Atom-Transfer Cyclization with CuSO(4)/KBH(4): A Formal "Activators Generated by Electron Transfer" Process Also Applicable to Atom-Transfer Polymerization

The 4-exo and 5-exo-trig atom-transfer cyclizations of 1, 8a-e, 9, 12, and 13 can be mediated with as little as 0.05 mol % of Cu(TPMA)SO(4)·5H(2)O in the presence of 2.5 mol % of borohydride salts in 10 min at room temperature in air. This formal "activators generated by electron transfer" (AGET) procedure utilizes a cheap and oxidatively stable copper source (CuSO(4)·5H(2)O) and can be carried out in environmentally benign solvents (EtOH). It is possible to alter the product distribution in the 5-endo radical-polar crossover reactions of 10a,b and 11 by tailoring the amount of borohydride. Cyclization onto alkynes 14 and 15 is also possible in only 20 min. Controlled radical polymerization of styrene, with increased rates over conventional atom-transfer radical polymerization (ATRP), can be carried out in a controlled fashion (Mn, PDI) using either CuBr or CuSO(4)·5H(2)O and Bu(4)NBH(4).     

Large antichains in the partition lattice

Consider the partially ordered set of all partitions of an n-element set, ordered by refinement. The sizes of the various ranks within this poset are the Stirling numbers of the second kind. We show that the ratio of the size of the largest antichain to the size of the largest rank exceeds n^1/35 for all n sufficiently large.     

Single‐crystal X‐ray diffraction and resonant X‐ray magnetic scattering at helium‐3 temperatures in high magnetic fields at beamline P09 at PETRA III

The resonant scattering and diffraction beamline P09 at PETRA III at DESY is equipped with a 14 T vertical field split‐pair magnet. A helium‐3 refrigerator is available that can be fitted inside the magnet's variable‐temperature insert. Here the results of a series of experiments aimed at determining the beam conditions permitting operations with the He‐3 insert are presented. By measuring the tetragonal‐to‐orthorhombic phase transition occurring at 2.1 K in the Jahn–Teller compound TmVO₄, it is found that the photon flux at P09 must be attenuated down to 1.5 × 10⁹ photons s^?1 for the sample to remain at temperatures below 800 mK. Despite such a reduction of the incident flux and the subsequent use of a Cu(111) analyzer, the resonant X‐ray magnetic scattering signal at the Tm L_III absorption edge associated with the spin‐density wave in TmNi₂B₂C below 1.5 K is intense enough to permit a complete study in magnetic field and at sub‐Kelvin temperatures to be carried out.     

Facile assembly of a Cu9 amido complex: a new tripodal ligand design that promotes transition metal cluster formation

A tripodal amido ligand with a central non-chelating phosphorus donor allows for the facile assembly of a pentane soluble organometallic copper cluster with a central copper atom surrounded by a nonplanar chain of eight copper atoms and two terminal amido-copper bonds.     

Density-functional geometry optimization of the 150,000-atom photosystem-I trimer

We present a linear-scaling method based on the use of density-functional theory (DFT) for the system-wide optimization of x-ray structural coordinates and apply it to optimize the 150,000 atoms of the photosystem-I (PS-I) trimer. The method is based on repetitive applications of a multilevel ONIOM procedure using the PW916-31G(d) DFT calculations for the high level and PM3 for the lower level; this method treats all atoms in the structure equivalently, a structure in which the majority of the atoms can be considered as part of some internal "active site." To obtain a realistic single structure, some changes to the original protein model were necessary but these are kept to a minimum in order that the optimized structure most closely resembles the original x-ray one. Optimization has profound effects on the perceived electronic properties of the cofactors, with, e.g., optimization lowering the internal energy of the chlorophylls by on average 53 kcal mol(-1) and eliminates the enormous 115 kcal mol(-1) energy spread depicted by the original x-ray heavy-atom coordinates. A highly precise structure for PS-I results that is suitable for analysis of device function. Significant qualitative features of the structure are also improved such as correction of an error in the stereochemistry of one of the chlorophylls in the "special pair" of the reaction center, as well as the replacement of a water molecule with a metal cation in a critical region on the C3 axis. The method also reveals other unusual features of the structure, leading both to suggestions concerning device functionality and possible mutations between gene sequencing and x-ray structure determination. The optimization scheme is thus shown to augment the molecular modeling schemes that are currently used to add medium-resolution structural information to the raw scattering data in order to obtain atomically resolved structures. System-wide optimization is now a feasible process and its use within protein x-ray data refinement should be considered.