Mechanistic Investigations into the Cation Radical Newman–Kwart Rearrangement

Abstract

Efforts to elucidate the governing factors in the cation radical Newman–Kwart rearrangement are described. Through a combination of spectroscopic and kinetic analyses, it has been shown that the reactive intermediate is a thione cation radical that has significant thiyl radical character. This intermediate undergoes similar chemistry, such as olefin stereomutation, that has been observed for thiyl radicals generated by other means. Moreover, kinetic studies demonstrate that the electronic dependence observed in the system is the result of rate-limiting intramolecular nucleophilic trapping of the thione cation radical. Importantly, the thiyl radical character of the reactive intermediate can be used to rationalize all reaction observations. This mechanistic model is in line with observed rate enhancement at higher dilutions that decreases the formation of an off-cycle intermediate. A thermochemical cycle was devised to predict which substrates will most easily undergo the cation-radical-mediated rearrangement.

Electrochemistry Broadens the Scope of Flavin Photocatalysis: Photoelectrocatalytic Oxidation of Unactivated Alcohols

“…In particular, when we increase the thiourea loading from 10 mol % to 20 mol % and 40 mol %, the generation of H 2 S is evident from the distinct odor of the completed reaction. Thiourea‐derived radicals have been reported to first dimerize and then decompose to the corresponding urea and H 2 S in the presence of H 2 O . Increasing the loading of TU‐2 led to a decrease in reaction rate (Figure ), a phenomenon that we can also attribute to the catalyst decomposition pathway.…”

“…Theformation of thiyl radicals is supported by alkene isomerization experiments (Scheme 7B). [25] Thus, when the b-methylstyrene 6,predominantly in the Z configuration (Z/E = 6:1), was added to the standard photoelectrochemical reaction, in the absence of any thiourea, 71 %o f6 was recovered with a Z/E ratio of 10:1. This result suggests that the radical-induced isomerization did not occur.T he marginal enrichment of the major Z isomer was likely due to the fact that the E isomer either undergoes preferential RFT*-mediated isomerization or decomposition.…”

Mapping the multi-step mechanism of a photoredox catalyzed atom-transfer radical polymerization reaction by direct observation of the reactive intermediates

“…17 Mechanistic studies have not kept up with the rate of synthetic innovation, but examples are emerging which contribute to the improvement of existing photoredox catalysis schemes or the design of new ones. [18][19][20][21][22][23][24][25][26][27][28] One technique wellsuited to study photochemical mechanisms is transient absorption spectroscopy, in which a laser pulse of chosen wavelength (the 'pump' pulse) selectively excites a PC and the resulting reaction intermediates and photoproducts are monitored in absorption with a probe laser pulse. The probe region most commonly spans either UV/visible or infrared (IR) wavelengths.…”

Photocatalytic Oxidative C–H Thiolation: Synthesis of Benzothiazoles and Sulfenylated Indoles

“…In the first map, we see a large concentration of electron density at the sulfur relative to the rest of the molecule A, implying it is the most favorable site for initial oxidation; this pathway is also supported by recently reported work from Nicewicz on allylic thioamides. 32,33 At this point, B will likely undergo radical cyclization to C. This is supported by CV scan-rate experiment; as we sweep from 0.1 V/s to 5 V/s, the second half-wave oxidation peak begins to diminish and completely disappears at the highest scan rate. One explanation for this observation is there is a new intermediate reaction between the first and second oxidation (i.e., thioamide cyclization) and that faster voltage sweeps can kinetically outpace the reaction, thereby hindering subsequent oxidation.…”