1200-10-8Relevant academic research and scientific papers
THE EFFECT OF ARYL SUBSTITUENTS ON ARYLCARBENE REACTIVITY
Tomioka, Hideo,Tabayashi, Kazuo,Ozaki, Yasuji,Izawa, Yasuji
, p. 1435 - 1440 (2007/10/02)
Substituted (p-MeO, p-Me, H, p-Cl, p-Br, m-Br, m-MeO, 3,4-Cl2, p-CO2Me, m-CN and p-CN) monophenylcarbenes are generated in a binary mixture of substrates (methanol, cis-4-methyl-2-pentene and cyclohexane) and the relative rate of O - H insertion into methanol to stereospecific cyclopropanation of the olefin to C - H insertion into cyclohexane are calculated from the ratios of products and substrates.It is found (i) that the reactivities of the substrates decrease in the order of methanol, olefin and cyclohexane and (ii) that electron-donating substituents generally lead to reaction with the more reactive substrates while the reaction with the less reactive substrates is favoured in the case of electron-withdrawing substituents.These results are interpreted in terms of the change in the electrophilicity of the singlet arylcarbene by the substituents rather than the change in the singlet-triplet equilibrium.
Effects of Substituents and Generation Methods on Insertion-Addition Selectivities of ''Arylcarbene'' in Alcohol-Olefin Binary Mixtures. Intervention of Reaction of Diazo Compounds Masquerading as Carbenes
Tomioka, Hideo,Suzuki, Shinji,Izawa, Yasuji
, p. 1047 - 1050 (2007/10/02)
A Hammett study of the insertion-addition selectivity (Ki/ka) and cyclopropanation stereoselectivity (kc/kt) of "arylcarbene" generated either photolytically or thermally in 2-propanol-ethyl vinyl ether binary mixtures showed that ρ values are highly sensitive to the generation method.Thus, plots of ki/ka and kc/kt vs. ? (?+) in the photolytic run gave ρ values of -0.96 (r = -0.96) and -0.15 (r = -0.95), respectively, whereas similar values are +1.4 (r = 0.93) and -1.1 (r = -0.96) in the thermal run.The results along with the effects of precursor, temperature, and sensitizer on the product distributions are interpreted as indicating that, while free carbene is involved in the photolytic run, the ground-state diazo compound is masquerading as carbene in its thermal reaction with the olefin.
Temperature and Substituent Effects on Regioselectivity in the Insertion of Arylcarbene into Alcohols
Tomioka, Hideo,Suzuki, Shinji,Izawa, Yasuji
, p. 3156 - 3162 (2007/10/02)
Photolysis of aryldiazomethanes in methanol, ethanol, and 2-propanol gave OH insertion products along with small amounts of CH insertion products at ambient temperature.However, the CH insertion products increased significantly at the expense of the ether as the temperature was lowered.The attempted sensitized decomposition of the diazomethane did not lead to an increase in the CH insertion products presumably because of a rapid singlet-triplet equilibrium.The key intermediate leading to the CH insertion is suggested to be ground-state triplet arylcarbene, based on the accumulated spectroscopic as well as chemical evidence for the intervention of the triplet arylcarbene in the low-temperature photolysis of aryldiazomethanes.Substituents on the phenyl ring also have an appreciable effect on the insertion selectivity.At room temperature, the OH/CH insertion selectivity increased with the electron-donating ability of the substituents.This is interpreted in terms of the substituent effect on the transition state of OH insertion, where there is a deficiency of electrons at the benzylic carbon atom, rather than on the stability of singlet carbene.At low temperature, both electron-donating and -withdrawing substituents facilitate OH insertion, indicating the change in substituents induces a concomitant change in the insertion mechanism, presumably due to decreasing nucleophilicity of carbene with increasing electron-withdrawing ability as well as decreasing proton-donor activity of alcohol with decreasing temperature.This may also reflect the effect of the substituent on the singlet-triplet energy gap.
