Friedel-Crafts alkylation reaction with fluorinated alcohols as hydrogen-bond donors and solvents

HFIP-mediated C-3-alkylation of indoles and synthesis of indolo[2,3-b]quinolines & related natural products

An expeditious metal free C-3 alkylation of indoles and its NIS-mediated deviation to indolo[2,3-b]quinolines is reported. This protocol, executed in aqueous HFIP has broad substrate scope and is well inclined towards the ideas of sustainable chemistry. Applications of these strategies in accessing bioactive natural products like vibrindole, norcryptotakeine, neocryptolepine and indenoindolone scaffolds has also been demonstrated.

Synthesis of 2-(2-Nitroalkyl)indoles by Rhodium(III)-Catalyzed C-H Alkylation

The Rh(III)-catalyzed addition of the indole C2-H bond to nitroalkenes under an ambient atmosphere is disclosed, providing direct access to a wide range of 2-(2-nitroalkyl)indoles (33 examples) with excellent chemo- and regioselectivity. In addition, pyrrole derivatives also successfully participated in this Friedel-Crafts alkylation reaction. Representative nitroalkane products could be converted into structurally diverse and valuable indole derivatives. Furthermore, a series of control experiments were conducted, and a plausible mechanism was proposed.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Site-selective C5-H and N-H alkylation of unprotected 8-aminoquinolines

8-Aminoquinolines are the building blocks of many pharmaceutical compounds, which has motivated the scientific community to develop new ways to derivatize these compounds. In this work, we performed a site-selective C5-H and N-H alkylation of 8-aminoquinolines using para-quinone methides under extremely mild conditions. C5-H alkylation was performed using protecting group-free 8-aminoquinolines and in metal-free conditions. N-H alkylations were also carried out under mild conditions. All corresponding alkylation products were obtained in high to excellent yields.

Studies on the Origin of the Stabilizing Effects of Fluorinated Alcohols and Weakly Coordinated Fluorine-Containing Anions on Cationic Reaction Intermediates

Many synthetic methods that use fluorinated alcohols as solvents have been reported, and the fluorinated alcohols have been found to be crucial to the success of these methods. In addition, there have been reports indicating that adding a weakly coordinated fluorine-containing anion, such as BF₄^-, PF₆^-, or SbF₆^-, to fluorinated alcohols can improve yields. The boosting effect of fluorinated alcohols is attributed mainly to hydrogen bond activation. A few studies have suggested that the very polar fluorinated alcohols can stabilize cationic reaction intermediates. However, how they do so and why weakly coordinated fluorine-containing anions improve yields have not been studied in depth. Here, we used quaternary ammonium cations, a quaternary phosphonium cation, and a triaryl-substituted carbocation as models for short-lived cationic intermediates and studied the possible interactions of these cations with fluorinated alcohols and BF₄^-, PF₆^-, or SbF₆^-. On the basis of the results, we propose that the C-F dipoles of fluorinated alcohols and the E-F dipoles (where E is B, P, or Sb) of weakly coordinated fluorine-containing anions stabilized these cations by intermolecular charge-dipole interactions. We deduced that in the same fashion the C-F and E-F dipoles can thermodynamically stabilize cationic reaction intermediates.

Cycloisomerization of Carbamoyl Chlorides in Hexafluoroisopropanol: Stereoselective Synthesis of Chlorinated Methylene Oxindoles and Quinolinones

Hexafluoroisopropanol (HFIP) was employed as an additive for the generation of 3-(chloromethylene)oxindoles via the chloroacylation of alkyne-tethered carbamoyl chlorides. This reaction avoids the use of a metal catalyst and accesses products in high yields and stereoselectivities. Additionally, this reaction is scalable and proved amenable to a series of product derivatizations, including the synthesis of nintedanib. The reactivity of alkene-tethered carbamoyl chlorides with hexafluoroisopropanol (HFIP) was harnessed towards the synthesis of 2-quinolinones.

Synthesis and practical applications of 2-(2-nitroalkyl)pyrroles

Functionalization of pyrroles introducing a 2-nitroalkyl moiety allows the formation of nitro-containing compounds to be used as pivotal intermediates for the synthesis of bioactive compounds. The reaction of pyrroles with nitroalkenes under the Friedel-Crafts conditions allows a direct entry to 2-(2-nitroalkyl)pyrroles. This approach can also be used for the preparation of enantioenriched derivatives exploiting asymmetric catalysis. In a complementary fashion, the Henry reaction between 2-formylpyrroles and nitroalkanes generates the corresponding nitroaldol products which upon dehydration and reduction of the intermediate 2-pyrrolylnitroethene efficiently afford 2-(2-nitroalkyl)pyrroles. This review article summarizes the most relevant procedures for the preparation of 2-(2-nitroalkyl)pyrroles during the last two decades as well as their significant practical applications.

Some applications of deep eutectic solvents in alkylation of heterocyclic compounds: A review of the past 10 years

This mini-review encapsulates the latest findings (past 10 years) in the field of the deep eutectic solvents (DESs) application in the alkylation/arylation of different heterocyclic compounds. These solvents have been developed to fulfill the green chemistry concept demands and have been proven excellent for the application in various fields. This review describes their application in different types of alkylation, C-, N-, O- and S-alkylation. P-alkylation has not yet been published within this scope. Not only have the authors in this study proven that DESs could be successfully applied for this specific type of reaction, but they have also offered an excellent insight into the mechanisms of their action.

Calix[6]arene-based Brønsted acids for molecular recognition and catalysis

We report the synthesis of a versatile trifluoromethylsulfonamide calix[6]arene derivative with Brønsted acid features which can influence both molecular recognition and catalytic application. Indeed, in low polarity media, the trifluoromethyl-containing supramolecular wheel is able to respond to the complexation with charged species as a function of its selective ion-pair recognition. In parallel, the enhanced acidity is the key to promote Michael additions of indoles to nitroalkenes under pseudo-physiological reaction conditions (H2O, 37 °C).

Trisulfonamide calix[6]arene-catalysed Michael addition to nitroalkenes

We describe the application of a novel family of trisulfonamide (TSA) calix[6]arenes in general acid catalysis. Hydrogen-bonding interactions between acidic TSA and methanol boosted the reactivity of the Michael addition of indoles to nitroalkene derivatives. The transformation occurs at a low catalyst loading of 5 mol%, allowing for the synthesis of nitroalkanes with good yields and functional group tolerance.

Solvent-mediated Highly Efficient Synthesis of [1,2,4]triazolo/benzimidazoloquinazolinone Derivatives

OBJECTIVE

An efficient and catalyst-free procedure for the synthesis of [1,2,4]triazolo/benzimidazolo quinazolinones has been developed in 2,2,2-trifluoroethanol or deep eutectic solvent(DESs) as a clean reaction media.

METHODS

All of the obtained products are known compounds and identified by IR, 1HNMR,13CNMR, and melting points.

RESULT

Various products were obtained in good to excellent yields under reaction conditions.

CONCLUSION

We have efficiently developed a practical and catalyst-free approach for the synthesis of [1,2,4]triazolo/benzimidazolo quinazolinones employing TFE as a clean and reusable media.

A hexameric resorcinarene capsule as a hydrogen bonding catalyst in the conjugate addition of pyrroles and indoles to nitroalkenes

A hexameric resorcinarene capsule (C) acts as a hydrogen bonding catalyst for the addition of nitroalkenes to pyrroles and indoles.