Synthesis, characterisation, and catalytic application of a soluble molecular carrier of sodium hydride activated by a substituted 4-(dimethylamino)pyridine

Recently main group compounds have stepped into the territory of precious transition metal compounds with respect to utility in the homogeneous catalysis of fundamentally important organic transformations. Inspired by the need to promote more sustainability in chemistry because of their greater abundance in nature, this change of direction is surprising since main group metals generally do not possess the same breadth of reactivity as precious transition metals. Here, we introduce the dihydropyridylsodium compound, Na-1,2-tBu-DH(DMAP), and its monomeric variant [Na-1,2-tBu-DH(DMAP)]·Me6TREN, and demonstrate their effectiveness in transfer hydrogenation catalysis of the representative alkene 1,1-diphenylethylene to the alkane 1,1-diphenylethane using 1,4-cyclohexadiene as hydrogen source [DMAP = 4-dimethylaminopyridine; Me6TREN = tris(N,N-dimethyl-2-aminoethyl)amine]. Sodium is appealing because of its high abundance in the earth's crust and oceans, but organosodium compounds have been rarely used in homogeneous catalysis. The success of the dihydropyridylsodium compounds can be attributed to their high solubility and reactivity in organic solvents.

Isolable rubidium and caesium derivatives of common organic carbonyl compounds

Light alkali metal (Li, Na, K) amides have a long history of synthetic utility, but heavier (Rb, Cs) congeners have barely been studied. This study reveals remarkable structurally complex outcomes of reacting AM(HMDS) (AM = Rb, Cs; HMDS = hexamethyldisilazide) with benzaldehyde and acetophenone. Though complicated, reactions give a diversity of eye-catching isolated products, an enolate with a hexagonal prismatic network, two dienolates with distinct extended ladder motifs, and two β-imino-alkoxides comprising zig-zag chains of metal-oxygen bonds in infinite cages.

Modification of a Common β-diketiminate NacNac Framework via Sequential Lithiation and Small Molecule Insertion

A widely utilised class of ligands in synthesis and catalysis, β-diketiminate (BDI) or NacNac compounds were initially considered innocent in the sense that they remained intact in all their applications. That changed when the γ-C-H unit of their NCCCN backbone was found to engage in reactions with electrophiles. Here, we show that this special reactivity can be used advantageously to prepare tripodal modifications of the common NacNac ligand derived from 2,6-diisopropylphenyl-β-methyldiketimine [NacNacH (Me, Dipp)]. Lithiation to give NacNacLi, followed by reactions with isocyanates, isothiocyanates and a carbodiimide, have afforded a series of tripodal NacNac variants having N,N,N,O; N,N,N,S; or N,N,N,N potential dentation sites, many of which have been crystallographically characterised. Distinct ligating modes of these new ligands have been elucidated through the crystal structures of their lithiated derivatives.

Three Oxidative Addition Routes of Alkali Metal Aluminyls to Dihydridoaluminates and Reactivity with CO2

Three distinct routes are reported to the soluble, dihydridoaluminate compounds, AM[Al(NONDipp )(H)2 ] (AM=Li, Na, K, Rb, Cs; [NONDipp ]2- =[O(SiMe2 NDipp)2 ]2- ; Dipp=2,6-iPr2 C6 H3 ) starting from the alkali metal aluminyls, AM[Al(NONDipp )]. Direct H2 hydrogenation of the heavier analogues (AM=Rb, Cs) produced the first examples of structurally characterized rubidium and caesium dihydridoaluminates, although harsh conditions were required for complete conversion. Using 1,4-cyclohexadiene (1,4-CHD) as an alternative hydrogen source in transfer hydrogenation reactions provided a lower energy pathway to the full series of products for AM=Li-Cs. A further moderation in conditions was noted for the thermal decomposition of the (silyl)(hydrido)aluminates, AM[Al(NONDipp )(H)(SiH2 Ph)]. Probing the reaction of Cs[Al(NONDipp )] with 1,4-CHD provided access to a novel inverse sandwich complex, [{Cs(Et2 O)}2 {Al(NONDipp )(H)}2 (C6 H6 )], containing the 1,4-dialuminated [C6 H6 ]2- dianion and representing the first time that an intermediate in the commonly utilized oxidation process of 1,4-CHD to benzene has been trapped. The synthetic utility of the newly installed Al-H bonds has been demonstrated by their ability to reduce CO2 under mild conditions to form the bis-formate AM[Al(NONDipp )(O2 CH)2 ] compounds, which exhibit a diverse series of eyecatching bimetallacyclic structures.

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.

Synthesis, Characterization, and Structural Analysis of [Al(NONDipp)(H)(SiH2Ph)] ( = Li, Na, K, Rb, Cs) Compounds, Made Via Oxidative Addition of Phenylsilane to Alkali Metal Aluminyls

We report the oxidative addition of phenylsilane to the complete series of alkali metal (AM) aluminyls [AM{Al(NON^Dipp)}]₂ (AM = Li, Na, K, Rb, and Cs). Crystalline products (1-AM) have been isolated as ether or THF adducts, [AM(L)_n][Al(NON^Dipp)(H)(SiH₂Ph)] (AM = Li, Na, K, Rb, L = Et₂O, n = 1; AM = Cs, L = THF, n = 2). Further to this series, the novel rubidium rubidiate, [{Rb(THF)₄}₂(Rb{Al(NON^Dipp)(H)(SiH₂Ph)}₂)]⁺ [Rb{Al(NON^Dipp)(H)(SiH₂Ph)}₂]^-, was isolated during an attempted recrystallization of Rb[Al(NON^Dipp)(H)(SiH₂Ph)] from a hexane/THF mixture. Structural and spectroscopic characterizations of the series 1-AM confirm the presence of μ-hydrides that bridge the aluminum and alkali metals (AM), with multiple stabilizing AM···π(arene) interactions to either the Dipp- or Ph-substituents. These products form a complete series of soluble, alkali metal (hydrido) aluminates that present a platform for further reactivity studies.

Structural characterization of three hydride-bridged sodium aluminate compounds

The synthesis and single-crystal structures of three hydride-bridged sodium aluminate compounds containing the utility amide HMDS [N(SiMe3)2 or C6H18NSi2] are reported. Both bis[bis(trimethylsilyl)amido-2κN]-μ-hydrido-hydrido-2κH-(N,N,N′,N′′,N′′-pentamethyldiethylenetriamine-1κ3 N,N′,N′′)(tetrahydrofuran-1κO)aluminiumsodium, [AlNa(C6H18NSi2)2H2(C4H8O)(C9H23N3)] or (HMDS)2Al(H)2Na(THF)(PMDETA), (1) (THF = tetrahydrofuran, C4H8O; PMDETA = N,N,N′,N′′,N′′-pentamethyldiethylenetriamine, C9H23N3) and tetrakis[bis(trimethylsilyl)amido]-3κ2 N,4κ2 N-tetra-μ-hydrido-tetrakis(tetrahydrofuran)-1κ2 O,2κ2 O-dialuminiumdisodium, [Al2Na2(C6H18NSi2)4H4(C4H8O)4] or [(HMDS)2Al(H)2Na(THF)2]2 (2), are dihydrides. However, 1 is a dinuclear Al—H—Na monomer with one bridging and one terminal hydride ligand whilst in 2 all the hydride ligands bridge between Al and Na atoms to give a dimeric structure with a core (AlHNaH)2 eight-membered ring. In contrast, the structure of bis[bis(trimethylsilyl)amido]-3κN,4κN-dihydrido-3κH,4κH-tetra-μ-hydrido-bis(N,N,N′,N′′,N′′-pentamethyldiethylenetriamine)-1κ3 N,N′,N′′;2κ3 N,N′,N′′-dialuminiumdisodium, [Al2Na2(C6H18NSi2)2H6(C9H23N3)2] or [(HMDS)Al(H)3Na(PMDETA)]2 (3), also contains a (AlHNaH)2 eight-membered ring but is a trihydride with two bridging and one terminal hydride ligand per Al centre. The (AlHNaH)2 eight-membered rings of 2 and 3 differ in their structural details. That of 2 is based around a twofold axis and has a larger Al...Al intra-ring distance than that found in centrosymmetric 3.

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].

Heavy Alkali Metal Manganate Complexes: Synthesis, Structures and Solvent-Induced Dissociation Effects

Rare examples of heavier alkali metal manganates [{(AM)Mn(CH₂ SiMe₃ )(N^'Ar )₂ }_∞ ] (AM=K, Rb, or Cs) [N^'Ar =N(SiMe₃ )(Dipp), where Dipp=2,6-iPr₂ -C₆ H₃ ] have been synthesised with the Rb and Cs examples crystallographically characterised. These heaviest manganates crystallise as polymeric zig-zag chains propagated by AM⋅⋅⋅π-arene interactions. Key to their preparation is to avoid Lewis base donor solvents. In contrast, using multidentate nitrogen donors encourages ligand scrambling leading to redistribution of these bimetallic manganate compounds into their corresponding homometallic species as witnessed for the complete Li - Cs series. Adding to the few known crystallographically characterised unsolvated and solvated rubidium and caesium s-block metal amides, six new derivatives ([{AM(N^'Ar )}_∞ ], [{AM(N^'Ar )⋅TMEDA}_∞ ], and [{AM(N^'Ar )⋅PMDETA}_∞ ] where AM=Rb or Cs) have been structurally authenticated. Utilising monodentate diethyl ether as a donor, it was also possible to isolate and crystallographically characterise sodium manganate [(Et₂ O)₂ Na(ⁿ Bu)Mn[(N^'Ar )₂ ], a monomeric, dinuclear structure prevented from aggregating by two blocking ether ligands bound to sodium.

Sigma/pi Bonding Preferences of Solvated Alkali-Metal Cations to Ditopic Arylmethyl Anions

Arylmethyl anions allow alkali‐metals to bind in a σ‐fashion to the lateral carbanionic centre or a π‐fashion to the aryl ring or in between these extremities, with the trend towards π bonding increasing on descending group 1. Here we review known alkali metal structures of diphenylmethane, fluorene, 2‐benzylpyridine and 4‐benzylpyridine. Next, we synthesise Li, Na, K monomers of these diarylmethyls using polydentate donors PMDETA or Me₆TREN to remove competing oligomerizing interactions, studying the effect that two aromatic rings has on negative charge (de)localisation via NMR, X‐ray crystallographic and DFT studies. Diphenylmethyl and fluorenyl anions maintain C(H)−M interactions regardless of alkali‐metal, although the adjacent arene carbons engage in interactions with larger alkali‐metals. Introducing a nitrogen atom into the ring (at the 2‐ or 4‐position) encourages relocalisation of negative charge away from the deprotonated carbon and onto nitrogen. Phenyl(2‐pyridyl)methyl moves from an enamide formation at one extremity (lithium) to an aza‐allyl formation at the other extremity (potassium), while C‐ or N‐coordination modes become energetically viable for Na and K phenyl(4‐pyridyl)methyl complexes.

Rubidium and caesium aluminyls: synthesis, structures and reactivity in C-H bond activation of benzene

Expanding knowledge of low valent aluminium chemistry, rubidium and caesium aluminyls are reported to complete the group 1 (Li-Cs) set of metal aluminyls. Both compounds crystallize as a contacted dimeric pair supported by M⋯π(arene) interactions with a pronounced twist between aluminyl units. Density functional theory calculations show symmetrical bonding between the M and Al atoms, with an Al centred lone-pair donating into vacant Rb and Cs orbitals. Interestingly, despite their structural similarity the Cs aluminyl enables C-H bond activation of benzene, but not the Rb aluminyl reflecting the importance of the alkali metal in these heterobimetallic systems.

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".

Expected and Unexpected Reactivities of Homoleptic LiNacNac and Heteroleptic NacNacMg(TMP) β-Diketiminates toward Various Small Unsaturated Organic Molecules

Homoleptic LiNacNac forms simple donor-acceptor complexes with N,N'-dicyclohexylcarbodiimide (CyN═C═NCy), triphenylphosphine oxide (Ph₃P═O), and benzophenone (Ph₂CO). These crystallographically characterized compounds could be regarded as model intermediates en route to reducing the N═C, P═O, and C═O bonds of unsaturated substrates. Heteroleptic NacNacMg(TMP) intriguingly functions as a TMP nucleophile both with t-BuNCO and t-BuNCS, producing a urea or thiourea derivative respectively attached to Mg, though the NacNac ligand in the former reaction also engages noninnocently with a second t-BuNCO molecule via insertion at the reactive NacNac backbone γ-carbon site.

When Electrons Step in: Polarizing Effects Explored with Triisobutylaluminum

High-resolution X-ray diffraction data of triisobutylaluminum were collected, and unexpected structural features were observed, hinting toward yet unnoticed polarization effects. To approach these, a multipole refinement using the Hansen and Coppens method, followed by a topological analysis using Bader's quantum theory of atoms in molecules, was employed. The electron localization function based on density functional theory calculations supported the experimental findings. Thereby, unobserved electron shifts within the isobutyl group become detectable. It is shown that the impact of this electron shift is dependent mainly on whether the ⁱBu substituent of the homoleptic triisobutylaluminum dimer [AlⁱBu₃]₂ (1) is connected by a directional (σ) or a multicenter (μ) bond to the metal. The effect found is assumed not only to be of paramount importance for organoaluminum compounds, widely used in synthesis and in the industrial value chain, but also to be present in organometallic chemistry in general.

Isomerization Reactions in Anionic Mesoionic Carbene-Borates and Control of Properties and Reactivities in the Resulting CoII Complexes through Agostic Interactions

We present herein anionic borate-based bi-mesoionic carbene compounds of the 1,2,3-triazol-4-ylidene type that undergo C-N isomerization reactions. The isomerized compounds are excellent ligands for CoII  centers. Strong agostic interactions with the "C-H"-groups of the cyclohexyl substituents result in an unusual low-spin square planar CoII  complex, which is unreactive towards external substrates. Such agostic interactions are absent in the complex with phenyl substituents on the borate backbone. This complex displays a high-spin tetrahedral CoII  center, which is reactive towards external substrates including dioxygen. To the best of our knowledge, this is also the first investigation of agostic interactions through single-crystal EPR spectroscopy. We conclusively show here that the structure and properties of these CoII complexes can be strongly influenced through interactions in the secondary coordination sphere. Additionally, we unravel a unique ligand rearrangement for these classes of anionic mesoionic carbene-based ligands.

Solvation Effects on the Structure and Stability of Alkali Metal Carbenoids

s‐Block metal carbenoids are carbene synthons and applied in a myriad of organic transformations. They exhibit a strong structure–activity relationship, but this is only poorly understood due to the challenging high reactivity and sensitivity of these reagents. Here, we report on systematic VT and DOSY NMR studies, XRD analyses as well as DFT calculations on a sulfoximinoyl‐substituted model system to explain the pronounced solvent dependency of the carbenoid stability. While the sodium and potassium chloride carbenoids showed high stabilities independent of the solvent, the lithium carbenoid was stable at room temperature in THF but decomposed at −10 °C in toluene. These divergent stabilities could be explained by the different structures formed in solution. In contrast to simple organolithium reagents, the monomeric THF‐solvate was found to be more stable than the dimer in toluene, since the latter more readily forms direct Li/Cl interactions which facilitate decomposition via α‐elimination.

A regioselectively 1,1',3,3'-tetrazincated ferrocene complex displaying core and peripheral reactivity

Regioselective 1,1',3,3'-tetrazincation [C-H to C-Zn(tBu)] of ferrocene has been achieved by reaction of a fourfold excess of di-t-butylzinc (tBu₂Zn) with sodium 2,2,6,6-tetramethylpiperidide (NaTMP) in hexane solution manifested in the trimetallic iron-sodium-zinc complex [Na₄(TMP)₄Zn₄(tBu)₄{(C₅H₃)₂Fe}], 1. X-ray crystallographic studies supported by DFT modelling reveal the structure to be an open inverse crown in which two [Na(TMP)Zn(tBu)Na(TMP)Zn(tBu)]²⁺ cationic units surround a {(C₅H₃)₂Fe}^4- tetraanion. Detailed C₆D₆ NMR studies have assigned the plethora of ¹H and ¹³C chemical shifts of this complex. It exists in a major form in which capping and bridging TMP groups interchange, as well as a minor form that appears to be an intermediate in this complicated exchange phenomenon. Investigation of 1 has uncovered two distinct reactivities. Two of its peripheral t-butyl carbanions formally deprotonate toluene at the lateral methyl group to generate benzyl ligands that replace these carbanions in [Na₄(TMP)₄Zn₄(tBu)₂(CH₂Ph)₂{(C₅H₃)₂Fe}], 2, which retains its tetrazincated ferrocenyl core. Benzyl-Na π-arene interactions are a notable feature of 2. In contrast, reaction with pyridine affords the crystalline product {[Na·4py][Zn(py*)₂(tBu)·py]}_∞, 3, where py is neutral pyridine (C₅H₅N) and py* is the anion (4-C₅H₄N), a rare example of pyridine deprotonated/metallated at the 4-position. This ferrocene-free complex appears to be a product of core reactivity in that the core-positioned ferrocenyl anions of 1, in company with TMP anions, have formally deprotonated the heterocycle.

Backbone Reactivity of Lithium β-Diketiminate (NacNac) Complexes with CO2 , tBuNCO and iPrNCO

Though alkali metal NacNac (β-diketiminate) complexes have been utilised in synthesis as NacNac-transfer agents, studies of them in their own right with small molecules are exceptionally rare. Here, the lithium compound of the common 2,6-diisopropylphenyl-β-methyldiketiminate [NacNac(Dipp, Me)] ligand is investigated with carbon dioxide and isocyanates. In all four cases reaction occurs at the backbone γ-C atom of the NacNac ligand, which redistributes electronically into a diimine. Insertion of CO₂ gives an eight-atom carboxylate (Li₂ O₄ C₂ ) ring at the γ-C site in a dimer. Insertion of tBuNCO gives a secondary amide at the γ-C site in a monomer with TMEDA chelating lithium. Double insertion of tBuNCO and (adventitious) oxygen gives a dimer with a (LiO)₂ central core involving the latter source. Insertion of less bulky (iPrNCO) gives a dimer with dimerisation through the C=O bonds of the emergent secondary amide function.

Selective mono- and dimetallation of a group 3 sandwich complex

The scandium Cp/COT hybrid sandwich compound [(η⁵-C₅H₅)Sc(η⁸-C₈H₈)] is resistant to metallation via conventional alkyllithium and lithium amide bases. In this work, clean, selective, stoichiometric and high-yielding mono- and dimetallation is accomplished using tandem trans-metal-trapping (TMT) involving LiTMP and iBu₂AlTMP with deprotonation occurring selectively at the Cp and Cp/COT rings respectively, providing the first examples of selective metallation of a sandwich complex featuring a group 3 element.

Lithium-Aluminate-Catalyzed Hydrophosphination Applications

Synthesized, isolated, and characterized by X‐ray crystallography and NMR spectroscopic studies, lithium phosphidoaluminate iBu₃AlPPh₂Li(THF)₃ has been tested as a catalyst for hydrophosphination of alkynes, alkenes, and carbodiimides. Based on the collective evidence of stoichiometric reactions, NMR monitoring studies, kinetic analysis, and DFT calculations, a mechanism involving deprotonation, alkyne insertion, and protonolysis is proposed for the [iBu₃AlHLi]₂ aluminate catalyzed hydrophosphination of alkynes with diphenylphosphine. This study enhances further the development of transition‐metal‐free, atom‐economical homogeneous catalysis using common sustainable main‐group metals.

Diverse outcomes of CO2 fixation using alkali metal amides including formation of a heterobimetallic lithium-sodium carbamato-anhydride via lithium-sodium bis-hexamethyldisilazide

Fixation of CO2 by lithium amides derived from pyrrole and diisopropylamine generates a lithium carbamate polymer and dodecamer respectively. Moving to lithium-sodium hexamethyldisilazide produces a more complicated, intriguing reaction, where unusually the bimetallic composition is maintained in the product but its composition contains both carbamato and anhydride functionalities.

Contrasting Synergistic Heterobimetallic (Na-Mg) and Homometallic (Na or Mg) Bases in Metallation Reactions of Dialkylphenylphosphines and Dialkylanilines: Lateral versus Ring Selectivities

A series of dialkylphenylphosphines and their analogous aniline substrates have been metallated with the synergistic mixed-metal base [(TMEDA)Na(TMP)(CH₂ SiMe₃ )Mg(TMP)] 1. Different metallation regioselectivities for the substrates were observed, with predominately lateral or meta-magnesiated products isolated from solution. Three novel heterobimetallic complexes [(TMEDA)Na(TMP)(CH₂ PCH₃ Ph)Mg(TMP)] 2, [(TMEDA)Na(TMP)(m-C₆ H₄ PiPr₂ )Mg(TMP)] 3 and [(TMEDA)Na(TMP)(m-C₆ H₄ NEt₂ )Mg(TMP)]  4 and two homometallic complexes [{(TMEDA)Na(EtNC₆ H₅ )}₂ ] 5 and [(TMEDA)Na₂ (TMP)(C₆ H₅ PEt)]₂  6 derived from homometallic metallation have been crystallographically characterised. Complex 6 is an unprecedented sodium-amide, sodium-phosphide hybrid with a rare (NaNNaP)₂ ladder motif. These products reveal contrasting heterobimetallic deprotonation with homometallic induced ethene elimination reactivity. Solution studies of metallation mixtures and electrophilic iodine quenching reactions confirmed the metallation sites. In an attempt to rationalise the regioselectivity of the magnesiation reactions the C-H acidities of the six substrates were determined in THF solution using DFT calculations employing the M06-2X functional and cc-pVTZ Dunning's basis set.

Transforming LiTMP Lithiation of Challenging Diazines through Gallium Alkyl Trans-Metal-Trapping

This study establishes a new trans‐metal‐trapping (TMT) procedure based on a mixture of LiTMP (the base) and tris(trimethylsilylmethyl)gallium [Ga(CH₂SiMe₃)₃, GaR₃] (the trap) that, operating in a tandem manner, is effective for the regioselective deprotonation of sensitive diazines in hydrocarbon solution, as illustrated through reactions of pyrazine, pyridazine, and pyrimidine, as well as through the N‐S heterocycle benzothiazole. The metallo‐activated complexes of all of these compounds were isolated and structurally defined.

Comparing Neutral (Monometallic) and Anionic (Bimetallic) Aluminum Complexes in Hydroboration Catalysis: Influences of Lithium Cooperation and Ligand Set

Bimetallic lithium aluminates and neutral aluminum counterparts are compared as catalysts in hydroboration reactions with aldehydes, ketones, imines and alkynes. Possessing Li–Al cooperativity, ate catalysts are found to be generally superior. Catalytic activity is also influenced by the ligand set, alkyl and/or amido. Devoid of an Al−H bond, iBu₂Al(TMP) operates as a masked hydride reducing benzophenone through a β‐Η transfer process. This catalyst library therefore provides an entry point into the future design of Al catalysts targeting substrate specific transformations.

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.

Trans-Metal-Trapping: Concealed Crossover Complexes En Route to Transmetallation?

Defined as the transfer of ligands from one metal to another, transmetallation is a common reaction in organometallic chemistry. Its chemical celebrity stems from its role in important catalytic cycles of cross-coupling reactions such as those of Negishi, Sonogashira, Stille, or Suzuki. This article focuses on trans-metal-trapping (TMT), which could be construed as partially complete transmetallations. On mixing two distinct organometallic compounds, of for example lithium with aluminium or gallium, the two metals meet in a crossover co-complex, but the reaction ceases at that point and full transmetallation is not reached. Though in its infancy, trans-metal-trapping shows promise in transforming failed lithiations into successful lithiations and in stabilising sensitive carbanions through cooperative bimetallic effects making them more amenable to onward reactivity.

Lithium diamidodihydridoaluminates: bimetallic cooperativity in catalytic hydroboration and metallation applications

Cooperativity between the Li and Al centres is implicated in catalytic hydroboration reactions of aldehydes and ketones with pinacolborane via heteroleptic lithium diamidodihydridoaluminates. In addition to implementing hydroalumination, these versatile heteroleptic ates can also perform as amido bases as illustrated with an acidic triazole.

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.

LiTMP Trans-Metal-Trapping of Fluorinated Aromatic Molecules: A Comparative Study of Aluminum and Gallium Carbanion Traps

Fluoroaromatic scaffolds pose a challenge to lithiation due to low stability of lithiated intermediates. Here we apply trans‐metal‐trapping (TMT) to a series of key fluorinated aromatics. In TMT, LiTMP performs the metalation, while an organometallic trap intercepts the emergent carbanion. This study contrasts the trapping abilities of iBu₂AlTMP and Ga(CH₂SiMe₃)₃, structurally mapping their TMT reactions and probing relative stabilities of metalated fluoroaromatic intermediates by NMR studies. Results show the installed Al−C(aryl) bonds are more prone to decomposition by benzyne formation and Li‐F liberation, than the Ga−C(aryl) species. The latter are thus better for onward reactivity as demonstrated in cross‐coupling reactions with benzoyl chloride that produce ketones.

Contrasting the group 6 metal-metal bonding in sodium dichromate(ii) and sodium dimolybdate(ii) polymethyl complexes: synthetic, X-ray crystallographic and theoretical studies

Extending the class of group 6 metal-metal bonded methylate compounds supported by alkali metal counter-ions, the first sodium octamethylmolybdate(ii) complex [(TMEDA)Na]₄Mo₂Me₈ and heptamethylchromate(ii) relations [(donor)Na]₃Cr₂Me₇ (donor is TMEDA or TMCDA) are reported. The former was made by treating [(Et₂O)Li]₄Mo₂Me₈ with four equivalents of NaOtBu/TMEDA in ether; whereas the latter resulted from introducing TMEDA or TMCDA to ether solutions of octamethyldichromate [(Et₂O)Na]₄Cr₂Me₈. X-ray crystallography revealed [(TMEDA)Na]₄Mo₂Me₈ is dimeric with square pyramidal Mo centres [including a short Mo-Mo interaction of 2.1403(3) Å] each with four methyl groups in a mutually eclipsed conformation. In dinuclear [(TMCDA)Na]₃Cr₂Me₇ trigonal bi-pyramidal Cr centres each bond to three terminal methyl groups and one common Me bridge, that produces a strikingly short Cr-Cr contact of 1.9136(4) Å. Broken symmetry density functional theoretical calculations expose the multiconfigurational metal-metal bonding in these compounds with a Mo-Mo bond order of 3 computed for octamethylmolybdate(ii). This is contrasted by the single Cr-Cr bond in heptamethylchromate(ii) where the singlet ground state is derived by strong antiferromagnetic coupling between adjacent metal ions.

Lithium Dihydropyridine Dehydrogenation Catalysis: A Group 1 Approach to the Cyclization of Diamine Boranes

In reactions restricted previously to a ruthenium catalyst, a 1‐lithium‐2‐alkyl‐1,2‐dihydropyridine complex is shown to be a competitive alternative dehydrogenation catalyst for the transformation of diamine boranes into cyclic 1,3,2‐diazaborolidines, which can in turn be smoothly arylated in good yields. This study established the conditions and solvent dependence of the catalysis through NMR monitoring, with mechanistic insight provided by NMR (including DOSY) experiments and X‐ray crystallographic studies of several model lithio intermediates.

Structural Diversity in Alkali Metal and Alkali Metal Magnesiate Chemistry of the Bulky 2,6-Diisopropyl-N-(trimethylsilyl)anilino Ligand

Bulky amido ligands are precious in s-block chemistry, since they can implant complementary strong basic and weak nucleophilic properties within compounds. Recent work has shown the pivotal importance of the base structure with enhancement of basicity and extraordinary regioselectivities possible for cyclic alkali metal magnesiates containing mixed n-butyl/amido ligand sets. This work advances alkali metal and alkali metal magnesiate chemistry of the bulky arylsilyl amido ligand [N(SiMe3 )(Dipp)]- (Dipp=2,6-iPr2 -C6 H3 ). Infinite chain structures of the parent sodium and potassium amides are disclosed, adding to the few known crystallographically characterised unsolvated s-block metal amides. Solvation by N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA) or N,N,N',N'-tetramethylethylenediamine (TMEDA) gives molecular variants of the lithium and sodium amides; whereas for potassium, PMDETA gives a molecular structure, TMEDA affords a novel, hemi-solvated infinite chain. Crystal structures of the first magnesiate examples of this amide in [MMg{N(SiMe3 )(Dipp)}2 (μ-nBu)]∞ (M=Na or K) are also revealed, though these breakdown to their homometallic components in donor solvents as revealed through NMR and DOSY studies.