Copper triflate catalyst

Takacs, J.M., Lawson, E.C., Reno, M.J., Youngman, M.A. and Quincy, D.A. (1997) Enantioselective Diels—Alder reactions Room temperature bis(oxazoline)-zinc, magnesium, and copper triflate catalysts. Tetrahedron Asymmetry, 8, 3073—3078. 

Salomon and his coworkers have over the years studied the influence of copper(I) triflate on the cyclization of non-conjugated dienes. In the present example the diene (170) is converted readily on irradiation in the presence of the catalyst and affords the alcohol (171). This is oxidised to the corresponding ketone. The intramolecular cyclization of the diene (172), using a copper triflate catalyst, affords the straight (2-1-2) adduct (173). This cyclization was used as an approach to the synthesis of RobustadiaJ B. However, it was shown that the proposed structure of the natural product was wrong and that the robustadials should have the camphahe moiety in their structure as shown in (174). ... 

One important point to stress from these results is the possibihty of using copper chloride instead of copper triflate to prepare the complexes. It is well known that in organic solvents there is a dramatic counteranion effect on the activity and enantioselectivity of these catalysts. On the other hand, the rapid anion exchange produced in the ionic hquid resulted in better performance of the complexes, as the bis(triflyl)imide behaves in a similar way to the triflate counteranion. 

Aziridination of alkenes can be carried out using N-(p- to I ucncsu I I o n y I i m i n o) phenyliodinane and copper triflate or other copper salts.257 These reactions are mechanistically analogous to metal-catalyzed cyclopropanation. Rhodium acetate also acts as a catalyst.258 Other arenesulfonyliminoiodinanes can be used,259 as can chloroamine T260 and bromoamine T.261 The range of substituted alkenes that react includes acrylate esters.262... 

The hetero Diels-Alder reactions discussed thus far use 2-10 mol% of catalyst. Jorgensen s group44b found that the reaction could be carried out even at very low catalyst loading. The catalyst can conveniently be prepared in situ by mixing the chiral ligand 83 and copper triflate in the reaction system. Scheme 5-35 shows that product 112 can be obtained with good yield and high enan-... 

Given the prevalence of bis(oxazoline)-copper catalysts as chiral Lewis acids, it seems appropriate to comment briefly on catalyst preparations, since differences arise in the nature of the catalyst complex. Triflate-derived catalysts are formed simply by combining the ligand and Cu(OTf)2 in a given solvent and stirring for an appropriate length of time (typically >2 h) to achieve complete dissolution and complexation, Scheme 14. The hydrated version is formed by addition of 2 equiv of water to this catalyst solution, followed by removal of solvent after 15 min of stirring. The hydrated triflate catalyst is bench stable for months. 

The first catalytic, asymmetric aziridination of an alkene in good yield and high enantioselectivity was recently reported56. Thus styrene (63) was treated with [N-(p-toluenesulphonyl)imino]phenyliodinane (64) and an asymmetric copper catalyst to yield (/ )-Ar-(p-toluenesulphonyl)-2-phenylaziridine [(/ )-65] in 97% yield with an ee of 61%, the catalyst being the complex formed in situ in chloroform from the chiral bis[(5 ) 4-ferf-butyloxazoline] [(S,S)-66] and copper triflate (CuOTf)56, the reaction proceeding by way of a nitrene transfer57. 

Complete conversion of the substrate was observed with reaction times as short as 2.5 h when copper triflate was the Lewis acid, and the selectivity was usually greater than 98% towards the desired compound. Upon reuse of the same catalyst system with fresh substrate, it was reported that the conversion was 80% and 76% for the second and third reuse of the catalyst. 

In certain cases, when the palladium or nickel catalyzed coupling is not efficient or fails completely, an alternate solution is provided by the use of copper based catalyst systems. The 5-iodouracil derivative shown in 7.77. was unreactive towards imidazole using either the Buchwald-Hartwig conditions or the copper(I) triflate promoted the carbon-nitrogen bond formation reported by Buchwald98 These latter conditions, however, were effective in coupling the iodouracil with a series of other amines (7.77.), The optimal catalyst system consisted of copper(I) triflate, phenantroline and dibenzylideneacetone (dba).99... 

Silver(I) triflate and copper triflate can be applied as catalysts A representative example is the preparation of alkynyl tosy lates by the catalytic decomposition of alkynyl lodomum salts in the presence of these salts [737] (equation 67)... 

The rhodium(II) catalysts and the chelated copper catalysts are considered to coordinate only to the carbenoid, while copper triflate and tetrafluoioborate coordinate to both the carbenoid and alkene and thus enhance cyclopropanation reactions through a template effect.14 Palladium-based catalysts, such as palladium(II) acetate and bis(benzonitrile)palladium(II) chloride,l6e are also believed to be able to coordinate with the alkene. Some chiral complexes based on cobalt have also been developed,21 but these have not been extensively used. 

For the transition-metal catalyzed decomposition of silyl-substituted diazoacetates 205 [silyl = SiMe3, SiEt3, SiMeiBu-i, SitPr-i SiPtnBiW, SiMe2SiMe3], copper triflate and dirhodium tetrakis(perfluorobutyrate) proved to be the best catalysts114. While these two catalysts induce the elimination of N2 at 20 °C even with bulky silyl substituents, dirhodium-tetraacetate even at 100 °C decomposes only the trimethylsilyl-and triethylsilyl-diazoacetates. When the decomposition reactions are carried out in... 

Efforts to trap the carbonyl ylide intermediate by intramolecular [3 + 2] cycloaddition to a C=C bond were unsuccessful. Rather, the decomposition of allyl (trimethylsilyl)diazoacetate (218) (equation 69) in the presence of aldehydes gave 1,3-dioxolan-4-oncs 219 their formation has been explained by 1,5-cyclization of the carbonyl ylide intermediate followed by a Claisen rearrangement122. With acetone as carbonyl component, the reaction proceeds analogously. Clean formation of 219 occurred only with Rh2(OOCC3F7)4 as catalyst, while the copper triflate catalyzed version led to a mixture of 219, an oxirane and the product of intramolecular carbenoid... 

The influence of Lewis acids on the 4 + 2-cycloaddition of (2ft,2/ft)-A,iV/-fumaro-ylbis[fenchane-8,2-sultam] with cyclopentadiene and cyclohexadiene was investigated by IR studies of the sultam compexes with various Lewis acids.101 The first enantios-elective silicon Lewis acid catalyst (91) catalysed the Diels-Alder cycloaddition of methacrolein and cyclopentadiene with 94% ee.102 [A1C13 + 2THF] is a new and efficient catalytic system for the Diels-Alder cycloaddition of a,/9-unsaturated carbonyl compounds with dienes under solvent-free conditions.103 Dendritic copper(II) triflate catalysts with a 2,2 -bipyridine core (92) increased the chemical yields of Diels-Alder adducts.104... 

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>. 

Very recently, an intramolecular version with alkynyl [3-diketones or (3-diketoe-sters has been reported (Scheme 10.23).42 With such substrates, the reaction required a mixture of silver tetrafluoroborate and copper triflate as catalysts. Here, also, nucleophilic addition of a ketoenol to a n complex seemed responsible for an exo-dig cyclization, but depending on substituents, exocyclic or endocyclic alkenes were obtained. 

Copper triflate has been found to promote the [3+2] cycloaddition of iV-tosylaziridines with nitriles <2006TL5399>. Ghorai has found that aziridine 149 and a substituted benzonitrile were dissolved and added to a suspension of copper triflate, resulting in the formation of good yield of the imidazoline 150. The mechanism proposed involves the addition of the nitrogen atom of the nitrile to the presumed copper aziridinium ion formed with the triflate catalyst. Subsequently, the iV-Ts moiety attacks the electrophilic carbon of the nitrilium ion to form the imidazoline. 

Various metal salts such as rare earth metal triflates and copper triflate can function as Lewis acids in aqueous media. They can effectively activate aldehydes and imines in the presence of water molecules, and the first successful examples of Lewis acid-catalyzed reactions in aqueous solution have been demonstrated. Water-soluble aldehydes such as foimaldehyde could be employed directly in these reactions. Moreover, the catalysts could be easily recovered after the reactions were completed and could be reused. There are many kinds of Lewis acid-promoted reactions in industrial chemistry, and treatment of large amounts of the acids left over after the reactions have induced some severe environmental problems. From the standpoints of their catalytic use and reusability, the Lewis acids described in this chapter are expected to be new types of catalysts providing some solutions for these problems. 

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