Iridium complexes hydrogen

The asymmetric hydrogenation of C=N (Eqn. (23)), in contrast with C=0 and C=C bonds, is much less developed. Hexahydrowoquinoline was used as its phosphoric acid salt. Iridium-ferrocenyl complexes were found to be sati.sfactory. After optimisiation, Meyer et al. (1997) were able to realize an enantioselectivity of 89% ee. 

Enantioselectivities of up to 47% ee were reported by Ruiz et al. in 1997 for the asymmetric hydrogenation of various prochiral dehydroamino acid derivatives and itaconic acid by using iridium cationic complexes of the novel chiral... 

Iridium(III) hydride forms complexes with DIOP, BDPP (2,4-bis(diphenyl-phosphino)pentane), NORPHOS, and BINAP ligands to produce amines in 11 -80% ee.679 Similar modest results are obtained in the reduction of N-arylketimines with an iridium(HI) complex with (2S,3 S) -C HIRA PHOS as the chiral ligand.680 The indium complexes with chiral phosphinodihydrooxazoles catalyze the enantioselective hydrogenation of imines in supercritical carbon dioxide with up to 80% ee, but generally lower ee values are observed in... 

The main species in solution has been identified to be the hydrido-alkynyl complex [IrH(C2Ph)(cod)(//2-iPrPCH2CH2OMe)]+BF4 (23). This is, however, only a sink that results from direct reaction of 22 with the 1-alkyne, draining the active catalyst from the system. The catalysis proceeds via the dihydrido-diene intermediate [IrH2(cod)(//2- PrPCH2CH2OMe)]+ BF4 (24), which reacts reversibly with the alkyne to yield the hydrido-iridium-styryl complex 25, followed by a rate-determining reaction of this hydrido-vinyl species with hydrogen to re-... 

The first application of a heterocyclic carbenoid achiral ligand for hydrogenation of alkenes was reported in 2001 by Nolan and coworkers. Both ruthenium [36] and iridium [37] complexes proved to be active catalysts. Turnover frequency (TOF) values of up to 24000 b 1 (at 373 K) were measured for a ruthenium catalyst in the hydrogenation of 1-hexene. 

Since there are unresolved issues in the fine detail of reaction mechanism, it is worth recalling an earlier publication on reactive intermediates in iridium hydrogenation [61]. In general, conventional Ir diphosphine complexes turnover slowly or not at all when enantioselective hydrogenation of standard substrates is attempted, and essentially all the practical and useful recent synthetic contri-... 

Brunner, Leitner and others have reported the enantioselective transfer hydrogenation of alpha-, beta-unsaturated alkenes of the acrylate type [50]. The catalysts are usually rhodium phosphine-based and the reductant is formic acid or salts. The rates of reduction of alkenes using rhodium and iridium diamine complexes is modest [87]. An example of this reaction is shown in Figure 35.8. Williams has shown the transfer hydrogenation of alkenes such as indene and styrene using IPA [88]. 

An iridium(I) complex with the l,2-bis(tcrt-butylmethylphosphino)ethane (4) and tetrakis(3,5-bis(trifluoromethyl)phenyl)borate as the counter anion catalyzes the hydrogenation of several acyclic aromatic Ai-arylimines under atmospheric hydrogen pressure at room temperature, giving the desired chiral amines with high-to-excellent enantioselectivities (up to 99%, Fig. 6) [19]. The authors also tested (S )-BINAP (Fig. 1) and (/ )-Ph-PHOX (PHOX = 2-[2-(diphenylphosphino) phenyl]-4,5-dihydrooxazole) hgands with lower enantioselectivities [19]. Both steric and electronic properties of the ligand and the combination with the BArF anion are in the base of the efficacy of this catalytic system. On the other hand, attempted hydrogenations of Ai-(2,2,2-trifluoro-l-phenylethylidene)aniline and M-(l,2,2-trimethyl-propylidene)aniline under the same conditions resulted in... 

The catalyst is also effective for the reduction of styrenes, ketones, and aldehydes. Cyclohexenone 16 was reduced to cyclohexanone 11 by transfer hydrogenation, and using a higher catalyst loading, styrene 17 was reduced to ethylbenzene 18. The elaboration of [Ir(cod)Cl]2 into the triazole-derived iridium carbene complex 19 provided a catalyst, which was used to reduce aUcene 20 by transfer hydrogenation [25]. 

In spite of the success of asymmetric iridium catalysts for the direct hydrogenation of alkenes, there has been very limited research into the use of alternative hydrogen donors. Carreira and coworkers have reported an enantioselective reduction of nitroalkenes in water using formic acid and the iridium aqua complex 69 [66]. For example, the reduction of nitroalkene 70 led to the formation of the product 71 in good yield and enantioselectivity (Scheme 17). The use of other aryl substrates afforded similar levels of enantioselectivity. 

The Dihydrido Iridium Triisopropylphosphine Complex [lrH2(NCMe)3(PPr3)]BF4 as Alkene Hydrogenation Catalysts... 

Dihydrido iridium Triisopropylphosphine Complexes as Imine Hydrogenation Catalysts... 

The dimerization of functional alkenes such as acrylates and acrylonitrile represents an attractive route to obtain bifunctional compounds such as dicarboxylates and diamine, respectively. The head-to-tail dimerizahon of acrylates and vinyl ketones was catalyzed by an iridium hydride complex generated in situ from [IrCl(cod)]2 and alcohols in the presence of P(OMe)3 and Na2C03 [26]. The reaction of butyl acrylate 51 in the presence of [IrCl(cod)]2 in 1-butanol led to a head-to-tail dimer, 2-methyl-2-pentenedioic acid dibutyl ester (53%), along with butyl propionate (35%) which is formed by hydrogen transfer from 1-butanol. In order to avoid... 

The transfer (de)hydrogenation is fully reversible-that is, the iridium pincer complexes catalyze the alkane dehydrogenation as well as the alkene hydrogenation. 

Attempts to turn this acetylene dimerization reachon into a catalyhc polymeriza-hon process have failed thus far. In the presence of excess phenylacetylene, the iridium(I) complex 23 activates another Caikynyi—H bond and hansforms, after a hydrogen shift, to the stable (vinyl)(alkynyl) iridium(III) system 27 (Equahon 12.10). 

A resolution of racemic CHIRAPHOS ligand has been achieved using a chiral iridium amide complex (Scheme 8.3). The chiral iridium complex (- -)-l reacts selectively with (S.S -CHIRAPHOS to form the inactive iridium complex 2. The remaining (R,R)-CHIRAPHOS affords the catalytically active chiral rhodium complex 3. The system catalyzes asymmetric hydrogenation to give the (5)-product with 87% ee. The opposite enantiomer (—)-l gives the (R)-product with 89.5% ee, which is almost the same level of enantioselectivity obtained by using optically pure (5,5)-CHlRAPHOS. 

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