Experimental and Computational Studies of Ruthenium(II)-Catalyzed Addition of Arene C−H Bonds to Olefins

Abstract

Hydroarylation reactions of olefins are catalyzed by the octahedral Ru(II) complex TpRu-(CO)(NCMe)(Ph) (1) (Tp ) hydridotris(pyrazolyl)borate). Experimental studies and density functional theory calculations support a reaction pathway that involves initial acetonitrile/ olefin ligand exchange and subsequent olefin insertion into the ruthenium-phenyl bond. Metal-mediated C-H activation of arene to form a Ru-aryl bond with release of alkyl arene completes the proposed catalytic cycle. The cyclopentadienyl complex CpRu(PPh 3 ) 2 (Ph) produces ethylbenzene and styrene from a benzene/ethylene solution at 90 °C; however, the transformation is not catalytic. A benzene solution of (PCP)Ru(CO)(Ph) (PCP ) 2,6-(CH 2 P t -Bu 2 ) 2 C 6 H 3 ) and ethylene at 90 °C produces styrene in 12% yield without observation of ethylbenzene. Computational studies (DFT) suggest that the C-H activation step does not proceed through the formation of a Ru(IV) oxidative addition intermediate but rather occurs by a concerted pathway.

Hydrogen−Deuterium Exchange between TpRu(PMe3)(L)X (L = PMe3 and X = OH, OPh, Me, Ph, or NHPh; L = NCMe and X = Ph) and Deuterated Arene Solvents: Evidence for Metal-Mediated Processes

“…For example, a combined experimental and computational 16 study suggests that the addition of H 2 across the Ru-N bond of (PCP)Ru(CO)(NH 2 ) (PCP ) 2,6-(CH 2 t Bu 2 ) 2 C 6 H 3 ) is thermally favorable, whereas the addition of a C-H bond of methane or benzene is disfavored. 20 …”

“…We have also previously reported calculations at the same level of density functional theory on benzene C-H activation from (Tab)Ru(CO)(Me)(η 2 -C 6 H 6 ) that indicate a nonoxidative addition process that resembles a σ-bond metathesis transition state with a close Ru-H contact in the transition state. 16 As a point of comparison, the values in brackets in Figure 8 represent the geometric details of the transition state of C-H activation for (Tab)Ru(CO)(Me)(η 2 -C 6 H 6 ) (i.e., transfer of a hydrogen from η 2 -C 6 H 6 to the methyl ligand). For the transition state starting from (Tab)Ru(PH 3 )(OH)(η 2 -C 6 H 6 ) and the corresponding portion of the four-centered active site, there are differences in the metric data compared with (Tab)Ru(CO)(Me)-(η 2 -C 6 H 6 ).…”

Combined Experimental and Computational Studies on the Nature of Aromatic C−H Activation by Octahedral Ruthenium(II) Complexes: Evidence for σ-Bond Metathesis from Hammett Studies

“…Transition State Geometries: Impact of Arene Substituent "X" Calculated transition states for C-H activation from TpRu(L)(η 2 -C,C-C 6 H 5 X)Me to form TpRu(L)(p-X-C 6 H 4 ) and methane are similar to previously calculated transition states for C-H activation by TpRu(L)(Me)(C 6 H 6 ) systems. 12,15 All first-order transition states for arene C-H activation show a large, single imaginary vibrational frequency, corresponding to the removal of an arene hydrogen and its transfer to the methyl substituent. The imaginary frequencies are in the range 954 to 1264 cm -1 for the CO ligand and 848 to 1104 cm -1 for the PMe 3 ligand.…”

“…12,57 However, heating THF-d 8 In contrast, the reactions of TpRu(PMe 3 )(NCMe)Me with meta-xylyl compounds that have electron-withdrawing groups in the 2-position (X ) NO 2 or Br) produce the corresponding TpRu(PMe 3 )(NCMe)(p-X-C 6 Me 2 H 2 ) complexes (eq 1). Monitoring these reactions by 1 H NMR spectroscopy in THF-d 8 (60°C ) with 5 equiv of the meta-xylyl substrate reveals that TpRu(PMe 3 )(NCMe)(p-NO 2 -C 6 H 2 Me 2 ) (1) is produced in 48% yield, while TpRu(PMe 3 )(NCMe)(p-Br-C 6 H 2 Me 2 ) (2) is produced in 33% yield.…”

Iridium-Catalyzed, Intermolecular Hydroetherification of Unactivated Aliphatic Alkenes with Phenols

“…It is a concerted reaction that combines an unsaturated ligand with an adjacent metal-ligand bond to form a product containing a new ligand with the unsaturated group formally inserted into the original covalent metalligand bond (Scheme 1). A variety of unsaturated ligands undergo migratory insertion, including carbon monoxide, carbon dioxide, alkenes, alkynes, ketones, aldehydes, and imines, and migratory insertion is a common step in numerous catalytic reactions, including hydroformylation, [1,2] hydrogenation, [3][4][5] polymerization, [6][7][8][9] hydroarylation, [10][11][12][13][14] difunctionalization of alkenes, [15][16][17][18] and the olefination of aryl halides (commonly termed the Mizoroki-Heck reaction). [19][20][21][22] In most cases, the unsaturated ligand inserts into a metalcarbon (M À C) or metal-hydrogen (M À H) bond.…”