Iridium allylic alkylation

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... 

The first iridium catalysts for allylic substitution were published in 1997. Takeuchi showed that the combination of [fr(COD)Cl]2 and triphenylphosphite catalyzes the addition of malonate nucleophiles to the substituted terminus of t -allyliridium intermediates that are generated from allylic acetates. This selectivity for attack at the more substituted terminus gives rise to the branched allylic alkylation products (Fig. 4), rather than the linear products that had been formed by palladium-catalyzed allylic substitution reactions at that time [7]. The initial scope of iridium-catalyzed allylic substitution was also restricted to stabilized enolate nucleophiles, but it was quickly expanded to a wide range of other nucleophiles. 

Iridium-Catalyzed Asymmetric Allylic Alkylation with Catalysts Derived from LI... 

This method can be extended for the synthesis of allyl alkyl ethers from alcohols with allyl acetate. Thus, the iridium cationic complex [lr(cod)2] BFc, catalyzes the allylation of alcohols 71 with allyl acetate 72 to afford allyl ethers 73 (Equation 10.14) [30]. 

For further details of this reaction, the reader is referred to Chapter 9. The catalytic allylation with nucleophiles via the formation of Ti-allyl metal intermediates has produced synthetically useful compounds, with the palladium-catalyzed reactions being known as Tsuji-Trost reactions [31]. The reactivity of Ti-allyl-iridium complexes has been widely studied [32] for example, in 1997, Takeuchi idenhfied a [lrCl(cod)]2 catalyst which, when combined with P(OPh)3, promoted the allylic alkylation of allylic esters 74 with sodium diethyl malonate 75 to give branched... 

Our study on the synthesis, structure and catalytic properties of rhodium and iridium dimeric and monomeric siloxide complexes has indicated that these complexes can be very useful as catalysts and precursors of catalysts of various reactions involving olefins, in particular hydrosilylation [9], silylative couphng [10], silyl carbonylation [11] and hydroformylation [12]. Especially, rhodium siloxide complexes appeared to be much more effective than the respective chloro complexes in the hydrosilylation of various olefins such as 1-hexene [9a], (poly)vinylsiloxanes [9b] and allyl alkyl ethers [9c]. 

The formation of a branched chiral product from the alkylation of monosubstituted substrates is not limited to the catalysis of metals described thus far. Allylic alkylation reactions catalyzed with rhodium [211] and iridium [212] complexes have been shown to occur at the more... 

Asymmetric allylboration, characteristics, 9, 197 Asymmetric allylic alkylation, allylic alcohols with copper, 11, 99 with iridium, 11, 105 with molybdenum, 11, 109 with nickel, 11, 102 with non-palladium catalysts, 11, 98 with platinum, 11, 103 reaction systems, 11, 112 with rhodium, 11, 104 with ruthenium, 11, 108 with tungsten, 11, 111... 

Combinations of eight different ligands and twelve different metal salts were screened for their efficiency to catalyze the allylation of /i-dicarbonyl compounds. The assay identified not only the well known catalyst system Pd(OAc)2 combined with a phosphine ligand but also the combination [ IrCl(cod) 2] and iPr-pybox or 1,10-phenanthroline as efficient catalysts. These are the first examples of non-phosphane iridium catalysts capable of allylic alkylations. 

Keywords Allylic substitution, Allylation, Allylic alkylation, Jt-Allyl complexes, Palladium, Molybdenum, Ruthenium, Iridium... 

Iridium catalysts generally favour chiral branched products in contrast to palladium catalysts, which typically give rise to linear achiral products. The catalyst system [Ir(COD)Cl]2-P(OPh)3 is effective in allylic alkylation and amination h... 

While the visual appearance of the products was sufficient to identify the active catalysts, the authors employed a parallel UV plate reader to quantify the results. The rhodium catalysts identified provided encouragement that metals other than palladium could be developed for this transformation, and in a subsequent experiment, the colorimetric technique was used to discover the first non-phosphine-based iridium catalyst for allylic alkylations. A high-throughput approach to the discovery of catalysts for asymmetric allylic alkylations is also described in Section 1.13.3.6. 

Siloxide complexes of rhodium and iridium are much more effective than the respective chloro-complexes in hydrosilylation of a variety of olefins such as 1-hexene, vinylsilanes, and allyl alkyl ethers (81). [ Rh(cod)()u-OSiMe3) 2] and [Rh(cod)(PCy3)3(OSilV[e3)] in particular are very active and selective catalysts for the hydrosilylation of glycidyl ether by triethoxysilane yielding commercially available silane coupling agent. 

You et al. reported the iridium-catalyzed enantioselective functionalization of indoles and pyrroles. In the event, indole 181 is transformed, via an intramolecular allylic alkylation, into a spirocyclic intermediate at C3, which then undergoes selective methylene migration to form 182 with no loss of enantiopurity (up to 99% ee is obtained).The indole ring can tolerate substitution with various halogens or methoxy groups, while pyrroles can be substituted with alkyl or aryl groups (13JA8169). 

Iridium-catalysed allylic alkylations of enantiomerically enriched monosubstituted allylic acetates have been found to proceed with up to 79% retention of configuration when triphenyl phosphite is used as ligand. ... 

Giacomina F, Riat D, Alexakis A (2010) o-Ethylenic allylic substrates as alternatives to cyclic substrates in copper- and iridium-catalyzed asymmetric allylic alkylation. Org Lett 12 1156-1159... 

A significant contribution to the use of iridium precursors for allylic alkylations has been provided by Takeuchi and co-workers, who demonstrated how the selectivity achieved by using iridium catalysts complements that obtained with palladium complexes. Fast combinatorial colorimetric screening has been used to individuate Ir(l) catalysts active for the allylic substitution reaction. Fundamental advancements in this field were achieved by Helmchen and co-workers who obtained high regio- and enantioselectivity in asymmetric allylic alkylations of achiral or racemic substrates with chiral phosphinooxazolines and phosphoramidites as... 

Scheme 5.27 Regioselective and enantioselective iridium-catalyzed allylic alkylation of silyl enol ethers 82.

In 2010, Ueda and Hartwig reported on an iridium-catalyzed asymmetric allylation of sodium sulfinates 345 to branched allylic sulfones 348 with high regioselectivities and enantioselectivities (Scheme 46.40). Notably, the reaction proceeded with a broad range of acyclic allylic carbonates 346 and aryl and alkyl sodium sulfinates 345. Most recently, Zhao et al. developed the catalytic asymmetric allylic alkylations of acyclic allylic carbonates 346 using sodium thiophenoxide and alkyl thiolates 348 to give good-to-excellent selectivities for branched products 350 with excellent enantioselectivities. 

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