Proton Affinities of Organocatalysts Derived from Pyridine N-oxide

Abstract

Abstract. Proton affinities of several efficient organocatalysts METHOX, QUINOX, ANETOX, KOTOX, FUREOX, and FUROOX bearing a pyridine N-oxide or 2,2′-bipyridyl N,N′-dioxide moiety were determined by using extended kinetic method and density functional theory calculations. Proton affinities are in the range of 1030 -1060 kJ mol -1 . Using isodesmic reactions, the effect of combining two pyridine N-oxide units in the neutral and the protonated molecule was studied: The combination of an unfavorable interaction in the former case and a favorable interaction in the latter accounts for the superbasic properties of 2,2′-bipyridyl N,N′-dioxides. Last but not least, the theoretically predicted pKa in ethanol are 0.1, -2.7, 0.9, 1.8, 1.9, and 2.3 for the METHOX, QUINOX, ANETOX, FUROOX, FUREOX, and KOTOX, respectively.

Exploiting the (–C–H···C–) Interaction to Design Cage-Functionalized Organic Superbases and Hyperbases: A Computational Study

“…39,40 There are several reports where B3LYP-D3 were employed in calculating PAs, and a reasonable agreement was found between the experimental and computated PA values. 41,42 The presence of an intramolecular The hydrogen bond under investigation is often represented by the symbols D−H•••A, where D stands for the donor atom and A for the acceptor atom. In the case of P-MHPNH, both the D and A atoms are expected to consist of just carbon centers.…”

“…In our study, the P-MHPN was optimized at M062 X /6-31G­(d) and B3LYP-D3/6-311+G­(d,p) levels, which have shown similarity with the available crystal structure. The superbase calculations with a variety of systems are performed with the B3LYP DFT functional, hence this functional was used for the study. , There are several reports where B3LYP-D3 were employed in calculating PAs, and a reasonable agreement was found between the experimental and computated PA values. , The presence of an intramolecular (–C–H···C–) interaction in the protonated P-MHPN helps in alleviating the steric strain in the unprotonated base. This naphthalene framework, which was employed to synthesize P-MHPN and anchored to P-ylide to generate the superbase, is similar to a class of nitrogen-based 1,8-bis­(dimethylamino)­naphthalene-type superbases, where the N–H···N hydrogen bonding plays a predominant role in achieving superbasicity. ,, Therefore, exploiting the possibility of intramolecular –C–H···C– interaction opens a new avenue for the design of neutral organic superbases.…”

Kinetics of reactions of NH4+ with some biogenic organic molecules and monoterpenes in helium and nitrogen carrier gases: A potential reagent ion for selected ion flow tube mass spectrometry

“…The proton affinities (PAs) were then calculated as the difference between the total enthalpies of the neutral molecules and the corresponding lowest energy (most stable) protonated structures, accounting for the thermal enthalpy of a free proton of 6.2 kJ/mol at 298.15 K. 30,37 To test whether the used B3LYP aug-cc-pVTZ D4 level of theory provides appropriate energetics, the method was benchmarked on the known PA of ammonia and the biogenic compounds. 8 For ammonia, the calculated value agreed to within 1 kJ/mol (854 kJ/mol).…”

Mass spectrometry in organic and bio‐organic catalysis: Using thermochemical properties to lend insight into mechanism

“…A series of chiral N ‐oxide catalysts based on C 2 ‐symmetric bipyridine scaffolds have been developed by the Kotora and Kočovský labs, and have been shown to be effective at catalyzing enantioselective allylation of aromatic aldehydes with allyltrichlorosilanes (Figure 12) (Malkov, Barłóg, et al, 2011; Malkov, Bell, et al, 2005; Malkov, Bell, et al, 2003; Malkov, Dufková, et al, 2003; Malkov, Ramírez‐López, et al, 2008; Malkov, Stončius, 2013). In 2014, the Roithova lab studied the PAs of these N ‐oxide catalysts (Váňa et al, 2014). Using a Thermo‐Finnigan TSQ Classic mass spectrometer, the authors determined the PAs of the catalysts using a combination of experimental (extended Cooks kinetic method), and computational (DFT calculations for the determination of theoretical PA and pK a values) methods.…”