Accessing Cationic α-Silylated and α-Germylated Phosphorus Ylides

The synthesis and full characterization of α‐silylated (α‐SiCPs; 1–7) and α‐germylated (α‐GeCPs; 11–13) phosphorus ylides bearing one chloride substituent R₃PC(R¹)E(Cl)R²₂ (R=Ph; R¹=Me, Et, Ph; R²=Me, Et, iPr, Mes; E=Si, Ge) is presented. The molecular structures were determined by X‐ray diffraction studies. The title compounds were applied in halide abstraction studies in order to access cationic species. The reaction of Ph₃PC(Me)Si(Cl)Me₂ (1) with Na[B(C₆F₅)₄] furnished the dimeric phosphonium‐like dication [Ph₃PC(Me)SiMe₂]₂[B(C₆F₅)₄]₂ (8). The highly reactive, mesityl‐ or iPr‐substituted cationic species [Ph₃PC(Me)SiMes₂][B(C₆F₅)₄] (9) and [Ph₃PC(Et)SiiPr₂][B(C₆F₅)₄] (10) could be characterized by NMR spectroscopy. Carrying out the halide abstraction reaction in the sterically demanding ether iPr₂O afforded the protonated α‐SiCP [Ph₃PCH(Et)Si(Cl)iPr₂][B(C₆F₅)₄] (6 dec) by sodium‐mediated basic ether decomposition, whereas successfully synthesized [Ph₃PC(Et)SiiPr₂][B(C₆F₅)₄] (10) readily cleaves the F−C bond in fluorobenzene. Thus, the ambiphilic character of α‐SiCPs is clearly demonstrated. The less reactive germanium analogue [Ph₃PC(Me)GeMes₂][B{3,5‐(CF₃)₂C₆H₃}₄] (14) was obtained by treating 11 with Na[B{3,5‐(CF₃)₂C₆H₃}₄] and fully characterized including by X‐ray diffraction analysis. Structural parameters indicate a strong C_Ylide−Ge interaction with high double bond character, and consequently the C−E (E=Si, Ge) bonds in 9, 10 and 14 were analyzed with NBO and AIM methods.

Enantiodivergent Cyclization by Inversion of the Reactivity in Ambiphilic Molecules

Inverting the reactivity of the functional groups in ambiphilic molecules provides a new synthetic strategy to perform late-stage enantiodivergence. Both enantiomers of the final compound can be obtained from a common chiral precursor. As a proof of concept, the synthesis of substituted five- and six-membered oxacycles is described. The key step is the cyclization of an ambiphilic linear precursor bearing a propargylic alcohol and an epoxide linked through an alkyl chain. Through a slight modification of these linear precursors and employing different reaction conditions, these functional groups can inverse their chemical reactivity, producing one enantiomer or another of the final product. This enantiodivergent cyclization involves three stereogenic centers that can undergo fully controlled retention or inversion of their configuration depending on the cyclization pathway that is activated. The cyclization provides late-stage enantiodivergence, enabling the synthesis of either enantiomers of the oxacycles from a common chiral substrate with total transfer of the enantiomeric purity.

Ambiphilic Frustrated Lewis Pair Exhibiting High Robustness and Reversible Water Activation: Towards the Metal-Free Hydrogenation of Carbon Dioxide

The synthesis and structural characterization of a phenylene-bridged Frustrated Lewis Pair (FLP) having a 2,2,6,6-tetramethylpiperidine (TMP) as the Lewis base and a 9-borabicyclo[3.3.1]nonane (BBN) as the Lewis acid is reported. This FLP exhibits unique robustness towards the products of carbon dioxide hydrogenation. The compound shows reversible splitting of water, formic acid and methanol while no reaction is observed in the presence of excess formaldehyde. The molecule is incredibly robust, showing little sign of degradation after heating at 80 °C in benzene with 10 equiv. of formic acid for 24 h. The robustness of the system could be exploited in the design of metal-free catalysts for the hydrogenation of carbon dioxide.