Room temperature decarboxylative cyanation of carboxylic acids using photoredox catalysis and cyanobenziodoxolones: a divergent mechanism compared to alkynylation

Abstract

Conversion of carboxylic acids to nitriles using photoredox catalysis and benziodoxolone reagents: divergent mechanism when compared to alkynylation!

C(sp3)−H Cyanation Promoted by Visible‐Light Photoredox/Phosphate Hybrid Catalysis

“…In combination with a visible‐light photoredox catalysis, structurally distinct HAT catalysts, such as thiols, thiophosphoric acids and imides, quinuclidines, sulfonamides, chloride and bromide ions, aryl carboxylates, and N‐hydroxy compounds were developed. The currently‐available HAT catalysts have limitations, such as: 1) limited scope of cleavable C−H bonds; 2) catalyst decomposition due to the inherent nucleophilicity and/or reactivity with carbon–carbon multiple bonds; and 3) insufficient stability, thus leaving room for improvement. HAT catalysts that can cleave C(sp 3 )−H bonds with a relatively high bond‐dissociation energy (BDE: 95–105 kcal mol −1 ) and high catalyst turnover are in high demand.…”

“…Based on the above‐discussed mechanistic insights, we propose a possible mechanism for the present reaction, as depicted in Scheme : 1) Photocatalyst 1 (*Ir III /Ir II +1.68 V vs. SCE) is excited by visible‐light irradiation; 2) Excited photocatalyst oxidizes the phosphate anion ( 2 a • / 2 a − +1.50 V vs. SCE), thus generating a phosphate radical. Without K 2 CO 3 , the reaction does not proceed, because the oxidation of 2 a requires +1.72 V (vs. SCE); 3) Resulting phosphate radical (BDE(O−H)=102.4 kcal mol −1 ) selectively abstracts the C−H bonds of 3 a (BDE(C−H)=95 kcal mol −1 ) to produce the corresponding carbon radical; 4) Resulting carbon radical is trapped by 4 a , producing cyanation product 5 a ; and 5) Thus‐generated p ‐toluenesulfonyl radical (Ts • /Ts − −0.50 V vs. SCE) is reduced by the photocatalyst (Ir III /Ir II =−0.69 V vs. SCE) to give a sulfinate anion (Ts − ), thereby closing the whole hybrid catalytic cycle.…”

Photoredox Imino Functionalizations of Olefins

“…Furthermore, by using hypervalent IBX reagents,23 we have developed unprecedented radical imino cyanation ( 5 l ), olefination ( 5 m ), and alkynylation ( 5 n ) processes. The successful formation of 5 l is interesting, as recent literature reports described the cyanating reagent to be able to react with only C(sp 3 )‐radicals α to a heteroatom 12i. The reagent used for the formation of 5 m 24 has not been used before in radical manifolds, and makes this example the first use of IBX reagents as vinylation partners in free‐radical processes.…”

New trends and applications in cyanation isotope chemistry

“…of the cyanation reagent 109 and obtained moderate to good yields. However, they observed a loss of stereoinformation using substrates such as chiral amino acids ( 105 , 106 ) . The cyanation reagent 1‐cyano‐1,2‐benziodoxol‐3‐(1H)‐one 109 was prepared from 1‐acetoxy‐1,2‐benziodoxol‐3‐(1H)‐one and TMSCN in 98% yield .…”

“…However, they observed a loss of stereoinformation using substrates such as chiral amino acids (105, 106). 63 The cyanation reagent 1-cyano-1,2-benziodoxol-3-(1H)-one 109 was prepared from 1-acetoxy-1,2-benziodoxol-3-(1H)-one and TMSCN in 98% yield. 64 This method could be potentially interesting for labelling more complex peptides.…”