Pivaloyl cation

Alkyl cations can also be generated by decarbonylation of tertiary acylium ions, like the pivaloyl cation 26 [Eq. (3.20)].91 This reaction corresponds to the reverse of Koch-Haaf acid synthesis, which is known to involve carbocation intermediates. Indeed the reaction of the terf-butyl cation with carbon monoxide gives the pivaloyl cation.91 135... 

Alkyl cations can also be generated by decarbonylation of tertiary acylium ions, like the Pivaloyl cation 1521). 

As expected, the reaction products after decompression and quenching with hydrochloric acid and ice water were pivalic acid, terr-butyl isobutenyl ketone (in the absence of a hydride donor, this unsaturated ketone replaced the saturated congener, and no isopropyl methyl ketone by rearrangement of the pivaloyl cation could result). An unexpected crystalline material also appeared in small yield when the aqueous solution was saturated with sodium chloride for facilitating the extraction of organic reaction products. This proved to be a pyrylium salt, as will be shown below. 

The pivaloyl cation that can be formed from Me3CCl, AICI3, and CO at high pressure undergoes decarbonylation at normal pressure when it is prepared from pivaloyl chloride and most Lewis acids. Only in the presence of SnCU can one diacylate isobutene (from Me3CCl) with pivaloyl chloride to form 3 as chlorostannate, which can then be converted into other salts on... 

In exceptional circumstances the acylium ion (or the polarised complex) can decompose to give an alkyl cation so that alkylation accompanies acylation. This occurs in the aluminium chloride-catalysed reaction of pivaloyl chloride which gives acylation with reactive aromatics such as anisole, but with less reactive aromatics such as benzene, the acylium ion has time to decompose, viz. 

If R is tertiary, RCOmay lose CO to give R, so that the alkylarene ArR is often a side product or even the main product. This kind of cleavage is much more likely with relatively unreactive substrates, where the acylium ion has time to break down. For example, pivaloyl chloride (McaCCOCl) gives the normal acyl product with anisole, but the alkyl product MesCPh with benzene. In the other mechanism an acyl cation is not involved, but the 1 1 complex attacks directly. 

Sometimes acylium ions lose carbon monoxide to generate an ordinary carbonium ion. It will be recalled that free acyl radicals exhibit similar behavior at high temperatures. Whether or not the loss of carbon monoxide takes place seems to depend on the stability of the resulting carbonium ion and on the speed with which the acylium ion is removed by competing reactions. Thus no decarbonylation is observed in Friedel-Crafts reactions of benzoyl chloride, the phenyl cation being rather unstable. But attempts to make pivaloyl benzene by the Friedel-Crafts reaction produce tert-butyl benzene instead. With compound XLIV cyclization competes with decarbonylation, but this competition is not successful in the case of compound XLV in which the ring is deactivated.263... 

The N-pivaloyl group is held (85JOM1) in a planar conformation in derivative 134 and a planar chelate ring, 135, is also established in the presence of a magnesium cation. 

We have already encountered selective derivatizations, for example with chloro-triphenylmethane and f-butylchlorodiphenylsilane which etherify selectively the primary alcohols. Pivaloyl chloride, McjCCOCl, also displays a preference for this position. Selective reactions can be observed with the mildly benzoylating reagent W-benzoylimidazole 5.4. When it is a matter of differentiating between secondary positions, it can be advantageous to go through intermediates which are formally alkoxides of the dibutyltin cation, Bu2Sn2. ... 

A second synthetic example illustrates several useful features of cation rearrangements. When 121 (OPiv is a pivaloyl ester) was treated with p-toluenesulfonic acid in methanol, protonation occurred at the more basic... 

A complex of pivaloyl chloride (Me3C-CO-Cl) and DMF quickly converts ketoximes to amides/lactams at room temperature. The complex is proposed to be of the Vilsmeier-Haack-type, that is, Cl-Me2 N=CH-02C-Bu-f, in equilibrium with a cation, Me2" N=CH-Cl, which reacts with the oxime. ... 

The oxidation of sterically hindered disilanes by trityl cation in pivaloyl nitrile was shown to yield silylium ions which form with the solvent silylnitrilium ions, e.g., 38 (Scheme 14) [53]. The generatitMi of silyl cations was, however, only observed for sterically overloaded disilanes, such as hexa-tcrt.-butyldisilane. Hexaethyldisilane for example was found to be inert under the applied reaction conditions [53], In view of the problems with bulky silanes in the standard hydride-transfer reaction, the oxidation of disilanes is a complementary addendum to the synthetic methodology. The very limited preparative access to stable radicals restricts the oxidation of silyl radicals to give silylium ions to only a few selected examples. The most prominent example is the oxidation of the stable radical 39 to the homoaromatic silylium ion 40 by trityl cation (Scheme 15) [23]. 

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