(CAAC)Copper Catalysis Enables Regioselective Three-Component Carboboration of Terminal Alkynes

Abstract

Cyclic­(alkyl)­(amino)­carbene (CAAC) ligands are found to perturb regioselectivity of the copper-catalyzed carboboration of terminal alkynes, favoring the less commonly observed internal alkenylboron regiosomer through an α-selective borylcupration step. A variety of carbon electrophiles participate in the reaction, including allyl alcohol derivatives and alkyl halides. The method provides a straightforward and selective route to versatile trisubstituted alkenylboron compounds that are otherwise challenging to access.

Facile and General Electrochemical Diselenylation of Terminal Alkynes

“…Alkynes are one of the most flexible building blocks and widely used as sustainable feedstocks for chemical synthesis, as well as in the production of numerous medications and materials. , Catalytic difunctionalization of these π systems is a direct and efficient method to access a variety of structurally diverse trisubstituted compounds. − Trisubstituted alkenes are pivotal structural motifs in the structure of numerous medicines − and materials, as well as helpful precursors for other functional groups and chiral centers . The methods employed, however, rely heavily on the use of organometallic species and require high-temperature or multistep reactions (Scheme a). − More recently, radical-mediated reactions have been devised, which provide an effective technique for difunctionalizing terminal alkynes. − In general, a radical is generated from various radical precursors, such as sulfonyl chlorides, α,β-unsaturated ketones, activated alkyl bromides, fluoroalkyl iodides, etc., − which is via a single electron transfer (SET) process employing transition metal species, − photocatalysts, − or electrocatalysis (Scheme b).…”

Recent Advances in Nonprecious Metal Catalysis

“…In a related work, Bertrand, Jazzar, Engle, and co-workers reported cyclic(alkyl)(amino)carbene (CAAC)Cu-catalyzed carboboration of terminal alkynes to form internal alkenylboron products, useful building blocks for organic synthesis, with high regioselectivity (Scheme 33). 65 This report is an extension of the author's previous work on (CAAC)Cu-catalyzed protoboration of terminal alkynes with a variety of bisboron nucleophiles. 66 The regioselectivity is complementary to the known Cu−boryl catalysis that favors the terminal alkenylboron products.…”

Chemoselective Coupling of π‐Systems to Access Metallated 1,4‐ or 1,5‐Skipped Dienes in Multicomponent Reactions

“…It is important to note that with terminal alkynes the vast majority of the allylboration process direct the boryl group to the terminal (β) position, restricting access to the opposite alkenylboron regioisomers. To perturb this general regioselectivity, Bertrand, Jazzar, and Engle research groups developed a novel methodology to enable α‐selective borylcupration by using cyclic(alkyl)(amino)‐carbene (CAAC)‐ligated copper catalyst (Scheme 5‐iv) [17] . The reaction of alkynes A 10 with allylphosphates B 10 in the presence of 6.0 mol% Et CAAC 5 ‐ligated Cu complex ( L CuCl) and 1.5 equiv.…”

“…To perturb this general regioselectivity, Bertrand, Jazzar, and Engle research groups developed a novel methodology to enable α-selective borylcupration by using cyclic(alkyl)(amino)-carbene (CAAC)-ligated copper catalyst (Scheme 5-iv). [17] The reaction of alkynes A 10 with allylphosphates B 10 in the presence of 6.0 mol% Et CAAC 5 -ligated Cu complex (LCuCl) and 1.5 equiv. of LiO-t Bu delivered the αborylated skipped dienes 20 in high regioselectivity.…”