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

Abstract

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. Mechanistic studies indicate that the B-chlorocatecholborane acts as a carbophilic Lewis acid toward the alkyne, providing a mechanistically distinct pathway for oxyboration that avoids B–O σ bond formation and enables this catalyst-free route.

Iridium‐Catalyzed Cycloisomerization of Alkynoic Acids: Synthesis of Unsaturated Lactones

“…[2] Owing to the great importance of these intriguing structures, much attention was paid to exploring synthetic methods, and tremendous progress have been achieved over the last decades. Many synthetic methodologies were well established to prepare lactones, [3][4][5][6][7][8][9][10][11][12][13][14] such as oxidative cyclization reaction of alkynylbenzaldehydes catalyzed by NHC (N-heterocyclic carbenes), [3f] bromolactonization of long-chain olefinic acids promoted by sulfur-based zwitterionic organocatalyst, [8] intermolecular [6 + 2] cyclization of amphoteric molecules with siloxy alkynes, [9] oxetene ring-opening reaction, [10] borylated lactones from esters with electrophilic oxyboration, [11] and intramolecular addition of carboxylic acids or esters to alkynes or alkenes promoted by transition metal, organic acids or bases. [3a-e,g,4,5,6a, [12][13][14] Among these synthetic routes to lactones, the cycloisomerization of alkynoic acids is a straightforward and attractive synthetic method to access lactones with great atom economy.…”

Divergent Elementoboration: 1,3‐Haloboration versus 1,1‐Carboboration of Propargyl Esters

“…The activation of carbon–carbon double and triple bonds by main group compounds has been a staple motif in the synthesis of a plethora of new element–carbon bonds such as C−C, C−H, C−N, C−B, and C−O bonds amongst many others . Seminal work by Wrackmeyer et al.…”

Palladium/Acid Relay Catalyzed Tandem Heck Coupling/6‐Endo Cyclization between ortho‐Halogenated Benzoates and Unactivated Terminal Alkenes for the Synthesis of 1‐Isochromanones

“…Once the Heck adduct A was formed, electrophilic addition of A with a proton cation was occurred to give a carbocation intermediate B (Route A), which was rapidly trapped by oxygen atom in carbonyl group through p-p conjugated effect [15] from esters to give oxonium intermediate C. Finally, elimination of the alkyl group from C, [7,16] to afford the desired product D. The intermediate E was trapped by HRMS experiment (see supporting information). Once the Heck adduct A was formed, electrophilic addition of A with a proton cation was occurred to give a carbocation intermediate B (Route A), which was rapidly trapped by oxygen atom in carbonyl group through p-p conjugated effect [15] from esters to give oxonium intermediate C. Finally, elimination of the alkyl group from C, [7,16] to afford the desired product D. The intermediate E was trapped by HRMS experiment (see supporting information).…”