the procedure is the need for IR diagnostic functions or changes in hybridization to be involved in the transformation to be investigated. For phenylacetylene oligomers, however, the TMS and terminal acetylene absorptions are ideal. 

Phenylacetylene is completely converted to ethylbenzene under the reaction conditions used. No hydrogenation of the phenyl group was detected. This shows a considerable degree of selectivity of the catalyst. This selectivity was further illustrated in the hydrogenation of diphenylacetylene which gave both stilbene (predominently trans-) and bibenzyl.8 Careful kinetic studies at 20 bar hydrogen and 373 °C show an induction time of 60 minutes and an... 

Kinetically stabilized silabenzene 32, the crystal structure of which has been discussed in Section 7.14.3, undergoes 1,2- and 1,4-addition reactions similar to those of the boron analogs, for example, with water to afford 1-hydroxy-silacyclohexadienes 81 and 82 (Scheme 4) <20050M6141>. Reaction of 32 with phenylacetylene results in two addition products, the silabarrelene derivative 83 formed by a [4+2] cycloaddition, and the 1,2-addition product 84. 

The product distribution from phenylacetylene in the absence of added salts shows that the bromination is not stereospecific (under kinetic control) although tram dibromide is obtained in higher yields than the cis isomer. The preferential formation of trans adduct as well as the formation of bromide-acetate (solvent-incorporated) adducts may be accounted for by the formation of intimate and solvent-separated ion pairs, approximately as outlined in Scheme 2. From tolyacetylene, solvent-incorporated products were not obtained (see Table 4). The reasons for such an abrupt change are not clear, although it has been suggested that in this case a more stable and discriminating ion is formed. 

Electrophilic addition of sulphenyl halides to alkenes occurs, by all the evidence, via cyclic thiiranium ions (Mueller, 1969) and a comparison of the rates of addition to the double and triple bond would be quite interesting. Unfortunately, direct kinetic data for strictly comparable and typical cases are not available. Phenylacetylene has been reported (Kharasch and Yannios, 1964) to react 102 times slower than styrene (in acetic acid at 25°) with 2,4-dinitrobenzenesulphenyl chloride. On the other hand, Thaler (1969), by means of competitive experiments carried out in dilute paraffin solutions at — 70°, estimated that methane-sulphenyl chloride adds to mono- (and di-)alkylacetylenes 50-100 times more slowly than to the corresponding alkenes (cis) (but only ca. twice slower than to trans dialkylethylenes). The paucity of information does not allow generalizations and further work in this area seems desirable also with respect to the much larger rate differences observed in those bromine additions to triple and double bonds which also occur via bridged species. 

In order to avoid complications in kinetics and analysis due to polymerization, multi-eth)myl monomers were not used during the initial rate studies. This study focussed instead on paru-substituted phenylacetylenes. 

As a rule, each phenylacetylene derivative was evaluated at an initial concentration of 125 mM reactions using phenylacetylene alone were conducted at concentrations of 125, 250, and 500 mM in order to establish the effect of eth myl concentration on the measured rate. Because more than one material is produced in even the simplest cyclotrimerization reaction, all reactions were followed only by measuring the disappearance of the starting material(s). Although attempts were made to fit the resulting data into the expected second- or third-order kinetics plots, it was finally concluded that the reactions were better described as zero-order. Accordingly, data were plotted on linear concentration and time scales. 

Because the EHT-MO calculations on the model cation [Ru(ti5-C5H5) (C=C=CH2 (C0)(PH3)]+ indicate that the C and Cp atoms of the allenylidene unit are electrophilic and nucleophilic centers, respectively, and the H-O hydrogen atoms of water and alcohols are electrophilic, it has been proposed that the transition states for the above mentioned additions require heteroatom-C interactions, which labilize the O-H bonds, favouring the migration of the H-O hydrogen atoms to the Cp atom of the allenylidene. Thus, the lower nucleophilicity of the H-C(sp)carbon atom of phenylacetylene and H-C(sp ) carbon atoms of methane and acetone could explain why the additions of the latter substrates to the allenylidene ligand are kinetically disfavored processes [23]. 

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