Zink‐katalysierte enantiospezifische sp3‐sp3‐Kreuzkupplung von α‐Hydroxyestertriflaten mit Grignard‐Reagentien

Total Synthesis of (-)-Cylindrocyclophane F: A Yardstick for Probing New Catalytic C-C Bond-Forming Methodologies

A short and efficient total synthesis of the C₂ -symmetric (-)-cylindrocyclophane F is presented, using a cross olefin metathesis dimerization strategy for construction of the [7,7]-paracyclophane macrocycle. The synthesis of the dimerization building block includes a Pd-catalyzed sp³ -sp² Negishi cross coupling of a sterically hindered Zn-reagent with an aromatic triflate, an enantiospecific Zn-catalyzed sp³ -sp³ cross coupling of an α-hydroxy ester triflate with a Grignard reagent and the application of an enantioselective Rh-catalyzed C-allylation of an electron rich arene.

A Zinc Catalyzed C(sp3 )-C(sp2 ) Suzuki-Miyaura Cross-Coupling Reaction Mediated by Aryl-Zincates

The Suzuki-Miyaura (SM) reaction is one of the most important methods for C-C bond formation in chemical synthesis. In this communication, we show for the first time that the low toxicity, inexpensive element zinc is able to catalyze SM reactions. The cross-coupling of benzyl bromides with aryl borates is catalyzed by ZnBr2 , in a process that is free from added ligand, and is compatible with a range of functionalized benzyl bromides and arylboronic acid pinacol esters. Initial mechanistic investigations indicate that the selective in situ formation of triaryl zincates is crucial to promote selective cross-coupling reactivity, which is facilitated by employing an arylborate of optimal nucleophilicity.

Alkynes as Electrophilic or Nucleophilic Allylmetal Precursors in Transition-Metal Catalysis

Diverse late transition metal catalysts convert terminal or internal alkynes into transient allylmetal species that display electrophilic or nucleophilic properties. Whereas classical methods for the generation of allylmetal species often form stoichiometric by-products, the recent use of alkynes as allylmetal precursors enables completely atom-efficient catalytic processes to be carried out, including enantioselective transformations.

Biocontrolled formal inversion or retention of L-α-amino acids to enantiopure (R)- or (S)-hydroxyacids

Natural L-α-amino acids and L-norleucine were transformed to the corresponding α-hydroxy acids by formal biocatalytic inversion or retention of absolute configuration. The one-pot transformation was achieved by a concurrent oxidation reduction cascade in aqueous media. A representative panel of enantiopure (R)- and (S)-2-hydroxy acids possessing aliphatic, aromatic and heteroaromatic moieties were isolated in high yield (67-85 %) and enantiopure form (>99 % ee) without requiring chromatographic purification.

Shedding new light on ZnCl2-mediated addition reactions of Grignard reagents to ketones: structural authentication of key intermediates and diffusion-ordered NMR studies

Building on recent advances in synthesis showing that the addition of inorganic salts to Grignard reagents can greatly enhance their performance in alkylation reactions to ketones, this study explores the reactions of EtMgCl with benzophenone in the presence of stoichiometric or catalytic amounts of ZnCl(2) with the aim of furthering the understanding of the role and constitution of the organometallic species involved in these transformations. Investigations into the metathesis reactions of three molar equivalents of EtMgCl with ZnCl(2) led to the isolation and characterisation (X-ray crystallography and (1)H and (13)C NMR spectroscopy) of novel magnesium "zinc-rich" zincate [{(THF)(6)Mg(2)Cl(3)}(+){Zn(2)Et(5)}(-)] (1), whose complicated constitution in THF solutions was assessed by variable-temperature (1)H DOSY NMR studies. Compound 1 reacted with one equivalent of benzophenone to yield magnesium magnesiate [{(THF)(6)Mg(2)Cl(3)}(+){Mg(2)(OC(Et)Ph(2))(2)Cl(3)(THF)}(-)] (3), whose structure was determined by X-ray crystallography. (1)H NMR monitoring of this reaction showed two equivalents of ZnEt(2) formed as a co-product, which together with the "magnesium only constitution" of 3 provides experimental insights into how zinc can be efficiently recycled in these reactions, and therefore used catalytically. The chemoselectivity of this reaction can be rationalised in terms of the synergic effect of magnesium and zinc and contrasts with the results obtained when benzophenone was allowed to react with EtMgCl in the absence of ZnCl(2), where the reduction of the ketone takes place preferentially. The reduction product [{(THF)(5)Mg(3)Cl(4){OC(H)Ph(CF(3))}(2)] (4) obtained from the reaction of EtMgCl with 2,2,2-trifluoroacetophenone was established by X-ray crystallography and multinuclear ((1)H, (13)C and (19)F) NMR spectroscopy. Compounds 3 and 4 exhibit new structural motifs in magnesium chemistry having MgCl(2) integrated within their constitution, which highlights the new role of this inorganic salt in providing structural support for the newly generated alkoxide ligand.

Iterative synthesis of (oligo)deoxypropionates via zinc-catalyzed enantiospecific sp3-sp3 cross-coupling

This group's recently reported mild zinc-catalyzed enantiospecific sp3-sp3 cross-coupling of lactic acid tert-butylester triflate with Grignard reagents was now applied to an iterative approach in the synthesis of a series of all four possible diastereomers of the shown trideoxypropionate. Oligodeoxypropionate structures are a common motif in a large number of biologically relevant natural products of polyketide origin thus making our approach a versatile tool in their synthesis.