Gary Scholes

CHEMISTRY OF DIENE AND ENONE IRON TRICARBONYL COMPLEXES

The way in which the stabilities and reactivities of polyolefins are modified by coordination to transition metals has received considerable attention in recent years. Preparations of stable transition metal complexes of exceedingly reactive polyolefins, which are not normally isolable at ambient temperatures, represent some of the most intriguing and synthetically useful applications to emerge from this area. For example, the iron tricarbonyl moiety has been used successfully to stabilize and render isolable as iron complexes such highly reactive polyolefins as cyclopentadienone,’ cyclobutadiene,2 7-n0rbornadienone,~ and trimethylenemethane.4 As demonstrated by the classic work of Pettit on cyclobutadieneiron tricarbonyl, such complexes can not only be used to “store” the ligand but, through oxidative cleavage of the complex, can also serve as a convenient source of the free polyolefin for use in synthesk2 One particular aspect of this area of chemistry that has occupied our attention over the last few years has been the modification, through binding to transition metals, of the thermal chemistry of cyclic polyolefins capable of undergoing electrocyclic ring-opening and ring-closing reactions. We have observed that binding a metal to polyolefinic systems capable of such valence tautomerism can have substantial effects on both the rates of interconversion of the valence isomers and on the position of the tautomeric equilibrium. The first example of such an observed ring closure was our observation5 that protonation of cyclooctatetraeneiron tricarbonyl at low temperatures led to the cyclooctatrienyliron tricarbonyl cation, 1, which undergoes ring closure at -60°C (AG’ = 15.7 kcal/mol) to yield the previously observed bicyclo[5. i.O]octadienyliron tricarbonyl cation, 2 (see SCHEME 1). In this particular case, a meaningful comparison with the free ligand system is difficult to make, because protonation of free cyclooctatetraene leads to the homotropylium ion.6 A more direct comparison can be made in the case of cis4-cyclononatetraene (see SCHEME 1). Whereas cis4-cyclononatetraeneiron tricarbony1 (3) is stable at room temperature and undergoes ring closure to cisdihydroindeneiron tricarbonyl(4) at 100°C (AG’ = 28.4 kcal/mol),’ free cyclononatetraene undergoes ring closure to cis-dihydroindene rapidly at room temperature (AG’ 23 kcal/mol).* Thus, the effect of binding iron to the 1,3-diene unit is to raise the free energy of activation for ring closure by about 5 kcal/mol. A system for which more quantitative data is available involves the equilibrium between 1,3,5-cyclooctatriene (5) and bicyclo[4.2.0]octadiene (6), established via electrocyclic ring-opening and ring-closing reactions (see SCHEME 1). Huisgen et al. .

Preparation of bicyclo[4.2.0]octa-2,4-dien-7-one via trapping with benzylideneacetoneiron tricarbonyl

The synthesis of bicyclo[4.2.0]octa-2,4-dien-7-one has been accomplished by trapping this unstable tautomer with benzylideneacetoneiron tricarbonyl to form the stable bicyclo[4.2.0]octa-2,4-dien-7-oneiron tricarbonyl which was then oxidatively cleaved with ceric ammonium nitrate at –30 °C to yield the title ketone.

Mechanism of decarboxylation of bicyclic acids by lead tetra-acetate

Oxidative decarboxylation of bicyclo[3.2.1]octane-2-carboxylic acids and bicyclo[2.2.2]octane-2-carboxylic acid by lead tetra-acetate gives mainly acetates by both a carbonium ion and a non-carbonium ion pathway. Analysis of products in the decarboxylation of these and other acids indicates the intermediacy of organolead intermediates, which in the non-carbonium ion route give acetates with retention of stereochemistry. This duality of mechanism is used to explain product distributions reported in earlier studies of related carboxylic acids.