Copper triflate Cu

Irradiation at 254 nm of solutions containing CuBr and norbomene results in dimerization of the olefm. Subsequent work with copper triflate [Cu(OTF)] in place of CuBr results in the formation of the exo,trans,exo dimer in 88% yield. Analysis of the quantum yield and other data from this photoreaction leads to the conclusion that a 1 2 ratio of Cu(I) alkene complex is the direct precursor of the dimer. A pathway involving a-bonded Cu-alkyl intermediates is a likely one (Scheme 7.1), although an alternative pathway has been considered where the copper ion acts purely as a template for the allowed photochemical [2+2] cycloaddition. 

The first example of an enantioselective thiadiene cycloaddition involved the reaction of 2,-4-diphenyl- 1-thiabuta-1,3-diene with l-propenoyl-l,3-oxazolidin-2-one. Stoichiometric quantities of a copper triflate bis-imine complex catalyst 428 and 4 A molecular sieves are necessary to achieve the highest enantioselectivity and the best endojexo ratio. The absolute configuration of the major endo isomer was determined by reduction of the acyloxazolidine side chain to the known (3/ ,4/ )-5-hydroxymethyl derivative (Scheme 137) <1997J(P1)2957>. The process is improved using a homochiral Cu triflate or Ni perchlorate bis(oxazoline) complex when catalytic amounts are adequate for a range of thiabutadienes <1999CC1001>. 

Abbreviations Ac acetyl Bn benzyl Cu(OTf)2 copper triflate Et ethyl HOTf trifluoromethanesulfonic acid KHMDS potassium hexamethyldisilazane Kim potassium imidazole / Hx / Hexyl Me methyl ... 

Cu(I) has also been used as a removable tether to preorganize alkene reactants such that they will undergo efficient [2 -I- 2] cyclization upon irradiation. Both diallyl ethers [137] and 1,6-heptadienes [138] will cyclize to from cyclobutane products when irradiated in the presence of copper triflate (CuOTf). Irradiation of diene 151 yields predominantly the exo product via transition state 152, which is favored over 153, which has unfavorable steric interactions between the methyl and R2 groups (see Scheme 60). 

Copper The catalytic activity of copper(II) triflate for cyclizations of alkenols or intermolecular additions of alcohols and carboxylic acids to norbomene has been reported [62, 63]. In dioxane at 80°C, high conversions were achieved at prolonged reaction times, and those were superior to those obtained with Lewis acids such as Yb(OTf)3, though the latter also displayed catalytic activity [62]. In a control experiment with triflic acid (10 mol%) only little product (29%) resulted with low stereoselectivity. However, it is now clear that this control experiment was flawed, as too much triflic acid and overly long reaction times had been applied. The previously mentioned study by Carpentier and coworkers on copper triflate catalyzed hydroaUcoxylations has established that Cu(OTf)2 decomposes to CuOTf and triflic acid when heated in organic solvents [50]. Triflic acid is catalytically active in hydroaUcoxylation at levels down to 0.1 mol%, if a polymerization inhibitor is present to prevent consumption of the olefinic substrate. Indeed, Cu (OTf)2 is an excellent reagent for releasing small amounts of triflic acid in this case, because the coreleased CuOTf acts as polymerization inhibitor for the acrylic substrate (Scheme 12) [50]. Other metal triflates like Sc(OTf)3 or Yb(OTf)3 displayed catalytic activity at the 1 mol% level in the reaction of Scheme 12. Additional experiments were presented to support the conclusion that triflic acid is the actual catalyst in this and other Lewis acid catalyzed hydroalkoxylations [50]. 

Catalysts. A variety of metal derivatives appear to catalyze the carbene transfer [6]. Copper metal (copper bronze) and many copper derivatives (CuCl, CuBr, CuS04, Cu(acac)2,. ..) are the most classical ones. However, except copper triflate and complexes V and VI, copper catalysts require relatively high reaction temperature (80-100°C) and generally lack selectivity abundant by-products being often formed. There are however exceptions. 

Liu and coworkers reported in the same year a three-component allylation of in situ generated imines [100,101]. In this case, 5mol% of copper(II) triflate (Cu(OTf)2) was used as a catalyst furnishing the corresponding Cbz-protected homoallylic amines in good to excellent yields (75-95%) from an aldehyde, a carbamate, and an allyltrimethyl silane. 

In addition to rare earth triflates, copper triflate was also found to be a stable Lewis acid in aqueous media. In a mixed aqueous solvent system (H20-EtOH-toIuene = 1 7 4), allylation of various aldehydes with tetraallyltin and aldol reactions with silyl enol ethers proceeded smoothly in the presence of Cu(OTf)2 (20 mol%) to give homoallylic alcohols and aldol adducts, respectively, in high yields (Schemes 3.9 and 3.10). 

Copper-catalyzed Direct Phenylation of Indoles. The site-selective direct phenylation of an indole bearing a tetrafluoro-phenyl substituent has been achieved in 64% yield using diphenyUodonium triflate, Cu(OTf)2, and 2,6-di-tert-butylpyridine in dichloromethane (eq 11). This coupUng occurs selectively at the C3 position of the indole ring under copper-catalyzed conditions, with no phenylation occurring at the C2 position of the indole ring or on the tetrafluorophenyl substituent. ... 

Copper triflate is an efficient catalyst in the cationic polymerization of styrene. However, its grafting onto a silica surface enhanced the overall reactivity of the catalyst providing faster conversions and polymers with higher molecular weight [98]. Only little leaching of species from the supported catalyst was reported. The initiation with Cu(OTl)2-silica catalyst occurs via a catalyst/cocatalyst process between the Lewis acid species and mostly the water present in the system. The latter originated from both the hydration water molecules of Cu(OTf)2 and the adventitious moisture. The surface silanol groups of the catalyst and the physisorbed solvent (methanol) from the catalyst preparation were also mentioned to play a cocatalytic role [98]. 

The Ohe group have investigated novel Cu-catalyzed cyclizations of cyano enones en route to highly substituted 3-pyrrohn-2-ones (2007JOMC579,2008JOC9174). Treatment of cyano enone 103 with copper triflate in the presence of furan gives 5,5-diaryl 3-pyrrohn-2-one 105 (Scheme 15). The reaction presumably proceeds via a dehydrative cycliza-tion to carbocationic intermediate 104, which is trapped by an aromatic nucleophile to give the observed product. 

A number of research groups have investigated the Cu-mediated conversion of C-B bonds in to C-CF3 groups. Clearly this has been driven by the ready availability of boronate starting materials. In 2010, Qing combined copper triflate with the Ruppert-Prakash reagent, KF and silver carbonate to effect the desired transformation. The conditions described display a broad scope of functional group tolerance (Scheme 15.97). 

Corey et al. [19] simultaneously reported similar studies using a2,2 -bis(oxazolyl)-6,6 -dimethyl-l,l -biphenyl as copper(I)triflate chelate. This hgand afforded a stable monomeric chiral Cu(I) complex providing a highly... 

Ghosh et al. [70] reviewed a few years ago the utihty of C2-symmetric chiral bis(oxazoline)-metal complexes for catalytic asymmetric synthesis, and they reserved an important place for Diels-Alder and related transformations. Bis(oxazoline) copper(II)triflate derivatives have been indeed described by Evans et al. as effective catalysts for the asymmetric Diels-Alder reaction [71]. The bis(oxazoline) Ugand 54 allowed the Diels-Alder transformation of two-point binding N-acylimide dienophiles with good yields, good diastereos-electivities (in favor of the endo diastereoisomer) and excellent ee values (up to 99%) [72]. These substrates represent the standard test for new catalysts development. To widen the use of Lewis acidic chiral Cu(ll) complexes, Evans et al. prepared and tested bis(oxazoHnyl)pyridine (PyBOx, structure 55, Scheme 26) as ligand [73]. 

Asymmetric conjugate addition of dialkyl or diaryl zincs for the formation of all carbon quaternary chiral centres was demonstrated by the combination of the chiral 123 and Cu(OTf)2-C H (2.5 mol% each component). Yields of 94-98% and ee of up to 93% were observed in some cases. Interestingly, the reactions with dialkyl zincs proceed in the opposite enantioselective sense to the ones with diaryl zincs, which has been rationalised by coordination of the opposite enantiofaces of the prochiral enone in the alkyl- and aryl-cuprate intermediates, which precedes the C-C bond formation, and determines the configuration of the product. The copper enolate intermediates can also be trapped by TMS triflate or triflic anhydride giving directly the versatile chiral enolsilanes or enoltriflates that can be used in further transformations (Scheme 2.30) [110],... 

The presence of Cu(I) salts promotes intermolecular photocycloaddition of simple alkenes. Copper(I) triflate is especially effective.182 It is believed that the photoreactive species is a 2 1 alkene Cu(I) complex in which the two alkene molecules are brought together prior to photoexcitation.183... 

Metal-Catalyzed. Cyclopropanation. Carbene addition reactions can be catalyzed by several transition metal complexes. Most of the synthetic work has been done using copper or rhodium complexes and we focus on these. The copper-catalyzed decomposition of diazo compounds is a useful reaction for formation of substituted cyclopropanes.188 The reaction has been carried out with several copper salts,189 and both Cu(I) and Cu(II) triflate are useful.190 Several Cu(II)salen complexes, such as the (V-f-butyl derivative, which is called Cu(TBS)2, have become popular catalysts.191... 

Smooth and efficient cyclopropanation also occurs with copper(II) triflate and diazomethane. Intra- and intermolecular competition experiments show that, in this case, the less substituted double bond reacts preferentially251. The same is true for CuOTf and Cu(BF4)2, whereas with CuX P(OMe)3 (X = Cl, I), CuS04 and cop-per(II) acetylacetonate, cyclopropanation of the more substituted double bond predominates. An example is given for cyclopropanation of 1. 

Copper(II) triflate has also been used for the carbenoid cyclopropanation reaction of simple olefins like cyclohexene, 2-methylpropene, cis- or rran.y-2-butene and norbomene with vinyldiazomethane 2 26,27). Although the yields were low (20-38 %), this catalyst is far superior to other copper salts and chelates except for copper(II) hexafluoroacetylaeetonate [Cu(hfacac)2], which exhibits similar efficiency. However, highly nucleophilic vinyl ethers, such as dihydropyran and dihydrofuran cannot be cyclopropanated as they rapidly polymerize on contact with Cu(OTf)2. With these substrates, copper(II) trifluoroacetate or copper(II) hexafluoroacetylaeetonate have to be used. The vinylcyclopropanation is stereospecific with cis- and rra s-2-butene. The 7-vinylbicyclo[4.1.0]heptanes formed from cyclohexene are obtained with the same exo/endo ratio in both the Cu(OTf)2 and Cu(hfacac)2 catalyzed reaction. The... 

Copper(II) triflate is quite inefficient in promoting cyclopropanation of allyl alcohol, and the use of f-butyl diazoacetate [164/(165+166) = 97/3%] brought no improvement over ethyl diazoacetate (67/6 %)162). If, however, copper(I) triflate was the catalyst, cyclopropanation with ethyl diazoacetate increased to 30% at the expense of O/H insertion (55%). As has already been discussed in Sect. 2.2.1, competitive coordination-type and carbenoid mechanisms may be involved in cyclopropanation with copper catalysts, and the ability of Cu(I) to coordinate efficiently with olefins may enhance this reaction in the intramolecular competition with O/H insertion. 

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