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

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195N-Acylaziridines are capable of various reactions which have close analogues in other fields of organic chemistry. However, their reactivities can in a unique manner depend on the nitrogen pyramid and its inversion which can be subject to steric respulsions of the N substituent by substituents of the aziridine carbons [1b]. This was already demonstrated by reactions of xanthyl anion X -with cis-trans pairs of 2,3-substituted N-benzoylaziridines, e.g. of 1a [2]. The trans isomers possess a flat and rapidly inverting nitrogen and hence an amide-like conformation resulting in an optimal leaving group for nucleophilic ring opening (NRO) [1b]. The cis isomers are characterized by a steep and slowly inverting pyramid that accentuates the ketonic character of the benzoyl group thereby drastically retarding NRO but increasing the ability to abstract an electron from X -.A more general and more typical reaction of ketones is addition of a nucleophile while carbonyl additions are more difficult to realize with amides. The comparatively easy carbonyl attack [2] places N-acylaziridines into a borderline region between ketones and amides and should make them sensitive to even moderate changes in the N pyramid. Carbonyl attack by a carbanion, as well as by other nucleophiles, was often described [1b] particularly when the nucleophile may be classified as hard and when lithium was the counter ion of a carbanion. An influence of the N pyramid was so far never detected and a search for a cis-trans reactivity difference with the carbanions used was not possible for various reasons: the primary adduct was known or expected to arise either reversibly or not at all or from both isomers. Such a difference was now found in a regioselectivity study on homolytic ring opening of aziridino ketyls (Scheme 1).Reactions of anthracenidyl (anthracenide) A ·-with other N-acylaziridines (cf. [1a, 3]) have clarified the mechanistic steps to final products in such reactions. Hence, electron transfer from A ·-to aziridines 1a generates the aziridino ketyls 2a that homolyze to amidatoalkyl radical 3a which is reduced by 2a to carbanion 4a. The latter abstracts a proton from THF (main reaction) or adds to C=O of cis-1a providing finally Abstract. In a multistep process, anthracenide and both 1-benzoyl-2-methyl-3-phenylaziridines 1a form carbanion 4a that abstracts a proton from the solvent THF. The steep N pyramid of cis-1a makes attack of 4a on C=O of cis-1a fast enough to compete with proton abstraction while C=O of trans-1a with its flat and rapidly inverting pyramid did not react with 4a. The initially generated ketyls of 1a show another cis-trans effect of steric repulsion: their homolytic ring opening forms benzylic radical 3a, the precursor of carbanion 4a, but this opening is regiospecific for the cis ketyl only. The trans ketyl forms some isomeric radical too. 16% 6a from cis-1a 0% 6a from trans-1a (27% 6b from 1b) 52% 5a from cis-1a 55% 5a from trans-1a (30% 5b from 1b) THF + cis-1a main or only homolysis

supporting

confidence: 77%

mentioning

confidence: 81%

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cis-trans Pair of aN-Benzoylaziridine: Dependence of Carbonyl Reactivity on Nitrogen Pyramid

Falkenstein¹,

Stamm²

2000

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“…Sequence of Michael addition, deprotonation, homolytic BFR, homolytic ring opening gives finally pyrrolidones 53a,b. [21,22]. Various paths to 53a,b may be considered.…”

Section: Synthesis Of 3-benzyl-2-oxopyrrolidinesmentioning

confidence: 99%

Benzylic Fragmentation (BFR), an Aromatic Analogue of E1cB Including a Homolytic Variant, as either Nucleofugal or Homolytic Path to Aryl Stabilized Carbanions or Ketyls, respectively

Stamm

2000

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Key aspects of the work of the Inner London Probation Service (ILPS) are described both in terms of organization and theoretical perspective. Issues of difference between sex offenders in particular and other offenders are explained in relation to legislation and practice. Some encouraging results from the third phase of the Sex Offender Treatment & Evaluation Programme (STEP) are outlined. The importance of progress in relation to adolescent sex offenders is stressed, and it is suggested that there is a need to broaden the approach in relation to sex offending to incorporate a public education dimension. Copyright © 1998 John Wiley & Sons, Ltd.

Nucleophilic ring opening of aziridines

Stamm

1999

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102

This account of research work on nucleophilic ring opening (NRO) of aziridines (Az's) demonstrates the broad synthetic scope of this reaction, inclusive secondary reactions, and it discusses the special mechanistic fundamentals and details as well as mechanistic variants. Formation of a C–C bond by NRO is the red thread in this report. A central mechanistic aspect is the quality of the leaving group in Az bases, aziridinium ions and activated Az's (acyl, sulfonyl; double activation). Factors controlling the regioselectivity of NRO are dealt with as well as the influence of the nitrogen pyramid. Competing reactions include carbonyl attack (acylated Az's) and electron transfer with cases of pseudo‐NRO (multistep radical paths).