Iridium-Catalysed Allylic Substitution: Stereochemical Aspects and Isolation of IrIII Complexes Related to the Catalytic Cycle

Bartels, Björn; García‐Yebra, Cristina; Röminger, Frank; Helmchen, Günter

doi:10.1002/1099-0682(200210)2002:10<2569::aid-ejic2569>3.0.co;2-5

Cited by 157 publications

(97 citation statements)

References 20 publications

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“…This is interesting due to reports on dimer opening by simple coordination of the double bond rather than via C-Cl activation [1,33]. However, it does not seem surprising since similar reactivities have been observed for reactions with allyl bromo/chloro derivatives [34,35]. With regard to 1 H NMR spectroscopy, the signals of [Ir(allyl)(Cl) 2 (COD)] at room temperature in CD 2 Cl 2 are particularly broad but measurements at À60°C reveal a very defined spectrum and in combination with 2D NMR experiments, two isomers in a ratio of 10:3 (different orientation of the allyl ligand) can be identified ( Fig.…”

Section: Reactions Of the Iridium Precursor And The Substratesmentioning

confidence: 54%

Mechanistic insights into the iridium-catalyzed hydrosilylation of allyl compounds

Riener

Meister

Gigler

et al. 2015

Journal of Catalysis

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Section: Reactions Of the Iridium Precursor And The Substratesmentioning

confidence: 54%

Mechanistic insights into the iridium-catalyzed hydrosilylation of allyl compounds

Riener

Meister

Gigler

et al. 2015

Journal of Catalysis

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“…13 Further studies showed that a catalyst generated from [Ir(COD)Cl] 2 and a chiral phosphoramidite ligand containing a dimethyl amino group14 was more active than that containing the phosphine-oxazoline, but enantioselectivities of allylic alkylation reactions with this second system varied from 4% to 86% ee (eq 5), and those of allylic aminations were 11–13% ee (eq 6). 14,15 …”

Section: Prior Studies On Rhodium- and Iridium-catalyzed Allylic Smentioning

confidence: 99%

Mechanistically Driven Development of Iridium Catalysts for Asymmetric Allylic Substitution

2010

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ConspectusEnantioselective allylic substitution reactions comprise some of the most versatile methods for preparing enantiomerically enriched materials. These reactions form products that contain multiple functionalities by creating carbon-nitrogen, carbon-oxygen, carbon-carbon, and carbon-sulfur bonds. For many years, the development of catalysts for allylic substitution focused on palladium complexes. However, studies of complexes of other metals have revealed selectivities that often complement those of palladium systems. Most striking is the observation that reactions with unsymmetrical allylic electrophiles that typically occur with palladium catalysts at the less hindered site of an allylic electrophile occur at the more hindered site with catalysts based on other metals. In this Account, we describe an iridium precursor and a phosphoramidite ligand that catalyze reactions with a particularly broad scope of nucleophiles.The active form of this iridium catalyst is not generated by the simple binding of the phosphoramidite ligand to the metal precursor. Instead, the initial phosphoramidite and iridium precursor react in the presence of base to form a metallacyclic species that is the active catalyst. This species is generated either in situ or separately in isolated form by reactions with added base. The identification of the structure of the active catalyst led to the development of simplified catalysts as well as the most active form of the catalyst now available, which is stabilized by a loosely bound ethylene. Most recently, this structure was used to prepare intermediates containing allyl ligands, the structures of which provide a model for the enantioselectivities discussed here.Initial studies from our laboratory on the scope of iridium-catalyzed allylic substitution showed that reactions of primary and secondary amines, including alkylamines, benzylamines, and allylamines, and reactions of phenoxides and alkoxides occurred in high yields, with high branched-to-linear ratios and high enantioselectivities. Parallel mechanistic studies had revealed the metallacyclic structure of the active catalyst, and subsequent experiments with the purposefully formed metallacycle increased the reaction scope dramatically. Aromatic amines, azoles, ammonia, and amides and carbamates as ammonia equivalents all reacted with high selectivities and yields. Moreover, weakly basic enolates (such as silyl enol ethers) and enolate equivalents (such as enamines) also reacted, and other research groups have used this catalyst to conduct reactions of stabilized carbon nucleophiles in the absence of additional base.One hallmark of the reactions catalyzed by this iridium system is the invariably high enantioselectivity, which reflects a high stereoselectivity for formation of the allyl intermediate. Enantioselectivity typically exceeds 95%, regioselectivity for formation of branched over linear products is usually near 20:1, and yields generally exceed 75% and are often greater than 90%. Thus, the development of iridium catalysts ...

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“…9,22 This overall stereochemical outcome could result from either two steps occurring by inversion of configuration or by all steps occurring with retention of configuration. Reactions of substrates that discourage additions or eliminations with inversion of configuration with the catalyst derived from [Ir(COD)Cl] 2 and P(OPh) 3 gave low yields of substitution product.…”

Section: Introductionmentioning

confidence: 99%

Origins of Enantioselectivity during Allylic Substitution Reactions Catalyzed by Metallacyclic Iridium Complexes

Madrahimov

Hartwig

2012

J. Am. Chem. Soc.

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In depth mechanistic studies of iridium catalyzed regioselective and enantioselective allylic substitution reactions are presented. A series of cyclometallated allyliridium complexes that are kinetically and chemically competent to be intermediates in the allylic substitution reactions was prepared and characterized by 1D and 2D NMR spectroscopies and solid state structural analysis. The rates of epimerization of the less thermodynamically stable diastereomeric allyliridium complexes to the thermodynamically more stable allyliridium stereoisomers were measured. The rates of nucleophilic attack by aniline and by N-methylaniline on the isolated allyliridium complexes were also measured. Attack on the thermodynamically less stable allyliridium complex was found to be orders of magnitude faster than attack on the thermodynamically more stable complex, yet the major enantiomer of the catalytic reaction is formed from the more stable diastereomer. Comparison of the rates of nucleophilic attack to the rates of epimerization of the diastereomeric allyliridium complexes containing a weakly-coordinating counterion showed that nucleophilic attack on the less stable allyliridium species is much faster than conversion of the less stable isomer to the more stable isomer. These observations imply that Curtin-Hammett conditions are not met during iridium catalyzed allylic substitution reactions by η3-η1-η3 interconversion. Rather, these data imply that when these conditions exist for this reaction, they are created by reversible oxidative addition and the high selectivity of this oxidative addition step to form the more stable diastereomeric allyl complex leads to the high enantioselectivity. The stereochemical outcome of the individual steps of allylic substitution was assessed by reaction of deuterium-labeled substrates. The reaction was shown to occur by oxidative addition with inversion of configuration, followed by an outer sphere nucleophilic attack that leads to a second inversion of configuration. This result contrasts the changes in configuration that occur during reactions of molybdenum complexes studied with these substrates previously. In short, these studies show that the factors that control enantioselectivity of iridium-catalyzed allylic substitution are distinct from those that control enantioselectivity during allylic substitution catalyzed by palladium or molybdenum complexes and lead to the unique combination of high regioselectivity, enantioselectivity, and scope of reactive nucleophile.

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Iridium-Catalysed Allylic Substitution: Stereochemical Aspects and Isolation of IrIII Complexes Related to the Catalytic Cycle

Cited by 157 publications

References 20 publications

Mechanistic insights into the iridium-catalyzed hydrosilylation of allyl compounds

Mechanistic insights into the iridium-catalyzed hydrosilylation of allyl compounds

Mechanistically Driven Development of Iridium Catalysts for Asymmetric Allylic Substitution

Origins of Enantioselectivity during Allylic Substitution Reactions Catalyzed by Metallacyclic Iridium Complexes

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