Photocycloaddition reactions Cycloadditions

Photocycloaddition Reactions ofEnones. Cyclic a,(3-unsaturated ketones are another class of molecules that undergo photochemical cycloadditions.188 The reactive... 

This article will only discuss two particular kinds of photocycloaddition reactions, the photodimerization or cross-cycloaddition of two olefins to yield a cyclobutane derivative, and the photoreaction of an olefin with a carbonyl compound to give an oxetane, Eq. 1 and Eq. 2. The inportance of substituent effects in reactions of these types is pointed... 

There has been considerable interest in various photocycloaddition reactions over the last years which not only broadened the number of useful photochemical applications but also revealed further mechanistic insight into these reactions [76,77]. Among these reactions, reports focusing on either the [2 -h 2] or the [4 -I- 2] cycloaddition, are numerous. Also the efforts toward the enantiodifferentiating photosensitization in photocyclization reactions have to be mentioned [78],... 

Step 3 (p. 303) is meant to include all of the competing processes other than cycloaddition which deactivate the excited state. These processes can vary in importance from trivial to predominant, depending upon the state, reactants, and conditions. They play a large role in determining the efficiency of any photochemical reaction. There are many factors which are known to affect these processes, but only a few examples pertinent to the photocycloaddition reaction will be discussed here. 

In a careful study of cyclopentenone photocycloaddition reaction, DeMayo and coworkers215 have noted that ketone sensitizers of triplet energy less than 71.2 kcal did not sensitize cycloaddition to cyclohexene. Triphenylene (Et = 66.6 kcal) and acenaphthene (ET = 59.3 kcal) were exceptional since they resulted in quantum yields of 0.10 and 0.21, respectively. This behavior, as well as the fact that 0.1 M cyclopentenone quenched acenaphthene fluorescence by 90% in an EPA (ether-isopentane-alcohol) glass at 77°K strongly implicate singlet energy transfer. 

Examples of this are the photocycloaddition reactions of 3-methylcyclohex-2-enone with ethene96 and isophoronc with l-propen-2-yl acetate.97 The cycloaddition of isophorone to methyl cyclobutene-1-carboxylate98 or of ( —)-piperitone (28) to 1,2-bis(trimethylsiloxy)cyclo-butene (29)99 affords tricyelo[4.4.0.02 5]deean-7-ones, used as starting compounds in fragmentation reactions. 

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). 

Photocycloaddition proceeds between allylsilane and A-methylphthalimide to yield a mixture of [2 + 2] and [4 + 2] adducts along with the allylated product291. Intramolecular cycloadditions of the vinylsilanes with the cyclopentenone moieties in 200 furnish good yields of cyclic products stereoselectively (equation 164)292. In the presence of 1,4-dicyanonaphthalene, diallylsilane 201 undergoes an intramolecular photocycloaddition reaction in an aromatic solvent to give a four-membered ring product (equation 165)293. 

Among the reactions applied in the synthesis of fullerene derivatives cycloaddition reactions such as [2 + 1]-, [2 + 2]-, [3 + 2] and [4 + 2] cycloadditions play a dominant role. In these reactions ring-fused fullerene derivatives are obtained, at least with incorporation of heteroatoms such as oxygen, nitrogen, or silicon. In this section photochemical reactions leading to cycloalkyl ring-fused fullerene adducts will be presented. Photocycloaddition reactions leading to C6o-fused heterocycles will be discussed later. 

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... 

No reaction is observed when 50 and 1,1-dimethoxyethene are allowed to stand in the dark for an equivalent period of time. Irradiation of 50 with 4 equiv. of cyclohexene in methylene chloride solvent gives a 51% isolated yield of the (2 + 2)-cyclo-adduct 52 assigned cis-anti-cis stereochemistry. These two cycloadditions were the first two well defined examples of the (2 + 2)-photocycloaddition reaction of an olefin to a carbon-nitrogen double bond. With furan the (2 + 2)-photocycloadduct 53 is formed, again regiospecifically. 

CONTENTS Preface, Mark Lautens. Photocyclization and Photocycloaddition Reactions of 4- and 2-Pyrones, Frederick G. West. Intramolecular [4+3] Cycloaddition Reactions, Michael Harmata. Lewis Acid Catalyzed [2+2] Cycloaddition Reactions of Vinyl Sulfides and Their Analogues Catalytic Asymmetric [2+2] Cycloaddition Reactions, Koichi Narasaka and Yujiro Hayashi. Vinylboranes as Diels-Alder Dienophiles, Daniel A. Singleton. Preparation and Exo-Selective [4+2] Cycloaddition Reactions of Cobaloxime-Substituted 1,3-Dienes, P... 

The photochemical cycloaddition of two different alkenes leads to multiply substituted cyclobutanes, and allows for a general access to this class of compounds. More specifically, ever since Ciamician observed the light-induced isomerization of carvone (1 —> 2) (Scheme 6.1) in 1908 [1], the inter- and intramolecular reaction between an a,(3-unsaturated carbonyl compound and an alkene has become the most intensively studied and most widely used class of [2 + 2]-photocycloaddition reactions [2-9]. 

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]. 

Despite the fact that the [2 + 2] -photocycloaddition reaction of enones has a history of more than 100 years, it has remained a vital and attractive reaction. The continuing interest and many applications to increasingly more complex targets not only bear testimony to its utility but also contradict the myth that photochemical reactions are nonselective and unpredictable. It would be desirable if this most useful chemistry could also be appreciated in the life sciences industry. The first blockbuster drug to be synthesized via a [2 + 2]-photocycloaddition is yet to be developed. Apart from the conventional evolution of the reaction, which involves an increase in scope and an improvement in its practical execution [154], it is expected that sensitization - as a means of catalyzing photochemical reactions in general [155-157] - will become a dominant factor in the development of catalytic enantioselective [2 + 2]-photo-cycloaddition variants. 

Copper(I) catalysis is very well established to promote intramolecular [2+2] photocycloaddition reactions of l,n-dienes (review [351]). The methodology recently enjoyed a number of applications [352-354], It is assumed that CuOTf, which is commonly applied as the catalyst, coordinates the diene and in this way mediates a preorganization. The Ghosh group recently reported a number of CuOTf-catalyzed photochemical [2+2] cycloaddition reactions, in which an organocopper radical complex was proposed as a cyclization intermediate (which should, however, have a formal Cu(II) oxidation state) (selected references [355-357]). A radical complex must, however, not be invoked, since the process may either proceed by a [2+2] photocycloaddition in the coordination sphere of copper without changing the oxidation state or according to a cycloisomerization/reductive elimination process. 

This review article deals with addition and cycloaddition reactions of organic compounds via photoinduced electron transfer. Various reactive species such as exdplex, triplex, radical ion pair and free radical ions are generated via photoinduced electron transfer reactions. These reactive species have their characteristic reactivities and discrimination among these species provides selective photoreactions. The solvent and salt effects and also the effects of electron transfer sensitizers on photoinduced electron transfer reactions can be applied to the selective generation of the reactive species. Examples and mechanistic features of photoaddition and photocycloaddition reactions that proceed via the following steps are given reactions of radical cations with nucleophiles reactions of radical anions with electrophiles reactions of radical cations and radical anions with neutral radicals radical-radical coupling reactions addition and cycloaddition reactions via triplexes three-component addition reactions. 

The importance of the ethylene ketal described above with respect to stereocontrol of the de-Mayo reaction is emphasized by later published works of two other teams. Fetizon and co-workers obviously followed a similar concept and carried out photocycloaddition reactions with 96a in model studies (Scheme 23) (Si). As can be seen from retroaldol product 98, exclusive a attack of the cyclohexene has taken place. Thus, the relative stereochemistry of the BC ring connection is opposite to that of taxane. Totally comparable results were obtained by Berkowitz et al. in the course of cycloaddition reactions of cyclopentene, cyclohexene, or those of a cyclohexenone ketal to the camphor derivative 96b (.84). 

The Patemo-Btichi photocycloaddition reaction - of various carbonyl compounds to furans was initially investigated by Sakurai in 1963 and was found to afford only the head-to-head photoproducts with high exo relative face selectivity. An NMR study by Whipple and Evanega later confirmed the exo mode of cycloaddition. Since the time of the origin report the photoreaction has been systematically studied by several groups and the 2,7-dioxabicyclo[3.2.0]hept-3-ene ring system has been exploited in several facets of synthesis. 

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