Non-classical carbonium ions

Fullerene derivatives such as CgoH, (p. 283), C60H2 (p. 287), and C61H2 (p. 287), and hyperco-ordinated non-classical carbonium ions (p. 290) have already been briefly mentioned. 

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

Aspects of bonding and structure/dynamics in selected carbonium ions were presented and discussed. These representative studies demonstrate the power of structural theory in the development of concepts that could lead to new and efficient processes, especially in the area of hydrocarbon chemistry and catalysis. There is no doubt that as newer theoretical and experimental techniques and models are introduced, they will be applied to the study of carbonium ions. A deeper understanding of structure/dynamics of hypervalent non-classical carbonium ions will not only deepen our knowledge of structural theory in chemistry, but could also help in the development of new processes and materials useful in our daily life. 

Cyclobutane has not been polymerised cationically (or by any other mechanism). Thermochemistry tells us that the reason is not thermodynamic it is attributable to the fact that the compound does not possess a point of attack for the initiating species, the ring being too big for the formation of a non-classical carbonium ion analogous to the cyclopropyl ion, so that there is no reaction path for initiation. The oxetans in which the oxygen atom provides a basic site for protonation, are readily polymerizable. Methylenecyclobutane polymerises without opening of the cyclobutane ring [72, 73]. 

This reaction with cyclic olefins has produced some interesting carbonium species a topical example is the C8H9 ligand. This has been complexed to iron as bicyclic cyclooctatrienyl (64, 144, 221), and to molybdenum as a monocyclic homocarbonium ion (247). The latter is an example of a non-classical carbonium ion. 

In the case of zeolites, an additional pathway has been proposed involving the formation of a non-classical carbonium ion by protonation of isobutane, which can alkylate the olefin with formation of a protonated cyclopropane intermediate (60,61) ... 

Some new examples of cyclosteroid formation by homo-allylic participation have appeared recently. Hydrolysis of the A -19-mesylate (4) proceeded with z -electron participation to give the 5j(5,i9-cyclo-6 -ol (5) [6sa] or the related A -olefin (6) [64] according to the reaction conditions. Further studies on these compounds [63,65,66,6 ]] revealed other complicated transformations which must proceed through carbonium ion intermediates. Recent work by Tadanier [66,6]] indicates that two distinct non-classical carbonium ions are involved. Buffered solvolysis of the A -ig-mesylate (4) gives the ion represented by resonance between the canonical structures (A), or by the non-classical structure (B). Stereoelectronic factors of the type discussed for i-steroids ensure 6jS-attachment of a nucleophile in forming the 5jS, 19-cyclosteroid (5). This appears to be the initial product of a kinetically-controlled process, for a further rearrangement occurs in weakly acidic... 

A polar-type transition state is also supported by the occurrence of Wagner-Meer-wein rearrangements accompanying the elimination of hydrogen chloride, from neo-pentyl - " - - , bornyl > andisobornylchlorides - ° In the case of neopentyl chloride the formation of 2-methylbut-l-ene and 2-methyl-but-2-ene has been explained in terms of a non-classical carbonium ion type transition state. 

Bicyclo[2,2,l]heptanes.—Reviews of interest in this section include Brown s on non-classical carbonium ions, Sorensen s on monoterpenoid rearrangements in superacids,and a discussion by Yates of the photochemical ring expansion of cyclic ketones, with particular emphasis on his own work. ... 

In contrast to classical tertiary and secondary cyclopropylmethyl cations (showing substantial charge delocalization into the cyclopropane ring but maintaining its identity) primary cyclopropylmethyl cations in contrast show compeltely cr-delocalized non-classical carbonium ion character (see subsequent discussion). 

The existence of non-classical carbonium ions has been subject for much debate. 

In 1972 Olah proposed a classification for carbocations dividing them into trivalent ( classical ) carbenium ions and penta- and tetracoordinate ( non-classical ) carbonium ions. 

A. The Deamination of 1,2,2-Triphenylethylamine The use of carbon-14 labels in two different positions of the molecule led to identification of open carboniiun ion intermediates during the reaction of several 1,2,2-tiiphenylethyl derivatives, and these experiments have already been discussed in Section II, A, 4 of this chapter. We shall now present some completely independent experiments of combined isotopic stereochemical nature which verify the results of the double-labeling experiments, and which leave little doubt that bridged, non-classical carbonium ions cannot explain either the stereochemical results or the isotope position rearrangements which take place upon deamination of 1,2,2-triphenylethylamine. 

Yet another review [7] concludes by asking the question "Is the expression aromatic actually useless " Since the article is written in German a possible answer supplied is "jein", and a parallel is drawn between the controversies concerning aromaticity and those debating classical and non-classical carbonium ions. It also comments that in reality either/or situations are seldom met. 

This stability of the non-classical carbonium ion or its open C3H7 fragment makes the m/z 43 fragment the base peak even for higher normal or branched alkanes. The assumption that alkanes, which are the simplest of all organic compounds, will hence produce easy-to-interpret MS was proved erroneous . 

Solvolyses of 8-X-3,4-tetrafluorobenzobicyclo[3,2,l]-octadienes and -octenes (X = leaving group) have been reported the syn-epimers react with retention of the skeleton and stereochemistry at C-8 by way of the non-classical carbonium ion intermediate, whereas the anti-compounds yield products with the benzobicyclo-[3,3,0]octane structure, presumably by a classical ion pathway. Although the epimeric tosylates (124) and (125) are solvolysed at different states and show different sensitivities to solvent variation, they produce the same product mixture, comprising... 

Four papers have appeared in which a Russian group report details of solvolytic studies on tetrafluorobenzobicyclo[2,2,2]-octenyl and -octadienyF " tosylates as well as on the addition of trifluoroacetic acid to the octadiene discussion centres on the role of non-classical carbonium ion intermediates. 

Microwave structural studies are reported for (34), (35), its dihydro-derivative and e.xo-5,6-dideuterio-(35), and 11 isotopic species of the tricyclohexane (36). The last paper offers an explanation of the mechanism of the conversion of (37) by LiAlH4 into the tricyclohexane (38) the bromine exo to the double bond is removed in the first of two steps which involve non-classical carbonium ion intermediates. 

Determination of the higher carbon Is binding energies by p.e. spectroscopy has been used to distinguish between classical and non-classical carbonium ions. The time-scale of n.m.r. spectroscopy makes a-distinction between a non-classical norbornyl carbonium ion and equilibration between two classical... 

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