Santiago Olivella

Mechanism of the Diels-Alder reaction: reactions of butadiene with ethylene and cyanoethylenes

PMO theory suggests that the effects of substituents on the rate of the Diels-Alder reaction between ethylene and 1,3-butadiene cannot be explained on the basis of a synchronous mechanism. Calculations are reported, using the RHF, UHF, and CI versions of MNDO and AMI, for the Diels-Alder reactions of 1,3-butadiene with ethylene, acrylonitrile, maleonitrile, fumaronitrile, and 1 , I -dicyanoethylene. The results confirm the PMO conclusions, indicating unambiguously that the reactions involving the cyanoethylenes cannot be synchronous. The evidence suggests that Diels-Alder reactions in general proceed via very unsymmetrical transition states, close to biradicals in structure and with energies differing little from those of the corresponding biradicals. The regioselectivities and rates of Diels-Alder reactions can be predicted on this basis, more simply and more reliably than they can in terms of frontier orbital theory. The mechanism of the simplest example. Le., the reaction of ethylene with butadiene, remains uncertain. The mechanism of the Diels-Alder (DA) reaction has been the subject of major interest and controversy, ever since it was discovered 60 years ago.’ The chemical evidence seemed initially to favor a two-step2 mechanism, involving a biradical or zwitterionic intermediate, since the regioselectivity of DA reactions can be interpreted very effectively on this basis (see below). It was, however, recognized that the observed stereochemistry is difficult to explain in such terms, DA reactions almost invariably involving exclusive cis addition to the dienophile. In 1938 Evans and Warhurst) pointed out that the cyclic transition state (TS) involved in a synchronous2 mechanism should be analogous to benzene and hence highly resonance stabilized. They suggested on this basis that DA reactions are in fact synchronous, unlike analogous dimerizations of olefins where the TS would be an analogue of cyclobutadiene. While this work was largely overlooked as a result of World War I1 and while good evidence for a nonconcerted* mechanism was later presented by Woodward and kat^,^ the investigations of pericyclic reactions by Woodward and HoffmannS revived the synchronous mechanism, and most organic chemists have assumed in recent years that DA reactions in general are not merely concerted2 but also synchronous. While the stereospecificity of DA reactions certainly suggests that they are concerted, this argument is not conclusive. The same would be true for a two-step mechanism if the intermediate biradicals or zwitterions collapse to the product faster than they isomerize by internal rotation. Several lines of evidence do, however, show that both of the new bonds are formed to significant extents in the TSs of certain specific DA reactions. These, and the properties involved (in parentheses), are as follows: (a) DA reaction of furan with maleic anhydride6 (kinetic isotope effects); (b) reverse DA conversion of dibenzotricyclo[2.2.2]octadiene to anthracene and ethylene’ (kinetic isotope effects); (c) DA reactions of diphenylisobenzofuran with methyl and menthyl furmarates8 (induced optical activity); (d) DA reactions of anthracene and ‘University of Texas at Austin. 1 University of Barcelona. its benzo derivatives with maleic anhydride’ (comparison of rates with localization and paralocalization energies). However, contrary to claims in the original paper^,^-^ this evidence does not show that any of the reactions in question are synchronous. The arguments to this effect have been given in detail elsewherelo and so need not be repeated here. The only new evidence, an addition” to (c), is also inconclusive.’* On the other hand, comparison of the rates of the DA reactions of 1,3( I ) Diels, 0.; Alder, K. Chem. Ber. 1929, 62, 554. (2) Problems have been caused in the past by loose terminology, the term “concerted” in particular having been used with a variety of meanings. The terminology used here, which now seems to be gaining general acceptance, is as follows. A concerted reaction is one which takes place in a single kinetic step. A two-step reaction is one which takes place in two distinct steps, via a stable intermediate. A synchronous reaction is a concerted reaction in which all the bond-breaking and bond-forming processes take place in parallel, all having proceeded to comparable extents in the TS. A two-stage reaction is concerted but not synchronous, some of the changes in bonding taking place mainly in the first half of the reaction, between the reactants and the TS, while the rest takes place in the second half, between the TS and the products. The new features are the precise definition of the term concerted and introduction of the term two-stage. The latter seems to have been first suggested by Goldstein and Thayer (Goldstein, M. J.; Thayer, J. L., J r . J . Am. Chem. Soc. 1965, 87, 1925). (3) Evans, M. G.; Warhurst, E. Trans. Faraday SOC. 1938,34,614. Evans, M . G. Ibid. 1939, 35, 824. (4) Woodward, R. B.; Katz, T. Tetrahedron 1959, 5, 70. (5) Woodward, R. B.; Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1969, ( 6 ) Seltzer, S. J . Am. Chem. Soc. 1965, 87, 1534. (7) Taagepera, M.; Thornton, E. R. J . Am. Chem. SOC. 1972, 94, 1168. (8) Tolbert, L. M.; Ali, M . B. J . Am. Chem. SOC. 1981, 103, 2104; 1982, (9) Dewar, M. J . S.; Pyron, R. S. J . Am. Chem. SOC. 1970, 92, 3098. ( I O ) Dewar, M. J. S.; Pierini, A. B. J . Am. Chem. Soc. 1984, 106, 203. ( 1 1 ) Tolbert, L. M.; Ali, M . B. J . Am. Chem. Soc. 1984, 106, 3804. (12) The earlier criticisms’0 apply equally ta.this work. The experimental results were explained in terms of compensation between two opposing steric effects. There is no reason to suppose that the cancellation may not be almost exact in any given case. The experiment by Tolbert and Ali was what may be termed a “one way” experiment, Le., an experiment with two possible outcomes, one of which leads to a definite conclusion, whereas the other does not. 8, 781. 104, 1742; 1984, 106, 3804. 0002-7863/86/ 1508-577 1

Theoretical Mechanistic Study of the Oxidative Degradation of Benzene in the Troposphere: Reaction of Benzene-HO Radical Adduct with O2.

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Ab initio calculations on the mechanism of the oxidation of the hydroxymethyl radical by molecular oxygen in the gas phase: a significant reaction for environmental science.

Competing pathways arising from the reaction of hydroxycyclohexadienyl radical (1) with O2, a key reaction in the oxidative degradation of benzene under tropospheric conditions, have been investigated by means of density functional theory (UB3LYP) and quantum-mechanical (UCCSD(T) and RCCSD(T)) electronic structure calculations. The energetic, structural, and vibrational results furnished by these calculations were subsequently used to perform conventional transition-state computations to predict the rate coefficients and evaluate the product yields. The trans stereoisomer of the peroxyl radical (4) produced by the O2 addition to position 2 of benzene ring in radical 1 is energetically more stable than the cis one, although the rate coefficients at 298 K for the formation of both isomers are predicted to be similar. The cyclization of the cis isomer of 4 to a bicyclic allyl radical (5) involves calculated barrier heights (ΔU(⧧), ΔE(⧧), ΔH(⧧), and ΔG(⧧)) significantly lower than those of the cyclization of the trans isomer of 4. This implies that the formation of the cis isomer of 4 can lead to irreversible loss of radical 1 and that the observed chemical equilibrium 1 + O2 ↔ 4 essentially involves the trans isomer of 4. Although the reaction enthalpies computed for the O2 addition to position 4 of benzene ring in radical 1, affording the cis and trans stereoisomers of a peroxyl radical (6), are similar to those for the addition to position 2, the latter addition mode is clearly preferred because it involves lower barrier heights. The barrier heights computed for the cyclization of either the cis or the trans isomers of 6 to a bicyclic radical bearing a peroxy bridge (7) are about twice those computed for the cyclization of either the cis or the trans isomers of 4. Thus, under tropospheric conditions, it is unlikely that the O2 addition to position 4 of the benzene ring in radical 1 can contribute to the formation of benzene oxidation products.

Ab-initio mechanistic studies of radical reactions. Directive effects in the addition of methyl radical to unsymmetrical fluoroethenes

The mechanism of the gas-phase reaction of *CH2OH+O2 to form CH2O+HO2* was studied theoretically by means of high-level quantum-chemical electronic structure methods (CASSCF and CCSD(T)). The calculations indicate that the oxidation of *CH2OH by O2 is a two-step process that goes through the peroxy radical intermediate *OOCH2OH (1), formed by the barrier-free radical addition of *CH2OH to O2. The concerted elimination of HO2* from 1 is predicted to occur via a five-membered ringlike transition structure of Cs symmetry, TS1, which lies 19.6 kcalmol(-1) below the sum of the energies of the reactants at 0 K. A four-membered ringlike transition structure TS2 of Cs symmetry, which lies 13.9 kcalmol(-1) above the energy of the separated reactants at 0 K, was also found for the concerted HO2* elimination from 1. An analysis of the electronic structures of TS1 and TS2 indicates that both modes of concerted HO2* elimination from 1 are better described as internal proton transfers than as intramolecular free-radical H-atom abstractions. The intramolecular 1,4-H-atom transfer in 1, which yields the alkoxy radical intermediate HOOCH2O*, takes place via a puckered ringlike transition structure TS3 that lies 13.7 kcalmol(-1) above the energy of the reactants at 0 K. In contrast with earlier studies suggesting that a direct H-atom abstraction mechanism might occur at high temperatures, we could not find any transition structure for direct H-atom transfer from the OH group of *CH2OH to the O2. The observed non-Arrhenius behavior of the temperature dependence of the rate constant for the gas-phase oxidation of *CH2OH is ascribed to the combined effect of the initial barrier-free formation of the *OO-CH2OH adduct with a substantial energy release and the existence of a low-barrier and two high-barrier pathways for its decomposition into CH2O and HO2*.

Reinvestigation of some thermally “forbidden” pericyclic reactions and biradical processes in the semiempirical TCSCF approach: Part 1. Disrotatory opening of cyclobutene, isomerization of bicyclo[2,1,0]pent-2-ene and dimerization of ethylene to cyclobutane☆

Addition of methyl radical to unsymmetrical fluoroethenes has been studied by ab-initio molecular orbital calculations. In agreement with experimental data, we find that the reaction rate decreases in going from ethene to mono- and 1,1 di-fluoroethene, but sharply increases in the case of trifluoroethene. Additions to the more fluorinated carbon atoms are always thermodynamically favoured, but addition to the CH2 end of mono- and 1,1 di-fluoroethene is kinetically favoured. The general trends of the potential energy barriers have been rationalized by means of the energy decomposition scheme proposed by Morokuma. Non potential energy effects have also been considered, but their role is negligible.

MINDO/3 comparison of the generalized SCF coupling operator and “half-electron” methods for calculating the energies and geometries of open shell systems

Abstract Two-configuration SCF MINDO/3 calculations on the three prototypical thermally “forbidden” pericyclic reactions of disrotatory openings of cyclobutene, isomerization of bicyclo[2,1,0]pent-2-ene and dimerization of ethylene to cyclobutane indicate that such a procedure provides a satisfactory description of both the biradical and biradicallike species involved in the pathway of these reactions and allows an efficient location of the stationary points on the corresponding potential energy surface. The present reinvestigation of the above three pericyclic reactions, shows that the structural and energetic results obtained by the MINDO/3 TCSCF procedure differe substantially from those reported in earlier MINDO/3 studies, based on either the single-configuration SCF or 2 × 2 CI wavefunctions.

Ab initio mechanistic studies of radical reactions. Addition of methyl radical to acetylene and ethylene

The MINDO/3 semi-empirical SCF MO method has been extended to the generalized coupling operator (GCO) treatment of open shell systems proposed by Hirao and Nakatsuji and has been used to calculate heats of formation and equilibrium geometries of various ground state radicals and the lowest singlet and triplet excited states of a series of closed shell ground state molecules. The results are compared with those from the “half-electron”(HE) method. While the heats of formation calculated by these methods are slightly different, the predicted equilibrium geometries do not differ appreciably. The generalized coupling operator method required more computing time in nearly all cases.

4-31G ab initio and MNDO semi-empirical calculations on bicyclic CN7– and N8 species, and n.m.r. and i.r. studies on 15N-labelled CN7–

Abstract Addition of methyl radicals to ethylene and acetylene has been studied by ab initio molecular-orbital calculations. In agreement with experimental data, we find that, although addition to ethylene is characterized by a lower activation energy, addition to acetylene is faster due to the opposite and larger trend of pre-exponential factors. The reaction barriers have been analyzed by the energy decomposition scheme proposed by Morokuma.

Mechanisms for the Reactions of Hydroxyl Radicals with Acrolein: A Theoretical Study.

Hepta-azapentalene anion and octa-azapentalene are predicted by both 4-31G ab initio and MNDO semi-empirical calculations to be true minima on the CN7– and N8 potential hypersurfaces, respectively; 15N scrambling from labelled 5-azidotetrazole anion is shown to occur by 15N n.m.r. and i.r. spectroscopy, all data pointing to the involvement of the bicyclic CN7– intermediate.

A MINDO/3 study on the monoelectronic reduction of carbon monoxide: The acetylenediolate dianion as the first member and as the common precursor of all the cyclic oxocarbon dianions

Three low-energy pathways for the reaction of HO(•) with acrolein, a key reaction in atmospheric environments, have been investigated by means of quantum-mechanical electronic structure methods (UQCISD and RQCISD(T)). The first step of all the reaction pathways studied involves the barrierless formation of a prereaction loosely bound complex in the entrance channel, lying a few kcal/mol below the energy of the reactants. The lowest-energy barrier pathway at 0 K is found to be the HO(•) abstraction of the aldehydic H-atom through a transition-state structure lying 1.1 kcal/mol below the energy of the reactants. The addition of HO(•) to the terminal carbon atom of the C═C double bond proceeds via a transition-state structure lying 0.7 kcal/mol below the energy of reactants at 0 K, whereas the HO(•) addition to the central carbon atom takes place via a transition-state structure lying 0.8 kcal/mol above the energy of the reactants at 0 K. On the basis of conventional transition-state theory calculations at 298 K, it is predicted that 74.5% of the HO(•) reaction with acrolein proceeds via abstraction of the aldehydic H-atom, 24.2% via HO(•) addition to the terminal carbon atom of the double bond, and 1.3% via HO(•) addition to the central carbon atom of the double bond. These results are in close agreement with available experimental data.