Iridium allyl

A fourth focus of catalytic chemistry in our laboratory has been iridium-catalyzed asymmetric allylic substitution. Dr. Toshimichi Ohmura had been studying additions to rhodium and iridium allyl and benzyl complexes in hopes of developing... 

Bergman has also found an iridium allyl hydride complex that reacts with arenes and alkanes, the allyl group being converted to an n-propyl group in the process. Butane and isobutane give methyl-substituted allyl derivatives under exchange with the coordinated allyl group (Eq. 23) [93]. 

Bergman has reported that at higher concentrations, the iridium allyl complex Cp Ir(r 3-allyl)H reacts with benzene to give a dinuclear product that has activated benzene. Furthermore, this dinuclear compound undergoes reversible benzene loss indicating that the dinuclear intermediate [Cp>fIr(r 1,r 3-al-lyl)IrCp ] is capable of reacting with aromatic C-H bonds (Eq. 27) [111]. 

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. ... 

Good fortune and design in the discovery of iridium-catalyzed asymmetric allylic substitution. 

Iridium-catalyzed transfer hydrogenation of aldehyde 73 in the presence of 1,1-dimethylallene promotes tert-prenylation [64] to form the secondary neopentyl alcohol 74. In this process, isopropanol serves as the hydrogen donor, and the isolated iridium complex prepared from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS is used as catalyst. Complete levels of catalyst-directed diastereoselectivity are observed. Exposure of neopentyl alcohol 74 to acetic anhydride followed by ozonolysis provides p-acetoxy aldehyde 75. Reductive coupling of aldehyde 75 with allyl acetate under transfer hydrogenation conditions results in the formation of homoallylic alcohol 76. As the stereochemistry of this addition is irrelevant, an achiral iridium complex derived from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and BIPHEP was employed as catalyst (Scheme 5.9). 

It is noteworthy that ZnEt2 has been used as a base in enantioselective allylic substitutions. A remarkable increase in ee was observed when ZnEt2 was used instead of KH, NaH, LiH, LDA, or BuLi in the Pd-catalyzed alkylations of allylic acetates by enolates of malonic esters and related compounds.403 In contrast, application of ZnEt2 was not as very effective as in similar iridium-catalyzed allylic alkylations.404... 

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-... 

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... 

A two-component bimetallic catalytic system has been developed for the allylic etherification of aliphatic alcohols, where an Ir(i) catalyst acts on allylic carbonates to generate electrophiles, while the aliphatic alcohols are independently activated by Zn(n) coordination to function as nucleophiles (Equation (48)).194 A cationic iridium complex, [Ir(COD)2]BF4,195 and an Ru(n)-bipyridine complex196 have also been reported to effectively catalyze the O-allylation of aliphatic alcohols, although allyl acetate and MeOH, respectively, are employed in excess in these examples. 

In the case of cyclopentenyl carbamate in which a directive group is present at the homoallyl position, the cationic rhodium [Rh(diphos-4)]+ or iridium [Ir(PCy3)(py)(nbd)]+ catalyst cannot interact with the carbamate carbonyl, and thus approaches the double bond from the less-hindered side. This affords a cis-product preferentially, whereas with the chiral rhodium-duphos catalyst, directivity of the carbamate unit is observed (Table 21.7, entry 7). The presence of a hydroxyl group at the allyl position induced hydroxy-directive hydrogenation, and higher diastereoselectivity was obtained (entry 8) [44]. 

One of the most active and selective catalysts in this kind of reaction is undoubtedly Ir/support, as recently demonstrated by Jacobs and coworkers [276], Therefore, by combining the carbonyl affinity of metallic iridium with the promotion effect of the H-fl zeolite, which is a strong Bronsted acid, one can reduce a large variety of unsaturated ketones and aldehydes to allylic alcohols with high conversions, selectivities, and diasteieoselectivities. 

The peaks in the m/z 50-57 range of the 1-butene El spectrum could be misinterpreted as a complex isotopic pattern if no formula were available on the plot (Fig. 3.8). However, there is no element having a comparable isotopic pattern and in addition, all elements exhibiting broad isotopic distributions have much higher mass. Instead, the 1-butene molecular ion undergoes H, H2 and multiple H2 losses. The m/z 57 peak, of course, results from In a similar fashion the peaks at m/z 39 and 41 appear to represent the isotopic distribution of iridium, but this is impossible due to the mass of iridium (cf. Appendix). However, these peaks originate from the formation of an allyl cation, CsHs, m/z 41, which fragments further by loss of H2 to form the CsHs" ion, m/z 39 (Chap. 6.2.4). 

Bis-allylic oxidation of 23 and related cyclohexa-1,4-dienes provides a convenient and general preparation of cyclohexa-2,5-dien-l-ones (Scheme 7). These cross-conjugated die-nones are substrates for a variety of photochemical rearrangement and intramolecular cycloaddition reactions. Amide-directed hydrogenations of dienones 24a and 24b with the homogeneous iridium catalyst afford cyclohexanones 25a and 25b, containing three stereogenic centers on the six-... 

Keywords Iridium, Enantioselective, Hydrogenation, Transfer hydrogenation, Allylation, Vinylation... 

Scheme 5 Enantioselective iridium-catalyzed hydrogenation of alkynes in the presence of N-arylsulfonyl imines to furnish trisubstituted allylic amines...

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