The synthetic efficacy of the dendrimer backbone rearrangement was examined by conducting heterogeneous Ramburg-Backlund rearrangements (alumina supported KOH, CBr2F2/THF/t-BuOH = 1/1/1) on dibenzylsulfone G3-CCSO2-dendrimer 141 and G3-CSO2C-dendrimer 142. Again, steric inhibition was found to play a prominent role in dictating the chemical reactivity of the sulfone moieties under heterogeneous conditions. The innermost sulfones in compound 141 were found to be relatively inert and at most only 1 out of the 3 sulfones could undergo the rearrangement, and the major product was the starting material. On the other hand, up to 3 or 4 of the intermediate layer sulfone moieties underwent the rearrangement reaction to give the triene-tris(sulfone) 145 or tetraene-bis(sulfone) 144 products. For the first time in the literature, conclusive mass spectral evidence was obtained to unveil the detailed outcome of such complex dendrimer backbone rearrangement reactions. / The synthetic efficiency of the dendrimer interior functional group conversion method was exemplified by converting the dibenzyl sulfide moieties in G3-CCS-dendrimer 68 and G3-CSC-dendrimer 69 to the corresponding dibenzyl sulfone under either heterogeneous (oxone in CH2Cl2) or homogeneous (35% H2O2 in CH2Cl2/HOAc) conditions. The effect of steric inhibition was prominent under the heterogeneous conditions as the solid oxone particles failed to penetrate into the dendrimer interior to initiate the oxidation reaction. On the other hand, dendrimers 68, 69 and 136 could be oxidized to the corresponding dibenzyl sulfones via the dibenzyl sulfoxides under homogeneous conditions. For the first oxidation to the dibenzyl sulfoxides, both compounds 68 and 69 proceeded with similar speed, indicating the similar steric environment between the innermost and intermediate layers of dibenzylsulfide moieties. On the other hand, the innermost dibenzyl sulfoxides in compound 68, in comparison to the dibenzyl sulfoxides in the middle layer in compound 69, underwent further oxidation to the dibenzyl sulfones with greater ease. This finding suggested that there were other factors, in addition to steric inhibition, in controlling the chemical reactivity of functional groups inside the dendrimer matrix. Our speculation was that the microenvironment polarity of the dendrimer interior could also play an important role in facilitating/retarding the diffusion of external chemical reagents into the interior of the dendrimer. / Two layer-block G3-CCS-dendrimer 68 and G3-CSC-dendrimer 69 with dibenzylsulfide functionalities embedded in the innermost and intermediate dendritic layers, respectively, were synthesized by a convergent method. The key steps involved in their constructions were Horner-Wadsworth-Emmons olefination and thiol-mediated alkylation reaction. A third target G3-SCC-dendrimer 70, with dibenzylsulfide moieties located in the outermost dendritic layer, could not be synthesized despite many attempts. A fourth model G3-SSS-dendrimer 136 containing 21 dibenzyl sulfone moieties in the innermost, middle and outermost layers was also prepared. All synthesized compounds were characterized by 1H and 13C nuclear magnetic resonance spectroscopy, mass spectrometry, elemental analysis and/or size exclusion chromatography. They were prepared in order to probe the feasibility of and the effect of dendritic shielding on two novel dendrimer synthesis methodologies, namely, dendrimer interior functional group conversion and dendrimer backbone rearrangement. / Cheng, Wing Sum. / Adviser: H. F. Chow. / Source: Dissertation Abstracts International, Volume: 73-01, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (leaves 117-124). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.
| Identifer | oai:union.ndltd.org:cuhk.edu.hk/oai:cuhk-dr:cuhk_344720 |
| Date | January 2010 |
| Contributors | Cheng, Wing Sum., Chinese University of Hong Kong Graduate School. Division of Chemistry. |
| Source Sets | The Chinese University of Hong Kong |
| Language | English, Chinese |
| Detected Language | English |
| Type | Text, theses |
| Format | electronic resource, microform, microfiche, 1 online resource (xvi, 124, [67] leaves : ill.) |
| Rights | Use of this resource is governed by the terms and conditions of the Creative Commons “Attribution-NonCommercial-NoDerivatives 4.0 International” License (http://creativecommons.org/licenses/by-nc-nd/4.0/) |
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