Copper triflate alkene dimerization

Copper(i) triflate, CuSOsF, is an efficient catalyst for the photodimerization of norbornene. The proposed mechanism of this reaction (Scheme 3), which is supported by quantum-yield studies, involves photoexcitation of a 2 1 alkene-CuSOsF complex followed by unimolecular collapse to products. An alternative mechanism involving photoexcitation of a 1 1 copper(i)-alkene complex and a subsequent termolecular interaction with two ground-state alkenes to give the dimer, a mechanism proposed for the photodimerization of norbomene-catalysed by copper(i) halides, is not consistent with the present studies. Photodimerizations catalysed by CuCl have only been observed with strained cycloalkenes. However, CuSOgF is an efficient catalyst for the photodimerization of simple alkenes e.g. cyclopentene. 

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. 

Interestingly, when using copper(I)triflate, the cyclopentadiene dimer 14 reacts in an intermolecular way, leading to the cydobutane 15 (reaction 5) [22], When the same substrate is transformed in the presence of the triplet sensitizer acetone, an intramolecular [2 + 2] cycloaddition takes place and the cage hydrocarbon compound 16 is formed. Obviously, the formation of a copper complex intermediate involving both alkene double bonds of the substrate is unfavorable in this case. 

Catalytic cyclopropanation of alkenes with diazomalonates is sometimes carried out with copper powder, but it appears that copper(I) halide/trialkyl phosphite complexes (for a procedure see Houben-Weyl Vol. E19b, p 1113), bis(acetylacetonato)copper(II), " ° and tet-raacetatodirhodium can be employed more advantageously (Table 13, entries 7-9). For the cyclopropanation of styrene with dicyclohexyl diazomalonate, however, copper(I) triflate was the catalyst of choice, while intramolecular C —H insertion at the cyclohexyl ring took place in the presence of tetraacetatodirhodium. A detailed comparison of copper catalysts for the cyclopropanation of cyclohexene, 1-methyl- and 1,2-dimethylcyclohexene, (Z)- and ( )-hept-2-ene with dimethyl diazomalonate, including competitive reaction pathways such as allylic C-H insertion and carbene dimer formation, is available. The catalyzed interaction between diazomalonic esters and enol ethers leads to cyclopropanes in some cases (e.g. ethoxymethylenecyclohexane to dimethyl 2-ethoxyspiro[2.5]octane-l,l-dicarboxylate ) and to different products in other cases (e.g. 1-methoxycyclohexene, 2-methoxy-3,4-dihydro-2/7-pyran ). This behavior is attributed to the occurence of stabilized dipolar intermediates in these reactions. 

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... 

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