Experimental and computational study of the conrotatory ring opening of various 3-chloro-2-azetines.
A combined experimental and theoretical study is presented on 2-azetines, a class of azaheterocyclic compounds, which are difficult to access but have shown a unique reactivity as strained cyclic enamines. New highly substituted 2-azetines bearing aryl substituents at the 2- and 4-position were synthesized from 3,3-dichloroazetidines. Whereas 2-aryl-3,3-dichloroazetidines gave stable 2-aryl-3-chloro-2-azetines upon treatment with sodium hydride in DMSO, 2,4-diaryl-3,3-dichloroazetidines showed a remarkably different reactivity in that they afforded benzimidoyl-substituted alkynes under similar mild treatment with base. The formation of the alkynes involves electrocyclic ring opening of intermediate 2,4-diaryl-3-chloro-2-azetines and elimination of hydrogen chloride. Ab initio theoretical calculations confirmed the experimental findings and demonstrated that the 4-aryl substituent is responsible for this remarkably enhanced reactivity of 2-azetines toward electrocyclic conrotatory ring opening by a significant decrease in reaction barrier of about 30 kJ/mol. This activation effect by an aryl group in the allylic position toward electrocyclic ring opening of unsaturated four-membered rings is of general importance since a similar increased reactivity of 4-aryloxetes, 4-arylthiete-1,1-dioxides, and 3-arylcyclobutenes has been reported in literature as well.
Department of Organic Chemistry, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium.
Sven
S
Mangelinckx
Veronique
V
Van Speybroeck
Peter
P
Vansteenkiste
Michel
M
Waroquier
Norbert
N
De Kimpe
D001385:Azetines / Q000138:chemical synthesis* / Q000737:chemistry; D017606:Chlorine Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D020650:Combinatorial Chemistry Techniques; D003201:Computers; D008958:Models, Molecular; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry
D001385:Azetines; D017606:Chlorine Compounds; D009584:Nitrogen