Generation and Study of the Reactivity of α-Ammonium Distonic Radical Cations in Solution

Sequential radical and cationic reactivity at separated sites within one molecule in solution

Distonic radical cations (DRCs) with spatially separated charge and radical sites are expected to show both radical and cationic reactivity at different sites within one molecule. However, such "dual" reactivity has rarely been observed in the condensed phase. Herein we report the isolation of crystalline 1λ2,3λ2-1-phosphonia-3-phosphinyl-cyclohex-4-enes 2a,b˙+, which can be considered delocalized DRCs and were completely characterized by crystallographic, spectroscopic, and computational methods. These DRCs contain a radical and cationic site with seven and six valence electrons, respectively, which are both stabilized via conjugation, yet remain spatially separated. They exhibit reactivity that differs from that of conventional radical cations (CRCs); specifically they show sequential radical and cationic reactivity at separated sites within one molecule in solution.

Merging of Light/Dark Palladium Catalytic Cycles Enables Multicomponent Tandem Alkyl Heck/Tsuji-Trost Homologative Amination Reaction toward Allylic Amines

A visible light-induced palladium-catalyzed homologative three-component synthesis of allylic amines has been developed. This protocol proceeds via a unique mechanism involving two distinct cycles enabled by the same Pd(0) catalyst: a visible light-induced hybrid radical alkyl Heck reaction between 1,1-dielectrophile and styrene, followed by the "in dark" classical Tsuji-Trost-type allylic substitution reaction. This method works well with a broad range of primary and secondary amines, aryl alkenes, dielectrophiles, and in complex settings. The regiochemistry of the obtained products is primarily governed by the structure of 1,1-dielectrophile. Involvement of π-allyl palladium intermediates allowed for the control of stereoselectivity, which has been demonstrated with up to 95:5 er.

Synthesis of Novel Quaternary Ammonium Salts and Their in Vitro Antileishmanial Activity and U-937 Cell Cytotoxicity

This work describes the synthesis of a series of quaternary ammonium salts and the assessment of their in vitro antileishmanial activity and cytotoxicity. A preliminary discussion on a structure-activity relationship of the compounds is also included. Three series of quaternary ammonium salts were prepared: (i) halomethylated quaternary ammonium salts (series I); (ii) non-halogenated quaternary ammonium salts (series II) and (iii) halomethylated choline analogs (series III). Assessments of their in vitro cytotoxicity in human promonocytic cells U-937 and antileishmanial activity in axenic amastigotes of L. (Viannia) panamensis (M/HOM/87/UA140-pIR-eGFP) were carried out using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide) micromethod. Antileishmanial activity was also tested in intracellular amastigotes of L. (V) panamensis using flow cytometry. High toxicity for human U937 cells was found with most of the compounds, which exhibited Lethal Concentration 50 (LC₅₀) values in the range of 9 to 46 μg/mL. Most of the compounds evidenced antileishmanial activity. In axenic amastigotes, the antileishmanial activity varied from 14 to 57 μg/mL, while in intracellular amastigotes their activity varied from 17 to 50 μg/mL. N-Chloromethyl-N,N-dimethyl-N-(4,4-diphenylbut-3-en-1-yl)ammonium iodide (1a), N-iodomethyl-N,N-dimethyl-N-(4,4-diphenylbut-3-en-1-yl)ammonium iodide (2a), N,N,N-trimethyl-N-(4,4-diphenylbut-3-en-1-yl)ammonium iodide (3a) and N,N,N-trimethyl-N-(5,5-diphenylpent-4-en-1-yl)ammonium iodide (3b) turned out to be the most active compounds against intracellular amastigotes of L. (V) panamensis, with EC₅₀ values varying between 24.7 for compound 3b and 38.4 μg/mL for compound 1a. Thus, these compounds represents new “hits” in the development of leishmanicidal drugs.

Crystal structures of three new N-halo-methyl-ated quaternary ammonium salts

In the crystals of the title N-halo-methyl-ated quaternary ammonium salts, C19H23IN(+)·I(-), (I) [systematic name: N-(4,4-di-phenyl-but-3-en-1-yl)-N-iodo-methyl-N,N-di-methyl-ammonium iodide], C20H25IN(+)·I(-), (II) [systematic name: N-(5,5-di-phenyl-pent-4-en-1-yl)-N-iodo-methyl-N,N-di-methyl-ammonium iodide], and C21H27IN(+)·I(-), (III) [systematic name: N-(6,6-di-phenyl-hex-5-en-1-yl)-N-iodo-methyl-N,N-di-methyl-ammonium iodide], there are short I⋯I(-) inter-actions of 3.564 (4), 3.506 (1) and 3.557 (1) Å for compounds (I), (II) and (III), respectively. Compound (I) crystallizes in the Sohncke group P21 as an 'enanti-opure' compound and is therefore a potential material for NLO properties. In the crystal of compound (I), mol-ecules are linked by C-H⋯I(-) and C-H⋯π inter-actions which, together with the I⋯I(-) inter-actions, lead to the formation of ribbons along [100]. In (II), there are only C-H⋯I(-) inter-actions which, together with the I⋯I(-) inter-actions, lead to the formation of helices along [010]. In (III), apart from the I⋯I(-) inter-actions, there are no significant inter-molecular inter-actions present.

Synthesis of Bridgehead Nitrogen Heterocycles via Cyclization of alpha-Ammonio 5-Hexenyl Radicals

Ring-closure of the 2,2-dimethyl-2-azonia-5-hexenyl radical (4) proceeds smoothly and efficiently to give the 5-exo isomer essentially quantitatively, in accordance with predictions based on MP4SDTQ/6-31G ab initio calculations on the thermodynamic stability of alpha-ammonio radicals. The corresponding 5-hexynyl radical species 15 and its 6-phenyl derivative 19 display similar behavior affording the analogous 5-exo-3-methylenepyrrolidinium salts in high yield. In none of these cases were the products of reduction were detected. All of the radical intermediates were generated conveniently by treatment of the iodomethyl and/or phenylselenomethyl salts with tributyltin hydride. Application of this procedure to monocyclic precursors such as 1-methyl-1-iodomethyl-4-methylene-1-azoniacyclohexyl iodide (31) provided an attractive entry into quaternary derivatives of the 1-azabicyclo[2.2.1]heptyl system in good yield via a three-step sequence from 1-methylpiperidone. Dequaternization of the bicyclic salts so obtained unexpectedly leads to rupture of one of the rings rather than loss of the N-methyl group. The 1-azabicyclo[2.2.1]heptane could be accessed readily via tin hydride-induced cyclization of the corresponding N-phenylethylammonium salt 54, followed by Hofmann elimination with potassium tert-butoxide.