Transition Metal‐Catalyzed Synthesis of Monocyclic Five‐Membered Aromatic Heterocycles

Abstract

Aromatic heterocycles, particularly furans and pyrroles, are structural motifs found in a vast number of biologically active natural and artificial compounds [1][2][3]. Other examples of practical applications of furans and pyrroles include dyes, polymers, and electronic materials [4]. Moreover, these heterocycles are employed as important intermediates in organic synthesis [5][6][7][8][9]. The successful application of furans and pyrroles in these and many other ways and their significance in applied and fundamental areas have placed them at the forefront of contemporary organic chemistry. A variety of methodologies and different protocols for their synthesis have been reported [1-3, 10-23] and become well established throughout decades. Among the variety of novel approaches for the synthesis of furans and pyrroles, transition metal-catalyzed transformations are arguably the most attractive methodologies . Several excellent reviews on the transition metal-catalyzed synthesis of monocyclic five-membered heterocycles have been published in the literature. Many of them were categorized by either the metal or the type of transformation. This chapter covers transition metal-catalyzed syntheses of furans and pyrroles. The main organization of this chapter is based on a heterocycle, wherein syntheses of a particular core are structured by the type of transformation and substrates engaged. Herein, we have tried to discuss equally the synthetic applicability of a method and mechanistic aspects and concepts implicated in the described transformations. A discussion of the mechanisms is given when needed to provide an idea about possible reaction pathways and the nature of the elementary processes involved in the catalytic transformation. It should be noted that the most essential and general catalytic reactions, as well as recent and conceptionally interesting transformations, are discussed in more detail in this chapter. Syntheses of furans and pyrroles via functionalization of the preexisting heterocyclic cores [37,[52][53][54][55][56][57][58][59][60] are not covered and, therefore, only reactions in which assembly of a heterocyclic ring occurs are described. In addition, the Pd-catalyzed synthesis of heterocycles is not covered herein, as this topic is covered in Chapter 6.

s-Block-Metal-Mediated Hydroamination of Diphenylbutadiyne with Primary Arylamines Using a Dipotassium Tetrakis(amino)calciate Precatalyst

“…The copper-catalyzed addition of primary amines to diphenylbutadiyne gives pyrroles in high yields. This reaction requires high catalyst loads such as 25 mol % of copper­(I) halide and enhanced reaction temperatures …”

RhII‐Catalyzed Cycloaddition of α‐Diazo Homophthalimides and Nitriles Delivers Oxazolo[5,4‐c]isoquinolin‐5(4H)‐one Scaffold

“…Metal‐catalyzed cycloaddition reaction of α‐diazo carbonyl compounds with nitriles is a versatile way of preparing 1,3‐oxazoles and represents only a fraction of the myriad ways diazo compounds can be employed in the synthesis of heterocycles via the formation of metal carbenoid species At the same time, compounds where an α‐diazo carbonyl moiety is incorporated in a heterocyclic system are exceedingly rare though their use in [3+2] cycloadditions with nitriles would be a facile entry into structurally diverse heterocycle‐fused 1,3‐oxazoles. Common illustrative examples of compounds having an α‐diazo carbonyl as a part of a heterocyclic system include 5‐diazo‐1,3‐disubstituted barbituric acids ( 1 ), 3‐diazooxindoles ( 2 ), 3‐diazopyrrolidine‐2,4‐diones ( 3 ) and 5‐diazo‐1,3‐dioxane‐4,6‐diones (Meldrum's acids) ( 4 ) .…”

Synthesis of Pyrroles through a 4π-Electrocyclic Ring-Closure Reaction of 1-Azapentadienyl Cations

“…This general importance is still inspiring synthetic chemists to find new ways to synthesize pyrrole and its derivatives. There is a whole range of methods available for the synthesis of pyrrole. , Multicomponent reactions and metal-catalyzed reactions are two of the predominantly applied strategies among others.…”