Butonium ions

A good correlation was found between the fraction of cyclized or rearranged products and the extent of participation k /kg. Similar results were observed by Santelli and Bertrand (92) on essentially the same systems. Jacobs and Macomber (91) as well as Santelli and Bertrand (92) suggest a bicyclo-butonium ion 125 as the first formed intermediate (with the predominance of... 

Figure 22, The butonium ions. Values on the bottom of the structures correspond to relative energies (AH) in kcal/mol.

Figure 23 shows the lowest energy species among the possible conformers for each butonium ion. 

Using methods such as those discussed for the norbornyl cation, nonclassical structures have now been established for a number of carbocations. " Representative examples are shown below. The 7-phenyl-7-norbornenyl cation 19 exists as a bridged strucmre 20, in which the formally empty p orbital at C7 overlaps with the C2—C3 double bond. This example is of a homoallylic cation. The cyclopropyl-carbinyl cation 21, historically one of the first systems where nonclassical ions were proposed, has been shown to exist in superacids mainly as the nonclassical bicyclo-butonium ion 22, although it appears as if there is a small amount of the classical 21 present in a rapid equilibrium. Cations 23 and 24 are examples of p-hydridobridged... 

Some of the evidence for the bisected cyclopropylcarbinyl cation follows. That the charge can be delocalized to both C2 and C3 simultaneously has been shown by the work of Schleyer and Van Dine.81 These workers studied the solvolysis of cyclopropylcarbinyl 3,5-dinitrobenzoates and found that methyl substituents accelerated the rate by an amount dependent only on the number of such substituents and not on their position. Thus 68, 69, and 70 react at almost the same rate. If the transition state for ionization were similar to the bicyclo-butonium ion, two methyl groups at C2 should accelerate the rate more than one at C2 and one at C3. A symmetrical transition state for ionization similar to the bisected cyclopropylcarbinyl cation (67) in which the charge is delocalized over all four carbon atoms best explains the results. 

There have been a very large number of investigations of carbocationic reactions of cyclobutyl, cyclopropylcarbinyl and allylcarbinyl derivatives under so-called stable-ion as well as solvolytic conditions. Bartlett (1965) has stated Among nonclassical ions the ratio of conceptual difficulty to molecular weight reaches a maximum with the cyclopropylcarbinyl-cyclobutyl system . The term nonclassical was first used by Roberts and Mazur (1951) to describe the nature of tricyclobutonium ion [32] suggested to be involved in reactions of cyclopropylcarbinyl derivatives. Later Roberts and coworkers (Mazur et al., 1959) favoured a set of rapidly equilibrating nonclassical bicyclo-butonium ions [34] instead of a single non-classical species. Essentially all experimental evidence on indicates that the species is a nonclassical... 

Ion 38 (Fig. 16) could be generated both from cyclobutyl and cyclopropylmethyl precursors. At lowest temperatures studied (= 140 °), ion 38 is still an equilibrating mixture of bisected o-delocalized cyclopropylcarbinyl cations 169 and the bicyclo-butonium ion 170. 

NMR studies in super acid media and theoretical studies have suggested that the bicyclo-butonium ion and the classical cyclopropylmethyl cation should be close in energy. 

Summary The l-(trimethylsilyl)bicyclobutonium ion and the 3-e c fe>-(/er/-butyldi-methylsilyl)bicyclobutonium ion were investigated by NMR spectroscopy in superacid solution and by quantum chemical ab initio calculations. The l-(trimethyl-silyl)bicyclobutonium ion undergoes a threefold degenerate methylene rearrangement. The 3-e /o-(/ert-butyldimethylsilyl)bicyclobutonium ion is the first static bicyclo-butonium ion. The NMR spectra of this carbocation are a direct proof for the hypercoordinated and puckered structure of bicyclobutonium ions. 

Different structures and various equilibria have been suggested to account for the observed nmr spectra. The scheme in (89) shows the relation and principal interconversion cycle of cations [124]-[I26] around a tricyclo-butonium ion structure [135] which is energetically disfavoured according to the Jahn-Teller theorem. 

These two observations exclude cyclopropylmethyl and cyclobutyl as the dominant structures in the equilibrium of C4H7 -cations. In a bicyclo-butonium ion [139], the pentacoordinated carbon is likely to be upheld as for the methyl-homologue. The vibrations for the endo-C—H bond at the pentacoordinated carbon are less conhned compared to the other endo-C— H bonds because bridging could drain electron density out of this bond. MlNDO/3 calculations of Dewar and Reynolds (1984) have shown a larger bond length and some involvement in multicentre bonding only for this endo-C—H bond. 

Although the l-t-butyl-l-azabicyclo[1.1.0]butonium ion (10) should also be highly strained, it and other such derivatives have been implicated as intermediates upon solvolysis of azetidine and aziridine precursors. Gaertner claimed that (11) and (12) are among the least reactive / -aminoalkyl chlorides known, yet isomerization of (11) = (12) occurred in either carbon tetrachloride or acetonitrile. With nucleophiles present, both gave products of simple displacement, hydrolysis, and rearrangement. The rate constants for solvolysis (in 50% aqueous ethanol) of (11) and (12) were... 

It will be recalled from Sec. VA, 2(a), that in solvolysis of cyclopro-pylmethyl benzenesulfonate, proceeding presumably via the bicyclo-butonium ion of Mazur et al. (115) ... 

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