Interrupted SNAr-Alkylation Dearomatization

Dearomatizations provide powerful synthetic routes to rapidly assemble substituted carbocycles and heterocycles found in a plethora of bioactive molecules. Harnessing the advantages of dearomatization typically requires vigorous reagents because of the difficulty in disrupting the stable aromatic core. A relatively mild dearomatization strategy is described that employs lithiated nitriles or isocyanides in a simple SNAr-type addition to form σ-complexes that are trapped by alkylation. The dearomatizations are diastereoselective and efficient and rapidly install two new carbon-carbon bonds, one of which is a quaternary center, as well as nitrile, isocyanide, and cyclohexadiene functionalities.

Stereoisomerism of bis(σ-Zincane) Complexes: Evidence for an Intramolecular Pathway

The first bis(σ-zincane) complexes, heterotri- metallic species [M(CO)₄ (η² -HZnBDI)₂ ], have been prepared (BDI=κ² -{2,6-(iPr)₂ C₆ H₃ NCMe}₂ CH). For M=Cr, a single stereoisomer is observed in solution and the solid-state. For M=Mo and W, cis and trans isomers were found to reversibly interconvert at 297 K. Despite the huge steric demands of the ligand on zinc, the cis isomer was found to be the most thermodynamically stable in all cases. The activation parameters for the isomerisation when M=Mo are ΔH^≠ =20.8 kcal mol^-1 and ΔS^≠ =-12.8 cal K^-1  mol^-1 . In combination with DFT calculations, the negative activation entropy suggests an intramolecular rotation mechanism for isomerisation.

Trajectory of Approach of a Zinc-Hydrogen Bond to Transition Metals

Through a dramatic advance in the coordination chemistry of the zinc-hydride bond, we describe the trajectory for the approach of this bond to transition metals. The dynamic reaction coordinate was interrogated through analysis of a series of solid state structures and is one in which the TM-H-Zn angle becomes increasingly acute as the TM-Zn distance decreases. Parallels may be drawn with the oxidative addition of boron-hydrogen and silicon-hydrogen bonds to transition metal centers.