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101.
Dieter Seebach Hans-Otto Kalinowski Bahram Bastani Gerhard Crass Hermann Daum Hennig Drr Nicolaas P. Dupreez Volker Ehrig Werner Langer Christa Nüssler Hoc-An Oei Manfred Schmidt 《Helvetica chimica acta》1977,60(2):301-325
Preparation of Auxiliaries for Asymmetric Syntheses from Tartaric Acid. Additions of Butyllithium to Aldehydes in Chiral Media. Chiral derivatives of the complexing 1,2-diheterosubstituted ethanes A–D are prepared from tartaric acid. The key starting materials are the succinic acid derivative 1 , the dioxolane 2a , and the diamide 3a . These are converted to the ethers, alkoxyamines, and alkylthio-amines listed in the first column of Table 2 which also contains the derivatives 21c, 22d , and 23d made from lactic acid, malic acid, and proline, respectively. It is shown that the highest optical yields (up to 40%) in reactions of butyllithium with aldehydes are obtained when mixtures of (?)-1,2,3,4-tetramethoxy-butane ( 4b ), (+)-2,3-dimethoxy-N,N,N′,N′-tetramethyl-1,4-butanediamine ( 17a ), and (?)-1,4-dimethoxy-N,N,N′,N′-tetramethyl-2,3-butanediamine ( 14c ) with pentane are used at temperatures down to ?150° and ratios of auxiliary/butyllithium of up to 10:1 (see equation (1), Tables 2–4). 相似文献
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103.
Ajellal N Lyubov DM Sinenkov MA Fukin GK Cherkasov AV Thomas CM Carpentier JF Trifonov AA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(18):5440-5448
A series of new bis(guanidinate) alkoxide Group 3 metal complexes [Ln((Me3Si)2NC(NiPr)2)2(OR)] (R=OtBu, Ln=Y, Nd, Sm, Lu; R=OiPr, Ln=Y, Nd, Lu) has been synthesized. X-ray structural determinations revealed that bis(guanidinate) tert-butoxides are monomeric complexes. The isopropoxide complex [Y((Me3Si)2NC(NiPr)2)2(OiPr)] undergoes slow decomposition in solution, to afford the unusual dimeric amido complex [(Y((Me3Si)2NC(NiPr)2)2(mu-N(iPr)C triple chemical bond N))2]. Complexes [Ln((Me3Si)2NC(NiPr)2)2(OR)] (R=OtBu, Ln=Y, Nd, Sm, Lu; R=OiPr, Ln=Y, Nd, Lu) are active catalysts/initiators for the ROP of rac-lactide and rac-beta-butyrolactone under mild conditions. Most of those polymerizations proceed with a significant degree of control. Bis(guanidinate) alkoxides appear to be well suited for achieving immortal polymerization of lactide, through the introduction of large amounts of isopropanol as a chain-transfer agent. The synthesized complexes are able to promote the stereoselective ROP of rac-beta-butyrolactone to afford syndiotactic poly(hydrobutyrate) through a chain-end control mechanism, while they are surprisingly non-stereoselective for the ROP of lactide under strictly similar conditions. 相似文献
104.
Ignatov SK Rees NH Merkoulov AA Dubberley SR Razuvaev AG Mountford P Nikonov GI 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(1):296-310
Reactions of imido complexes [M(Cp)(=NR')(PR'3)2] (M=V, Nb) with silanes afford a plethora of products, depending on the nature of the metal, substitution at silicon and nitrogen and the steric properties of the phosphine. The main products are [M(Cp)(=NR')(PR3)(H)(SiRnCl3-n)] (M=V, Nb; R'=2,6-diisopropylphenyl (Ar), 2,6-dimethylphenyl (Ar')), [Nb(Cp)(=NR')(PR'3)(H)(SiPhR2)] (R2=MeH, H2), [Nb(Cp)(==NR')(PR'3)(Cl)(SiHRnCl2-n)] and [Nb(Cp)(eta 3-N(R)SiR2--H...)(PR'3)(Cl)]. Complexes with the smaller Ar' substituent at nitrogen react faster, as do more acidic silanes. Bulkier groups at silicon and phosphorus slow down the reaction substantially. Kinetic NMR experiments supported by DFT calculations reveal an associative mechanism going via an intermediate N-silane adduct [Nb(Cp){=N(-->SiHClR2)R'}(PR'3)2] bearing a penta-coordinate silicon centre, which then rearranges into the final products through a Si--H or Si--Cl bond activation process. DFT calculations show that this imido-silane adduct is additionally stabilized by a Si--HM agostic interaction. Si--H activation is kinetically preferred even when Si--Cl activation affords thermodynamically more stable products. The niobium complexes [NbCp(=NAr)(PMe3)(H)(SiR2Cl)] (R=Ph, Cl) are classical according to X-ray studies, but DFT calculations suggest the presence of interligand hypervalent interactions (IHI) in the model complex [Nb(Cp) (==NMe)(PMe3)(H)(SiMe2Cl)]. The extent of Si--H activation in the beta-Si--HM agostic complexes [Cp{eta 3-N(R')SiR2--H}M(PR'3)(Cl)] (R'=PMe3, PMe2Ph) primarily depends on the identity of the ligand trans to the Si--H bond. A trans phosphine leads to a stronger Si--H bond, manifested by a larger J(Si--H) coupling constant. The Si--H activation diminishes slightly when a less basic phosphine is employed, consistent with decreased back-donation from the metal. 相似文献
105.
Khalimon AY Holland JP Kowalczyk RM McInnes EJ Green JC Mountford P Nikonov GI 《Inorganic chemistry》2008,47(3):999-1006
Reaction of Mo(NAr)2Cl2(DME) (Ar=2,6-C6H3iPr2, DME=1,2-dimethoxyethane) with NaBH4 and PMe3 in THF formed the paramagnetic Mo(V) d1 borohydride complex Mo(NAr)2(PMe3)2(eta2-BH4) (1). Compound 1, which was characterized by EPR spectroscopy and X-ray diffraction analysis, provides a rare example both of a paramagnetic bis(imido) group 6 compound and a structurally characterized molybdenum borohydride complex. Density functional theory calculations were used to determine the electronic structure and bonding parameters of 1 and showed that it is best viewed as a 19 valence electron compound (having a primarily metal-based SOMO) in which the BH4- ligand behaves as a sigma-only, 2-electron donor. 相似文献
106.
107.
108.
Dr. Iryna D. Alshakova Hayden C. Foy Prof. Travis Dudding Prof. Georgii I. Nikonov 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(50):11734-11744
This work unveils the reactivity patterns, as well as ligand and additive effect on alkali-metal-base-catalyzed transfer hydrogenation of ketones. Crucially to this reactivity is the presence of a Lewis acid (alkali cation), as opposed to a simple base effect. With aryl ketones, the observed reactivity order is Na+>Li+>K+, whereas for aliphatic substrates it follows the expected Lewis acidity, Li+>Na+>K+. Importantly, the reactivity pattern can be drastically changed by adding ligands and additives. Kinetic, labelling, and competition experiments as well as DFT calculations suggested that the reaction proceeds through a concerted direct hydride-transfer mechanism, originally suggested by Woodward. The lithium cation was found to be intrinsically more active than heavier congeners, but in the case of aryl ketones a decrease in reaction rate was observed at ≈40 % conversion with lithium cations. Noncovalent-interaction analysis revealed that this deceleration effect originated from specific noncovalent interactions between the aryl moiety of 1-phenylethanol and the carbonyl group of acetophenone, which stabilize the product in the coordination sphere of lithium and thus poison the catalyst. The ligand/additive effect is a complicated phenomenon that includes a combination of several factors, such as the decrease of activation energy by ligation (confirmed by distortion/interaction calculations of N,N,N’,N’-tetramethylethylenediamine, TMEDA) and the change in relative stabilization of reagents and substrates in the solution and the coordination sphere of the metal. Finally, we observed that lithium-base-catalyzed transfer hydrogenation can be further facilitated by the addition of an inexpensive and benign reagent, LiCl, which likely operates by re-initiating the reaction on a new lithium center. 相似文献
109.