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The irradiations of 1,1-dimethyl- (8), 1,1-di-(tri-deuteriomethyl)- (d6-8), 1,1,2.2-tetramethyl- (9) and cis- and trans-1,2-dimethyl-1,2-dihydronaphthalenes (cis- and trans-10) were investigated in 2,2-dimethylbutane/pentane at -100° using a mercury high-pressure lamp, and with mercury high- and low-pressure lamps at room temperature. The results were compared with one another, and those of the individual compounds are collected in schemes 2 and 4-7. The most important results are the following: 1.The 1,2-dihydronaphthalenes undergo a conrotatory ring opening to the o-quinodimethanes on irradiation with high- or low-pressure lamps at room temperature or at -100°. Thermal reactions ([1,7a]H-shifts, electrocyclisations) are suppressed at -100°. The o-quinodimethanes formed from 8 (scheme 2), 9 (scheme 5) or cis-10 (scheme 6) undergo on irradiation with the high-pressure lamp, [1,5]H-shifts or photochemical Diels-Alder reactions after renewed photochemical excitation, to yield the benzobicyclo[3.1.0]hex-2-ene derivatives. These Diels-Alder reactions do not proceed stereospecifically, and therefore are not orbital symmetry controlled reactions. 2. If the 1,2-dihydronaphthalenes are irradiated at room temperature with either a high- or a low-pressure lamp, then the initially formed o-quinodimethanes undergo thermal [1,7a]H-shifts, in preference to all other reactions, as long as this is sterically possible; the resulting products can undergo secondary photochemical transformations. Such o-quinodimethanes are formed on irradiation of 8, 9 and cis-10. From trans-10, an o-quinodimethane mixture is formed, of which one component (cis,cis-22) undergoes thermal [1,7a]H-shifts, while the other (trans,trans-22) suffers a thermal disrotatory electrocyclisation to give cis-10. If a high-pressure lamp is used in the last experiment, then the competing photochemical Diels-Alder cyclisation to bicyclic compounds of the type 23 (scheme 7) can result in the trans,trans-22. As was shown by Salisbury [3], and confirmed by ourselves in other cases [2], photochemical Diels-Alder reactions or [1,5]H-shifts in the o-quinodimethanes require light of wavelength 400 nm (high-pressure lamp). The present photochemical investigations amplify and confirm our earlier conclusions concerning the photochemistry of the 1,2-dihydronaphthalenes [2].

1-cis, 2-cis-Dipropenylbenzene (cis, cis-1) isomerises thermally at 215–235° with 1st order kinetics to give trans, cis-1 and vice versa. At equilibrium 89% trans, cis- and 11% cis, cis-1 are present. It is shown by thermal rearrangement of cis, cis-2′, 2″-d2-1 that the isomerisation is attributable to aromatic [1, 7a]-sigmatropic H-shifts. trans, trans-1 rearranges thermally at 225–245° to yield 2, 3-dimethyl-1, 2-dihydronaphthalene (2). The formation of 2 can be visualized by disrotatory ring closure followed by an aromatic [1, 5s]-sigmatropic H-shift. 2 is also formed when, cis, cis- or trans, cis-1 are heated for 153 h at 225°. Besides 2 a small amount (3%) of 1-ethyl-1, 2-dihydronaphthalene (5) is formed. The rearrangement of trans, trans-1 and trans, trans-2′, 2″-d2-1 shows a secondary isotope effect kH/kD = 0,90.

Irradiation of 1-alkyl-substituted 1,2-dihydronaphthalenes (10, 11, 12) with a low-pressure mercury lamp yields by ring opening omega-vinyl-o-quinodimethanes, which undergo [1,7]H-shifts to give 1,2-divinylbenzenes (8, 18, 23; cf. schemes 2, 3 and 4). In a further photoreaction of the divinylbenzenes, benzobicyclo[3.1.0]hex-2-enes (17, 19, 22) are formed. 2-Alkyl-substituted 1,2-dihydronaphthalenes (13, 14, 15, 16) are transformed by irradiation into o-vinyl-o-quinodimethanes, which show [1,7] H-shifts to yield in this case 2-(buta-1',3’-dienyl)toluenes (9, 25, 26, 27; cf. schemes 6 and7). The irradiation of 1-methyl-(10) and 1-ethyl-1,2-dihydronaphthalene (11) with a high-pressure mercury lamp produces, besides the products of irradiation using the low-pressure lamp, 2-ethyl-allenylbenzene (24), and (from 11) 4-exo-ethyl-benzobicyclo[3.1.0]hex-2- ene (exo-20) and 2-propyl-allenylbenzene (21), respectively (cf. scheme 5). Obviously, these products arise from a photoreaction of the primarily formed omega-vinyl-o-quinodimethanes a.

The acid catalysed dienol-benzene rearrangement of methyl substituted o- and p-propargylcyclohexadienols (18–22, 34 and 35) was investigated. In the first step water is eliminated to yield the corresponding methyl propargyl benzonium ions (cf. scheme 6, a), which undergo [1s, 2s] sigmatropic rearrangements to give propargylbenzenes (28, 29, 30, 38) and [3s, 4s] sigmatropic rearrangements to give allenylbenzenes (24–27, 40) (cf. schemes 2, 3, 5, 6). [3s, 3s] sigmatropic rearrangements occur only to a small extend. In the rearrangement of 2-propargyl-2,4,6-trimethylcyclohexa-3,5-dien-1-ol (18) a [1s, 2s] sigmatropic methyl shift is observed (4%).

Benzonitrile p-nitrobenzylide (5) undergoes 1,3-dipolar cyclo-additions in the presence of 3-phenyl-2H-azirines (1), yielding in benzene at 0° 2-(p-nitrophenyl)-4,5-diphenyl-1,3-diazabicyclo[3.1.0]hex-3-enes (7, scheme 2). Under the basic conditions of the reaction mixture, 7 a and 7 b are partially converted to 2-(p-nitrophenyl)-4,5-diphenyl-1,6-dihydropyrimidines (8a, b) which are dehydrogenated by oxygen to the corresponding pyrimidines 9a and 9b, respectively. 3-Phenyl-2H-azirines (1) form, on heating at 145° in xylene in the presence of the azalactone 32 (2,4-diphenyl-Δ2-oxazolin-5-one), 4-(aziridin-2′-yl)-2,4-diphenyl-Δ2-oxazolin-5-ones (33, scheme 11). 33 arises from an ene reaction of the enol form of 32 with 1. Similar ene reactions are observed with the azirines 1 and dimedone (37, scheme 12). Under the ene reaction conditions (xylene, 145°), the non-isolated intermediate primary adducts (38a and 38b) undergo rearrangements of the vinylcyclopane-cyclopentene type to give 6,6-dimethyl-4-oxo-1,3-diphenyl-4, 5, 6, 7-tetrahydroisoindole (40) and 6, 6-dimethyl-4-oxo-3-phenyl-4, 5, 6, 7-tetrahedroindole (42), respectively.

1-Mesityl allene (1), 1-mesityl-3-methyl allene (2) and 1-mesityl-3,3-dimethyl allene (3) rearrange thermally at 150–190° in decane via [1,5s]sigmatropic H-shifts to yield the o-quinodimethanes 4, which cyclise to give the 1,2-dihydronaphthalenes 5 and 6 and/or undergo [1,7 a]sigmatropic H-shifts to give 1-mesityl-(Z)-buta-1, 3-dienes (Z)-7 and (Z)-8, respectively (Schemes 1,3,4 and 5) in almost quantitative yields. The activation parameters of these isomerisations are given in Table 1. 1-Mesityl-1-methyl allene (9) isomerises at 190° to give 4,5,7-trimethyl-1,2-dihydronaphthalene (17) in 50% yield (Scheme 6). 2′-Isopropylphenyl allene (10) in decane rearranges at 170° to 1-(Z)-propenyl-2-isopropenyl-benzene ((Z)-19, Scheme 7). Deuterium labelling experiments show that the rate determining step is an aromatic [1,5s]sigmatropic hydrogen shift from an sp3- to an sp-hybridised carbon atom. The primary kinetic isotopic effect (kH/kD) is 3.45, while the secondary βisotopic effect is 1.20 (Scheme 7 and Table 2).