Hetero-Diels-Alder reactions of alpha-aryl-beta-monohalo-alpha-nitrosoethylenes: diastereoselective synthesis of 6-substituted 3-aryl-4-halo-5,6-dihydro-4H-1,2-oxazines.

Treatment of alpha-aryl-beta-bromo(or chloro)-alpha-nitrosoethylene, prepared in situ from alpha-monobromo(or chloro)ketoximes and sodium carbonate in ether at rt, with allytrimethylsilane afforded exclusively trans-(4S,6S)- and trans-(4R,6R)-3-aryl-4-halo-6-[(trimethylsilyl)methyl]-5,6-dihydro-4H-1,2-oxazines 10 albeit in low yields. Similar treatment of beta-halo-alpha-nitrosoethylenes with ethyl vinyl ether, however, gave single stereoisomers, i.e., cis-(4S,6S)- and cis-(4R,6R)-6-ethoxy-4-halo-5,6-dihydro-4H-1,2-oxazines 11, in moderate to good yields. The result is in contrast to the reported predominant formation of trans-11a by a radical reaction. On the other hand, similar reactions with tert-butyl vinyl ether at 30 degrees C gave diastereomeric mixtures of cis-(4S,6S)-, cis-(4R,6R)-, trans-(4S,6R)-, and trans-(4R,6S)-6-(tert-butoxy)-4-halo-5,6-dihydro-4H-1,2-oxazines 12. In contrast to compounds 11, the major isomers have (4S,6R) and (4R,6S) configurations. The tendency of a [4 + 2] cycloaddition reaction is consistent with that observed in the Diels-Alder reaction with inverse-electron demand. The stereochemistries of compounds 10-12 were assigned on the basis of the (1)H NMR coupling constants, which were unambiguously determined by the decoupling experiments. All reactions leading to compounds 10-12 proceed with very high regioselectivity. Diastereoselectivity and high regioselectivity are understood in terms of the frontier orbital method. It has been found that cis-12g is isomerized to a mixture of stereoisomers in favor of the trans-isomer in the presence of HClO(4) (72%) in CHCl(3) at rt.     

Synthesis and biological activities of optical isomers of 2-(4-diphenylmethyl-1-piperazinyl)ethyl 5-(4,6-dimethyl-2-oxo-1,3,2-dioxaphosphorinan-2-yl)-1,4-dihydro-2,6-dim ethyl-4-(3-nitrophenyl)-3-pyridinecarboxylate dihydrochloride (NIP-101).

Six optical isomers of 2-(4-diphenylmethyl-1-piperazinyl)ethyl 5-(4,6-dimethyl-2-oxo-1,3,2-dioxaphosphorinan-2-yl)-1,4-dihydro-2, 6-dimethyl-4-(3-nitrophenyl)-3-pyridinecarboxylate dihydrochloride (NIP-101, 1.2HCl.2H2O), a potent calcium antagonist, were successfully prepared by using optically active (2R,4R)-(-)- and (2S,4S)-(+)-2,4-pentanediols, and cis-2,4-pentanediol and optically active (S)-(+)-2-methoxy-2-phenylethanol. Their proton nuclear magnetic resonance investigations demonstrate that the 1,3,2-dioxaphosphorinane group is conformationally constrained around the C-P bond. Calcium-antagonistic and hypotensive activities of the optical isomers were examined and found to depend mainly on the absolute configuration at a stereogenic center in the 1,4-dihydropyridine ring rather than the configuration of the 1,3,2-dioxaphosphorinane moiety.     

A spirocyclic P-S cage cation: synthesis and formation of P7S6I2+

Upon ionization of the P4S3I2 molecule with Ag[Al(OR)4], a highly reactive sulfonium cation P4S3I+ is generated (NMR simulated and assigned). At -80 degrees C this cation reacts with additional P4S3I2 to give either an iodophosphonium P4S3I3+ cation (NMR simulated and assigned) and P4S3 or to give several isomers of a metastable compound that is probably P8S3I3+. This mixture decomposes at 0 degrees C to give only three isomers of the spirocyclic P7S6I2+ cage cation (31P NMR simulated and assigned, X-ray of one isomer, IR assigned). The oxidation of the [Ag(P4S3)2]+ complex by I2 also resulted in the formation of P7S6I2+, but with more by-products. The spirocyclic 15-atom cage of P7S6I2+ has no precedent and contains the first phosphonium center bonded only to P and S atoms. This structural element gives the first experimental clue as to how formal charge-bearing elements in the still unknown class of binary P-Ch (Ch = chalcogen) or homopolyatomic P cations may be constructed.     

Synthesis, cytotoxic effect, and structure-activity relationship of Pd(II) complexes with coumarin derivatives

We report the influence of the substituent at the N atom of the ligands on the synthesis, biological activity, and stability of Pd(II) complexes of the general formula PdL(2). The compounds adopt a cis or trans configuration with respect to the substituent at the nitrogen atom. Sterically hindered substituents promote the formation of trans isomers, whereas when the nitrogen atom is unsubstituted, cis isomers are formed. The compounds were characterized by elemental analysis, infrared and 1H NMR spectroscopies, and electrospray mass spectrometry. The complexes were also studied using X-ray diffraction and computational DFT methods. Both complexes cis-3a and trans-3c exhibit square-planar geometries around the Pd(II) atom. The cytotoxic effects of these complexes were examined on two human leukemia cell lines, HL-60 and NALM-6. Pd complex cis-3a showed significant cytotoxic activity. The effects exhibited by this complex were comparable to those reported for carboplatin. Loigand 2a was not cytotoxic. Computational analysis carried out at the PB/B3LYP/LACVP**//mPW1PW91/LanL2DZ level showed excellent correlation between the energy difference of the cis and trans isomers and the cytotoxic activity, rendering computations a useful predictive tool for the design of new drugs.     

Synthesis of heteroaryl imines: theoretical and experimental approach to the determination of the configuration of C=N double bond

The reaction between an iminophosphorane with furan-2-carbaldehyde, thiophene-2-carbaldehyde, furan-3-carbaldehyde, and thiophene-3-carbaldehyde at 60 degrees C gives the corresponding trans imines in 53-84% yields, while the same reaction at 100 degrees C gives a mixture of the corresponding trans and cis imines. Whether the iminophosphorane reacted with 5-nitrofuran-2-carbaldehyde or 5-nitrothiophene-2-carbaldehyde only the trans imines were obtained in 85-89% yields. The irradiation of the imines obtained from thiophene-2-carbaldehyde and thiophene-3-carbaldehyde gave the corresponding photocyclization products. Cis/trans stereochemistry of the imines can be assigned simulating the UV-vis spectra. In the case of the imine from furan-2-carbaldehyde the computed spectra are characterized by an intense absorption at 361 and 357 nm respectively for the trans-1 and trans-2 structures. No other absorptions of comparable intensity have been predicted: the agreement with the experimental spectrum can be considered good. Furthermore, the experimental weak peaks at 280 and 270 nm can be associated to the computed transitions at 278 and 260 nm for the trans-1 isomer. Several minima of the energy surface can be assigned to the cis isomer, and they all present a very similar energy. The structures of the cis-1 and cis-2 isomers present quite coincident computed electronic spectra. In both cases, the computed spectrum shows two principal features. For the cis-1 structure, the first characteristic absorption is located at 414 nm and the second one at 284 nm. For the cis-2 structure, the first feature is located at 412 nm and the second one at 286 nm. The second transition is computed somewhat more intense. The experimental spectrum could be the consequence of similar populations of the planar cis structure (cis-3) and nonplanar cis structures (cis-1, cis-2, and their enantiomers).     

Isolation and structural analysis of the cyclic fatty acid monomers formed from eicosapentaenoic and docosahexaenoic acids during fish oil deodorization

Long-chain polyunsaturated fatty acids (LC-PUFAs) present in fish oils are thermolabile molecules. Among the degradation reactions encountered, thermal cyclization occurs during refining or other heat treatments. Numerous studies have been carried out in the past to quantify and determine the structures of cyclic fatty acid monomers (CFAMs) formed from oleic, linoleic and linolenic acids in heated vegetable oils. Recently, much attention have been given to LC-PUFAs due to their potential health benefits. However, data on quantification of CFAMs formed from these fatty acids, such as eicosapentaenoic acid (EPA, cis-5, cis-8, cis-11, cis-14, cis-17 20:5) and docosahexaenoic acid (DHA, cis-4, cis-7, cis-10, cis-13, cis-16, cis-19 22:6), the two main LC-PUFAs in fish oils, are scarce. In the present study, structural analyses of CFAMs formed from EPA and DHA during the deodorization of fish oil are presented. Fish oil sample was deodorized at 250 degrees C for 3 h under a pressure of 1.5 mbar in a laboratory deodorizer. The CFAMs formed during heat treatment of fish oil were isolated by a combination of saponification, esterification, urea fractionations and column chromatography. Structural analyses of C20- and C22-CFAMs were achieved by gas-chromatography electronic-ionization mass-spectrometry (GC-EI-MS) of their 4,4-dimethyloxazoline (DMOX) derivatives. We identified seven out of 13 possible structures of hydrogenated CFAMs formed from EPA, and nine out of 16 possible structures of CFAM formed from DHA. Major CFAMs from both EPA and DHA were cyclohexyl isomers. All possible cyclohexyl isomers were found but only nine out of 18 of the cyclopentyl isomers were present in concentration sufficient for identification. Chemical mechanisms involved in the formation of polyunsaturated LC-PUFAs have been investigated. The results have shown that general principle involved in the cyclization of LC-PUFAs is same as that for the thermal cyclization of oleic, linoleic and alpha-linolenic acids.     

Research on 1-aza two-ring systems

The physical properties and chromatographic behavior of homologs of pyrrolizidine were investigated with the aid of squalane, polyethylene glycol 20,000, and triethanolamine as liquid stationary phases. It was found that the properties of pairs of unstrained isomers are subject to the conformational rule. The retention times of the trans isomers of unstrained 1-methyl- and 3-alkylpyrrolizidines are shorter on all three stationary phases than the retention times of the cis isomers. The less significant (than in the case of other epimers) difference in the physical properties of cis- and trans-3,8-H-3-tert-butylpyrrolizidines and the peculiarity of their chromatographic behavior are explained by the distinct conformational heterogeneity of the ring fusion type of cis-3,8-H-3-tert-butylpyrrolizidine. With respect to all of the examined physical constants, strained cis-3,8-H-cis-5,8-H-3,5-dimethylpyrrolizidine and its epimer do not comply with the conformational rule. In the liquid phase at ambient temperature and at the boiling point and in solutions in squalane, polyethylene glycol 20,000, and triethanolamine at –100°C, cis-3,8-H-cis-5,8-H-3,5-dimethylpyrrolizidine contains significant amounts of the trans-fused form, which affect the entire set of properties. An empirical dependence of the properties of the pyrrolizidines on their properties can be used in the conformational assignment of analogous compounds.See [1] for communication 18.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 58–64, January, 1979.     

Synthesis and fungicidal activity study of novel daphneolone analogs with 2,6-dimethylmorpholine

A series of novel daphneolone analogs was designed and synthesized on the basis of natural product 1,5-diphenyl-2-penten-1-one(I) from Stellera chamaejasme L. as lead compound, whereby 2,6-dimethylmorpholine moiety was introduced to replace 1-phenyl group. Their structures were confirmed by IR,1H NMR, and HRMS(ESI) or elemental analysis,13 C NMR for some representative compounds. The two isomers of target compounds were separated and identified by NOESY technique and chemical method.All of the synthesized compounds have been evaluated for anti-plant pathogenic fungi activities. The results showed that some compounds exhibited moderate to good antifungal activities against tested fungi at the concentration of 50 mg/L. Among them, compound 7d, with a 4-bromine-substituted phenyl group and cis-2,6-dimethylmorpholine moiety, displayed best activity with an EC50 of 23.87 mmol/L against Valsa mali, superior to lead compound I. In addition, preliminary structure–activity relationship analysis indicated that, between two isomers of target compounds, the antifungal activities of the isomer with cis-2,6-dimethylmorpholine were better than the trans-isomer.     

Theoretical and experimental NMR study of protopine hydrochloride isomers

The 1H and 13C NMR chemical shifts of cis- and trans-protopinium salts were measured and calculated. The calculations of the chemical shifts consisted of conformational analysis, geometry optimization (RHF/6-31G** method) and shielding constants calculations (B3LYP/6-31G** method). Based on the results of the quantum chemical calculations, two sets of experimental chemical shifts were assigned to the particular isomers. According to the experimental results, the trans-isomer is more stable and its population is approximately 68%.     

Synthesis and structure of nongeminally substituted cyclic phosphazenes with haloalkyl and thioester functional groups

Nongeminally substituted cyclic phosphazenes with various haloalkyl substituents were prepared using deprotonation-substitution reactions at the methyl groups of the cis isomers of nongeminally substituted cis-[Me(Ph)P=N]3, 2. Treatment of 2 with n-BuLi followed by reaction with organic halogenated reagents (RX=C2Cl6, BrC(O)CMe2Br, and ICH2COOEt) at low temperature afforded the various cyclic derivatives cis-[(XCH2)(Ph)PN]3 (3, X=Cl, 4, Br, and 5, I). The mono- and dibromoalkyl derivatives, cis-[Ph3(BrCH2)Me2P3N3], 6, and [Ph3(BrCH2)2MeP3N3], 7, were also isolated along with 4 when the electrophile was dibromoethane. Reaction of cis-[Ph(BrCH2)PN]3, 4, with KSC(O)Me gave cis-[Ph(MeC(O)SCH2)PN]3, 8. The structures of all the cis cyclic phosphazenes were determined by NMR spectroscopy and X-ray diffraction. All retained the basketlike shape with the hydrophobic phenyl groups opposite the haloalkyl groups on the P3N3 ring. Thermal analysis of the new cyclic trimers indicates that ring-opening polymerization does not occur. The melting points and the thermal stabilities of haloalkyl cyclophosphazenes were higher than those of the parent compound 2.     

Bis(1 degree-amino)cyclodistib(III)azanes: the first structural characterization of cis and trans isomers of a single cyclodipnict(III)azane

The dichlorocyclodistib(III)azane [ClSb(mu-NtBu)]2 (1) has been shown to exist as the cis isomer in the solid state. A series of bis(1 degree-amino)cyclodistib(III)azanes [R'NHSb(mu-NtBu)]2 (2, R' = tBu; 3, R' = Dipp; 4, R' = Dmp) has been prepared by the reaction of 1 with 2 equiv. of LiNHR'. On the basis of NMR solution spectra, all three derivatives are formed as a mixture of cis and trans isomers. In the case of 3, the structures of both the cis and trans isomers have been determined by X-ray crystallography; cis-3 adopts an endo, endo arrangement for the amido protons of the DippNH groups. Isomerization of trans-3 into cis-3 occurs slowly in solution. Deprotonation of 2 with 2 equiv. of nBuNa or trans-3 with nBuLi produces [Na2Sb2(mu-NtBu)4] (5) and [Li2Sb2(mu-NtBu)2(mu-NDipp)2] (6), whose solvated cubane structures were established by X-ray crystallography. In contrast, the reaction of cis-3 with 2 equiv. of nBuLi produces the tricyclic compound [Li2Sb(mu-NtBu)2(mu-NDipp)(mu-NHDipp)] (7).     

Synthesis,structure, and spectroscopic,photochemical, redox,and catalytic properties of ruthenium(II) isomeric complexes containing dimethyl sulfoxide,chloro, and the dinucleating bis(2-pyridyl)pyrazole ligands

Two isomeric Ru(II) complexes containing the dinucleating Hbpp (3,5-bis(2-pyridyl)pyrazole) ligand together with Cl and dmso ligands have been prepared and their structural, spectroscopic, electrochemical, photochemical, and catalytic properties studied. The crystal structures of trans,cis-[Ru(II)Cl(2)(Hbpp)(dmso)(2)], 2a, and cis(out),cis-[Ru(II)Cl(2)(Hbpp)(dmso)(2)], 2b, have been solved by means of single-crystal X-ray diffraction analysis showing a distorted octahedral geometry for the metal center where the dmso ligands coordinate through their S atom. 1D and 2D NMR spectroscopy corroborates a similar structure in solution for both isomers. Exposure of either 2a or 2b in acetonitrile solution under UV light produces a substitution of one dmso ligand by a solvent molecule generating the same product namely, cis(out)-[Ru(II)Cl(2)(Hbpp)(MeCN)(dmso)], 4. While the 1 e(-) oxidation of 2b or cis(out),cis-[Ru(II)Cl(2)(bpp)(dmso)(2)](+), 3b, generates a stable product, the same process for 2a or trans,cis-[Ru(II)Cl(2)(bpp)(dmso)(2)](+), 3a, produces the interesting linkage isomerization phenomenon where the dmso ligand switches its bond from Ru-S to Ru-O (K(III)(O)(-->)(S) = 0.25 +/- 0.025, k(III)(O)(-->)(S) = 0.017 s(-1), and k(III)(S)(-->)(O) = 0.065 s(-1); K(II)(O)(-->)(S) = 6.45 x 10(9), k(II)(O)(-->)(S) = 0.132 s(-1), k(II)(S)(-->)(O) = 2.1 x 10(-11) s(-1)). Finally complex 3a presents a relatively high activity as hydrogen transfer catalyst, with regard to its ability to transform acetophenone into 2-phenylethyl alcohol using 2-propanol as the source of hydrogen atoms.     

Asymmetric allyl-metal bonding in substituted (eta3-allyl) palladium complexes: X-ray structures of cis- and trans-4-acetoxy-

Enantiomerically pure cis and trans isomers of 4-acetoxy-[eta3(1,2,3)-cyclohexenyl]palladium chloride dimers (cis-1 and trans-1) were prepared from enantiomerically pure trans-1-acetoxy-4-chloro-2-cyclohexene. X-ray analyses of these complexes show that in the trans complex (trans-1) the six-membered ring prefers a chair conformation, whereas in the cis complex (cis-1) the cyclohexenyl ring has a boat conformation. According to the X-ray structure of trans-1 the Pd-C3 bond is shorter than the other allylic terminal palladium-carbon bond (Pd-C1). On the other hand, in cis-1 the Pd-C3 and Pd-C1 bond lengths are identical within the experimental error. The calculated structures (B3PW91/LANL2DZ + P) of trans-1 and cis-1 also display differences in the allylpalladium bonding. The asymmetric allylpalladium bonding in trans-1 is explained on the basis of pi-sigma electronic interactions between the 4-acetoxy substituent and the allyl-metal moiety.     

Separation of positional isomers of oxidation catalyst precursors

A series of polypyridyl ruthenium complexes of the general formula [Ru(tpy)(bpy')Cl]+ where tpy is 2,2':6',2"-terpyridine and bpy' is 4-carboxy-4'-methyl-2,2'-bipyridine (4-CO2H-4'-Mebpy), a proline derviative (4-CO-Pra-(Boc)(OMe)-4'-Mebpy), or 4-((diethoxyphosphinyl)methyl)-4'-methyl-2,2'-bipyridine (4-CH2PO3Et2-4'-Mebpy) are prepared. For each complex, two isomers exist, and these are separated chromatographically. The structure of the hexafluorophosphate salt of cis-[Ru(tpy)(4-CO2H-4'-Mebpy)Cl]+, cis-1, is determined by X-ray crystallography. The salt crystallizes in the monoclinic space group Cc with a = 12.4778(6) A, b = 12.6086(6) A, c = 20.1215(9) A, beta = 107.08200(1) degrees, Z = 4, R = 0.058, and Rw = 0.072. The structures of the remaining complexes are assigned by 1H NMR comparisons with cis-1. The complexes are potentially important precursors for the incorporation of RuIV=O2+ oxidants into polymers or peptides or for their adsorption onto oxide surfaces. Preliminary electrochemical results for the isomers of [Ru(tpy)(4-CH2PO3H2-4'-Mebpy)(H2O)]2+, 4, adsorbed on ITO (In2O3:Sn) surfaces add support to a recently proposed electron-transfer mechanism involving cross-surface proton-coupled electron transfer.     

Simultaneous determination of cis- and trans-resveratrol in wines by capillary zone electrophoresis

A capillary zone electrophoresis method for determining cis- and trans-resveratrol isomers is proposed. Optimal conditions for the quantitative separation were investigated. A background electrolyte solution consisting of 40 mM borate buffer adjusted to pH 9.5, hydrodynamic injection and 5 kV of separation voltage were used. Good linearity and precision were obtained for the two isomers. Detection limits of 0.06 mg L-1 for trans-resveratrol and 0.08 mg L-1 for cis-reveratrol were obtained. The developed method is rapid and sensitive and it has been applied to determine cis- and trans-resveratrol in several red wines. The samples were purified and enriched by passing them through a preconditioned C18 cartridge and eluting the isomers with acetonitrile-water (3 + 7).     

Diosmium(III) compounds supported by 2-anilinopyridinate and novel alkynyl derivatives

The reaction between Os(2)(OAc)(4)Cl(2) and Hap (Hap is 2-anilinopyridine) under prolonged refluxing conditions resulted in an Os(III)(2) compound, Os(2)(ap)(4)Cl(2) (1), that can be crystallized as either the cis-(2,2) isomer from a CH(3)OH-CH(2)Cl(2) solution or the (3,1) isomer from a hexanes-CH(2)Cl(2) solution. Compound 1 undergoes facile reactions with LiC(2)Y to yield a series of Os(2)(ap)(4)(C(2)Y)(2) compounds with Y as Ph (2), ferrocenyl (3), SiMe(3) (4), and C(2)SiMe(3) (5). X-ray diffraction study of compound 2 reveals solvent-dependent isomerism similar to that of the parent compound 1. Compound 1 has Os-Os distances of 2.3937(8) and 2.3913(8) Angstroms for the cis-(2,2) and (3,1) isomers, respectively, and is paramagnetic (S = 1). Both the ethynyl derivatives 2-4 and butadiynyl derivative 5 are diamagnetic and have Os-Os distances of 2.456(1), 2.471(1), and 2.481(1) Angstroms for the cis-(2,2) and (3,1) isomers of 2 and (3,1) isomer of 4, respectively. Compounds 1-5 exhibit multiple one-electron redox couples in their cyclic voltammograms, including a reversible Os(2)(8+/7+) couple for 2. Resonance Raman spectra of both compounds 1 and 2 are reported.     

Resolution of racemic gossypol

The resolution of gossypol for the purpose of comparing the biologic action of (-)-gossypol with its naturally occurring antipode has attracted the attention of scientists. Recently the 1st successful resolution accomplished by chromatographic separation of the hexa-acetates of the condensation products of (+)-gossypol with (S)-1-methylphenethylamine followed by acid hydrolysis of the 2 main hexa-acetate, to give (+)- and (-)-gossypol with identical mp and alpha D values, was reported. Treatment of racemic gossypol with either (R)- or (S)-methylphenethylamine afforded yellow solids with mp 197-200 degrees C and m/z 753 (M + 1)+. By quick chromatography of the condensation products of either the (S) or (R) amine with (+)-gossypol on a silica column with diethyl ether-light petroleum as eluant, pure compounds were obtained. The compounds were assigned as (S)-amino-(+)-gossypol and (R)-amino-(-)-gossypol. These gossypols contained less than 5% of their antipodes. Although different in their antifertility action, the (+ or -), (+), and (-) isomers of gossypol demonstrate the same degree of inhibitory activity against the enzyme lactate dehydrogenase-X. Comparison of the antifertility action of the 3 isomers in male rats revealed that (-)-gossypol in half the dose of the (+ or -) isomer showed comparable activity, while the (+)-gossypol proved to be inactive.     

Determination of the absolute configuration of flexible molecules by ab initio ORD calculations: a case study with cytoxazones and isocytoxazones

Ab initio calculations of the optical rotatory power of the natural cytokine modulator cytoxazone 1 and its trans-diastereomer 2, as well as the structural isomers cis-3 and trans-4 isocytoxazones, have been performed at four different wavelengths (589, 546, 435, and 405 nm) by Density Functional Theory. The calculation of ORD curves provides a reliable method for the assignment of absolute configuration of these conformationally flexible molecules. The absolute configurations of isocytoxazones has been established as (+)-(4R,5S)-cis-3 and (+)-(4S,5S)-trans-4.     

Room-temperature photochromism in cis- and trans-[Ru(bpy)2(dmso)2]2+

We report on phototriggered Ru-S --> Ru-O and thermal Ru-O --> Ru-S intramolecular linkage isomerizations in cis- and trans-[Ru(bpy)2(dmso)2]2+. The cis complex features only S-bonded sulfoxides (cis-[S,S]), whereas the trans isomer is characterized by S- and O-bonded dmso ligands. Both cis-[S,S] and trans-[S,O] exhibit photochromism at room temperature in dmso solution and ionic liquid (IL). Rates of reaction in IL were monitored by UV-visible spectroscopy and are similar to those reported in dmso solution (k(O-->S) ranges from approximately 10(-3) to 10(-4) s(-1)). Cyclic voltammetric measurements of cis-[S,S] and trans-[S,O] are consistent with an electrochemically triggered linkage isomerism mechanism. While both cis-[S,S] and trans-[S,O] are photochromic at room temperature, neither complex is emissive. However, upon cooling to 77 K, cis-[S,S] exhibits LMCT (ligand-to-metal charge transfer) emission typical of many ruthenium polypyridine complexes. In contrast to cis-[S,S], trans-[S,O] does not show any detectable emission even at 77 K.     

Synthesis and Aza-[2,3]-Wittig Rearrangements of Vinylaziridines: Scope and Limitations

cis- and trans-2,3-Trisubstituted vinylaziridines have been prepared from cis- and trans-epoxy alcohols, respectively, and used as substrates in the aza-[2,3]-Wittig rearrangement. Five different anion-stabilizing groups have been investigated for their efficiency to promote the rearrangement, and it was found that N-tert-butyl acetyl vinylaziridines were superior in this reaction, affording the corresponding cis-2,6-tetrahydropyridines (>90%) as single isomers when treated with LDA. Similarly, the corresponding (Z)-propenylaziridines gave trans,trans-2,3,6-trisubstituted tetrahydropyridines as the sole products while the (E)-propenylaziridines afforded the cis,cis-2,3,6-derivatives with equally high selectivity. The scope and limitations of the process have been investigated by varying the structure of the substrate, and the mechanism of the rearrangement has been probed to some extent; the mechanistic picture is more complex than assumed previously.