Bridged carbocations,

A bridged carbocation with a two-electron, three-centre bond was proposed as early as 1939 (Nevell et al., 1939) for the 2-norbornyl cation [lO ] as a reactive intermediate in the solvolysis of 2-norbornyl system (see also Winstein and Trifan, 1949). It has now been isolated as the SbFe salt and the bridged structure is accounted for using solid-state nmr studies... 

This is clearly demonstrated in the pinacolinic deamination (cf. p. 114) of an optically active form of the amino-alcohol (50). Such reactions proceed from a conformation (antiperiplanar 50a or 50b) in which the migrating (Ph) and leaving (NH2 as N2 cf. p. 114) groups are TRANS to each other. Rearrangement via a bridged carbocation would necessarily lead to 100% inversion at the migration terminus in the product ketone (5lab), whichever initial conformation, (50a) or (50b), was involved ... 

Examples of the behavior of other substituted vinyl substrates upon exposure to the action of trifluoroacetic acid and triethylsilane are known. For example, -butyl vinyl ether, when reacted at 50° for 10 hours, gives -butyl ethyl ether in 80% yield (Eq. 65).234 In contrast, -butyl vinyl thioether gives only a 5% yield of n-butyl ethyl sulfide product after 2 hours and 15% after 20 horns of reaction.234 It is suggested that this low reactvity is the result of the formation of a very stable sulfur-bridged carbocation intermediate that resists attack by the organosilicon hydride (Eq. 66). 

Bicyclobutonium ions, are bridged carbocations with a pentacoordinated y-carbon which were first discussed as short lived intermediates in the solvolysis reaction of cyclopropylmethyl and cyclobutyl compounds (56, 57, 58, 59, 60, 61). 

A cation in which significant positive charge is located on at least one carbon atom, itself having an even number of electrons. Both carbenium ions and pentavalent species (such as the methanonium ion, CH5+) are carbo-cations. However, radical cations and carbynium ions are not considered to be carbocations. See Carbenium Ion Bridged Carbocation Carbonium Ion... 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS BRIDGED CARBOCATION yS-y Bridge oxygen,... 

PHYSICAL ORGANIC CHEMISTRY NO-MENCATURE CARBOCATION CARBENIUM ION CARBONIUM ION BRIDGED CARBOCATION CARBOHYDRATE MODIFICATION REACTIONS... 

The stereochemical outcome of Friedel-Crafts alkylation may be either inversion (nucleophilic displacement) or racemization (involvement of a trivalent flat carbocation). Most transformations were shown to occur with complete racemization. In a few instances, inversion or retention was observed. For example, the formation of (—)-l,2-diphenylpropane [(—)-28] from (—)-26 was interpreted to take place through the 27 nonsymmetrically Jt-bridged carbocation, ensuring 50-100% retention of configuration 136... 

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