Sonia M. Horvat

An ab initio and DFT study of homolytic substitution reactions of acyl radicals at sulfur, selenium, and tellurium

Ab initio calculations using the 6-311G**, cc-pVDZ, aug-cc-pVDZ, and (valence) double-ζ pseudopotential (DZP) basis sets, with (MP2, ROMP2, QCISD, CCSD(T)) and without (HF) the inclusion of electron correlation, and density functional (BHandHLYP and B3LYP) calculations predict that homolytic substitution reactions of acetyl radicals at the sulfur, selenium and tellurium atoms in dimethyl sulfide, dimethyl selenide, and dimethyl telluride adopt an almost collinear arrangement of attacking and leaving radicals at the chalcogen atom. Energy barriers (ΔE‡) for these reactions range from 76.5 (attack at S, BHandHLYP/6-311G**) to 35.5 kJ mol−1 (attack at Te, BHandHLYP/DZP). While the calculated energy barriers for the forward and reverse energy barriers for substitution of acetyl radical at the sulfur atom are comparable, the reverse reactions are favoured by 3–14 kJ mol−1 for attack at selenium and by 20–25 kJ mol−1 for attack at tellurium.

Methyl radical also reacts by the frontside mechanism: An ab initio study of some homolytic substitution reactions of methyl radical at silicon, germanium and tin

Ab initio calculations using 6-311G**, cc-pVDZ, aug-cc-pVDZ, and a (valence) double-zeta pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, and density functional (B3LYO) calculations predict that homolytic substitution reactions of the methyl radical at the silicon atom in disilane can proceed via both backside and frontside attack mechanisms. At the highest level of theory (CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ), energy barriers (delta E) of 47.4 and 48.6 kJ mol-1 are calculated for the backside and frontside reactions respectively. Similar results are obtained for reactions involving germanium and tin with energy barriers (delta E) of between 46.5 and 67.3, and 41.0 and 73.3 kJ mol-1 for the backside and frontside mechanisms, respectively. These data suggest that homolytic substitution reactions of methyl radical at silicon, germanium, and tin can proceed via either homolytic substitution mechanism.

Understanding Homolytic Translocation Chemistry—An Ab Initio Study of Some 1,5-Transfers of Silyl, Germyl and Stannyl Groups of Synthetic Significance

Ab initio molecular orbital and density functional calculations predict that 1,5-homolytic translocation reactions involving silyl, germyl and stannyl groups between two alkyl carbon atoms, between alkoxy oxygen atoms, and between alkyl and allylic carbon atoms proceed via concerted mechanisms involving frontside substitution at the higher heteroatom involved; CCSD(T)/DZP//B3LYP/DZP calculations predict energy barriers ranging from 69 to 114 kJ mol–1 depending on the system involved.

Rate coefficients for intramolecular homolytic substitution of oxyacyl radicals at sulfur

It is predicted on the basis of ab initio and density functional calculations that intramolecular homolytic substitution of oxyacyl radicals at the sulfur atom in ω-alkylthio-substituted radicals do not involve hypervalent intermediates. With tert-butyl as the leaving radical, free energy barriers ΔG‡ (G3(MP2)-RAD) for these reactions range from 45.8 kJ mol–1 for the formation of the five-membered cyclic thiocarbonate (8) to 56.7 kJ mol–1 for the formation of the six-membered thiocarbonate (9). Rate coefficients in the order of 104–106 s–1 and 101–104 s–1 for the formation of 8 and 9, respectively, at 353.15 K in the gas phase are predicted at the G3(MP2)-RAD level of theory.

An ab initio and DFT study of some homolytic substitution reactions of methoxycarbonyl radicals at silicon, germanium, and tin

Homolytic substitution reactions of methoxycarbonyl radicals at the silicon, germanium, and tin atoms in various dialkylsilanes, dialkylgermanes and dialkylstannanes have been investigated using computational techniques. Ab initio and DFT calculations predict that attack of methoxycarbonyl radical at the silicon containing molecules can proceed via both backside and frontside attack mechanisms. At the (CCSD(T)/DZP//BHandHLYP/DZP) level of theory, energy barriers (ΔE‡) of 93.9 and 103.3 kJ mol−1 are calculated for the backside and frontside reactions, respectively. Similar results are obtained for reactions involving germanium and tin with calculated energy barriers (ΔE‡) of 89.2 and 99.2 kJ mol−1 for the backside and frontside attack (respectively) at germanium (CCSD(T)/DZP//BHandHLYP/DZP), and 71.8 (backside) and 71.9 kJ mol−1 (frontside) for the analogous reactions at tin. These data suggest that both homolytic substitution mechanisms are feasible for homolytic reactions of methoxycarbonyl radicals at silicon, germanium, and tin. These homolytic substitution reactions are also predicted to be endothermic at all levels of theory with the reverse reaction favoured by 11–37 kJ mol−1 for attack at silicon, 7–31 kJ mol−1 for attack at germanium, and by 9–30 kJ mol−1 for attack at tin, depending on the level of theory.

Polysilane and related radical rearrangements: an ab initio study of (1,2)-silyl, germyl and stannyl translocations in radicals derived from trisilanes and related species

Ab initio molecular orbital calculations using a (valence) double-ζ pseudopotential basis set (DZP) with (MP2, QCISD) and without (SCF) the inclusion of electron correlation predict that the transition states (5, 7) involved in homolytic (1,2)-translocation reactions of silyl (SiH3), germyl (GeH3) and stannyl (SnH3) groups between silicon and other group (IV) centres proceed via homolytic substitution mechanisms involving frontside attack at the heteroatom undergoing translocation. At the highest level of theory (CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ), an energy barrier (ΔE‡) of 135.9 kJ mol−1 is calculated for the translocation of SiH3 between silicon centres; this value is 143.8 kJ mol−1 at the CCSD(T)/DZP//MP2/DZP level. Similar results are obtained at the CCSD(T)/DZP//MP2/DZP level of theory for reactions involving germanium and tin with values of ΔE‡ of 146.5 and 129.1 kJ mol−1 respectively for the rearrangements of trigermapropyl and tristannapropyl radicals respectively. These data strongly suggest that homolytic (1,2)-translocation reactions are unlikely to be involved in the free-radical degradation of polysilanes, polygermanes and polystannanes. CCSD(T)/DZP//MP2/DZP calculated energy barriers associated with mixed systems range from 108.1 kJ mol−1 for the (1,2)-translocation of SnH3 from tin to silicon, to 181.0 kJ mol−1 for the similar migration of SiH3 from silicon to tin. The mechanistic implications of these observations are discussed.

Homolytic substitution chemistry: from James Franz to antihypertensives

There was a time when free radicals were scorned by organic chemists and when “practically every organic text book written” contained a statement that free radicals were “incapable of an independent existence”.

Homolytic 1,5-transfer of chiral organosilicon groups from an enoxy oxygen to an alkoxy oxygen—implications for mechanism

Reaction of the optically active silanes, ((Ssi)-(-)-6), formed by treatment of racemic 2-methylenecycloheptanone oxide with LDA followed by (R)-(+)-chloromethyl(1-naphthyl)-phenylsilane, with tributyltin hydride under standard radical conditions affords (2R/2S)-[(S)-(methyl(1-naphthyl)-phensylsilyloxy)methyl]cycloheptanone, (Ssi)-(-)-7, providing strong evidence that homolytic 1,5-transfers of organosilicon groups from enoxy oxygen to alkoxy oxygen proceed with retention of configuration, most likely through a frontside attack mechanism rather than via a hypervalent intermediate.

Reactions of β-trimethylstannylcyclohexanones with peracids: investigations into the stannyl-directed Baeyer–Villiger reaction

The trimethylstannyl substituent raises the migratory aptitude of a primary β-carbon to be above that of a not otherwise activated secondary or tertiary carbon. This is apparent from the exclusive formation of the alkene acids 9–11 from Baeyer–Villiger reaction of the β-stannyl cyclohexanones 3–5. The stereoelectronic requirements of the stannyl-directed Baeyer–Villiger reaction were investigated using the axial β-trimethylstannylcyclohexanone 20.