Chemo- and regioselective [3 + 2]-cycloadditions of thiocarbonyl ylides: crystal structures of trans-8-benzoyl-1,1,3,3-tetramethyl-7-trifluoromethyl-5-thiaspiro[3.4]octan-2-one and trans-3-benzoyl-2,2-diphenyl-4-(trifluoromethyl)tetrahydrothiophene

The title compounds were prepared via chemo- and regioselective [3 + 2]-cycloadditions. The thiophene ring in each crystal structure has an envelope conformation. The largest differences between the two molecular structures is in the bond lengths about the quaternary C atom of the thiophene ring. In the spirocyclic structure, the C—C bonds to the spiro C atom in the cyclobutane ring are around 1.60 Å and weak intermolecular C—H⋯X (X = S, O) interactions link the molecules into extended ribbons. In the other structure, weak C—H⋯π interactions link the molecules into sheets.


Structural commentary
Compounds 1a and 1b crystallized as racemates with the benzoyl and trifluoromethyl substituents on the thiophene ring in a trans configuration ( Figs. 1 and 2). The thiophene ring in each case has an envelope conformation with the sulfur atom as the envelope flap. For 1a, the ring puckering parameters (Cremer & Pople, 1975) for the atom sequence S1,C2-C5 are Q(2) = 0.5164 (14) Å , '(2) = 359.73 (18) and atom S1 is 0.853 (1) Å from the mean plane through the other four ring atoms. The corresponding puckering parameters for 1b are Q(2) = 0.5714 (16) Å , '(2) = 349.86 (19) with atom S1 being 0.921 (1) Å from the mean plane through the other four ring atoms. These parameters show that the thiophene ring is slightly more distorted from an ideal envelope conformation in 1b than in 1a.
The most significant differences in the bond lengths within the two molecules appears at the spiro C atom, C2 (Table 1). The C2-C13 and C2-C14 bonds involving the cyclobutane ring in 1a, at around 1.60 Å , are significantly longer than is usual for an alkyl C-C bond and 0.058 (3)  View of the molecule of 1a showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are represented by circles of arbitrary size.

Figure 2
View of the molecule of 1b showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are represented by circles of arbitrary size. phenyl rings in 1b. In concert, the S1-C2 and C2-C3 bonds are around 0.034 (2) Å shorter and the C3-C4 bond 0.018 (3) Å longer in 1a than in 1b; all other related bond lengths in the two molecules are comparable. Despite these variations and the acute 'bite angle' of the cyclobutane ring at C2 of the thiophene ring [89.45 (12) compared with 110.44 (14) for the diphenyl-substituted 1b], the intra-ring bond angles in the thiophene rings of the two compounds are not very different. The above-mentioned differences in ring puckering presumably allow the bond-length variations not to impinge on the intra-ring angles. The Cambridge Structural Database (CSD, Version 5.39 with August 2018 updates; Groom et al., 2016) contains one other example of a 2-cyclobutane-substituted thiophene ring (Seyfried et al., 2006) and six examples of a 2,2-diphenyl-substituted thiophene ring (Huisgen et al., 1986;Seyfried et al., 2006;Augustin et al., 2017). These seven structures display exactly the same relative patterns of bond lengths as that described above.
The carbonyl group in 1b is significantly twisted out of the plane of the benzoyl ring, with the O1-C6-C7-C8 torsion angle being À9.1 (2) and À29.5 (3) in 1a and 1b, respectively. The O1-C6-C3-C4 torsion angles also differ by about 41 , so that the carbonyl group is more slanted with respect to the mean plane of the thiophene ring in 1b than in 1a.

Supramolecular features
In 1a, there are three unique potentially significant weak supramolecular contacts ( Table 2). One of the methylene H atoms at C5 interacts with the carbonyl O atom of a neighbouring molecule related by a centre of inversion, while the methine H atom at the CF 3 -substituted C4 of this second molecule interacts with the S atom of the first molecule, thus forming centrosymmetric molecular pairs with a total of four interactions between them. Graph-set motifs (Bernstein et al., 1995) C 2 2 (8) (two different ones), C 2 2 (9) and C 2 2 (12) can be discerned here. The third interaction is a C-HÁ Á ÁS interaction between the para-H atom at C10 of the benzoyl ring and the S atom of a molecule related by one unit-cell translation parallel to the [001] direction. This forms a chain of molecules with a graph-set descriptor of C(9). The combination of these interactions leads to double-stranded chains of molecules, or ribbons, running parallel to the [001] direction (Fig. 3). Within these ribbons, there is also a potentialinteraction between adjacent parallel benzoyl rings, where the centroidcentroid distance is 3.8740 (10) Å and the perpendicular distance between the ring planes is 3.4342 (7) Å , although the offset of the rings is rather large at 1.79 Å , so that the separation may be a fortuitous consequence of the alignment resulting from the other interactions.

Figure 3
The ribbons formed by the weak intermolecular C-HÁ Á ÁX (X = S, O) interactions in 1a. Most H atoms have been omitted for clarity. Table 3 Weak C-HÁ Á Á interactions (Å , ) found in 1b.
with the benzoyl ring of a neighbouring molecule related by a glide plane to give chains of molecules parallel to the [001] direction, while one of the methylene H atoms at C5 interacts with one of the phenyl rings in the molecule related by one unit cell translation parallel to the [100] direction. Together, these interactions link the molecules into sheets which lie parallel to the (010) plane (Fig. 4). Within these sheets, there is a potential intermolecular C-HÁ Á ÁF interaction involving another phenyl ring H atom (C15-H15Á Á ÁF3 ii ), albeit with a rather sharp C-HÁ Á ÁF angle of 121 [H15Á Á ÁF3 ii = 2.53 Å , C15Á Á ÁF3 ii = 3.132 (2) Å ; symmetry code as in Table 4].

Database survey
The CSD contains crystal structure data with atomic coordinates for 3225 monomeric organic compounds with the string thiophene in the compound name, of which 70 are named as

Synthesis and Crystallization
The title compounds were prepared according to the reaction sequence presented in Fig. 5 and fully described with full spectroscopic data by Mlostoń et al. (2016). Thermal decomposition of 1,3,4-thiadiazolines 2a and 2b in THF solution in the presence of (E)-4,4,4-trifluoro-1-phenylbut-2-en-1-one (3) leads to the tetrahydrothiophenes 1a and 1b, respectively, as the product of the The sheets formed by the weak intermolecular C-HÁ Á Á interactions in 1b. The relevant centroids are shown as red spheres. Most H atoms have been omitted for clarity. tetramethylcyclobutanone, decomposes at 318 K, the less stable precursor 2b, derived from thiobenzophenone, already extrudes N 2 at 228 K. The 1 H NMR analysis showed that only one product was formed in each case. After chromatographic purification, the isolated products were crystallized from petroleum ether by slow evaporation of the solvent.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The methyl H atoms were constrained to an ideal geometry (C-H = 0.98 Å ) with U iso (H) = 1.5U eq (C) while each group was allowed to rotate freely about its parent C-C bond. All other H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms with C-H distances in the range 0.95-1.00 Å and U iso (H) = 1.2U eq (C). For 1a, one low angle reflection was omitted from the final cycles of refinement because its observed intensity was much lower than the calculated value.

Special details
Experimental. Data collection and full structure determination done by Prof. Anthony Linden: anthony.linden@chem.uzh.ch Solvent used: petroleum ether Cooling Device: Oxford Cryosystems Cryostream 700 Crystal mount: on a glass fibre Client: Grzegorz Mloston Sample code: MG-1225 (HG1704) Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.