Rate Constants and Arrhenius Parameters for the Reaction of Acyl Radicals with Bu3SnH and (Me3Si)3SiH

Abstract

The kinetics of hydrogen abstraction from BuaSnH and (MeaSi)aSiH by a variety of acyl radicals have been studied in nonpolar solvents over a range of temperatures by using competing decarbonylation reactions. The relative preexponential factors are essentially independent of the nature of the acyl radical and of the hydride; i.e. logíAd/Amn) a* 4.8. On the other hand, the relative activation energies, i.e. E¿ -£mh, decrease by ca. 2 kcal mol-1 on going from primary to secondary and also on going from secondary to tertiary substituents at the carbonyl moiety for a particular hydride. As one goes from BusSnH to (MesSihSiH for a particular acyl radical, the Ed -Emu also decreases by ca. 2 kcal mol-1. Combining our relative kinetic data with the gas-phase Arrhenius parameters for the propanoyl radical decarbonylation allows for the evaluation of the absolute Arrhenius expressions for the reactions of acyl radicals with BuaSnH and (MesSihSiH and for the decarbonylation of secondary and tertiary acyl radicals. Rate constants at 80 °C for the reactions of (Mes-Si)8Si* and BusSn* radicals with acyl derivatives have also been estimated.

Chemistry of Acyl Radicals

“…A combination of the relative kinetic data with the Arrhenius parameters for the propanoyl radical decarbonylation allows the corresponding data for the decarbonylation of secondary and tertiary alkyl-substituted acyl radicals to be obtained (Table 8) as well as the activation param- eters for the reaction of acyl radicals with Bu 3 SnH and (TMS) 3 SiH (Scheme 1). 75 In this respect, the absolute rate constant for the reaction of the propanoyl radical with Bu 3 SnD was found to be 3.0 × 10 5 M -1 s -1 at 23°C. 55 On the basis of the kinetic data in Scheme 1, we calculate a deuterium kinetic isotope effect of 1.4.…”

“…2 kcal mol -1 on going from primary to secondary and also on going from secondary to tertiary substituents at the carbonyl moiety for a particular hydride. 75 When going from Bu 3 SnH to (TMS) 3 SiH for a particular acyl radical, the E d -E MH also decreases by ca. 2 kcal mol -1 .…”

“…189 The rate constant for reaction of the (TMS) 3 Si • radical with acyl chlorides is estimated to be 7 × 10 5 M -1 s -1 at 80°C. 75 Kagan and co-workers studied the reaction of acyl chlorides with samarium diiodide and dicyclopentadienylsamarium. 116,117 When phenylacetyl chloride was exposed to dicyclopentadienylsamarium at room temperature, an 85% yield of dibenzyl was obtained (Scheme 11), which strongly supports the notion that acyl radicals are intermediates in this chemistry (see section IX.B.1).…”

Dual Hypervalent Iodine(III) Reagents and Photoredox Catalysis Enable Decarboxylative Ynonylation under Mild Conditions

“…Heterocyclic thiophenes,f urans,a nd indoles reacted smoothly to give ynones [17][18][19].N otably,a lkyl-substituted ketoacids decarboxylated to give coupling adducts:P rimary and secondary alkyl ketoacids gave ynones without decarbonylation in 66 %a nd 82 % yields,r espectively (20,21), while the tertiary alkyl ketoacid gave the dual decarboxylative-decarbonylative alkyne coupling adduct 22 in 73 %y ield. [15] In addition to aryl-and alkyl-substituted a-ketoacids,c arbamoyl ketoacids readily reacted to give ynamides:P rimary carbamoyl ketoacids reacted to give,inyields of 61-69 %, primary ynamide products, which could not be obtained by transition-metalcatalyzed aminocarbonylation because of the catalytic inhibition of primary amines by transition metals (23)(24)(25)(26)(27)(28).…”

Silyl Radicals

“…Phenyl selenoesters have been reported to undergo reduction to the corresponding aldehydes and/or alkanes in the presence of TMS 3 SiH under free-radical conditions 77 . The decrease of aldehyde formation through the primary, secondary and tertiary substituted series, under the same conditions, indicated that a decarbonylation of acyl radicals takes place.…”