Reactions of Dienes, Trienes, and Higher Polyenes

Reviews have dealt with the selective isomerization of alkenes, dienes, and trienes with infrared lasers, euid with the photochemical reactivity of cumulenes.  

The formation of 3-anilinoalkenes (91) results on irradiation of acetonitrile solutions of cyclohexarl,3-diene and aniline or A methyl aniline. Similar addition occurs with the 

The wavelength (185-254 nm) dependent photochemistry of cyclo-octa- and cyclo-hepta-1,3-dienes has been studied. cis,cis -Cyclohepta-l,3-diene is converted to the trans -cIs -isomer (92) when irradiated in a matrix at low temperature. On warming to temperatures above -78°C this intermediate ring closes to afford bicyclo(3,2,0)hept-6-ene, the same product obtained by irradiation at room temperature.  

The ionone series of compounds has been fruitful for Jeger smd his coworkers. The area is clearly not exhausted and they have studied the behaviour of the epoxy dienes (93) under acetone-sensitized irradiation. Irradiation of the enol benzoate (94) in the presence of acid yields the isoquinolinone (95) in good yield by a mechanism involving cyclization, elimination and oxidation.  

Matrix-isolated diene (96) undergoes photochemical transformation into the isomeric compounds (97) and (98) as the primary photoproducts. Maier and his coworkers have observed the photoisomerization of the cyclobutadiene dimer (99) into the pentacyclic compound (100). The u.v. irradiation of the Dewar benzene (101) a.R ords the paracyclophane (102, 13%). The prismane (103) can be produced by the irradiation of the Dewar benzene (104). The intermediacy of the benzene (105) in the transformation is likely.  

The irradiation of the strained diene (87a) in alcoholic solution affords the rearranged product (88). The authors interpret this as evidence for the isomerization of the starting material (87a) into the 4-paracyclophane (89) which thermally adds solvent to afford the isolated product. A more detailed report of this work has included the photoisomerizations of (87 b.c) which yield mixtures of (90) and (91) on irradiation in ethanol. Spectroscopic evidence confirms the paracyclophanes (89) as the intermediates in the formation of the alcohol addition products (88), (90), and (91).  

Several reports over the past few years have dealt with the photochemical cyclization of dienes or enones of the type represented by (92). The problem with such systems is that it is often difficult to predict whether straight addition to the biradical (93) or crossed addition, biradical (94), will be preferred. Ohsaku has studied this problem and has published a theoretical treatment. 

A study of the photochromic behaviour of the stilbene analogue (103) and its reversible cyclization to (104) has been made.  

A high yield of the enone (109) is obtained from the diene 

A study of the photoisomerization of the dienes (113) has sought to establish the involvement of zwitterions in the isomerization process and the influence that substitution can have on the reaction. The results obtained indicate that isomerization about the C-2 and C-4 double bonds does occur and that the polarity of the substituent plays an important role in determining the position of the isomerization in the molecule. Thus in (113a) the effect of the ester group is dominant and Influences the involvement of the twisted zwitterion (114) leading to isomerization of the C-2 bond. The introduction of fluoro substituents on C-2 or C-4 or the use of the aldehyde instead of the ester in (113b-e) changes the 

5-migration take place. The triene (127) is photo-chemically 

Various transpositions occur such as illustrated for the indene 71 72 

Pyrex in methylene dichloride solution gives the cyclobutenes 

The irradiation of the methyl substituted cyclic allene (163) has been studied to evaluate the influence of substitution on the reactions encountered. This work follows on an earlier study of the photoreactivity of the unsubstituted allene. Direct irradiation using light of wavelength 220nm in pentane brought about the formation of the products shown in scheme 7 in the yields shown below the appropriate structure. The products obtained from this reaction were compared with those from a thermally generated carbene intermediate and the conclusion reached is that carbene intermediates are involved in the photochemical reaction. This is different from the unsubstituted case 80 

A description of the electron-transfer induced ring-closure of 

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In the reaction of cycloocta-1,5-diene (Mi) with oct-4-ene (M2), the proportion of any given linear polyene formed at equilibrium increases to a maximum and then declines as the ratio [Mi]/[M2] is increased. If the double bonds were distributed at random between Mi, M2, and the linear polyenes, the optimum content of double bonds in linear dienes should occur at [M2]/[Mi] = 2, and for trienes at [M2]/ [Ml] = 1. Experimentally, the maximum proportion of double bonds in the linear diene and triene species is found to occur at the somewhat higher values of [M2]/ [Ml] = 2.5-3 and 2-2.5, respectively (Pinazzi 1977a). This is a consequence of the... 

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