Zinc-catalyzed cycloisomerizations. Synthesis of substituted furans and furopyrimidine nucleosides

5-Endo-dig cycloisomerization of 1,4- and 1,2,4- mostly aryl-substituted but-3-yn-1-ones in the presence of a catalytic amount of zinc chloride etherate (10 mol %) in dichloromethane at room temperature gave 2,5-di- and 2,3,5-trisubstituted furans in high yields (85-97%). DSC studies confirmed that a solely thermal process does not take place. A relevant catalytic process, employing mu-oxo-tetranuclear zinc cluster Zn4(OCOCF3)6O, yielded bicyclic furopyrimidine nucleosides, when starting from acetyl-protected 5-alkynyl-2'-deoxyuridines (85-86%). Furopyrimidine was deprotected or simultaneously converted into pyrrolopyrimidine nucleoside. The time/concentration dependence for the reaction of 1-phenyl-4-(4-methylphenyl)butynone to 2-(4-methylphenyl)-5-phenylfuran displayed first-order kinetics with the rate dependent on catalyst concentration. The plot of ln k(obs) versus ln[ZnCl2] indicated first-order cycloisomerization, as referred to ZnCl2 concentration, using both NMR and UV-vis reaction monitoring. The crystal structure of propyl furopyrimidine nucleoside (orthorhombic, P2(1)2(1)2(1), a/b/c = 5.684(2)/6.682(2)/36.02(2) A, Z = 4) shows C2'- endo deoxyribose puckering, and the base is found in the anti position in crystalline form.     

Efficient and rapid synthesis of phenolic analogs of 4-phenylbutanoic acid using microwave-assisted Michael addition as a key reaction

The addition of phenols to acrylonitrile in the presence of aqueous benzyltrimethylammonium hydroxide or tetramethylammonium hydroxide under microwave irradiation yielded the corresponding Michael adducts. The obtained adducts were easily transformed to phenolic analogs of 4-phenylbutanoic acids via the hydrolysis of nitrile groups.     

Asymmetric Allylic Alkylation of β‐Ketoesters with Allylic Alcohols by a Nickel/Diphosphine Catalyst

Asymmetric allylic alkylation of β‐ketoesters with allylic alcohols catalyzed by [Ni(cod)₂]/(S)‐H₈‐BINAP was found to be a superior synthetic protocol for constructing quaternary chiral centers at the α‐position of β‐ketoesters. The reaction proceeded in high yield and with high enantioselectivity using various β‐ketoesters and allylic alcohols, without any additional activators. The versatility of this methodology for accessing useful and enantioenriched products was demonstrated.     

Studies of the Electronic Effects of Zinc Cluster Catalysts and Their Application to the Transesterification of β‐Keto Esters

The electronic effects of tetranuclear zinc cluster catalysts on transesterification were investigated by changing the carboxylate ligands in the clusters. High catalyst activity crucially depended on the balance between Lewis acidity and Brønsted basicity of the catalyst; this was consistent with the dual activation of both the electrophile and nucleophile by the cooperative zinc centers. In addition, tetranuclear zinc cluster catalysts achieved the transesterification of β‐keto esters with unprecedented levels of broad substrate generality, in which a newly developed pentafluoropropionate‐bridged zinc cluster and 4‐dimethylaminopyridine additive greatly improved the reactivity of sterically congested α‐ and α,α‐disubstituted β‐keto esters.     

Catalysis of a peptidic micellar assembly covalently immobilized within mesoporous silica channels: importance of amphiphilic spatial design

A mesostructured silica/organic composite 1-MS, constructed from a rodlike micelle of amino acid amphiphile 1 that has a condensable head group and that can be used as a template, was found to be able to catalyze the acetalization of cyclohexanone, in ethanol at 25 degrees C (50% in 12 h), whereas no reaction took place with unfunctionalized mesoporous silica. In sharp contrast, hydrolytic removal of the C16 alkyl tail of immobilized 1 resulted in the complete disappearance of the catalytic activity, which suggests the importance of a hydrophobic inner domain for the admission of cyclohexanone. Unsupported peptide amphiphile 2, under identical conditions to those above, was inefficient for acetalization regardless of the absence (2% in 24 h) or presence of mesoporous silica (7% in 24 h). Reference composite 2-MS, which is a noncovalently immobilized peptidic micelle, was virtually inactive (1% in 24 h). These observations indicate the importance of covalent immobilization of the peptidic rod micelle for catalysis. Mesostructured silicate 3-MS hybridized with a nonpeptidic, ammonium ion amphiphile (3) showed a certain catalytic activity, but the yield (12% in 24 h) of the acetal was much lower than that achieved by using 1-MS as the catalyst. Amorphous silica with immobilized 1 on its surface was much less active than 1-MS for acetalization (5% in 24 h).     

Direct use of allylic alcohols for platinum-catalyzed monoallylation of amines

A new direct catalytic amination of allylic alcohols promoted by the combination of platinum and a large bite-angle ligand DPEphos was developed in which the allylic alcohol was effectively converted to a pi-allylplatinum intermediate without the use of an activating reagent. The use of the DPEphos ligand was essential for obtaining high catalyst activity and high monoallylation selectivity of primary amines, allowing the formation of a variety of monoallylation products in good to excellent yield.     

Tetraplatinum precursors for supramolecular assemblies: syntheses, crystal structures, and stereoselective self-assemblies of [Pt4(micro-OCOCH3)6(kappa4-N4-DArBp)] (DArBp = 1,3-bis(arylbenzamidinate)propane)

Treatment of the cisoid-capped complex [Pt(4)(micro-OCOCH(3))(6)(kappa(4)-N(4)-DAr'Bp)] (), which was obtained from the reaction of [Pt(4)(micro-OCOCH(3))(8)] (1) with Na(2)D(t)BuPhBp (D(t)BuPhBp = 1,3-bis(para-tert-butylphenylbenzamidinate)propane), with an equimolar amount of 1,1'-ferrocenedicarboxylic acid undergoes a stereoselective self-assembly to afford an indented-quadrangle tetramer [Pt(4)(micro-OCOCH(3))(4)(kappa(4)-N(4)-D(t)BuPhBp){Fe(C(5)H(4)COO)(2)}](4) as a single diastereomer.     

Cerium(IV) Hexanuclear Clusters from Cerium(III) Precursors: Molecular Models for Oxidative Growth of Ceria Nanoparticles

Reactions of cerium(III) nitrate, Ce(NO₃)₃?6 H₂O, with different carboxylic acids, such as pivalic acid, benzoic acid, and 4‐methoxybenzoic acid, in the presence of a tridentate N,N,N‐donor ligand, diethylenetriamine (L¹), under aerobic conditions yielded the corresponding cerium hexamers Ce₆O₈(O₂CtBu)₈(L¹)₄ ( 1 ), Ce₆O₈(O₂CC₆H₅)₈(L¹)₄ ( 2 ), and Ce₆O₈(O₂CC₆H₄‐4‐OCH₃)₈(L¹)₄ ( 3 ). Hexamers 1 , 2 , and 3 contain the same octahedral Ce^IV₆O₈ core, in which all interstitial oxygen atoms are connected by μ₃‐oxo bridging ligands. In contrast, treatment of the Ce^IV precursor (NH₄)₂Ce(NO₃)₆ (CAN) with pivalic acid and the ligand L¹ under the same conditions afforded Ce₆O₄(OH)₄(O₂CtBu)₁₂(L¹)₂ ( 4 ), exhibiting a deformed octahedral Ce^IV₆O₄(OH)₄ core containing μ₃‐oxo and μ₃‐hydroxo moieties in defined positions. In contrast to the formation of 1 – 3 , the use of N‐methyldiethanolamine (L) in the reaction with Ce(NO₃)₃?6 H₂O and pivalic acid afforded a previously reported Ce^III dinuclear cluster, Ce₂(O₂CtBu)₆L₂, even in the presence of dioxygen. ESI‐MS analysis of the reaction mixture clearly indicated the importance of the ligand L¹ in promoting oxidation of the Ce^III aggregates, [Ce_n(O₂CtBu)_3n(L¹)₂], which is necessary for the formation of Ce^IV hexamers.     

Direct substitution of the hydroxy group with highly functionalized nitrogen nucleophiles catalyzed by Au(III)

A direct catalytic substitution of various allylic and benzylic alcohols with synthetically useful, but acid-sensitive Boc, Bus, and Dios protected amine nucleophiles, which have not been well utilized for Lewis acid catalysis, with various functionalities (OTBS, OTHP, etc.) was efficiently catalyzed by 1 mol% of Au(III) under mild conditions.     

First isolation of fully delocalized mixed-valent imido-bridged [Ti2]7+ complexes by one-electron reduction of [(C5R5)TiCl]2(μ-NAr)2

Mixed-valent imido-bridged dinuclear titanium complexes, [Cp(2)Co][{(C(5)R(5))TiCl}(2)(μ-NAr)(2)] (R = H, Me; Ar = 3,5-(CF(3))(2)C(6)H(3)), were prepared by one-electron reduction of the corresponding [(C(5)R(5))TiCl](2)(μ-NAr)(2). One unpaired electron is delocalized in the central Ti(μ-NR)(2)Ti core as the first example of a fully delocalized mixed-valent imido-bridged [Ti(2)](7+) species.