1. Tunable Ligands for Rhodium-Catalyzed Hydroformylation
Rhett C. Smith, Department of Chemistry, Clemson University
Hydroformylation is elegant in its simplicity – an alkene reacts with CO and H2 to form a synthetically versatile aldehyde with complete atom economy. Even the simplest substrate, a terminal olefin, produces linear (l) and branched (b) aldehydes. The l-aldehyde is typically desired, and we have focused on improving regioselectivity through rational catalyst design. The most active mononuclear (pro)catalysts are rhodium complexes of bidentate P-donor ligands. The trigonal bipyramidal complex that functions as the progenitor of the active species can exist as e,e- and e,a-isomers, and the e,e isomer predominantly yields l-aldehydes. The ability of the ligand to accommodate a trans-spanning mode in square planar complexes generated from e,e- species during catalysis is also beneficial. We have successfully prepared a family of TERPHSPAN diphosphine ligands and crystallographically characterized their square planar and trigonal bipyramidal complexes (shown below), demonstrating that these ligands indeed take on the desired geometries for hydroformylation to yield linear aldehydes.
We have subsequently tested TERPHSPAN ligands under various conditions for hydroformylation of styrene, a substrate that typically produces branched aldehydes preferentially. Using TERPHSPAN diphosphines we have accomplished hydroformylation to the aldehyde mixtures in which the linear aldehyde is the major product.
The Rh complexes of TERPHSPAN diphosphines have also proven useful for the conjugate addition of boronic acids to unsaturated carbonyl compounds, including reactions involving sterically encumbered coupling partners. P2 P1 O Cl Rh Ir Cl P1 P2
P–M–P angle = 171°
P–M–P angle = 105°