Pericyclic reactions ethylene

The Diels-Alder reaction is the best known and most widely used pericyclic reaction. Two limiting mechanisms are possible (see Fig. 10.11) and have been vigorously debated. In the first, the addition takes place in concerted fashion with two equivalent new bonds forming in the transition state (bottom center, Fig. 10.11), while for the second reaction path the addition occurs stepwise (top row, Fig. 10.11). The stepwise path involves the formation of a single bond between the diene (butadiene in our example) and the dienophile (ethylene) and (most likely) a diradical intermediate, although zwitterion structures have also been proposed. In the last step, ring closure results with the formation of a second new carbon carbon bond. Either step may be rate determining. 

Zecchina A, Groppo E, Damin A, Prestipino C (2005) Anatomy of Catalytic Centers in Phillips Ethylene Polymerization Catalyst. 16 1-35 von Zezschwitz P, de Meijere A (2006) Domino Heck-Pericyclic Reactions. 19 49-90 Zhu G, see Negishi E (2006) 19 1-48 Ziegler T, see Michalak A (2005) 12 145-186... 

This lack of mixing of DA and D+A for two ethylenes, in contrast to the situation for ethylene and butadiene, where mixing can take place, will be utilized (Section 3) to analyse the relationship of allowed and forbidden pericyclic reactions in configuration terms. 

Application of CM theory to explain pericyclic reactions was first attempted by Epiotis and coworkers (Epiotis, 1972, 1973, 1974 Epiotis and Shaik, 1978b Epiotis et al 1980). The following analysis is a much-simplified treatment of that approach. Let us compare, therefore, the CM analysis for the [4 + 2] allowed cycloaddition of ethylene to butadiene to give cyclohexene with the [2 + 2] forbidden dimerization of two ethylenes to give cyclobutane. For simplicity only the suprafacial-suprafacial approach is considered, although this simplification in no way weakens the argument. 

In a pericyclic reaction, electron counting can be effected in several ways, all equivalent. For example, in the Diels-Alder reaction, one can count the number of conjugated atoms in butadiene and in ethylene, or the number of bonds made (two o and one n bonds) or broken (three n bonds) in the process. In all cases, a total of six intervening electrons are obtained. 

The reaction of butadiene and ethylene has been studied at many computational levels and serves as a prototype for pericyclic reactions [6,7], The concerted transition structure with Cs symmetry, shown in Fig. 5, is found to be lowest in energy. The calculated activation energy varies widely, from over 45 kcal/mol at the Hartree-Fock limit to 17.6 kcal/mol, with MP2/6-31G calculations. Inclusion of dynamic correlation energy, such as provided by QCISD(T) calculations [35], is necessary to give accurate activation energies near the experimental values of 24 kcal/mol [6, 36]. 

When ultraviolet light rather than heat is used to induce pericyclic reactions, our predictions generally must be reversed. For example, the [2 + 2] cycloaddition of two ethylenes is photochemically allowed. When a photon with the correct energy... 

Though any group could be added in theory, the addition of a CH2 group ensures that the other product will be gaseous ethylene so the thermodynamics are favourable. The Grubbs catalyst (Cy = cyclohexyl) is greatly to be preferred because of its stability and wide scope. This is not strictly a pericyclic reaction but in concept it is very like a cycloaddition followed by a reverse cycloaddition. 

We are now equipped to consider a pericyclic reaction to see how best we can accommodate the principle of conservation of orbital symmetry. For this, we shall associate the relevant reactant orbitals and the product orbitals with a certain symmetry element. Let us first consider the n2 + n2 [2 + 2] reaction of two simple ethylene molecules to form cyclobutane as shown below. We create two a bonds in the product at the expense of two n bonds in the reactants. The energy level... 

Advances in computational chemistry allow for the determination of stationary points by various approximations to the Schrodinger equation [4,35 43], Complete discussions and excellent reviews of the different methods can be found in the literature [6,33,44,45]. Over the years, the Diels-Alder reaction between 1,3-butadiene and ethylene has become a prototype reaction to evaluate the accuracy of many different levels of theory. A level of theory involves the specific combination of a computational method and basis set. For example, the RHF/3-21G level of theory involves the restricted Flartree-Fock method with the 3-21G basis set. Ken Flouk and his research group have pioneered many ideas concerning the fundamental ideas of pericyclic reactions by combining theory and experiment [3,4,37,38,46 48], For the Diels-Alder... 

Other pericyclic reactions of alkynes that have been studied computationally include the addition of singlet methylene to acetylene [109], the addition of carbon monosulfide to acetylene [110], the [2 + 2] dimerization [100, 111], and the dihydrogen transfer reaction between acetylene and ethylene [112, 113]. 

MOs, while tlie two 7t c orbitals lead to the tt and tt MOs. In the initial stage of (he dimerization, the interaction between two ethylencs is weak so that 7t+ and tt. lie far below the n+ and tt levels, so that only 7t+ and rr are occupied. Of the a orbitals of cyclobutane described earlier, only those related to the tt., 7t1 and nl levels by symmetry are shown in Figure 11.1. Not all the occupied MOs of the reactant lead to occupied orbitals in the product. In particular, tt. correlates with one component of the empty set in cyclobutane. The tt+ combination ultimately becomes one component of the filled set in cyclobutane. So the reaction is symmetry forbidden. The reader should carefully compare the correlation diagram for ethylene dimerization here with the Ho + O2 reaction in ITgure 5.8. flie two correlation diagrams are very similar, as they should be, since in this instance the spatial dfstributions of tt and n " are similar to those of and respectively, in H2. These two reactions are probably the premier examples of symmetry-forbidden reactions. A related symmetry-allowed example is the concerted cycloaddition of ethylene and butadiene, the Diels-Alder reaction. We shall not cover the orbital symmetry rules for organic, pericyclic reactions. There are several excellent reviews that the reader should consult.But it should be pointed out that the orbital symmetry rules have stereochemical implications in terms of the reaction path and products formed. The development of these rules by Woodward and Hoffmann... 

When ultraviolet light rather than heat is used to induce pericyclic reactions, our predictions are generally reversed. For example, the [2 -i- 2] cycloaddition of two ethylenes is photochemically allowed. When a photon with the correct energy strikes ethylene, one of the pi electrons is excited to the next higher molecular orbital (Figure 15-21). This higher orbital, formerly the LUMO, is now occupied It is the new HOMO, the HOMO of the excited molecule. 

To develop the concepts related to understanding all pericyclic reactions, we will study two prototype reactions, shown in Eqs. 15.1 and 15.2. Both are cycloadditions, reactions in which two (or more) molecules combine to make a new ring system. We will develop the nomenclature more fully below, but for now it is convenient to refer to the dimerization of ethylene to give cyclobutane as a [2-1-2] cycloaddition, and the combination of butadiene and ethylene to give cyclohexene as a 4-l-2] cycloaddition. We assume a pericyclic transition state, and that the two partners approach each other in a symmetrical fashion, forming a symmetrical cyclic transition state, and then go on to product. "Symmetrical" means that the trajectory for approach of the reactants and the geometry of the transition state have a particular symmetry element, such as a o plane, C axis, S axis, etc. We will refer to this symmetry element in many of the theoretical models used to analyze pericyclic reactions. 

As always, stereochemistry has proven to be a crucial indicator of mechanism. Many examples of highly stereospecific SET reactions have been found. An example is the Diels-Alder reaction of the l,2-di(aryloxy)-ethylenes shown below. Mixing the dienophile with cyclopentadiene and the very convenient SET reagent tris(p-bromophenyl)aminiiim (1 ) gives the cycloaddition adducts with high stereospecificity (first two examples below). The observation of several cases like this led many to conclude that the SET reactions really were concerted, pericyclic processes. However, more recent work has found clear exceptions. The deuterated 4-methoxy-styrene shown adds to cyclopentadiene under the same conditions with extensive loss of stereochemistry (third example). These systems are more complicated than conventional pericyclic reactions. 

Hydroboration attracted several theoretical studies in 1978. Publications which have appeared include an ab initio study of the reaction of BHs with ethylene, a CNDO/2 study of the nature of the ethylene-borane complex, the transition state in the hydroboration reaction, a MNDO study of hydroboration of alkenes and alkynes, and a MNDO study of hydroboration and borohydride reduction with implications concerning cyclic conjugation and pericyclic reactions. Finally, HaBCHO was one of the molecules which has been studied with regard to banana bonds of the carbonyl group. ... 

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