Aluminum-Hydride-Catalyzed Hydroboration of Carbon Dioxide

Zinc-Catalyzed Hydroboration of Carbon Dioxide Amplified by Borane-Tethered Heteroscorpionate Bis(Pyrazolyl)methane Ligands

The borane-functionalized (BR2) bis(3,5-dimethylpyrazolyl)methane (LH) ligands 1a (BR2: 9-borabicyclo[3.3.1]nonane or 9-BBN), 1b (BR2: BCy2), and 1c (BR2: B(C6F5)2) were synthesized by the allylation-hydroboration of LH. Metalation of 1a,b with ZnCl2 yielded the heteroscorpionate dichloride complexes [(1a,b)ZnCl2] 3a,b. The reaction of 1a with ZnEt2 led to the formation of the zwitterionic complex [Et(1a)ZnEt(THF)] 5. The reaction of complex 3a with two equivalents of KHBEt3 under a carbon dioxide (CO2) atmosphere gave rise to the formation of the dimeric bis(formate) complex [(1a)Zn(OCHO)2]2 8, in which its borane moieties intermolecularly stabilize the formate ligands of opposite metal centers. The allylated precursor Lallyl and its zinc dichloride, diethyl and bis(formate) complexes [(Lallyl)ZnCl2] 2, [(Lallyl)ZnEt2] 4, and [(Lallyl)Zn(OCHO)2] 7 were also isolated. The catalyst systems composed of 1 mol % of 3a or 3b and two equivalents of KHBEt3 hydroborated CO2 at 1 bar with pinacolborane (HBPin) to the methanol-level product H3COBPin (and PinBOBPin) in yields of 42 or 86%, respectively. The catalyst systems using the unfunctionalized complex [(LH)ZnCl2] 6 and KHBEt3 or KHBEt3/nOctBR2 (BR2: 9-BBN or BCy2) hydroborated CO2 to H3COBPin but in 2.5- to 6-fold lower activities than those exhibited by 3a,b/KHBEt3. The hydroboration of CO2 using 8 as a catalyst led to yields of 39-43%, comparable to those obtained with 3a/KHBEt3. The results confirmed that the catalytic intermediates benefit from the incorporated boranes' intra- or intermolecular stabilizations.

Quo Vadis CO2 Activation: Catalytic Reduction of CO2 to Methanol Using Aluminum and Gallium/Carbon-based Ambiphiles

We report on so-called "hidden FLPs" (FLP: frustrated Lewis pair) consisting of a phosphorus ylide featuring a group 13 fragment in the ortho position of a phenyl ring scaffold to form five-membered ring structures. Although the formation of the Lewis acid/base adducts was observed in the solid state, most of the title compounds readily react with carbon dioxide to provide stable insertion products. Strikingly, 0.3-3.0 mol% of the reported aluminum and gallium/carbon-based ambiphiles catalyze the reduction of CO2 to methanol with satisfactory high selectivity and yields using pinacol borane as stoichiometric reduction equivalent. Comprehensive computational studies provided valuable mechanistic insights and shed more light on activity differences.

Quest for Active Species in Al/B-Catalyzed CO2 Hydrosilylation

We demonstrate the catalytic role of aluminum and boron centers in aluminum borohydride [(2-Me2CH2C6H4)(C6H5)Al(μ-H)2B(C6H5)2] (6) during carbon dioxide (CO2) hydrosilylation. Preliminary investigations into CO2 reduction using [(2-Me2NCH2C6H4)(H)Al(μ-H)]2 (1) and [Ph3C][B(3,5-C6H3Cl2)4] (2) in the presence of Et3SiH and PhSiH3 resulted in CH2(OSiR3)2 and CH3OSiR3, which serve as formaldehyde and methanol surrogates, respectively. In pursuit of identifying the active catalytic species, three compounds, B(3,5-C6H3Cl2)3 (3), [(2-Me2NCH2C6H4)(3,5-C6H3Cl2)Al(μ-H)2B(3,5-C6H3Cl2)2] (4), and [(2-Me2NCH2C6H4)2Al(THF)][B(3,5-C6H3Cl2)4] (5), were isolated. Among compounds 2-5, the highest catalytic conversion was achieved by 4. Further, 4 and 6 were prepared in a straightforward method by treating 1 with 3 and BPh3, respectively. 6 was found to be in equilibrium with 1 and BPh3, thus making the catalytic process of 6 more efficient than that of 4. Computational investigations inferred that CO2 reduction occurs across the Al-H bond, while Si-H activation occurs through a concerted mechanism involving an in situ generated aluminum formate species and BPh3.

Diverse Cooperative Reactivity at a Square Planar Aluminium Complex and Catalytic Reduction of CO2

The use of a sterically demanding pincer ligand to prepare an unusual square planar aluminium complex is reported. Due to the constrained geometry imposed by the ligand scaffold, this four-coordinate aluminium centre remains Lewis acidic and reacts via differing metal-ligand cooperative pathways for activating ketones and CO₂ . It is also a rare example of a single-component aluminium system for the catalytic reduction of CO₂ to a methanol equivalent at room temperature.

Evaluating the Catalytic Activities of PNCNP Pincer Group 10 Metal Hydride Complexes: Pd-Catalyzed Reduction of CO2 to the Formic Acid Level with NH3·BH3 and NaBH4 under Ambient Conditions

In order to develop efficient protocols for CO₂ reduction with less expensive and more convenient hydrogen sources, the catalytic reactivities of group 10 metal hydride complexes supported by a PNCNP pincer ligand, [2,6-(^tBu₂PNH)₂C₆H₃]MH (M = Ni, 1a; Pd, 1b; Pt, 1c), against the hydroboration of CO₂ with NH₃·BH₃ and NaBH₄ have been explored. Both 1a and 1b readily react with CO₂ at room temperature to form the corresponding formato complexes, [2,6-(^tBu₂PNH)₂C₆H₃]MOC(O)H (M = Ni, 2a; Pd, 2b), in nearly quantitative yields. Treatment of NH₃·BH₃ with CO₂ (1 atm) in 1,4-dioxane or THF at room temperature in the presence of 0.05-1.0 mol % of 1b followed by hydrolysis of the resulting mixtures produces formic acid in 105-186% yields, and initial turnover frequencies of up to 2000 h^-1 are observed. In the presence of 1.0 mol % of 1b, NaBH₄ reacts with CO₂ (1 atm) in THF at room temperature to form NaB[OC(O)H]₄ (3) in 87% isolated yield. In situ NMR spectroscopy indicates that the reactions proceed through the insertion of the C═O bond in CO₂ into the Pd-H bond in 1b to form 2b, which sequentially reacts with the hydrides in NH₃·BH₃ or NaBH₄ to produce boron formato species and regenerate 1b. This work represents one of the rare examples of catalytic transfer hydrogenation of CO₂ with NH₃·BH₃ to the formic acid level under very mild conditions without any additives and also the first example of 4 equiv of CO₂ uptake by NaBH₄ in a reaction.

Aluminium-Catalyzed Selective Hydroboration of Esters and Epoxides to Alcohols: C-O Bond Activation

In this work, the molecular aluminium dihydride complex bearing an N, N'-chelated conjugated bis-guanidinate (CBG) ligand is used as a catalyst for reducing a wide range of aryl and alkyl esters with good tolerance of alkene (C=C), alkyne (C≡C), halides (Cl, Br, I and F), nitrile (C≡N), and nitro (NO₂ ) functionalities. Further, we investigated the catalytic application of aluminium dihydride in the C-O bond cleavage of alkyl and aryl epoxides into corresponding branched Markovnikov ring-opening products. In addition, the chemoselective intermolecular reduction of esters over other reducible functional groups, such as amides and alkenes, has been established. Intermediates are isolated and characterized by NMR and HRMS studies, which confirm the probable catalytic cycles for the hydroboration of esters and epoxides.

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 Boron-Nitrogen Double Transborylation Strategy for Borane-Catalyzed Hydroboration of Nitriles

Organoborane-catalyzed hydroboration of nitriles provides N,N-diborylamines, which act as efficient synthons for the synthesis of primary amines and secondary amides. Known nitrile hydroboration methods are dominated by metal catalysis. Simple and metal-free hydroboration of nitriles using diborane [H-B-9-BBN]₂ as a catalyst and pinacolborane as a turnover reagent is reported. The reaction of monomeric H-B-9-BBN with nitriles leads to the hydrido-bridged diborylimine intermediate; a subsequent sequential double hydroboration-transborylation pathway involving B-N/B-H σ bond metathesis is proposed.

Organoaluminum hydrides catalyzed hydroboration of carbonates, esters, carboxylic acids, and carbon dioxide

The reductive functionalization of the CO unit of carbonates, carboxylic acids, esters, and CO₂, respectively has received great attention since its introduction. This method is often used industrially for the synthesis of high value-added energy products in chemistry. This opens up a new way forward to reduce greenhouse gases and the consumption of traditional energy sources. Herein, we report an earth-abundant, cheap, and readily available aluminum dihydride, which can catalyze the reduction of a range of carbonates, esters, carboxylic acids, and CO₂, respectively in the presence of pinacolborane as a reducing agent. Moreover, we demonstrate that the reaction can proceed to obtain good yield products under mild conditions, with low catalyst loading and solvent-free reactions. The mechanism of the catalytic reduction of carbonates has been investigated.

Hydroboration of CO2 to Methyl Boronate Catalyzed by a Manganese Pincer Complex: Insights into the Reaction Mechanism and Ligand Effect

The conversion of carbon dioxide to fuels, polymers, and chemicals is an attractive strategy for the synthesis of high-value-added products and energy-storage materials. Herein, the density functional theory method was employed to investigate the reaction mechanism of CO₂ hydroboration catalyzed by manganese pincer complex, [Mn(Ph₂PCH₂SiMe₂)₂NH(CO)₂Br]. The carbonyl association and carbonyl dissociation mechanisms were investigated, and the calculated results showed that the carbonyl association mechanism is more favorable with an energetic span of 27.0 kcal/mol. Meanwhile, the solvent effect of the reaction was explored, indicating that the solvents could reduce the catalytic activity of the catalyst, which was consistent with the experimental results. In addition, the X ligand effect (X = CO, Br, H, PH₃) on the catalytic activity of the manganese complex was explored, indicating that the anionic complexes [Mn^I - Br]^- and [Mn^I - H]^- have higher catalytic activity. This may not only shed light on the fixation and conversion of CO₂ catalyzed by earth-abundant transition-metal complexes but also provide theoretical insights to design new transition-metal catalysts.

Hydroboration of CO2 catalyzed by heteroscorpionate zwitterionic zinc and magnesium hydride complexes

The heteroscorpionate zinc hydride complex LZnH 2, (L = (MePz)₂CP(Ph)₂NPh, MePz = 3,5-dimethylpyrazolyl), its formate complex 3, and magnesium hydride complex LMgH 5 with the same ligand were synthesized and detected for the catalytic hydroboration reaction of CO₂. With BH₃·SMe₂ as the reductant, zinc-based hydride complex 2 and formate complex 3 show a similar capability of hydroboration of CO₂, featuring excellent reactivity and selectivity. The conversion of BH₃·SMe₂ reached 84%, the highest TON of 252 compared to other zinc catalysts was achieved at room temperature and borate ester products at reduction levels of CH₃OH were obtained. Magnesium-based hydride complex 5 showed inferior activity for the hydroboration reduction of CO₂.

s-Block Metal Catalysts for the Hydroboration of Unsaturated Bonds

The addition of a B-H bond to an unsaturated bond (polarized or unpolarized) is a powerful and atom-economic tool for the synthesis of organoboranes. In recent years, s-block organometallics have appeared as alternative catalysts to transition-metal complexes, which traditionally catalyze the hydroboration of unsaturated bonds. Because of the recent and rapid development in the field of hydroboration of unsaturated bonds catalyzed by alkali (Li, Na, K) and alkaline earth (Mg, Ca, Sr, Ba) metals, we provide a detailed and updated comprehensive review that covers the synthesis, reactivity, and application of s-block metal catalysts in the hydroboration of polarized as well as unsaturated carbon-carbon bonds. Moreover, we describe the main reaction mechanisms, providing valuable insight into the reactivity of the s-block metal catalysts. Finally, we compare these s-block metal complexes with other redox-neutral catalytic systems based on p-block metals including aluminum complexes and f-block metal complexes of lanthanides and early actinides. In this review, we aim to provide a comprehensive, authoritative, and critical assessment of the state of the art within this highly interesting research area.

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 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...

Pincer-Supported Gallium Complexes for the Catalytic Hydroboration of Aldehydes, Ketones and Carbon Dioxide

Gallium hydrides stabilised by primary and secondary amines are scarce due to their propensity to eliminate dihydrogen. Consequently, their reactivity has received limited attention. The synthesis of two novel gallium hydride complexes HGa(THF)[ON(H)O] and H₂Ga[μ²‐ON(H)O]Ga[ON(H)O] ([ON(H)O]²⁻=N,N‐bis(3,5‐di‐tert‐butyl‐2‐phenoxy)amine) is described and their reactivity towards aldehydes and ketones is explored. These reactions afford alkoxide‐bridged dimers through 1,2‐hydrogallation reactions. The gallium hydrides can be regenerated through Ga−O/B−H metathesis from the reaction of such dimers with pinacol borane (HBpin) or 9‐borabicyclo[3.3.1]nonane (9‐BBN). These observations allowed us to target the catalytic reduction of carbonyl substrates (aldehydes, ketones and carbon dioxide) with low catalyst loadings at room temperature.

Reduction of Carbon Dioxide with Ammonia-Borane under Ambient Conditions: Maneuvering a Catalytic Way

In the development of alternatives to the traditional catalytic hydrogenation of CO₂ with gaseous H₂, employing nongaseous H₂ storage compounds as potential reductants for catalytic transfer hydrogenation of CO₂ is promising. Ammonia-borane, due to its high hydrogen storage capacity (19.6 wt %), has been used for catalytic transfer hydrogenation of several organic unsaturated compounds. However, a similar protocol involving catalytic transfer hydrogenation of less reactive CO₂ with NH₃BH₃ is yet to be realized experimentally. Herein, we demonstrate the first catalytic CO₂ transfer hydrogenation process for generating formate salt with NH₃BH₃ under ambient conditions (1 atm and 30 °C) employing a cationic "Ir(III)-abnormal NHC" catalyst via an electrophilic NH₃BH₃ activation route. It exhibited an initial turnover frequency of 686 h^-1 and a high turnover number (TON) of ≈1300 in just 4 h. Most significantly, the catalyst was durable enough to maintain long-term activity, and upon only periodic recharging of NH₃BH₃, it furnished a total TON of >4200 in 10 h.

Reactions of a diborylstannylene with CO2 and N2O: diboration of carbon dioxide by a main group bis(boryl) complex

The reactions of the boryl-substituted stannylene Sn{B(NDippCH)2}2 (1) with carbon dioxide have been investigated and shown to proceed via pathways involving insertion into the Sn-B bond(s). In the first instance this leads to formation of the (boryl)tin(ii) borylcarboxylate complex Sn{B(NDippCH)2}{O2CB(NDippCH)2} (2), which has been structurally characterized and shown to feature a κ2 mode of coordination of the [(HCDippN)2BCO2]- ligand at the metal centre. 2 undergoes B-O reductive elimination in hexane solution (in the absence of further CO2) to give the boryl(borylcarboxylate)ester {(HCDippN)2B}O2C{B(NDippCH)2} (3) i.e. the product of formal diboration of carbon dioxide. Alternatively, 2 can assimilate a second equivalent of CO2 to give the homoleptic bis(borylcarboxylate) Sn{O2CB(NDippCH)2}2 (4), which can be prepared via an alternative route from SnBr2 and the potassium salt of [(HCDippN)2BCO2]-, and structurally characterized as its DMAP (N,N-dimethylaminopyridine) adduct. Structural and reactivity studies also point to the possibility for extrusion of CO from the [(HCDippN)2BCO2]- fragment to generate the boryloxy system [(HCDippN)2BO]-, a ligand which can be generated directly from 1via reaction with N2O. The initially formed unsymmetrical species Sn{B(NDippCH)2}{OB(NDippCH)2} has been shown to be amenable to crystallographic study in the solid state, but to undergo ligand redistribution in solution to generate a mixture of 1 and the bis(boryloxy) complex Sn{OB(NDippCH)2}2.