Rhodium-Mediated C–C Bond Activation of 2-(2′,6′-Dialkylarylazo)-4-methylphenols. Elimination and Migration of Alkyl Groups

Abstract

Upon reaction of 2-(2′,6′-dimethylphenylazo)-4-methylphenol with [Rh(PPh 3 ) 3 Cl], the azo ligand undergoes an interesting rhodium-assisted C-C bond activation at one ortho position of the 2′,6′dimethylphenyl fragment, leading to the elimination of the methyl group from that ortho position. A similar elimination of an ethyl group takes place when 2-(2′,6′-diethylphenylazo)-4-methylphenol is allowed to react with [Rh(PPh 3 ) 3 Cl]. However, when 2-(2′,6′-diisopopylphenylazo)-4-methylphenol reacts with [Rh(PPh 3 ) 3 Cl], an interesting migration of an isopropyl group from its original location (say the 2′ position) to the corresponding third position (say the 4′ position) takes place. In all the three complexes the modified azo ligands bind to rhodium as dianionic C,N,O-donors, and two triphenylphosphines and a chloride are also coordinated to the metal center. Structures of all three complexes have been determined by X-ray crystallography. These complexes show characteristic 1 H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on these complexes shows a Rh(III)-Rh(IV) oxidation on the positive side of SCE and a reduction of the coordinated azo ligand on the negative side.

Rhodium complexes of 1,3-diaryltriazenes: Usual coordination, N–H bond activation and, N–N and C–N bond cleavage

“…This distortion is particularly attributable to the strain imposed by formation of the four-membered chelate rings with relatively small bite angles (61.5(4)°and 58.8(3)°) by the triazene ligands. The Rh-P and Rh-C distances are all quite normal, and so are the Rh-N distances [2][3][4][5]9,12], except the Rh(1)-N(1B) distance, which is slightly longer than usual. The observed elongation of the Rh(1)-N(1B) bond, which is trans to the Rh(1)-C(1C) bond, may be due to the trans effect of the coordinated 4-methylphenyl fragment [43][44][45].…”

“…[Rh(PPh 3 ) 3 Cl], is famous for its catalytic application in bringing about numerous interesting chemical transformations [1], we have found it to be a useful synthon for the preparation of mixed-ligand complexes of various types [2][3][4][5][6][7][8][9][10][11][12][13]. Besides its demonstrated ability to accommodate new ligands, it has also been found to mediate many interesting reactions, such as C-H and C-C bond activation of organic molecules [2,5,9,12], activation of molecular oxygen [13], and transformation of coordinated ligands [5,9,11]. These interesting observations have prompted us to further continue our exploration on reaction of the Wilkinson's catalyst with organic ligands, and the present study has originated from this exploration.…”

Interaction of 2-(2′,6′-dialkylphenylazo)-4-methylphenols with iridium. C–H activation and migration of alkyl group

“…[Ir(PPh 3 ) 3 Cl] was synthesized by following a reported procedure [46]. The 2-(2 0 ,6 0 -diethylphenylazo)-4-methylphenol and 2-(2 0 ,6 0 -diisopropylphenylazo)-4-methylphenol were prepared following reported procedures [53]. Purification of dichloromethane and acetonitrile, and preparation of tetrabutylammonium perchlorate (TBAP) for electrochemical work were performed following reported procedures [67,68].…”

“…Metal promoted C-H and C-C bond activation of organic molecules has thus been of significant current interest [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. We have been active, for quite some time, in the synthesis of organometallic complexes via platinum-group metal mediated C-H bond activation of organic ligands in general [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] and of 2-(arylazo)-4-methylphenols (I) in particular [44][45][46][47][48][49], and the present work has emerged from our continued interest in this area. In this study iridium has been selected as the platinum-group metal for promoting C-H or C-C bond activation and [Ir(PPh 3 ) 3 Cl] has been utilized as the iridium starting material.…”

“…Initially, reaction of 2-(2 0 ,6 0 -dimethylphenylazo)-4-methylphenol (L 1 ) was carried out with [Ir(PPh 3 ) 3 Cl], where one of the two ortho methyl groups was found to undergo C-H activation to afford organoiridium complexes (1 and 2) [52]. Although an ortho methyl group in ligand L 1 was observed to undergo interesting elimination [53], migration [51] and oxidative migration [50], such a metal-carbon bond formation via methyl C-H bond activation of ligand L 1 was not experienced. It is also noteworthy that methyl C-H activation is less common than phenyl C-H activation [54][55][56][57][58][59][60][61][62].…”