Adamantane and related ring systems

Adamantane and related ring systems
The thesis is an account of
(a) the synthesis of derivatives of adamantane and an exploratory investigation of the chemistry of some of these compounds.
(b) their use as models for correlating structure and nuclear magnetic resonance (NMR) chemical shifts and for determining values of coupling constants needed when estimating conformational equilibria in certain acylic compounds, and
(c) their use in determining the magnitude and sign of the electric dipole moment of a C-methyl group relative to a C-H group in a saturated system.
Section (a) was described in four parts. Firstly, the oxidation of adamantane and its bridgehead methyl derivatives was studied. Adamantane (1), 1,3-dimethyl-(2), and 1,3,5-trimethyladamantane (3) were oxidised with concentrated sulphuric acid. Adamantane gave 2- adamantanone (4), but no products were isolated from (2) and (3). The monohydroxy bridgehead derivatives of (2) and (3), however, gave the 6-oxo- and the 4-oxo-derivatives, respectively, after treatment with sulphuric acid.
Ozonolysis of (2) gave the 2-oxo-, 4-oxo-, and 5-hydroxy derivatives, and of (3) gave the 4-oxo and 7-hydroxy derivatives. Aerial oxidation gave the highest yields of monoketones, and the 2-oxo-, 4-oxo-, 6-oxo- (total 11%), and 5-hydroxy derivatives of (2), and the 2-oxo-, 4-oxo- (total 10%), and 7-hydroxy derivatives of (3) were obtained. Separation of these ketones was achieved by alumina chromatography, and their structures were established from studies of their infrared, mass and NMR spectra. The oxidation of (3) with chromic acid produced mainly the 7-hydroxy derivative and its acetate.
1,3,5,7-Tetramethyladamantane (5) gave the 2-acetoxy-(6) 2-oxo-(7), and 2,4-dioxo-(8) (but not the 2,6-dioxo-(9)), derivatives with chromic acid. Oxidation of the ketone (7) with chromic acid gave the 2,4-dione (8) and 4-oxa-1,3,6,8-tetramethylhomoadamantan-5-one (10), whereas the acetate (6) gave material, which was hydrolysed and oxidised to give the 2,6-dione (9) among other products. The ketone (7) and the three possible monoacetoxyketones, (11) (12) and (13), were identified in the product of oxidation of the acetate (6).
Secondly, some derivatives of adamantane (1) were prepared to provide compounds suitable for the studies described in sections (b) and (c). Adamantanone (4) was converted via 2-methyleneadamantane (14) into the epoxide (15), and thence into the 2-aldehyde (16). A previously described preparation of (14) gave mainly a dimer. The 1,3-dioxane (17) was made from the 2-aldehyde [see section (b)]. Reduction of the ketone (4) gave 2-adamantanol, which was converted via the tosylate into diethyl 2-adamantylmalonate (18), which was reduced to the propan-1,3-diol (19). The 1,3-dioxane (20) was prepared from the diol (19) with paraformaldehyde. Treatment of the ketone (4) in methanol with an excess of diazomethane gave almost exclusively 4-homoadamantanone (21) with traces of higher homologues, which were formed in much larger amounts when the reaction was catalysed by aluminium chloride in ether. The 1,3-dibromide of (1) was converted into the 1,3-dicarboxylic acid, which was used to make the 1,3-dinitrile [see section (c)].
Thirdly, some bridgehead derivatives of (3) were prepared providing further examples of the correlation between substituents and changes in chemical shift. The hydrocarbon (3) was used for the synthesis of the 7-bromide, the 7-acetate, the 7-carboxylic acid and its methyl ester, and the 7-nitrile [see section (c)].
Fourthly, exploratory studies of the chemistry of the 2-oxo-(7) and 2,4-dioxo-(8) derivatives of (5) were made. The monoketone (7) was reduced to the alcohol (22), which was converted into the 2-acetate (6), the 2-bromide, the 2-carboxylic acid and its methyl ester (23), the 2-methyl ketone and the 2-nitrile. The ketone (7) was converted via the 2-methylene derivative (24) into the epoxide, and thence to the 2-aldehyde (25), from which the 0-methyl oxime was made [see section (b)] . The methyl ester (23) was reduced to the primary alcohol (26), and the solvolysis of its tosylate gave the olefin (24), but not homoadamantanoid derivatives. The ketone (7) was treated with diazomethane, and the product mixture was analysed by gas liquid chromatography. 1,3,6,8-Tetramethylhomoadamantan-4-one (27) was isolated and a bishomologue (28) was detected.
The 2,4-dione (8) was treated with (i) methyl magnesium iodide giving syn-cis-2,4-dihydroxy-1,2,3,4,5,7-hexamethyladamantane (29), (ii) phenyl lithium giving syn-cis-2,4-dihydroxy- 2,4-diphenyl-1,3,5,7-tetramethyladamantane (30) and 4-axial-hydroxy-4-phenyl-1,3,5,7-tetramethyladamantan-2- one (31), and (iii) lithium aluminium hydride giving a mixture of isomeric diols, which was carbonylated with formic acid and converted into the lactone (32), and into an acid or acids which by treatment with acetic anhydride gave the cyclic anhydride (33) in low yield. The hydroxy-ketone (31) was reduced to syn-cis-2,4-dihydroxy-2-phenyl-1,3,5,7- tetrarnethyladamantane (34), which had a most unusual NMR spectrum, which included one hydroxyl singlet resonance, and another hydroxyl signal coupled to the secondary alcohol proton in a well resolved AB quartet. Very large intra-molecular hydrogen-bonded spectral shifts were observed for the diols, (29), (30) and (34), the value for (30) being 172 cm-1.
Section (b) was described in 3 parts. Firstly, derivatives of adamantane were used as models for correlating structure and NMR chemical shifts. NMR spectra were run on hydrocarbons (2) (3) and (5), and on all their monoketone derivatives,in carbon tetrachloride and tetramethylsilane as solvent. Assignments of protons were mostly straightforward, and in other cases, solvent shifts in benzene were used. The effects on the chemical shifts of other protons in the molecule of changing a methylene group to a carbonyl group were tabulated and compared with the chemical shift differences calculated due to magnetic anisotropy using a modified form of McConnell's equation. The values of carbonyl anisotropies used had been derived from a series of steroids and the calculated shifts were found to be much smaller than those measured experimentally for the adamantanes, which was explained by the flattened geometry of the steroids leading to overestimates in the 'geometric factor', and consequently underestimates in the anisotropies derived. More importantly, however, it was found the effect on the chemical shift of a C-H bond caused by changing a C-H bond or bonds by other groups (e.g. carbonyl or bromide) at a point distant from the first C-H bond is also dependent on the environment of that C-H bond.
Secondly, NMR chemical shifts were used to determine the conformation of the carbonyl group in 1,3,5,7-tetramethyladamantane- 2-carboxylic acid and related compounds. In all cases except the 2-aldehyde (25), it was found that the carbonyl group pointed into the ring. The reverse conformation was predominant for the aldehyde (25).
trans can be estimated than has been generally possible with the systems studied previously, which should lead to more confident predictions about conformation equilibria in such systems.
Section (c) described the use of adamantane derivatives in determining the magnitude and sign of the electric dipole movement of a C-CH3 group relative to a C-H group in a saturated system. Previous studies, which were not unambiguous, had suggested that a methyl group is electron-withdrawing relative to hydrogen when attached to (sp3) carbon. The dipole moments of two pairs of bromides, nitriles, and monoketones were measured. The structure of the molecules in each pair was identical except for the substitution in one at bridgehead positions of two or three methyl groups. The moments induced in the methyl groups by the polar group or groups were taken into account giving a 'corrected' dipole moment for the methyl substituted compounds. A comparison between the 'corrected' moments and the dipole moments of the non-methyl analogues led directly to values of the dipole moment of a methyl group relative to hydrogen in a saturated system. The average value obtained was 0.082±0.02 D, the negative end of the dipole being on the methyl group.