2-Phenyl-2-propyl cation

Calculate energies for reaction of 2-methyl-2-propyl chloride (to 2-methyl-2-propyl cation), 2-phenyl-2-propyl chloride (to 2-phenyl-2-propyl cation) mdphenyl chloride (to phenyl cation). (The energy of chloride is given at right.) Assuming that reaction (1) is normal , what is the effect of a benzene ring Does it facilitate or hinder loss of chloride ... 

LUMO of 2-phenyl-2-propyl cation reveals location of positive charge. 

It is known that for SnI ewaction (substitution-nucleophilic-unimolecular) [140-141] a key intermediate is a carbocation, therefore the more reactive substrate will be the one that can produce the most stable carbocation. The reactivity of methyl, ethyl, 2-propyl, and 2-methyl-2-propyl tosylates under SnI reaction conditions is inversely proportional to the calculated hydride affinity of the corresponding carbocations. The calculated values were in agreement with the experimental findings which were obtained through solvolysis rate measurement of these tosylates under SnI conditions [142, 143]. Correlation of the cation stability-hydride and affinity-solvolytic rate of the reaction under Sn 1 reaction conditions was observed for the allyl cation (allyl, 3-penten-2-yl, and 2-methyl-3-butene-2-yl cations)[144] and the benzyl cation (benzyl, 1-phenylethyl, and 2-phenyl-2-propyl cations) [145] series. The most reactive substrates were the ones that formed the carbocations with the lowest hydride affinity. 

Several benzylic cations are stable enough to be isolable. For example, the X-ray structure of the 2-pheny 1-2-propyl cation (as its SbFg salt) was obtained in 1997 and shows the phenyl-C bond (1.41 A) to be intermediate in length between those of pure single (1.54 A) and double bonds (1.33 A), in addition to the expected planar framework and trigonal arrangement of all sp carbons (Figure 22-2), as expected for a delocalized benzylic system. 

Catalyst Cation. The logarithms of extraction constants for symmetrical tetra- -alkylammonium salts (log rise by ca 0.54 per added C atom. Although absolute numerical values for extraction coefficients are vastly different in various solvents and for various anions, this relation holds as a first approximation for most solvent—water combinations tested and for many anions. It is important to note, however, that the lipophilicity of phenyl and benzyl groups carrying ammonium salts is much lower than the number of C atoms might suggest. Benzyl is extracted between / -propyl and -butyl. The extraction constants of tetra- -butylammonium salts are about 140 times larger than the constants for tetra- -propylammonium salts of the same anion in the same solvent—water system. 

Experience in PTC with cationic catalysts showed that, in general, the most suitable compounds have symmetrical structures, are lipophilic, and have the active cationic charge centrally located or sterically shielded by substituents. For anionic catalysis sodium tetraphenylborate fulfills these conditions, but it is not stable under acidic conditions. However, certain derivatives of this compound, namely sodium tetra-kis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB, 12.162) and sodium tetrakis[3,5-bis-(l,l,l,3,3,3-hexafluoro-2-methoxy-2-propyl)phenyl]borate (HFPB) are sufficiently stable to be used as PTC catalysts for azo coupling reactions (Iwamoto et al., 1983b 1984 Nishida et al., 1984). These fluorinated tetraphenylborates were found to catalyze strongly azo coupling reactions, some of which were carried out with the corresponding diazotization in situ. 

A final type of cyclopropenium cyanines is found in the azatriapentafulvalenium ions 145,146, and 147/148, which have been prepared93,112,113 from diphenyl and di-n-propyl cyclopropenone or ethoxy cation 75 and indole derivatives as well as from diphenyl methylthio cyclopropenium cation and phenyl-substituted pyr-roles114> ... 

Still another barrier to employing the shifts to resolve the norbomyl problem stems from recent studies of H. C. Brown and Peters (1973). These authors have measured the relative rates of solvolysis of 2-methyl-, 2-t-butyl-, 2-phenyl- and 2-cyclopropyl-2-propyl p-nitrobenzoate in 80% aqueous acetone to gather information about the electron-releasing ability of these groups. Upon comparing their results with the C shifts of the carbonium carbon of the same cation, they found no correlation. Although it might be... 

A further [3 4- 2)-cycloaddition process takes place when imines react with 2-azaallenyl radical cations (23) derived from azirines by photolysis (Scheme 4.2.10) [31, 32]. Yields of (24) are variable, e.g. 2,4,5-triphenyl-1-propyl- (87%), 4-butyl-1,5-dipropyl- (40%), 4-phenyl-1,5-dipropyl- (35%),... 

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