Hydrogen atom abstraction from C–H bonds of benzylamides by the aminoxyl radical BTNO: A kinetic study

Abstract

The aminoxyl radical BTNO (benzotriazole-N-oxyl; >N-O*) is generated from HBT (1-hydroxybenzotriazole; >N-OH) by oxidation with a Ce(IV) salt. BTNO presents a broad absorption band with lambda(max) 474 nm that lends itself to investigate the kinetics of H-abstraction from H-donor substrates by spectrophotometry. Thus, rate constants (k(H)) of H-abstraction by BTNO from CH(2)-groups alpha to the nitrogen atom in X-substituted-(N-acetyl)benzylamines (X-C(6)H(4)CH(2)NHCOCH(3)) have been determined in MeCN solution at 25 degrees C. Correlation of the k(H)(X) data with the Hammett sigma(+) parameters gives a small value for rho (-0.65) that is compatible with a radical H-abstraction step. The sizeable value (k(H)/k(D)=8.8) of the kinetic isotope effect from a suitably deuteriated amide substrate further confirms H-abstraction as rate-determining. Evidence is acquired for the relevance of stereoelectronic effects that speed up the H-abstraction whenever the scissile C-H bond is co-linear with either the nitrogen lone-pair of the amide moiety or an adjacent aromatic group. An assessment of the dissociation energy value of the benzylic C-H bond in ArCH(2)NHCOMe is accordingly reported.

Practical N-Hydroxyphthalimide-Mediated Oxidation of Sulfonamides to N-Sulfonylimines

“…Product 4 in this experiment was formed by selective radical/radical cross-coupling between the PINO radical and the benzylic radical which came from hydrogen abstraction of mesitylene, and this explanation is supported by reported literature [40]. On the basis of the above observation and other literature reports [42,43,44], we propose a mechanism where the key step is the hydrogen abstraction reaction from starting materials by the PINO radical (Figure 5b). Then radical 5 oxidizes the α-sulfonamido carbon radical 6 to obtain the final product.…”

Absolute Rate Constants for Hydrogen Atom Transfer from Tertiary Amides to the Cumyloxyl Radical: Evaluating the Role of Stereoelectronic Effects

“…In a kinetic study of the reactions of the benzotriazole-N-oxyl radical (BTNO) with acetamides, 43 significantly higher rate constants for HAT (k H ) from the benzylic C−H bonds α to nitrogen were measured for N-acetyltetrahydroisoquinoline in comparison to N-benzylacetamide (Scheme 3).…”

“…Despite this fact, limited information is available on the role of stereoelectronic effects on HAT from amides to alkoxyl radicals; to the best of our knowledge, there is only one kinetic study that deals with such effects, and it involves an aminoxyl radical (see below). 43 The operation of stereoelectronic effects has been proposed in order to account for the peculiar inter-and intramolecular selectivities observed in HAT reactions from amides and lactams to a variety of hydrogen-abstracting species. For example, in product studies of the reactions of 1-methyl-2-pyrrolidone and other lactams with the tert-butoxyl radical, 44 the ethyl radical, 45 and photoexcited tetrabutylammonium decatungstate, 46 preferential formation of products derived from HAT from the ring C−H bonds adjacent to nitrogen (Scheme 2, a) as compared to the exocyclic methyl C−H bonds (Scheme 2, b) was observed, as quantified by the product ratios derived from the two competitive pathways that are between 6 and 13.…”

Hydrogen abstraction from H‐donor substrates by the 6‐CF3‐benzotriazol‐N‐oxyl radical (TFNO)

“…[37] Our finding of an appreciable rate increase when passing from Ph 2 CH 2 to Ph 2 CHOH (Table 2) is likely due to the profitable stereoelectronic interaction from the oxygen lone-pair of the alcohol with the scissile α C-H bond, an interaction that clearly lacks in the hydrocarbon counterpart, thereby confirming the analogous finding in the kinetic study with BTNO. [18] By exploiting the higher reactivity of TFNO than BTNO in HAT reactions, we have been able to extend the previous kinetic investigation on the stereoelectronic effects [40,41] to additional and significant substrates. Table 5 gives the results for H-abstraction by TFNO, the investigated substrates being organised pairwise with their experimental k H and BDE C-H values (whenever available).…”

“…[40,41] Figure 8. Enforced co-linearity of a benzydrylic radical with the p-orbitals of the adjacent aromatic rings, as due to the planar 5-membered ring interconnecting the two π-systems (the case of entry 8 in Table 5 is shown).…”