Abstract
The gas-phase basicity of nitriles can be enhanced by a push–pull effect. The role of the intercalated scaffold between the pushing group (electron-donor) and the pulling (electron-acceptor) nitrile group is crucial in the basicity enhancement, simultaneously having a transmission function and an intrinsic contribution to the basicity. In this study, we examine the methylenecyclopropene and the N-analog, cyclopropenimine, as the smallest cyclic π systems that can be considered for resonance propagation in a push–pull system, as well as their derivatives possessing two strong pushing groups (X) attached symmetrically to the cyclopropene scaffold. For basicity and push–pull effect investigations, we apply theoretical methods (DFT and G2). The effects of geometrical and rotational isomerism on the basicity are explored. We establish that the protonation of the cyano group is always favored. The push–pull effect of strong electron donor X substituents is very similar and the two π-systems appear to be good relays for this effect. The effects of groups in the two cyclopropene series are found to be proportional to the effects in the directly substituted nitrile series X–C≡N. In parallel to the basicity, changes in electron delocalization caused by protonation are also assessed on the basis of aromaticity indices. The calculated proton affinities of the nitrile series reported in this study enrich the gas-phase basicity scale of nitriles to around 1000 kJ mol−1.
Highlights
The two DFT levels of theory for B3LYP/311+G(d,p) and B3LYP/6-311++G(d,p), referred to as DFT1 and DFT2, respectively, applied for selected derivatives lead to very similar basicity parameters (PA and GB) for the cyano and imino N sites, and to analogous DFT gas-phase basicity scales
On the basis of our DFT calculations, we considered all compounds studied as functional nitriles in the gas phase, because protonation occurs on the nitrile group
Using the electron acceptor nitrile functional group as a probe, we explored the efficiency of the methylenecyclopropene (CPC) and cyclopropenimine (CPN) π-systems (3 with Z: CH and N, respectively, in Figure 1) as resonance transmission scaffolds for strong electron donor substituents
Summary
Nitriles are usually considered weak bases because of the unfavorable sp hybridization of the nitrogen atom and the high s character, being adverse to electron pair sharing with acids. Their gas-phase basicity, as experimentally measured or calculated, may be increased by the push–pull effect. This effect occurs when a strong electrondonating group pushes its electron density toward the electron-accepting cyano group through a conjugated system [1].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
