Copper-Catalyzed Synthesis of Tetrasubstituted Enynylboronates via Chemo-, Regio-, and Stereoselective Borylalkynylation

Abstract

An efficient, catalytic method for accessing tetrasubstituted enynylboronates has been established via copper-catalyzed chemo-, regio-, and stereoselective borylalkynylation of internal alkynes. In this protocol, a range of symmetrical and unsymmetrical internal alkynes with aryl, heteroaryl, and alkyl substituents afforded fully substituted enynylboron compounds in good yields and with high levels of regio- and stereoselectivity, up to a ratio of >20:1. The enynylboron products could be further utilized in transforming the C-B bond into C-C bonds by coupling reactions.

Synthesis of Conjugated Boron‐Enynes via cis‐Alkynylboration of Terminal Alkynes

“…Among the methods reported, copper‐catalyzed carboboration reaction of alkynes with bis(pinacolato)‐diboron [B 2 (pin) 2 ] and subsequent interception of the putative β‐ boryl alkenyl−copper intermediate species with carbon based electrophiles, which appears to be the most straightforward and effective method to obtain tri‐ and tetra‐substituted alkenylboronates, have received increasing interest . Copper‐catalyzed carboboration of internal alkynes involving boracarboxylation, alkylboration,– arylboration and alkynylboration have been extensively established for synthesis of tetra‐substituted alkenylboronates, but only few examples of carboboration of terminal alkynes were reported. For examples, Tortosa's and Yoshida's group pioneered Cu‐catalyzed methylboration and benzylboration of activated terminal alkynes (Scheme , a) .…”

“…On the other hand, enynes are widely used for designing nanostructures, polymer synthesis and organic synthesis . Although, transition‐metal‐catalyzed cross‐dimerization of alkynes, Suzuki, Sonogashira‐coupling reaction and alkynylboration of internal alkynes have been developed toward the synthesis of enynes –. The search for new synthetic approaches to enyne compounds is still a challenging issue in chemical synthesis.…”

Stereoselective Synthesis of Borylated 1,3‐Dienes by Synergistic Cu/Pd Catalysis

“…This transformation entails the catalytic formation of a nucleophilic β‐boryl‐substituted alkenylcopper intermediate and subsequent electrophilic trapping. So far, protocols involving alkyl, aryl, allyl, alkynyl and carbonyl electrophiles have been described. The use of an alkenyl (pseudo)halide in this type of carboboration reaction might enable the syn ‐selective formation of 1‐borylated 1,3‐dienes; however, to the best of our knowledge, the Cu‐catalyzed alkenylboration of alkynes has not been reported to date.…”

Copper‐Catalyzed Preparation of Alkenylboronates and Arylboronates

“…In 2018, the Yun Group developed a novel catalytic carboboration method to trap the in situ generated β-borylalkenyl-copper species with an C(sp)-based alkynyl electrophile selectively towards accessing a tetrasubstituted enynylboronate (Scheme 29a). [160] (IMes)CuCl-catalyzed borylalkynylation reactions of a variety of internal alkynes with triisopropylsilylethynyl bromide and B 2 pin 2 furnished the corresponding fully substituted enynylboronates in good yields with high levels of chemo-, regio-and stereoselectivities under multicomponent and tandem reaction conditions. The proposed mechanism for borylalkynylation reaction involves regioselective addition of the in situ formed LCu-Bpin to the CÀ C triple bond of the nonsilylated internal alkyne selectively in presence of a silylated alkynyl bromide (chemoselectively) to generate the required β-borylalkenylcopper species.…”