Neighboring group reactions

THERMAL DECOMPOSITION OF O- SUBSTITUTED /-BUTYL PERBENZOATES IN CHLOROBENZENE AT 60 °C 

FUNCTIONAL GROUP PROXIMITY EFFECTS IN THE DECOMPOSITION OF /-BUTYL PERESTERS IN CHLOROBENZENE 

From the observation that was retained in the carbonyl group during the thermal decomposition of t-butyl o-(2,2-diphenylvinyl) perbenzoate, it was concluded that neighboring group participation occurred at the peroxide oxygen of the perester °. The rate acceleration and the observation that the activation parameters fall into the range expected for the loss of rotational freedom of three bonds led Fisher and Martin to suggest the activated complex (XVII) for the decomposition of /-butyl 8-(phenylthio)per-I-naphthoate (XVIII). Presumably, 

The influence of the alkyl group (R) upon the rate of decomposition of a few peresters has been studied and the data are presented in Table 104. Both the kinetic 

EFFECT OF THE ALKYL GROUP (R) ON THE RATE OF PERACETATE DECOMPOSITION  

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The decomposition of 5-phenyl-5-pentenoyl peroxide was previously suggested to proceed by a neighboring group reaction where charge development occurred in the transition state. Consistent with this proposal are the solvent effects upon (X), X = H. Table 88 shows these data along with 5-phenylpentanoyl peroxide (XI) for comparison. As would be expected from the proposed mechanism, polar solvents increase the rate of decomposition of (X), X = H, and decrease the activation energy. 

In contrast, sulfur mustard forms the less stable episulfon-ium ion more slowly than this ion reacts with biologlci nucleophiles. Thus, the neighboring-group reaction is rak limiting, and the kinetics are first order. "... 

Other examples of neighboring-group reactions include the competition between water [Eq. (8)] and ammonia [Eq. (9)] loss in amino alcohols and amino acids, which has been probed as a function of chain length [35]. 

Baker and coworkers first reported the neighboring-group reactions of the acetamido group in the carbohydrate field in their several syntheses of... 

The method used most frequently for preparation of acyloxonium salts is the neighboring-group reaction. The esterified chlorohydrin (4) reacts with antimony pentachloride (or, better, with silver fluoroborate) to give the salt 2. The fluoro or bromo analogs react similarly with boron trifluoride or silver fluoroborate. Even the ether 6 can be transformed into 2 by use of antimony pentachloride or triethyloxonium fluoroborate. ... 

Evidence that the reaction of the diol ester 5 with antimony pen-tachloride genuinely involves a neighboring-group reaction is provided by the stereospecificity of this reaction with esters of cyclic diols. trans -l,2-Cyclopentanediol diacetate (14) and Irons-1,2-cyclo-hexanediol diacetate (17) react " with antimony pentachloride to give the cis-acetoxonium salts 15 and 18, whereas the cis-diol esters (16 and 19) merely give difficultly soluble adducts from which unchanged starting material can be recovered after hydrolytic treatment. 

Esters of acyclic 1,2,3-trioIs, and esters of cyclic (l,3/2)-trioIs react, via a neighboring-group reaction, with antimony pentachloride to give acyloxonium salts in good yield. Thus, glycerol triesters (27)... 

The formation of acyloxonium derivatives (35) of (l,2/3)-cycIo-hexanetriol from tri-0-acetyI-(l,3/2)-cyclohexanetriol and antimony pentachloride does not proceed in a manner analogous to the pathway 30 - 31. In this triol system, in its favored conformation, all three substituents are equatorially disposed. The neighboring-group reaction leading to splitting out of an acetate anion requires, however, a diaxial orientation of the substituents reacting. It is understandable that this reaction should proceed considerably more slowly, because of the expenditure of energy required for conversion of the triol into its all-axial conformation. 

A fluoroborate salt of the ion 43 can be obtained from the acetal 42 by treatment with triphenylmethyl fluoroborate. As indicated in the cycle of reactions, the cation 43 can rearrange successively, by neighboring-group reactions, into subsequent ions all of the ions are structurally identical with 43. The formula scheme indicates that, after each rearrangement step, the stereochemistry is so disposed that a new neighboring-group reaction can follow. The cycle is completed after ten steps, and the starting point is reached. 

It was further proved that, together with the rearrangement cycle, side reactions occur that involve 1,3-neighboring-group reactions and configurational inversion. Such processes lead to other isomers of the cyclopentanepentol system. Analysis of an isomerized and hydrolyzed reaction-mixture showed that only 3% of (l,2,3/4,5)-cyclopentanepentol and 0.5% of (l,2,3,4/5)-cyclopentanepentol were formed the first of these isomers can arise by a 1,3-neighboring-group reaction. 

Methyl 2-0 - benzoyl - 4,6 - O - benzylidene - /3 - D - arabinopyranos ide (113) presumably reacts with N-bromosuccinimide to give the benz-oxonium ion 114 that, by a neighboring-group reaction, could rearrange to the ion 117. The observed products from 113 were the 4-bromo derivative 115 and a 3-bromo derivative, in 1 2 ratio. Unfortunately, it was not established whether the 3-bromo derivative has the structure 116 or 118, or whether it is a mixture of the two. A 2-bromo derivative 119, whose presence would have been indicative... 

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