Iodide effects in transition metal catalyzed reactions

Abstract

The unique properties of I(-) allow it to be involved in several different ways in reactions catalyzed by the late transition metals: in the oxidative addition, the migration, and the coupling/reductive elimination steps, as well as in substrate activation. Most steps are accelerated by I(-)(for example through an increased nucleophilicity of the metal center), but some are retarded, because a coordination site is blocked. The "soft" iodide ligand binds more strongly to soft metals (low oxidation state, electron rich, and polarizable) such as the later and heavier transition metals, than do the other halides, or N- and O-centered ligands. Hence in a catalytic cycle that includes the metal in a formally low oxidation state there will be less tendency for the metal to precipitate (and be removed from the cycle) in the presence of I(-) than most other ligands. Iodide is a good nucleophile and is also easily and reversibly oxidized to I(2). In addition, I(-) can play key roles in purely organic reactions that occur as part of a catalytic cycle. Thus to understand the function of iodide requires careful analysis, since two or sometimes more effects occur in different steps of one single cycle. Each of these topics is illustrated with examples of the influence of iodide from homogeneous catalytic reactions in the literature: methanol carbonylation to acetic acid and related reactions; CO hydrogenation; imine hydrogenation; and C-C and C-N coupling reactions. General features are summarised in the Conclusions.

New Synthesis of Furans: the Rhodium‐Catalysed Carbonylative Addition of Arylboronic Acids to Propargylic Alcohols/ Cyclisation Sequence

“…The structure of 2-phenylfuran was confirmed by 1 H and 13 C NMR spectroscopy of the product isolated by silica gel column chromatography.…”

“…Although this effect cannot be easily rationalised, it is noteworthy that the use of an iodide ligand has led to important improvements in several catalytic reactions. [13] Although no oxidation reaction is involved in our case, we believe that the action of iodide as ligand versus rhodium is the source of the improved selectivity. Yields and conversions were determined by GC using undecane as internal standard.…”

Platinum‐Catalyzed Intermolecular Hydroamination of Alkenes: Halide‐Anion‐Promoted Catalysis

“…[62] The beneficial effect of molecular iodine (generally used in large amounts) has also been observed in some transition-metal-catalyzed reactions. [69] This proved to be the case also for the Pt II -catalyzed hydroamination of 1-hexene.…”

“…Such halide ion effects have already been observed, but are generally poorly understood, [59][60][61] except in some cases. [62] A possible explanation for the promoting role of nBu 4 PBr is outlined below (Scheme 3). For simplicity, the alkene considered is ethylene and the [nBu 4 P] + cation has been omitted.…”

Palladium‐catalyzed unstrained C(sp3)―N bond activation: the synthesis of N,N‐dimethylacetamide by carbonylation of trimethylamine

“…For example, adding iodide ions result in pronounced promotion in an amazing variety of Pd-catalyzed C-C coupling reactions and CH 3 OH carbonylation reactions. [23] Some researchers have investigated the beneficial influence of iodine-containing promoters in the carbonylation of Me 3 N. One report in the literature achieved 72% conversion of Me 3 N and 56% yield of DMAc when using RhCl 3 as a catalyst in the presence of MeI as a promoter, but obtained only a small amount of DMAc without MeI.…”