s-Block-Metal-Mediated Hydroamination of Diphenylbutadiyne with Primary Arylamines Using a Dipotassium Tetrakis(amino)calciate Precatalyst

Ion-bearing stairs: alkali metal complexes of 1,2-diaza-4-phospholides

In this work, eight alkali metal complexes with 1,2-diaza-4-phospholide ligands were prepared and characterized by X-ray single-crystal structural analysis and NMR spectroscopy. Their structures showed varied coordination motifs: (i) a dimeric 1,2-diaza-4-phospholide lithium complex with exo-bidentate bridging coordination (4) consists of two lithium atoms that are linked via two μ₂-bridging, κN,κN'-coordinated ligands; (ii) the polymeric chain 1,2-diaza-4-phospholide potassium complex (5) showed an ion-bearing stair-shaped chain structure running through axis a, where the steps are η² interactions, and there is a transition platform between every two stairs; (iii) the polymeric chain 1,2-diaza-4-phospholide potassium complex (6) also presented a polymeric chain structure in the solid state but displayed a head-to-tail arrangement of two 1,2-diaza-4-phospholides; (iv) in comparison to 6, the 1,2-diaza-4-phospholide sodium complex (7) displayed a tetrameric structure, in which the sodium ions are arranged in a distorted tetrahedral fashion and each of them occupies a vertex of the tetrahedron; (v) the polymeric chain 1,2-diaza-4-phospholide potassium complex (8) presented a solvent-free chain structure, in which potassium ions each is η⁵-bonded by two 1,2-diaza-4-phospholides and η²-coordinated by another, consisting of a stair-shaped chain structure running through axis a but without significant intermolecular contacts between the adjacent stairs in comparison to that of 5; (vi) the polymeric chain 1,2-diaza-4-phospholide sodium complex (9) presented a solvent-free chain structure, in which sodium ions each is η¹(N),η²(N,N),η¹(P)-bonded by three 1,2-diaza-4-phospholides, consisting of a chain structure running through axis a; and (vii) the treatment complex 8 with elemental sulphur or selenium in the presence of crown ether gave rare thiophosphonato potassium [η³(S,P,S)-3,5-tBu₂dp-(μ-K)(S₂)([18]crown-6)] (10) or a selenophosphonato potassium [η³(Se,P,Se)-3,5-tBu₂dp-(μ-K)(Se₂)([18]crown-6)] (11). Both of the complexes crystallized in the orthorhombic space group Pnma as pale-yellow (or red) crystals. The X-ray diffraction analysis revealed 10 or 11 as a terminal complex with the η¹,η¹-X,X-coordination mode (X = S and Se). The ¹H DOSY NMR spectroscopy study of the species 8 in DMSO-d₆ suggested that polymeric complexes (4-9) in the solid state should dissociate into the related monomers in the solutions when the donor solvents were used.

Catalytic Hydrofunctionalization Reactions of 1,3-Diynes

Metal-catalyzed hydrofunctionalization reactions of alkynes, i.e., the addition of Y–H units (Y = heteroatom or carbon) across the carbon–carbon triple bond, have attracted enormous attention for decades since they allow the straightforward and atom-economic access to a wide variety of functionalized olefins and, in its intramolecular version, to relevant heterocyclic and carbocyclic compounds. Despite conjugated 1,3-diynes being considered key building blocks in synthetic organic chemistry, this particular class of alkynes has been much less employed in hydrofunctionalization reactions when compared to terminal or internal monoynes. The presence of two C≡C bonds in conjugated 1,3-diynes adds to the classical regio- and stereocontrol issues associated with the alkyne hydrofunctionalization processes’ other problems, such as the possibility to undergo 1,2-, 3,4-, or 1,4-monoadditions as well as double addition reactions, thus increasing the number of potential products that can be formed. In this review article, metal-catalyzed hydrofunctionalization reactions of these challenging substrates are comprehensively discussed.

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".

Generation of Aryllithium Reagents from N-Arylpyrroles Using Lithium

Treatment of 1-aryl-2,5-diphenylpyrroles with lithium powder in tetrahydrofuran at 0 °C results in the generation of the corresponding aryllithium reagents through reductive C–N bond cleavage.

Calcium catalyzed enantioselective intramolecular alkene hydroamination with chiral C2-symmetric bis-amide ligands

The chiral building block (R)-(+)-2,2'-diamino-1,1'-binaphthyl, (R)-BINAM, which is often used as backbone in privileged enantioselective catalysts, was converted to a series of N-substituted proligands R1-H2 (R = CH2tBu, C(H)Ph2, PPh2, dibenzosuberane, 8-quinoline). After double deprotonation with strong Mg or Ca bases, a series of alkaline earth (Ae) metal catalysts R1-Ae·(THF)n was obtained. Crystal structures of these C2-symmetric catalysts have been analyzed by quadrant models which show that the ligands with C(H)Ph2, dibenzosuberane and 8-quinoline substituents should give the best steric discrimination for the enantioselective intramolecular alkene hydroamination (IAH) of the aminoalkenes H2C[double bond, length as m-dash]CHCH2CR'2CH2NH2 (CR'2 = CPh2, CCy or CMe2). The dianionic R12- ligand in R1-Ae·(THF)n functions as reagent that deprotonates the aminoalkene substrate, while the monoanionic (R1-H)- ligand formed in this reaction functions as a chiral spectator ligand that controls the enantioselectivity of the ring closure reaction. Depending on the substituent R in the BINAM ligand, full cyclization of aminoalkenes to chiral pyrrolidine products as fast as 5 minutes was observed. Product analysis furnished enantioselectivities up to 57% ee, which marks the highest enantioselectivity reported for Ca catalyzed IAH. Higher selectivities are impeded by double protonation of the R12- ligand leading to complete loss of chiral information in the catalytically active species.

Aminopotassiation by Mixed Potassium/Lithium Amides: A Synthetic Path to Difficult to Access Phenethylamine Derivates

Insights gained from a comparison of aminometalation reactions with lithium amides, potassium amides and mixed lithium/potassium amides are presented. A combination of structural characterization, DFT calculations and electrophile reactions of aminometalated intermediates has shown the advantages of using a mixed metal strategy. While potassium amides fail to add, the lithium amides are uncontrollable and eliminated, yet the mixed K/Li amides deliver the best of both systems. Aminopotassiation proceeds to form the alkylpotassium species which has enhanced stability over its lithium counterpart allowing for its isolation and thereby its further characterization.

Main group bimetallic partnerships for cooperative catalysis

Over the past decade s-block metal catalysis has undergone a transformation from being an esoteric curiosity to a well-established and consolidated field towards sustainable synthesis. Earth-abundant metals such as Ca, Mg, and Al have shown eye-opening catalytic performances in key catalytic processes such as hydrosilylation, hydroamination or alkene polymerization. In parallel to these studies, s-block mixed-metal reagents have also been attracting widespread interest from scientists. These bimetallic reagents effect many cornerstone organic transformations, often providing enhanced reactivities and better chemo- and regioselectivities than conventional monometallic reagents. Despite a significant number of synthetic advances to date, most efforts have focused primarily on stoichiometric transformations. Merging these two exciting areas of research, this Perspective Article provides an overview on the emerging concept of s/p-block cooperative catalysis. Showcasing recent contributions from several research groups across the world, the untapped potential that these systems can offer in catalytic transformations is discussed with special emphasis placed on how synergistic effects can operate and the special roles played by each metal in these transformations. Advancing the understanding of the ground rules of s-block cooperative catalysis, the application of these bimetalic systems in a critical selection of catalytic transformations encompassing hydroamination, cyclisation, hydroboration to C-C bond forming processes are presented as well as their uses in important polymerization reactions.

Alkali-metal organomagnesiate complexes as catalysts for highly chemoselective crossed-Tishchenko reactions

Five heterobimetallic magnesiates bearing bidentate pyrrolyl ligand have been synthesized and their structural features were provided. As catalyst for cross-coupling Tishchenko reaction, they exhibited higher catalytic activities and chemoselectivity.

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.

Alkali metal and stoichiometric effects in intermolecular hydroamination catalysed by lithium, sodium and potassium magnesiates

Main group bimetallic complexes, while being increasingly used in stoichiometric deprotonation and metal-halogen exchange reactions, have not yet made a significant impact in catalytic applications. This paper explores the ability of alkali metal magnesiates to catalyse the intermolecular hydroamination of alkynes and alkenes using sytrene and diphenylacetylene as principle setting model substrates. By systematically studying the role of the alkali-metal and the formulation of the heterobimetallic precatalyst, this study establishes higher order potassium magnesiate [(PMDETA)₂K₂Mg(CH₂SiMe₃)₄] (7) as a highly effective system capable of catalysing hydroamination of styrene and diphenylacetylene with several amines while operating at room temperature. This high reactivity contrasts with the complete lack of catalytic ability of neutral Mg(CH₂SiMe₃)₂, even when harsher reaction conditions are employed (24 h, 80 °C). A pronounced alkali metal effect is also uncovered proving that the alkali metal (Li, Na, or K) is not a mere spectating counterion. Through stoichiometric reactions, and structural and spectroscopic (DOSY NMR) investigations we shed some light on the potential reaction pathway as well as the constitution of key intermediates. This work suggests that the enhanced catalytic activity of 7 can be rationalised in terms of the superior nucleophilic power of the formally dianionic magnesiate {Mg(NR₂)₄}^2- generated in situ during the hydroamination process, along with the ability of potassium to engage in π-interactions with the unsaturated organic substrate, enhancing its susceptibility towards a nucleophilic attack by the amide anion.

Influence of 18-Crown-6 Ether Coordination on the Catalytic Activity of Potassium and Calcium Diarylphosphinites in Hydrophosphorylation Reactions

The addition of 18-crown-6 ether (1,4,7,10,13,16-hexaoxacyclooctadecane) to tetranuclear [(thf)K(OPAryl₂)]₄ and [(thf)₄Ca(OPAryl₂)₂] yields the corresponding mononuclear complexes [(18C6)K(OPAryl₂)] [Aryl = Ph (1a), Mes (1b)] and [(18C6)Ca(OPAryl₂)₂] [Aryl = Ph (2a), Mes (2b)]. The metathesis reaction of [(thf)K(OPAryl₂)]₄ with CaI₂ yields the calciate [K₂Ca(thf)_x{OPMes₂}₄]. The addition of dimesitylphosphane oxide and crystallization from a hexane solution gives [K₂Ca{OPMes₂}₄{Mes₂P(O)H}] (3). The complexes [(thf)K(OPMes₂)]₄, [(thf)₄Ca(OPMes₂)₂], 1b, 2b, and the calciate 3 are tested as catalysts in the hydrophosphorylation of isopropylisocyanate with dimesitylphosphane oxide, quantitatively yielding N-isopropyl(dimesitylphosphoryl)formamide. The potassium complexes are more efficient catalysts than the calcium congeners, and coordination of 18-crown-6 decelerates the catalytic conversion.

Calcium, Strontium and Barium Homogeneous Catalysts for Fine Chemicals Synthesis

The large alkaline earths (Ae), calcium, strontium and barium, have in the past 15 years yielded a brand new generation of heteroleptic molecular catalysts for the production of fine chemicals. However, the integrity of these complexes is often plagued by ligand redistribution equilibria in solution. This personal account retraces the paths followed in our research group towards the design of stable heteroleptic alkalino-earth complexes, including the use of intramolecular noncovalent Ae···H-Si and Ae···F-C interactions. Their implementation as homogenous precatalysts for reactions such as the intramolecular and intermolecular hydroamination and hydrophosphination of activated alkenes, the hydrophosphonylation of ketones, and the dehydrogenative coupling of amines and hydrosilanes that enable the efficient and controlled formations of CP, CN, or SiN σ-bonds, is presented in a synthetic perspective that highlights their overall outstanding catalytic performance.

Rare-earth metal diisopropylamide-catalyzed intramolecular hydroamination

Well-defined rare-earth metal diisopropylamide complexes provide an exemplary case study to investigate the effect of donor solvent, alkali metal, chloro co-ligands, and in situ catalyst formation.

Hydroamination of diphenylbutadiyne with secondary N-methyl-anilines using the dipotassium tetrakis(2,6-diisopropylanilino)calciate precatalyst

The approved precatalyst [K2Ca{N(H)Dipp}4] was employed to study the hydroamination of diphenylbutadiyne with N-methyl-anilines in tetrahydrofuran at room temperature. The hydroamination occurs regioselectively within a few hours yielding (N-methyl)-(N-aryl)-1,4-diphenylbut-1-ene-3-yne-1-ylamine with phenyl (1a), 4-tolyl (1b) and 4-fluorophenyl groups (1c). In all cases a mixture of E- and Z-isomers is obtained. The second hydroamination step requires drastically extended reaction times and is successful only for the reaction of diphenylbutadiyne with N-methyl-aniline and N-methyl-4-fluoroaniline giving 1,4-diphenyl-1,4-bis(N-methyl-anilino)buta-1,3-diene [R = H (2a) and F (2c)]; a mixture of E,E-, E,Z- and Z,Z-isomers is obtained. The X-ray structures of E-1a, E-1b and E-1c show a slightly shortened N-C bond to the alkene moieties. Due to enhanced steric strain the anilino units of Z,Z-2c and Z,Z-3 turn away from the butadiene unit and consequently, the lone pair at the planar nitrogen atoms slightly interacts with the adjacent aryl groups.