Stereospecific Synthesis of cis-2,5-Disubstituted Pyrrolidines via N,O-Acetals Formed by Hydroamination Cyclization–Hydroalkoxylation of Homopropargylic Sulfonamides in HFIP

HFIP-Mediated, Highly Chemo-, Regio-, and Stereoselective Hydrofunctionalizations of Ynamides: Access to Stereodefined Alkenes Bearing Drugs and Natural Products

We disclose a sustainable and versatile synthetic strategy for the highly chemo-, regio-, and stereoselective hydrofunctionalizations of ynamides, through its activation by a solvent, HFIP, to access a wide variety of stereodefined ketene N,N, N,O, N,S, and N,Se acetals in high yield at room temperature. The reaction proceeded through the formation of the reactive keteniminium ion intermediate, formed via protonation at the β-carbon of ynamide by HFIP, followed by an attack of a nucleophile (syn-addition) at the α-carbon. When ynamides are treated with only HFIP at room temperature, the HFIP addition products of ynamides are formed in a 100% atom-economic fashion; however, in the presence of a stronger N-/O-/S-/Se-based nucleophile, the corresponding syn-hydroheterofunctionalized products are formed. Notably, HFIP played multiple roles, such as a reagent, in particular, a Brønsted acid, nucleophile, as well as solvent. Interestingly, HFIP is found to be unique for this transformation. Notably, this strategy is utilized for the late-stage functionalization of several marketed drugs and natural products, and it also enables the connection of two different drugs or a drug and a natural product through chemical bonds. Significantly, HFIP was recovered after the reaction and reused for consecutive reactions.

Regio- and Stereoselective Hexafluoroisopropoxylation and Trifluoroethoxylation of Allenamides

Incorporating fluorinated moieties into organic molecules is an attractive strategy to enhance drug-like properties. Herein, we have developed a simple and self-promoted protocol for hexafluoroisopropoxylation and trifluoroethoxylation of allenamides with fluorinated alcohols such as HFIP and TFE. The reaction provided the fluoroalkoxylated products in a regio- and stereoselective manner in good to moderate yields under mild conditions.

Mild Method for Deprotection of the N-Benzyloxycarbonyl (N-Cbz) Group by the Combination of AlCl3 and HFIP

Herein, we report our findings on the novel ability of aluminum chloride (AlCl3) in fluorinated solvent 1,1,1,3,3,3-hexafluoroisopropanol [HFIP] to selectively deprotect the N-benzyloxycarbonyl group (N-Cbz). The salient features of this method are good functional group tolerance including other reducible groups, cost-effectiveness, easy-to-handle, safe protocol, amenable to scale-up, high yields, and ambient temperature reactions. The methodology would serve as an excellent alternative to the use of pyrophoric hydrogen gas and metal catalyst reagents that pose severe safety and environmental concerns. The most notable feature of this methodology is the orthogonal deprotection of the N-Cbz group in the presence of O- and N-Bn protecting groups, hence, expanding the scope for designing synthetic routes to target compounds requiring multiple functional group transformations.

Synthesis of an Indazole/Indazolium Phosphine Ligand Scaffold and Its Application in Gold(I) Catalysis

Advances in ligand development have allowed for the fine-tuning of gold catalysis. To contribute to this field, we designed an indazole phosphine ligand scaffold that allows facile introduction of cationic charge through methylation. With minimal changes to the structure upon methylation, we could assess the importance of the electronic effects of the insertion of a positive charge on the catalytic activity of the resulting gold(I) complex. Using the benchmark reactions of propargyl amide cyclization and enyne cyclization with and without hexafluoroisopropanol (HFIP), we observed marked differences in the catalytic activities of the neutral and cationic gold species.

Hydrogen bonding-enabled gold catalysis: ligand effects in gold-catalyzed cycloisomerizations in hexafluoroisopropanol (HFIP)

Gold catalysis has witnessed immense evolution in recent years, yet it still requires the use of activators to render the common [AuCl(L)] complexes catalytically active. Herein, the H-bonding donor properties of hexafluoroisopropanol (HFIP) are utilized for Au-Cl bond activation and the ancillary ligand and counteranion effects on cycloisomerization reactions are showcased in HFIP as solvent.

Hydroarylation of Enamides Enabled by HFIP via a Hexafluoroisopropyl Ether as Iminium Reservoir

Here we describe that HFIP greatly expands the scope with respect to both reaction partners of the Brønsted acid-catalyzed hydroarylation of enamides. The reaction is fast and practical and can...

Direct activation of alcohols via perrhenate ester formation for an intramolecular dehydrative Friedel–Crafts reaction

A general and highly efficient intramolecular dehydrative Friedel–Crafts reactions via Re2O7 mediated hydroxyl group activation is described for the syntheses of tetrahydronaphthalene, tetrahydroquinoline, tetrahydroisoquinoline, chromane, and isochromane derivatives.

C-H bond cleavage-enabled aerobic ring-opening reaction of in situ formed 2-aminobenzofuran-3(2H)-ones

A C-H bond cleavage-enabled aerobic ring-opening reaction of 2-aminobenzofuran-3(2H)-ones formed in situ by hemiacetals with a variety of amines is reported. This simple one-pot reaction provides an alternative approach to obtain o-hydroxyaryl glyoxylamides in excellent yields of up to 97%. Alkylamines react with hemiacetals via a catalyst-free dehydration condensation to generate 2-aminobenzofuran-3(2H)-ones. The in situ formed semicyclic N,O-acetals undergo the same amine-initiated C-H bond hydroxylation in air under mild conditions to afford ring-opening products. Similarly, arylamines were investigated as substrates for a two-step tandem process involving a DPP-catalyzed condensation followed by a Et₂NH-mediated C-H hydroxylation. Unlike the previously reported functionalization of N,O-acetals via a C-O or C-N cleavage, the aerobic oxidative C-H hydroxylation in this reaction, which is promoted by using stoichiometric amounts of alkylamines as both a Lewis base and a reductant at room temperature under atmospheric air, proceeds via α-carbonyl-stabilized carbanion intermediates from the C-H cleavage of N,O-acetals.

TMSOTf-mediated synthesis of skipped dienes through the addition of olefins to imines and semicyclic N,O-acetals

A novel approach to skipped dienes has been developed through the TMSOTf-mediated one-pot addition-substitution of olefins 2a, 2f and 2g with imines 1a-1g, and a series of aryl substituted skipped dienes 3aa-3gf were accordingly obtained in 62%-94% yields. Moreover, semicyclic N,O-acetals 5 and 7 could also undergo this transformation to produce the corresponding skipped dienes 6aa and 6af-6al and 8ba and 8bf-8bk in moderate to good yields.

Metal-free synthesis of 1,4-benzodiazepines and quinazolinones from hexafluoroisopropyl 2-aminobenzoates at room temperature

Herein, we describe the novel reactivity of hexafluoroisopropyl 2-aminobenzoates. The metal-free synthesis of 1,4-benzodiazepines and quinazolinones from hexafluoroisopropyl 2-aminobenzoates has been developed at room temperature. These procedures feature good functional group tolerance, mild reaction conditions, and excellent yields. The newly formed products can readily be converted to other useful N-heterocycles. Moreover, the products and their derivatives showed potent anticancer activities in vitro by MTT assay.

A metal-free synthesis of 1,4-benzodiazepines and quinazolinones from hexafluoroisopropyl 2-aminobenzoates has been developed at room temperature.

Exploiting hexafluoroisopropanol (HFIP) in Lewis and Brønsted acid-catalyzed reactions

Hexafluoroisopropanol (HFIP) is a solvent with unique properties that has recently gained attention for promoting a wide range of challenging chemical reactions. It was initially believed that HFIP was almost exclusively involved in the stabilization of cationic intermediates, owing to its high polarity and low nucleophilicity. However, in many cases, the mechanism of action of HFIP appears to be more complex. Recent findings reveal that many Lewis and Brønsted acid-catalyzed transformations conducted in HFIP additionally involve cooperation between the catalyst and HFIP hydrogen-bond clusters, akin to Lewis- or Brønsted acid-assisted-Brønsted acid catalysis. This feature article showcases the remarkable versatility of HFIP in Lewis and Brønsted acid-catalyzed reactions, with an emphasis on examples yielding mechanistic insight.