Acetolysis rate

Although not competitive synthetically with the methods discussed above two interesting applications of ring closure reactions for the synthesis of homoadamantane derivatives have been reported. The first involves trans-annular cyclopropyl participation in the solvolysis of 48 acetolysis gives 3-homoadamantyl acetate as the only observed product (Eq. (32)) 112 Comparison of the acetolysis rate of 48 (2.14 x 10 4 sec"1,25°C) with that of exo-3-bicyclo[3.3.1]nonyl tosylate (5.82 x 10"s sec. 1,25°C) U3>114)... 

Another instructive example of electrophilic or H-bonding assistance of protic solvents (or co-solvents) in SnI reactions is the accelerated acetolysis rate of 2-bromo-2-methylpropane upon the addition of phenols to a tetrachloromethane/acetic acid solution of the reactant [582] see reference [582] for further examples. The usefulness of phenol as a solvent for SnI solvolysis reactions, in particular phenolysis of 1-halo-l-phenylethanes, has been stressed by Okamoto [582], In spite of its low relative permittivity (fir = 9.78 at 60 °C), its low dipolarity fi = 4.8 10 Cm = 1.45 D), and its low nucleophihcity, it represents a solvent of high ionizing power due to its electrophilic driving force. 

The proposal that cyclopropyl participation in solvolysis of the cis-3-bicyclo[3.1.0]hexyl tosylate (108) proceeds via a trishomocyclopropenyl cation intermediate (109) has been firmly established by product, deuterium scrambling and salt effect studies However, because the cyclopropyl participation in this system requires an unfavorable conformation, it is not apparent from simple kinetic studies. Thus, the cis-3-bicyclo[3.1.0]hexyl tosylate (108) has an acetolysis rate which is only nine times faster than that of the non-participating trans isomer, 110. However, when the 3-bicyclo[3.1.0]hexyl moiety is incorporated into a tricyclic or tetracyclic structure which locks it into a chair form, dramatically large rate enhancements are found. Some examples from the literature which are provided in the review by Haywood-Farmer are given below. These... 

Application of the Taft treatment to cyclic and bicyclic systems suffers from the availability of a values only for noncyclic substrates. The Foote-Schleyer correlation238, 239 was proposed as a means for estimating acetolysis rates of cyclic substrates ... 

Fig. 19. a) Bridgehead derivatives used in the calculation b) An example of correlation between -log of the experimental tosylate acetolysis rate constants at 70° and the calculated hydrocarbon-carbonium ion strain energy... 

Table 4. Calculation of the Acetolysis Rates of Arenesulphonates at 25 °C and Estimation of the Magnitude of Anchimeric Assistance...

The configuration of chlorine affects little the acetolysis rate the acetolysis rate ratios of 75, 77 and 124,125 are only 2. However the exo endo rate ratio for the two anti-epimers 77 and 125 and for the two syn-epimers 75 and 124 are 80 and 246, respectively these values are quite comparable with the exo endo rate ratio for unsubstituted 2-norbomyl tosylates. The absolute values of rates for the four epimers are tially lower (about 300 times) than those for the respective unsubstituted 2-norbomyl tosylates as expected from the inductive effect of the chlorine at C . All four epimeric tosylates yield qualitatively the same mixtures of products, identical by GLC from anti-chloro tosylates 77 and 125, quantitatively rather different from syn-isomers 76 and 124. 

These data also agree with Brown s results on the solvolysis of 2-aryl-2-bicyclo-[2,l,l]hexyl-p-nitrobenzoates for which the value of was found to be —4.31, i.e. still more negative than that of 2-aryl-2-endo-norbomyl-p-nitrobenzoates (q = —3.72). The acetolysis rate of the secondary tosylate 142 is 3 times as low as that of 2-endo-norbomyl tosylate and 1000 times as low as that of 2-exo-epimer i.e. there are no data in favour of kinetic acceleration due to a-participation. 

Due to the negative inductive effect the introduction of a double bond decreases the acetolysis rates of both 2-exo- and 2-endo-norbomenyl brosylates as compared with saturated analogues (the numbers stand for the relative acetolysis rates of brosylates) ... 

As we pointed out earlier, in bicyclic olefines of the type with decreasing number of methylene groups in the syn-located bridge the rate of solvolysis increases (in passing from n = 2 to n = 1-10 -fold Coates attempted to investigate a similar effect of shortening the syn-bridge by the homocyclopropyl participation. For this purpose the authors have shown that the acetolysis rate of 4-tosyloxy-exo-... 

TABLE 8.7 Acetolysis Rate Constants and Activation Parameters for Primary Tosylates... 

Svensson has reported detailed studies of the solvolysis of rigid cyclobutylcarbinyl systems. The reactivity of the cis-brosylate (800) is 133 times lower than that of its trans-epimer to acetolysis. Rate and product results are compatible with the... 

Rate data for the acetolysis of norbornyl tosylates with substituents at the 3-, 5-, 6-, and 7-positions afford a linear log versus logkg plot with slope 1.11 0.08 (r = 0.98X indicating that the transition states derived from the exo- and ewdo-isomers are equally affected by inductive variations. Hence, it is concluded that o-delocali-zation in the transition state for exo-solvolysis is absent. Studies on the acetolysis rates of the cis,exo- and cis,endo-2,3-norhorna.ne ditosylates (82) and (83), the acetoxy-tosylates (84) and (85), and the norborn-5-ene ditosylates (86) and (87) have been reported. Whereas it was found that 7t-participation is enhanced by the presence of an adjacent electron-withdrawing group in (86) vs. (87) relative to the analogous mono-tosylates, no such response was found for the saturated systems (82) vs. (83) or (84) vs. (85) relative to 2-exo- and -endo-norbomyl tosylate. It is therefore concluded that a-participation is unimportant under these conditions of inductively enhanced electron demand. Essentially opposite conclusions have been reached from the results obtained on the solvolysis rates of the p-nitrobenzoates (88)—(91). In contrast... 

Fig. 4. Correlation of acetolysis rates of arenesulfonates and carbonyl frequencies of the corresponding ketones.The numbers represent the following 1-10, cyclohexyl through cyclopentadecyl, respectively 11, 2-propyl 12, 2-butyl 13, 3-methyl-2-butyl 14, 3,3-dimethyl-2-butyl 15, 7-norbornyl 16, enifo-8-bicyclo[3.2.1]octyl 17, 2-adamantyl 18, a-nopinyl 19, jS-nopinyl 20, l,4-a-5,8-j5-dimethanperhydro-9-anthracyl. Reprinted by permission of the American Chemical Society.

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