Arene C–H activation using Rh(i) catalysts supported by bidentate nitrogen chelates

Abstract

The RhIJI) complexes [( The synthesis of catalysts for the selective and efficient functionalization of C-H bonds remains a challenge. [1][2][3][4][5][6][7][8][9] In their seminal work, Shilov and co-workers found that Pt salts can activate and functionalize the C-H bonds of methane, resulting in oxidation to methanol and methyl halides when Pt IV is used as oxidant. [10][11][12] Numerous studies have sought to understand and improve the Shilov catalyst, 13-18 and several electrophilic late transition metal and main group reagents have been developed for light alkane functionalization. 19-25These electrophilic catalysts are often inhibited by Lewis bases and, as a result, typically require superacidic media (e.g., oleum). The use of strong acid (HX) can lead to the formation of RX, and the electron withdrawing group "X" can protect the functionalized product from over-oxidation (e.g., MeOSO 3 H formation in H 2 SO 4 ); but, product extraction from strong acids can be problematic. Less electronegative metal centres should be less susceptible to inhibition by Lewis bases. Thus, moving to the left in the transition metal series provides a strategy to attenuate inhibition of catalysis in weaker acids. Earlier transition metal complexes can activate hydrocarbons.26-33 However, transition metals earlier than group 10 are likely to be more susceptible to oxidation in acidic media, which could place the catalyst in an oxidation state that is incapable of C-H activation (Scheme 1). Thus, a desirable but challenging aspect of developing catalysts using acidic solvents is maintaining efficient C-H activation. Rh catalysts provide a possible alternative to later metal and main group counterparts since C-H activation by RhIJI) complexes has been reported,

Computational Studies of Heteroatom‐Assisted CH Activation at Ru, Rh, Ir, and Pd as a Basis for Heterocycle Synthesis and Derivatization

“…More recently, further computational studies of the C-H activation of benzene at Ir [71] and Rh [72] have appeared, while Jones and coworkers compared aryl C(sp 2 )-H bond activation and alkyl C(sp 3 )-H bond activation …”

DFT Mechanistic Study of Methane Mono-Esterification by Hypervalent Iodine Alkane Oxidation Process

“…[44] For instance, we have characterized a number of rhodium complexes with C-H activation ability in TFAH. [45][46][47][48][49] Recently, we have reported the ability of hypervalent iodine complexes to oxidize light alkanes (methane, ethane and propane) under mild conditions. Hypervalent iodine complexes were reported to activate C-H bonds in an electrophilic 2-electron oxidation.…”

“…Hence, it is expected the same mechanism with improved yield due perhaps to the improved acidity of the system. − and HSO 4 − , like Cl − and I − , are weak bases in TFAH, but S 2 O 7 2− and SO 4 2− are favorable in reacting with TFAH and are thus protonated (equations[45][46][47][48].…”

Electrophilic RhI catalysts for arene H/D exchange in acidic media: Evidence for an electrophilic aromatic substitution mechanism

“…Recently, a number of catalysts based on Ir and Rh have been shown to be active for benzene C−H activation [50][51][52][53][54][55][56][57][58][59][60][61]. Rhodium complexes are particularly attractive due to the possibility of C−H activation by Rh I [62] or Rh III [55]. Further, oxidation of the rhodium center can be achieved with air recyclable Cu(II) salts, which provides a strategy for indirect use of dioxygen or air as the terminal oxidant [60,63].…”

“…Recently, two Rh I complexes bearing neutral bidentate nitrogen donors were shown to be catalysts for the rapid H/D exchange between benzene and trifluoroacetic acid [62]. The most active catalyst for aromatic H/D exchange, (FlDAB)Rh(COE)(TFA) (FlDAB = N,N'-bis-(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene, COE = cyclooctene, TFA = trifluoroacetate) possesses easily modulated aryl groups and therefore was chosen for a study of the impact of substituent variation on catalytic benzene H/D exchange reactions.…”