Activation of benzene by a tetrakis(trimethylphosphine)osmium(II) system. Mechanism of activation

Abstract

The intermolecular activation of the carbon-hydrogen bond in benzene has been investigated by the thermolysis of c;í-L4Os(H)R (L = P(CH3)3; R = CH3, 1; R = CH2C(CH3)3, 2; R = CH2Si(CH3)3, 3). Thermolysis of 2 at 70 °C in benzene results in activation of the arene C-H bonds to afford L4Os(H)(C6H5), 6, quantitatively. This reaction is shown to proceed, in the case of 2, via initial reversible L dissociation to form an unsaturated intermediate, L3Os(H)Np, 9, which is configurationally stable during its lifetime and which rapidly and reversibly activates the C-H bonds of its own neopentyl ligand. Unsaturated intermediate 9 reacts with benzene by oxidative addition of Ar-H to form an Os(IV) intermediate, L3Os(H)2(Np)(Ar), which then undergoes reductive elimination of NpH. The activation step exhibits a kinetic isotope effect, *c6h6/*C6D6! of 3.32 ± 0.30. The thermolyses of 3 in benzene were shown to proceed by the same mechanism of initial dissociation of L, followed by oxidative addition of benzene, reductive elimination of SiMe4, and uptake of L. The unsaturated intermediate

Transition metal-catalyzed olefin arylation via C–H bond activation of arenes

“…Therefore, the attempt on direct formation of styrene from benzene and ethylene using aromatic C-H bond activation leading to C-C bond formation is a considerably attractive field, which has enough room to be developed from the industrial point of view. As described in Section 1, much work related to homogeneous C-H bond activation of aromatic compounds by discrete transition metal complexes has been reported [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Group VIII metal complexes can also be employed for olefin oxidation reactions such as Pd-catalyzed partial oxidation of ethylene, as is done in the well-known Wacker reaction to form acetaldehyde [61,62] and oxidative vinylation of acetic acid to produce vinyl acetate [63].…”

“…Significant efforts have been directed at homogeneous C-H bond activation of aromatic compounds by discrete transition metal complexes [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. However, among the numerous stoichiometric studies, relatively small numbers of organic syntheses are reported.…”

Synthesis, characterization, and crystal structures of the osmium triflate complexes Cp∗Os(P–P)(OTf) and [Cp∗Os(P–P)(OH2)][OTf]

“…We have previously described the conversion of the bromo complexes Cp * Os(P-P) 2 Br to their corresponding hydrides Cp * Os(P-P)H, where P-P = bis(dimethylphosphino)methane (dmpm), dppm = bis(diphenylphosphino)-methane (dppm), or 1,2-bis(dimethylphosphino)ethane (dmpe) [16]. Treatment of these hydride complexes with one to two equivalents of methyl trifluoromethanesulfonate in pentane results in loss of methane and generation of the triflate compounds Cp * Os(dmpm)(OTf) (1), Cp * Os(dppm)-(OTf) (2), and Cp * Os(dmpe)(OTf) (3), in 59%, 69%, and 86% yield, respectively.…”

“…Treatment of these hydride complexes with one to two equivalents of methyl trifluoromethanesulfonate in pentane results in loss of methane and generation of the triflate compounds Cp * Os(dmpm)(OTf) (1), Cp * Os(dppm)-(OTf) (2), and Cp * Os(dmpe)(OTf) (3), in 59%, 69%, and 86% yield, respectively. To obtain high yields of these compounds, it is important to limit the amount of MeOTf added and to isolate the reaction products promptly (i.e., after at most a few hours).…”

“…A particularly clean method is the addition of triflic acid to a transition metal alkyl complex, with loss of alkane. For the platinum metals, this route has been used to prepare triflate complexes of Ru [2][3][4], Os [5,6], Rh [7], Ir [8], and Pt [9][10][11]. Formation of triflate complexes by addition of triflic acid to metal hydrides (with loss of H 2 ) is also possible; all the examples of this approach of which we are aware involve the element Ru [12][13][14].…”