Intramolecular ketenimine-ketenimine [2 + 2] and [4 + 2] cycloadditions

Bis(ketenimines), in which the two heterocumulenic functions are placed in close proximity on a carbon skeleton to allow their mutual interaction, show a rich and not easily predictable chemistry. Intramolecular [2 + 2] or [4 + 2] cycloadditions are, respectively, observed when both ketenimine functions are supported on either ortho-benzylic or 2,2'-biphenylenic scaffolds. In addition, nitrogen-to-carbon [1,3] and [1,5] shifts of arylmethyl groups in N-arylmethyl-C,C-diphenyl ketenimines are also disclosed.     

Synthesis of 3‐Fluoropyrrolidines and 4‐Fluoropyrrolidin‐2‐ones from Allylic Fluorides

Various 3‐fluoropyrrolidines and 4‐fluoropyrrolidin‐2‐ones were prepared by 5‐exo‐trig iodocyclisation from allylic fluorides bearing a pending nitrogen nucleophile. These bench‐stable precursors were made accessible upon electrophilic fluorination of the corresponding allylsilanes. The presence of the allylic fluorine substituent induces syn‐stereocontrol upon iodocyclisation with diastereomeric ratios ranging from 10:1 to > 20:1 for all N‐tosyl‐3‐fluoropent‐4‐en‐1‐amines and amides. The sense and level of stereocontrol is strikingly similar to the corresponding iodocyclisation of structurally related allylic fluorides bearing pending oxygen nucleophiles. These results suggest that the syn selectivity observed upon ring closure involves I₂–π complexes with the fluorine positioned inside.     

Torque on a sphere inside a rotating cylinder

Summary The couple on a sphere in the centre of a finite rotating circular cylinder is measured over a wide range of Reynolds numbers for both Newtonian and non-Newtonian fluids. Wall effects are calculated. Experimental results are compared with Collins' analysis. Secondary flow is made visible. For non-Newtonian fluids the apparatus determines accurately the zero shear rate viscosity.

---

Résumé Le couple sur une sphère dans un cylindre circulaire rotatoire est mesuré pour des divers nombres de Reynolds et pour des fluides Newtoniens et non-Newtoniens. L'effet des parois sur le couple est calculé. Les resultats obtenus sont comparés avec l'analyse de Collins. Pour les fluides non-Newtoniens l'appareil determine la viscosité de zéro cisaillement.

     

Tandem [1,5]-H shift/6π-electrocyclizations of ketenimines bearing 1,3-oxathiane units. Computational assessment of the experimental diastereoselection

N-Aryl ketenimines bearing a 1,3-oxathiane function at the ortho position of the keteniminic nitrogen atom convert into spiro[1,3-oxathiane-2,4′(3′H)quinolines] under mild thermal treatment. These cyclization processes are interpreted in terms of a two-step tandem sequence involving a [1,5]-H migration followed by a 6π-electrocyclic ring closure. Moreover, the cyclization of 1,3-oxathiane-ketenimines having two different substituents at the terminal carbon atom of the ketenimine moiety provided spiroquinolines bearing two stereocenters, the C3 and C4 atoms, with moderate diastereoselectivity. A DFT study support that the mechanism of these conversions consists of a [1,5]-H shift/6π-electrocyclization sequence, in which the [1,5]-H shift is the rate-limiting step. A quantitative kinetic analysis of the cyclization of an oxathiane-ketenimine with a prochiral ketenimine function explains the sense and degree of the experimentally observed diastereoselectivity.     

Hydricity-promoted [1,5]-H shifts in acetalic ketenimines and carbodiimides

2-monosubstituted 1,3-dioxolanes and dithiolanes act as hydride-releasing fragments, transferring intramolecularly their acetalic H atom to the central carbon of ketenimine functions. The presumed products of these migrations, o-quinomethanimines, undergo in situ 6pi-electrocyclization. A computational study supports this mechanism and the hydride-shift character of the first step. Carbodiimides were also suitable substrates, although less reactive. [reaction: see text].     

Banach Spaces which are Radon Spaces with the Weak Topology

In this paper, Schachermayer's Theorem [6] is improved by showingthat a WCG Banach space whose density is of measure zero isa Radon space with the weak topology (weak-Radon). We also considerother conditions under which a Banach space is a weak-Radonspace.     

Lattice-universal Orlicz spaces on probability spaces

Lattice-universal Orlicz function spacesL ^F _α,β[0, 1] with prefixed Boyd indices are constructed. Namely, given 0<α<β<∞ arbitrary there exists Orlicz function spacesL ^F _α,β[0, 1] with indices α and β such that every Orlicz function spaceL ^G [0, 1] with indices between α and β is lattice-isomorphic to a sublattice ofL ^F _α,β[0, 1]. The existence of classes of universal Orlicz spacesl ^Fα,β(I) with uncountable symmetric basis and prefixed indices α and β is also proved in the uncountable discrete case. Partially supported by BFM2001-1284.