Metal-assisted reactions. Part 18. Reaction of titanium tetrachloride with cyclic sulphides: novel bicyclic disulphur compounds

Abstract

In marked contrast to the reaction of tetrahydrofuran with Ti CI,, tetrahydrothiophene did not afford ring-opened products. Instead, small quantities of ring-dimers were produced, possibly via initial chlorination and dehydrochlorination to produce 2,3-dihydrothiophene. An improved synthesis of 2,3-di hydrothiophene is reported.Reaction of a wide variety of cyclic ethers with TiCl, has been shown to produce mainly dimeric dichloro compounds or chloro alcohols, depending on the method of work-up.2 For example, tetrahydrofuran readily yields either 1,9-dichloro-5-oxanonane or 9-chloro-5-oxanonan-1-01, together with some simple ring-opened 1,4-dichlorobutane or 4-chlorobutan-1-01. Titanium halides also form stable complexes with cyclic sulphide~.~ Attempted ring-opening and dimerization of tetrahydrothiophene with TiCl, did not give ring-opened dimers but did afford a small quantity of a ring-dimer.Merhod-Te t ra h y d ro t hiop hene was refl uxed with TiCl , fo r several hours after which water was added and the organic product was distilled to yield a small quantity of a dimeric compound, CSH14S2. By 'H n.m.r. spectroscopy and synthesis this dimer of tetrahydrothiophene was shown to be 3-(tetrahydro-2-thieny1)tetrahydrothiophene (1) (Scheme); a possible route to this compound involves the intermediate, 2,3-dihydrothiophene. The latter was synthesized in good yield by pyrolysis of 2-benzoyloxytetrahydrothiophene. By introducing 2,3-dihydrothiophene into a reacting mixture of tetrahydrothiophene and TiCl,, an increased yield of the dimer (1) was obtained, together with a number of other products (2)--(5), some of which were synthesized independently. bicyclic structure (6). Structure (1) for the dimer was shown to be correct by independent synthesis. Refluxing a solution of 2-chlorotetrahydrothiophene in benzene afforded the known dimer of 2,3-dihydrothiophene, uiz. 4-(tetrahydro-2-thienyl)-2,3-dihydrothiophene (7).5 Catalytic reduction of sulphurcontaining compounds is hindered by poisoning effects and so the 2,3-dihydrothiophene dimer (7) was reduced to 3-(tetrahydro-2-thieny1)tetrahydrothiophene (1) by use of a mixture of triethylsilane and trifluoroacetic acid.6 Analytical g.c. showed that the reduced product had been obtained as a mixture (cu. 1 : 1) of two diastereoisomers which could be separated by preparative scale g.c. Similarly, the dimer (1) obtained from the reaction of TiCl, with tetrahydrothiophene was also obtained as a diastereoisomeric mixture (cu. 2: 3). The 'H n.m.r. and mass spectra of both diastereoisomers were very similar.Formation of the dimer (1) probably resulted from a small amount of radical chlorination of tetrahydrothiophene by TiCl, (Scheme) followed by elimination of HCl to give 2,3dihydro-thiophene; further radical reaction could give the product (1) as shown in the Scheme. A radical process is preferred as an explanation because an ionic process would require a hydride ion transfer in order to form the fully reduced species (1); ionic dimerization of 2,3-dihydrothiophene is known t...