Pyramidal carbocation

Pertinent reviews this year deal with a reappraisal of the structure of the norbornyl cation under stable ion conditions, theoretical approaches to the structure of carbocations, pyramidal mono- and di-cations, and dynamic n.m.r. studies of carbonium ion rearrangements. ... 

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). 

What is the preferred geometry about the radical center in free radicals Carbocation centers are characterized by a vacant orbital and are known to be planar, while carbanion centers incorporate a nonbonded electron pair and are typically pyramidal (see Chapter 1, Problem 9). 

The SnI reactions do not proceed at bridgehead carbons in [2.2.1] bicyclic systems (p. 397) because planar carbocations cannot form at these carbons. However, carbanions not stabilized by resonance are probably not planar SeI reactions should readily occur with this type of substrate. This is the case. Indeed, the question of carbanion stracture is intimately tied into the problem of the stereochemistry of the SeI reaction. If a carbanion is planar, racemization should occur. If it is pyramidal and can hold its structure, the result should be retention of configuration. On the other hand, even a pyramidal carbanion will give racemization if it cannot hold its structure, that is, if there is pyramidal inversion as with amines (p. 129). Unfortunately, the only carbanions that can be studied easily are those stabilized by resonance, which makes them planar, as expected (p. 233). For simple alkyl carbanions, the main approach to determining structure has been to study the stereochemistry of SeI reactions rather than the other way around. What is found is almost always racemization. Whether this is caused by planar carbanions or by oscillating pyramidal carbanions is not known. In either case, racemization occurs whenever a carbanion is completely free or is symmetrically solvated. 

An essential requirement for such stabilisation is that the carbocation should be planar, for it is only in this configuration that effective delocalisation can occur. Quantum mechanical calculations for simple alkyl cations do indeed suggest that the planar (sp2) configuration is more stable than the pyramidal (sp3) by = 84 kJ (20 kcal) mol-1. As planarity is departed from, or its attainment inhibited, instability of the cation and consequent difficulty in its formation increases very rapidly. This has already been seen in the extreme inertness of 1-bromotriptycene (p. 87) to SN1 attack, due to inability to assume the planar configuration preventing formation of the carbocation. The expected planar structure of even simple cations has been confirmed by analysis of the n.m.r. and i.r. spectra of species such as Me3C SbF6e they thus parallel the trialkyl borons, R3B, with which they are isoelectronic. 

A series of hypercoordinated square-pyramidal carbocations were optimized at the MP2/6-31G(d) level and the 13C NMR chemical shifts of the carbocations were calculated using IGLO-HF and GIAO-MP2 methods.86... 

Compounds containing a pyramidally arranged (hence, chiral) sulfur to which are linked three alkyl or aryl groups, resulting in a net positive charge on the sulfur. A biologically important example is S-adenosyl-L-methi-onine chloride. Sulfonium salts can also be utilized as analogs or mimics of carbocation intermediates in enzyme-catalyzed reactions. For example, methyl-(4-meth-ylpent-3-en-l-yl)vinylsulfonium perchlorate proved to be an excellent inhibitor (Ki = 2.5 tM) of the enzyme that catalyzes the formation of the bicyclic (+)- -pinene ... 

CH)5+-Type Cations. The close relationship between carbocations and boranes led Williams1077 to suggest the square-pyramidal structure 615 for the (CH)5+ cation based on the square pyramidal structure of pentaborane. Stohrer and Hoffmann1078 subsequently came to the same conclusion concerning the preferred square-pyramidal structure for the (CH)5+ cation using extended Hiickel MO calculations. 

Various studies indicate that although radicals tend to be pyramidal, the pyramids are shallow and the barriers to inversion are low. This means that the stereochemical results of radical reactions are very similar to those of carbocations, in other words, stereochemistry is usually lost at a reactive center. 

This section on single electron ionization to form cation radicals is logically concluded by a brief discussion of the possibility of ionization to form dications and of their chemistry. Though quantitation is absent, there are three pieces of indirect data that relate to this latter class of cations. The first is the existence and the high stabilityof the pyramidal carbocation, [C(Me)]6 (41). This species may be viewed as formally the dication of hexamethylbenzvalene where the five-fold symmetry in the dication is thwarted by Jahn-Teller effects for the neutral and radical cation forms. No coproportionation reaction of this dication and any neutral benzvalene has been reported to our knowledge. 

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