A process for prepn. of 1-R1-2-R2-3-R3-4-R4-5-R5-6-R6-pentalenes (1; R1-R6 = C1-40 organyl, halo; preferably R1-R6 = C1-4 alkyl; preferably R5 = R5, R1 = R4, R3 = R6; more preferably R1-R6 are the same, R1-R6 = Me), 1-R1-2-R2-3-R3-4-R4-5-R5-6-R6-1,2-dihydropentalenes (2, same R), alkali and alk. earth salts of 1,2-dihydropentalenes MSx[1·H]n (M = Group IA, IIA metal, S = chelating solvent, preferably 1,2-dimethoxyethane, TMEDA; x = 0-8; n = 1,2) and Group IIIB-VIII transition metal complexes, comprising η3-η8 dihydropentalenide ligands, including sandwich, ansa-sandwich and binuclear sandwich compds., useful as catalysts for hydrogenation, Fischer-Tropsch reaction, hydroformylation, hydroamination, hydrosilylation, polymn., oxidn., cyclotrimerization, and cycloaddn. reactions, is described. The process comprises electrophilic alkylation of 3,7-dioxobicyclo[3.3.0]octane-2,4,6,8-tetracarboxylate ester, decarboxylation yielding the 2-R1-4-R3-6-R4-8-R6-bicyclo[3.3.0]octane-3,7-dione (3), dehydrogenation to the substituted 1,2,4,5-tetrahydropentalene-2,5-dione (4), redn. to 1,3,4,6-tetrahydropentalene-2(2H),5(5H)-dione (5), alkylation of the dione 5 by an organometallic reagent, preferably by RCeCl2 at -70°, yielding 1-R1-2-R2-3-R3-4-R4-5-R5-6-R6-1,2,3,4,5,6-hexahydropentalene-2,5-diol (6), dehydration of the diol 6, preferably by reaction with conc. H2SO4 to give the 1,2- or 1,4-dihydropentalenes (1a) followed by optional deprotonation, transmetalation, stannylation or complexation reactions. In an example, 1,2,3,4,5,6-hexamethyl-1,2-dihydropentalene (1aa)was prepd. in a multistep procedure starting from tetra-Me 3,7-dihydroxybicyclo[3.3.0]octa-2,6-diene-2,4,6,8-tetracarboxylate by MeI/K2CO3 methylation, decarboxylation, Br2 oxidn. to 2,4,6,8-tetramethylbicyclo[3.3.0]octa-2,5-diene-3,7-dione, Zn redn. to 2,4,6,8-tetramethylbicyclo[3.3.0]octa-1(5)-ene-3,7-dione and methylation by MeCeCl2 in a 60% yield. In another example, the iron complex [Fe2(CO)4(μ-CO)(μ-η5:η5-Pn*)] (H2Pn* = 1aa) was prepd. by reaction of 1,3,4,5,6-pentamethyl-2-methylene-1,2-dihydropentalene with Fe2(CO)9 in toluene for 24 h at reflux in a 45% yield. [on SciFinder(R)]
