Figure 1. An in-sequence diboration cross-coupling reaction converts alpha-olefins into new organic compounds. Source: James P. Morken.
The two catalytic reactions, developed in conjunction with Massachusetts Institute of Technology (MIT), Cambridge, Mass., when combined in a sequential process, result in unprecedented reaction with high levels of purity and selectivity (>95:5 enantiomer ratio), says James P. Morken, lead researcher and professor of chemistry at Boston College.The team first devised a catalytic enantioselective conversion of alpha-oefins into new boron compounds, then paired it with a palladium-catalyzed reaction developed by MIT. The combination of the diboration and cross-coupling reactions transforms the alpha-olefins into an array of chiral products. The shape of the chiral phosphonite ligand that is part of the platinum catalyst controls the formation of the enantiomer in the diboration, Morken notes. "Fortunately, the chiral phosphonite is easy to prepare and it is now commercially available …," he adds. More details appear in a recent Nature
article.Low loadings of catalysts and reagents, which are available on a multi-kilo scale, make the reaction amenable to large-scale synthesis. The dual-catalyst approach also provides an expansive substrate scope and can address a broad range of alcohol and amine synthesis. "The reaction can be used to produce an array of different products all from the same alkene starting material. This allows rapid exploration of chemical structure space," Morken explains.Despite these benefits, Morken says challenges remain. "We always worry about catalyst efficiency — can loadings be lowered? And about green chemistry aspects — can we avoid the use of noxious reagents, minimize solvent use, and avoid purification of intermediates?"Morken now is capitalizing on the reactivity of the 1,2-diboron intermediate to participate in other transition metal-catalyzed processes, and has had some success in cross-coupling other electrophiles that are of use in target-oriented synthesis. "In this way, one can imagine an entire suite of stereoselective catalytic transformations of alpha-olefins that aren't presently available," he adds.Future research could include expanding the cross-coupling chemistry in many other directions. "For example, a process that involves C-N bond formation at the terminal carbon, followed by oxidation would provide a route to simple amino amino-alcohols, which are important building blocks," Morken notes.Morken also hopes to try the dual-catalyst system on a pilot plant scale as soon as someone wants to. "We have run the reactions on a multi-gram scale and would love an excuse to run them on an even larger scale," he says.Eager to help people use the reaction if it solves their problems, Morken hasn't claimed any intellectual property for commercial use of the diboration chemistry. "The diboration cross-coupling tandem reaction sequence can target a lot of compounds that we already prepare by existing technology. It just gets to those same targets from different starting materials and often-times that can result in much shorter synthesis sequences. So, if anyone thinks that this technology could be of use in their enterprise, we would be more than happy to help in any way."