Steric effects rearrangements

EE rearrangements Steric effects are particularly strong in the dissociations of EE ions produced by chemical ionization of multifunctional compounds. This selectivity results from two additional effects (i) stabilization of MH by intra-... 

The mechanism and stereochemistry of the orthoester Claisen rearrangement is analogous to the Cope rearrangement. The reaction is stereospecific with respect to the double bond present in the initial allylic alcohol. In acyclic molecules, the stereochemistry of the product can usually be predicted on the basis of a chairlike TS.233 When steric effects or ring geometry preclude a chairlike structure, the reaction can proceed through a boatlike TS.234... 

A number of steric effects on the rate of rearrangement have been observed and can be accommodated by the chairlike TS model.242 The Zf-silyl ketene acetals... 

I. Dostrovsky, E. D. Hughes, and C. K. Ingold, XXXII. The role of steric hindrance, magnitued of steric effect, range of occurrence of steric and polar effects, and place of the Wagner rearrangement in nucleophilic substitution and elimination, Chem. Soc. 173 (1946). 

The role of steric effects is unclear but the anomeric effect could also contribute to an increase in electron density at nitrogen. X-ray data for the two TV-acyloxy-TV-alkoxyamides, a urea and a carbamate outlined above show clear evidence, both from bond lengths and conformations, of an anomeric interaction RO-N bonds are short when compared to alkoxyamines. This interaction is responsible for SN1, SN2, homolytic and rearrangement reactions of /V-acyloxy-TV-alkoxyamides (vide infra) and has also been supported computationally. Acyloxylation of the hydroxamic esters results in both pyramidalisation as well as anomeric donation from the... 

The use of the zinc-copper couple to effect the reduction of the methanesulfonate 168 with rearrangement furnished 169 (Scheme 20.34) [10]. Treatment of 168 with methylmagnesium bromide in the presence of copper(I) cyanide to induce an SN2 -type reaction produced the methylated adduct 170. The half-life of the Myers-Saito cyclization of 169 is 66 h at 37 °C, whereas that of 170 is 100 min. The faster rate of cyclization for 170 has been attributed to a steric effect favoring the requisite s-cis or twisted s-cis conformation. 

The Role of Steric Hindrance (Section G) Magnitude of Steric Effects. Range of Occurrence of Steric and Polar Effects, and Place of the Wagner Rearrangement in Nucleophilic Substitution and Elimination," JCS 149 (1946) 173194. 

An increase of char yield is generally reflected as an improvement in oxygen index. In the styrylpyridine based polyesters and polycarbonate an intermolecular thermally induced Diels-Alder reaction has occurred through the double bond, this increased the char yield and decreased the flammability. The Fries rearrangement, as well as dimerization and isomerization, occurred simultaneously during the UV irradiation of p-VPPB, but no dimerization or isomerization occurred for p,p -BVPDPC, probably due to steric effects. 

Steric effects, molecular rearrangements, anchimeric assistance, and the use of tracer techniques have provided useful information about the nature of catalytic sites of aluminas and have led to a unified mechanism of their action. 

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