TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preparation (claimed): Preparing compound containing pentalene unit having general formula (I) involves a reductive homocoupling reaction of a halogenated alkoxy-aromatic or alkoxy-heteroaromatic compound having general formula (II).Z=a halogen atom selected from bromine, chlorine, iodine, in the presence of at least one catalyst containing palladium, preferably selected from compounds of palladium in oxidation state 0 or +2 such as, for example, palladium (II) chloride (PdCl2), palladium (II) acetate (Pd(OAc)2), bis (dibenzylidene) palladium(0) (Pd2(dba)3 where dba is C6H5CH=CHCOCH=CHC6H5), bis(acetonitrile)-palladium(II) chloride (Pd(CH3CN)2Cl2), bis(triphenylphosphine) palladium (II) chloride, (Pd(PPh3)2CI2), bis-(triphenylphosphine) palladium (II) acetate (Pd(PPh3)2(OAc)2), tetrakis (triphenyl phosphine) palladium (0) (Pd(PPh3)4), or its mixtures.The process is carried out in the presence of at least one weak organic base, preferably selected from: carboxylates of alkaline or of alkaline-earth metals such as potassium acetate, sodium acetate, cesium acetate, magnesium acetate, calcium acetate, potassium propionate, sodium propionate, cesium propionate, magnesium propionate, calcium propionate, or its mixtures; carbonates of alkaline or of alkaline-earth metals such as lithium carbonate, potassium carbonate, sodium carbonate, cesium carbonate, magnesium carbonate, calcium carbonate, or its mixtures; bicarbonates of alkaline or of alkaline-earth metals such as lithium bicarbonate, potassium bicarbonate, sodium bicarbonate, cesium bicarbonate, magnesium bicarbonate, calcium bicarbonate, or its mixtures; or its mixtures. The process is carried out in the presence of at least one organic reducing agent, preferably selected from hydroquinone, pyrocatechol, and para-hydroxyaminobenzene. The process is carried out in the presence of at least one polar aprotic organic solvent, preferably selected from: diethyl ether (Et2O), tetrahydrofuran (THF), 1,4-dioxane, dimethylsulfoxide (DMSO), N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), or its mixtures. It is carried out in the presence of at least one ligand of the catalyst containing palladium, preferably selected from: tri(tert.-butyl) phosphine, triphenylphosphine, tricyclohexyl-phosphonium tetrafluoroborate, 2-dicyclohexyl-phosphine-2'-(N,N-dimethyl-amino) biphenyl (DavePhos), di-tert.-butyl (methyl) phosphonium tetra- fluoroborate, tri-tert.-butyl (methyl) phosphonium tetra-fluoroborate, or its mixtures. Preferred Composition: The halogenated alkoxy-aromatic or alkoxy-heteroaromatic compound having general formula (II) and the catalyst containing palladium are used in a molar ratio ranging from 100:0.1 to 100:6. The halogenated alkoxy-aromatic or alkoxy-heteroaromatic compound having general formula (II) and the weak organic base are used in a molar ratio ranging from 1:2.2 to 1:20. Halogenated alkoxy-aromatic or alkoxy-heteroaromatic compound having general formula (II) and the organic reducing agent are used in molar ratios ranging from 1:1.5 to 1:10. Halogenated alkoxy-aromatic or alkoxy-heteroaromatic compound having general formula (II) is used at a molar concentration ranging from 0.1 mmole to 10 mmoles. Halogenated alkoxy-aromatic or alkoxy-heteroaromatic compound having general formula (II) is used in the dipolar aprotic organic solvent in such a quantity so as to have a molar concentration in the solvent ranging from 0.05 mmole to 2 mmoles. Halogenated alkoxy-aromatic or alkoxy-heteroaromatic compound having general formula (II) and the ligand are used in molar ratios ranging from 100:1 to 100:10. Preferred Conditions: The process is carried out at a temperature ranging from 80 degrees C to 170 degrees C. It is carried out for a time ranging from 12 hours to 72 hours. Preferred Definitions:    R1 and R2=the same as each other, selected from branched 2-12C alkyl groups, and are preferably a 2-ethylhexyloxyl group;    R3=selected from heteroaryl groups optionally substituted, and is preferably a thienyl group;    n=1; and    X=Y', and represents a CH group.    EXAMPLE - No suitable example given.