CuOTf elimination

Thioacetals eliminate to vinylsulfides in the presence of CuOTf (Scheme 46).192 Cu1 and Cu11 triflates are mild Lewis acids for Friedel-Crafts acylation and alkylation reactions. CuOTf effectively catalyzes the reaction of anisole with selenoesters.193,194 Copper(II) sulfate promotes epoxide ring opening reactions in the presence of pyridine,195 with retention of configuration being observed. Cu(OTf)2 is a catalyst for the ring opening of aziridine by aniline.196... 

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. 

Copper-mediated sulfide elimination can lead to ring opening reactions. The hydroxy thioacetal 83, when treated first with n-BuLi and then with CuOTf afforded 84 in 92 % yield via a Grob fragmentation (Sch. 21) [47]. Two equivalents of the Lewis acid were required for the reaction because one equivalent complexes the alk-oxide and a second is necessary to activate the sulfide for elimination. Ring enlargement occurred upon treatment of 85 with CuOTf in benzene through the intermediacy of the thio-substituted epoxide 86 [48]. a-Phenylthiocyclohexanone 87 was obtained in 83 % yield. 

CuOTf-promoted elimination of thiophenol was exploited in two syntheses of 1-phenylthio-1,3-butadiene, one a C-C connective route from allyl bromide and bis(phenylthio)-methyllithium, and another from crotonaldehyde (eq 49). A topologically analogous C-C connective strategy provides 2-methoxy-1-phenylthio-1,3-butadiene from acrolein (eq 50). That the phenylthio rather than the methoxy substituent in 2-methoxy-1-phenylthio-1,3-butadiene controls the orientation of its Diels-Alder cycloadditions is noteworthy (eq 50). 

Syntheses of 1-phenylthio-1,3-butadienes from carboxylic esters (eq 52) and carboxylic acids (eq 53) are achieved by CuOTf-promoted elimination of thiophenol from intermediate thioacetals. ... 

Elimination of Benzylic Phenyl Thioethers. That C-S bond activation by CuOTf is not limited to substrates that can generate sulfur-stabilized carbocation intermediates is illustrated by a C-C connective synthesis of trans-stilbene (eq 56). The elimination of thiophenol under mild conditions is favored by benzylic stabilization of a carbocation intermediate or an E2 transition state with substantial carbocationic character. 

Heterocyclization of y-Keto Dithioacetals. A C-C connective synthesis of furans is completed by a CuOTf-promoted heterocyclization of y-keto thioacetals (eq 54). Rather than simple /3-elimination to generate a vinyl sulfide (eq 46), a presumed y-keto carbocation intermediate is captured intramolecularly by an intimately juxtaposed carbonyl oxygen nucleophile. 

The a-[bis(phenylthio)methyl]alkanol derived from cyclohep-tanone does not undergo ring expansion upon treatment with CuOTf and EtN(i-Pr)2 in benzene. Instead, 1,3-elimination of thiophenol delivers an epoxy thioether intermediate that undergoes a rearrangement involving 1,2-shift of a phenylsulfenyl group to produce an a-phenylsulfenyl aldehyde (eq 62). ... 

Either concerted electrocyclic reactions of the complexed olefin(s) or a sequence involving the formation of Cu-bound alkyl radicals produced through 7t/a-rearrangement, ring closure to an intermediate cupracycle followed by reductive elimination has been considered. In particular in the framework of the ionic Cu model, approximated closest by CuOTf, ethylene-7i mixes into Cu-3d to an appreciable amount. Excitation then has more resemblance to an intraligand transition and thus parallels the photochemistry of the pure olefins. 

Elimination of Thiophenol from Thioacetals. Conversion of thioacetals to vinyl sulfides is accomplished under exceptionally mild conditions by treatment with (CuOTf)2 C6H6 (eq 45). The reaction involves an a-phenylthio carbocation intermediate. Three factors contribute to the effectiveness of this synthetic method the Lewis acidity of a copper(I) cation that is unencumbered by a strongly coordinated counter anion, the solubility of the copper(I) triflate-benzene complex, and the insolubility of CuSPh in the reaction mixture. An analogous elimination reaction provides an effective route to phenylthio enol ethers from ketones (eq 46). ... 

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