Ligand Influence on Carbonyl Hydroboration Catalysis by Alkali Metal Hydridotriphenylborates [(L)M][HBPh3 ] (M=Li, Na, K)

The Reduction of Metallabenzenes: Different Scenarios Highly Dependent on the Central Group 14 Elements, Si vs. Ge

In order to synthesize a silabenzenyl anion, in which the anionic carbon atom of a phenyl anion was replaced with a silicon atom, the reductive dearylation reaction of 1-Tbt-2-tert-butyl-silabenzene (Tbt=2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) with KC8 was attempted. Unexpectedly, this reaction resulted in the formation of a dianion species without the elimination of the Tbt group. This is totally different from the reactions of Tbt-substituted germa- or stannabenzene with KC8 , which resulted in the formation of the corresponding heavy analogues of phenyl anion, together with the elimination of Tbt group. Experimental and theoretical investigation revealed that one of the protons of the o-benzyl positions of the Tbt group was abstracted by the negatively charged silicon atom of an in-situ generated intermediate. Compared with the previously reported germanium system, the contrasting results indicate that the central heavy group 14 element has a great influence on the elimination step of the Tbt group.

Reactivity of a series of triaryl borates, B(OArx)3, in hydroboration catalysis

In this paper, we compare the reactivity of a series of triaryl borates B(OArx)3 as catalysts for the hydroboration of alkenes and alkynes. It was observed that commercially available B(OPh)3 performed the poorest, whereas catalysts with o-F atoms appeared to perform much better.

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.

Hexamethyldisilazane Lithium (LiHMDS)-Promoted Hydroboration of Alkynes and Alkenes with Pinacolborane

Lithium-promoted hydroboration of alkynes and alkenes using commercially available hexamethyldisilazane lithium as a precatalyst and HBpin as a hydride source has been developed. This method will be appealing for organic synthesis because of its remarkable substrate tolerance and good yields. Mechanistic studies revealed that the hydroboration proceeds through the in situ-formed BH₃ species, which acts to drive the turnover of the hydroboration of alkynes and alkenes.

Lithium and magnesium complexes from the employment of pyridyl-pendanted unsymmetrical β-diketiminates: syntheses and utilization as catalysts for the hydroboration of carbonyl compounds

The push for environmentally benign and sustainable chemical processes has reinforced the demand to displace transition metals with cheap, nontoxic and naturally abundant metals. To fulfil this requirement, we endeavored...

Deaggregation of Zinc Dihydride by Lewis Acids Including Carbon Dioxide in the Presence of Nitrogen Donors

Thermally sensitive polymeric zinc dihydride [ZnH₂]_n can conveniently be prepared by the reaction of ZnEt₂ with [AlH₃(NEt₃)]. When reacted with CO₂ (1 bar) in the presence of chelating N-donor ligands L_n = N,N,N',N'-tetramethylethylenediamine (TMEDA), N,N,N',N'-tetramethyl-1,3-propanediamine (TMPDA), N,N,N',N″,N''-pentamethyldiethylenetriamine (PMDTA), and 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane (Me₄TACD), insertion into the Zn-H bond readily occurred. Depending on the denticity n, formates [(L_n)Zn(OCHO)₂] were isolated and structurally characterized, either as a molecule (L_n = TMEDA, TMPDA, PMDTA) or a charge-separated ion pair [(L_n)Zn(OCHO)][OCHO] (L_n = Me₄TACD). The reaction of [ZnH₂]_n with the mild Lewis acid BPh₃ in the presence of chelating N-donor ligands L_n gave a series of hydridotriphenylborates, either as a contact ion pair [(L₂)Zn(H)(HBPh₃)] (L₂ = TMEDA, TMPDA) or a separated ion pair [(L_n)Zn(H)][HBPh₃] (L_n = PMDTA, Me₄TACD). In the crystal, the contact ion pair [(TMEDA)Zn(H)(HBPh₃)] showed a bent Zn-H-B bridge indicative of a delocalized Zn-H-B interaction. In contrast, a linear Zn-H-B bridge for [(TMPDA)Zn(H)(HBPh₃)] was observed, suggesting a contact ion pair. In THF solution, both complexes show an exchange with free BPh₃ as well as [HBPh₃]^-. DFT calculations suggest the presence of [HBPh₃]^- anion with a highly polarized B-H bond that interacts with the Lewis acidic zinc hydride cation [(L₂)Zn(H)]⁺. The hydridotriphenylborates [(L_n)Zn(H)(HBPh₃)] underwent CO₂ insertion to give (formato)zinc (formoxy)triphenylborate complexes [(L_n)Zn(OCHO)][(OCHO)BPh₃] (L_n = TMPDA, PMDTA, Me₄TACD). For L_n = TMEDA, a dinuclear complex [(L_n)₂Zn₂(μ-OCHO)₃][(OCHO)BPh₃] was isolated. Hydridotriphenylborates [(L_n)Zn(H)(HBPh₃)] catalyzed the hydrosilylation of CO₂ (1 bar) by ⁿBuMe₂SiH in THF at 70 °C to give formoxysilane and (methoxy)silane.

Lithium compound catalyzed deoxygenative hydroboration of primary, secondary and tertiary amides

A selective and efficient route for the deoxygenative reduction of primary to tertiary amides to corresponding amines has been achieved with pinacolborane (HBpin) using simple and readily accessible 2,6-di-tert-butyl phenolate lithium·THF (1a) as a catalyst. Both experimental and DFT studies provide mechanistic insight.

O,N-Heterocyclic germylenes as efficient catalysts for hydroboration and cyanosilylation of benzaldehyde

Novel O,N-heterocyclic germylenes were examined as catalysts for cyanosilylation and hydroboration of benzaldehyde.

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.

Emergence of a New [NNN] Pincer Ligand via Si-H Bond Activation and β-Hydride Abstraction at Tetravalent Cerium

The cerium(IV) pyrazolate complexes [Ce(Me2 pz)4 ]2 and [Ce(Me2 pz)4 (thf)] initiate β-hydride abstraction of the bis(dimethylsilyl)amido moiety, to afford a heteroleptic cerium(IV) species containing a dimethylpyrazolyl-substituted silylamido ligand, namely [Ce(Me2 pz)3 (bpsa)] (bpsa=bis((3,5-dimethylpyrazol-1-yl)dimethylsilyl)amido; Me2 pz =3,5-dimethylpyrazolato), along with some cerium(III) species. Remarkably, the nucleophilic attack of the pyrazolyl at the silicon atom and concomitant Si-H-bond cleavage is restricted to the tetravalent cerium oxidation state and appears to proceed via the formation of a transient cerium(IV) hydride, which engages in immediate redox chemistry. When [Ce(Me2 pz)4 ]2 is treated with [Li{N(SiMe3 )2 }], that is, in the absence of the SiH functionality, any redox chemistry did not occur. Instead, the ceric ate complex [LiCe2 (Me2 pz)9 ] and the stable mixed-ligand ceric species [Ce(Me2 pz)2 {N(SiMe3 )2 }2 ] were obtained.

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.

Alkali Metal Triphenyl- and Trihydridosilanides Stabilized by a Macrocyclic Polyamine Ligand

Potassium silanide [KSiH₃]_∞ contains 4.2 wt % of hydrogen and has been intensely studied as hydrogen storage material. The macrocyclic ligand Me₄TACD (1,4,7,10‐tetramethyl‐1,4,7,10‐tetraaminocyclododecane, L) stabilizes the full range of triphenylsilyl complexes [(L)MSiPh₃]_n (M=Li–Cs), which react with H₂ or PhSiH₃ to form molecular [(L)MSiH₃]_n that can be isolated in soluble form and fully characterized.

Lithium compounds as single site catalysts for hydroboration of alkenes and alkynes

Readily accessible lithium compounds have been employed to catalyze the hydroboration of alkene and alkynes including terpenes using HBpin with anti-Markovnikov selectivity. The mechanism is proposed on the basis of experimental and DFT studies.

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.

Catalyst-free and solvent-free hydroboration of ketones

The hydroboration of a wide range of ketones with HBpin under catalyst-free and solvent-free conditions was reported in high yields.

The directions of an external electric field control the catalysis of the hydroboration of C–O unsaturated compounds

EEF facilitates hydroboration by reducing its barrier in a specific direction; as illustrated in the image, the monkey easily picks the peach on the lower branch.

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.

Cobalt-Catalyzed Hydroboration of Alkenes, Aldehydes, and Ketones

An operationally convenient and general method for hydroboration of alkenes, aldehydes, and ketones employing Co(acac)₃ as a precatalyst is reported. The hydroboration of alkenes in the presence of HBpin, PPh₃, and NaO ^tBu affords good to excellent yields with high Markovnikov selectivity with up to 97:3 branched/linear selectivity. Moreover, Co(acac)₃ could be used effectively to hydroborate aldehydes and ketones in the absence of additives under mild reaction conditions. Inter- and intramolecular chemoselective reduction of the aldehyde group took place over the ketone functional group.

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.

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.

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.

Easily accessible lithium compound catalyzed mild and facile hydroboration and cyanosilylation of aldehydes and ketones

Simple and readily accessible lithium compounds are found to be efficient single site catalysts for cyanosilylation and hydroboration of a range of aldehydes and ketones under ambient conditions.

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.

A study of the Group 1 metal tetra-aza macrocyclic complexes [M(Me4cyclen)(L)]+ using electronic structure calculations

Metal-cyclen complexes have a number of important applications. However, the coordination chemistry between metal ions and cyclen-based macrocycles is much less well studied compared to their metal ion-crown ether analogues. This work, which makes a contribution to address this imbalance by studying complex ions of the type [M(Me₄cyclen)(L)]⁺, was initiated by results of an experimental study which prepared some Group 1 metal cyclen complexes, namely [Li(Me₄cyclen)(H₂O)][BAr^F] and [Na(Me₄cyclen)(THF)][BAr^F] and obtained their X-ray crystal structures [J. M. Dyke, W. Levason, M. E. Light, D. Pugh, G. Reid, H. Bhakhoa, P. Ramasami, and L. Rhyman, Dalton Trans., 2015, 44, 13853]. The lowest [M(Me₄cyclen)(L)]⁺ minimum energy structures (M = Li, Na, K, and L = H₂O, THF, DEE, MeOH, DCM) are studied using density functional theory (DFT) calculations. The geometry of each [M(Me₄cyclen)(L)]⁺ structure and, in particular, the conformation of L are found to be mainly governed by steric hindrance which decreases as the size of the ionic radius increases from Li⁺ → Na⁺ → K⁺. Good agreement of computed geometrical parameters of [Li(Me₄cyclen)(H₂O)]⁺ and [Na(Me₄cyclen)(THF)]⁺ with the corresponding geometrical parameters derived from the crystal structures [Li(Me₄cyclen)(H₂O)]⁺[BAr^F]^- and [Na(Me₄cyclen)(THF)]⁺[BAr^F]^- is obtained. Bonding analysis indicates that the stability of the [M(Me₄cyclen)(L)]⁺ structures originates mainly from ionic interaction between the Me₄cyclen/L ligands and the M⁺ centres. The experimental observation that [M(Me₄cyclen)(L)]⁺[BAr^F]^- complexes could be prepared in crystalline form for M⁺ = Li⁺ and Na⁺, but that experiments aimed at synthesising the corresponding K⁺, Rb⁺, and Cs⁺ complexes failed resulting in formation of [Me₄cyclenH][BAr^F] is investigated using DFT and explicitly correlated calculations, and explained by considering production of [Me₄cyclenH]⁺ by a hydrolysis reaction, involving traces of water, which competes with [M(Me₄cyclen)(L)]⁺ formation. [Me₄cyclenH]⁺ formation dominates for M⁺ = K⁺, Rb⁺, and Cs⁺ whereas formation of [M(Me₄cyclen)(L)]⁺ is energetically favoured for M⁺ = Li⁺ and Na⁺. The results indicate that the number and type of ligands, play a key role in stabilising the [M(Me₄cyclen)]⁺ complexes and it is hoped that this work will encourage experimentalists to prepare and characterise other [M(Me₄cyclen)(L)]⁺ complexes.