Dihydrothiophene complexes

Reduction of TpW(NO)(PMe3)(Br) (Tp=hydrido-tris(pyrazolyl)borate) with sodium in the presence of thiophene yielded a mixture of tungsten-containing complexes, exhibiting distinct chemical properties, which could be partially hydrogenated to dihydrothiophene complexes with combined yields of 22% in a 1 1 ratio of two diastereoisomers (Scheme 130). The coordinate binding fashion is less common for 16-electron metal complex than the or S-bound form [144]. 

Figure 34. Surface pressure - area isotherms for monolayers of Ci 8TCNQ (a), the mixture of the dihydrothiophene and G 8TCNQ (b), and the complex (c), spread on distilled water, as compared with that on the aqueous subphase with 10 5M LiTCNQ (c ).

Figure 35. Fluorescence microscopic pictures for LB films of a mixture and the complex of 2,5-dihydrothiophene and Ci sTCNQ. 

The reaction of o-halomethylene benzyl halides, l,4-dihalobut-2-enes, cyclo-2-hexenols or 2,5-dihydrothiophene-l,1-dioxides with Na2Fe(CO)4 or Fe2(CO)9 results in the formation of (diene)Fe(CO)3 complexes108. In each case, the precursor is transformed in situ into the free diene ligand, followed by complexation. 

A novel route to 2,3-dihydrothiophenes involved a titanocene-promoted carbene formation and subsequenct intramolecular cyclization onto a thiol ester <99SL1029>. Treatment of thioacetal 9 with the low-valent titanium complex 10 gave 2,3-dihydrothiophene 12 by intramolecular olefination of the thiol ester of titanium-carbene intermediate 11. Another metal-mediated cyclization onto the thiophene ring system involved the palladium-catalyzed cyclization of 1,6-diynes <99T485>. For example, treatment of thioether 1,6-diyne 13 with Pdlj in the presence of CO and Oj in methanol followed by treatment with base gave 14. 

The conversion of thiophene to THT, via 2,3-dihydrothiophene, is much more difficult. A major difference is that after the first hydride migration a thioallyl ligand is formed (Scheme 74). The poor catalytic activity has been attributed to the fact that THT is a good rt-donor and is not easily displaced by the thiophene so it traps all the available catalytically active species as the bis-THT complex <1998ACR109>. 

Simple protonation of (8) with a strong acid at 25 °C yields the 2,3-dihydrothiophene (2,3-DHT) complex (equation 24) -SH groups on M0S2 have been proposed to be strong acids and may therefore be relevant. The stepwise addition of H (equation 19) andH+ (equation 24) to jj -thiophene forms the basis of an important mechanism proposed for thiophene HDS (see Section 2.3). 

Coordinated 2,3-dihydrothiophene (DHT) in a platinum hydride complex (32) rearranges slowly, by inserting into the M—H bond, to give 33. A... 

Catalytic reactor studies of 2,3-dihydrothiophene (2,3-DHT) have shown it to undergo desulfurization this supports the proposal that it might be an intermediate in the HDS of thiophene " (Scheme 17). Because there is also substantial conversion of this partly hydrogenated thiophene to tetrahydrothiophene (THT), it is of interest to investigate the reactivity of 2,3-DHT with metal hydride complexes to see whether hydrogen is transferred from the metal to the unsaturated heterocycle. 

Di-iron nonacarbonyl reacts with 2,5-dihydrothiophene to give the mononuclear complex [Fe(CO)4(C4H6S)j in which the heterocycle donates via the sulfur atomJ l Subsequent thermolysis brings about C S bond cleavage giving butadiene and an insoluble black material presumed to be FeS. Reaction of 2,5-dihydrothiophene with [Re2(CO)9(MeCN)j leads to a dirhenium product which features S-bound thiophene. This decomposes readily at 110 °C and butadiene is detected among the decomposition products. Because 2,5-dihydrothiophene does not decompose when heated at 120 °C over a period of three days, it can be concluded that the S-coordination of dihydrothiophene promotes decomposition with liberation of butadiene. 

A series of benzobithiophene derivatives are electron accq>tors in charge-transfer complexes. Of the three isomers reported, 114 is the most stable and soluble <93SM1910, 94JOC3077>. [3]RadiaIene derivative 115 was also prepared as an electron acceptor. Extensive delocalization of these molecules produces powerfiil acceptors with 1 to be more positive by 0.2-0.25 V than the reference compound 2,S-bis(dicyanomethylene)-2,S-dihydrothiophene <94CC519>. A related tris[5-(3,5-di-/-butyl-4-hydroxyphenyl)-2-thienyl]cyclopropenylium ion is also reported <93CL911>. 

Dihydrothiophene forms a sulfur-coordinated platnium complex (286) on reaction with HPt(acetone)(PEt3)2. This then slowly rearranges to (287), hydrogenation of which gives butane (Equation (27)) <890M1856>. 

Investigations of Mn-thiophene complex ( -C4H4S)Mn(CO)3 showed that bubbling HCl gas into a solution of 125 in hexane gave a yellow and air-stable precipitate 126, then released the partially reduced product 2,3-dihydrothiophene... 

In 2006, Morita and Krause discovered a highly stereoselective cycloisomerization of a-thioallenes 243 to 2,5-dihydrothiophenes 244 in the presence of 5 mol% of AuCl (Scheme 46.29). The computational investigation by Ando in 2010 indicated that the complexation of a-thiollene with AuCl occurred preferentially at the distal double bond, followed by the C—S bond formation 245, the proton transfer from the sulfur to 3-carbon atom rather than C-AuCl bond 246. The subsequent 1,2-hydride shift 247 followed by a removal of AuCl led to 2,5-... 

Electrochemical Reactions.—An electrochemical method for the one-step preparation of 2,5-dihydrothiophen-2-carboxylic acid has been developed. A stable cation radical has been obtained from the electrochemical oxidation of 2-/7-nitrophenyl-3,4,5-triphenylthiophen. The reduction of 2-acetylthiophen, 2-benzoylthiophen, and 2,5-diformylthiophen by electrochemical methods on a mercury electrode has been investigated and compared in the latter case with chemical reductions. Electrochemical reactions have been carried out with some complex thiophens. " ... 

Other Routes to ni" Complexes. Alternative access to Fe(CO)3 complexes are afforded by a variety of more unusual starting materials cyclopropenes, dibromides, dihydrothiophene dioxides, alkynes/CO, allenes, and cyclohexadienones are all discussed below. 

Reaction with 2,5-Dihydrothiophene 1,1-Dioxide. 2,5-Dihydrothiophene 1,1-dioxides are known to be converted into 1,3-dienes after thermal displacement of sulfur dioxide. Reaction in situ with Fe2(CO)9 offers a general preparation of highly functionalized tricarbonyl(Ti -buta-l,3-diene)iron(0) complexes (50) (eq 31). ... 

Reaction of dialkyl disulfide with allenes is catalyzed by a rhodium-phosphine complex and trifluoromethanesulfonic acid giving ( )-2-alkylthio-l,3-dienes [78]. AuBr3-catalyzed regioselective hydrothiolation of aromatic allenes with arenethiols affords the corresponding dithioketals in good yields under mild conditions [79]. Intramolecular hydrothiolation of a-thioallenes to 2,5-dihydrothiophenes is successfully catalyzed by AuCl (Scheme 24) [80]. 

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