Alkoxide‐Promoted Selective Hydroboration of N‐Heteroarenes: Pivotal Roles of in situ Generated BH₃ in the Dearomatization Process

Abstract

While numerous organo(metallic)catalyst systems were documented for dearomative hydroboration of N‐aromatics, alkoxide base catalysts have not been disclosed thus far. Described herein is the first example of alkoxide‐catalyzed hydroboration of N‐heteroaromatics including pyridines, providing a broad range of reduced N‐heterocycles with high efficiency and selectivity. Mechanistic studies revealed an unprecedented counterintuitive dearomatization pathway, in which 1) pyridine‐BH3 adducts undergo a hydride attack by alkoxyborohydrides, 2) in situ generated BH3 serves as a catalytic promoter, and 3) 1,4‐dihydropyridyl borohydride is in a predominant resting state.

Double Hydroboration of Quinolines via Borane Catalysis: Diastereoselective One Pot Synthesis of 3‐Hydroxytetrahydroquinolines

“…On the other hand, a number of elegant catalytic hydroboration methodologies for generating six‐membered N ‐heteroarenes have been developed by using organohydroboranes such as pinacolborane to selectively afford N ‐borylated 1,2‐ or 1,4‐dihydro‐products (Scheme c) . For example, in situ generated metal hydride species based on Mg, Rh, Zn, La, and Th were found to promote 1,2‐hydroboration of pyridines– while 1,4‐selectivity was observed via an ionic outer‐sphere pathway catalysed by Lewis or Brønsted acids –. Despite these recent advances in the dearomative hydroboration of N ‐heteroarenes, to our best knowledge, no catalytic procedures have been revealed thus far for the sp 3 C−B bond‐forming hydroboration of N ‐heteroaromatics …”

nBuLi-promoted anti-Markovnikov selective hydroboration of unactivated alkenes and internal alkynes

“…For example, it is highly desirable to develop simple base catalysed hydroboration methods [8][9][10][11][12] of alkenes with comparable efficiency from a practical point of view due to the economic and environmental benefits of base catalysts. However, the scopes of known chemistries on base-promoted hydroboration are limited mainly to carbonyl compounds, 9 pyridine 10 or terminal alkyne 11 (Scheme 1B).…”

Hydrosilylation and hydroboration in a sustainable manner: from Earth-abundant catalysts to catalyst-free solutions

“…At almost the same time, the Chang group reported a similar procedure utilizing less sensitive potassium alkoxide as the catalyst (Scheme 17). 125 Here, the reduction occurs selectively at the 1,4-position under an argon atmosphere, and in the presence of expensive 18-crown-6 ether. In addition, the authors disclosed detailed mechanistic studies.…”