Four-center reaction orbital symmetry

Four-center reactions have two kinds of barriers an inherent one due to conservation of orbital symmetry [14] and an additional one, kinematic in origin [11] which requires vibrational excitation of the reactants (and leads to vibrationally excited products). For a homogeneous reaction, the energy required to overcome both barriers needs to be provided by the atoms of the cluster. Since the reaction shown in Fig. 2 occurs quite soon after impact, the energy is provided by those rare gas atoms which have already rebounded from the surface and are moving towards the reactants, which are still moving forward. Figure 4 identifies the key rare gas atoms that are involved. 

Notwithstanding the fact that certain four-center additions to alkenes, e.g. equation (94) or Fig. 22, are forbidden, bromine addition to an alkene does occur. The orbital symmetry arguments which forbid cis addition to isolated bonds favor trans addition in equation (144), in which j = 4, X may be a nucleophile or radical and Y an electrophile or radical. Therefore, additions of molecular bromine or in general X—Y to alkenes, should proceed in at least two steps. Otherwise, separated X and Y, with one electron pair between them, add in concerted fashion to the alkene. Equation (144) is effectively the prototype of numerous ionic and radical a-w exchange reactions. A wealth of information has been recorded in excellent reviews covering special aspects of this general process, e.g. electrophilic additions (de la Mare and Bolton,... 

This so-called stereoelectronic factor operates to maximize or minimize orbital overlap, as the case requires, to obtain the most favorable energy. This was evident from the three- and four-center systems we have discussed by the VB and HMO methods. It was also implicit in favored anti-1,2-additions, 1,3-cyclizations (Fig. 23), fragmentations (e.g. (174)), etc. Here we have selected several reaction types to illustrate the principle. In this and other sections, we show that the tendency for reaction centers to be collinear or coplanar stems largely from orbital symmetry (bonding), but may also derive from steric and electrostatic effects, as well as PLM. 

Ionic mechanisms based on betaine intermediates or TS are difficult to reconcile with the absence of solvent effects on lithium-free nonstabilized ylide reactions (Table 12) or reactivity-selectivity considerations (15). Also, there is no apparent reason why the reactants should prefer to form a high-energy intermediate such as 93 when the direct conversion to a more stable oxaphosphetane 97 is possible. Orbital symmetry should not interfere with the four-center process since phosphorus can provide 3d orbitals of appropriate symmetry for a 2s - - 2s cycloaddition. Nevertheless, the betaine mechanism has persisted in the literature because there was no direct evidence against the formation of 93 as a transient intermediate until recently (229). 

Now we allow reaction to occur to give 2 HD molecules. We form products also in the (ai) bi) b2) configuration, one molecule of HD being in the excited S state. The symmetries of all the orbitals containing electrons are matched up, and the exchange is allowed with a four center transition state. 

The criteria of allowedness discussed in the preceding two sections do not require the explicit consideration of orbital symmetry, in the sense that the symmetry elements retained along the reaction path do not enter directly into the analysis consequently, they were not drawn in the figures. However, it is easy to ascertain from Fig. 1.1, for example, that two ethylene molecules in either the coplanar or [s -f s] orientation have three perpendicular mirror planes one common to the four carbon atoms, another reflecting one molecule into the other, and a third bisecting both of them three twofold axes of rotation (one at the intersection of each pair of mirror planes) and a center of inversion at the point where the three rotational axes intersect. After both molecules have been twisted so as to react in the [a + a] mode (Fig. 1.1c), only the rotational axes remain, whereas the off-orthogonal orientation of Fig. 1.4b retains a single twofold rotational axis and no other element of symmetry. 

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