Photocycloaddition reactions 1.3- dienes

Irradiation of benzothiazoline-2-thiones (34) and acrylonitrile affords 2-alkyhdenethiazoles (35) (Scheme 6). On the other hand, two products (36,37) are formed by the photoaddition of (34) with 1,1-disubstituted alkenes such as methacrylonitrile and methyl methacrylate. Aminothietanes (38) is proposed as intermediates in these photoreactions. Thioformaldehyde formed by the decomposition of the intermediate has been detected by its trapping by a diene. Photocycloaddition reaction of N-unsubstituted... 

Preparation of oxetanes by [2- -2]-photocycloaddition reactions of ketones and dienes 98S683. 

The literature of diene and polyene photochemistry provides many cases of synthetically useful reactions. As a result, certain arbitrary decisions have been made regarding what is covered in this chapter. For example, intramolecular [2 + 2]-photocycloaddition reactions of a, >-dienes can be formally included under the general rubric of diene photochemistry. However, we have chosen to restrict our discussion to dienes and polyenes which constitute a self-contained chromophore, viz. conjugated, cross-conjugated and 1,4-diene systems. Likewise, arene-olefin photocycloadditions will not be considered. These two broad classes of photoreactions have been applied extensively in synthesis, and have been the subject of recent reviews3,4. 

One of the limitations of the photocycloaddition reaction is that the unsaturated system may itself act as a quencher. Conjugated dienes fall within this category since they quench the n,ir triplet of some carbonyl compounds. For the photoreduction of benzophenone in benzhydrol, the ratio, kqlka, for m-piperylene is 750 (Table II), which indicates that this diene is indeed an efficient quencher for the reaction. 

An interesting extension of the forementioned generalizations has recently been reported.43 When a dilute solution of 2,3-dimethyl-l,3-butadiene 11a is irradiated in the presence of a large excess of benzo-phenone, a product 16 arising from the photocycloaddition reaction is observed. This anomalous result has been explained as involving the 77,77- triplet of the diene, formed by triplet-triplet transfer from the n,n triplet of benzophenone, which then reacts with ground-state benzophenone. The critical steps are shown in structures 14-16. 

With conjugated dienes, photocycloaddition of carbonyl compounds occurs at one of the double bonds to give vinyloxetanes. An interesting example is the reaction of acetone with 2-methyl-l,3-butadiene, which gave the two oxetanes (60) and (61) in a ratio of 3 1 and a total yield of about 20% (72JA8761). Other alkenes which have been used for photosynthesis of oxetanes include enol ethers, ketene acetals, enamines, allenes and diketene, with the reaction of the last compound with benzaldehyde illustrated in equation (105) (75CPB365). 

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... 

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... 

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. 

Mattay et al. employed asymmetric copper(I)-catalyzed intramolecular [2 + 2]-photocycloaddition reactions in a synthetic approach to (+)- and (— )-grandisol [56]. Racemic dienol 33 was irradiated in the presence of CuOTf and a chiral ligand to yield mainly cyclobutanes 34 and ent-34 as a mixture of enantiomers. Other 1,6-dienes were also employed. A number of chiral nitrogen-containing bidentate ligands were tested, the most effective of which, (4S,4 S)-4,4 -diisopropyl-2,2 -bisoxazoline (35) and (4R,47 )-4,4 -diethyl-2,2 -bisoxazoline (36), ensured a minor enantiomeric excess of <5% ee (Scheme 12). The coordination of the diene to the chiral Cu(I) complex under formation of a complex of type 37 was proved by CD analysis. The authors suggest a lower reactivity of the chiral complex compared to the copper ion coordinated to solvent molecules as the reason for the low enantioselectivities observed. 

The full account has appeared of the photocycloaddition reactions between the 3,5-dioxo-A" -compounds (208) and cyclopentene, dihydropyran, and various dienes. Although the 6,7-methano-3-oxo-A -compounds (209) and (210) were... 

Full details of the effects of acid and solvent polarity on various photocycloaddition reactions of ethylenes and acetylenes to benzene have been published and a mechanistic rationale of the observations has been presented. Thus the largely homopolar concerted meta cycloaddition of ethylenes to benzene, which yields the tricyclo[3.3.0.0 "]oct-3-ene system (36), has little sensitivity to either solvent polarity change or the presence of a proton donor, but the onho process leading to bicyclo[4.2.0]octa-2,4-diene derivatives (37)... 

Photocycloaddition Reactions of 12-Dienes andArene Substrates 5.222 Dimerization ofo-Xylylene and 22-Dimethylenefurans... 

The photocycloaddition reactions of 3-aryl-1,2-benzisothiazole with alkenes is regio- and stereospecific and yields 2,3-dihydro-1,4-benzothiazepines (Scheme 10) <8IT3377>. When electron-rich alkynes are used, then substituted 3,4-benzo-2,6-thiazabicyclo[3.2.3]hepta-3,6-dienes are the major product (Scheme 10) <83JOCi275>. 

Benzene in the Si (n,n ) excited state is no longer aromatic and is capable of undergoing various chemical reactions not observed in the ground state. Three basic types of photocycloaddition reaction of an alkene to the excited benzene can be de-scribed 802,804,809 816 (a) bicyclo[4.2.0]octa-2,4-dienes (e.g. 191) are formed by ortho-photocycloaddition (1,2- or [2 + 2] photocycloaddition), (b) tricyclo[3.3.0.02 8]oct-3-enes (e.g. 192) are obtained by mcia-photocycloaddition (1,3- or [3 + 2]... 

There are several reports this year of photocycloaddition reactions between polycyclic aromatic hydrocarbons and 1,3-dienes. Anthracene (58 R = H) or 9-cyanoanthracene (58 R = CN) give [4 -I- 4] and [4 -I- 2]adducts on irradiation with buta-1,3-diene, but the different product ratios and temperature effects are used to support the previously proposed biradical mechanism for the reaction. Another report from the same group deals with anthracene-hexa-2,4-diene and 9-phenylanthracene-penta-1,3-diene or cyclohexa-1,3-diene systems in the case of 9-phenylanthracene (59) and cyclohexa-1,3-diene, two [2 -I- 2]cycloadducts involving a terminal aromatic ring are isolated, as well as the more usual [4 + 2] and [4 -f- 4]adducts involving the central ring. Irradiation of substituted anthracenes... 

Unless the indole nitrogen atom is substituted by an acyl group, the photocycloaddition reactivity of indoles is very limited. In the absence of an N-acyl group, the only reactions reported are the triplet-sensitized 2-1-2 addition of alkynes to AT-methylindoles and the electron-transfer-sensitized 4-1-2 addition of dienes to indoles. When an AT-acyl substituent is present, the C2-C3 bond appears to be activated and will undergo photocycloaddition reactions with a variety of partners such as alkenes, carbonyl groups, and the C2-C3 bond of ground-state N-acylindoles. 

Photochemical [3 + 2] photocycloaddition of alkenes to benzene ring is a useful tool for construction of 5-membered ring compounds in a one-step. Penkett reported the synthesis of the [5.5.5.5] fenestrane structures (47), (48), (50) by arenyl-diene (either 46 or 49) double [3 + 2] photocycloaddition reactions. Penifulvins (53), natural products, were prepared by use of an intramolecular [3 + 2] photocycloaddition of (51) as a key step." ... 

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