Directed Functionalization of Halophenyl-2-oxazolines with TMPMgCl·LiCl

Are benzoylium and nitrilium ions produced from substituted 2-arene-2-oxazolines during mass spectrometry? A study based on density functional theory calculations, quantum theory of atoms in molecules, and electrospray ionization tandem mass spectrometry

RATIONALE

Oxazolines are important compounds for drug development, synthesis, and pharmaceutical applications. Interest in analyzing and developing methods to characterize reaction products from these small heterocyclics has led us to study the gas-phase reactivity and fragmentation of seven 2-arene-2-oxazolines compounds using computational chemistry combined with mass spectrometry.

METHOD

Protonation sites were investigated using computed proton affinity, gas-phase basicity, and some quantum chemistry descriptors of reactivity; the B3LYP/6-31+G(d,p) computational model was used. Fragmentation mechanisms were suggested by employing data from collision-induced dissociation (CID), energy-resolved plots from MS/MS spectra, multiple-stage experiments, and survival-yield method.

RESULTS

Protonation studies based on quantum theory of atoms in molecules (QTAIM) and computational thermochemistry were useful to describe the reactivity of the investigated 2-arene-2-oxazolines, which can be protonated at the nitrogen atom. Three major fragmentation pathways were identified for the protonated molecules: formation of (a) benzoylium or (b) nitrilium ions through elimination of 71 and 72 u from the protonated molecules, respectively, and (c) elimination of 54 u from [M+H]⁺ . These pathways were exploited by the density functional theory calculations combined with QTAIM studies.

CONCLUSIONS

Our results can help in identifying 2-arene-2-oxazoline derivatives using electrospray ionization tandem mass spectrometry (ESI-MS/MS), which can be applied for monitoring reactions through the identified diagnostic ions (product ions). Also, we can suggest that benzoylium and nitrilium ions emerge during fragmentation under CID conditions.

Synthesis of 7-Chloroquinoline Derivatives Using Mixed Lithium-Magnesium Reagents

We have prepared a library of functionalized quinolines through the magnesiation of 7-chloroquinolines under mild conditions, employing both batch and continuous flow conditions. The preparation involved the generation of mixed lithium-magnesium intermediates, which were reacted with different electrophiles. Mixed lithium-zinc reagents allowed the synthesis of halogenated and arylated derivatives. Some of the synthesized 4-carbinol quinolines have shown interesting antiproliferative properties, their hydroxyl group being a suitable amino group bioisostere. We also report a two-step approach for optically active derivatives.

Regioselective Functionalization of Ester-, Amide-, Carbonate-, and Carbamate-Substituted 2-Phenyl-2-oxazolines with Mixed Lithium-Magnesium Amides

We have prepared novel highly functionalized benzene derivatives by regioselective metalation of ester-, amide-, carbamate-, and carbonate-substituted 2-phenyl-2-oxazolines with mixed lithium-magnesium amides followed by reaction with different electrophiles. While a complementary metalation site can be accessed by using different bases, steric and electronic effects promoted by the aromatic ring substituents also play an important role in reaction regioselectivity. Computational calculations of the aromatic hydrogen pK_a values have helped us to rationalize the metalation preference by the complex-induced proximity effect concept.

Organic Synthesis: New Vistas in the Brazilian Landscape

In this overview, we present our analysis of the future of organic synthesis in Brazil, a highly innovative and strategic area of research which underpins our social and economical progress. Several different topics (automation, catalysis, green chemistry, scalability, methodological studies and total syntheses) were considered to hold promise for the future advance of chemical sciences in Brazil. In order to put it in perspective, contributions from Brazilian laboratories were selected by the citations received and importance for the field and were benchmarked against some of the most important results disclosed by authors worldwide. The picture that emerged reveals a thriving area of research, with new generations of well-trained and productive chemists engaged particularly in the areas of green chemistry and catalysis. In order to fulfill the promise of delivering more efficient and sustainable processes, an integration of the academic and industrial research agendas is to be expected. On the other hand, academic research in automation of chemical processes, a well established topic of investigation in industrial settings, has just recently began in Brazil and more academic laboratories are lining up to contribute. All these areas of research are expected to enable the future development of the almost unchartered field of scalability.

Base-Controlled Regioselective Functionalization of Chloro-Substituted Quinolines

We prepared a number of di- and trifunctionalized quinolines by selective metalation of chloro-substituted quinolines with metal amides followed by reaction with different electrophiles. Metalation of the C-3 position of the quinolinic ring with lithium diisopropylamide at -70 °C is easy to achieve, whereas reaction with lithium-magnesium and lithium-zinc amides affords C-2 or C-8 functionalized derivatives in a regioselective fashion. These complementary methods could be rationalized by DFT calculations and are convenient strategies toward the synthesis of bioactive quinoline derivatives such as chloroquine analogues.

Regioselective ortho-functionalization of bromofluorenecarbaldehydes using TMPMgCl·LiCl

A highly regioselective functionalization of 7-bromofluorene-2-carbaldehydes, potent organic chromophores, in position C3 using a mild ortho-metallation strategy (DoM) with TMPMgCl·LiCl has been developed.

Unified Protocol for Cobalt-Catalyzed Oxidative Assembly of Two Aryl Metal Reagents Using Oxygen as an Oxidant

The first cobalt-catalyzed oxidative cross-coupling reaction of two aryl metal reagents is described. An equivalent amount of two aryl Grignard or lithium reagents, after mediation by an equivalent amount of simple ClTi(OEt)3, was facilely assembled under the catalysis of 1 mol % of CoCl2/10 mol % of DMPU using oxygen. The cross-couplings between various aryl metal reagents, especially between two structurally similar aryl Grignard reagents, proceeded smoothly and selectively and, thus, provided a highly general and efficient method for the construction of biaryl compounds.

Zincation of 4,4-dimethyloxazoline using TMPZnCl·LiCl. A new preparation of 2-aryloxazolines

4,4-Dimethyloxazoline is directly zincated using TMPZnCl·LiCl and further functionalized by Pd-catalyzed cross-couplings, as well as Cu-mediated acylation and allylation reactions.