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The problem with which this paper is concerned has as its basis one of the most important properties of the conjugated system of double bonds found in some organic molecules, namely, the ability to exhibit stereoisomerism.

Yes / The stereochemical outcome of the epoxidation of Δ14–15 cholestanes with mCPBA is controlled by the steric bulk of a C17 substituent. When the C17 is in the β configuration, the epoxide is formed in the α face, whereas if the C17 is trigonal (flat) or the substituent is in the α configuration, the epoxide is formed in the β face. The presence of a hydroxyl substituent at C20 does not influence the stereochemical outcome of the epoxidation. / We thank University of Bradford for a bursary (MA).

The purpose of this investigation is to study the Birch reduction products of [2.2lparacyclophane and of [2.2]paracyclophane- 2-carboxylic acid. The tetrahydro Birch reduction product of [2.2]paracyclophane is shown to be dl stereoisomer, with the olefins of the upper deck only partially overlapping with the olefins of the lower deck. This stereochemical elucidation is accomplished by means of a complete proton nmr analysis of the tetraepoxide derivative. A proposed mechanism supported by VB (valence bond) and MO (molecular orbital) theories for this Birch reduction process is presented.

The reaction of tert-butyllithium with chloromethylphenylvinylsilane at low temperatures in hexane gave a 48% yield of a mixture of the five isomers of 1,3-dimethyl-1,3-diphenyl-2,4-dineopentyl-1,3-disilacyclobutane, formed by the head-to-tail dimerization of both E- and Z-1-methyl-1-phenyl-2-neopentylsilenes, along with an acyclic dimer. These were separated and their stereochemistry was established by ('1)H- and ('13)C-NMR spectroscopy. The E- and Z-silenes were also trapped as their {4 + 2} cycloadducts with cyclopentadiene, 2,3-dimethyl-1,3-butadiene and anthracene, which also were separated and stereochemically characterized. A consistent mole ratio of 70:30 for the E- and Z-silene adducts is interpreted as evidence for stereochemical induction in the silene generation reaction. It is also suggested that the dimerization of the silenes to give the 1,3-disilacyclobutanes occurs by a nonstereospecific stepwise pathway. When E- or Z-1-methyl-1-phenyl-2-neopentylsilene was generated by the retro-Diels-Alder flow vacuum thermolysis of its corresponding cyclopentadiene or anthracene adduct at temperatures between 400 and 600(DEGREES)C and then trapped with 2,3-dimethyl-1,3-butadiene, the stereochemical distribution of the products is independent of the stereochemistry of the silene precursor, indicating that the silene is not configurationally stable towards cis-trans isomerization at these temperatures. Evidence that the intermolecular ene reaction and the {4 + 2} cycloaddition which occur with 2,3-dimethyl-1,3-butadiene are concerted is presented. When either the E- or Z-silene, generated by the sealed tube thermolysis of its anthracene adduct by 300(DEGREES)C, was trapped with trimethylmethoxysilene, the diastereomer obtained depended on the stereochemistry of the silene precursor, showing that the silene is configurationally stable towards cis-trans isomerization up to 300(DEGREES)C. The temperature dependence of the ratio of the two diastereomers obtained when the silene formed from the pure E- or Z-anthracene adduct was trapped at higher temperatures permitted the determination of an activation energy for the silene isomerization. The activation energies for the E- and Z- and Z- to E-silene isomerization are 45 (+OR-) 6 and 20 (+OR-) 4 kcal mol('-1), respectively. The significance of these values is discussed.

A scanning force microscope is an instrument which can image the forces present on a sample with high resolution. These forces include magnetic, electric, and atomic forces. The scanning force microscope uses either a tunneling, capacitive or optical method of sensing the motion of a lever-tip mechanical system that reacts to the forces present on a sample. There are four optical methods used in scanning force microscopy: heterodyne, homodyne, deflection and laser feedback. In this thesis, two implementations of the laser feedback method of detecting lever motion are described: a fiber coupled laser diode feedback and a tightly coupled laser diode feedback. The theory of interactions between a tip and the magnetic or electric fields at the surface of a sample are presented, along with the theory of the laser diode feedback. The limiting noises inherent in the system are discussed and representative values are given. Results of the sensitivity of the two systems are presented, and images of electric forces acting on the tip from interdigitated electrodes are demonstrated.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

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