Cations 1-adamantyl

S 1-Adamantyl Cation What is required in order to compute very accurate NMR chemical shifts Harding et al. take on the interesting spectrum of 

1-adamantyl cation to try to discern the important factors in computing its and 

A better geometry does improve the agreement. The RMSE of the chemical shifts, computed at MP2/tzp with the HF, MP2, and CCSD(T) geometries, are 9.55, 5.62, and 5.06ppm, respectively. Unfortunately, the computed chemical shifts at CCSD(T)/qz2p//CCSD(T)/cc-pVTZ are still in error the RMS is 4.78 ppm for the carbon shifts and 0.26ppm for the proton shifts. Including a correction for 

we can offer no definitive answer to our opening question, other than that extreme cantion mnst be nsed when attempting to compute highly accurate chemical shifts  

4 OPTICAL ROTATION, OPTICAL ROTATORY DISPERSION, ELECTRONIC CIRCULAR DICHROISM, AND VIBRATIONAL CIRCULAR DICHROISM 

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Electrostatic potential map of adamantyl cation shows most positively-charged regions (in blue) and less positively-charged regions (in red). 

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. 

Another possible explanation is that 2-methyl-2-propyl cation allows better access to solvent than adamantyl cation. Examine hydrates of 2-methyl-2-propyl and adamantyl cations. How many water molecules does each accomodate Calculate hydration energies for the two cations. (The energy of water is provided at left.)... 

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... 

Scheme 7.7, ORTEP adapted from reference 32), with [BCCgFj) or [CBnHgBrg] as the counterions. The X-ray structure clearly shows evidence of an sp hybridized center with a C -C -C angle of 178.8° and an abnormally short C -C double bond distance of 1.22 A (compared to 1.32 A, Table 7.1), and a nearly normal C -CA (sp -sp) distance of 1.45 A. The most striking feature of 8, however, is the very long C -Si distance of 1.97 A (compared to 1.87 A, Table 7.1), which is attributed to hyperconjugation, as shown in the scheme. Elongation of the bonds a- to the cation center is a characteristic of hyperconjugation, also observed in the structure of the adamantyl cation. ...

The trishomoaromatic 1,3-dehydro-5-adamantyl cation [52] has been invoked to explain the extremely high reactivity of the bromide [53] (Scott and Pincock, 1973). Recent calculations suggest that [52] does indeed enjoy homoaromatic stabilization to the extent of 25.4 kcal mol1 relative to the 1-adamantyl cation and 1,3-dehydroadamantane (Bremer et al., 1987). 

Azoadamantane exposed to 2 mol equivalents of T CIO4 at room temperature rapidly and quantitatively evolved nitrogen, and thianthrene and products derived from the adamantyl cation were obtained. Equations (38)-(40) (AA, azo-adamantane Ad, adamantane) make clear why 2 mol equivalents of the radical oxidant are required (85JA2561). The comparable interaction of T with phenylazotriphenylmethane and di-ter/-butyl diazene, using a 2 1 ratio of radical cation to substrate, also leads to the formation of thianthrene and nitrogen (85PS111). 

Aside from tert- mty cation, where the experimental X-ray structure is clearly suspect, and aside from semi-empirical methods applied to cations with multi-center bonding, all models perform quite well for all systems. Subtle (and not so subtle) effects such as the alternation in bond distances in benzyl and in heptamethylbenzenium cations, or the greatly elongated carbon-carbon bonds in the derivative of adamantyl cation are generally well reproduced. In fact, except for systems capable of multi-center bonding, the differences among the various methods are fairly modest. (Of course, this is due in part to "lack of precision" in the experimental data.)... 

The remarkable effect of hyperconjugation is seen in the structure of the 3,5,7-trimethyl-1-adamantyl cation (28), also obtained as its Sb2Fjj salt." ° The cation shows a signihcant flattening at Cl, with short C —Co. bonds (1.44 A) and relatively long Co.—Cp bonds (1.61 A). These point to signihcant C—C hyperconjugation, as shown in Eq. 9. 

The reaction of adamantane with lower alkenes (ethylene, propylene, and butylenes) in the presence of superacids [CF3SO3H or CF3SO3H—B(0S02CF3)3] shows the involvement of both alkylations.34 In the predominant reaction the 1-adamantyl cation formed through protolytic C—H bond ionization of adamantane adamanty-... 

Nonplanar carbenium ions, such as the 1-adamantyl cation, display shielded C+ carbon nuclei compared with open-chain trialkylcarbenium ions (e.g. the triethylcarbenium ion in Table 4.76). Deshielding of a and jt carbons relative to the parent adamantane is attributed to charge delocalization via a bonds [493]. 

The 2-methyl, 2-cyclopropyl and 2-phenyl substituted 8,9-dehydro-2-adamantyl cations 25 were prepared from their respective alcohols using fluorosulfuric acid in sulfuryl chloride fluoride at low temperatures (equation 27). The relative extent of charge delocalization in these cations was estimated by comparing their NMR spectra. The ions are nonequilibrating static cations, as shown by their proton NMR spectra56. 

The tertiary 2-methyl-8,9-dehydro-2-adamantyl cation 25, R = Me is stable up to 10 °C, and shows a much deshielded absorption for the cationic center (<5I3C 274.4), which is relatively shielded compared with the 2-methyl-2-adamantyl cation (<513C 323) showing some charge delocalization into the cyclopropane ring. The C8 and C9 carbons are also deshielded (<5I3C 100.7), comparable to other static cyclopropylcarbinyl cations. 

The l,2-dimethyl-8,9-dehydro-2-adamantyl cation 2656, as expected, is also a static cation and stable up to -10 °C. Its NMR behavior (<5I3C of C+ = 266.3) can be compared with the l,a,a-trimethylcyclopropylcarbinyl cation 20a (<513C of C+ = 276.6)54. The introduction of the 1-methyl group causes shielding of the cationic centers in both cases (the cationic center is shielded with respect to the demethylated analogue by 8 ppm in the former case, and it is shielded by 3.4 ppm in the latter case). 

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