Acidities and homolytic bond dissociation energies of the αC—H bonds in ketones in DMSO

Abstract

Can. J. Chem. 68, 1714 (1990). Equilibrium acidities in DMSO are reported for nine cycloalkanones, acetone, acetophenone, and 19 of their a-substituted derivatives. Oxidation potentials in DMSO for the conjugate bases of most of these ketones are also reported. Combination of these Eox(A-) and pKHA values gives estimates of the homolytic bond dissociation energies (BDEs) of the acidic C-H bonds in the ketones. The ABDEs, relative to the BDE of CH3-H, or a parent ketone, provide a measure of the radical stabilization energies (RSEs) of the corresponding radicals. The effects of successive a-Me and a-Ph substitutions on RSEs, relative to those of CH3COCH2-H or PhCOCH2-H, are similar to those reported in the gas phase for methane. The RSE for the MeCHCOPh radical, relative to CH3* is 17 kcal/mol, which is smaller than the sum of the RSEs of the MeCH2* and PhCOCH2* radicals relative to CH3* (7 + 12 = 19), contrary to the prediction of the captodative postulate. When G in PhCOCH2G is PhCO, CH3C0, or CN the ABDEs (relative to PhCOCH2-H) are 0, 1, and 3 respectively; for MeCOCH2S02Ph, PhCOCH2S02Ph, and P~C O C H~N M~~' the ABDEs are -5, -2, and -4, respectively. The BDEs in C5, C6, C7. C8, ClO, and C12cycloalkanones are within k2.5 kcal/mol of that of 3-pentanone. Acetophenones bearing meta or para substituents all have BDEs of 93-94 kcal/mol. Ketone radical cations, [RCOR']", appear to be superacids with estimated pKHA+* values below -25.

Substituent and charge distribution effects on the redox potentials of radicals. Thermodynamics for homolytic versus heterolytic cleavage in the 1-naphthylmethyl system

“…1 and for the reduction of 1 (p-= 13.0 * 1. all cases. While most of these redox reactions are irreversible (Table l), Bordwell et al (9)(10)(11)(12) have shown that for a series of structurally related species, the changes in redox properties closely approximate thermodynamic changes; i. e., tammetry) also have been shown to be close to EO (16,17,21).…”

“…Recently, Arnold and co-workers (6-8) and Bordwell et al (9)(10)(11)(12) used this approach for the determination of pK,'s of radical cations and (or) the energies for the homolytic cleavage of C-H bonds. More recently, Arnett et al (1 3-15) combined calorimetric and electrochemical measurements to obtain data for the heterolytic C-C cleavage reactions.…”

“…deprotonation reactions (pKals) while reactions [lo] and [12] are hydride affinities. The latter two reactions are of particular importance to the chemistry of NADH/NAD' and its analogs (1 8-20).…”

Quantum-chemical study of C— H bond dissociation enthalpies of various small non-aromatic organic molecules

“…In the case of C26-H of desmosterol and Z-but-2-ene, the inductive effect of geminal alkyl group diminishes the stabilizing mesomeric effect of double bond and results in the higher BDE value of the sterol. (Bordwell 1990). …”

C‐Glycosidations of a 2‐Ketohexosyl Bromide with Electrophilic, Radical, and Nucleophilic Anomeric Carbons

“…This course is in fact not unexpected, since even the prototype nucleophilic glucopyranosyl radical, generated from acetobromoglucose, [17] produces the hydrogen transfer product 1,5-anhydroglucose tetraacetate in a yield of over 20% on reaction with acrylonitrile, [17b] and the 2-oxoglycosyl radical 8 is clearly less nucleophilic, if not essentially electrophilic due to its push-pull Ϫ or capto-dative [18] Ϫ substitution: an electron-withdrawing group such as a carbonyl moiety next to a carbon radical entails a stabilization by some 10Ϫ12 kcal/mol in relation to a ''pure'' C-radical, [19] and the additional presence of a cycloalkoxy group allows for a variety of mesomeric forms (i.e., 8a o 8b o 8c, Scheme 3). This is likely to result in a planar arrangement of O5ϪC1ϪC2ϪC3 (i.e., a 4 H 5 half chair form rather than a mere 4 C 1 -type chair geometry as in 8a).…”