Zwitterion‐Initiated Hydroboration of Alkynes and Styrene

Abstract

The hydroboration of alkynes and styrene with HBpin has been developed using tris(pentaflurophenyl)borane (B(C6F5)3) as the initiator of catalysis. The hydroboration is proposed to be initiated by Lewis acid activation of the alkyne by (B(C6F5)3) to form a highly reactive zwitterionic species which subsequently react with HBpin to give the alkenyl boronic ester. This zwitterion has also showed potential to be a competent catalyst for the hydroboration of styrene. The zwitterionic intermediate is analogous to that proposed in the Piers borane‐catalysed hydroboration and 1,1‐carboboration of alkynes with B(C6F5)3.

Reaction mechanism study on reactions of phenylacetylenes with HSnEt3promoted by B(C6F5)3with and without DABCO

“…Zwitterionic intermediate 5 was reported by Thomas et al in the reaction of B(C 6 F 5 ) 3 with phenylacetylene and was used as an initiator to achieve the hydroboration of phenylacetylene. 15 Zwitterionic intermediate 5 will be transformed into the complexes 6 and 6′ via the interaction of HSnEt 3 with the carbon cationic center of 5 . Afterward, hydride transfers from HSnEt 3 to the carbon cationic center through TS4 (Δ G ‡ = 25.7 kcal mol −1 ) or TS4′ (Δ G ‡ = 29.2 kcal mol −1 ) to form intermediates 7 and 7′ .…”

Metal‐Free Catalytic Hydroboration of Unsaturated Compounds: A Greener Strategy for the Synthesis of Organoboranes

“…catalyst (Scheme 3b). [42] Subsequently, HBpin reacted with the zwitterion (10) to deliver alkenylboronates (11). Another metalfree catalytic approach was realized by Melen et al where tris (2,4,6-trifluorophenyl)borane (2,4, ) was used as an extremely versatile catalyst.…”

Boron-Based Lewis Acid Catalysis: Challenges and Perspectives

“…In the presence of BCF as a catalyst, a variety of carbazoles 35 functionalisation occurs with good to excellent yields (up to 97%) and high selectivity. Similarly, the same research group employed a similar approach to improve the efficiency of well-known transformations via BCF catalysis [42]. They developed a protocol to perform the hydroboration of alkynes 47 and styrene with HBpin 41 using BCF as the initiator of catalysis (Scheme 15).…”

“…This zwitterion is a strong enough Lewis acid to catalyse the hydroboration of alkenes, offering a promising methodology to explore further reactivity of the archetypal boron Lewis acid. Similarly, the same research group employed a similar approach to improve the efficiency of well-known transformations via BCF catalysis [42]. They developed a protocol to perform the hydroboration of alkynes 47 and styrene with HBpin 41 using BCF as the initiator of catalysis (Scheme 15).…”