Photochemisches Verhalten von 1- und 2-alkylierten 1, 2-Dihydronaphthalinen bei tiefen Temperaturen

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].