Chiral discrimination in electronic energy-transfer processes in solution. Effects of temperature and solution properties on chirality-dependent rate parameters

In this thesis, chiral cobalt(III) complexes with en ligands (en = ethylenediamine) and ruthenium complexes with 2-guanidinobenzimidazole (GBI) and N-(2-benzimidazolyl)thiourea (BITU) ligands are developed. Their efficiency as catalysts for Friedel Crafts and Michael reactions are assayed. Chapter 1 provides an overview of the development of bifunctional thiourea catalysts and analyzes crystal structures of previously reported [Co(diamine)3]3+ complexes as well as GBI systems and corresponding metal complexes with respect to their ability to form hydrogen bonds. Chapter 2 gives an overview of the stereochemistry of [Co(diamine)3]3+ complexes and describes the syntheses of cobalt complexes with substituted en ligands and their applications as catalysts for Michael reactions. The complexes Δ[Co(en)3]3+ 3BArf–∙14H2O (Δ13+ 3BArf–∙14H2O) and [Co((R,R)-chxn)3]3+ 3BArf–∙11H2O (BArf– = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; chxn = 1,2-diaminocyclohexane) are applied as catalysts for the Michael reaction of 2-cyclopentenone and dimethyl malonate with NEt3 as basic additive. The influence of the catalyst structure, catalyst loading, amount of base, solvent, and reaction temperature on reactivity and enantioselectivity is investigated as well as a competing side reaction, which leads to catalyst deactivation. Under optimized conditions with Δ[Co(en)3]3+ 3BArf–∙14H2O as catalyst, the Michael product is obtained in 78% yield with 33% ee. A second generation of cobalt(III) catalysts with one or more (S)-N1,N1-dimethylpropane-1,2,3-triamine ((S)-23; (S)-enCH2NMe2) ligands is synthesized. The tris(ammonium) salt ((S)-[23∙3H]3+ 3Cl–∙H2O) is prepared in ten steps from L-asparagine in 20% overall yield. The racemic precursors [Co(en)2(CO3)]+ Cl– and 3Na+ [Co(CO3)3]3–∙3H2O are treated with 1 equiv or 3 equiv of (S)-[23∙3H]3+ 3Cl–∙H2O to give Δ[Co(en)2(S)-enCH2NMe2H]4+ 4Cl–∙3H2O (Δ(S)-[26∙H]4+ 4Cl–∙3H2O, 19%)/Λ[Co(en)2(S)-enCH2NMe2H]4+ 4Cl–∙4H2O (Λ(S)-[26∙H]4+ 4Cl–∙4H2O, 6%) and [Co((S)-enCH2NMe2H)3]6+ 6Cl–∙6H2O ((S,S,S)-[28∙3H]6+∙6Cl–∙6H2O , 62%), respectively. Deprotonations of Δ(S)-[26∙H]4+ 4Cl–∙3H2O and (S,S,S)-[28∙3H]6+ 6Cl–∙6H2O and subsequent anion metatheses with Na+ BArf– yield the salts (Δ[Co(en)2(S)-enCH2NMe2]3+ 3BArf–∙12H2O (Δ(S)-[26]3+ 3BArf–∙12H2O, 84%) and [Co((S)-enCH2NMe2)3]3+ 3BArf–∙5H2O ((S,S,S)-[28]3+ 3BArf–∙5H2O, 82%). The Michael reaction of 2-cyclopentenone/trans-β-nitrostyrene and diethyl malonate catalyzed by Δ13+ 3BArf–∙14H2O, Δ(S)-[26]3+ 3BArf–∙12H2O or (S,S,S)-[28]3+ 3BArf–∙5H2O (7-10 mol%) in the presence of base affords the corresponding Michael products in good (88%) to quantitative yields with up to 30% ee. The Michael reactions of 2-cyclopentenone/-hexenone and 2-nitropropane are distinctly accelerated by the cobalt complexes (3 mol%) in presence of trans-2,5-dimethylpiperazine (1.0 equiv), but the products are racemic. Chapter 3 describes the syntheses of achiral and chiral substituted GBI and BITU ligands, their coordination to ruthenium, and their application as catalysts for Michael and Friedel Crafts reactions. Reactions of N-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-1-carbothioamide and N-(2-benzimidazolium)-S-methylisothiourea iodide (3H+ I) with amines (a, benzylamine (PhCH2NH2); b, (S)-1-phenylethylamine ((S)-(PhCH(CH3)NH2); c, (1R,2R)-N1,N1-dimethyl-1,2-diaminocyclohexane (chxnNMe2) afford the N13 substituted GBI ligands (R-GBI) ((X)-4; (X)/R = a, -/PhCH2; b, (S)/PhCH(CH3); c, (R,R)/chxnNMe2) and the N13 substituted BITU ligands (R-BITU) ((X)-5; (X)/R = b, (S)/PhCH(CH3); c, (R,R)/chxnNMe2) in 46-71% yields. The precursor [(η5-C5H5)Ru(CO)(NCCH3)2]+ PF6– and GBI or ligands 4a-c are reacted to give the corresponding complexes [(η5-C5H5)Ru(CO)(R-GBI)]+ PF6– in 47-81% yields. The complexes and ligands are tested as catalysts (10 mol%) for the Friedel Crafts reaction of trans-β-nitrostyrene with N-methyl indole to give racemic products in good yields (82-98%). The Michael reaction of trans-β-nitrostyrene and nitroethane is screened with ligands and ruthenium complexes. The best results are obtained for [(η5-C5H5)Ru(CO)((R,R)-chxnNMe2GBI)]+ PF6 ((R,R)-11c+ PF6–, 52% yield; dr = 72:28; ee syn: 22%; ee anti: 61%). The Michael reaction of trans-β-nitrostyrene and diethyl malonate is also investigated. Again, complex (R,R)-11c+ PF6– gives the best results. The Michael product is obtained in moderate to good yields (60-95%) with high enantioselectivities (93-96% depending upon batch).

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Werner-type Cobalt Complexes and Ruthenium Complexes with Substituted 2-Guanidinobenzimidazole Ligands as Catalysts for Michael and Friedel Crafts Reactions

B. Bosnich

Papers

Inorganic Chemistry Concepts Vol. 4, Inorganic Molecular Dissymmetry