Exploration of Lewis acidity and fluorophilicity of germanium compounds and their applications in C-F bond activation

Abstract

The elusive branched fluoro-oligogermane (Ph3Ge)3GeF which is the only crystallographically characterized Ge-F containing compound with unsupported Ge- Ge bonds, was successfully synthesized and its chemical/electrochemical properties studied by NMR, UV-Vis spectroscopy, CV, DPV, and DFT calculations. The key to access (Ph3Ge)3GeF is the germylium intermediate (Ph3Ge)3Ge+ that is a strong Lewis acid and is able to activate the C-X bond in CH2X2 (X=Cl, Br, I).

The potential of germyliums as Lewis acids is explored in hydrodefluorination reactions. In a transition-metal-free approach, the germylium [Ph3Ge]+ generated from Ph3GeH and a catalytic amount of [Ph3C][B(C6F5)4] is able to convert aryl and aliphatic acid fluorides directly to their corresponding aldehydes without decarbonylation. The catalyst is also capable of performing the hydrodefluorination of aliphatic organofluorines.

In the early attempts at C - F amination of organofluorines, it was observed that germanium amides Ph3GeNR2 exhibit a frustrated-Lewis pair-type reactivity. However, when germanium amides Ph3GeNR2 (R=TMS, Me, iPr) are reacted with acyl fluorides, it results in the direct amidation reactions to form important tertiary amides. Experimental and computational studies suggest a o-bond metathesis pathway for this reaction.