2,5-Bis(1,1,3,3-tetramethylbutyl)thiophene

There are two independent molecules in the asymmetric unit of the title compound, C20H36S. Crystals are non-merohedrally twinned by twofold rotation about [001]. The bulky octyl groups of each molecule are on the same side of the thiophene plane and are approximately parallel. S—C distances are in the range 1.729 (4)–1.745 (3) Å, and the C—S—C angles are 92.98 (18) and 93.08 (17)°. The CH2 groups of the octyl groups are involved in weak C—H⋯S intramolecular interactions.

There are two independent molecules in the asymmetric unit of the title compound, C 20 H 36 S. Crystals are non-merohedrally twinned by twofold rotation about [001]. The bulky octyl groups of each molecule are on the same side of the thiophene plane and are approximately parallel. S-C distances are in the range 1.729 (4)-1.745 (3) Å , and the C-S-C angles are 92.98 (18) and 93.08 (17) . The CH 2 groups of the octyl groups are involved in weak C-HÁ Á ÁS intramolecular interactions.

S1. Comment
Kutz & Corson (1946) first reported acid catalyzed alkylation of thiophene with olefins and alcohols. They suggested that the mono-alkylation reaction occurred at the 2-position of the thiophene ring. Higher boiling liquid products were also isolated, which they believed to be di-alkylated thiophenes. Caeser (1948) synthesized 2,5-di-(1,1,3,3-tetramethylbutyl)thiophene by reacting diisobutylene with thiophene and isolated it as a low-melting solid (b.p. 146-147°C, m.p. 36-37°C). He correctly proposed the structure from its physical properties as well as from the work by Kutz & Corson (1946) on the mono-alkyl derivatives.
The two independent molecules of the asymmetric unit are shown in Fig. 1. Their conformations are quite similar, having both octyl chains on the same side of the thiophene plane. A least-squares fit, overlaying the thiophene and three central C atoms of the octyl groups yields an average deviation (11 atoms) of 0.182 Å, as shown in Fig. 2 (Duchamp, 2005). The thiophene rings in both molecules exhibit maximum deviation 0.011 (4) Å from planarity; it is pertinent for C2 in one molecule and C23 in the other. Some geometric features of the thiophene rings are given in Abstract. The molecules deviate slightly from mirror symmetry, as described by the S-C-C-C torsion angles about the bonds attaching the octyl groups to the thiophenes. Torsion angle magnitudes are about 12° larger on one side of the molecule than on the other: S1-C1-C5-C8 = -59.2 (4) and S2-C24-C33-C36 = -57.1 (4)° vs. S1-C4-C13-C16 = 47.8 (4) and S2-C21-C25-C28 = 44.7 (4)°.
No otherwise unsubstituted thiophenes having tertiary C atoms adjacent to both S atoms are present in the Cambridge Structural Database (Version 5.29 of November 2007; Allen, 2002) except for several macrocyclic molecules: AMADIB, FOJPOK, FOJPUK, FOJQAX, FOJQEB, LOHVEJ, XEMDOJ, XEMFAX, and YOZHEA. The structure of tetra-t-butylthiophene has been reported (Krebs et al., 1992). It has its thiophene twisted out of planarity as a result of the four bulky substituents.
Weak intramolecular C-H···S interactions involving the CH 2 groups of the octyl substituents and the thiophene S atoms are listed in Tab. 1. The C-H···S angles are quite small for this type of interaction, near the tetrahedral angle.

S2. Experimental
Thiophene (42.4 g, 0.50 mol) and diisobutylene (60.8 g, 0.54 mol) were mixed at 20°C, and then the catalyst system containing triethylaluminum (30 ml, 1.0 M solution in heptane, 0.03 mol)/hydrogen chloride (120 ml, 1.0 M solution in ether, 0.12 mol) was carefully added to control the exothermic reaction, under a nitrogen atmosphere (Elnagar et al., 2006). The resulting reaction mixture was heated for 1 h at 80° C. After workup with 12% NaOH solution, the crude product (67.2 g, 87% yield based on diisobutylene) was obtained as a liquid. It solidified upon standing at room temperature. The solidified material was recrystallized from 20% aqueous 2-propanol to obtain colorless plate-like crystals with a melting point range of 36.9-38.1° C. 1

S3. Refinement
Though all the H atoms were observable in the difference electron density map, they were situated into the idealized positions. The C-H distances were 0.95 for thiophene C, 0.98 for methyl and 0.99 Å for CH 2 , and thereafter treated as riding. U iso for H was assigned as 1.2 × U eq of the carrier atoms except for the methyls (1.5). The crystal was a nonmerohedral twin with a twinning operation being rotation by 180° about [0 0 1]. The twin law was (-1 0 -0.935, 0 -1 0, 0 0 1), determined by ROTAX (Cooper et al., 2002). (-0.935 ~2a(cosβ)/c.) The number of the reflections in the first and the second domain of the non-merohedral components was 9704 and 2651 respectively. Four domain states were taken into account: two for the non-merohedral components while each moreover had an inversion counterpart. Refinement yielded component proportions 0.80 (2): 0.16 (2) and both inversion-related components 0.02 (2). 2248/609 Friedel pairs were present in the data set for the major/minor component. The largest residual peak was located 1.55 Å from as H20A.

Figure 1
The molecular structure of the title molecules. The displacement ellipsoids shown at the 50% probability level. The H atoms are shown with arbitrary radius.  Overlay of two independent molecules of the title structure (Duchamp, 2005). The H atoms are not shown.