Carbocations hypervalent

In this chapter, we will consider examples of RIs characterized by a hypervalent or valency-deficient carbon, such as carbocations, carbenes, carbanions, and carbon radicals. In the first part, we will consider examples that take advantage of stabilization and persistence to determine their structures by single crystal X-ray diffraction. In the second part we will describe several examples of transient reactive intermediates in crystals. ... 

Hypervalent carbocations have received some attention this year.14 The concept of three-centre, two-electron bonding in these entities is supported by a topological bifurcation analysis of the electronic structure of CH5 15 this and the related species CHg+ and CH3+ are also the subject of a review.16 The CHj + H2 reaction has been studied theoretically.17... 

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... 

A number of intermediates have been suggested in the epoxidation by hypervalent Fe 0X0 [296,526,533,538,558,559] and Mn oxo [536,537,539,558,559] porphyrins (Fig. 1.25) 1 cation radical, 2 metal oxo carbon radical, 3 metal oxo carbocation, and 4 metallaoxetane. Another possibility is the direct oxygen transfer by concerted bond shifts that does not involve an intermediate. 

The 2-norbornyl cation is part of the group of hypervalent carbon compounds, which are discussed in the books of Olah et al. (1987) and Minkin et al. (1987). Hanack (1990) edited a volume of Houben-Weyl on carbocations. Berndt (1993) described interesting correlations between nonclassical carbocations of the type discussed above with related compounds among methylidene-boranes (-CH2 = B-). 

Table 2.1 Relationship between hypovalent and hypervalent carbocations.

The crucial step in acid-catalyzed conversions of hydrocarbons is the formation of the intermediate trivalent or classical sp hybridized carbocation (car-benium ion). In the case of saturated hydrocarbons, this is interpreted by the interaction of the proton of the superacid and the bonding electron pair of the C—H a bond (a similar interaction between the proton and the C—C a bond would result in the cleavage of the carbon-carbon bond). This is based on the concept of a-basicity developed by Olah (65), which describes the ability of a bonds to share their bonded electrons with electrophiles. A hypervalent, pentacoordinate non-classical carbocation (carbonium ion) is formed, which possesses a three-center, two-electron (3c-2e) bond. This is transformed to the classical trivalent carbocation by the loss of hydrogen, that is, protolysis (protolytic cleavage) of the carbon-hydrogen bond occurs. The process is illustrated by the conversion of hexane to yield the 3-hexyl cation through the pentacoordinate carbonium ion (1) (eq. 45). 

Scheme Ic). This type of mechanism is common in the synthesis of biaryls by the coupling of two electron-rich aryl moieties (see Sect. 4.2.1) [28]. Kita and coworkers discovered that the fluorinated alcohols 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) are excellent solvents in many reactions with At2IX and other hypervalent iodine reactions because of their ability to stabilize carbocation radicals in SET reactiOTis [29, 30]. 

Under radical initiation conditions, typically peroxides, hypervalent iodine reagents can be homolytically cleaved to iodine-centered radicals. These iodine centered radicals abstract a hydrogen atom from a labile benzylic C—H bond to yield a resonance-stabilized benzylic radical. At this point in the mechanism, researchers seem divided on the next step. Some propose a second single electron transfer (SET) to form a benzylic carbocation, ° which undergoes ionic reactions to form product. Others suggest radical combination to form an alkyl halide or organic peroxide which reacts further under the reaction conditions to form product. 

---