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 B-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 ΔG = 27.

Catalyst-Free Formal Thioboration to Synthesize Borylated Benzothiophenes and Dihydrothiophenes.

The first ring-forming thioboration reaction of C-C π bonds is reported. This catalyst-free method proceeds in the presence of a commercially available external electrophilic boron source (B-chlorocatecholborane) in good to high yields. The method is scalable and tolerates a variety of functional groups that are intolerant of other major borylation methods.

Catalyst-Free Synthesis of Borylated Lactones from Esters via Electrophilic Oxyboration.

A catalyst-free oxyboration reaction of alkynes is developed. The resulting borylated isocoumarins and 2-pyrones are isolated as boronic acids, pinacolboronate esters, or potassium organotrifluoroborate salts, providing a variety of bench-stable organoboron building blocks for downstream functionalization. This method has functional group compatibility, is scalable, and proceeds with readily available materials: B-chlorocatecholborane and methyl esters.

Oxyboration: Synthesis of Borylated Benzofurans.

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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.

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 E,E, Z,Z, or E,Z stereochemistry. Various functional groups are tolerated including aryl bromides, and applications are illustrated.