I-butyl cation

Step through the sequence of structures depicting rotation about the C i - C+ bond in 2-methyl-2-butyl cation. Plot energy (vertical axis) vs. CCCC dihedral angle (horizontal axis). What is the preferred conformation, with the ethyl group in plane or perpendicular to the plane ... 

Less reactive electrophilic reagents like those involved in acylation or alkylation apparently do not react with phenyl-substituted pyrylium salts the p-acylation of a phenyl group in position 3 of the pyrylium salt obtained on diacylation of allylbenzene (Section II, I), 3, a), and the p-l-butylation of phenyl groups in y-positions of pyrylium salts prepared by dehydrogenation of 1,5-diones by means of butyl cations (Section II, B, 2, f) probably occur in stages preceding the pyrylium ring closure. 

In view of the chemical nature of alkylaluminums and methyl halides, complexation is most likely to be rapid and complete, i. e. K is large. Indeed Me3 Al and a variety of Lewis bases were found to complex rapidly2. Initiation, i.e., f-butyl cation attack on monomer, is also rapid since it is an ion molecule reaction which requires very little activation energy. Thus, it appears that Rj t. and hence initiator reactivity are determined by the rate of displacement Ri and ionization R2. 

The photophysical properties of magnesium(II) tetra-(i-butyl)phthalocyanine (27) have been studied in solution, in micelles and in liposomes cation radical formation (CBr4 as electron acceptor) has been detected with UV excitation, or by a two-photon excitation using a pulsed laser in the therapeutic window at 670 nm.118 The Mg11 complex of octa(tri-z -propylsilylethy-nyl)tetra[6,7]quinoxalinoporphyrazine (28) has been prepared as a potential PDT sensitizer. The synthesis is shown in Figure 8. Compound (28) has Amax 770 nm (e = 512,000 M-1 cm-1), d>f = 0.46 and d>A = 0.19 (all in THF, under air).119... 

This assignment was queried [16-18] and detailed investigations [19a] showed that the principal originator of this spectrum is the 3-(l-methyl, 3-phenylindanyl) cation (I), and that, according to the conditions of the experiment, other ions of the diphenylmethyl type may contribute. The spectrum of the 1-phenylethyl ion has still not been identified but that of the homologous 1,3-diphenyl-n-butyl cation (II) was shown to have only a single important absorption, at 315 pm [19a]. 

The principal components of the trityl cation in zeolite HY are <5 = 282 ppm and <5j = 55 ppm. It is instructive to tabulate all of the 13C principal component data measured for free carbenium ions in zeolites as well as for a few carbenium ions characterized in other solid acid media (Table III). The zeolitic species, in addition to the trityl cation (119), are the substituted cyclopentenyl cation 8 (102), the phenylindanyl cation 13, and the methylindanyl cation 12 (113). Values for the rert-butyl cation 2 and methylcyclopentyl cation 17 (prepared on metal halides) (43, 45) are included for comparison. Note that the ordering of isotropic chemical shifts is reasonably consistent with one s intuition from resonance structures i.e., the more delocalized the positive charge, the smaller the isotropic shift. This effect is even more apparent in the magnitudes of the CSA. Since... 

Figure 24 reports 13C MAS spectra of the ferf-butyl cation (43) and the methylcyclopentyl cation 17 (45) on the solid metal halides A1C13 and AlBr3 the asymmetry parameters, CSAs, and isotropic shifts (Table III) are unambiguous for the species indicated. Repeated attempts in various laboratories to observe the ferf-butyl cation as a persistent species in a zeolite have thus far been unsuccessful. Detailed theoretical work will be required to determine whether or not the ferf-butyl cations are local minima (i.e., true intermediates) on typical reaction pathways in zeolites. The ease with which these cations form in true superacids (liquid or solid) should be contrasted with the history of negative observations in zeolites. 

Several classes of synthesized calixarenes bearing several moieties (ether, ester, and amide derivatives), were tested for the extraction of strontium picrates (from aqueous solutions into dichloromethane).128 Only a few of them show appreciable extraction levels. The p-i-butyl calix[6]arene hexa(di-/V-ethyl)amide (CA4) shows a very high extraction level of alkaline earth cations with respect to alkali metal cations. Moreover, dealkylation of the calix[6]arene hcxa(di-/V-cthyl)amidc (CA5) decreases the extraction of both sodium and strontium. As this decrease is much more important for sodium than for strontium, the Sr/Na selectivity, which increases from 3.12 to 9.4, is better than that achieved for DC18 derivative under the same conditions (8.7). These results were confirmed by extraction of strontium (5 x 10 4 M) from 1 M HN03 solutions, where it was found that p-t-butyl calix[4]arene tetra(di-N-ethyl) amide (CA2) (10 2 M in NPOE) extracts only sodium (DNa = 12.3, DSl < 0.001). 

Structural modifications of the reactive intermediates also alter selectivity. The alkylating agent in the isopropylation of toluene, approximating the i-propyl cation, yields 28.5% ro-i-propyltoluene (Condon, 1948, 1949). The reaction of toluene with t-butyl halides under Friedel-Crafts conditions results in the formation of only 7% ro-t-butyltoluene (Schlatter and Clark, 1953). More precisely, the parajmeta ratio is greater for the more selective tertiary ion than for the more reactive secondary species. These results are in agreement with the expectation of a depressed reactivity for the t-butyl cation as compared to the less stable i-propyl cation. 

Attempting Friedel-Crafts alkylation with primary halides often gives the wrong product by rearrangement of the intermediate cation. If we want to make /-butylbenzene 16, it seems obvious that we should alkylate benzene with an i-butyl halide, e.g. 18 and AICI3. 

Why are intermediates (8) and (9) more stable than intermediates (8 ) and (9/) This can be explained by the inductive effect (I effect) and the hyperconjugation effect. The methyl group has an electron donation ability through the a bond. So, the tert-butyl cation and the terf-butyl radical can be stabilized by the inductive effect of the methyl group (Figure 1.4). Normally, the inductive effect is increased in the following order ... 

Figure 3.53 shows an addition of a carboxybc acid to isobutene, which takes place via the tert-butyl cation. This reaction is a method for forming ferf-butyl esters. Because the acid shown in Figure 3.53 is a /i-hydroxycarboxylic acid whose alcohol group adds to an additional isobutene molecule, this also shows an addition of a primary alcohol to isobutene, which takes place via the ferf-butyl cation. Because neither an ordinary carboxylic acid nor, of course, an alcohol is sufficiently acidic to protonate the alkene to give a carbenium ion, catalytic amounts of a mineral or sulfonic acid are also required here. 

According to Section 5.1.1, electrophilic i/ ,vo-substitutions via sigma complexes occur, for example, when a proton reacts with the substructure Csp2—tert-Bu or C-sp2—S03H of appropriately substituted aromatic compounds. After expulsion of a /erf-butyl cation or an HS03+ ion, an aromatic compound is obtained, which has been defunctionalized in the respective position. 

Methyl vinyl ketone (entry 3) and the tert-butyl cation (entry 4) are also reactive toward complex 3. The naphthalenium complexes resulting from the addition of these electrophiles will add the conjugate base of dimethyl malonate (generated in situ from a combination of dimethyl malonate (DMM) and diisopropylethylamine (DIEA)) to complete the tandem additions. Oxidation of the resulting complexes yields cis-l,4-dihydronaphthalenes. The entire sequence of complexation, tandem addition, and demetalation employed for all entries in Table 4 can be performed using bench-top conditions (i.e., a non-inert atmosphere). 

Production of t-Butyl Cations. Table I outlines six methods for production of t-butyl cations. The first two listed... 

Main Olefin Reactions. Although some olefins react to form t-butyl cations, i.e. to initiate the alkylation steps as shown in Table I, most olefins react by Reactions G through K, as summarized in Table II. Reaction G-1 is the reaction in which either 2-butene or Isobutylene reacts with a t-C Hn to form a TMP . This reaction is of major importance, and it is widely accepted as being a key step in alkylation (9,10). It should be emphasized, however, that the reaction as postulated here can be but is not necessarily part of a chain sequence of reactions. 

Olah (17a) has also reported the alkylation reactions (at -10 with 1 1 HS03F-SbF5) of n-butane with ethylene to yield 38 weight percent of hexanes and of n utane with propylene to yield 29 weight percent of heptanes. The former reaction has also been reported by Parker (31) at 60 , but the product in this case more nearly resembles polyethylene degradation products. In our work with 10 1 HF-TaF5 at 40 , in a flow system, ethylene (14.1 wt.%) reacted with rv-butane to form 3-methyl-pentane as the initial product of 94% selectivity (Scheme 6, path a). The alternative, i.e., the direct reaction of ethylene with a secondary-butyl cation (path b), can be ruled out since butane does not ionize under these conditions (vide supra). 

Sometimes intermediates like [6] undergo hydride shift to another comer protonated cyclopropane (7). This is another mechanism by which carbon scrambling can be achieved, i.e. through a combination of methide- and hydride shifts. Together with 1,2-hydride shifts, such a mechanism accounts for the complete degeneracy of the 2-butyl cation [7]. Vinyl cations substituted... 

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