Nucleophilic Iron Catalysis in Transesterifications: Scope and Limitations

Magens, Silja; Plietker, Bernd

doi:10.1021/jo1004636

Cited by 53 publications

(19 citation statements)

References 45 publications

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“…14,15 The use of the diphenylmethyl imidate 2 allows for formation of diphenylmethyl esters without relying on toxic and energetic diazo reagents. In addition, no strong oxidizing agents or toxic metal salts are needed for this new esterification.…”

Section: Scheme 4 Deprotection Of Diphenylmethyl Ester 23 Without Racmentioning

confidence: 99%

Convenient Formation of Diphenylmethyl Esters Using Diphenylmethyl Trichloroacetimidate

Adhikari

Shah

Howard

et al. 2013

Synlett

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Diphenylmethyl trichloroacetimidate is a useful reagent for the protection of carboxylic acids as their corresponding diphenylmethyl esters. These esterifications proceed rapidly without the need for an added catalyst or promoter. A variety of carboxylic acid substrates undergo esterification in excellent yields with the trichloroacetimidate reagent, including substrates possessing acid-or base-sensitive functionality. Protection of a carboxylic acid with a highly enolizable α-stereocenter using diphenylmethyl imidate was also accomplished without racemization.Esters are common protecting groups for carboxylic acids and are often utilized in multistep organic synthesis. A popular choice is the diphenylmethyl (DPM) ester, as it can be removed using aqueous base, by hydrogenation, or under acidic conditions. 1 This flexibility is advantageous, as the different conditions provide options should incompatibilities with other functional groups be encountered during removal. Diphenylmethyl esters are also popular because they do not introduce new stereogenic centers or overly complicate NMR spectra. For these reasons, the diphenylmethyl protecting group has been used extensively in the synthesis of peptides, 2 β-lactam antibiotics 3 and complex natural products. 4Formation of diphenylmethyl esters with simple substrates is typically effected using acid catalysis with diphenylmethanol, 5 however, these conditions do not tolerate complex substrates with delicate functionality. With sensitive substrates, diphenylmethyl esters are typically prepared by exposing the carboxylic acid to diphenyldiazomethane. 6 The unstable and toxic nature of this reagent 7 has led to a number of surrogate reactions based on the in situ formation of diphenyldiazomethane from diphenylmethyl hydrazone. Unfortunately, these procedures typically depend on strong oxidizing reagents or environmentally hazardous metal salts. 3b,7a,8 The scope of these esterifications is also restricted due to the powerful oxidizing conditions required to form the diphenyldiazomethane. Given the limitations associated with known methods, especially with carboxylic acids possessing sensitive functionality, esterification reagents that show improved convenience, practicality and safety are desirable. Ideally, any new reagent for the generation of diphenylmethyl esters would: 1) form the ester under mild conditions at room temperature, 2) not require stoichiometric amounts of toxic metal salts or powerful oxidizing agents, 3) react selectively with the carboxylic acid without disturbing other functionality in the molecule, and 4) be relatively stable with a long shelf-life.Recently, we began to evaluate alternatives for the formation of diphenylmethyl esters under mild conditions from the corresponding trichloroacetimidate. Trichloroacetimidates are excellent alkylating agents under acidic conditions, as the alkylation is driven by the loss of the imidate and the formation of trichloroacetamide. This process provides a substantial thermodynamic driving force, facilitatin...

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Section: Scheme 4 Deprotection Of Diphenylmethyl Ester 23 Without Racmentioning

confidence: 99%

Convenient Formation of Diphenylmethyl Esters Using Diphenylmethyl Trichloroacetimidate

Adhikari

Shah

Howard

et al. 2013

Synlett

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“…[1] Clearly, most of the work in organometallic catalysis has been performed by applying noble metals based on palladium, rhodium, iridium, and ruthenium complexes. [6][7][8][9][10][11][12][13][14][15][16][17] To date, most of the work in this field has demonstrated that iron complexes can be used in the same way as noble metal catalysts. In this respect, homogeneous catalysis with iron complexes offers a highly attractive substitute.…”

Section: Introductionmentioning

confidence: 99%

Benign Catalysis with Iron: Unique Selectivity in Catalytic Isomerization Reactions of Olefins

et al. 2012

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The use of noble metal catalysts in homogeneous catalysis has been well established. Due to their price and limited availability, there is growing interest in the substitution of such precious metal complexes with readily available and bio-relevant catalysts. In particular, iron is a "rising star" in catalysis. Herein, we present a general and selective iron-catalyzed monoisomerization of olefins, which allows for the selective generation of 2-olefins. Typically, common metal complexes give mixtures of various internal olefins. Both bulk-scale terminal olefins and functionalized terminal olefins give the corresponding products under mild conditions in good to excellent yields. The proposed reaction mechanism was elucidated by in situ NMR studies and supported by DFT calculations and extended X-ray absorption fine structure (EXAFS) measurements.

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“…Recently, we were able to show that Fe–acyl complexes derived from reaction of an ester with a low‐valent iron species, such as [Bu 4 N][Fe(CO) 3 (NO)] (abbreviated to TBAFe), are intermediates in transesterifications under neutral conditions 8. Whereas initial studies concentrated on the use of enol acetates,8a further investigations showed that p ‐chloro aryl esters are potent acyl donors allowing for the successful transesterification of a variety of structurally labile acid residues to various alcohols 8b…”

Section: Methodsmentioning

confidence: 99%

Fe‐Catalyzed Thioesterification of Carboxylic Esters

Magens

Plietker

2011

Chemistry A European J

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Second nature: Starting from shelf-stable aryl esters and thiols, a variety of carboxylic acid esters were transformed into the corresponding thioesters with no racemization of labile stereocenters. The method was successfully applied in a native chemical-ligation-type peptide formation, which suggests that the thiol may act as a co-catalyst for future 1,2-additions of pronucleophiles to carboxylic esters.

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Nucleophilic Iron Catalysis in Transesterifications: Scope and Limitations

Cited by 53 publications

References 45 publications

Convenient Formation of Diphenylmethyl Esters Using Diphenylmethyl Trichloroacetimidate

Convenient Formation of Diphenylmethyl Esters Using Diphenylmethyl Trichloroacetimidate

Benign Catalysis with Iron: Unique Selectivity in Catalytic Isomerization Reactions of Olefins

Fe‐Catalyzed Thioesterification of Carboxylic Esters

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