An Actinide Complex with a Nucleophilic Allenylidene Ligand

The reaction of [Cp3Th(3,3-diphenylcyclopropenyl)] (Cp = η5-C5H5) with 1 equiv of lithium diisopropylamide (LDA) results in cyclopropenyl ring opening and formation of the thorium allenylidene complex, [Li(Et2O)2][Cp3Th(CCCPh2)] ([Li(Et2O)2][1]), in good yield. Additionally, deprotonation of [Cp3Th(3,3-diphenylcyclopropenyl)] with 1 equiv of LDA, in the presence of 12-crown-4 or 2.2.2-cryptand, results in the formation of discrete cation/anion pairs, [Li(12-crown-4)(THF)][Cp3Th(CCCPh2)] ([Li(12-crown-4)(THF)][1]) and [Li(2.2.2-cryptand)][Cp3Th(CCCPh2)] ([Li(2.2.2-cryptand)][1]), respectively. Interestingly, the complex [Li(Et2O)2][1] undergoes dimerization upon standing at room temperature, resulting in the formation of [Cp2Th(μ:η1:η3-CCCPh2)]2 (2), via loss of LiCp. The reaction of [Li(Et2O)2][1] with MeI results in electrophilic attack at the Cγ carbon atom, leading to the formation of a thorium acetylide complex, [Cp3Th(C≡CC(Me)Ph2)] (3), which can be isolated in 83% yield upon workup, whereas the reaction of [Li(Et2O)2][1] with benzophenone results in the formation of 1,1,4,4-tetraphenylbutatriene (4) in 99% yield, according to integration against an internal standard. Density functional theory (DFT) calculations performed on [1]- and 2 reveal significant electron delocalization across the allenylidene ligand. Additionally, calculations of the 13C NMR chemical shifts for the Cα, Cβ, and Cγ nuclei of the allenylidene ligand were in good agreement with the experimental shifts. The calculations reveal modest deshielding induced by spin-orbital effects originating at Th due to the involvement of the 5f orbitals in the Th-C bonds. According to a DFT analysis, the cyclopropenyl ring-opening reaction proceeds via [Cp3Th(η1-3,3-Ph2-cyclo-C3)]- (IM), which features a carbanion character at Cβ.

Synthesis of trifluoromethyl N,N-aminals from nitrogen containing heterocycles by using a plasma flow microreactor

Trifluoromethyl N,N-aminals are important precursors allowing access to fluorinated building blocks. In this work, the direct synthesis of trifluoromethyl N,N-aminals from nitrogen containing heterocycles is reported using argon plasma in a continuous flow microreactor without any additives or metal catalysts. Their transformation to N-trifluoroethyl amines is also reported.

Synthesis of enantiopure α-Tfm-proline and α-Tfm-pipecolic acid from oxazolo-pyrrolidines and -piperidines

Enantiopure α-Tfm-proline and α-Tfm-pipecolic acid were synthesized starting from commercially available diesters and ethyl trifluoroacetate. A Strecker type reaction on intermediate chiral Tfm-oxazolo-pyrrolidine and -piperidine provided the corresponding nitrile precursor of enantiopure (R) and (S) α-Tfm-proline and α-Tfm-pipecolic acid. The C-terminal peptide coupling reaction of α-Tfm-pipecolic acid has been successfully achieved.

Kitamura Electrophilic Fluorination Using HF as a Source of Fluorine

This review article focused on the innovative procedure for electrophilic fluorination using HF and in situ generation of the required electrophilic species derived from hypervalent iodine compounds. The areas of synthetic application of this approach include fluorination of 1,3-dicarbonyl compounds, aryl-alkyl ketones, styrene derivatives, α,β-unsaturated ketones and alcohols, homoallyl amine and homoallyl alcohol derivatives, 3-butenoic acids and alkynes.

Synthesis of Substituted α-Trifluoromethyl Piperidinic Derivatives

A comprehensive survey of pathways leading to the generation of α-trifluoromethyl monocyclic piperidinic derivatives is provided (65 references). These compounds have been synthesized either from 6-membered rings e.g., pipecolic acid or lactam derivatives by introduction a trifluoromethyl group, from pyridine or pyridinone derivatives by reduction, and from 5-membered rings e.g., prolinol derivatives by ring expansion, from linear amines by cyclization or from dienes/dienophiles by [4 + 2]-cycloaddition.

Stereoselective access to fluorinated and non-fluorinated quaternary piperidines: synthesis of pipecolic acid and iminosugar derivatives

The preparation of optically pure quaternary piperidines, both fluorinated and non-fluorinated, has been achieved from a chiral imino lactone derived from (R)-phenylglycinol. In the case of the fluorinated derivatives, the addition of (trifluoromethyl)trimethylsilane (TMSCF(3)) followed by iodoamination and migration of the CF(3) group allowed access to four derivatives of α-(trifluoromethyl)pipecolic acid. A theoretical study of the CF(3)-group rearrangement has been carried out to help establish the reaction mechanism of this uncommon transformation. Moreover, a route to trifluoromethyl-substituted iminosugars was also developed through the diastereoselective dihydroxylation of suitable synthetic intermediates. Conversely, alkylation of the starting substrate and subsequent cross-metathesis and aza-Michael reactions led to α-alkyl derivatives of the target compounds.

An Overview of Stereoselective Synthesis of α-Aminophosphonic Acids and Derivatives

An overview of all methodologies published during the last few years focused to the stereoselective (diastereoselective or enantioselective) synthesis of α-aminophosphonic acids and derivatives is reported. The procedures have been classified according a retrosynthetic strategy and taking into account the formation of each one of the bonds connected to the chiral centre.

Metal Vinylidenes and Allenylidenes in Catalysis: Applications in Anti-Markovnikov Additions to Terminal Alkynes and Alkene Metathesis

The involvement of a catalytic metal vinylidene species was proposed for the first time in 1986 to explain the regioselective formation of vinyl carbamates directly from terminal alkynes, carbon dioxide, and amines. Since this initial report, various metal vinylidenes and allenylidenes, which are key activation intermediates, have proved extremely useful for many alkyne transformations. They have contributed to the rational design of new catalytic reactions. This 20th anniversary is a suitable occasion to present the advancement of organometallic vinylidenes and allenylidenes in catalysis.

Allenylidene-to-Indenylidene Rearrangement in Arene−Ruthenium Complexes: A Key Step to Highly Active Catalysts for Olefin Metathesis Reactions

The allenylidene-ruthenium complexes [(eta6-arene)RuCl(=C=C=CR2)(PR'3)]OTf (R2 = Ph; fluorene, Ph, Me; PR'3 = PCy3, P(i)Pr3, PPh3) (OTf = CF3SO3) on protonation with HOTf at -40 C are completely transformed into alkenylcarbyne complexes [(eta6-p-cymene)RuCl([triple bond]CCH=CR2)(PR3)](OTf)2. At -20 degrees C the latter undergo intramolecular rearrangement of the allenylidene ligand, with release of HOTf, into the indenylidene group in derivatives [(eta6-arene)RuCl(indenylidene)(PR3)]OTf. The in situ-prepared indenylidene-ruthenium complexes are efficient catalyst precursors for ring-opening metathesis polymerization of cyclooctene and cyclopentene, reaching turnover frequencies of nearly 300 s(-1) at room temperature. Isolation of these derivatives improves catalytic activity for the ring-closing metathesis of a variety of dienes and enynes. A mechanism based on the initial release of arene ligand and the in situ generation of the active catalytic species RuCl(OTf)(=CH2)(PR3) is proposed.

Synthetic applications of aliphatic unsaturated α-H-α-amino acids

This article provides an overview of the literature concerning synthetic applications of unsaturated aliphatic amino acids in the period May 2000 to December 2004.