The X-ray Structures of 2- and 3-Sulfolene and Two Halogenated Derivatives

The structures of the isomeric 2,5-dihydrothiophene 1,1-dioxide [orthorhombic, a = 11.340(2), b = 7.0887(15), c = 6.2811(13) Å, space group Pnma] and 2,3-dihydrothiophene 1,1-dioxide [orthorhombic, a = 6.3903(13), b = 7.2783(16), c = 11.075(2) Å, space group Pnma] have been determined and show perfectly planar rings with the expected bond lengths and angles. In contrast, the halogenated derivatives 3,3,4,4-tetrachlorotetrahydrothiophene 1,1-dioxide [monoclinic, a = 11.8716(8), b = 6.5579(4), c = 11.4802(8) Å, β = 97.705(17), space group P21/c] and 2,3-dibromotetrahydrothiophene 1,1-dioxide [orthorhombic, a = 5.2502(3), b = 11.3561(6), c = 24.9802(17) Å, space group Pbca] both show twisted conformations. The degree of planarity is compared with that in the structures of comparable 5-membered ring cyclic sulfones and C–H…O hydrogen bonding patterns are discussed for all four structures. The two isomeric sulfolenes are perfectly planar while tetrachloro- and dibromo-derivatives adopt twisted structures.


Experimental
Compound 1 was obtained commercially and converted into the isomer 2 by base-induced isomerisation using the published method [6]. Photochemical chlorination of 1 gave 3 [7,8] while addition of bromine to 2 gave the trans-dibromide 4 [9,10]. All four compounds had melting points and spectroscopic data in agreement with published values.

Results and Discussion
The structures of 3-sulfolene 1 and 2-sulfolene 2 are shown in Fig. 2 and both are perfectly planar with equivalent oxygen atoms located equidistant above and below the plane containing the ring atoms. The bonds lengths and angles (Table 2) are as expected and in good agreement with those observed for acyclic sulfones [1] and the average bond lengths of 1.786 Å for C-S and 1.436 Å for S=O for sulfones in general [12]. The structure of 1 was in fact investigated at a very early stage (Ref Code ZZZGBM) [13], but the methods of that time only allowed some rough information on the unit cell dimensions to be obtained. There is a more recent structure determination, which for some reason is not included in the CSD, that gives a very similar result to ours (space group Pnma, a = 11.484, b = 7.262, c = 6.316Å) [14]. Whilst wishing to give full credit to this earlier determination, we feel it is important to finally document the structure of this fundamental compound in the CSD.
For comparison a range of simple analogues of 1 (Fig. 3) [15][16][17][18][19][20][21][22] and 2 ( Fig. 4) [15,[23][24][25][26] that have been crystallographically characterised are shown with CSD reference codes, literature references and, as a measure of planarity, the sum of the five in-ring torsion angles. As compared to the structures of 1 and 2 which have all torsion angles zero, we can see that introducing a substituent has a range of effects on the degree of planarity from no effect (IPRNSO) to small (GAMKEK), moderate (BAHQEG, MIXYUN, XOJFUX, WASBOI) and fairly large (BAHQIK, VUFKIS, XUTVUF, VAGXAC). However the presence of a ring double bond in all these compounds limits the possible degree of non-planarity. As might be expected, coordination of the oxygen of 1 to MoCl 5 results in significant lengthening of that S-O bond and movement of sulfur out of the plane of the ring carbons [22].
The structures of both 1 and 2 show a range of weak C-H…O hydrogen bonds and these are listed in Table 3. As shown in Fig. 5, the different position of the double bond between 1 and 2 leads to different higher level motifs with 1 forming R 2 2 (8) dimers while 2 displays R 2 1 (4) interactions. The structures of halogenated derivatives 3 and 4 are shown in Fig. 6 and, in contrast to those of 1 and 2, these are significantly twisted with C(3) 0.432 Å above and C(4) 0.261 Å below the plane defined by S(1), C(2) and C(5) in 3. In the case of 4 there is again a twisted conformation with C(3) 0.489 Å above and C(4) 0.249 Å below the plane defined by S(1), C(2) and C(5), although in this case there is a small torsion angle of − 9.6(2) for C(2)-S(1)-C(5)-C(4) and it is perhaps more accurate to describe it as an envelope conformation with C(3) at the flap. With no double bond present, the deviation from planarity can be much larger and the sum of in-ring torsion angles is 149.07° for 3 and 160.96° for 4. These values can be compared to the four values between 172° and 190° that occur in the structure of ZUFWIG (Fig. 7), the isomer of 4.
The parent tetrahydrothiophene dioxide (Ref. Code BUGHOA), although solid at room temperature, forms a plastic phase with disorder from which no detailed information on the conformation can be gained [27], however the isomeric trans-3,4-dibromodihydrothiophene dioxide (Ref Code ZUFWIG) obtained by reaction of 1 with bromine has been found to exhibit an unusual form of disorder in the crystal with the large bromine atoms and the CH 2 SO 2 CH 2 group remaining relatively fixed but the two CH(-Br) centres occupying positions above and below the mean molecular plane to give two alternative forms which pack at random (Fig. 7) [28]. No such problems occur in the less symmetrical isomer 4. The structures of 3 and 4 also contain a series of weak hydrogen bonding interactions ( Fig. 8; Table 3

Conclusion
The crystal and molecular structures of the isomeric 2,5-and 2,3-dihydrothiophene 1,1-dioxides 1 and 2 have been fully documented for the first time and show similar perfectly planar rings with the expected bond lengths and angles. In contrast the 3,3,4,4-tetrachloro-and 2,3-dibromotetrahydrothiophene 1,1-dioxides 3 and 4 are distinctly non-planar with no sign of the disorder that occurs in the previously determined structure of the 3,4-dibromo isomer of 4.  (3)

Conflict of interest
The authors declare no conflict of interest.
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