Date of Award

1993

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

George G. Stanley

Abstract

The tetraphosphine ligand Et$\sb2$PCH$\sb2$CH$\sb2$P(Ph)CH$\sb2$P(Ph)CH$\sb2$CH$\sb2$PEt$ \sb2$ (to be referred to as et,ph-P4) was designed to both bridge and chelate two metal centers and hold them in general proximity to investigate bimetallic cooperativity in homogeneous catalysis. The internal phosphines of et,ph-P4 are chiral, thereby generating a pair of diastereomers: the racemic, and the meso. These are produced in approximately equal amounts from the ligand synthesis. This project has concentrated on the separation and isolation of diastereomerically pure forms of et,ph-P4 to determine individual hydroformylation catalytic activity of each bimetallic complex. Complexes of Ni(NCS)$\sb2$ with et,ph-P4 were effective in accomplishing this separation by utilizing solubility differences between the two diastereomeric nickel complexes. The reaction of one equivalent of Ni(NCS)$\sb2$ with et,ph-P4 in EtOH precipitates pure meso- (Ni(NCS)($\eta\sp3$-et,ph-P4)) (NCS) leaving racemic- (Ni(NCS)($\eta\sp3$-et,ph-P4)) (NCS) in solution. The meso- (Ni(NCS)($\eta\sp3$-et,ph-P4)) (NCS) readily crystallizes in CHCl$\sb3$/EtOH solution. Two equivalents of Ni(NCS)$\sb2$ reacts with et,ph-P4 to produce dimeric racemic- and meso-Ni$\sb2$(NCS)$\sb4$(et,ph-P4). The pure racemic- and meso-et,ph-P4 ligands were decomplexed from the metal complexes by refluxing with excess NaCN in a heptane/water solvent mixture. Pure meso et,ph-P4 converts to a mixture of both diastereomers when heated close to 150$\sp\circ$C for 20 minutes. This can provide additional yields for the racemic diastereomer if desired. Catalytic results for the hydroformylation of 1-hexene using the bimetallic rhodium complexes racemic- and meso- (Rh$\sb2$(nbd)$\sb2$(et,ph-P4)) (BF$\sb4)\sb2$ support our molecular modeling predictions that the racemic- (Rh$\sb2$(nbd)$\sb2$(et,ph-P4)) (BF$\sb4)\sb2$ catalyst system is the more active species. Molecular modeling studies further indicated a distinct energy difference between the two faces of a pro-chiral alkene coordinating to the (S,S) or R,R)-bimetallic catalyst. The racemic-et,ph-P4 was then separated into individual (S,S) and (R,R)-et,ph-P4 enantiomers by chiral-column HPLC. Chiral rhodium catalysts of each enantiomer afforded asymmetric hydroformylation of vinyl acetate to give the appropriate optically active 2-acetoxypropanal with high enantiomeric excess and regioselectivity. Both chiral catalysts have been isolated and each gives enantioselective hydroformylation of vinyl acetate to the opposite chiral product. Model reaction of Ni(NCS)$\sb2$ with crude Et$\sb2$PCH$\sb2$CH$\sb2$PPh$\sb2$ in EtOH yields two products, Ni(NCS)$\sb2$(Et$\sb2$PCH$\sb2$CH$\sb2$PPh$\sb2$) and Ni(NCS)$\sb2$ (Ph$\sb2$PCH$\sb2$CH$\sb2$P(Et)-CH$\sb2$CH$\sb2$PPh$\sb2$) which have different solubilities in heptane, acetone and acetonitrile. X-ray crystal structure analyses of Ni(NCS)$\sb2$(Et$\sb2$PCH$\sb2$CH$\sb2$PPh$\sb2$), Ni(NCS)$\sb2$ (Ph$\sb2$PCH$\sb2$CH$\sb2$P(Et)CH$\sb2$CH$\sb2$PPh$\sb2$), meso- (Ni(NCS)($\eta\sp3$-et,ph-P4)) (NCS), meso-Ni$\sb2$(NCS)$\sb4$(et,ph-P4), and dimethylammonium (hydrogen-$\mu$-oxo-bis$\{$phenylphosphinate$\}$) are presented.

Pages

107

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