The “super-basic” butyllithium/potassium tert-butoxide mixture and other lickor-reagents

The reactivity pattern of the butyllithium/potassium tert-butoxide mixture differs significantly from that of butylpotassium and thus disproves the claimed identity of these two reagents.     

2,3,5,6-tetramethylmorpholine. V. Mechanism study of cyclization of 3,3′-iminobis-2-butanols

The mechanism of cyclization of 3,3′-iminobis-2-butanols to 2,3,5,6-tetramethylmorpholines in sulfuric acid is studied. Contrary to our prior suggestions, the ring closure seems to be exclusively a normal S-type substitution. The partial inversion before the cyclization is discussed.     

Synthesis and 270 MHz Nmr structural studies of the isomers of 2,3,5-trimethyl- and 2,3,6-trimethylmorpholine

The four isomers of 2,3,5-trimethylmorpholine and those of 2,3,6-trimethylmorpholine have been synthesized from appropriately substituted ethanolamines and separated by means of preparative gas-liquid chromatography. The configuration of each isomer was determined from 270 MHz FT-nmr studies of the magnitudes of the methine-methine and methine-methylene proton coupling constants. The evaluated chemical shifts of axial methyl groups are discussed.     

Lower bounds for the economic lot scheduling problem using aggregation

The problem considered in this paper is to choose cyclical production patterns for several products which are produced on a common production facility. By aggregating data, dynamic programming can be used to obtain lower bounds which, in general, are much stronger than those obtained when the capacity constraint is not recognized.Lower bounds are of interest when generating possible solutions and also in connection with branch-and-bound methods.     

Ni0]-catalyzed Co-oligomerization of 1,3-butadiene and ethylene: a theoretical mechanistic investigation of competing routes for generation of linear and cyclic C10-olefins

A detailed theoretical investigation of the mechanism for the [Ni(0)]-catalyzed co-oligomerization of 1,3-butadiene and ethylene to afford linear and cyclic C(10)-olefins is presented. Crucial elementary processes have been carefully explored for a tentative catalytic cycle, employing a gradient-corrected density functional theory (DFT) method. The favorable route for oxidative coupling starts from the prevalent [Ni(0)(eta(2)-butadiene)(2)(ethylene)] form of the active catalyst through oxidative coupling between the two eta(2)-butadienes. The initial eta(3),eta(1)(C(1))-octadienediyl-Ni(II) product is the active precursor for ethylene insertion, which preferably takes place into the syn-eta(3)-allyl-Ni(II) bond of the prevalent eta(3)-syn,eta(1)(C(1)),Delta-cis isomer. The insertion is driven by a strong thermodynamic force, giving rise entirely to eta(3),eta(1),Delta-trans-decatrienyl-Ni(II) forms, with the eta(3)-anti,eta(1),Delta-trans isomer almost exclusively generated. Occurrence of allyl,eta(1),Delta-cis isomers, however, is precluded on both kinetic and thermodynamic grounds, thereby rationalizing the observation that cis-DT and cis,cis-CDD are never formed. Linear and cyclic C(10)-olefins are generated in a highly stereoselective fashion, with trans-DT and cis,trans-CDD as the only isomers, along competing routes of stepwise transition-metal-assisted H-transfer (DT) and reductive CC elimination under ring closure (CDD), respectively, that start from the prevalent eta(3)-anti,eta(1),Delta-trans-decatrienyl-Ni(II) species. The role of allylic conversion in the octadienediyl-Ni(II) and decatrienyl-Ni(II) complexes has been analyzed. As a result of the detailed exploration of all important elementary steps, a theoretically verified, refined catalytic cycle is proposed and the regulation of the selectivity for formation of linear and cyclic C(10)-olefins is elucidated.     

The structure of alpha-Zn4Sb3: ordering of the phonon-glass thermoelectric material beta-Zn4Sb3

beta-Zn4Sb3 is an outstanding thermoelectric material mainly due to its extraordinarily low thermal conductivity, which is similar to that of glasses. Recently it was proposed that interstitial Zn atoms are responsible for this peculiar behavior. Here we report on the crystal and electronic stucture of the low-temperature polymorph alpha-Zn4Sb3. During the reversible phase transition the intricate disorder in beta-Zn4Sb3 disappears, and all Zn atoms localize completely. The electronic structure of alpha-Zn4Sb3 corresponds to that of a narrow-gap semiconductor.     

Use of electrospray ionization mass spectrometry for the investigation of radical cation chain reactions in solution: detection of transient radical cations

Electrospray ionization mass spectrometry (ESI-MS) coupled to a microreactor is an excellent tool for the investigation of reactions in solution. Here, we report the first results of our investigations into preparatively interesting electron-transfer-initiated chain reactions in solution which proceed via radical cations as reactive intermediates. The tris(p-bromophenyl)aminium hexachloroantimonate (1)-mediated [2+2] cycloaddition of trans-anethole (2) to give 1,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane (3) was investigated. The reaction proceeds as a radical cation chain reaction via transient intermediates 2 ⁺ and 3 ⁺ that could be detected and characterized unambiguously directly in the reacting solution by ESI-MS/MS. The identity of the intermediates was confirmed by comparison with authentic MS/MS spectra of 2 ⁺ and 3 ⁺ obtained by atmospheric pressure chemical ionization mass spectrometry (APCI-MS). In addition, substrate and product can be monitored easily in the reacting solution by APCI-MS.Part of this work was presented at the 36th Conference of the German Society for Mass Spectrometry (DGMS).