Rajendra P. Singh

Nitrogen-Rich Heterocycles

Many advantages accrue from nitrogen-rich heterocyclic compounds compared to traditional molecular energetic compounds. Utilization of heterocyclic nitrogen-containing cations and anions in energetic salts gives rise to lower vapor pressures, higher heats of formation and higher densities. Additionally, smaller amounts of hydrogen and carbon contribute to a better oxygen balance than normally is found with their carbocyclic analogues. Nitrogen-rich compounds are promising high energetic materials that may be more acceptable than their alternatives for both industrial and military uses since a higher percentage of their decomposition products will be dinitrogen.

New dense fluoroalkyl-substituted imidazolium ionic liquids

New mono and polyfluoroalkyl-substituted ionic liquids have been synthesized and characterized. The neat reaction of 1-methylimidazole (1a) with 1,1,1-trifluoro-3-iodopropane (2a) at 80°C formed the quaternary salt, 1-methyl-3-trifluoropropylimidazolium iodide (3), in 90% yield. Similarly, sodium imidazole (1b) gave good yields of the quaternary salts, 1,3-bis(trifluoropropyl)imidazolium iodide (3b) with an excess of 2a, and 1,3-bis(monofluoropropyl)imidazolium bromide (3c) with an excess of 1-fluoro-3-bromopropane (2b). In water as solvent, metathesis reactions of 3 with KPF6, LiN(SO2CF3)2 and KSO3CF3, and of 3b and 3c with KPF6 and LiN(SO2CF3)2 led to 4a–c, 4d,e, and 4f,g, respectively, in good isolated yields. Some of the di(polyfluoroalkyl)imidazolium salts are liquids with densities in excess of 1.8 g/cm3.

Chemistry of tetravalent nickel and related species. 3. Characterization and cyclic voltammetry of new [NiN6] species based on tridentate ligands

High-spin octahedral (NiN 6 core;Dg≈ 1250cm -1 ) nickel(II) complexes of type Ni(HRRI) 2 X 2 (where HRRT is a tridentate ligand of class N.β-aminoethylisonitroso ketimine and X = CI0 4 . NO 3 ) are readily oxidized by concentrated nitric acid to yield the diamagnetic nicket(IV) species Ni(RRT).K.. Cyclic voltammcry (CV) of Ni(RRT) 2 (CIO 4 ) 2 has been thoroughly done in well-buffered aqueous media over a wide range of pH. Below pH 6.0. CV data show a single reversible two-electron, two-proton transfer process followed by an irreversible chemical decomposition of the nickel(II) species. The rate constant for this decomposition reaction has been determined to be 0.16s -1 (283 K) from CV data. In the pH range 7.0-9.0. two distinct reversible onc-elcctron processes are observed corresponding to Ni(IV)-Ni(III) and Ni(III- Ni(II) couples. The first of these has no proton involvement while the other involves one proton. The E°′ 298 values for the various couples have been accurately determined front CV data. Representative results are as follows: Ni(Me 2 T) 2 2+ -Ni(HMe 2 T) 2 2+ ,0.71 V; Ni(Me 2 T) 2 2+ -Ni(Me 2 T)2 + ,0.40 V; Ni(Me 2 T)2 + -Ni(HMe 2 T)(Me 2 T) + ,0.66 V; Ni(Me 2 T)2 + -Ni(Me 2 T)2,0.07 V (estimated). The nickel(II) species involved in each of the above couples is what is expected from acid dissociation (oxime protons) data of Ni(Hme 2 T) 2 2+ (pK 1 =7.80;pK 2 =10.00).

Nucleophilic trifluoromethylation and difluorination of substituted aromatic aldehydes with Ruppert’s and Deoxofluor™ reagents

Efficient, high yield syntheses of 2,2,2-trifluoro-1-(N,N-dialkylaminophenyl)-ethanols (3a, b) and 2,2,2-trifluoro-1-(hydroxyaryl)ethanols (6c–g) by reacting Ruppert’s reagent, (trifluoromethyl)trimethylsilane (TMSCF3), with appropriate substrates in the presence of a catalytic amount of cesium fluoride are described. A versatile route to the synthesis of substituted aryl difluoromethane derivatives (8h–l, 10m, 12n, o, 14p) and the reactivity of substituted aromatic aldehydes towards bis(2-methoxyethyl)aminosulfur trifluoride (Deoxofluor™) were also investigated.

4-Fluoropyrrolidine-2-carbonyl fluorides: useful synthons and their facile preparation with 4-tert-butyl-2,6-dimethylphenylsulfur trifluoride.

4-Fluoropyrrolidine derivatives are useful in medicinal chemistry applications such as dipeptidyl peptidase IV inhibitors. As attractive synthons for these, N-protected (2S,4S)-4-fluoropyrrolidine-2-carbonyl fluorides were synthesized in high yield by double fluorination of N-protected (2S,4R)-4-hydroxyproline with 4-tert-butyl-2,6-dimethylphenylsulfur trifluoride (Fluolead). The 4-fluoropyrrolidine-2-carbonyl fluorides were converted to useful intermediates such as 4-fluoropyrrolidine-2-carboxamides, -N-methoxy-N-methylcarboxamide (Weinreb amide), -carboxylate methyl esters, and -carbonitriles in excellent yields. The crystalline N-Fmoc-cis-4-fluoropyrrolidine-2-carbonyl fluoride 2a is a particularly useful synthon due to its high yield of preparation and easy isolation as an enantiomerically pure compound by crystallization. Thus, the methodology using the synthons prepared by the stereospecific double fluorination has enabled a significant decrease in the synthetic steps needed for the preparation of the 4-fluoropyrrolidine derivatives useful for medicinal applications.

Nucleophilic fluorination of amino alcohols and diols using Deoxofluor

Abstract Various fluorinated chiral compounds were synthesized using bis(2-methoxyethyl)aminosulfur trifluoride (Deoxofluor) as a nucleophilic fluorinating reagent. Reactions of Deoxofluor (1) with amino alcohols (2a–d) and diols (2e–g) in methylene chloride at room temperature led to the formation of the corresponding fluoro derivatives (3a–g) in good yields.

New energetic polynitro cyclic esters: ammonium, hydrazinium, and hydroxylammonim salts of polynitramines.

Reaction of 2,2-dinitro-1,3-propanediol (1) with oxalyl dichloride or malonyl dichloride in refluxing ether led to the formation of cyclic dinitro-containing esters (2, 3) in very good yields. Compounds 2 and 3 were also isolated in similar yields by the treatment of 1 with oxalic acid or malonic acid in trifluoroacetic anhydride at room temperature. Nitration of 3 with fuming nitric acid resulted in the corresponding trinitro cyclic ester 4 in 70% isolated yield. Treatment of 1 with a large excess of methanolic ammonia gave impure 2,2-dinitro-1,3-diaminopropane (5). Polynitraamines, 7 and 11, were treated with aqueous ammonia, hydrazine monohydrate or hydroxylamine in methanol at room temperature to obtain their corresponding salts 8-10 and 12-14, respectively, in excellent isolated yields. All new compounds were characterized by IR, NMR spectroscopy ((1)H, (13)C, (15)N), DSC, and elemental analysis. Their energetic properties, such as impact sensitivity, detonation velocity, and detonation pressure were also determined and compared with existing energetic compounds, such as PETN (pentaerythritol tetranitrate), RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), and TNT (trinitrotoluene).

Synthesis and X-ray crystal structure of PhC(CF3)(OH)CONEt2, and CH3C(CF3)(OH)CON[CH2CH(CH3)2]2

Abstract α-Hydroxy α-trifluoromethylated amides, PhC(CF3)(OH)CONEt2, and CH3C(CF3)(OH)CON[CH2CH(CH3)2]2, were prepared under mild conditions and in excellent yields by direct nucleophilic trifluoromethylation of the corresponding α-keto amides, PhCOCONEt2, and CH3COCON[CH2CH(CH3)2]2, respectively, with trimethyl(trifluoromethyl)silane. Their structures were confirmed by single crystal X-ray analysis. In each case, the structure was solved by direct methods and was refined by full-matrix least-square procedures. The presence of intermolecular hydrogen bonding has been observed in all the three cases based on infrared spectroscopy and X-ray crystal structures. In PhC(CF3)(OH)CONEt2 and CH3C(CF3)(OH)CON[CH2CH(CH3)2]2 intermolecular hydrogen bonding occurred between the hydrogen of the OH group of one molecule and oxygen of the CO group of a second molecule whereas in the hydrogen of the OH group was found to be hydrogen bonded with the oxygen of the morpholine ring of a second molecule.

The first application of SelectfluorTM in electrophilic fluorination of amines: a new route to –NF2, –NHF, and >NF compounds

Reactions of primary amines, RNH2 [R = (CH3)3C–, (CH3)2CH–, CH3(CH2)2–, (CH3)2CHCH2–, CH3(CH2)3–] and secondary amines, R2NH [(R = CH3CH2–, CH3(CH2)2–, (CH3)2CHCH2–], with Selectfluor™ in acetonitrile or DMF or DMA result in formation of the corresponding RNF2, RNHF, and R2NF in high yields.

Reinvestigation of the synthesis of tris[benzoyl(thiobenzoyl)methanato-S]arsenic(III): synthesis and X-ray structure of 3,3′-thiodi(1,3-diphenylprop-2-en-1-one)

Tris[benzoyl(thiobenzoyl)methanato-S]arsenic(III), prepared by the reaction of arsenic trichloride with benzoyl(thiobenzoyl)methane, appears to be essentially tetrahedral, involving only sulphur-bonded ligands. Under reflux, the Z,Z isomer or 3,3′-thiodi(1,3-diphenylprop-2-en-1-one) is obtained exclusively as confirmed by a single-crystal X-ray analysis.