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Rh–POP Pincer Xantphos Complexes for C–S and C–H Activation. Implications for Carbothiolation Catalysis

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journal contribution
posted on 17.12.2015, 07:26 authored by Peng Ren, Sebastian D. Pike, Indrek Pernik, Andrew S. Weller, Michael C. Willis
The neutral Rh­(I)–Xantphos complex [Rh­(κ3-P,O,P-Xantphos)­Cl]n, 4, and cationic Rh­(III) [Rh­(κ3-P,O,P-Xantphos)­(H)2]­[BArF4], 2a, and [Rh­(κ3-P,O,P-Xantphos-3,5-C6H3(CF3)2)­(H)2]­[BArF4], 2b, are described [ArF = 3,5-(CF3)2C6H3; Xantphos = 4,5-bis­(diphenylphosphino)-9,9-dimethylxanthene; Xantphos-3,5-C6H3(CF3)2 = 9,9-dimethylxanthene-4,5-bis­(bis­(3,5-bis­(trifluoromethyl)­phenyl)­phosphine]. A solid-state structure of 2b isolated from C6H5Cl solution shows a κ1-chlorobenzene adduct, [Rh­(κ3-P,O,P-Xantphos-3,5-C6H3(CF3)2)­(H)21-ClC6H5)]­[BArF4], 3. Addition of H2 to 4 affords, crystallographically characterized, [Rh­(κ3-P,O,P-Xantphos)­(H)2Cl], 5. Addition of diphenyl acetylene to 2a results in the formation of the C–H activated metallacyclopentadiene [Rh­(κ3-P,O,P-Xantphos)­(ClCH2Cl)­(σ,σ-(C6H4)­C­(H)CPh)]­[BArF4], 7, a rare example of a crystallographically characterized Rh–dichloromethane complex, alongside the Rh­(I) complex mer-[Rh­(κ3-P,O,P-Xantphos)­(η2-PhCCPh)]­[BArF4], 6. Halide abstraction from [Rh­(κ3-P,O,P-Xantphos)­Cl]n in the presence of diphenylacetylene affords 6 as the only product, which in the solid state shows that the alkyne binds perpendicular to the κ3-POP Xantphos ligand plane. This complex acts as a latent source of the [Rh­(κ3-P,O,P-Xantphos)]+ fragment and facilitates ortho-directed C–S activation in a number of 2-arylsulfides to give mer-[Rh­(κ3-P,O,P-Xantphos)­(σ,κ1-Ar)­(SMe)]­[BArF4] (Ar = C6H4COMe, 8; C6H4(CO)­OMe, 9; C6H4NO2, 10; C6H4CNCH2CH2O, 11; C6H4C5H4N, 12). Similar C–S bond cleavage is observed with allyl sulfide, to give fac-[Rh­(κ3-P,O,P-Xantphos)­(η3-C3H5)­(SPh)]­[BArF4], 13. These products of C–S activation have been crystallographically characterized. For 8 in situ monitoring of the reaction by NMR spectroscopy reveals the initial formation of fac3-8, which then proceeds to isomerize to the mer-isomer. With the para-ketone aryl sulfide, 4-SMeC 6H4COMe, C–H activation ortho to the ketone occurs to give mer-[Rh­(κ3-P,O,P-Xantphos)­(σ,κ1-4-(COMe)­C6H3SMe)­(H)]­[BArF4], 14. The temporal evolution of carbothiolation catalysis using mer3-8, and phenyl acetylene and 2-(methylthio)­acetophenone substrates shows initial fast catalysis and then a considerably slower evolution of the product. We suggest that the initially formed fac-isomer of the C–S activation product is considerably more active than the mer-isomer (i.e., mer-8), the latter of which is formed rapidly by isomerization, and this accounts for the observed difference in rates. A likely mechanism is proposed based upon these data.

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