Chiral terpyridine-metal complexes

: syntheses, characterizations and applications in asymmetric cyclopropanation

Abstract

Terpyridine−metal complexes have been of great interest ever since its discovery. This thesis describes the synthesis of new chiral terpyridine−metal complexes and the use of these complexes in asymmetric cyclopropanation. Synthesis of polypyridine−Re(I) complexes and their applications in asymmetric catalytic cyclopropanation are presented in chapter 2. Twelve catalyst precursors, [Re(L)(CO)3Br] (L = polypyridine ligands), were prepared in good yields (85−95%) by reactions of the polypyridine ligands (L = 1, 13a, 13b, 16a−16e, 17a, 17b, 18a and 18b) with Re(CO)5Br and they were characterized by NMR, ESI-MS and elemental analyses. Two of the complexes, one based on a C2-bipyridine (18a) and another one based on a C2-terpyridine (13b), were structurally characterized by X-ray crystallography. Each of them adopted distorted octahedral geometry with one bromide, two pyridine units and three facial carbonyl ligands. Cyclopropanation catalysts were generated by reacting catalyst precursors with AgOTf and they gave good yields (68−99%) for cyclopropanation of aliphatic and aromatic alkenes with alkyl diazoacetate. With chiral C2-terpyridines as ligands, enantiomeric excesses up to 73% and diastereoselectivities up to 83:17 were resulted. Hammett study, using competition experiments, gave a good linear plot with a negative + value of −0.57, suggesting that the catalytic intermediate is electrophilic in nature. Re(I) carbene species, observed by 1H NMR and ESI-MS when the catalyst was reacted with EDA, is proposed to be the active intermediate in cyclopropanation. Synthesis of double stranded helical terpyridine−Cu(I) and quinquepyridine−Cu(I) complexes and their applications in asymmetric cyclopropanation are presented in chapter 3. Five air stable terpyridine−Cu(I) helicates, [Cu2(L)2]X2 (L = C2-terpyridines, X = OTf¯ and ClO4¯), were prepared in good yields (75−88%) by reaction of Cu(I) starting materials with C2-terpyridines (13a−c) and four air stable quinquepyridine−Cu(I) helicates, of formula [Cu3(L)2]X3 and [Cu2(L)2]X2 (L = C2-quinquepyridine, X = OTf¯, PF6¯ and ClO4¯) were prepared in good yields (60−80%) by reaction of Cu(I) starting materials with C2-quinquepyridine (19a,b). These complexes were characterized by NMR, circular dichroism, ESI-MS, IR and elemental analyses. Two of the complexes, one based on a terpyridine (13c) and another one based on a quinquepyridine (19b) were structurally characterized by X-ray crystallography and they were shown to be double stranded dinuclear helicates. In the structure of terpyridine−Cu(I), each Cu(I) center adopts a distorted trigonal planar geometry with coordination to two pyridine units from one terpyridine and one pyridine unit from another terpyridine, while pseudo-trigonal bipyramidal geometry on Cu(I) centers from the quinquepyridine complex with three pyridine units from one quinquepyridine and two pyridine units from another quinquepyridine was observed. All the terpyridine−Cu(I) helicates and trinuclear quinquepyridine−Cu(I) complex are active catalysts for asymmetric cyclopropanation. With [Cu2(13b)2]X2, in a low catalyst loading (0.2 mol%), high TONs (up to 404), high enantiomeric excesses (up to 88%) and short reaction times (30−60 mins) were achieved for aliphatic and aromatic alkenes with EDA. Syntheses of several new ruthenium complexes, based on C2- and new C1-terpyridine ligands and their use as catalysts in cyclopropanation are presented in chapter 4. Three C1-terpyridine ligands (20a, 20b and 21) were prepared in good yields (44−63%) by Kröhnke condensation of 6-(1-pyridinoacetyl)-2,2’-bipyridine iodide with different chiral unsaturated ketones. These ligands together with their analogue C2-terpyridines (13a, 13b and 14) and one achiral terpyridine (1) formed fifteen ruthenium(II) complexes of cis- or trans-[Ru(L)(X)Cl2] (L = terpyridine ligands, X = PPh3, DMSO or CO) in moderate to good yields (26−97%). All of the complexes were characterized by NMR, ESI-MS, IR and elemental analyses. One complex, cis-[Ru(20a)(X)Cl2], was structurally characterized by X-ray crystallography. It was shown to be in distorted octahedral geometry with one DMSO ligand, two cis-chloride ligands and three pyridine units. Active catalysts for cyclopropanation were generated by the reactions of ruthenium complexes with AgOTf. Yields of cyclopropane were in the range of 54−99%. The best enantioselectivities of 74% ee and 72% ee for the cis-isomers of cyclopropanes were obtained with catalysts derived from C2-terpyridine 13b and C1-terpyridine 20b respectively. Hammett study through competition experiments showed a large negative value of −1.82 which suggested that a significant positive charge was built up in the transition state. Syntheses of terpyridine metal complexes with metal ions of Co(II), Fe(II) and Fe(III) and applications of them in asymmetric catalytic cyclopropanation are presented in chapter 5. Complexes of [Co(L)Cl2] (L = 13b, 13c, 14, 20a and 20b), [Fe(L)Cl2] (L = 13a−c, 14, 20a and 20b) and [Fe(L)Cl3] (20a and 20b) were prepared and characterized with NMR, ESI-MS and elemental analyses. Structural characterization with X-ray crystallography showed that intermediate geometries between square pyramidal and trigonal bipyramidal were adopted by the [Co(L)Cl2] (L = 13b, 14 and 20a), while distorted octahedral geometry was adopted by [Fe(20a)Cl3]. For catalytic cyclopropanation, all of the complexes were active catalysts after reaction with AgOTf. Good yields (up to 83%) and high enantiomeric excesses (up to 83%) of the cyclopropanes with the same configuration were obtained by Co(II) and Fe(II) complexes with C2-symmetric terpyridines. For the Hammett studies, Co(II) and Fe(II) complexes gave non-linear U-shape plots.

Date of Award	16 Feb 2009
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Hoi Lun KWONG (Supervisor)