On the Basicity of Organic Bases in Different Media

Abstract

The basicities of simple organic bases – aliphatic and aromatic amines, amidines, phosphazenes, as well as saturated and unsaturated nitrogen heterocycles – are examined in acetonitrile, dimethyl sulfoxide, tetrahydrofuran, water and the gas phase. The basicities (pKaH values) of conjugate acids of a large variety of bases in these media are presented and discussed. Equations employing easily usable structural descriptors have been derived for approximately converting basicities from acetonitrile to other solvents. Recommendations are given on their practical use and a number of pKaH values that are experimentally unavailable are estimated from these relationships. An important part of the minireview is a large compilation of pKaH and GB values of the compounds in solvents and the gas phase, respectively, as well as the revised basicity scale in acetonitrile, now containing more than 270 pKaH values.

Proton-coupled electron transfer across benzimidazole bridges in bioinspired proton wires

“…For example, the value of E drop or E 0 drop resulting from changing the TPAs between 4 and 1 or 4 0 and 1 0 is 380 mV, qualitatively in agreement with the DpK a in acetonitrile of a tertiary amine and a benzimidazole substituted with an electron-withdrawing group. 36 In our theoretical treatment, the change in the redox potential due to incorporation of a second benzimidazole and different TPAs can be expressed by the thermodynamics of the overall PCET process (see analysis Page S62 †). The calculations show that upon oxidation of the phenol, each benzimidazole and TPA contributes a term corresponding to DG < 0 for proton transfer.…”

Geminal Dimethyl Substitution Enables Controlled Polymerization of Penicillamine-Derived β-Thiolactones and Reversed Depolymerization

“…To our gratification, the ROP of N ene -PenTL, catalyzed by a weak organobase, triethylamine (TEA, pKa DMSO = 9.0), 52 afforded the desired polymer product PN ene -PenTE ( 1 H NMR in Figure S18) with a considerably larger Mn and narrower dispersity (entry 3, Table 1; Mn = 19.4 kg/mol, Đ~1.10) compared to similar PTEs synthesized previously from CysTLs (Mn~8.8 kg/mol, Đ~2.4) at the same M/I ratio. 47 Replacing TEA with 0.1 equivalent of 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU), a stronger base with a pKa DMSO of 12, 52 greatly accelerated the ROP reaction (entry 4-7, Table 1 and (Fig.…”

“…For example, DBU-catalyzed formation of PN C8 -PenTE ( 1 H NMR in Figure S19) at a M/I ratio of 100/1 exhibited a Mn of 20.1 kg/mol and Đ of 1.21 (entry 8, Table 1). By employing tBuP4, a phosphazene superbase with a pKa DMSO of 30.3, 52 the Mn could be further boosted to 52.4 and 70.6 kg/mol at a M/I ratio of 250/1 and 350/1, respectively, while maintaining a Đ less than 1.30 (entry 9-10, Table 1). Copolymerization of N C8 -PenTL and N Boc -PenTL mixture gave a random copolymer with a Mn of 14.1 kg/mol and a Đ 1.24 (entry 11, Table 1).…”

N‐Heterocyclic Carbene Organocatalysis: With or Without Carbenes?

“…However, the effect of the enzyme cannot explain how NHC organocatalytic reactions can be possible in organic syntheses in the absence of any biomolecule. Imidazolium (in water: p K a =19–24, in DMSO: p K a =19–24, in MeCN: p K a =33.6), triazolium (in water: p K a =14.9–17.4) and thiazolium (in water: p K a =17–19, in DMSO: p K a =14.5, in MeCN: p K a =25.6) salts have all been applied as catalysts, and despite their high p K a values, triethylamine (in water: p K a =10.65, in DMSO: pKa=9.0, in THF: p K a =12.5, in MeCN: p K a =12.5) has been observed to be basic enough to deprotonate them in a quantity, which is sufficient to exhibit reasonable to excellent catalytic activity…”