Regiochemistry of S-alkyl-substituted alkynes in Pauson-Khand reactions. Is a correlation with X-ray data and charge distribution calculations of the Co(2)(CO)(6)-alkyne complexes possible?

[reaction: see text] Pauson-Khand reactions (PKR) of RSCtbd1;CR' (6-10) yielded in all cases as the main product the regioisomer with the alkyl-S group disposed alpha to the CO group (16a-22a). Correlation of these results with X-ray data and charge distribution calculations of the corresponding dicobalthexacarbonyl complexes proves that a recently postulated "trans effect" in these complexes is not suitable for predicting the regiochemical outcome of the PKR unambiguously.     

Schlüsselreaktionen einer synthese des sibirosamins

[2,3]-Sigmatropic rearrangement and vic. $\text{cis}$-oxyamination of 3-$\text{C}$-methyl-ald-m-enopyranosides are the key steps for the synthesis of a sibirosamine precursor.     

Buchbesprechungen

Ohne Zusammenfassung     

Characterization and control of phase fluctuations in elongated Bose–Einstein condensates

Quasi-one-dimensional Bose–Einstein condensates (BECs) in elongated traps exhibit significant phase fluctuations even at very low temperatures. We present recent experimental results on the dynamic transformation of phase fluctuations into density modulations during time of flight and show the excellent quantitative agreement with the theoretical prediction. In addition we confirm that, under our experimental conditions, in the magnetic trap density modulations are strongly suppressed even when the phase fluctuates. We also discuss our theoretical results on control of the condensate phase by employing a time-dependent perturbation. Our results set important limitations on future applications of BECs in precision atom interferometry and atom optics, but at the same time suggest pathways to overcome these limitations. Received: 17 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +49-511/762-3023, E-mail: Helge.Kreutzmann@ITP.uni-hannover.de     

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.     

The Structuring Causes of Behavior: Has Dretske Saved Mental Causation?

Fred Dretske’s account of mental causation, developed in Explaining Behavior and defended in numerous articles, is generally regarded as one of the most interesting and most ambitious approaches in the field. According to Dretske, meaning facts, construed historically as facts about the indicator functions of internal states, are the structuring causes of behavior. In this article, we argue that Dretske’s view is untenable: On closer examination, the real structuring causes of behavior turn out to be markedly different from Dretske’s meaning facts. Our argument proceeds in three steps. First, we set forth the problem of meaning individuation: We argue that the proposal that meaning facts are structuring causes of behavior commits Dretske to a very fine-grained individuation of meanings that is deeply counterintuitive. In a second step, we show that even these finely individuated meaning facts cannot do the job that they are supposed to do, since information facts—which are constitutive of, but distinct from Dretske’s meaning facts—are better candidates for the role of structuring causes. Finally, we argue that it is not even information facts, but facts of co-instantiation which are the real structuring causes of behavior. In concluding, we briefly consider the options that are left for Dretske if our arguments succeed.     

Mechanism of multi‐metal(loid) methylation and hydride generation by methylcobalamin and cob(I)alamin: a side reaction of methanogenesis

Metal(loid)s are subject to many transformation processes in the environment, such as oxidation, reduction, methylation and hydride generation, predominantly accomplished by prokaryotes. Since these widespread processes affect the bioavailability and toxicity of metal(loid)s to a large extent, the investigation of their formation is of high relevance. Methanogenic Archaea are capable of methylating and hydrogenating Group 15 and 16 metal(loid)s arsenic, selenium, antimony, tellurium, and bismuth due to side reactions between central methanogenic cofactors, methylcobalamin (CH₃Cob(III)) and cob(I)alamin (Cob(I)). Here, we present systematic mechanistic studies on methylation and hydride generation of Group 15 and 16 metal(loid)s by CH₃Cob(III) and Cob(I). Pentavalent arsenical species showed neither methylation nor reduction as determined by using a newly developed oxidation state specific hydride generation technique, which allows direct determination of tri‐ and pentavalent arsenic species in a single batch. In contrast, efficient methylation of trivalent species without a change in oxidation state indicated that the methyl transfer does not proceed via a Challenger‐like oxidative methylation, but via a non‐oxidative methylation. Our findings also point towards a similar mechanism for antimony, bismuth, selenium, and tellurium. Overall, we suggest that the transfer of a methyl group does not involve a free reactive species, such as a radical, but instead is transferred either in a concerted nucleophilic substitution or in a caged radical mechanism. For hydride generation, we propose the intermediate formation of hydridocobalamin, transferring a hydride ion to the metal(loid)s. Copyright © 2012 John Wiley & Sons, Ltd.