Part 1. Some aspects of NMR spectroscopy: A review of coupling mechanisms is given and notable examples of long range coupling in the literature are discussed. Methyl protons can undergo long range coupling with protons which are 4s bonds away, and the mechanism of this process as related to the known examples is discussed. The effects of electronegative substituents on long range coupling is also outlined. A Discussion of the present work follows, wherein the description of the preparation of many bornane derivatives, including several deuterated derivatives, is presented. The line widths (Wh/2) of the methyl groups were measured. It was found that the width of the methyl groups decreased in the order C8>C9>C10, and the C8 and C9 methyl signals were always appreciably broader than the C10. Long range coupling between the protons of the C8 and C9 methyl groups is postulated as being responsible for those two being broader than C10. Long range coupling between the C8 methyl protons and the C5 and C6 endo protons, and between the C9 methyl and the C3 endo proton is tentatively ascribed to the differences in the widths of the C8 and C9 methyl groups. Although the line widths of the tertiary methyls were found to vary with solvent, the relative widths of the three methyl signals of the bornane derivatives was found to be the same. Thus measurement of line widths is a method of correlating the methyl signals in spectra run in different solvents. The effects on the chemical shifts of the tertiary methyl groups of bornanes when various substituents are placed in the basic skeleton, is also described. The presence of an ethylenedioxy group at C2 or C3 results in a desheilding of C8 and a shielding of C9, while introduction of hydroxyl groups has no uniform effect because of different orientations of the hydroxyl group from compound to compound. An introduction to the mechanism of benzene induced solvent shifts is given. Benzene appears to form a stereospecific complex with ketones, and other molecules, which can undergo a dipole-induced dipole-interaction. The effects of this interaction on the n.m.r. spectra of various ketones, quinones and aromatic compounds is described. The present work describes the measurement of solvent shifts at various concentrations of ketones and ketals in benzene. Previous similar work is discussed in the light of the results obtained, and it has been shown that some assumptions previously used in correlating the data with the interactions, have been unjustified. The solvent shifts versus the concentration have been plotted to give curves which have been interpreted as indicating specific solvent-solute interactions, i.e. 1:1, 1:2, etc., and not random associations. This has been shown by comparing calculated curves with the experimental results, with which agreement is good. The third part of this section of the work deals with the spectra of some hydroxymethylene ketones, and commences with an introduction to the general behaviour of 1,3-diketones in solution. The rates of exchange of enolizable protons is discussed, and their use in determining the relative proportions of enol forms is outlined. The present work describes the behaviour of 3-formylbornan-2-one in carbontetrachloride-D2O solution, and an explanation for the preference for one enol form is given. Also discussed is the coupling between ‘formyl’ and hydroxyl protons in hydroxymethylene ketones, and it is shown that the coupling depends strongly on the H-O-C-H angle, which can be altered by slight changes in molecular geometry, thus leading to changes in coupling constants. No change in coupling constants caused by temperature differences was observed, and it is speculated that exchange processes do not affect the value of the coupling constant, but only whether coupling occurs. Part 2: Acid catalysed rearrangements in cyclic systems A comprehensive review of the dienone-phenol rearrangement is given and it is shown that any cross-conjugated cyclohexadienone will rearrange to an aromatic product, the substitution of which depends strongly on (i) the substitution pattern of the original dienone, (ii) peculiarities in the molecular structure, and (iii) the rearrangement conditions. Rearrangements of some dienols and some cross-conjugated methylenecyclohexadienes or ‘semibenzenes’ are described, and in general these follow the pattern of the dienone-phenol rearrangement. Rearrangement of some steroidal α-substituted ketones is also mentioned briefly. Two cross-conjugated methylenecyclohexadienes were synthesised by reaction of methylmagnesium iodide on the corresponding dienones – this reaction had previously been reported to give only the products of rearrangement of the intermediate semibenzenes. The semibenzenes were rearranged using perchloric acid in acetic anhydride-ethyl acetate to give the 1,4-disubstituted aromatic product. The attacking species was shown to be a proton, and the reaction, unlike the dienone-phenol rearrangement, showed a clear preference for methylene rather than methyl migration. Several methyl substituted steroidal 2α-hydroxy-4-en-3-ones were prepared by the rearrangement of the corresponding 4,5-epoxy-3-ones with sulphuric acid in aqueous acetone, herein shown to be a general method. The hydroxyenones were rearranged using three sets of conditions: Toluene-p-sulphonic acid in methanol, toluene-p-sulphonic acid in benzene, and perchloric acid in acetic anhydride-ethyl acetate. Only one compound, 2α-hydroxycholest-4-en-3-one gave an aromatic product; the other substrates underwent rearrangement through the various enols, to give attack usually at C6 of the carbon skeleton. A mechanism is given and discussed for each rearrangement. No general pattern seemed to emerge. Part 3: The Clemmensen reduction of 1,3-diketones A review of the Clemmensen reduction is given with a specific reference to reduction of difunctional compounds, where rearrangements may occur. The present work shows that in the Clemmensen reduction of 1,3-diketones, the intermediate cyclopropanediol which is first formed, undergoes acidolysis to give an α-hydroxy ketone and a β-hydroxy ketone, each of which may then undergo further Clemmensen reduction to give the observed rearranged and unrearranged monoketones. The intermediacy of a β-hydroxy ketone was proved by observing the rate of formation of the products, and by measuring the rate of reduction of an independently synthesised α-hydroxy ketone. Reduction of two acyclic ketones was shown to give a ring expanded product only in the case of 2-acetylcyclopentanone. Part 4: Intramolecular hydrogen abstraction reactions from aromatic alkyl groups A short resume of intramolecular hydrogen reactions using lead tetraacetate or the hypoiodite process is given, with some detail being applied to the conditions, and the stereochemistry needed for the reaction, and to fragmentation processes which may occur. Brief mention is also made of the new reaction; hydrogen abstraction using decomposition of a hypobromite with silver ions. The lead tetraacetate reaction was applied without success to abstraction of hydrogen from an aromatic methyl group, utilizing an α-benzylic hydroxyl group. Fragmentation reactions were generally obtained. Application of the bromine-silver ion method gave good yields of cyclic ethers in two cases, indicating that an abstraction reaction had taken place. The conditions for such a reaction seem to be critical: A tertiary hydroxyl group is required, and the abstraction seems only to take place from a primary centre.
| Identifer | oai:union.ndltd.org:ADTP/247090 |
| Date | January 1968 |
| Creators | Baker, Kenneth Matthew |
| Publisher | ResearchSpace@Auckland |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated., http://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm, Copyright: The author |
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