Addition photochemical cycloaddition

Benzene rings can undergo photochemical cycloaddition with alkenes. The major product is usually the 1,3 addition product, 116 (in which a three-membered ring has also been formed), though some of the 1,2 product (117)... 

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. 

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

Several different methods of sidewall functionalisation, such as fluorination, radical addition, nucleophilic addition, electrophilic addition and cycloaddition, have been developed (Tasis et al., 2006). The sidewalls of vertically aligned CNTs have been functionalised with DNA using azide units as photoactive components. The azi-dothymidine reacted photochemically with sidewalls of CNTs utilising [2+1] cycloaddition. The oligonucleotides were grown in situ on the sidewalls of CNTs and the DNA-modified CNTs were obtained after the deprotection of the nucleic acid (Moghaddam et al., 2004). 

The most widely exploited photochemical cycloadditions involve irradiation of dienes in which the two double bonds are fairly close and result in formation of polycyclic cage compounds. Some examples are given in Scheme 6.7. Copper(I) triflate facilitates these intramolecular additions, as was the case for intermolecular reactions. 

Only very few examples of alkene-]2-i-2] cycloadditions are known ]345, 347, 348]. By using a large excess of the moderate electron-rich alkene p-propenyl-anisol ]348] or even less electron-rich alkyl-subshtuted 1,3-butadienes [347] no thermal [2-1-2] cycloaddition occurs, but a photochemical cycloaddition can be enforced. The mechanism is proven to be stepwise via a biradical or dipolar intermediate ]347-351], comparable to the addition of the alkynes. During the addihon of cis- and trons-alkenes the existence of this relahvely long lived intermediate leads to a loss of stereochemical integrity. Addihon of ds-4-propenylanisol or trans-4-propenylanisol results in both cases exclusively in the trans-adduct (Scheme 4.61). 

Photochemical cycloaddition of cyclohex-2-ene-l,4-dione (31) and acenaphthylene (32) in dichloromethane gives the iM-,un//,c -adduct 33 in 15% yield, in addition to large amounts of acenaphthylene dimers, which are insoluble and precipitate during the photolysis.100... 

The major classes of photochemical reaction for aromatic compounds are nucleophilic substitution and a range of processes that lead to non-aromatic products—valence isomerization, addition or cycloaddition reactions, and cyclization involving 6-electron systems. These five general categories of reaction will be described in the following sections, together with a few examples of more specific processes. 

The fact that molecules in L-B films keep fairly rigid relative orientations (at least within a microdomain) is potentially useful for regio- and stereoselective addition reactions (Figure 8.23). Here a derivative of cirfnamic acid forms the head-to-head dimer in the photochemical cycloaddition reaction. 

X. Photochemical cycloaddition of maleimide to certain benzene derivatives. Zhournal Organicheskoi Khimii, 14, 1522-1529 (b) Zhubanov, B.A., Almabekov, O.A., and Ismailova, Zh.M. (1981) Photoinitiated addition of maleic anhydride to monoalkylbenzenes. Zhournal Organicheskoi Khimii, 17, 996-999. 

Among the variety of terpenes which were synthesized with an intramolecular meta photocycloaddition as key step, triquinane derivatives were particularly well studied. Two isomers of these compounds are readily accessible via intramolecular meta cycloaddition in position 1,3 (Sch. 9). The following cyclopropanation reaction of intermediate O controls which of the angular or linear isomers is formed (Sch. 17) [67]. As this step is almost always unselective, both isomers are concomitantly formed. However, one of the isomers can be obtained predominantly via an additional photochemical equilibration step (compare Sch. 11). The triquinane frame is obtained by rupture of the distant C-C bond of the cyclopropane in connection to the 10-membered ring moiety. 

An additional example is a two-step approach to the preparation of 1,5-cyclo-decadiene. By a photochemical cycloaddition of a substituted cyclobutene and 2-cyclohexenone, a strained tricyclo[4.4.0.02,5]decane system is generated. Thermolysis of the tricycle gave 1,5-cyclodecadiene [52]. 

Among these reactions, the photochemical cycloadditions of C=C bom which can create up to four asymmetric carbons during the photochemical sti are particularly interesting, and numerous synthetic applications of this react have been reported. Advances in the understanding of the origin of asymmefa induction, during addition of alkenes with carbonyl derivatives, cyclic enom and aromatic compounds, will be discussed in detail. 

A selective sampling of the photochemical cycloaddition and cyclization chemistry of 2H-azirines has been outlined in this chapter. Some photochemical sequences increase molecular complexity more than others, but each seems to provide complex heterocyclic structures in a very efficient manner. Indeed, many of these photoreactions rapidly construct hetero-polycyclic systems that are difficult to produce in other ways. In contrast to their photochemical behavior, the major thermal reaction of 2H-azirines generally involves C(2)-N bond cleavage to form vinyl nitrenes which further react by either insertion into an adjacent C-H bond or else undergo addition across a neighboring rc-bond. The 27i-electrons of the carbon-nitrogen double bond of 2H-azirines can also participate in thermal symmetry-allowed [4- -2]-cycloadditions with a variety of substrates. It is clear from the above discussion that the chemistry of 2H-azirines is both mechanistically complex and... 

Fluorine-containing aldehydes, ketones and acid fluorides undergo photoinitiated cycloaddition reactions with fluorinated alkenes to give oxetanes.90 91 The addition of hexafluoroacetone (11) to fluoroalkenes can also be performed in the presence of the Lewis acid, aluminum chloride fluoride.92-1 22-1 23 Unlike the photochemical cycloaddition of hexafluoroacetone with trifluoroethene, the Lewis acid catalyzed addition is regioselective.92 Fluorooxetanes (c. g., 14) have also been synthesized by the addition of formaldehyde to fluoroalkenes in hydrogen fluoride.94 Examples of the formation of fluorooxetanes by [2 + 2]-cycloaddition reactions arc-given in Table 8. 

Ortho addition to the arene ring also occurs in cage systems where geometrical constraints disallow other orientations of addition a common example is the intramolecular photochemical cycloaddition which can occur in... 

Cyclopropanes exhibit similar modes of reactivity. [2Dipolar additions with electron-deficient alkenes and electron-donor-substituted cyclopropanes, additions of electron-rich alkenes to electron-deficient cyclopropanes, a number of radicaloid reactions and intramolecular photochemical cycloadditions are known, which may be described by the general scheme H-2 3. 

Photochemical cycloadditions of homodienes are extremely rare. An unusual photocycloaddition of the 3-OXO-4,6-diene steroid 28 to 2,3-dimethylbutadiene has been reported.The reaction has been described as a [(27H- 27t) -t- (2n + 2a)] cycloaddition, but can also be explained as a sequence of two independent processes, a [(2n + 2n) + (2n + 2a)] and a [27c- -27t] addition (both photochemically allowed). 

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