Organoboronic Esters Research Articles

Catalytic Asymmetric Synthesis of Phosphine Boronates.

The first catalytic enantioselective synthesis of ambiphilic phosphine boronate esters is presented. The asymmetric boration of α,β-unsaturated phosphine oxides catalyzed by a copper bisphosphine complex affords optically active organoboronate esters that bear a vicinal phosphine oxide group in good yields and high enantiomeric excess. The synthetic utility of the products is demonstrated through stereospecific transformations into multifunctional optically active compounds.

Copper(i)-catalyzed amidation reaction of organoboronic esters and isocyanates

A simple and efficient methodology for the preparation of amides from easily available organoboronic esters and isocyanates with a ligand-free copper(i) catalyst.

ChemInform Abstract: Carboxylation of Organoboronic Esters with Potassium Methyl Carbonate under Copper Catalysis.

In the presence of a copper catalyst, potassium methyl carbonate serves as a versatile carboxylating agent of allyl- and ­arylboronic esters for the preparation of carboxylic acids.

Carboxylation of Organoboronic Esters with Potassium Methyl Carbonate under Copper Catalysis

In the presence of a copper catalyst, potassium methyl carbonate serves as a versatile carboxylating agent of allyl- and ­arylboronic esters for the preparation of carboxylic acids.

Regio‐ and Enantioselective Monoborylation of Alkenylsilanes Catalyzed by an Electron‐Donating Chiral Phosphine–Copper(I) Complex

AbstractAn asymmetric monoborylation of alkenylsilanes catalyzed by a copper(I) complex with the chiral bisphosphine ligand BenzP* is reported. The reaction proceeded with excellent regioselectivity and high enantioselectivity to afford the corresponding optically active organoboronate esters with a stereogenic CB bond containing a vicinal silyl group. The synthetic utility of the product is demonstrated through stepwise transformations to multifunctional optically active compounds in a stereospecific manner.magnified image

Highly Chemoselective and Enantiospecific Suzuki-Miyaura Cross-Couplings of Benzylic Organoboronic Esters

The use of potassium carbonate in addition to silver oxide­ is shown to increase the enantiospecificity of the Suzuki–Miyaura­ cross-coupling reaction of chiral secondary benzylic boronic esters. From mechanistic studies, it is shown that the reaction is compatible with Mizoroki–Heck coupling partners, even when they are present in considerable excess. This unique chemoselectivity provides the opportunity to carry out sequential reactions.

Evaluation of TlOH Effect for Pd<sup>0</sup>-Mediated Cross-Coupling of Methyl Iodide and Excess Boronic Acid Ester toward Fabrication of [<sup>11</sup>C]CH<sub>3</sub>-Incorporated PET Tracer

The use of thallium(I) hydroxide (TlOH) as a base is known to extremely accelerate the Suzuki-Miyaura cross-coupling reaction using organoboronic acid or organoboronic acid ester as a substrate. Here, we investigated the effects of TlOH by comparing with other conventional bases such as KOH, K2CO3, and CsF for Pd0-mediated rapid cross-coupling reactions between CH3I and organoborane reagents, such as phenyl-, (Z)-4-benzyloxy-2-butenyl-, and benzylboronic acid pinacol esters under the conditions CH3I/borane/Pd0/base (1:40:1:3) in THF/H2O or DMF/H2O for 5 min with an aim to fabricate a PET tracer efficiently. Consequently, however, the use of TlOH was much less efficient than the other bases for the acceleration of cross-coupling reactions. Thus, it was reconfirmed that the milder and non-toxic conditions using K2CO3 or CsF so far developed by our group were most appropriate for the rapid C-methylations.

PH-Responsive N-heterocyclic carbene copper(i) complexes: syntheses and recoverable applications in the carboxylation of arylboronic esters and benzoxazole with carbon dioxide

A pH-controlled monophasic/biphasic switchable system has been developed as a green and novel strategy for homogeneous catalyst recycling, which has been successfully applied to the Cu-NHC-catalyzed carboxylation of organoboronic esters and benzoxazole with carbon dioxide. Additionally, the present strategy could also be extended to the Ag-NHC-catalyzed carboxylation of terminal alkyne. The tertiary amine-functionalized catalysts could be used for at least four times with a slight loss of activity.

The Boron Imprint into Combined Catalytic Organic Reactions

Combined reactions through organoboron intermediates belong to the class of sequential reactions whereby the advantages are clear in terms of efficiency and selectivity. The goal to prepare target molecules from simple substrates requires strategic steps and very selective formation of new bonds. The addition of boryl units to unsaturated substrates can be performed chemo-, regio- and stereoselectively, therefore the organoboron intermediates might be easily transformed into the functionalized molecules, with total retention of configuration. Taking advantage of these protocols, here we describe the most recent advances in combined reactions through B chemistry. Keywords: Cross-coupling, bis(pinacolato)diboron, bis(catecholato)diboron, boration, dimerization, cyclization, Organic Reactions, organic synthesis, building blocks, organoboronic esters, reductive elimination, oxidative addition, Transition metal-catalyzed, atom-economical manner, crystalline materials

Nickel-Catalyzed Suzuki–Miyaura Reaction of Aryl Fluorides

Two protocols for the nickel-catalyzed cross-coupling of aryl fluorides with aryl boronic esters have been developed. The first employs metal fluoride cocatalysts, such as ZrF(4) and TiF(4), which enable Suzuki-Miyaura reactions of aryl fluorides bearing electron-withdrawing (ketones, esters, and CF(3)), aryl and alkenyl groups as well as those comprising fused aromatic rings, such as fluoronaphthalenes and fluoroquinolines. The second protocol employs aryl fluorides bearing ortho-directing groups, which facilitate the difficult C-F bond activation process via cyclometalation. N-heterocycles, such as pyridines, quinolines, pyrazoles, and oxazolines, can successfully promote cross-coupling with an array of organoboronic esters. A study into the substituent effects with respect to both coupling components has provided fundamental insights into the mechanism of the nickel-catalyzed cross-coupling of aryl fluorides.

Transition‐Metal‐Catalyzed Synthesis of Diboranes(4)

The diboranes(4) bis(catecholato)diborane (B(2)Cat(2)) and bis(pinacolato)diborane (B(2)Pin(2)) are important precursors for organoboronic esters, which are versatile reagents for the formation of carbon-carbon bonds. A new catalytic synthesis for these compounds starts from catecholborane or pinacolborane and gives the dehydrocoupling products B(2)Cat(2) and B(2)Pin(2) with turnover numbers of up to 11,600 (see scheme).

ChemInform Abstract: Photochemically Promoted Transition Metal‐Free Cross‐Coupling of Acylsilanes with Organoboronic Esters.

Intermolecular carbon−carbon bond formation between acylsilanes and organoboronic esters was achieved by photoirradiation under almost neutral, transition metal-free conditions. In this reaction, siloxycarbenes generated by photoisomerization of acylsilanes reacted with boronic esters to give the formal B−C bond insertion intermediates, which underwent unique rearrangement to afford the cyclic α-alkoxyboronic esters. Acidic treatment of the resulting crude products under air furnished the cross-coupled ketones in good yields.

Photochemically Promoted Transition Metal-Free Cross-Coupling of Acylsilanes with Organoboronic Esters

Intermolecular carbon-carbon bond formation between acylsilanes and organoboronic esters was achieved by photoirradiation under almost neutral, transition metal-free conditions. In this reaction, siloxycarbenes generated by photoisomerization of acylsilanes reacted with boronic esters to give the formal B-C bond insertion intermediates, which underwent unique rearrangement to afford the cyclic α-alkoxyboronic esters. Acidic treatment of the resulting crude products under air furnished the cross-coupled ketones in good yields.

ChemInform Abstract: Carboxylation of N—H/C—H Bonds Using N‐Heterocyclic Carbene Copper(I) Complexes.

Transition-metal-mediated carboxylation of N H and C H bonds represents a nascent area in organic chemistry, because these reactions enable the efficient construction of valuable synthons. Palladium-catalyzed N-carbonylation–oxidation sequences are well-documented, but they often require high catalyst loadings and the use of either gaseous carbon monoxide or Group VI metal–carbonyl complexes. An analogous transformation sequence is also promoted by molybdenum and tungsten carbonyl amine species under forcing temperatures. Important advances in C-carboxylation reactions have been made using ruthenium and nickel complexes; however, examples under mild conditions are elusive. The carboxylation of allylstannanes, organozincs, and organoboronic esters have been described as a new method to improve functional group tolerance, but the stoichiometric consumption of an organometallic reagent remains a disadvantage. The reactivity of allylstannanes and organozinc compounds necessitates handling under an inert atmosphere, while organoboronic esters are expensive. A protocol has recently been developed for the C-carboxylation of simple aromatic groups under very mild reaction conditions. In this case the strongly basic [Au(IPr)(OH)] (IPr=1,3-bis(diisopropyl)phenylimidazol-2-ylidene) complex (pKaDMSO= 30.3(2)) was used, [11] which contains an N-heterocyclic carbene (NHC) ligand [Eq. (1)].

A Nickel Catalyst for the Addition of Organoboronate Esters to Ketones and Aldehydes

A Ni(cod)(2)/IPr catalyst promotes the intermolecular 1,2-addition of arylboronate esters to unactivated aldehydes and ketones. Diaryl, alkyl aryl, and dialkyl ketones show good reactivity under mild reaction conditions (< or = 80 degrees C, nonpolar solvents, no strong base or acid additives). A dramatic ligand effect favors either carbonyl addition (IPr) or C-OR cross-coupling (PCy(3)) with aryl ether substrates. A Ni(0)/Ni(II) catalytic cycle initiated by the oxidative cyclization of the carbonyl substrate is proposed.

A new approach to carbon–carbon bond formation: development of aerobic Pd-catalyzed reductive coupling reactions of organometallic reagents and styrenes

Alkenes are attractive starting materials for organic synthesis and the development of new selective functionalization reactions is desired. Previously, our laboratory discovered a unique Pd-catalyzed hydroalkoxylation reaction of styrenes containing a phenol. Based upon deuterium labeling experiments, a mechanism involving an aerobic alcohol oxidation coupled to alkene functionalization was proposed. These results inspired the development of a new Pd-catalyzed reductive coupling reaction of alkenes and organometallic reagents that generates a new carbon–carbon bond. Optimization of the conditions for the coupling of both organostannanes and organoboronic esters is described and the initial scope of the transformation is presented. Additionally, several mechanistic experiments are outlined and support the rationale for the development of the reaction based upon coupling alcohol oxidation to alkene functionalization.

Cross Coupling Reactions of Chiral Secondary Organoboronic Esters With Retention of Configuration

We report the first example of a coupling reaction of chiral secondary boronic esters generated by the hydroboration of vinyl arenes. In order for the reaction to take place in high yields, the use of silver oxide as a base and the presence of at least 8 equiv of triphenyl phosphine per Pd are required. The reaction proceeds with >90% retention of configuration in all cases except one. Remarkably, the linear boronate ester does not react under these conditions.

Carboxylation of Organoboronic Esters Catalyzed by N‐Heterocyclic Carbene Copper(I) Complexes

Copper complexes with a CO2 fixation: Copper(I) complexes serve as excellent catalysts for the carboxylation of aryl- and alkenylboronic esters with CO2, affording a variety of functionalized carboxylic acid derivatives (see scheme). Important active intermediates such as the copper(I) aryl and carboxylate complexes, [(IPr)CuR] and [(IPr)CuOCOR] (R=4-MeOC6H4, IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), are isolated and structurally characterized.

4,4,5,5‐Tetraphenyl‐1,3,2‐dioxaborolane: A Bulky Borane for the Transition Metal Catalysed Hydroboration of Alkenes

Abstract4,4,5,5‐Tetraphenyl‐1,3,2‐dioxaborolane (HBBzpin, 3) has been prepared in high yield by the addition of H3B·SMe2 to benzopinacol. HBBzpin is a relatively stable solid that reacts with a variety of alkenes under catalytic conditions to give air‐ and chromatography‐stable organoboronate esters. Reactions of vinylarenes in the presence of catalytic amounts of [Cp*IrCl2]2 gave the corresponding terminal products selectively. Addition of HBBzpin to RhCl(PPh3)3 gave Rh(H)Cl(BzBpin)(PPh3)2 (11) as the only new rhodium‐containing product. The complex 11 has been characterized by a number of physical and analytical methods, including a single‐crystal X‐ray diffraction study. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

First Catalytic Functionalization of Unreactive Aryl C-N Bonds in Anilines

The first catalytic functionalization of unreactive aryl C-N bonds with the help of a low-valent transition metal is demonstrated in this article. Hereby, the C-N bond is activated via an oxidative addition to the Ru complex. The subsequent coupling reaction proceeds through trans­metallation between the Ru-amide complex and an organoboronic ester. Cyclopropyl rings are transferred without ring opening.