Cross-metathesis of 1,3-dienes with electron-deficient olefins

Cross-metathesis reactions between 1,3-dienes and electron-deficient olefins have been investigated. Terminal monosubstituted 1,3-dienes afforded low yields of the desired CM products due to competing cleavage of the internal double bond. This undesired cleavage is successfully suppressed in the case of 1,3-dienes containing a sterically more congested internal double bond. Methyl vinyl ketone as the coupling partner was shown to provide the best yields.     

Electrochemical iodofluorination of electron-deficient olefins

Electron-deficient olefins like α,β-unsaturated ester, amide, and phosphonate reacted with iodonium cation species generated by the anodic oxidation of iodide anion in Et₃N-5HF/CH₃NO₂ to form corresponding iodofluorinated compounds in good to moderate yields. The reaction proceeds at room temperature under air atmosphere and constant current conditions. The iodofluorinated products were shown to be useful fluorine-containing building blocks.     

Efficient epoxidation of electron-deficient olefins with a cationic manganese complex

The complex [MnII(R,R-mcp)(CF3SO3)2] is an efficient and practical catalyst for the epoxidation of electron-deficient olefins. This catalyst is capable of epoxidizing olefins with as little as 0.1 mol % catalyst in under 5 min using 1.2 equiv of peracetic acid as the terminal oxidant. A wide scope of substrates are epoxidized including terminal, tertiary, cis and trans internal, enones, and methacrylates with >85% isolated yields.     

Electrochemical fluoro-selenenylation of electron-deficient olefins

Electrochemical fluoro-selenenylation of electron-deficient olefins like α,β-unsaturated ester, carboxylic acid, amide, and phosphonate was successfully carried out by the anodic oxidation of diphenyl diselenide in the presence of olefins in Et₃N·5HF/CH₃NO₂. The anodically generated benzeneselenenyl fluoride [PhSeF] equivalent was stable in the electrolytic solution, which resulted in the efficient fluoro-selenenylation. The fluoro-selenenylation products were shown to be potential useful fluoro-building blocks.     

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.     

Highly efficient epoxidation of electron-deficient olefins with tetrabutylammonium peroxydisulfate

The epoxidation of α,β-unsaturated carbonyl compounds such as enones and enals was efficiently achieved with tetrabutylammonium peroxydisulfate in the presence of equimolar amounts of hydrogen peroxide and NaOH in acetonitrile or methanol at 25 °C in excellent yields. Base-sensitive substrate such as cinnamaldehyde could be successfully epoxidized in short reaction time under mild reaction conditions.     

Copper hydride-catalyzed reduction of electron-deficient olefins

Copper hydride derived from CuF(PPh₃)₃·2MeOH-bis[(2-diphenylphosphino)phenyl] ethersilane can reduce electron-deficient olefins selectively and efficiently.     

Enantioselective 1,3-dipolar cycloadditions of diazoacetates with electron-deficient olefins

[reaction: see text] A general strategy for highly enantioselective 1,3-dipolar cycloaddition of diazoesters to beta-substituted, alpha-substituted, and alpha,beta-disubstituted alpha,beta-unsaturated pyrazolidinone imides is described. Cycloadditions utilizing less reactive alpha,beta-disubstituted dipolarophiles require elevated reaction temperatures, but still provide the corresponding pyrazolines with excellent enantioselectivities. Finally, an efficient synthesis of (-)-manzacidin A employing this cycloaddition methodology as a key step is illustrated.     

Samarium triiodide-catalyzed conjugate addition of indoles with electron-deficient olefins

The SmI₃-catalyzed reaction of indoles with electron-deficient olefins generated the corresponding Michael adducts in high yields. The substitution on the indole nucleus occurred exclusively at the 3-position and N-alkylation products have not been observed.     

Ruthenium-catalyzed cross-metathesis between diallylsilanes and electron-deficient olefins

Diallysilanes can be selectively coupled with electron-deficient olefins under cross-metathesis (CM) conditions using the Hoveyda-Grubbs catalyst to produce mono- and/or bis-CM compounds in good yield. The formation of the mono- and the bis-CM compounds depends on the nature of the electron-deficient olefin.     

Synthesis of catalytically active polymer-bound schiff base manganese complexes for selective epoxidation of unfunctional olefins

An unsymmetric tetradentate Schiff base with one vinyl group was first synthesized and copolymerized with styrene in toluene. Metalation of the copolymer with Mn(OAc)₂ ·4H₂O and LiCl-H₂O was quantitatively incorporated by the functional moiety determined by ICP-AES technology The linear polymer-bound manganese complexes were used in homogeneous condition as catalysts in acetonitrile for selective epoxidation of unfunctional olefins(i.e. styrene, α -methylstyrene and cyclohexene) at room temperature in the presence of iodosylbenzene (PhIO) as the terminal oxidant. The efficacy of epoxidation using the polymeric catalysts was comparable to the monomolecular analogs. Polymer effect was discussed.     

Brønsted acid-promoted aziridination of electron-deficient olefins

A combined theoretical and experimental approach was used to systematically study the Brønsted acid-promoted aziridination of electron-deficient olefins. It was found that Brønsted acid-promoted aziridination of electron-deficient olefins proceeded through the attack of the internal nitrogen of the azide to the terminal carbon of protonated olefin, which afforded an acyclic adduct that subsequently discharged N₂ to produce the aziridine ring. The basicity of the electron-deficient olefins is an important parameter to determine the efficiency of Brønsted acid-promoted aziridination. More basic carbonyl compounds including vinyl ketones and acrylamides were predicted to be readily activated by Brønsted acid such as TfOH, whereas less basic carbonyl compounds were predicted to be poor substrates. Significantly, all these theoretical predictions were demonstrated to be consistent with the experimental data. Furthermore, a systematic evaluation of TfOH-promoted aziridination of acrylamides was performed, which established a new, single-step method for the preparation of a number of aziridine-2-carboxamides.     

Development of asymmetric phosphine-promoted annulations of allenes with electron-deficient olefins and imines

Asymmetric annulation of allenes with electron-deficient olefins and imines is one of the most important reactions for the synthesis of optically active carbo- and heterocycles, which are useful building blocks for the synthesis of natural products and medicinally important substances. The use of chiral phosphines as enantioselective catalysts can be envisaged for such cyclizations. This article focuses on the important developments concerning asymmetric annulations of allenes with unsaturated partners in the recent decades and on the perspectives that these new developments offer.     

Chemoselective cross-metathesis reaction between electron-deficient 1,3-dienes and olefins

Chemoselective cross-metathesis reactions between methyl sorbate or 1,3-dienic amides and various olefins in the presence of the Grubbs–Hoveyda catalyst have been investigated. Cross-metathesis reactions turned out to be more chemoselective with 1,3-dienic amides than with 1,3-dienic esters.     

Synthesis of new schiff base ester liquid crystals with a benzothiazole core

A series of new calamitic liquid crystals, 6-methoxy-2-(4-alkanoyloxybenzylidenamino)benzothiazoles, comprising a benzothiazole core, terminal methoxy group and a Schiff base linkage were synthesised and characterised. This series comprises 12 members wherein members differ by the length of the alkanoyloxy chain (C _n-1H_2n-1COO-, where n?=?2–8, 10, 12, 14, 16, 18). Their mesomorphic properties were studied by using differential scanning calorimetry, optical polarising microscopy and powder X-ray diffraction techniques. The short chain derivatives (n?=?2 and 3) were non-mesogenic compounds, while an enantiotropic nematic phase was present throughout the remaining members of the series. The smectic C phase emerged from the decanoyloxy derivative onwards.     

Synthesis of phosphiranes via organoiron-catalyzed phosphinidene transfer to electron-deficient olefins

Herein is reported the structural characterization and scalable preparation of the elusive iron–phosphido complex FpP(^tBu)(F) (2-F, Fp = (Fe(η⁵-C₅H₅)(CO)₂)) and its precursor FpP(^tBu)(Cl) (2-Cl) in 51% and 71% yields, respectively. These phosphide complexes are proposed to be relevant to an organoiron catalytic cycle for phosphinidene transfer to electron-deficient alkenes. Examination of their properties led to the discovery of a more efficient catalytic system involving the simple, commercially available organoiron catalyst Fp₂. This improved catalysis also enabled the preparation of new phosphiranes with high yields (^tBuPCH₂CHR; R = CO₂Me, 41%; R = CN, 83%; R = 4-biphenyl, 73%; R = SO₂Ph, 71%; R = POPh₂, 70%; R = 4-pyridyl, 82%; R = 2-pyridyl, 67%; R = PPh₃⁺, 64%) and good diastereoselectivity, demonstrating the feasibility of the phosphinidene group-transfer strategy in synthetic chemistry. Experimental and theoretical studies suggest that the original catalysis involves 2-X as the nucleophile, while for the new Fp₂-catalyzed reaction they implicate a diiron–phosphido complex Fp₂(P^tBu), 4, as the nucleophile which attacks the electron-deficient olefin in the key first P–C bond-forming step. In both systems, the initial nucleophilic attack may be accompanied by favorable five-membered ring formation involving a carbonyl ligand, a (reversible) pathway competitive with formation of the three-membered ring found in the phosphirane product. A novel radical mechanism is suggested for the new Fp₂-catalyzed system.

The iconic cyclopentadienyliron dicarbonyl dimer ([CpFe(CO)₂]₂ or Fp₂) catalyzes phosphinidene group transfer from anthracene to activated alkenes, forming new phosphiranes diastereoselectively and with good isolated yields.

Phosphiranes, the phosphorus analogues of aziridines, have been used as ligands,^1–8 as polymer precursors,^9,10 and more recently as precursors to P,N-bidentate ligands via ring-opening reactions using amide nucleophiles.¹¹ Given the possibility of chirality at both phosphorus and carbon in the three-membered ring,^7,12 such ring-opening reactions have the potential to enable preparation of enantiomerically pure ligands from phosphiranes.In contrast to the well-established alkene aziridination reactions that are facilitated by transition-metal catalysts,^13,14 only a handful of transition-metal promoted phosphirane syntheses, namely “phosphiranation” reactions, have been reported,^15–19 among which there are only two examples of catalytic alkene phosphiranation processes yielding free (unprotected) phosphiranes.^20,21 We recently reported a catalytic method for preparing phosphiranes using a phosphinidene group-transfer strategy, mirroring the analogous aziridination reactions.²⁰ The system consisted of dibenzo-7-phosphanorbornadienes (RPA, A = C₁₄H₁₀, anthracene), a class of compounds capable of transferring phosphinidene groups to unsaturated molecules upon loss of an aromatic moiety,^22–30 as the phosphinidene source and styrene as the receptor (Fig. 1A). In addition, both a source of the cyclopentadienyliron dicarbonyl cation ([CpFe(CO)₂]⁺, Fp⁺) and the fluoride anion were required as co-catalysts for phosphinidene group transfer. The reaction was postulated to proceed via an iron–phosphido intermediate (FpP(^tBu)(F), 2-F, Fig. 1A) based on evidence provided by stoichiometric reactions, deuterium labeling studies, and a Hammett analysis. However, the intermediate 2-F eluded isolation and full characterization due to its instability and high solubility in organic solvents.Open in a separate windowFig. 1(A) Organoiron- and fluoride-catalyzed synthesis of phosphiranes by phosphinidene transfer and the proposed intermediate. (B) This work: organoiron-catalyzed phosphinidene transfer to electron-deficient olefins (EWG = electron-withdrawing group).Herein, we report the isolation and structural characterization of 2-F, thus providing a more complete picture of the catalytic cycle. In addition, we report the discovery of a more efficient catalytic system involving a simple, commercially available organoiron catalyst, the iconic cyclopentadienyliron dicarbonyl dimer ([CpFe(CO)₂]₂ or Fp₂) for this phosphiranation reaction (Fig. 1B). This improved system was also used to prepare new phosphiranes bearing electron-withdrawing substituents, expanding the list of chemically accessible phosphiranes that may serve as useful ligands and synthetic building blocks.The iron–phosphido complex 2-F was identified in our previous work as a plausible intermediate in the catalytic cycle,²⁰ and therefore became the target of an independent synthesis. This species was previously generated in situ by treating [Fp(^tBuPA)][BF₄] with a source of fluoride to elicit anthracene elimination, though at the time it eluded purification and complete characterization. A new, scalable preparation of 2-F was achieved by utilizing the readily available ^tBuPCl₂ and K[Fp]³¹ which, when combined in an equimolar ratio in THF, afforded 2-Cl in 71% yield (Scheme 1). Such halide displacement reactions at phosphorus were previously used to prepare analogous metal–phosphido complexes.^32–34 In a subsequent step, halogen exchange with tetramethylammonium fluoride ([Me₄N]F) in CH₂Cl₂ led to the isolation of 2-F in 51% yield (Scheme 2). In addition to characterization by NMR spectroscopy, both 2-Cl and 2-F were structurally characterized by X-ray crystallography (Fig. 2). Both compounds exhibit a pyramidal geometry at the phosphorus atom, with the sum of bond angles being 316° and 314° for 2-Cl and 2-F, respectively. The Fe–P distances in 2-Cl and 2-F are in the range of a Fe–P single bond, similar to other reported Fp–phosphido complexes.^33,35,36Open in a separate windowScheme 1Synthesis of 2-Cl and 2-F starting from K[Fp] and ^tBuPCl₂.Open in a separate windowScheme 2Stoichiometric (top) and catalytic (bottom) synthesis of 1b.Open in a separate windowFig. 2Solid-state structures of 2-Cl (left) and 2-F (right), with thermal ellipsoids shown at the 50% probability level and hydrogen atoms omitted for clarity. Selected bond lengths (Å) for 2-Cl: Fe1–P1: 2.2935(3); P1–Cl1: 2.1315(3). 2-F: Fe1–P1: 2.2898(4); P1–F1: 1.6455(11).With proposed catalytic intermediate 2-F in hand, it was tested as a catalyst in the phosphiranation reaction of ^tBuPA with styrene resulting in isolation of 1a with a yield similar to that originally reported, supporting the relevance of 2-F to the catalytic cycle. The chloride analogue 2-Cl was also found to catalyze the phosphiranation reaction, albeit with a lower yield (33 while FpP(Ph)₂ could catalyze the isomerization of dimethyl maleate to dimethyl fumarate via reversible nucleophilic addition to the double bond.³⁷ Similar reactions have also been reported for an iridium phosphido complex.³⁸ When 2-Cl was treated with the electron-deficient olefin methyl acrylate, clean conversion to a single new compound 3 was observed (Fig. 3 top). Compound 3 features a five-membered organometallic ring, resulting from addition of methyl acrylate across the nucleophilic phosphorus center and into one carbonyl ligand of the Fp group. Two isomers were identified by NMR spectroscopy, and the major isomer was characterized by X-ray crystallography (Fig. 3 bottom). The formation of 3 from methyl acrylate raises the possibility that an analogous species may play a role in the catalytic cycle of the reaction employing styrene. Heating 2-Cl or 2-F in the presence of excess styrene, however, did not lead to any similar species, although it cannot be ruled out as a short-lived intermediate under the conditions of catalysis.Selected catalyst screening for styrene phosphiranation^a

Phase-transfer catalytic aza-Michael addition of tert-butyl benzyloxycarbamate to electron-deficient olefins

A highly efficient phase-transfer catalytic aza-Michael addition of tert-butyl benzyloxycarbamate to a wide range of electron-deficient olefins is presented (90-99%).     

Novel chlorotitanium complexes containing chiral tridentate schiff base ligands for ring-opening polymerization of lactide

The selective synthesis of tri- and dichlorotitanium species containing chiral tridentate Schiff base ligands, derived from (1R,2S)-(-)-1-aminoindanol, was achieved by changing the solvent. Single-crystal X-ray analyses reveal that chlorotitanium complexes are monomeric and octahedral with the meridional occupation of the Schiff base. Although the chlorotitanium complexes lack typical initiating groups such as alkoxides or amides, they are effective catalysts for the controlled ring-opening polymerization of L-lactide (L-LA) as shown by the linearity of the molecular weight vs [L-LA]/[Ti] ratio plot as well as very narrow polydispersity index values.