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Reactions with Aziridines, XXXII[1]. Mechanistic Aspects in Nucleophilic Opening of Three‐Membered Rings. Alcoholyses of Activated Aziridines
Author(s) -
Bucholz B.,
Stamm H.
Publication year - 1986
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.198600003
Subject(s) - chemistry , nucleophile , ether , steric effects , catalysis , medicinal chemistry , carbenium ion , leaving group , racemization , selectivity , ring (chemistry) , photochemistry , stereochemistry , organic chemistry
Nucleophilic ring opening of three‐membered rings is discussed in terms of leaving group basicity. Reactions of activated (weakly basic leaving groups) 2,2‐dimethylaziridines 5 and 2‐phenylaziridines 6 with alcohols ROH are studied with reference to the site selectivity of ring opening and competing side‐reactions. Abnormal opening (at the substituted carbon) of 5 proceeds in neutral ROH without catalyst when the activation is strong (tosyl) whilst no reaction or mainly carbonyl attack is observed with weak activation (CONHAr). NaClO 4 catalyzes the abnormal opening of 5. EtO − gives exclusively normal opening of 5 whilst, in contrast to the literature, MeO − gives also a substantial amount of abnormal ether. Attack on 6 always occurs at both positions with the predominance of the abnormal reaction being greater for MeO − than for EtO − . These reactions are considered S N 2 with steric and benzylic differentiation between normal and abnormal attack depending on the size of ROH. Acid catalyzed openings of 5 and 6 proceed exclusively abnormally in the ether‐forming reaction as well as in side‐reactions. Competition experiments and increase of the side‐reactions with the size of ROH demonstrate a deceleration of the ether‐forming reaction with the size of ROH. Obviously, the side‐reactions originate from a carbenium ion whilst the ethers are mainly formed by a borderline mechanism. R(‐)6b and acidic MeOH give only 8% racemization in the abnormal ether. An order of activation is proposed for activated aziridines under acid catalysis (double activation).