Abstract
Transition metal-catalysed C–H hydroxylation is one of the most notable advances in synthetic chemistry during the past few decades and it has been widely employed in the preparation of alcohols and phenols. The site-selective hydroxylation of aromatic C–H bonds under mild conditions, especially in the context of substituted (hetero)arenes with diverse functional groups, remains a challenge. Here, we report a general and mild chelation-assisted C–H hydroxylation of (hetero)arenes mediated by boron species without the use of any transition metals. Diverse (hetero)arenes bearing amide directing groups can be utilized for ortho C–H hydroxylation under mild reaction conditions and with broad functional group compatibility. Additionally, this transition metal-free strategy can be extended to synthesize C7 and C4-hydroxylated indoles. By utilizing the present method, the formal synthesis of several phenol intermediates to bioactive molecules is demonstrated.
Highlights
Transition metal-catalysed C–H hydroxylation is one of the most notable advances in synthetic chemistry during the past few decades and it has been widely employed in the preparation of alcohols and phenols
The ability to prepare synthetically relevant scaffolds via regio-controlled C–H hydroxylation under mild conditions by using cheap oxidants and avoiding the use of transition metals would be of great importance
We found that the boron-mediated directed C–H hydroxylation of N-phenylpyrrolidinone (29a) in the presence of BBr3 could provide the desired product 29c with a 79% yield
Summary
Transition metal-catalysed C–H hydroxylation is one of the most notable advances in synthetic chemistry during the past few decades and it has been widely employed in the preparation of alcohols and phenols. Diverse (hetero)arenes bearing amide directing groups can be utilized for ortho C–H hydroxylation under mild reaction conditions and with broad functional group compatibility This transition metal-free strategy can be extended to synthesize C7 and C4-hydroxylated indoles. Substrates bearing a chelating functional group can coordinate with the metal catalyst and undergo further C–H functionalization[32,33,34] In this context, several groups have explored transition-metal-catalysed directed aromatic C–H hydroxylation using organic oxidants, hydrogen peroxide or molecular oxygen (Fig. 1b)[35,36,37,38,39,40,41,42,43,44,45,46]. Alternative pathway for synthesizing phenols and has exciting possibilities because of the superior practicality, low cost, and environmental friendliness of this alternate method
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