Metal-mediated functionalization of tetrahydronaphthalene, octahydroanthracene, and tetrahydroquinoline: stereochemical control of nucleophilic addition to cationic cyclopentadienyl(η⁶-arene)iron(II) complexes via benzylic substitution

Treatment of the cationic tetrahydronaphthalene complex [Fe(cp)(η6-C10H12)][PF6](1; cp =η5-C5H5) with hydride or with carbon nucleophiles affords mixtures of isomeric neutral η5-cyclohexadienyl complexes with low to moderate regioselectivity. Substitution at all four benzylic positions of the tetrahydronaphthalene ligand in compound (1) is achieved by treatment with an excess of ButOK as base and in the presence of organic halide; by contrast with complex (1), the product tetra substituted tetrahydronaphthalene complexes, [Fe(cp)(C10H8R4)][PF6](7; R = Me), (8; R = CH2:CHCH2), and (10; R = CH2Ph) exhibit a high degree of selectivity in the subsequent formation of substituted cyclohexadienyl complexes via addition of nucleophiles. With ButOK–PhCH2Br, trisubstitution can be effected, giving the complex [Fe(cp)(C10H9Bzl3)][PF6](9; Bzl = CH2Ph) and leaving a single benzylic hydrogen which is substituted on further treatment with base and organic halide, to give diastereoisomeric mixtures of [Fe(cp)(C10H8BzI3R)][PF6], (11; R = CH2:CHCH2), (12; R = Me), and (13; R = COPh). Under slightly different reaction conditions mono benzylic substitution is also possible, affording the compounds [Fe(cp)(C10H11R)][PF6], (17; R = Me) and (18; R = COPh). Octahydroanthracene and tetrahydroquinoline iron analogues [Fe(cp)(Ar)][PF6], (14; Ar = C14H18) and (37; Ar = C9H11N) of compound (1) have been synthesized and some deprotonations and substitutions have been cursorily examined. Hydrocarbon decomplexation from derivatives of (1) may be achieved by pyrolytic sublimation, completing Fe(cp) mediated synthesis of polysubstituted tetrahydronaphthalene derivatives including inter alia 1,1,4,4-tetramethyl-6-phenyltetrahydronaphthalene and 1,1,4,4-tetramethyl-6,7-diphenyltetrahydronaphthalene.