CuOTf complexes

Tanner et al. (58) investigated the use of chelating diaziridines (85) as ligands for transition metals. The cyclopropanation of styrene using CuOTf complexes of phenyl-substituted aziridine (85a) proceeds in modest enantioselectivity and dias-tereoselectivity, but improved enantioselectivity is observed with complexes derived from benzyl-substituted bis(aziridine) (85b), Eq. 42 (59). Complexes derived... 

Andrus also reported the synthesis and use of biphenyl-derived bis(oxazoline) (154) as a ligand for Cu(I) (110). In the presence of this catalyst, cyclohexene is oxidized in comparable yield and selectivity as 55c CuOTf complexes. The ni-trobenzoate perester was found to be a more reactive oxidant than perbenzoate, although the reaction still requires 5 days to proceed to completion, Eq. 93. 

CuOTf-l- CuOTf complex of Cu(OTf)2 -I-196b, 196c or 196d 196f, 196g or 196h 196i... 

Lactonization Cydization of the thioester 1 cannot be effected with mercuric trifluoroacetate or coppcr(I) trifluroacctatc. The usual reagents used for this reaction (6, 582. 7, 444) are ineffective, but cydization is effected with CuOTf complexed With benzene. Two isomeric lactones (2 and 3) are obtained in 62% yield. One of IhcNC is the acetate, of the pyrrolizidine alkaloid crobarbatine. Unfortunately dcucctylation of these products is accompanied by further hydrolysis to the pyrrolizidine unit (retronecine). 

Shibasaki has reported the preparation of y3-lactams by cyclization of thiophenyl esters in the presence of CuOTf [39]. The /3-aminothioester 60 was cyclized in 80 % yield to afford 61 (Sch. 16). The cyclization of 62 was noted to proceed with replacement of the trimethylsilyl group with a phenylsulfide group via the intermediacy of a PhSSPh-CuOTf complex which was formed in situ. Optimized conditions afforded 63 in 63 % yield, with some of the protonated product 64. 

Aziridines. A CuOTf complex of ferrocenyldiimine is effective for nitrene transfer from TsN=IPh (and also carbene from diazoacetic esters) to alkenes. 

Cycloaddition.—Herndon has reviewed substituent effects in photocycloaddi-tions. Copper(i) triflate is found to be an effective catalyst for the photodimerization of norbornene to the usual [2 + 2] dimer. Photoexcitation of a 2 1 alkene-CuOTf complex is suggested. The photodimerization of other simple alkenes is also... 

An isolable CuOTf complex of a highly strained alkene, trans-cycloheptene, is produced by UV irradiation of a hexane solution of cis-cycloheptene in the presence of CuOTf (eq 14).i Photocycloaddition of cycloheptene is also catalyzed by CuOTf. Surprisingly, the major product is not a trans,anti,trans dimer analogous to that formed from cyclohexene (eq 12) but rather a trans,anti,trans,anti,trans trimer (eq 15). ... 

Dissolution of the frans-cycloheptene-CuOTf complex in cycloheptene and evaporation of the solvent delivers a tris alkene complex of CuOTf containing one frans-cycloheptene and two cis-cycloheptene ligands. Heating frans-cycloheptene-CuOTf in neat c/s-cycloheptene delivers the trans,anti,trans,anti,trans trimer (eq 16). Experiments with c/s-cycloheptene-<4 show that the cyclotrimerization involves only frans-cycloheptene molecules, although the reaction is accelerated by the presence of cis-cycloheptene. A likely explanation for these observations is concerted template cyclotrimerization of a tns-trans-cycloheptene-CuOTf complex formed by ligand redistribution (eq 16). ... 

Chiral bis(oxazoline)-CuOTf complexes also have been found to catalyze enantioselective cyclopropanation with diazomethane (eq 76) and (trimethylsilyl)diazomethane, the latter giving rise to higher diastereoselectivities particularly in the case of CuPFe. Copper catalysts are, however, of no help with donor-acceptor diazo reagents such as aryl- and vinyldiazoesters, which are preferentially transferred with very good selectivities in the presence of Rh2(DOSP)4. ... 

Recently, it was found that chiral bis(oxazoline)-CuOTf complexes are also efficient catalysts in the case of a-diazo -keto sulfones whose bulk has a significant influence on the enantio-selectivity (eq 78). The modest results observed with the parent a-diazo 8-keto esters could be attributed to the lack of this steric element. 

Chiral biaryl Schiff base-CuOTf complexes also efficiently catalyze asymmetric aziridination. ortho Substituents prove once more to be crucial since ligands derived from 2,6-disubstituted benzaldehyde and particularly from 2,6-dichlorobenzaldehyde provide, by reaction with CuOTf, monomeric species of high reactivity and selectivity. Under these conditions, asymmetric aziri-dination of trans- and c/s alkenes occurs with very good enantio-selectivities (eqs 92 and 93). 

Catalytic Nitrene Transfer to Heteroatoms. The experimental procedure described above for the copper-catalyzed aziridination of olefins can be applied to the imidation of sulfides, where CuOTf in conjunction with PhI=NTs mediates the formation of siilfimides in good yields (eq 95). Spontaneous [2,3] sigma-tropic rearrangements occur in the case of allylic sulfides. Chiral bis(oxazoline)-CuOTf complexes catalyze both reactions with acceptable enantioselectivities (eq 96). Chloratnine-T is also a suitable but less efficient nitrene precursor. Selenides undergo the same catalytic asymmetric imidation to afford selenimides albeit with lower yields and enantioselectivities. ... 

Metal Catalysis Arai and Yokoyama have developed a chiral imidazoline-aminophenol-CuOTf complex-catalyzed three-component tandem reaction to create acyclic products 146 with three contiguous stereocenters (Scheme 6.20) [47]. Product 146 is mainly formed along with 147, as the minor product. The yield can be enhanced by the addition of HFIP, as weU as the stereoselectivity of the major product. Interesting fuUy substituted tetrahydro-p-carbolines (THBCs) 148 can be obtained after reduction of the nitro group of 146 and subsequent Pictet-Spengler cyclization [48]. 

Codimerization of norbomene with allylalcohol gave a cis/trans-mixturc of hydroxymethylcyclobutanes, whereas QC, formed in situ by irradiation of the NBD-CuOTf complex, adds the alcohol across one of the three... 

Copper(i) co-ordinates with both double bonds but not the hydroxy-group of (CH2=CHCH2)a CHOH in the CuOTf complex. Irradiation of the CT bond of the complex leads to the formation of endo- and eAru-bicyclo-[3,2,0]heptane-3-ol. 

Irradiation of cyclohexene in the presence of CuOTf produces the dimers 14 (49%) and 15 (8%), along with the cyclohexylcyclohexene 16 (24%) (Scheme 5)T The stereochemical outcome in Cu(I)-catalyzed dimerization of cyclohexene may be the result of cis-tram isomerization on irradiation of the initially formed Cu(I)-cyclohexene complex to the trans-cyclohexene-CuOTf complex 13, followed by a concerted ground state 2 + 2 cycloaddition of the highly strained tram-cyclohexene to another cyclohexene. Cycloheptene, on the other hand, produces the aU tra s-fused trimer 17 (Scheme 6) as the sole product. A 1 3 CuOTf-trans-cycloheptene complex has been proposed to be the precursor of this product. No dimerization reaction has been observed for cyclooctene and acychc olefins.However, mixed photocycloaddition occurs with cyclooctene if the other olefin is sufficiently reactive. Thus, cyclooctene adds to norbornene to produce the cyclobutane derivative 18 in 40% yield. GDdimerization was also observed when a mixture of cyclohexene and cycloheptene was irradiated in the presence of CuOTf to yield the adduct 19 (Scheme 6). ... 

A complex of a chiral, nonracemic bis(oxazoline) with CuOTf is a highly effective catalyst for asymmetric cyclopropanation of alkenes. Copper(ll) triflate complexes do not catalyze the reaction unless they are first converted to Cu by reduction with a diazo compound or with phenylhydrazine. CuOTf complexes are uniquely effective. Thus the observed enantioselectivity and catalytic activity, if any, are much lower with other Cu or Cu" salts including halide, cyanide, acetate, and even perchlorate. Both enantiomers of the bis(oxazoline) ligand are readily available. Spectacularly high levels of asymmetric induction are achieved with both mono- (eq 8) and 1,1-disubstituted alkenes (eq 9). 

V,M-Diallylamides are recovered unchanged when irradiated in the presence of CuOTf. This is because the amide chro-mophore interferes with photoactivation of the Cu -alkene complex. Thus CuOTf-alkene complexes containing one, two, three, or even four coordinated C=C bonds exhibit UV absorption at 235 5 nm (e ax 2950 450). The CuOTf complex of ethyl A(Ai-diallylcarbamate exhibits Xmax =233.4 nm (emax 2676) but the free ligand is virtually transparent at this wavelength. Consequently, UV irradiation of ethyl V,V-diallylcarbamates in the presence of CuOTf delivers bicyclic (eq 35) or tricyclic (eq 36)... 

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