Hydroxyl and ester functionalized N-heterocyclic carbene complexes of iridium(i): Efficient catalysts for transfer hydrogenation reactions

Hydroxyl and ester-functionalized iridium(i) complexes of N-heterocyclic carbenes (3a-e) were obtained by transmetalation reactions from the in situ prepared silver(i)-NHC complexes and characterized by IR, NMR, mass spectroscopies, and elemental analysis. X-ray diffraction studies on single crystals of 3a, 3c and 3d verify the square planar geometry at the iridium center. Ester functionalized iridium(i)-NHC complexes were found to be highly active and selective catalysts for the transfer hydrogenation reactions of various aldehydes and ketones. The influence of different ester substituents on the reactivity of the complexes was studied and the complex with a pivaloyl substituent (3d) showed the best activity (TON up to 10 000). © 2014 the Partner Organisations

Acceptorless Dehydrogenative Oxidation of Secondary Alcohols Catalysed by Cp*IrIII–NHC Complexes

A series of new IrIIIcomplexes with carbene ligands that contain a range of benzyl wingtip groups have been prepared and fully characterised by NMR spectroscopy, HRMS, elemental analysis and X-ray diffraction. All the complexes were active in the acceptorless dehydrogenation of alcohol substrates in 2,2,2-trifluoroethanol to give the corresponding carbonyl compounds. The most active complex bore an electron-rich carbene ligand; this complex was used to catalyse the highly efficient and chemoselective dehydrogenation of a wide range of secondary alcohols to their respective ketones, with turnover numbers up to 1660. Mechanistic studies suggested that the turnover of the dehydrogenation reaction is limited by the H2-formation step. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimEngineering and Physical Sciences Research Council, EPSRC: EP/K039687/

Iridium(I)-Catalyzed Alkylation Reactions to Form ?-Alkylated Ketones

A highly effective and green procedure for the formation of ?-alkylated ketones has been disclosed via the reaction of primary alcohols with secondary alcohols and ketones by using [IrCl(COD)(NHC)] complexes as a catalyst. Various ?-alkylated ketones were obtained in high yields from the alkylation of alcohol with alcohol and ketone with alcohol through a borrowing hydrogen reaction by using 0.05-0.5 mol % iridium(I) and a catalytic amount of KOH (5-10 mol %) as the base under air atmosphere and within very short reaction times. © 2018 American Chemical Society

Saponins from Cephalaria aristata C. Koch

WOS: 000343058200003One new oleanane-type saponin, 3-O-beta-D-glucopyranosyl-(1 -> 4)-beta-D-xylopyranosyl-(1 -> 3)-alpha-D-rhamnopyranosyl-(1 -> 2)-alpha-L-arabinopyranosylhederagenin 28-O-beta-D-glucopyranosyl-(1 -> 6)-beta-D-glucopyranosyl ester (1) was isolated from the MeOH extract of whole plant parts of Cephalaria aristata C. Koch along with three known oleanane-type saponins, 3-O-alpha-L-rhamnopyranosyl-(1 -> 2)-alpha-L-arabinopyranosyl hederagenin 28-O-(beta-D-glucopyranosyl-(1 -> 6)-beta-D-glucopyranosyl) ester (2), 3-O-beta-D-glucopyranosyl-(1 -> 4)-beta-D-xylopyranosyl-(1 -> 3)-alpha-L-rhamnopyranosyl-(1 -> 2)-alpha-L-arabinopyranosyl hederagenin (3) and 3-O-alpha-L-rhamnopyranosyl-(1 -> 2)-alpha-L-arabinopyranosylhederagenin (4). Also three triterpenoids and a steroid glucoside oleanolic acid (5), beta-amyrin (6), 20-beta-hydroxyursolic acid (7) and 29-hydroxystigmast-5-en-3-O-beta-D-glucopyranoside (8). Their structures were established by the extensive use of 1D-and 2D-NMR experiments along with ESIMS and HRMS analysis