Transition Metal-Free Generation of N-Unsubstituted Imines from Benzyl Azides: Synthesis of N-Unsubstituted Homoallylic Amines

Iron-Catalyzed Denitrogenative Annulation Reactions between α-Azido Acetamides and Cyclic Ketones

We report an FeCl2-catalyzed annulation reaction between α-azido acetamides and cyclic ketones. Two types of α,β-unsaturated γ-lactam products can be obtained, depending on the reaction conditions. When α-azido acetamides were reacted with cyclohexanone, 8-amino-5,6,7,8-tetrahydro-1H-indol-2(4H)-ones were obtained when a primary amine was present in the reaction system; conducting the reaction in the presence of 2-aminobenzenesulfonic acid, on the contrary, resulted in the formation of 5,6-dihydro-1H-indol-2(4H)-ones. Cycloheptanone and cyclooctanone reacted in the same way as cyclohexanone. The reactions proceed via the intermediacy of 2-iminoacetamides, which are formed by FeCl2-facilitated dinitrogenation of α-azido acetamides. These reactions constitute a new strategy for expanding the synthetic dimensions of organic azides.

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.

Formal α-Allylation of Primary Amines by a Dearomative, Palladium-Catalyzed Umpolung Allylation of N-(Aryloxy)imines

N-(Aryloxy)imines, readily accessible by condensation/tautomerization of (pseudo)benzylic primary amines and 2,6-di-tert-butyl-1,4-benzoquinone, undergo efficient allylation to afford a wide range of homoallylic primary amines following hydrolytic workup. Deprotonation of N-(aryloxy)imines generates a delocalized 2-azaallyl anion-type nucleophile that engages in dearomative C-C bond-forming reactions with allylpalladium(II) electrophiles generated from allylic tert-butyl carbonates. This reactivity umpolung enables the formal α-allylation of (pseudo)benzylic primary amines. Mechanistic studies reveal that the apparent regioselectivity of the desired bond-forming event is a convergent process that is initiated by unselective allylation of N-(aryloxy)imines to give several regioisomeric species, which subsequently rearrange via stepwise [1,3]- or concerted [3,3]-sigmatropic shifts, ultimately converging to provide the desired regioisomer of the amine products.

Scandium(III) Triflate Catalyzed Direct Synthesis of N-Unprotected Ketimines

N-Unprotected ketimines are useful substrates and intermediates for synthesizing valuable nitrogen-containing compounds, but their potential applicability is limited by the available synthetic methods. To address this issue, we report a scandium(III) triflate catalyzed direct synthesis of N-unprotected ketimines. Using commercially available reagents and Lewis acid catalysts, ketones were directly transformed into the corresponding N-unprotected ketimines in high yields with broad functional group tolerance, even in multigram scales. The reactions were readily applicable for one-pot synthesis of important compounds such as a glycine Schiff base without isolation of N-unprotected ketimine intermediates. Preliminary mechanistic studies to clarify the reaction mechanism are also described.

Allylic azides: synthesis, reactivity, and the Winstein rearrangement

Organic azides are useful synthetic intermediates, which demonstrate broad reactivity. Unlike most organic azides, allylic azides can spontaneously rearrange to form a mixture of isomers. This rearrangement has been named the Winstein rearrangement. Using allylic azides can result in low yields and azide racemization in some synthetic contexts due to the Winstein rearrangement. Effort has been made to understand the mechanism of the Winstein rearrangement and to take advantage of this process. Several guiding principles can be used to identify which azides will produce a mixture of isomers and which will resist rearrangement. Selective reaction conditions can be used to differentiate the azide isomers in a dynamic manner. This review covers all aspects of allylic azides including their synthesis, their reactivity, the mechanism of the Winstein rearrangement, and reactions that can selectively elaborate an azide isomer. This review covers the literature from Winstein's initial report to early 2019.

In-flow photooxygenation of aminothienopyridinones generates iminopyridinedione PTP4A3 phosphatase inhibitors

A continuous flow photooxygenation of 7-aminothieno[3,2-c]pyridin-4(5H)-ones to produce 7-iminothieno[3,2-c]pyridine-4,6(5H,7H)-diones has been developed, utilizing ambient air as the sole reactant. N-H Imines are formed as the major products, and excellent functional group tolerance and conversion on gram-scale without the need for chromatographic purification allow for facile late-stage diversification of the aminothienopyridinone scaffold. Several analogs exhibit potent in vitro inhibition of the cancer-associated protein tyrosine phosphatase PTP4A3, and the SAR supports an exploratory docking model.

One pot conversion of benzophenone imine into the relevant 2-aza-allenium

Interaction of common imines with WCl₆ in organic solvents revealed very unusual features in the context of imine chemistry. Ph₂C[double bond, length as m-dash]NH was converted in one pot into the 2-aza-allenium species [Ph₂C[double bond, length as m-dash]N[double bond, length as m-dash]CPh₂]⁺via N₂ release, with [Ph₂C[double bond, length as m-dash]NH₂]⁺ being a co-product. PhCH[double bond, length as m-dash]N^tBu underwent C-H bond activation resulting in the formation of [C[triple bond, length as m-dash]N] containing derivatives, together with [PhCH[double bond, length as m-dash]NH^tBu]⁺.