Mechanistic Investigations of Imine Hydrogenation Catalyzed by Dinuclear Iridium Complexes

Abstract

Treatment of [Ir2(mu-H)(mu-Pz)2H3(NCMe)(PiPr3)2] (1) with one equivalent of HBF4 or [PhNH=CHPh]BF4 affords efficient catalysts for the homogeneous hydrogenation of N-benzylideneaniline. The reaction of 1 with HBF4 leads to the trihydride-dihydrogen complex [Ir2(mu-H)(mu-Pz)2H2(eta2-H2)(NCMe)(PiPr3)2]BF4 (2), which has been characterized by NMR spectroscopy and DFT calculations on a model complex. Complex 2 reacts with imines such as tBuN=CHPh or PhN=CHPh to afford amine complexes [Ir2(mu-H)(mu-Pz)2H2(NCMe){L}(PiPr3)2]BF4 (L = NH(tBu)CH2Ph, 3; NH(Ph)CH2Ph, 4) through a sequence of proton- and hydride-transfer steps. Dihydrogen partially displaces the amine ligand of 4 to form 2; this complements a possible catalytic cycle for the N-benzylideneaniline hydrogenation in which the amine-by-dihydrogen substitution is the turnover-determining step. The rates of ligand substitution in 4 and its analogues with labile ligands other than amine are dependent upon the nature of the leaving ligand and independent on the incoming ligand concentration, in agreement with dissociative substitutions. Water complex [Ir2(mu-H)(mu-Pz)2H2(NCMe)(OH2)(PiPr3)2]BF4 (7) hydrolyzes N-benzylideneaniline, which eventually affords the poor hydrogenation catalyst [Ir2(mu-H)(mu-Pz)2H2(NCMe)(NH2Ph)(PiPr3)2]BF4 (11). The rate law for the catalytic hydrogenation in 1,2-dichloroethane with complex [Ir2(mu-H)(mu-Pz)2H2(OSO2CF3)(NCMe)(PiPr3)2] (8) as catalyst precursor is rate = k[8]{p(H2)}; this is in agreement with the catalytic cycle deduced from the stochiometric experiments. The hydrogenation reaction takes place at a single iridium center of the dinuclear catalyst, although ligand modifications at the neighboring iridium center provoke changes in the hydrogenation rate. Even though this catalyst system is also capable of effectively hydrogenating alkenes, N-benzylideneaniline can be selectively hydrogenated in the presence of simple alkenes.

Mechanistic analysis of the transition metal-catalyzed hydrogenation of imines and functionalized enamines

“…These reports by James [24][25][26][27][28][29][30][31][32][33], Bianchini [34,35], Oro [36][37][38][39] …”

“…In addition, Oro et al have studied the reactivity of the neutral triflate species 24 [Ir 2 (-H)(-pz) 2 [39]. They have substituted acetonitrile with L CO, 2 -C 2 H 4 or pyridine and evidenced on the catalytic hydrogenation rates a strong influence of the L ligand in trans-position to the bridging hydride.…”

“…This latter complex hydrogenates the resulting iminium molecule affording [Ir 2 (-H)(-pz) 2 (H) 2 (CH 3 CN){ 1 -NHPh(CH 2 Ph)}(PiPr 3 ) 2 ] + 20. From the isolation and characterization of these complexes, the authors proposed the simplified catalytic cycle shown in Scheme 12 [39].…”

“…The hydrogenation of imines has been also achieved by the use of a dinuclear iridium cationic precursor [39]. It results from the pro- tonation by HBF 4 of the neutral tetrahydride complex […”

Imine Hydrogenation

“…Finally, the hydrogenated product is produced by simultaneous H + /H − transfer to the imine from the NH moiety of the arenecoordinated amine ligand and the metal -H, respectively. A close ionic outer -sphere mechanism has also been postulated by the same authors for the dinuclear [Ir 2 ( µ -H)( µ -Pz) 2 H 3 (NCMe)(P -i -Pr 3 ) 2 ] in an attempt to explain the effi ciency of the hydrogenation of imines in a versatile dinuclear environment [77] . Despite the apparent coincidence of mechanistic features between the mononuclear and the dinuclear catalytic precursor, the postulated catalytic cycles have no elementary step in common, which illustrates the variety of mechanistic resources available for the ionic hydrogenation of imines.…”