Transition‐Metal‐Free Heterocyclic Carbon‐Boron Bond Formation

Abstract

Heteroaryl boronic acids and esters are extremely important and valuable intermediates because of their wide application in the synthesis of marketed drugs and bioactive compounds. Over the last couple of decades, the construction of highly important heteroaryl carbon‐boron bonds has created huge attention. The transition‐metal‐free protocols are more green, less sensitive to air and moisture, and also economically advantageous over the transition‐metal‐based protocols. The transition‐metal‐free C−H borylation of heteroarenes and C−X (X=halogen) borylation of heteroaryl halides represents an excellent approach for their synthesis. Also, various cyclization and alkyne activation protocols have been recently established for their synthesis. The goal of this review article is to summarize the existing literature and the current state of the art for transition‐metal‐free synthesis of heteroaryl boronic acid and esters.

Transition-metal-free chemoselective reduction of α,β-unsaturated ketones using H2O as a hydrogen source

“…Metal-free catalysis has been the most important and interesting research topic in organic chemistry due to it being environmentally benign. 5 Recently, great efforts have been devoted to developing metal-free catalytic reduction methods using various hydrogen donors, such as NaHSe, Na 2 S·9H 2 O, and HCOOH/H 2 . 6 For example, Xia and co-workers have disclosed a chemoselective electrochemical 1,4-reduction of the carbon–carbon double bond in alkenones using methanol and ammonium chloride as hydrogen sources (Scheme 1b).…”

Selective Synthesis of Boron-Functionalized Indenes and Benzofulvenes by BCl3-Promoted Cyclizations of ortho-Alkynylstyrenes

“…In the past decade, metal-free C–B bond formation strategies have emerged as more sustainable and practical alternatives, overcoming many of these issues. In this context, borylative heterocyclizations provide a straightforward and regioselective access to heterocycles bearing easily functionalizable boron substituents in their structure. ,, This innovative methodology simultaneously forms a heterocycle and a C–B bond via an intramolecular cyclization triggered by a nucleophilic attack on a triple bond activated by a boron electrophile. Ingleson developed the analogous carbocyclizations to synthesize B-containing dihydronaphtalenes, using alkynyl substrates with an arene acting as an internal nucleophile and simple BCl 3 as activator .…”