a-two-step-protocol-for-umpolung-functionalization-ketones-via

A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species

&#945;-Functionalization of ketones via umpolung of enolates by hypervalent iodine reagents is an important concept in synthetic organic chemistry. ...

Ariel University

Nitroxides (nitroxyl radicals) hold a unique place in science due to their stable radical nature. We have recently reported the first design concept providing a general solution to the problem of designing and preparing monocyclic α-hydrogen nitroxides. The initial studies were limited to aryl derivatives. We now report a wider study showing that alkyl substituents may be employed as well. In addition, we report several additional examples of aryl substituents and reveal some of the structural limitations with regard to nitroxide stability as a function of the α-carbon substituent. 

Synthesis and stability of cyclic α-hydrogen nitroxides.

Identifying the mechanism of a catalytic reaction is paramount for designing new and improved catalysts. Several alternative catalytic cycles for the copper/2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)-catalyzed aerobic oxidation of alcohols to the corresponding aldehydes or ketones were examined using DFT at the SMD(CH CN)-RIJCOSX-DSD-PBEB95/def2-TZVP//DF-PBE /def2-SVP level of theory. A catalytic cycle in which TEMPO remains coordinated to copper throughout was identified as the most likely mechanism. There are three components to the catalytic cycle: 1) hydrogen transfer from the alkoxyl ligand to coordinated TEMPO, 2) oxygen activation with formation of a peroxo complex, and 3) alcohol activation with transfer of the OH proton to the peroxo ligand. The oxidation takes place via a six-membered intramolecular hydrogen-transfer transition state. Importantly, this is not the rate-determining step, which instead involves oxygen activation and/or the initial alcohol activation.

Mechanism of the Copper/TEMPO-Catalyzed Aerobic Oxidation of Alcohols.

Enolonium species/iodo(III)enolates of carbonyl compounds have been suggested to be intermediates in a wide variety of hypervalent iodine induced chemical transformations of ketones, including α-C-O, α-C-N, α-C-C, and α-carbon-halide bond formation, but they have never been characterized. We report that these elusive umpoled enolates may be made as discrete species that are stable for several minutes at -78 °C, and report the first spectroscopic identification of such species. It is shown that enolonium species are direct intermediates in C-O, C-N, C-Cl, and C-C bond forming reactions. Our results open up chemical space for designing a variety of new transformations. We showcase the ability of enolonium species to react with prenyl, crotyl, cinnamyl, and allyl silanes with absolute regioselectivity in up to 92 % yield.

Enolonium Species-Umpoled Enolates.

Herein it is shown, for the first time, that enolonium species are powerful electrophiles capable of reacting with aromatic compounds in an intermolecular manner to afford α-arylated ketones. The reaction is compatible with a variety of functional groups, is of wide scope with respect to aromatic compounds and ketone, and even works for polymerization-prone substrates such as substituted pyrroles, thiophenes, and furans. Only 1.6 to 5 equiv of the commodity aromatic substrates is needed.

Transition-Metal-Free Intermolecular α-Arylation of Ketones via Enolonium Species.

Enolonium species, resulting from the umpolung of ketone enolates by Koser's hypervalent iodine reagents activated by boron trifluoride, react with a variety of nitrogen heterocycles to form α-aminated ketones. The reactions are mild and complete in 4-5 h. Additionally, α-azidation of the enolonium species takes place using trimethylsilyl azide as a convenient source of azide nucleophile.

α-N-Heteroarylation and α-Azidation of Ketones via Enolonium Species.

Due to their closely matched reactivity, the coupling of two dissimilar ketone enolates to form a 1,4-diketone remains a challenge in organic synthesis. We herein report that umpolung of a ketone trimethylsilyl enol ether (1 equiv) to form a discrete enolonium species, followed by addition of as little as 1.2-1.4 equivalents of a second trimethylsilyl enol ether, provides an attractive solution to this problem. A wide array of enolates may be used to form the 1,4-diketone products in 38 to 74% yield. Due to the use of two TMS enol ethers as precursors, an optimization of the cross-coupling should include investigating the order of addition.

Article	Year
Synthesis and stability of cyclic α-hydrogen nitroxides. Organic & biomolecular chemistry\| PubMed ID: 26356203	2015
Mechanism of the Copper/TEMPO-Catalyzed Aerobic Oxidation of Alcohols. Chemistry (Weinheim an der Bergstrasse, Germany)\| PubMed ID: 27862437	2017
Enolonium Species-Umpoled Enolates. Angewandte Chemie (International ed. in English)\| PubMed ID: 28128488	2017
Transition-Metal-Free Intermolecular α-Arylation of Ketones via Enolonium Species. Organic letters\| PubMed ID: 29140710	2017
α-N-Heteroarylation and α-Azidation of Ketones via Enolonium Species. The Journal of organic chemistry\| PubMed ID: 29334466	2018
Cross-coupling of dissimilar ketone enolates via enolonium species to afford non-symmetrical 1,4-diketones. Beilstein journal of organic chemistry\| PubMed ID: 29977370	2018

Alex M. Szpilman

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