Mechanism, neighboring group groups

Series 7 gives a linear plot with p = —5.27, consistent with the S l mechanism. Series 8, however, shows a discontinuity, which Gassmann and Fentiman interpreted as a change in mechanism. Compounds in series 8 are capable of intramolecular assistance (neighboring group participation) by electron donation from the double bond to stabilize the cation, as in 9. 

Benzofuroxan 79 can be generated from 2-nitrophenyl azide 80 (Scheme 49). Neighboring-group assistance within the pyrolysis leads to a one-step mechanism with an activation barrier of 24.6 kcal/mol at the CCSD(T)/6-31 lG(2d,p) level [99JPC(A)9086]. This value closely resembles the experimental one of 25.5 kcal/mol. Based on the ab initio results for this reaction, rate constants were computed using variational transition state theory. 

The Prins reaction often yields stereospecifically the and-addition product this observation is not rationalized by the above mechanism. Investigations of the sulfuric acid-catalyzed reaction of cyclohexene 8 with formaldehyde in acetic acid as solvent suggest that the carbenium ion species 7 is stabilized by a neighboring-group effect as shown in 9. The further reaction then proceeds from the face opposite to the coordinating OH-group " ... 

Figure 25.6 Mechanism of the Koenigs-Knorr reaclion, showing the neighboring-group effect of a nearby acetate.

El reaction and, 392 E2 reaction and, 386 S l reaction and, 373-374 Sjxj2 reaction and, 362-363 Kishner, N. L. 715 Knoevenagel reaction, 913 Knowles, William S., 734, 1027 Kodel, structure of, 1222 Koenigs-Knorr reaction, 990 mechanism of, 990 neighboring-group effect in, 990-991... 

Although 17 is symmetrical, intermediates in most neighboring-group mechanisms are not, and it is therefore possible to get not a simple substitution product but a rearrangement. This will happen if Y attacks not the carbon atom from which X left, hut the one to which Z was originally attached ... 

For an example, see 10-9.) When this happens internally, we have an example of a neighboring-group mechanism at a carbonyl carbon. For example, the hydrolysis of phthalamic acid (77) takes place as follows ... 

For example, substrates of the type RSCH2CH2X are so prone to the neighboring-group mechanism that ordinary Sn2 reactions have only recently been observed Sedaghat-Herati, M.R. McManus, S.P. Harris, J.M. J. Org. CherrL, 1988, 53, 2539. 

See Glaser, R. Horan, C.J. Nelson, E.D. Hall, M.K. J. Org. Chem., 1992,57, 215 for the influence of neighboring group interactions on the electronic stmcture of diazonium ions. Aryl iodonium salts Ar2l also undergo substitutions by this mechanism (and by a free-radical mechanism). 

The bromine atom is much more selective than the chlorine atom. As indicated on page 906, it is often possible to brominate tertiary and benzylic positions selectively. High regioselectivity can also be obtained where the neighboring-group mechanism (p. 899) can operate. 

This intermediate is similar to those encountered in the neighboring-group mechanism of nucleophilic substitution (see p. 404). The attack of W on an intermediate like 2 is an Sn2 step. Whether the intermediate is 1 or 2, the mechanism is called AdE2 (electrophilic addition, bimolecular). 

Explain the difference with respect to the size of the neighboring groups on substitution in an octahedral complex by associative and dissociative mechanisms. 

A reaction that offers a closer parallel to the neighboring group effect of carbonium ion chemistry is a possible concerted mechanism for the Michael rearrangement ... 

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