Palladium-catalyzed cyclopropanation of electron-deficient olefins with aryldiazocarbonyl compounds

A concise and efficient protocol for the preparation of cyclopropanes from various aryldiazocarbonyl compounds and electron-deficient olefins catalyzed by Pd(OAc)₂ is reported.     

Enantioselective synthesis of chiral carbocyclic pyrimidine nucleosides via asymmetric cyclopropanation

An efficient method to construct chiral cyclopropyl pyrimidine carbocyclic nucleoside analogues bearing a quaternary center was developed via asymmetric Michael-initiated cyclopropanation. The axis chirality was observed in cyclopropyl pyrimidine carbocyclic nucleoside analogues for the first time, which was caused by the rotationally restricted NC bond in N-COPh moiety. Using (DHQD)₂AQN as the organocatalyst, diverse cyclopropyl pyrimidine carbocyclic nucleoside analogues were generated in 76–93% yields and 73–96% ee.     

Efficient and regioselective chromium(0)-catalyzed reaction of 2-substituted furans with diazo compounds: stereoselective synthesis of (2E,4Z)-2-aryl-hexadienedioic acid diesters

Pentacarbonyl(η²-cis-cyclooctene)chromium(0) (1) catalyzes efficiently reactions of diazo compounds with electron-rich furans. The reaction of 2-methoxyfuran (2) with alkyl α-diazoarylacetate (3a-g) furnishes the (2E,4Z)-2-aryl-hexadienedioic acid diesters (4a-g) in excellent yields. These reactions are highly regioselective. The cyclopropanation intermediates formed from 1 and diazo compounds 3a-g always arise from a carbene addition to the less substituted CC bond of 2. The resulting cyclopropanation product undergoes a ring opening reaction to form the corresponding (2E,4Z)-2-aryl-hexadienedioic acid diesters (4a-g). The pentacarbonylchromium(0)-catalyzed reactions of 2-alkylfuran (5a-b) with ethyl α-diazophenylacetate (3a) and 9-diazo-9H-fluorene (3h) produce the 1(E),3(E)-butadienes (6a-d) in very good yields.     

Diruthenium(I,I) saccharinate complexes: Synthesis, molecular structure, and evaluation as catalysts for carbenoid reactions of diazoacetates

The dinuclear ruthenium complexes [Ru₂(μ-sac)₂(CO)₆] (1), [Ru₂(μ-sac)₂(CH₃CN)₂(CO)₄] (3), [Ru₂(μ-sac)₂(CO)₅(PPh₃)] (4) and [Ru₂(μ-sac)₂(CO)₄(PPh₃)₂] (5) as well as the tetranuclear ruthenium complex [Ru₂(μ-sac)₂(CO)₅]₂ (2) (sac = saccharinate, C₇H₄NO₃S⁻) were synthesized starting from Ru₃(CO)₁₂ and saccharin. X-ray crystal structure analysis of 1, 3A × p-xylene, 4 × CH₂Cl₂ and 5 × 3CH₂Cl₂ showed that the core is bridged through the amidate moieties of the two saccharinate ligands, with a head-tail arrangement in complexes 1, 3A and 5, and a head-head arrangement in 4. For complex 3, an equilibrium mixture of the head-head regioisomer 3A and a second species 3b exists in solution. Complexes 1 and 2 are suitable catalysts for the cyclopropanation of nucleophilic alkenes (styrene, cyclohexene and 2-methyl-2-butene) with methyl diazoacetate.     

Rh(II) acetate catalyzed cyclopropanation of styrenes with enaldiazo esters: diastereoselective synthesis of enal-cyclopropanes

An efficient Rh(II) acetate catalyzed highly diastereoselective cyclopropanation of styrenes with enaldiazo esters has been developed (up to >95:5 dr and 62–82% yield). The reaction is proposed to involve diacceptor electrophilic rhodium enalcarbenoids and constitutes the first direct synthesis of enal-cyclopropanes with an all carbon γ-quaternary stereocenter.     

An efficient route for the construction of cyclopenta[b]quinoline derivatives via intramolecular cyclopropanation

A one-pot process to introduce diazoacetoacetate functionality into quinoline was identified with excellent yield and regioselectivity. An intramolecular cyclopropanation of the resulting adducts gave tetracyclic cyclopenta[b]quinoline derivatives in nearly quantitative yields. A synthetic utility of the tetracyclic derivatives was examined by a simple ring opening reaction to afford cyclopenta[b]quinoline in a good yield.     

New chiral Schiff base–Cu(II) complexes as cyclopropanation catalysts

A series of chiral Schiff base ligands 1–4, derived from (1R,2S)-(+)-cis-1-amino-2-indanol and other chiral amines with substituted salycilaldehydes were synthesized and transformed to the corresponding Cu(II) complexes. Molecular structures of six Cu(II) complexes were determined by X-ray crystallographic studies. The structures show the metal ion in a distorted square planar geometry with dimeric or monomeric structures, depending on the ligand denticity. The potential use of these complexes in asymmetric Cyclopropanation was explored.     

Chiral copper-bipyridine complexes: Synthesis, characterization and mechanistic studies on asymmetric cyclopropanation

Chiral bipyridine ligands of different steric properties when reacted with CuCl₂ formed orange, yellow or green solids of new copper(II) complexes, [Cu(L)Cl₂] (L = L2-6), in good yield. Together with [Cu(L1)Cl₂], these complexes were characterized in solution by UV-Vis spectroscopy and cyclic voltammetry. The complexes give d-d transitions between 860 and 970 nm, and exhibit one quasi-reversible Cu(II)/Cu(I) couple between +0.405 V and +0.516 V versus NHE. Two of the copper(II) complexes, [Cu(L5)Cl₂] and [Cu(L6)Cl₂], and a copper(I) complex of L1, [Cu(L1)Cl], were characterized by X-ray crystallography. The triflate derivatives of both the Cu(I) and Cu(II) complexes are active catalysts towards the cyclopropanation of ethyl diazoacetate with styrene. The asymmetric induction suffers when the size difference between the alkyl and alkoxyl groups was minimized. The mechanism of the cyclopropanation was studied with kinetic and competition experiments. The rate is first order in catalyst and ethyl diazoacetate, inverse order with styrene and is strongly affected by the counterion.     

Highly diastereoselective synthesis of spiro[cyclopropane-1,3′-indolin]-2′-ones via catalyst-free cyclopropanation using ethyl diazoacetate

Substituted 3-methyleneindolin-2-ones were efficiently cyclopropanated with ethyl diazoacetate to yield spiro[cyclopropane-1,3′-indolin]-2′-ones in a catalyst-free and highly diastereoselective reaction in a mixture of ethanol–acetonitrile (1:1 v/v) as solvent.     

Synthesis of 2,3-benzoxepins by sequential cyclopropanation/ring-enlargement reactions of benzopyrylium triflates with diazoesters

Functionalized 2,3-benzoxepins were prepared by cyclopropanation of benzopyrylium triflates with diazoesters and subsequent TFA-mediated ring enlargement.     

Palladium-catalyzed cyclization/cyclopropanation reaction for the synthesis of fused N-containing heterocycles

Palladium-catalyzed cyclization/cyclopropanation can be used to convert a range of substituted cyclic N-aryl allyl/methallyl amines efficiently and selectively to the corresponding fused tetrahydropyridine/cyclopropane-fused isoquinoline derivatives via β-hydrogen elimination or a domino sequence.     

Efficient cyclopropanation of active methylene compounds. A serendipitous discovery

Cyclopropanation of active methylene compounds has been achieved in good yields with ethylene carbonate as the cyclopropanating agent in the presence of a simple base.     

Cu(I) catalysed cyclopropanation of olefins: Stereoselectivity studies with Arylid-Box and Isbut-Box ligands

In our quest to find new ligands for highly stereoselective reactions, we tested a variety of chiral non-racemic pseudo-C₂ symmetric bis-oxazolines derived from malonic acid containing an arylidene bridge unit (and appropriately termed Arylid-Box) in the catalytic asymmetric cyclopropanation (CAP) reaction of styrene and ethyl diazoacetate using between only 1-2 mol% of a Cu(I) pre-catalyst. Some very good e.e.s (up to 86%), were obtained. It was possible to isolate 10a′-[Cu(CH₃CN)₄]PF₆ which existed as a bench stable solid that proved to be more efficient than the catalyst prepared in situ. Cu(I) pre-catalysts were used for the first time in the CAP reaction with the Isbut-Box ligands 13a and 13b and, although, the e.e.s were better for ligand 13a using these pre-catalysts, in the case of ligand 11b this was not the case. Spectroscopic studies (UV-Vis and ¹H NMR) were carried out to gain an insight into the nature of the catalytic species at work so that the conditions could be optimised giving better results. Some theoretical studies were conducted to try to explain the better enantioselectivities obtained using Evans’ tert-Box-Cu(I) complex over our complex.     

Efficient aldehyde olefination reactions catalyzed by an iron porphyrin complex in an ionic liquid

The commercially available complex Fe(TPP)Cl is an active and highly (E)-selective catalyst for the olefination of a variety of aldehydes in the presence of PPh₃ and diazoacetate in the ionic liquid (bmim)(PF₆). Dependent on the reactivity of the applied aldehyde, the reaction can be carried out at a reaction temperature of 50-80 °C. After 0.5-24 h quantitative olefin yields are reached with a broad variety of different aldehydes. Due to the application of an ionic liquid as reaction medium the products can be easily removed from the catalyst by a simple extraction and the catalyst is conveniently reusable without significant activity loss. Spectroscopic investigations indicate that the reaction mechanism includes the quantitative formation of a phosphorus ylide, which then reacts further in a Wittig reaction under formation of an olefin.     

Modular furanoside phosphite ligands for asymmetric Pd-catalyzed allylic substitution.

A series of diphosphite, phosphine-phosphite, and thioether-phosphite ligands 1-5 with a furanoside backbone have been used in the enantioselective palladium-catalyzed allylic substitution of rac-1,3-diphenyl-2-propenyl acetate giving low to high enantioselectivies (from close to 0% to 97% ee). The modular nature of these ligands enables systematic investigations of the effect of the ligand structure on the enantioselectivity. The enantioselectivity is mainly determined by the configuration of the stereogenic center C-3 of the furanose backbone. From this we conclude that the attack of the nucleophile takes place trans toward the donating group at the stereogenic C-5 atom. Systematic variation of the donor group attached to the carbon atom C-5 indicated that the presence of a bulky phosphite functionality has a positive effect on enantioselectivity. Thus, the highest ee's are obtained using the bulky diphosphite ligand 1b containing a xylofuranoside backbone.     

Ruthenium titanocene and ruthenium titanium half-sandwich bimetallic complexes in catalytic cyclopropanation

The reaction of the phosphine functionalised titanium half-sandwich complexes 7, 9 and 10 with the binuclear complex [(p-cymene)RuCl₂]₂ allowed the access to three new early-late bimetallic complexes (p-cymene)[(μ-η⁵:η¹-C₅H₄(CH₂)_nPR₂)TiX₃]RuCl₂ (11-13). The structure of 11 (n = 0, X = Cl) has been confirmed by X-ray diffraction. The ruthenium titanium half-sandwich bimetallic complexes so formed and the ruthenium titanocene analogues 4-6 catalyse the addition of ethyl diazoacetate to styrene with high selectivity toward cyclopropanation versus metathesis contrary to the monometallic complexes (p-cymene)RuCl₂PR₃.     

Palladium-mediated synthesis of novel meso-chiral porphyrins: cobalt-catalyzed cyclopropanation

A series of novel meso-chiral porphyrins were effectively synthesized from reactions of 5,15-dibromo-10,20-diarylporphyrins with readily available chiral alcohols and amides via palladium-mediated C-N and C-O bond formations. Cobalt complexes of these chiral porphyrins were prepared and shown to be effective catalysts for cyclopropanation of styrene with ethyl diazoacetate under mild and practical conditions, affording the desired cyclopropane ester as a trans-dominant form in excellent yields. Due to orientation and flexibility of the chiral appendages, only low enantioselectivity was observed.     

Application of carbene chemistry in the synthesis of organofluorine compounds

The methods for the synthesis of fluorine-containing organic compounds are highly demanded due to the importance of those compounds in various fields. On the other hand, carbenes are in general highly reactive species, though which various transformations are possible. Carbene chemistry has been recently explored in the development of new methodologies for the synthesis of organofluorine compounds. This short report summarizes the recent advances in this area.     

Desymmetrization of meso [3.2.1]oxabicyclic systems using metal-catalysed asymmetric intramolecular C-H insertion

Diazoketone derivatives of meso 8-oxabicyclo[3.2.1]octen-3-ones were desymmetrized by an intramolecular C-H insertion mediated by chiral copper and dirhodium catalysts to generate oxatricyclic systems with up to 5 contiguous stereogenic centres and 50% ee.