Bridgehead 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]-... 

The effect of structure on relative reactivity may be seen particularly clearly when a halogen atom is located at the bridgehead of a bicyclic system. Thus the following rates were observed for solvolysis in 80% aqueous ethanol at 25° ... 

All are tertiary halides so that attack by the S mode would not be expected to occur on (16) or (17) any more than it did on (8) (cf. p. 82). Sn2 attack from the back on the carbon atom carrying Br would in any case be prevented in (16) and (17) both sterically by their cagelike structure, and also by the impossibility of forcing their fairly rigid framework through transition states with the required planar distribution of bonds to the bridgehead carbon atom (cf. p. 84). Solvolysis via rate-limiting formation of the ion pair (SN1), as happens with (8) is... 

Table 5.5 Approximate Solvolysis Rates of Bridgehead Systems Relative to I-Butyl...

Their approach in looking into the problem further was to find structures in which specific covalent bonding to the back side of the carbon undergoing substitution is difficult or impossible. As models for reactions at tertiary carbon they chose bridgehead substitutions. We have seen in Section 5.2 that rates in these systems are retarded, in some cases by many powers of ten, because of the increase in strain upon ionization. But the important point in the present context is that it is impossible for a solvent molecule to approach from the back side of a bridgehead carbon the only possibilities are frontside attack, known to be strongly disfavored (Section 4.2), or limiting SW1 solvolysis with nonspecific solvation. 

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 ... 

Early work in the area of physical organic chemistry revealed that the bridgehead positions of many polycyclic hydrocarbons are unusually inert to solvolysis and to nucleophilic attack 187>. It is now realized that the solvolytic reactivities of bridgehead derivatives of a variety of polycyclic hydrocarbons vary widely. This is illustrated below for the adamantane family. The reactivities range from 3-homoadamantyl (93), the reactivity of which is nearly the same as that found for typical tertiary acyclic analogues (e.g. f-butyl), to 7-methyl-3-noradamantyl (94) 36 9S>10Si 187 ... 

Since the solvolysis reactions of bridgehead substrates are mechanistically uncomplicated (i.e. competing elimination is highly unlikely187 ns-H9) an(j backside solvent participation is impossible), they provide ideal models for limiting carbonium ion behavior. Adamantyl derivatives have become the substrates of choice for this purpose due to their availability and convenient reactivity. 

Finally, de Meijere and coworkers ° have studied the reactivities of some bridgehead substituted trishomobarrelene and trishomobullvalene derivatives. These are triscyclopropylcarbinyl systems. The relative rates of solvolysis below are for reactions in 80% aqueous dioxane at 25°C . In both systems the cyclopropyl substitution results in... 

The mechanism of such solvolysis of 11,11-dihalo[4.4. IJpropellanes leading to bridgehead olefin formation has been discussed in the light of various alternatives. ... 

The latter substrates were deuteriated and mass spectral fragmentation of the products of solvolysis was studied . The authors concluded that both unassisted and Ag" -assisted solvolysis were stereoretentive and that the intermediate cations and the bridgehead olefins do not interconvert. They believe that a perpendicular cation, 58, does not intervene but the precise structure of the intermediates is still uncertain. 

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