Umpolung α-Silylation of Cyclopropyl Acetates via Low-Temperature Catalytic C-C Activation

Advances in Catalytic C–F Bond Activation and Transformation of Aromatic Fluorides

The activation and transformation of C–F bonds in fluoro-aromatics is a highly desirable process in organic chemistry. It provides synthetic methods/protocols for the generation of organic compounds possessing single or multiple C–F bonds, and effective catalytic systems for further study of the activation mode of inert chemical bonds. Due to the high polarity of the C–F bond and it having the highest bond energy in organics, C–F activation often faces considerable academic challenges. In this mini-review, the important research achievements in the activation and transformation of aromatic C–F bond, catalyzed by transition metal and metal-free systems, are presented.

Catalytic Net Oxidative C-C Activation and Silylation of Cyclopropanols with a Traceless Acetal Directing Group

Redox-neutral carbon-carbon (C-C) bond activation and functionalization strategies of cyclopropanols that give metallo homoenolate have offered merits to construct a range of useful β-functionalized ketones in an inverse-polarity fashion. Discovery and identification of oxidative C-C activation reactions of cyclopropanols that generate metallo enolate-homoenolate would provide an opportunity to afford α,β-difunctionalized ketones. We report catalytic, net oxidative C-C activation, and silylation of cyclopropanols with traceless acetal directing groups under consecutive Ir and Rh catalysis in regio-, stereo-, and chemo-selective fashion. In detail, Ir-catalyzed hydrosilylation of cyclopropyl acetates provides the acetal directing group in quantitative yield. Rh-catalyzed proximal C-C silylation of the resulting cyclopropyl silyl acetal produces the metallo enolate-homoenolate equivalent, dioxasilepine, which uniquely holds an interconnected β-silyl moiety and Z-vinyl acetal. Upon sequential treatment of a silaphile that removes the acetal directing group and electrophile, the seven-membered silicon-containing heterocycle, serving as the ketone α,β-dianion equivalent, delivers α,β-difunctionalized ketones. Scope of the hitherto unexplored reactivity of cyclopropanols toward net oxidative C-C silylation and the versatility of the resulting dioxasilepines were demonstrated. These include late-stage, net oxidative C-C silylation of biologically relevant molecules and facile production of a range of α,β-difunctionalized ketones. Preliminary mechanistic studies suggest that the C-C activation harnessing the electron-rich Wilkinson-type catalyst is likely the turnover-determining step and a Rh-π interaction is the key to the efficient metal insertion to the proximal C-C bond in cyclopropanols.

SuFEx-enabled, chemoselective synthesis of triflates, triflamides and triflimidates

Sulfur(vi) Fluoride Exchange (SuFEx) chemistry has emerged as a next-generation click reaction, designed to assemble functional molecules quickly and modularly. Here, we report the ex situ generation of trifluoromethanesulfonyl fluoride (CF₃SO₂F) gas in a two chamber system, and its use as a new SuFEx handle to efficiently synthesize triflates and triflamides. This broadly tolerated protocol lends itself to peptide modification or to telescoping into coupling reactions. Moreover, redesigning the S^VI–F connector with a S Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O → SNR replacement furnished the analogous triflimidoyl fluorides as SuFEx electrophiles, which were engaged in the synthesis of rarely reported triflimidate esters. Notably, experiments showed H₂O to be the key towards achieving chemoselective trifluoromethanesulfonation of phenols vs. amine groups, a phenomenon best explained—using ab initio metadynamics simulations—by a hydrogen bonded termolecular transition state for the CF₃SO₂F triflylation of amines.

Triflyl fluoride gas (CF₃SO₂F) and its aza analogues are reported as new SuFEx activators. These S^VI–F reagents react efficiently with a variety of nucleophiles, yet the presence of water grants complete chemoselectivity to phenols.

An asymmetric Salamo-based Zn complex supported on Fe3O4 MNPs: a novel heterogeneous nanocatalyst for the silyl protection and deprotection of alcohols under mild conditions

In this study, a magnetic asymmetric Salamo-based Zn complex (H2L = salen type di-Schiff bases)-supported on the surface of modified Fe3O4 (Fe3O4@H2L-Zn) as a new catalyst was designed and characterized via numerous analytical techniques such as FT-IR spectroscopy, XRD, EDS, ICP-AES, SEM, TEM, TGA and VSM. An efficient and sustainable synthetic protocol has been presented for the synthesis of silyl ether substructures via the silyl protection of alcohols under mild conditions. The synthetic protocol involves a two-component solvent-free reaction between various hydroxyl-bearing substrates and hexamethyldisilazane (HMDS) as an inexpensive silylating agent using Fe3O4@H2L-Zn MNPs as a magnetically separable, recyclable and reusable heterogeneous catalyst. Fe3O4@H2L-Zn MNPs were also applied for the removal of silyl protecting groups from hydroxyl functions using water in CH2Cl2 under green conditions. The catalyst demonstrated good to excellent catalytic yield efficiency for both the reactions compared to the commercial metal-based catalysts under green conditions for a wide range of substrates.