Selective Hydroboration Research Articles

Aluminum Monohydride Catalyzed Selective Hydroboration of Carbonyl Compounds.

The well-defined aluminum monohydride compound [{(2,4,6-Me3-C6H2)NC(Me)}2(Me)(H)]AlH·(NMe2Et) (1) catalyzes hydroboration of a wide range of aldehydes and ketones under mild reaction conditions. Moreover, compound 1 displayed chemoselective hydroboration of aldehydes over ketones at rt.

Highly selective Markovnikov hydroboration of alkyl-substituted terminal alkenes with a phosphine-copper(i) catalyst.

A new method has been developed for the Markovnikov hydroboration of alkyl-substituted terminal alkenes. Notably, the use of a bulky bisphosphine-copper(i) catalyst system resulted in high regioselectivity to afford secondary alkylboronates from the corresponding terminal alkenes (branch/linear = 92 : 8-97 : 3). This method also exhibited good functional group compatibility.

ChemInform Abstract: Ruthenium Catalyzed Selective Hydroboration of Carbonyl Compounds.

AbstractThe Ru‐catalyzed hydroboration of aldehydes (I) and (III) followed by hydrolysis of the intermediate boronate esters with HCl affords primary alcohols (II) and (IV), resp., in high yields.

A binary catalyst system of a cationic Ru-CNC pincer complex with an alkali metal salt for selective hydroboration of carbon dioxide.

Binary catalyst systems comprising a cationic Ru-CNC pincer complex and an alkali metal salt were developed for selective hydroboration of CO2 utilizing pinacolborane at r.t. and 1 atm CO2, with the combination of [Ru(CNCBn)(CO)2(H)][PF6] and KOCO2tBu producing formoxyborane in 76% yield. A bicyclic catalytic mechanism was proposed and discussed.

Ruthenium Catalyzed Selective Hydroboration of Carbonyl Compounds.

Using the [Ru(p-cymene)Cl2]2 (1) complex, catalytic hydroboration of aldehydes and ketones with pinacolborane under neat and mild conditions is reported. At rt, chemoselective hydroboration of aldehydes over the ketones is also attained. Mechanistic studies confirmed the immediate formation of monohydride bridged dinuclear complex [{(η(6)-p-cymene)RuCl}2(μ-H-μ-Cl)] (1b) from the reaction of 1 with pinacolborane, which catalyzed the highly efficient hydroboration reactions. The catalytic cycle containing mononuclear Ru-H species and intramolecular 1,3-hydride transfer is postulated.

Cobalt catalyzed z-selective hydroboration of terminal alkynes and elucidation of the origin of selectivity.

A bis(imino)pyridine cobalt-catalyzed hydroboration of terminal alkynes with HBPin (Pin = pinacolate) with high yield and (Z)-selectivity for synthetically valuable vinylboronate esters is described. Deuterium labeling studies, stoichiometric experiments, and isolation of catalytically relevant intermediates support a mechanism involving selective insertion of an alkynylboronate ester into a Co-H bond, a pathway distinct from known precious metal catalysts where metal vinylidene intermediates have been proposed to account for the observed (Z) selectivity. The identity of the imine substituents dictates the relative rates of activation of the cobalt precatalyst with HBPin or the terminal alkyne and, as a consequence, is responsible for the stereochemical outcome of the catalytic reaction.

Carboxylic acid-catalyzed highly efficient and selective hydroboration of alkynes with pinacolborane.

We have demonstrated for the first time that carboxylic acids are able to catalyze the direct hydroboration of various terminal and internal alkynes with pinacolborane without using any metal catalysts. This unprecedented catalytic hydroboration exhibits a broad functional groups compatibility, giving the corresponding alkenyl diboronates and monoboronates in good to high yields with exclusive regio- and stereoselectivities.

ChemInform Abstract: [(NHC)Fe(CO)4] Efficient Pre‐Catalyst for Selective Hydroboration of Alkenes.

[(IMes)Fe(CO)4] [IMes=1,3-bis(2,4,6-trimethylphenyl) imidazol-2-ylidene] complex was found to be an efficient pre-catalyst for the hydroboration of functional alkenes in the presence of pinacolborane at room temperature. Notably, UV irradiation (350 nm) is important to promote this catalytic transformation. Interestingly, high chemo- and regioselectivities were observed as only anti-Markovnikov boronate derivatives were obtained and various functional groups can be tolerated.

Multinuclear Magnesium Hydride Clusters: Selective Reduction and Catalytic Hydroboration of Pyridines

Multinuclear magnesium hydride complexes react with pyridine, forming 1,2- and 1,4-dihydropyridide (DHP) complexes. Reaction of PARA3Mg8H10 with pyridine initially formed 1,2-DHP and 1,4-DHP product mixtures which converted at 60 °C into PARA-[Mg(1,4-DHP)]2·(pyridine)2 (PARA = [(2,6-iPr2C6H3)NC(Me)C(H)C(Me)N]2-(p-C6H4)). Reaction of [NN-(MgH)2]2 with pyridine gave exclusive formation of the 1,2-DHP product NN-[Mg(1,2-DHP)]2·(pyridine)2 (NN = [(2,6-iPr2C6H3)NC(Me)CHC(Me)N−]2). Both products were characterized by crystal structure determinations. The unusual preference for 1,2-addition is likely caused by secondary intramolecular interactions based on mutual communication between the metal coordination geometries: an extended network of C–H···C π-interactions and π-stacking interactions is found. Whereas PARA3Mg8H10 is hardly active in magnesium-catalyzed hydroboration of pyridines with pinacolborane, [NN-(MgH)2]2 shows efficient coupling. However, the regioselectivity of the stoichiometric reaction is not translated to the catalytic regime. This result is taken as an indication for a potential alternative mechanism in which magnesium hydride intermediates do not play a role but the hydride is transferred from an intermediate borate complex.

(NHC)Fe(CO)4] Efficient Pre‐catalyst for Selective Hydroboration of Alkenes

Abstract[(IMes)Fe(CO)4] [IMes=1,3‐bis(2,4,6‐trimethylphenyl) imidazol‐2‐ylidene] complex was found to be an efficient pre‐catalyst for the hydroboration of functional alkenes in the presence of pinacolborane at room temperature. Notably, UV irradiation (350 nm) is important to promote this catalytic transformation. Interestingly, high chemo‐ and regioselectivities were observed as only anti‐Markovnikov boronate derivatives were obtained and various functional groups can be tolerated.

Selective reduction of CO2 to a methanol equivalent by B(C6F5)3-activated alkaline earth catalysis

Magnesium and calcium hydrido-tris(pentafluorophenyl)borate derivatives catalyse the completely selective reductive hydroboration of CO2 with pinacolborane (HBpin) to provide the methanol equivalent, CH3OBpin.

A trans‐Selective Hydroboration of Internal Alkynes

A <i>trans</i>‐Selective Hydroboration of Internal Alkynes

A trans‐Selective Hydroboration of Internal Alkynes

A <i>trans</i>‐Selective Hydroboration of Internal Alkynes

Highly Selective Copper‐Catalyzed Hydroboration of Allenes and 1,3‐Dienes

The highly selective copper-catalyzed hydroboration of allenes has been developed. Allylboranes and alkenylboranes were selectively prepared by the judicious choice of catalytic species (copper hydride and boryl copper). Furthermore, two types of alkenylboranes could be selectively synthesized by the choice of an appropriate ligand. Mechanistic studies confirmed that the protonation of a (Z)-σ-allyl copper species, which was isolated and structurally characterized by single-crystal X-ray diffraction, was a key step in these reactions. Besides allenes, this method is also applicable to the selective hydroboration of 1,3-diene derivatives to afford allylboranes and homoallylboranes.

Stereoselective synthesis of the C15–C30 subunit of dolabelides A–D

A highly stereoselective synthesis of the C15–C30 subunit of the cytotoxic macrolides dolabelides A–D has been accomplished by utilizing anti selective aldol reaction, Wittig olefination for the introduction of a trisubstituted (E)-double bond, selective hydroboration, Brown asymmetric allylation, Wilkinson hydrogenation and stereoselective keto reduction for the installation of C19 hydroxy group as key steps.

Selective hydroboration of dieneamines. Formation of hydroxyalkylphenothiazines as MDR modulators

N-dienylphenothiazines synthesized from tetrazolo[1,5-a]pyridinium salts by treatment with phenothiazine were subjected to catalytic hydrogenation to yield N-butylphenothiazines, whereas transformation of these dienes with borane dimethyl sulfide (BH3×Me2S) resulted in selective hydroboration of one double bond and full reduction of the other double bond to give 2-hydroxybutylphenothiazines. Position of the hydroxyl group was supported by NMR spectroscopy and verified by X-ray analysis. Comparison of MDR modulatory activity of the new derivatives revealed that the hydroxybutyl compounds are promising candidates for development of novel MDR inhibitors.

ChemInform Abstract: Thirty Six Years of Research on the Selective Reduction and Hydroboration

AbstractReview: 199 refs.

Thirty Six Years of Research on the Selective Reduction and Hydroboration

From 1975 to 2011, for thirty six years, the author and his collaborators have developed a variety of reducing and hydroborating agents, and applied them to organic synthesis, which involves the 1,2-reduction of <TEX>${\alpha}$</TEX>,<TEX>${\beta}$</TEX>-unsaturated carbonyl compounds, stereoselective reduction of cycloalkanones, regioselective ring-opening of epoxides, partial reduction of carboxylic acid derivatives to aldehydes, regioselective addition to carbon-carbon multiple bonds, etc. by utilizing metal hydrides and the newly-devised the Meerwein-Ponndorf-Verley (MPV) type reagents. Such developments provide a new synthetic methodology making possible valuable selective reductions and hydroborations, not practical previously.

Selective Hydroboration of 1,3-Enynes Containing Internal Alkynes

A regio- and stereoselective synthesis of alkyl- and alkenylboronates is disclosed. The authors utilize a copper-phosphine catalyst and obtain boronic esters selectively under substrate and ligand control. The utility of the products is further exemplified by palladium-catalyzed cross-coupling reactions and Diels-Alder cyclo­addition. The observed products of this hydroboration differ from the previous reports of similar ­hydroboration reactions with enynes. Notably, internal alkynes can be used effectively, which are a difficult substrate class for this transformation.

Iridium Phosphane Complexes Containing Arylspiroboronate Esters for the Hydroboration of Alkenes

AbstractIridium phosphane arylspiroboronate esters have been prepared by addition of B2cat3 (cat = 1,2‐O2C6H4) to the iridium precursors Ir(acac)(coe)2/PR3 (acac = acetylacetonato, coe = cis‐cyclooctene) or Ir(acac)(coe)2/P‐P (chelating bidentate ligands) at elevated temperatures. These novel complexes have been characterized by a number of physical methods including X‐ray diffraction studies for the PCy3 (2) and dppp (6, 1,3‐diphenylphosphanopropane) derivatives. The arylspiroboronate esters, as seen with the more common rhodium complexes, are bound to the metal centre via one of thebenzene catecholato rings. These organometallic complexes can be used as precatalysts for the selective hydroboration of vinylarene derivatives with HBpin (pin = 1,2‐O2C2Me4) to give the corresponding linear hydroboration products. Reactions with methyl oleate proceed at elevated temperatures to give the isomerised/hydroboration product selectively along with equal amounts of hydrogenation product.