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Zum Mechanismus der Cyclopropan‐“Walk”‐Umlagerung: Synthese und Eigenschaften eines optisch aktiven Diazoalkan‐Addukts von Toluol; eine hochstereoselektive N 2 ‐Eliminierung
Author(s) -
Klärner FrankGerrit,
Band Rudiger,
Glock Volker,
König Wilfried A.
Publication year - 1992
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19921250132
Subject(s) - chemistry , diradical , cyclopropane , electrocyclic reaction , cycloheptatriene , stereochemistry , stereoselectivity , nitrene , medicinal chemistry , photochemistry , bicyclic molecule , singlet state , ring (chemistry) , catalysis , organic chemistry , physics , nuclear physics , excited state
Mechanism of the Cyclopropane “Walk” Rearrangement: Synthesis and Properties of an Optically Active Diazoalkane‐Toluene Adduct; a Highly Stereoselective N 2 Elimination The synthesis and the thermal and photochemical behavior of the optically active 2‐diazo‐1‐methoxypropane‐toluene adduct 3 are reported. The N 2 elimination of 3 ocurs with one and the same stereochemical course upon thermolysis as well as direct photolysis: 7‐(Methoxymethyl)‐1,7‐dimethyl‐1,3,5‐cycloheptatriene ( 1 ) is formed with retention of configuration at C‐7 and 7‐(methoxymethyl)‐2,7‐dimethyl‐1,3,5‐cycloheptatriene ( 2 ) with inversion of configuration at C‐7. These stereochemical findings are explained by an intermediary singlet diradical 1 [ 4 ] in which the ring closure between C‐1 and C‐7 or C‐5 and C‐7 is faster than the rotation around the single bond between C‐6 and C‐7. In the case of the thermally induced formation of 2 a concerted N 2 elimination has to be considered as a competitive pathway. The results presented here are well in accord with the stereochemical analysis of the cyclopropane “walk” rearrangement 1 ⇋ 2 reported previously and are therefore a good evidence that the diradical intermediate 4 is also involved in the “walk” rearrangement. In the photochemically induced N 2 elimination of 3 , sensitized by benzophenone and leading again to 1 and 2 , a surprisingly high degree of stereoselectivity is observed. Thus, the intermediary triplet diradical 3 [ 4 ] must have a much shorter life time than ordinary 1,3‐diradicals in the triplet state.