Title

Controlling selectivity in photochemical reactions through confinement and non bonding interactions

Date of Award

2007

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

First Committee Member

V. Ramamurthy, Committee Chair

Abstract

Work presented in this thesis is the consolidation of our experiments aimed at achieving selectivity in photochemical reactions by making use of confinement provided by the host and the weak intermolecular interactions existing between the host and the guest(s) molecules. Two supramolecules namely Pd-nanocage (a water soluble self assembled coordination cage) and commercially available Y zeolites were used as hosts to conduct photochemical reactions. The cavities provided by these host molecules were used as a reaction vessel to conduct photochemical reactions.Chapter 1 gives a brief overview of supramolecular organic chemistry. The supramolecules reported in the literature which are related to the work presented in this thesis are discussed. Research conducted by using these supramolecules with the aim of achieving selectivity in photochemical reactions is also discussed.Chapter 2 presents the utility of hydrophobic cavity provided by water soluble octahedral Pd-nanocage to conduct photodimerization reaction of coumarins. With the help of five different coumarin molecules we have shown that syn head-head dimer is formed as the major product when the reaction is conducted within Pd-nanocage. The weak intermolecular forces existing between the host and the guest(s) (C-H---pi, pi---pi) and the compactness of the dimer are held responsible for such behaviour.Chapter 3 presents the utility of hydrophobic cavity provided by Pd-nanocage in conducting photodimerization of trans-cinnamic acid esters. This work is an extension of the work presented in Chapter 2. Unlike coumarins, trans-cinnamic acid esters have two reaction pathways namely isomerization and dimerization. Isomerization is the major pathway during solution phase irradiation of trans-cinnamic acid esters. Irradiation of the host-guest complexes of trans-cinnamic acid methyl esters with the Pd-nanocage resulted in the selective formation of syn head-head dimer in addition to the corresponding cis isomer. The obtained results suggest that the guest molecules are preoriented in a selective fashion within the Pd-nanocage. Weak intermolecular forces between the host and the guest(s) (C-H---pi, pi---pi) are likely to be responsible for the lack of mobility of the reactant olefins during their short excited state life time.Chapter 4 deals with photodimerization of anthracene derivatives and triplet sensitized photodimerization of acenaphthalene within Pd-nanocage. Among the anthracene derivatives studied, only 9-anthraldehyde (4b) dimerized within Pd-nanocage yielding head-tail dimer. The other anthracene derivatives inspite of long wavelength absorption were photoinactive within Pd-nanocage. In the case of triplet sensitized photodimerization of acenapthalene, water soluble xanthene dyes were used as sensitizers. The triplet sensitized photodimerization of acenapthalene within Pd-nanocage yielded the syn dimer exclusively.Chapter 5 discusses the results obtained during stereoselective photocyclization of pyridones within zeolites. Two approaches namely chiral inductor approach and chiral auxiliary approach were employed with the aim of achieving stereoselectivity during photocyclization of pyridones. Enantioselectivity of around 55% and diastereoselectivity of around 88% were obtained. The charge compensating cations of the zeolites, their interaction with the guest molecules and the confinement provided by the zeolite interior are held responsible for the observed stereoselectivity.Chapter 6 deals with probing the cation organic interaction existing between organic molecules and the charge compensating cations of the zeolites by solid state NMR experiments. Static, magic angle spinning and cross-polarization magic angle spinning experiments were employed to study the cation-organic interaction. These experiments suggested the existence of an interaction between the organic molecules and the charge compensating cations of the zeolites.

Keywords

Chemistry, Organic

Link to Full Text

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