Secondary deuterium isotope effects in radical-forming reactions. IV. Concerted perester decompositions

Abstract

The secondary deuterium isotope effects on the rates of decomposition of /-butyl peresters of phenylacetic (1), hydratropic (2), , -dimethylphenylacetic (3), and pivalic acids have been determined by infrared, galvinoxyl, and double-labeling methods. The a effects for 1 and 2 are 1.03-1.06 per deuterium. The ß effects for the series are 1.014-1.020 per deuterium. The implications of these effects with respect to the detailed mechanisms of the reactions are discussed. Preliminary investigations2 of secondary deuterium kinetic isotope effects on the decomposition of /-butyl perphenylacetate (1) and /-butyl perhydratropate (2) were carried out as control experiments to test the validity of our expectation that concerted decomposition rates would be sensitive to deuteration. This expectation was based on the idea that the benzylic carbon atoms of the peresters would approach sp2 hybridization in the transition states. The bending frequencies associated with the a-hydrogen atoms might therefore be expected to be reduced.3 The studies of Seltzer on the a effects in azo compound decom-position4 as well as the ß scission of cumyloxy radical5 have indicated that these a effects are of the order of 1.10-1.15 per deuterium atom for these radical-forming reactions. The observed a effects in the preliminary studies were within this expected range while that for the decomposition of acetyl peroxide was much smaller. We therefore concluded that the small value for the latter reaction must be interpreted in terms of a nonconcerted process.The ß effect in the perhydratropate case was also ex-(1)

The mechanism of the photo-decarboxylation of alkyl- and arylmethyl-carboxylates using visible light and iron(III) tetra(2-N-methylpyridyl)porphyrin pentachloride in aqueous solution

“…Our results (Table 2) show that within experimental error (±3%) the two propionic acids react at the same rate (k H /k D ~1.0), suggesting that the rate of iron() porphyrin formation depends on the rate of homolytic cleavage of the iron-ligand bond [step (ii)] and not decarboxylation to give the alkyl radical. In agreement with this conclusion Koenig and Wolff 25 Further support for the photo-reaction proceeding via step (ii) rather than step (v) comes from a plot of the log k obs values for the alkylcarboxylic acid reactions against the Taft σ* constant of the alkyl group which gives a good linear correlation (r = 0.980) with a ρ* value of Ϫ6.35 ± 0.735 (Fig. 2).…”

Peroxyester Decarboxylation Studied by Picosecond Transient IR Spectroscopy

“…It is suggested that thermal decomposition of tert -butyl peroxyesters occurs by concerted bond cleavage if a highly stable radical may be formed . The solvent viscosity dependence, secondary deuterium isotope effects, and pressure dependence of thermal decomposition rates support this assumption for a series of tert -butyl peroxyesters, such as tert -butyl diphenylperacetate, tert -butyl p -methoxyphenylperacetate, and tert -butyl p -methylphenylperacetate. However, intermediate aroyloxy radicals were observed following laser flash photolysis of, e.g., tert -butyl peroxyesters R−C(O)O−O t Bu, with R = phenyl and 9-methylfluorene …”

Ultrafast Decarboxylation of Carbonyloxy Radicals: Influence of Molecular Structure

“…This aspect has been addressed by several groups. [6][7][8][9][10][11][12] The question about the mechanism is obviously related to the time scale of observation. On the basis of thermal experiments, concerted bond breaking and stepwise dissociation via intermediate radicals on a picosecond or nanosecond time scale cannot be safely distinguished.…”