Mechanistic studies on rhodium-N-heterocyclic carbene catalysts

Resumen del trabajo presentado a la 11th Inorganic Chemistry Conference y al 1st Meeting of the Inorganic and Bioinorganic Chemistry Division of SPQ, celebradas en Sintra (Portugal) del 7 al 8 de octubre de 2016.-- Prémio Luso-Espanhol de Química 2015.The catalytic activity of a set of rhodium complexes with N-heterocyclic carbene (NHC) ligands in three specific homogeneous reactions, vinyl selective H/D exchange, alkyne hydrothiolation and alkyne hydrophosphination, has been studied. The high steric hindrance and powerful electron-donor capacity of the bulky NHC´s used, along with ancillary N-donor ligands, seems to be determinant to get selective transformations and to facilitate valuable information about the mechanism of the mentioned reactions. Rhodium(III)-NHC complexes containing quinolinato or acetonitrile ligands are active and selective catalysts for the H/D exchange of aromatic α-olefins, using CD3OD as deuterium source. Most of these complexes resulted to be selective in the vinylic-H/D exchange of styrene without the concomitant deuteration of the aromatic region, being able to deuterate the vinylic β-positions with very high selectivity. The proposed mechanism implies an initial H/D exchange, a 1,2 or 2,1 insertion of the coordinated olefin on the Rh-D bond, to give linear or branched alkyl products, followed by rotation and β-elimination. Interestingly, the steric constraints exerted by the bulky IPr NHC ligand (IPr = 1,3-bis(2,6-diisopropyl-phenyl)imidazol-2-carbene) control the rotation of the alkyl intermediate, which in turn determines the selectivity towards H/D exchange at the β-position of aromatic α-olefins. Rhodium(I) compounds of formula [Rh(μ-X)(IPr)(η2-olefin)]2 (X = Cl, OH), RhCl(IPr)(py)(η2-olefin) and Rh(oq)(IPr)(η2-olefin) (py = pyridine, oq = quinolinolate) are very active catalysts for alkyne hydrothiolation under mild conditions, presenting high selectivity towards α-vinyl sulphides. Several intermediates relevant for the catalytic process have been detected. Most of the studied rhodium carbene catalysts have in common a mechanism that proceed via oxidative addition of the S-H bond to rhodium(I) intermediates and successive alkyne insertion into the Rh-S, or Rh-H, bond followed by reductive elimination steps. The [Rh(μ-Cl)(IPr)(η2-cyclooctene)]2 complex has resulted to be an efficient catalyst precursor for the addition of diphenylphosphine to terminal alkynes. Interestingly, this complex is able to catalyze the regioselective double hydrophosphination of a wide range of terminal aromatic or aliphatic alkynes. The distinctive stereoelectronic properties of the NHC ligand prevent the catalyst poisoning by diphosphine coordination thereby allowing for the closing of a productive catalytic cycle. The process is initiated by ligand exchange between diphenylphosphine and cyclooctene and cleavage of the chlorido bridges to give a mononuclear phosphine complex, followed by oxidative addition of the P-H bond to generate a terminal phosphido Rh(III) hydride species.Peer Reviewe