Efficient Construction of α-Spirocyclopropyl Lactones: Iridium-Salen-Catalyzed Asymmetric Cyclopropanation

Stereoconvergent Approach to the Enantioselective Construction of α-Quaternary Alcohols by Radical Epoxide Allylation

We describe a highly stereoconvergent radical epoxide allylation towards diastereomerically and enantiomerically enriched α-quaternary alcohols in two steps from olefins. Our approach combines the stereospecifity and enantioselectivity of the Shi epoxidation with the unprecedented Ti(III)-promoted intramolecular radical group transfer allylation of epoxides. A directional isomerization step via configurationally labile radical intermediates enables the selective preparation of all-carbon quaternary stereocenters in a unique fashion.

Metal-free β,γ-C(sp3)-H difunctionalization of propanols: DMP-initiated asymmetric spirocyclopropanation

A DMP-initiated metal-free effective β,γ-asymmetric spirocyclopropanation of propanols strategy using oxidative iminium activation is described. This process has been realized by a synergistic amine-catalyzed one-pot cascade oxidation-Michael addition cyclopropanation for "one-pot" access to various spirocyclopropyl propionaldehydes/propanols from diverse 3-arylpropanols and α-brominated active methylene compounds under mild conditions and with high enantioselectivity (ee up to >99%).

Highly stereoselective synthesis of spirocyclopropylthiooxindoles and biological evaluation

We present a novel highly stereoselective Rh-catalyzed cyclopropanation of diazothiooxindoles with a broad range of α-functionalized styrenes, enabling facile access of chiral spirocyclopropylthiooxindoles in high to excellent enantiomeric excess.

Asymmetric Catalysis Using Chiral Salen-Metal Complexes: Recent Advances

Chiral salen-metal complexes are among the most versatile asymmetric catalysts and have found utility in fields ranging from materials chemistry to organic synthesis. These complexes are capable of inducing chirality in products formed from a wide variety of chemical processes, often with close to perfect stereoinduction. Salen ligands are tunable for steric as well as electronic properties, and their ability to coordinate a large number of metals gives the derived chiral salen-metal complex very broad utility in asymmetric catalysis. This review primarily summarizes developments in chiral salen-metal catalysis over the last two decades with particular emphasis on those applications of importance in asymmetric synthesis.

Catalytic enantioselective synthesis of cyclopropanes featuring vicinal all-carbon quaternary stereocenters with a CH2F group; study of the influence of C–F⋯H–N interactions on reactivity

A highly diastereo- and enantioselective synthesis of cyclopropanes with adjacent all-carbon quaternary stereocenters featuring a CH₂F group is reported.

Stereoselective cis-Vinylcyclopropanation via a Gold(I)-Catalyzed Retro-Buchner Reaction under Mild Conditions

A highly stereoselective gold(I)-catalyzed cis-vinylcyclopropanation of alkenes has been developed. Allylic gold carbenes, generated via a retro-Buchner reaction of 7-alkenyl-1,3,5-cycloheptatrienes, react with alkenes to form vinylcyclopropanes. The gold(I)-catalyzed retro-Buchner reaction of these substrates proceeds by simple heating at a temperature much lower than that required for the reaction of 7-aryl-1,3,5-cycloheptatrienes (75 °C vs 120 °C). A newly developed Julia–Kocienski reagent enables the synthesis of the required cycloheptatriene derivatives in one step from readily available aldehydes or ketones. On the basis of mechanistic investigations, a stereochemical model for the cis selectivity was proposed. An unprecedented gold-catalyzed isomerization of cis- to trans-cyclopropanes has also been discovered and studied by DFT calculations.

Ir-Catalyzed enantioselective group transfer reactions

This tutorial review highlights the emergence of iridium complexes as catalysts for enantioselective group transfer reactions.

Catalytic asymmetric synthesis of polysubstituted spirocyclopropyl oxindoles: organocatalysis versus transition metal catalysis

Recent progress in catalytic asymmetric synthesis of spirocyclopropyl oxindoles via organocatalysis and transition metal catalysis are summarized and discussed.

Enantioselective iron-catalyzed intramolecular cyclopropanation reactions

An iron-catalyzed asymmetric intramolecular cyclopropanation was realized in high yields and excellent enantioselectivity (up to 97% ee) by using the iron complexes of chiral spiro-bisoxazoline ligands as catalysts. The superiority of iron catalysts exhibited in this reaction demonstrated the potential abilities of this sustainable metal in asymmetric carbenoid transformation reactions.

An efficient organocatalytic enantioselective synthesis of spironitrocyclopropanes

An organocatalytic asymmetric synthesis of spironitrocyclopropanes has been demonstrated starting from 2-arylidene-1,3-indandiones and bromonitroalkanes catalyzed by a cinchona-derived bifunctional organocatalyst. The products were obtained with excellent enantioselectivities, diastereoselectivities and with good yields.

Self-assembly of Zn(salphen) complexes: steric regulation, stability studies and crystallographic analysis revealing an unexpected dimeric 3,3'-t-Bu-substituted Zn(salphen) complex

The self-assembly features of a series of (non)symmetrical Zn(salphen) complexes have been studied in detail by X-ray crystallography, NMR and UV-vis techniques. The combined data demonstrate that the stability of these dimeric assemblies and the relative position of each monomeric unit within the dinuclear structure depend on the location and combination of the aromatic ring substituents.

Enantioenriched synthesis of cyclopropenes with a quaternary stereocenter, versatile building blocks

Ir(salen) complexes were found to catalyze enantioselective cyclopropenation efficiently. Cyclopropenation can be carried out using either a donor/acceptor- or an acceptor/acceptor-substituted diazo compound such as α-aryl-α-diazoacetates, α-phenyl-α-diazophosphonate, 2,2,2-trifluoro-1-phenyl-1-diazoethane, and α-cyano-α-diazoacetamide as carbenoid precursors. The reactions provide highly enantioenriched cyclopropenes (84-98% ee) with a functionalized quaternary carbon as versatile building blocks.

Iridium(III)-catalyzed enantioselective Si-H bond insertion and formation of an enantioenriched silicon center

Iridium(III)-salen complexes were found to efficiently catalyze enantioselective carbene Si-H bond insertion. Highly enantioselective Si-H insertion with alpha-alkyl-alpha-diazoacetates (>or=97% ee) was achieved for the first time by using the iridium complex 4 {(aR,S), Ar = 4-TBDPSC(6)H(4)} bearing a concave-shaped salen ligand as the catalyst. Formation of a chiral silicon center was also achieved for the first time by the Si-H insertion into prochiral silanes: the reactions between prochiral silanes and tert-butyl alpha-diazopropionate in the presence of complex 5 {(aR,S), Ar = Ph} proceeded with high stereoselectivity (84-99% de, 94-->99% ee). The Si-H insertion into trisubstitued silanes with alpha-aryl-alpha-diazoacetates proceeded with almost complete enantioselectivity (>or=99% ee) by using complex 1 {(aR,R), Ar = Ph} as catalyst.

Iridium(III) catalyzed diastereo- and enantioselective C-H bond functionalization

Iridium(III)-salen complexes were found to efficiently catalyze the enantioselective C-H carbene insertion. The insertion reaction at the alpha-position of tetrahydrofuran or at the methylene of 1,4-cyclohexadiene proceeded with high enantio- (up to 99%) and diastereoselectivity (up to >20:1 dr; in the former reaction) by using an appropriate combination of complex (1, 2, or 3) and alpha-diazoacetate (alpha-aryl-alpha-diazoacetates or alpha-diazopropionate) as the carbene source.