- A library approach to the development of BenzaPhos: Highly efficient chiral supramolecular ligands for asymmetric hydrogenation
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A library of chiral supramolecular ligands, named BenzaPhos, of straightforward preparation (two steps from commercially or readily available starting materials) and modular structure, was designed and synthesized. The ligands were screened in the search for new rhodium catalysts for the enantioselective hydrogenation of several benchmark and industrially relevant substrates. Once a series of hits were identified, structural modifications were introduced on three of the best ligands and a small second-generation library was created. Members of the latter library showed outstanding levels of activity and enantioselectivity in the hydrogenation of challenging olefins, such as enamide S4 and β-dehydroamino ester S5 (>99 % ee: best value ever reported in both cases). A series of control experiments were undertaken to clarify the role of hydrogen bonding in determining the catalytic properties of the new ligands. The results of these experiments, together with those of computational studies carried out on four dihydride complexes involved in the catalytic hydrogenation of substrate S4, strongly suggest that a substrate orientation takes place in the catalytic cycle by formation of a hydrogen bond between the ligand amide oxygen atom and the substrate amide NH atom. As simple as selective: BenzaPhos ligands, benzamide-containing chiral monophosphites of modular structure and trivial synthesis, have been screened in the Rh-catalyzed hydrogenation of olefins (see scheme), giving excellent enantioselectivities with three benchmark and two industrially relevant substrates. Control experiments and computational studies suggest an important role of ligand-substrate hydrogen bonding in the stereodiscriminating step of the catalytic cycle. Copyright
- Pignataro, Luca,Bovio, Chiara,Civera, Monica,Piarulli, Umberto,Gennari, Cesare
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supporting information; experimental part
p. 10368 - 10381
(2012/10/08)
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- Synthesis of oxazines and thiazines by cyclodehydration of hydroxy amides and thioamides
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Dihydro-1,3-oxazines and -thiazines were obtained by cyclodehydration of hydroxy amides and thioamides with PEG-linked Burgess reagent or under Mitsunobu conditions. Yields were generally higher with polymer-Burgess reagent, but both conditions failed to
- Wipf, Peter,Hayes, Gregory B.
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p. 6987 - 6998
(2007/10/03)
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- A novel electrochemical method for benzoylation of aminoalcohols with methyl benzoates at room temperature
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A novel electrochemical method of benzoylating aminoalcohols 2 by use of methyl benzoates 1 at room temperature was developed. 2-Aminoethanols 2 (n=0, R3=H) and 3-aminopropanols 2 (n=1, R3=H) gave the corresponding benzamides 4, as a result of electrochemical transesterification followed by O,N-acyl migration.In contrast, o- and m-aminobenzylalcohols (7a and 7b) and trans-4-(aminomethyl)cyclohexanemethanol (8a) afforded the corresponding benzoic esters, since their acyl migration is hindered.A local anesthetic, procaine, was prepared by using this electrochemical reaction.A mechanism for electrochemical transesterification, which involves a formation of alcoholate anion by a one-electron reduction of alcohol, is presented.
- Arai, Kenji,Tamura, Shohei,Masumizu, Tatsuya,Kawai, Ken-Ichi,Nakajima, Shoichi,Ueda, Akira
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p. 903 - 907
(2007/10/02)
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- ELECTROCHEMICAL AMIDATION OF ESTERS
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Amidation reactions between ester and amine, both aliphatic and aromatic, which do not happen non-electrochemically at room temperature, took place merely by passing electric current in cathodic compartment.
- Arai, Kenji,Shaw, Chu-hei,Nozawa, Koohei,Kawai, Ken-ichi,Nakajima, Shoichi
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p. 441 - 442
(2007/10/02)
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