Photodimerization 1,3 dienes

Intermolecular photocycloadditions of alkenes can be carried out by photosensitization with mercury or directly with short-wavelength light.179 Relatively little preparative use has been made of this reaction for simple alkenes. Dienes can be photosensitized using benzophenone, butane-2,3-dione, and acetophenone.180 The photodimerization of derivatives of cinnamic acid was among the earliest photochemical reactions to be studied.181 Good yields of dimers are obtained when irradiation is carried out in the crystalline state. In solution, cis-trans isomerization is the dominant reaction. 

Rubin and co-workers have investigated the photodimerization of the 4,6-diene-3-ketosteroid (75)(86) ... 

Dienes (17) and (18) may not actually be photoproducts but may arise from thermal rearrangement of m-l,2-dialkenylcyclobutanes.<8,9) As in the photodimerization of butadiene, the product distributions of sensitized... 

Photodimerization reactions of some other simple alkenes and dienes follow/39-30 36-182 Although not a dimerization reaction, photochemical ring closures to yield cyclobutane derivatives are analogous and are included in this section 31-35 ... 

Irradiation of cycloocta-2,4-dien-l-one (55) in pentane gives a racemic photodimer, anti-tricyclofSAO.O Jhexadeca- , 11 -diene-3,16-dione (60) in 10% yield along with polymeric materials 34). Efficient and enantioselective photodimerization of 58 was achieved by irradiation of the 2 1 inclusion complex 59 formed between 2 a and 5813). When a solution of 2a and an equimolar amount of 58 in ether-hexane (1 1) was kept at room temperature for 12 h, 59 was obtained as colorless needles of mp 105 to 108 °C. Irradiation of 59 in the solid state for 48 h gave (—)-60 of 78 % ee in 55 % yield. 

III. PHOTOCYCLOADDITIONS INVOLVING DIENES AND POLYENES A. [2 + 2]-Photodimerization of 1,3-Dienes... 

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). 

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

Figure 1 summarizes the chemical structures of the topochemically polymerizable 1,3-diene monomers providing stereoregular 1,4-trans polymer (Scheme 6) [ 16]. Most of the polymerizable monomers contain benzyl, naphthylmethyl, and long alkyl-chain substituents in their chemical structures. The (ZyZ)-, (E,Z)-, and ( , )-muconic and sorbic acids as well as the other diene carboxylic acids are used as the ester, amide, and ammonium derivatives. In contrast to this, the carboxylic acids themselves have crystal structures unfavorable for polymerization while they undergo [2-1-2] photodimerization, as has already been described in the preceding sections.

The synthesis of cis-1,4 polymers was also tried by e use of monomers with an s-cis conformation. The solid-state photopolymerization of pyridone derivatives, which is a six-membered cyclic diene amide and is a tautomer of 2-hydroxypyridine, was attempted [100]. Pyridones make hydrogen-bonded cocrystals with a carboxylic acid in the crystalline state. Because the cyclic structure fixes its s-cis conformation, if the polymerization proceeds, a cis-2,5 polymer would be obtained. Actually, however, the photopolymerization did not occur, contrary to our expectation, but [4-1-4] photodimerization proceeded when the carbon-to-carbon distance for the dimerization was small (less than 4 A) [101]. A closer stacking distance of the 2-pyridone moieties might be required for the topochemical polymerization of cychc diene monomers. 

In order to overcome this lack of selectivity, photodimerizations have been performed in micelles,16,17 in supercritical fluids,18 in inclusion compounds19 and in the solid state.20 Nevertheless, such reactions are difficult to run on a preparative scale, and better results can be obtained by careful choice of an appropriate solvent. Enantioselcctive gas chromatography combined with GC/MS analysis proved to be a very efficient tool for the direct assignment of constitution and configuration of the photocyclodimers formed.21 In this manner, /ram-1,2-di-vinylcyclobutane has been prepared by sensitized irradiation of buta-1,3-diene.22... 

Photodimerization involves 1 1 adduct formation between an excited nd a ground state molecule. Olefinic compounds, aromatic hydrocarbons, conjugated dienes, oc-unsaturated compounds are known to dimerize when Cxpdsed to suitable radiation. Photodimerization of olefinic compounds Can occur by either (a) 1,2-1,2 addition, (b) 1,2-1,4 addition or... 

Conjugated dienes take part readily in triplet-sensitized photodimerization. and the products obtained from buta-1.3-diene (2.691 include a (4 + 2) adduct as well as slereoisomeric (2 + 2) adducts. The reaction is non-concerted. and a rationalization for the products is provided on the basisof the formation of a biradical intermediate as shown (which is the most stable of the three possible biradicals that might be formed in the first step), by the attack of triplet diene on ground-state diene. Cross-addition takes place in some systems, such as myrcene (2.70) where a triplet diene group attacks the alkene within the same molecule direct irradiation of myrcene gives mainly... 

Cyclic dienes such as cyclopentadiene and 1,3-cyclohexadiene also are dimerized very efficiently with triplet sensitizers.285 A noteworthy feature of these photodimerizations is that they do not produce much of the endo- 1,4-adducts, the principal products of thermal dimerization. 

Die von Stobbe und Farber 295) vorgeschlagene tricyclische Struktur 9 fiir das Photodimere des Dimethylesters der 3-Oxo-penta-l,4-dien-l,5-... 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

Numerous photodimerization studies of 1,3-cyclohexadiene 36 have been reported (Sch. 9). Thermal cycloaddition yields a 4 1 mixture of endo/ exo [4+2] adducts 37 and 38 in modest yield. Irradiation of the diene in cyclohexane near its 2max of 254 nm yields very little dimer, but irradiation at 313 nm leads to a mixture of dimers, favoring the [2+2] adducts 39 [37]. The use of y-radiation produces similar mixtures [38,39]. A triplet sensitizer leads to largely the [2+2] adducts plus exo 38 and little of the endo [4+2] isomer 37 [40]. When the photochemistry is conducted in the presence of the electron acceptors anthracene 41, LiC104-42 or pyrylium 43, only [4+2]... 

Irradiation of the 2 1 host-guest crystals of cyclohex-2-enone 81 with the axle-wheel-type host compound (— )-5 as an aqueous suspension caused regio- and enantioselective [2 + 2] photodimerization to afford the (— )-anti-head-to-head dimer 82 of 48% ee in 75% chemical yield (Scheme 18) [86]. Similarly, solid-state photolysis of the 3 2 complex of cycloocta-2,4-dien-l-one 83 with (R,R)-( — )-4 gave the (-)-anti-head-to-head dimer 84 in moderate optical yield [87]. 

The only reaction in which a chromene appears to react as a hetero-diene and not like a styrene is the addition of acetylene dicarboxylate to lapachenol, followed by thermal decomposition of the adduct (Scheme 16). The ready photodimerization of a chromene has been reported recently. (See also cannabicyclol, Section V,B )... 

The 1,4-dicyanonaphthalene-sensitized photocyclodimerization of the vinyl ethers (192), yielding (193) has been described. 9,10-Dicyanoanthracene-sensitized dimerization of cyclohexa-1,3-diene affords the two [4+2] adducts (194) and (195) in a total yield of 60% and in a ratio of 4 1. This is to be contrasted with the previous report of cyclohexadiene dimerization where [2 + 2] dimers were also obtained." A detailed study of the photodimerization of some acenaphthylenes (196) has been reported. " The structures and configurations of the products have been elucidated by spectroscopic methods." ... 

Quina, F H., and Wliitten, D. G. Photochemical Reactions in Organized Monolayer Assemblies. 4. Photodimerization, Photoisomerization, and Excimer Formation with Surfactant Olefins and Dienes in Monolayer Assemblies, Crystals, and Micelles. J. Am. Chem. Soc. 99, 877 (1977). 

Photodimerization of simple 1,3-dienes in a 4 1 + 4 r cycloaddition process is typically an inefficient process . This is not surprising, given the highly ordered transition state for [4+4]-cycloadditions, and the predominance of the unreactive s-trans conformation . As a result, as noted above [2 + 2]-cycloadducts are often the major product, accompanied by varying amounts of vinylcyclohexenes and cyclooctadienes. Crossed photocycloadditions employing 1,3-dienes with substituents at the 2- or 3-positions can furnish greater amounts of cyclooctadiene products (equation This presumably results from a perturbation of the diene conformational equilibration to provide a higher proportion of the s-cis conformer. 

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