Remarkable catalytic effect of H+ in Michael-type additions of anilines to 3-butyn-2-one

Nitrogen heterocycles form peptide nucleic acid precursors in complex prebiotic mixtures

The ability to store information is believed to have been crucial for the origin and evolution of life; however, little is known about the genetic polymers relevant to abiogenesis. Nitrogen heterocycles (N-heterocycles) are plausible components of such polymers as they may have been readily available on early Earth and are the means by which the extant genetic macromolecules RNA and DNA store information. Here, we report the reactivity of numerous N-heterocycles in highly complex mixtures, which were generated using a Miller-Urey spark discharge apparatus with either a reducing or neutral atmosphere, to investigate how N-heterocycles are modified under plausible prebiotic conditions. High throughput mass spectrometry was used to identify N-heterocycle adducts. Additionally, tandem mass spectrometry and nuclear magnetic resonance spectroscopy were used to elucidate reaction pathways for select reactions. Remarkably, we found that the majority of N-heterocycles, including the canonical nucleobases, gain short carbonyl side chains in our complex mixtures via a Strecker-like synthesis or Michael addition. These types of N-heterocycle adducts are subunits of the proposed RNA precursor, peptide nucleic acids (PNAs). The ease with which these carbonylated heterocycles form under both reducing and neutral atmospheres is suggestive that PNAs could be prebiotically feasible on early Earth.

"On water" reaction of deactivated anilines with 4-methoxy-3-buten-2-one, an effective butynone surrogate

Poorly nucleophilic aromatic amines (nitroanilines, chloroanilines, etc.) react readily and selectively with trans-4-methoxy-3-buten-2-one, a convenient, effective and inexpensive surrogate for 3-butyn-2-one, to afford (Z)-enaminones. The efficiency of the reaction mostly lies in the use of water as a solvent, which enhances the reaction rate by a 45 to 200-fold factor with regard to other media.

Conjugate Addition of 3-Buytn-2-one to Anilines in Ethanol: Alkene Geometric Insights through In Situ FTIR Monitoring

A convenient, mild and effective conjugate addition of 3-butyn-2-one to a variety of anilines in ethanol is reported. The reaction was monitored and characterized through in situ FTIR, and the dynamics of the facile E/Z alkene geometry interconversion of the resultant aniline-derived enaminones was explored through NMR, FTIR and X-ray crystallography. A straightforward purification protocol that employs direct Kugelrohr distillation was identified, and the method was further extended to other amines and ynones, allowing rapid access to these interesting compounds.

Electronic and solvent effects on kinetics of SNAr substitution reactions of substituted anilines with 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole in MeOH-Me2SO mixtures of varying composition: one reaction with two mechanistic pathways

ABSTRACT

The kinetics and mechanism of the aromatic nucleophilic substitution reactions of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole with para-X-substituted anilines (X = OH, OMe, Me, H, F, I, Cl) were studied in MeOH-Me2SO mixtures and pure Me2SO at 25.0 °C. The second-order rate coefficients depend on the substitutent in aniline and give good Hammett and Brønsted correlations; a polar SNAr reaction is proposed for the reaction in different MeOH-Me2SO mixtures. The measured rate coefficients of the reaction demonstrated dramatic variations for aniline donor with the increasing dimethyl sulfoxide composition in MeOH-Me2SO mixtures. In this case, the Hammett and Brønsted plots are biphasic and concave upwards with a break point at 4-methylaniline. These results indicate a change in mechanism from the polar (SNAr) for less basic nucleophiles (X = 4-Cl, 4-I, 4-F, and H) to the single electron transfer (SET) for more basic nucleophiles (X = 4-OH, 4-OMe and 4-Me). The changes of the structure of the transitions states with substituents and solvent are in accordance with the results of kinetics studies. The solvation model described is well supported by the solvatochromism exhibited by aniline in the solvent mixture under investigation. These results provide an ideal framework for understanding the paramount importance of the specific molecular structure of solvent molecules in determining chemical reactivity versus solvent effects.

GRAPHICAL ABSTRACT

Single electron transfer in SNAr process: Reactions of 2,4,6-tris(trifluoromethanesulfonyl)anisole with anilines in dimethyl sulfoxide

Second-order rate constants ( k1) have been measured spectrophotometrically for reactions of 2,4,6-tris(trifluoromethanesulfonyl)anisole 1 with 4-X-substituted anilines (X = OH, OCH3, CH3, H, I and Cl) 2a–f in dimethyl sulfoxide (DMSO) at 25 °C. The Hammett plot is nonlinear, i.e. the anilines having an electron-donating substituent exhibit positive deviations from the log k1 versus correlation. The Hammett plot consists of two intersecting straight lines, i.e. ρ=– 1.40 for electron-withdrawing substituents (X = I and Cl) while ρ=– 3.26 for electron-donating substituents (X = OH, OCH3 and CH3). The Brønsted-type plot is also nonlinear with a change in slope from a small value (βnuc=0.50) to a large value (βnuc = 1.29 at p Ka=4.58). This change could be interpreted in terms of a change in mechanism from a polar SNAr process to a single electron transfer. On the other hand, the experimental data show that the rate constants ( k1) values exhibit good correlation with the oxidation potentials ( E°) of the aniline nucleophiles and are consistent with the suggested mechanism.