Catalysis of ethylene dimerization by complexation with metal radical cations: an ab initio study (original) (raw)
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Ab Initio Study of Radical Reactions: Cyclization Pathways for the Butylbenzene Radical (II)
The Journal of Physical Chemistry A, 2001
Ab initio density functional theory calculations are presented on some model reactions involved in coke formation during the thermal cracking of hydrocarbons. The reactions under consideration are different cyclization pathways for the butylbenzene radical, which can lead to a further growth of the coke layer. This study enables us to gain more microscopic insight into the mechanistic and kinetic aspects of the reactions. Special attention is paid to the exact treatment of internal rotations and their impact on the kinetic parameters. Pre-exponential factors are very sensitive to the accuracy of constructing the microscopic partition functions. In particular, the relative importance of cyclization toward five and six-membered rings is studied on the basis of the calculated rate constants and concentration profiles of the reactants. The influence of the size of the ring and of the relative stability of the primary and secondary butylbenzene radical on the cyclization reaction is discussed. The activation energy for the formation of six-membered rings is approximately 30 kJ/mol lower than that for five-ring formation. The predicted values for the kinetic parameters enable us to validate some basic assumptions on coke formation. The calculations as presented here are especially important for complex reaction schemes, for which experimental data are not always available.
Journal of the American Chemical Society, 1992
The ring opening and isomerization of cyclopropane are shown by ab initio calculations to be catalyzed by complexation with Be'+. The reactions proceed via a common metallacyclobutane radical cation intermediate formed in a reaction that is endothermic by 12.3 kcal mol-l relative to the cyclopropane-Be'+ complex. From this intermediate, three alternative reaction paths were examined, two isomerizations to give a propene-Be*+ complex and a CC bond cleavage forming a carbeneethylene-Be'+ species. These reactions are found to be exothermic by 5.4 and 5.2 kcal mo1-l relative to the cyclopropane-Be'+ complex, respectively. The isomerization of cyclopropane to give propene is calculated to be exothermic by 8.0 kcal mol-l (exptl 7.9 kcal mol-I). Remarkable parallels to transition-metal catalyzed reactions were found for all the reactions examined.
Journal of Organic Chemistry, 2008
The electrophilic/nucleophilic character of a series of captodative (CD) ethylenes involved in polar cycloaddition reactions has been studied using DFT methods at the B3LYP/6-31G(d) level of theory. The transition state structures for the electrophilic/nucleophilic interactions of two CD ethylenes toward a nucleophilically activated ethylene, 2-methylene-1,3-dioxolane, and an electrophilically activated ethylene, 1,1-dicyanoethyelene, have been studied, and their electronic structures have been characterized using both NBO and ELF methods. Analysis of the reactivity indexes of the CD ethylenes explains the reactivity of these species. While the electrophilicity of the molecules accounts for the reactivity toward nucleophiles, it is shown that a simple index chosen for the nucleophilicity, Ν, based on the HOMO energy is useful explaining the reactivity of these CD ethylenes toward electrophiles. † Dedicated to Prof. K. N. Houk on the occasion of his 65th birthday. ‡ Universidad de Valencia.
Organometallics, 1999
A number of substituted-titanocene-alkynyl-alkenyl complexes, [(η 5 -C 5 Me 4 R 1 ) 2 Ti(η 1 -Ct CR 2 )(η 1 -(E)-CHdCHR 2 )] (R 1 ) H, Me, Ph, Bz; R 2 ) CMe 3 , SiMe 3 , ferrocenyl; A type complexes), were obtained by reacting the corresponding bis(trimethylsilyl)ethyne complexes [(η 5 -C 5 Me 4 R 1 ) 2 Ti(η 2 -Me 3 SiCtCSiMe 3 )] with 1-alkynes R 2 CtCH in the dark at 60°C. The complexes undergo a coupling of the carbyl ligands upon exposure to sunlight to give titanocene complexes with 1,4-disubstituted but-1-en-3-ynes, [(η 5 -C 5 Me 4 R 1 ) 2 Ti(3,4-η-R 2 Ct CCHdCHR 2 )] (B type complexes). In contrast to A type complexes that do not react further with an excess of tert-butylethyne and (trimethylsilyl)ethyne in the dark, B type complexes induce rapid dimerization of these terminal alkynes to 2,4-disubstituted but-1-en-3-ynes (head-to-tail dimers). This implies that the known dimerization of 1-alkynes in the presence of [(η 5 -C 5 Me 4 R 1 ) 2 Ti(η 2 -Me 3 SiCtCSiMe 3 )] (R 1 ) H, Me) performed in diffuse daylight is initiated by the B type complexes originating from photoinduced conversion of the initially formed A type products. Titanocene complexes with 2,4-disubstituted but-1-en-3-ynes, [(η 5 -C 5 Me 4 R 1 ) 2 Ti(3,4-η-R 2 CtCC(R 2 )dCH 2 )] (R 1 ) H, Me, Ph; R 2 ) SiMe 3 ), were also prepared and their participation in the catalytic cycle was demonstrated. Mereiter, K.; Zobetz, E.; Schmid, R.; Kirchner, K. Organometallics 1996, 15, 5275. (d) Rh: Schä fer, M.; Mahr, N.; Wolf, J.; Werner, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 1315. (e) Rh: Kovalev, I. P.; Yevdakov, K. V.; Strelenko, Yu. A.; Vinogradov, M. G.; Nikishin, G. I. J. Organomet. Chem. 1990, 386, 139. (f) Pd: Trost, B. M.; Sorum, M. T.; Chan, C.; Harms, A. E.; Rü hter, G. J. Am. Chem. Soc. 1997, 119, 698. (g) Os: Barbaro, P.; Bianchini, C.; Peruzzini, M.; Polo, A.; Zanobini, F.; Frediani, P. Inorg. Chim. Acta 1994, 220, 5. (h) Ir: Jun, C.-H.; Lu, Z.; Crabtree, R. H. Tetrahedron Lett. 1992, 47, 7119. (4) (a) Sc: Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E.
An MC-SCF study of the thermal cycloaddition of two ethylenes
Journal of The American Chemical Society, 1985
stable species on the potential energy surface of CH302+. dications is not questioned by the result. It should also be mentioned that in line with the theoretical results, neither spontaneous nor collision-induced dissociation of CH302+-dications to either CH2+./OH+ and HC+/H20+-(eq 3 and 4, Scheme I) has been observed.
Chemistry-a European Journal, 2004
An alternative skeletal rearrangement of the quadricyclane radical cation (Q.+) explains the side products formed in the one-electron oxidation to norbornadiene. First, the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation, with an activation energy of 14.9 kcal mol−1, is formed. Second, this species can further rearrange to 1,3,5-cycloheptatriene through two plausible paths, that is, a multistep mechanism with two shallow intermediates and a stepwise path in which the bicyclo[3.2.0]hepta-2,6-diene radical cation is an intermediate. The multistep rearrangement has a rate-limiting step with an estimated activation energy of 16.5 kcal mol−1, which is 2.8 kcal mol−1 lower in energy than the stepwise mechanism. However, the lowest activation energy is found for the Q.+ cycloreversion to norbornadiene that has a transition structure, in close correspondence with earlier studies, and an activation energy of 10.1 kcal mol−1, which agrees well with the experimental estimate of 9.3 kcal mol−1. The computational estimates of activation energies were done using the CCSD(T)/6–311+G(d,p) method with geometries optimized on the B3LYP/6–311+G(d,p) level, combined with B3LYP/6–311+G(d,p) frequencies.
2011
The n-butyramido, isobutyramido, benzamido, and furancarboxamido functions profoundly modulate the electronics of the stilbene olefinic and NH groups and the corresponding radical cations in ways that influence the efficiency of the cyclization due presumably to conformational and stereoelectronic factors. For example, isobutyramido-stilbene undergoes FeCl(3) promoted cyclization to produce only indoline, while n-butyramidostilbene, under the same conditions, produces both indoline and bisindoline.
The Journal of Physical Chemistry, 1991
A new method has been developed for obtaining the reorganization energy for the changes in the intramolecular degrees of freedom during an electron-transfer reaction. This method is based on the use of analytic gradient and Hessian formalism and is an improvement over previous efforts to obtain the intramolecular reorganization energy for larger organic molecules. This approach is expected to be useful, and a preliminary application is presented. The differences in reorganization energies measured for alkyl and benzyl radicals are discussed. Experimental ittvestigations have shown the existence of significant differences in reorganization energies for self-exchange reactions of aliphatic (50 & 10 kcal mol-') and benzylic (15 f 5 kcal mol-') radicals. Most puzzling is why these large differences appear for self-exchange reactions of alkyl anions/radicals and benzyl anion/radicals. The focus is on current experimental reactions of interest: self-exchange reaction with (i) benzyl anion and benzyl radical and (ii) butyl anion and butyl radical, as donor and acceptor. The difference in reorganization energy is investigated in terms of the conventional Marcus theory dividing the reorganization energy into two separate contributions, the solvent reorganization and the intramolecular reorganization.
Catalysis Today, 2018
Six first-row transition metal cations (Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+) were evaluated as catalysts for ethene dimerization to 1-butene. This is an important reaction in the chemistry of CC bond formation and in the conversion of natural gas to higher hydrocarbons. Two related classes of transition metal cation catalysts were investigated: 1) single transition metal cations supported on zirconium oxide nodes of the metal-organic framework NU-1000 and 2) small