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

2006

Chem. Commun.

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Free radical chemistry has come a long way in a relatively short period of time. Armed with mechanistic and rate constant data, the synthetic practitioner can now apply free radical chemistry to the synthesis of many different classes of target molecule with confidence. This Feature Article highlights progress made in the understanding and application of free radical reactions at main group higher heteroatoms and demonstrates how this knowledge can be used to construct interesting higher heterocycles, many of which exhibit biological activity, through the use of intramolecular homolytic substitution chemistry.

Section: Discussionmentioning

confidence: 99%

Taming the free radical shrew ? learning to control homolytic reactions at higher heteroatoms

2006

Chem. Commun.

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“…11,16,17 In addition to the computational results mentioned above that suggests the involvement of frontside transition states, we have also reported that the 1,5-translocation of a chiral organosilane from an enoxy oxygen (6) to alkoxy oxygen (7) proceeds with retention of configuration, a result consistent with the involvement of a frontside transition state (Scheme 2). 18 As part of our on-going interest in homolytic substitution involving the main group higher heteroatoms and in order to provide further insight into mechanistic details of homolytic substitution of oxyacyl radicals at the group 14 heteroatoms, we now report the results of a computational investigation into the homolytic substitution of methoxycarbonyl radicals with dimethylsilane, dimethylgermane and dimethylstannane.…”

Section: Introductionmentioning

confidence: 99%

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

Horvat

2014

New J. Chem.

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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 (DE ‡ ) of 93.9 and 103.3 kJ mol À1 are calculated for the backside and frontside reactions, respectively. Similar results are obtained for reactions involving germanium and tin with calculated energy barriers (DE ‡ ) of 89.2 and 99.2 kJ mol À1 for the backside and frontside attack (respectively) at germanium (CCSD(T)/DZP//BHandHLYP/DZP), and 71.8 (backside) and 71.9 kJ mol À1 (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 À1 for attack at silicon, 7-31 kJ mol À1 for attack at germanium, and by 9-30 kJ mol À1 for attack at tin, depending on the level of theory.

“…The concept and utilization of homolytic substitution (S H 2) reactions has advanced in leaps and bounds since the discovery of free radicals by Gomberg at the beginning of the 20th century and as such are now regularly used in organic synthesis. , Work in our laboratories has been focused on the design, application, and understanding of free radical homolytic substitution chemistry with the aim of developing novel synthetic methodology. , To that end, we have published several ab initio studies with the aim of increasing our understanding of factors that affect and control the mechanism of homolytic substitution at several main group higher heteroatoms. − It is generally agreed that homolytic substitution by an attacking radical (R) involves the approach of the radical at the heteroatom (Y) along a trajectory opposite the leaving group (Z). This backside mechanism can proceed either via a transition state 1 in which the attacking and leaving radicals adopt a collinear (or nearly so) arrangement, resulting in Walden inversion, or with the involvement of a hypervalent intermediate 2 , which may or may not undergo pseudorotation prior to dissociation. , …”

Section: Introductionmentioning

confidence: 99%

Ab Initio and DFT Study of Homolytic Substitution Reactions of Acyl Radicals at Silicon, Germanium, and Tin

Horvat

2009

Organometallics

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Ab initio calculations using the 6-311G(d,p), 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) calculations predict that homolytic substitution reactions of acetyl radicals at the silicon atoms in dimethylsilane 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 (ΔE ⧧) of 110.4 and 107.5 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 (ΔE ⧧) of 97.6−191.7 and 100.8−171.3 kJ mol−1 for the backside and frontside mechanisms, respectively. These data suggest that both homolytic substitution mechanisms are feasible for homolytic reactions of acetyl radicals at silicon, germanium, and tin. BHandHLYP calculations provide geometries and energy barriers for backside and frontside transition states in good agreement with those obtained by traditional ab initio techniques.