Iridium enrichment

Evidence also exists for a terrestrial source of the iridium enrichment as volcanic ejecta is enriched in this rare element. Thus, the enriched sediment layer could also have been caused by an abrupt and large increase in volcanic activity. Evidence for this is suggested by high levels of volcanic ash, soot, and shocked minerals in the iridium-enriched layer. Other geochemical characteristics of this sediment layer appear to have been caused by acid rain and tsunamis, both of which are by-products of volcanic activity. 

Zoller, W.H., Parrington, J.R. and Phelan Kotra, J.M. (1983) Iridium enrichment in airborne particles from Kilauea volcano January 1983. Science, 222(4628), 1118-21. 

The unique chemical composition of cosmogenous debris has provided some insight into why approximately 70% of the species of organisms on Earth were driven extinct over a relatively short time interval approximately 66 million years ago. Evidence for this mass extinction has been observed in marine sediments throughout all the ocean basins. In a contemporaneous layer deposited at the end of the Cretaceous period, the hard parts of many species of marine plankton abruptly vanished from the sedimentary record. This sedimentary layer is also characterized by a large enrichment in the rare element iridium. 

Since meteorites are known to have high levels of iridium, some scientists have suggested that the sediment enrichment was produced by the impact of a very large comet, which either disintegrated in Earth s atmosphere or exploded upon impact. This hypothesis assumes that a huge amount of airborne debris was produced and distributed... 

The control of enantioselectivity in the reduction of carbonyl compounds provides an opportunity for obtaining the product alcohols in an enantiomerically enriched form. For transfer hydrogenation, such reactions have been dominated by the use of enantiomerically pure ruthenium complexes [33, 34], although Pfaltz and coworkers had shown by 1991 that high levels of enantioselectivity could be obtained using iridium(I) bis-oxazoline complexes [35]. 

Marr and coworkers have used the combination of the iridium W-heterocyclic carbene complex 100 (0.1 mol%) with the lipase Novozyme 435 to convert racemic alcohols 101 into enantiomerically enriched esters 102 with excellent yield and enantioselectivities (Scheme 25) [85]. Peris and coworkers have used complex 25... 

A wide range of carbon, nitrogen, and oxygen nucleophiles react with allylic esters in the presence of iridium catalysts to form branched allylic substitution products. The bulk of the recent literature on iridium-catalyzed allylic substitution has focused on catalysts derived from [Ir(COD)Cl]2 and phosphoramidite ligands. These complexes catalyze the formation of enantiomerically enriched allylic amines, allylic ethers, and (3-branched y-8 unsaturated carbonyl compounds. The latest generation and most commonly used of these catalysts (Scheme 1) consists of a cyclometalated iridium-phosphoramidite core chelated by 1,5-cyclooctadiene. A fifth coordination site is occupied in catalyst precursors by an additional -phosphoramidite or ethylene. The phosphoramidite that is used to generate the metalacyclic core typically contains one BlNOLate and one bis-arylethylamino group on phosphorus. 

Alternative synthetic approaches include enantioselective addition of the organometallic reagent to quinoline in the first step of the synthesis [16], the resolution of the racemic amines resulting from simple protonation of anions 1 (Scheme 2.1.5.1, Method C) by diastereomeric salts formation [17] or by enzymatic kinetic resolution [18], and the iridium-catalyzed enantioselective hydrogenation of 2-substituted quinolines [19]. All these methodologies would avoid the need for diastereomer separation later on, and give direct access to enantio-enriched QUINAPHOS derivatives bearing achiral or tropoisomeric diols. Current work in our laboratories is directed to the evaluation of these methods. 

Based on the concept mentioned above, Brown realized the asymmetric deactivation of a racemic catalyst in asymmetric hydrogenation (Scheme 9.18) [35]. One enantiomer of (+)-CHIRAPHOS 28 was selectively converted into an inactive complex 30 with a chiral iridium complex 29, whereas the remaining enantiomer of CHIRAPHOS forms a chiral rhodium complex 31 that acts as the chiral catalyst for the enantioselective hydrogenation of dehydroamino acid derivative 32 to give an enantio-enriched phenylalanine derivative... 

Like iridium, arsenic is enriched in Cretaceous-Tertiary boundary shales from New Zealand (Brooks et al., 1984 Strong et al., 1987). The iridium is believed to have originated from an asteroid impact that caused the massive extinction at the end of the Cretaceous period about 65 million years ago. In contrast, most of the arsenic in the boundary shales probably had a terrestrial origin (Strong et al., 1987). The extinction of marine organisms, especially plankton, from the impact may have been responsible for increased anoxic conditions in the oceans, which led to the precipitation of arsenic in the marine deposits (Brooks et al., 1984), 541. 

Figure 24 Concentration profiles of siderophile elements in a radially zoned Fe,Ni grain in the CBb chondrite, QUE 94411 (a) electron microprobe data (b) and (c) trace element data from laser ablation ICPMS (Campbell et ai, 2001). The nickel, cobalt, and chromium profiles can be matched by nonequilibrium nebular condensation assuming an enhanced dust-gas ratio of —36 X solar, partial condensation of chromium into silicates, and isolation of 4% of condensates per degree of cooling (Petaev etal, 2001). Concentrations of the refractory siderophile elements, osmium, iridium, platinum, ruthenium, and rhodium, are enriched at the center of the grain by factors of 2.5-3 relative to edge concentrations, which are near Cl levels after normalization to iron (reproduced by permission of University of Arizona on behalf of The Meteoritical Society from Meteorit. Planet. ScL, 2002, 37, pp. 1451-1490).

Some HSE ratios in upper mantle rocks often show significant deviations from chondritic ratios. For example, Schmidt et al. (2000) reported a 20-40% enhancement of ruthenium relative to iridium and Cl-chondrites in spinel Iherzolites from the Zabargad island. Data by Pattou et al. (1996) on Pyrenean peridotites, analyses of abyssal peridotites by Snow and Schmidt (1998), and data by Rehkamper et al. (1997) on various mantle rocks suggest that higher than chondritic Ru/lr ratios are widespread and may be characteristic of a larger fraction, if not of the whole of the upper mantle. A parallel enrichment is found for rhodium in Zabargard rocks (Schmidt et al, 2000). There are. 

As even the optimized hydrogenation conditions gave rise to not more than 89% enantioselectivity it became very important to find an efficient purification procedure which would allow the enrichment of the desired (-(-enantiomer. A rapid screening of several weak and strong acids led to an acceptable solution. The acetates of both the racemate 11 as well as the (-)-enantiomer 12 are readily crystalline salts but the acetate of the desired (S)-enantiomer is less soluble in toluene at 0°C than that of the racemate by a factor of 10. Thus it could be crystallized in overall yield of 84%, content 99.1%, 98.9% ee. Finally it was noticed that traces of iridium (70-200 ppm) in the acetate salt hindered the following reaction in the synthetic sequence, by leading to decomposition of the formic acid (used as for-mylation reagent). A treatment of the octahydroisoquinoline solution with charcoal prior to precipitation of the acetate salt lowers the Ir content to 20-40 ppm. This new quality can then be formylated without problems under the standard conditions. 

In Table 4.1 chemisorption data on alumina-supported platinum-iridium catalysts and related catalysts containing platinum or iridium alone show the effect of varying the temperature of calcination of the catalyst (in air or oxygen-helium mixture) on the metal dispersion (40,41). Data are presented for chemisorption of carbon monoxide, hydrogen, and oxygen. The final three catalysts in the table contained more metal than the first three. They also contained 0.1 wt% Fe (enriched with 57Fe) incorporated as a probe for Moss-bauer spectroscopy experiments (41). The presence of the iron is ignored in the discussion of the chemisorption results. 

Figure 4.32 Mossbauer spectra at 25°C on alumina-supported platinum, iridium, and platinum-iridium catalysts (samples B, C, and D, respectively) containing a small amount of Fe (0.1 wt%) enriched with 57Fe as a Mossbauer probe (3,41). (Sample A is a reference material containing only the enriched Fe on alumina.) (Reprinted with permission from Academic Press, Inc.)...

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-... 

SrgIrHg from Ir and SrHg powder at 1033 K under hydrogen EugIrDg from Eu isotope enriched intermetallic Eulrg at 673 K under deuterium, or by reaction of binary europium hydride (natural isotope mixture) with iridium metal... 

While the necessary ores are found in any place on the earth where there has been volcanic action, the best source of material is the Maui craters in Hawaii where the magma is highly enriched with iridium. Iridium ore is a pink-orange color and that with rhodium is more of a grey color. The metals are already monatomic but purification is a lengthy process with 32 steps and it takes 100kg of ore to get 1kg of precipitate. 

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