1,2-Hydroboration of Pyridines by Organomagnesium

Synthesis and reactivity of a heteroleptic magnesium hydride on a dearomatized picolyl-based NNN-chelator

Heteroleptic magnesium hydrides are important for their stoichiometric and catalytic reduction chemistry. Their primary nucleophilic site is typically the hydride, while the ancillary ligands commonly used are mostly spectators. Chemically non-innocent ligands in comparison are rarely applied on MgH as their reactivity can be complex. Milstein et al. have recently reported Mg-mediated alkyne hydrogenation by using their metal-ligand cooperation (MLC) concept on a dearomatized picolyl-based PNP pincer that is non-innocent with a nucleophilic nature. A '(PNP)MgH' is noted as the active catalyst in hydrogenation but without structural validation. Inspired by the same, we report herein a novel NNN-chelator (MesL) with a dearomatized picolyl moiety and its well-defined MgH. Having two prominent nucleophilic sites, the present MgH shows metal-ligand competition while reacting with certain electrophiles. It also distinguishes nonpolar alkynes and polar carbonyls by cleanly inserting itself into the former but not the latter. The nucleophilicities of the two sites are also probed by DFT methods and compared with Milstein's (PNP)MgH. Although the present system shows no MLC-type H2 activation, the addition of a CS2 molecule in that way is realized.

Heteroleptic Magnesium n-Butyl on a Chemically Non-innocent 2-Anilidomethylpyridine Ligand Leading to Diverse Magnesium Hydrides

Molecular magnesium hydrides and hydride-rich clusters are of significant interest for applications ranging from catalysis and small molecule activation to hydrogen storage. Here, we investigate the 2-anilidomethylpyridine framework NNL as an ancillary support for magnesium organometallics with a special emphasis on hydrides. The proligand NNLH (N-[2,6-bis(1-methylethyl)phenyl]-α,6-diphenyl-2-pyridinemethanamine) gives [(NNL)Mg(nBu)(thf)] (1) by nbutane elimination from Mg(nBu)2(thf)n. A stronger donor such as DMAP replaces the THF from 1 to give [(NNL)Mg(nBu)(dmap)] (2). Both are air-sensitive, and 1 is adventitiously oxidized into [(NNL)Mg(μ-OnBu)]2 (32). The homoleptic [(NNL)2Mg] (8) is made from 1 and a second equiv of NNLH. 1's terminal nBu group is selectively protonated by HN(SiMe3)2 to give [(NNL)MgHMDS] (4; HMDS = N(SiMe3)2), whereas Ph3SiOH partially protonates the backbone anilide as well to give a mixture of [(NNL)Mg(OSiPh3)(thf)] (5) and free NNLH. Like HN(SiMe3)2, aprotic MeOTf also reacts by selectively abstracting the nBu group from 1 to give [(NNL)Mg(μ:κ2-O,O'-OTf)(thf)]2 (62). Interestingly, screening the common synthetic routes for magnesium hydrides leads to diverse outcomes upon varying the Mg precursors and hydride sources. 1 and PhSiH3 give the hydride cluster [{(NNL)2Mg2(μ-H)}2(μ-H)4Mg] (7), whereas 2 and PhSiH3 give the molecular complex [(NNLde)Mg(dmap)2] (9) with a dearomatized pyridyl backbone. 1 and HBpin (pinacolborane) give a product mixture, from which a different hydride cluster [(NNL)2Mg2(μ-H)}2(μ:κ2-O,O'-O2C2Me4)] (10) is identified, showing a rare instance of complete deborylation of a HBpin molecule. 1 and HBcat (catecholborane) also give a product mixture, one of which is the borylated ligand [(NNL)Bcat] (11). HBpin with 4 as the Mg precursor takes the ligand borylation route more selectively to give [(NNL)Bpin] (12). Last, 1 reacts with iPrNH2BH3 to give [(NNL)Mg{NH(iPr)BH3}] (13), which shows a slow and fractional conversion into the dinuclear mixed hydrido amidoborane [(NNL)2Mg2(μ-H){(μ-NH(iPr)BH3}] (14) by partial β-hydride elimination. In comparison, [(NNL)Mg(iPrNHBH3)(dmap)] (15) arising from the DMAP-bound 2 and iPrNH2BH3 is stable toward such elimination.

Synthesis and utility of N-boryl and N-silyl enamines derived from the hydroboration and hydrosilylation of N-heteroarenes and N-conjugated compounds

Catalytic hydroboration and hydrosilylation have emerged as promising strategies for the reduction of unsaturated hydrocarbons and carbonyl compounds, as well as for the dearomatization of N-heteroarenes. Various catalysts have been employed in these processes to achieve the formation of reduced products via distinct reaction pathways and intermediates. Among these intermediates, N-silyl enamines and N-boryl enamines, which are derived from hydrosilylation and hydroboration, are commonly underestimated in this reduction process. Because these versatile intermediates have recently been utilized in situ as nucleophilic reagents or dipolarophiles for the synthesis of diverse molecules, an expeditious review of the synthesis and utilization of N-silyl and N-boryl enamines is crucial. In this review, we comprehensively discuss a wide range of hydrosilylation and hydroboration catalysts used for the synthesis of N-silyl and N-boryl enamines. These catalysts include main-group metals (e.g., Mg and Zn), transition metals (e.g., Rh, Ru, and Ir), earth-abundant metals (e.g., Fe, Co, and Ni), and non-metal catalysts (including P, B, and organocatalysts). Furthermore, we highlight recent research efforts that have leveraged these versatile intermediates for the synthesis of intriguing molecules, offering insights into future directions for these invaluable building blocks.

Low oxidation state and hydrido group 2 complexes: synthesis and applications in the activation of gaseous substrates

Numerous industrial processes utilise gaseous chemical feedstocks to produce useful chemical products. Atmospheric and other small molecule gases, including anthropogenic waste products (e.g. carbon dioxide), can be viewed as sustainable building blocks to access value-added chemical commodities and materials. While transition metal complexes have been well documented in the reduction and transformation of these substrates, molecular complexes of the terrestrially abundant alkaline earth metals have also demonstrated promise with remarkable reactivity reported towards an array of industrially relevant gases over the past two decades. This review covers low oxidation state and hydrido group 2 complexes and their role in the reduction and transformation of a selection of important gaseous substrates towards value-added chemical products.

Cobalt-catalyzed double hydroboration of pyridines

Cobalt(ii) complexes were prepared from a modular phosphinopyridonate platform and applied to the hydroboration of pyridines. The synthetically useful, yet challenging, double hydroboration toward tetrahydropyridine derivatives was successfully performed with high activity and regiocontrol. This new method enabled the direct synthesis of N-heterocyclic allylic boronates from commercial pyridines and pinacolborane (HBpin). One-pot acetylation afforded the bench-stable borylated N-acetyl tetrahydropyridines in good yields. The synthetic utility of this procedure was demonstrated by a gram-scale double hydroboration-acetylation sequence followed by chemical diversification. Mechanistic experiments indicated metal-ligand cooperativity involving ligand-centered C-H activation and the intermediacy of a cobalt(iii) hydride species.

Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines

Dearomatization reactions have become fundamental chemical transformations in organic synthesis since they allow for the generation of three-dimensional complexity from two-dimensional precursors, bridging arene feedstocks with alicyclic structures. When those processes are applied to pyridines, quinolines, and isoquinolines, partially or fully saturated nitrogen heterocycles are formed, which are among the most significant structural components of pharmaceuticals and natural products. The inherent challenge of those transformations lies in the low reactivity of heteroaromatic substrates, which makes the dearomatization process thermodynamically unfavorable. Usually, connecting the dearomatization event to the irreversible formation of a strong C-C, C-H, or C-heteroatom bond compensates the energy required to disrupt the aromaticity. This aromaticity breakup normally results in a 1,2- or 1,4-functionalization of the heterocycle. Moreover, the combination of these dearomatization processes with subsequent transformations in tandem or stepwise protocols allows for multiple heterocycle functionalizations, giving access to complex molecular skeletons. The aim of this review, which covers the period from 2016 to 2022, is to update the state of the art of nucleophilic dearomatizations of pyridines, quinolines, and isoquinolines, showing the extraordinary ability of the dearomative methodology in organic synthesis and indicating their limitations and future trends.

Cobalt-Catalyzed Regioselective 1,2-Hydroboration of N-Heteroarenes

Regioselective hydroboration of pyridines to 1,2-dihydropyridines remains a significant challenge for the synthesis of valuable nitrogenous bioactive molecules. Herein, we report a base free ligand-controlled cobalt-catalyzed 1,2-hydroboration of pyridines and quinolines with very low catalyst loading under neat reaction conditions. The choice of sterically demanding N-heterocyclic ligands led to the 1,2-regioselectivity and the scope was demonstrated by the N-heterocycles having a variety of functional groups. The preliminary mechanistic studies corroborate that the two ligands followed a distinct catalytic cycle with Co(I) as an active species.

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.

A Heterogeneous Iridium Catalyst for the Hydroboration of Pyridines

Sulfated zirconium oxide (SZO) capped with silylium-like ions reacts with (cod)Ir(py)Cl (cod = 1,5-cyclooctadiene; py = pyridine) to form [Ir(cod)py][SZO] (1) and Me₃SiCl. 1 can also be formed in reactions of phosphonium functionalized SZO and [Ir(cod)(OSi(O^tBu)₃]₂, which forms [Ir(cod)P(^tBu)₂Ph][SZO] (2), followed by reaction with pyridine to form 1. FTIR and ¹⁵N{¹H} MAS NMR spectroscopy are consistent with coordination of pyridine in 1 to an electrophilic iridium. 1 is moderately active in the dearomative hydroboration of pyridine. The primary product of this reaction is 1,2-dihydropyridine, which converts to the 1,4-dihydropyridine product at long reaction times. 1 catalyzes the dearomative hydroboration of a variety of substituted pyridines and is also reactive toward pyrazines and N-methylimidazole.

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.

Catalytic hydroboration of carbonyl derivatives by using phosphinimino amide ligated magnesium complexes

Reduction of carbonyl derivatives by using Earth-abundant, cheap, and environmentally benign metal-based catalysts through an atom-efficient method is a challenging task. Herein, we report the synthesis and characterization of dinuclear magnesium complexes 1-3 chelated by a phosphinimino amide skeleton. In combination with pinacolborane (HBpin) as a reducing agent, complex 1 bearing an ortho-methyl substituent on the phenyl ring of the ligand showed excellent reduction capability for a broad range of carbonyl derivatives under mild reaction conditions. Aldehydes, ketones, and acrolein substrates were efficiently reduced to the corresponding alkoxy-borane products with a record high TOF. Besides, acrolein derivatives were exclusively reduced to 1,2-regioselective products. Using two equiv. of HBpin, ester substrates were reduced to two kinds of alkoxy-borane products. Carbonate reduction accomplished by using complex 1 and three equiv. of HBpin afforded diols and a methanol precursor, respectively. When chiral substrates such as (S)-1,2-propanediol carbonate and L-lactide or polymeric P(L-LA) were employed, the chirality was almost retained in their reductive products.

Synthesis of Cationic Silaamidinate Germylenes and Stannylenes and the Catalytic Application for Hydroboration of Pyridines

The N-heterocyclic germylenes and stannylenes LSi(NAr)₂EX (L = PhC(NtBu)₂, Ar = 2,6-iPr₂C₆H₃; E = Ge, Sn; X = Cl, CF₃SO₃, BPh₄) supported by the bulky silaamidinate ligand [LSi(NAr)₂]^- have been synthesized and fully characterized. The germylene triflate LSi(NAr)₂GeOTf (3b) and dimeric borate [LSi(NAr)₂Ge]₂ClBPh₄ (3a) enabled highly regio- and chemoselective catalytic hydroboration of pyridines and may represent the most active catalytic system for the transformation. DFT calculations disclosed that the cationic germylene [LSi(NAr)₂Ge]⁺ with a low-lying LUMO energy initiated the catalytic process. In contrast, the analogous amidinate germylene triflates are almost inactive, indicating the silaamidinate ligand is essential for the stabilization of cationic species.

Manganese-Catalyzed Hydroborations with Broad Scope

Reductive transformations of easily available oxidized matter are at the heart of synthetic manipulation and chemical valorization. The applications of catalytic hydrofunctionalization benefit from the use of liquid reducing agents and operationally facile setups. Metal‐catalyzed hydroborations provide a highly prolific platform for reductive valorizations of stable C=X electrophiles. Here, we report an especially facile, broad‐scope reduction of various functions including carbonyls, carboxylates, pyridines, carbodiimides, and carbonates under very mild conditions with the inexpensive pre‐catalyst Mn(hmds)₂. The reaction could be successfully applied to depolymerizations.