The electron transfer reactions of cyano substituted pyridines and quinolines with thermally generated diphenyl ketyl

Abstract

Electron transfer reactions of cyano substituted pyridines and quinolines with thermally generated diphenyl ketyl are reported. When E1/2(‐) for the heterocycle is less negative than ‐2V, electron transfer occurs from the ketyl to the heterocyclic base. In some cases the products obtained from the thermal reaction are the same as those obtained from the photochemical reaction with benzophenone and alcohols. In other cases different products are formed. Two bases, 2‐pyridinecarbonitrile and 2,4‐pyridinedicarbonitrile, undergo regiospecific reactions in which the course of the reaction is determined by the acidity of the medium. A mechanism in which the heterocyclic nitrogen is involved in the substitution process at the 2 position is proposed.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

“…The aminium radical cation undergoes deprotonation with another equivalent of tertiary amine to give triethylammonium cation and the neutral α-amino radical. This proposal is supported by SV quenching studies, demonstrating that neither the arylidene malonate ( E p/2 red = −1.57 V vs SCE in MeCN) 874 nor the 4-cyanopyridine ( E 1/2 red = −1.87 V vs SCE in MeCN) 878 quench the catalyst fluorescence. Hydrogen bonding of the in situ generated ammonium salt (p K a = 18.5 in MeCN) 879 to the malonate substrate facilitates PCET mediated by reduction from the photocatalyst radical anion, leading to regeneration of the photocatalyst ground state and generation of a stabilized vinylogous ketyl radical after PT from the ammonium salt.…”

Reductive Arylation of Arylidene Malonates Using Photoredox Catalysis

“…As aforementioned, 1,4-DCB is unreactive under these reaction conditions, which was surprising because of the potential difference between 1,4-DCB and 4-CN pyridine (E 1/2 red = −1.67 V vs SCE for 1,4-DCB and E 1/2 red = −1.87 V vs SCE for 4-CN-pyridine). 32 However, it is noteworthy that NHEt 3 + , which is likely being formed under the reaction conditions, (pK a = 9.00 in DMSO) 33 is sufficiently acidic to protonate 4-CN-pyridine, therefore allowing for a shift in the reduction potential through Brønsted acid activation and resulting in SET instead occurring on the 4-CN-pyridinium ion. 34 To investigate this, we conducted cyclic voltammetry (CV) experiments with stoichiometric concentrations of NHEt 3 Cl with 4-CN-pyridine, and found there was a notable shift in reduction potential for the 4-CN-pyridinium species (E 1/2 red = −1.51 V), allowing for SET from the DPAIPN radical anion to be slightly exergonic.…”

Metal-free deoxygenative coupling of alcohol-derived benzoates and pyridines for small molecules and DNA-encoded libraries synthesis

“…These findings, together with the control experiments in the absence of light and NaOAc ( Table 1 , entries 9 and 10), suggest that the HE, in combination with NaOAc, is the main photoactive species in the reaction: 15 upon absorption of visible light, the excited HE–NaOAc complex ( E red = −2.28 V vs. SCE for HE) 17 donates an electron to 4-cyano-2-fluorobenzoate 3, while 4-cyanopyridine 9 ( E red = −1.87 V vs. SCE) 18 can also be reduced by excited HE alone ( Table 1 , entry 10). 13 b , c The resulting radical anion of benzoate 3 is proposed to undergo β-scission fragmentation, providing the desired alkyl radical along with a benign carboxylate byproduct ( Fig.…”