Mechanistic study of homoleptic trisamidolanthanide-catalyzed aldehyde and ketone hydroboration. Chemically non-innocent ligand participation.

Carbonyl bond hydroboration is a valuable synthetic route to functionalized alcohols but relies on sometimes unselective and sluggish reagents. While rapid and selective aldehyde and ketone hydroboration mediated by trisamidolanthanide catalysts is known, the origin of the selectivity is not well-understood and is the subject of this contribution. Here the aldehyde and ketone HBpin hydroboration reaction mechanisms catalyzed by La[N(SiMe3)2]3 are investigated both experimentally and theoretically. The results support initial carbonyl oxygen coordination to the acidic La center, followed by intramolecular ligand-assisted hydroboration of the carbonyl moiety by bound HBpin. Interestingly, ketone hydroboration has a higher energetic barrier than that of aldehydes due to the increased steric encumbrance and decreased electrophilicity. Utilizing NMR spectroscopy and X-ray diffraction, a bidentate acylamino lanthanide complex associated with the aldehyde hydroboration is isolated and characterized, consistent with the relative reaction rates. Furthermore, an aminomonoboronate-lanthanide complex produced when the La catalyst is exposed to excess HBpin is isolated and characterized by X-ray diffraction, illuminating unusual aminomonoboronate coordination. These results shed new light on the origin of the catalytic activity patterns, reveal a unique ligand-assisted hydroboration pathway, and uncover previously unknown catalyst deactivation pathways.

Selective Lanthanide-Organic Catalyzed Depolymerization of Nylon-6 to ϵ-Caprolactam.

Nylon-6 is selectively depolymerized to the parent monomer ϵ-caprolactam by the readily accessible and commercially available lanthanide trisamido catalysts Ln(N(TMS)2 )3 (Ln=lanthanide). The depolymerization process is solvent-free, near quantitative, highly selective, and operates at the lowest Nylon-6 to ϵ-caprolactam depolymerization temperature reported to date. The catalytic activity of the different lanthanide trisamides scales with the Ln3+ ionic radius, and this process is effective with post-consumer Nylon-6 as well as with Nylon-6+polyethylene, polypropylene or polyethylene terephthalate mixtures. Experimental kinetic data and theoretical (DFT) mechanistic analyses suggest initial deprotonation of a Nylon terminal amido N-H bond, which covalently binds the catalyst to the polymer, followed by a chain-end back-biting process in which ϵ-caprolactam units are sequentially extruded from the chain end.

Chemodivergent Organolanthanide-Catalyzed C-H α-Mono-Borylation of Pyridines.

Chemodivergent synthetic methodologies enable the efficient introduction of structural diversity into high-value organic products via simple chemical alterations. In this regard, C-H activation and functionalization of pyridinoid azines are important transformations in the synthesis of many natural products, pharmaceuticals, and functional materials. Reflecting on azinyl nitrogen lone-pair steric repulsion, its tendency to irreversibly coordinate metal ion catalysts, and the electron deficiency of pyridine, C-H functionalization at the important α-position remains challenging. Thus, developing earth-abundant catalysts for α-selective azine mono-functionalization is an attractive target for chemical synthesis. Here, the selective organolanthanide-catalyzed α-mono-borylation of a diverse series of 18 pyridines is reported using Cp*2LuCH(TMS)2 (Cp* = η5-C5Me5) (TMS = SiMe3) and affording valuable precursors for subsequent functionalization. Experimental and theoretical mechanistic data reported here support the intermediacy of a C-H-activated η2-lanthanide-azine complex, followed by intermolecular α-mono-borylation via σ-bond metathesis. Notably, varying the lanthanide identity and substrate substituent electronic character promotes marked chemodivergence of the catalytic selectivity: smaller/more electrophilic lanthanide3+ ions and electron-rich substrates favor selective α-C-H functionalization, whereas larger/less electrophilic lanthanide3+ ions and electron-poor substrates favor selective B-N bond-forming 1,2-dearomatization. Such lanthanide series catalytic chemodivergence is, to our knowledge, unprecedented.

The design of a readily attachable and cleavable molecular scaffold for ortho-selective C-H alkenylation of arene alcohols.

We describe herein the design of a novel molecular scaffold that can induce facile oxidative olefinations when attached to alcohols. Benzylic, homo-, and bishomobenzylic alcohols are utilized. The scaffold can act as a protecting group for the alcohol in other transformations, and it is recoverable in excellent yield. The overall sequence can also be telescoped without purifications of intermediates, representing a net alcohol-based directed ortho-alkenylation.