Dihydrido Iridium Triisopropylphosphine Complexes: From Organometallic Chemistry to Catalysis

Abstract

2.1 IntroductionThe impressive developments of organometallic chemistry have allowed the preparation of a wide variety of soluble metal complexes useful for organic transformations under mild conditions. There is no question that much of the general knowledge on homogeneous catalysis has stemmed from studies of organometallic chemistry and homogeneous hydrogenation. Following the discovery of Wilkinson ' s hydrogenation catalyst, RhCl(PPh 3 ) 3 , in 1964 [1] -which is by far the most -studied homogeneous catalyst -Schrock and Osborn reported the catalytic potential of [Rh(diene)L 2 ] + complexes as hydrogenation catalysts. Under hydrogen, the diene is hydrogenated to generate the reactive [RhH 2 S 2 (PR 3 ) 2 ] + species which, in some cases, can be isolated relatively easily from coordinating solvents (S) such as acetone or ethanol [2] . In contrast to Wilkinson ' s catalyst, a large number of donor ligands can be used, while the use of a PR 3 : Rh ratio of 2 : 1 avoids the need for phosphine dissociation which is required with Wilkinson ' s catalyst.Interestingly, cationic diene iridium complexes of general formula [Ir(COD)(PR 3 ) 2 ] + (COD = 1,5 -cyclo -octadiene), mainly developed by Crabtree, may be even more active than the cationic diene rhodium complexes, particularly when dichloromethane ( DCM ) is used as solvent. The solvent is less coordinating and must be easily displaced by the substrates. Furthermore, its polarity favors the solubility of the ionic catalyst. As in the case of rhodium systems, [IrH 2 S 2 (PR 3 ) 2 ] + species can be observed by hydrogenation of the diene [3,4] . Another important discovery was the very active mixed ligand complex [Ir(COD)(py)(PCy 3 )] + (py = pyridine), usually known as Crabtree ' s catalyst, that in DCM as solvent effectively hydrogenates tetrasubstituted amido -alkenes [3] . The related benzonitrile (bzn) derivatives [Ir(COD)(bzn)(PR 3 )] + (PR 3 = PCy 3 or the chiral phosphines, neomenthyldiphenylphosphine or phenyl -( o -methoxyphenyl)methylphosphine) are also very active catalysts for the hydrogenation of prochiral didehydro amino acid derivatives [5] , although poor enantiomeric excess ( ee ) values are obtained with chiral monodentate phosphines. The effectiveness of the [Ir(COD)(L)(PR 3 )] + cations for

Iridium-Catalyzed Synthesis of Chiral 1,2,3-Triazoles Units and Precise Construction of Stereocontrolled Oligomers

“…Iridium-catalyzed transformations have evolved as one significant part in organic synthesis [1,2]. Especially noteworthy is the cycloaddition of unsaturated motifs under iridium catalysis, some of which afford carbocyclic or heterocyclic skeletons in unique manners [3].…”

“…As a continuation of our research in the precise synthesis of macromolecules [31][32][33][34], here we report the construction of an improved stereocontrolled polytriazole architecture from novel chiral triazole units that are prepared by IrAAC (Scheme 1B). To demonstrate the tolerance of this stereoregular skeleton for functionalities, three kinds of oligomers bearing different side chains were precisely fabricated, the structure and monodispersity of which were well characterized by 1 H NMR, size-exclusion chromatography (SEC) and mass spectrometry (MS) analysis. It is noteworthy that in comparison with the previous work, this newly introduced architecture was established through one metal-free exponential growth strategy at a higher efficiency.…”

Theoretical Studies on the Mechanism of Iridium-Catalyzed Alkene Hydrogenation by the Cationic Complex [IrH2(NCMe)3(PiPr3)]+

“…The rate-determining step of the process seems to be the reductive elimination of ethane, which is the only reaction that requires temperatures above 273 K to proceed . When the alkene used was propene, a very stable allyl-hydrido complex, [IrH(η 3 -C 3 H 5 )­(NCMe) 2 ­(P i Pr 3 )]BF 4 , was fully characterized, suggesting that this compound could be just a resting state (Scheme b) and that a possible path involving the reaction of [IrH( n Pr)­(NCMe) 3 ­(P i Pr 3 )]BF 4 with hydrogen via iridium(V) species could be an alternative pathway . According to Scheme , all the species detected contain the iridium center in oxidation states I and III, although, on the basis of some experimental observations, we have recently speculated about the possibility of a cycle involving Ir(III)/Ir(V) intermediates .…”

“…When the alkene used was propene, a very stable allyl-hydrido complex, [IrH(η 3 -C 3 H 5 )­(NCMe) 2 ­(P i Pr 3 )]BF 4 , was fully characterized, suggesting that this compound could be just a resting state (Scheme b) and that a possible path involving the reaction of [IrH( n Pr)­(NCMe) 3 ­(P i Pr 3 )]BF 4 with hydrogen via iridium(V) species could be an alternative pathway . According to Scheme , all the species detected contain the iridium center in oxidation states I and III, although, on the basis of some experimental observations, we have recently speculated about the possibility of a cycle involving Ir(III)/Ir(V) intermediates . Although three intermediate species have been identified working at three different temperatures, the mechanistic details for the ethene hydrogenation using this catalyst are still unknown.…”

“…Although three intermediate species have been identified working at three different temperatures, the mechanistic details for the ethene hydrogenation using this catalyst are still unknown. In the past few years, theoretical studies on alkene hydrogenation reactions proposed different possible mechanism through Ir(I)/Ir(III) or Ir(III)/Ir(V) intermediates …”