N,S-Dimethyldithiocarbamyl oxalates as precursors for determining kinetic parameters for oxyacyl radicals

N,S-Dimethyldithiocarbamyl oxalates (e.g.6, 10) are novel, readily prepared precursors to alkyloxyacyl radicals 1 that are more suitable for kinetic studies than existing precursors; 10 has allowed the determination of accurate rate data for the cyclization of the butenyloxyacyl radical 5 (kc = 1.2 X 107 s-1 at 21 °C)

Steric trends and kinetic parameters for radical reductions involving alkyldiphenyltin hydrides

Absolute rate constants and Arrhenius parameters for hydrogen atom abstraction by primary alkyl radicals from methyldiphenyl-, ethyldiphenyl-, butyldiphenyl-, isopropyldiphenyl-, cyclohexyldiphenyl- and (trimethylsilyl)methyldiphenyltin hydride were determined in tert-butylbenzene through utilization of the ‘5-hexenyl radical clock’ reaction. At 80 °C, rate constants (kH) for all hydrides were found to lie in the range (8.2–11.5) × 106 lmol−1 s−1, with similar Arrhenius expressions for all reactions studied [viz. log kH = (8.92–8.97)−(3.03–3.24)/2.3RT]. The nature of the alkyl substituent appears to have a subtle effect on the function of the hydride such that the order or reactivity of stannanes (RPh2SnH) is Me > Et > Bu > i-Pr > c-Hex ≥ Me3SiCH2; this trend can be directly traced to steric effects operating in the transition states for hydrogen transfer from tin to carbon. The implications of these observations are discussed

Stannanes as free-radical reducing agents: an ab initio study of hydrogen atom transfer from some trialkyltin hydrides to alkyl radicals

Ab initio molecular orbital calculations using a (valence) double-ξ pseudopotential (DZP) basis set, with (MP2, QCISD) and without (SCF) the inclusion of electron correlation, predict that hydrogen atoms, methyl, ethyl, isopropyl and tert-butyl radicals abstract hydrogen atoms from stannane and trimethyltin hydride via transition states in which the attacking and leaving radicals adopt a colinear arrangement. Transition states in which (overall) Sn–C separations of 3.50 Å have been calculated; these distances appear to be independent of the nature of the attacking radical and alkyl substitution at tin. At the highest level of theory (QCISD/DZP//MP2/DZP), energy barriers (ωE1‡) of 18–34 kJ mol-1 are predicted for the forward reactions, while the reverse reactions (ωE2‡) are calculated to require 140–170 kJ mol-1. These values are marginally affected by the inclusion of zero-point vibrational energy correction. Importantly, QCISD and MP2 calculations predict correctly the relative order of radical reactivity toward reduction by stannanes: tert-butyl > isopropyl > ethyl. By comparison, SCF/DZP, AM1 and AM1(CI = 2) calculations perform somewhat more poorly in their prediction of relative radical reactivity

Equilibria in free-radical chemistry: An ab initio study of hydrogen atom transfer reactions between silyl, germyl, and stannyl radicals and their hydrides

Ab initio calculations using a (valence) double-ζ pseudopotential (DZP) basis set, with (MP2, QCISD) and without (SCF) the inclusion of electron correlation, predict that the reactions of silyl, germyl, and stannyl radicals with silane, germane, stannane, trimethylsilane, trimethylgermane, and trimethylstannane proceed via transition states of C3v or D3d symmetry in which the attacking and leaving radical centers adopt a collinear arrangement. For reactions involving •SiH3, •GeH3 and •SnH3, energy barriers of between 23.4 (•SiH3 + SnH4) and 86.0 (•SnH3 + SiH4) kJ•mol-1 are predicted at the QCISD/DZP//MP2/DZP (+ ZPVE) level of theory. Specifically, the identity exchange reaction involving silane and the silyl radical is predicted to involve an energy barrier of some 53.6 kJ•mol-1 at the highest level of theory, in good agreement with the available experimental data. The similar reactions involving germyl (•GeH3 + GeH4) and stannyl radicals (•SnH3 + SnH4) are predicted to have energy barriers of 47.0 and 38.9 kJ•mol-1, respectively, at the same level of theory. Inclusion of alkyl substitution on one of the heteroatoms in each reaction serves to alter the position of the hydrogen atom undergoing translocation in the transition state when compared with the unsubstituted series; the reactions of H3Y• with Me3XH become “later” when compared with the analogous parent reaction (H3Y• with XH4). Energy barriers of between 23.8 (•SiH3 + Me3SnH) and 98.3 (•SnH3 + Me3SiH) kJ•mol-1 are predicted at the MP2/DZP (+ ZPVE) level of theory. The mechanistic implications of these computational data are discussed

Silanes and germanes as free-radical reducing agents: an ab initio study of hydrogen atom transfer from some trialkylsilanes and germanes to alkyl radicals

Ab initio molecular orbital calculations using a (valence) double-ζ pseudopotential (DZP) basis set, with (MP2, QCISD) and without (SCF ) the inclusion of electron correlation predict that hydrogen atoms, methyl, ethyl, isopropyl and tert-butyl radicals abstract hydrogen atom from silane, methylsilane, dimethylsilane, trimethylsilane, trisilylsilane and the analogous germanes via transition states in which the attacking and leaving radicals adopt colinear (or nearly so) arrangements. Except for reactions involving trisilylsilane which are predicted at the MP2/DZP level to involve transition states with Si–C distances of about 3.19 Å, transition states which have (overall) Si–C and Ge–C separations of 3.12–3.15 and 3.24–3.26 Å respectively are calculated; these distances appear to be independent of the number of methyl substituents on the group(IV) element, but appear to be slightly sensitive to the nature of the attacking radical, with marginally earlier transition states calculated as the degree of alkyl substitution on the attacking radical is increased. At the highest level of theory (QCISD/DZP//MP2/DZP), energy barriers (ΔE1‡) of 27–57 (Si) or 26–44 (Ge) kJ mol–1 are predicted for the forward reactions, while the reverse reactions (ΔE2‡) are calculated to require 85–134 (Si) or 102–138 (Ge) kJ mol–1. These values are marginally affected by the inclusion of zero-point vibrational energy correction. Importantly, QCISD and MP2 calculations appear to predict correctly the relative ordering of activation energies for alkyl radical reduction by silanes: tertiary < secondary < primary; SCF/DZP, AM1 and AM1 (CI = 2) calculations perform somewhat more poorly in their prediction of relative radical reactivity

Role of catechol in the radical reduction of B-alkylcatecholboranes in presence of methanol

Mechanistic investigations on the previously reported reduction of B-alkylcatecholboranes in the presence of methanol led to the disclosure of a new mechanism involving catechol as a reducing agent. More than just revising the mechanism of this reaction, we disclose here the surprising role of catechol, a chain breaking antioxidant, which becomes a source of hydrogen atoms in an efficient radical chain proces

Radical Cyclisation of α-Halo Aluminium Acetals: A Mechanistic Study

International audienceα‐Bromo aluminium acetals are suitable substrates for Ueno–Stork‐like radical cyclisations affording γ‐lactols and acid‐sensitive methylene‐γ‐lactols in high yields. The mechanistic study herein sets the scope and limitation of this reaction. The influence of the halide (or chalcogenide) atom X (X=Cl, Br, I, SPh, SePh) in the precursors α‐haloesters, as well as influence of the solvent and temperature was studied. The structure of the aluminium acetal intermediates resulting from the reduction of the corresponding α‐haloesters has been investigated by low‐temperature 13C‐INEPT diffusion‐ordered NMR spectroscopy (DOSY) experiments and quantum calculations, providing new insights into the structures of these thermally labile intermediates. Oxygen‐bridged dimeric structures with a planar Al2O2 ring are proposed for the least hindered aluminium acetals, while monomeric structures seem to prevail for the most hindered species. A comparison against the radical cyclisation of aluminium acetals derived from allyl and propargyl alcohols with the parent Ueno–Stork has been made at the BHandHLYP/6‐311++G(d,p) level of theory, highlighting mechanistic similarities and differences