Cycloadditions copper triflate

Oxa-l -silabicyclo[ . 1,0 alkanes (n = 3 111 n = 4 113) were the only products isolated from the photochemical, thermal or transition-metal catalyzed decomposition of (alkenyloxysilyl)diazoacetates 110 and 112, respectively (equation 28)62. The results indicate that intramolecular cyclopropanation is possible via both a carbene and a carbenoid pathway. The efficiency of this transformation depends on the particular system and on the mode of decomposition, but the copper triflate catalyzed reaction is always more efficient than the photochemical route. For the thermally induced cyclopropanation 112 —> 113, a two-step noncarbene pathway at the high reaction temperature appears as an alternative, namely intramolecular cycloaddition of the diazo dipole to the olefinic bond followed by extrusion of N2 from the pyrazoline intermediate. A direct hint to this reaction mode is the formation of 3-methoxycarbonyl-4-methyl-l-oxa-2-sila-3-cyclopentenes instead of cyclopropanes 111 in the thermolysis of 110. 

Cycloaddition of the carbene derived from 205 to bis(trimethylsilyl)acetylene yields the expected cyclopropene in low yield both photochemically (20%) and under catalysis by copper triflate at 80 °C (10-13%)119. The latter version of the reaction is accompanied by [3 + 2] cycloaddition of the diazo compound to the alkyne, and the photochemical route yields a by-product which obviously comes from carbenic C,H insertion at a SiMe3 group of the alkyne. 

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 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>. 

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. 

Silyldiazoacetate 344 decomposes in the presence of copper triflate with the exclusive formation of the unusual carbene dimer 345 (31%). The heterocyclic framework is obtained by ketene cycloaddition to a ketocar-bene dipole through intermediate 346 (92MI6). 

In recent years some work has been done to link oleochemicals with petrochemicals via oligomerization. One possibility is the Dids-Alder reaction of linoleic acid esters with dienophiles, for instance with quinones or ,/Tun saturated aldehydes and ketones [80]. Using scandium or copper triflates as catalysts the reaction can be carried out at very mild temperature conditions (25-40°C) with good yields (< 94%). For the first time in oleochemistry it was possible to carry out Diels-Alder cycloadditions with low catalyst concentrations instead of stoichiometric amounts of Lewis acids. The most successful way to recycle the catalyst was the successive extraction of the triflates with water. After removing the water and drying in vacuum the catalyst was used three times without any loss of yield. 

Some new combinations of chiral ligands with different Lewis acids have been lately evaluated in catalytic asymmetric 1,3-DC reactions of nitrones. When the complex derived from copper(II) triflate and bis(oxazoline) 72 was used as chiral catalyst in the cycloaddition of nitrone 66 and crotonate 68, both endo and exo isomers were obtained with very high enantioselectivities (7 3 dr > 99% ee). In this reaction, the presence of molecular sieves 4 A (MS) was crucial as in their absence the nitrone decomposed and almost no cycloadduct was obtained <04TL9581>. Sibi et al. found that square planar complexes derived from copper triflate and some chiral bisoxazolines favour the COZ-exo approach in the 1,3-DC of nitrone... 

The copper(I) catalysed photo-cycloaddition reactions of the dienes (160) have been studied. This process provides an efficient route to the (2+2) cycloadducts (160a) in the ratios and yields shown under the appropriate structures. These adducts are key components in an approach to the synthesis of A -capnellene. Copper triflate controlled photocycloadditions have also been used as a key step in the synthesis of some cyclopentanes. The reaction involves the cyclization of the dienes (161) under the copper(I) controlled conditions into the adducts (162) and this is followed by thermal reactions that bring about rearrangement and ring opening. 

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. 

Davies and colleagues266 studied the use of copper(II) complexes of chiral bis(oxazoli-dine) 430 as catalysts in the cycloadditions of cyclopentadiene to substituted /V-acryloyl-l,3-oxazolidin-2-ones. They observed high endo and enantioselectivities. Again, the highest enantioselectivities were observed using SbEfi as the counterion, although differences were small this time ee values of 92 and 95% were obtained for the triflate and S h I Y> based catalysts, respectively. 

The most widely exploited photochemical cycloadditions involve irradiation of dienes in which the two double bonds are fairly close and result in formation of polycyclic cage compounds. Some examples are given in Scheme 6.7. Copper(I) triflate facilitates these intramolecular additions, as was the case for intermolecular reactions. 

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... 

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. 

A study on fulleroisoxazolines showed that these adducts undergo retro-cycloaddition by heating in the presence of copper(II) triflate as a catalyst and an excess of a dipolarophile to trap nitrile oxide. The electronic nature of the isoxazoline substituents strongly influences the reaction outcome <07JOC3840>. The fulleroisoxazolines 50 functionalized with electron-donor groups have been synthesized and their photophysical properties analyzed <07EJO2175>. 

A direct and efficient route to imidazoline and pyrrolidine derivatives using copper(ll) triflate-mediated [3+2] cycloaddition of various aryl, alkyl, and cycloalkyl iV-tosylaziridines with nitriles and olefins as dipolarophiles has been reported <2006TL5399>. Formation of bicyclic imidazoline 334 with a /ra j-ring junction as a single product from aziridine 333 suggested that the reaction proceeded through an SN2-type pathway (Scheme 86). 

Salomon has recendy investigated the coi er(I) triflate catalyzed photocycloadditions of allylic alcohols and allylic ethers in an effort to improve the selectivity in photocycloadditions of unactivated al-kenes (equations 103-105,108). ° One particularly interesting case is illustrated in equation (104), where Ae more sterically hindered product is produced as a result of the photocycloaddition of a rigidly held tridentate copper complex. McMurry has also utilized a copper(I) catalyzed [2 + 2] photocycloaddition in the synthesis of -panisene (equation 106)."° Interestingly, the noncatalyzed cycloaddition shown in equation (107), which might also produce a precursor for p-panisene, was unsuccessful. 

More modern studies have made use of copper(I) triflate (CuOTf) as the reagent. This compound is well known to form complexes with dienes and it provides a template on which cycloadditions can be effected. Several examples of this type of cyclization have been reported and cycloadditions based on this approach provide a useful route to cyclobutane derivatives. Thus, a new stereochemical synthesis of grandisol has been developed using the copper(I)-catalysed cycloaddition of the dienol 95 to afford the isomeric bicyclo-heptenols 96 exo endo ratio in this cyclization is solvent-dependent. The racemic... 

Finally, in order to overcome the problems of short wavelength excitation, energy wasting by Z,E-isomerisation and unfavorable entropy effects, the intramolecular [2+2] cycloaddition of 1,6-dienes can be efficiently catalyzed by means of copper(I)triflate. J. Mattay made use of this procedure for the synthesis of the pheromone grandisol (cf. chapter 1.2). The photochemical key step is the excitation of the preformed copper (I) complex with the 1,6-hexadiene acting as a bidentate ligand. 

Ghorai, M.K., Ghosh, K. and Das, K. (2006) Copper(II) triflate promoted cycloaddition of a-alkyl or aryl substituted A-tosylaziridines with nitriles a highly efficient synthesis of substituted imidazolines. Tetrahedron Letters, 47, 5399-5403. 

Copper(I) triflate-catalyzed decomposition of diazoacetates 1 leads to silaheterocycles 2, 3, and 4. Vinylcarbenes or vinylcarbene metal complexes may be assumed as reaction intermediates. With copper(I) chloride as catalyst, the dediazotization of 1 leads to the bicyclic fiiran 5, which formally represents the result of an intramolecular [3+2] cycloaddition of an acylcarbene to the C-C triple bond [5]. 

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