Ir-olefin complexes

The oxidative functionalization of olefins through ir-olefin complexes of palladium also has a long history, including the industrial production of acetaldehyde and vinyl acetate. Related reactions, including the conversion of olefins to vinyl ethers and enamines, have been studied in more recent times for fine chemical synthesis. These oxidative C-0 and C-N bond formations have been conducted with a variety of oxidants, including Oj, and have been studied as both intermolecular and intramolecular processes. 

The bonding and structures of ir-olefin complexes of the transition elements have already been discussed in some detail in Chapter 5, and their preparation in Chapter 6. We have shown above (p 189) that 2-electron ligands can be formed from and can themselves be converted into 1-electron ligands. The following example shows how one may proceed from a coordinated olefin to an enyl or 3-electron system ... 

The reaction mechanism clearly involves the oxidative addition of aniline to an unsaturated Ir(I) complex (Scheme 4-4). Interestingly, the azametallacyclobutane intermediate could be characterized by single-crystal X-ray diffraction [141]. This result confirms that insertion of an olefin into the M-H bond is less favorable than insertion into the M-N bond [142]. 

Under Lewis-acid-catalyzed conditions, electron-rich arenes can be added to alkenes to generate Friedel-Crafts reaction products. This subject will be discussed in detail in Chapter 7, on aromatic compounds. However, it is interesting to note that direct arylation of styrene with benzene in aqueous CF3CO2H containing H2PtCl6 yielded 30-5% zram-PhCH CHR via the intermediate PhPt(H20)Cl4.157 Hydropheny-lation of olefins can be catalyzed by an Ir(III) complex.158... 

Electron-rich olefins such as 36 have been used by Lappert in the synthesis of a great number of mono-, bis-, tris-, and tetrakiscarbene complexes from various transition metal species (62). Ru, Os, and Ir carbene complexes have been prepared from reactions with these olefins, e.g.,... 

The calculated thermodynamic parameters in Table X are for 150 °C, a value close to typical operating temperatures, and without pressure corrections, there is no H2 and both alkane and alkene could be at unit concentration. The bottom part of the table is the same as Table VI. The free-energy profile of the transfer reaction mechanism is shown in Fig. 4. In the first step of the mechanism (IVR), sacrificial alkene (in this case ethylene) was bound to the (L)Ir(H)2 complex 4 to form an olefin complex 7. As mentioned above, the forward reaction of this step has no... 

Although the reaction responsible for the generation of the hydride is not specified, it is assumed that it arises from a disproportionation of iron carbonyl complexes. The hydride presumably adds after ir-complexing to form the c-bonded complex which then splits out the metal hydride in either direction. The ir-complexed olefin may then be displaced by another olefin or undergo another hydride addition-elimination sequence. The second path involves olefin complexing with the deficient Fe(CO)3 species and formation of a jr-allyliron hydride intermediate ... 

Thus the two plausible catalytic cycles have been considered, one via an Ir dihydride complex A and the other via an IrH2(ri -H2) complex B (Fig. 3). The first is analogous to the well-established mechanism for rhodium diphosphine-catalyzed hydrogenation of olefins going through Ir(I) and Ir(III) intermediates [26-29]. 

Interestingly, Cp Ir(NHC) complexes were also shown to be efficient catalysts for the deuteration of organic molecules using CD3OH or (CD3)2CO as deuterium sources (Scheme 3.17). A wide set of organic molecules (including ketones, alcohols, olefins and ethers) were deuterated in high yields [15]. 

Once the N—H bond has been oxidatively added to the Ir(I) complex (in the context of the CCM cycle, vide supra), the resultant Ir(III) intermediate is a Lewis acid that is thought to coordinate the olefin. A synergistic effect between the coordinated electrophilically activated olefin and the highly nucleophilic nature of the amido function is believed to facilitate the C—N bond formation within the coordination sphere of the Ir center (see 56). Alkyl-amino-Ir(III) complexes, such as the key intermediate 24 of the CCM system (as described in Section 6.2.1) are of paramount importance to better understand Ir-catalyzed hydroaminations. Complex... 

Cycloisomerization represents another approach for the construction of cyclic compounds from acyclic substrates, with iridium complexes functioning as efficient catalysts. The reaction of enynes has been widely studied for example, Chatani et al. reported the transformation of 1,6-enynes into 1-vinylcyclopentenes using [lrCl(CO)3]n (Scheme 11.26) [39]. In contrast, when 1,6-enynes were submitted in the presence of [lrCl(cod)]2 and AcOH, cyclopentanes with two exo-olefin moieties were obtained (Scheme 11.27) [39]. Interestingly, however, when the Ir-DPPF complex was used, the geometry of olefinic moiety in the product was opposite (Scheme 11.28) [17]. The Ir-catalyzed cycloisomerization was efficiently utilized in a tandem reaction along with a Cu(l)-catalyzed three-component coupling, Diels-Alder reaction, and dehydrogenation for the synthesis of polycyclic pyrroles [40]. 

We are currently trying to answer specifically the question of whether ir-bonded complexes do occur in certain cases where insertion reactions are observed. I think they do because I believe that the same factors which favor stabilization of this type of transition state will also tend to favor formation of 7r-bonded olefin complexes, which are only slightly removed from this. At the moment Bern Tinker is examining the insertion of olefins in mercuric complexes to see whether there is any indication of 7r-bonded intermediates. In his paper, Dr. Heck referred to some unpublished work relevant to this theme. I would certainly be interested in anything more he can tell us about that. 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

There is often a striking similarity between olefins and carbon monoxide as ligands, and one of the most common ways of preparing olefin complexes is by replacement of one or more carbon monoxide ligands in a metal carbonyl by olefins. Both olefin and carbonyl complexes frequently obey the simple E.A.N. rule, each CO ligand and C C bond contributing two ir-electrons to the metal atom, to enable it to attain the electronic configuration of the next inert gas in the Periodic Table. 

The first class of complexes are often analogous to olefin complexes (see Section III). Thus the acetylene is intact in the complex, it can be recovered unchanged in many cases, and it is ir-bonded to the metal through its CjC bond, as shown by X-ray structural determinations and changes... 

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