X-ray structure, of carbocations

A new Y solvolysis scale has been developed for benzylic species with extensive charge delocalization, based upon the solvolyses of some benzhydryl bromides and /-butyl(2-naphthyl)methyl bromides.39 Chlorides have negative salt effects on the ionization of benzhydryl bromide in 7-butyrolactone.40 The X-ray structure of the dimerization product of l,8-bis(dhnethylammonio)-4-naphthyl(phenyl)methyl carbocation has been determined it appears to be formed via a 4n + 2n-cycloaddition mechanism 41... 

The first X-ray structure of an a-silyl-substituted carbocation (33) is reported its pXR+ value is predicted to be 4.73 The trimesitylsilylium cation is proposed to be a nearly free, tricoordinate species.74 The dimethylsilylium cation undergoes isomer interconversion via (34), according to high-level calculations the most stable structure is... 

Hojo and coworkers subsequently synthesized the related 171 carbocation. The X-ray structure of 171 indicated, however, that the two S-fC distances are not identical, that is, it cannot be regarded as an Sn2 transition state. According to theoretical calculations, Sn2 transition states should be trigonal bipyramidal around the central carbon. Recently, Yamamoto, Akiba, and coworkers have prepared the interesting hypervalent compounds 172 and... 

Figure 15-21 X-ray structure of the carbocation arising from attack of bromine on hexamethyl-benzene. This cation can be isoiated because it is unusually stabilized by the methyl substituents and because it lacks a proton on the sp -hybridized carbon, thus disabling bromoarene formation.

NotE, howEVEr, that an X-ray structurE analysis of thE stablE, ctystallinE carbocation 3,5,7-trimEthyladamantyl showEd thE 3-coordinatE C(l) atom as a considErably flattEHEd pyramid 21pm abovE thE planE of thE 3 adjacEnt C atoms and with bond anglES 120°, 118° and 116° (X = 354°). T. LaubE, Angew. Chem. Int. Edn. Engl. 25, 349-51 (1986). 

The isomerization of isopentenyl diphosphate to dimethylally diphos phate is catalyzed by JPP isomerase and occurs through a carbocation pathway Protonation of the IPP double bond by a hydrogen-bonded cysteine residue ir the enzyme gives a tertiary carbocation intermediate, which is deprotonated b a glutamate residue as base to yield DMAPP. X-ray structural studies on the enzyme show that it holds the substrate in an unusually deep, well-protectec pocket to shield the highly reactive carbocation from reaction with solvent 01 other external substances. 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

Thus, X-Ray investigations of MYKO 63 molecular structures make a remarkable flexibility of its aziridino wings conspicuous. Having in mind that these wings may generate the suitable carbocations for dialkylating DNA, we may assume that MYKO 63, owing to its versatile character, will have the capability of easily... 

A book containing authoritative reviews of many aspects of carbocation chemistry has been published.1 These reviews are up-to-date to the end of 1991, as the book is the result of a symposium held in honour of George Olah in 1992 it features many of the world s leading carbocation chemists. At least one book review of it has also appeared.2 Olah has an introductory chapter concerning his decades-long search for stable long-lived carbocations in superacid media,3 and other review chapters include ones on carbocations at surfaces and interfaces,4 on the X-ray structural analyses that have been performed on many carbocation salts and related compounds in recent years,5 and on natural product chemistry in superacids.6 Other review chapters will be referred to below, as appropriate. 

This chapter deals with silyl-substituted carbocations. In Section II results of quantum chemical ab initio calculations of energies and structures of silyl-substituted carbocations are summarized1. Throughout the whole chapter results of ab initio calculations which relate directly to the experimental observation of silyl-substituted carbocations and their reactions are reviewed. Section m reports on gas phase studies and Section IV on solvolytic investigations of reactions which involve silyl-substituted carbocation intermediates and transition states. Section V summarizes the structure elucidation studies on stable silyl-substituted carbocations. It includes ultra-fast optical spectroscopic methods for the detection of transient intermediates in solution, NMR spectroscopic investigations of silyl-substituted carbocations in superacids and non-nucleophilic solvents, concomitant computational studies of model cation and X-ray crystallography of some silyl-substituted carbocations which can be prepared as crystals of salts. 

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... 

A recent X-ray characterization of the cyclopentyl cation 33 generated with methyl carboranes has shown a perfect planar structure around the carbocation center [bond angles CH3-C+-CH2= 124.9 and 125.2°, CH2-C+-CH2= 109.9° bond distances CH3-C+ = 1.46 A, CH2-C+ = 1.45 A).123... 

The JV-acetyl derivatives 37 and 38 were transformed efficiently into the corresponding indolines. One notable point was that the alcohol 39 was produced as a single diastereoisomer and the stereochemistry was established by performing a single-crystal X-ray structure determination on a derivative of this compound. Since the final step in the formation of the alcohol featured attack on a carbocation by a water molecule, it is clear that this carbocation is much more easily attacked from one face than the other. 

The elusiveness of "free" silyl cations in condensed phases stands in remarkable contrast to the behavior of the isoelectronic carbocations [11] which Olah s school has explored so extensively in super acid media [12]. Laube s splendid recent series of carbocation X-ray structures [13, 14] illustrate what to expect truly "free" cations will be separated from the nearest atoms of the counteranions by 3 A or so. Despite the larger size of silicon, the Si-ligand distances in the Lambert-Reed structures are only modestly longer than typical covalent bonds, and are in the range expected for dative bonds [IS]. 

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