Photochemical 2 + 2 allowed

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... 

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,... 

Bridging of a different type has been observed in 3-oxido-l-phenylpyri-dinium (213) which on irradiation in ethyl acetate affords the isomer 214 by way of a photochemically allowed disrotatory ring closure.158 Similarly, stable diaziridines (215) have been isolated on irradiation of 3-oxidopyrida-zinium betaines (216).159 Other related transformations in 2-alkylcin-nolinium-4-olates160 and in 5-oxidopyridazinium betaines161 have been... 

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... 

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 photoprocess involves intramolecular twist mechanism without bond cleavage. The reactive intermediate is a triplet state of pseudo-tetrahedral geometry. The transformation is photochemically allowed but thermally disallowed from symmetry considerations. 

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. 

The formation of the oxepin is reasonably explained by an electrocyclic ring opening of rearranged epoxide 299 in a thermal reaction. As mentioned above, two routes to 299 are possible. If the rearrangement is concerted, a 1,5-sigmatropic reaction with inversion of the reaction center (oxygen) in 299 is photochemically allowed. It is possible to separate a nonconcerted process... 

There are two different methods of splitting a thymine dimer photochemical and enzymatic. In the photochemical method, the sample containing thymine dimers is irradiated with UV light. Splitting of the thymine cyclobutane dimer follows the same symmetry rules as its formation it is thermally forbidden but photochemically allowed. When a dimer absorbs a photon of suitable wavelength (2 240 nm), it reacts with a quantum yield of nearly 100% forming two thymines [60]. In the enzymatic method an enzyme recognizes thymine dimers and repairs them. It may require the absorption of a photon, or it may happen in the dark. 

Photochemically allowed processes have al8o been renorted H 31 (Wehrli et al., 1963) ... 

Just as there are secondary interactions in cycloadditions, so too are there ancillary orbital-symmetry effects in sigmatropic reactions. In process (79), Jones and Jones (1967) find no products of [1,3] hydrogen or methyl shifts, e.g. 1,4,7-trimethylheptatriene, which ostensibly (Table 5) are photochemically allowed. They point out that in the first... 

In this reaction, light of appropriate energy is used to selectively excite 1,3-cyclohepta-diene. The diene closes to a cyclobutene by a disrotatory motion. Although the product, because of its strained cyclobutene ring, is much less stable than the reactant, it is unable to revert back to the diene by an allowed pathway. It does not absorb the light used in the reaction, so the photochemically allowed disrotatory pathway is not available. A conrotatory opening is thermally allowed but results in a cycloheptadiene with a trans double bond. Such a compound is much too strained to form. Therefore, the product can... 

Both of the overlaps where the new sigma bonds are forming are bonding interactions, so the [2 + 2] cycloaddition reaction is photochemically allowed—that is, it does occur when the compounds absorb light. 

If another pi bond is added to either component of the previous cycloaddition, an additional node is introduced into either the HOMO or the LUMO, resulting in the reaction being thermally forbidden and photochemically allowed. Recognition of this pattern enables the preferences for cycloadditions to be summarized in the chart below. 

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