45312-B3
Microwave-Promoted Assembly of Organometallic Hosts: Responsive Systems for Small Molecule Binding
Throughout the past year we have continued to explore the scope and utility of a microwave–mediated approach to the construction of eighteen–electron cobalt metallocenes. A series of experiments were designed to delineate the upper and lower steric limits of the synthesis — proving that 1–naphthyl resides at the upper limit — and to explore the possibility of participation of a (so–called) specific microwave effect in these preparations. Such effects have been invoked as an explanation for the very high yields obtained from certain microwave transformations and are described in terms of specific vibrations of key bonds during chemical reactions. Our experiments provided no evidence for the presence of such an effect. Rather, we were able to conclude that the very rapid heating provided by microwave irradiation favored production of the desired complexes at the expense of alternative reaction pathways. This outcome was most dramatically demonstrated with the preparation of a crowded tetranaphthyl complex that we could not prepare using traditional thermal heating methods. We have also continued to explore the question of whether non aryl–substituted acetylenes could be successfully employed in complex formation. Microwave irradiation of mixtures of dimethyl acetylenedicarboxylate and cyclopentadienyl cobalt dicarbonyl under our standard microwave conditions gave a 30% average yield of a dinuclear cobalt complex as the sole isolable organometallic product. This complex is air and light stable and the x–ray crystal structure was readily obtained. This complex joins a small number of dinuclear complexes of this class that have previously been prepared and studied because of their relevance to the mechanism of acetylene cyclotrimerization. Indeed, the only other significant material obtained from the synthesis of this material is the benzene hexaester that results from cobalt–mediated cyclotrimerization of the starting acetylene. It appears therefore that production of significant amounts of an intermediate complex is the result of maintaining a low relative ratio of acetylene to cobalt starting material. Further, ready access to significant quantities of the intermediate suggests possibilities for (i) probing elements of the mechanism of acetylene cyclotrimerization that continue to prove of interest and significance, and (ii) new synthetic methods that exploit the "trapped diene" nature of the intermediate in syntheses of, for example, crowded hexasubstituted benzenes. The straightforward access to cobalt metallocenes provided by our method has opened routes to complexes which we had targeted for projects involving dynamic assembly via reversible covalent bond formation. Ester, acid, and hydrazide appended cobalt metallocene scaffolds have been prepared via this approach and are currently being employed as components of dynamic combinatorial mixtures. The final year of the current award will be focussed on examining the nature of these dynamic systems and probing their response to the addition of templating species.
