Copper photocycloaddition

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... 

The thermo- and photocycloaddition of alkenes will be discussed in Chapter 12, on pericyclic reactions. On the other hand, transition-metals have effectively catalyzed some synthetically useful cycloaddition reactions in water. For example, Lubineau and co-worker reported a [4 + 3] cycloaddition by reacting a,a-dibromo ketones with furan or cyclopen-tadiene mediated by iron or copper, or a-chloro ketones in the presence of triethylamine (Eq. 3.48).185... 

One of the exciting areas that has gained importance over the recent decade is the photochemical cyclization of non-conjugated dienes in the presence of species that can act as templates. One such species that has been used is copper(I) salts. The earliest example of the use of copper salts in the intramolecular photocycloaddition of non-conjugated dienes is that described for cycloocta-1,5-diene. When this is irradiated in the presence... 

Besides the l-(but-3-enyl)allyl alcohols discussed above, 2- and 3-(pent-4-enyl)allyl alcohols also undergo clean photobicyclization in the presence of copper trifluoromethanesulfonate as catalyst. However, no intramolecular photocycloaddition product could be isolated from irradiation of a homologous 2-(hex-5-enyl)allyl alcohol.8... 

Copper-catalyzed [2 + 2] photocycloadditions are related to the latter reactions. These transformations have been extensively studied, frequently in the context of application to organic synthesis [21], When irradiated in the presence of copper(I) triflate, norbomene 12 was effidently transformed into its dimer 13 (Scheme 5.3, reaction 4) [22]. Although complexes such as III are involved in the reaction mechanism [22, 23], it is unclear whether MLCT (metal to ligand charge transfer) or LMCT (ligand to metal charge transfer) excitation induces the transformation. 

Ghosh, S., Raychaudhuri, S.R., and Salomon, R.G. (1987) Copper(I) catalysis of olefin photoreactions. 15. Synthesis of cyclobutanated butyrolactones via copper(I)-catalyzed intermolecular photocycloadditions of homoallyl vinyl or diallyl ethers. Journal of Organic Chemistry,... 

Galoppini, E., Chebolu, B., Gilardi, R., and Zhang, W. (2001) Copper(I)-catalyzed [2 + 2] photocycloadditions with tethered linkers synthesis of syn-photodimers of dicyclopentadienes. Journal of Organic Chemistry, 66, 162-168. 

Malik, C.K., Vaultier, M., and Ghosh, S. (2007) Copper(I)-catalyzed [2 + 2] photocycloaddition of nonconjugated alkenes in room-temperature ionic liquids. Synthesis, 1247-1250. 

Copper(I) catalysis is very well established to promote intramolecular [2+2] photocycloaddition reactions of l,n-dienes (review [351]). The methodology recently enjoyed a number of applications [352-354], It is assumed that CuOTf, which is commonly applied as the catalyst, coordinates the diene and in this way mediates a preorganization. The Ghosh group recently reported a number of CuOTf-catalyzed photochemical [2+2] cycloaddition reactions, in which an organocopper radical complex was proposed as a cyclization intermediate (which should, however, have a formal Cu(II) oxidation state) (selected references [355-357]). A radical complex must, however, not be invoked, since the process may either proceed by a [2+2] photocycloaddition in the coordination sphere of copper without changing the oxidation state or according to a cycloisomerization/reductive elimination process. 

Scheme 9 Early example of copper catalyzed intramolecular photocycloaddition.

Scheme 15 Copper catalyzed photocycloaddition of diallyl ether.

Norbornene structures can be linked by long chain diesters. The copper catalyzed photocycloaddition of these alkenes gives rise to the cyclobutane shown in Sch. 26 [38]. The intramolecular reaction is stereoselective. 

Grandisol has previously been synthesized using a number of photochemical procedures. Two recent reports demonstrate the utility of the copper catalyzed photocycloaddition (see Sch. 28) in the synthesis of the four member ring observed in the boll weevil pheromone. The first example [41] is a chiral synthesis and the second pathway [42] provides the racemic product. 

Sensitized intramolecular reaction of two 1,3-dienes (86) (Sch. 17) yields predominantly the [2+2] adduct 87, with small amounts of [4+4] adduct 89 and little, if any, [4+2] product 88 [58,59], consistent with Hammond results for intermolecular reactions of acyclic dienes (Sch. 4). Benzophenone-sensitized reaction yields a mixture of two isomers of 87. Heating this mixture to 200 °C converts both isomers of 87 to cyclo-octadiene 89 [58]. Unsensitized photoreaction of 86 in the presence of copper(I) triflate gives a significant amount of [4+2] adduct 88. Extended irradiation time converts much of 87 and 89 into 88, as well as producing secondary products [59]. Copper triflate-mediated photocycloaddition of a related tethered diene-monoalkene, gave only the [2+2] adduct [59]. 

It is well known that copper salts can catalyze the photocycloaddition of dienes [56]. The reaction can be very diastereoselective and useful as an access to polycyclic molecules, especially 1 - vinyl-2-allylcyclanes [57]. The reaction of vinylglycine, cinnamyl derivatives 76, and analogs is also catalyzed by Cu(I) produced in situ by the reduction of cupric triflate. The coordination of the double bonds to the copper ion during the [2 4- 2]-photocycloaddition process increase ... 

Mattay et al. employed asymmetric copper(I)-catalyzed intramolecular [2 + 2]-photocycloaddition reactions in a synthetic approach to (+)- and (— )-grandisol [56]. Racemic dienol 33 was irradiated in the presence of CuOTf and a chiral ligand to yield mainly cyclobutanes 34 and ent-34 as a mixture of enantiomers. Other 1,6-dienes were also employed. A number of chiral nitrogen-containing bidentate ligands were tested, the most effective of which, (4S,4 S)-4,4 -diisopropyl-2,2 -bisoxazoline (35) and (4R,47 )-4,4 -diethyl-2,2 -bisoxazoline (36), ensured a minor enantiomeric excess of <5% ee (Scheme 12). The coordination of the diene to the chiral Cu(I) complex under formation of a complex of type 37 was proved by CD analysis. The authors suggest a lower reactivity of the chiral complex compared to the copper ion coordinated to solvent molecules as the reason for the low enantioselectivities observed. 

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. 

An example of complexation of Ji bonds is illustrated in the isomerization reaction shown in Sch. 39. Use of copper chloride enables the efficient conversion of the cis olefin 168 to the trans olefin 169 [77]. (CuOTf)2(C6Hg) can be readily prepared (or purchased) and has been used for [2 + 2] photocycloaddition. An example of norbor-nene dimerization is shown below in which the Cu(I) forms a n complex with two molecules of the olefin and enables a facile cycloaddition.[51]... 

K. Avatschi, S. R. Raychaunduri, and R. G. Salomon, Copper(I) catalysis of olefin photoreactions. 13. Synthesis of bicyclic vinylcyclobutanes via copper(I)-catalyzed intramolecular 2.pi. + 2.pi. photocycloadditions of conjugated dienes to alkenes, J. Org. Chem. 49,4322-4324 (1984). 

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