Deprotonation of tertiary amine cation radicals. A direct experimental approach

Abstract

the proposed concerted unimolecular fragmentation step explains the effects of structure, conformation, counterion, and solvent on regioselectivity without any need to resort to the hypothesis of an unsubstantiated * intermediate. Experimental SectionSamples of compounds 1-3 and 5 were kindly provided by Prof. A. Maercker, 4 was purchased from Aldrich, and 6 and 7 were synthesized and purified as described in the literature. 39•40 Tetrahydrofuran (THF) and 1,2-dimethoxyethane (DME) were distilled from a Na-K alloy in the vacuum line directly into the sample tube.

Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

“…Significantly, single-electron oxidation of an amine dramatically lowers the bond strength of the α -C–H bonds; Dinnocenzo has estimated the bond dissociation energy of this bond to be 47 kcal/mol, or over 30 kcal/mol less than that of the parent amine (ΔH BDE ° = 80 kcal/mol for the α -C–H bond of triethylamine). 36 The radical 22 may thus abstract a hydrogen atom at one of the α -positions of 20 , converting 22 to the reduced product 23 and the amine radical cation to iminium ion 24 . Although deuterium-labeling studies implicated the amine radical cation as the primary source of the hydrogen atom added in 23 , the formate ion and Hantzsch ester employed in the reaction are also likely hydrogen atom donors.…”

“…As described in section 3.2, oxidation of an amine has the effect of dramatically lowering the bond dissociation energy of the C–H bonds at the amine α -positions. 36 If a good hydrogen atom acceptor is present in the reaction, it may abstract a hydrogen atom from one of these positions, converting the aminium radical cation 119 to the iminium ion 120 . At the same time, however, single-electron oxidation to the radical cation also significantly lowers the p K a of the protons at the α -positions: aminium radical cations have been estimated to have p K a values between 3 and 13.…”

The chemistry of amine radical cations produced by visible light photoredox catalysis

“…The acidifying effect of one-electron oxidation on the α-C–H bond remains debatable [56–60]. The rate for deprotonation of amine radical cation 2 has been measured experimentally by several groups, and a broad range has been obtained [61–62]. α-Amino radical 3 is strongly reducing [45,63], thus making the second one-electron oxidation facile.…”

Discovery of an α-Amino C–H Arylation Reaction Using the Strategy of Accelerated Serendipity

“…The resultant Ir IV (ppy) 3 ( 6 ) is a strong oxidant ( E 1/2red = +0.77 V versus SCE in CH 3 CN) (30, 31) and would be capable of undergoing a single-electron transfer event with amine 7 , generating amine radical cation 8 , as well as re-forming Ir III (ppy) 3 ( 3 ) and thereby completing the photoredox cycle. The C–H bonds adjacent to the nitrogen atom in 8 are weakened by about 40 kcal/mol and so could undergo deprotonation by NaOAc (where OAc is an acetoxy group) to give α-amino radical 9 (19, 36). A radical-radical coupling reaction could then unite intermediates 5 and 9 , representing the key bond-forming step (15–19, 37–43).…”