Macrobicycles 1-3 have been prepared by palladium-catalyzed double macrocyclization. The planar geometry of this system is such that the para axis of the inner ring exactly matches the inner diameter of the macrocycle. Molecular models suggest that the inner ring of 1 should be able to rotate freely about its long axis, acting like the spindle of a turnstile. For large spindle substituents, this rotation will become hindered while for intermediate sized substituents, a pair of energetically equivalent conformational states should rapidly interconvert on an experimentally observable time scale. Substituted derivatives 2 and 3 possess diastereotopic methylene protons which become operationally enantiotopic upon fast rotation. Based on variable-temperature 'H NMR and longitudinal T\ relaxation experiments, examples of what are believed to be freely rotating 2 and conformationally locked 3 spindles are reported. These molecules represent the first iteration in the development of phenylacetylene macrocycles possessing conformational bistability that may eventually lead to new types of solids or liquid crystals that respond rapidly to external electric fields.Recently, several examples that may be considered mechanical molecular devices have been reported including Stoddart's rotaxane based molecular shuttle,2 Feringa's chiroptical molecular switch,3 Jprgenson's light activated bianthrone molecules,4 Kelly's molecular "brake",5 6*and Malthete and Collet's conformationally invertable columnar mesophases.6,7 Despite these and other impressive examples, two formidable obstacles toward the continued development of molecular-based devices present themselves to modem chemistry. These include the development of general schemes for the predictable assembly of multimolecular aggregates in all
Organometallics due to allyl q3 to q1 isomerization. 1991,10, [924][925][926][927][928][929][930] Supplementary Material Available: Tables of thermal parameters and hydrogen atom coordinates (Tables I11 and IV) (2 pages); listings of observed and calculated structure factors (Tables V and VI) (11 pages). Ordering information is given on any current masthead page.The catalytic dehydrogenative coupling of secondary silanes has been achieved with the combination of Cp2ZrC12 and 2 equiv of nBuLi in toluene at 90 "C. The products produced from PhMeSiH, contain disilane through pentasilanes after 2 days, and longer time periods produce oligomeric mixtures up to the octamer. Disilane through tetrasilane have been isolated as distillable, air-stable liquids and are produced as mixtures of stereoisomers. The distribution of oligomers has been determined under various conditions of temperature, time, solvent, and Si/Zr and Zr/Li ratios as well as with a combination of Cp,ZrCl, with Grignard reagents. Disilane is formed from Ph2SiH2 even at 110 "C, although silafluorene condenses rapidly even at room temperature to a mixture of oligomers. The combination of Cp2TiC12/nBuLi with PhMeSiH, provides oligomers from disilane through hexasilane, whereas Cp2HfC1, produces disilane and trisilane. A possible mechanism for the coupling reaction is presented.
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Addition of resolved-Ru(phenanthroline)32+ to aqueous solutions of racemic complexes of Ln(2,6-pyridinedicarboxylate)33- (where Ln = Eu(III) and Tb(III) results in efficient quenching of lanthanide emission. The terbium complex is quenched more than the europium complex illustrating the importance of electronic overlap. The emission is also circularly polarized indicating that the quenching mechanism is enantioselective. Comparison with previous data allows for the identification of the enantiomer in excess, and, thus, of the favored enantioselective energy transfer pair.
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