Chemistry of amido radicals: Stereochemical controls in II radical reactions

Abstract

. Can. J. Chem. 54, 3517 (1976).The acyclic amido radicals generated from the corresponding N-bromo.AT-iert-alkylamides reacted like ,?'-radicals, rather than 0-radicals, and preferentially abstracted a 6-hydrogen of the alkyl chain, rather than a r-iiydrogen of the acyl chain. On the basis that, in these amido radicals, the conforn~ation of the transition state of the hydrogen transfer is a five-membered ring and the stereoelectronic requiren~ent for the transfer demands a colinear arrangement of the radical containing orbital and the C-PI bond, the results are satisfactorily rationalized by the n electron configuration of the amido radicals. It is also established that the chain length of these N-bromoamide photodecompositions increases as the intran~olecular hydrogen transfer of the corresponding amido radicals becomes easier; the hydrogen abstraction step must be the rate determining step in the chain reaction. The spontaneous formations of r-iminolactones from r-bromoamides indicate that the intramolecular nucleopliilic displacement by the 0-center of the amido group is facilitated since its transition state conformation readily accommodates the stereoelectronic requirements for the nucleophilic substitution. A colinear approach of thep-orbital of the more nucleophilic N-center to the rear side of the yC-Br bond is shown to be prohibitively strained. Chem. 54, 3517 (1976).Les radicaux amido acycliques, gknCrCs B partir des N-bromo N-tert alkylamides correspondantes, reagissent comrne les radicaux N-alkyles plutBt que comme des radicaux 0-alkyles; ils rkagissent prCf6rentiellement pour enlever un hydrogkne en 6 de la chaine aIkylCe plut6t qu'un hydrogkne en 7 de la chaine acylCe. En faisant l'hypothbse que, dans ces radicaux amido, la conformation de 1'Ctat de transition du transfert d'hydrogene est un cycle a cinq membres et que les conditioils stCrCoClectroniques pour le trarsfert demandent un arrangement colinCaire de l'orbitale contenant le radical et du lien C-H, on peut rationaliser d'une fason satisfaisante les resultats par la configuration Clectronique IT des radicaux amido. On a aussi Ctabli que la longueur de la chaine de ces photodkcompositions de N-bromoamides augmente B mesure que le rransfert d'hydrogbne intramolCculaire des radicaux arnido correspondants devient plus facile; I'6tape de l'enlkvement de l'hydrogene doit &tre l'ktape dCterminante dans la reaction en chaine.Les reactions spoi~tanCes de formation de r-iminolactones B partir de r-bromoan~ides indiquent que le dkplacement nuclCophile intramolCculaire par l'intermediaire du centre oxygCnC du groupe amido est facilite puisque la conformation de son Ctat de transition accommode facilement les demandes stCrCoClectrorliques de la substitution nuclCopbile. La tension provoquCe par une approche colineaire de l'orbitale p du centre azote, qui est plus nuclkophile, vers la face arrikre du lien -y-C-Br est prohibitive.[Traduit par le journal]In recent. years, the existence of anlido radi-(81, and reaction patterns (9-13). These results cal...

Chemistry of amidyl radicals: intramolecular reactivities of alkenyl amidyl radicals

“…Recently, Ingold's group (26)(27)(28)(29) has extended the esr observations reported earlier by Danen and Gellert (12), and proven convincingly that amidyl radicals (RCONR) possess n structure in N-alkyl or N-acyl chains and investigate competing intramolecular H-abstraction from the C(5)-hydrogens and additions to the double bonds. It should be noted that intramolecularly, amidyl radicals preferentially abstract the C-5 hydrogens on the alkyl side chain of the N-center rather than those from the acyl side chain (9,14), and do so from the nitrogen center but not from the amide oxygen center (14,15).…”

“…The to publish this work now. major interest has been the electronic configuration from which amidyl radicals (RCONR) (12)(13)(14)(15)(16)(17)(18)(19)(20) react with substrates and its…”

Reaction ofγ-Hydroxy-N-[1-(dimethylcarbamoyl)ethyl]butanamides under the ‘Direct Amide Cyclization’ Conditions

“…These interactions link To isolate the crucial intermediate 12a, extraction with aq. Na 2 CO 3 solution [26] and deprotection by treatment with polyvinylpyridine were attempted, but neither procedure gave satisfactory results. Although esterification of 12a with CH 2 N 2 yielded the corresponding crude methyl ester hydrochloride 14a (Scheme 5), the product was also not stable in polar solvents and isomerized partially to the corresponding chloro ester.…”

Selectivities of .pi.- and .sigma.-succinimidyl radicals in substitution and addition reactions. Response to Walling, El-Taliawi and Zhao

“…But nonetheless the subject is of considerable interest since the possibility of a and u radicals in amidyl systems has not been adequately recognized. The extensive work of Lessard and cow o r k e r~~~ is recognizably concerned with a highly selective type of amidyl radical, with little doubt the a state.35 When comparisons are made between the Lessard amidyl chemistry and the amidyl radical chemistries described by Johnson and Greene36 and Joseph et al, 37 it becomes apparent that amidyl radicals show a variety of behaviors, depending on what scavengers are present, in the latter two studies, including H abstractions from unactivated methyl groups. Clearly the amidyl system also shows a and u behaviors.…”