Water in Stereoselective Organocatalytic Reactions

Gruttadauria, Michelangelo; Giacalone, Francesco; Noto, Renato

doi:10.1002/adsc.200800731

Cited by 306 publications

(90 citation statements)

References 165 publications

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“…As a result, the IL immobilized catalyst is regenerated and can be used for further reactions. Water is known to be the cheapest, environmentally benign, safe and most abundant compound on the earth's surface; and it has attracted a great deal of attention from a green chemistry perspective [33][34][35][36][37][38][39][40][41][42][43]. In addition, water also holds some unique physical and chemical properties, such as high surface tension, hydrogen bonding capability, high dielectric constant and high cohesive energy density [44][45][46].…”

Section: Introductionmentioning

confidence: 99%

Ionic Liquid Immobilized Organocatalysts for Asymmetric Reactions in Aqueous Media

Qiao

Headley

2013

Catalysts

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Ionic liquids are organic salts with melting points typically below ambient or reaction temperature. The unique combination of physical properties of ionic liquids, such as lack of measurable vapor pressure, high thermal and chemical stability, make them ideal to be used as reusable homogenous support for catalysts. In addition, the solubility of ionic liquids in various reaction media can be controlled and easily fine-tuned by modification of the structures of their cations and anions. As a result, ionic liquid immobilized organocatalysts are very effective in aqueous media and can be separated easily from organic solvents, as well as aqueous phases by simply adjusting the polarity of the media. Ionic liquid immobilized organocatalysts are not only very versatile compounds that are effective catalysts for a wide spectrum of reactions, but are also environmentally friendly and recyclable organocatalysts. Herein, we provide a summary of the past decade in the area of asymmetric catalysis in aqueous media for a wide variety of reactions in which ionic liquid and related ammonium salt immobilized organocatalysts are used.

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Section: Introductionmentioning

confidence: 99%

Ionic Liquid Immobilized Organocatalysts for Asymmetric Reactions in Aqueous Media

Qiao

Headley

2013

Catalysts

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“…reaction conditions, RPKA shows the change in rate with respect to [1] is constant, which is consistent with constant [3] total , [Ce IV ], and [H 2 O] throughout the reaction, providing the straight line defined by Equation (6).…”

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confidence: 86%

“…This finding is consistent with the behavior of other organocatalytic systems that proceed through enamine formation. [6] Additionally, the [8b]…”

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confidence: 99%

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Mechanistic Complexity in Organo–SOMO Activation

et al. 2010

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Singly occupied molecular orbital (SOMO) activation provides a pathway for asymmetric a-addition to aldehydes. [1] The scope of SOMO activation includes the allylation, enolation, vinylation, styrenation, chlorination, polyene cyclization, and arylation [2] of a range of aldehydes. This union of organocatalysis with single-electron oxidative coupling is an intricate process involving the complex balance of catalyst, oxidant, radicophile, base, temperature, heterogeneous reaction conditions, and H 2 O. To examine the role of each component in this complex process, a series of spectroscopic and kinetic studies were carried out to study the asymmetric allylation of 1 shown in Equation (1).[1a] The data described herein show three important features: 1) Oxidation of the intermediate enamine is rapid and preferential to oxidation of the catalyst, 2) H 2 O concentration is critical for catalytic efficiency, and 3) the kinetic role of ceric ammonium nitrate (CAN) is masked by a physical phase-transfer process.The mechanism of the system is challenging to study since it is heterogeneous, but findings from initial work show that the presence of H 2 O is important for reaction success.[1a]Additionally, one of the key design features of the process involves reaction of the catalyst (3) with the substrate to form an intermediate enamine that is preferentially oxidized in the presence of catalyst or substrate. Previous studies show that enamines [3] are more readily oxidized than amines. [4] To further explore the selectivity of the oxidation, a series of amines and enamines derived from imidazolidinone catalysts were studied using stopped-flow spectrophotometry in an attempt to determine the impact of the structure on the rate of a single-electron oxidation. A range of single-electron oxidants based on Ce IV , Cu II , and Fe III were explored. In each case, enamine oxidation was faster than the mixing time of the stopped-flow spectrophotometer even at reduced temperatures (À10 8C). Conversely, oxidation of the catalyst was slower than the time-scale of the stopped-flow system (1000 s). Although these findings did not provide the structure-reactivity relationships initially desired, they indicated that oxidation of the enamine is significantly more rapid under homogeneous conditions than oxidation of the catalyst.Since the SOMO system is mechanistically complex, reaction progress kinetic analysis (RPKA) was employed to study the reaction in Equation (1). This approach, described by Blackmond, provides an elegant method for the study of mechanistic properties under synthetic conditions.[5] To utilize RPKA, two factors are necessary: 1) a detailed understanding of the stoichiometry of the reagents used, and 2) an understanding of the concept of excess. Excess (e) is defined as the difference in initial stoichiometric concentrations of a reagent (CAN) and the monitored substrate (1) [Eq 2].The "same excess" protocol [5] determines whether the active catalyst concentration remains constant throughout the reaction. Initially, t...

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“…7 Most of these systems are highly hydrophobic molecules that diminished the contact with bulk water and the transition states, with actually the process taking place in a highly concentrated organic phase. 8 Recently, we have shown that prolinamides derived from 1,1'-binaphthyl-2,2´-diamine (binam) 2 9 and 3…”

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confidence: 99%