Photodimerization of 1,3-Dienes

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). 

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.

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... 

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... 

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]... 

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." ... 

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. 

Rate data have been obtained from a detailed analysis of the previously reported 1420 maximum and fall off in the photodimerization of acenaphthylene with increasing concentrations of dissolved ethyl iodide.1420 The heavy-atom solvent dibromomethane facilitates the cross cycloaddition of acenaphthylene to trans- and cw-penta-1,3-diene by inducing intersystem crossing to the triplet state of the aromatic hydrocarbon. The intermediacy of a biradical is proposed and the stereospecificity can be understood in terms of the maintenance of the stereochemical integrity of the allylic radical units.143... 

During nearly forty years of study, the photocycloaddition of 2-pyri-dones has consistently exhibited versatile and reliable [4+4] reactivity. When dimerization is not desired, intramolecular reactions efficiently steer two pyridones to react. Alternatively, intermolecular cross reaction with an excess of another four-electron reactant can be extended to other heterocycles and to simple 1,3-dienes. It is perhaps surprising how few photoreactions of 2-pyridone fail to yield [4+4] products. Failure of the [4+4] reaction can result from a tethering unit that prevents the normal head-to-tail reactivity and introduces strain (Figures 14 and 22) or from steric hindrance caused by excessive substitution at the reacting carbons (Figure 23). A 4-alkoxy substrate shuts down the normally dominant photodimerization reaction and provides opportunities for other [4+-2] or [4+4] reactions (Figures 6 and 7). 

As expected, benzoxazoles do not react as 1,3-dienes. However, (2 + 2)-cycloadditions have been observed, as shown by the photodimerization of 2-phenylbenzoxazole (11) giving rise to the dimer 12 [283] ... 

Cyclo-octadienone was reported earlier to give two cyclobutane-containing photodimers. It has more recently been shown that the trans-isomer (137) is an intermediate in this photodimerization and can be trapped by 2,3-dimethyl-buta-1,3-diene to give (138). Irradiation of either cis,cis- or trans,trans-... 

In addition to dimerizations, 2-pyridones undergo [4-f4]-photocycloadditions with other aromatics and with 1,3-dienes (Schemes 9 and 10). Triazolopyridine 71 will photodimerize, but when mixed with pyridone 18 yields significant amounts of cross product 72. Intermolecular reactions of pyridones with... 

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. 

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). 

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. 

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]. 

Interratingly, irradiation for 6 h of the 1 1 complex 61 compc d of 8 a and 58 which had been prepared by keeping a solution of 8a and an equimolar amount of 58 in benzene-hexane at room temperature for 12 h (colorless needles of mp 133 to 136 °C) gave the optically active photocyclization product (—)-bk yclo[4.2.0]oct-7-en-2-one (62) Qa]p —60.6 (c 0.18, CHCI3)] in quantitative yield The photocyclization reaction is in contrast to the photodimerization of 58 in 59. In the case of 61, the photodimerization of 58 is probably prevented by steric hindrance. It is almost certain that one optical conformer of 58 is included in 61, but a real proof of this fact requires an X-ray structural study in the future. However, the formation of the optically active 62 is valuable, because photoreaction of 58 in solution does not give any intramolecular photocyclization product It has been reported that irradiation of bicyclo[5.1.0]octa-3,5-dien-2-one (63) in methanol leads to a mixture of racemic tricyclo[4.2.0.0 ]oct-7-en-2-one (65), 37, and cyclohepta-l,3,5-triene Control of the reaction was also tried in expectation... 

Slobodin reported the photodimerization reaction of allene. Irradiation of 1,1-dimethylallene 49 affords dimers 63 and 64. The ring-opening reaction of cyclic bisallene 65 was reported by Kaupp to yield butatriene 66 on irradiation in a low-temperature matrix,and the reverse reaction from butatriene 67 to cyclic bisallene 68 in the solid state was also reported by Berkovitch-YeUin and co-workers. The photochemical racemization of optically active penta-2,3-diene 69 in hexane was reported by Rodriguez and Morrison. Kuhn and Schulz reported the photochemical ds,trans-isomerization reaction between cis-butatriene 70 and trans-butatriene 71 and also reported a 39.5% 60.5% cis-trans ratio at equilibrium upon irradiation at 313 nm. ... 

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