On-line solvent evaporator for coupled LC systems

The mobile phase of a fraction eluted from a first LC column is removed by an on-line evaporator in order to reconcentrate the solute material or to exchange the eluent before performing a subsequent LC separation. Evaporation essentially occurs by concurrent evaporation, i.e. the solvent evaporates at a rate equal to the flow rate of the incoming eluent, and is driven by the overflow principle, i.e. vapors leave the tube as a result of the expansion resulting from evaporation. The liquid is introduced into a small tube (e.g., 4 cm × 1.3 mm i.d.) which is packed, e.g., with a coarse silica gel. The outlet of the evaporator is connected to vacuum in order to enable evaporation at reduced temperature and to increase retention of the volatile components. With normal phase eluents, evaporation rates may approach 1 ml/min; n-dodecane was the most volatile n-alkane fully retained by the evaporator.     

A Facile Conversion of Tertiary Amines into [2-(dialkylamino)vinyl]triphenylphosphonium salts

Halogenation of Et₃N, (i-Pr)₂EtN, and N-ethylmorpholine or of enamines with dichlorotriphenylphosphorane gives in up to 75% yield the corresponding [2-(dialkylamino)vinyl]triphenylphosphonium chlorides, which can be readily converted into the corresponding stable crystalline tetraphenylborates (Schemes 2 and 3).     

Helly-type theorems for homothets of planar convex curves

Helly's theorem implies that if is a finite collection of (positive) homothets of a planar convex body , any three having non-empty intersection, then has non-empty intersection. We show that for collections of homothets (including translates) of the boundary , if any four curves in have non-empty intersection, then has non-empty intersection. We prove the following dual version: If any four points of a finite set in the plane can be covered by a translate [homothet] of , then can be covered by a translate [homothet] of . These results are best possible in general.

     

Mechanisms of Cobalt/Phosphine-Catalyzed Cross-Coupling Reactions

>The combination of CoCl₂ with bidentate phosphines is known to catalyze challenging cross-coupling and Heck-type reactions, but the mechanisms of these valuable transformations have not been established. Here, we use electrospray-ionization mass spectrometry to intercept the species formed in these reactions. Our results indicate that a sequence of transmetalation, reductive elimination, and redox disproportionation convert the cobalt(II) precatalyst into low-valent cobalt complexes. These species readily transfer single electrons to alkyl bromides, which thereupon dissociate into alkyl radicals and Br⁻. In cross-coupling reactions, the alkyl radicals add to the cobalt catalyst to form observable heteroleptic complexes, which release the coupling products through reductive eliminations. In the Heck-type reactions, the low abundance of newly formed ionic species renders the analysis more difficult. Nonetheless, our results also point to the occurrence of single-electron transfer processes and the involvement of radicals in these transformations.     

The platinum catalyst [bpyrPtCl2] in superacidic solution

The methane oxidation catalyst [bpyrPtCl(2)] (bpyr = bis-pyrimidine) dissolves in superacidic HF/SbF(5) solution under formation of a dinuclear cation [H(2)bpyrPt(mu-Cl)(2)PtbpyrH(2)](6+). Two crystal forms are isolated, [Pt(2)Cl(2)bpyr(2)H(4)](6+)(SbF(6)(-))(4)(Sb(2)F(11)(-))(2).2HF (I) (triclinic, Ponemacr;, a = 814.8(2) pm, b = 1444.8(3) pm, c = 2300.5(5) pm, alpha = 89.627(4) degrees, beta = 84.285(4) degrees, gamma = 84.665(4) degrees, Z = 2) and [Pt(2)Cl(2)bpyrH(4)](6+)(Sb(2)F(11)(-))(6).4HF (II) (triclinic, Ponemacr;, a = 879.4(2) pm, b = 1170.4(3) pm, c = 1789.9(5) pm, alpha = 95.37(2) degrees, beta = 99.97(2) degrees, gamma = 100.41(2) degrees, Z = 1). The cation in I has an angle of 148.4(1) degrees between the two square plane platinum environments, while the cation in II is fully planar. The non-platinum-bound nitrogen atoms are all protonated in the superacidic medium.