Ir-AUyl complexes

One example of a DyKAT is the asymmetric allylic alkylation outlined in Equation 14.18, in which the two diastereomeric ir-aUyl complexes epimerize via the a-allyl intermediate, as shown previously in Figure 14.15. 

In addition, terminal alkenes can epitnerize via 7i-a-7i-isomerization (Scheme 12.5). This isomerization is an important mechanistic feature in ir-allyl-paUadium chemistry and results in a fast interconversion of ir-aUyl complexes into o-complexes and vice versa. At the state of the o-aUyl complex, rotations around o-bonds are possible, and therefore, the thermodynamically most stable complexes are formed If chiral allylic substrates with a terminal aUcene moiety are used, this isomerization results in a loss of stereogenic information. 

Ir -aUyl complexes can also act as nucleophiles in additing to various electrophiles such as aldehydes, ketones, or imines. Krische et al. [143d] have developed a broad new family of enantioselective allylations via hydrogenations and transfer... 

The generally accepted mechanism for Pd-catalyzed allylic substitution involves association of the palladium(0) catalyst to the substrate, and oxidative addition to provide a ir-aUyl complex. The equilibrium between the neutral 7r-allyl complex and the more reactive cationic 7r-allyl complex depends on the nature/concentration of phosphine Ugand. Nucleophilic addition to the ligand involves direct attack on the ligand when stabilized enolates are employed. After dissociation of the product, the palladium is able to continue in the next catalytic cycle (Scheme 2). In general, the reaction proceeds via a Pd(0)/Pd(II) shuttle, although a Pd(II)/Pd(IV) pathway is also possible. 

Although the combination of [Ir(COD)Cl]2 and LI was shown to catalyze the alkylation, amination, and etherification of allyiic esters to form the branched substitution product in high yield and enantioselectivity, the identity of the active catalyst in these reactions had not been identified. The combination of [Ir(COD) Cl]2 and LI forms the square-planar [Ir(COD)(Cl)Ll] (4) (Scheme 11) [45]. However, this complex does not react with allyiic carbonates to form an appreciable amount of an aUyl complex, and the absence of this reactivity suggested that the mechanism or identity of the active catalyst was more complex than that from simple addition of the allyiic ester to the square-planar complex containing a k -phosphoramidite ligand. 

On the basis of the computed kinetic and thermodynamic data, it is very interesting to have a comparison with the recent experimental observation of the most stable intermediates. Using online IR and NMR techniques in the cobalt-catalyzed hydromethoxycarbonylation of butadiene. Tuba et al. [77] observed several key intermediates and found that (1) the aUyl complex ( 7 -C4H7)Co(CO)3 (11a) is quite stable at 100°C, and no further reactions could be detected after the formation for several hours under 75 bar of CO at 100°C in MeOH (2) the but-2-enyl complex (CH3CH=CHCH2)Co(CO)4 (12a) is very unstable, and can be easily converted to the acyl complex (CH3CH=CHCH2CO)Co(CO)4 with CO... 

The [Ir(cod)2]BARF complex also showed high catalytic activity in the hydrogena-tive coupling of alkyne with aldimines to lead to reductive couphng products, aUyl amines [69]. 

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