((2-Hydroxynapthalen-1-yl)methylene)aniline derived Schiff base adducts of MTO: Synthesis and catalytic application

Equimolar reactions of the derivatives of ((2-hydroxynapthalen-1-yl)methylene)aniline with methyltrioxorhenium (MTO) lead to complexes 1–4, where MTO is coordinated via the oxygen of the former hydroxyl-group to MTO. The resulting complexes are very stable but not particularly catalytically active if no electron acceptor resides on the Schiff base. The electron withdrawing groups placed on the Schiff base ligand have the effect of increasing the catalytic activity but somewhat decrease the complex stability.     

(N‐Salicylidene)aniline Derived Schiff Base Complexes of Methyltrioxorhenium(VII): Ligand Influence and Catalytic Performance

It′s a fine line : Methyltrioxorhenium(VII) (MTO) readily forms 1:1 adducts with several N‐(salicylidene)aniline derived Schiff bases. Steric and electronic effects of the ligands play a major role when it comes to stability and catalytic activity. Good donors on the Schiff base ligands, in general, lead to shorter Re? O(Schiff base) bridges and lower catalytic activity, while it is the opposite for acceptor ligands on the Schiff bases.

     

Enantioselective α‐Benzylation of Acyclic Esters Using π‐Extended Electrophiles

The first asymmetric cooperative Lewis base/palladium catalyzed benzylic alkylation of acyclic esters is reported. This reaction proceeds via stereodefined C1‐ammonium enolate nucleophiles. Critical to its success was the identification of benzylic phosphate electrophiles, which were uniquely reactive. Alkylated products were obtained with very high levels of enantioselectivity, and this method has been applied toward the synthesis of the thrombin inhibitor DX‐9065a.     

The First Enantioselective Aza-Diels–Alder Reactions of Imino Dienophiles on Use of a Chiral Zirconium Catalyst

Chiral dihydropyridone derivatives 3 are obtained in high yields and with good enantioselectivities by the title reaction of aldimines such as 1 with Danishefsky's diene 2 . 6,6′-Dibromo-1,1′-binaphthol complexes with Group 4 metals serve as catalysts; zirconium proved to be especially effective.     

Towards the Design of New Materials: Regio- and Stereoselective (Cyclo-) Polymerization of 1-Alkynes and 1,6-Heptadiynes

Summary. The metathesis polymerization, respectively cyclopolymerization, of 1-alkynes and 1,6-heptadiynes using well-defined group VIA transition metal initiators is summarized. For purposes of comparison, selected quaternary catalytic systems used as alternatives as well as the properties of the resulting materials will be presented. Special consideration is given to the mechanistic advancements that have been made during the last decade, which allow the tailor-made synthesis of conjugated materials with interesting optical and electronic properties. E-mail: michael.r.buchmeiser@uibk.ac.at Received November 20, 2002; accepted November 21, 2002     

Traversing Steric Limitations by Cooperative Lewis Base/Palladium Catalysis: An Enantioselective Synthesis of α‐Branched Esters Using 2‐Substituted Allyl Electrophiles

Cooperative catalysis enables the direct enantioselective α‐allylation of linear prochiral esters with 2‐substituted allyl electrophiles. Critical to the successful development of the method was the recognition that metal‐centered reactivity and the source of enantiocontrol are independent. This feature is unique to simultaneous catalysis events and permits logical tuning of the supporting ligands without compromising enantioselectivity.     

π‐Hole Bonds: Boron and Aluminum Lewis Acid Centers

MP2/aug‐cc‐pVTZ calculations were performed on complexes of boron and aluminum trihydrides and trihalides with hydrogen cyanide (ZH₃‐NCH and ZX₃‐NCH; Z=B, Al; X=F, Cl). The complexes are linked through the B???N and Al???N interactions, which are named as triel bonds and which are classified as π‐hole bonds. It was found that they possess numerous characteristics of typical covalent bonds, since they are ruled mainly by processes of the electron charge shift from the Lewis base to the Lewis acid unit. Other configurations of the ZH₃‐NCH and ZX₃‐NCH complexes linked by the dihydrogen, hydrogen, and halogen bonds were found. However, these interactions are much weaker than the corresponding π‐hole bonds. The quantum theory of atoms in molecules and the natural bond orbital approaches were applied to characterize the complexes and interactions analyzed. The crystal structures of triel trihydrides and triel trihalides were also analyzed for comparison with the results of calculations.     

A Stable Homodinuclear Biscobalt(III)–Schiff Base Complex for Catalytic Asymmetric 1,4‐Addition Reactions of β‐Keto Esters to Alkynones

Two metal cooperation : A homodinuclear Co₂–Schiff base complex Co₂(OAc)₂– 1 promoted the asymmetric 1,4‐addition of β‐keto esters to alkynones under solvent‐free conditions in air (see scheme). The reactions proceeded without air or moisture sensitivity in high yields and with high enantioselectivities (99–91 % ee) at room temperature under highly concentrated conditions (neat–20 M ) with 0.1–2.5 mol % catalyst loading.

     

Epoxidation reactions with urea–hydrogen peroxide catalyzed by methyltrioxorhenium(VII) on niobia

Soybean oils (oleic, linoleic, and linolenic acids and their methyl esters) are epoxidized readily with urea–hydrogen peroxide (UHP) when methyltrioxorhenium(VII) supported on niobia is used as the catalyst in chloroform. Simple alkenes are epoxidized by the same method. The epoxide and not a diol is produced.     

Tandem Gold Self‐Relay Catalysis for the Synthesis of 2,3‐Dihydropyridin‐4(1 H)‐ones: Combination of σ and π Lewis Acid Properties of Gold Salts

The dual ability of gold salts to act as π‐ and σ Lewis acids has been exploited in a tandem self‐relay catalysis. Thus, triphenylphosphanegold(I) triflate mediated the intramolecular carbonyl addition of the amide functionality of homoprogargyl amides to a triple bond. The formation of a σ complex of the gold salt with the intermediate oxazine promoted a nucleophilic addition followed by a Petasis–Ferrier rearrangement. This tandem protocol, catalyzed by the same gold salt under the same reaction conditions, gave rise to the efficient synthesis of 2,3‐dihydropyridin‐4‐(1 H)‐ones, which contain a cyclic quaternary α‐amino acid unit. The asymmetric version was performed by generating the starting materials from the corresponding sulfinylimines.     

Bis[N,N′‐diisopropylbenzamidinato(−)]silicon(II): Lewis Acid/Base Reactions with Triorganylboranes

Reaction of the donor‐stabilized silylene 1 (which is three‐coordinate in the solid state and four‐coordinate in solution) with BEt₃ and BPh₃ leads to the formation of the Lewis acid/base complexes 2 and 3 , respectively, which are the first five‐coordinate silicon compounds with an Si?B bond. These compounds were structurally characterized by crystal structure analyses and by multinuclear NMR spectroscopic studies in the solid state and in solution. Additionally, the bonding situation in 2 and 3 was analyzed by quantum chemical studies.     

Synthesis of 2′‐Deoxy‐1′‐homo‐N‐nucleosides with Anti‐Influenza Activity by Catalytic Methyltrioxorhenium (MTO)/H2O2 Oxyfunctionalization

This paper describes a new route for the synthesis of 1′‐homo‐N‐nucleoside derivatives by means of either methyltrioxorhenium (MTO) or supported MTO catalysts, with H₂O₂ as the primary oxidant. Under these selective conditions, the oxyfunctionalization of the heterocyclic ring and the N heteroatom oxidation were operative processes, regardless of the type of substrate used, that is, purine or pyrimidine derivatives. In addition, the oxidation of 1′‐homo‐N‐thionucleosides, showed the occurrence of site‐specific oxidative nucleophilic substitutions of the heterocyclic ring. The MTO/H₂O₂ system showed, in general, high reactivity under both homogeneous and heterogeneous conditions, affording the final products with high conversion values of substrates and from medium to high yields. Many of the novel 1′‐homo‐N‐nucleoside analogues were active against the influenza A virus, without any cytotoxic effects, retaining their activity in both protected and unprotected forms.     

Structure–Reactivity Relationship in the Frustrated Lewis Pair (FLP)‐Catalyzed Hydrogenation of Imines

The autoinduced, frustrated Lewis pair (FLP)‐catalyzed hydrogenation of 16‐benzene‐ring substituted N‐benzylidene‐tert‐butylamines with B(2,6‐F₂C₆H₃)₃ and molecular hydrogen was investigated by kinetic analysis. The pK_a values for imines and for the corresponding amines were determined by quantum‐mechanical methods and provided a direct proportional relationship. The correlation of the two rate constants k₁ (simple catalytic cycle) and k₂ (autoinduced catalytic cycle) with pK_a difference between imine and amine pairs (ΔpK_a) or Hammett's σ parameter served as useful parameters to establish a structure–reactivity relationship for the FLP‐catalyzed hydrogenation of imines.     

Isothiourea‐Mediated Asymmetric O‐ to C‐Carboxyl Transfer of Oxazolyl Carbonates: Structure–Selectivity Profiles and Mechanistic Studies

The structural motif within a series of tetrahydropyrimidine‐based isothioureas necessary for generating high asymmetric induction in the asymmetric Steglich rearrangement of oxazolyl carbonates is fully explored, with crossover and dynamic ¹⁹F NMR experiments used to develop a mechanistic understanding of this transformation.     

Synthesis,Coordination, and Spectroscopic Properties of 2,2′‐Bipyridyldimesitylpalladium(II) and 6‐Mesityl‐2,2′‐bipyridyldimesitylpalladium(II)

The synthetic route to the dimesitylpalladium(II) complex [(bpy)PdMes₂] ( 1 ) (Mes = mesityl = 2,4,6‐trimethyl phenyl) does not only give the desired compound but also the 6‐mesityl‐2,2′bipyridyldimesitylpalladium [(6‐Mes‐bpy)PdMes₂] ( 2 ) complex and the free ligand 6,6′‐dimesityl‐2,2′‐bipyridine in reasonable yields. Single crystals of 2 were examined by X‐Ray diffraction. The compound reveals a sterically crowded molecular structure. An intramolecular π‐stacking interaction was found between the mesityl substituent on the bipyridine ligand and the adjacent mesityl ligand. The electrochemical behaviour of 1 and 2 together with a related compound was examined at various temperatures showing two reversible reduction reactions and reversible one‐electron oxidation steps at low temperatures. The latter are assigned to Pd^II/Pd^III couples.