Synthesis of Triborylalkenes from Terminal Alkynes by Iridium‐Catalyzed Tandem C?H Borylation and Diboration

A two‐step reaction to convert terminal alkynes into triborylalkenes is reported. In the first step, the terminal alkyne and pinacolborane (HBpin) are converted into an alkynylboronate, which is catalyzed by an iridium complex supported by a SiNN pincer ligand. In the second step, treatment of the reaction mixture with CO generates a new catalyst which mediates dehydrogenative diboration of alkynylboronate with pinacolborane. The mechanism of the diboration remains unclear but it does not proceed via intermediacy of hydroboration products or via B₂pin₂.     

Organosulfide‐Catalyzed Diboration of Terminal Alkynes under Light

An efficient metal‐free diboration of terminal alkynes is reported. In the presence of a catalytic amount of organosulfides under light, the addition of bis(pinacolato)diboron (B₂pin₂) to terminal alkynes takes place efficiently to produce the corresponding double borylation products in good yields. Mechanistic studies indicate that this metal‐free sulfide‐catalyzed diboration of alkynes likely occurs by generation of a boryl‐centered radical with the aid of light and a sulfide, since such a radical was detected in the reaction mixture by electron spin resonance (ESR) spectroscopy. The present form of catalysis (sulfide/light) is thought to be unprecedented and provides a new means of preparation for organoboranes without heavy metal contamination in the products, which is highly desired in the preparation of drugs and electronic materials.     

Copper‐Catalyzed Triboration: Straightforward,Atom‐Economical Synthesis of 1,1,1‐Triborylalkanes from Terminal Alkynes and HBpin

A convenient and efficient one‐step synthesis of 1,1,1‐triborylalkanes was achieved via sequential dehydrogenative borylation and double hydroborations of terminal alkynes with HBpin (HBpin=pinacolborane) catalyzed by inexpensive and readily available Cu(OAc)₂. This process proceeds under mild conditions, furnishing 1,1,1‐tris(boronates) with wide substrate scope, excellent selectivity, and good functional‐group tolerance, and is applicable to gram‐scale synthesis without loss of yield. The 1,1,1‐triborylalkanes can be used in the preparation of α‐vinylboronates and borylated cyclic compounds, which are valuable but previously rare compounds. Different alkyl groups can be introduced stepwise via base‐mediated deborylative alkylation to produce racemic tertiary alkyl boronates, which can be readily transformed into useful tertiary alcohols.     

Diastereo‐ and Enantioselective Synthesis of (E)‐δ‐Boryl‐Substituted anti‐Homoallylic Alcohols in Two Steps from Terminal Alkynes

We report the highly diastereo‐ and enantioselective preparation of (E)‐δ‐boryl‐substituted anti‐homoallylic alcohols in two steps from terminal alkynes. This method consists of a cobalt(II)‐catalyzed 1,1‐diboration reaction of terminal alkynes with B₂pin₂ and a palladium(I)‐mediated asymmetric allylation reaction of the resulting 1,1‐di(boryl)alk‐1‐enes with aldehydes in the presence of a chiral phosphoric acid. Propyne, which is produced as the byproduct during petroleum refining, could be used as the starting material to construct homoallylic alcohols that are otherwise difficult to synthesize with high stereocontrol.     

Iron‐Catalyzed Diboration and Carboboration of Alkynes

An iron‐catalyzed diboration reaction of alkynes with bis(pinacolato)diboron (B₂pin₂) and external borating agents (MeOB(OR)₂) affords diverse symmetrical or unsymmetrical cis‐1,2‐diborylalkenes. The simple protocol for the diboration reaction can be extended to the iron‐catalyzed carboboration of alkynes with primary and, unprecedentedly, secondary alkyl halides, affording various tetrasubstituted monoborylalkenes in a highly stereoselective manner. DFT calculations indicate that a boryliron intermediate adds across the triple bond of an alkyne to afford an alkenyliron intermediate, which can react with the external trapping agents, borates and alkyl halides. In situ trapping experiments support the intermediacy of the alkenyl iron species using radical probe stubstrates.     

Cobalt‐Catalyzed Alkyne Hydrosilylation and Sequential Vinylsilane Hydroboration with Markovnikov Selectivity

A pyridinebis(oxazoline) cobalt complex is a very efficient precatalyst for the hydrosilylation of terminal alkynes with Ph₂SiH₂, providing α‐vinylsilanes with high (Markovnikov) regioselectivity and broad functional‐group tolerance. The vinylsilane products can be further converted into geminal borosilanes through Markovnikov hydroboration with pinacolborane and a bis(imino)pyridine cobalt catalyst.     

Mechanistic study on the reaction of pinB-BMes2 with alkynes based on experimental investigation and DFT calculations: gradual change of mechanism depending on the substituent

Transition metal-free direct and base-catalyzed 1,2-diborations of arylacetylenes using pinB-BMes₂ provided a syn/anti-isomeric mixture of diborylalkenes. The kinetic analysis showed that the reaction rate and isomer ratio were affected by reaction conditions and substituents on the aryl ring. DFT calculations indicated that direct addition proceeded via the interaction of acetylene-π with the BMes₂ fragment. In contrast, for the base-catalyzed diboration, the previously isolated sp²–sp³ diborane and borataallene were confirmed as stable intermediates by calculations. The whole reaction pathways can be divided into the Bpin-migration and deprotonation steps, where the borataallene should be considered as a common intermediate. It should be noted that the deprotonation step is reversible and affords the kinetically less favoured isomer under the thermodynamic conditions. As a result, the composition of isomeric products, in the base-catalyzed diboration, is attributed to the small difference of activation barriers between direct and base-catalyzed systems.

Combination of kinetic and DFT studies revealed a subtle balance for substituent effect toward the regioselectivity of the product in metal-free and base-catalyzed diboration of arylacetylenes.     

Organophosphorus-catalyzed relay oxidation of H-Bpin: electrophilic C–H borylation of heteroarenes

A nontrigonal phosphorus triamide (1, P{N[o-NMe-C₆H₄]₂}) is shown to catalyze C–H borylation of electron-rich heteroarenes with pinacolborane (HBpin) in the presence of a mild chloroalkane reagent. C–H borylation proceeds for a range of electron-rich heterocycles including pyrroles, indoles, and thiophenes of varied substitution. Mechanistic studies implicate an initial P–N cooperative activation of HBpin by 1 to give P-hydrido diazaphospholene 2, which is diverted by Atherton–Todd oxidation with chloroalkane to generate P-chloro diazaphospholene 3. DFT calculations suggest subsequent oxidation of pinacolborane by 3 generates chloropinacolborane (ClBpin) as a transient electrophilic borylating species, consistent with observed substituent effects and regiochemical outcomes. These results illustrate the targeted diversion of established reaction pathways in organophosphorus catalysis to enable a new mode of main group-catalyzed C–H borylation.

A nontrigonal phosphorus triamide (1, P{N[o-NMe-C₆H₄]₂}) is shown to catalyze C–H borylation of electron-rich heteroarenes with pinacolborane (HBpin) in the presence of a mild chloroalkane reagent.     

MNDO study of reaction paths: Diboration of acetylene

The reaction of B₂H₄ with acetylene has been studied by the MNDO method. It is shown that the reaction is exothermic and proceeds in two steps. The first step is the formation of a three-center -complex and this is the rate-determining step of the reaction. The second step is the rearrangement of the -complex to the product and this step requires a very small amount of activation energy. The activation barrier for the diboration reaction is 12.8 kcal/mol.The proposed mechanism is significantly different from those proposed earlier and explains all experimental data relating to this reaction.     

One-pot synthesis of α,α-difluoroimines from alkynes through tandem catalytic diboration/fluorination/imination reaction

Tandem catalytic diboration/fluorination/imination of arylacetylenes leads to the formation of α,α-difluoroimines, where the adjacent CN and C-F₂ bonds are formed simultaneously. The convenient one-pot protocol involves a Pt(0)-catalyzed diboration of terminal or internal arylalkynes followed by electrophilic fluorination with Selectfluor in the presence of primary amines and a dehydrating agent. A plausible mechanism for the three consecutive steps (diboration/fluorination/imination) is suggested in accordance with the electronic properties of the substrates. Alkynes/catalytic diboration/alkenyl diboronate esters/Selectfluor/electrophilic fluorination/α,α-difluoroimines.     

Zirconium‐Catalyzed Atom‐Economical Synthesis of 1,1‐Diborylalkanes from Terminal and Internal Alkenes

A general and atom‐economical synthesis of 1,1‐diborylalkanes from alkenes and a borane without the need for an additional H₂ acceptor is reported for the first time. The key to our success is the use of an earth‐abundant zirconium‐based catalyst, which allows a balance of self‐contradictory reactivities (dehydrogenative boration and hydroboration) to be achieved. Our method avoids using an excess amount of another alkene as an H₂ acceptor, which was required in other reported systems. Furthermore, substrates such as simple long‐chain aliphatic alkenes that did not react before also underwent 1,1‐diboration in our system. Significantly, the unprecedented 1,1‐diboration of internal alkenes enabled the preparation of 1,1‐diborylalkanes.     

Rhodium‐Catalyzed Dehydrogenative Borylation of Aliphatic Terminal Alkenes with Pinacolborane

Aliphatic terminal alkenes react with pinacolborane at ambient temperature to afford dehydrogenative borylation compounds as the major product when iPr‐Foxap is used as the ligand with cationic rhodium(I) in the presence of norbornene, which acts as the sacrificial hydrogen acceptor. The reaction is applied to the one‐pot syntheses of aldehydes and homoallylic alcohols from aliphatic terminal alkenes.     

Microwave-assisted one-pot diboration/Suzuki cross-couplings. A rapid route to tetrasubstituted alkenes

Internal and terminal alkynes undergo rapid platinum(0)-catalyzed diboration with bis(pinacolato)diboron in dioxane to yield cis-1,2-bis(boryl)alkenes under sealed vessel microwave conditions. Subsequent addition of aryl bromides, base and a palladium catalyst to the reaction vial followed by resubjection to microwave conditions provides tetrasubstituted ethylenes in high yields via Suzuki cross-coupling of the boron intermediates.     

Nitrite oxidation on RuO<Subscript>2</Subscript> electrodes

An electrochemical kinetic investigation of nitrite oxidation to nitrate on RuO₂ is discussed. The process is studied by cyclic voltammetry, steady-state measurements and potential step measurements. The overall oxidation reaction is a two-electron process where the first step involves a reversible charge transfer: NO₂⁻ ⇔ NO₂ + e⁻ The one-electron oxidation of nitrite yields adsorbed NO₂ which is further oxidized to adsorbed (NO₂)⁺ and subsequently desorbed via a chemical reaction. In the general case, fit of experimental data is obtained with adsorption described by a Temkin isotherm unless the electrode is pre-treated at a cathodic potential where the (NO₂)_ads is removed. This treatment lowers the degree of coverage by intermediates but not the nature of the slow step. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 1, pp. 142–149. The text was submitted by the authors in English.     

Modular Synthesis of Furans with up to Four Different Substituents by a trans‐Carboboration Strategy

Propargyl alcohols, on treatment with MHMDS (M=Na, K), B₂(pin)₂, an acid chloride and a palladium/copper co‐catalyst system, undergo a reaction cascade comprised of trans‐diboration, regioselective acylation, cyclization and dehydration to give trisubstituted furylboronic acid pinacol ester derivatives in good yields; subsequent Suzuki coupling allows a fourth substituent of choice to be introduced and hence tetrasubstituted (arylated) furans to be formed. In terms of modularity, the method seems unrivaled, not least because each product can be attained by two orthogonal but convergent ways (“diagonal split”). This asset is illustrated by the “serial” formation of a “library” of all twelve possible furan isomers that result from systematic permutation of four different substituents about the heterocyclic core.     

Ruthenium catalyzed hydroboration of terminal alkynes to z-vinylboronates

The nonclassical ruthenium hydride pincer complex [Ru(PNP)(H)(2)(H(2))] 1 (PNP = 1,3-bis(di-tert-butyl-phosphinomethyl)pyridine) catalyzes the anti-Markovnikov addition of pinacolborane to terminal alkynes yielding Z-vinylboronates at mild conditions. The complex [Ru(PNP)(H)(2)(HBpin)] 2 (HBpin = pinacolborane), which was identified at the end of the reaction and prepared independently, is proposed as the direct precursor to the catalytic cycle involving rearrangement of coordinated alkyne to Z-vinylidene as a key step for the apparent trans-hydroboration.     

Diverse Reactivity of an Electrophilic Phosphasilene towards Anionic Nucleophiles: Substitution or Metal–Amino Exchange

The reaction of MesLi (Mes=2,4,6‐trimethylphenyl) with the electrophilic phosphasilene R₂(NMe₂)Si‐RSi=PNMe₂ ( 2 , R=Tip=2,4,6‐triisopropylphenyl) cleanly affords R₂(NMe₂)Si‐RSi=PMes and thus provides the first example of a substitution reaction at an unperturbed Si=P bond. In toluene, the reaction of 2 with lithium disilenide, R₂Si=Si(R)Li ( 1 ), apparently proceeds via an initial nucleophilic substitution step as well (as suggested by DFT calculations), but affords a saturated bicyclo[1.1.0]butane analogue as the final product, which was further characterized as its Fe(CO)₄ complex. In contrast, in 1,2‐dimethoxyethane the reaction of 1 with 2 results in an unprecedented metal–amino exchange reaction.     

DFT study on RuII‐catalyzed cyclization of terminal alkynals to cycloalkenes

The reaction mechanism of [CpRu(MeCN)₃]PF₆‐catalyzed cyclization of terminal alkynals 1 to cycloalkenes 2 was investigated by means of density functional methods combined with polarizable continuum models. Calculations indicate that the coordination of the cationic catalyst [CpRu(CH₃CN)₂]⁺ to the carbon–carbon triple bond of the substrate 1 enhances the electrophilicity of alkynyl group, and the subsequent nucleophilic attack of the carbonyl oxygen to the electron‐deficient alkyne forms ate complex IM2 , which would further isomerize into 2H‐oxete complex IM3 . Then a replacement of MeCN by AcOH occurs, followed by two proton‐migrations, which leads to a Fischer‐type carbene complex IM6 . Finally, a decarbonylation takes place leading to cycloalkene 2 . The terminal alkynal is activated by its combination with ruthenium, which leads to a decrease in the natural bond orbital energy of π*_(C1?C2). The four‐membered ring formation is the rate‐controlling step. However, AcOH helps proton shift through coordination with metal center and decreases the reaction energy barriers. Throughout the reactions, all the Ru^II complexes obey the 18‐electron‐rule. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Magnesium Boryl Reactivity with 9‐BBN and Ph3B: Rational B−B′ Bond Formation and Diborane Isomerization

Reactions of a magnesium‐based pinacolatoboryl nucleophile with the electrophilic organoboranes, 9‐BBN and Ph₃B, provide facile B−B′ single bond formation. Although the Ph₃B derivative is thermally stable, when heated, the unsymmetrical diborane(5) anion derived from 9‐BBN is found to isomerize to two regioisomeric species via a proposed mechanism involving dehydroboration of the borabicyclo[3.3.1]nonane and syn‐diboration of the resultant alkenyl carbocycle.     

A convenient access to chiral and racemic (Z)-1-bromo-3-benzoxy-1-butene

The conversion of propargylic trimethylsilyl ether 3 into (Z)-1-bromo-3-benzoxy-1-butène 1 was achieved by hydroboration with pinacolborane followed by successive treatment with bromine and DBU. The key step of the chiral synthesis involves the asymmetric reduction of the α,β-unsaturated ketone 7 with (−)-diisopinocampheylchloroborane [(−)-Ipc₂BCl].