Optimal orbital overlap of breaking and forming bonds requires rear-side approach of the shifting alkyl group to the site of the leaving chloride anion, in a manner similar to a SN2 reaction. The chief protagonists favoring the nonclassical view were S. Winstein and J. D. Roberts. Finally, reaction # 5 clearly shows the influence of reaction conditions on product composition, but explaining the manner in which different conditions perturb the outcome is challenging. If water had been the solvent, the resulting 1º-enamine would have rearranged to an imine and hydrolyzed to an aldehyde. In this manner a neighboring aromatic ring accelerates the rate-determining (endothermic) ionization step, an influence called anchimeric assistance (Greek: anchi = neighbor). Once the peracid has added to the carbonyl group, the rearrangement may be facilitated by an intramolecular hydrogen bond, in the manner depicted in brackets on the right. As anticipated, the charged tricoordinate carbon atom exhibited a 13C signal over 300ppm downfield from TMS. Equations for the latter two solvolyses are shown in the following diagram. Threo isomers cannot assume such a conformation. The diastereomeric configurations are called erythro and threo, according to their correlation with the tetroses erythrose and threose. The following diagram shows three examples of neighboring double bond interaction, the first being one of the most striking cases of anchimeric assistance on record. S. J. Kwon, D. Y. Kim, Org. Lett., 2000, 2, 1609-1611. Curtius Route: An acyl azide (RCON3) is prepared in one of two ways. The rearrangement that occurs under base catalyzed elimination conditions reflects the eclipsed configuration of the two-carbon bridge bearing the chlorine atom. Such an application, which proceeds by way of a cyanohydrin intermediate, is shown in the first example below. Rh-catalyzed [5 + 2] cycloadditions of 1-(2-methyoxyethoxy)-1-vinylcyclopropane Five more examples will be displayed above by clicking on the diagram. This tenet simplified the elucidation of the numerous substitution, addition and elimination reactions that characterized the behavior of common functional groups. Solvolysis of the exo and endo-2-norbornyl sulfonate esters disclosed differences that suggested anchimeric assistance for the exo-isomer. The product of the rearrangement is a lactam (a cyclic amide), which can be hydrolyzed to an omega-amino acid. For a nitrogen cation to initiate rearrangement it must have an unfilled valence shell, and two such azacations are shown in the center of the diagram (pink box). Loss of water from the 3º-carbinol site, followed by a reversible 1,2-hydride shift, generates the conjugate acid of the ketone product. ring-expanded silyl enol ethers. The primary opposition came from H. C. Brown, who espoused a more conventional rationalization. Excess base generates a conjugate base of the product. Each shift involves a separate transition state in which the positive charge is delocalized over the migration terminus, origin and migrating group. A relationship of this kind corresponds to the rearrangement of neopentyl chloride. Addition of HCl to camphene, in a similar fashion, initially produces an unstable chloride (camphene hydrochloride) which quickly isomerizes to isobornyl chloride, a stereoisomer of bornyl chloride. 12, 3598-3601. However, this is an oversimplification which ignores the fact that these reactions take place in nonpolar solvents, and are unlikely to involve discrete, unassociated carbocations. The product from ethyl bromide is ethanol, the simple and direct substitution product, but neopentyl bromide yields 2-methyl-2-butanol instead of the expected neopentyl alcohol. Since many simple nitrogen compounds are bases, they form "onium" cations when protonated. The general equations on the left below illustrate that the end product from the Arndt-Eistert reaction may be a carboxylic acid, an ester or an amide. The rigid configuration of the phenonium cation shown above imposes a structural constraint that is nicely demonstrated by the rates of rearrangement of some fused ring bicyclic compounds. 1R & 2R) is generally: 3º-alkyl > 2º-alkyl ~ benzyl ~ phenyl > 1º-alkyl > methyl. Click below for an article detailing these procedures. Chem. Some of the stereoelectronic effects that influence these reactions will be shown by clicking on the above diagram a second time. A curved arrow representation of the five sequential ring expansion steps will be added to the equation by clicking on the diagram. If angle strain , torsional strain or steric crowding in the reactant structure may is relieved by an alkyl or aryl shift to a carbocation site, such a rearrangement is commonly observed. In this equation a discrete oxacation is drawn as an intermediate, but it is more likely that the rearrangement is concerted, as will be shown by clicking on the equation. Please account for both products. If an open carbocation intermediate were formed in these reactions, mixtures of erythro and threo acetates would be expected from both tosylates, but only trace amounts of the opposite diastereomer were found among the products.