Organic Reducing Agents: Some Radical Chain Reactions of Ketyl and 1,3-Dioxolanyl Radicals with Activated Bromides
摘要:
The radical chain reduction of primary and secondary alpha-bromo esters by 2-propanol and 2-methyldioxolane has been shown to be a kinetically viable process with average chain lengths of >10 at temperatures between 30 and 60 degrees C. For electron-deficient bromides, these simple organic reagents may effectively replace the more commonly used (and more expensive) organosilicon or tin hydides. Some mechanistic insights have been obtained from a combination of kinetic and thermodynamic measurements. The rate constants for the reaction of the 2-hydroxy-2-propyl and the 2-methyl-1,3-dioxolan-2-yl radicals with a primary bromo ester have been estimated to be 8 x 10(3) M(-1) s(-1) and 6 x 10(3) M(-1) s(-1), respectively, at 30 degrees C in acetonitrile. The electrochemical oxidation potentials of a number of electron-rich radicals derived from hydrogen atom abstraction from alcohols and dioxolanes, determined using photomodulation voltammetry, provide an assessment of the thermochemisty of the putative outer sphere electron-transfer reaction between the radical and the alpha-bromo ester (the value for 1,3-dioxolane has been redetermined and corrects an erroneous value previously published). From these data, it is shown that the order of reactivity of the organic reducing agents follows the same trend as the oxidation potentials of the corresponding radicals. Rate constants for the outer sphere electron-transfer reduction of the bromo esters by electrochemically generated radical anions were used to establish a Marcus-type relationship between the rate constants and the standard potential of the reducing agent. Comparison of these rate constants with those for the reactions of the electron-rich radicals suggest that the reactions of the methanol; and ethanol-derived radicals do not proceed via outer sphere electron transfer as had been previously suggested but have significant bonding (i.e., atom transfer) character at the transition state. Electron transfer in the reactions of the 2-propanol- and 2-methyl-1,3-dioxolane-derived radicals cannot be ruled out. Nevertheless, the order of reactivity of the organic reducing agents follows the same trend as the oxidation potentials of the corresponding radicals suggesting that these potentials can be used as a predictive tool for the design of new reagents.
Ruthenium(II)-carboxylate-catalyzed C4/C6–H dual alkylations of indoles
作者:Isaac Choi、Valentin Müller、Lutz Ackermann
DOI:10.1016/j.tetlet.2021.153064
日期:2021.5
C4/C6–H double alkylations of indoles accomplished by arene-ligand-free ruthenium(II)-carboxylate catalysis. Base-assisted C2–H ruthenation enabled the C4/C6–H dialkylations with ample substrate scope. The robust C4/C6–H alkylations were demonstrated by gram-scale syntheses and late-stage diversifications. Mechanistic studies unraveled the unique features of ruthenium(II)-catalyzed C4/C6–H indole functionalizations
Carbon-Centered Radical Addition to OC of Amides or Esters as a Route to CO Bond Formations
作者:Dong Liu、Shan Tang、Hong Yi、Chao Liu、Xiaotian Qi、Yu Lan、Aiwen Lei
DOI:10.1002/chem.201404607
日期:2014.11.17
typical applied unsaturated substrates include alkenes, alkynes and imines. Carbonyl is perhaps the most common unsaturated group in nature. This work demonstrates a novel CO bond formation through carbon‐centered radical addition to the carbonyloxygen of amide or ester, in which amide and ester groups are easily activated through the radical process. EPR spectroscopy and radical clock experiments
Organic reducing agents. Reduction of electron deficient bromides by 1,2,2,6,6-pentamethylpiperidine (PMP)/mercaptoethanol
作者:Maryam Amoli、Mark S. Workentin、Danial D.M. Wayner
DOI:10.1016/0040-4039(95)00697-b
日期:1995.6
1,2,2,6,6-pentamethylpiperidine (PMP) is shown to be an effective reducing agent for the radical chain conversion of primary bromoesters to the corresponding esters. The problem of inefficient reduction of tertiary bromoesters in these reactions has been overcome by the addition of an alkyl thiol which mediates the hydrogen atom transfer between the two hindered alkyl centers.