Reversible 1,4-insertion of pyridine into a highly polar metal-carbon bond: effect of the second metal

Salicylaldimines: Formation via Ring Contraction and Synthesis of Mono- and Heterobimetallic Alkali Metal Heterocubanes

The formation of salicylaldimine derivatives via ring contraction as byproducts in 2-aminotropone syntheses has been investigated. Salicylaldiminate (SAI) complexes of the alkali metals Li-K have been synthesized and transformed into heterobimetallic complexes. Important findings include an unusual double heterocubane structure of the homometallic sodium SAI, an unprecedented ligand-induced E/Z isomerization of the aldimine functional group in the homometallic potassium SAI, and the first example of a structurally authenticated mixed-metal SAI based on s-block central atoms. Rapid equilibria have been shown to play a crucial role in the solution phase chemistry of mixed-metal SAIs. Analytical techniques applied in this work include (heteronuclear) NMR spectroscopy, VT- and DOSY NMR spectroscopy, high-resolution mass spectrometry, single-crystal X-ray diffraction analysis, and DFT calculations.

s-Block cooperative catalysis: alkali metal magnesiate-catalysed cyclisation of alkynols

Through mixed metal cooperativity, alkali metal magnesiates efficiently catalyse the cyclisation of alkynols.

Mixed s-block metal organometallic reagents have been successfully utilised in the catalytic intramolecular hydroalkoxylation of alkynols. This success has been attributed to the unique manner in which these reagents can overcome the challenges of the reaction: namely OH activation and coordination to and then addition across a C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond. In order to optimise the reaction conditions and to garner vital catalytic system requirements, a series of alkali metal magnesiates were enlisted for the catalytic intramolecular hydroalkoxylation of 4-pentynol. In a prelude to the main investigation, the homometallic magnesium dialkyl reagent MgR₂ (where R = CH₂SiMe₃) was utilised. This reagent was unsuccessful in cyclising the alcohol into 2-methylenetetrahydrofuran 2a or 5-methyl-2,3-dihydrofuran 2b, even in the presence of multidentate Lewis donor molecules such as N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA). Alkali metal magnesiates M^IMgR₃ (when M^I = Li, Na or K) performed the cyclisation unsatisfactorily both in the absence/presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA) or PMDETA. When higher-order magnesiates (i.e., M^I₂MgR₄) were employed, in general a marked increase in yield was observed for M^I = Na or K; however, the reactions were still sluggish with long reaction times (22–36 h). A major improvement in the catalytic activity of the magnesiates was observed when the crown ether molecule 15-crown-5 was combined with sodium magnesiate Na₂MgR₄(TMEDA)₂ furnishing yields of 87% with 2a : 2b ratios of 95 : 5 after 5 h. Similar high yields of 88% with 2a : 2b ratios of 90 : 10 after 3 h were obtained combining 18-crown-6 with potassium magnesiate K₂MgR₄(PMDETA)₂. Having optimised these systems, substrate scope was examined to probe the range and robustness of 18-crown-6/K₂MgR₄(PMDETA)₂ as a catalyst. A wide series of alkynols, including terminal and internal alkynes which contain a variety of potentially reactive functional groups, were cyclised. In comparison to previously reported monometallic systems, bimetallic 18-crown-6/K₂MgR₄(PMDETA)₂ displays enhanced reactivity towards internal alkynol-cyclisation. Kinetic studies revealed an inhibition effect of substrate on the catalysts via adduct formation and requiring dissociation prior to the rate limiting cyclisation step.

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.

Accessible heavier s-block dihydropyridines: structural elucidation and reactivity of isolable molecular hydride sources

Transmetallation of lithiodihydropyridines with Group 1 alkoxides provides facile access to reactive MH (M = Na, K) sources, which show significant structural diversity due in part to the distinct ways that Na/K engage with the σ (green) and π (red) donor systems of the DHP ligands.

The coordination chemistry of organo-hydride donors: new prospects for efficient multi-electron reduction

In biological reduction processes the dihydronicotinamides NAD(P)H often transfer hydride to an unsaturated substrate bound within an enzyme active site. In many cases, metal ions in the active site bind, polarize and thereby activate the substrate to direct attack by hydride from NAD(P)H cofactor. This review looks more widely at the metal coordination chemistry of organic donors of hydride ion--organo-hydrides--such as dihydronicotinamides, other dihydropyridines including Hantzsch's ester and dihydroacridine derivatives, those derived from five-membered heterocycles including the benzimidazolines and benzoxazolines, and all-aliphatic hydride donors such as hexadiene and hexadienyl anion derivatives. The hydride donor properties--hydricities--of organo-hydrides and how these are affected by metal ions are discussed. The coordination chemistry of organo-hydrides is critically surveyed and the use of metal-organo-hydride systems in electrochemically-, photochemically- and chemically-driven reductions of unsaturated organic and inorganic (e.g. carbon dioxide) substrates is highlighted. The sustainable electrocatalytic, photochemical or chemical regeneration of organo-hydrides such as NAD(P)H, including for driving enzyme-catalysed reactions, is summarised and opportunities for development are indicated. Finally, new prospects are identified for metal-organo-hydride systems as catalysts for organic transformations involving 'hydride-borrowing' and for sustainable multi-electron reductions of unsaturated organic and inorganic substrates directly driven by electricity or light or by renewable reductants such as formate/formic acid.