Carbon–Hydrogen Bond Activation of Arenes by a [Bis(oxazolinyl)phenyl]rhodium(III) Acetate Complex

Abstract

Thermolysis of the rhodium(III) complex [(dm‐Phebox‐dm)Rh(OAc)2(H2O)] [1; dm‐Phebox‐dm = 2,6‐bis(4,4‐dimethyloxazolinyl)phenyl] in various arenes results in the formation of the corresponding aryl complexes [(dm‐Phebox‐dm)Rh(Ar)(κ2‐OAc)] [Ar = C6H5 (2), 3,5‐Me2C6H3 (3), 3,4‐Me2C6H3 (4), C6H4Me (5), C6H4CF3 (6), C6H4OMe (7), C6H4COMe (8), C6H4Cl (9)]. The reaction of 1 with monosubstituted benzenes such as toluene, anisole, acetophenone, trifluorotoluene, and chlorobenzene produces a mixture of meta‐ and para‐activated complexes, the ratio of which depends on the substituents on the benzene ring. The relative rate of the reaction of 1 with monosubstituted benzenes C6H5X was determined to be: X = OMe (1.8) > COMe (1.6) > CF3 (1.2), Cl (1.2) > CH3 (1). The acetato ligand of 1 appears to be essential for C–H bond activation of arenes as no reaction of the RhIII complex [(dm‐Phebox‐dm)RhCl2(H2O)] is observed in chlorobenzene at 120 °C. The first‐order rate constants obtained by thermolysis of 1 in [D8]toluene at 97.1–116.5 °C yielded the following activation parameters: ΔH‡ = 22(2) kcal mol–1, ΔS‡ = –24(5) cal mol–1 K–1. The kinetic isotope effect for the C–H bond activation of toluene by 1 was determined to be kH/kD = 5.4 at 100 °C. These data suggest that the rate‐determining step involves the C–H bond cleavage with a rigid, cyclic transition structure. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Asymmetric β-functionalization of secondary amine with CH2Cl2 on a chiral NCN pincer Rh complex

“…The RheC bond of the ylide fragment was placed at the axial position. This structural feature is similar to that of other alkyl and aryl complexes with NCN pincer ligands [8,10,13]. The Rh1eC27 bond length (2.079(9) Å) is comparable to that of 2.…”

Oxidatively Induced Reductive Elimination: Exploring the Scope and Catalyst Systems with Ir, Rh, and Ru Complexes

“…It is interesting that, unlike the aryl group transfer to Rh-OTFA (Scheme a), copper additive was not required in the transmetalation of the methyl group. Although the exact mechanism of this methyl transmetalation is not fully understood at the present stage, we tentatively assume that it may be attributed to the relatively electron-rich property of the methyl group. , It should be mentioned that the transfer of a methyl group from boronic esters to a Cp*-rhodacycle has not been reported, and to our best knowledge, this represents the first example of isolating a rhodacycle methyl complex.…”

“…Unlike Ir-Ar and Rh-Ar , the aryl group of Ru-Ar is slightly bent toward the p -cymene ligand, showing ∠M–C­( ipso )–C­(−CF 3 ) = 166.3° . It should be emphasized that, to our best knowledge, Rh-Ar represents the first example of such isolated transmetalated species obtained from Cp*Rh­(III) derivatives, while Rh­(III)-catalyzed direct C–H arylation with aryl nucleophiles has been known. ,,, Likewise, to our best knowledge, transmetalation of an aryl group on the cyclometalated Ru­(II) center has not been documented despite the widely reported Ru-catalyzed biaryl coupling reactions. , …”

Bis(oxazolinyl)phenyl transition-metal complexes: asymmetric catalysis and some reactions of the metals

“…Treatment of the acetate complex 18, which was prepared from a 4,6-dimethyl derivative of the bis(oxazolinyl)benzene 1, in benzene and m-xylene solutions at 90 and 140 1C gave formation of the corresponding aryl complexes 37 in 60 and 56% yields, respectively (Scheme 16). 25 The acetate moiety plays an important role for the C-H bond cleavage during the rate determining step. Similarly, phenylacetylene reacted with the acetate complex 18 at 60 1C to form the acetylide complex 38.…”