Insights into the making of a stable silylene

Reduction of Cl2Si[(NR)2C6H4-1,2] (R = CH2Bu(t)) with potassium is known to lead to the stable silylene Si[(NR)2C6H4-1,2] (1). However, silylene is now shown to react further with an alkali metal (Na or K) to yield the (1)(2)2-, c-(1)(3)-*, c-(1)(3)2- or c-(1)(4)2- derivatives. Reduction of Cl2Si[(NR)2C6H4-1,2] (R = CH2CH3 or CH2CHMe2) with potassium does not lead to an isolable silylene, but such a silylene is proposed to be an intermediate and, as for 1, reacts further to afford the potassium salts of c-[Si{(NR)2C6H4-1,2}]4-* and c-[Si{(NR)2C6H4-1,2}](4)2-. The pathways leading to the anionic cyclotri- and cyclotetrasilanes are discussed and supported experimentally; including by X-ray structures of relevant intermediates.     

Theoretical understanding on the v(1)-SO4(2-) band perturbed by the formation of magnesium sulfate ion pairs

The factors determining the spectroscopic characteristics of the v(1)-SO4(2-) band of the MgSO4 ion pairs are discussed via ab initio calculation, including coupling effect, hydrogen bonding effect, and direct contact effect of Mg2+ with SO4(2-). With the calculation of the heavy water hydrated contact ion pairs (CIP), the overlap between the librations of water and the v(1)-SO4(2-) band can be separated, and thus the coupling effect is abstracted, and this coupling effect leads to a blue shift for the v(1)-SO4(2-) band of 5.6 cm(-1) in the monodentate CIP and 3.6 cm(-1) in the bidentate CIP. The hydrogen bonding between each water molecule without relation to Mg2+ and the sulfate ion makes the v(1)-SO4(2-) band blue shift of 3.7 cm(-1). When the outer-sphere water around Mg2+ are hydrogen bonded between SO4(2-) and Mg2+, it will make the largest disturbance to the v(1)-SO4(2-) band. Moreover, the inner-sphere water can affect the v(1)-SO4(2-) band conjunct with the direct contact of Mg2+ with SO4(2-), showing a blue shift of 14.4 cm(-1) in the solvent-shared ion pair, 22.6 cm(-1) in the monodentate CIP, 4.3 cm(-1) in the bidentate CIP, and 21.4 cm(-1) in the tridentate CIP. At last, the Raman spectral evolution in the efflorescence production process is tried to be rationalized. The shoulder at 995 cm(-1) is attributed to the monodentate CIP with 2-3 outer-sphere water molecules, whereas the new peak at 1021 cm(-1) at high concentration is assigned to the formation of aqueous triple ion.     

Imidazolyl derivatives of the chroman ring 3

The synthesis of 2-methyl-3-(1-methyl-1H-imidazol-2-yl)-4H-1-benzopyran-4-ones 4 is described starting from 2-acetoxybenzoyl chlorides and 1,2-dimethylimidazole. Chromones 4 undergo alkaline ring opening to the corresponding 1-(2-hydroxyphenyl)-2-(1-methyl-1H-imidazol-2-yl)ethenols 5 which give ring closure to 2-substituted 3-(1-methyl-1H-imidazol-2-yl)-4H-1-benzopyran-4-ones or 2,3-dihydro-3-(1-methyl-1H-imidazol-2-yl)-4H-1-benzopyran-4-ones. The corresponding chromanols and chromenes can be easily obtained from chromones 4 .     

Facilitated sulfate transfer across the nitrobenzene-water interface as mediated by hydrogen-bonding ionophores.

Facilitated SO4(2-) transfers by hydrogen bond-forming ionophores are investigated across the nitrobenzene (NB)-water interface by using polarography with a dropping electrolyte electrode. Bis-thiourea 1, alpha,alpha'-bis(N'-p-nitrophenylthioureylene)-m-xylene, is found to significantly facilitate the transfer of the highly hydrophilic SO4(2-) whereas its counterpart, N-(p-nitrophenyl)-N'-propylthiourea (ionophore 2), cannot. In contrast to the predominant formation of a 1:1 complex with SO4(2-) in the bulk NB phase, the SO4(2-) transfer assisted by 1 is indeed based on the formation of a 1:2 complex between SO4(2-) and ionophore, even under the condition of [SO4(2-)]aq > [1]org. Such an exclusive formation of the 1:2 (SO4(2-) to ionophore) complex at the NB-water interface is not observed with structurally similar bis-thiourea 3, alpha,alpha'-bis(N'-phenylthioureylene)-m-xylene, where p-nitrophenyl moietes of bis-thiourea 1 are simply replaced by phenyl groups. The facilitated transfer of SO4(2-) with bis-thiourea 1 is further compared to that of HPO4(2-) and H2PO4- across the NB-water interface, which was previously shown to be assisted by 1 through the formation of the 1:1 and 2:1 (anion to ionophore) complexes, respectively. On the basis of these examinations, unique binding behaviors of hydrogen bond-forming ionophores at the NB-water interface are discussed, with a view towards development of ionophore-based anion-selective chemical sensors.     

A Raman and infrared spectroscopic study of the uranyl silicates--weeksite, soddyite and haiweeite: part 2

Raman spectroscopy has been used to study the molecular structure of a series of selected uranyl silicate minerals including weeksite K2[(UO2)2(Si5O13)].H2O, soddyite [(UO2)2SiO4.2H2O] and haiweeite Ca[(UO2)2(Si5O12(OH)2](H2O)3 with UO2(2+)/SiO2 molar ratio 2:1 or 2:5. Raman spectra clearly show well resolved bands in the 750-800 cm(-1) region and in the 950-1000 cm(-1) region assigned to the nu1 modes of the (UO2)2+ units and to the (SiO4)4- tetrahedra. Soddyite is characterized by Raman bands at 828.0, 808.6 and 801.8 cm(-1), 909.6 and 898.0 cm(-1), and 268.2, 257.8 and 246.9 cm(-1), attributed to the nu1, nu3, and nu2 (delta) (UO2)2+, respectively. Coincidences of the nu1 (UO2)2+ and the nu1 (SiO4)4- is expected. Bands at 1082.2, 1071.2, 1036.3, 995.1 and 966.3 cm(-1) are attributed to the nu3 (SiO4)4-. Sets of Raman bands in the 200-300 cm(-1) region are assigned to nu2 (delta) (UO2)2+ and UO ligand vibrations. Multiple bands indicate the non-equivalence of the UO bonds and the lifting of the degeneracy of nu2 (delta) (UO2)2+ vibrations. The (SiO4)4- tetrahedral are characterized by bands in the 470-550 cm(-1) and in the 390-420 cm(-1) region. These bands are attributed to the nu4 and nu2 (SiO4)4- bending modes. The minerals show characteristic OH stretching bands in the 2900-3500 and 3600-3700 cm(-1).     

Cyclodextrin complexation of a stilbene and the self-assembly of a simple molecular device

(E)-4-tert-Butyl-4'-oxystilbene, 1(-), is thermally stable as the (E)-1(-) isomer but may be photoisomerized to the (Z)-1(-) isomer as shown by UV-vis and (1)H NMR studies in aqueous solution. When (E)-1(-) is complexed by alphaCD two inclusion isomers (includomers) form in which alphaCD assumes either of the two possible orientations about the axis of (E)-1(-) in alphaCD.(E)-1(-) for which (1)H NMR studies yield the parameters: k(1)(298 K)= 12.3 +/- 0.6 s(-1), DeltaH(1)(++)= 94.3 +/- 4.7 kJ mol(-1), DeltaS1(++)= 92.0 +/- 5.0 J K(-1) mol(-1), and k(2)(298 K)= 10.7 +/- 0.5 s(-1), DeltaH(2)(++)= 93.1 +/- 4.7 kJ mol(-1), DeltaS2(++)= 87.3 +/- 5.0 J K(-1) mol(-1) for the minor and major includomers, respectively. The betaCD.(E)-1(-) complex either forms a single includomer or its includomers interchange at the fast exchange limit of the (1)H NMR timescale. Complexation of 1(-) by N-(6(A)-deoxy- alpha-cyclodextrin-6(A)-yl)-N'-(6(A)-deoxy- beta-cyclodextrin-6(A)-yl)urea, results in the binary complexes 2.(E)-1(-) in which both CD component annuli are occupied by (E)-1(-) and which exists exclusively in darkness and 2.(Z)-1(-) in which only one CD component is occupied by (Z)-1(-) and exists exclusively in daylight at lambda > or = 300 nm. Irradiation of solutions of the binary complexes at 300 and 355 nm results in photostationary states dominated by 2.(E)-1(-) and 2.(Z)-1(-), respectively. In the presence of 4-methylbenzoate, 4(-), 2.(Z)-1(-) forms the ternary complex 2.(Z)-1(-).4(-) where 4(-) occupies the second CD annulus. Interconversion occurs between 2.(Z)-1(-).4(-) and 2.(E)-1(-)+4(-) under the same conditions as for the binary complexes alone. Similar interactions occur in the presence of 4-methylphenolate and 4-methylphenylsulfonate. The two isomers of each of these systems represent different states of a molecular device, as do the analogous binary complexes of N,N-bis(6(A)-deoxy- beta-cyclodextrin-6(A)-yl)urea, 3, [3.(E)-1(-) and 3.(Z)-1(-), where the latter also forms a ternary complex with 4(-).     

A Raman and infrared spectroscopic study of the uranyl silicates--weeksite, soddyite and haiweeite

Raman spectroscopy has been used to study the molecular structure of a series of selected uranyl silicate minerals, including weeksite K2[(UO2)2(Si5O13)].H2O, soddyite [(UO2)2SiO4.2H2O] and haiweeite Ca[(UO2)2(Si5O12(OH)2](H2O)3 with UO2(2+)/SiO2 molar ratio 2:1 or 2:5. Raman spectra clearly show well resolved bands in the 750-800 cm-1 region and in the 950-1000 cm-1 region assigned to the nu1 modes of the (UO2)2+ units and to the (SiO4)4- tetrahedra. For example, soddyite is characterized by Raman bands at 828.0, 808.6 and 801.8 cm-1 (UO2)2+ (nu1), 909.6 and 898.0 cm-1 (UO2)2+ (nu3), 268.2, 257.8 and 246.9 cm-1 are assigned to the nu2 (delta) (UO2)2+. Coincidences of the nu1 (UO2)2+ and the nu1 (SiO4)4- is expected. Bands at 1082.2, 1071.2, 1036.3, 995.1 and 966.3 cm-1 are attributed to the nu3 (SiO4)4-. Sets of Raman bands in the 200-300 cm-1 region are assigned to nu2 (delta) (UO2)2+ and UO ligand vibrations. Multiple bands indicate the non-equivalence of the UO bonds and the lifting of the degeneracy of nu2 (delta) (UO2)2+ vibrations. The (SiO4)4- tetrahedral are characterized by bands in the 470-550 cm-1 and in the 390-420 cm-1 region. These bands are attributed to the nu4 and nu2 (SiO4)4- bending modes. The minerals show characteristic OH stretching bands in the 2900-3500 cm-1 and 3600-3700 cm-1.     

Observation of the first hydration layer of isolated cations and anions through the FTIR-ATR difference spectra

The attenuated total reflectance-Fourier transform infrared (ATR-FTIR) difference spectra of the dilute aqueous (NH4)2SO4, Na2SO4, MgSO4, ZnSO4, NaClO4, and Mg(ClO4)2 solutions by pure water were obtained at various concentrations. In the difference spectra of aqueous (NH4)2SO4 solutions, a peak at approximately 3039 cm(-1), two shoulders at approximately 3155 and approximately 2894 cm(-1), and a peak at approximately 1445 cm(-1) were ascribed to N-H stretching and bending vibrations, respectively. A small negative peak was resolved at approximately 3660 cm(-1) in the difference spectra of (NH4)2SO4, which is the sole contribution of SO4(2-) either in the O-H stretching or in the O-H bending region. The positive peaks of the difference spectra in the O-H stretching region for Na2SO4, MgSO4, and ZnSO4 systems, which constantly appeared at approximately 3423, approximately 3136, and approximately 3103 cm(-1) respectively, were suggested to be the contribution of the interactions between metal cations (Na+, Mg2+, and Zn2+) and water molecules, especially from the first hydrated layer of the cations. In the region of 800-1200 cm(-1), the normally infrared-prohibited nu1 (SO4(2-)) band was observed as a weak peak at approximately 981 cm(-1) even at very dilute concentrations (0.10 mol dm(-3)) due to the disturbance of the water molecules hydrated with SO4(2-), even though such a feature may increasingly result from associated ions with increasing concentration. The spectra of the water molecules directly influenced by ClO4-, i.e., mostly the first layer of hydrated water, in NaClO4 and Mg(ClO4)2 solutions were obtained by subtracting the corresponding spectra of the same metal sulfate solutions at the same concentrations from the perchlorate solutions. A positive peak at approximately 3583 +/- 6 cm(-1) and a negative peak at approximately 3184 +/- 25 cm(-1) were obtained as the result of the subtraction. The positive peak was attributed to the water molecules weakly hydrogen-bonded with ClO4-, while the negative one to the reduction of water molecules with fully hydrogen-bonded five-molecule tetrahedral nearest neighbor structure on the introduction of ClO4-.     

Cheletropic decomposition of cyclic nitrosoamines revisited: the nature of the transition states and a critical role of the ring strain

The cheletropic decompositions of 1-nitrosoaziridine (1), 1-nitroso-Delta(3)-pyrroline (2), 7-nitroso-7-azabicyclo[2.2. 1]hepta-2,5-diene (3), and 6-nitroso-6-azabicyclo[2.1.1]hexa-4-ene (4) have been studied theoretically using high level ab initio computations. Activation parameters of the decomposition of nitrosoaziridine 1 were obtained experimentally in heptane (DeltaH()(298) = 18.6 kcal mol(-)(1), DeltaS()(298) = -7.6 cal mol(-)(1) K(-)(1)) and methanol (20.3 kcal mol(-)(1), 0.3 cal mol(-)(1) K(-)(1)). Among employed theoretical methods (B3LYP, MP2, CCD, CCSD(T)//CCD), the B3LYP method in conjunction with 6-31+G, 6-311+G, and 6-311++G(3df,2pd) basis sets gives the best agreement with experimental data. It was found that typical N-nitrosoheterocycles 2-4 which have high N-N bond rotation barriers (>16 kcal mol(-)(1)) extrude nitrous oxide via a highly asynchronous transition state with a planar ring nitrogen atom. Nitrosoaziridine 1, with a low rotation barrier (<9 kcal mol(-)(1)) represents a special case. This compound can eliminate N(2)O via a low energy linear synperiplanar transition state (DeltaH()(298) = 20.6 kcal mol(-)(1), DeltaS()(298) = 2.5 cal mol(-)(1) K(-)(1)). Two higher energy transition states are also available. The B3LYP activation barriers of the cheletropic fragmentation of nitrosoheterocycles 2-4 decrease in the series: 2 (58 kcal mol(-)(1)) > 3 (18 kcal mol(-)(1)) > 4 (12) kcal mol(-)(1). The relative strain energies increase in the same order: 2 (0 kcal mol(-)(1)) < 3 (39 kcal mol(-)(1)) < 4 (52 kcal mol(-)(1)). Comparison of the relative energies of 2-4 and their transition states on a common scale where the energy of nitrosopyrroline 2 is assumed as reference indicates that the thermal stability of the cyclic nitrosoamines toward cheletropic decomposition is almost entirely determined by the ring strain.     

Sesquiterpenoids of the Sponge Dysidea fragilis of the North-Brittany Sea

The title sponge is shown to contain eight new sesquiterpenoids for which a common, unusual biogenetic origin is postulated. The compounds are shown to be: (–)-(1R*,4R*)-3-(3′-furyl)methyl-2-p-menthen-7-yl acetate ((–)- 8b ); two diols separated as the monoacetates (–)-(1S*,4R*)-3-(3′-furyl)methyl-l-hydroxy-2-p-menthen-7-yl acetate ((–)- 13a ) and the (–)-(1R*,4R*)-epimer (–)- 13b , the two C(4)-epimeric 4-ethoxy-3-(1′(7′),2′-p-menthadien-3′-yl)methyl-2-buten-4-olides ((+)- 14a and (–)- 14b ), (–)-3-(3′-furyl)methyl-7-nor-2-p-menthen-l-one ((–)- 11 ), (–)-(3Z)-1-(3′-furyl)-4,8-dimethylnona-3, 7-dien-2-yl acetate ((–)- 17 ), and (+)-3-(5′,7′-seco-2′(10′)-pinen-7′-yl)methylfuran ((+)- 15 ).     

Tandem ireland-claisen rearrangement ring-closing alkene metathesis in the construction of bicyclic beta-lactam carboxylic esters

4-Alkenyl-2-azetidinone systems were converted to the corresponding ethyl 2-?4-alkenyl-2-oxo-1-azetidinyl-4-pentenoates. In addition, 4-(2-propenyl-1-oxy)-, 4-(2-propenyl-1-thio)-, 4-?N-(2-propenyl)-(4-toluenesulfonyl)- and (3S, 4R)-4-(2-propenyl)-3-?(1R)-1-(tert-butyldimethylsilyloxy)ethyl-++ +azeti din-2-one were converted into beta-lactam dienes via sequential N-alkylation, Ireland-Claisen ester enolate rearrangement and esterification. Ring-closing metathesis using the Schrock ?(CF(3))(2)MeCO(2)Mo(=CHCMe(2)Ph)(=NC(6)H(3)-2,6-iso-Pr(2)) (1) or Grubbs Cl(2)(Cy(3)P)(2)Ru=CHPh (2) carbenes gave a series of ?5.2.0 and ?6.2.0 bicycles. Subsequent elaboration of the analogous (2R,7R, 8S)-tert-butyl 8-?(1R)-(tert-butyldimethylsilyloxy)ethyl-1-aza-9-oxobicyclo++ +?5.2. 0non-4-ene-2-carboxylate (15), via selenation and desilylation, gave (+)-(2S,7R,8S)-tert-butyl 8-?(1R)-hydroxyethyl-1-aza-9-oxobicyclo?5.2.0nona-2, 4-diene-2-carboxylate (18), a novel type of bicyclic beta-lactam. Diels-Alder cycloaddition further afforded tetracyclic systems exemplified by tert-butyl (1R,4S,5R,7S)-4-?(1R)-1-hydroxyethyl-3,9, 11-trioxo-10-phenyl-2,8,10,12-tetraazatetracyclo?5.5.2.0.(2, 5)0(8, 12)tetradec-13-ene-1-carboxylate (19).     

Influence of the counterion and the solvent molecules in the spin crossover system [Co(4-terpyridone)2]Xp.nH2O

A series of new complexes belonging to the [Co(4-terpyridone)2]X(p) x nS family (4-terpyridone = 2,6-bis(2-pyridyl)-4(1H)-pyridone) have been synthesized and characterized, using X-ray single crystal determination and magnetic susceptibility studies, to be X = [BF4]- (p = 2) and S = H2O for polymorphs 1 and 2, X = [BF4]- (p = 1) and [SiF6]2- (p = 0.5) and S = CH(3)OH for 3, X = [SiF6]2- (p = 1) and S = 3CH3OH and H2O for 4, X = [Co(NCS)4]2- (p = 1) and S = 0.5CH3OH for 5, X = I- (p = 2) and S = 5H2O for 6, X = [PF6]- (p = 1) for 7, and X = [NO3]- (p = 2) for 8. Compounds 1-7 can be grouped in three sets according to the space group in which they crystallize: (i) P1 triclinic (1, 3), (ii) P2(1) monoclinic (2), and (iii) P2(1)/c monoclinic (4-7). The tridentate 4-terpyridone ligands coordinate the Co(II) ions in a mer fashion defining essentially tetragonally compressed [CoN6] octahedrons. The Co-N axial bonds involving the pyridone rings are markedly shorter than the Co-N equatorial bonds collectively denoted as Co-N(central) and Co-N(distal), respectively. The differences in the average Co-N(central) or Co-N(distal) distances observed for 1-7 reflect the different spin states of Co(II). Complexes 7 and 4' are fully high spin (HS), while 5 and 6 are low spin (LS). However, the counterion [Co(NCS)4]2- in complex 5 is high spin. Complexes 1, 2, 3, and 8 exhibit spin-crossover behavior in the 400-100 K temperature region. Compounds 1 and 2 are polymorphs, and interestingly, 1 irreversibly transforms into 2 above 340 K because of a crystallographic phase transition which involves a drastic modification of the crystal packing. The relevant thermodynamic parameters associated with the spin transition of polymorph 2 have been estimated using the regular solution theory leading to DeltaH = 3.04 kJ mol(-1), DeltaS = 20 J K(-1) mol(-1), and Gamma = 0.95 kJ mol(-1).     

New glucose esters from the fresh leaves of Jacaranda mimosaefolia

From the fresh leaves of Jacaranda mimosaefolia were isolated Phytoquinoids 1-4 established as beta-D-glucopyranose 2-benzeneacetatel,6-bis(1-hydroxy-4-oxo-2,5-cyclohexadiene-1-acetate), for which the name Jacaranoside is proposed; beta-D-glucopyranose 2-(4-hydroxybenzeneacetate) 1,6-bis(1-hydroxy-4-oxo-2,5-cyclohexadiene-1-acetate), for which the name Jacarandol is proposed; beta-D-glucopyranose 2-benzeneacetate 1-(1-hydroxy-4-oxo-2,5-cyclohexadiene-1-acetate) and beta-D-glucopyranose 1,6-bis (1-hydroxy-4-oxo-2,5-cyclohexadiene-1-acetate) respectively.     

Further studies on the structure of polysaccharides from the lichen Flavoparmelia caperata (L.) Hale

A water-soluble polysaccharide called PC-2 was previously isolated from Flavoparmelia caperata (L.) Hale and assigned to alpha(1-3)(1-4)glucan. However, PC-2 separated into three components, PC-2A, PC-2B, and PC-2C (a single peak in HPLC, respectively) on further purification. Methylation analysis and (1)H- and (13)C-NMR spectroscopic studies suggested that PC-2A is composed of repeating units of [alpha-D-Glc1-3](3), : [alpha-D-Glc1-4](2), while PC-2B and PC-2C are partly branched galactoglucomannans consisting of (1-3)- and (1-4)-linked alpha-D-glucopyranosyl units as the main chain. In addition we confirmed that the polysaccharide fraction PB-2 from the lichen of the same genus, Flavoparmelia baltimorensis (Gyelink & Foriss) Hale, is identical to PC-2 based on the chemical and spectroscopic data.     

Influence of H2S and thiols on the binding of alkenes and alkynes to ReS4-: the spectator sulfido effect

The three-component reaction of ReS4- (1), H2S, and unsaturated substrates (un = alkene, alkyne) affords the ReV derivatives Re(S)(S2un)(SH)2-. These adducts arise via the addition of H2S to intermediate dithiolates ReS2(S2C2R4)- and dithiolenes ReS2(S2C2R2)-. The species [ReS[S2C2(tms)2](SH)2]-, [ReS(S2C7H10)(SH)2]- (3), and [ReS(S2C2H4)(SH)2]- are prepared according to this route. Similarly, the selenolate-thiolate complex [ReS(S2C7H10)(SeH)(SH)]- (5) is produced by the reaction of [ReS2(S2C7H10)]- with H2Se. The corresponding reactions using benzenethiol in place of H2S afford the more thermally robust adducts [ReS[S2C2(tms)2](SH)(SPh)]-, [ReS(S2C7H10)(SH)(SPh)]- (7), and [ReS(S2C2H4)(SH)(SPh)]-. Norbornanedithiolato compounds 3, 5, and 7 are obtained as pairs of isomers that differ in terms of the relative orientation of the norbornane bridgehead relative to the Re=S unit. The reaction of [ReS(S2C7H10)(SD)2]- (3-d2) with H2S to give 3 is proposed to proceed via elimination of D2S and subsequent addition of H2S. Variable-temperature 1H NMR measurements on the equilibrium of [ReS(S2C6H12)(SPh)(SH)]- with 1,1-hexene, and PhSH gave the following results: deltaH = -7 (+/- 1) kJ x mol(-1); deltaS = 23 (+/- 4) J x mol(-1) x K(-1). Solutions of ethanedithiol and 1 react with C2(tms)2 and C2H4 to give [ReS[S2C2(tms)2](S2C2H4)]- and [ReS(S2C2H4)2]-, respectively, concomitant with loss of H2S. The pathway for the ethanedithiol reaction is examined using 2-mercaptoethanol, affording [ReS[S2C2(tms)2](SC2H4OH)]-, which does not cyclize. Treatment of a solution of diphenylbutadiyne and 1 with PhSH gives two isomers of the dithiolene [ReS(SH)(SPh)[S2C2Ph(C2Ph)]]-. The corresponding reaction of ethanedithiol, diphenylbutadiyne, and 1 affords the 1,4-diphenylbutadiene-1,2,3,4-tetrathiolate complex [[ReS(S2C2H4)]2(S4C4Ph2)]2-.     

High functionalization of 5-nitro-1H-imidazole derivatives: the TDAE approach

We report herein the synthesis of substituted 2-[4-(1,2-dimethyl-5-nitro-1H-imidazol-4-yl)phenyl]-1-arylethanols, ethyl 3-[4-(1,2-dimethyl-5-nitro-1H-imidazol-4-yl)-phenyl]-2-hydroxypropanoate and 2-[4-(1,2-dimethyl-5-nitro-1H-imidazol-4-yl)benzyl]-2-hydroxy-acenaphthylen-1(2H)-one from the reactions of 4-[4-(chloromethyl)phenyl]-1,2-dimethyl-5-nitro-1H-imidazole with various aromatic carbonyl and a-carbonyl ester derivatives using tetrakis(dimethylamino)ethylene (TDAE) methodology.     

Influence of chirality of the preceding acyl moiety on the cis/trans ratio of the proline peptide bond

We report that the cis/trans ratio of the proline peptide bond can be strongly influenced by the chirality of the acyl residue preceding proline. Acyl moieties derived from (2S)-2,6-dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid (8) and (2R)-3-methoxy-2-methyl-2-(4-methyl-2-nitrophenoxy)-3-oxopropanoic acid (5) in acyl-Pro molecules influence isomerization of the proline peptide bond constraining the omega dihedral angle to the trans orientation. Structures of benzyl (2S)-1-([(2S)-2,6-dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazin-2-yl]carbonyl)-2-pyrrolidinecarboxylate (3) derived from 2D (1)H NMR conformational analysis and crystallographic data exhibit only the trans conformation of proline peptide bond. On the other hand the diastereomer 4, which contains an (R) acyl moiety, exhibits two sets of signals in (1)H NMR spectra. The signals were assigned to trans (72%) and cis (28%) conformers. Crystallographic analysis of 4 showed that only the cis conformation is present in the crystalline state. The (1)H NMR chemical shift pattern of three sets of signals observed in 2 was observed also in benzyl (2S)-1-[(2R/S)-3-methoxy-2-methyl-2-(4-methyl-2-nitrophenoxy)-3-oxopropanoyl]-2-pyrrolidinecarboxylate. (R)-Carboxylic acid 5, after coupling with (S)-ProOBn, yielded benzyl (2S)-1-[(2R)-3-methoxy-2-methyl-2-(4-methyl-2-nitrophenoxy)-3-oxopropanoyl]-2-pyrrolidinecarboxylate (6), which in DMSO-d(6) exhibited only the trans conformation of the proline peptide bond. These results suggest that in these particular cases acyl-Pro peptide bond isomerization is strongly influenced by the stereochemistry of the acyl residue preceding proline. (2S)-2,6-Dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid (8) and (2R)-3-methoxy-2-methyl-2-(4-methyl-2-nitrophenoxy)-3-oxopropanoic acid (5) are promising chiral peptidomimetic building blocks that can be used as acyl moieties to force the proline peptide bond into the trans conformation in a variety of acyl-Pro molecules.     

Rovibrationally selected and resolved state-to-state photoionization of ethylene using the infrared-vacuum ultraviolet pulsed field ionization-photoelectron method

By preparing ethylene [C2H4(X1Ag)] in selected rotational levels of the nu11(b1u), nu2+nu12(b1u), or nu9(b2u) vibrational state with infrared (IR) laser photoexcitation prior to vacuum ultraviolet (VUV) laser photoionization, we have recorded rotationally resolved pulsed field ionization-photoelectron (PFI-PE) spectra for C2H4+(X2B3u) in the energy region of 0-3000 cm(-1) above the ionization energy (IE) of C2H4(X1Ag). Here, nu2(ag), nu9(b2u), nu11(b1u), and nu12(b1u) represent the C-C stretching, CH2 stretching, CH2 stretching, and CH2 bending modes of C2H4(X1Ag), respectively. The fully rovibrationally resolved spectra have allowed unambiguous symmetry assignments of the observed vibrational bands, which in turn have provided valuable information on the photoionization dynamics of C2H4. The IR-VUV photoionization of C2H4(X1Ag) via the nu11(b1u) or nu2+nu12(b1u) vibrational states is found to predominantly produce vibrational states of C2H4+(X2B3u) with b1u symmetry, which cannot be observed in single-photon VUV-PFI-PE measurements of C2H4(X1Ag). The analysis of the observed IR-VUV-PFI-PE bands has provided the IE(C2H4) = 84,790.2(2) cm(-1) and accurate vibrational frequencies for the nu4+(au)[84.1(2) cm(-1)], nu12+(b1u)[1411.7(2) cm(-1)], nu4+ +nu12+(b1g)[1482.5(2) cm(-1)], nu2+(ag)[1488.3(2) cm(-1)], nu2+ + nu4+(au)[1559.2(2) cm(-1)], 2nu4+ + nu12 +(b1u)[1848.5(2) cm(-1)], 4nu4+ + nu12 +(b1u)[2558.8(2) cm(-1)], nu2+ + nu12 +(b1u)[2872.7(2) cm(-1)], and nu11+(b1u)[2978.7(2) cm(-1)] vibrational states of C2H4+(X2B3u), where nu4+ is the ion torsional state. The IE(C2H4) and the nu4+(au), nu2+(ag), and nu2+ + nu4+ (au) frequencies are in excellent accord with those obtained in previous single-photon VUV-PFI-PE measurements. The other ion vibrational frequencies represent new experimental determinations. We have also performed high-level ab initio anharmonic vibrational frequency calculations for C2H4(X1Ag) and C2H4+(X2B3u) at the CCSD(T)/aug-cc-pVQZ level for guidance in the assignment of the IR-VUV-PFI-PE spectra. All theoretical vibrational frequencies for the neutral and ion, except the ion torsional frequency, are found to agree with experimental vibrational frequencies to better than 1%.     

An efficient multigram synthesis of the potent histamine H3 antagonist GT-2331 and the reassessment of the absolute configuration

GT-2331 is a potent histamine H(3) antagonist which has entered clinical trials. Efficient multigram syntheses of this compound and its enantiomer are described. The literature reports that GT-2331 is the dextrorotatory (+), more potent, enantiomer of 4-[2-(5,5-dimethylhex-1-ynyl)cyclopropyl]-1H-imidazole with the absolute configuration of (1R,2R)-1. However, we found that the dextrorotatory, more potent, enantiomer of 4-[2-(5,5-dimethylhex-1-ynyl)cyclopropyl]-1H-imidazole has the (1S,2S) absolute configuration. We suggest a reconsideration of the absolute configuration of GT-2331.     

Theoretical study of the effects of F to Cl chemical substitution on the electronic structure and the luminescence properties of Cs2GeF6:Os4+ and Cs2ZrCl6:Os4+ materials

It has been experimentally determined that Cs2ZrCl6:Os4+ shows luminescence and up-converted luminescence from the highest t(2g) (4) excited level 2 A1g(1A1g), whereas Cs2GeF6:Os4+ 2 A1g(1A1g) does not luminescence at all. Ab initio quantum chemical calculations on these materials are presented here and show that the variation of the energy gap between the t2g 4 and t2g 3 eg 1 manifolds with F to Cl chemical substitution is a key factor to interpret the experimental findings. This energy gap is calculated to be some 1500 cm(-1) (approximately 2nua1g) in the fluoride host, whereas it is about 3300 cm(-1) (approximately 9nua1g) in the chloride host. The calculated values for the ground state totally symmetric vibrational frequency nu(a1g) are 626 cm(-1) (Cs2GeF6:Os4+) and 355 cm(-1) (Cs2ZrCl6:Os4+), in good agreement with the available experimental data. Geometrical structure of (OsX6)2- clusters (X=F,Cl) embedded in Cs2GeF6 and Cs2ZrCl6 lattices is calculated as well. New assignments for some spectral features based in the results of our calculations are proposed.