Copper Hydride Complexes Research Articles

Copper-Catalyzed Hydrofunctionalization of Alkynes

We have developed a general approach to the catalytic hydrofunctionalization of alkynes, involving hydrocupration and electrophilic functionalization of the resulting alkenylcopper intermediate. Using this approach with different electrophiles we have developed three transformations: semireduction, hydrobromination, and hydroalkylation of alkynes. In the context of a brief history of copper hydride complexes in organic chemistry, in this account we describe the key developments that inspired and enabled our research. We also describe the reaction development, the scope, and the mechanism of these transformations. In the process, we highlight some of the challenges inherent to our approach and summarize new insights that allowed us to successfully address them. 1 Introduction 2 Semireduction 3 Hydrobromination 4 Hydroalkylation 5 Conclusions

A Comparison of the Stability and Reactivity of Diamido- and Diaminocarbene Copper Alkoxide and Hydride Complexes.

The mononuclear N-heterocyclic carbene (NHC) copper alkoxide complexes [(6-NHC)CuOtBu] (6-NHC = 6-MesDAC (1), 6-Mes (2)) have been prepared by addition of the free carbenes to the tetrameric tert-butoxide precursor [Cu(OtBu)]4, or by protonolysis of [(6-NHC)CuMes] (6-NHC = 6-MesDAC (3), 6-Mes (4)) with tBuOH. In contrast to the relatively stable diaminocarbene complex 2, the diamidocarbene derivative 1 proved susceptible to both thermal and hydrolytic ring-opening reactions, the latter affording [(6-MesDAC)Cu(OC(O)CMe2C(O)N(H)Mes)(CNMes)] (6). The intermediacy of [(6-MesDAC)Cu(OH)] in this reaction was supported by the generation of Cu2O as an additional product. Attempts to generate an isolable copper hydride complex of the type [(6-MesDAC)CuH] by reaction of 1 with Et3SiH resulted instead in migratory insertion to generate [(6-MesDAC-H)Cu(P(p-tolyl)3)] (9) upon trapping by P(p-tolyl)3. Migratory insertion was also observed during attempts to prepare [(6-Mes)CuH], with [(6-Mes-H)Cu(6-Mes)] (10) isolated, following a reaction that was significantly slower than in the 6-MesDAC case. The longer lifetime of [(6-Mes)CuH] allowed it to be trapped stoichiometrically by alkyne, and also employed in the catalytic semi-reduction of alkynes and hydrosilylation of ketones.

Mechanism of Copper-Catalyzed Hydroalkylation of Alkynes: An Unexpected Role of Dinuclear Copper Complexes.

This article describes a mechanistic study of copper-catalyzed hydroalkylation of terminal alkynes. Relying on the established chemistry of N-heterocyclic carbene copper hydride (NHCCuH) complexes, we previously proposed that the hydroalkylation reaction proceeds by hydrocupration of an alkyne by NHCCuH followed by alkylation of the resulting alkenylcopper intermediate by an alkyl triflate. NHCCuH is regenerated from NHCCuOTf through substitution with CsF followed by transmetalation with silane. According to this proposal, NHCCuH must react with an alkyne faster than with an alkyl triflate to avoid reduction of the alkyl triflate. However, we have determined that NHCCuH reacts with alkyl triflates significantly faster than with terminal alkynes, strongly suggesting that the previously proposed mechanism is incorrect. Additionally, we have found that NHCCuOTf rapidly traps NHCCuX (X = F, H, alkenyl) complexes to produce (NHCCu)2(μ-X)(OTf) (X = F, H, alkenyl) complexes. In this article, we propose a new mechanism for hydroalkylation of alkynes that features dinuclear (NHCCu)2(μ-H)(OTf) (X = F, H, alkenyl) complexes as key catalytic intermediates. The results of our study establish feasible pathways for the formation of these intermediates, their ability to participate in the elementary steps of the proposed catalytic cycle, and their ability to serve as competent catalysts in the hydroalkylation reaction. We also provide evidence that the unusual reactivity of the dinuclear complexes is responsible for efficient hydroalkylation of alkynes without concomitant side reactions of the highly reactive alkyl triflates.

Facile insertion of carbon dioxide into Cu₂(μ-H) dinuclear units supported by tetraphosphine ligands.

Reactions of meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm) with Cu(I) species in the presence of NaBH4 afforded di- and tetranuclear copper hydride complexes, [Cu2(μ-H)(μ-dpmppm)2]X (1) and [Cu4(μ-H)2(μ4-H)(μ-dpmppm)2]X (2) (X=BF4, PF6). Complex 1 undergoes facile insertion of CO2 (1 atm) at room temperature, leading to a formate-bridged dicopper complex [Cu2(μ-HCOO)(dpmppm)2]X (3). The experimental and DFT theoretical studies clearly demonstrate that CO2 insertion into the Cu2(μ-H) unit occurred with the flexible dicopper platform. Complex 2 also undergoes CO2 insertion to give a formate-bridged complex, [Cu4(μ-HCOO)3(dpmppm)2]X, during which the square Cu4 framework opened up to a linear tetranuclear chain.

NHC–copper hydrides as chemoselective reducing agents: catalytic reduction of alkynes, alkyl triflates, and alkyl halides

The NHC–copper-catalyzed Z-selective semi-reduction of terminal and internal alkynes, as well as the NHC–copper-catalyzed reduction of primary alkyl triflates and primary and secondary alkyl iodides and bromides are described. The high chemoselectivity demonstrated in these examples illustrates the mild nature of copper hydride complexes as reducing agents, which have applications in synthetic chemistry beyond their traditional role in the reduction of activated alkenes and carbonyl compounds.

Influence of the structure of the diphosphine ligand on the copper fluoride and copper hydride complexes

The influence of the structure of diphosphine ligands on copper fluoride and copper (I) hydride complexes has been investigated by a theoretical DFT study. For both complexes, two stable structures appear. A near “symmetric” one, with both phosphorus lone pairs coordinated to the copper atom, and an “asymmetric” with a single phosphorus atom linked to the copper atom. Three interactions influence the stability of one situation over the other. Although the higher ligand deformation energy observed for the near symmetric complexes is compensated by the increased stability due to the second metal-phosphorus bond, a supplementary hydrogen bond interaction observed for the copper fluoride complexes, leads to a more important stabilization of the asymmetric complexes. The presence of multiple stable catalytic structures is most likely of importance for the reactivity of the catalytic reactions involving copper hydride or copper fluoride catalysts.

ChemInform Abstract: Copper(I) Hydride Reagents and Catalysts

AbstractReview: 113 refs.

ChemInform Abstract: Highly Chemoselective Catalytic Hydrogenation of Unsaturated Ketones and Aldehydes to Unsaturated Alcohols Using Phosphine-Stabilized Copper(I) Hydride Complexes.

Abstract A base metal hydrogenation catalyst composed of the phenyldimethylphosphine-stabilized copper(I) hydride complex provides for the highly chemoselective hydrogenation of unsaturated ketones and aldehydes to unsaturated alcohols, including the regioselective 1,2-reduction of α,β-unsaturated ketones and aldehydes to allylic alcohols. The active catalyst can be derived in situ by phosphine exchange using commercial [(Ph 3 P)CuH] 6 or from the reaction of copper(I) chloride, sodium tert -butoxide, and dimethylphenylphosphine under hydrogen. The catalyst derived from 1,1,1-tris(diphenylphosphinomethyl)ethane is mechanistically interesting but less synthetically useful.

Highly Chemoselective Copper-Catalyzed Conjugate Reduction of Stereochemically Labile α,β-Unsaturated Amino Ketones

Highly chemoselective conjugate reduction of chiral alpha,beta-unsaturated amino ketones has been developed by using triisopropyl phosphite ligated copper hydride complex. The highlights of the method are wide substrate compatibility and exceptional chemoselectivity.

A New and Effective Method for Providing Optically Active Monosubstituted Malononitriles: Selective Reduction of α,β‐Unsaturated Dinitriles Catalyzed by Copper Hydride Complexes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

A new and effective method for providing optically active monosubstituted malononitriles: selective reduction of α,β-unsaturated dinitriles catalyzed by copper hydride complexes

A copper hydride complex, which was generated in situ from CuO t-Bu, BINAP, and PhSiH 3, was found to be a highly effective catalyst for the reduction of the C C of α,β-unsaturated dinitriles. In addition to CuO t-Bu, some air and moisture stable Cu salts were also effective as catalyst precursors in this reduction. Herein, we have developed an effective method for providing optically active monosubstituted malononitriles.

Synthesis, Structure, and Alkyne Reactivity of a Dimeric (Carbene)copper(I) Hydride

The monomeric, two-coordinate carbene complex (IPr)CuO-t-Bu (1) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) reacts readily with silanes such as triethoxysilane, forming a dimeric copper(I) hydride complex (2) with a very short copper−copper distance. Hydrocupration of 3-hexyne by 2 affords a monomeric copper(I) vinyl complex.

Highly Chemoselective Catalytic Hydrogenation of Unsaturated Ketones and Aldehydes to Unsaturated Alcohols Using Phosphine-Stabilized Copper(I) Hydride Complexes

A base metal hydrogenation catalyst composed of the phenyldimethylphosphine-stabilized copper(I) hydride complex provides for the highly chemoselective hydrogenation of unsaturated ketones and aldehydes to unsaturated alcohols, including the regioselective 1,2-reduction of α,β-unsaturated ketones and aldehydes to allylic alcohols. The active catalyst can be derived in situ by phosphine exchange using commercial [(Ph 3P)CuH] 6 or from the reaction of copper(I) chloride, sodium tert-butoxide, and dimethylphenylphosphine under hydrogen. The catalyst derived from 1,1,1-tris(diphenylphosphinomethyl)ethane is mechanistically interesting but less synthetically useful.

Reduction of .alpha.,.beta.-unsaturated nitriles with a copper hydride complex

Reduction of .alpha.,.beta.-unsaturated nitriles with a copper hydride complex

ChemInform Abstract: A STEREOSPECIFIC SYNTHESIS OF CIS‐ AND TRANS‐1‐ALKENYL SULFIDES FROM 1‐ALKYNYL SULFIDES AND A COPPER(I) HYDRIDE COMPLEX OR LITHIUM TETRAHYDRIDOALUMINATE

1-Alkynyl sulfides R1C≡C-S-R21 are easily converted with a copper(I) hydride complex in tetrahydrofuran into cis-1-alkenyl sulfides R1CHCH-S-R22 in yields >90%. Treatment of compounds 1 with LiAlH4 in the same solvent leads to the isomeric trans-1-alkenyl sulfides R1CH CH-S-R23 in almost quantitative yield.

A stereospecific synthesis of cis‐ and trans‐1‐alkenyl sulfides from 1‐alkynyl sulfides and a copper(I) hydride complex or lithium tetrahydridoaluminate

Abstract1‐Alkynyl sulfides R1C≡C‐S‐R2 1 are easily converted with a copper(I) hydride complex in tetrahydrofuran into cis‐1‐alkenyl sulfides R1CHCH‐S‐R2 2 in yields >90%. Treatment of compounds 1 with LiAlH4 in the same solvent leads to the isomeric trans‐1‐alkenyl sulfides R1CH CH‐S‐R2 3 in almost quantitative yield.