4/7-1,3-Dithiin, conformations

The results are critically dependent on the level of theory. However, a stepwise mechanism with closed shell structures along the reaction path was found to be lower in energy than a concerted reaction. An all-cw conformer of 172 is reported to be a transition state rather than an intermediate. Similarities of the conformational isomers of the intermediate 2-butenedithial 172 with the dinitrosoethylenes discussed in Section IV,c are evident. 3,6-Diamino-substituted dithiins are predicted to be more stable in the open-chain bisthioamide structure [95JST51]. The... 

Tricyclic dithiine derivatives of tetrathiafulvalene (TTF) 32 have been prepared for their increased electropolymerization potential <2000CC1005>. The effect of different tricyclic heterocycles upon the redox properties of TTF analogues was explored, and showed that the furan derivative 33 had an unusual nonplanar conformation that allowed for the attainment of higher oxidation states at relatively low oxidation potentials <2004JMC2822>. 

In the synthesis of l,3-dithiole-2-thione derivatives as intermediates for electropolymerization precursors, the bicyclic 462 was found to be inert to normal cyclization conditions <1999JOC6418>. This is believed to be due to steric hindrance, from the boat conformation of the dithiin ring. Cyclization was achieved, albeit in only moderate yields, by heating with P2S3 to give the thiophene 463a or HBr/AcOH for the furan 463b (Equation 125). 

Henning et al. (2006) used spectroelectrochemical and DFT methods to follow conformational transition of 3,6-diphenyl-l,2-dithiin by one-electron oxidation. The primary cation-radical is flattened partially. This cation-radical was fixed at 223 K. Heating up to 293 K provided this cation-radical with an additional energy. It resulted in the formation of an entirely planar structure with complete spin delocalization within the molecular framework. The transition is depicted in Scheme 6.22. 

As in the case of the 1,2-dioxins, the 1,2-dithiins exist in various states of saturation, oxidation, and benzoannelation (cf. Scheme 1, 17-27) and they have been studied in detail both theoretically and experimentally. Not only were the conformations of the ring and attached substituents investigated, but the valence isomerism of 1,2-dithiin by both NMR and high-level ab initio molecular orbital (MO) calculations and the dithiol/disulfide equilibrium by MP2 calculations were also examined. The latter equilibrium has been applied successfully as a luminescent molecular switch (cf. Section 8.10.2.1). Finally, as a very interesting 1,2-dithiin derivative, the synthesis, structure, and reactivity of the (-l-)-camphor-derived analog 25 and its sulfoxide 26 and sulfone 27 have been reported. Both the synthesis and the antimalarial activity of the 2,3-dioxabicyclo[3.3.1]nonane pharmacophore 28, which contains the 1,2-dioxane moiety, have been reviewed recently <2006BML2991>. 

The rotational spectrum of 1,2-dithiin was measured using a pulsed-beam microwave spectrometer in the 8-18 GHz range <1996JSP(180)139> by Stark effect measurements, the electric dipole moment was also determined (/ta = 1.85 D). The molecule proved to be of C2 symmetry with a twisted conformation about the S-S bond and a C-S-S-C dihedral angle of 53.9... 

At the MP2 level of theory, the half-chair conformer of 4//-l,3-dithiin 55 is 2.0kcalmoP more stable than the boat conformer (Equation 4) <1998JCC1064> and has a calculated twist angle of 33.2° the relative stability of the halfchair conformer was attributed to absent lone-pair-lone-pair repulsions and a decrease of torsional strain owing to an... 

A Newman-Kwart rearrangement features in a synthesis of 3,8-diiododibenzo-l,2-dithiin which possesses a screw-boat conformation. The substituents allow elaboration of the molecule, though preferably prior to formation of the thiin ring <06TL9135>. 

The central 1,4-dithiin ring in 33 adopts a boat conformation, as is the case for TH, with a 48° fold along the S-S vector in the solid state as shown by X-ray crystallographic analysis [92]. Both 33 and 34 undergo reversible one-electron oxidation to the corresponding radical cation with E1/2=0.93 and 0.77 V, respectively, vs Ag/AgCl as shown by cyclic voltammetry in acetonitrile [92]. There is also an irreversible second oxidation for both compounds at E1/2 ca. 1.5 V. 

Compound 37a and its derivatives adopt a boat conformation in the solid state as revealed by X-ray crystallographic structure studies [98-100]. However, the 1,4-dithiin ring in 38 is planar in its complex with acridine [101]. This facilitates 7T-7T stacking of 38 and acridine although their interaction is weak. Furthermore, calculations suggest that there is little energy difference between planar and boat conformers [102]. Thus conformational analysis of 1,4-dithiin is similar to that of thianthrene. 

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