Synthesis of N-Unprotected Diaryl Ketimines and Alkyl Ketimines from Ketones and Ammonia Using Porous Solid Acids with Analysis of Their Adsorption Behavior

Abstract

The most atom-efficient synthetic method for N-unprotected ketimines (N-H ketimines) from ketones is the dehydration condensation reaction with ammonia (NH3). However, until now, few synthetic methods for N-H ketimines with high versatility have been known. In this study, we examined various solid acids and found that N-H diaryl ketimines with various functional groups on the aryl groups could be synthesized in high yields from diaryl ketones and NH3 under solvent-free conditions using silica-alumina (SiO2-Al2O3) or proton-exchanged Y-type zeolite (H-Y). Solid-state 13C and 15N NMR measurements indicated that the N-H ketimine formed in the pores of acidic zeolite was coordinated to NH4+ species on the pore surface. By quantum chemical calculations we also discussed the reason why the dehydration-condensation reaction between ketone and NH3, which is an endothermic reaction in a vacuum, was biased toward the product side in the actual experiment. In addition, this synthetic method can be applied to synthesize N-H alkyl ketimines with α-acidic hydrogens, which are less stable and more sensitive to hydrolysis and oligomerization than diaryl ketimines.

Highly Active and Sulfur-Tolerant Ruthenium Phosphide Catalyst for Efficient Reductive Amination of Carbonyl Compounds

“…The condensation of carbonyl compounds with NH 3 (Reaction A in Figure b) is well-established to be facilitated by acid catalysts . Thus, we carried out TPD of NH 3 to investigate the acidic properties of Ru 2 P/C and Ru/C (Figure a).…”

“…The condensation of carbonyl compounds with NH 3 (Reaction A in Figure 4b) is well-established to be facilitated by acid catalysts. 69 Thus, we carried out TPD of NH 3 to investigate the acidic properties of Ru 2 P/C and Ru/C (Figure 5a). The NH 3 -TPD profile of Ru 2 P/C showed one major peak at 119 °C, indicating the existence of weak acidic sites in Ru 2 P/C.…”