Pen Yuan Lin

Homolytic aziridine opening (aza variant of cyclopropylcarbinyl-homoallyl rearrangement) by addition of tributyltin radical to N-acylaziridines. Factors contributing to the regioselectivity

Abstract AIBN initiated reaction of N-acylaziridines 1 with Bu3SnH in refluxing benzene provided products 5 and 8 of reductive ring opening. Yields (practically quantitative in most cases) fell drastically with steric hindrance of the addition of Bu3Sn. to the acyl oxygen of 1. They depended to some extent on the experimental conditions for hydrogen capturing when aziridine homolysis provided a primary radical 3 or 6. The regioselectivity of (probably reversible) ring homolysis can be understood in terms of the stability of the arising radical (3, 6), of stereoelectronic control (e.g. 1i as compared to 1h) and of frontier orbital interactions (1j). A possible difference in bond lengths as explanation for the formation of the primary radical from 1j did not find support from an X-ray structure analysis of N-tosyl-2-methyl-aziridine 11. Isomeric products were obtained only twice (1i, 1j) with a dependence of the ratio 5j:8j on concentration and hydrogen isotope of Bu3SnH. No such dependence was found for the ratio 5:14 (reduction without and with an intervening cyclization of 3 leading to a pyrrolidone) obtained from the N-cinnamoylaziridine 11. This ratio (1:9 for 11 and 1:3 for 1n) must reflect the E-Z isomers in 3. The observed preference for the formation of E-3 from 2 can be explained by stereoelectronic and steric effects. A cinnamoyl double bond in 5 was saturated depending on experimental conditions.

Acylated 2,2-dimethylaziridines: regioselectivity of ring opening by sodium thiophenolate; borderline SN2 due to planarization of nitrogen pyramid☆

Abstract 2,2-Dimethylaziridines 1 carrying a COR group on nitrogen are opened by PhS⊖ with a regioselectivity RS = abnormal:normal ⩾ 20 in MeOH and 5–16 in THF. Practically no influence of COR on RS was found except for the poorest oxidant 1g (R = tBu) that gave the highest RS (95, MeOH). This rules out an SET mechanism for the abnormal opening. Absence of homolytic ring opening is directly shown by the failure to detect products resulting from cyclization of a homolytically formed radical when COR is cinnamoyl (1f). Direct nucleophilic attack is indicated by suppression of any reaction with PhS⊖ when the steric hindrance is increased (N-benzoylaziridine 13). The observed SN2 borderline behaviour with 1 is explained by reaction in a flattened nitrogen conformation of the otherwise poorly reactive 1. Carboxamide resonance in this (nearly) planar conformation generates a substantial positive charge on N which will shift the mechanism closer to SN1 by partial heterolysis of the N-CMe2 bond prior to attack by PhS⊖.

Three positional isomers of substituted triphenylmethanes from reactions of trityl anion with 1-acyl-2,2-dimethylaziridines

Ring opening of aziridines 4a–d in reactions with trityl anion Tr– proceeds exclusively by cleavage of the NCMe2 bond. Substitution of the benzylic carbon of Tr– leads to ‘central’ products 10a–d in yields of 0-5%. This is ascribed to an SN2 reaction with borderline character, as is well known from reactions of aziridines 4a–d with other nucleophiles. All remaining ring-opening reactions result from single-electron transfer (SET). This is direct SET from Tr– to aziridines 4a–c. For compound 4d (acyl = cinnamoyl), the SET reaction is of the innersphere type and proceeds via Michael addition, at least in part. Homolytic ring opening of the generated aziridino ketyls 5 forms the tertiary amidatoalkyl radicals 6. Main reaction of radicals 6a–c is transfer of a hydrogen atom from one of its two methyl groups to die generated trityl radical Tr˙. Methallylamides 7 and enamides 8 are the final products. ortho-Substituted triphenylmethanes 12 and/or its olefinic precursors 13 arise in ∼ 20% yield. A mechanism for the formation of these unique products is proposed that first converts the radicals 6 into the corresponding carbanions 16 which undergo an SN2′ reaction with one allylic system TrCHCHCH* of the dimer 14 of Tr˙. The leaving group Tr– is eliminated from this partial structure when carbanions 16 attack the marked carbon converting it finally into the substituted ortho carbon of compounds 12. Addition of radicals 6 to Tr– is probably the way to the para-substituted triphenylmethanes 11, which arise in yields of only 0–1% from aziridines 4a,b (acyl = 4-benzoyl, pivaloyl). Higher yields of para-substituted compounds 11 are obtained from aziridines 4c (acyl = 4-phenylbenzoyl) and 4d. This is ascribed, at least for substrate 4c, to a chain reaction because ketyl 5c must be formed more rapidly than ketyls 5a,b. A substantial part of radical 6d cyclizes, ending up as the triphenylmethane compound 26 that carries a pyrrolidone ring in the para position.

Neglected Aspects of Anthracenide (Anthracenidyl) Chemistry — Reactions with two N-Benzoylaziridines

Reaction of anthracenide A·- with N-benzoylaziridines 1a,b forms charged radicals 3a,b by single electron transfer and homolytic ring opening. Reactions follow that are known or expected as e.g. coupling with position 9 of A·- forming dihydroanthracene anions 9a,b that yield amidoethylated dihydroanthracenes 10a,b, or react with 1a,b giving finally 9,10-bis-amidoethylated dihydroanthracenes 11a,b. Results depend on experimental conditions and on the counter ions Na+ or Li+. Coupling is not regiospecific: contributions by positions 2 and 1 reach 29% or 4%, respectively, of total coupling with the primary radical 3a; much higher contributions are possible with Li. Product 21s (probably 3,3′-disubstituted tetrahydrobianthryl) may arise by hydrogen detachment from the first intermediate (29) of coupling with position 2 and dimerization of the formed 2-substituted A·- (30). Coupling products may be fully aromatized or may be hydroxylated in one of the benzylic positions. With counter ion Li+ a non-SET reaction of 1a with the dimer of A·- is indicated by the isolation of 9-benzoyl-dihydroanthracene 15 and by 19% yield of 16a (aromatized 10a). Reaction of 3b with anthracene is indicated by 10,10′-disubstituted tetrahydrobianthryl 37.

Aziridines. Part 60. Electron transfer from radical anions to N-alkanoylaziridines. Exocyclic cleavage of an aziridino ketyl

Reactions of non-aromatic N-acylaziridines with radical anions yield products arising from the intermediate β-amidatoalkyl radicals that are generated by homolytic ring opening of the first formed aziridino ketyls. Reduction of these radicals and their combination with the radical anion show a dependence on the nature of the radical anion (naphthalenide, anthracenide) similar to the known reactions of these radical anions with alkyl halides, i.e.(nearly) no reduction by anthracenide. This contrasts with the published 37% reduction in the reaction of anthracenide with an N-benzoylaziridine. Very rapid mixing of the reagents by an injection technique changes the product mixture in a manner that points to a reduction by the aziridino ketyl which has a very short lifetime if derived from a non-aromatic acyl group. It is shown that an aziridino ketyl can undergo exocyclic cleavage of the bond next to the ketyl carbon provided that the eliminated carbanion (R– of the acyl group RCO) is stabilized such as by two phenyl groups in the reported example.

Radical combination in the ortho position of trityl radical observed in single-electron transfer reactions of trityl anion

Single-electron transfer reactions between trityl anion and 1-acyl-2,2-dimethylaziridines provide, among other products, the methallyl amides 7 and the triphenylmethanes 8 carrying an amidoethyl chain attached with the tertiary carbon ortho to the triphenylmethane.