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Müller-Bunz, Helge
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Müller-Bunz, Helge
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Müller-Bunz, Helge
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Now showing 1 - 10 of 36
- PublicationPalladium carbene complexes for selective alkene di- and oligomerization(ACS, 2012-01-13)
; ; ; ; ; ; ; A series of palladium complexes were synthesized that comprise three sterically different C,N-bidentate coordinating NHC-pyridine ligands (NHC = N-heterocyclic carbene). In one set, the pyridine and the carbene are linked by a flexible CH2 group (a), in the other two sets, the two ligand units are directly linked and feature a shielding mesityl substituent on the carbene and either an unsubstituted pyridine (b) or a xylyl-substituted pyridine unit (c). Investigation of the reactivity of cationic complexes [Pd(C^N)Me(NCMe)]+, 6, analogues to Brookhart’s α-diimine system, towards alkenes showed a strong correlation between the catalytic activity and selectivity and the ligand setting. While 6a was inactive in ethylene conversion, 6b afforded low-molecular weight olefins (oligomerization), and 6c produced exclusively butene (dimerization). With styrene as substrate, exclusive dimerization occurred with all three complexes. Steric and electronic factors were identified that govern the disparate activity and selectivity, and that allow for efficient tailoring of the catalytic performance.552Scopus© Citations 57 - PublicationNovel approach to the synthesis of aliphatic and aromatic alpha-keto acids(Elsevier, 2012-09)
; ; ; ; A new practical and efficient synthesis of alpha-keto acids was accomplished starting from the synthon 1,4-diacetylpiperazine-2,5-dione. The synthesis encompasses both aromatic and aliphatic substrates proving to be versatile and innovative with excellent carbon economy and recycling of the glycine by-product.Scopus© Citations 14 487 - Publication
1015Scopus© Citations 343 - PublicationMesoionic oxides: facile access from triazolium salts or triazolylidene copper precursors and, catalytic relevanceReaction of CsOH with triazolium salts affords mesoionic compounds containing an exocyclic oxygen; the same product is obtained by reaction of the corresponding Cu(I) triazolylidenes with CsOH and represents an unusual reactivity pattern of N-heterocyclic carbene precursors that has implications for carbene copper-catalyzed reactions.
380Scopus© Citations 37 - PublicationSynthesis and Structural Elucidation of 1,2‐Disubstituted 3‐FluoropiperidinesThe work described details the reaction between Selectfluor® and a series of 1‐carbonyloxy and 1‐sulfonyl 2‐piperidines in order to generate 3‐fluoro‐2‐methoxypiperidines 3a–f. Their subsequent reaction with allyltrimethylsilane, in the presence of BF3 and TiCl4, is then reported. Studies involving a combination of single‐crystal X‐ray crystallography and NMR spectroscopy indicate that the allylation process is cis‐selective for both carbamate and sulfonamide variants and that optimal levels of diastereoselectivity are obtained using the N‐2‐nitrobenzene sulfonyl (2‐Ns) group. In this manner, the synthesis of a series of 2‐allyl 3‐fluoro‐substituted piperidines (5a, c–f) was achieved. The conversion of both the cis and trans‐N‐tosyl adducts (5d) into 3‐fluorinated analogues of the natural products pelletierine (10) and coniine (11) is subsequently detailed.
325Scopus© Citations 6 - Publication[Ru(bpy)3]2+ analogues containing a N-heterocyclic carbene ligandA synthetic procedure is described that provides access to [Ru(bpy)3]2+ analogues in which one bpy ligand is replaced by a C,N-bidentate coordinating carbene-benzimidazole ligand (bpy = 2,2’-bipyridine). These new complexes were prepared by first installing the chelating carbene ligand onto a Ru(cymene) platform and subsequent ligand substitution using bpy or terpy (terpy = 2:2’,6’:2’’-terpyridine). The carbene ligand significantly affects the optical properties of the complex and lowers the ruthenium(II) oxidation potential substantially. Such modifications may be advantageous for the development of new classes of photosensitizer materials.
964Scopus© Citations 52 - PublicationPreparation and characterization of 4-dimethylaminopyridine-stabilized palladium nanoparticles(American Chemical Society, 2007-12-04)
; ; 4-Dimethylaminopyridine (DMAP)-stabilized palladium nanoparticles with a mean diameter of 3.4 +/- 0.5 mn are prepared from the aqueous phase reduction of Na2PdCl4 using NaBH4 in the presence of DMAP. TEM and UV-vis spectroscopy characterization of the nanoparticle dispersion shows no obvious change in the nanoparticles several months after preparation. H-1 NMR spectroscopy of the nanoparticles shows that the nanoparticle dispersion also contains a boron/DMAP complex and two palladium/DMAP complexes. One of the palladium complexes crystallizes out of the dispersion and is identified as Pd(DMAP)(4)(OH)(2) by X-ray crystallography. Following extensive analysis, it is believed that the palladium/DMAP complexes are formed following the oxidation of the palladium nanoparticles. The prepared nanoparticle dispersion promotes selective hydrogen/deuterium (H/D) exchange on the carbon atoms alpha to the endocyclic nitrogen atom on the DMAP-stabilizing ligands through reaction with D2O. This activity is attributed to the presence of the nanoparticles rather than to the presence of the oxidized palladium/DMAP complexes.785Scopus© Citations 42 - PublicationPEPPSI-Type Palladium Complexes Containing Basic 1,2,3-Triazolylidene Ligands and Their Role in Suzuki-Miyaura Catalysis(Wiley, 2012-05-07)
; ; ; ; ; A series of PEPPSI-type palladium(II) complexes was synthesized that contain 3-chloropyridine as an easily removable ligand and a triazolylidene as a strongly donating mesoionic spectator ligand. Catalytic tests in Suzuki–Miyaura cross-coupling reactions revealed the activity of these complexes towards aryl bromides and aryl chlorides at moderate temperatures (50 °C). However, the impact of steric shielding was the inverse of that observed with related normal Nheterocyclic carbenes (imidazol-2-ylidenes) and sterically congested mesityl substituents induced lower activity than small alkyl groups. Mechanistic investigations, including mercury poisoning experiments, TEM analyses, and ESI mass spectrometry, provide evidence for ligand dissociation and the formation of nanoparticles as a catalyst resting state. These heterogeneous particles provide a reservoir for soluble palladium atoms or clusters as operationally homogeneous catalysts for the arylation of aryl halides. Clearly, the substitution of a normal N-heterocyclic carbene for a more basic triazolylidene ligand in the precatalyst has a profound impact on the mode of action of the catalytic system.590Scopus© Citations 152 - PublicationCarbene transfer from triazolylidene gold complexes as a potent strategy for inducing high catalytic activity(American Chemical Society, 2013-09-04)
; ; ; ; A series of gold(I) complexes [AuCl(trz)] were synthesized that contain 1,2,3-triazolylidene (trz) ligands with variable wingtip groups. In the presence of AgBF4, these complexes undergo ligand redistribution to yield cationic complexes [Au(trz)2]BF4 in high yields as a result of efficient carbene transfer. Identical reactivity patterns were detected for carbene gold complexes comprised of Arduengo-type IMes ligands (IMes=N,Nâ ²-dimesityl-imidazol-2-ylidene). Reaction of cationic complexes [Au(trz)2]+ with [AuCl(trzâ ²)] afforded the heteroleptic complex [Au(trz)(trzâ ²)]+ and [AuCl(trz)] (trz, trzâ ²=triazolylidene ligands with different wingtip groups). Carbene transfer occurs spontaneously, yet is markeldy rate-enhanced in the presence of Ag+. The facile carbene transfer was exploited as a catalyst activation process to form active gold species for the aldol condensation of isocyanides and aldehydes to form oxazolines. The catalytic activity is strongly dependent on the presence of Ag+ ions to initiate catalyst activation. High turnovers (105) and turnover frequencies (10 4 h-1) were accomplished. Structural analysis at early stages of the reaction support the critical role of triazolylidene dissociation to activate the precatalyst and dynamic light scattering revealed the presence of nanoparticles (±100 nm diameter) as potential catalytically active species. Furthermore, the triazolylidene scaffold had no impact on the diastereoselectivity of the oxazoline formation, and chiral triazolylidenes did not induce any asymmetry in the product. The facile dissociation of carbenes from [AuCl(carbene)] in the presence of Ag+ ions suggests a less stable Au-Ccarbene interaction than often assumed, with potential implications for gold-catalyzed reactions that employ a silver salt as (putative) halide scavenger.485Scopus© Citations 117 - PublicationDirect evidence of a multicentre halogen bond: unexpected contraction of the P–XXX–P fragment in triphenylphosphine dihalidesTriphenylhalophosphonium halides, Ph3PX2, form crystals comprising bridged linear cations [Ph3P-X-X-X-PPh3]+ where the X3 bridge is shortened from 6.56 Å in Cl-Cl-Cl to 6.37 Å in the Br-Br-Br system. It is proposed that this structure is stabilized by five-center/six-electron (5c-6e) hypervalent interactions.
494Scopus© Citations 27