The first part of this review article deals with the structures of enolates. The development of research in this field during the last half century will be illustrated by highlightening seminal ...contributions, while, by no means, an attempt of comprehensiveness is made: the choice of spotlights has, admittedly, a personal touch. Aside from derivatization of lithium enolates and their crystal structures that meanwhile are classics, more recent solution studies are presented. In the second part, it will be shown by contributions of our laboratory that, despite their complicated structures, lithium enolates are reliable ‘workhorses’ in synthesis with emphasis being given to stereoselective CC bond forming reactions due to the Pd‐catalyzed allylic alkylation of non‐stabilized, preformed enolates.
Either as the free alcohol or deprotonated, a carbinol residue bearing gemial, identical aryl residues can at first sight be recognized as an achiral structural unit. When incorporated, however, into ...a chiral molecule, the two aryl groups become diastereotopic. Thus, they might contribute to an enhancement in stereoselectivity and do so in a variety of reactions. This Minireview highlights developments in stereochemistry when the diaryl(oxy)methyl group is involved, with emphasis given to the beneficial effect of this moiety. Of particular focus are auxiliaries, the stoichiometric use of metal complexes, metal and organocatalysts, and finally chiral dopants for liquid crystals, all featuring the diaryl(oxy)methyl group.
Not chiral, but temporarily stereogenic: The introduction of a diaryl(oxy)methyl group into chiral auxiliaries, catalysts, and dopants can be rewarding: As a temporary stereogenic unit, it has a beneficial effect in a variety of asymmetric syntheses and has helped a series of well‐known, efficient, and reliable “work horses” to succeed.
Boron‐Based Enantiomerism Braun, Manfred
European journal of organic chemistry,
April 8, 2024, Letnik:
27, Številka:
14
Journal Article
Recenzirano
Odprti dostop
Boron‐based enantiomerism is fragile due to the inherent tendency of a dissociation of a ligand from tetra‐coordinate chiral boron complexes under formation of the achiral tri‐coordinate species. ...This review will present the different approaches in overcoming the inherent tendency of racemization in boron‐stereogenic compounds. When embedded in an environment of chiral ligands or substituents, configurationally stable boron stereogenic centers can form in a diastereoselective manner. Compounds incorporating boron as the exclusive stereogenic center are obtained by resolution of the racemic mixtures. The recently developed – much more efficient – methods of a catalytic, enantioselective creation of boron stereogenic compounds are highlighted in this review. Finally the chiroptical properties of enantiomerically pure boron complexes that makes them promising materials or devices are addressed.
Flat, tri‐coordinate boron rears its had, becomes tetra‐coordinate and chiral! When treated in an appropriate manner, it forms enantiomers that are configurationally stable. Only since recently, boron‐stereogenic complexes are accessible by catalytic enantioselective syntheses. What makes this particularly exciting is that tetra‐coordinate boron compounds are considered promising light‐emitting materials.
Palladium catalysis enables efficient and versatile domino reactions that are initiated by an intramolecular Mizoroki‐Heck reaction and then terminated by a Suzuki‐Miyaura coupling. The ...stereochemistry of this cascade is concerned under three aspects: Firstly, as stereoselective formation of carbon−carbon‐double bonds, when the initial carbometalation occurs at an alkyne moiety. Secondly as diastereoselective and thirdly enantioselective formation of stereogenic carbon centers when the intramolecular Heck step is directed to an alkene moiety. Valuable protocols lead to – otherwise not easily accomplished – stereo control in the formation of tri‐ and tetra‐substituted double bonds according to the first conception. The diastereoselective versions permit the formation of multiple stereocenters in mono‐ and polycycles. Enantioselective protocols were developed only recently. The fact that the newly created stereocenters are highly‐substituted, tertiary or quaternary ones, makes the methods that are presented in this review useful and versatile tools in synthesis.
Exocyclic tetra‐substituted carbon‐carbon double bonds in uniform configuration and creation of tertiary or quaternary stereogenic centers in a diastereoselective or enantioselective manner – these are structural motives that are accessible by a cascade consisting of a Heck reaction and subsequent Suzuki‐Miyaura coupling with suitable palladium catalysts that guarantee a high degree of stereoselectivity.
Benzyl- and methyl-protected 2,4-dihydroxyacetophenones are added under ruthenium catalysis to 4-methoxy- and 3,4-dimethoxystyrene in a completely regioselective manner. Thus, oxygenated ...dihydrostilbenes are obtained that feature the skeleton of scorzodihydrostilbenes - antioxidative agents that were recently isolated from
. Selective deprotection liberates the corresponding phenols, among them the aglycon of scorzodihydrostilbenes B and D.
The structural motif of α,α‐difluoro‐substituted carboxyl and carbonyl groups with hydroxy‐ or amino‐substituents at the stereogenic carbon in β‐position is found frequently in drugs and biologically ...active compounds. A straightforward method for obtaining those targets in high enantiomeric purity is given by asymmetric difluoro‐Reformatsky and imino‐difluoro‐Reformatsky reactions that lead to α,α‐difluorinated β‐hydroxy‐ and β‐amino‐carboxylates or β‐lactams. Three concepts that open a route to this type of compounds are discussed here. Firstly, chiral substrates are able to induce stereoselectivity in the formation of the newly created stereogenic center. Secondly, covalently bound and removable chiral auxiliaries are used in asymmetric syntheses and, thirdly, chiral additives and ligands at the metal direct the addition of the zinc enolate, the Reformatsky reagent, to aldehydes or imines in an enantioselective manner. The review closes with a short discussion of more recently developed difluoro‐aldol reactions that may be considered as alternatives to the Reformatsky procedure.
Convenient, versatile, reliable, and functional‐group tolerant: The Reformatsky reaction! Over 130 years old, it is still a good choice when synthesis requires the introduction of the difluoroacetate enolate into aldehydes and imines. Appropriately chosen chiral substrates, auxiliaries, or additives enable the creation of the new stereogenic center and make asymmetric syntheses of difluorinated biologically active targets accessible.
High yields and enantioselectivities are obtained in a dynamic kinetic asymmetric transformation in which the CO bond of alcohols, silyl ethers, and acetals are substituted by a CC bond. For ...example, the racemic indanyl silyl ether 1 is converted into (S)‐2 in the presence of the chiral Lewis acidic TiIV complex 3.
Doubly deprotonated carboxylic acids undergo smooth palladium‐catalyzed carbon alkylations with the allylic substrates methyl allyl carbonate and (E)‐methyl (pent‐3‐en‐2‐yl) carbonate to give ...γ,δ‐unsaturated carboxylic acids. A diastereoselective and enantioselective protocol leads to (2S,3R)‐hexenoic acid in 87% ee.
Enantiomerically and diastereomerically pure chelated boronate–imine complexes 5 and a library of boronate–amine complexes 8 have been synthesized by taking advantage of a modular concept based on ...aromatic aldehydes, aminoethanols and boronic acids. The configuration of the amine complexes 8 featuring stable stereogenic boron and nitrogen centers are assigned based upon crystal structure analyses of the representative compounds 8aaa and 8acj and the comparison of the CD spectra of all complexes 8. They serve as colorless, stable dopants for nematic liquid crystals and provide high helical twisting power. The interaction of boronates 8 with nematic compounds ZLI‐1840 and 5‐CB featuring a benzonitrile moiety is studied by 19F and 1H NMR spectroscopy. A strong π‐stacking between the arylboronate residue and the nematic compound as well as a hydrogen bond are indicated by the spectroscopic data. Based thereupon, a model is proposed that correlates the configuration of the dopant with the sign of the helix formed by twisting the nematic phase.
Induced helicity results upon doping nematic mesophases with novel boronate–amine complexes that feature configurationally stable stereogenic boron and nitrogen centers. π‐Stacking and hydrogen bonding, as indicated by NMR spectroscopy, offer a rationale for the efficient twisting of the nematic compounds and the sign of the induced helix.
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