A highly regio-, diastereo- and enantioselective intramolecular cyclopropanation reaction of a racemic α-diazo ketone catalyzed by chiral ortho-metalated dirhodium(II) compounds

A series of racemic dirhodium(II) compounds with two ortho-metalated aryl phosphine ligands in a head-to-tail arrangement Rh₂(O₂CR)₂(pc)₂ (pc=ortho-metalated aryl phosphine) (1a–k) were tested in the regio- and stereoselective cyclopropanation of racemic 1-diazo-6-methyl-3-(2-propenyl)-5-hepten-2-one 2, which possesses two different reactive CC double bonds for a five-membered-ring formation. The complexes Rh₂(O₂CCH₃)₂(pc)₂ {pc=[(C₆H₄)P(C₆H₅)₂], [(p-CH₃C₆H₃)P(p-CH₃C₆H₄)₂], and [(C₆H₄)P(C₆H₅)(C₆F₅)]} (1a–d) successfully enhanced the cyclopropanation of trisubstituted versus monosubstituted CC bonds to give an 80:20 selectivity ratio. The reaction occurred with excellent diastereoselectivity; the syn-products were the only stereoisomers observed in the whole series of the catalysts. Enantioenriched products were obtained when enantiomerically pure dirhodium(II) complexes were used.     

Chemoselective aromatic C-H insertion of α-diazo-β-ketoesters catalyzed by dirhodium(II) carboxylates

The ability of α-diazo-β-ketoesters bearing a substituent on the benzylic position to undergo aromatic C-H insertion is described. Good to excellent yields of the aromatic C-H insertion products were observed with Rh(2)(tpa)(4) or Rh(2)(esp)(2) catalysts. This is an attractive strategy to prepare tetralins carrying a methyl group on the benzylic position, a structural motif found in several types of natural products.     

Enantioselective SiH insertion of methyl phenyldiazoacetate catalyzed by dirhodium(II) carboxylates incorporating N-phthaloyl-(S)-amino acids as chiral bridging ligands

Enantioselective insertion reactions of methyl phenyldiazoacetate into the SiH bond of silanes were effected by employing dirhodium(II) carboxylates incorporating N-phthaloyl-(S)-amino acids as chiral bridging ligands. The use of dirhodium(II) tetrakis[N-phthaloyl-(S)-phenylalaninate] as a catalyst produced methyl (2S)-(dimethylphenylsilyl)phenylacetate in 74% ee, whereas catalysis with dirhodium(II) tetrakis[N-phthaloyl-(S)-phenylglycinate] afforded its enantiomer in 72% ee.     

Enantio- and diastereoselective hetero-Diels–Alder reactions between 4-methyl-substituted Rawal’s diene and aldehydes catalyzed by chiral dirhodium(II) carboxamidates: catalytic asymmetric synthesis of (−)-cis-aerangis lactone

The first catalytic asymmetric hetero-Diels–Alder (HDA) reaction between 3-tert-butyldimethylsilyloxy-1-dimethylamino-1,3-pentadiene (4-methyl-substituted Rawal’s diene) and aldehydes is described. With 3 mol % of dirhodium(II) tetrakis[N-benzene-fused-phthaloyl-(S)-piperidinonate], Rh₂((S)-BPTPI)₄, the cycloaddition reaction proceeded exclusively in an endo fashion and gave, after a novel sequential treatment with dimethyl acetylenedicarboxylate and acetyl chloride, the corresponding 2,3-cis-disubstituted dihydropyranones with up to 98% ee and perfect diastereoselectivity. The utility of this catalytic protocol was demonstrated by an asymmetric synthesis of the (−)-cis-aerangis lactone.     

Acylative kinetic resolution of racemic aromatic β-hydroxy esters catalyzed by chiral nucleophilic N-(1-arylethyl)benzoguanidines

An efficient acylative kinetic resolution of racemic aromatic β-hydroxy esters with cyclohexanecarboxylic anhydride was achieved using newly designed (R)-N-methylbenzoguanidine ((R)-NMBG) derivatives. A series of (R)-NMBG derivatives was synthesized by modifying the original (R)-NMBG catalyst with the introduction of branched N-substituents containing a stereogenic center, and their catalytic performance was evaluated. (R,R)-N-(1-(β-1-Naphthyl)ethyl)benzoguanidine [(R,R)-NβNpEtBG] was found to function as an efficient acyl transfer catalyst for the reaction of a broad variety of substrates, regardless of the substituent type and substitution pattern.     

Development and outlook of chiral carbene–gold(I) complexes catalyzed asymmetric reactions

Great progress has been made in developing of homogeneous Au-catalyzed reactions in the past decade. The unique versatility and efficiency of gold complexes have been obtained including carbene ligated gold complexes. Due to the special linear coordination mode of gold(I) complex, Au catalyzed asymmetric reactions have become a huge challenge. Chiral carbene–gold complexes also have been applied in asymmetric reactions. Major breakthrough in this field has been obtained by the Toste group recently and more are expected in the future. This digest, by highlighting recent works, aims to make further progress in this fascinating research field.     

Enantioselective Diels–Alder reactions of chiral racemic acyclic dienes with (SS)-2-(p-tolylsulfinyl)-1,4-naphthoquinone

Enantiopure sulfinylnaphthoquinone (+)-1 reacted with racemic acyclic dienes 2a–f bearing a stereogenic allylic center, through a tandem cycloaddition/pyrolytic sulfenic acid elimination, to afford enantio-enriched compounds 4a–f and 5a–f with good like:unlike selectivities (ca. 75:25) and good enantiomeric excesses (68–82%), arising from the partial kinetic resolution of the racemic dienes.     

Stereoselective intramolecular cyclization of γ-allylbenzamide via π-allylpalladium complex catalyzed by Pd(0)

An efficient procedure for synthesizing oxazines was developed by the palladium(0)-catalyzed intramolecular cyclization of a benzamide through a π-allylpalladium (II) complex. Interestingly, the diastereoselectivity of oxazine ring formation was dominantly controlled by the bulkiness of various protecting groups on the secondary alcohols.     

Reactivity of thio (amino) derivatives of the ω-(4-hydroxyaryl)alkyl type in radical reactions

The enthalpies, activation energies, and rate constants of the reactions of thio (amino) alkylphenols of different structures were calculated and compared with those of the reactions of alkyl-substituted phenols, alkoxyl and alkyl radicals, hydroperoxides, and nitrogen dioxide, as well as the reactions of phenoxyl radicals with molecules of the substrate being oxidized. The calculation was performed by the intersecting parabolas method using O-H bond energy data for phenols. The correlation between the molecular structure of the thio (amino) alkylphenols and their reactivity in radical reactions is considered.     

Asymmetric Diels–Alder reactions of N-allenoyloxazolidinones catalyzed by Cu(II)–bis(oxazoline) complexes

Catalytic asymmetric Diels–Alder reactions of N-allenoyloxazolidinones were investigated. Various chiral metal–bis(oxazoline) and metal–pyridinebis(oxazoline) complexes were screened. Cu(SbF₆)₂(H₂O)₂(t-BuBox) was found to be the most effective catalyst, giving the product in high yield, enantioselectivity, and endo:exo selectivity. The relative reactivity between N-allenoyloxazolidinones and N-alkenoyloxazolidinones was also investigated. A model for stereoinduction was proposed to account for the enantioselectivity and endo:exo selectivity.     

Study of chiral recognition mechanism of O,O-diethyl (p-methylbenzenesulfonamindo)-aryl(alkyl)-methylphosphonates by HPLC with a series of CSPs

Five chiral stationary phases (CSPs) were used to separate the enantiomers of a series of O,O-diethyl (p-methyl-benzenesulfonamindo)- aryl(alkyl)-methylphosphonates. A chiral recognition mechanism was presented to explain the resolution of these compounds. Results show that CSP with strong π-acceptor 3,5-dinitrobenzoyl group and high steric hindrance has the best resolution ability in chiral separation of O,O-diethyi (p-methyl-benzenesulfonamindo)- aryl(alkyl)-methylphosphonates. When a CSP has just a strong π-acceptor 3,5-dinitrobenzoyl or high steric hindrance it does not have good chiral resolution ability. The chiral recognition is more difficult when the CSP has more than one asymmetric center.     

Enantioselective aerobic oxidation of α-hydroxy-ketones catalyzed by oxidovanadium(V) methoxides bearing chiral, N-salicylidene-tert-butylglycinates

Chiral oxidovanadium(V) methoxides prepared from 3,5-disubstituted-N-salicylidene-l-tert-butylglycines and vanadyl sulfate in air-saturated MeOH serve as highly enantioselective catalysts for asymmetric aerobic oxidations and kinetic resolution of alkyl, aryl, and heteroaryl α-hydroxy-ketones with differed α-substituents at ambient temperature in toluene or TBME (tert-butyl methyl ether). The best scenarios involve the use of complexes which bear the tridendate templates derived from 3,5-diphenyl- or 3-o-biphenyl-5-nitro-salicyaldehyde. The kinetic resolution selectivities of the aerobic oxidation process are in the range of 12 to >1000 based on the selectivity factors (k(rel)).     

Radiation-induced chemical reactions of carbon monoxide and hydrogen mixture—IV. On the water produced by the addition of small amounts of ammonia

The isotope content in water obtained from irradiated CO-H₂ mixture with added small amounts of NH₃ or their deuterated equivalents was compared. Compared with that in water obtained from CO-D₂ mixture with added NH₃, D content in water produced from the mixture with the substitution of ND₃ for NH₃ was increased remarkably. The NH₃ added to the mixture contributes to the water formation under irradiation.     

Enantioselective intermolecular iodoacetalization of enol ethers catalyzed by chiral Co(III)-complex-templated Brønsted acids

The chiral Co(III)-complex-templated Brønsted acids were used as efficient bifunctional phase-transfer catalysts for the asymmetric intermolecular iodoacetalization of enol ethers, such as 3,4-dihydro-2H-pyran, 2,3-dihydrofuran, ethyl vinyl ether with alcohols and NIS, furnishing the 3-iodoacetals in high yield with up to 83:17 er.     

Enantioselective conjugate addition of diethylzinc to cyclohexenone catalyzed by a chiral aminophosphine–copper(II) triflate

Enantioselective conjugate addition of diethylzinc to cyclohexenone and 4,4-dimethylcyclohexenone was catalyzed by the combination of 0.1 equiv. of a chiral monodentate aminophosphine 3 and 0.05 equiv. of copper(II) triflate to give the corresponding adducts 4 in up to 70% ee.