Thiophenes metalation

A copper-mediated cyclization of metallated thiophenes has been utilized to prepare polycyclic thiophenes and thiophene cyclophanes. Treatment of dibromide 106 in succession with M-butyllithium (halogen-metal exchange), zinc chloride (transmetallation), and copper chloride gave 7//-cyclopcnta[ 1,2-fc 4,3-6 dithiophene (107) <00H(52)761>. This conversion has also been achieved using a palladium-mediated cyclization performed in the presence of hexamethylditin . Copper-mediated cyclizations have also been applied to the syntheses of cyclopenta[2,l-6 3,4-A ]dithiophen-4-one (108) (three steps from 73) <00S1253> and cyclophane 109 <00CC2329>. 

Metallated thiophenes can also be readily prepared by halogen-metal exchange. Thus, 2-bromothiophene is rapidly metallated with n-butyllithium at low temperatures (e.g. -78°C) to give 2-lithiothiophene (Figure 18). This method has been shown to be applicable to the preparation of polylithiothiophenes. Thus, di-, tri- and tetra-brominated thiophenes afford the corresponding di-, tri- and tetra-lithiated thiophenes upon reaction with n-butyllithium (30 e.g. Figure 18). 

The yield of metallated thiophene is ca. 50 % when ArH is naphthalene. The reactivity of ArH is ArH = PhPh > naphthalene > PhCH=CHPh > phenanthrene > anthracene. The normal two-electron reaction using [ArH] Li changes to a one-electron process in the presence of Ph2C=CH2 or PhCMe=CHj, and yields of 2-thienyl-Li also increase to >90%. 

Metal complexes containing q -bonded thiophenes, where the ring formally donates 6 electrons and occupies three coordination sites (Lj-type ligand) are the most numerous and stable of the transition metal-thiophene derivatives, examples being available for Cr, Mn, Re, Fe, Ru, Rh, and Ir. Curiously, the synthesis of the first n-thiophene metal complex, viz. Cr(CO)3(q -T) reported by Fischer as early as 1958 [67] represents still today the only example available for a Group 6 metal it-bonded to thiophene its X-ray structure was solved by Dahl in 1965 albeit with a strong rotational disorder for the... 

C=C)-bonded metal thiophene complexes ] (S,C=C)-bonded metal thiophene complexes j" -Bonded metal thiophene complexes j -Bonded metal thiophene complexes... 

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