Fluorescence spectra and structure of the difluoro(dibenzoylmethanato)boron monomers and dimers absorbed on silica gel

It has been shown that difluoro(dibenzoylmethanato)boron ((dbm)BF₂) can be absorbed on silica gel in the form of fluorescent monomers and dimers with the emission properties that change in the presence of vapors of volatile organic compounds, such as ethanol, acetone, toluene, and meta-xylene. Fluorescence quenching was observed for the (dbm)BF₂ monomers and dimers in the case of ethanol and acetone, whereas the formation of fluorescent exciplexes with monomers and enhancement of the dimer fluorescence were observed in the case of toluene and meta-xylene. Results of the quantum-chemical calculations of the structure of the (dbm)BF₂ monomer complex with the matrix and toluene and (dbm)BF₂ dimers with matrix are presented.     

Development of a Simple Adjustable Zinc Acid/Base Hybrid Catalyst for C−C and C−O Bond‐Forming and C−C Bond‐Cleavage Reactions

A newly designed zinc Lewis acid/base hybrid catalyst was developed. By adjusting the Lewis acidity of the zinc center, aldol‐type additions of 2‐picolylamine Schiff base to aldehydes proceeded smoothly to afford syn‐aldol adduct equivalents, trans‐N,O‐acetal adducts, in high yields with high selectivities. NMR experiments, including microchanneled cell for synthesis monitoring (MICCS) NMR analysis, revealed that anti‐aldol adducts were formed at the initial stage of the reactions under kinetic control, but the final products were the trans‐(syn)‐N,O‐acetal adducts that were produced through a retro‐aldol process under thermodynamic control. In the whole reaction process, the zinc catalyst played three important roles: i) promotion of the aldol process (C?C bond formation), ii) cyclization process to the N,O‐acetal product (C?O bond formation), and iii) retro‐aldol process from the anti‐aldol adduct to the syn‐aldol adduct (C?C bond cleavage and C?C bond formation).     

Synthesis of polymers bearing proline moieties in the side chains and their application as catalysts for asymmetric induction

Novel polyphenylacetylene and polystyrene derivatives carrying L ‐proline moieties at the side chains were synthesized by the rhodium‐catalyzed and radical polymerizations of the corresponding monomers. The polyphenylacetylene derivatives showed Cotton effects at the absorption region of the main chain, indicating that the polymers adopt helical conformations with predominantly one‐handed screw sense. The polymers catalyzed the asymmetric aldol reactions of acetone with aromatic aldehydes, and cyclohexanone with p‐nitrobenzaldehyde. The enantioselectivities largely depended on the reaction conditions. In the asymmetric aldol reaction of acetone with aromatic aldehydes, the R‐enantiomeric products were predominantly obtained except the cases with the polymer catalyst in CHCl₃. The ee of the products became higher as the reaction temperature was decreased. The polymeric catalysts were recoverable from the reaction mixture by filtration, and the recovered ones catalyzed the asymmetric aldol reaction of acetone with p‐nitrobenzaldehyde without decreasing the product yield and ee. The ee was improved using the copolymers of L ‐proline‐based and nonchiral monomers as catalysts. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Asymmetric phase-transfer reactions under base-free neutral conditions

Although quaternary onium salt-catalyzed phase-transfer reactions are generally believed to require base additives, we discovered even without any base additives conjugate additions of 3-substituted oxindoles proceeded smoothly in the presence of lipophilic quaternary onium bromide under water-organic biphasic conditions. The mechanism of this novel base-free neutral phase-transfer reaction system was investigated, and the assumed catalytic cycle was presented together with interesting effects of water and lipophilicity of the phase-transfer catalyst. The base-free neutral phase-transfer reaction system could be applied to highly enantioselective conjugate additions, aldol reaction, sulfenylation, and chlorination under the influence of chiral bifunctional onium bromides as key catalysts.     

Cu-Catalyzed Asymmetric Kinetic Boron Conjugate Addition of γ-Substituted α,β-Unsaturated γ-Lactams

Chiral α,β-unsaturated γ-lactams bearing simple γ- substituents are found in biologically active molecules and natural products, however, their synthesis still remains difficult. Herein, we report an efficient kinetic resolution (KR) of γ-substituted α,β-unsaturated γ-lactams via a Cu-catalyzed asymmetric boron conjugate addition, which also leads to the efficient synthesis of chiral β-hydroxy-γ-lactams with β,γ-stereogenic carbon centers. The KR proceeded smoothly with a wide range of γ-alkyl or aryl substituted substrates including those bearing aromatic heterocycles and different N-protected substrates in up to 347 of s value. Their highly versatile transformations, synthetic utility in biologically active molecules, and inhibitory activities against cisplatin-sensitive ovarian cancer cell A2780 have also been demonstrated. Differing from the well-known mechanism involving Cu−B species in Cu-catalyzed boron conjugate additions, our mechanistic studies using density functional theory (DFT) calculations and experiments indicate that a Lewis acid Cu^I-catalyzed mechanism is the likely pathway in the catalytic reaction.     

Formation of Dimers of Some 2‐Substituted Indan‐1‐one Derivatives during Base‐Mediated Cross‐Aldol Condensation

Unexpected dimers of some 2‐substituted indan‐1‐one derivatives were isolated during aldol condensation of indan‐1‐one with various aldehydes in the presence of KOH (see Scheme). Monomeric products, usually expected from aldol condensation, further underwent a base‐catalyzed nucleophilic addition reaction to their dimeric form in some cases. The structures of these dimers were characterized by using various spectral techniques and in one case, structural details were determined from a high‐resolution crystallographic analysis.     

Effects of Remote Substitution on the Photo-oxidation of Exocyclic s-cis-Butadienes Grafted onto Bicyclo [2.2.1]heptanes. Thermal and Rhodium Catalyzed Rearrangements of 3,6-Dihydro-1,2-dioxines

The rates of photo-oxidation of exocyclic S-cis-butadienes grafted onto bicyclo-[2.2.1]heptanes and 7-oxabicyclo[2.2.1]heptanes ( 1–6 ) are dependent upon remote modifications of the bicyclic skeletons. They correlate with the rates of Diels-Alder additions of these dienes to strong dienophiles. The 2,3-dimethylidenenorbornane ( 1 ), 5,6-dimethylidene-2-norbornene ( 2 ) and 2,3-dimethylidene-7-oxanorbornane ( 3 ) gave the corresponding endo-peroxides (3,6-dihydro-1,2-dioxines) 7–9 in good yield. The 2, 3, 5, 6-tetramethylidene-7-oxanorbornane ( 4 ) gave the mono-endo-pe-roxide 6 , the latter did not react with a second equivalent of oxygen. Similarly, 5, 6-dimethylidene-7-oxa-2-norbornene ( 5 ) was unreactive toward photo-oxidation. Thermal isomerization of the endo-peroxides 7 and 9 gave, the trans-diepoxides 10 and 14 , respectively, with high stereoselectivity. The endo-peroxides 6 , 7 and 9 were cleanly isomerized into the corresponding α, β-unsaturated γ-hydroxy aldehydes in the presence of catalytic amounts of Rh₂(CO)₄Cl₂.     

Synthetic and Organocatalytic Studies of Quinidine Analogues with Ring‐Size Modifications in the Quinuclidine Moiety

Six highly enantiopure analogues of [2.2.2] were synthesized with five‐ or seven‐membered rings in the (original) quinuclidine skeleton. Five of these compounds were prepared through epoxide opening by a secondary cyclic amine, providing the nor‐ and homoquinuclidine moieties through five‐ and six‐membered ring formation. This method failed in the case of seven‐membered ring formation, so for that particular ring size a different synthetic route starting from 3‐quinuclidone was applied. The six novel analogues were examined as organocatalysts in four asymmetric conjugate addition reactions and the results compared with those of known cinchona alkaloid catalysts. This study shows that modification of the quinuclidine ring can have a substantial influence on catalyst activity and enantioselectivity. To acquire more insight into the characteristics of the new catalysts, the pK_aH values were determined by means of fluorescence spectroscopy. Furthermore, relative reaction rates of conjugate thiol additions reactions catalyzed by these quinidine analogues were measured through polarimetry.     

Magnesium Catalysis for the Hydroboration of Carbodiimides

A β‐diketiminato magnesium alkyl complex, [CH{C(Me)NDipp}₂}MgnBu] (Dipp=2,6‐iPr₂C₆H₃), was shown to be an effective pre‐catalyst for the first reported catalytic hydroboration of alkyl‐ and aryl‐substituted carbodiimides with pinacol borane (HBpin). The catalytic reactions proceed under mild conditions to afford the corresponding N‐borylated formamidine compounds in good yields. The reactions were observed to proceed through the intermediacy of magnesium amidinate and formamidinatoborate intermediates and an example of one of these latter species has been structurally characterised by an X‐ray diffraction analysis. Crucially, no formation of the N‐boryl formamidine products was observed in the absence of additional equivalents of the carbodiimide and HBpin substrates. This observation, supported by the evolution of a sigmoidal kinetic profile for the hydroboration of dicyclohexylcarbodiimide, has been rationalised as the consequence of an allosteric effect of the pinacol borane and carbodiimide on the magnesium formamidinatoborate intermediates.     

Amino acid catalyzed direct asymmetric aldol reactions: a bioorganic approach to catalytic asymmetric carbon-carbon bond-forming reactions.

Direct asymmetric catalytic aldol reactions have been successfully performed using aldehydes and unmodified ketones together with commercially available chiral cyclic secondary amines as catalysts. Structure-based catalyst screening identified L-proline and 5,5-dimethyl thiazolidinium-4-carboxylate (DMTC) as the most powerful amino acid catalysts for the reaction of both acyclic and cyclic ketones as aldol donors with aromatic and aliphatic aldehydes to afford the corresponding aldol products with high regio-, diastereo-, and enantioselectivities. Reactions employing hydroxyacetone as an aldol donor provide anti-1,2-diols as the major product with ee values up to >99%. The reactions are assumed to proceed via a metal-free Zimmerman-Traxler-type transition state and involve an enamine intermediate. The observed stereochemistry of the products is in accordance with the proposed transition state. Further supporting evidence is provided by the lack of nonlinear effects. The reactions tolerate a small amount of water (<4 vol %), do not require inert reaction conditions and preformed enolate equivalents, and can be conveniently performed at room temperature in various solvents. In addition, reaction conditions that facilitate catalyst recovery as well as immobilization are described. Finally, mechanistically related addition reactions such as ketone additions to imines (Mannich-type reactions) and to nitro-olefins and alpha,beta-unsaturated diesters (Michael-type reactions) have also been developed.     

Catalytic,Conjugate Reduction‐Aldol Addition Reaction of β′‐Oxoalkyl α,β‐Unsaturated Carboxylates

Intramolecular conjugate reduction‐aldol addition reactions of β′‐oxoalkyl α,β‐unsaturated carboxylates were performed in the presence of copper catalysts generated in situ from copper salts, phosphine ligands and silanes. Moderate to good yields and high diastereoselectivities were obtained in 15 min to 3 h using bis[(2‐diphenylphosphino)phenyl] ether as the ligand.     

钌1,2-萘醌-1-肟配合物对aldol C—C成键反应的催化作用

在钌1,2-萘醌-1-肟(1-nqo)配合物cis,cis-[Ru(1-nqo)₂(CO)(NCMe)] (1)或trans,trans-[Ru(1-nqo)₂(PBu₃)₂] (2)的作用下, 氰基乙酸乙酯与取代苯甲醛发生aldol C—C成键反应. 根据GC-MS检测及HPLC分离结果, 对二苯甲醛的二个醛基可分别或同时与氰基乙酸乙酯发生aldol反应. ¹H NMR表征结果证明, 二种产物的双键构型均为反式. 其它取代苯甲醛的反应均给出单一反式aldol产物, 这表明该催化反应具有立体选择性. 配合物1的催化活性稍差, 产率不超过60%, 而配合物2的催化活性要高于1, 最高产率达99%.     

Intermediates Formed in the Reactions of Organocuprates with α,β‐Unsaturated Nitriles

Conjugate additions of organocuprates are of outstanding importance for organic synthesis. To improve our mechanistic understanding of these reactions, we have used electrospray ionization mass spectrometry for the identification of the ionic intermediates formed upon the treatment of LiCuR₂ ? LiCN (R=Me, Bu, Ph) with a series of α,β‐unsaturated nitriles. Acrylonitrile, the weakest Michael acceptor included, did not afford any detectable intermediates. Fumaronitrile (FN) yielded adducts of the type Li_n?1Cu_nR_2n(FN)_n^?, n=1–3. When subjected to fragmentation in the gas phase, these adducts were not converted into the conjugate addition products, but re‐dissociated into the reactants. In contrast, the reaction with 1,1‐dicyanoethylene furnished the products of the conjugate addition without any observable intermediates. Tri‐ and tetracyanoethylene proved to be quite reactive as well. The presence of several cyano groups in these substrates opened up reaction pathways different from simple conjugate additions, however, and led to dimerization and substitution reactions. Moreover, the gas‐phase fragmentation behavior of the species formed from these substrates indicated the occurrence of single‐electron transfer processes. Additional quantum‐chemical calculations provided insight into the structures and stabilities of the observed intermediates and their consecutive reactions.     

Group 13 Precursor Structures and Their Effect on Oxide Nanocrystal Formation

Precursor structures (PSs) in solution are expected to influence both nanocrystal formation mechanisms, as well as crystallization of specific polymorphs. Herein, Group 13 PS structures determined by pair distribution function and extended X-ray absorption fine structure analysis are reported. Corner-sharing octahedral dimers form from the metal nitrates dissolved in either water, isopropanol, or ethanol at room temperature contradicting previous studies that suggested monomers or larger Keggin clusters. Because all crystalline indium oxides have octahedral coordination, crystals can easily nucleate from the observed PSs. Similarly, MOOH (M=Al and Ga) with octahedral M coordination is expected to form readily from the PSs, whereas formation of γ-M₂O₃ requires a partial conversion to tetrahedral M coordination. This explains the long-standing observation of initial AlOOH formation as a bottleneck for γ-Al₂O₃ synthesis. Different indium polymorphs crystallize from the various solvents, and thus there is no obvious link between the PSs and observed polymorphism.     

Chiral Imidazolium Prolinate Salts as Efficient Synzymatic Organocatalysts for the Asymmetric Aldol Reaction

Chiral imidazolium l-prolinate salts, providing a complex network of supramolecular interaction in a chiral environment, have been studied as synzymatic catalytic systems. They are demonstrated to be green and efficient chiral organocatalysts for direct asymmetric aldol reactions at room temperature. The corresponding aldol products were obtained with moderate to good enantioselectivities. The influence of the presence of chirality in both the imidazolium cation and the prolinate anion on the transfer of chirality from the organocatalyst to the aldol product has been studied. Moreover, interesting match/mismatch situations have been observed regarding configuration of chirality of the two components through the analysis of results for organocatalysts derived from both enantiomers of prolinate (R/S) and the trans/cis isomers for the chiral fragment of the cation. This is associated with differences in the corresponding reaction rates but also to the different tendencies for the formation of aggregates, as evidenced by nonlinear effects studies (NLE). Excellent activities, selectivities, and enantioselectivities could be achieved by an appropriate selection of the structural elements at the cation and anion.     

含有酯基二茂铁亚胺环钯化合物催化Suzuki和Heck反应

合成并表征了系列含有酯基二茂铁亚胺环钯化合物.结果表明,该系列环钯催化剂在纯水或有机溶剂中均能高效催化Heck和Suzuki偶联反应.     

Anthracene cycloadducts as highly selective chiral auxiliaries

A new chiral auxiliary was designed and easily prepared from a Diels-Alder cycloadduct of an enantiomerically pure anthracene with maleimide. Excellent diastereoselectivities in Diels-Alder reactions, conjugate additions, and aldol reactions employing these auxiliaries are now reported.     

Multinuclear enantiopure titanium self‐assembly complexes—synthesis,characterization and application to organic synthesis

Hexa‐ and nonanuclear titanium complexes were obtained by self‐assembly of titanium(IV)‐tert‐butoxide and D ‐mandelic acid. Suitable single crystals of these complexes were characterized by X‐ray structure analysis. When used with these complexes, aldol adducts were isolated with a high degree of regioselectivity in direct aldol additions of aromatic and aliphatic aldehydes to functionalized unsymmetrical ketones. High syn‐diastereoselectivities were obtained in aldol additions of enolizable aldehydes with hydroxyacetone and methoxyacetone. Copyright © 2007 John Wiley & Sons, Ltd.     

Catalytic enantioselective intermolecular reductive aldol reaction to ketones

We describe the first example of a catalytic enantioselective intermolecular reductive aldol reaction. Three types of reactions were studied: (1) reactions between acetophenone and methyl acrylate; (2) reactions between symmetric ketones and β-substituted α,β-unsaturated esters; and (3) reactions between acetophenone derivatives and an allenic ester. Although only moderate enantioselectivity was obtained in the first reaction type, high to excellent enantioselectivity was realized in the enantio-induction at the α-position in the second reaction type and at the δ-position in the third reaction type. Specifically, the third reaction type afforded the corresponding tertiary alcohols with up to 99% ee. Pre-activation of the nucleophile by silyl enolate formation is not necessary in these one-pot catalytic enantioselective reductive aldol reactions.     

Asymmetric Catalytic Assembly of Triple-Columned and Multilayered Chiral Folding Polymers Showing Aggregation-Induced Emission (AIE)

Unprecedented chiral multilayer folding 3D polymers have been assembled and regulated by uniform and differentiated aromatic chromophore units between naphthyl piers. Screening catalysts, catalytic systems and monomers were proven to be crucial for asymmetric catalytic Suzuki–Miyaura polycouplings for this assembly. X-ray crystallography of the corresponding dimers and trimers revealed the absolute configuration and the intermolecular packing pattern. Up to 61 960 M_w/41 900 M_n and m/z 4317 for polymers and oligomers, as confirmed by gel permeation chromatography (GPC) and MALDI-TOF MS, indicated that these frameworks were composed of multiple stacked layers. The resulting multiple π-assemblies exhibited remarkable optical properties in aggregated states (photoluminescence in solids and aggregation-induced emission in solutions), as well as reversible redox properties in electrochemical performance.