A Capstone Review on Synthetic Methods and Applications of Schiffs Bases

Abstract

Different drugs, dyes, agrochemicals, a few to list, require to unify two or more structural features. A unique and easily feasible solution is to employ Schiff’s bases. The ability of -C=N- part of Schiff’s bases enhances flexibility of the molecules and acts as a bridge for joining two distinct structural features. The synthetic procedures for the preparation of Schiff’s bases have been discussed with suitable examples. In addition, some useful applications of Schiff’s bases have been reported in the present work. The work is an attempt to understand the synthesis of Schiff’s bases and their uses.

Introduction

I. INTRODUCTION

Hugo Schiff, a German scientist, is credited with being the first to characterise the products of the reaction between primary amines and carbonyl compounds in 1864, giving rise to the phrase “Schiff's base” [1]. Schiff bases are a vast group of compounds considered by the existence of a double bond connecting the carbon and nitrogen atoms, the flexibility of which is produced in the several ways to combined with different alkyl or aryl substituents. This kind of compounds can be both found in nature and prepared in a laboratory [1].

Schiff bases, also known as azomethines or imines, are compounds that in a broad sense possess the general formula R3R2C=NR1. The substituents R2 and R3 may be alkyl, phenyl, heteroaryl, hydrogen. The substituent R1 at the nitrogen of imino (C=N) may be an alkyl, phenyl, heteroaryl, hydrogen or a metal (mostly Si, Al, B, Sn). A Schiff base resulting from aniline, where R3 is a phenyl or a substituted aryl, can be called an ‘anil’. Therefore, Schiff bases can be considered as a nitrogen equivalent of an aldehyde or a ketone in which the carbonyl group (C=O) has been substituted by an imine or azomethine group. The name “Organic Bases” first appeared in a German paper entitled “Eine neue Reihe organischer Basen” (“A New Series of Organic Bases”) [2], though, they are not used as bases in the conventional sense, the designation of these compounds as bases, has persisted till this date.

The Schiff bases act as L-type ligands (like amines, amides, and phosphines) or ligands having two electron donors, which do not undergo electron modifications on their valence shells, in order to form co-ordination compounds with ions of transition elements [3]. The d-block metal ion forms coordination-bond by the electron-donating ligand atom during the coordination compound formation, which alters the metal's steric and electronic environment. As a result, reactivity of the ion of metal is stabilised and controlled, which is valuable for less stable ions at elevated oxidation states (O.S.) [3,4]. In the latter half of the 19th century, profound interest in applying transition metal complexes with different Schiff bases to medicinal and similar applications started to grow.

Schiff bases are referred to as auxiliary ligands because, unlike reactive ligands, they do not go through irreversible transformations themselves. Instead, they modify the structure and reactivity of the transition metal ion in the centre of the complex [3,4].

II. SYNTHETIC STRATEGIES FOR SCHIFF BASES

Hugo Schiff [2] first described the interaction of aniline with aldehydes in 1864, which is when the chemistry of Schiff's bases first emerged. The method involved Dean Stark apparatus for removal of water molecule to favour condensation to give imines.

Conclusion

The present work is an attempt to highlight the importance of Schiff’s bases and their applications. The synthetic protocols mostly involve dehydration using an amine with an appropriate aldehyde or ketone. Organometallic reagents and molecular sieves have appeared as attractive alternatives with improved yields. Further, Schiff’s bases are largely used as a part pharmacophore for different drugs, as the group enhances flexibility and allows the introduction of novel groups or rings. In majority of drugs, the -C=N- moiety of Schiff’s bases acts as a bridge or linker to club two different structural features. The work could be beneficial to organic and synthetic chemists.

References

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Copyright © 2022 Nilesh S. Kadu, Vijay H. Masand. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.