Characterization and Dynamics of Substituted Ruthenacyclobutanes Relevant to the Olefin Cross-Metathesis Reaction

The reaction of the phosphonium alkylidene [(H_(2)IMes)RuCl2═CHP(Cy)_3)]^(+) BF_(4)^− with propene, 1-butene, and 1-hexene at −45 °C affords various substituted, metathesis-active ruthenacycles. These metallacycles were found to equilibrate over extended reaction times in response to decreases in ethylene concentrations, which favored increased populations of α-monosubstituted and α,α′-disubstituted (both cis and trans) ruthenacycles. On an NMR time scale, rapid chemical exchange was found to preferentially occur between the β-hydrogens of the cis and trans stereoisomers prior to olefin exchange. Exchange on an NMR time scale was also observed between the α- and β-methylene groups of the monosubstituted ruthenacycle (H_(2)IMes)Cl_(2)Ru(CHRCH_(2)CH_(2)) (R = CH_3, CH_(2)CH_3, (CH_2)_)_(3)CH_3). EXSY NMR experiments at −87 °C were used to determine the activation energies for both of these exchange processes. In addition, new methods have been developed for the direct preparation of metathesis-active ruthenacyclobutanes via the protonolysis of dichloro(1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)(benzylidene) bis(pyridine)ruthenium(II) and its 3-bromopyridine analogue. Using either trifluoroacetic acid or silica-bound toluenesulfonic acid as the proton source, the ethylene-derived ruthenacyclobutane (H_(2)IMes)Cl_(2)Ru(CH_(2)CH_(2)CH_(2)) was observed in up to 98% yield via NMR at −40 °C. On the basis of these studies, mechanisms accounting for the positional and stereochemical exchange within ruthenacyclobutanes are proposed, as well as the implications of these dynamics toward olefin metathesis catalyst and reaction design are described

NMR and computational studies on the reactions of enamines with nitroalkenes that may pass through cyclobutanes

The addition of aldehyde enamines to nitroalkenes affords cyclobutanes in all solvents, with all of the pyrrolidine and proline derivatives tested by us and with all of the substrates we have examined. Depending on the temperature, concentration of water, solvent polarity, and other factors, the opening and hydrolysis of such a four-membered ring may take place rapidly or last for several days, producing the final Michael-like adducts (4-nitrobutanals). Thirteen new cyclobutanes have now been characterized by NMR spectroscopy. As could be expected, s-trans-enamine conformers give rise to all-trans-(4S)-4-nitrocyclobutylpyrrolidines, while s-cis-enamine conformers afford all-trans-(4R)-4-nitrocyclobutylpyrrolidines. These four-membered rings can isomerize to adduct enamines, which should be hydrolyzed via their iminium ions. MP2 and M06-2X calculations predict that one iminium ion is more stable than the other iminium species, so that protonation of the adduct enamines can be quite stereoselective; in the presence of water, the so-called syn adducts (e.g., OCH-*CHR-*CHPh-CH2NO2, with R and Ph syn) eventually become the major products. Why one syn adduct is obtained with aldehydes, whereas cyclic ketones (the predicted ring-fused cyclobutanes of which isomerize to their enamines more easily) produce the other syn adduct, is also explained by means of molecular orbital calculations. Nitro-Michael reactions of aldehyde enamines that "stop" at the nitrocyclobutane stage and final enamine stage do not work catalytically, as known, but those of cyclic ketone enamines that do not work stop at the final enamine stage (if their hydrolysis to the corresponding nitroethylketones is less favorable than expected). These and other facts are accounted for, and the proposals of the groups led by Seebach and Hayashi, Blackmond, and Pihko and Papai are reconciled

Crossed Beam Imaging Of The Reaction Dynamics Of Halogen Atoms With Selected Hydrocarbons

This dissertation presents results of applying dc slice imaging in crossed molecular beams to probe the dynamics of the reactions of halogen atoms (chlorine and fluorine) with polyatomic hydrocarbons and alcohols such as deuterated propanes, butane isomers, pentane, alkenes and propanol. The full velocity-flux contour maps of the radical products were measured with 157nm single photon ionization at various collision eneriges. Secondary and tertiary abstractions were found in Cl with normal and deuterated propanes and butane isomers and show distinct differences. The differences were explained in terms of the nature of abstraction sites, energy disposal of the radical product, and kinetic isotope effects. For Cl reaction with butene isomers, the coupling of translational energy and center of mass angular distributions reflect the energetics of competition between direct abstraction and addition/elimination pathways in accordance with ab initio thermochemical data. A possible Cl atom roaming mediating the indirect mechanism is suggested and further addressed with investigations of Cl + isobutene reactions at various collision energies. For reaction of chlorine atoms with butenes, the combined experimental theoretical calculations result shows that Cl addition-HCl elimination occurs from an abstraction-like Cl-H-C geometry, rather than a conventional three-center or four-center transition state. This geometry is accessed exclusively by Cl atom roaming from the initial adduct. For fluorine atom reaction with linear alkanes, i.e. propane, n-butane and n-pentane, little effect of reaction exoergicity appears in the reduced translational energy distributions. The fraction of available energy in translation for pentane is smaller than the other two. Sharp forward scattering were found in the center of mass angular distributions of all targets and the backscattering decreases in with the size of the molecule increasing. The analyzed data were compared with corresponding theoretical studies. For fluorine atom reaction with 1-propanol, the translational energy distribution and center of mass angular distributions is quite similar to the results of F + n-butane; it is possible that the greater fraction of collision energy in translation comes from the existence of O-H group. The product scattering distributions of fluorine reaction with 1-butene and 1-hexene provide evidence of a long-lived complex mediated mechanism

Helical and higher structural ordering in poly(2 methoxystyrene) biocompatibility and cell adhesion studies of poly(2 mehtoxystyrene), 2004

Poly(2 methoxystyrene) has been synthesized by a variety of polymerization pathways such as anionic, free radical, and helix sense anionic. The polymers were casted on glass slides and cell adhesion, cell morphology, and cell growth were observed using HeLa Ovarian Cancer cell culture. Tissue culture modified poly(styrene) (TCPS) was used as a control. HeLa cell growth and adhesion was established onto all of the polymers but at different levels of preference. In all cases, the HeLa cells showed the best growth on ( ) and (+) optically active helical poly(2 methoxystyrene) (+) and ( ) P2MS. Remarkably, the HeLa Ovarian Cancer cells displayed a chiral preference for (+) optically active helical P2MS

Highly Strained Organophosphorus Compounds

In our research on small, strained organophosphorus ring systems we became interested in the synthesis and applications of species that are even more strained than the parent phosphirane, by introducing an exocyclic double bond (methylenephosphirane), and by cyclopropyl spirofusion to the edge (e.g. phospha[2]triangulane) and side (phosphabicyclo[1.1.0]butane). These highly strained phosphorus heterocycles are fascinating species from a fundamental point of view. Moreover, they show intriguing reactivity and remarkably facile rearrangements resulting in a large spectrum of novel organophosphorus compounds. Thus, our interest not only lies in the efficient synthesis of these highly strained phosphacycles, but also in employing them as attractive building blocks for the synthesis of phosphine ligands. To achieve this aspiration, reactive intermediate chemistry is used, supported by high-level theoretical calculations, to develop new methodologies and versatile approaches toward applicable and processable building blocks creating unprecedented entries in organophosphorus chemistry.Lammertsma, K. [Promotor]Ehlers, A.W. [Copromotor]Schakel, M. [Copromotor

Synthesis and Reactions of Bicyclo[1.1.0]butanes

This dissertation describes methods for the synthesis of bicyclo[1.1.0]butanes and their reactions. The bicyclo[1.1.0]butyl skeleton was first assembled by intramolecular, amide directed Simmons-Smith cyclopropanation of propargyl amides and cyclopropenes. However, a more general approach applicable to the synthesis of amines, alcohol, esters, and amides was developed. This method is based on the generation of bicyclo[1.1.0]butyllithium from gem-dibromocyclopropanes via sequential transmetallation reactions with alkyllithium reagents and addition to electrophilic acceptors.Studies on the reaction of bicyclo[1.1.0]butanes were focused on thermal and metal-catalyzed transformations. Bicyclo[1.1.0]butanes activated by an aromatic group underwent highly chemo- and diastereoselective intramolecular pericyclic reactions under mild conditions. The outcome of these processes was controlled by the selection of the allylating reagents, and N-allyl amides gave exclusively formal ene products. On the other hand, cinnamyl amides participated in an intramolecular cycloaddition reaction. The postulated presence of radical in the mechanism of these reactions was supported by chemical and spectroscopic (ESR) studies.In the metal-catalyzed cycloisomerization reactions, an intramolecular cyclopropanation of allyl amides catalyzed by complexes of Rh(I) is described. These reactions proceeded via metal carbene intermediates, which were selectively generated by applying phosphine additives with different steric and electronic properties. Based on the mechanistic proposal, the synthesis of pyrroles from bicyclo[1.1.0]butanes was achieved using a Rh(I)/bidentate ligand catalytic system and hindered amides. Finally, cycloisomerization reactions of propargyl amides and ethers as well as electron-deficient bicyclo[1.1.0]butanes catalyzed by Pt(II) are described. These reactions proceeded via a series of carbene intermediates to give polycyclic nitrogen- and oxygen-containing heterocycles, which could be of utility in research and development

Synthesis and characterization of molecules to study the conformational barriers of fluorocarbon chains

Fluorocarbons are known to be stiffer than their hydrocarbon analogues, a property that underlines the extensive industrial application of fluorocarbon materials. Although there has been previous studies on the rotational barrier of molecules having fluorocarbon centers, a detailed systematic study is necessary to quantify flurocarbon stiffness. The molecules, Pyrene-(CF2)n-Pyrene, Pyrene-(CF2)n-F, Pyrene-(CH2)n-Pyrene and Pyrene-(CH2)n-H were therefore synthesized to enable the determination of the barrier to rotation of the carbon backbone in fluorocarbons. Conformational studies will be completed with steady-state and time-dependent emission spectroscopy