Alkoxyboration: Ring-Closing Addition of B–O σ Bonds across Alkynes

For nearly 70 years, the addition of boron–X σ bonds to carbon–carbon multiple bonds has been employed in the preparation of organoboron reagents. However, the significantly higher strength of boron–oxygen bonds has thus far precluded their activation for addition, preventing a direct route to access a potentially valuable class of oxygen-containing organoboron reagents for divergent synthesis. We herein report the realization of an alkoxyboration reaction, the addition of boron–oxygen σ bonds to alkynes. Functionalized <i>O</i>-heterocyclic boronic acid derivatives are produced using this transformation, which is mild and exhibits broad functional group compatibility. Our results demonstrate activation of this boron–O σ bond using a gold catalysis strategy that is fundamentally different from that used previously for other boron addition reactions

Mechanistic Studies of Formal Thioboration Reactions of Alkynes

Several formal heteroborylative cyclization reactions have been recently reported, but little physical–organic and mechanistic data are known. We now investigate the catalyst-free formal thioboration reaction of alkynes to gain mechanistic insight into <i>B</i>-chloro­catechol­borane (ClBcat) in its new role as an alkynophilic Lewis acid in electrophilic cyclization/dealkylation reactions. In kinetic studies, the reaction is second-order globally and first-order with respect to both the 2-alkynylthioanisole substrate and the ClBcat electrophile, with activation parameters of Δ<i>G</i>^‡ = 27.1 ± 0.1 kcal mol^–1 at 90 °C, Δ<i>H</i>^‡ = 13.8 ± 1.0 kcal mol^–1, and Δ<i>S</i>^‡ = −37 ± 3 cal mol^–1 K^–1, measured over the range 70–90 °C. Carbon kinetic isotope effects supported a rate-determining Ad_E3 mechanism wherein alkyne activation by neutral ClBcat is concerted with cyclative attack by nucleophilic sulfur. A Hammett study found a ρ⁺ of −1.7, suggesting cationic charge buildup during the cyclization and supporting rate-determining concerted cyclization. Studies of the reaction with tris­(pentafluoro­phenyl)­borane (B­(C₆F₅)₃), an activating agent capable of cyclization but not dealkylation, resulted in the isolation of a postcyclization zwitterionic intermediate. Kinetic studies via UV–vis spectroscopy with this boron reagent found second-order kinetics, supporting the likely relevancy of intermediates in this system to the ClBcat system. Computational studies comparing ClBcat with BCl₃ as an activating agent showed why BCl₃, in contrast to ClBcat, failed to mediate the complete the cyclization/demethylation reaction sequence by itself. Overall, the results support a mechanism in which the ClBcat reagent serves a bifunctional role by sequentially activating the alkyne, despite being less electrophilic than other known alkyne-activating reagents and then providing chloride for post-rate-determining demethylation/neutralization of the resulting zwitterionic intermediate

Cyclization Strategies to Polyenes Using Pd(II)-Catalyzed Couplings of Pinacol Vinylboronates

As a complement to Pd(0)-catalyzed cyclizations, seven Pd(II)-catalyzed cyclization strategies are reported. α,ω-Diynes are selectively hydroborated to bis(boronate esters), which cyclize under Pd(II)-catalysis producing a diverse array of small, medium, and macrocyclic polyenes with controlled <i>E,E</i>, <i>Z,Z</i>, or <i>E,Z</i> stereochemistry. Various functional groups are tolerated including aryl bromides, and applications are illustrated

Cyclization Strategies to Polyenes Using Pd(II)-Catalyzed Couplings of Pinacol Vinylboronates

As a complement to Pd(0)-catalyzed cyclizations, seven Pd(II)-catalyzed cyclization strategies are reported. α,ω-Diynes are selectively hydroborated to bis(boronate esters), which cyclize under Pd(II)-catalysis producing a diverse array of small, medium, and macrocyclic polyenes with controlled <i>E,E</i>, <i>Z,Z</i>, or <i>E,Z</i> stereochemistry. Various functional groups are tolerated including aryl bromides, and applications are illustrated