Photochemically allowed reaction

Fig. 21 Energy diagram for ground and excited state curves for (a) a thermally forbidden, photochemically allowed reaction, and (b) a thermally allowed, photochemi-cally forbidden reaction...

The HOMO of the excited ethylene molecule has the same symmetry as the LUMO of a ground-state ethylene. An excited molecule can react with a ground-state molecule to give cyclobutane (Figure 15-22). The [2 + 2] cycloaddition is therefore photo chemically allowed but thermally forbidden. In most cases, photochemically allowed reactions are thermally forbidden, and thermally allowed reactions are photochemically forbidden. 

In order to avoid solvent problems we again chose the pericyclic reactions as the source of our new reaction, since these reactions are usually not solvent dependent. In consideration of the known thermal lability of this class of compounds we only took the photochemically allowed reaction categories into account (those with 4n electrons in the transition state. Figure 10). Photolysis at low temperatures might be the suitable condition for the formation. 

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

The latter product is an example of a product formed by a concerted, photochemically allowed, electrocyclic reaction. A hydrogen-atom migration from a cyclopropyldimethyl... 

The Woodward-Hoffmann allowed reactions can be classified according to how many electrons are involved, and whether the reaction occurs thermally or photochemically, as shown in Table 15.1. 

Reaction type Number of electrons Thermally allowed Photochemically allowed... 

Direct aromatization of the quinonoid intermediates is a photochemically allowed but thermally forbidden rearrangement (Scheme 5.6). When phenylethyl radicals are generated photochemically at 20 °C there is evidence95 of a-o coupling by way of the aromatized product 7. The products derived from these pathways can be trapped in thermal reactions by radical98 or acid1 catalyzed... 

This type of orientation of the newly formed bonds is called antarafacial, and the reaction would be a [ 2s + 4a] cycloaddition (a stands for antarafacial). We can easily show by the frontier-orbital method that this reaction (and consequently the reverse ring-opening reactions) are thermally forbidden and photochemically allowed. Thus in order for a reaction to proceed,... 

Photochemical [2h-2] cycloadditions of olefins occur with retention of configuration according to the Woodward-Hoffmarm rule [6,7], These are excited-state reactions in the delocalization band of the mechanistic spectrum. A striking example of the symmetry-allowed reaction was observed when the neat cis- and tran -butenes were irradiated (delocahzation band in Scheme 3) [8],... 

A number of photochemically induced 4 + 2 cycloadditions have been observed/131-150 This reaction can be either a concerted ( 4, + 20) or a ( 40 4- 2S) addition. The stereochemical consequences of the allowed reactions are as follows ... 

Various types of photochemically induced 1,3-shifts have been observed in nitrogen containing heterocycles. Concerted [1,3] suprafacial sigmatropic reactions are photochemically allowed processes, but many of the reported transformations especially those which arise by n->n excitation un-... 

If j is of the form 4n 1, suprafacial migrations are photochemically allowed. For antarafacial migrations, the restrictions are reversed. See also Antarafacial Suprafacial Reactions... 

The reason that thermally allowed reactions are photochemically forbidden and vice versa also becomes more readily apparent (van der Lugt and Oosterhoff, 1969). As we have just noted the existence of a large ground state barrier for a particular reaction (i.e. the reaction is thermally forbidden ) implies that there is another potential surface of the same symmetry that has a well not far above this barrier (as in Fig. 21a). Photoinduced reactions may occur through the funnelling of the reaction complex from the excited surface well on to the ground state surface in a radiationless transition. Since the... 

If the reverse back reaction is prevented or is forbidden by other considerations, the energy remains stored in the photoproducts. Some simple photorearrangement reactions which are governed by Woodward-Hoffman rules have been found useful. These rules provide the stereochemical course of photochemical rearrangement based on symmetry properties of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecule (Section 8.6). A reaction which is photochemically allowed may be thermally forbidden. Front the principle of microscopic reversibility, the same will be true for the reverse reaction also. Thermally forbidden back reaction will produce. ble - photoproducts. Such electrocyclic rearrangements are given in . ..ure... 

It is possible to show very generally that for two olefins, having w, and m2 n electrons, coming together to form a cyclic olefin with (m] + m2 - 4)/2 n bonds, as shown below, the reaction will be thermally allowed when ms + m2 = 4n + 2 (e.g., 4 + 2 = 6 in the case of the Diels-Alder reaction). On the contrary, when m, + m2 = 4n (e.g., 2 4- 2 = 4 for the ethylene dimerization) the reaction is thermally forbidden but photochemically allowed. For a discussion of this generalization the article of Woodward and Hoffmann should be consulted. 

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