Synthesis and Diatropicity of trans- 10b-Methyl-10c-undecyl- and trans-10b-Methyl-10c-pentadecyl-10b,10c-dihydropyrene. An Empirical Approach to a Semiquantitative Assessment of the Diamagnetic Ring Current in a [14]Annulene

Abstract

trans-10b-Methyl-10c-undecyl- and trans-10b-methyl-10c-pentadecyl-10b,10c-dihydropyrene, 3b and 3c, were obtained from the valence isomerization of their corresponding [2.2]cyclophanedienes 28a and 28b. The long internal alkyl chain in each case was previously introduced via a Wittig reaction followed by reduction. The cyclophanedienes were synthesized from their corresponding dithia[3.3]cyclophanes via a Wittig rearrangement-Hofmann elimination sequence. Proton chemical shifts of H1′ to H9′ of the alkyl chains in 3b and 3c, respectively, are identical, suggesting that the C1′ -C9′ section of the long alkyl chain in 3b or 3c has similar averaged conformational movement. Application of the Johnson-Bovey (J-B) model to 3b does not give a satisfactory correlation between the proton ring current shift and the out-of-plane distance. A direct empirical plot of the out-of-plane distance, D, against the proton ring current shifts, Δδ, however, gives a curve similar to that obtained from the J-B model of benzene. This curve in fact corresponds to a linear relationship between D and log |Δδ|. Calculated ring current shifts of several derivatives of 10b,10cdihydropyrene based on the above empirical correlation agree well with the reported experimental values. Our model shows that the experimental detectable limit for the diatropicity of 10b,10cdihydropyrene is about 14 Åfrom the mean molecular plane. A similar empirical treatment based on the carbon chemical shifts of the n-undecyl chain in 3b gives unsatisfactory results. Our model does not support the prediction that the magnitude of ring current effect on proton and carbon chemical shifts is essentially the same at the sample position in space relative to the mean plane of delocalization of an aromatic π-electron cloud.