Stoichiometric and Catalytic C−C and C−H Bond Formation with B(C₆F₅)₃ via Cationic Intermediates

Abstract

This work showcases a new catalytic cyclization reaction using a highly Lewis acidic borane with concomitant C-H or C-C bond formation. The activation of alkyne-containing substrates with B(C F ) enabled the first catalytic intramolecular cyclizations of carboxylic acid substrates using this Lewis acid. In addition, intramolecular cyclizations of esters enable C-C bond formation as catalytic B(C F ) can be used to effect formal 1,5-alkyl migrations from the ester functional groups to unsaturated carbon-carbon frameworks. This metal-free method was used for the catalytic formation of complex dihydropyrones and isocoumarins in very good yields under relatively mild conditions with excellent atom efficiency.

B(C6F5)3-catalyzed dehydrogenative cyclization of N-tosylhydrazones and anilines via a Lewis adduct: a combined experimental and computational investigation

“…[26][27][28] Consequently, B(C 6 F 5 ) 3 catalyzed cyclization leading to important heterocyclic scaffolds is rare. [29][30][31][32] In this context, 1,2,4-triazoles are omnipresent heterocyclic motifs in numerous biologically active compounds 33,34 and they also have widespread applications in organic light emitting diodes, 35,36 organic photovoltaic cells, electroluminescent devices, 37,38 bi-stable resistive memory devices, 39 pesticides, and medicines. 40 Given their applications, a number of strategies have been implemented for the synthesis of 1,2,4-triazoles.…”

A Comparative Assessment of Modern Cyclization Methods of Substituted Alkynyl Esters, Ethers, and Acids

“…Expounding on this, our group used B(C6F5)3 to affect similar rearrangements as observed with Takaki, with stoichiometric experiments generating the pyrone and isocoumarin products in excellent yields (generally >95%) in as little as 10 minutes. [35] This holds true for a number of substituents on the ether linkage such as benzyl, ethylphenyl and benzhydrol moieties. We then attempted the corresponding carboxylic acid which interestingly did produce the unsaturated 2-pyrone with selective transfer of the proton to the γ-position.…”

Boron‐Based Catalysts for C−C Bond‐Formation Reactions

“…The group of Melen very recently described that, in the presence of a catalytic amount of 1 , the intramolecular cyclization of alkyne‐containing carboxylic esters 58 proceeded through alkyl group migration to afford dihydropyrones or isocoumarins products 59 (Scheme ) . In the screening of other acid catalysts, such as BPh 3 , BF 3 ⋅ OEt, and triflic acid, a trace amount of product was obtained.…”