Friedel–Crafts Acylation Reactions of BN-Substituted Arenes

Abstract

The acylation reaction of BN-arenes has been studied using BN-arene and acyl chloride in good to excellent yields, which led to the first synthesis of indanone BN-analogue. The BN-aromatic ketone products have been characterized by H NMR spectroscopy with their molecular structures unambiguously confirmed by X-ray crystallography. The annulation reaction of BN-arenes promoted by AgBF occurs in a completely regioselective manner and a mechanism for this transformation is proposed.

Late-stage functionalization of BN-heterocycles

“…Acylation of the C4 position with acyl chlorides proceeds in good to excellent yield with either AlCl 3 or AgBF 4 as the Lewis-acid promoter (Scheme 27). 9 Aroyl and alkanoyl chlorides were effective substrates in the acylation reaction and all furnished C4 BN-naphthyl ketones. Dimethylcarbamoyl chloride was the exception, which yielded the C2 isomer 54p.…”

“…8 In another publication by the Fang group, the C3 position of 5 underwent electrophilic attack with NIS activated by AlCl 3 in high yield (Scheme 1d). 9 While the C3 position of the 1,2-azaborine attacks halogen electrophiles, the C5 position tends to engage in Friedel-Crafts reactivity. Ashe also reported the first example of C5 substitution; Friedel-Crafts acetylation of compound 1 proceeds in poor yield in the presence of acetic anhydride and SnCl 4 (Scheme 2a).…”

“…Fang built on the work of Ashe to develop an improved Friedel-Crafts acylation method for N-methyl, B-mesityl-substituted azaborine 5 (Scheme 2b). 9 The mesityl (Mes = 2,4,6-trimethylphenyl) group is known to provide robustness to the azaborine heterocycle against unwanted side reactions such as oxidative degradation. AgBF 4 activates various acylchlorides at 50 °C in CH 2 Cl 2 toward Friedel-Crafts reaction with 1,2-azaborines 5 or 7.…”

“…As part of their acylation work, the Fang group investigated the reaction between 5 and methacryloyl chloride, which produced BN-indanone analogue 10 (Scheme 3a). 9 According to the proposed mechanism, 5 first undergoes C5 Friedel-Crafts acylation followed by a Nazarov cyclization at the C6 position. In another report, Fang and co-workers developed a procedure for the regioselective nitration of 5 and 6 (Scheme 3b).…”

“…15 The scope with respect to the nucleophiles is quite broad, including various N-,O-based nucleophiles and alkyl, alkenyl, and aryl organometallic reagents (Scheme 9a). In addition to the N-Et derivatives 1, 15 examples of N-Me-(compounds 5), 9,16 and N-Bn-substituted 1,2-azaborine (compounds 24) 17 have been reported. Substitution at boron is also possible with nucleophiles generated in situ with non-nucleophilic bases such as KH and Et 3 N (Scheme 9b).…”