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971.
Dr. Lingfang Wang Dr. Christos E. Kefalidis Dr. Sourisak Sinbandhit Dr. Vincent Dorcet Prof. Dr. Jean‐François Carpentier Prof. Dr. Laurent Maron Dr. Yann Sarazin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(40):13463-13478
The tin(II) complexes {LOx}Sn(X) ({LOx}?=aminophenolate ancillary) containing amido ( 1 – 4 ), chloro ( 5 ), or lactyl ( 6 ) coligands (X) promote the ring‐opening polymerization (ROP) of cyclic esters. Complex 6 , which models the first insertion of L ‐lactide, initiates the living ROP of L ‐LA on its own, but the amido derivatives 1 – 4 require the addition of alcohol to do so. Upon addition of one to ten equivalents of iPrOH, precatalysts 1 – 4 promote the ROP of trimethylene carbonate (TMC); yet, hardly any activity is observed if tert‐butyl (R)‐lactate is used instead of iPrOH. Strong inhibition of the reactivity of TMC is also detected for the simultaneous copolymerization of L ‐LA and TMC, or for the block copolymerization of TMC after that of L ‐LA. Experimental and computational data for the {LOx}Sn(OR) complexes (OR=lactyl or lactidyl) replicating the active species during the tin(II)‐mediated ROP of L ‐LA demonstrate that the formation of a five‐membered chelate is largely favored over that of an eight‐membered one, and that it constitutes the resting state of the catalyst during this (co)polymerization. Comprehensive DFT calculations show that, out of the four possible monomer insertion sequences during simultaneous copolymerization of L ‐LA and TMC: 1) TMC then TMC, 2) TMC then L ‐LA, 3) L ‐LA then L ‐LA, and 4) L ‐LA then TMC, the first three are possible. By contrast, insertion of L ‐LA followed by that of TMC (i.e., insertion sequence 4) is endothermic by +1.1 kcal mol?1, which compares unfavorably with consecutive insertions of two L ‐LA units (i.e., insertion sequence 3) (?10.2 kcal mol?1). The copolymerization of L ‐LA and TMC thus proceeds under thermodynamic control. 相似文献
972.
Yanxia Zhao Dr. Yibo Lei Qingsong Dong Prof. Biao Wu Prof. Xiao‐Juan Yang 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(36):12059-12066
The reactions of dialumane [L(thf)Al? Al(thf)L] ( 1 , L=[{(2,6‐iPr2C6H3)NC(Me)}2]2?) with stilbene and styrene afforded the oxidation/insertion products [L(thf)Al(CH(Ph)? CH(Ph))AlL] ( 2 ) and [L(thf)Al(CH(Ph)? CH2)Al(thf)L] ( 3 ), respectively. In the presence of Na metal, precursor 1 reacted with butadienes, possibly through the reduced “dialumene” or the “carbene‐like” :AlL species, to yield aluminacyclopentenes [LAl(CH2C(Me)?C(Me)CH2)Na]n ( 4 a ) and [Na(dme)3][LAl(CH2C(Me)?CHCH2)] ( 4 b , dme=dimethoxyethane) as [1+4] cycloaddition products, as well as the [2+4] cycloaddition product 1,6‐dialumina‐3,8‐cyclodecadiene, [{Na(dme)}2][LAl(CH2C(Me)?C(Me)CH2)2AlL] ( 5 ). The possible mechanisms of the cycloaddition reactions were studied by using DFT computations. 相似文献
973.
R. J. W. Cremlyn N. Akhtar I. Khattak 《Phosphorus, sulfur, and silicon and the related elements》2013,188(2):205-209
Abstract The preparation of cholesteryl phosphorodichloridite (2) is described; this compound with aniline (2 mol. equiv.) gave the N-phenylphosphoramidochloridite (5) and the latter by condensation with water afforded the N-phenyl-amidophosphite (6). Similarly the N-phenylphosphoramidochloridite (5) with morpholine gave the morpholidite (7); phenylhydrazine gave the hydrazinophosphite (8) and ethanol the amidoethyl phosphite (9). Cholesteryl phosphorodichloridite (2) by reaction with aniline (4 mol. equiv.) gave the N,N 1?diphenylphosphorodiamidite (10). The reaction of cholesteryl phosphorodichloridite (2) with methanol and ethanol are discussed in relation to the analogous reactions with cholesteryl phosphorodichloridate. Boiling ethanol gave cholesterol as the only isolatable product but at room temperature a low yield of the diethylphosphite (11; R=Et) was obtained. The yield of the phosphite was greatly increased in the presence of base. Similarly the dichloridite 2 with boiling water gave cholesterol (1), but at room temperature cholesteryl phosphite 3 was isolated: the mechanistic basis for these different results is briefly discussed. trans-4-t-Butylcyclohexanol with phosphorus trichloride gave the phosphorodichloridite, which was characterised by conversion to the corresponding N,N 1?diphenylphosphorodiamidite. 相似文献
974.
975.
Dr. Christian Amatore Dr. Gaëtan Le Duc Dr. Anny Jutand 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(31):10082-10093
In Suzuki–Miyaura reactions, anionic bases F? and OH? (used as is or generated from CO32? in water) play multiple antagonistic roles. Two are positive: 1) formation of trans‐[Pd(Ar)F(L)2] or trans‐[Pd(Ar)‐ (L)2(OH)] (L=PPh3) that react with Ar′B(OH)2 in the rate‐determining step (rds) transmetallation and 2) catalysis of the reductive elimination from intermediate trans‐[Pd(Ar)(Ar′)(L)2]. Two roles are negative: 1) formation of unreactive arylborates (or fluoroborates) and 2) complexation of the OH group of [Pd(Ar)(L)2(OH)] by the countercation of the base (Na+, Cs+, K+). 相似文献
976.
Hannelore Goossens Nicola Piens Dr. Saron Catak Prof. Dr. Michel Waroquier Prof. Dr. Karl W. Törnroos Prof. Dr. Veronique Van Speybroeck Prof. Dr. Matthias D'hooghe Prof. Dr. Norbert De Kimpe 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(10):3383-3396
The reactivity of 3‐hydroxy‐4‐(1,2‐dihydroxyethyl)‐β‐lactams with regard to the oxidant sodium periodate was evaluated, unexpectedly resulting in the exclusive formation of new 2‐hydroxy‐1,4‐oxazin‐3‐ones through a C3? C4 bond cleavage of the intermediate 4‐formyl‐3‐hydroxy‐β‐lactams followed by a ring expansion. This peculiar transformation stands in sharp contrast with the known NaIO4‐mediated oxidation of 3‐alkoxy‐ and 3‐phenoxy‐4‐(1,2‐dihydroxyethyl)‐β‐lactams, which exclusively leads to the corresponding 4‐formyl‐β‐lactams without a subsequent ring enlargement. In addition, this new class of functionalized oxazin‐3‐ones was further evaluated for its potential use as building blocks in the synthesis of a variety of differently substituted oxazin‐3‐ones, morpholin‐3‐ones and pyrazinones. Furthermore, additional insights into the mechanism and the factors governing this new ring‐expansion reaction were provided by means of density functional theory calculations. 相似文献
977.
Prof. Dr. Lukas Hintermann Dr. Jens Ackerstaff Dr. Florian Boeck 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(7):2311-2321
Cinchona alkaloids catalyze the oxa‐Michael cyclization of 4‐(2‐hydroxyphenyl)‐2‐butenoates to benzo‐2,3‐dihydrofuran‐2‐yl acetates and related substrates in up to 99 % yield and 91 % ee (ee=enantiomeric excess). Catalyst and substrate variation studies reveal an important role of the alkaloid hydroxy group in the reaction mechanism, but not in the sense of a hydrogen‐bonding activation of the carbonyl group of the substrate as assumed by the Hiemstra–Wynberg mechanism of bifunctional catalysis. Deuterium labeling at C‐2 of the substrate shows that addition of RO? H to the alkenoate occurs with syn diastereoselectivity of ≥99:1, suggesting a mechanism‐based specificity. A concerted hydrogen‐bond network mechanism is proposed, in which the alkaloid hydroxy group acts as a general acid in the protonation of the α‐carbanionic center of the product enolate. The importance of concerted hydrogen‐bond network mechanisms in organocatalytic reactions is discussed. The relative stereochemistry of protonation is proposed as analytical tool for detecting concerted addition mechanisms, as opposed to ionic 1,4‐additions. 相似文献
978.
Xiu‐Ling Liu Dr. Jian‐Bo Wang Yao Tong Prof. Dr. Qin‐Hua Song 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(39):13216-13223
The photochemical reaction of a pyrimidine and a ketone occurs either as a Paternò–Büchi (PB) reaction or as energy transfer (ET) from the triplet ketone to the pyrimidine. It is rare for the two types of reactions to occur concurrently, and their competitive mechanism remains unknown. In this work, two classes of products, regioisomeric oxetane(s) ( 2 , 3 ) from a PB reaction and three isomeric dimers of 5‐fluoro‐1,3‐dimethyl uracil (FDMU) ( 4 – 6 ) from a photosensitized dimerization of FDMU, are obtained through the UV irradiation of FDMU with various benzophenones (BPs). The ratio of the two products (oxetanes to dimers) reveals that the two competitive reactions depend strongly on the triplet energy levels (ET) of the BPs. The BPs with higher ET values lead to higher proportions of dimers, whereas those with lower ET values give higher proportions of oxetane(s), with the generation of just two regioisomeric oxetanes for the BP with the lowest ET of the eight BPs investigated. The ratio of the two oxetanes ( 2 : 3 ) decreases with the BP ET value. The competitive mechanism for the two types of photochemical reactions is demonstrated through quenching experiments and investigation of temperature effects. Kinetic analysis shows that the rate constants of the two [2+2] photocycloadditions are comparable. Furthermore, in combination with the results of previous studies, we have gained insight into the dependence of the photochemical type and the regioselectivity in the PB reaction on the triplet energy gaps (ΔE) between the pyrimidines and ketones. For ketones with higher ET values than the pyrimidines, the photochemical reaction is a photosensitized dimerization of the pyrimidine. In the opposite case, a PB reaction occurs, and the lower the ET of the ketones, the lower the ratio of oxetanes ( 2 : 3 ). When the ET of values of the ketones are close to those of the pyrimidines, the two reactions occur concurrently, and the higher the ET of the ketones, the higher the proportion of the dimers. The ratio of oxetanes ( 2 : 3 ) decreases with the ET value of the BPs. 相似文献
979.
Christoph Schädle Dr. Andreas Fischbach Dr. Eberhardt Herdtweck Prof. Dr. Karl W. Törnroos Prof. Dr. Reiner Anwander 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(48):16334-16341
Yttrocene‐carboxylate complex [Cp*2Y(OOCArMe)] (Cp*=C5Me5, ArMe=C6H2Me3‐2,4,6) was synthesized as a spectroscopically versatile model system for investigating the reactivity of alkylaluminum hydrides towards rare‐earth‐metal carboxylates. Equimolar reactions with bis‐neosilylaluminum hydride and dimethylaluminum hydride gave adduct complexes of the general formula [Cp*2Y(μ‐OOCArMe)(μ‐H)AlR2] (R=CH2SiMe3, Me). The use of an excess of the respective aluminum hydride led to the formation of product mixtures, from which the yttrium‐aluminum‐hydride complex [{Cp*2Y(μ‐H)AlMe2(μ‐H)AlMe2(μ‐CH3)}2] could be isolated, which features a 12‐membered‐ring structure. The adduct complexes [Cp*2Y(μ‐OOCArMe)(μ‐H)AlR2] display identical 1J(Y,H) coupling constants of 24.5 Hz for the bridging hydrido ligands and similar 89Y NMR shifts of δ=?88.1 ppm (R=CH2SiMe3) and δ=?86.3 ppm (R=Me) in the 89Y DEPT45 NMR experiments. 相似文献
980.
Venkataramana Imandi Sooraj Kunnikuruvan Dr. Nisanth N. Nair 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(15):4724-4731
The complete reaction mechanism and kinetics of the Wacker oxidation of ethene in water under low [Cl?], [PdII], and [CuII] conditions are investigated in this work by using ab initio molecular dynamics. These extensive simulations shed light on the molecular details of the associated individual steps, along two different reaction routes, starting from a series of ligand‐exchange processes in the catalyst precursor PdCl42? to the final aldehyde‐formation step and the reduction of PdII. Herein, we report that hydroxylpalladation is not the rate‐determining step and is, in fact, in equilibrium. The newly proposed rate‐determining step involves isomerization and follows the hydroxypalladation step. The mechanism proposed herein is shown to be in excellent agreement with the experimentally observed rate law and rate. Moreover, this mechanism is in consensus with the observed kinetic isotope effects. This report further confirms the outer‐sphere (anti) hydroxypalladation mechanism. Our calculations also ratify that the final product formation proceeds through a reductive elimination, assisted by solvent molecules, rather than through β‐hydride elimination. 相似文献