Magnetic deflection of neutral metal clusters in a beam

A new apparatus for measuring the magnetic properties of metal clusters has been constructed. The technique involves the conventional Stern-Gerlach deflection scheme together with modern pulsed laser vaporization source technology and time of flight mass spectrometry. High field seeking monodirectional deflections have been measured for cobalt clusters containing between 40 and 400 atoms. The measured magnetic moments per atom are found to be lower than the known values for the bulk. Special attention has been given to velocity measurements of the metal clusters and the carrier gas atoms in the beam. The residence time of the particles in the source cavity has been measured.     

Irreducible Components of Fixed Point Subvarieties of Flag Varieties

Suppose G is a connected reductive algebraic group, P is a parabolic subgroup of G, L is a Levi factor of P, and e is a regular nilpotent element in Lie L. We assume that the characteristic of the underlying field is good for G. Choose a maximal torus, T, and a Borel subgroup, B, of G, so that T?B∩L, B ? P and e ∈ Lie B. Let β be the variety of Borel subgroups of G and let ??_e be the subset of ?? consisting of Borel subgroups whose Lie algebras contain e. Finally, let W be the Weyl group of G with respect to T. For ω ∈ W let ??_ω be the B-orbit in ?? containing ^ωB. We consider the intersections ??_ω ∩ ??_e. The main result is that if dim ??_ω ∩ ??_e = dim ??_e, then ??_ω ∩ ??_e is an affine space. Thus, the irreducible components of ??_e are indexed by Weyl group elements. It is also shown that if G is of type A, then this set of Weyl group elements is a right cell in W.     

Simple preparation of alpha-diazo esters

[reaction: see text] The TiCl4-mediated reaction of an ester with benzoyl chloride results in high yields of the alpha-benzoylated ester. Diazo transfer of the benzoylated ester utilizing p-acetoamidobenzenesulfonyl azide affords the alpha-diazo ester in good yield. Using this simplified procedure, it is easy to prepare gram quantities of alpha-diazo esters.     

Reactivity of secondary metallocene alkyls and the question of dormant sites in catalytic alkene polymerization

The reactivity of [rac-(C2H4(1-indenyl)2)Zr(n-butyl)][MeB(C6F5)3] (4), [rac-(C2H4(1-indenyl)2)Zr(sec-butyl)][MeB(C6F5)3] (5), and [rac-(C2H4(1-indenyl)2)Zr(polypropenyl)][MeB(C6F5)3] with propene, ethene, and hydrogen was studied by low-temperature (<-40 degrees C) 1H and 13C NMR spectroscopy in toluene solutions. In contrast with previous suggestions that 2 degrees zirconium alkyl species such as 5 are dormant sites, these measurements demonstrate reactivity of 2 degrees zirconium alkyls with propene and ethene comparable to the 1 degrees zirconium alkyl species 4 and [rac-(C2H4(1-indenyl)2)Zr(polypropenyl)][MeB(C6F5)3]. Because 2,1-insertion of propene is an infrequent event, these results preclude significant accumulation of catalyst in the form of 2 degrees zirconium alkyls for this metallocene and counterion. The reactivity of 5 with hydrogen is at least 2 orders of magnitude faster than other 1 degrees zirconium alkyls. Such high reactivity accounts for the puzzlingly high fraction of butyl end groups in prior hydrooligomerization studies and implies that catalyst responsivity to H2 as a molecular weight control agent correlates with the regioselectivity of the catalyst.     

Direct observation of insertion events at rac-(C2H4(1-indenyl)2)Zr(MeB(C6F5)3)-polymeryl intermediates: distinction between continuous and intermittent propagation modes

The propagating species for 1-hexene, propene, and ethene polymerization as catalyzed by [rac-(C2H4(1-indenyl)2)Zr(CH3)][CH3B(C6F5)3] has been intercepted at T < -40 degrees C and characterized by NMR methods. Observation of the propagating species permits direct monitoring of initiation, propagation, and termination processes by NMR. Detailed examination of alkene incorporation into Zr-polymeryl species using 13C-labeled 1-alkenes supports a continuous insertion mechanism whereby the anion re-coordinates after each alkene insertion.     

Enantioselective construction of carbobicyclic scaffolds

Allylic and benzylic Grignard reagents smoothly open phenylalkynyl-activated cyclic trisubstituted epoxides at the more substituted carbon atom to give secondary alcohols with a chiral quaternary center. These alcohols are good substrates for the construction of enantiomerically pure carbobicyclic scaffolds through intramolecular alkylation.     

Synthesis of (-)-calicoferol B

The first total synthesis of (-)-calicoferol B (III) is described. The cyclozirconation product I, prepared in enantiomerically pure form, was converted into the CD ring chiron II. This was coupled with the aromatic A-ring, and then the side chain was constructed with control of relative and absolute configuration to complete the total synthesis of III. The first total synthesis of (-)-calicoferol B (1) is described. The cyclozirconation product 8, prepared in enantiomerically pure form, was converted into the CD ring chiron 6. This was coupled with the aromatic A-ring, and then the side chain was constructed with control of relative and absolute configuration to complete the total synthesis of 1.