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Front Matter |
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Abstract
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Abstract
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| 3 |
Introduction |
Brian Capon,Samuel P. McManus |
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Abstract
The most widely investigated substituent effects on organic reactions are electronic effects transmitted through the carbon skeleton and steric effects. Substituents, however, may influence reactivity in other ways. When a substituent stabilizes a transition state or intermediate by becoming bonded to the reaction center this effect is called .,.* and if such participation leads to an enhanced reaction rate, the group is said to provide . (derived from the Greek : ., “adjacent”; ., “part”)..
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Some Factors that Influence Anchimeric Assistance |
Brian Capon,Samuel P. McManus |
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Abstract
It is obvious that a number of requirements are necessary for anchimeric assistance to occur. First, the neighboring group must be configurationally and conformationally situated to provide proper orbital overlap in the transition state. Second, the reaction that involves neighboring group participation must not be very much slower than any competing reactions. Thus, solvolyses of tertiary halides and esters usually occur with greatly reduced anchimeric assistance (as measured by . ./. .) even when a good neighboring group is appropriately situated,. because the introduction of the extra α substituent lowers the free energy of the transition state for forming a carbocation while hardly affecting the free energy of the transition state of the neighboring group process. In fact the transition state for the pathway that involves neighboring group participation may be of higher energy with a tertiary than with a secondary halide owing to steric crowding.
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Some Experimental Methods Used in the Study of Neighboring Group Participation |
Brian Capon,Samuel P. McManus |
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Abstract
The study of neighboring group participation involves, for the most part, the same experimental techniques required in the study of other chemical mechanisms. A discussion of common techniques is beyond the scope of this monograph, and a general knowledge of physical organic chemistry is assumed; for appropriate general and special treatments of specific physical organic methods, see Ref. 1. The purpose of this chapter is to describe the use of various experimental techniques in defining the presence (and extent) or absence of neighboring group participation.
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Abstract
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Participation by Oxygen Groups |
Brian Capon,Samuel P. McManus |
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Abstract
The absence of a steady trend in the rates of solvolysis of a series of .-comethoxyalky1 .-bromobenzenesulfonates (see Table 1) indicates the importance of an effect by the methoxy groups other than the inductive effect. In particular the high rates measured for the 4-methoxybutyl and 5-methoxy-pentyl compounds [(.) and (.)] suggest nucleophilic participation. Winstein and his co-workers. formulated these reactions as proceeding through the cyclic oxonium ions (.) (MeO-5) and (.) (MeO-6 participation). Hence the primary methoxyl group provides assistance when a five- or six-membered cyclic oxonium ion is possible, but methoxy group assistance is not readily apparent when this intermediate would have seven, three, or four members.
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Participation by Sulfur Groups |
Brian Capon,Samuel P. McManus |
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Abstract
Neighboring divalent sulfur in thioether groups is extremely active in reactions involving carbocation and free-radical intermediates. Measures of anchimeric assistance show these groups to be far superior to comparable oxygen-containing compounds. While the majority of cases reported involve S-3 participation, larger rings form as well.
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Participation by Nitrogen Groups |
Brian Capon,Samuel P. McManus |
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Abstract
In view of the well-known nucleophilicity of the amino group it is not surprising that a considerable number of examples of participation by amines have appeared. Yet the assignment of a mechanism is not always simple because the interpretation of kinetic studies is often complicated by the appearance of competitive reactions. Competing with intramolecular displacement by the amino group (ring closure) are dimerization, polymerization, elimination, fragmentation, and solvent displacement, although all these processes would generally not occur in a single system.
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Back Matter |
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Abstract
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发表于 2025-3-21 18:45:37
