Synthesis,characterization and antifungal testing of 3,4-dihydropyrimidin-2(1H)-(thio)ones containing boronic acids and boronate esters

The addition of formylphenylboronic acid derivatives to thiourea and ethyl acetoacetate proceeds in the presence of an additional Lewis acid catalyst to give the corresponding 3,4-dihydropyrimidin-2(1H)-(thio)ones (Biginelli products) in moderate yield. Compounds were tested for antifungal activity against pure cultures of Aspergillus niger, Aspergillus flavus, Candida albicans and Saccharomyces cerevisiae but, unfortunately, none showed any appreciable activity.      

Synthesis and Complexation Study of New Aminoalkynyl−amidinate Ligands

The current library of amidinate ligands has been extended by the synthesis of two novel dimethylamino-substituted alkynylamidinate anions of the composition [Me₂N−CH₂−C≡C−C(NR)₂]⁻ (R = ⁱPr, cyclohexyl (Cy)). The unsolvated lithium derivatives Li[Me₂N−CH₂−C≡C−C(NR)₂] ( 1 : R = ⁱPr, 2 : R = Cy) were obtained in good yields by treatment of in situ-prepared Me₂N−CH₂−C≡C−Li with the respective carbodiimides, R−N=C=N−R. Recrystallization of 1 and 2 from THF afforded the crystalline THF adducts Li[Me₂N−CH₂−C≡C−C(NR)₂] ⋅ nTHF ( 1 a : R = ⁱPr, n=1; 2 a : R = Cy, n=1.5). Precursor 2 was subsequently used to study initial complexation reactions with selected di- and trivalent transition metals. The dark red homoleptic vanadium(III) tris(alkynylamidinate) complex V[Me₂N−CH₂−C≡C−C(NCy)₂]₃ ( 3 ) was prepared by reaction of VCl₃(THF)₃ with 3 equiv. of 2 (75 % yield). A salt-metathesis reaction of 2 with anhydrous FeCl₂ in a molar ratio of 2 : 1 afforded the dinuclear homoleptic iron(II) alkynylamidinate complex Fe₂[Me₂N−CH₂−C≡C−C(NCy)₂]₄ ( 4 ) in 69 % isolated yield. Similarly, treatment of Mo₂(OAc)₄ with 3 or 4 equiv. of 2 provided the dinuclear, heteroleptic molybdenum(II) amidinate complex Mo₂(OAc)[Me₂N−CH₂−C≡C−C(NCy)₂]₃ ( 5 ; yellow crystals, 50 % isolated yield). The cyclohexyl-substituted title compounds 2 a , 4 , and 5 were structurally characterized through single-crystal X-ray diffraction studies.     

Energetics and mechanism of ammonia synthesis through the Chatt Cycle: conditions for a catalytic mode and comparison with the Schrock Cycle

Through a series of DFT calculations the energy profile of the Chatt cycle is evaluated. This is the counterpiece of our earlier investigations of the Schrock cycle (Angew. Chem. 2005, 117, 5783; Angew. Chem. Int. Ed. 2005, 44, 5639), applying the same quantumchemical methodology and approximations. As for the Schrock cycle, decamethylchromocene acts as reductant. The protonation reactions are considered to be mediated by HBF4/diethyl ether or lutidinium. For all protonation and reduction steps the corresponding free reaction enthalpy changes are calculated. The derived energy profile and corresponding reaction mechanism bear strong similarities to the Schrock cycle. In particular, the most endergonic reaction is the first protonation of the N2 complex and the most exergonic reaction is the cleavage of the N--N bond. If lutidinium is employed as acid and Cp2*Cr as reductant, the reaction course involves steps that are not thermally allowed. For HBF4/diethyl ether as the acid and Cp2*Cr as reducant, however, a catalytic cycle consisting of thermally allowed reactions is principally feasible. This cycle involves a Mo I-fluoro complex as dinitrogen intermediate. It is shown that regeneration to the Mo 0-bis(dinitrogen) complex is thermally not accessible in this system. Moreover, the Mo I fluoro-dinitrogen complex is labile towards disproportionation. The implications of these results with respect to the realization of a catalytic system on the basis of Mo and W phosphine complexes are discussed.     

Origin of diastereocontrol in the oxy-Michael reactions of delta-lactol anions: a computational and experimental study

The diastereoselectivity in the alkylation and Michael addition of "naked" 6-substituted delta-lactolates has been studied by density functional (B3LYP) calculations with ab initio (MP2) energy refinements. The resulting proposed model for the origins of stereocontrol in this reaction has been tested by experiment. The reactions lead to a high cis diastereoselectivity across the THP ring due to the preference for both the alkoxide and the 6-substituent to sit equatorial in the alkylation transition structure. In the oxy-Michael addition of these lactolates to beta-substituted nitroolefins, we propose that the high diastereoselectivity beta- to the nitro group is a result of a combination of steric, stereoelectronic and solvation factors.     

Critical dynamics of ballistic and Brownian particles in a heterogeneous environment

The dynamic properties of a classical tracer particle in a random, disordered medium are investigated close to the localization transition. For Lorentz models obeying Newtonian and diffusive motion at the microscale, we have performed large-scale computer simulations, demonstrating that universality holds at long times in the immediate vicinity of the transition. The scaling function describing the crossover from anomalous transport to diffusive motion is found to vary extremely slowly and spans at least five decades in time. To extract the scaling function, one has to allow for the leading universal corrections to scaling. Our findings suggest that apparent power laws with varying exponents generically occur and dominate experimentally accessible time windows as soon as the heterogeneities cover a decade in length scale. We extract the divergent length scales, quantify the spatial heterogeneities in terms of the non-Gaussian parameter, and corroborate our results by a thorough finite-size analysis.     

The [12]annulene global minimum

A new global minimum for [12]annulene has been computationally located. This mono-trans minimum 5 (CCCCCT) is computed to be 1.5 kcal/mol more stable (CCSD(T)/cc-pVDZ//BHHLYP/6-311+G**) than the known tri-trans isomer 1 (CTCTCT) and 2.4 kcal/mol lower than the di-trans isomer 4 (CCTCCT), for which there is indirect evidence. The barriers for several rearrangements of 5 were all computed to be above 15 kcal/mol, indicating that direct experimental characterization of 5 should be possible. The computed barriers for the dynamic processes (including conformational automerization) coupled with computed 1H NMR shift values should aid in the future characterization of this [12]annulene isomer.     

Azo supramolecules on Au(111) with controlled size and shape

Using a subtle balance between weak intermolecular C-H...N hydrogen bonds and molecule-surface interactions, supramolecules of azobenzene-related molecules on Au surfaces were prepared. For analysis, modeling based on first-principles calculations was performed.     

Ligand localized triplet excited states in platinum(II) bipyridyl and terpyridyl peryleneacetylides

An investigation of the photophysics of two complexes, [Pt((t)Bu3tpy)(C triple bond C-perylene)]BF4 (1) and Pt((t)Bu2bpy)(C triple bond C-perylene)2 (2), where (t)Bu3tpy is 4,4',4'-tri( tert-butyl)-2,2':6',2'-terpyridine, (t)Bu2bpy is 4,4'-di( tert-butyl)-2,2'-bipyridine, and C triple bond C-perylene is 3-ethynylperylene, reveals that they both exhibit perylene-centered ligand localized excited triplet states ((3)IL) upon excitation with visible light. These complexes do not display any significant photoluminescence at room temperature but readily sensitize (1)O2 in aerated CH2Cl2 solutions, as evidenced by its characteristic emission near 1270 nm. The transient absorption difference spectra were compared to bi- and tridentate phosphine peryleneacetylides intended to model the (3)IL peryleneacetylide excited states in addition to the related phenylacetylide-bearing polyimine analogues, with the latter model being the respective triplet charge-transfer ((3)CT) excited states. The transient difference spectra of the two title compounds display excited-state absorptions largely attributable to perylene localized (3)IL states yet exhibit somewhat attenuated excited-state lifetimes relative to those of the phosphine model chromophores. The abbreviated lifetimes in 1 and 2 may suggest the involvement of the energetically proximate (3)CT triplet state exerting an influence on excited-state decay, and the effect appears to be stronger in 1 relative to 2, consistent with the energies of their respective (3)CT states.     

Tetradehydroadamantane-1,3,5,7-di- and tetracations and their helium and hydride inclusion complexes: spherical aromaticity and evidence for a bonding interaction between carbon and helium

The unusual stability of the experimentally known 1,3-dehydro-5,7-adamantyl dication was previously explained by four-center two-electron aromaticity with three-dimensional (tetrahedral) topology. Magnetic criteria (ACID and ring-current analysis) now demonstrate that there is also a very strong contribution from hyperconjugation with all six methylene bridges. The delocalized system of electrons thus includes all valence electrons, and the structure, therefore, should rather be described as a spherically aromatic 50-electron system. The corresponding Td-symmetric tetracation with 48 electrons is antiaromatic and not a minimum structure. With a He atom or a hydride ion at the center of the cage, the tetracation is predicted to form a kinetically stable complex. Magnetic criteria demonstrate that the antiaromaticity is greatly reduced, and a bond analysis hints at bonding interactions between He (and H-) and the carbon atoms of the adamantane cage.     

Photochemical upconversion approach to broad-band visible light generation

The sensitized triplet-triplet annihilation (TTA) of 9,10-dimethylanthracene (DMA) upon selective excitation of [Ru(dmb)3]2+ (dmb = 4,4'-dimethyl-2,2'-bipyridine) at 514.5 nm in dimethylformamide (DMF) resulted in upconverted and downconverted DMA excimer photoluminescence. The triplet excited state of [Ru(dmb)3]2+ is efficiently quenched by 11 mM DMA in DMF resulting in photon upconversion but no excimer formation. The bimolecular quenching constant of the dynamic quenching process is 1.4 x 109 M-1 s-1. At 90 mM DMA, both upconversion and downconversion processes are readily observed in aerated DMF solutions. The TTA process was confirmed by the quadratic dependence of the upconverted and downconverted emission emanating from the entire integrated photoluminescence profile (400-800 nm) of DMA measured with respect to incident light power. Time-resolved emission spectra of [Ru(dmb)3]2+ and 90 mM DMA in both aerated and deaerated DMF clearly illustrates the time-delayed nature of both types of singlet-state emission, which interestingly shows similar decay kinetics on the order of 14 mus. The emission quantum yields (Phi) measured using relative actinometry increased with increasing DMA concentrations, reaching a plateau at 3.0 mM DMA (Phi = 4.0%), while at 90 mM DMA, the overall quantum yield diminished to 0.5%. The dominant process occurring at 3.0 mM DMA is upconversion from the singlet excited state of DMA, whereas at 90 mM DMA, both upconversion and excimeric emission are observed in almost equal portions, thereby resulting in an overall broad-band visible light-emission profile.