Lithium Dihydropyridine Dehydrogenation Catalysis: A Group 1 Approach to the Cyclization of Diamine Boranes

Abstract

In reactions restricted previously to a ruthenium catalyst, a 1‐lithium‐2‐alkyl‐1,2‐dihydropyridine complex is shown to be a competitive alternative dehydrogenation catalyst for the transformation of diamine boranes into cyclic 1,3,2‐diazaborolidines, which can in turn be smoothly arylated in good yields. This study established the conditions and solvent dependence of the catalysis through NMR monitoring, with mechanistic insight provided by NMR (including DOSY) experiments and X‐ray crystallographic studies of several model lithio intermediates.

A Highly Active Bidentate Magnesium Catalyst for Amine‐Borane Dehydrocoupling: Kinetic and Mechanistic Studies

“…The catalyst also works in THF, albeit at a significantly reduced rate (Figure S16), which is attributed to co‐ordination of the solvent to the catalyst active site . A similar solvent effect was seen with a lithium‐based dehydrocoupling catalyst studied by Mulvey and co‐workers …”

“…[43] As imilar solvent effect was seen with al ithium-based dehydrocoupling catalyst studied by Mulvey and co-workers. [44] Reactions with the related amine-borane iPr 2 NH·BH 3 afforded the dehydrogenated compound iPr 2 N=BH 2 as the dominant product (> 95 %, Scheme 1d). This reactionp roceeded (albeit slowly)a tr oom temperature ( Figure S21 and S22) and increasing the temperature to 60 8Cg ave near total conversion in less than 1hour (5 mol %c atalyst loading, Figure S23).…”

Magnesocenophane‐Catalyzed Amine Borane Dehydrocoupling

“…In previously reported lithium‐ and magnesium‐based systems, strong solvent effects were observed, insofar as dehydrocoupling reactions were slower in donor solvents, such as thf, but faster in non‐coordinating solvents, such as aromatic hydrocarbons . This can be rationalized in terms of coordination of the solvent to the metal atom of the catalyst, essentially blocking binding of the substrate.…”

NHI- and NHC-Supported Al(III) Hydrides for Amine–Borane Dehydrocoupling Catalysis

“…As such, catalysts from across the periodic table have shown their potential in the catalytic release of H2 from amine-boranes. Whilst transition metals dominate the field [22], examples from s-block [23][24][25][26], f-block [27,28], and metal-free [29][30][31] based systems have also contributed to furthering our understanding of these complex mechanisms. Central to most of the metal-based systems is the formation of metal hydrides [22] and their subsequent reactivity in enabling turnover.…”