1-Alkali-metal-2-alkyl-1,2-dihydropyridines: Soluble Hydride Surrogates for Catalytic Dehydrogenative Coupling and Hydroboration Applications

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

Metallated dihydropyridinates: prospects in hydride transfer and (electro)catalysis

Hydride transfer (HT) is a fundamental step in a wide range of reaction pathways, including those mediated by dihydropyridinates (DHP-s). Coordination of ions directly to the pyridine ring or functional groups stemming therefrom, provides a powerful approach for influencing the electronic structure and in turn HT chemistry. Much of the work in this area is inspired by the chemistry of bioinorganic systems including NADH. Coordination of metal ions to pyridines lowers the electron density in the pyridine ring and lowers the reduction potential: lower-energy reactions and enhanced selectivity are two outcomes from these modifications. Herein, we discuss approaches for the preparation of DHP-metal complexes and selected examples of their reactivity. We suggest further areas in which these metallated DHP-s could be developed and applied in synthesis and catalysis.

Alkali Metal Dihydropyridines in Transfer Hydrogenation Catalysis of Imines: Amide Basicity versus Hydride Surrogacy

Catalytic reduction of a representative set of imines, both aldimines and ketimines, to amines has been studied using transfer hydrogenation from 1,4-dicyclohexadiene. Unusually, this has been achieved using s-block pre-catalysts, namely 1-metallo-2-tert-butyl-1,2-dihydropyridines, 2-tBuC5 H5 NM, M(tBuDHP), where M=Li-Cs. Reactions have been monitored in C6 D6 and tetrahydrofuran-d8 (THF-d8 ). A definite trend is observed in catalyst efficiency with the heavier alkali metal tBuDHPs outperforming the lighter congeners. In general, Cs(tBuDHP) is the optimal pre-catalyst with, in the best cases, reactions producing quantitative yields of amines in minutes at room temperature using 5 mol % catalyst. Supporting the experimental study, Density Functional Theory (DFT) calculations have also been carried out which reveal that Cs has a pathway with a significantly lower rate determining step than the Li congener. In the postulated initiation pathways DHP can act as either a base or as a surrogate hydride.

Intermediates, Isolation and Mechanistic Insights into Zinc Hydride-Catalyzed 1,2-Regioselective Hydrofunctionalization of N-Heteroarenes

The conjugated bis-guanidinate-supported zinc hydride [{LZnH}₂; L = {(ArHN) (ArN)-C═N-C═(NAr) (NHAr); Ar = 2,6-Et₂-C₆H₃}] (I)-catalyzed highly demanding exclusive 1,2-regioselective hydroboration and hydrosilylation of N-heteroarenes is demonstrated with excellent yields. This protocol is compatible with many pyridines and N-heteroarene derivatives, including electron-donating and -withdrawing substituents. Catalytic intermediates, such as [(LZnH) (4-methylpyridine)] IIA, [(L'ZnH) (4-methylpyridine) IIA', where L' = CH{(CMe) (2,6-Et₂C₆H₃N)}₂)], LZn(1,2-DhiQ) (isoquinoline) III, [L'Zn(1,2-DhiQ) (isoquinoline)] III', and LZn(1,2-(3-MeDHQ)) (3-methylquinoline) V, were isolated and thoroughly characterized by NMR, HRMS, and IR analyses. Furthermore, X-ray single-crystal diffraction studies confirmed the molecular structures of compounds IIA', III, and III'. The NMR data proved that the intermediate III or III' reacted with HBpin and gave a selective 1,2-addition hydroborated product. Stoichiometric experiments suggest that V and III independently reacted with silane, yielding selective 1,2-addition of mono- and bis-hydrosilylated products, respectively. Based on the isolation of intermediates and a series of stoichiometric experiments, plausible catalytic cycles were established. Furthermore, the intermolecular chemoselective hydroboration reaction over other reducible functionalities was studied.

Hydrocarbon Soluble Alkali-Metal-Aluminium Hydride Surrog[ATES]

A series of group 1 hydrocarbon-soluble donor free aluminates [AM(^t BuDHP)(TMP)Al(ⁱ Bu)₂ ] (AM=Li, Na, K, Rb) have been synthesised by combining an alkali metal dihydropyridyl unit [(2-^t BuC₅ H₅ N)AM)] containing a surrogate hydride (sp³ C-H) with [(ⁱ Bu)₂ Al(TMP)]. These aluminates have been characterised by X-ray crystallography and NMR spectroscopy. While the lithium aluminate forms a monomer, the heavier alkali metal aluminates exist as polymeric chains propagated by non-covalent interactions between the alkali metal cations and the alkyldihydropyridyl units. Solvates [(THF)Li(^t BuDHP)(TMP)Al(ⁱ Bu)₂ ] and [(TMEDA)Na(^t BuDHP)(TMP)Al(ⁱ Bu)₂ ] have also been crystallographically characterised. Theoretical calculations show how the dispersion forces tend to increase on moving from Li to Rb, as opposed to the electrostatic forces of stabilization, which are orders of magnitude more significant. Having unique structural features, these bimetallic compounds can be considered as starting points for exploring unique reactivity trends as alkali-metal-aluminium hydride surrog[ATES].

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.

s-Block Metal Catalysts for the Hydroboration of Unsaturated Bonds

The addition of a B-H bond to an unsaturated bond (polarized or unpolarized) is a powerful and atom-economic tool for the synthesis of organoboranes. In recent years, s-block organometallics have appeared as alternative catalysts to transition-metal complexes, which traditionally catalyze the hydroboration of unsaturated bonds. Because of the recent and rapid development in the field of hydroboration of unsaturated bonds catalyzed by alkali (Li, Na, K) and alkaline earth (Mg, Ca, Sr, Ba) metals, we provide a detailed and updated comprehensive review that covers the synthesis, reactivity, and application of s-block metal catalysts in the hydroboration of polarized as well as unsaturated carbon-carbon bonds. Moreover, we describe the main reaction mechanisms, providing valuable insight into the reactivity of the s-block metal catalysts. Finally, we compare these s-block metal complexes with other redox-neutral catalytic systems based on p-block metals including aluminum complexes and f-block metal complexes of lanthanides and early actinides. In this review, we aim to provide a comprehensive, authoritative, and critical assessment of the state of the art within this highly interesting research area.

Deoxygenative hydroboration of carboxamides: a versatile and selective synthetic approach to amines

Deoxygenative reduction of amides is considered as an attractive method for preparation of synthetically valuable amines. However, the low electrophilicity of the amide carbonyl group, high thermodynamic stability and kinetic inertness of the amides make their reduction a challenging task. Until recently, most efforts for catalytic deoxygenation of amides to amines were concentrated on hydrogenation and hydrosilylation approaches, which mainly employed precious metal catalysts and often required harsh reaction conditions and showed insufficient selectivities. Moreover, these reactions are mostly limited to secondary and tertiary amides, whereas direct reduction of primary amides to primary amines remained arduous. In contrast, deoxygenative hydroboration of amides, although it appeared less then a decade ago, has already proved advantageous in terms of the amide scope, reaction conditions and selectivity of transformations. This article provides an overview of the developments in hydroboration of amides, focusing on mechanistic aspects of these transformations and advantages of hydroboration compared to hydrogenation and hydrosilylation approaches.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Alkali-Metal Mediation: Diversity of Applications in Main-Group Organometallic Chemistry

Organolithium compounds have been at the forefront of synthetic chemistry for over a century, as they mediate the synthesis of myriads of compounds that are utilised worldwide in academic and industrial settings. For that reason, lithium has always been the most important alkali metal in organometallic chemistry. Today, that importance is being seriously challenged by sodium and potassium, as the alkali-metal mediation of organic reactions in general has started branching off in several new directions. Recent examples covering main-group homogeneous catalysis, stoichiometric organic synthesis, low-valent main-group metal chemistry, polymerization, and green chemistry are showcased in this Review. Since alkali-metal compounds are often not the end products of these applications, their roles are rarely given top billing. Thus, this Review has been written to alert the community to this rising unifying phenomenon of "alkali-metal mediation".

Lithium compound catalyzed deoxygenative hydroboration of primary, secondary and tertiary amides

A selective and efficient route for the deoxygenative reduction of primary to tertiary amides to corresponding amines has been achieved with pinacolborane (HBpin) using simple and readily accessible 2,6-di-tert-butyl phenolate lithium·THF (1a) as a catalyst. Both experimental and DFT studies provide mechanistic insight.

CeO2–nanocubes as efficient and selective catalysts for the hydroboration of carbonyl groups

An efficient and reusable CeO2 nanocatalyst has been developed for the selective hydroboration of carbonyl compounds, including aromatic, heteroaromatic, aliphatic, and (hetero)aliphatic aldehydes and ketones.

Asymmetric ketone hydroboration catalyzed by alkali metal complexes derived from BINOL ligands

The ability of alkali metal complexes featuring functionalized BINOL-derived ligands to catalyze ketone hydroboration reactions was explored. The reduced products were formed in excellent yields and with variable enantioselectivities dependent upon the nature of the ligand and the alkali metal cation.

Homo- and heterodehydrocoupling of phosphines mediated by alkali metal catalysts

Catalytic chemistry that involves the activation and transformation of main group substrates is relatively undeveloped and current examples are generally mediated by expensive transition metal species. Herein, we describe the use of inexpensive and readily available tBuOK as a catalyst for P-P and P-E (E = O, S, or N) bond formation. Catalytic quantities of tBuOK in the presence of imine, azobenzene hydrogen acceptors, or a stoichiometric amount of tBuOK with hydrazobenzene, allow efficient homodehydrocoupling of phosphines under mild conditions (e.g. 25 °C and < 5 min). Further studies demonstrate that the hydrogen acceptors play an intimate mechanistic role. We also show that our tBuOK catalysed methodology is general for the heterodehydrocoupling of phosphines with alcohols, thiols and amines to generate a range of potentially useful products containing P-O, P-S, or P-N bonds.

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

A magnesium complex (1) featuring a bidentate aminopyridinato ligand is a remarkably selective catalyst for the dehydrocoupling of amine-boranes. This reaction proceeds to completion with low catalyst loadings (1 mol %) under mild conditions (60 °C), exceeding previously reported s-block systems in terms of selectivity, rate, and turnover number (TON). Mechanistic studies by in situ NMR analysis reveals the reaction to be first order in both catalyst and substrate. A reaction mechanism is proposed to account for these findings, with the high TON of the catalyst attributed to the bidentate nature of the ligand, which allows for reversible deprotonation of the substrate and regeneration of 1 as a stable resting state.

Crystal structure of diethyl 2-(4-methoxyphenyl)-1-phenyl-1,2-dihydropyridine-3,5-dicarboxylate, C24H25NO5

C24H25NO5, triclinic, P1̄, a = 9.2625(8) Å, b = 10.5974(10) Å, c = 11.7431(11) Å, α = 77.836(8)°, β = 87.564(8)°, γ = 66.951(9)°, V = 1035.83(17) Å3, Z = 2, R gt(F) = 0.0404, wR ref(F 2) = 0.0985, T = 107.5 K.

Lithium compounds as single site catalysts for hydroboration of alkenes and alkynes

Readily accessible lithium compounds have been employed to catalyze the hydroboration of alkene and alkynes including terpenes using HBpin with anti-Markovnikov selectivity. The mechanism is proposed on the basis of experimental and DFT studies.

Recent advances in the catalytic hydroboration of carbonyl compounds

The latest development in the catalytic hydroboration of CO groups is summarized in this review. Access to borate ester intermediates provides a pathway to convert them into the corresponding valuable functionalized alcohols.

Alkali metal complexes as efficient catalysts for hydroboration and cyanosilylation of carbonyl compounds

We report here reactions between the N-adamantyliminopyrolyl ligand 2-(AdN[double bond, length as m-dash]CH)-C4H3NH (L-H) and alkali metal hexamethyldisilazides [MN(SiMe3)2] (M = Li, Na and K) to afford the dimeric [{2-(AdN[double bond, length as m-dash]CH)-C4H3NLi(THF)}2] (1), [{2-(AdN[double bond, length as m-dash]CH)-C4H3N}{Na(THF)1.5}2] (2) and polymeric [{2-(AdN[double bond, length as m-dash]CH)-C4H3NK(THF)}n] (3) complexes at ambient temperature. A one-pot reaction between L-H, [KN(SiMe3)2] and alkaline earth metal diiodide (AeI2) in a 2 : 2 : 1 molar ratio, which resulted in the formation of a heteroleptic Ae metal complex [κ2-{2-(AdN[double bond, length as m-dash]CH)-C4H3N}2Ae(THF)2] [Ae = Mg (4), Ca (5)], is also reported. The solid-state structures of complexes 1, 3 and 4 were established through single-crystal X-ray diffraction analysis. The alkali and alkaline earth metal complexes 1-5 were utilised as precatalysts for the catalytic hydroboration of pinacolborane (HBpin) with aldehydes and ketones, and potassium complex 3 was identified as a competent catalyst under mild conditions. Additionally, cyanosilylation of carbonyl compounds was explored with trimethylsilyl cyanide and aldehydes/ketones, using the alkali metal precatalyst 3 under mild conditions. In both catalytic processes, the potassium catalyst 3 exhibited high tolerance towards a number of functional groups.

Efficient hydroboration of carbonyls by an iron(ii) amide catalyst

An easily prepared iron(ii) amide precatalyst enables the selective hydroboration of carbonyls with HBpin (pinacolborane) in the absence of any additive. The reactions proceed with low catalytic loading (1-3 mol%) under mild reaction conditions and display wide functional group compatibility. Aldehydes are selectively hydroborated in the presence of other reducible functional groups, such as ketones, alkenes, nitriles, esters, amides, acids and halides.

Donor-influenced Structure-Activity Correlations in Stoichiometric and Catalytic Reactions of Lithium Monoamido-Monohydrido-Dialkylaluminates

A series of heteroleptic monoamido‐monohydrido‐dialkylaluminate complexes of general formula [iBu₂AlTMPHLi⋅donor] were synthesized and characterised in solution and in the solid state. Applying these complexes in catalytic hydroboration reactions with representative aldehydes and ketones reveals that all are competent, however a definite donor substituent effect is discernible. The bifunctional nature of the complexes is also probed by assessing their performance in metallation of a triazole and phenylacetylene and addition across pyrazine. These results lead to an example of phenylacetylene hydroboration, which likely proceeds via deprotonation, rather than insertion as observed with the aldehydes and ketones. Collectively, the results emphasise that reactivity is strongly influenced by both the mixed‐metal constitution and mixed‐ligand constitution of the new aluminates.

1,4-Dihydropyridyl complexes of magnesium: synthesis by pyridine insertion into the magnesium–silicon bond of triphenylsilyls and catalytic pyridine hydrofunctionalization

Magnesium triphenylsilyl complexes [Mg(SiPh₃)₂(THF)₂] and [(Me₃TACD)Mg(SiPh₃)] ((Me₃TACD)H = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane) serve as precursors for 1,4-dihydropyridyl complexes of magnesium which are active in the hydroboration of pyridine.

Easily accessible lithium compound catalyzed mild and facile hydroboration and cyanosilylation of aldehydes and ketones

Simple and readily accessible lithium compounds are found to be efficient single site catalysts for cyanosilylation and hydroboration of a range of aldehydes and ketones under ambient conditions.

Reusable Fe2O3-nanoparticle catalysed efficient and selective hydroboration of carbonyl compounds

We present readily accessible Fe₂O₃ nanoparticles as an efficient catalyst for the selective hydroboration of carbonyl compounds, which represents the first example of the use of nanoparticles as a catalyst for this process.

Dehydrogenation of dimethylamine-borane mediated by Group 1 pincer complexes

Alkali metal carbazolido complexes are precatalysts for the dehydrogenation of Me₂NH·BH₃, where the cation plays a vital role in the reaction outcome.

Small bite-angle 2-phosphinophosphinine ligands enable rhodium-catalysed hydroboration of carbonyls

2-Phosphinophosphinine ligands generate Rh catalysts for the hydroboration of ketones and imines in contrast to standard phosphine ligands.

1-Alkali-metal-2-alkyl-1,2-dihydropyridines: Soluble Hydride Surrogates for Catalytic Dehydrogenative Coupling and Hydroboration Applications

Equipped with excellent hydrocarbon solubility, the lithium hydride surrogate 1-lithium-2-tert-butyl-1,2-dihydropyridine (1tLi) functions as a precatalyst to convert Me2 NH⋅BH3 to [NMe2 BH2 ]2 (89 % conversion) under competitive conditions (2.5 mol %, 60 h, 80 °C, toluene solvent) to that of previously reported LiN(SiMe3 )2 . Sodium and potassium dihydropyridine congeners produce similar high yields of [NMe2 BH2 ]2 but require longer times. Switching the solvent to pyridine induces a remarkable change in the dehydrocoupling product ratio, with (NMe2 )2 BH favoured over [NMe2 BH2 ]2 (e.g., 94 %:2 % for 1tLi). Demonstrating its versatility, precatalyst 1tLi was also successful in promoting hydroboration reactions between pinacolborane and a selection of aldehydes and ketones. Most reactions gave near quantitative conversion to the hydroborated products in 15 minutes, though sterically demanding carbonyl substrates require longer times. The mechanisms of these rare examples of Group 1 metal-catalysed processes are discussed.