Edwin D. Stevens

A sterically demanding nucleophilic carbene : 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) : thermochemistry and catalytic application in olefin metathesis

Abstract The sterically demanding nucleophilic carbene ligand 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr, 4) has been synthesized. The reaction of [Cp*RuCl]4 (5; Cp*=η5-C5Me5) with this ligand affords a coordinatively unsaturated Cp*Ru(IPr)Cl (6) complex. Solution calorimetric results in this system provide information concerning the electron donor properties of the carbene ligand. Steric parameters associated with this ligand are determined from the X-ray crystal structure study. The carbene ligand reacts with RuCl2(C(H)Ph)(PCy3)2 (1) to yield a mixed carbene–phosphine ruthenium complex RuCl2(C(H)Ph)(IPr)(PCy3) (9). A single-crystal X-ray diffraction study has been performed on 9. The thermal stability of 9 has been studied at 60°C and its catalytic activity has been evaluated for the ring closing metathesis of diethyldiallylmalonate.

[(NHC)CuX] complexes: Synthesis, characterization and catalytic activities in reduction reactions and Click Chemistry. On the advantage of using well-defined catalytic systems

The preparation of three series of [(NHC)CuX] complexes (NHC = N-heterocyclic carbene, X = Cl, Br, or I) is reported. These syntheses are high yielding and only use readily available starting materials. The prepared complexes were spectroscopically and structurally characterized. Notably, two of them present a bridging NHC ligand between two copper centers in the solid state, an extremely rare coordination mode for these ligands. These complexes were then applied to two distinct organic reactions: the hydrosilylation of ketones and the 1,3-dipolar cycloaddition of azides and alkynes. In both transformations, outstanding catalytic systems were found for preparing the corresponding products in excellent yields and short reaction times. Most remarkably, the screening of well-defined systems in the hydrosilylation reaction allowed for the identification of a pre-catalyst previously overlooked since, originally, catalytic species were in situ generated. Under such conditions, major formation of [(NHC)(2)Cu](+) species, inactive in this reduction reaction, occurred instead of the expected copper hydride. These results highlight one of the most important advantages of employing well-defined complexes in catalysis: gaining an improved control of the nature of the catalytically relevant species in the reaction media.

Catalytic activity of Pd(II) and Pd(II)/DAB-R systems for the Heck arylation of olefins

Abstract Palladium-catalyzed reactions of aryl bromides with various olefins involving Pd(II)/diazabutadiene (DAB-R) systems have been investigated. The scope of a coupling process using Pd(II) sources and an α-diimine as ligand in the presence of Cs2CO3 as base was tested using various substrates. The Pd(OAc)2/DAB-Cy (1, DAB-Cy=1,4-dicyclohexyl-diazabutadiene) system presents the highest activity with respect to electron-neutral and electron-deficient aryl bromides in coupling with electron rich olefins. The synthesis and X-ray characterization of a Pd(II)-diazabutadiene ligand is reported. Extensive optimization experiments showed that another Pd(II) source, Pd(acac)2 (acac=acetylacetonate), proved to activate aryl bromides at high temperatures, low catalyst loadings when the appropriate concentration of nBu4NBr additive was employed. The effect of the DAB-Cy ligand is important at very low catalyst loadings and high temperatures. Pd(acac)2 and Pd(acac)2/DAB-Cy precatalysts were very effective for the arylation of various olefins with aryl bromides with respect to reaction rate, catalyst loadings, and functional group tolerance.

A [(NHC)CuCl] complex as a latent Click catalyst

A latent catalyst for the [3+2] cycloaddition reaction of azides and alkynes has been developed in accordance with the principles of Click chemistry.

Structural characterization and catalytic activity of the rhodium–carbene complex Rh(PPh3)2(IMes)Cl (IMes=bis(1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene)

Abstract The rhodium–carbene complex Rh(PPh3)2(IMes)Cl (2) is an active catalyst for the hydroboration of simple olefins at room temperature. The reactivity of 2 was also tested in the methylenation of aldehydes. The crystal structure of 2 is also reported.

Preparation of dibarbiturates of oxindole by condensation of isatin and barbituric acid derivatives

A very efficient, simple, and high yield procedure for the isatin condensation with barbituric acid derivatives was described. The condensation reaction is very selective, which is corroborated by following the reaction by NMR, and the structure of the product was confirmed by X-ray structural analysis.

Unusual reactivities of N-heterocyclic carbenes upon coordination to the platinum(II)–dimethyl moiety

Unsaturated NHCs of varying steric bulk undergo a series of unusual oxidative addition and reductive elimination processes upon binding to the Pt(Me)(2) fragment.

Mono C-alkylation and mono C-benzylation of barbituric acids through zinc/acid reduction of acyl, benzylidene, and alkylidene barbiturate intermediates

Abstract Through systematic exploration of reaction conditions, very efficient preparative procedures for obtaining large quantities of substituted 5-alkyl and 5-benzylbarbituric acids were developed. The procedure involves a two step preparation in which the second step is zinc dust/acid reduction. For preparation of 5-alkylbarbiturates, the first step is the preparation of either 5-acyl or 5-alkylidenebarbiturate. If 5-benzylbarbiturate is the target product, then the first step includes the preparation of 5-benzylidene. Regardless of the nature of the first step, all reactions presented synthetic yields around 90% and isolation and purification involves only crystallization.

Synthesis and structural analysis of a series of D-glucose derivatives as low molecular weight gelators.

Low molecular weight gelators are an interesting new type of compounds that are important in supramolecular chemistry and advanced materials. Previously, we had synthesized several acyl derivatives of methyl 4,6-O-benzylidene-alpha-D-glucopyranoside and found that a number of terminal acetylene-containing esters are good gelators. To understand the structure requirement of the acyl chains, we synthesized a series of analogs containing different functional groups including aryl, alkenyl, and halogen derivatives. X-ray crystal structures of a monoester and a diester derivative were also obtained to help understand the relationship between structure and gelation. For good gelation properties, the carboxyl derivatives should possess alkyl groups containing a terminal acetylene group and aryl derivatives.

A new facile method for preparation of heterocyclic α-iminonitriles and α-oxoacetic acid from heterocyclic aldehydes, p-aminophenol, and sodium cyanide

Very efficient, simple, and high yield procedures for the transformation of heterocyclic aldehydes into heterocyclic methylidene-p-hydroxyanilines, heterocyclic α-iminonitriles, and finally into heterocyclic α-oxoacetic acids were described. Considering that many of these compounds have biological activity, the synthetic methodology was optimized using readily available, inexpensive starting materials, and the purification of the product involved only simple crystallization.