Bipyridinium and Phenanthrolinium Dications for Metal-Free Hydrodefluorination: Distinctive Carbon-Based Reactivity.

The development of novel Lewis acids derived from bipyridinium and phenanthrolinium dications is reported. Calculations of Hydride Ion Affinity (HIA) values indicate high carbon-based Lewis acidity at the ortho and para positions. This arises in part from extensive LUMO delocalization across the aromatic backbones. Species [C10 H6 R2 N2 CH2 CH2 ]2+ (R=H [1 a]2+ , Me [1 f]2+ , tBu [1 g]2+ ), and [C12 H4 R4 N2 CH2 CH2 ]2+ (R=H [2 a]2+ , Me [2 b]2+ ) were prepared and evaluated for use in the initiation of hydrodefluorination (HDF) catalysis. Compound [2 a]2+ proved highly effective towards generating catalytically active silylium cations via Lewis acid-mediated hydride abstraction from silane. This enabled the HDF of a range of aryl- and alkyl- substituted sp3 (C-F) bonds under mild conditions. The protocol was also adapted to effect the deuterodefluorination of cis-2,4,6-(CF3 )3 C6 H9 . The dications are shown to act as hydride acceptors with the isolation of neutral species C16 H14 N2 (3 a) and C16 H10 Me4 N2 (3 b) and monocationic species [C14 H13 N2 ]+ ([4 a]+ ) and [C18 H21 N2 ]+ ([4 b]+ ). Experimental and computational data provide further support that the dications are initiators in the generation of silylium cations.

Synthesis of Fluorinated Aminoalkylboronic Acids from Amphoteric α-Boryl Aldehydes.

Our ongoing search for underdeveloped functional group combinations has brought to light α-fluorinated aminoalkylboronic acids, a new class of molecules featuring the B-CF linkage. These compounds can now be generated from secondary amines and α-boryl aldehydes through electrophilic fluorination of boryl enamines or enamides. Fluorinated ß-aminoalkylboronic acids show no signs of degradation under ambient conditions. We present evidence for the involvement of chair-like motifs, favored over the acyclic forms by up to 1.7±0.1 kcal mol-1 in water and held together by an amine-boronate hydrogen bond. Fluorinated ß-aminoalkylboronic acids are stable over a wide pH range and are characterized by a pKa of 3.4, which is the lowest of any alkylboronic acid.

Synthesis of Urea Derivatives from CO2 and Silylamines.

A series of thirty-three N,N'-diaryl, dialkyl, and alkyl-aryl ureas have been prepared in pyridine or toluene by reaction of silylamines with CO2 . This protocol is shown to provide facile access to 13 C-labeled ureas, as well as chiral and macrocyclic ureas. These reactions proceed through initial generation of the corresponding silylcarbamates, which subsequently react with silylamine under thermal conditions to afford the thermodynamically favored urea and disilyl ether.

Palladium-Catalyzed Hydride Addition/C-H Bond Activation Cascade: Cycloisomerization of 1,6-Diynes.

The use of ammonium halide salts as metal hydride precursors in a new Pd-catalyzed cycloisomerization of 1,6-diynes, which affords unexplored silylated 2-azafluorenes, is reported. This cascade process includes the addition of a Pd-hydride species to a π-system, intramolecular carbopalladation, and C(sp2)-H bond activation. A variety of functional groups are tolerated, and the synthetic utility of the resulting products has been demonstrated by a series of derivatizations.

Enantioselective Intramolecular Copper-Catalyzed Borylacylation.

An enantioselective copper-catalyzed intramolecular borylacylation is reported. The reaction proceeds through an initial enantioselective borylcupration of the styrene, followed by a nucleophilic attack on the tethered carbamoyl chloride. The products, chiral borylated 3,3-disubstituted oxindoles, were generated in excellent yields and enantioselectivities. The versatile carbon-boron bond provides a platform for a wide array of diversification.