Catalytic Arene H/D Exchange with Novel Rhodium and Iridium Complexes

Abstract

Three novel pendant acetate complexes, [Rh­(bdmpza)­Cl3]−M+, [Rh­(bdmpza)­Cl2(py)], and [Ir­(bdmpza)­Cl3]−M+ (bdmpza = bis­(3,5-dimethylpyrazol-1-yl) acetate, M+ = Li+, Na+), were synthesized. Abstraction of halide from these complexes with silver salts yielded species capable of C–H activation of arenes. The catalytic H/D exchange reaction between benzene and trifluoroacetic acid-d was optimized, and these conditions were used to evaluate H/D exchange in other arenes. Branched alkyl substituents in alkyl aromatics showed an affinity toward deuterium exchange in the β-alkyl position only. DFT calculations were performed to determine the mechanism of H/D exchange.

Highlights of C (sp2)–H hydrogen isotope exchange reactions

“…In contrast to ruthenium, there have only been a couple of examples of Rh‐catalysed HIE published since the 2010 special edition of this journal. One such example on the use of Rh catalysts for C–H activation of arenes was reported by Rinehar et al In this case, the complexes [Rh (bdmpza)Cl 3 ] − Na + and [Rh (bdmpza)Cl 2 (py)] (bdmpza = bis(3,5‐dimethylpyriazol‐1‐yl acetate) (Figure 28) were shown in the presence of silver salts and CF 3 CO 2 D to be capable of promoting H–D exchange in simple arene substrates.…”

C−H Functionalisation for Hydrogen Isotope Exchange

“…[132,133,134] More specifically, the bis(3,5dimethylpyrazol-1-yl) acetate (bdmpza) ligand, 141, delivered an anionic Rh(III)-centred pre-catalyst, 142, that was able to deuterate both aryl and β-alkyl positions of substrates bearing no strong directing group (143→143a; Scheme 43). [139] Under identical conditions, the parent RhCl3.H2O was more selective for aryl deuteration (143→143b). A combined experimental and DFT investigation into the operative reaction mechanism using142 revealed that aryl labeling was kinetically favoured over alkyl labeling by 9.0 kcal/mol, via the shared catalyst resting state, 144.…”

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

“…Developing reactivity with earlier transition metals (i.e., earlier than the Ni, Pd and Pt triad) could provide more tolerance of Lewis basic groups, but major challenges include avoiding oxidation of the metal to higher valent states that are incapable of C-H activation and developing complexes that possess electrophilic hydrocarbyl ligands (after C-H activation) [42][43][44][45][46][47][48][49][50][51][52][53]. 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].…”