Gas-Phase Anionic σ-Adduct (Trans)formations in Heteroaromatic Systems

Abstract

Anions of nitroderivatives of thiophene and furan were subjected to the reactions with selected C-H acids in the gas phase. Various structures and reaction pathways were proposed for the observed ionic products. In general, the reactions of heteroaromatic anions with C-H acids may be divided into three groups, depending on the proton affinity difference between C-H acid's conjugate base and heteroaromatic anion (ΔPA). The proton transfer from C-H acid to heteroaromatic anion is a dominant process in the reactions for which ΔPA < 0 kcal mol(-1), whereas the reactions with high ΔPA (ΔPA > 16 kcal mol(-1)) do not lead to any ionic products. The formation of σ-adducts and products of their further transformations according to the VNS, SNAr, cine, and tele substitution mechanisms have been proposed for reactions with moderate ΔPA. The other possible mechanisms as SN2 reaction, nucleophilic addition to the cyano group, ring-opening pathway, and halogenophilic reaction have also been discussed to contribute in the reactions between heteroaromatic anions and C-H acids.

Graphical Abstract

ᅟ

Scheme 1

Formation and further transformations of σ^X and σ^H-adducts in solution

Scheme 2

Nitrothiophene and nitrofuran anions studied in the present work

Figure 1

CID mass spectra (collision energy, CE = 5 eV) of heteroaromatic anions: (a) 2-nitrothiophene 5-anion, (b) 3-nitrothiophene 5-anion, (c) 2-nitrofuran 5-anion, (d) 2-bromo-3-nitrothiophene 5-anion, and (e) 3-bromo-2-nitrothiophene 5-anion

Scheme 3

Reaction of 2-nitrothiophene 5-anion with chloroacetonitrile and methyl chloroacetate according to S_N2 mechanism

Scheme 4

Reaction of 2-nitrothiophene 5-anion with chloroacetonitrile and methyl chloroacetate according to proton transfer followed by σ^H-adduct formation and elimination of HCl

Scheme 5

Reaction of 2-nitrothiophene 5-anion with acetonitrile

Figure 2

CID spectra (CE = 15 eV) of anions generated from deprotonation of (a) 2-thiophenoacetonitrile, (b) 3-thiophenoacetonitrile, and (c) [A – HNO₂]^¯ ion formed upon reaction of 2-nitrothiophene 5-anion with acetonitrile

Figure 3

Gas-phase reaction of 2-nitrothiophene 5-anion with CD₃CN

Scheme 6

Proposed mechanism for the [A – HNO₂]^¯ ion formation in the reaction of 2-nitrothiophene 5-anion with CD₃CN

Scheme 7

Formation of σ^H-adduct in the reaction of 3-nitrothiphene 5-anion with acetonitrile

Scheme 8

Proposed reaction pathway for the formation of bromochloroacetonitrile anion according to the halogenophilic mechanism

Scheme 9

Proposed pathway for the reaction between 3-bromo-2-nitrothiophene 5-anion with chloroacetonitrile leading to the formation of Br⁻ anion

Scheme 10

Proposed pathways for the reaction of 3-bromo-2-nitrothiphene 5-anion with chloroacetonitrile and methyl chloroacetate leading to the elimination of HCl molecule