Abstract
The addition of AlCl3 to four-coordinate boranes of the general formula (C–N-chelate)BCl2 results in halide abstraction and formation of three-coordinate borenium cations of the general formula [(C–N-chelate)BCl]+. The latter react with both arylstannanes and arylsilanes by boro-destannylation and -desilylation, respectively, to form arylated boranes. Catalytic quantities of AlCl3 were sufficient to effect high-yielding arylation of (C–N-chelate)BCl2. Boro-destannylation is more rapid than boro-desilylation and leads to double arylation at the boron center, whereas in reactions with arylsilanes either single or double arylation occurs dependent on the nucleophilicity of the arylsilane and on the electrophilicity of the borenium cation. The electrophilicity of the borenium cation derived from 2-phenylpyridine was greater than that of the benzothiadiazole analogues, enabling the boro-desilyation of less nucleophilic silanes and the direct electrophilic borylation of 2-methylthiophene.
Highlights
Four-coordinate boron compounds containing a chelating πconjugated C/N donor and two exocyclic aromatic moieties, termed (C−N-chelate)BAr2 (e.g., 1-BAr2 right Scheme 1), haveScheme 1
Instead functionalization of boraneLewis adducts such as (2-phenylpyridyl)BBr2 (1-BBr2, Scheme 1) requires organozinc or organoaluminum reagents to achieve high-yielding transmetalation.[3,4]. These nucleophiles are highly sensitive to protic species (ROH), and the synthesis of organozinc reagents often results in mixtures containing ionic species and coordinated etherate solvent, which can complicate transmetalation.[6]
The process is potentially catalytic in the halophile, as the byproduct from transmetalation will react as a functional equivalent of [R3Si]+ or [R3Sn]+, abstracting halide to generate further equivalents of borenium cations for subsequent transmetalation (Scheme 2)
Summary
Four-coordinate boron compounds containing a chelating πconjugated C/N donor and two exocyclic aromatic moieties, termed (C−N-chelate)BAr2 (e.g., 1-BAr2 right Scheme 1), have. Installation of aromatic moieties at three-coordinate boron species is generally achieved by reaction with either arylithium or aryl Grignard reagents.[5] reaction of these reagents with Lewis base adducts of boranes often gives the desired product in poor yield.[4] Instead functionalization of boraneLewis adducts such as (2-phenylpyridyl)BBr2 (1-BBr2, Scheme 1) requires organozinc or organoaluminum reagents to achieve high-yielding transmetalation.[3,4] these nucleophiles are highly sensitive to protic species (ROH), and the synthesis of organozinc reagents often results in mixtures containing ionic species (termed zincates) and coordinated etherate solvent, which can complicate transmetalation.[6] Alternative nucleophiles are required that are readily synthesized, are well-defined, can be handled in air, and enable the boron-containing products to be isolated, preferably without column chromatography. Is reported catalytic (in AlCl3 activator) borenium cation mediated borylation as a simple method to functionalize (C−N-chelate)BCl2 species based on benzothiadiazole (BT) and pyridyl with aryl and heteroaryl groups
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