Leaving group mobilities

Elimination of hydrogen halides from polyfluoroalkanes by bases also usually involves earbanion intermediates (ElcB mechanism) [<8/l, and orientation is there tore governed by relative C H acidities and leaving group mobility Some examples are shown in equations 16-18 [145]... 

Table 7. Leaving group mobilities in reactions of l-X-4-nitrobenzenes with...

In principle, this aspect of the reactivity of aza-activated compounds might be deduced from data of the kind shown in Table VII, but experimentally it is easier to obtain by direct comparison of the mobilities of the leaving group from the non-equivalent positions of a given... 

Reactions of propynyl alcohols and their derivatives with metal hydrides, such as lithium aluminum hydride, constitute an important regio- and stereoselective approach to chiral allenes of high enantiomeric purity63-69. Formally, a hydride is introduced by net 1,3-substitution, however, when leaving groups such as amines, sulfonates and tetrahydropyranyloxy are involved, it has been established that the reaction proceeds by successive trans-1,2-addition and preferred anti-1,2-elimination reactions. The conformational mobility of the intermediate results in both syn- and ami- 1,2-elimination, which leads to competition between overall syn- and anti-1,3-substitution and hence lower optical yields and/or a reversal of the stereochemistry. 

In conformationally mobile systems, both syn and anti eliminations are theoretically possible. The anti elimination should be favored electronically over the syn elimination because the electron pair of the C-H bond is anti-periplanar to the leaving group. It has also been suggested (84, 85) that the syn elimination might require a configurational inversion at the C-H bond, so the electron pair of that bond becomes antiperi planar to the C-X bond (297-298). 

The key feature of this reaction mechanism is the fact that a certain proton mobility is induced on the surface silanols by the interaction with ammonia.23 The excellent leaving group, H20, is subsequently replaced by NH3. The rate-limiting step in this reaction is the formation and desorption of the water molecule. This mechanism could explain why Fink was able to reach a much higher degree of ammoniation, using a flow system, compared to other researchers using a static system. 

Sulfanilamido-6-(p-tolyl8ulfonyl)pyridazine, deactivated by anionization, seems to be about as reactive (150°, 12 hr) toward alkoxide ions as is the 6-chloro analog. In 3-chloro-6-methyl-sulfonylpyridazine the chloro group is preferentially displaced by sulfanilamide anion and by alkylamines and aniline. However, in this substrate as well as in 4-chloro-6-methylsulfonylpyrimidine, the relative reactivity involves their mutual activation as well as their mobilities as leaving groups. 

Also the product ratio observed for donor molecules with other leaving groups than protons can be understood under the above mentioned aspects. So for the compounds of trimethylbenzylsi-lane and of sulphide type, the product distribution of FET depends on the mobility of the critical bond (bending frequency and activation barrier of this motion). Details of these observations should be taken from the publications cited in the last column of Table 1. 

This single-step mechanism appears reasonable, because carboxylate is a fair leaving group and hydroxide is a very good nucleophile. However, labeling studies rule out this mechanism under common reaction conditions. The two-step mechanism must be favored because the higher mobility of the 7T electrons of the carbonyl group makes the carbonyl carbon especially electrophilic. 

The mobilities of some common functional groups X relative to chlorine (Cl = 1) for the reaction of 1-X-4-nitrobenzenes with methoxide ion in methanol at 50° have been determined by Miller and co-workers (Table 7). These data indicate that the leaving group... 

Leaving group (X) Mobility relative to Cl = I AE (kcal mole ) logioB... 

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