Anchimeric assistance. See

The question as to whether 9 is an intermediate or a transition state has been much debated. When R is aryl or vinyl, then 9 is probably an intermediate and the migrating group lends anchimeric assistance (see p. 414 for resonance stabilization of this intermediate when R is aryl). When R is alkyl, 9 is a... 

The direct interaction of the reaction center of a molecular entity with a lone pair of electrons of an atom within that same molecular entity that is not associated with the reaction center or, interaction of the reaction center with the reaction center nor conjugated with the reaction center. Rate acceleration by such a process is referred to as anchimeric assistance. See Intramolecular Catalysis Synartetic Acceleration. 

Introduction to of electron-releasing substituents sharply decreases the exo endo rate ratio there is no such effect if two geminal methyl groups are introduced to C. The value 3 for the exo endo rate ratio resembles greatly the usual epimeric rate ratio for secondary substrates without anchimeric assistance (see above). Introduction of an electron-releasing substituent to levels the o-participation in the solvolysis of the secondary 2-exo-epimer, but it does not affect the rate ratio of the tertiary analogues for which the solvolysis rate is predominantly determined by steric factors, for example ... 

We refer to this mechanism as the Bordwell-Shiner-Sneen mechanism for frangomeric assistance. Such a mechanism is also possible for anchimeric assistance (see p. 78>... 

Based on kinetic investigations the solvolysis of ochloroalkyl sulphoxides 506 in 80% ethanol was found to proceed via a cyclic intermediate formed via anchimeric assistance of the sulphinyl oxygen atom601,602. For a solvolysis of 4-halogenothian-l-oxides see Reference 603 (equation 309). 

This would render an overall process of Sn2 addition in an inverted fashion. In addition, from the model, it appeared that if such an anchimeric assistance indeed existed, Sn2 addition could proceed faster, whereas the SN2 addition pathway could be slowed or even suppressed because the ct c o (of the oxonium bond) is not aligned with the system of the C5a-C5 exo-cyclic olefin in the conformation assumed by 90 (see model). The following experiment would actually support this assertion. 

Extensions of the electrophilic activation of the alkyne moiety as well as an alkene moiety have been developed and applied. The applications include various reactions, for instance, Friedel-Crafts type alkylations,323 anchimeric assistance of heteroatomic moiety generally followed by rearrangements (see below), implementation of more sophisticated functional groups such as ynamides and allenynes, which are discussed below. 

Dimethylaminoethane-2-ol (20) is a compound that, by virtue of its nucleophilic center (Me2NH+C2H40), is employed to convert protected segments bound to supports as benzyl esters into acids by transesterification into dimethylaminoethyl esters [C(=0)0C2H4NMe2] that are hydrolyzable by a dimethylformamide-water (1 1) mixture. Compound 20 readily forms esters from acid chlorides. The hydrolysis and esterification are facilitated by anchimeric assistance by the adjacent nitrogen atom (see Section 2.10). The amino alcohol also reacts with dichloromethane. 

For a number of years, a storm of controversy raged over this proposal, with H. C. Brown as the chief opponent. Brown ruled out anchimeric assistance as an explanation for the rate acceleration of the exo derivative, arguing that exo was normal, but that endo was unusually slow because of a steric effect. The racemization and isotopic tracer results, he proposed, could be explained by a rapid equilibrium between the classical ions 15 and 17 (see Scheme 1.3), with a steric effect responsible for the exo addition of nucleophiles. In terms of the cation, the question revolves around the issue whether the classical ions 15 and 17 should be joined by the equilibrium depiction (the rapidly rearranging scenario) or with a... 

The direction of oxirane ring-opening in the stereoisomeric compounds 289 depends on several factors for example, favored axial attack of the nucleophile, anchimeric assistance of neighboring groups, and conformational control of the approaching nucleophile. The examples shown in Scheme 2 provide support for the importance of each factor. There are, however, exceptions to these rules (see, for example, Ref. 184). Therefore, the results of oxirane ring-opening in 289 cannot always be foreseen. 

For other evidence for anchimeric assistance by cyclopropyl, see Sargent Lowry Reich J. Am. Chem. Soc. 1967, 89, 5985 Battistc Haywood-Farmer Malkus Seidl Winstein J. Am. Chem. Soc. 1970, 92. 2144 Coates Yano Tetrahedron Lett. 1972, 2289 Masamune Vukov Bennett Purdham J. Am. Chem. Soc. 1972, 94, 8239 Gassman Creary J. Am. Chem. Soc. 1973, 95. 2729 Costanza Geneste Lamaty Roque Bull. Soc. Chim. Fr. 197S, 2358 Takakis Rhodes Tetrahedron Lett. 1983, 24, 4959,... 

Winstein also found that the rate is highly dependent on the electron-donating ability of R—much more so than the rates of other intramolecular rearrangements (see Table 6.3). He therefore postulated that the migrating alkyl group provides anchimeric assistance to the departure of the nitrobenzoate anion and that the transition state for the rearrangement can be represented by a bridged structure such as 130. 

Other evidence to support the selection of 2-adamantyl tosylate as model compound has been discussed in Section 2 (p. 8), and the main conclusion can be summarized as follows. Independent evidence and the results in Fig. 15 firmly establish that nucleophilic solvent assistance cannot be appreciable for solvolyses of 2-adamantyl tosylate, since its response to solvents of widely varying nucleophilicities is almost exactly the same as 1-bicyclo [2,2,2] octyl tosylate [49] for which rearside attack is impossible if solvolyses of 2-adamantyl tosylate are anchimerically assisted, the extent of anchimeric assistance does not appear to be dependent on solvent (cf. Pritt and Whiting, 1975). In addition to these conclusions, the results in Fig. 14 suggest that internal return from intermediate contact ion pairs does not occur to a detectable extent for solvolyses of 2-adamantyl tosylate (see also Bentley and Schleyer, 1976). 

The acid catalyzed hydration of compounds 18 to give both the expected /l-hydroxy ester 19 and the unsaturated ester 20 proceeds at a rate which is larger by a factor of 50-100 than calculated on the basis of the Taft p o relationship (see 7b and 8 of Table 1) (Hekkert and Drenth, 1961). The rate enhancement has been explained in terms of anchimeric assistance efieets due to the hydroxyl group, and a four membered ring structure of type 21 has been proposed for the cationic intermediate. 

The study of the addition of trifluoroacetic acid to several 5-substituted 1-pentyne derivatives 22 led Peterson and Duddey (1963, 1966) and Peterson and Bopp (1967) to the conclusion that for some of the compounds investigated the intermediate resulting from protonation of the triple bond may be a five-membered ring cation of type 23, rather than a linear vinyl cation. This is indicated by the observation of rearranged products deriving from 1-5 shift when Y = Cl, F, OMe, and OAc (see scheme 3). Kinetio data indicate that Y-participation is important in the transition state of the reaction. Anchimeric assistance effects, expressed in terms of the ratio of assisted to unassisted mechanisms, kjjkg (Heck and Winstein, 1957), have been estimated to be 3 4 for Y = Cl and 6 5 for Y = OMe. The entire kinetic picture almost matches... 

Another way to obtain chiral clusters is to use mixed-metal systems (see Section II,A). Addition of acid to one of the two diastereomers of [WCo(/i-HC2CH(OH)Et)(CO)5Cp] leads to formation of two propargylium cations which do not isomerize in solution on the NMR time scale.75 Similarly, protonation of the separate diastereomers of [MoCo(/i-2-propynylborneol) (CO)5Cp] leads to formation of propargylium cations which are nonflux-ional. It has been shown that the diastereomer ratios for metal-stabilized cationic clusters can be directly correlated with the ionization process, and it is proposed that the elimination of water is anchimerically assisted by... 

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