Synthesis and characterization of cationic tungsten(V) methylidynes

Cationic tungsten(V) methylidynes [L4W(X)[triple bond]CH]+[B(C6F5)4]- [L = PMe3, 0.5dmpe (dmpe = Me2PCH2CH2PMe2), X = Cl, OSO2CF3] have been prepared in high yield by a one-electron oxidation of the neutral tungsten(IV) methylidynes L4W(X)[triple bond]CH with [Ph3C]+[B(C6F5)4]-. The ease and reversibility of the one-electron oxidation of L4W(X)[triple bond]CH were demonstrated by cyclic voltammetry in tetrahydrofuran (E1/2 is approximately -0.68 to -0.91 V vs Fc). The paramagnetic d1 (S = 1/2) complexes were characterized in solution by electron spin resonance (g = 2.023-2.048, quintets due to coupling to 31P) and NMR spectroscopy and Evans magnetic susceptibility measurements (mu = 2.0-2.1 muB). Single-crystal X-ray diffraction showed that the cationic methylidynes are structurally similar to the neutral precursor methylidynes. In addition, the neutral (PMe3)4W(Cl)[triple bond]CH was deprotonated with a strong base at the trimethylphosphine ligand to afford (PMe3)3(Me2PCH2)W[triple bond]CH, a tungsten(IV) methylidyne complex that features a (dimethylphosphino)methyl ligand.     

Microwave-assisted asymmetric organocatalysis. A probe for nonthermal microwave effects and the concept of simultaneous cooling

A series of five known asymmetric organocatalytic reactions was re-evaluated at elevated temperatures applying both microwave dielectric heating and conventional thermal heating in order to probe the existence of specific or nonthermal microwave effects. All transformations were conducted in a dedicated reactor setup that allowed accurate internal reaction temperature measurements using fiber-optic probes. In addition, the concept of simultaneous external cooling while irradiating with microwave power was also applied in all of the studied cases. This method allows a higher level of microwave power to be administered to the reaction mixture and, therefore, enhances any potential microwave effects while continuously removing heat. For all of the five studied (S)-proline-catalyzed asymmetric Mannich- and aldol-type reactions, the observed rate enhancements were a consequence of the increased temperatures attained by microwave dielectric heating and were not related to the presence of the microwave field. In all cases, in contrast to previous literature reports, the results obtained either with microwave irradiation or with microwave irradiation with simultaneous cooling could be reproduced by conventional heating at the same reaction temperature and time in an oil bath. No evidence for specific or nonthermal microwave effects was obtained.     

Development of a simple and sensitive high-performance liquid chromatography method for determination of glucosamine in pharmaceutical formulations

A column high-performance liquid chromatography (HPLC) method was developed for the determination of glucosamine in dosage forms. Glucosamine was derivatized by addition of a solution containing orthophthaldialdehyde. The HPLC separation was achieved on a Spherimage 80 ODS2 column (250 x 4 mm id, 5 microm particle size) using an isocratic mobile phase containing phosphate buffer-methanol (90 + 10, v/v, pH 6.50) and methanol-tetrahydrofuran (97 + 3, v/v) in proportions of 85 + 15 at a flow rate of 1 mL/min, followed by fluorescence detection. The method was validated for specificity, linearity, accuracy, precision, limit of detection (LOD), and limit of quantitation (LOQ). The detector response for glucosamine HCI was linear over the concentration range of 0.1-20 microg/mL with a correlation coefficient of 0.9980. The accuracy was between 99.4 and 100.8%. The LOD and the LOQ were 0.009 and 0.027 microg/mL, respectively. The method was applied to determination of glucosamine in solid dosage forms.     

Cubic and spirocyclic radicals containing a tetraimidophosphate dianion [P(NR)3(NR')]*2-

The reaction of Cl(3)PNSiMe(3) with 3 equiv of LiHNR (R = (i)Pr, Cy, (t)Bu, Ad) in diethyl ether produces the corresponding tris(amino)(imino)phosphoranes (RNH)(3)PNSiMe(3) (1a, R = (i)Pr; 1b, R = Cy; 1c, R = (t)Bu; 1d, R = Ad); subsequent reactions of 1b-d with (n)BuLi yield the trilithiated tetraimidophosphates {Li(3)[P(NR)(3)(NSiMe(3))]} (2a, R = Cy; 2b, R = (t)Bu; 2c, R = Ad). The reaction of [((t)BuNH)(4)P]Cl with 1 equiv of (n)BuLi results in the isolation of ((t)BuNH)(3)PN(t)Bu (1e); treatment of 1e with additional (n)BuLi generates the symmetrical tetraimidophosphate {Li(3)[P(N(t)Bu)(4)]} (2d). Compounds 1 and 2 have been characterized by multinuclear ((1)H, (13)C, and (31)P) NMR spectroscopy; X-ray structures of 1b,c were also obtained. Oxidations of 2a-c with iodine, bromine, or sulfuryl chloride produces transient radicals in the case of 2a or stable radicals of the formula {Li(2)[P(NR)(3)(NSiMe(3))]LiX.3THF}* (X = Cl, Br, I; R = (t)Bu, Ad). The stable radicals exhibit C(3) symmetry and are thought to exist in a cubic arrangement, with the monomeric LiX unit bonded to the neutral radical {Li(2)[P(NR)(3)(NSiMe(3))]}* to complete the Li(3)N(3)PX cube. Reactions of solvent-separated ion pair {[Li(THF)(4)]{Li(THF)(2)[(mu-N(t)Bu)(2)P(mu-N(t)Bu)(2)]Li(THF)(2)} (6) with I(2) or SO(2)Cl(2) produce the persistent spirocyclic radical {(THF)(2)Li(mu-N(t)Bu)(2)P(mu-N(t)Bu)Li(THF)(2)}* (10a); all radicals have been characterized by a combination of variable concentration EPR experiments and DFT calculations.     

Imino Diels-Alder-Based Construction of a Piperidine A-Ring Unit for Total Synthesis of the Marine Hepatotoxin Cylindrospermopsin

The synthesis of a piperidine A-ring precursor to the alkaloid cylindrospermopsin (1) is described. The initial approach to the A-ring precursor focused on the imino Diels-Alder reaction of diene 8 with ethyl (N-tosylimino)acetate (9) to form the cycloadduct 10 as a single stereoisomer. However, all attempts to convert ester 10 to a requisite diene such as 5 were unsuccessful. An alternative strategy involved the Diels-Alder cycloaddition of N-tosylimine 9 with oxygenated diene 19 under either thermal or Lewis acid-catalyzed conditions to produce a mixture of cis and trans enones 20 and 21. Although the undesired cis-enone 20 was the major product under all reaction conditions, it could be converted to the desired trans enone 21 by acid-catalyzed isomerization. Copper-mediated conjugate addition of vinylmagnesium bromide to cis-enone 20 followed by stereoselective ketone reduction with L-Selectride produced alcohol 23, whose structure was confirmed by X-ray crystallography. Similarly, trans-enone 21 was converted to alcohol 25 whose structure and stereochemistry were also established by X-ray analysis. Alcohol 25 was then protected as the silyl ether 26, which was hydroborated at the terminal olefin to produce primary alcohol ester 28having the stereochemistry and functionality needed for cylindrospermopsin.     

Experimental and Theoretical Investigations of the Formation of the Diazene PhSN=C(H)N=NC(H)=NSPh from HCN(2)(SPh)(3) by a Thiyl-Radical-Catalyzed Mechanism: Identification of the HC(NSPh)(2)(*) Radical and X-ray Structures of HCN(2)(SPh)(3) and PhSN=C(H)N=NC(H)=NSPh

The reaction of HCN(2)(SiMe(3))(3) with benzenesulfenyl chloride in a 1:3 molar ratio produces HCN(2)(SPh)(3) (4) as thermally unstable, colorless crystals. The decomposition of (4) in toluene at 95 degrees C was monitored by UV-visible, (1)H NMR and ESR spectroscopy. The major final products of the decomposition were identified as PhSN=C(H)N=NC(H)=NSPh (5) and PhSSPh. The structures of 4 and 5 were determined by X-ray crystallography. The crystals of 4 are monoclinic, space group P2(1)/a, with a = 9.874(2) ?, b = 19.133(2) ?, c = 10.280(2) ?, beta = 113.37(1) degrees, V = 1782.8(5) ?(3), and Z = 4. The final R and R(w) values were 0.042 and 0.049, respectively. The crystals of 5 are monoclinic, space group P2(1)/n, with a = 5.897(6) ?, b = 18.458(10) ?, c = 7.050(8) ?, beta = 110.97(5) degrees, V = 716(1) ?(3), and Z = 2. The final R and R(w) values were 0.075 and 0.085, respectively. The diazene 5 adopts a Z,E,Z structure with weak intramolecular S.N contacts of 2.83 ?, giving rise to four-membered NCNS rings. During the thermolysis of 4 at 95 degrees C in toluene a transient species (lambda(max) 820 nm) was detected. It decomposes with second-order kinetics to give 5 (lambda(max) 450 nm). The ESR spectrum of the reaction mixture consisted of the superposition of a three-line 1:1:1 spectrum (g = 2.0074, A(N) = 11.45 G), attributed to (PhS)(2)N(*), upon a doublet of quintets (1:2:3:2:1) with g = 2.0070, A(N) = 6.14 G, A(H) = 2.1 G assigned to the radical HCN(2)(SPh)(2)(*). Density functional theory (DFT) calculations for the models of the radical showed the E,Z isomer to have the lowest energy. Thermochemical calculations indicate that the decomposition of HCN(2)(SH)(3) into the diazene (Z,E,Z)-HSN=C(H)N=NC(H)=NSH (and 2 HSSH) is substantially more exothermic (DeltaH = -176.1 kJ mol(-)(1)) than the corresponding formation of the isomeric eight-membered ring (HC)(2)N(4)(SH)(2) (DeltaH = -40.6 kJ mol(-)(1)). These calculations also indicate that the diazene is formed by a mechanism in which the RS(*) radical acts as a catalyst.     

Coinage Metal Complexes of a Tellurium Diimide: cis→trans Isomerization and Metal–Metal Interactions

The different coordination behavior of the ligand tBuN=Te(μ-NtBu)₂Te=NtBu (L) towards Cu⁺ and Ag⁺ results from a cis→trans isomerization. The two Cu⁺ ions in [Cu₂L₃]²⁺ (shown schematically) bridge trans and cis isomers of the ligand, whereas the Ag⁺ ions in [Ag₂L₂]²⁺ link two trans ligands and exhibit a weak Ag⋅⋅⋅Ag interaction.     

Syntheses and X-ray structures of monocyclic,bicyclic, and spirocyclic gallium and indium boraamidinates

The reactions of [Li(2)[PhB(N(t)Bu)(2)]](2) with GaCl(3) in various stoichiometries yield [Li(thf)(4)][PhB(mu-N(t)Bu)(2)GaCl(2) x GaCl(3)] (1), [PhB(mu-N(t)Bu)(2)GaCl](2) (2), and [mu-Li(OEt(2))[PhB(N(t)Bu)(2)]Ga] (3a), a series of complexes in which the three chloride ligands are successively replaced by the dianion [PhB(N(t)Bu)(2)](2-). The X-ray structures of 1, 2, and 3a show that the boraamidinate ligand adopts an N,N'-chelating mode. In the ion-separated complex 1, one of the nitrogen atoms is coordinated to a GaCl(3) molecule. The related indium complexes [mu-LiCl(thf)(2)][PhB(mu-N(t)Bu)(2)InCl](2) (4) and [mu-Li(OEt(2))[PhB(mu-N(t)Bu)(2)]In] (3b) were obtained in a similar manner. Complex 4 is the indium analogue of 2 with the incorporation of a bissolvated LiCl molecule. In 3a and 3b the spirocyclic [[PhB(mu-N(t)Bu)(2)](2)M](-) (M = Ga, In) anions are N,N'-chelated to the [Li(OEt(2))](+) counterion. Prolonged reactions result in the formation of [PhB(mu-N(t)Bu)(2)GaCl][(t)BuN(H)GaCl(2)] (5) and [[PhB(mu-N(t)Bu)(2)InCl][(t)BuN(H)InCl(2)][mu-LiCl(OEt(2))(2)]] (6), respectively. The X-ray structures of 5 and 6 reveal bicyclic structures which formally involve the entrapment of the monomers (t)BuN(H)MCl(2) by a four-membered BN(2)M ring (M = Ga, In). The synthesis and X-ray structure of Cl(2)Ga[mu-N(H)(t)Bu](2)GaCl(2) are also reported.     

Isolation and characterization of a monomeric cationic titanium hydride

Methyl cations 1-Cp and 1-Cp*, stabilized by the tri-tert-butylphophinimine ligand and either C5H5 or C5Me5, were generated from the neutral dimethyl precursors and [Ph3C]+[B(C6F5)4]-. Reaction of these compounds with H2 resulted in contrasting reactions. For 1-Cp, hydrogenolysis of the Ti-CH3 group led to rapid reduction to Ti(III) and production of a cationic Ti(III) dimer, 2, presumably formed upon loss of H2 from a transiently generated Ti(IV) hydride. Compound 2 was characterized crystallographically and via its cleavage with donor solvents such as THF to form the monomeric [Cp(L)Ti(THF)2]+[B(C6F5)4]-, 3. In contrast, 1-Cp* reacted rapidly with H2 to form a cationic Ti(IV) hydride species, 4, which was resistant to reduction. While only moderately stable in solution under H2, a stable, isolable THF adduct preciptitated upon addition of THF, giving 4.THF, which was fully characterized, including via X-ray crystallography. Naked hydride 4 was very reactive toward haloarene solvents such as bromobenzene, giving the cationic bromide [Cp*(L)TiBr]+[B(C6F5)4]-, 5, which was fully characterized as its THF adduct 5.THF. The contrasting behavior of 1-Cp and 1-Cp* is a result of the greater steric protection and electron donation provided by the Cp* ligand relative to the Cp donor.     

Osmium (VI) catalyzed chemoselective oxidation of allylic and benzylic alcohols

A mild and highly chemoselective approach to oxidation of allylic, electron rich/deficient benzylic, and heterocyclic alcohols employing catalytic quantities of K₂[OsO₂(OH)₄] (3 mol %) and chloramine-T (50 mol %) is described. The protocol offers short reaction times (25 min–2 h), controlled oxidation, and tolerance to a variety of substrates. A systematic mechanistic study based on the LC-ESI-MS/MS reveals the presence of imidotriooxoosmium species which further reacts with alcohol to give the oxidized product.