Photocycloaddition reactions intermolecular

Photocycloaddition Reaction Intermolecular Reaction Intramolecular Reaction 74-3... 

Stereoselective intermolecular [2- -2]-photocycloaddition reactions of unsaturated heterocycles with formation of fused systems 98S683. 

The synthetic applications 440) and mechanistic aspects 4411 of intermolecular photocycloaddition reactions of arenes to olefins have been reviewed recently. Intramolecular cycloadditions442a,b) have been studied in the context of the photochemical behaviour of bichromophoric molecules, as to investigate interchromophoric interactions in polyfunctional molecules. Three types of addition products can be formed in the photocycloaddition of benzene to an alkene (4.37)441. ... 

The N=N double bon d does take part in a few photocycloaddition reactions to give cyclic compounds with two adjacent nitrogen atoms in the ring. Intermolecular (2 + 2 cycloadditions are not known, but some intramolecular examples of this reaction are reported for quite complex compounds 15.27 in which the reacting groups are held fairly rigidly in an orientation suitable for reaction. A (4 + 2) cycloaddition takes place when naphthalene is irradiated with an electron-deficient cvdrc azo-compound (5.28). 

Demonstration of the unique synthetic utility of the [2 + 2] photocycloaddition reaction of enones to alkenes and the success in controlling the stereoselectivity, to some extent, in the intermolecular additions (discussed above) prompted further studies and development of new synthetic applications in the intramolecular photoadditions during the last decade. In most cases that have been studied, the alkene was tethered to the cyclic enone by three carbon units or two carbons and one heteroatom. 

Intermolecular and intramolecular photocycloaddition and photoaddition to aromatic rings in the electron-donor and electron-acceptor systems were discussed in this chapter. The highly stereoselective and regioselective photocycloaddition is a synthetically useful method for the construction of polycyclic carbon-skeleton compounds, including natural products. New aspects for the stereoselective intermolecular and intramolecular photocycloaddition reactions via exciplexes in less... 

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... 

The photocycloaddition chemistry of oc,P-unsaturated 8-lactones is similar to the chemistry of y-lactones. Complications arise as with cyclohexenones because anti-addition to the a,P-unsaturated double bond can occur, particularly in the intermolecular addition mode. Even if one product prevails, intermolecular [2 + 2]-photocycloaddition reactions are often sluggish. Despite the fact that alkene 92, for example, was employed in a twofold excess relative to dihydropyranone 91, the reaction delivered only 32% of the desired product 93 (43% based on recovered starting material Scheme 6.33). The relative product configuration, which was established by X-ray crystallography, came as a surprise because the lactone apparently... 

Recently, pyrone [2 + 2]-photocycloaddition reactions were used to construct macrocyclic compounds. A dipyrone was irradiated in the presences of a,(0-diolefins yielding 18- to 25-membered rings by a sequential intermolecular and intramolecular cycloaddition [146]. 

In enantioselective photocycloaddition reactions, 4-alkoxyquinolones perform in superior fashion to l,5-dihydropyrrol-2-ones and 5,6-dihydro-lff-pyridin-2-ones. Both, intermolecular and intramolecular reactions were performed with excellent enantioselectivity in the presence of the chiral template 115, or of its enantiomer ent-115 [147, 148], The well-established photocycloaddition reactions [149, 150] enabled access to a variety of chiral dihydroquinolones. 4-Methoxyquinolone (157) produced, upon direct irradiation in the presence of allyl acetate, the formal HT product 158 in 80% yield and with 92% ee (Scheme 6.56) [151]. 

Bach, T. and Bergmann, H. (2000) Enantioselective intermolecular [2 + 2]-photocycloaddition reactions of alkenes... 

Bach, T., Bergmann, H., Grosch, B., and Harms, K. (2002) Highly enantioselective intra- and intermolecular [2 + 2] photocycloaddition reactions of 2-quinolones mediated by a chiral lactam host host-guest interactions, product configuration, and the origin of the stereoselectivity in solution. Journal of the American Chemical Society, 124, 7982-7990. 

The intermolecular [2 + 2] photocycloaddition reactions of 2-quinolones such as 59 mediated by a chiral lactam host 60 or ent-60 were reported with enantioselectivities of 93%. The intermolecular version of this reaction was also shown to be highly enantioselective. One example shown in Scheme 9 afforded cycloadduct 63 in 92% ee with a diastereoselectivity of >95 5 <02JA7982>. 

The scope of this approach was widened by the observation of excellent enantioselectivities in intermolecular [2+ 2]-photocycloaddition reactions with various alkenes [62,71]. In the presence of an excess amount of alkene, 4-me thoxy-2-quinolone (57) was converted with high chemo- and regioselectivity to the exo and endo cyclobutanes 59 and 60. With 4-penten-1-ol (58a), allyl acetate (58b), methyl acrylate (58c), and vinyl acetate (58d), the exo diastereomers 59a-d were formed with high simple diastereoselectivity and in high yields (80-89%), Under optimized irradiation conditions (2.4 eq. of host 44 or ent-44, — 60°C), high enantiomeric excesses were achieved in all instances, as depicted in Scheme 22. These enantiomeric excesses are unprecedented for an intermolecular photochemical reaction. 

Brandes S, Selig P, Bach T (2004) Stereoselective intra- and intermolecular [2+2]-photocycloaddition reactions of 4-(2,-aminoethyl)quinolones. Syn-lett 2588-2590... 

Bach, T. Stereoselective intermolecular [2+2] photocycloaddition reactions and their application in synthesis. Synthesis 1998, 683-703. 

Intermolecular Cycloaddition - Reviews pertinent to the area discussed in this chapter have commented upon photocycloaddition reactions and also have highlighted methods for the synthesis of macrocyclic ring systems. ... 

Arene-alkene photocycloaddition reactions have been reviewed in detail. This review includes a tabular survey, synthetic applications and exhaustive bibliography. Many of the reactions considered in this review were however performed to determine answers to mechanistic questions, therefore they are not necessarily optimized and chemical yields are not indicated. Tables 2 and 3 bring together, from the references cited in the review , those intermolecular photochemical meta cycloadditions which are principally synthetic methods. More concise general treatments of this reaction have since been published, as have other papers mainly concerned with its mechanistic aspects. """ ... 

Addition reactions.- A wide variety of intermolecular and intramolecular [ 2 + jj2] photocycloaddition reactions have again been described in nitrogen-containing systems. [ 2 + 2] Photo-dimerisation has been reported for (E)-1,2-bis(1, 3 -benzoxazol-2 -... 

There are only a limited number of intermolecular [2 + 2] photocycloaddition reactions known to occur in solution because alkene excited singlet state lifetimes are very short (on the order of 10 ns).708 Direct irradiation of neat but-2-ene, for example, yields tetramethylcyclobutane with stereochemistry suggesting the concerted mechanism (Scheme 6.45).709 Inefficient dimerization (

with efficient E Z isomerization (

[Pg.257]

This reaction pathway is usually favoured when an aromatic moiety and an alkene bear electron-withdrawing and electron-donating substituents, respectively (or vice versa). This addition involves a charge transfer and the course of the reaction is sensitive to the solvent polarity. Such a mechanism may resemble that of [2 + 2] photocycloaddition of alkenes to aji-unsaturated carbonyl compounds (Section 6.3.2). Scheme 6.81 shows examples of two intermolecular processes and one intramolecular [2 + 2] photocycloaddition reaction (a) crotononitrile (196) is added to anisole (197) to yield several stereoisomers of 198 in 38% chemical yield and with high regioselectivity, which is linked to bond polarization in the exciplex 818 (b) hexafluorobenzene (199) reacts with 1-ethynylbenzene (200) to form the bicyclo[4.2.0]octa-2,4,7-triene 201 in 86% yield 819 and (c) irradiation of 202 in methanol leads to the single photoproduct 203. 820... 

Langer, K., Mattay, J., Copper(I) catalyzed Intra and Intermolecular Photocycloaddition Reactions of Alkenes. In Horspool, W. M., Song, P. S. (eds), CRC Handbook of Organic Photochemistry and Photobiology, CRC Press, Boca Raton, FL, 1995, pp. 84 104. 

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