Anchimeric assistance measurement

Intramolecular reactions are much faster than intermolecular reactions, and the anchimeric assistance, measured by the ratio of the rate constant of the unimolecular reaction to that of the analogous bimolecular reaction, depends strongly on the size of the ring that should be formed in the unimolecular process. The ratio of these constants can reach values of up to 10 mol/L. Thus, intramolecular trapping confirms the presence of radicals as short-lived intermediates but does not prove that chain growth proceeds via radical coupling. 

Unequivocal characterization of the transition structures involved in the anchimeric assistance to H2O loss from 26 and 28 (A = H) is obtained by the measurement of the relevant deuterium primary and secondary kinetic effects. 

It appears that the measured ability of Z to supply anchimeric assistance decreases in the following order OMe > SO3CF3 > C104 > F > Cl, I > Me, in agreement with MNDO calculations of the relative stability for acyclic... 

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 kinetics study has been performed on the aqueous iodine oxidation of the thiolactam (78). The rate of oxidation of (78) was found to be approximately 100 times that of a simple cyclic thioether, and this enhancement was ascribed to transannular anchimeric assistance provided by the nitrogen of the amide group <87JOC258i>. The thermal decomposition of the fused dioxetane (79) has been monitored by chemiluminescence decay, and this has been shown to follow first-order kinetics. The activation parameters have been calculated <83CL431>. Second-order rate constants for the quatemization of octahydro-lfl-azonine (9) and other azacycloalkanes with iodomethane in acetonitrile and methanol have been measured <68CCC1429>. The kinetics of the hydrogen-deuterium... 

From this discussion it is seen that in order to make a quantitative measure of anchimeric assistance it is necessary to know k or the (often hypothetical) rate constants for reaction of the substrates without any form of nucleophilic assistance or with nucleophilic assistance from the solvent. The problems in estimating these constants will be discussed in Chapter 3. 

The above constants are usually only used in discussions of nucleophilic substitution reactions at saturated carbon. In other reactions a quantity known as the effective molarity is frequently used as a measure of anchimeric assistance. This can be determined only if the rate constant for an analogous intermolecular reaction has been measured. Consider two analogous reactions, one intramolecular [Eq. (10)] and one intermolecular [Eq. (11)]. These are, respectively, first- and second-order pro-... 

To measure anchimeric assistance it is necessary to estimate the reaction rate of the hypothetical reaction in the absence of anchimeric assistance. 

The study of a series of exo- and emfo-2-aryl-2-norbornyl p-nitro-benzoates is said to offer no evidence favoring a participation in the secondary 2-norbornyl system [cf. the p values of (33), (34), and (35)]. However, this treatment suffers from the same handicap as found in the 5-norbornen-2-yl system that is, the amount of anchimeric assistance present in the secondary derivative (if any) is small, and hence all the aryl groups commonly used are apparently capable of leveling the proposed a participation.f Nevertheless, cr contributions can be measured by the... 

MeO-5 participation also occurs in the reaction of (77) with potassium mercaptoacetate in anhydrous methanol. The products obtained were the benzofuran derivative (78), S-methylmercaptoacetic acid, and a small amount of (79). No kinetic studies were performed to measure anchimeric assistance, but based on studies of similar systems, assistance would... 

Measurements of the temperature coefficient of activation/ the volume of activation/ and the entropy of activation have also been reported for the cyclization of 4-chloro-l-butanol. While there are some differences of opinion as to the interpretation of these values/ support of the kinetic evidence of a weakly anchimerically assisted process is indicated. 

Anchimeric assistance in oxymercurationand oxythallation reactions has been substantiated by the kinetic measurements of Halpern et al (see Table 18). When log k values for the alkenols in each reaction were plotted versus Taft s a constants (see Chapter 3) only l-penten-5-ol and l-hexen-6-ol fell off the lines (p in each case 3.2) defined by CH2=CHR, where R = alkyl or H. The positive deviation by l-penten-5-ol and 1-hexen-6-ol is good evidence for anchimeric assistance. Isolation of the furan and pyran derivatives expected from HO-5 and HO-6 involvement substantiate the claim of anchimeric assistance in oxymercuration. 

Scheme 19). The trans-diitthtris lit afford very abundant [MH-ROH]+ ions upon Cl in contrast to the corresponding cis counterparts, suggesting anchimeric assistance in the elimination of alcohol from the MH+ ions of the trans-6 tt tTs. Collision induced dissociation (CID) measurements of the [MH-ROH]+ ions, obtained from various suitably deuterium... 

Table 52.4 lists measured rate constants for triplet state y-hydrogen abstraction by 8-substituted vale-rophenones. Here, the substituents are not conjugated with the developing y-radical site so their effects are entirely inductive. They indicate a p, value of -1.85, which demonstrates the highly electron-deficient nature of the carbonyl n,K state. It should be noted that I, Br, and RSO substituents enhance reactivity by what has been suggested to be anchimeric assistance. ... 

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