Butonium cations

Figure 23. Relative enthalpy (AH) for the isomeric butonium cations, calculated by MP4SDTQ/6-311++G //MP2(full)/6-31G (82).

The pulsed electron beam MS technique was also used by Hiraoka and Kebarle842 to study the C4H + cations. In the ion-molecule reaction of ethane and the ethyl cation, two species were observed and identified as the 2-//-n-butoniu m cation 469 and the 2-C-w-butonium cation 470. C—C protonated ion 470 formed first rearranges to C—H protonated ion 469 (energy barrier = 9.6 kcal mol-1) and then dissociation to sec-C4H9+ + H2 takes place. 

An experimental study also exists with respect to the C4H1C cations. Using the pulsed electron-beam MS technique, Hiraoka and Kebarle observed two species in the ion-molecule reaction of ethane and the ethyl cation. They identified them as the 59 2-H-n-butonium and the 60 2-C-n-butonium cation. C-C protonated ion 60 formed first rearranges to C-H protonated ion 59 (energy barrier = 9.6 kcal mol ) before dissociation to xcc-QH/ -1- If. 

Ab initio topological analysis of the proponium cations showed that the most stable structure is the C-proponium ion lying, however, only 0.3 kcal mol below the van der Waals complex of isopropyl cation and H2 (MP4/6-311-H-G / MP2/6-31G level of theory). Similar topological studies of the stability of the different n-butonium cations formed by the protonation of the nonequivalent bonds of n-butane indicated the MP2 level stability order... 

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. 

Three cations, an //-protonated (471) and two C-protonated (470 and 472) isomers were found by Collins and O Malley831 by DFTand MP2 calculations. Ion 472 is more stable than ion 470 by about 5 kcal mol-1. Eleven stable isomeric protonated butane cations including rotamers were found by extensive studies by Mota and coworkers843 [MP4SDTQ(fc)/6-311++G //MP2(full)6 - 31G level]. The stability order 2-C-w-butonium (470) > 1-C-w-butonium (472) > 2-/7-w-butonium (469) > l-//-/ -butonium (471) was interpreted in terms of charge delocalization in the involved 3c-2e bonds. Cation 471 prefers to rearrange to cation 472, whereas cation... 

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. 

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. 

---