Catalytic Asymmetric Diels–Alder Reaction of Quinone Imine Ketals: A Site‐Divergent Approach

The catalytic asymmetric Diels–Alder reaction of quinone imine ketals with diene carbamates catalyzed by axially chiral dicarboxylic acids is reported herein. A variety of primary and secondary alkyl‐substituted quinone derivatives which have not been applied in previous asymmetric quinone Diels–Alder reactions could be employed using this method. More importantly, we succeeded in developing a strategy to divert the reaction site in unsymmetrical 3‐alkyl quinone imine ketals from the inherently favored unsubstituted C?C bond to the disfavored alkyl‐substituted C?C bond.     

Au6Pd/resin催化伯醇和仲醇氧化偶联-转移加氢反应制备酮类化合物

由简单小分子通过 C–C键偶联来构筑复杂多样的大分子是有机合成的重要方向.传统的 C–C键偶联反应一般使用卤代烃和金属有机化合物为底物,具有原子效率低、有害废弃物排放等缺点.因此,迫切需要发展一种绿色高效的 C–C键偶联方法.其中,以醇类化合物作为底物通过“氢转移”(脱氢/aldol缩合/加氢)实现 C–C键偶联的途径受到广泛关注.该方法具有诸多优点：(1)醇类化合物来源广泛、价格低廉、相对安全;(2)只产生 H2和 H2O,没有其它副产物.但由于醇类化合物(特别是仲醇)脱氢困难,该偶联反应条件一般比较苛刻.我们使用 O2来辅助仲醇脱氢,采用离子交换树脂负载的 Au6Pd纳米颗粒为催化剂,实现了温和条件下伯醇和仲醇的偶联反应.而且发现在氧化气氛下,反应过程中发生了“氢转移”现象,产物为饱和酮类化合物.通过设计对照实验并结合 XAFS(X–射线吸收光谱)表征结果,我们揭示了在 Au6Pd/resin催化剂上发生“氢转移”反应的机理. AuPd/resin催化剂采用离子交换–NaBH4还原法制备. TEM照片显示 Au, Pd以及双金属 AuPd纳米颗粒均匀分散在载体上,平均粒径为2–4 nm,而且随着 Au/Pd比例减小, AuPd纳米颗粒的粒径逐渐减小. XRD谱图显示,随着 Au/Pd比例减小, Au(111)衍射峰逐渐向高角度发生偏移,说明 AuPd形成了合金.我们以苯甲醇和(±)-1-苯乙醇氧化偶联为探针反应考察了催化剂的催化性能.结果显示,以 Au/resin和 Pd/resin为催化剂时,产物为不饱和酮.而以 AuPd/resin为催化剂时,转化率显著提高,说明 AuPd之间存在明显的协同作用.而且随着 Au/Pd比例增加,产物逐渐由不饱和酮转变为饱和酮,当 Au/Pd≥6时,产物完全为饱和酮,说明反应过程中发生了“氢转移”.为验证这一推测,我们以苯甲醇和查尔酮为底物在相同条件下反应.结果显示,以 Au/resin和 Pd/resin为催化剂时,查尔酮没有转化.而以 AuPd/resin为催化剂时,查尔酮大部分转化为饱和酮(转化率为91%),验证了反应中发生了“氢转移”的推测.为研究“氢转移”发生的机理,我们采用 XAFS对催化剂价态进行了表征. Pd元素 K边 X射线吸收谱图显示,随着催化剂中 Au/Pd比例的增加,E0值逐渐减小,说明 Pd价态逐渐降低. EXAFS拟合数据表明,随催化剂中 Au/Pd比例增加, Pd–O配位数逐渐减小.基于以上结果推断,在 AuPd/resin催化剂中,随着 Au/Pd比例的增加, Pd的抗氧化能力显著增强,更多的 Pd以 Pd(0)形式存在.结合文献报道结果,我们认为正是催化剂中的 Pd(0)夺取了醇的βC–H后生成了 Pd–H,而 Pd–H是“氢转移”反应的催化剂.另一方面,有文献报道,在氧化气氛下, O2也可以辅助脱除醇的βC–H.为区分 Pd(0)和 O2在脱除醇βC–H中的作用,我们对 Au6Pd/resin在惰性气氛下对伯醇(苯甲醇)或仲醇((±)-1–苯乙醇)转化的催化性能进行了考察.结果显示,苯甲醇可以转化为苯甲酸(收率为23%),而(±)-1–苯乙醇则完全没有转化.这说明伯醇可以直接被催化剂(Pd(0))活化,而仲醇的活化则必须有 O2参与.综上,我们提出伯醇和仲醇氧化偶联反应的机理： Au6Pd/resin催化伯醇转化为醛(同时产生 Pd–H物种),而 O2辅助活化仲醇转化为酮.醛和酮发生 aldol缩合生成α,β不饱和酮,该中间物种被 Pd–H加氢生成饱和产物.     

Copper‐Catalyzed Oxidative Reaction of β‐Keto Sulfones with Alcohols via C−S Bond Cleavage: Reaction Development and Mechanism Study

A Cu‐catalyzed cascade oxidative radical process of β‐keto sulfones with alcohols has been achieved by using oxygen as an oxidant. In this reaction, β‐keto sulfones were converted into sulfinate esters under the oxidative conditions via cleavage of C?S bond. Experimental and computational studies demonstrate that a new pathway is involved in this reaction, which proceeds through the formation of the key four‐coordinated Cu^II intermediate, O?O bond homolysis induced C?S bond cleavage and Cu‐catalyzed esterification to form the final products. This reaction provides a new strategy to sulfonate esters and enriches the research content of C?S bond cleavage and transformations.     

Solvent‐free reactions of alcohols with β‐dicarbonyl compounds catalyzed by iron(III) chloride

The direct substitution of alcohols and β‐dicarbonyl compounds was catalyzed with FeCl₃ under solvent‐free conditions. The catalyst loading could be decreased to 0.01 mol% at high activities. It was shown that the reaction proceeded in two steps via the etherification of the alcohols. This method provided an easy and practical procedure for C? C bond formation.Copyright © 2007 John Wiley & Sons, Ltd.     

Intermolecular σ‐Bond Cross‐Exchange Reaction between Cyclopropenones and (Benzo)silacyclobutanes: Straightforward Access towards Sila(benzo)cycloheptenones

Described herein is the first intermolecular σ‐bond exchange reaction between the C?C bond of cyclopropenones and C?Si bond of (benzo)silacyclobutanes and it proceeds smoothly by treatment with either 1 mol % of a palladium or 2 mol % of a nickel catalyst. This reaction constitutes an unprecedented route for the synthesis of various sila(benzo)suberones. And it is also the first example of a σ‐bond exchange reaction involving cyclopropenones.     

Oxidation and β‐Alkylation of Alcohols Catalysed by Iridium(I) Complexes with Functionalised N‐Heterocyclic Carbene Ligands

The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for C?C bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)‐cyclooctadiene complexes having a NHC ligand with a O‐ or N‐functionalised wingtip efficiently catalysed the oxidation and β‐alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex [Ir(NCCH₃)(cod)(MeIm(2‐ methoxybenzyl))][BF₄] (cod=1,5‐cyclooctadiene, MeIm=1‐methylimidazolyl) having a rigid O‐functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF₀) of 1283 h^?1, and also in the β‐alkylation of 2‐propanol with butan‐1‐ol, which gives a conversion of 94 % in 10 h with a selectivity of 99 % for heptan‐2‐ol. We have investigated the full reaction mechanism including the dehydrogenation, the cross‐aldol condensation and the hydrogenation step by DFT calculations. Interestingly, these studies revealed the participation of the iridium catalyst in the key step leading to the formation of the new C?C bond that involves the reaction of an O‐bound enolate generated in the basic medium with the electrophilic aldehyde.     

Palladium(II)‐Catalyzed ortho‐Arylation of Aromatic Alcohols with a Readily Attachable and Cleavable Molecular Scaffold

A palladium(II)‐catalyzed C?H arylation process of alcohols has been developed. The strategy utilizes a novel quinoline‐based hemiacetal scaffold that can direct the selective C?H bond functionalization. This reaction provides a useful method to construct biaryl compounds of benzyl alcohols in good to excellent yields. The new molecular scaffold can be readily attached, removed, and recovered.     

Olefin Cyclopropanation by Radical Carbene Transfer Reactions Promoted by Cobalt(II)/Porphyrinates: Active‐Metal‐Template Synthesis of [2]Rotaxanes

A Co^II/porphyrinate‐based macrocycle in the presence of a 3,5‐diphenylpyridine axial ligand functions as an endotopic ligand to direct the assembly of [2]rotaxanes from diazo and styrene half‐threads, by radical‐carbene‐transfer reactions, in excellent 95 % yield. The method reported herein applies the active‐metal‐template strategy to include radical‐type activation of ligands by the metal‐template ion during the organometallic process which ultimately yields the mechanical bond. A careful quantitative analysis of the product distribution afforded from the rotaxane self‐assembly reaction shows that the Co^II/porphyrinate subunit is still active after formation of the mechanical bond and, upon coordination of an additional diazo half‐thread derivative, promotes a novel intercomponent C?H insertion reaction to yield a new rotaxane‐like species. This unexpected intercomponent C?H insertion illustrates the distinct reactivity brought to the Co^II/porphyrinate catalyst by the mechanical bond.     

CC Coupling of Ketones with Methanol Catalyzed by a N‐Heterocyclic Carbene–Phosphine Iridium Complex

An N‐heterocyclic carbene–phosphine iridium complex system was found to be a very efficient catalyst for the methylation of ketone via a hydrogen transfer reaction. Mild conditions together with low catalyst loading (1 mol %) were used for a tandem process which involves the dehydrogenation of methanol, C?C bond formation with a ketone, and hydrogenation of the new generated double bond by iridium hydride to give the alkylated product. Using this iridium catalyst system, a number of branched ketones were synthesized with good to excellent conversions and yields.     

Photocatalytic Dehydrogenative Cross‐Coupling of Alkenes with Alcohols or Azoles without External Oxidant

Direct cross‐coupling between alkenes/R‐H or alkenes/RXH is a dream reaction, especially without external oxidants. Inputting energy by photocatalysis and employing a cobalt catalyst as a two‐electron acceptor, a direct C−H/X−H cross‐coupling with H₂ evolution has been achieved for C−O and C−N bond formation. A new radical alkenylation using alkene as the redox compound is presented. A wide range of aliphatic alcohols—even long chain alcohols—are tolerated well in this system, providing a new route to multi‐substituted enol ether derivatives using simple alkenes. Additionally, this protocol can also be used for N ‐vinylazole synthesis. Mechanistic insights reveal that the cobalt catalyst oxidizes the photocatalyst to revive the photocatalytic cycle.     

非过渡金属催化体系NHPI/DDQ/NaNO2催化分子氧选择氧化醇

考察了N-羟基邻苯二甲酰亚胺(NHPI),2,3-二氯-5,6-二氰基-1,4-苯醌(DDQ)与NaNO2组成的非金属催化体系,催化分子氧选择氧化醇的反应性能.结果表明,该体系可有效地催化芳香醇等生成相应的醛(酮).在80℃反应6h,苯甲醇转化率达到65%,苯甲醛选择性为99%.此外,该催化体系也能有效地催化其它醇的选...     

Polymerization of Acetonitrile via a Hydrogen Transfer Reaction from CH3 to CN under Extreme Conditions

Acetonitrile (CH₃CN) is the simplest and one of the most stable nitriles. Reactions usually occur on the C≡N triple bond, while the C?H bond is very inert and can only be activated by a very strong base or a metal catalyst. It is demonstrated that C?H bonds can be activated by the cyano group under high pressure, but at room temperature. The hydrogen atom transfers from the CH₃ to CN along the CH???N hydrogen bond, which produces an amino group and initiates polymerization to form a dimer, 1D chain, and 2D nanoribbon with mixed sp² and sp³ bonded carbon. Finally, it transforms into a graphitic polymer by eliminating ammonia. This study shows that applying pressure can induce a distinctive reaction which is guided by the structure of the molecular crystal. It highlights the fact that very inert C?H can be activated by high pressure, even at room temperature and without a catalyst.     

CC or co bond cleavage in a phenolic lignin model compound: selectivity depends on vanadium catalyst

The aerobic oxidation of a phenolic lignin model compound with a vanadium catalyst results in the oxidative cleavage of the C?C bond between the aryl ring and the adjacent hydroxy-substituted carbon atom. Labeling experiments indicate key mechanistic differences to a previously reported related C?O bond cleavage reaction. The selectivity in C?C versus C?O bond cleavage depends on the choice of the vanadium catalyst.     

Unique feature of chain reactions of thiols with quinone imines: thiols as reactants and simultaneously catalysts at the rate-determining step of chain propagation

The isomerization of radical adducts, formed due to the addition of thiyl radicals to the cyclohexadiene ring of quinone imine, to phenoxyl and aromatic aminyl radicals is considered by quantum chemical methods (DFT/PBE and CCSD). Isomerization via the intramolecular transfer of the highly mobile H atom of the C—H bond to the O or N atoms from the position of PhS? radical addition to the cyclohexadiene ring of quinone imine cannot virtually occur because of the high activation energy comparable or even exceeding the C—H bond dissociation energy. An alternative bimolecular mechanism involving the thiol molecule, which is inserted into the transition state thus extending it to be favorable for the reaction to occur, was proposed. After the reaction, the thiol is regenerated, i.e., acts as both the reactant and catalyst of the chain reaction of quinone imine with thiol. The reasons for the high rate of the H atom transfer via this mechanism are considered.     

Dehydrative Direct CH Allylation with Allylic Alcohols under [Cp*CoIII] Catalysis

The unique reactivity of [Cp*Co^III] over [Cp*Rh^III] was demonstrated. A cationic [Cp*Co^III] catalyst promoted direct dehydrative C? H allylation with non‐activated allyl alcohols, thus giving C2‐allylated indoles, pyrrole, and phenyl‐pyrazole in good yields, while analogous [Cp*Rh^III] catalysts were not effective. The high γ‐selectivity and C2‐selectivity indicated that the reaction proceeded by directing‐group‐assisted C? H metalation. DFT calculations suggested that the γ‐selective substitution reaction proceeded by C? H metalation and insertion of a C? C double bond, with subsequent β‐hydroxide elimination. The [Cp*Co^III] catalyst favored β‐hydroxide elimination over β‐hydride elimination.     

Rh‐Catalyzed C?C Cleavage of Benzyl/Allylic Alcohols to Produce Benzyl/Allylic Amines or other Alcohols by Nucleophilic Addition of Intermediate Rhodacycles to Aldehydes and Imines

We report three transformations: 1) direct transformation from biarylmethanols into biarylmethylamines; 2) direct transformation from one biarylmethanol into another biarylmethanol; 3) direct transformation from allylic alcohols into allylic amines. These transformations are based on pyridyl‐directed Rh‐catalyzed C? C bond cleavage of secondary alcohols and subsequent addition to C?X (X=N or O) double bonds. The reaction conditions are simple and no additive is required. The driving force of C? C bond cleavage is the formation of the stable rhodacycle intermediate. Other directing groups, such as the pyrazolyl group, can also be used although it is not as efficient as the pyridyl group. We carried out in‐depth investigations for transformation 1 and found that: 1) the substrate scope was broad and electron‐rich alcohols and electron‐deficient imines are more efficient; 2) as the leaving group, aldehyde had no significant impact on either the C? C bond cleavage or the whole transformation; 3) mechanistic studies (intermediate isolation, in situ NMR spectroscopic studies, competing reactions, isotopic labeling experiments) implied that: i) The C? C cleavage was very efficient under these conditions; ii) there is an equilibrium between the rhodacycle intermediate and the protonated byproduct phenylpyridine; iii) the addition step of the rhodacycle intermediate to imines was slower than the C? C cleavage and the equilibrium between the rhodacycle and phenylpyridine; iv) the whole transformation was a combination of two sequences of C? C cleavage/nucleophilic addition and C? C cleavage/protonation/C? H activation/nucleophilic addition, with the latter being perhaps the main pathway. We also demonstrated the first example of cleavage of an C(alkenyl)? C(benzyl) bond. These transformations showed the exchange (or substitution) of the alcohol group with either an amine or another alcohol group. Like the “group transplant”, this method offers a new concept that can be used to directly synthesize the desired products from other chemicals through reorganization of carbon skeletons.     

Extended Reaction Scope of Thiamine Diphosphate Dependent Cyclohexane‐1,2‐dione Hydrolase: From CC Bond Cleavage to CC Bond Ligation

ThDP‐dependent cyclohexane‐1,2‐dione hydrolase (CDH) catalyzes the C? C bond cleavage of cyclohexane‐1,2‐dione to 6‐oxohexanoate, and the asymmetric benzoin condensation between benzaldehyde and pyruvate. One of the two reactivities of CDH was selectively knocked down by mutation experiments. CDH‐H28A is much less able to catalyze the C? C bond formation, while the ability for C? C bond cleavage is still intact. The double variant CDH‐H28A/N484A shows the opposite behavior and catalyzes the addition of pyruvate to cyclohexane‐1,2‐dione, resulting in the formation of a tertiary alcohol. Several acyloins of tertiary alcohols are formed with 54–94 % enantiomeric excess. In addition to pyruvate, methyl pyruvate and butane‐2,3‐dione are alternative donor substrates for C? C bond formation. Thus, the very rare aldehyde–ketone cross‐benzoin reaction has been solved by design of an enzyme variant.     

Reactions in the Photocatalytic Conversion of Tertiary Alcohols on Rutile TiO2(110)

According to textbooks, tertiary alcohols are inert towards oxidation. The photocatalysis of tertiary alcohols under highly defined vacuum conditions on a titania single crystal reveals unexpected and new reactions, which can be described as disproportionation into an alkane and the respective ketone. In contrast to primary and secondary alcohols, in tertiary alcohols the absence of an α‐H leads to a C?C‐bond cleavage instead of the common abstraction of hydrogen. Surprisingly, bonds to methyl groups are not cleaved when the alcohol exhibits longer alkyl chains in the α‐position to the hydroxyl group. The presence of platinum loadings not only increases the reaction rate but also opens up a new reaction channel: the formation of molecular hydrogen and a long‐chain alkane resulting from recombination of two alkyl moieties. This work demonstrates that new synthetic routes may become possible by introducing photocatalytic reaction steps in which the co‐catalysts may also play a decisive role.     

Oxidative Interception of the Hydroamination Pathway: A Gold‐Catalyzed Diamination of Alkenes

A complimentary diamination of alkenes by using homogeneous gold catalysts is described. The reaction is one of very few examples of homogeneous gold oxidation catalysis and proceeds with high selectivity under mild conditions. Individual steps of the suggested catalytic cycle were investigated on isolated model gold complexes, and new pathways for gold‐catalyzed amination reactions were established. The key step is an intramolecular alkyl–nitrogen bond formation from a gold(III) intermediate. This process validates the concept of reductive elimination from high oxidation catalyst states for this type of C? N bond forming reactions.     

Efficient ruthenium-catalyzed aerobic oxidation of alcohols using a biomimetic coupled catalytic system

Efficient aerobic oxidation of alcohols was developed via a biomimetic catalytic system. The principle for this aerobic oxidation is reminiscent of biological oxidation of alcohols via the respiratory chain and involves selective electron/proton transfer. A substrate-selective catalyst (ruthenium complex 1) dehydrogenates the alcohol, and the hydrogens abstracted are transferred to an electron-rich quinone (4b). The hydroquinone thus formed is continuously reoxidized by air with the aid of an oxygen-activating Co[bond]salen type complex (6). Most alcohols are oxidized to ketones in high yield and selectivity within 1-2 h, and the catalytic system tolerates a wide range of O(2) concentrations without being deactivated. Compared to other ruthenium-catalyzed aerobic oxidations this new catalytic system has high turnover frequency (TOF).