Hydroboration Of Terminal Alkynes Research Articles

Counterion Variation: A Useful Lever for Maximizing the Regioselectivity in the Hydroboration of Terminal Alkynes

Counterion Variation: A Useful Lever for Maximizing the Regioselectivity in the Hydroboration of Terminal Alkynes

Copper(I)–Thiazol-2-ylidenes: Highly Reactive N-Heterocyclic Carbenes for the Hydroboration of Terminal and Internal Alkynes. Ligand Development, Synthetic Utility, and Mechanistic Studies

In the last 15 years, copper-catalyzed borylative transformations utilizing boryl–copper have been established as a powerful activation mode in organic synthesis and catalysis, enabling direct transformations of various π-systems. Although many of these transformations use NHC (N-heterocyclic carbene) ligands, these studies have been almost exclusively limited to the derivatives of imidazol-2-ylidenes. However, the molecular properties of N-aryl-imidazol-2-ylidenes, such as IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), are limited by (1) the high degree of heteroatom stabilization and (2) symmetrical substitution of the nitrogen atoms. Herein, we report a study on Cu(I)–thiazol-2-ylidenes, thiazole analogues of imidazol-2-ylidenes, which (1) feature a distinct half-umbrella shape of the coordinating ligand and (2) exhibit lower heteroatom stabilization of the ancillary ligand through reduced π-donation from sulfur. We present the development of a family of stable Cu(I)–thiazol-2-ylidenes, where the combined sterics of thiazol-2-ylidenes lead to monomers [Cu(NHC)X] or bridged-halo dimers [Cu(NHC) (μ-X)]2, their crystallographic characteristics, and application to the hydroboration of alkynes to afford trisubstituted vinylboronates by β-hydroboration of internal alkynes or terminal vinylboronates by β-hydroboration of terminal alkynes. Application to the late-stage modification and detailed mechanistic studies on the catalyst structure and activation are presented. Most crucially, Cu(I)–thiazol-2-ylidenes show a much higher β-selectivity in the hydroboration of alkynes than that of classical imidazol-2-ylidenes, affording vinylborons in excellent yields at ambient conditions. The unique “half-umbrella” shape of thiazol-2-ylidenes reverses the α/β regioselectivity observed with imidazol-2-ylidenes in the hydroboration of terminal alkynes. Kinetic studies demonstrate that Cu(I)–thiazol-2-ylidenes supersede imidazol-2-ylidenes. Considering the significant utility of borylative transformations of π-systems, we anticipate that Cu(I)–thiazol-2-ylidenes will advance the synthetic transformations of boryl–copper in organic synthesis and catalysis.

An “On‐Demand”, Selective Dehydrogenative Borylation or Hydroboration of Terminal Alkynes Using Zn2+‐based Catalyst

AbstractAn air‐stable dicationic Zn2+ complex (1) in a tripod‐type ligand with non‐bound phosphorus base and three pyridinyl “arms” (TPPh) was synthesized. Remarkably, while 2 mol% of 1 at room temperature selectively catalyzed dehydrogenative borylation of terminal alkynes with HBPin, a lower loading of 1 (0.5 mol%) at 90 °C selectively promoted hydroboration reaction of the same alkynes skipping the dehydrogenative borylation step. The mode of action of 1 was proposed based on experimental observations as well as the mechanism of dehydrogenative borylation was studied by DFT computations.

Cobalt‐Catalyzed One‐Pot Asymmetric Difunctionalization of Alkynes to Access Chiral gem‐(Borylsilyl)alkanes

AbstractEnantioselective cobalt‐catalyzed one‐pot hydrosilylation and hydroboration of terminal alkynes has been developed employing a cobalt catalyst generated from Co(acac)2 and (R,R)‐Me‐Ferrocelane. A variety of terminal alkynes undergo this asymmetric transformation, affording the corresponding gem‐(borylsilyl)alkane products with high enantioselectivity (up to 98 % ee). This one‐pot reaction combines (E)‐selective hydrosilylation of alkynes and consecutive enantioselective hydroboration of (E)‐vinylsilanes with one chiral cobalt catalyst. This protocol represents the most straightforward approach to access versatile chiral gem‐(borylsilyl)alkanes from readily available alkynes with commercially available cobalt salt and chiral ligand.

Cobalt-Catalyzed One-Pot Asymmetric Difunctionalization of Alkynes to Access Chiral gem-(Borylsilyl)alkanes.

Enantioselective cobalt-catalyzed one-pot hydrosilylation and hydroboration of terminal alkynes has been developed employing a cobalt catalyst generated from Co(acac)2 and (R,R)-Me-Ferrocelane. A variety of terminal alkynes undergo this asymmetric transformation, affording the corresponding gem-(borylsilyl)alkane products with high enantioselectivity (up to 98 % ee). This one-pot reaction combines (E)-selective hydrosilylation of alkynes and consecutive enantioselective hydroboration of (E)-vinylsilanes with one chiral cobalt catalyst. This protocol represents the most straightforward approach to access versatile chiral gem-(borylsilyl)alkanes from readily available alkynes with commercially available cobalt salt and chiral ligand.

(Z)-Selective Hydrosilylation and Hydroboration of Terminal Alkynes Enabled by Ruthenium Complexes with an N-Heterocyclic Carbene Ligand.

Metal-catalyzed trans-1,2-hydrosilylations and hydroborations of terminal alkynes that generate synthetically valuable (Z)-alkenylsilanes and (Z)-alkenylboranes remain challenging due to the (E)-selective nature of the reactions and the formation of the thermodynamically unfavorable (Z)-isomer. The development of new, efficient catalytic systems for the (Z)-selective hydrosilylation and hydroboration of terminal alkynes is thus highly desirable from a fundamental perspective as it would deepen our understanding of the metal-catalyzed (Z)-selective hydrosilylation and hydroboration of terminal alkynes. This personal account describes our research for developing a ruthenium complex that can efficiently catalyze the hydrosilylation and hydroboration of terminal alkynes, and for exploring the factors controlling (Z)-selectivity of the reactions. Our effort into the activation of B-protected boronic acids, R-B(dan) (dan=naphthalene-1,8-diaminato), that was believed not to participate in Suzuki-Miyaura cross-coupling, is also discussed.

Cobalt‐Catalyzed Markovnikov‐Type Selective Hydroboration of Terminal Alkynes

AbstractA cobalt‐catalyzed Markovnikov‐type hydroboration of terminal alkynes with HBpin to access α‐alkenyl boronates with good regioselectivity and atom economy is reported. A new ligand has been developed for the cobalt hydride catalyst that has been used for a unique Markovnikov selective insertion of terminal alkynes into metal hydride bond. This operationally simple protocol exhibits excellent functional group tolerance to deliver valuable alkene derivatives.

Novel Synthesis of β-Keto Cycloacetals via Organoboranes

(Z)-1-Bromo-1-alkenylboronate esters were easily synthesized from the hydroboration of terminal alkynes with dibromoborane dimethyl complex under nitrogen atmosphere followed by treatment with propne-1,3-diol. These boronate esters are readily reacted with 1,3-dioxan-2-ylethyl)magnesium bromide in ether and tetrahydrofuran at -78 ºC for 1 h. The resulting "ate" complex upon addition of sodium methoxide in methanol undergo intramolecular nucleophilic substitution reactions to produce the corresponding (E)-alkenylboronate esters containing (1,3-dioxan-2-ylethyl) moiety in 70-82% isolated yields. The resulting intermediates are oxidized with sodium acetate and hydrogen peroxide to provide the corresponding β-keto cycloacetals in good yields (68-84%).

Synthesis, characterization, and catalytic performance of Aluminum and Tin Compounds with β-diketiminato ligand

Two new tin (II) and tin (IV) compounds [LSnCl] (1) and [LSnCl3] (2), bearing the β-diketiminato ligand LH (L = HC(CMeNAr)2, Ar = 2,6-Et2C6H3) were synthesized by the reactions of LH with n-BuLi, then respectively with SnCl2 and SnCl4. Two new aluminum alkyl compounds, [LAlMe2] (3) and [LAlEt2] (4), were synthesized in high yield from the reactions of LH with AlMe3 and AlEt2, respectively. Compound [LAlI2] (5) was synthesized from the reaction of 3 with I2 in good yield. All these main group metal compounds bearing β-diketiminato ligand LH were fully characterized by 1H NMR and 13C NMR spectroscopy, and elemental analysis. Meanwhile, compounds 1, 2, 3, and 5 were characterized by single crystal X-ray structural analysis. Moreover, the well-defined compounds 1–4 were investigated to show good catalytic activity in hydroboration of terminal alkynes.

Stable, Yet Highly Reactive Nonclassical Iron(II) Polyhydride Pincer Complexes: Z-Selective Dimerization and Hydroboration of Terminal Alkynes.

The synthesis, characterization, and catalytic activity of nonclassical iron(II) polyhydride complexes containing tridentate PNP pincer-type ligands is described. These compounds of the general formula [Fe(PNP)(H)2(η2-H2)] exhibit remarkable reactivity toward terminal alkynes. They efficiently promote the catalytic dimerization of aryl acetylenes giving the corresponding conjugated 1,3-enynes in excellent yields with low catalyst loadings. When the reaction is carried out in the presence of pinacolborane, vinyl boronates are obtained. Both reactions take place under mild conditions and are highly chemo-, regio-, and stereoselective with up to 99% Z-selectivity.

Aluminum Hydrides Stabilized by N‐Heterocyclic Imines as Catalysts for Hydroborations with Pinacolborane

The catalytic activity of the NHI‐substituted aluminum hydrides {LMesNAlH2}2 (1a), {LDipNAlH2}2 (1b), {LMesNAl(H)OTf}2 (2a), and {LDipNAl(H)OTf}2 (2b) in the hydroboration of terminal alkynes (for 1), as well as carbonyl compounds (for 2) with pinacolborane (4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolane) is investigated (LMesN = 1,3‐dimesityl‐imidazolin‐2‐imino, Mes = 2,4,6‐trimethylphenyl, LDipN = 1,3‐bis(2,6‐diisopropylphenyl)‐imidazolin‐2‐imino, Tf = triflyl). With the implementation of 1 as a catalyst, the hydroboration of selected terminal arylalkynes requires elevated temperature (80 °C) to proceed. The less sterically congested 1a produces faster conversions than the bulkier 1b. With the use of 2 as a catalyst, the hydroboration of carbonyl compounds occurs at room temperature. An increased steric hindrance of the catalyst (2a vs. 2b) does not mitigate the rate of conversion to a relevant degree.

ChemInform Abstract: An Aluminum Dihydride Working as a Catalyst in Hydroboration and Dehydrocoupling.

AbstractAn aluminum dihydride complex catalyzes both the hydroboration of terminal alkynes and the dehydrocoupling of boranes with amines, thiols, and phenols, respectively, to form compounds with B—N, B—S, and B—O under elimination of H2.

ChemInform Abstract: Cobalt Catalyzed Z‐Selective Hydroboration of Terminal Alkynes and Elucidation of the Origin of Selectivity.

AbstractAnti‐Markovnikov hydroboration of terminal alkynes in the presence of a bis(imino)pyridine cobalt catalyst affords (Z)‐vinylboronate esters.

Synthesis of E-Alkyl Alkenes from Terminal Alkynes via Ni-Catalyzed Cross-Coupling of Alkyl Halides with B-Alkenyl-9-borabicyclo[3.3.1]nonanes.

The first Ni-catalyzed Suzuki-Miyaura coupling of alkyl halides with alkenyl-(9-BBN) reagents is reported. Both primary and secondary alkyl halides including alkyl chlorides can be coupled. The coupling method can be combined with hydroboration of terminal alkynes, allowing the expedited synthesis of functionalized alkyl alkenes from readily available alkynes with complete (E)-selectivity in one pot. The method was applied to the total synthesis of (±)-Recifeiolide, a natural macrolide.

ChemInform Abstract: Straightforward Iron‐Catalyzed Synthesis of Vinylboronates by the Hydroboration of Alkynes.

AbstractThe iron‐catalyzed hydroboration of terminal alkynes (I) affords selectively the corresponding (E)‐isomer (III) in high yields.

Ruthenium-Catalyzed Hydroboration of Terminal Alkynes

Ruthenium-Catalyzed Hydroboration of Terminal Alkynes

ChemInform Abstract: Highly Selective Methods for Synthesis of Internal (α‐) Vinylboronates Through Efficient NHC—Cu‐Catalyzed Hydroboration of Terminal Alkynes. Utility in Chemical Synthesis and Mechanistic Basis for Selectivity.

AbstractTerminal aliphatic or aromatic alkynes can easily undergo hydroboration in the presence of NHC—Cu complexes.

ChemInform Abstract: Catalytic Enantioselective Synthesis of Secondary Allylic Alcohols from Terminal Alkynes and Aldehydes via 1‐Alkenylboron Reagents.

A practical one-pot method has been developed for preparing enantioenriched secondary allylic alcohols starting from terminal alkynes and aldehydes. Hydroboration of terminal alkynes with dicyclohexylborane and subsequent reaction of the resulting alkenylboron reagents with aldehydes in the presence of a catalytic amount (5 mol %) of 3-(3,5-diphenylphenyl)-H8-BINOL and excess titanium tetraisopropoxide afforded the corresponding allylic alcohols in high enantioselectivities up to 94% ee.

Catalytic Enantioselective Synthesis of Secondary Allylic Alcohols from Terminal Alkynes and Aldehydes via 1-Alkenylboron Reagents

A practical one-pot method has been developed for preparing enantioenriched secondary allylic alcohols starting from terminal alkynes and aldehydes. Hydroboration of terminal alkynes with dicyclohexylborane and subsequent reaction of the resulting alkenylboron reagents with aldehydes in the presence of a catalytic amount (5 mol %) of 3-(3,5-diphenylphenyl)-H(8)-BINOL and excess titanium tetraisopropoxide afforded the corresponding allylic alcohols in high enantioselectivities up to 94% ee.

Transition Metal Catalyzed Hydrosilylation and Hydroboration of Terminal Alkynes in Ionic Liquids

AbstractFor Abstract see ChemInform Abstract in Full Text.