Transition state geometry in the scission and formation of cyclopentane and cyclohexane oxiranes. Use of the dilatometer in studying mixed reactions of different orders

Abstract

Can. J. Chem. 57.2444 ( 1979. Two methods, one more accurate, the other more convenient, were developed by which the dilatometer could be used with concurrent reactions of the second and third order. The more accurate method, employing a dilatometer of high specific surface area and equipped with a stable and responsive thermometer, was used in studying as many as six simultaneous oxirane scissions in acid solution. Interlocking double comparisons were made between cyclohexane and cyclopentane ring systcms, between systems possessing or lacking a methoxyl substituent and between others having the substituent in varying orientations, between charged and uncharged nucleophiles, and between transition states of different degrees of protonation. Searching experimental checks were therefore possible in four main areas: (i) a quantitative dissection of the inductive effect from steric and conformational effects, and from dipole-ion or dipole-dipole interactions with the nucleophile, (if) a quantitative demonstration of three main differences between transition states based on 5-and 6-membered rings, (iii) a proof that .,ytr-1,3-effects in the transition states are very charge dependent, being large and positive with unfavourably arranged dipoles, and small or perhaps negative with favourable arrangements, and (iu) a rejection of certain proposed ion-pair alternatives to the A2 mechanism. On a developpi deux methodes, I'une precise et l'autre plus commode, faisant appel a un dilatomttre pour Ctudier des reactions concurrentes du deuxieme et du troisieme ordre; la methode la plus precise, qui fait appel a un dilatornetre a grande surface specifique equip6 d'un thermometre stable et sensible a Cte utilisee pour etudier jusqu'a 6 reactions de scissions simultanees d'oxiranne en solution acide. On a fait des comparaisons de recouvrement double entre les systemes cyclopentanique et cyclohexanique, entre des systemes avec ou sans substituants methoxyles, entre d'autres systemes portant le substituant dans diverses orientations entre des nucleophiles charges ou non et entre des Ctats de transition a divers degres de protonation. I1 a ete possible de faire des verifications experimentales dans quatre domaines principaux; (i) une dissection quantitative entre I'effet inductif et les effets stCriques et conformationnels ou les interactions dipole-ion ou dipole-dipole avec le nuclCophile, (ii) une demonstration quantitative de trois differences principales entre les Ctats de transition portts par des cycles a 5 et a 6 chainons, (iii),une preuve que les effets syn-1,3, dans les Ctats de transition dependent beaucoup sur la charge; les effets Ctant importants et positifs lorsque les dipoles sont dans un arrangement non favorable et petits ou peut &tre negatifs lorsque les arrangements sont favorables et (iu) un rejet de certaines paires d'ions proposees comme alternative au nitcanisme A2.[Traduit par le journal] Introduction measurement of certain polar effects in a particularly Theories of oxirane scission (1, 2) attempt ex-simpl...

Epoxide ring-opening approach to the synthesis of diverse trisubstituted cyclopentanes

“…It was hypothesized earlier, that the direction of a nucleophilic attack towards oxirane carbon atoms of 3-substituted cyclopentane oxiranes is defined by the structure of the corresponding transition states. 24,25 In the case of 1 and 2, the mechanism for the epoxide ring-opening reactions has been proposed (Scheme 3). The geometry of transition states 11 and 12, related to the nucleophilic attack at C1 or C2 carbon atom, suggests a distorted twist conformation.…”