Iridium isomerization

The pivaloylamidobenzyl group was stable to acetic acid-water-90°, MeOH-NaOMe, iridium-induced allyl isomerization, and to many of the Lewis acids used in glycosylation. ... 

The rhodium and iridium complexes of dibenzothiophene (L) reveal an interesting case of linkage isomerism (91IC5046). Thus, the ti S) coordinated species [MCp LCb] on thermolysis with silver tetrafluoroborate afford the Ti -coordinated dicationic species. 

Another example is the hydrogenation of the homoallylic eompound 4-methyl-3-cyclohexenyl ethyl ether to a mixture of 4-methylcyclohexyl ethyl ether and methylcyclohexane. The extent of hydrogenolysis depends on both the isomerizing and the hydrogenolyzing tendencies of the catalysts. With unsupported metals in ethanol, the percent hydrogenolysis decreased in the order palladium (62.6%), rhodium (23 6%), platinum (7.1%), iridium (3.9%), ruthenium (3.0%) (S3). 

Other halides can be introduced by metathesis. Figure 2.55 summarizes some of the complexes isolable with dimethylphenylphosphine, similar in general to the corresponding iridium complexes (section 2.13.3), including the photochemical isomerization of the mer-isomer. 

The two Mossbauer levels of Pt, 99 keV and 130 keV, are populated by either EC of Au(fi/2 = 183 days) or isomeric transition of Pt(fi/2 = 4.1 days). Only a few authors, e.g., [323, 324] reported on the use of Pt, which is produced by thermal neutron activation of " Pt via " Pt(n, y) Pt. The source used in the early measurements by Harris et al. [322, 325] was carrier-free Au diffused into platinum metal. Walcher [326] irradiated natural platinum metal with deuterons to obtain the parent nuclide Au by (d, xn) reactions. After the decay of short-lived isotopes, especially Au(fi/2 = 6.18 days), Au was extracted with ethyl acetate, and the Au/Pt source prepared by induction melting. Buym and Grodzins [323] made use of (a, xn) reactions when bombarding natural iridium with... 

In considering ways to rationalize the differences in product distribution in Table IV, it is speculated that the ability of the surface atoms of certain metals to exhibit different valence states may be important (23). It has previously been suggested by Boudart and Ptak (27) that the ability of platinum and iridium to catalyze the isomerization of neopentane could be a consequence of the variable surface valency of these metals proposed by... 

Jonathan Bould has now shown that ortho-cycloboronation occurs essentially quantitatively in certain reactions using [IrCl(PPh3)3] as the source of iridium (7.) Thus, as shown in Scheme 1 reaction with nido-B9Hi2 in CH2CI2 solution at room temperature yields two isomeric forms of the yellow nido iridium(lll) compound [ (Hlri H-j2) (C6HI4PPI12) (PPh3) ] by the unusually clean reaction ... 

The proposed mechanism of the bond shift isomerization of neopentane is shown in Scheme I Cl-3). There are now good models for each step in the proposed sequence, but no simple transition metal complex can accomplish all steps since there cannot be sufficient co-ordination sites. The first steps involve a,y-dinstallation of the alkane, for which there are good precedents in both platinum and iridium chemistry (4, 5, 6). The... 

Successive hydrogen transfers within 60, followed by coordination of olefin and then H2 (an unsaturate route), constitute the catalytic cycle, while isomerization is effected through HFe(CO)3(7r-allyl) formed from 59. Loss of H2 from 60 was also considered to be photoinduced, and several hydrides, including neutral and cationic dihydrides of iridium(III) (385, 450, 451), ruthenium(II) (452) and a bis(7j-cyclopentadienyltungsten) dihydride (453), have been shown to undergo such reductive elimination of hydrogen. Photoassisted oxidative addition of H2 has also been dem-... 

Isomerization of [(, )-3-alkoxy-l-propenyl]boronates to the corresponding 7-alkoxyallylboronates was catalyzed by ruthenium or iridium complexes.64 [IrH2(TFlF)2(PPh2Me)2]PF6 was the most efficient catalyst for selective preparation of (ftfy-alkoxyallylboronates (Scheme 37). 

The stereoselective isomerization of allyl silyl ethers to (E)- or (Z)-silyl enol ethers was carried out in the presence of a cationic iridium(i) catalyst. The complex, prepared in situ by treating [Ir(cod)2]PFf,/2PPi3 with hydrogen was... 

A polymer-supported iridium catalyst 4 has been prepared and used in the isomerization of the double bonds in aryl allyl ethers and aryl allylic compounds with excellent trans-scIcctivity and without conventional workup procedures (Scheme 45).73... 

The isomerization of propargylic alcohols to a,/3-enones was also developed by using an iridium catalyst. A variety of secondary propargylic alcohols can be isomerized to the corresponding a,/3-unsaturated ketones in the presence of 1 mol% IrHs(Pr 3P)2 (Scheme 54).93... 

Quickly, it became clear that iridium and rhodium do not cleanly fit the Chalk-Harrod mechanism as does platinum. For electron-rich silanes and relatively unhindered terminal alkynes, the major product is the (Z)-vinylsi-lane (Scheme 3, B) from apparent unusual trans-addition to the alkyne.22 This observation was followed by important and confusing discoveries. First, rhodium, under appropriate conditions, will catalyze the isomerization of the (Z)-vinylsilane product B to the (ft)-vinylsilane product A.23 Second, rhodium can also catalyze the reverse, contra-thermodynamic reaction of the (ft)-vinylsilane A to the (Z)-vinylsilane B.24... 

Cyclic imines do not have the problem of syn/anti isomerism and therefore, in principle, higher enantioselectivities can be expected (Fig. 34.8). Several cyclic model substrates 6 were hydrogenated using the Ti-ebthi catalyst, with ee-val-ues up to 99% (Table 34.5 entry 5.1), whereas enantioselectivities for acyclic imines were <90% [20, 21]. Unfortunately, these very selective catalysts operate at low SCRs and exhibit TOFs <3 h-1. In this respect, iridium-diphosphine catalysts, in the presence of various additives, seem more promising because they show higher activities. With several different ligands such as josiphos, bicp, bi-... 

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