Jim Green
Research Interests
Our work is focused on metal mediated and catalyzed organic synthetic methods. We have a particular interest in situations where the metal fragment imparts reactivity that would be difficult in its absence, and in molecules that would not exist without stabilization by the transition metal unit. The chemistry is then developed to a point where it can be used in biologically interesting or structurally interesting natural products. In other cases the existence of the complex itself is an important finding, and we target issues such as its stability or reactivity. Some specific examples of our research interests include:
1) The use of equivalents of γ-carbonyl cations in organic synthesis. These umpolung synthons are fundamentally difficult, due to the destabilization of the cation by the electron withdrawing group. Nevertheless, we have had excellent success using Co2(CO)6 complexes of γ-alkoxyalkynoates (1) as precursors to these cations, by way of Nicholas reaction chemistry. The alkynedicobalt unit stabilizes the cation extensively, and nucleophiles react reliably at the γ-site. The product complexes are easily handled under conventional conditions, and decomplexation of the alkyne from the Co2(CO)8 is straightforward. We have used this method recently in the synthesis of natural products velloziolide (2) and microstegiol (3). We are now developing catalytic versions of γ-carbonyl cation equivalents, that don’t get their stabilization by Co2(CO)6 complexation.
2) Cycloheptyne-Co2(CO)6 complexes in organic synthesis. Cycloheptynes themselves are unstable compounds, due to the angle strain imposed by the seven membered ring on the triple bond. However, complexation of that alkyne by Co2(CO)6 bends the formal C-C≡C bond angle to ca. 140 o, making seven membered ring formation possible (4). Removal of the Co2(CO)6 unit in conjunction with reduction to the cycloheptene is also facile, and therefore this chemistry has given us access into a number of cycloheptane based ring systems. In particular, we have employed these compounds in the enantioselective total synthesis of several antitumour allocolchicines, including NSC 51046 (5). Many other cycloheptyne and cyclooctyne based systems are being targeted.
3) Fundamental properties of cycloheptyne-Co2(CO)6 complexes. The stabilization of propargylic cations by complexation to (di)cobalt has several unsolved questions. We have prepared the precursor to and generated 6, the ‘dehydro’ version of classically aromatic tropylium ion, and looked at its reactions and measures of its stability. We are pursuing higher homologues of this dehydrotropylium ion complex. By our evaluation, 6 possesses aromatic stabilization, but to a reduced degree relative to tropylium ion itself.