Friedel–Crafts acylation of tricarbonyl(cyclo-octatetraene)iron. Preparation and reactions of (8-acylbicyclo[3.2.1]octadienylium)tricarbonyliron hexafluorophosphates

Abstract

The acylation of the title complex [Fe(C₈H₈)(CO)₃](1a) has been found to give cations of molecular formula [Fe{C₈H₈(COR)}(CO)₃]⁺(3a; R = Me, 3b; R = Ph) which contain a co-ordinated bicyclo[3.2.1]octadienyl ligand in which the C(8) acyl group is stereospecifically endo to the C(6),C(7) double bond [in contrast, [Ru(C₈H₈)(CO)₃](1b) is unreactive towards acylating reagents]. Reaction of (3a) with BH₄^– or CN^– gives, as major products, complexes of the type [Fe{2–4-η,6-σ-C₈H₈X(COMe)}(CO)₃](6a; X = H, 7; X = CN), while I^– displaces co-ordinated CO to give [Fe{2–4:6–7-η-C₈H₈(COMe)}(CO)₂l](9). The addition of R′S^–(R′= Prⁱ, Bu^t, or Ph) to (3a) or (3b) affords the diene complexes [Fe{3–4:6–7-η-C₈H₈(SR′)(COR)}(CO)₃](11). The stereochemistry of addition of R′S^– is exclusively exo_M. The thioethers (11) react with HPF₆ to regenerate their parent cations and this facility has been utilised to permit nucleophilic addition to the acyl carbonyl while retaining the co-ordinated dienyl in ‘protected’ form. Subsequent removal of the thio-substituent leads to some novel co-ordinated cyclic ehters. Oxidative demetallation of (11) liberates a substituted bicyclo[3.2.1] octadiene in high yield. A possible mechanism for the rearrangement of the cyclo-octatetraene during the acylation reaction is discussed.