Dual role tertiary amines in photocatalytic cyclizations: towards sustainable syntheses of 1,3-dinitrogen heterocycles

A novel photocatalytic cascade has been engineered for the transformation of diketimines into 1,3-dinitrogen heterocycles, ingeniously repurposing methylamine a typically utilized sacrificial additive as a dual-function reagent, serving simultaneously as an electron donor and a methyl source. This innovation champions atom economy and underscores a commitment to sustainable chemical synthesis.

Frustrated Lewis Pair-Type Reactivity of Intermolecular Rare-Earth Aryloxide and N-Heterocyclic Carbene/Olefin Combinations

This work reports the cooperative reactivity of rare-earth aryloxide complexes with N-heterocyclic carbene (NHC) or N-heterocyclic olefin (NHO), showcasing their synergistic effect on the activation of H2 and diverse organic substrates. Reactions of RE(OAr)3 (RE=La, Sm, and Y; Ar=2,6-tBu2-C6H3) with unsaturated NHC ItBu (:C[N(R)CH]2, R=tBu) isolated abnormally bound RE metal NHC complexes RE/aNHC. In contrast, no metal-NHO adducts were formed when RE(OAr)3 were treated with NHO (R2C=C[N(R)C(R)]2, R=CH3). Both RE/aNHC and RE/NHO Lewis pairs enabled cooperative H2 activation. Furthermore, RE(OAr)3 were found to catalyze the hydrogenation of the exocyclic C=C double bond of NHO under mild conditions. Moreover, treatment of the La/aNHC complex with benzaldehyde produced a La/C4 1,2-addition product. The La/NHO Lewis pair could react with (trimethylsilyl)diazomethane and α, β-conjugated imine, affording an isocyanotrimethylsilyl lanthanum amide complex and a La/C 1,4-addition product, respectively.

Double Bond Cleavage in Small Molecules Using a Geminal Sc/P Lewis Pair

Frustrated Lewis pairs (FLPs) have proven capable of cleaving the H-H σ-bond and binding a variety of unsaturated small molecules. In contrast, examples of FLP-mediated complete rupture of double-bonded substrates remain scarce. Herein, we present a geminal Sc/P Lewis pair, i.e., (ArO)2ScN(tBu)PPh2 (Ar = 2,6-tBu2-C6H3), that exhibits typical FLP-type 1,2-addition reactivity toward CO2. Notably, it enables the complete cleavage of a series of double bonds, such as the N═N bond in azobenzene or pyridazine, the N═O bond in nitrosobenzene, and the N═S and S═O bonds in N-sulfinylaniline, to yield the corresponding metallacyclic products. Moreover, the first rare-earth metal sulfur monoxide adduct could be obtained through the bond cleavage of PhNSO, demonstrating the capability of rare-earth metal complexes to capture reactive species.

Frustrated Lewis pair chemistry of hydride sponges

An improved synthesis for 1,8-bis(dimethylboranyl)naphthalene (1, hydride sponge) was developed avoiding the use of tin(IV) reagents. The related 1,2-bis(dimethylboranyl)benzene (2) was prepared. 1 combined with 1,8-bis(dimethylamino)naphthalene (3, proton sponge) is a Frustrated Lewis pair (FLP) that forms adducts [1-EH_n-1][3-H] with the protic compounds EH_n = H₂O, NH₃, H₂S, PH₃, H₂Se, HCN. Their structures show the chelation of the deprotonated substrates, except for CN^- (binds to one B atom of 1). The mechanisms of formation of [1-EH_n-1][3-H] were explored by NMR spectroscopy. Similar reactions took places for the FLP system 2 + 3, but the adducts [2-EH_n-1][3-H] are less stable; only the PH₃ adduct was isolated and AsH₃ forms a chelated adduct [2-NC(CD₃)AsH₂][3-H]. FLP 1 + 3 does not react with molecular hydrogen, but the formal adduct [1-H][3-H] was isolated via salt metathesis from K[1-H] and [3-H]Cl; it is stable towards water, atmospheric oxygen and up to 100 °C; its thermal decomposition proceeds without formation of H₂. The stabilities of both, the mixture 1 + 3 + H₂ and the formal adduct [1-H][3-H], allow concluding that hydrogen activation of FLP 1 + 3 is kinetically prevented.

Dihydrogen cleavage by a dimetalloxycarbene-borane frustrated Lewis pair

A frustrated Lewis pair of dititanoxycarbene [(Ti(N[^tBu]Ar)₃)₂(μ-CO₂)] (Ar = 3,5-Me₂C₆H₃) and B(C₆F₅)₃ cleaved dihydrogen under ambient conditions to give the zwitterionic formate [(Ti(N[^tBu]Ar)₃)₂(μ-OCHO-ηO:ηO')(B(C₆F₅)₃)] and the hydrido borate [Ti(N[^tBu]Ar)₃][HB(C₆F₅)₃].

Hydrosilylative reduction of carbon dioxide by a homoleptic lanthanum aryloxide catalyst with high activity and selectivity

An efficient tandem hydrosilylation of CO2, which uses a combination of a simple, homoleptic lanthanum aryloxide and B(C6F5)3, was performed. Use of a less sterically hindered silane led to an exclusive reduction of CO2 to CH4, with a turnover frequency of up to 6000 h-1 at room temperature. The catalytic system is robust, and 19 400 turnovers could be achieved with 0.005 mol% loading of lanthanum. The reaction outcome depended highly on the nature of the silane reductant used. Selective production of the formaldehyde equivalent, i.e., bis(silyl)acetal, without over-reduction, was observed when a sterically bulky silane was used. The reaction mechanism was elucidated by stoichiometric reactions and DFT calculations.

Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization

N-heterocyclic carbenes (NHCs) are ubiquitous ancillary ligands employed in metal-catalyzed homogeneous reactions and polymerization reactions. Of significance is the use of NHCs as the supporting ligand in second- and third-generation Grubbs catalysts for their application in olefin metathesis and ring-opening metathesis polymerization. While the applications of transition metal catalysts ligated with NHCs in polymerization chemistry are well-documented, the use of analogous rare earth (Ln = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) catalysts in this area remains under-developed, despite the unique role of rare earth elements in regio- and stereo-specific (co)polymerization reactions. By using hetero-atom-tethered chelating NHCs and, more recently, the employment of other structurally related NHCs, NHC-ligated Ln complexes have proven to be promising and fruitful catalysts for selective polymerization reactions. This review summarizes the recent developments in the coordination chemistry of Ln complexes containing NHCs and their catalytic performance in polymerization.