A Convenient Approach for the Synthesis of Multidentate N-Heterocyclic Carbene Ligand Precursors.

Multidentate bis-NHCs ligands with various spacers are expected to combine the advantages of coordination chemistry and catalysis. These offer opportunities to construct various organometallic frameworks involving transition metals and main group elements. Therefore, developing a general procedure for synthesizing a variety of carbene salt precursors using a convenient technique is key in this context.

Catalyst-Free Diborylation and Silaboration of Azoarenes: A Simple Photoinduced Approach.

The diborylation of the N=N double bond of azoarenes has been achieved using a commercially available diboron reagent, Bcat [bis(catecholato)diboron]. By utilizing the photo-switchable nature of azoarenes under blue-LED light irradiation, an uncatalyzed diborylation and silaboration yielded a broad range of functionalized hydrazine derivatives. The mechanistic origin validates the importance of cis configuration, which is corroborated by theoretical calculations.

Synthesis of α-Trifluoromethylated Ketones from α,β-Unsaturated Ketones via Catecholboron Enolates.

Herein, we report a protocol for the synthesis of α-trifluoromethylated ketones through trifluoromethylation of the corresponding catecholboron enolates of α,β-unsaturated ketones. The reaction of the 1,4-hydroborated product of enones with the Togni II reagent affords the α-trifluoromethylated ketones without any catalyst or additive. Moreover, the protocol has been employed on a series of α,β-unsaturated ketones, including chalcones, substrate-bearing heterocycles, and bioactive molecules.

Regioselective Hydroboration of Unsymmetrical Internal Alkynes Catalyzed by a Cobalt Pincer-NHC Complex.

Highly regioselective hydroboration of unsymmetrical internal alkynes remains a significant challenge for synthesizing valuable alkenylboronate esters. Herein, we describe an easily synthesizable pincer NHC-based Co complex as a catalyst for the -α selective hydroboration of unactivated internal alkynes and the -β selective hydroboration of activated internal alkynes with pinacolborane. The reaction showcases high chemo-, regio-, and stereoselectivity, and the catalyst displays high efficiency and very low loading under base-free reaction conditions.

Chemistry of the Au-Thiol Interface through the Lens of Single-Molecule Flicker Noise Measurements.

Chemistry of the Au-S interface at the nanoscale is one of the most complex systems to study, as the nature and strength of the Au-S bond change under different experimental conditions. In this study, using mechanically controlled break junction technique, we probed the conductance and analyzed Flicker noise for several aliphatic and aromatic thiol derivatives and thioethers. We demonstrate that Flicker noise can be used to unambiguously differentiate between stronger chemisorption (Au-SR) and weaker physisorption (Au-SRR') type interactions.

A Metal-Free and Operationally Simple Radical Trifluoromethylative Borylation of Unactivated Alkenes.

We herein describe a protocol to synthesize trifluoromethylated alkyl boronates from alkenes by the mutual activation of the Togni II and the bis(catecholato)diboron reagents in the absence of any catalyst and additives. This reaction enables synthesizing a series of trifluoromethylated alkyl boronates using unactivated alkenes, including natural products and drug derivatives, in a regioselective manner. Moreover, the synthetic utility of the boronic ester present in the product allows access to a range of trifluoromethyl containing compounds.

Synthesis of α-Seleno Boronates via Diboration of Carbonyl Compounds.

A method has been described for accessing α-seleno alkyl boronates. The selenoboration was achieved via the diboration of carbonyl compounds to give α-oxyl boronates, which then undergo 1,2-metalate rearrangement in the presence of lithium selenolates and trifluoroacetic anhydride (TFAA). A variety of structurally diverse substrates were compatible with this protocol and efficiently provides difunctionalized products from simple starting materials.

Approach to the Synthesis of -Thiolated Alkylboronates via Cobalt-Catalyzed Diboration of Aldehydes.

A new method has been developed for the sequential -thioborylation of readily available aldehydes via the cobalt-catalyzed diboration reaction. The -heterocyclic carbene (NHC)-cobalt complex has been used as a catalyst for the diboration of aldehydes to generate α-oxyl boronic esters, which react with lithium thiolates to form a tetracoordinate boronate species, which undergoes 1,2-metalate rearrangement in the presence of trifluoroacetic anhydride. The stepwise functionalization of the boryl and thiol moiety of the products enriches the chemical toolbox of diverse organic synthesis.

Cobalt-Catalyzed Regioselective 1,2-Hydroboration of N-Heteroarenes.

Regioselective hydroboration of pyridines to 1,2-dihydropyridines remains a significant challenge for the synthesis of valuable nitrogenous bioactive molecules. Herein, we report a base free ligand-controlled cobalt-catalyzed 1,2-hydroboration of pyridines and quinolines with very low catalyst loading under neat reaction conditions. The choice of sterically demanding N-heterocyclic ligands led to the 1,2-regioselectivity and the scope was demonstrated by the N-heterocycles having a variety of functional groups.

Homolytic cleavage of diboron(4) compounds using diazabutadiene derivatives.

Diazabutadiene derivatives have been identified as a distinct class of reagents, capable of cleaving B-B bonds of diboron(4). The cleavage is accompanied by the formation of a new C[double bond, length as m-dash]C bond and the product geometry is highly dependent on the substituents on the DAB units. Preliminary mechanistic investigations suggest a concerted mechanism and the absence of any radical intermediates.

Iron-Based Catalyst for Borylation of Unactivated Alkyl Halides without Using Highly Basic Organometallic Reagents.

The mild borylation of alkyl bromides and chlorides with bis(neopentylglycolato)diborane (Bneop) mediated by iron-bis amide is described. The reaction proceeds with a broad substrate scope and good functional group compatibility. Moreover, sufficient catalytic activity was obtained for primary and secondary alkyl halides.

Cobalt(I)-Catalyzed Borylation of Unactivated Alkyl Bromides and Chlorides.

A cobalt-complex-catalyzed borylation of a wide range of alkyl halides with a diboron reagent (Bpin or Bneop) has been developed under mild reaction conditions, demonstrating the first cobalt-mediated cross-coupling with alkyl electrophiles. This protocol allows alkyl boronic esters to be accessed from alkyl halides, including alkyl chlorides, which were used rarely as coupling partners. Mechanistic studies reveal the possible involvement of an alkyl radical intermediate in this cobalt-mediated catalytic cycle.

Zinc-Catalysed Hydroboration of Terminal and Internal Alkynes.

A regioselective hydroboration of alkynes has been developed by using commercially available zinc triflate as a catalyst, in the presence of catalytic amount of NaBHEt . The reaction tolerates a wide range of terminal alkynes having several synthetically useful functional groups and proceeds regioselectively to furnish hydroborated products in moderate to excellent yields. This system shows moderate chemoselectivity towards terminal C≡C bond over terminal and internal C=C bond and internal C≡C bond.

Markovnikov-Selective Co(I)-Catalyzed Hydroboration of Vinylarenes and Carbonyl Compounds.

An NHC-supported Co(I) catalyst has been developed for selective Markovnikov hydroboration of vinylarenes under mild reaction conditions. The hydroboration allows highly selective synthesis of a wide range of secondary and tertiary alkyl boronates in excellent yields. Our protocol also enables hydroboration of aldehydes and ketones with diverse functional groups to access the corresponding borate esters.

Lewis acid catalysis: regioselective hydroboration of alkynes and alkenes promoted by scandium triflate.

The first commercially available scandium-catalysed selective hydroboration of alkynes and alkenes with HBpin (pin = OC-Me2CMe2O) in the presence of a catalytic amount of NaHBEt3 has been developed. This protocol can be applicable to a wide range of substrates including aromatic, aliphatic with cyclic and acyclic side chains, and heteroaryl systems with broad functional-group compatibility. Mechanistic studies revealed that the reaction occurs in a syn fashion via the σ-bond metathesis between the alkenyl scandium species and HBpin.

Spontaneous Metal-Free Transfer Hydrogenation of Iminoboranes with Ammonia Borane and Amine Boranes.

The first spontaneous, metal-free, room-temperature hydrogen transfer from ammonia borane to an iminoborane is reported. Mechanistic studies of the reaction indicate a concerted transfer of H and H from donor to acceptor with an activation energy far below those of comparable concerted transfer hydrogenations. This reaction was employed in the syntheses and isolation of new B,N-disubstituted aminoboranes, a comparatively rare subset within the aminoborane family.

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.

A nano-catalytic approach for C-B bond formation reactions.

Herein, we present a chronological survey of the metal/metal oxide nanoparticle-catalysed borylation reactions. Transition metal-catalysed borylation is considered to be one of the most efficient methods for the synthesis of organoboron derivatives. Considering chemical and pharmaceutical processes, the major drawbacks of homogeneous catalysis are metal contamination in products and inability to recover catalysts for reuse, which limit its application industrially, in biomolecules, and materials science.

Synthesis of Functionalized 1,4-Azaborinines by the Cyclization of Di-tert-butyliminoborane and Alkynes.

Di-tert-butyliminoborane is found to be a very useful synthon for the synthesis of a variety of functionalized 1,4-azaborinines by the Rh-mediated cyclization of iminoboranes with alkynes. The reactions proceed via [2 + 2] cycloaddition of iminoboranes and alkynes in the presence of [RhCl(PiPr3)2]2, which gives a rhodium η(4)-1,2-azaborete complex that yields 1,4-azaborinines upon reaction with acetylene. This reaction is compatible with substrates containing more than one alkynyl unit, cleanly affording compounds containing multiple 1,4-azaborinines.

The reactivities of iminoboranes with carbenes: BN isosteres of carbene-alkyne adducts.

The first examples of adducts of cyclic alkyl(amino) carbenes (CAAC) and N-heterocyclic carbenes (NHCs) with iminoboranes have been synthesized and isolated at low temperature (-45 °C). The adducts show short BN bonds and planarity at boron, mimicking the structures of the isoelectronic imine functionality. When di-tert-butyliminoborane was reacted with 1,3-bis(isopropyl)imidazol-2-ylidene (IPr), the initially formed Lewis acid-base adduct quickly rearranged to form a new carbene substituted with an aminoborane at the 4-position.

Direct synthetic route to functionalized 1,2-azaborinines.

A new catalytic synthetic route to functionalized 1,2-azaborinines has been developed by the [2+2]/[2+4] cycloaddition reactions of di-tert-butyliminoboranes and alkynes in presence of a rhodium catalyst. The first examples of ferrocene-functionalized azaborinines have been synthesized using this strategy. Moreover, the regioselectivity of this reaction can be controlled by the formation of an intermediate rhodium 1,2-azaborete complex, which results in the isolation of the first azaborinine boronic ester.

A fine tuning of metallaborane to bridged-boryl complex, [(Cp*Ru)2(μ-H)(μ-CO)(μ-Bcat)] (cat = 1,2-O2C6H4; Cp* = η5-C5Me5).

Room temperature photolysis of [(Cp*RuCO)2BH4(Bcat)], 3, generated from the reaction of arachno-[(Cp*RuCO)2B2H6], 1, with HBcat (cat = 1,2-O2C6H4), yielded a rare homodinuclear bridged-boryl complex, [(Cp*Ru)2(μ-H)(μ-CO)(μ-Bcat)], 4, confirmed by X-ray diffraction.

Supraicosahedral polyhedra in metallaboranes: synthesis and structural characterization of 12-, 15-, and 16-vertex rhodaboranes.

Syntheses and structural characterization of supraicosahedral rhodaborane clusters are reported. Reaction of [(Cp*RhCl2)2], (Cp* = η(5)-C5Me5) with [LiBH4·thf] followed by thermolysis with excess of [BH3·thf] afforded 16-vertex closo-[(Cp*Rh)3B12H12Rh{Cp*RhB4H9}], 1, 15-vertex [(Cp*Rh)2B13H13], 2, 12-vertex [(Cp*Rh)2B10Hn(OH)m], (3a: n = 12, m = 0; 3b: n = 9, m = 1; 3c: n = 8, m = 2) and 10-vertex [(Cp*Rh)3B7H7], 4, and [(Cp*Rh)4B6H6], 5. Cluster 1 is the unprecedented 16-vertex cluster, consists of a sixteen-vertex {Rh4B12} with an exo-polyhedral {RhB4} moiety.

Synthesis and characterization of hypoelectronic tantalaboranes: comparison of the geometric and electronic structures of [(Cp*TaX)2B5H11] (X = Cl, Br, and I).

Mild thermolysis of tantalaborane [(Cp*Ta)(2)B(5)H(11)], 1 (Cp* = η(5)-C(5)Me(5)) in presence of halogen sources affords the open cage clusters [(Cp*TaX)(2)B(5)H(11)], 2-4 (2: X = Cl; 3: X = Br; and 4: X = I) in good yields. In contrast, the tetraborohydride cluster, [(Cp*Ta)(2)B(4)H(9)(μ-BH(4))], 5, under the same reaction conditions forms the B-H substituted cluster [(Cp*Ta)(2)B(4)H(8)I(μ-BH(4))], 6. All the new metallaboranes have been characterized by mass spectrometry, (1)H, (11)B, (13)C NMR spectroscopy, and elemental analysis, and the structural types were established by crystallographic analysis of clusters 3, 4, and 6.