Iridium dihydride species

In these reactions, the major diastereomer is formed by the addition of hydrogen syn to the hydroxyl group in the substrate. The cationic iridium catalyst [Ir(PCy3)(py)(nbd)]+ is very effective in hydroxy-directive hydrogenation of cyclic alcohols to afford high diastereoselectivity, even in the case of bishomoallyl alcohols (Table 21.4, entries 10-13) [5, 34, 35]. An intermediary dihydride species is not observed in the case of rhodium complexes, but iridium dihydride species are observed and the interaction of the hydroxyl unit of an unsaturated alcohol with iridium is detected spectrometrically through the presence of diastereotopic hydrides using NMR spectroscopy [21]. 

Scheme 3 Selective formation of specific iridium dihydride species due to electronic and steric differentiation...

Complexes 6 undergo the second migratory insertion in this scheme to form the acyl complexes 7. Complexes 7 can react either with CO to give the saturated acyl intermediates 8, which have been observed spectroscopically, or with H2 to give the aldehyde product and the unsaturated intermediates 3. The reaction with H2 involves presumably oxidative addition and reductive elimination, but for rhodium no trivalent intermediates have been observed. For iridium the trivalent intermediate acyl dihydrides have been observed [29], The Rh-acyl intermediates 8 have also been observed [26] and due to the influence of the more bulky acyl group, as compared to the hydride atom in 2e and 2a, isomer 8ae is the most abundant species. 

The reaction with dihydrogen could not be tracked down in this study. Iridium forms much more stable complexes than rhodium and therefore it has been studied to obtain more details about the complexes and the mechanism of rhodium hydroformylation. Using an iridium complex, Eisenberg and Deutsch [34] succeeded in the identification by NMR of all species involved in the catalytic cycle the ethyl complex (i) (Fig. 6.8) the propionyl complex (j) (including X-ray structure determination), and, moreover, the dihydride adduct (k). The latter... 

As recounted, these studies demonstrate that two of the three expected intermediates in asymmetric hydrogenation may be directly observed, but the expected dihydride is too fleeting. There are two further experiments which are pertinent to this issue. A related diphosphine-iridium alkene complex reacts with dihydrogen at low temperatures and a series of alkene dihydrides are observed prior to the formation of the expected alkyl hydride. Based on the H-NMR chemical shifts of the respective Ir-H species, the initial addition (or to be more correct the initially observed species) possesses H trans to alkene and H trans to phosphine only at higher temperatures does this rearrange to the expected H trans to amide and H trans to phosphine structure (Fig. 9a) [36]. A more directly relevant experiment involves para-enriched hydrogen, and in the illustrated case a transient dihydride is observed. A problem is that the spectral characteristics are not entirely in accord with expectations for the proposed structure (the supposed trans-P-Rh-H coupHng is 4 Hz rather than ca. 120 Hz), but the presence of some transient Rh dihydride is definitive based on the evi-... 

The diruthenium(O) complexes [Ru2(CO) J,-(RO)2PN(Et)P(OR)2 2] (3, R = Me, Pr) (Scheme 2) have been reported to react with dihydrogen affording l,2-dihydrides.[13] Despite the formal similarity between these compounds and the previously described iridium and rhodium dppm derivatives, they display different coordination environments around the metal atoms. This could be related to the fact that, unlike the typical planar structure of most mononuclear d species, the unsaturated zerovalent ruthenium fragments adopt a structure which resembles a trigonal bipyramid with an empty equatorial site. [14]... 

The signal enhancements of PHIP make possible the observation of previously unknown species. An excellent example is the addition ofp-Viz to /r< //i -RhCl(GO)(PPh3)2 18. While its iridium analog (Vaska s complex) oxidatively adds H2 to form a dihydride complex, the corresponding reaction with 18 had not been reported. However, a NMR spectrum of complex 18 under 3 atm P-H2 revealed two hydride signals at —19.31 and —19.66 ppm. Significant to the interpretation of this result were the similarity of chemical shifts of the two hydride resonances and the lack of additional splitting in the and P NMR spectra when was used. Both of these results... 

Binuclear iridium hydrides like 8 often form as off-loop species that can result in decreasing of the catalytic activity. Formation of trinuclear complexes completely deactivating the catalyst has been also observed. Nevertheless, the dimeric hydrides formed reversibly before the elimination of the proton are catalytically active, since they can recover mononuclear dihydrides via reversible dissociation. ... 

The mechanism of the iridium-catalyzed hydroformylation has been studied by NMR spectroscopy. Species including iridium acyl and alkyl dihydride intermediates were detected. The results confirmed that the CO-deficient atmosphere favors hydrogenation over carbonylation [76]. 

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