Carboboration of Alkynes with Cyclodextrin‐Encapsulated N‐Heterocyclic Carbene Copper Complexes

Abstract

The copper‐catalyzed carboboration of various alkynes was investigated with a modified N‐heterocyclic carbene‐capped α‐cyclodextrin copper(I) complex in which the reactive copper center is deeply encapsulated in the cyclodextrin (CD) cavity. The methylborylation of terminal alkynes was found to give linear (L) (E)‐vinyl boron isomers as the major isomers, as expected from the previously proposed “perpendicular” approach of the alkyne to the Cu–B bond, and methylation of the vinyl boron copper intermediate. Under similar conditions, the intramolecular carboboration reaction with terminal alkynes functionalized by alkyl halides, led to exocyclic vinyl boronate species as the major isomer. However, an endocyclic (Z)‐isomer was also observed in some cases. This isomer was not previously observed and is unexpected considering the “classical” mechanism. The direct generation of boron functionalized (Z)‐alkenes by carboboration of alkynes is unprecedented.

Mapping C−H⋅⋅⋅M Interactions in Confined Spaces: (α‐ICyDMe)Au, Ag, Cu Complexes Reveal “Contra‐electrostatic H Bonds” Masquerading as Anagostic Interactions**

“…The spectroscopic signatures of which interactions have been used to get deeper insights into their electronic origin [18,30–32] . Recently, the Sollogoub group reported synthesis, [33] NMR analysis, [34] and catalytic applications [33–37] of several NHC‐MX complexes with M=Au, Ag, Cu embedded in a cyclodextrin cavity (Figure 1). [38] These well‐defined NHC‐capped cyclodextrins (ICyDs) are characterized by a network of introverted cavity protons, which show intriguing behavior by 1 H NMR spectroscopy.…”

Indol-2-Ylidene (IdY): A New Ambiphilic N-Heterocyclic Carbene Derived from Indole

“…Having confirmed the ambiphilic nature and considerable steric bulkiness of IdY, we seeked to demonstrate the utility of IdYmetal complexes in organic synthesis. Following the previous work in the field, NHC complexes of large buried volume [25] and strong π-accepting NHC ligands [26] find application in regioselective carboboration of alkynes. Inspired by these works, we investigated the benzylboration of p-tolyl acetylene in presence of 6 as a catalyst.…”

Synthetic Application of Cyclodextrins in Combination with Metal Ions, Complexes, and Metal Particles

“…In a related recent study, they have used complex 120α in carboboration. [217] In methylboration of alkynes with methyl iodide, the perpendicular approach allows the formation of the linear products with very high selectivity of > 91 : 9 (Scheme 73). A reaction at lower temperature (20°C) resulted in lower conversion and yield but it did not affect selectivity.…”