13 Jan 17:49
by Kohei Moriya, Dorian Didier, Meike Simon, Jeffrey M. Hammann, Guillaume Berionni, Konstantin Karaghiosoff, Hendrik Zipse, Herbert Mayr, Paul Knochel
Secondary alkyllithium reagents bearing an OTBS group (TBS=tert-butyldimethylsilyl) at the 3-position can be prepared stereoconvergently through an I/Li exchange from a diastereomeric mixture of the corresponding secondary alkyl iodides. These lithium reagents react with a range of electrophiles, including carbon electrophiles, with retention of configuration to yield various 1,3-difunctionalized derivatives with good diastereoselectivities. Kinetic studies show that the 3-siloxy group strongly accelerates the epimerization at the lithium-substituted carbon atom. This method offers a new way to construct chiral open-chain molecules with excellent stereoselectivity.
Stereoconvergent or not? Secondary alkyllithium reagents bearing an OTBS group (TBS=tert-butyldimethylsilyloxy) at the 3-position were prepared stereoconvergently through an I/Li exchange of a diastereomeric mixture of the corresponding secondary alkyl iodides. These lithium reagents react with a range of electrophiles, including carbon electrophiles, with retention of configuration to yield 3-functionalized derivatives with good diastereoselectivity.
07 Mar 18:09
by Janina Bucher, Thomas Wurm, Kumara Swamy Nalivela, Matthias Rudolph, Frank Rominger, A. Stephen K. Hashmi
Differently substituted terminal alkynes that bear sulfonate leaving groups at an appropriate distance were converted in the presence of a propynyl gold(I) precatalyst. After initial formation of a gold acetylide, a cyclization takes place at the β-carbon atom of this species. Mechanistic studies support a mechanism that is related to that of dual gold-catalyzed reactions, but for the new substrates, only one gold atom is needed for substrate activation. After formation of a gold vinylidene complex, which forms a tight contact ion pair with the sulfonate leaving group, recombination of the two parts delivers vinyl sulfonates, which are valuable targets that can serve as precursors for cross-coupling reactions, for example.
Gold vinylidene intermediates are generated by the cyclization of gold acetylides that carry a sulfonate leaving group. This result demonstrates for the first time that the formation of these species is not restricted to a dual activation mode. The cyclization products obtained herein contain a vinyl sulfonate moiety, which makes them useful building blocks for cross-coupling reactions.
05 Dec 11:38
by Pierre Queval, Claire Jahier, Mathieu Rouen, Isabelle Artur, Jean-Christophe Legeay, Laura Falivene, Loic Toupet, Christophe Crévisy, Luigi Cavallo, Olivier Baslé, Marc Mauduit
A low-cost, modular, and easily scalable multicomponent procedure affording access in good yields and excellent selectivity (up to 93 %) to a wide range of (a)chiral unsymmetrical 1-aryl-3-cycloalkyl-imidazolium salts is disclosed. Electronic and steric properties of the corresponding unsymmetrical unsaturated N-heterocyclic carbene (U2-NHC) ligands were evaluated and evidenced strong electron donor ability, high steric discrimination, and modular steric demand.
A low-cost, modular, and easily scalable multicomponent procedure, affording access to a wide range of (a)chiral unsymmetrical 1-aryl-3-cycloalkyl-imidazolium salts in good yields and excellent selectivities, is disclosed. Electronic and steric properties of the corresponding unsymmetrical unsaturated N-heterocyclic carbene (U2-NHC) ligands were evaluated and evidenced strong electron-donor ability, high steric discrimination, and modular steric demand.
27 Nov 18:43
by Günter Seidel, Barbara Gabor, Richard Goddard, Berit Heggen, Walter Thiel, Alois Fürstner
Carbophilic catalysts that are based on AuI allow a host of different nucleophiles to be added across various π systems.1–3 Although many of these reactions are thought to proceed via gold carbenoids, the challenge to observe and characterize these putative intermediates has basically been unmet.4 The current mechanistic interpretation therefore largely relies on indirect evidence and computational data, some of which are subject to debate.5 In an attempt to fill this gap, we pursued a potential route to gold carbenoids by formal transmetalation of chromium or tungsten Fischer carbene complexes with [LAu]+. Whereas this transformation proceeds with exceptional ease as long as a stabilizing heteroelement is present on the carbene center, it stops half-way in its absence. Rather unusual bimetallic arrays are formed, which allow the charge density to delocalize over several positions. The obvious difficulty of releasing an “unstabilized” gold carbenoid has potential mechanistic implications for the understanding of π-acid catalysis in general.
I do not want to be free! Whereas prototype Fischer-type chromium carbene complexes transfer their organic ligand to gold with exceptional ease, chromium complexes devoid of the heteroelement substituent do not want to release unstabilized gold carbenoids of the type that is often invoked in mechanistic discussions of π-acid catalysis. Instead, unusual bimetallic arrays are formed in which charge density gets delocalized over several positions.
10 Oct 17:54
by Pascal Nösel, Laura Nunes dos Santos Comprido, Tobias Lauterbach, Matthias Rudolph, Frank Rominger and A. Stephen K. Hashmi
Journal of the American Chemical Society
03 Sep 17:30
by Mattia Silvi, Indranil Chatterjee, Yiankai Liu, Paolo Melchiorre
All about topology control: The title reaction yields valuable tetrahydrofuran spirooxindoles (see scheme; TMS=trimethylsilyl), and exemplifies a rare asymmetric 1,6-addition to linear 2,4-dienals proceeding with high δ-site- and stereoselectivity. A steering group at the β-dienal position ensured molecular preorganization of the catalytically active vinylogous iminium ion intermediate for highly predictable reaction outcomes.