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Title: Ruthenium-catalyzed β-alkylation of secondary alcohols with primary alcohols, α-alkylation of arylacetonitriles with primary alcohols and Markovnikov and anti-Markovnikov functionalization of terminal alkynes
Authors: Cheung, Hung-wai
Keywords: Hong Kong Polytechnic University -- Dissertations
Alcohols -- Synthesis.
Carbonyl compounds -- Synthesis.
Issue Date: 2010
Publisher: The Hong Kong Polytechnic University
Abstract: A one-pot catalytic β-alkylation of secondary alcohols with primary alcohols to give higher alcohols with water as the only by-product is a highly desirable process. Cyclopentadienyl (Cp) ruthenium complexes [CpRu(PPh₃)₂(CH₃CN)]⁺BF₄⁻ (M1), CpRu(PPh₃)₂Cl (M2) and CpRu(dppm)Cl (M3) and hydrotris(pyrazolyl)borato (Tp) ruthenium complexes TpRu(PPh₃)₂Cl (M4) and [TpRu(dppm)(CH₃CN)]⁺BF₄⁻ (M5) are found to be active catalysts for the β-alkylation of secondary alcohols with primary alcohols. Mechanistic aspects of the M1-M5-catalyzed reactions were investigated; the crucial hydrido complexes CpRu(PPh₃)₂H (M6), TpRu(PPh₃)₂H (M8), CpRu(dppm)H (M10) and TpRu(dppm)H (M11), which were formed by alkoxide attack at the metal center and subsequent β-elimination, were identified. Aldehydes and ketones were generated concomitantly. The reaction pathway is broadly similar to that of the common mechanism for the one-pot catalytic β-alkylation of secondary alcohols with primary alcohols involving alcohol oxidation, ketone alkylation and ketone reduction. Carbonyl complexes CpRu(PPh₃)(CO)Ph (M7) and TpRu(PPh₃)(CO)Ph (M9) resulting from aldehyde decarbonylation were formed in some cases, and surprisingly, they are also found to be active for the catalytic processes. Although not observed, the anionic metal hydride complex is believed to be the key intermediate of the catalytic process. The aminocyclopentadienyl-ruthenium complexes [(CpNMe₂)Ru(PPh₃)₂(CH₃CN)]⁺BF₄⁻ (M16) and [(CpNEt₂)Ru(PPh₃)₂(CH₃CN)]⁺BF₄⁻ (M17) were prepared by protonation of the hydride precursors, (CpNMe₂)Ru(PPh₃)₂H (M14) and (CpNEt₂)Ru(PPh₃)₂H (M15), with HBF4·Et₂O in the presence of acetonitrile; the hydride complexes M14 and M15 were formed by reacting CpRu(PPh₃)₂Cl (M2) with LiNR2 (R = CH₃, C₂H₅) in THF. X-ray crystallography study of M16 shows that the nitrogen atom of the aminocyclopentadienyl ligand is nearly coplanar with its substituents, which indicates delocalization of the lone pair electrons. M16 and M17 are found to be moderately active catalysts for α-alkylation of arylacetonitriles with primary alcohols; on the other hand, the analogous unsubstituted cyclopentadienyl ruthenium complex [CpRu(PPh₃)₂(CH₃CN)]⁺BF₄⁻ (M1) shows very low catalytic activity. On the basis of experimental results and theoretical calculations, rationalization for the much higher catalytic activity of the aminocyclopentadienyl complexes over that of the unsubstituted Cp complex is provided. In the catalytic systems with the aminocyclopentadienyl-ruthenium complexes M16 and M17, it is possible to regenerate the active solvento species via protonation of the metal hydride intermediate and subsequent ligand substitution; this process is, however nonfacile in the catalytic system with the unsubstituted cyclopentadienyl ruthenium complex M1.
Tp-ruthenium (II) diphosphinoamino complex TpRu(4-CF₃C6H₄N(PPh₂)₂)OTf (M24) is prepared by chloride abstraction from its chloride precursor TpRu(4-CF₃C₆H₄N(PPh₂)₂)Cl (M21) using AgOTf in THF as the solvent; the chloride precursor M21 is formed by reacting TpRu(PPh₃)₂Cl (M4) with 4-CF₃C₆H₄N(PPh₂)₂ in toluene. The molecular structures of M21 and M22 are determined by X-ray crystallography. M24 is found to be an active catalyst for the 2-alkenylation of 1,3-dicarbonyl compounds with terminal alkynes in Markovnikov manner. The ³¹P{¹H}NMR monitoring experiment shown that, M24 reacts with terminal alkyne to give the vinylidene complex [TpRu(4-CF₃C₆H₄N(PPh₂)₂)(=C=CHR)]⁺OTf⁻. Under the reaction conditions, the vinylidene complex is deprotonated by adventitious water in the substrates to afford the alkynyl complex TpRu(4-CF₃C₆H₄N(PPh₂)₂)(C≡CR), which is the dominant metal-containing species throughout the catalytic process; the alkynyl complex by itself, however, is not the active species as demonstrated by an independent experiment. It is proposed that, triflate acid (H₃O⁺OTf⁻) generated by the deprotonation of the vinylidene moiety of [TpRu(4-CF₃C₆H₄N(PPh₂)₂)(=C=CHR)]⁺OTf⁻ by H₂O reacts with other species presence in the catalytic system to form a new acidic species suspected to be [diagram: see article file for the details of the abstract] (B⁺). To a very small extent, the alkynyl complex is partially protonated by B⁺ to generate very minute amount of the vinylidene complex, which at the reaction temperature equilibrates with its η² -alkyne tautomer; the latter is immediately attacked by the 1,3-dicarbonyl compound or its enol form originated from B⁺ and in close vicinity. The molecular structures of the intermediates in the M24-catalyzed 2-alkenylation of acetylacetone with phenylacetylene are determined by X-ray crystallography. Secondary amines also add to terminal alkynes yielding the corresponding enamines under the influence of M24. Similarly, displacement of the triflate ligand in M24 by the terminal alkyne affords the vinylidene complex [TpRu(4-CF₃C₆H₄N(PPh₂)₂)(=C=CHR)]⁺OTf⁻-. In the presence of amine the vinylidene complex is expected to be in equilibrium with the alkynyl complex TpRu(4-CF₃C₆H₄N(PPh₂)₂)(C≡CR); the equilibrium lies predominately to the side of the latter, which is the most stable metal-containing species throughout the catalysis. The vinylidene complex undergoes nucleophilic attack by the amine at the α-carbon to afford an α-aminovinylruthenium species. Protonlysis of the intermediate regenerates the catalytic cycle. In contrast to the result of the M24-catalyzed 2-alkenylation of 1,3-dicarbonyl compounds with terminal alkynes to yield Markovnikov products, M24 catalyzes hydroamination of terminal alkynes to enamines in anti- Markovnikov fashion.
Description: xxxviii, 300 p. : ill. ; 30 cm.
PolyU Library Call No.: [THS] LG51 .H577P ABCT 2010 Cheung
Rights: All rights reserved.
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