Transition-Metal-Catalyzed Functionalization of Alkynes with Organoboron Reagents: New Trends, Mechanistic Insights, and Applications

Abstract

Catalytic functionalization of alkynes with organoboron reagents provides a straightforward access to stereochemically defined multisubstituted alkenes, which are structural motifs commonly found in bioactive compounds and organic materials. Recent progress has substantially broadened the scope of this field on several fronts. Strategies for regioselectivity control in the 1,2-migratory insertion across unsymmetrical internal alkynes, as well as for the direct access to products with anti-insertion stereochemistry, have been devised. The alkenyl-to-aryl 1,4-metal migration upon metal insertion has been recently exploited in powerful cascade sequences leading to complex polycyclic scaffolds, including the development of enantioselective processes. Elegant enantiospecific and dynamic kinetic resolution methods have been developed for accessing chiral allenes from propargylic alcohol derivatives. Mechanistic manifolds have emerged based on single-electron transfer (SET) that have provided a fresh impetus for alkyne 1,2-difunctionalization with complementary stereoselectivity to processes relying on 1,2-insertion of R–M species. Herein, we discuss the most recent advances in transition-metal-catalyzed functionalization of alkynes using organoboron reagents, categorized according to the type of mechanistic outcome. Emphasis is placed on mechanistic aspects, synthetic utility, limitations, and challenges for future research.

Nickel‐Catalyzed Arylative Cyclizations of Alkyne‐ and Allene‐Tethered Electrophiles using Arylboron Reagents

“…Because of their generally good chemical stability, low toxicity, and widespread availability, arylboron compounds are commonly used as pronucleophiles in transition-metal-catalyzed domino carbometalation-cyclization reactions of alkyne- [60] or allene-tethered electrophiles. Arylative cyclization reactions of alkyne-tethered electrophiles involving an arylboron species have been described using rhodium, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] palladium, [32][33][34][35][36][37][38][39][40][41] and copper catalysis.…”

Relay Rhodium(I)/Acid Catalysis for Rapid Access to Benzo‐2H‐Pyrans and Benzofurans

“…Based on our experimental results and literature report, [13c,18] a plausible mechanism for the syntheses of benzo‐2 H ‐pyran 4 a and benzofuran 5 a was proposed in Scheme 5. First, the rhodium(I) catalyst reacted with arylboronic acid 2 a to form the ortho ‐hydroxylated phenylrhodium species.…”

Interplay between the Directing Group and Multifunctional Acetate Ligand in Pd-Catalyzed anti-Acetoxylation of Unsymmetrical Dialkyl-Substituted Alkynes

“…Interestingly, the SO 2 Py directing group is capable to override the inherent electronic bias due to conjugation imposed by the aromatic substituent in internal aryl-substituted alkynes, since this class of alkynes typically undergoes insertion with opposite regioselectivity. 9 As illustrated for products Z - 13 – 16 , the excellent β-regioselectivity was maintained for some representative aryl alkynes regardless of the electronic character of the aromatic ring. In all cases studied, the corresponding acetoxylation products were isolated as single regioisomers in synthetically useful yields, even for substrates containing strong electron-withdrawing groups ( Z - 15 and Z -16 , 52 and 50%, respectively).…”