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)‐ (d 6 – 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, 7 a ] 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].