The use of 10b, 10c-dimethyldihydropyrene (DHP) as a probe to study the Mills-Nixon effect




Lau, Yee-Kwan (Danny)

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The syntheses of many cycloalkene and cycloalkeneone annelated DHPs have been achieved. A late ring formation approach was used to synthesize the dicycloalkeneone annelated DHP 63 (Scheme 4) and the unsymmetrical dicycloalkeneone annelated DHPs 70a and 70b (Scheme 5). To synthesize the other ring annelated DHPs, a more versatile early ring approach was employed. Through an asymmetrical coupling followed by a series of standard transformations, the cyclopentene-, cyclohexene-, cyclopentenone- and cyclohexenone-annelated DHPs 64 , 118 , 130 , 131 were synthesized (Schemes 13 and 15). Similarily, the dicycloalkene and dicycloalkenone annelated DHPs such as 41 , 42, 123, 124 and 136 were obtained by a symmetrical coupling followed by a series of standard transformations (Schemes 12, 14 and 16). Other than the cycloalkene or cycloalkenone annelated DHPs, the acyclic tetra-substituted DHPs 142 and 143 (Scheme 17) were also synthesized as model compounds. As well, the asymmetrical DHPs 148 and 149 (Scheme 18), having a benzene and a cyclopentenone annelation or with a benzene and a cyclopentenone annelation, were also synthesized to test the annelation effect with a combination of a benzene and a five-membered ring. In the cycloalkene and cycloalkenone annelated DHP series, it was demonstrated that the [pi]-bond fixation effect could be indirectly probed by the internal methyl proton chemical shifts. These are based on the ring current of DHP and the magnitude of bond fixation depends on the annelating ring size, the coplanarity of the carbonyl group with the [pi]-system of DHP (for the cycloalkenone annelated DHPs) and the relative arrangement of the annelated rings (cisoid versus transoid for the diannelated compounds). Thus, when the ring size varied from four to seven in the cycloalkene- and dicycloalkenene annelated DHP series, the cyclohexene ring has the strongest bond fixation effect. When the ring size varied from five to seven in the cycloalkenone- and the dicycloalkenone-annelated DHP series, the cyclopentenone annelated DHPs have the strongest bond fixation effect. In the mono-cycloalkenone annelated DHP series (ring size =5 to 7), the Kekulé structures of the cycloalkenone annelated DHPs were determined by the vicinal coupling constant (3Jhh) to adopt an endocyclic structure (the double bond appears at the ring junction between the DHP and the annelating ring). For the diannelated DHP derivatives, the cisoid arrangement of ring annelation always has a stronger bond fixation effect compared to that of a transoid arrangement in almost all cases. In this thesis work, the use of DHP as a sensitive NMR probe was successfully demonstrated in that its internal methyl proton chemical shift responds to a change of ring current caused by different ring annelations. It is so sensitive, that even the very small perturbation on ring annelation (by cycloalkanes) can be sensed. DHP is a better NMR probe molecule than benzene because the chemical shifts of the internal methyl protons of DHP are less seriously affected by any effects such as geometrical distortion, rehybridization, steric compression, hyperconjugation and through space effects as probed by the ¹H and ¹³C-NMR spectroscopies in benzocycloalkenes. The ring annelation effect probed by DHP is closer to a pure [pi]-effect due to a change in ring current which is different from benzene, which is a mixture of both α- and [pi]-effects...



Ring formation (Chemistry), Organic compounds