Adamantyl carbocations

One possible explanation is that adamantyl cation, an intermediate in the reaction, is particularly unstable because it cannot accomodate a planar carbocation center (see Chapter 1, Problem 9). Examine the geometry of adamantyl cation. Does it incorporate a planar carbocation center Compare electrostatic potential maps of adamantyl cation and 2-methyl-2-propyl cation. Which cation better delocalizes the positive charge Assuming that the more delocalized cation is also the more stable cation, would you expect adamantyl tosylate to react slower or faster than tcrf-butyl tosylate Calculate the energy of the reaction. 

There is evidence that the configuration of the molecule may be important even where the leaving group is gone long before migration takes place. For example, the 1-adamantyl cation (17) does not equilibrate intramolecularly, even at temperatures up to 130°C, though open-chain (e.g., 5 50 and cyclic tertiary carbocations... 

The activation energies for the fragmentation of the carbene in CH2C12 were calculated by the B3LYP/6-31G method to be 14.6, 2.2, and —0.95 for the bicyclo[2.2.1]heptyl, bicyclo[2.2.2]octyl, and adamantyl systems, respectively. Are the product trends consistent with these computational results, which presumably reflect the relative stability of the carbocation formed by the fragmentation ... 

That maximum acceleration occurs when the vacant p orbital is parallel to the plane of the cyclopropyl ring can be seen from the solvolysis of spiro[cyclo-propane-l,2 -adamantyl] chloride (71). The carbocation formed by departure of Cl is unable to adopt the geometry of the bisected cyclopropylcarbinyl cation, but can orient its empty p orbital properly to form the bicyclobutonium ion. This compound solvolyzes 103 times more slowly than 1-adamantyl chloride.82 On the other hand, 72 solvolyzes 10s times faster than 73. The cation from 72 does have its p orbital parallel to the plane of the ring as in the bisected cyclopropylcarbinyl cation.83... 

The solvolysis mechanisms of 2,2-dimethyl-3-pentyl- and l-(l-adamantyl)-propyl sulfonates appeal- to involve partial reversible ionization to the ultimate ion pair followed by competing elimination and solvent separation, substitution products being formed from the separated ions.27 The lifetimes of simple tertiary carbocations may be some 100 times shorter than previously thought several 3-(4-methoxyphenyl)-l,l-dimethylpropyl species hydrolyse in 50% aqueous TFE with rate constants estimated at some 3.5 x 1012 s-1.28 Much elimination was also observed.28 Two studies concerning proposed carbocation intermediates in enzymatic processes are reported.29,30... 

The method should work better for adamantyl chlorides or norbomyl chloride for which Sn2, and, presumably, preassociation processes are precluded. Speculative values of pATr1 and pATR for the relevant carbocations 54-56 are listed below based on a solvolysis rate constant in water for the... 

To study the possible stabilizing effect of [3-silyl cations, Olah and co-workers334 prepared the 2- [(1 -trimethylsilyl)vinyl]-2-adamantyl cation 132 [Eq. (3.43)] as well as the parent silicon-free carbocation. In contrast to the above observations, NMR data [the (Cl ), (C2), and (C2 ) carbons are more deshielded in 132 than in the parent ion] showed that cation 132 is destabilized compared with the silicon-free analog. Furthermore, at — 100°C the C(l) and C(3) carbons were found to be equivalent, whereas in the parent ion they were nonequivalent. This indicates a rapid rotation about the C(l)-C(3) bond in 132, which can be rationalized by assuming the intermediacy of the [3-silyl-stabilized cation 133. The difference between cation 132 and those having [3-silyl-stabilization discussed above may be the orthogonal arrangement of the [3-C-Si bond and the p-orbital of the carbocation center. 

Cyclobutyl, substituted cyclobutyl, and related carbocations were reviewed.23 A study of the photophysical properties and photochemistry of 1-adamantyl aryl ethers in methanol solvent showed that, although the majority of the ethers underwent photolysis by homolytic pathways, irradiation of the 4-cyanophenol ether resulted, in part, in the methyl ether, implicating an ionic mechanism with the 1-adamantyl cation as an intermediate.24 Labelling experiments demonstrated that the fragmentation of 7-norbomyloxychlorocarbene to 7-norbornyl chloride proceeds with both retention and inversion.25 While an ion pair [Cl CO R+] is a possible intermediate, computational studies suggest that the fragmentations proceed via transition states that lead to either retention or inversion. 

All available experimental results [87] suggest that carbocations are just weakly coordinated to counterion or solvent. For trialkyl and triaryl substituted carbenium ions generated under superacid conditions, this has been demonstrated in a series of X-ray diffraction experiments in particular carried out by Laube and co-workers. [98-102] For example, for the 3,5,7-trimethyl-l-adamantyl cation and its counterion Sb2Fn, the closest C-F distance was measured to be 2.88 A suggesting that interactions between cation and anion are rather weak. [98]... 

In an interesting study on the rotational reorientation of the 1-adamantyl cation in different superacid solutions, Kelly and Leslie confirmed that interactions between carbocation and the surrounding medium are also weak. 

DEGENERATE CARBOCATION REARRANGEMENTS 8,9-dehydro-2-adamantyl and related cations... 

Diem et al. investigated the photolysis of adamantyl iodide as a po ible source of carbocationic initiation for the polymerisation of isobutene. Althou no polymer was obtained with this simple tystem (or with f-butyl iodide as photolyte), addition of iodine scavengers such as zinc, zinc fodide or both together gave some polymerisation, indicating that the carbocations produced in the photolysis of alkyl iodides possess a modest initiating power if generated in the presence of isobutene. 

---