Π-allyl Complexes Research Articles

The mechanism of enantioselective palladium(0)-catalyzed allylic alkylation with chiral oxazolinylpyridines: a DFT study

The density functional computations of the asymmetric allylic alkylation of (±)tans-1,3-dimethylallyl formate, 2, with malonaldehyde catalyzed by chiral Pd–oxazolinylpyridine are performed. All the structures are optimised completely at the B3LYP/LANL2DZ+P level. As illustrated, this allylic alkylation is endothermic and goes mainly through association of Pd–oxazolinylpyridine with trans-1,3-dimethylallyl formate, oxidative addition of HCOO− in 2 to Pd, nucleophilic addition of malonaldehyde anion to the π-allyl cation complex, and dissociation of the Pd–oxazolinylpyridine–product complex to generate the product with regeneration of the catalyst. The turnover-limiting step for this reaction is the nucleophilic addition of malonaldehyde anion. The main substitution product predicted theoretically is (S)-trans-1,3-dimethylallyl malonaldehyde. The transition states for the oxidative addition and the nucleophilic addition involve a twisted papilionaceous Pd–C–C–C four-membered ring.

Allylic Amination via Decarboxylative C-N Bond Formation

This manuscript details the development of a palladium-catalyzed allylic amination that proceeds via decarboxylation of allylic carbamates. Both saturated and aromatic heterocycles undergo decarboxylative rearrangement ingood yields. The mechanism of allylation of heteroaromatic amines involves the formation of π-allyl palladium complexes followed by decarboxylation of the carbamate. Finally, the heteroaromatic anion equivalent is allylated to provide allylic amines.

Synthesis of α,α-disubstituted amino acids based on tandem reaction of dehydroamino acid derivatives

The formation of all-substituted sp3-hybridized carbon-center was investigated via tandem reaction of dehydroamino acid derivatives. The diethylzinc-promoted reaction of dehydroamino acid derivatives with acid anhydride or π-allyl palladium complex proceeded smoothly to afford α,α-disubstituted amino acids via a radical and anionic carbon–carbon bond-forming processes. The tandem reductive reaction of N-phthaloyl dehydroalanine also proceeded effectively by using Bu3SnH and Pd(PPh3)4.

Diastereoselective Oxidative Addition of Allyl Chloride to Planar-Chiral Cyclopentadienyl−Ruthenium Complexes

The reaction of planar-chiral cyclopentadienyl−ruthenium complexes with allyl chloride at room temperature resulted in the diastereoselective formation of π-allyl−ruthenium complexes, in which the chirality at the Ru center depended on the substituent at the 4-position of the cyclopentadienyl group. Epimerization at the Ru center of π-allyl complexes at 90 °C suggested that the diastereoselectivity was under kinetic control.

A Novel Entry to C‐Glycals via Diethylzinc‐Mediated Umpolung of π‐Allyl Palladium Derived from 1‐exo‐Methylene 2,3‐Anhydrofuranoses.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Enantioselective catalytic allylation of carbonyl groups by umpolung of pi-allyl palladium complexes.

Diethylzinc mediates the asymmetric allylation of aldehydes through Umpolung of a π-allyl palladium complex. An investigation of the palladium source and the chiral monophosphane ligands have shown that the leaving group has a dramatic effect on the enantioselectivity of the reaction. The procedure furnished products in satisfactory yields and up to 70% ee .

Method of Establishing the Lewis Acidity of a Metal Fragment Based on the Relative Binding Strengths of Ar-BIAN Ligands (Ar-BIAN = Bis(aryl)acenaphthenequinonediimine)

The relative coordination strengths of a series of differently substituted Ar-BIAN ligands (Ar-BIAN = bis(aryl)acenaphthenequinonediimine) to a series of palladium complexes in both the formal 0 and 2 oxidation states have been determined. In all cases a good to excellent linearity of log Keq with respect to the Hammett σ constants of the substituents on the aryl fragments of the ligands was observed. The resulting ρ constant is proposed to be a good indication of the Lewis acidity of the metal fragment, a physical quantity for which experimental parameters have been determined only for a limited class of compounds. The obtained parameters allow a comparison not only of different olefin complexes among themselves but also with respect to different metal fragments such as Pd(OAc)2, Pd(Me)Cl, and a π-allyl complex. The Lewis acidity of the olefin complexes is extremely variable and ranges from the less acidic (Pd(Ar-BIAN)(DMFU); DMFU = dimethyl fumarate) to two of the most acidic (Pd(Ar-BIAN)(TCNE) and Pd(A...

Construction of Consecutive Chiral Non‐racemic Quaternary and Tertiary Carbon Centers: A Short Synthetic Route to (‐)‐Acetomycin.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthetic Analogues of the Antibiotic Pestalone.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Palladium catalyzed transformations of monoterpenes: stereoselective deuteriation and oxidative dimerization of camphene

Camphene undergoes a highly regio and stereoselective palladium catalyzed deuteriation in deuteriated acetic acid solutions of Pd(OAc)2. NMR reveals that an outward oriented vinylic hydrogen is selectively exchanged for 2 H, resulting in 90% camphene-d1 (ca. 100% stereoselectivity) and 10% camphene-d2 at 75% conversion of camphene (6 h, 25 C). Neither p-allyl nor p-olefin palladium complexes are formed in detectable concentrations during the reaction, whereas palladium hydride (singlet at )6.86 ppm) and palladium deuteride (singlet at )6.78 ppm) intermediates have been detected by 1 H and 2 H NMR, respectively. At higher temperature, oxidative coupling of camphene readily occurs giving the (E; E)-diene, i.e., bis(3,3-dimethyl-2-norbornylidene)ethane, which formally originates by abstracting the outward oriented vinylic hydrogens and coupling the resulting fragments of two camphene molecules. The reaction is catalytic at palladium in the Pd(OAc)2–LiNO3(cat)–O2 and Pd(OAc)2–benzoquinone systems. Similar mechanisms for the deuteriation and oxidative coupling of camphene are proposed, which involve the formation of r-vinyl palladium hydride intermediates. No deuteriation neither oxidative coupling of limonene, myrcene and b-pinene were observed under the same conditions. 2003 Elsevier B.V. All rights reserved.

Water-Dispersible Tetrablock Copolymer Synthesis, Aggregation, Nanotube Preparation, and Impregnation

A tetrablock copolymer polyisoprene-block-poly(tert-butyl acrylate)-block-poly[(2-cinnamoyloxyethyl methacrylate)-ran-(2-hydroxyethyl methacrylate)]-block-poly(solketal methacrylate), or PI-b-PtBA-b-P(CEMA-r-HEMA)-b-PSMA, was synthesized and characterized. After PSMA hydrolysis to poly(glyceryl methacrylate), or PGMA, the tetrablock formed cylindrical aggregates in water with a PGMA corona and an insoluble part consisting of PI and P(CEMA-r-PHEMA) core−shell cylinders that sandwich a PtBA thin layer. Water-dispersible nanotubes were obtained after the cross-linking of the P(CEMA-r-HEMA) layer and cleavage of the PI cores with ozone. Both nanotubes containing some ozonized PI fragments and the cross-linked PI-b-PtBA-b-P(CEMA-r-HEMA)-b-PGMA cylindrical aggregates sorbed Pd2+ from dimethylformamide as a result of the π-allyl complex formation between Pd2+ and the PI double bonds. The sorbed Pd2+ was reduced to Pd nanoparticles, and the particles were made to overlap by catalyzing further Pd deposition on the...

A novel entry to C-glycals via diethylzinc-mediated umpolung of π-allyl palladium derived from 1- exo-methylene 2,3-anhydrofuranoses

The reaction of aldehydes with allylzinc reagents resulting from the umpolung of π-allyl palladium complexes from 1-exo-methylene 2,3-anhydrofuranoses provides a novel entry to C-glycals.

Palladium‐Catalyzed C—C Bond Formation: Synthesis of 1,1‐Dialkylbuta‐1,3‐dienes and β‐Phenylstyrenes via Organoboron Intermediates.

AbstractFor Abstract see ChemInform Abstract in Full Text.

π-Allyl Nickelate(II) and Palladate(II) Complexes

The homoleptic M−π-allyl complexes M(η3-C3H5)2 (M = Ni, Pd) react with LiC3H5 and Li2C4H8 in the presence of tmeda to give the ionic π-allyl nickelates(II) and palladates(II) [Li(tmeda)2]+[(η3-C3H5)M(η1-C3H5)2]- (M = Ni, 3a; M = Pd, 3b) and [Li(tmeda)2]+[(η3-C3H5)M(C4H8)]- (M = Ni, 4a; M = Pd, 4b). Complexes 3a,b are formed by reversible addition of a further allyl group, while the formation of 4a,b occurs by replacement of one allyl group by the chelating dianionic butane-1,4-diyl ligand. The solid nickel complexes 3a and 4a are temporarily stable at ambient temperature, whereas the palladium derivatives 3b and 4b decompose above 0 °C. All the complexes are ionic, with square-planar metalate(II) anions containing one π-allyl and two σ-organyl ligands, and there is no direct interaction between the Li atom and the anion, as exemplified by low-temperature X-ray structure analyses of 3a and 4a. CP-MAS 13C NMR spectroscopy shows that the structure of 3a is fluxional in the solid state above −93 °C.

Synthetic analogues of the antibiotic pestalone

Abstract The first synthetic study towards the natural product pestalone ( 1 ) is described culminating in the preparation of selected n −1 analogues. Pestalone is a chlorinated and prenylated benzophenone antibiotic, which is of interest due to a strong activity against methicillin-resistant staphylococcus aureus strains (MRSA). Key step of the synthesis is the nucleophilic addition of a highly functionalized aryllithium building block to a 2-prenylated 3,5-dialkoxy-benzaldehyde followed by oxidation. For the introduction of the prenyl sidechain by aryl-allyl coupling, different procedures were evaluated, among them the Stille reaction and a nickel π-allyl complex coupling.

Absolute configuration, coordination and stereochemical influence of 1,3,5,7-tetramethyl-2,6,9-trioxo-bicyclo[3,3,1]nona-3,7-diene in Rh(I) and cationic Pd(II) complexes

Relevant stereochemical and coordination features of 1,3,5,7-tetramethyl-2,6,9-trioxo-bicyclo[3,3,1]nona-3,7-diene (TOND), a chiral molecule of C 2 symmetry are described. The X-ray crystal structure of [RhCl{(S)-CHPhMeNH 2 }{(+)-TOND}] has ascertained that the absolute configuration of (+)-TOND is R , R . Furthermore, the synthesis of stable cationic Pd(II) π-allyl complexes of general formula [Pd(η 3 -allyl)(TOND)][BF 4 ] has allowed to probe the ability of this ligand to afford stereoselective coordination of prochiral fragments. The X-ray molecular structure of the representative compound [Pd(η 3 -crotyl)(TOND)][BF 4 ] has been determined. Finally, the influence of TOND on the stereochemistry of prochiral nitrogen donors of diamine and phosphamine chelates has been explored in rhodium complexes of general formula [Rh(chelate)(TOND)][BF 4 ]. The configurations of the nitrogen donors have resulted as stereospecifically selected by the presence of TOND.

On the regiochemistry of nucleophilic attack on 2-halo pi-allyl complexes. 4. The effect of silver acetate and nucleophile concentrations in competitive nucleophilic attack with malonate and phenoxide nucleophiles.

2,3-Dibromo-1-propene or its allyl carbonate analogue are ionized under Pd catalysis to generate the 2-bromo Pd-pi-allyl complex (triphenylphosphine ligand), which alkylates with malonate nucleophile at the terminal position. The presence of acetate ion in the reaction mixture results in some malonate attack being redirected to the central carbon. The acetate ion can come from the ionization of 1-acetoxy-2-bromo-2-propene or by the addition of silver acetate to the reaction mixture. The addition of phenoxide ion to the reaction also causes the same regiochemical phenomena, although harder anions such as methoxide exert no such effect.

Pd‐Catalyzed Regioselective Acylation of α,β‐Unsaturated Ketone Derivatives by Acylzirconocene Chloride as an Acyl Group Donor.

AbstractFor Abstract see ChemInform Abstract in Full Text.

On the regiochemistry of nucleophilic attack on 2-halo π-allyl complexes. Part 3: The electronic effect of phenoxide ion and the ligand

Abstract 2,3-Dibromo-1-propene was subjected to competitive attack by malonate and different phenoxide nucleophiles in the presence of Pd substituted with monodentate or bidentate ligands. The presence of phenoxide promotes central carbon attack on the initially-formed 2-bromo Pd-π-allyl complex by malonate in the presence of mono or bidentate ligands on Pd. However, bidentate ligands on Pd disfavour the attack of phenoxide, either at the central position or the terminal position of either of the two π-ally complexes formed during the course of these di-additions.

Pd-catalyzed regioselective acylation of α,β-unsaturated ketone derivatives by acylzirconocene chloride as an acyl group donor

Acylzirconocene chlorides reacted with α,β-unsaturated ketones (α,β-enone or -ynone) to give regioselectively 1,2- or 1,4-products under Pd-catalyzed conditions. In the reactions of α,β-enones, excellent regioselectivity was attained by the choice of the Pd(II) catalyst system. Thus, the 1,2- or 1,4-products were selectively prepared by choosing PdCl2(PPh3)2 or Pd(OAc)2–BF3·OEt2, respectively. The presence of a monodentate phosphine ligand brought about a 1,2-addition product, selectively. In these Pd-catalyzed reactions, the Pd(0) species was considered to be an active catalyst, which was generated in situ through transmetalation of acyl groups from zirconium to Pd(II) followed by reductive elimination of Pd(0). α,β-Ynones also reacted with acylzirconocene chloride to give regioselectively 1,4-products under the Pd-catalyzed conditions. In contrast to the reactions of α,β-enones, the presence of a triphenylphosphine ligand brought about the selective formation of 1,4-adduct. By virtue of the phosphine ligand-effect in the addition of an acyl anion to α,β-enones, enantioselective 1,2-addition of acylzirconocene chloride to cyclic α,β-enones was carried out in modest optical purity (∼66% ee) by the use of 5 mol% Pd(OAc)2-(R)-MOP (Pd/P=1:2) catalyst. The origin of the enantioselectivity was deduced by considering an (R)-MOP coordinated π-allylic acyl Pd complex.