Inductive and polarizability effects of an exocyclic diene-iron tricarbonyl group. The acetolyses of exo- and endo-irontricarbonyl complexes of 5,6-dimethylidene-2-exo-norbornyl and 2,3-dimethylidene-7-anti-norbornyl parabromobenzenesulfonates

The exo- and endo-irontricarbonyl complexes of 5,6-dimethylidene-2-exo-norbornyl alcohols 10x, 10n, p-bromobenzenesulfonates 11x, 11n, acetate 12x and of the 2,3-dimethylidene-7-anti-norbornyl alcohols 17x, 17n, p-bromobenzenesulfonates 19x, 19n and acetates 20x, 20n have been prepared. The SN1 buffered acetolyses of 11x, 19x and 19n gave 12x, 20x and 20n, respectively (retention of configuration). The first-order rate constants of the acetolyses have been evaluated and compared with those of the acetolyses of the uncomplexed 5,6-dimethylidene-2-exo-norbornyl (14) and 2,3-dimethylidene-7-anti-norbornyl p-bromobenzenesulfonates (18). A rate retardation effect of ca. 1.5 · 105 was measured for 11x 12x (65°) compared with the acetolysis of 14. The retardation effect is larger (> 5 · 107) with 11n. Contrastingly, the acetolysis 19x 20x was slightly accelerated with respect to that of the uncomplexed p-bromobenzenesulfonate 18. An unsignificant rate-retardation effect was measured for the acetolysis 19n 20n. The results are interpreted in terms of competitive inductive destabilization and charge-induced dipole stabilizing interaction by the exocyclic diene-iron tricarbonyl fragment. PMO. arguments give a rationale for the difference in polarizability between the diene-Fe(CO)3 group in 19 and that in the endo-7-norbornadienyl-iron tricarbonyl system.