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41.
42.
Dieter Seebach 《Angewandte Chemie (International ed. in English)》1988,27(12):1624-1654
The chemistry of lithium enolates is used to demonstrate that complex structures held together by noncovalent bonds (“supramolecules”) may dramatically influence the result of seemingly simple standard reactions of organic synthesis. Detailed structural data have been obtained by crystallographic investigations of numerous Li enolates and analogous derivatives. The most remarkable features of these structures are aggregation to give dimers, tetramers, and higher oligomers, complexation of the metal centers by solvent molecules and chelating ligands, and hydrogen-bond formation of weak acids such as secondary amines with the anionoid part of the enolates. The presence in nonpolar solvents of the same supramolecules has been established by NMR-spectroscopic, by osmometric, and by calorimetric measurements. The structures and the order of magnitude of the interactions have also been reproduced by ab-initio calculations. Most importantly, supramolecules may be product-forming species in synthetic reactions of Li enolates. A knowledge of the complex structures of Li enolates also improves our understanding of their reactivity. Thus, simple procedures have been developed to avoid complications caused by secondary amines, formed concomitantly with Li enolates by the common methods. Mixtures of achiral Li enolates and chiral Li amides can give rise to enantioselective reactions. Solubilization by LiX is observed, especially of multiply lithiated compounds. This effect is exploited for alkylations of N-methylglycine (sarcosine) CH2 groups in open-chain oligopeptides. Thus, the cyclic undecapeptide cyclosporine, a potent immunosuppressant, is converted into a THF-soluble hexalithio derivative (without epimerization of stereogenic centers) and alkylated by a variety of electrophiles in the presence of either excess lithiumdiisopropyl amide or of up to 30 equivalents of lithium chloride. Depending on the nature of the LiX additive, a new stereogenic center of (R) or (S) configuration is created in the peptide chain by this process. A structure-activity correlation in the series of cyclosporine derivatives thus available is discussed. 相似文献
43.
η5-C5H5V(NO)2CO is prepared in 40% yield by the photo-reaction between η5-C5H5V(CO)4 and [Co(NO)2Br]2.η5-C5H5V(NO)2CO reacts by an SN1 mechanism with various phosphines PZ3 to yield η5-C5-H5V(NO)2PZ3. The phosphine complexes are also obtained by photo-induced ligand interchange between η5-C5H5V(CO)3PZ3 and [Co(NO)2Br]2, or η5-C5H5V(CO)4 and Co(NO)2Br(PZ3). In all cases, the main cobalt species formed is Co(NO)(CO)3. While the one-bond vanadiumphosphorus coupling constants of most of the phosphine complexes are virtually the same (ca 410 Hz),the chemical shift values δ(51V) (?1328 to ?973 ppm rel. VOCl3) decrease in the order PF3 > CO > P(OR)3 > P(alkyl)3 > PPh3 > PPh(NEt2)2, reflecting the decreasing π-acceptor ability of the ligands. δ(51V) also decreases in the series of alkylphosphines PR3 (R = Me, Et, Prn, Bui, Pri, BUt) as the cone angle of PR3increases. 相似文献
44.
Dieter Seebach 《Angewandte Chemie (International ed. in English)》1979,18(4):239-258
The past decade of organic chemistry may be charcterized as a period of violent development of new synthetic methods. This was accompained by a systematization of the analysis of synthetic problems (synthetic strategy). The planning of the synthesis of an organic target molecule is greatly facilitated by distinguishing between reagents X(C)n … with normal reactivity (acceptor properties at C1,3,5…, donor properties at X, C2,4…) and with reactivity umpolung (acceptor properties at X, C2,4…, donor properties at C1,3,5…). In this context, reactivity umpolung turned out to be useful as a heuristic principle, as a classification scheme, and as an aid for locating so-called strategic bonds (synthon, transform, and antithesis according to E. J. Corey). There are six principal methods of umpolung: 1 2n-oxidation, heteroatom exchange and modification, homologation and its reversal, the cyclopropane “trick”, use of acetylenes, and redox reactions; under certain circumstances none of these techniques is necessary in cases where direct umpolung is possible. Throughout the article, normal reactivity is indicated by green print; reactivity umpolung by red print. 相似文献
45.
A. v. Richthofen Rainer Cremer Ralph Domnick Dieter Neuschütz 《Analytical and bioanalytical chemistry》1997,358(1-2):308-311
Ti-Al-O layers were deposited on Si-<100> wafers at 500 °C by means of reactive magnetron sputtering ion plating (R-MSIP).
An Al-target was sputtered in rf-mode and a Ti-target in dc-mode simultaneously by an oxygen/argon plasma. The influence of
the Al- and Ti-sputter powers on composition, structure, and morphology of the Ti-Al-O layers and the binding states of the
components were investigated. The analysis with EPMA, XPS, AES and TEM yielded the following results: Ti-Al-O coatings with
different Ti, Al, and O contents in the range of TiO2 to Al2O3 were grown. TEM structure analysis revealed: the pure TiO2 film consisted of the tetragonal phases rutile and anatase; the two structures were found in the titanium-rich Ti-Al-O film,
too, but with significant smaller lattice constants. The aluminium-rich Ti-Al-O film displayed the same cubic structure of
γ-Al2O3 as determined for the pure Al2O3 film, but the lattice constant is significant lower. Evaluation of the TEM pattern of the film with a Ti/Al ratio of 0.8
indicates a hexagonal structure with lattice constants similar to those of κ′-Al2O3. All films are nanocrystalline and not textured.
Received: 24 June 1996 / Revised: 27 December 1996 / Accepted: 4 January 1997 相似文献
46.
alpha-(N-Carbamoylalkyl)cuprates couple with enol triflates derived from carbocyclic and heterocyclic (i.e., piperidinones) beta-keto esters. Product yields are higher with the alkyl(cyano)cuprates [i.e., RCu(CN)Li, 56-93%] than with the dialkylcuprate reagents (i.e., R(2)CuLi.LiCN). An enol nonaflate works as well as the corresponding enol triflate. A facile synthetic route to gamma-amino alpha,beta-enoates not readily prepared from gamma-keto-alpha,beta-enoates is thus established. The gamma-amino-alpha,beta-enoates, available via N-Boc deprotection, can be cyclized to annulated pyrrolin-2-ones. 相似文献
47.
Zijp EJ van der Vlugt JI Tooke DM Spek AL Vogt D 《Dalton transactions (Cambridge, England : 2003)》2005,(3):512-517
Chiral aminophosphines Ph2PN(R)(CH2)nN(R)PPh2 1-4 [n= 2, R = CH(CH3)(Ph) 1; n= 3, R = CH(CH2CH3)(Ph) 2, n= 2, R = CH(CH3)(1-naphthyl) 3; n= 2, R = CH(CH3)(C6H11) 4] were synthesized by the reaction of ClPPh2 with the appropriate easily accessible enantiopure amine building blocks. For compounds 1 and 2, the corresponding selenides 5 and 6 were prepared to determine the electronic character of the phosphine moieties. By reaction of 1 with either PdCl2(cod) or PdCl(CH3)(cod) the cis-complexes 7 and 8 were obtained. The molecular structure for complex 7, cis-[PdCl2(1)], was determined by X-ray crystallography. Reaction of PtCl2(cod) with 1 or 2 yielded the corresponding monomeric cis-isomers 9 and 10. The rhodium derivative [RhCl(CO)(1)] (11) was obtained as a mixture of cis and trans-isomers. Preliminary results in the rhodium catalyzed hydroformylation of styrene and vinyl acetate, with ee's up to 51% and high regioselectivities, showed the potential of these chiral aminophosphines for homogeneous catalysis. 相似文献
48.
Treatment of transition-metal—ammonia complexes with ketones yields complexes with RR′CNH ligands. Of particular interest is the stabilization of dialkylketimines such as e.g. (CH3)2CNH and C6H10NH in [M(CO)5{NHC(CH3)2}] or [M(CO)5 {NHC6H10}] (M = Cr, Mo, W). The principle of synthesis may be applied to a wide range of different metals and types of complexes, as can be shown by the synthesis of [C5H5Mn(CO)2 {NHC(CH3)2}], [C5H5Fe(CO)2{NHC(CH3)2}]PF6, [M(CO)4L2] (M = Cr, Mo, W; L = (CH3)2CNH, C6H10NH) and [W(CO)3(diphos){NHC(CH3)}2]. Treatment of [Cr(CO)5NH3] with urotropine gives [Cr(CO)5 {N4(CH2)6}] which is also obtained from [Cr(CO)5THF] and urotropine. The methods of preparation, reactions and spectroscopic properties of the complexes are reported. 相似文献
49.
The 13C-NMR spectra of the synthetic membrane modifying nonadecapeptide Boc-(Aib-l-Ala)5-Gly-Ala-Aib-Pro-Ala- Aib-Aib-Glu(OBz)-Gln-OMe (Aib = α-aminoisobutyric acid), and of synthetic intermediates were used for conformational analysis in solution. The assignments of the 13C-NMR signals of Aib are based on the magnetic nonequivalence (MNE) of the geminal Cβ-signals in asymmetric environment resulting in a shift difference of 0.2–0.5 ppm due to neighbouring chiral residues. More than 4 ppm MNE are observed due to α-helical conformation and about 2.5 ppm for Aib situated in the corners of a rigid β-turn. The Ala-Cα signal is also sensitive to different secondary structures. The Cα signal for C-terminal alanine is found at 49–50 ppm, and for alanine within unordered oligopeptides it absorbs at 50–51 ppm. α-Helical environment shifts the Ala-Cα signal to lower field down to 54 ppm. In methanolic solution the nonadecapeptide shows a α-helical N-terminal region. For the C-terminus beginning with proline-14 no periodically ordered conformation is observed, and we suggest a sequence of β-turns. Furthermore the typical E/Z isomerism of the prolyl-peptide bond can be observed on proline itself and on its neighbour alanine. 相似文献
50.
Preparation of 1,2-Diarylethylenediamines by Aminative Reductive Coupling of Aromatic Aldehydes with Low-Valent Titanium Reagents In a novel McMurry- Type one-pot reaction, aromatic aldehydes and secondary amines are poupled of give the N, N, N′, N′-tetraalkyl-1,2-diarylethylendiamines 1–22 (Table 3). To this end, a lithium dialkylamide is added to an aromatic aldehyde to give the adduct B which is then treated with 1 equiv. of TiC14 to yield a coloured suspension of a reagent synthetically equivalent to a iminium salt ( C/D in Scheme 4). After treatment with a low-valent Ti reagent which is prepared by reduction of TiC14 with either K or, preferably, Mg, the coupling products are isolated in 23 to 81% yield as a 1:1 mixture of the diastereoisomers (meso- and rac-form). These are separated either by chromatography or by crystallization and characterized. 相似文献