Triplet state 2 + 2 thermal cycloaddition

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

In the thermal reaction the [4 + 2] or Diels-Alder adduct is the major product, whereas in the photochemical reaction [2 + 2] cycloadditions dominate. Because the photochemical additions are sensitized by a ketone, C6H5-COCH3, these cycloadditions occur through the triplet state of 1,3-butadiene and, as a result, it is not surprising that these cycloadditions are stepwise, nonstereospecific, and involve diradical intermediates. 

Continuing work 158) on photoreactions of electron-rich olefins with biacetyl shows that the complexity of product mixtures obtains in these reactions also. Effects of solvent polarity provide further support for the importance of ionic intermediates in these reactions. The reactions of biacetyl with 1,1-diethoxyethylene are proposed to proceed via the triplet state (in contrast to reactions with dioxoles). The reversal of regiospecifity between thermal and photochemical cycloaddition of this olefin with biacetyl is nicely explained by the assumption of excited state electron transfer from olefin to dione to give the corresponding radical ions. 

Cycloadditions represent an important class of photochemical reactions. We discussed thermal cycloadditions extensively in Chapter 15, with the prototype being the [4+2] cycloaddition of the Diels-Alder reaction. Orbital symmetry reasoning would lead us to expect that photochemical cycloadditions should be typified by a [2+2] reaction. Indeed, formal [2+2] photocycloadditions are common. However, most photochemical cycloadditions involve triplet states and biradical intermediates. Concerted photochemical cycloadditions are rare. As such, orbital symmetry arguments are not directly relevant, and instead we must focus on potential biradical intermediates and possible funnels and other surface crossing points. Some photochemical cycloadditions do proceed via singlet states, and usually these involve the formation of exciplexes. 

Dicyclopropylethylene has been used as a substrate to detect the triplet state in carbene cycloaddition. This depends on the fact that the rearrangement of the intermediate cyclopropylmethyl radical to the allyl-methyl radical is a very fast process. Thus in the addition of thermally generated fluorenylidene to cyclopropylethylene, (557) and (558) are among the products obtained. The ratio of (558) (557) is increased upon dilution, as expected for a carbene decaying to a triplet (biradical) ground state. 

Four-membered rings can be synthesised by [2 + 2] cycloadditions. However, thermal [2 + 2] cycloadditions occur only with difficulty they are not concerted but involve diradicals. Photochemical [2 + 2] reactions are common and although some of these may occur by a stepwise mechanism many are concerted. An example of a [2 + 2] reaction is the photodimerisation of cyclopent-2-enone. This compound, as the neat liquid, or in a variety of solvents, on irradiation with light of wavelength greater than 300 nm (the n - n excited state is involved) is converted to a mixture of the head-to-head (48) and head-to-tail (49) dimers, both having the cis,anti,cis stereochemistry as shown. It is believed that the reaction proceeds by attack of an n - n triplet excited species on a ground state molecule of the unsaturated ketone (Section 2.17.5, p. 106). In the reaction described (Expt 7.24) the cyclopent-2-enone is irradiated in methanol and the head-to-tail dimer further reacts with the solvent to form the di-acetal which conveniently crystallises from the reaction medium as the irradiation proceeds the other dimer (the minor product under these conditions) remains in solution. The di-acetal is converted to the diketone by treatment with the two-phase dilute hydrochloric acid-dichloromethane system. 

Furans give 2,5-cycloadducts on irradiation with alkynes (equation 75) . Thiophens probably behave similarly , but the product isolated is a substituted benzene which arises by extrusion of sulphur from the adduct (equation 76). The photochemical reaction with thiophen involves a triplet excited state of the thiophen, but both furan and thiophen cycloadditions can also be brought about thermally , (compare the pyrrole reaction in equation 74). 

Diyls are short-lived intermediates and in most cases are not directly observed during cycloaddition reactions. The course of the reaction is complicated by the fact that diyls possess several close-lying electronic states which might be populated thermally singlet diradical, triplet diradical, and zwitterionic states1. These various electronic states differ in reactivity as well as in selectivity. Triplet reactions are generally slower by several orders of magnitude and are less selective. 

An exception, with respect to the photosensitised reactions mentioned so far, is represented by the Diels-Alder addition of maleic anhydride to anthracene in dioxane, carried out under irradiation ( = 365 nm) without sensitisers, at 26-45°C. The rate of removal of anthracene was found to be linearly dependent on the total light absorption. As maleic anhydride quenches the fluorescence (transition from excited singlet to ground state) of anthracene, and in agreement with the kinetic evidence, a mechanism was suggested by which singlet anthracene is responsible for cycloaddition rather than triplet anthracene. According to this mechanism, the reaction of the excited diene with maleic anhydride has a rate coefficient of about 3x 10 l.mole . sec, i.e. of the same order as the reported frequency factor of the thermal reac-... 

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