In situ generation of N-unsubstituted imines from alkyl azides and their applications for imine transfer via copper catalysis

One-Pot Knoevenagel/Imination/6π-Azaelectrocyclization Sequence for the Synthesis of Disubstituted Nicotinonitriles

We report on a copper-catalyzed three-component reaction for the synthesis of disubstituted nicotinonitriles using 3-bromopropenals, benzoylacetonitriles, and ammonium acetate (NH₄OAc). The Knoevenagel-type condensation of 3-bromopropenals with benzoylacetonitriles gives δ-bromo-2,4-dienones that contain strategically placed functional groups that react with the ammonia generated in situ to give the corresponding azatrienes. These azatrienes can then be transformed into trisubstituted pyridines under the reaction conditions via a reaction sequence involving 6π-azaelectrocyclization and aromatization.

Sc(OTf)3-Mediated One-Pot Synthesis of 3,4-Disubstituted 1H-Pyrazoles and 3,5-Disubstituted Pyridines from Hydrazine or Ammonia with Epoxides

Here, we report the synthesis of 3,4-disubstituted 1H-pyrazoles and 3,5-disubstituted pyridines from the reaction of epoxides with hydrazine and ammonia, respectively. Both reactions utilize Sc(OTf)₃ as a Lewis acid. The pyrazole synthesis utilizes N-bromosuccinimide to convert the intermediate pyrazolines to the pyrazoles, whereas the pyridine synthesis utilizes FeCl₃ as a cocatalyst.

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.

Iron(II) Mediated Deazotation of Benzyl Azide: Trapping and Subsequent Transformations of the Benzaldimine Fragment

The mono-benzaldimine (HN═CHPh) complex [(^tBupyrpyrr₂)Fe(HN═CHPh)] (1-HN═CHPh) has been prepared by reaction of [(^tBupyrpyrr₂)Fe(OEt₂)] (1-OEt₂) (^tBupyrpyrr₂ = 2,6-bis(3,5-di-tert-butyl-pyrrolyl)pyridine) with one equivalent of benzyl azide. Compound 1-HN═CHPh retains the cis-divacant octahedral coordination geometry akin to 1, as established by single crystal X-ray diffraction study. A bis-HN═CHPh complex [(^tBupyrpyrr₂)Fe(HN═CHPh)₂] (2) was also prepared by the addition of two equivalents of benzyl azide to 1, and its molecular structure exhibits the two HN═CHPh ligands coordinated trans to each other, thereby forming a square pyramidal coordination geometry at the Fe^II center. Reaction of 1 with excess benzyl azide yields [(^tBupyrpyrr₂)Fe(HN═CHPh)₂·PhCHNCH(NH₂)Ph] (2-PhCHNCH(NH₂)Ph), which contains an unstable benzylideneamino phenyl methanamine fragment, effectively hydrogen bonded to 2. Thermolysis of 2 or 2-PhCHNCH(NH₂)Ph releases the HN═CHPh self-coupling products hydrobenzamide (A), N-benzylidine benzylamine (B), and benzonitrile (C). Under catalytic conditions, free HN═CHPh (cis/trans-HN═CHPh mixture) is produced using 2.5 mol % of 1 in 90% spectroscopic yield. These studies provide a clearer understanding for the conversion of the HN═CHPh in 2 or 2-PhCHNCH(NH₂)Ph to the C-C and C-N coupled products. Reduction of 1-HN═CHPh with KC₈ yields the reductively coupled benzylamide complex [K(OEt₂)]₂[(^tBupyrpyrr₂)₂Fe₂(μ₂-NHCHPhCHPhNH)] (3) as the result of a new C-C bond formed between two radical benzylamide fragments.

Generation of N-H Imines from α-Azidocarboxylic Acids through Ru-Catalyzed Decarboxylation

A new method for the synthesis of N-H imines from α-azidocarboxylic acids was developed, which proceeds through decarboxylative C-C bond cleavage catalyzed by a commercial diruthenium complex ([CpRu(CO)₂]₂) under visible light irradiation at room temperature within several minutes. The reactive products undergo condensation, which forms cyclic trimers (2,4,6-trialkylhexahydro-1,3,5-triazines) or linear N,N'-bis(arylmethylidene)arylmethanediamines in quantitative yields. Alternatively, the N-H imines can be trapped with benzylamine and 2-(aminomethyl)aniline, providing stable N-benylimines and tetrahydroquinazolines, respectively. Subsequent oxidation of tetrahydroquinazolines produced quinazolines.

Sulfonium ion-promoted traceless Schmidt reaction of alkyl azides

Schmidt reaction by sulfonium ions is described. General primary, secondary, and tertiary alkyl azides were converted to the corresponding carbonyl or imine compounds without any trace of the activators. This bond scission reaction through 1,2-migration of C-H and C-C bonds was accessible to the one-pot substitution reaction.

Theoretical studies on the N–X (X = Cl, O) bond activation mechanism in catalytic C–H amination

A favorable S_N2-type N–Cl bond cleavage mechanism are proposed for Rh-catalysed C–H amination, which also works for N–O bond cleavage in Rh, Ru, and Pd analogous systems. These results could provide new understanding of C–H amination.

Enantioselective intramolecular C-H amination of aliphatic azides by dual ruthenium and phosphine catalysis

By combining a chiral-at-metal ruthenium catalyst with catalytic amounts of tris(p-fluorophenyl)phosphine (both 1 mol%), the challenging catalytic enantioselective ring-closing C(sp³)-H amination of unactivated aliphatic azides has been achieved with high enantioselectivities.

The catalytic enantioselective intramolecular C(sp³)-H amination of aliphatic azides represents an efficient method for constructing chiral saturated cyclic amines which constitute a prominent structural motif in bioactive compounds. We report a dual catalytic system involving a chiral-at-metal bis(pyridyl-NHC) ruthenium complex and tris(4-fluorophenyl)phosphine (both 1 mol%), which facilitates the cyclization of aliphatic azides to chiral α-aryl pyrrolidines with enantioselectivities of up to 99% ee, including a pyrrolidine which can be converted to the anti-tumor alkaloid (R)-(+)-crispine. Mechanistically, the phosphine activates the organic azide to form an intermediate iminophosphorane and transfers the nitrene unit to the ruthenium providing an imido ruthenium intermediate which engages in the highly stereocontrolled C–H amination. This dual catalysis combines ruthenium catalysis with the Staudinger reaction and provides a novel strategy for catalyzing enantioselective C–H aminations of unactivated aliphatic azides.

Copper-catalyzed nitrene transfer/cyclization cascade to synthesize 3a-nitrogenous furoindolines and pyrroloindolines

Copper-catalyzed nitrene transfer for amination/cyclization of tryptophols and tryptamines to generate the corresponding indole alkaloids in good to excellent yields was successfully developed.