作者:Scott A. Sadler、Hazmi Tajuddin、Ibraheem A. I. Mkhalid、Andrei S. Batsanov、David Albesa-Jove、Man Sing Cheung、Aoife C. Maxwell、Lena Shukla、Bryan Roberts、David C. Blakemore、Zhenyang Lin、Todd B. Marder、Patrick G. Steel
DOI:10.1039/c4ob01565g
日期:——
The iridium-catalysed CâH borylation is a valuable and attractive method for the preparation of aryl and heteroaryl boronates. However, application of this methodology for the preparation of pyridyl and related azinyl boronates can be challenged by low reactivity and propensity for rapid protodeborylation, particularly for a boronate ester ortho to the azinyl nitrogen. Competition experiments have revealed that the low reactivity is due to inhibition of the active catalyst through coordination of the azinyl nitrogen lone pair at the vacant site on the iridium. This effect can be overcome through the incorporation of a substituent at C-2. Moreover, when this is sufficiently electron-withdrawing protodeborylation is sufficiently slowed to permit isolation and purification of the C-6 boronate ester. Following functionalization, reduction of the directing C-2 substituent provides the product arising from formal ortho borylation of an unhindered pyridine ring.
铱催化的C-H硼化是一种有价值且有吸引力的方法,用于制备芳基和杂芳基硼酸酯。然而,这种方法在制备吡啶基及相关氮杂基硼酸酯时可能面临反应活性低和快速质子脱硼化的挑战,尤其是当硼酸酯位于氮杂基氮原子的邻位时。竞争实验揭示,低反应活性是由于氮杂基氮原子的孤对电子在铱的空位上配位,抑制了活性催化剂。通过在C-2位引入取代基可以克服这一效应。此外,当该取代基足够吸电子时,质子脱硼化过程会被足够减缓,从而允许分离和纯化C-6硼酸酯。经过功能化后,导向C-2取代基的还原提供了源自未受阻吡啶环形式邻位硼化的产物。