A selected diphosphine binuclear gold(I) chloride complex was combined with a silver salt to catalyze efficiently, for the first time, the asymmetric intramolecular hydroamination of alkenes with high conversions and enantioselectivities, in mild conditions and in presence of water. Both enantiomers of the products could be obtained by controlling the molecular ion-pairs through the solvent polarity. The gold(I) cationic active species was characterized unambiguously at the solid state by X-ray analysis and in solution by DOSY 1 H NMR experiments. No contribution of silver chloride was observed on the bonding mode of the catalyst.The hydroamination of unactivated alkenes is the shortest synthetic route to secondary and tertiary amines. [1] For the enantioselective synthesis of optically pure amines, the most studied and privileged hydroamination method is metal catalysis. [1c, 2-6] Throughout recent years, the usefulness of gold has been applied to various CÀC multiple bond substrates like alkynes, alkenes, allenes, and dienes for both intra-and intermolecular hydroamination reactions. [7] As high temperatures, long reaction times, and strict conditions are generally required, the gold catalyzed hydroamination of alkenes has been scarcely studied in its asymmetric version. [7, 8] To the best of our knowledge, only four specific reports have been published so far. First, binuclear gold(I) catalysts were found to be active for intermolecular hydroamination of ethylene and 1-alkenes with cyclic ureas leading to high yields and enantioselectivities. [8d] Second, binuclear gold(I) species catalyzed intramolecular hydroamination of N-alkenyl ureas at room temperature with good yields and average enantioselectivities. [8c] A third work reported on the preparation and the use of several monodentate axially-chiral N-heterocyclic carbene and phosphine ligands for mononuclear gold(I) catalyzed intramolecular hydroamination of N-alkenyl tosylates. Moderate yields and enantioselectivities were obtained at quite high temperatures and reaction times. [8b] Finally, following our ongoing interest in hydroamination reactions, [9] we recently reported on mononuclear and binuclear gold(I) complexes and found phosphoramidite ligands could lead to valuable catalysts for the intramolecular hydroamination of several alkenes at mild temperatures, with good conversions and average enantioselectivities. [8a] Like many organometallics, most gold complexes require the combined use of silver salts to be activated. Indeed, reactive gold cationic species can be generated by abstraction of halides from neutral complexes. However, the existence of these so-called "naked gold complexes" [AuL] is still discussed for lack of structural evidence as isolable and well-defined species. [10, 11c] Recent works have reported on the influence of silver salts on gold catalyzed reactions. [11] Impact on conversions and selectivities were observed and the interference of silver and chloride has been highlighted by several bonding modes for cationic m...
The intramolecular gold-catalyzed asymmetric hydroamination of allenes was studied by screening a series of mononuclear gold(I) and -(III) complexes in combination with silver salts. Among the various chiral monophosphine and diaminocarbene ligands tried, the best catalysts arose from mononuclear gold(I) complexes synthesized from BINOL-based phosphoramidite ligands. The latest were improved by addition of bulky substituents at specific positions of the BINOL scaffold. The resulting gold(I) complexes were combined with selected silver salts to afford efficient catalysts for intramolecular hydroamination of allenes at room temperature or below, with good conversions and enantioselectivities.
The selective and asymmetric addition of amines across alkene C=C bonds (hydroamination of alkenes) is one of the most important tool to synthesise chiral amines. Though this reaction remains a challenge due to its high energy gap and the difficult control of the enantioselection, some chiral organometallic catalysts can provide promising activities and enantioselectivities. This minireview highlights the recent use of chiral organometallic complexes in homogeneous catalysed asymmetric hydroaminations of non-activated alkenes from 2013 to 2016.
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