Nonclassical carbonium ion

Penta- (or higher) coordinate ( nonclassical carbonium ions contain five or (higher) coordinate carbon atoms. They cannot be described by two-electron two-center single bonds alone but also neces-... 

Studies of solvolysis of similar polyfluonnated polycyclic aromatic systems, such as 2,3-(tetrafluorobenzo)bicyclo[2 2 2]octadienes and related compounds, proved the ionic mechanism of this rearrangement [55, 36, 37] (equation 9) Possible nonclassical carbonium ion involvement has been discussed [5S, 39, 40, 41]... 

An alternative explanation for the enhanced rates made its appearance. It was proposed that cr-participation in certain nonclassical ions provided a more satisfactory interpretation. This stimulated a detailed study of the norbornyl system, considered to provide the best available case for such nonclassical carbonium ion intermediates. The results failed to confirm the presence of significant -participation and supported the conclusion that the phenomena must be largely, if not entirely, steric in origin. 

The consideration of HO—LU interaction is useful also in the interpretation of the stability of "nonclassical carbonium ions. For instance, the 7-norbornenyl cation would be stabilized by the symmetry-allowed... 

A detailed study of the mechanism of glycal formation and subsequent transformations in the light of modem concepts of nonclassical carbonium ions might prove useful. Many years ago, use was made of glycals for the... 

In this section we shall discuss carbocations in which at least one carbon-through a three-center bond (see Section 5.3) is coordinated to four orJive-atoxns. By Olah s terminology these are carbonium ions as opposed to tricoordinated carbenium ions. 61 By older terminology the more highly coordinated carbocations are called nonclassical carbonium ions to differentiate them from the tricoordinated classical carbonium ions. ... 

In contrast to classical tertiary and secondary cyclopropylmethyl cations (showing substantial charge delocalization into the cyclopropane ring but maintaining its identity), primary cyclopropylmethyl cations show completely o-delocalized nonclassical carbonium ion character (see Section 3.5.2.5). Also, some of the secondary cyclopropylmethyl cations undergo rapid degenerate equilibrium (see later discussion). 

Spherical-domain models of three-center bonds in localized-molecular-orbital models of a nonclassical carbonium ion, B4CI4, and TaeClfJ have been described 49,52) a drawing of a spherical-domain model of the methyl lithium tetramer, (LiCH, is shown in Fig. 31. Large, outer circles represent domains of electron-pairs of C—H bonds. Solid circles represent domains of Li+ ions. Shaded circles represent 4-center lithium-lithium-lithium-carbon bonds — i.e., electron-pair domains that touch, simultaneously, three lithium ions and the kernel of a carbon atom. The... 

Since our present interest is the reaction of r-bonded systems, the carbon-carbon double bond of alkenyl monomers, with electrophiles to give classical carbenium ions (or esters) as end products, we will concentrate on these entities. Whether nonclassical carbonium ions ( TT-complexes ) are involved in these reactions as hi -energy transition states, is still uncertain, and this interesting aspect will be discussed specifically in later sections (see Chaps. Ill and IV). 

The same products, but in different ratios, occur in the gas phase eliminations of HCl from bornyl and isobornyl chlorides and from the pyrolysis reaction of bornyl and isobornyl benzoates and methyl xanthates . The isobornyl reaction is appreciably faster than that of the bornyl ester ((iso-B/B) = 6.8 at 345 °C), but proceeds at a slightly slower rate than that of cyclohexyl acetate, (CH/iso-B) = 2.1 at 600 °K. By analogy with the nonclassical carbonium ion interpretation of the solvolysis rates of bornyl and isobornyl chlorides, the participation of nonclassical carbonium ion intimate ion-pairs, e.g. 

The transannular cyclizations described in Sections 1.10.1-1.10.6 are unique to medium-sized ring systems. Although transannular electrophilic processes are known with smaller rings, i.e. 3- to 7-rings, many of these processes can best be reviewed in terms of nonclassical carbonium ion phenomena. Some selected examples are collected in equations (73-77). 

The low-temperature stability of bicyclo[2.2.1]heptane derivatives is well documented by the huge literature on liquid-phase norbornyl carbonium chemistry under such conditions, no ring expansion is observed, but instead, multiple Wagner-Meerwein rearrangements take place, preserving the bicyclo[2.2.1]heptane structure in the course of nucleophilic substitution or elimination reactions. This remarkable behavior is at the origin of the nonclassic carbonium ion concept (28). 

In contrast to the rather well-defined trivalent ( classical ) carbenium ions, nonclassical ions 8 have been more loosely defined. In recent years, a lively controversy has centered on the classical-nonclassical carbonium ion problem9. The extensive use of dotted lines in writing carbonium ion structures has been (rightly) criticized by Brown9d, who carried, however, the criticism to question the existence of any a-delocalized (nonclassical) ion. For these ions, if they exist, he stated . .. a new bonding concept not yet established in carbon structures is required. ... 

Cycloheptenyl tosylate undergoes cyclization in the presence of acetic acid to form the bicyclo[4.1.0]heptanyl acetate 3 stereoselectively80. These processes can be best interpreted via a nonclassical carbonium ion phenomena. 

The displacement of the 7-chloro substituent by cyanide or methoxide ions is thought to proceed via the mechanism sketched in Chart IV in which a nonclassic carbonium ion, X (see XXIII suggested by the same authors as an intermediate in the synthesis of ibogaine, Section I,C) is suggested as the true intermediate rather than the more highly strained neutral possibility XI. 

Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent (classical) carbenium ions from three-center bound penta- of tetracoordinated (nonclassical) carbonium ions. Role of carbocations in electrophilic reactions. Olah, G.A. 

Perhaps the "classic" example of a nonclassical carbocation is the 2-norbornyl cation, which was at the center of what has been called "the most heated chemical controversy in our time." In Chapter 8 we will review the experimental evidence, largely based on solvolysis reactions, that led to the proposal of the nonclassical carbonium ion structure shown in Figure 5.48. However, this description was not accepted by all researchers, and an alternative model for the 2-norbomyl cation was a pair of rapidly equilibrating classical (carbenium) ions, as shown in Figure 5.49. Many papers relating to the development of contrasting ideas in this area were published in a reprint and commentary volume by Bartlett. ... 

Nonclassical Carbonium Ions and the Norbomyl Cation Problem... 

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