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991.
Azomethine Ylides from Nitrones: Using Catalytic nBuLi for the Totally Stereoselective Synthesis of trans‐2‐Alkyl‐3‐oxazolines 下载免费PDF全文
Veronica Juste‐Navarro Dr. Ignacio Delso Prof. Tomás Tejero Prof. Pedro Merino 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(33):11527-11532
The cycloaddition of azomethine ylide N‐oxides (nitrone ylides) with aldehydes provides 3‐oxazolines in a completely stereoselective manner in the presence of a catalytic amount of n‐butyllithium. The process involves an initial nucleophilic attack on the aldehyde, followed by intramolecular oxygen addition to the nitrone moiety and lithium‐assisted elimination of water, regenerating the catalytic species. Various Li‐based catalytic systems are possible and the in situ generated water is required for continuing the catalytic cycle. The best results are observed with 20 mol % of n‐butyllithium, whereas the use of stoichiometric amounts inhibit the rate of catalysis. Experimental, spectroscopic, and computational mechanistic studies have provided evidence of lithium‐ion catalysis and rationalized several competing catalytic pathways 相似文献
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Dr. David Cantillo Prof. Martín Ávalos Prof. Reyes Babiano Prof. Pedro Cintas Prof. José L. Jiménez Prof. Juan C. Palacios 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(28):8795-8799
What accounts for a particular chiral selection in the case of a few sugars of prebiotic relevance, thereby mirroring the asymmetry observed in nature? By using first‐principles calculations, the generation of pentoses from glycolaldehyde (the initial product of the autocatalytic formose reaction), which has been detected in outer space), has been modeled by using L ‐Val‐L ‐Val as a primeval catalyst. Our theoretical study provides insight into the mechanism of this reaction and satisfactorily explains a few key molecular events. Our rationale agrees with the reported experimental data and shows that the D ‐configuration is only favored for ribose. L ‐pentoses are usually favored in the presence of L ‐configured dipeptides, as observed experimentally, although no chiral selection could be observed in the case of xylose. These results confirm that a prebiotic sugar soup could be fine‐tuned in the presence of shorter peptides as catalysts and that D ‐ribose would have also resulted in an advantageous imbalance for further amplification and chemical evolution. 相似文献
995.
Eutectic mixtures as a green alternative for efficient catalyst recycling in atom transfer radical polymerizations 下载免费PDF全文
Pedro Maximiano Patrícia V. Mendonça Madson R. E. Santos João R. C. Costa Tamaz Guliashvili Arménio C. Serra Jorge F. J. Coelho 《Journal of polymer science. Part A, Polymer chemistry》2017,55(3):371-381
A new solvent mixture, based on ethanol/reline (EM: eutectic mixture), was investigated for the supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) of methyl acrylate (MA) near room temperature, for the first time, affording complete catalyst recovery and reuse. The kinetic results revealed that the polymerizations were controlled, with polymers having narrow molecular weight distributions (? < 1.2). The “living” character of the resultant PMA was confirmed by the synthesis of a well‐defined PMA‐b‐PBA block copolymer. Remarkably, it was demonstrated that the Cu(0)/CuBr2/Me6TREN (Me6TREN: tris[2‐(dimethylamino)ethyl]amine) could be recovered from the final reaction mixture and reused for new successful SARA ATRP of MA, suggesting that the reported system could be very attractive from both the economic and environmental perspectives. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 371–381 相似文献
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Ambient temperature SARA ATRP for meth(acrylates), styrene,and vinyl chloride using sulfolane/1‐butyl‐3‐methylimidazolium hexafluorophosphate‐based mixtures 下载免费PDF全文
1000.
We develop an integral version of Deligne cohomology for smooth proper real varieties. For this purpose the role played by singular cohomology in the complex case has to be replaced by the ordinary bigraded
Gal(\mathbbC/\mathbbR){Gal(\mathbb{C}/{\mathbb{R}})}-equivariant cohomology of Lewis et al. (Bull Am Math Soc (N.S.) 4(2):208–212, 1981), the equivariant counterpart of singular cohomology. The theory is aimed at giving more precise information about the 2-primary
components of regulators. We establish basic properties and give a geometric interpretation for the groups in dimension 2
in weights 1 and 2. 相似文献