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26 Jan 12:51

Cooperative Reductive Elimination: The Missing Piece in the Oxidative-Coupling Mechanistic Puzzle

by Ignacio Funes-Ardoiz, Feliu Maseras

Abstract

The reaction between benzoic acid and methylphenylacetylene to form an isocoumarin is catalyzed by Cp*Rh(OAc)2 in the presence of Cu(OAc)2(H2O) as an oxidant and a leading example of oxidative-coupling reactions. Its mechanism was elucidated by DFT calculations with the B97D functional. The conventional mechanism, with separate reductive-elimination and reoxidation steps, was found to yield a naphthalene derivative as the major product by CO2 extrusion, contradicting experimental observations. The experimental result was reproduced by an alternative mechanism with a lower barrier: In this case, the copper acetate oxidant plays a key role in the reductive-elimination step, which takes place through a transition state containing both rhodium and copper centers. This cooperative reductive-elimination step would not be accessible with a generic oxidant, which, again, is in agreement with available experimental data.

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Joint effort: The reaction between benzoic acid and methylphenylacetylene to form an isocoumarin is catalyzed by Cp*Rh(OAc)2 in the presence of Cu(OAc)2(H2O) as the oxidant. Its mechanism was elucidated by DFT calculations with the B97D functional, which showed that the overall transformation proceeds by cooperative reductive elimination with a transition state containing both rhodium and copper centers.

06 Apr 22:53

Stereocontrolled Synthesis of syn-β-Hydroxy-α-Amino Acids by Direct Aldolization of Pseudoephenamine Glycinamide

by Ian B. Seiple, Jaron A. M. Mercer, Robin J. Sussman, Ziyang Zhang, Andrew G. Myers

Abstract

β-Hydroxy-α-amino acids figure prominently as chiral building blocks in chemical synthesis and serve as precursors to numerous important medicines. Reported herein is a method for the synthesis of β-hydroxy-α-amino acid derivatives by aldolization of pseudoephenamine glycinamide, which can be prepared from pseudoephenamine in a one-flask protocol. Enolization of (R,R)- or (S,S)-pseudoephenamine glycinamide with lithium hexamethyldisilazide in the presence of LiCl followed by addition of an aldehyde or ketone substrate affords aldol addition products that are stereochemically homologous with L- or D-threonine, respectively. These products, which are typically solids, can be obtained in stereoisomerically pure form in yields of 55–98 %, and are readily transformed into β-hydroxy-α-amino acids by mild hydrolysis or into 2-amino-1,3-diols by reduction with sodium borohydride. This new chemistry greatly facilitates the construction of novel antibiotics of several different classes.

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On aldol: Enolization of (R,R)- or (S,S)-pseudoephenamine glycinamide with lithium hexamethyldisilazide (LiHMDS) in the presence of LiCl followed by addition of either an aldehyde or ketone substrate affords aldol addition products which are stereochemically homologous with L- or D-threonine, respectively. These products can be obtained in stereoisomerically pure form in yields of 55–98 %, and are readily transformed into β-hydroxy-α-amino acids by mild hydrolysis or into 2-amino-1,3-diols by reduction.

29 Sep 22:06

Iron-Catalyzed Oxidative CH/CH Cross-Coupling: An Efficient Route to α-Quaternary α-Amino Acid Derivatives

by Kaizhi Li, Guangying Tan, Jingsheng Huang, Feijie Song, Jingsong You
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Fully loaded: A coordinating activation strategy has been developed to furnish α-quaternary α-amino acids through the iron(III)-catalyzed oxidative functionalization of α-C(sp3)[BOND]H bonds of α-tertiary α-amino acid esters. The reaction exhibits a broad substrate scope for both α-amino acids and nucleophiles (Nu) as well as good functional-group tolerance (see scheme, DTBP=di-tert-butyl peroxide, DCE=1,2-dichloroethane).