Radical pivaloyl

The high reactivity of protonated heteroaromatic bases towards acyl radicals is shown by the success of the reaction with the pivaloyl radical, which usually undergoes rapid decarbonylation [Eq. (33)]. 

Protonated heteroaromatic bases are therefore more reactive than simple olefins toward acyl radicals. The radical addition of pivalaldehyde to olefins is, in fact, characterized by a radical chain, whose propagation is determined by decarbonylation of the pivaloyl radical and addition of <-butyl radical to the olefin. The synthetic interest is great in the case of substrates with only one reactive position, such as benzothiazole, ... 

In designing multicomponent coupling reactions, the nature of the individual components is obviously a key factor. Generally speaking, carbon radical species, such as alkyl radicals, aryl radicals, vinyl radicals, and acyl radicals are all classified as nucleophilic radicals, which exhibit high reactivity toward electron-deficient alkenes [2]. To give readers some ideas about this, kinetic results on the addition of tert-butyl and pivaloyl radicals are shown in Scheme 6.2. These radicals add to acrylonitrile with rate constants of 2.4 x 106 M-1 s 1 and 5 x 105 M-1 s-1 at... 

An acyl radical is also nucleophilic. For example, the rate constant of (CH3)3CC0 (te/T-butylcarbonyl radical, pivaloyl radical) with acrylonitrile is 4.8 X 105 M-1 s-1 (25 °C), and so its addition reaction proceeds effectively [72]. 

Decarbonylation of the pivaloyl radical, to afford the stable terf-butyl radical is rapid. 

Few examples of direct comparisons of rates of reaction of different radicals with a common species are in the literature. In one, the (nondelocalized) tert-butyl radical was found to react more rapidly than pivaloyl radical with an electron-deficient partner, acrylonitrile, in 2-propanol. This is not a good analogy to the comparison between 1-phenylethyl and 2-phenylpropanoyl being made here because we suspect that 1-naphthoxy is more electron-rich than acrylonitrile, polyethylene is much less polar than 2-propanol, and the odd-electron in a 1-phenylethyl radical is delocalized, (a) Jent, F. Paul, H. Roduner, E. Heming, M. Fischer, H. Int. J. Chem. Kinet. 1986, 18, 1113. 

The decarbonylation of pivaloyl radicals in aqueous solution, k - 2.5 X 10 s 147) is somewhat slower than in most non-aqueous solvents 148), but sufficiently rapid to compete with the reaction with O2 in Eq. (21) under most conditions. In the absence of O2, most of the acyl radicals produced in Eq. (19) will dissociate CO, Eq. (20), and react with CraqOO Eq. (21). A small fraction may disappear in bimolecular radical self-reactions. 

TABLE 48.4 Rate Constants for Decarbonyiation of Phenylacetyl and Pivaloyl Radicals at Ambient Temperatures in Different Solvents... 

The high decarbonylation efficiency of di-t-butyl ketone 8 impHes that the acyl and alkyl fragments of the primary pair can separate and remain separated as free radicals long enough for the pivaloyl radical to lose CO with a time constant of -1.2 to 1.5 ps." " In contrast, the decarbonylation efficiency of the cyclic ketone analogs in Scheme 5 is remarkably reduced by the average proximity of the two radical termini, which facilitates faster radical-radical reactions. 

Schuh, H., Hamilton, E. J., Paul, H. and Fisher, H., Reaction Rates of t-Butyl and Pivaloyl Radicals... 

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 reaction of phosphorous acid with pivaloyl chloride in pyridine has been studied three eq. of pivaloyl chloride gave the triacyl phosphite (152). Thiophos-phinltes or monothiophosphites (153) rearrange spontaneously to the thiophosphoryl isomer (154) in the presence of oxygen. The rearrangement was inhibited by the addition of radical inhibitors. Irradiation of a mixture of naphthalene or phenanthrene and trialkyl phosphites in the presence of 1,3-dicyanobenzene gave several isomeric monophosphonates. ... 

Dimethylpyrrolidine and its analogs are efficient stereocontrol elements, since the C2 axis of the pyrrolidine makes the conformation about the carbonyl-nitrogen bond irrelevant. In radicals such as 27, both conformations provide essentially the same stereochemical environment for the radical center. An example of the use of dimethylpyrrolidine, is shown in Fig. 4. In this transformation, a /cr/-butyl ETOC ester is reacted with the acryloyl carboxamide of dimethylpyrrolidine and the addition product is isolated in excellent yield and selectivity. The propagation sequence involves addition of a tert-h xty radical to the acrylamide, trapping of the adduct radical, 27, by the PTOC ester, and decarboxylation of the pivaloyl carboxy radical. This transformation may be initiated thermally or photochemically, and photoinitiation at reduced temperatures gives product with higher diastereoselectivity. A... 

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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