Electronically Unsaturated Three-Coordinate Aluminum Hydride and Organoaluminum Cations

Alkylaluminum Complexes Featuring Bridged Bis-Formylfluorenimide Ligands for Hydroboration of Aldehyde, Ketone, and Imines

Three bis-formylfluorenimide ligands with different bridging groups were designed and synthesized, leading to the successful preparation of six novel alkylaluminum complexes through their reaction with alkylaluminum reagents (AlMe3 or AlEt3). Complexes 1 and 2 were obtained by the reaction of 1,2-propylene-bridged diamine (L1) with AlMe3 or AlEt3. By reacting 1,2-cyclohexylene-bridged diamine (L2) with AlMe3 or AlEt3 to obtain complexes 3 and 4. The above ligands formed a bidentate four-coordinate structure with alkylaluminum, which involved the elimination of one alkyl group as the ligand reacted with alkylaluminum. The complexes 5 and 6 were synthesized through the reaction of 1,2-phenylene-bridged diamine (L3) with AlEt3 in toluene or tetrahydrofuran. Notably, L3 exhibited unique reactivity compared with the other ligands, which formed a tridentate four-coordinated structure when reacting with alkylaluminum. The formation of the tridentate complex resulted from the introduction of a benzimidazole derivative or tetrahydrofuran (THF) molecule along with the elimination of two alkyl groups during its coordination with alkylaluminum. All complexes were characterized via 1H NMR, 13C NMR, and elemental analysis, with structural determination confirmed through X-ray. Furthermore, the catalytic activity in the hydroboration reaction of aldehyde, ketone, and imines was investigated with these complexes as catalysts. Among them, complex 1 demonstrated excellent catalytic performance (up to 99% yield) and broad substrate compatibility (more than 30 substrates) at low catalyst loading (1 mol %) under mild reaction conditions.

Highly Electrophilic Mononuclear Cationic Aluminium Alkoxide Complexes: Syntheses, Reactivity and Catalytic Applications

Herein, we report the synthesis of β-diketiminate-supported aluminium complexes bearing terminal alkoxide and mono-thiol functional groups: LAlOMe(Et) (2), LAlOtBu(Et) (3), and LAlSH(Et) (4), (L=[HC{C(Me)N-(2,6-iPr2 C6 H3 )}2 ]). Complexes 2 and 3 are further used as synthons to generate the fascinating cationic aluminium alkoxide complexes, [LAlOMe(μ-OMe)-Al(Et)L][EtB(C6 F5 )3 ] (5), [LAlOMe(OEt2 )][EtB(C6 F5 )3 ] (6), and [LAlOtBu(OEt2 )][EtB(C6 F5 )3 ] (8). These electrophilic cationic species are well characterized by spectroscopic and crystallographic techniques. The assessment of Lewis acidity by the Gutmann-Beckett method revealed superior Lewis acidity of the cations substituted with electron-demanding alkoxy groups in comparison to the known methyl analogue [LAlMe][B(C6 F5 )4 ]. This has been further endorsed by computational calculations to determine the NBO charges and hydride ion affinity for complexes 6 and 8. These complexes are also capable of activating triethylsilane in stoichiometric reactions. The applicability of these complexes has been realized in the hydrosilylation of ethers, carbonyls, and olefines. Additionally, the solid-state structure of a new THF stabilized aluminium halide cation [LAlCl(THF)][B(C6 F5 )4 ] (11) has also been reported.

Group 13 exchange and transborylation in catalysis

Catalysis is dominated by the use of rare and potentially toxic transition metals. The main group offers a potentially sustainable alternative for catalysis, due to the generally higher abundance and lower toxicity of these elements. Group 13 elements have a rich catalogue of stoichiometric addition reactions to unsaturated bonds but cannot undergo the redox chemistry which underpins transition-metal catalysis. Group 13 exchange reactions transfer one or more groups from one group 13 element to another, through σ-bond metathesis; where boron is both of the group 13 elements, this is termed transborylation. These redox-neutral processes are increasingly being used to render traditionally stoichiometric group 13-mediated processes catalytic and develop new catalytic processes, examples of which are the focus of this review.

A cyclic divalent N(I) species isoelectronic to carbodiphosphoranes

The formation of a rare type of diphosphazenium cation is described. Its synthesis features a unique oxidative dealkylation of an iminophosphorane-phosphole by a silver(I) salt. DFT study of this compound reveals the low valent character of the N(I) center.

Aluminium complexes: next-generation catalysts for selective hydroboration

Organoboranes obtained from hydroboration reactions are one of the important classes of compounds that could be used to provide valuable synthons for follow-up transformations such as various functional group incorporation or C-C bond forming reactions. For decades, various transition metals were utilised as catalysts in such transformations. Recently Earth-abundant and less toxic main group metals have revived their importance in hydroboration chemistry, among which the suitable candidates are aluminium complexes as catalysts. In this regard, the development of aluminium complexes to achieve more robust catalytic systems with greater efficiency is appreciable.

Hydroboration and reductive amination of ketones and aldehydes with HBpin by a bench stable Pd(II)-catalyst

A palladium(II) complex [(κ⁴-{1,2-C₆H₄(NCH-C₆H₄O)₂}Pd] (1) supported by a dianionic salen ligand [1,2-C₆H₄(NCH-C₆H₄O)₂]^2- (L) was synthesised and used as a molecular pre-catalyst in the hydroboration of aldehydes and ketones. The molecular structure of Pd(II) complex 1 was established by single-crystal X-ray diffraction analysis. Complex 1 was tested as a competent pre-catalyst in the hydroboration of aldehydes and ketones with pinacolborane (HBpin) to produce corresponding boronate esters in excellent yields at ambient temperature under solvent-free conditions. Further, the complex 1 proved to be a competent catalyst in the reductive amination of aldehydes with HBpin and primary amines under mild and solvent-free conditions to afford a high yield (up to 97%) of corresponding secondary amines. Both protocols provided high conversion, superior selectivity and broad substrate scope, from electron-withdrawing to electron-donating and heterocyclic substitutions. A computational study based on density functional theory (DFT) revealed a reaction mechanism for Pd-catalysed hydroboration of carbonyl species in the presence of HBpin. The protocols also uncovered the dual role of HBpin in achieving the hydroboration reaction.

Hydroboration of carbon dioxide with pinacolborane catalyzed by various aluminum hydrides: A comparative mechanistic study

In this work, density functional theory (DFT) calculations was performed to probe the catalytic viability of various neutral, cationic and anionic aluminum hydrides (AlH) in the hydroboration of CO2 with...

Aluminum-Hydride-Catalyzed Hydroboration of Carbon Dioxide

This study describes the first use of a bis(phosphoranyl)methanido aluminum hydride, [ClC(PPh₂NMes)₂AlH₂] (2, Mes = Me₃C₆H₂), for the catalytic hydroboration of CO₂. Complex 2 was synthesized by the reaction of a lithium carbenoid [Li(Cl)C(PPh₂NMes)₂] with 2 equiv of AlH₃·NEtMe₂ in toluene at -78 °C. 2 (10 mol %) was able to catalyze the reduction of CO₂ with HBpin in C₆D₆ at 110 °C for 2 days to afford a mixture of methoxyborane [MeOBpin] (3a; yield: 78%, TOF: 0.16 h^-1) and bis(boryl)oxide [pinBOBpin] (3b). When more potent [BH₃·SMe₂] was used instead of HBpin, the catalytic reaction was extremely pure, resulting in the formation of trimethyl borate [B(OMe)₃] (3e) [catalytic loading: 1 mol % (10 mol %); reaction time: 60 min (5 min); yield: 97.6% (>99%); TOF: 292.8 h^-1 (356.4 h^-1)] and B₂O₃ (3f). Mechanistic studies show that the Al-H bond in complex 2 activated CO₂ to form [ClC(PPh₂NMes)₂Al(H){OC(O)H}] (4), which was subsequently reacted with BH₃·SMe₂ to form 3e and 3f, along with the regeneration of complex 2. Complex 2 also shows good catalytic activity toward the hydroboration of carbonyl, nitrile, and alkyne derivatives.

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.

Hydrosilylation of Carbonyls Catalyzed by Hydridoborenium Borate Salts: Lewis Acid Activation and Anion Mediated Pathways

The electronically unsaturated three-coordinated hydridoborenium cations [LBH]⁺[HB(C₆F₅)₃]^- (1) and [LBH]⁺[B(C₆F₅)₄]^- (2), supported by a bis(phosphinimino)amide ligand, were found to be excellent catalysts for hydrosilylation of a range of aliphatic and aromatic aldehydes and ketones under mild reaction conditions (L = [{(2,4,6-Me₃C₆H₂N)P(Ph₂)}₂N]). The key steps of the catalytic cycle for hydrosilylation of PhCHO were monitored via in situ multinuclear NMR measurements for catalysts 1 and 2. The combined effect of carbonyl activation via the Lewis acidic hydridoborenium cation and the hydridic nature of the borate counteranion in 1 makes it a more efficient catalyst in comparison to that of carbonyl activation via the predominant Lewis acid activation pathway operating with catalyst 2. The catalytic cycle of 1 showed hydride transfer from the borate moiety [HB(C₆F₅)₃]^- to PhCHO in the first step, forming [PhCH₂-O-B(C₆F₅)₃]^-, which subsequently underwent σ-bond metathesis with Et₃SiH to form the product, PhCH₂-O-SiEt₃. Quantum chemical calculations also support the borate anion mediated mechanism with 1. In contrast, the reaction catalyzed by 2 proceeds predominantly via the Lewis acid activation of the carbonyl group involving [LB(H)←OC(H)Ph]⁺[B(C₆F₅)₄]^- as the transition state and [LBOCH₂Ph]⁺[B(C₆F₅)₄]^- as the intermediate.

Mono- and Bimetallic Aluminum Alkyl, Alkoxide, Halide and Hydride Complexes of a Bulky Conjugated Bis-Guanidinate(CBG) Ligand and Aluminum Alkyls as Precatalysts for Carbonyl Hydroboration

Tetra-aryl-substituted symmetrical conjugated bis-guanidine (CBG) ligands such as L^1-3 (3H) [L(3H) = {(ArHN)(ArHN)C═N-C═NAr(NHAr)}; Ar = 2,6-Me₂-C₆H₃ (L¹(3H)), 2,6-Et₂-C₆H₃ (L²(3H)), and 2,6-ⁱPr₂-C₆H₃ (L³(3H))] have been employed to synthesize a series of four- and six-membered aluminum heterocycles (1-8) for the first time. Generally, aluminum complexes bearing N,N'- chelated guanidinate and β-diketiminate/dipyrromethene ligand systems form four- and six-membered heterocycles, respectively. However, the conjugated bis-guanidine ligand has the capability of forming both four- and six-membered heterocycles possessing multimetal centers within the same molecule; this is due to the presence of three acidic protons, which can be easily deprotonated (at least two protons) upon treatment with metal reagents. Both mono- and dinuclear aluminum alkyls and mononuclear aluminum alkoxide, halide, and hydride complexes have been structurally characterized. Further, we have demonstrated the potential of mononuclear, six-membered CBG aluminum dialkyls in catalytic hydroboration of a broad range of aldehydes and ketones with pinacolborane (HBpin).

Organoaluminum cations for carbonyl activation

In search of stable, yet reactive aluminum Lewis acids, we have isolated an organoaluminum cation, [(Me2NC6H4)2Al(C4H8O)2]+, coordinated with two labile tetrahydrofuran ligands. Its catalytic performance in aldehyde dimerization reveals turn-over frequencies reaching up to 6000 h-1, exceeding that of the reported main group catalysts. The cation is further demonstrated to catalyze hydroelementation of ketones. Mechanistic investigations reveal that aldehyde dimerization and ketone hydrosilylation occur through carbonyl activation.

Catalytic CO2 Reduction with Boron- and Aluminum Hydrides

The previously reported dimeric NHI aluminum dihydrides 1 a,b, as well as the bis(NHI) aluminum dihydride salt 9 +[OTs]-, the bis(NHI) boron dihydride salt 10 +[OTs]-, and the "free" bis(NHI) ligand 12 were investigated with regard to their activity as a homogenous (pre)catalyst in the hydroboration (i. e. catalytic reduction) of carbon dioxide (CO2) in chloroform under mild conditions (i. e. room temperature, 1 atm; NHI=N-heterocyclic imine, Ts=tosyl). Borane dimethylsulfide complex and catecholborane were used as a hydride source. Surprisingly, the less sterically hindered 1 a exhibited lower catalytic activity than the bulkier 1 b. A similarly unexpected discrepancy was found with the lower catalytic activity of 10 + in comparison to the one of the bis(NHI) 12. The latter is incorporated as the ligand to the boron center in 10 +. To elucidate possible mechanisms for CO2 reduction the compounds were subjected to stoichiometric reactivity studies with the borane or CO2. Aluminum carboxylates 4, 6, and 7 + with two, four, and one formate group per two aluminum centers were isolated. Also, the boron formate salt 11 +[OTs]- was characterized. Selected metal formates were subjected to stoichiometric reactions with boranes and/or tested as a catalyst. We conclude that each type of catalyst (1 a,b, 9 +, 10 +, 12) follows an individual mechanistic pathway for CO2 reduction.

Reversible borohydride formation from aluminium hydrides and {H(9-BBN)}2: structural, thermodynamic and reactivity studies

A series of novel β-diketiminate stabilised aluminium borohydrides of the type (Nacnac)Al(R){H₂(9-BBN)} has been synthesised offering variation in both the auxiliary R substituent and in the Nacnac backbone itself. A number of these complexes show unusual dissociation of the borane from the aluminium hydride in solution under ambient conditions. The lability of the borane is shown (by variable temperature NMR analyses) to be influenced by the electronic character of both the aluminium-bound R substituent and the Nacnac ligand itself, such that electron-withdrawing substituents lead to greater dissociation of the borane. Comparison of these complexes with related systems featuring the tetrahydroborate [BH₄]^- ligand illustrates the impact of the boron-bound substituents on the ability of the borane fragment to dissociate from the aluminium hydride. This dissociative behaviour is shown to be highly influential on the ability of the borohydride complexes to reduce carbon dioxide in a stoichiometric manner.

Aluminum containing molecular bowls and pyridinophanes: use of pyridine modules to access different molecular topologies

Molecular topologies varying from simple complexes to pyridinophanes (neutral and cationic) and to bicyclic pyridinophane containing organoaluminum (Al-Me) species were synthesized by varying the relative stoichiometry of bis(trimethylsilyl)-N,N'-2,6-diaminopyridine (bap) and the reactive partner (AlMe₃). The ultimate goal of these reactions was to systematically design cyclic structures containing group 13 elements. To highlight the reaction potential of these shapes, the bowl-shaped pyridinophane was reacted with the Lewis acid, B(C₆F₅)₃, to generate a stable cationic derivative. An unprecedented bicyclic pyridinophane, [2,6-(Me₃SiN)₂C₅H₃N]₃Al₂, was obtained from the reaction of bap with AlH₃·NMe₂Et. The formation of [2,6-(Me₃SiN)₂C₅H₃N]₃Al₂ is in contrast to the known reaction between BH₃·SMe₂ and bap that afforded the syn-tetraazadibora[3.3](2,6)pyridinophane. Quantum chemical calculations have been performed to rationalize the preference for the formation of B-pyridinophane and Al-bicyclic pyridinophane and can be attributed to the nature of B-N and Al-N bonds.

Ge(ii) cation catalyzed hydroboration of aldehydes and ketones

The catalytic utility of a germylene cation 4 is reported. In the presence of compound 4, a variety of aldehydes and ketones can be hydroborylated using HBpin.

Expanding the limits of catalysts with low-valent main-group elements for the hydroboration of aldehydes and ketones using [L†Sn(ii)][OTf] (L† = aminotroponate; OTf = triflate)

A triflatostannylene [L^†Sn][OTf] (2) is found to be an efficient catalyst with low-valent main-group element for the hydroboration of aldehydes and ketones.

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.

Cationic Complexes of Boron and Aluminum: An Early 21st Century Viewpoint

Boron and aluminum are lighter Group 13 elements, found in daily life commodities, and considered environmentally benign. Nevertheless, they markedly differ in their elemental properties (e.g., metal character, atomic radius). The use of Lewis acidic complexes of boron and aluminum for methods of bond activation and catalysis (e.g., hydrogenation of unsaturated substrates, polymerization of olefins and epoxides) is quickly expanding. The introduction of cationic charge may boost the metalloid-centered Lewis acidity and allow for its fine-tuning particularly with regard to preference for "hard" or "soft" Lewis bases (i.e., substrates). Especially the isolation of low-coordinate cations (number of ligand atoms smaller than four) demands elaborate techniques of thermodynamic and kinetic stabilization (i.e., electronic saturation and steric shielding) by a ligand system. Furthermore, the properties of the solvent and the counteranion must be considered with care. Here, selected examples of boron and aluminum cations are described.

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.

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.

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.