Polar effects in free-radical reactions. Homolytic heteroaromatic substitutions by alkyl bromides

Synthesis of Functionalized Six-Membered-Ring Azahelicenes

Functionalization, namely the introduction of side groups onto the molecular scaffold of a helicene, may have either the purpose of modifying the electronic properties of the parent helicene, e.g., by adding electron-withdrawing or electron-donating groups, or the scope of providing the helicene with a “handle”, which can be reacted to bind the molecule to another molecule or to a solid structure, such as a carbon or metal surface, or again to allow for complexation of the helicene with metal ions. The possible approaches are two-fold: the synthesis of the helicene can be performed using starting materials that already contain a side group, or the side group can be introduced after the synthesis of the parent helicene. As azahelicenes are helicenes bearing one or more nitrogen atom(s) in the molecular framework, parent azahelicenes can be functionalized on carbon atoms by exploiting the presence of the electron-withdrawing nitrogen atom. Moreover, they can be transformed into quaternary salts, whose properties are quite different from those of the parent azahelicenes in terms of the solubility and electronic properties. This review aims to provide a survey of the different synthetic methods available to attain this fascinating class of compounds.

Visible Light-Mediated Direct C-H Aroylation and Alkylation of Heteroarenes

Herein, we describe the photocatalytic generation of nucleophilic aroyl radicals from simple aroyl chlorides as a universal and efficient cross-coupling strategy for the direct aroylation of heteroarenes. Furthermore, visible light-mediated direct alkylation of heteroarenes has also been achieved using unactivated bromoalkanes as radical precursors. These two strategies feature high functional group tolerance, exclusive regioselectivity for reaction at the more electrophilic position of heteroarenes, easily accessible substrates, and mild reaction conditions. Moreover, mechanism studies of two reactions are carried out to support our hypotheses.

Functionalization of nanostructured gold substrates with chiral chromophores for SERS applications: The case of 5-Aza[5]helicene

Nanostructured gold thin films can be fabricated by controlled pulsed laser deposition to get efficient sensors, with uniform morphology and optimized plasmon resonance, to be employed as plasmonic substrates in surface enhanced Raman scattering spectroscopy. By attaching 5-aza[5]helicen-6-yl-6-hexanethiol to such gold nanostructures, used in a previous work for label-free drug sensing with biomedical purposes, we successfully prepared functionalized substrates with remarkable surface enhanced Raman scattering activity. The long-term motivation is to develop probes for drug detection at low concentrations, where sensitivity to specific chiral targets is required.

Versatile cross-dehydrogenative coupling of heteroaromatics and hydrogen donors via decatungstate photocatalysis

A facile sunlight-induced derivatization of heteroaromatics via photocatalyzed C-H functionalization in amides, ethers, alkanes and aldehydes is described. Tetrabutylammonium decatungstate (TBADT) was used as the photocatalyst and allowed to carry out the process under mild conditions.

A new era for homolytic aromatic substitution: replacing Bu3SnH with efficient light-induced chain reactions

Herein is a pertinent review of recent photochemical homolytic aromatic substitution (HAS) literature. Issues with using the reductant Bu3SnH in an oxidative process where the net loss of a hydrogen atom occurs is discussed. Nowadays more efficient light-induced chain reactions are used resulting in HAS becoming a synthetic mechanism of choice rivaling organometallic, transition-metal and electrophilic aromatic substitution protocols. The review includes aromatic substitution as part of a tandem or cascade reaction, Pschorr reaction, as well as HAS facilitated by ipso-substitution, and Smiles rearrangement. Recently visible-light photoredox catalysis, which is carried out at room temperature has become one of the most important means of aromatic substitution. The main photoredox catalysts used are polypyridine complexes of Ru(ii) and Ir(iii), although eosin Y is an alternative allowing metal-free HAS. Other radical initiator-free aromatic substitutions have used 9-mesityl-10-methylacridinium ion and N,N-bis(2,6-diisopropylphenyl)perylene-3,4,9,10-bis(dicarboximide) as the photoredox catalyst, UV-light, photoinduced electron-transfer, zwitterionic semiquinone radical anions, and Barton ester intermediates.

Expedient synthesis of 3-phenylbicyclo[1.1.1]pentan-1-amine via metal-free homolytic aromatic alkylation of benzene

The inquisitiveness on the use of the BCP motif as a modern lead optimization tactic entails reliable synthetic access. In that direction, we disclose a new and versatile approach to 3-phenylbicyclo[1.1.1]pentan-1-amine, via metal-free homolytic alkylation of benzene.

Tandem reactions via Barton esters with intermolecular addition and vinyl radical substitution onto indole

A one-pot initiator-free Barton ester decomposition with tandem radical addition onto alkyl propiolates or phenylacetylene with aromatic substitution of the resultant vinyl radical allows convenient access to new 9-substituted 6,7-dihydropyrido[1,2-a]indoles. Propyl radical cyclizations compete when forming the expanded 7,8-dihydro-6H-azepino[1,2-a]indole system. 2-Thiopyridinyl S-radical is incorporated into aromatic adducts when using unsubstituted indole-1-alkanoic acid precursors. X-ray crystallography on substitution products allows selectivity of the radical addition onto less reactive internal alkynes to be determined.

Barton esters for initiator-free radical cyclisation with heteroaromatic substitution

S-(1-Oxido-2-pyridinyl)-1,1,3,3-tetramethylthiouronium hexafluorophosphate (HOTT) facilitates the first examples of efficient radical cyclisation with (hetero)aromatic substitution via Barton ester intermediates. Cyclopropyl and alkyl radicals allow access to five, six and seven-membered alicyclic-ring fused heterocycles with and without an additional fused cyclopropane, including the skeleton of the anti-cancer agent, cyclopropamitosene, expanded, and diazole analogues. Radical initiators are not required for cyclisation from carboxylic acid precursors.

Direct alkylation of heteroaryls using potassium alkyl- and alkoxymethyltrifluoroborates

A direct alkylation of various heteroaryls using stoichiometric potassium alkyl- and alkoxymethyltrifluoroborates has been developed. This method leads to the synthesis of complex substituted heterocycles, which have been obtained with yields up to 89%.

A New Radical-Based Route to Calothrixin B

[structure: see text] A high-yielding totally regioselective intramolecular homolytic acylation of a quinoline ring constitutes the key step in a new synthesis of the pentacyclic indolo[3,2-j]phenanthridine alkaloid calothrixin B.

Substituted 4-methylquinolines as a new class of anti-tuberculosis agents

We report synthesis and anti-tuberculosis activities of a series of novel ring-substituted quinolines. The most effective compound of the series 3d (MIC=6.25 microg/mL, Mycobacterium tuberculosis H37Rv strain) was synthesized in one step; thus is an attractive lead molecule for anti-tuberculosis drug development. The results of this study represent the discovery of ring-substituted 4-methylquinolines as new class of potential anti-tuberculosis agents.