Crystal structures, carbocations

For a review of crystal-structure determinations of triarylmethyl cations and other carbocations that can be isolated in stable solids, see SundaraUngam, M. Chwang, A.K. in Olah Schleyer, Ref. 2, vol. 5, p. 2427. 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

Recentiy published crystal structures of antibody 4C6, an antibody that catalyzes another cationic cyclization reaction (Figure 6), revealed that this antibody has exquisite shape complementarity to its eliciting hapten 5. The active site contains multiple aromatic residues which shield the high-energy intermediate from solvent and stabilize the carbocation intermediates through cation-7r interactions. 

The hrst X-ray crystal structure of a carbocation salt was reported in 1965. Triphenylmethyl perchlorate (27) has a planar central carbon. The three phenyl rings are each twisted 30°, so that overall the cation has a propellor shape. Disordered perchlorate anions sit above and below the central carbon, with a C—Cl separation of 4.09 A. 

X-ray diffractometry is the most powerful method to determine atomic coordinates of molecules in the solid state. X-ray crystal structure analysis was, however, rarely applied in the early years of development of persistent, long-lived alkyl carbocations and studies were only performed to investigate structures of carbocations of aryl derivatives and aromatic systems.65 This is due to the low thermal stability of alkyl carbocations and to the difficulties in obtaining single crystals of carbocations suitable for analysis. Since then, however, methods and instrumentation have improved significantly and X-ray crystal structure analysis has become a powerful tool to solve structural problems of carbocations.65,66... 

Crystal structure determinations of trichodiene synthases were reported, including structures of ternary complexes incorporating inorganic pyrophosphate and an aza analogue of the bisabolyl carbocation intermediate.220,221 The conformation and orientation of the carbocation analogue lead to the conclusion that carbocation intermediates... 

The lipase-catalysed hydrolysis of methyl 2-fluoro-2-arylpropionates was proposed to proceed via a mechanism whereby, after ester hydrolysis, the enzyme facilitates the elimination of fluoride ion with the formation of a carbocation stabilized by the adjacent C02 group.230 Determination of the crystal structure of human sialidase Neu2, an enzyme that catalyses the hydrolysis of sialic acids, reveals a tyrosine residue that is positioned in the active site to stabilize the carbocation proposed as an intermediate in the hydrolysis.231 ll-Fluoro-all-frans-retinol is found to undergo isomerization to its 11 -cis form in the presence of visual cycle enzymes, in contrast to a previous study where no isomerization was reported.232 The result of the prior study was taken as evidence for a carbonium ion pathway in the isomerization. Although the authors of the present study do not rule out such a mechanism, they suggest that the isomerization mechanism remains unknown. Data obtained in a study of the oxidation of... 

Unlike mononuclear carbonylates, reaction of carbocation reagents with anionic polynuclear carbonylates often results in coordination at the bridging carbonyl oxygen rather than at the metal center. This type of interaction was first observed for the reaction of [HFe3(CO)II] with CH3S03F (88) [Eq. (12)], and a higher yield route is indicated in Eqs. (13) and (14). The X-ray crystal structure of this molecule is shown in Fig. 13. 

The 1-adamantyl carbocation provides another example of C-C hyperconjugation. The Ca-C(3 bond is shortened by 0.06 A in the crystal structure. The NMR spectrum also shows characteristics of delocalization of the positive charge. A computational study also indicates delocalization of the positive charge. ... 

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