Copper hydride species

Copper hydride species, notably Stryker s reagent [Ph3PCuH]6, are capable of promoting the conjugate reduction of a,( >-unsalurated carbonyl compounds [42], Taking advantage of this trustworthy method, Chiu et al. demonstrated in 1998 an intramolecular reductive aldol reaction in the synthesis of novel terpenoid pseudolaric acids isolated from Chinese folk medicine (Scheme 28) [43]. Two equivalents of [Ph3PCuH]6 enabled cycli-zation of keto-enone 104 to provide the bicyclic diastereomers 105 (66%) and 106 (16%). The reaction also was applied to the transformation of 107... 

Copper hydride species generated in situ from hydrophenylsilane and a CuF complex initiates reductive aldol reaction by forming copper enolates (rather than enol silyl ethers). For accomplising a chiral reaction the ferrocenyl ligand 73 is added. 

The stabilized copper hydride species 1 is obtained by mixing PMHS with Cu(OAc)2 2H2O in r-BuOH and toluene in the presence of l,2-bis(diphenylphosphino)-benzene at room temperature. It can be used in lieu of the Stryker reagent. 

Alternatively, carbene-complexed copper hydride species is involved in the transformation of alkynyl epoxides to allenyl carbinols using Cul, t-BuONa, PMHS and a imidazo-lium salt. ... 

In closing this section, it should be noted that this far there is no report of NHC-copper-mediated reduction of unbiased C-C multiple bonds, a somewhat surprising feature considering the high efficiency of NHC-supported copper hydride species towards other n bonds. 

Once the pre-catalyst formed, this species reacts with a hydrosilane via a a bond metathesis reaction to yield the active copper hydride species. As shown in Scheme 3, the formation of a cr bond between the copper and hydrogen atoms occurs through transmetalation [46, 72, 110] passing by a 4-center transition state. 

Once activated, the copper hydride species enters the catalytic cycle. The proposed mechanism for the hydrogenation of ketones occurs through a two-step cycle as presented in Scheme 4. A first step concerns the formation of a copper alkoxide, through a a metathesis similar to that observed for the activation of the pre-catalyst. In a second step, another 4-center transition state between this alkoxide and a hydrosilane leads to a silylated ether, and the reactivated catalyst. The alcohol can be recovered through hydrolysis of the silylated ether. Due to the fact the alkoxide complex is not observed experimentally, the reduction of the ketone is suggested to be the rate-limiting step. 

A highly enantioselective reduction of o /3-unsaturated nitriles has been conducted by using a Cu(OAc)2-josiphos complex as the catalyst under hydrosilylation conditions. This reaction provides access to valuable /3-aryl-substituted chiral nitriles in good yields and with excellent enantioselectivities by employing a stable catalytic pre- cursor and a readily available commercial bisphosphine ligand. The active reducing species is believed to be copper hydride.315... 

The use of copper catalysts in the presence of silanes provides another possibility to achieve 1,4-addition selectively. The active species is hkely to be a copper hydride, which reacts to give a copper enolate, which undergoes a a-bond metathesis step with the hydrosilane (Scheme 7). ... 

Sodium hydride reduction of quinoline in HMPA leads to a 2 3 mixture of 1,2-dihydroquinoline (82) and 1,4-dihydroquinoline (83) isolated as the A-methoxycarbonyl derivatives. In situ produced copper hydride reagents react with pyridinium species with high regioselectivity generating 1,4-dihydropyridine... 

Extensive studies have been performed to enable understanding of the mechanism of hydrogenation of o-fructose on copper catalysts. Experiments performed with deuterium showed clearly that the enediol form is not involved in hydrogenation and D-fructose is assumed to be preferentially hydrogenated via its furanose form by attack of a copper hydride-like species at the anomeric carbon, with inversion of configuration [22]. The proposed mechanism could explain the diaster-... 

Mechanistic studies have been carried out for neutral and cationic Cu systems [12,13b]. The proposed mechanism for [Cu(Cl)(NHC)j complexes involves the formation of [Cu(0 Bu)(NHC)] by reaction of the chloride complex with the base (Scheme 8.3). [Cu(H)(NHC)j would be formed in situ by o-bond metathesis between the terf-butoxide copper complex and the hydrosilane. The hydride copper complex is highly unstable (observable by NMR) however, it is the active species. Hence, by addition of the hydride species to the carbonyl, a second o-bond metathesis with the silane affords the expected silyl ether and regenerates the active catalyst. In the case of cationic derivatives, dissociation of one NHC occurs as the first step, which is displaced by the fert-butoxide moiety, and is the direct precursor of the active species. The hydrosilane is activated by the nucleophilic NHC, leading to the formation of the silyl ether. The activation of the silane appears to be the decisive step for this transformation. 

Mechanistic insight highlighted the in situ formation of the hydride species via o-bond metathesis between [Cu(0 Bu)(IPr)] and the silane, followed by conjugate reduction leading to the formation of a copper enolate intermediate (Scheme 8.8). However, when an alcohol is introduced in the process, protonation of copper acetal occurs leading to the product and the alkoxide copper complex. 

Following stoichiometric reactions, a plausible mechanism was described involving the formation of a hydride species (Scheme 8.39). The reaction of [Cu(H)(NHC)j with an alkyne affords the corresponding copper alkenyl complex, followed by CO2 insertion into the Cu—C bond. Recently, Lin and coworkers reported DFT studies on this transformation, which confirmed the previously reported mechanism [97]. 

In another recent work, Chattaraj and coworkers [68] investigated the possible stable geometries of Be ion and some of the bimetallic species containing the dianion. Besides, orbital analyses and NICS values, which are normally used for the prediction of aromatic behavior, the aromaticities of MBej and M2Be3 (M Li, Na, and Cu) are studied using some other aromaticity indexes such as chemical hardness, polarizability, and electrophilicity. All the species are found to possess 7t-aromaticity. As mentioned earlier, Tsipis et al. [54] studied a new class of cyclic copper hydrides or hydrocoppers (Cu H n = 3-6), which are the cyclic hydrocarbon analogs. 

Regioselectivity of the copper(I)-catalysed hydroboration of unsymmetrical internal alkynes r1C=CR2 (R1 = Ar, COjR, amide, CHjOR, CH2NR2, CH2CH2OR R = alkyl, SiMe3) has been shown to be controlled by the choice of the catalytic species (copper hydride or boryl copper). Thus, the reaction with PinBH (Pin = pinacol), catalysed by CuCl chelated to an electron-rich diphosphine, affords R C(BPin)=CHR in the presence of t-BuONa switching to pinjBj and electron-poor diphosphine results in the formation of the opposite isomer, in both cases with >99 1 regioselectivity. ... 

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