RESUMEN
The utilisation of the PNP iridium pincer complex [Ir(PNP)(COE)][BF4] [PNP = 2,6-bis{(di-tert-butylphosphino)methyl}pyridine; COE = cyclooctene] in the sp(3) C-H activation of methyl propanoate and other related esters was explored. In particular, this study provides further insight into the factors that govern the regioselectivity of such reactions. These included factors such as the steric demands of the substrate, the formation of favourable ring systems as well as the electronic effects that may influence the pKa values of protons. In particular, the effects of water on the outcome of these reactions were of great interest, since earlier literature reports have shown the presence of water to promote selective C-H activation in the α-position of ketones.
RESUMEN
The complex [Ru(Triphos)(TMM)] (Triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane, TMM = trimethylene methane) provides an efficient catalytic system for the hydrogenation of a broad range of challenging functionalities encompassing carboxylic esters, amides, carboxylic acids, carbonates, and urea derivatives. The key control factor for this unique substrate scope results from selective activation to generate either the neutral species [Ru(Triphos)(Solvent)H2] or the cationic intermediate [Ru(Triphos)(Solvent)(H)(H2)](+) in the presence of an acid additive. Multinuclear NMR spectroscopic studies demonstrated together with DFT investigations that the neutral species generally provides lower energy pathways for the multistep reduction cascades comprising hydrogen transfer to CâO groups and C-O bond cleavage. Carboxylic esters, lactones, anhydrides, secondary amides, and carboxylic acids were hydrogenated in good to excellent yields under these conditions. The formation of the catalytically inactive complexes [Ru(Triphos)(CO)H2] and [Ru(Triphos)(µ-H)]2 was identified as major deactivation pathways. The former complex results from substrate-dependent decarbonylation and constitutes a major limitation for the substrate scope under the neutral conditions. The deactivation via the carbonyl complex can be suppressed by addition of catalytic amounts of acids comprising non-coordinating anions such as HNTf2 (bis(trifluoromethane)sulfonimide). Although the corresponding cationic cycle shows higher overall barriers of activation, it provides a powerful hydrogenation pathway at elevated temperatures, enabling the selective reduction of primary amides, carbonates, and ureas in high yields. Thus, the complex [Ru(Triphos)(TMM)] provides a unique platform for the rational selection of reaction conditions for the selective hydrogenation of challenging functional groups and opens novel synthetic pathways for the utilization of renewable carbon sources.
RESUMEN
The coordination chemistry and solution behaviour of Rh(i) and Ru(ii) complexes derived from mixed anhydride ligands of carboxylic acids and phosphorus acids were explored. Similar to the free ligand systems, mixed anhydride complexes rearranged in solution via a number of pathways, with the pathway of choice dependent on the mixed anhydride employed, the auxiliary ligands present as well as the nature of the metal centre. Plausible mechanisms for some of the routes of rearrangement and by-product formation are proposed. Where stability allowed, new complexes were fully characterised, including solid state structures for four of the unrearranged mixed anhydride complexes and two of the interesting rearrangement products.
RESUMEN
Hydrogenation of amides in the presence of [Ru(acac)3] (acacH=2,4-pentanedione), triphos [1,1,1-tris- (diphenylphosphinomethyl)ethane] and methanesulfonic acid (MSA) produces secondary and tertiary amines with selectivities as high as 93% provided that there is at least one aromatic ring on N. The system is also active for the synthesis of primary amines. In an attempt to probe the role of MSA and the mechanism of the reaction, a range of methanesulfonato complexes has been prepared from [Ru(acac)3], triphos and MSA, or from reactions of [RuX(OAc)(triphos)] (X=H or OAc) or [RuH2(CO)(triphos)] with MSA. Crystallographically characterised complexes include: [Ru(OAc-κ(1)O)2(H2O)(triphos)], [Ru(OAc-κ(2)O,O')(CH3SO3-κ(1)O)(triphos)], [Ru(CH3SO3-κ(1)O)2(H2O)(triphos)] and [Ru2(µ-CH3SO3)3(triphos)2][CH3SO3], whereas other complexes, such as [Ru(OAc-κ(1)O)(OAc-κ(2)O,O')(triphos)], [Ru(CH3SO3-κ(1)O)(CH3SO3-κ(2)O,O')(triphos)], H[Ru(CH3SO3-κ(1)O)3(triphos)], [RuH(CH3SO3-κ(1)O)(CO)(triphos)] and [RuH(CH3SO3-κ(2)O,O')(triphos)] have been characterised spectroscopically. The interactions between these various complexes and their relevance to the catalytic reactions are discussed.
Asunto(s)
Amidas/química , Aminas/química , Catálisis , Complejos de Coordinación/química , Hidrogenación , Espectroscopía de Resonancia Magnética , Oxidación-Reducción , Rutenio/químicaRESUMEN
Simple mixed anhydrides are known to pose synthetic difficulties relating to their thermal lability and ways to stabilise such mixed anhydride systems by relying on either electronic or steric effects were therefore explored. Thus, a series of acyloxyphosphines and acylphosphites derived from either propanoic acid or phenylacetic acid were prepared and their in solution stability assessed. These compounds were, where stability allowed, fully characterised using standard analytical techniques. NMR studies, in particular, unearthed interesting coupling behaviour for a number of the acyloxyphosphines and acylphosphites as well as their rearrangement products which could be linked to their chiral nature. Furthermore, the crystal structures for three of the prepared mixed anhydrides were determined using X-ray crystallography and are reported herein.
RESUMEN
Ylideneamine functionalised heterocyclic ligands, 1,3-dimethyl-1,3-dihydro-benzimidazol-2-ylideneamine (I), 3-methyl-3H-benzothiazol-2-ylideneamine (II) or 3,4-dimethyl-3H-thiazol-2-ylideneamine (III), were employed in the preparation of a series of both charged and neutral gold(I) complexes consisting either of a Au(C(6)F(5)) fragment (1-3), a [Au(PPh(3))](+) unit (4-6) or a [Au(NHC)](+) unit (7) coordinated to the imine nitrogen of the neutral ylideneamine ligand. These complexes were fully characterised by various techniques including X-ray diffraction. In addition, the antitumour and antimalarial potential of selected compounds were assessed in a preliminary study aimed at determining the medicinal value of such compounds. Complexation of the azol-2-ylideneamine ligands with [Au(PPh(3))](+) increases their antitumour as well as antimalarial activity.