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

Abstract

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 Bu^tOK 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 Bu^tOK–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.