Cyclic (Alkyl)(amino)carbene Ligands Enable Cu‐Catalyzed Markovnikov Protoboration and Protosilylation of Terminal Alkynes: A Versatile Portal to Functionalized Alkenes**

Abstract

Regioselective hydrofunctionalization of alkynes represents a straightforward route to access alkenyl boronate and silane building blocks. In previously reported catalytic systems, high selectivity is achieved with a limited scope of substrates and/or reagents, with general solutions lacking. Herein, we describe a selective copper-catalyzed Markovnikov hydrofunctionalization of terminal alkynes that is facilitated by strongly donating cyclic (alkyl)(amino)carbene (CAAC) ligands. Using this method, both alkyl-and aryl-substituted alkynes are coupled with a variety of boryl and silyl reagents with high α-selectivity. The reaction is scalable, and the products are versatile intermediates that can participate in various downstream transformations.Preliminary mechanistic experiments shed light on the role of CAAC ligands in this process. File list (2) download file view on ChemRxiv Manuscript.pdf (2.91 MiB) download file view on ChemRxiv Supporting Info.pdf (7.13 MiB)

Origins of Internal Regioselectivity in Copper-Catalyzed Borylation of Terminal Alkynes

“…The characteristic feature of the B­(aam) moiety to be installed is slightly residual Lewis acidity that allows its direct use for SMC under standard conditions, and thus, this was applied to the two-step total synthesis of biologically active iso -Combretastatin A4 conducted all in one-pot. In view of the versatility of the boryl-substituted alkenylcopper intermediate, which can be captured by various electrophiles, the present results would open new avenues for catalytic borylations with rare regioselectivity. , …”

Recent Advances in the Domain of Cyclic (Alkyl)(Amino) Carbenes

“…The further one-electron reduction of 76 produces a novel dimeric borylketyl radical anion (77) by intramolecular Dipp group transfer to the carbonyl carbon. [138] On further reduction with KC 8 it gives a dimeric compound (78) with OÀ KÀ O linkage and on hydrolysis, it converts into a desired protonated dimer (79) (Scheme 34).…”

“…have synthesized a series of cAAC−Cu and NHC−Cu metal complexes for the selective hydroboration and hydrosilylation of terminal aliphatic and aromatic alkynes (Scheme 11). They found that cAAC−Cu complex ( 24 and 25 ) gave the better selectivity of the Markonikov selective hydrofunctionalized product with good to excellent yield [78] . The reaction condition is highly tolerant for both alkyne as well as a variety of boryl and silyl reagents.…”

Recent Advances in the Functionalization of Terminal and Internal Alkynes

“…In 2021, the same group reported an interesting copper‐catalyzed Markovnikov protoboration and protosilyation of terminal alkynes through strongly donating cyclic (alkyl)(amino)carbene (CAAC) ligands (Scheme 17). [21] A variety of CAAC‐ligated Cu complexes were examined but EtCAAC 6 (L 4 ) and PhEtBiCAAC (L 6 ) ligands were found to be efficient for this transformation. The initial reaction was performed using 5‐phenyl‐1‐pentyne as the model substrate and B 2 pin 2 as the boron coupling partner.…”