Enhanced Basicity of Push–Pull Nitrogen Bases in the Gas Phase

Abstract

Nitrogen bases containing one or more pushing amino-group(s) directly linked to a pulling cyano, imino, or phosphoimino group, as well as those in which the pushing and pulling moieties are separated by a conjugated spacer (C═X), where X is CH or N, display an exceptionally strong basicity. The n-π conjugation between the pushing and pulling groups in such systems lowers the basicity of the pushing amino-group(s) and increases the basicity of the pulling cyano, imino, or phosphoimino group. In the gas phase, most of the so-called push-pull nitrogen bases exhibit a very high basicity. This paper presents an analysis of the exceptional gas-phase basicity, mostly in terms of experimental data, in relation with structure and conjugation of various subfamilies of push-pull nitrogen bases: nitriles, azoles, azines, amidines, guanidines, vinamidines, biguanides, and phosphazenes. The strong basicity of biomolecules containing a push-pull nitrogen substructure, such as bioamines, amino acids, and peptides containing push-pull side chains, nucleobases, and their nucleosides and nucleotides, is also analyzed. Progress and perspectives of experimental determinations of GBs and PAs of highly basic compounds, termed as "superbases", are presented and benchmarked on the basis of theoretical calculations on existing or hypothetical molecules.

Stabilizing a different cyclooctatetraene stereoisomer

“…There is a long history of reducing the strength of the π-component of double bonds by a substitution pattern such as DDC=CAA, where D is a π-donor (NH 2 , OH, halogen), and A a π-acceptor (CN, COH, BH 2 , CF 3 ). The donor and acceptor substituents stabilize respective partial positive and negative ion character on the carbons they bond to, and thereby entail a zwitterionic contribution at the expense of the covalent π-bond (34)(35)(36)(37). The argument given is basically a valence bond theory one; a molecular orbital explanation of the effect of these "push-pull" systems may also be provided.…”

Gas phase basicities of polyfunctional molecules. Part 6: Cyanides and isocyanides

“…25 kJ/mol clearly demonstrates a π‐donating effect of the amino nitrogen toward the cyano group. This constitutes the simplest illustration of the so‐called “push‐pull” effect (see Scheme ).…”

“…A number of molecules studied here include species characterized by large proton affinity and gas‐phase basicity values, due, in particular to the so‐called “push‐pull effect” where a π‐donnor subtituent is directly linked to the nitrile group . Accordingly, some of these molecules are denoted as “superbases,” that is are stronger bases than the classical proton sponge, 1,8‐bis(dimethylamino)naphthalene (see parts 2 and 5 of this review), which is characterized by a proton affinity (PA) of 1015 kJ/mol, and a gas‐phase basicity (GB) of 980 kJ/mol.…”

“…A noticeable exception of this rule is provided by X = N(CH 3 ) 2 where the CN bond of the cyano group is slightly elongated upon protonation. This is a consequence of an efficient n‐electrons donation effect of the amino nitrogen atom which, combined with the acceptor effect of the cyano group, constitutes a typical case of the so‐called “push‐pull” effect . This effect may be illustrated by Scheme where valence bond structures Nc and Pd account for the extended electron delocalization.…”

A density functional theory study on the superacidity of sulfuric, fluorosulfuric, and triflic acid derivatives with two cyclopentadiene rings: ion pairs formation in the gas phase

“…As such, a spontaneous proton transfer can be achieved by decreasing the corresponding acidity of the proton donor and accomplishing an increase in the PA or basicity of the proton‐accepting site that matches the mentioned ∆ H acid value and can occur when there is no significant transition state between the reactants and products. Because of the above‐mentioned applications and challenges, the design and synthesis of compounds with high acidity (superacids) and high basicity (superbases) is an active field in chemistry attracting the attention of both experimental and theoretical researchers …”