An examination of hyperconjugative and electrostatic effects in the hydride reductions of 2-substituted-4-tert-butylcyclohexanones.

To better understand electronic effects on the diastereoselectivity of nucleophilic additions to the carbonyl group, a series of 2-X-4-tert-butylcyclohexanones (X = H, CH(3), OCH(3), F, Cl, Br) were reacted with LiAlH(4). Reduction of ketones with equatorial substituents yields increasing amounts of axial alcohol in the series for X [H < CH(3) < Br < Cl < F << OCH(3)]. These data cannot be explained by steric or chelation effects or by the theories of Felkin-Anh or Cieplak. Instead, an electrostatic argument is introduced: due to repulsion between the nucleophile and the X group, axial approach becomes energetically less favorable with an increase in the component of the dipole moment anti to the hydride approach trajectory. The ab initio calculated diastereoselectivities were close to the experimental values but did not reproduce the relative selectivity ordering among substituents. For reduction of ketones with axial substituents, increasing amounts of axial alcohol are seen in the series for X [Cl < Br < CH(3) < OCH(3) < H < F]. After some minor adjustments are made, this ordering is consistent with both the electrostatic model and Felkin-Anh theory. Cieplak theory cannot account for these data regardless of adjustments. Ab initio calculated diastereoselectivities were reasonably accurate for the nonpolar substituents but were poor for the polar substituents.

Dendrimeric organochalcogen catalysts for the activation of hydrogen peroxide: improved catalytic activity through statistical effects and cooperativity in successive generations.

Dendrimeric polyphenylsulfides, -selenides, and -tellurides are prepared in high yield using propyloxy spacers to connect the phenylchalcogeno groups to the dendrimeric core. The selenides and tellurides catalyze the oxidation of bromide with hydrogen peroxide to give positive bromine species that can be captured by cyclohexene in two-phase systems. The corresponding sulfides show no catalytic activity. The increase in the rate of catalysis followed statistical effects for 1, 6, and 12 phenyltelluro groups. However, the increase in the rate of catalysis exceeds statistical contributions for the first few generations with 1, 3, 6, and 12 phenylseleno groups and suggested cooperativity among phenylseleno groups. The increase in catalytic rate was lost upon replacing all but one phenylseleno group with phenoxy groups. On the basis of H2O2 consumed, the dendrimer with 12 phenylseleno groups has a turnover number of >60 000 mol of H2O2 consumed per mole of catalyst.

Optical fingerprint recognition using a waveguide hologram.

We demonstrate the fabrication of a waveguide hologram and describe how a plane-wave output beam can be generated for the illumination of a finger for fingerprint image capture. Also, when using a diverging beam in the substrate as a reference wave, one obtains a set of gratings written in the hologram. The reflected light from the finger is spatially filtered by this hologram, providing bandpassed information back through the waveguide when it is viewed through the plate.

Vernier scaling in shortened optical correlators.

Vernier transform scaling is achieved in a shortened optical correlator by placing the input spatial light modulator (SLM) between the telephoto transform lens pair. Translating the SLM changes the Fourier transform scale but not the position of the transform plane.

Low reflectance terminations and connections for duplex fiber-optic telecommunication links.

The signal-to-noise ratio of full duplex fiber-optic telecommunication links using a single fiber with directional couplers is shown to be extremely sensitive to reflections from fiber end terminations and fiber-to-fiber connections. This paper will examine a series of experimental measurements made to determine the reflectance coefficients of several different end terminations and types of connection.

Multimode fiber-optics coupler with low insertion loss.

A fiber-optics coupler is described which is simple to fabricate, is directional, leaves the fiber intact, and has an insertion loss of <0.05 dB. The coupler utilizes the energy contained in the leaky modes of the fiber. The coupling ratio may be varied by changing the type of light source and input conditions. Possible applications are to optical data bus links of up to a few hundred meters or as power or mode monitors.

PLZT Matrix-Type Block Data Composers.

The characteristics of PLZT matrix-type block data composers operated in the strain-biased, scattering, differential phase and edge-effect modes are described, and a comparison is given of these four modes of operation.