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

Abstract

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]­[BArF 4], 2a, and [Rh­(κ3-P,O,P-Xantphos-3,5-C6H3(CF3)2)­(H)2]­[BArF 4], 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)2(κ1-ClC6H5)]­[BArF 4], 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)]­[BArF 4], 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)]­[BArF 4], 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)]­[BArF 4] (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)]­[BArF 4], 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 fac-κ3-8, 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)]­[BArF 4], 14. The temporal evolution of carbothiolation catalysis using mer-κ3-8, 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.

mer, fac, and Bidentate Coordination of an Alkyl-POP Ligand in the Chemistry of Nonclassical Osmium Hydrides

“…21 Similarly to chloride, the oxygen atom of the mer-coordinate diphosphine of the latter has tendency to be displaced by the hydride ion. The addition of KH to 7 in tetrahydrofuran leads to the anionic pentahydride [OsH 5 {xant(P i Pr 2 ) 2 }] - (8), containing a κ 2 -P,P-bidentate diphosphine. In the presence of the crown ether 18-crown-6, yellow crystals of the [K (18-crown-6)] + -salt were obtained in 66% yield.…”

“…Weller and co-workers have demonstrated that 4,5-bis(diphenylphosphino)-9,9dimethylxanthene (xantphos) has the ability of changing its coordination fashion in several relevant rhodium-and iridium-mediated processes, 6 such as the hydroacylation 7 and carbothiolation 8 of alkenes and alkynes and the dehydrocoupling of amineboranes. 9 Transition metal polyhydrides are compounds having enough hydrogen atoms bonded to the metal center of a L n M fragment to form at least two different types of ligands.…”

Reactivity of Xantphos‐Type Rhodium Complexes Towards SF4: SF3 Versus SF2 Complex Generation

“…The structure of the cationic part in 6 exhibits a slightly distorted square‐pyramidal coordination geometry at the rhodium atom with the λ 4 ‐trifluorosulfanyl ligand located in a trans position to the ether function of the phosphine with O1−Rh1−S1a and O1−Rh1−S1b angles of 169.78(17)° and 170.6(2)°, respectively. The Rh−S bond lengths (Rh1−S1a 2.195(9) Å, Rh1−S1b 2.164(11) Å) are significantly smaller when compared to the one in [Rh(SH)( t Bu xanPOP)] (Rh−S 2.2865(4) Å or in various cationic xantphos‐type Rh (III) −S complexes as for instance [Rh(xanPOP)(C 6 H 4 COMe)(SMe)][BAr F 4 ] (Rh−S 2.3373(14) Å, xanPOP=4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene, Ar F =3,5‐(CF 3 ) 2 C 6 H 3 ) [8,11] . The SF 3 ligand displays an Rh⋅⋅⋅F interaction of 2.418(3) Å, which results in a small F1−S1a−Rh angle of 72.3(3)° as well as the significantly elongated bond between the sulfur atom and the fluorine atom F1 of 1.880(7) Å.…”

Cobalt-catalyzed nucleophilic addition of the allylic C(sp3)–H bond of simple alkenes to ketones

“…First, methylcobalt(I) I should be generated from Co(acac) 2 , Xantphos, and AlMe 3 [ 28 – 29 ]. Oxidative addition of the allylic C(sp 3 )–H bond to I would proceed to afford η 3 -allylcobalt(III) intermediate II , which is tautomerized to η 1 -allylcobalt(III) III by the assistance of the oxygen atom in the Xantphos ligand [ 30 ]. When using α-olefin as a substrate, the cobalt atom should be located at the terminal position due to the avoidance of steric repulsion between the bulky Xantphos ligand and an alkyl substitution (similar to the case of nucleophilic η 1 -allylpalladium species [ 31 – 39 ]), whereas the cobalt atom preferred to reside at the internal position when allylarenes and 1,4-dienes were employed in our previous studies [ 28 – 29 ].…”