Jerome R. Robinson

NiXantphos: A Deprotonatable Ligand for Room-Temperature Palladium-Catalyzed Cross-Couplings of Aryl Chlorides

Although the past 15 years have witnessed the development of sterically bulky and electron-rich alkylphosphine ligands for palladium-catalyzed cross-couplings with aryl chlorides, examples of palladium catalysts based on either triarylphosphine or bidentate phosphine ligands for efficient room temperature cross-coupling reactions with unactivated aryl chlorides are rare. Herein we report a palladium catalyst based on NiXantphos, a deprotonatable chelating aryldiphosphine ligand, to oxidatively add unactivated aryl chlorides at room temperature. Surprisingly, comparison of an extensive array of ligands revealed that under the basic reaction conditions the resultant heterobimetallic Pd–NiXantphos catalyst system outperformed all the other mono- and bidentate ligands in a deprotonative cross-coupling process (DCCP) with aryl chlorides. The DCCP with aryl chlorides affords a variety of triarylmethane products, a class of compounds with various applications and interesting biological activity. Additionally, the DCCP exhibits remarkable chemoselectivity in the presence of aryl chloride substrates bearing heteroaryl groups and sensitive functional groups that are known to undergo 1,2-addition, aldol reaction, and O-, N-, enolate-α-, and C(sp2)–H arylations. The advantages and importance of the Pd–NiXantphos catalyst system outlined herein make it a valuable contribution for applications in Pd-catalyzed arylation reactions with aryl chlorides.

Synthesis, Bonding, and Reactivity of a Cerium(IV) Fluoride Complex

Oxidation of Ce[N(SiMe3)2]3 in the presence of PF6(-) or BF4(-) afforded isolation of CeF[N(SiMe3)2]3. Structural and electrochemical characterization shows that this compound is in its tetravalent oxidation state and contains a terminal fluoride ligand. Spectroscopy and density functional theory have been used to characterize the Ce-F bond as ionic, which is reinforced by an initial reactivity study that demonstrates the nucleophilicity of the fluoride ligand.

Efficient catalytic cycloalkane oxidation employing a “helmet” phthalocyaninato iron(III) complex

We have examined the catalytic activity of an iron(III) complex bearing the 14,28-[1,3-diiminoisoindolinato]phthalocyaninato (diiPc) ligand in oxidation reactions with three substrates (cyclohexane, cyclooctane, and indan). This modified metallophthalocyaninato complex serves as an efficient and selective catalyst for the oxidation of cyclohexane and cyclooctane, and to a far lesser extent indan. In the oxidations of cyclohexane and cyclooctane, in which hydrogen peroxide is employed as the oxidant under inert atmosphere, we have observed turnover numbers of 100.9 and 122.2 for cyclohexanol and cyclooctanol, respectively. The catalyst shows strong selectivity for alcohol (vs. ketone) formation, with alcohol to ketone (A/K) ratios of 6.7 and 21.0 for the cyclohexane and cyclooctane oxidations, respectively. Overall yields (alcohol + ketone) were 73% for cyclohexane and 92% for cyclooctane, based upon the total hydrogen peroxide added. In the catalytic oxidation of indan under similar conditions, the TON for 1-indanol was 10.1, with a yield of 12% based upon hydrogen peroxide. No 1-indanone was observed in the product mixture.

Asymmetric allylation of ketones and subsequent tandem reactions catalyzed by a novel polymer-supported titanium-BINOLate complex.

By using a novel, simple, and convenient synthetic route, enantiopure 6-ethynyl-BINOL (BINOL = 1,1-binaphthol) was synthesized and anchored to an azidomethylpolystyrene resin through a copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction. The polystyrene (PS)-supported BINOL ligand was converted into its diisopropoxytitanium derivative in situ and used as a heterogeneous catalyst in the asymmetric allylation of ketones. The catalyst showed good activity and excellent enantioselectivity, typically matching the results obtained in the corresponding homogeneous reaction. The allylation reaction mixture could be submitted to epoxidation by simple treatment with tert-butyl hydroperoxide (TBHP), and the tandem asymmetric allylation epoxidation process led to a highly enantioenriched epoxy alcohol with two adjacent quaternary centers as a single diastereomer. A tandem asymmetric allylation/Pauson-Khand reaction was also performed, involving simple treatment of the allylation reaction mixture with Co2(CO)8/N-methyl morpholine N-oxide. This cascade process resulted in the formation of two diastereomeric tricyclic enones in high yields and enantioselectivities.

Exchange Processes in Shibasaki’s Rare Earth Alkali Metal BINOLate Frameworks and Their Relevance in Multifunctional Asymmetric Catalysis

Shibasakis rare earth alkali metal BINOLate (REMB) catalysts (REMB; RE = Sc, Y, La - Lu; M = Li, Na, K; B = 1,1-bi-2-naphtholate; RE/M/B = 1/3/3) are among the most successful enantioselective catalysts and have been employed in a broad range of mechanistically diverse reactions. Despite the phenomenal success of these catalysts, several fundamental questions central to their reactivity remain unresolved. Combined reactivity and spectroscopic studies were undertaken to probe the identity of the active catalyst(s) in Lewis-acid (LA) and Lewis-acid/Brønsted-base (LA/BB) catalyzed reactions. Exchange spectroscopy provided a method to obtain rates of ligand and alkali metal self-exchange in the RE/Li frameworks, demonstrating the utility of this technique for probing solution dynamics of REMB catalysts. Isolation of the first crystallographically characterized REMB complex with substrate bound enabled stoichiometric and catalytic reactivity studies, wherein we observed that substrate deprotonation by the catalyst framework was necessary to achieve selectivity. Our spectroscopic observations in LA/BB catalysis are inconsistent with previous mechanistic proposals, which considered only tris(BINOLate) species as active catalysts. These findings significantly expand our understanding of the catalyst structure in these privileged multifunctional frameworks and identify new directions for development of new catalysts.

Constructor graph description of hydrogen bonding in a supra­molecular assembly of N,N′-bis­(2-hydr­oxy-1-methyl­ethyl)phthalamide

Hydrogen bonds of four types (N-H...O=C, N-H...OH, O-H...O=C and O-H...OH) connect molecules of the title compound, C(14)H(20)N(2)O(4), in the crystal into sheets folded into a zigzag pattern. The intermolecular interactions are discussed in terms of the first- through fourth-level graph sets, and a constructor graph helps visualize the supramolecular assembly.