Novel p-quinone methides have been synthesized by the dealkylation of 5-(p-alkyloxyaryl)- 10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-ols and related compounds. Aspects of the dealkylation reaction were investigated using computational methods in order to identify possible intermediates and postulate reasons for the observed reactivity patterns. This included studying the effect of varying the size of the central B-ring, changing the alkyloxy group, and altering the substitution pattern on the parent alcohols. We have assessed the relative energies of reaction intermediates and have also evaluated the influence of factors such as charge delocalisation, LUMO properties of the carbocations and thermodynamic factors on the dealkylation reaction. The use of different dealkylating reagents was also briefly investigated. Demethylation of 1,3- dimethyl-11-(4-methoxyphenyl)-6,11-dihydrodibenzo[b,e]oxepin-11-ol with pyridine hydrochloride led to acid-catalyzed ring-contraction of the parent alcohol to form a novel substituted anthraquinone, 9-(4-hydroxyphenyl)-1,3-dimethyl-anthracen-10-(9H)-one, in good yield. The general reactivity of the p-quinone methides of interest to us was explored by subjecting these compounds to reaction with a range of nucleophiles (bases, Grignard reagents and alcohols). A range of aryl Grignard reagents were reacted with the p-quinone methides, with the main product isolated in almost all cases being the aryl-coupled 1,2-addition product. The nucleophilic addition reactions of alcohols were supported by a computational study and a probable reaction mechanism has been postulated. A base-catalyzed rearrangement is proposed to account for the formation of products in which dehydrogenation of the ethane bridge was observed. These studies showed that in these p-quinone methides, chemical reactivity is strongly influenced by steric crowding, resulting in reversal of the normal 1,2- vs. 1,6- selectivities expected for nucleophilic addition. The ketalization process was explored further using diols and thiols. Products analogous to those obtained with the monohydric alcohols resulted from the diols, along with a series of novel bis-ethers. A range of miscellaneous reactions of 4-(dibenzo[a,d]cycloheptan-5-ylidene)cyclohexa-2,5,- dienone and related systems were investigated. Functionalization by epoxidation, dichlorocarbenation and Diels-Alder reactions, photochemical and [2+2] cycloaddition were attempted and reduction and oxidation reactions were also explored. Photochemical demethylation of an ortho-methoxyl substituent on the p-quinone methide system was observed to occur in good yield. The p-quinone methides underwent reductive coupling in the presence of Zn/AlCl3. The electronic spectra of highly conjugated carbocations were obtained and their potential as novel dyes evaluated. A low-temperature Grignard exchange reaction followed by spontaneous cyclization upon workup, was successful in synthesizing the lactone, spiro[10,11- dihydro-5H-dibenzo[a,d]cyclohepten-(3’,4’H)-phenyl-5,2’(5’H)-furan-5’-one], in one step from the starting ketone. A novel seven-membered Malachite Green dye analogue, 11-(4- dimethylamino-phenyl)-3-morpholin-4-yl-6,11-dihydro-dibenzo[b,e]oxepin-11-ol, was also synthesized and its electronic spectra compared to that of the unannulated Malachite Green dye series. All novel compounds synthesized were characterized using NMR, IR and HRMS-analysis.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nmmu/vital:10408 |
| Date | January 2007 |
| Creators | Taljaard, Jana Heloïse |
| Publisher | Nelson Mandela Metropolitan University, Faculty of Science |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Doctoral, PhD |
| Format | xxi, 229 leaves, pdf |
| Rights | Nelson Mandela Metropolitan University |
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