1-Bromoadamantane, solvolysis

The apocamphyl structure is particularly rigid, and bridgehead caibocationa become accessible in more flexible structures. The relative solvolysis rates of the bridgehead bromides 1-bromoadamantane, l-bromobicyclo[2.2.2]octane, and l-bromobicyclo[2.2,l]-... 

In the adamantyl framework the /-silicon effect was studied by Grob and coworkers96,97. The first-y-trimethylsilyl substituent (cf 245) enhances the solvolysis rate of 1-bromoadamantane 246 by a factor of only 8.6 and the second trimethylsilyl group in... 

Previously, it has been noted that the solvolysis rates of both 3-methyl-l-bromoadamantane and 3,5-dimethyl-1-bromoadamantane are slower than 1-bromoadamantane itself. This result was felt to be inconsistent with any mechanism which distributed positive charge to all bridgehead positions of the adamantyl cation under solvolysis conditions since the introduction of tertiary resonance contributors (cf. Eq. (49)) should enhance rather than retard solvolysis rates 56,164 ... 

The relative solvolysis rate constants of 1-bromoadamantane, 74, and 76 explain this behavior 266> ... 

As indicated in Table 16, the amino substituted adamantyl bromide solvoly-ses are significantly enhanced relative to the corresponding alkyl substituted adamantyl bromides. The effect is even more dramatic when one recalls that the crj substituent paramter for -N (CH3)2 and -C6H5 are nearly identical 304 The solvolysis rate of 3-phenyl-l-bromoadamantane 2981 is nearly 104 times slower than that of 3-dimethylamino-l-bromoadamantane 303) at 50 °C, however. 

Comparison of the solvolysis rate constants of 2-chloro-2-methylpropane obtained in water and in benzene solution reveals a rate acceleration of ca. 10 with increasing solvent polarity [47] f The solvolysis rate of 1-bromoadamantane in... 

The apocamphyl case is an extreme one, and bridgehead carbonium ions in other systems are more accessible. Inclusion of more atoms in the bridge gives a more flexible molecule and allows carbonium ion formation to proceed with a somewhat lower activation energy. Thus, the relative solvolysis rates of the bridgehead bromides 1-bromoadamantane, l-bromobicyclo[2.2.2]octane, and 1-bromobicyclo[2.2.1]heptane in 80% ethanol at 25°C are 1,10", and Under the same conditions, the rate of solvolysis of tert-butyl bromide is 1000 times that of 1-bromoadamantane. The 1-adamantyl cation is sufficiently stable to be generated in antimony pentafluoride in concentrations sufficient for observation by NMR. ... 

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