Organometallics

1. Steric Acceleration of the Pauson-Khand (PK) Reaction

The PK reaction has developed over the last few years as an excellent method for cyclopentenone synthesis.  However, despite significant advances in the scope of metals that will promote or catalyze this reaction, there are still substrate limitations.   One of the most notable limitations is found in the ring sizes that can be prepared through the intramolecular variant.  When this project was initiated, only [3.3.0] and [4.3.0] bicyclic systems were accessible, given the plethora of natural products that possess 7-, 8-, or 9-membered rings annulated to a five membered ring, this limitation is extremely unfortunate.  During the intervening period there have been sporadic reports of successful, albeit generally low yielding, examples of medium-sized rings through the intramolecular PK reaction.  Our approach to this problem has explored the concept of steric buttressing to increase the reactive conformer population.  This approach has been successful, albeit in an unanticipated manner.  For example, the aryl enyne depicted in the figure undergoes a PK-cyclization to provide the bridged cycloadduct (9-membered ring) rather than the anticipated angularly fused product (8-membered).  This appears to be the first example of such regiochemistry in a PK reaction.

2. Catalytic Chemistry of Metal Complexes with Fluorinated Trispyrazoylborate Ligands

In a collaborative project with Prof Dias in this department, we have become interested in studying the catalytic properties of transition metal complexes prepared from the fluorinated variants of the hydrotrispyrazoylborato (TP) ligand.  These ligand systems, popularized by Trofiemenko, have become widely employed in coordination chemistry.  The Dias group has demonstrated that the introduction of fluorine onto the ligand confers exceptional stability on complexes derived from these systems, in addition, the high electronegativity of the fluoro substituents renders the metal center highly electrophilic.  Perhaps even more interesting is that a number of these complexes exhibit useful catalytic activity.  We have recently demonstrated that the copper complexes derived from these ligands are excellent nitrene transfer reagents.  However, perhaps the most interesting result that we have obtained in this area is with the related silver complexes.  Very recently we have found that this complex will catalyze the net insertion of a carbene into either a C-Cl bond or a C-Br bond.  This same complex also catalyzes carbene transfer reactions, including C-H insertions and Büchner reactions, and most recently the addition to and rearrangement of allylic and propargylic halides.