A combined theoretical and experimental study of efficient and fast titanocene-catalyzed 3-exo cyclizations

The mechanism of titanocene mediated 3-exo cyclizations was investigated by a combined theoretical and experimental study. A gradient corrected density functional theory (DFT) method has been scaled against titanocene dichloride, the parent butenyl radical, and in bond dissociation energy (BDE) calculations. The BP86 method using density fitting, and a basis set of triple-zeta quality emerged as a highly reliable tool for studying titanocene mediated radical reactions. The computational results revealed important kinetic and thermodynamic features of cyclopropane formation. Surprisingly, the beta-titanoxy radicals, the first intermediates of our investigations, were demonstrated to possess essentially the same thermodynamic stabilization as the corresponding alkyl radicals by comparison of the calculated BDEs. In contrast to suggestions for samarium mediated reactions, the cyclization was shown to be thermodynamically favorable in agreement with earlier kinetic studies. It was established that stereoselectivity of the cyclization is governed by the stability of the intermediates and thus the trans disubstituted products are formed preferentially. The observed ratios of products are in good to excellent agreement with the DFT results. By a combination of computational and experimental results, it was also shown that for the completion of the overall cyclopropane formation the efficiency of the trapping of the cyclopropylcarbinyl radicals is decisive.     

Syntheses and solid state structures of cyclic diynes with two chalcogen centres--a competition between weak interactions

In this paper we report the synthesis of cyclic diynes of the general formula mX2n. The letters m and n indicate the lengths of chains between two C triple bond C-X units where X indicates S, Se, or Te. The lengths of the bridges vary between m=4-8 and n=2-6. Structural investigations by means of X-ray diffraction reveal for most systems a chair-like conformation in the solid state. For 5S(2)2, 5S(2)3, 7S(2)5, 5Se(2)2 and 5Te(2)3 tubular structures were encountered. These structures can be traced back to weak XX or weak C-Hpi interactions.     

Über die dilatometrisch indizierten Titrationen

Dilatometrical indication can be employed to the neutralizing reactions of polybasic acids and is demonstrated by the following acid-base couples: H₂SO₄/NaOH, H₃PO₄/NaOH and H₅JO₆/NaOH. With amounts between 0.5 and 4 mMol several, frequently all, equivalence points can be determined with a deviation of less than 0.2%. The determination of the changes of molar volume of the reactions depending on the concentrations allow conclusions as to the course of the reactions.     

Mechanistic studies of UV assisted [4 + 2] cycloadditions in synthetic efforts toward vibsanin E

Quantum chemical DFT calculations at the B3LYP/6-31G(d) level have been used to study the stereochemical course of the photochemical cycloaddition of enone 9 with dienes. The observed products of this photochemically induced cycloaddition showed a stereoselectivity, which is opposite to what would be expected by FMO considerations. The quantum chemical calculations revealed that the unusual stereoselectivity of the reaction can be rationalized by assuming a stereospecific photochemical cis-trans isomerization of enone 9 to trans isomer 9a followed by a thermal Diels-Alder reaction of the diene onto the highly reactive trans enone. The photochemical reaction step involves the selective formation of a twisted triplet intermediate, which accounts for the selectivity of the reaction.     

Asymmetric Synthesis of Densely Functionalized Medium‐Ring Carbocycles and Lactones through Modular Assembly and Ring‐Closing Metathesis of Sulfoximine‐Substituted Trienes and Dienynes

An asymmetric synthesis of densely functionalized 7–11‐membered carbocycles and 9–11‐membered lactones has been developed. Its key steps are a modular assembly of sulfoximine‐substituted C‐ and O‐tethered trienes and C‐tethered dienynes and their Ru‐catalyzed ring‐closing diene and enyne metathesis (RCDEM and RCEYM). The synthesis of the C‐tethered trienes and dienynes includes the following steps: 1) hydroxyalkylation of enantiomerically pure titanated allylic sulfoximines with unsaturated aldehydes, 2) α‐lithiation of alkenylsulfoximines, 3) alkylation, hydroxy‐alkylation, formylation, and acylation of α‐lithioalkenylsulfoximines, and 4) addition of Grignard reagents to α‐formyl(acyl)alkenylsulfoximines. The sulfoximine group provided for high asymmetric induction in steps 1) and 4). RCDEM of the sulfoximine‐substituted trienes with the second‐generation Ru catalyst stereoselectively afforded the corresponding functionalized 7–11‐membered carbocyles. RCDEM of diastereomeric silyloxy‐substituted 1,6,12‐trienes revealed an interesting difference in reactivity. While the (R)‐diastereomer gave the 11‐membered carbocyle, the (S)‐diastereomer delivered in a cascade of cross metathesis and RCDEM 22‐membered macrocycles. RCDEM of cyclic trienes furnished bicyclic carbocycles with a bicyclo[7.4.0]tridecane and bicyclo[9.4.0]pentadecane skeleton. Selective transformations of the sulfoximine‐ and bissilyloxy‐substituted carbocycles were performed including deprotection, cross‐coupling reaction and reduction of the sulfoximine moiety. Esterification of a sulfoximine‐substituted homoallylic alcohol with unsaturated carboxylic acids gave the O‐tethered trienes, RCDEM of which yielded the sulfoximine‐substituted 9–11‐membered lactones. RCEYM of a sulfoximine‐substituted 1,7‐dien‐10‐yne showed an unprecedented dichotomy in ring formation depending on the Ru catalyst. While the second‐generation Ru catalyst gave the 9‐membered exo 1,3‐dienyl carbocycle, the first‐generation Ru catalyst furnished a truncated 9‐membered 1,3‐dieny carbocycle having one CH₂ unit less than the dienyne.     

Measuring nitrate concentration in wastewaters with high chloride content

The presence of chloride ions in wastewaters in concentrations above 50 mg/L poses interference in several methods used for the measurement of nitrate-nitrogen. The aim of this study was to characterise the reliability and costs of some commonly available methods for the analysis of nitrate concentration in landfill leachate which contains high chloride levels. To investigate the effect of chloride interference, several widely used methods [ion chromatography (IC), continuous flow analysis (CFA), the German standard method (DIN), cuvette test (CUV), standard addition method (SAM) and reflectometric test (REF)] were used to measure the nitrate concentration in synthetic solutions containing varying concentrations of chloride and nitrate-nitrogen. Nitrate recoveries of the various methods were found to decrease in the following rank order: CUV (>95%) > IC (>90%) > CFA (89%) > DIN (88%) > REF (70%) > SAM (<80%). In the second part of the study, the same methods were used to measure nitrate concentrations in samples of biologically nitrified landfill leachate with and without chloride elimination. For leachate samples without chloride elimination, CUV results were well correlated (linear regression) with IC results (slope = 1.02/R² = 0.99) but to lesser extents with results obtained by CFA (0.91/0.86), DIN (0.89/0.97) and REF (0.86/0.77), and not correlated with SAM (0.74/–1.3). The incurred measurement costs per sample (in Euros) for the methods were as follows: CFA (<0.1) < DIN (0.6) < REF (0.7) < SAM (3) < CUV (3.8) < IC (15). Cuvette tests are recommended as the method of choice due to their accuracy and lower cost than IC.