Synthesis and X-ray structure determination of highly active Pd(II), Pd(I), and Pd(0) complexes of di(tert-butyl)neopentylphosphine (DTBNpP) in the arylation of amines and ketones.

The air-stable complex Pd(η(3)-allyl)(DTBNpP)Cl (DTBNpP = di(tert-butyl)neopentylphosphine) serves as a highly efficient precatalyst for the arylation of amines and enolates using aryl bromides and chlorides under mild conditions with yields ranging from 74% to 98%. Amination reactions of aryl bromides were carried out using 1-2 mol % Pd(η(3)-allyl)(DTBNpP)Cl at 23-50 °C without the need to exclude oxygen or moisture. The C-N coupling of the aryl chlorides occurred at relatively lower temperature (80-100 °C) and catalyst loading (1 mol %) using the Pd(η(3)-allyl)(DTBNpP)Cl precatalyst than the catalyst generated in situ from DTBNpP and Pd(2)(dba)(3) (100-140 °C, 2-5 mol % Pd). Other Pd(DTBNpP)(2)-based complexes, (Pd(DTBNpP)(2) and Pd(DTBNpP)(2)Cl(2)) were ineffective precatalysts under identical conditions for the amination reactions. Both Pd(DTBNpP)(2) and Pd(DTBNpP)(2)Cl(2) precatalysts gave nearly quantitative conversions to the product in the α-arylation of propiophenone with p-chlorotoluene and p-bromoanisole at a substrate/catalyst loading of 100/1. At lower substrate/catalyst loading (1000/1), the conversions were lower but comparable to that of Pd(t-Bu(3)P)(2). In many cases, the tri-tert-butylphosphine (TTBP) based Pd(I) dimer, [Pd(µ-Br)(TTBP)](2), stood out to be the most reactive catalyst under identical conditions for the enolate arylation. Interestingly, the air-stable Pd(I) dimer, Pd(2)(DTBNpP)(2)(µ-Cl)(µ-allyl), was less active in comparison to [Pd(µ-Br)(TTBP)](2) and Pd(η(3)-allyl)(DTBNpP)Cl. The X-ray crystal structures of Pd(η(3)-allyl)(DTBNpP)Cl, Pd(DTBNpP)(2)Cl(2), Pd(DTBNpP)(2), and Pd(2)(DTBNpP)(2)(µ-Cl)(µ-allyl) are reported in this paper along with initial studies on the catalyst activation of the Pd(η(3)-allyl)(DTBNpP)Cl precatalyst.

A highly efficient, practical, and general route for the synthesis of (R3P)2Pd(0): structural evidence on the reduction mechanism of Pd(II) to Pd(0).

A highly efficient, practical, and general method was developed to synthesize a family of (R(3)P)(2)Pd(0) complexes, using a stoichiometric amount of phosphine ligands and readily available Pd(II) precursors. The stepwise pathway of reducing Pd(II) to Pd(0) was established by isolating two key intermediates. Both [t-Bu(2)(4-Me(2)NC(6)H(4))P](2)Pd and (t-Bu(2)NpP)(2)Pd are new compounds. Preliminary studies on [t-Bu(2)(4-Me(2)NC(6)H(4))P](2)Pd have indicated that it is a very active catalyst (84-95% isolated yield) in the Cu-free Sonogashira coupling involving aryl and heteroaryl chlorides at 0.5 mol % catalyst loading.

alpha-arylation of ketones using highly active, air-stable (DtBPF)PdX2 (X = Cl, Br) catalysts.

alpha-Arylation of various ketones with aryl chlorides and bromides using the well-defined and air-stable (DtBPF)PdX2 (X = Cl, Br) catalysts gave 80-100% yield of the coupled products under relatively mild conditions at low catalyst loadings. The X-ray structure of (DtBPF)PdCl2 revealed the largest P-Pd-P bite angle (104.2 degrees ) for a ferrocenyl bisphosphine ligand. 31P NMR monitoring of (DtBPF)PdCl2-catalyzed reaction of 4-chlorotoluene with propiophenone indicated that DtBPF remained coordinated in a bidentate mode during the catalytic cycle.

[(Bisphosphine) Ru(II) diamine] complexes in asymmetric hydrogenation: expanding the scope of the diamine ligand.

[(Bisphosphine) RuCl 2 (1,2-diamine)] complexes are powerful catalysts in the asymmetric hydrogenation of unfunctionalized ketones. We sought to expand the scope and applicability of these complexes by exploring changes to the diamine structural motif. Via introduction of 1,3- and 1,4-diamines, the catalytic activity was significantly altered such that new classes of ketones could be considered for [(bisphosphine) RuCl 2 (diamine)] asymmetric hydrogenation.

Asymmetric hydrogenation of isobutyrophenone using a [(Diphosphine) RuCl2 (1,4-diamine)] catalyst.

[reaction: see text] The use of three chiral 1,4-diamines in the [(diphosphine) RuCl(2) (diamine)] catalyst system is demonstrated in the hydrogenation of acetophenone. The use of a 1,4-diamine offers unique properties that allow tuning of the catalyst system. These include the first example of the use of a racemic diamine in combination with a chiral phosphine, which gives 95% ee in the hydrogenation of isobutyrophenone.

Reactivity of a N-heterocyclic carbene, 1,3-di-(1-adamantyl) imidazol-2-ylidene, with a pseudo-acid: structural characterization of Claisen condensation adduct.

The reactivity of a N-heterocyclic carbene (NHC) with a pseudo-acid (ester in this case) is described. The product results from an unusual C-H bond activation. The structure of the product has been established by a single crystal diffraction study.

New strategy for the construction of a monotetrahydrofuran ring in Annonaceous acetogenin based on a ruthenium ring-closing metathesis: application to the synthesis of Solamin.

An original convergent total synthesis of Solamin (type A annonaceous acetogenin) was achieved. The central THF core was obtained by means of a ring-closing metathesis (RCM) reaction using a ruthenium imidazolylidene complex. The RCM substrate was prepared from a vinyl-substituted epoxide by reaction with an allyl alcohol, both synthesized from propargylic alcohol. The flexibility of the strategy should be useful in preparing various natural and unnatural annonaceous acetogenins.

Efficient transesterification/acylation reactions mediated by N-heterocyclic carbene catalysts.

Imidazol-2-ylidenes, a family of N-heterocyclic carbenes (NHC), are efficient catalysts in the transesterification involving numerous esters and alcohols. Low catalyst loadings of aryl- or alkyl-substituted NHC catalysts mediate the acylation of alcohols with enol acetates in short reaction times at room temperature. Commercially available and more difficult to cleave methyl esters react with primary alcohols in the presence of alkyl-substituted NHC to efficiently form the corresponding esters. While primary alcohols are selectively acylated over secondary alcohols with use of enol esters as acylating agents, methyl and ethyl esters can be employed as protective agents for secondary alcohols in the presence of the more active alkyl-substituted NHC catalysts. The NHC-catalyzed transesterification protocol was simplified by generating the imidazol-2-ylidene catalysts in situ.

N-heterocyclic carbenes as versatile nucleophilic catalysts for transesterification/acylation reactions.

[reaction: see text] Imidazol-2-ylidenes, a family of N-heterocyclic carbenes (NHC), are efficient catalysts in the transesterification between esters and alcohols. Low catalyst loadings of aryl- or alkyl-substituted NHC catalysts mediate the acylation of alcohols with vinyl acetate in convenient reaction times at room temperature. Commercially available and more difficult to cleave methyl esters react with numerous alcohols in the presence of alkyl-substituted NHC to form efficiently the corresponding esters in very short reaction times.