Highly efficient multimetallic catalysts for sequential reactions

Abstract

This thesis describes investigations into metal-catalysed sequential reactions using both single metal and bimetallic Rh(I) and Ir(I) complexes containing N-donor ligands as catalysts. Two types of bimetallic catalyst systems were developed including the combined use of single metal complexes and bimetallic complexes. All catalyst systems were highly effective for catalysing the synthesis of heterocyclic compounds using sequential reactions. The complexes [M(bpm)(CO)2]BArF4 (M = Rh, Ir; bpm = bis(1-pyrazolyl)methane; BArF4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) were found to be highly efficient for the dihydroalkoxylation reaction forming spiroketals and fused bicyclic acetals. The catalysed dihydroalkoxylation reactions for a series of alkynediols showed that the Rh(I) complex favours the formation of 6-membered spiroketals while the Ir(I) complex favours the formation of 5-membered O-rings. A dual metal (Rh(I) and Ir(I)) catalyst system was used here for the first time to promote the formation of mixed 5,6- and 6,5-spiroketals and was found to be a more effective catalyst system than the individual single metal complexes. A new bitopic ligand [1,8-bis(bis(1-pyrazolyl)methyl)anthracene] was synthesised. A series of homobimetallic complexes [M2(COD)2(L)][BArF4]2 and [M2(CO)4(L)][BArF4]2 (M = Rh, Ir) each containing two bis(1-pyrazolyl)methane motifs bridged by one of three well-defined rigid aromatic scaffolds, where L is m-C6H4[CH(pz)2]2 (Lm), p-C6H4[CH(pz)2]2 (Lp) and anthracene-dibpm (LAnt), were synthesised. The solid-state structures of the three sets of homobimetallic complexes showed very different spatial relationships between the ligand pairs in each. For the catalysed dihydroalkoxylation reaction, the homobimetallic complexes are more active catalysts that the single metal complexes. The order of catalytic activity of the bimetallic complexes were found to be M2(LAnt) > M2(Lm) > M2(Lp). The order of activity can be attributed to the degree of bimetallic cooperativity, where the anthracene-linked complexes achieve the greatest level of cooperativity in solution and the two metal centers of the anthracene system are closest together in space. For the catalysed tandem hydroamination/hydrosilation of 4-phenyl-3-butyn-1-amine with silanes, a combination of the dirhodium and diiridium complexes were found to be a more efficient catalyst than the single metal complexes. A series of chiral ligands and their complexes were synthesised and tested for the catalysed dihydroalkoxylation reaction but showed no significant stereoselectivity in the products formed.