Cyclohexadiene photocycloaddition reactions

The dimerization of thioformylketene was investigated by B3LYP and G3MP2B3 methods. The 4 + 4-pathway has the lowest energy barrier and calculations suggest that the reaction is pseudopericyclic.231 The stereospecific intramolecular 4 + 4-cycloaddition reaction between cyclohexadiene iron tricarbonyl complex and appended dienes (198) generates cyclooctadiene tricyclic adducts (199) (Scheme 56).232 The first example of an asymmetric intermolecular 4 + 4-photocycloaddition reaction in solution between 9-cyanoanthracene and chiral 2-methoxy-l-naphthamides has been reported. The frozen chirality is effectively transferred to the optically active product.233... 

Transient absorption spectra of singlet diradical in addition to exciplex have been observed in the course of the photocycloaddition reaction of 9-cyanoan-thracene (CNA) with 2,5-dimethyl-2,4-hexadiene (DMHD), while no intermediate was detected in CNA-l,3-cyclohexadiene (CHD). The cycloadduct with CHD shows an efficient adiabatic photodissociation into CNA and CHD immediately after excitation. 

The literature of mechanistic aromatic photochemistry has produced a number of examples of [4 + 4]-photocycloadditions. The photodimerization of anthracene and its derivatives is one of the earliest known photochemical reactions of any type97. More recently, naphthalenes98, 2-pyridones" and 2-aminopyridinium salts100 have all been shown to undergo analogous head-to-tail [4 + 4]-photodimerization. Moreover, crossed [4+4]-photocycloaddition products can be obtained in some cases101. Acyclic 1,3-dienes, cyclohexadienes and furan can form [4 + 4]-cycloadducts 211-214 with a variety of aromatic partners (Scheme 48). 

In the formation of tetraenes from bicyclo[4.2.0]octa-2,4-dienes, two bonds are broken. This may occur in one concerted reaction which can be regarded as a retro [2 + 2] cycloaddition. It is also possible that the central bond, being part of a cyclohexadiene system, is the first one to break in a thermal, concerted disrota-tory process that leads to a 1,3,5-cyclooctatriene derivative. Ring opening of the cyclooctatriene then might take place photochemically, again disrotatory, to produce a tetraene. This two-step sequence was first observed by Mirbach et al. [114] in their study of the photocycloaddition of the two parent molecules benzene and ethene. The same explanation for the formation of a tetraene was given by Nuss et al. [160] in their report on the intramolecular ortho photocycloaddition of ( )-6-(2-methoxyphenyl)-5,5-dimethyl-2-hexenenitrile (see Scheme 40). 

Mixed photocycloadditions of anthracene and conjugated polyenes yield products that correspond to a concerted reaction path, as well as others that are Woodward-Hoffmann-forbidden and presumably result from noncon-certed reactions. For example, the reaction of singlet-excited anthracene with 1,3-cyclohexadiene yields small quantities of the [ 4 +, 2J product 72 in addition to the allowed 4, +, 4J product 71. 

Synthetic application of Paterno-Bilchi reaction of simple dienes with carbonyl compounds is rare. While seemingly an extension of the photocycloaddition of olefins and carbonyl compounds, the reaction between dienes and carbonyls is often complicated by the fact that triplet excited states of carbonyl compounds are quenched by dienes, although the formation of oxetanes can be observed during these reactions. Recall also that the photosensitized dimerization of diene triplet excited states is also a well known reaction vide infra), these two observations would seem to naturally limit the synthetic potential of this process. Kubota and coworkers found that irradiation of propanal in the presence of 1,3-cyclohexadiene produced oxetanes 164a and 164b in a 4 1 ratio (Scheme 37). ... 

Photo-[4+4] cycloaddition of 2-pyridone is not restricted to reaction with other pyridones. Sato, Ikeda, and Kanaoka found that cyclic and acyclic-1,3-dienes photoreact with 2-pyridones (Figure 8), ° reactions similar to the [4+4] photocycloaddition of 1,3-dienes with other aromatic molecules. Cyclopentadiene is abetter substrate than cyclohexadiene, as it is for the Diels-Alder reaction. A mixture of trans (24) and cis (25) isomers is formed in all cases. Use of acyclic... 

Similar to fiiran, photocycloaddition of 1,3-cyclohexadiene 124 to propanal gives exclusively exo-oxetane 125 from the reaction of singlet excited propanal to a ground-state 1,3-cyclohexadiene [70]. 

Shima, K., Kubota, T., and Sakurai, H., Organic photochemical reactions. XXIV. Photocycloaddition of propanal to 1,3-cyclohexadiene, Bull. Chem. Soc. Jpn., 49, 2567, 1976. 

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