Nucleophilic Attack on Nitrogen in Tetrazines by Silyl-Enol Ethers
作者:Simon D. Schnell、Mauro Schilling、Jan Sklyaruk、Anthony Linden、Sandra Luber、Karl Gademann
DOI:10.1021/acs.orglett.0c04113
日期:2021.4.2
The nucleophilic addition of silyl-enol ethers to nitrogen in 3-monosubstituted s-tetrazines mediated by BF3 is reported. The preference for this azaphilic addition over the usually observed inverse electron demand Diels–Alder reactions was evaluated theoretically and corroborated by experiments. The substrate dependency of this unusual reaction was rationalized by determination of the activation barriers
Oxiranes (epoxides) and oxetanes having a carbonyl functional group are chemoselectively isomerized to different heterocyclic compounds via Lewis acid-promoted 1,6- and 1,7-intramolecular nucleophilic attacks of the carbonyl oxygen on the electron-deficient carbon neighboring the oxonium oxygen: for example, cyclic imides to bicyclic acetals, esters to bicyclic orthoesters, sec-amides to 4,5-dihydrooxazole or 5,6-dihydro-4H-1,3-oxazines, and tert-amides to bicyclic acetals or azetidines. The intramolecular attack of a 1,5-positioned carbonyl oxygen predominantly results in a propagating-end isomerization polymerization. On the other hand, cyclic ethers having a 1,8- or farther positioned carbonyl group undergo conventional ring-opening polymerization. A tetrahydrofuran (oxolane) ring does not open, even with a 1,6-positioned carbonyl group. (C) 2002 Elsevier Science Ltd. All rights reserved.