Access to luminescent mesophases of gold(I) chiral complexes by thermal or mechanic stimuli. The role of the tertiary carbon

Chiral gold(I) isocyanide complexes [Au(p-C6F4OR1)(p-CNC6H4(O2CC6H4(OR2)] with R1, R2 or both being (R)-2-octyl, display an interesting and unprecedented response to mechanical or thermal stress. For all the complexes, heating leads to...

Synthesis of β-Nitro Ketone from Geminal Bromonitroalkane and Silyl Enol Ether by Visible Light Photoredox Catalysis

Various β-nitro ketones, including those bearing a β-tertiary carbon, were prepared from geminal bromonitroalkanes and trimethylsilyl enol ethers of a broad range of ketones under visible light photoredox catalysis, which...

Thermal Analysis of Plastics Used in the Food Industry

Fires in landfills, where used plastic packaging waste is discarded, have shown how great a fire hazard these types of materials pose. In this study, the course of thermo-oxidation of samples made of polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET) based plastics was determined. Based on an analysis of the dissociation energy of bonds between atoms in a polymer molecule, the mechanisms responsible for the character and course of degradation were observed. It was found that the degradation rate of PP and PS could be a result of the stability of C-H bonds on the tertiary carbon atom. In the case of PS, due to facilitated intramolecular hydrogen transfer, stabilization of hydroperoxide, and formation of a stable tertiary alcohol molecule, the onset of degradation is shifted towards higher temperatures than in the case of PP. Notably, the PP fragmentation occurs to a greater extent due to the easier course of β-scission. In addition, it was found that during a fire, the least amount of heat would be generated by thermo-oxidation of PS-based plastics. This is a result of the formation of a styrene molecule during decomposition that, due to the high stability of bonds in the aromatic ring, escapes from the combustion zone without oxidation. It has been proven that the greatest thermal effect accompanies PET decomposition, during which a phenyl radical is produced, where the C-H bonds break more easily in comparison with the bonds of an intact ring.

Rhodium-catalyzed intermolecular enantioselective Alder–ene type reaction of cyclopentenes with silylacetylenes

The Alder–ene type reaction between alkenes and alkynes provides an efficient and atom-economic method for the construction of C-C bond, which has been widely employed in the synthesis of natural products and other functional molecules. The intramolecular enantioselective Alder-ene cycloisomerization reactions of 1,n-enynes have been extensively investigated. However, the intermolecular asymmetric version has not been reported, and remains a challenging task. Herein, we describe a rhodium-catalyzed intermolecular enantioselective Alder-ene type reaction of cyclopentenes with silylacetylenes. A variety of chiral (E)-vinylsilane tethered cyclopentenes bearing one quaternary carbon and one tertiary carbon stereocenters are achieved in high yields and enantioselectivities. The reaction undergoes carbonyl-directed migratory insertion, β-H elimination and desymmetrization of prochiral cyclopentenes processes.

Ring-Opening Fluorination of Bicyclic Azaarenes

We have discovered a ring-opening fluorination of bicyclic azaarenes. Upon treatment of bicyclic azaarenes such as pyrazolopyridines with electrophilic fluorinating agents, fluorination of the aromatic ring is followed by a ring-opening reaction. Although this overall trans-formation can be classified as an electrophilic fluorination of an aromatic ring, it is a novel type of fluorination that results in construction of tertiary carbon–fluorine bonds. The present protocol can be applied to a range of bicyclic azaarenes, tolerating azines and a variety of functional groups. Additionally, mechanistic studies and enantioselective fluorination have been examined.

Ring-Opening Fluorination of Bicyclic Azaarenes

We have discovered a ring-opening fluorination of bicyclic azaarenes. Upon treatment of bicyclic azaarenes such as pyrazolopyridines with electrophilic fluorinating agents, fluorination of the aromatic ring is followed by a ring-opening reaction. Although this overall trans-formation can be classified as an electrophilic fluorination of an aromatic ring, it is a novel type of fluorination that results in construction of tertiary carbon–fluorine bonds. The present protocol can be applied to a range of bicyclic azaarenes, tolerating azines and a variety of functional groups. Additionally, mechanistic studies and enantioselective fluorination have been examined.

Tandem Friedel-Crafts-Alkylation-Enantioselective-Protonation by Artificial Enzyme Iminium Catalysis

The incorporation of organocatalysts into protein scaffolds, i.e. the production of organocatalytic artificial enzymes, holds the promise of overcoming some of the limitations of this powerful catalytic approach. In particular, transformations for which good reactivity or selectivity is challenging for organocatalysts may find particular benefit from translation into a protein scaffold so that its chiral microenvironment can be utilised in catalysis. Previously, we showed that incorporation of the non-canonical amino acid para-aminophenylalanine into the non-enzymatic protein scaffold LmrR forms a proficient and enantioselective artificial enzyme (LmrR_pAF) for the Friedel-Crafts alkylation of indoles with enals. The unnatural aniline side-chain is directly involved in catalysis, operating via a well-known organocatalytic iminium-based mechanism. In this study, we show that LmrR_pAF can enantioselectively form tertiary carbon centres not only during C-C bond formation, but also by enantioselective protonation. Control over this process is an ongoing challenge for small-molecule catalysts for which general solutions do not exist. LmrR_pAF can selectively deliver a proton to one face of a prochiral enamine intermediate delivering product enantiomeric excesses and yields that rival the best organocatalyst for this transformation. The importance of various side-chains in the pocket of LmrR is distinct from the Friedel-Crafts reaction without enantioselective protonation, and two particularly important residues were probed by exhaustive mutagenesis. This study shows how organocatalytic artificial enzymes can provide solutions to transformations which otherwise require empirical optimisation and design of multifunctional small molecule catalysts.

Asymmetric Aldol Reaction of Alkenyl Esters with α-Ketoesters Catalyzed by Chiral Tin Alkoxides

A catalytic enantioselective aldol reaction of alkenyl esters with α-ketoesters was achieved using an (R)-BINOL-derived chiral tin dibromide possessing a 4-t-butylphenyl group at 3- and 3’-positions as the chiral precatalyst in the presence of sodium methoxide and methanol. Optically active aldol products possessing a chiral tertiary carbon with up to 92% ee were diastereoselectively obtained in moderate to high yields not only from cyclic alkenyl esters but also from acyclic ones under the influence of the in situ generated chiral tin methoxide.