Anchimeric assistance reactions

Reaction of the cis tosylate is much slower than that of cyclohexyl tosylate, and this we can readily understand powerful electron-withdrawal by acetoxy slows down formation of the carbonium ion in the SnI process. Reaction of the trans tosylate, although much faster than that of its diastereomer, is still somewhat slower than that of cyclohexyl tosylate. But should not the anchimerically assisted reaction be much faster tlian the unassisted reaction of the unsubstituted tosylate The answer is, not necessarily. We must not forget the electronic effect of the acetoxy substituent. Although SN2-like, attack by acetoxy has considerable SnI character (see Sec. 17.15) deactivation by electron withdrawal tends to offset activation by anchimeric assistance. The cis tosylate is electronically similar to the trans, and is a much better standard by which to measure anchimeric assistance. (This point will be discussed further in the next section.)... 

A related, but distinct, question is whether there is an energy minimum on the reaction path of an anchimerically assisted reaction, i.e., whether there is a bridged intermediate. A variety of energy profiles must be considered for the rearrangement... 

From the rate data presented in the first part of this chapter it is clear that R2N-5 and R2N-6 anchimerically assisted reactions are even more favored than the reasonably facile R2N-3 and R2N-4 processes discussed above. It is not unexpected then that neighboring amino groups have been observed to intervene intramolecularly in a wide variety of reactions when five- or six-membered rings result. A few illustrative examples will be presented below in order to show the scope and limitations of these common reactions. 

Allenmark s work on anchimerically assisted reactions of sulphoxides" continues with a study of the rates of reduction of the syn- and anti-forms of e fo-cis-3-benzenesulphinylbicyclo[2,2,l]heptane-2-carboxylic acids. The syn-was more reactive than the an/i-isomer by a factor of more than 3.2 x 10 . The relative ease of formation of the acyloxysulphonium ion resulting from nucleophilic attack by the carboxy-group on sulphur accounts for this result, and also for the relative racemization rates of jS-carboxy-substituted sulphoxides. Pummerer reaction of one enantiomer of a benzyl o-carboxyphenyl sulphoxide in the presence of DCCI leads to 2-phenyl-3,l-benzoxathian-4-one (47) in an optically active form, indicating the transfer of chirality from S to C. ... 

The behaviour of /S-oxovinylazides is quite similar to those above. The Z isomer (556), formed from the /S-halo carbonyl compound and sodium azide, is unstable losing N2 and forming the isoxazole (557) in an anchimerically assisted concerted reaction (75AG(E)775, 78H(9)1207). At moderate temperatures (50-80 °C) the E isomer formed acylazirines which at higher temperatures rearranged to oxazoles and isoxazoles. 

At present, this rule fails only when functional neighboring substituents, capable of anchimeric assistance and in a convenient position with respect to the developing positive charge, can compete with bromine in the charge stabilization of the cationic intermediate (ref. 15). For example, the reaction of some unsaturated alcohols (ref. 16) goes through five- or six-membered cyclic oxonium ions, rather than through bromonium ions. 

An interesting alcoholysis of epoxides has been reported by Masaki and coworkers <96BCSJ195>, who examined the behavior of epoxides in the presence of a catalytic amount of the Tt-acid tetracyanoethylene (TCNE, 85) in alcoholic media. Ring-opening is very facile under these conditions, typically proceeding via normal C-2 attack, as exemplified by styrene oxide (86). Certain epoxy ethers (e.g., 89) undergo C-1 attack due to anchimeric assistance. Analysis of the reaction mixtures revealed the presence of captodative ethylenes (e.g., 85) formed in situ, whieh were shown to be aetive in eatalyzing the reaction. The proposed mode of catalysis is represented by the intermediate 87. The affinity of these captodative olefins for... 

Within the past several years, we have examined the synthesis and reactions of several classes of polymers related to PECH. We have adopted three simple approaches to the preparation of polymeric substrates more reactive than PECH toward nucleophilic substitution. We have i). removed the 8-branch point by extension of the side chain, ii). replaced the chloride leaving group by a more reactive bromide and iii). replaced the backbone oxygen atom by a sulfur atom that offers substantial anchimeric assistance to nucleophilic... 

Three different mechanisms of perester homolytic decay are known [3,4] splitting of the weakest O—O bond with the formation of alkoxyl and acyloxyl radicals, concerted fragmentation with simultaneous splitting of O—O and C—C(O) bonds [3,4], and some ortho-substituted benzoyl peresters are decomposed by the mechanism of decomposition with anchimeric assistance [3,4]. The rate constants of perester decomposition and values of e = k l2kd are collected in the Handbook of Radical Initiators [4]. The yield of cage reaction products increases with increasing viscosity of the solvent. 

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. 

FIGURE 8.21 Proposed mechanism for the BOP-Cl-mediated reaction of a carboxylate anion with a methylamino group. Formation of a mixed anhydride is followed by aminolysis that is facilitated by anchimeric assistance provided by the oxygen atom of the ring carbonyl.101 (van der Auwera Anteunis, 1987). BOP-C1 = fcw(2-oxo-3-oxazolidino)phosphinic chloride. 

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