Sulfur-Bridged pen-Naphthalenes: Synthesis, Conformational Analysis, and Photoelectron Spectroscopy of the Mono-, Di-, and Trisulfides of 1,8-Dimethylnaphthalene
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The sulfur-bridged 1,8-peri-naphthalenes 3H,7H-naphtho[1,8-de]-1,2-dithiepin (8) and 4H,8H-naphtho[1,8-ef]-1,2,3-trithiocin (9) have been prepared. Their variable-temperature 1H NMR spectra reveal the presence of conformations undergoing rapid equilibration. For the disulfide (8) line-shape analysis yields a single interconversion with ∆H* = 9.3 kcal/mol and ∆S* = -1.2 cal/(mol K). Two processes are resolvable for the trisulfide (9), a relatively rapid one with ∆H* = 15.4 kcal/mol and ∆S* = -1.1 cal/(mol K) and a slower exchange with ∆G* = 17.9 kcal/mol. Interpretation of the DNMR data was assisted by molecular mechanics calculations and photoelectron spectroscopic measurements. It is concluded that the disulfide (8) exists as an unsymmetrical twist-boat conformer with an S-S dihedral angle of ca. 40°. On the NMR time scale, racemization occurs via a boat conformation (Cs symmetry) with a predicted barrier of ∆E = 7.5 kcal/mol. The calculations suggest, however, an accompanying but lower energy transformation (∆E = 5.4 kcal/mol) which interconverts superimposable enantiomers by way of a twist transition state of C2 symmetry. For the trisulfide (9) the force field calculations indicate the boat structure to be 0.6 kcal/mol more stable than the chair conformation. Boat-boat interchange is suggested to take place through an intermediate twist form (∆E = 13.6 kcal/mol) and is lower in energy than the corresponding boat-chair equilibration.
|Journal||Journal of the American Chemical Society|
|Number of pages||10|
|Publication status||Published - 1 Jan 1981|