A simple, modular method for the synthesis of 3,4,5-trisubstituted pyrazoles
Mark McLaughlin1, Karen Marcantonio, Cheng-Yi Chen
1Process Research, Merck Research Laboratories, Rahway, NJ 07065, USA. mark_mclaughlin@merck.com
Abstract:
A modular approach for the regiocontrolled preparation of pyrazoles bearing substituents on all three carbon atoms is described. Central to this method is the use of a switchable metal-directing group (MDG) to enable sequential direct lithiation of the 3- and 5-positions of the pyrazole ring. Pyrazole boronic esters obtained from these lithiated intermediates can undergo efficient Suzuki cross-coupling under the developed nonaqueous conditions, which minimize undesirable protolytic deboronation. Halogenation of the 4-position provides the means for substitution at the remaining carbon atom.
Related Concept Videos
Diazonium Group Substitution with Halogens and Cyanide: Sandmeyer and Schiemann Reactions
Preparation of 1° Amines: Hofmann and Curtius Rearrangement Overview
Five-Membered Heterocyclic Aromatic Compounds: Overview
Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic SN1
In the Sandmeyer reaction, for example, the diazonio group is replaced by a chloro, bromo, or cyano...
Diazonium Group Substitution: –OH and –H
Preparation of 1° Amines: Azide Synthesis
Azide ions act as good nucleophiles and react with unhindered alkyl halides to form alkyl azides. Alkyl azides do not participate in further nucleophilic substitution reactions, thereby eliminating the chances of polyalkylated products. Alkyl azides are reduced by hydride-based reducing agents, like lithium aluminum...


