Uncovering the Synchronous Role of Bis-borane with Nucleophilic Solvent as Frustrated Lewis pair in Metal-free Catalytic Dehydrogenation of Ammonia-borane

Metal free catalysis has emerged as a viable alternative for transition metal based catalysts for enabling different chemical processes, particularly for de/hydrogenation reactions. Herein, employing theoretical studies we reveal the unexpected Frustrated Lewis Pair like reactivity of a boron based catalyst, 9,10-dichlorodiboraanthracene and the ethereal solvent to enable dehydrogenation of ammonia-borane (NH3BH3, AB) under mild conditions. The mechanistic channels thus uncovered reveal that the boron catalyst abstracts a hydride from NH3BH3 followed by crucial stabilisation of the NH3BH2+ moiety by the nucleophilic action of the solvent. H2 is released by the combination of hydride and proton from the borohydride moiety and the solvated NH3BH2+ respectively. Catalysis becomes unfeasible if the Lewis base-like action of the ethereal solvent is not taken into consideration. Thus it is suggested that the clandestine partnership of the Lewis Acid, Boron catalyst and the Lewis Base, ethereal solvent, i.e. FLP like action enables dehydrogenation of NH3BH3 in the instant case

Mechanistic Details of Ru–Bis(pyridyl)borate Complex Catalyzed Dehydrogenation of Ammonia–Borane: Role of the Pendant Boron Ligand in Catalysis

The role of pendant boron ligands in ammonia–borane (AB) dehydrogenation has been investigated using hybrid density functional theory for two very efficient ruthenium-based catalysts developed by Williams and co-workers. Our findings reveal that the catalytic action initiates through opening of the labile metal–ligand bridging group associated with the boron-based pendant ligand arm for both catalysts. In case of the hydroxyl-bridged catalyst, the ligand (B–OH moiety) backbone plays an active role along with the metal center to perform concerted dehydrogenation of ammonia–borane by overcoming a free energy activation barrier of 24.3 kcal/mol, and this dehydrogenation step is the rate-determining step of the catalytic cycle. However, for the trifluoroacetate-bridged complex, H₂ is released in a stepwise fashion with active participation of the solvent. It involves formation of a boronium cation with a rate-determining free energy activation barrier of 23.7 kcal/mol for the solvent-assisted B–H bond-breaking step, while the pendant boron ligand acts as a spectator. Overall, our detailed theoretical study illustrates that the chemical nature of the pendant boron ligand is decisive in the AB dehydrogenation pathway. Further computational investigations indicate that greater amounts of hydrogen are released from AB by the dual participation of free NH₂BH₂ and the Ru catalysts

Unraveling the Crucial Role of Metal-Free Catalysis in Borazine and Polyborazylene Formation in Transition-Metal-Catalyzed Ammonia–Borane Dehydrogenation

Though the recent scientific literature is rife with experimental and theoretical studies on transition-metal (TM)-catalyzed dehydrogenation of ammonia–borane (NH₃·BH₃) due to its relevance in chemical hydrogen storage, the mechanistic knowledge is mostly restricted to the formation of aminoborane (NH₂BH₂) after 1 equiv of H₂ removal from NH₃·BH₃. Unfortunately, the chemistry behind the formation of borazine and polyborazylene, which happens only after more than 1 equiv of H₂ is released from ammonia–borane in these TM-catalyzed homogeneous reactions, largely remains unknown. In this work we use density functional theory to unravel the curious function of “free NH₂BH₂”. Initially, free NH₂BH₂ molecules form oligomers such as cyclotriborazane and <i>B</i>-(cyclodiborazanyl)­aminoborohydride. We show that, through a web of concerted proton and hydride transfer based dehydrogenations of oligomeric intermediates, cycloaddition reactions, and hydroboration steps facilitated by NH₂BH₂, the development of the polyborazylene framework occurs. The rate-determining free energy barrier for the formation of a polyborazylene template is predicted to be 25.7 kcal/mol at the M05-2X­(SMD)/6-31++G­(d,p)//M05-2X/6-31++G­(d,p) level of theory. The dehydrogenation of BN oligomeric intermediates by NH₂BH₂ yields NH₃·BH₃, suggesting for certain catalytic systems that the role of the TM catalyst is limited to the dehydrogenation of NH₃·BH₃ to maintain optimal amounts of free NH₂BH₂ in the reaction medium to enable polyborazylene formation. TM catalysts that fail to produce borazine and polyborazylene falter because they rapidly consume NH₂BH₂ in TM-catalyzed polyaminoborane formation, thus preventing the chain of events triggered by NH₂BH₂

Rh(II)-Catalysed N2-Selective Arylation of Benzotriazoles and Indazoles using Quinoid Carbenes via 1,5-H Shift

A Rh(II)-catalyzed highly selective N2-arylation of benzotriazole is developed with wide scope and good functional group tolerance. The reaction is also extended on indazole and substituted 1,2,3-triazole scaffolds. In addition, late-stage arylation of benzotriazoles tethered with bioactive molecules is realized under the developed conditions. Control experiments and DFT calculations reveal that presumably, the reaction proceeds via nucleophilic addition of N2 (of 1H tautomer) center to metal-carbene followed by 1,5-H shift. This differs from classical X-H insertion into carbene centers and subsequent 1,2-H shift

Reactivity of Biomimetic Iron(II)-2-aminophenolate Complexes toward Dioxygen: Mechanistic Investigations on the Oxidative C–C Bond Cleavage of Substituted 2‑Aminophenols

The isolation and characterization of a series of iron­(II)-2-aminophenolate complexes [(6-Me₃-TPA)­Fe^II(X)]⁺ (X = 2-amino-4-nitrophenolate (4-NO₂-HAP), <b>1;</b> X = 2-aminophenolate (2-HAP), <b>2</b>; X = 2-amino-3-methylphenolate (3-Me-HAP), <b>3</b>; X = 2-amino-4-methylphenolate (4-Me-HAP), <b>4</b>; X = 2-amino-5-methylphenolate (5-Me-HAP), <b>5</b>; X = 2-amino-4-<i>tert</i>-butylphenolate (4-<i>^t</i>Bu-HAP), <b>6</b> and X = 2-amino-4,6-di-<i>tert</i>-butylphenolate (4,6-di-<i>^t</i>Bu-HAP), <b>7</b>) and an iron­(III)-2-amidophenolate complex [(6-Me₃-TPA)­Fe^III(4,6-di-<i>^t</i>Bu-AP)]⁺ (<b>7</b>^<b>Ox</b>) supported by a tripodal nitrogen ligand (6-Me₃-TPA = tris­(6-methyl-2-pyridylmethyl)­amine) are reported. Substituted 2-aminophenols were used to prepare the biomimetic iron­(II) complexes to understand the effect of electronic and structural properties of aminophenolate rings on the dioxygen reactivity and on the selectivity of C–C bond cleavage reactions. Crystal structures of the cationic parts of <b>5</b>·ClO₄ and <b>7</b>·BPh₄ show six-coordinate iron­(II) centers ligated by a neutral tetradentate ligand and a monoanionic 2-aminophenolate in a bidentate fashion. While <b>1</b>·BPh₄ does not react with oxygen, other complexes undergo oxidative transformation in the presence of dioxygen. The reaction of <b>2</b>·ClO₄ with dioxygen affords 2-amino-3<i>H</i>-phenoxazin-3-one, an auto-oxidation product of 2-aminophenol, whereas complexes <b>3</b>·BPh₄, <b>4</b>·BPh₄, <b>5</b>·ClO₄ and <b>6</b>·ClO₄ react with O₂ to exhibit C–C bond cleavage of the bound aminophenolates. Complexes <b>7</b>·ClO₄ and <b>7</b>^Ox·BPh_<b>4</b> produce a mixture of 4,6-di-<i>tert</i>-butyl-2<i>H</i>-pyran-2-imine and 4,6-di-<i>tert</i>-butyl-2-picolinic acid. Labeling experiments with ¹⁸O₂ show the incorporation of one oxygen atom from dioxygen into the cleavage products. The reactivity (and stability) of the intermediate, which directs the course of aromatic ring cleavage reaction, is found to be dependent on the nature of ring substituent. The presence of two <i>tert</i>-butyl groups on the aminophenolate ring in <b>7</b>·ClO₄ makes the complex slow to cleave the C–C bond of 4,6-di-<i>^t</i>Bu-HAP, whereas <b>4</b>·BPh₄ containing 4-Me-HAP displays fastest reactivity. Density functional theory calculations were conducted on [(6-Me₃-TPA)­Fe^III(4-<i>^t</i>Bu-AP)]⁺ (<b>6</b>^Ox) to gain a mechanistic insight into the regioselective C–C bond cleavage reaction. On the basis of the experimental and computational studies, an iron­(II)-2-iminobenzosemiquinonate intermediate is proposed to react with dioxygen resulting in the oxidative C–C bond cleavage of the coordinated 2-aminophenolates

Reactivity of Biomimetic Iron(II)-2-aminophenolate Complexes toward Dioxygen: Mechanistic Investigations on the Oxidative C–C Bond Cleavage of Substituted 2‑Aminophenols

The isolation and characterization of a series of iron­(II)-2-aminophenolate complexes [(6-Me₃-TPA)­Fe^II(X)]⁺ (X = 2-amino-4-nitrophenolate (4-NO₂-HAP), <b>1;</b> X = 2-aminophenolate (2-HAP), <b>2</b>; X = 2-amino-3-methylphenolate (3-Me-HAP), <b>3</b>; X = 2-amino-4-methylphenolate (4-Me-HAP), <b>4</b>; X = 2-amino-5-methylphenolate (5-Me-HAP), <b>5</b>; X = 2-amino-4-<i>tert</i>-butylphenolate (4-<i>^t</i>Bu-HAP), <b>6</b> and X = 2-amino-4,6-di-<i>tert</i>-butylphenolate (4,6-di-<i>^t</i>Bu-HAP), <b>7</b>) and an iron­(III)-2-amidophenolate complex [(6-Me₃-TPA)­Fe^III(4,6-di-<i>^t</i>Bu-AP)]⁺ (<b>7</b>^<b>Ox</b>) supported by a tripodal nitrogen ligand (6-Me₃-TPA = tris­(6-methyl-2-pyridylmethyl)­amine) are reported. Substituted 2-aminophenols were used to prepare the biomimetic iron­(II) complexes to understand the effect of electronic and structural properties of aminophenolate rings on the dioxygen reactivity and on the selectivity of C–C bond cleavage reactions. Crystal structures of the cationic parts of <b>5</b>·ClO₄ and <b>7</b>·BPh₄ show six-coordinate iron­(II) centers ligated by a neutral tetradentate ligand and a monoanionic 2-aminophenolate in a bidentate fashion. While <b>1</b>·BPh₄ does not react with oxygen, other complexes undergo oxidative transformation in the presence of dioxygen. The reaction of <b>2</b>·ClO₄ with dioxygen affords 2-amino-3<i>H</i>-phenoxazin-3-one, an auto-oxidation product of 2-aminophenol, whereas complexes <b>3</b>·BPh₄, <b>4</b>·BPh₄, <b>5</b>·ClO₄ and <b>6</b>·ClO₄ react with O₂ to exhibit C–C bond cleavage of the bound aminophenolates. Complexes <b>7</b>·ClO₄ and <b>7</b>^Ox·BPh_<b>4</b> produce a mixture of 4,6-di-<i>tert</i>-butyl-2<i>H</i>-pyran-2-imine and 4,6-di-<i>tert</i>-butyl-2-picolinic acid. Labeling experiments with ¹⁸O₂ show the incorporation of one oxygen atom from dioxygen into the cleavage products. The reactivity (and stability) of the intermediate, which directs the course of aromatic ring cleavage reaction, is found to be dependent on the nature of ring substituent. The presence of two <i>tert</i>-butyl groups on the aminophenolate ring in <b>7</b>·ClO₄ makes the complex slow to cleave the C–C bond of 4,6-di-<i>^t</i>Bu-HAP, whereas <b>4</b>·BPh₄ containing 4-Me-HAP displays fastest reactivity. Density functional theory calculations were conducted on [(6-Me₃-TPA)­Fe^III(4-<i>^t</i>Bu-AP)]⁺ (<b>6</b>^Ox) to gain a mechanistic insight into the regioselective C–C bond cleavage reaction. On the basis of the experimental and computational studies, an iron­(II)-2-iminobenzosemiquinonate intermediate is proposed to react with dioxygen resulting in the oxidative C–C bond cleavage of the coordinated 2-aminophenolates

Cis<i>–</i>Trans Conformational Analysis of δ‑Azaproline in Peptides

The cis–trans isomerization and conformer specificity of δ-azaproline and its carbamate-protected form in linear and cyclic peptides were investigated using NMR and α-chymotrypsin assay. Comparisons of the chemical shift value of the α-hydrogen in each case of δ-azaproline-containing peptides with conformer-specific locked diketopiperazines reveal the fact that an upfield chemical shift value corresponds to cis conformer and a downfield value corresponds to a trans conformer. δ-Azaproline adopts cis-conformation in simple amides, dipeptides, and tripeptides whereas its carbamate-protected form adopts trans-conformation. In the case of longer, linear or cyclic peptides, vice versa results are obtained. Interestingly, in all these peptides exclusively one conformer, either cis or trans, is stabilized. This cis–trans isomerization is independent of both temperature and solvents; only the δ-nitrogen protecting group plays key role in the isomerization. δ-Azaproline is conformer-specific in either of its protected or deprotected forms, which is a unique property of this proline. Unlike other covalently modified proline surrogates, this isomerization of δ-azaproline can be tuned easily by a protecting group. The mechanism of cis–trans isomerization of δ-azaproline during deprotection and reprotection is supported by theoretical calculations

A “Hemilabile” Palladium–Carbon Bond: Characterization and Its Implication in Catalysis

An unusual palladium–carbon bond was identified in the crystal structure of a Pd­(II) complex (<b>VI</b>) derived from 1-(2-diphenylphosphinophenyl)-2,5-dimethyl-1<i>H</i>-pyrrole (<b>L4</b>). Theoretical calculations indicate that the Pd–C bond is covalent in nature. The complex exhibits fluxional behavior in solution at ambient temperaturethe Pd binds alternately to C2 and C5 of pyrrole, generating equivalent, enantiomeric structures. Theory predicts that the transition state of this interconversion probably occurs through an unsaturated 14e complex, where the ligand binds the metal in a monodentate fashion. Comparison with related structures reported earlier has been made, and the possible implication of such unusual bonding in the context of catalysis of coupling reactions is discussed