71117-29-8Relevant academic research and scientific papers
Visible Light Photocatalytic Reduction of O-Thiocarbamates: Development of a Tin-Free Barton-McCombie Deoxygenation Reaction
Chenneberg, Ludwig,Baralle, Alexandre,Daniel, Marion,Fensterbank, Louis,Goddard, Jean-Philippe,Ollivier, Cyril
supporting information, p. 2756 - 2762 (2016/02/19)
The Barton-McCombie deoxygenation is one of the most important transformations in the toolbox of organic chemists which has been the subject of a number of methodological developments. In this study, we report a photocatalyzed redox deoxygenation of secondary and tertiary alcohols from thiocarbamate precursors under visible light activation. The iridium complex Ir(ppy)3 proved to be the most efficient catalyst in the presence of Hünig's base as sacrifial electron donor. A mechanistic investigation is presented based on fluorescence quenching experiments and cyclic voltammetry.
A non-cope among the cope rearrangements of 1,3,4,6-tetraphenylhexa-1,5- dienes
Doering,Birladeanu, Ludmila,Sarma, Keshab,Blaschke,Scheidemantel, Ursula,Boese,Benet-Bucholz, Jordi,Klaerner,Gehrke, Jan-Stephan,Zimny, Bernd Ulrich,Sustmann,Korth, Hans-Gert
, p. 193 - 203 (2007/10/03)
The set of 1,3,4,6-tetraphenylhexa-1,5-dienes (1) represents a perturbation of Cope's rearrangement by four radical-stabilizing phenyl groups all positioned to drive the transition region toward the homolytic- colligative end of the mechanistic spectrum. The appearance of (Z)-isomers being suppressed thermodynamically by a steric interaction of +2.6 kcal mol- 1 per cis double bond, an equilibration that is stereochemically not of any Cope type, emerges as the predominant reaction. It is an interconversion of rac-(E,E)-1 and meso-(E,E)-1 (48:52; 77.3-115.3 °C) with the following values of the enthalpy, entropy, and volume of activation: ΔH(+) = 30.7 ± 0.2 kcal mol-1, ΔS(+) = +2.1 ± 0.4 cal mol-1 K-1, and ΔV(+) = +13.5 ± 0.1 cm-3 mol-1, respectively. Structures have been established by X- ray crystallographic analysis; a possible relationship between dihedral angle and bond lengths in the styrene portions is proposed. The entropy of activation is incompatible with a chair or boat Cope rearrangement; the volume of activation is neither low enough for a pericyclic Cope ('concerted') mechanism nor high enough for a homolytic-colligative mechanism involving full dissociation as the rate-determining step. Trapping and a crossover experiment give some but only partial support to the intermediacy of free radicals. At higher temperatures, however, electron spin resonance experiments demonstrate an equilibrium with kinetically free (E,E)-1,3- diphenylallyl radicals. These observations are rationalized in terms of geometric reorganization within the confines of a 'cage'. Resolution by chiral chromatography of rac-(E,E)-1 allows recognition of a fast racemization (40-65 °C), of which ΔH(+) (21.3 ± 0.1 kcal mol-1), DS(+) (-13.2 ± 0.3 cal mol-1 K-1), and ΔV(+) (-7.4 ± 0.4 cm-3 mol-1) are consistent with a pericyclic Cope rearrangement. Enriched (Z)-isomers undergo Cope rearrangements in accord with the known influence of axiality and the chair/boat alternative on the energy of the transition region.
Van der Waals complexes, intermediates in unimolecular reactions
Roth, Wolfgang R.,Hunold, Frank
, p. 1917 - 1928 (2007/10/03)
Thermolysis of rac- and meso-1,1′,2,2′,3,3′-hexamethyl-1,1′-biindenyl and (1E,5E)-1,3,4,6-tetraphenyl-1,5-hexadiene in supercritical CO2 in the presence of oxygen allows to identify three types of intermediates, the free radicals (2 and 5) and the equilibrating complexes I and II consisting of radical pairs. From the oxygen dependence of the trapping and rearrangement rates it is shown, that the complex I is the primary thermolysis product, which in contrast to the complex II does not react with oxygen. The temperature dependance of these rates allows to calculate the enthalpy well of the complex II, which agrees well with predictions of our reaction force field. VCH Verlagsgesellschaft mbH, 1996.
