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Abstract
The first step of electrophilic aromatic substitution reactions generally involves the formation of charge transfer complexes. Tetracyanoethylene (TCNE) in the presence of aromatic amines forms π-type complexes that subsequently lead to tricyanovinylation reaction. Although this kind of reaction was investigated in the past, the spectroscopic characterization of the charge transfer complexes is still lacking, since in many cases the first step of the reaction is too fast to allow their characterization by conventional techniques. In this paper, it is shown the UV-Vis and resonance Raman characterization of the charge transfer complexes formed between TCNE and substituted anilines. The magnitude of the charge transfer could be linearly correlated with the electronic absorpion energies, as well as to the Raman shifts observed for the characteristic ν(C≡N) mode of TCNE. In addition, the results were correlated to ionization potentials of amines, and theoretical calculations by density functional theory/time-dependent density functional theory (DFT/TDDFT) methods were performed to support the experimental data.
Highlights
Molecular charge transfer complexes play a key role as intermediate species in a wide variety of organic reactions involving nucleophiles and electrophiles species.[1,2] The typical reaction is the electrophilic aromatic substitution, where the formation of a charge transfer complex comprises the first step of the reaction mechanism.[2]
The complex TCNE-ANI absorbs at 587 nm, while the TCNE-TMA, with three methyl groups, at 707 nm, i.e., as the basicity of the electron donor increases, the charge transfer transition energy decreases
The charge transfer complexes formed by TCNE and methyl substituted anilines were characterized by UV‐Vis, resonance Raman and density functional theory (DFT) calculations
Summary
Molecular charge transfer complexes play a key role as intermediate species in a wide variety of organic reactions involving nucleophiles and electrophiles species.[1,2] The typical reaction is the electrophilic aromatic substitution, where the formation of a charge transfer complex comprises the first step of the reaction mechanism.[2] Tetracyanoethylene (TCNE) is a characteristic strong electron acceptor due to the presence of four cyano groups in its structure, which leads to a deficient electron density at ethylenic group[3] increasing its susceptibility to be attacked by nucleophilic species in vinylic substitution reactions.[4,5] When aromatic amines acting as nucleophilic species interact with TCNE, the electrophilic aromatic substitution reaction called tricyanovinylation takes place.[6] The tricyanovinylation reaction was investigated extensively in the past and the reaction mechanism consists essentially of three steps: the formation of a π charge transfer complex, followed by the formation of a σ-complex (Wheland complex) and the final product.[7,8,9,10] In the presence of secondary or tertiary aromatic amines the substitution of the tricyanovinyl group
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