Directed Synthesis and Chemistry of Unsymmetric Dicationic Diboranes and Their Use in Frustrated Lewis Pair-like Chemistry

The chemistry of dicationic diboranes with two B^II atoms that are engaged in direct B−B bonding is by enlarge unexplored, although these molecules have intriguing properties due to their combined Lewis acidic and electron-donor properties. Unsymmetric dicationic diboranes are extremely rare, but especially attractive due to their polarized B−B bond. In this work we report the directed synthesis of several stable unsymmetric dicationic diboranes by reaction between the electron-rich ditriflato-diborane B₂(hpp)₂(OTf)₂ (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidinate) and phosphino-pyridines, establishing B−N and B−P bonds with the diborane concomitant with triflate elimination. In the case of 2-((ditertbutylphosphino)methyl)pyridine, the B−N bond is formed instantly, but the B−P bond formation requires (due to steric constraints) several days at ambient conditions for completion, creating an intermediate that could be used for frustrated Lewis pair (FLP)-like chemistry. Here we test its reaction with an aldehyde, and propose a new type of FLP-like chemistry.     

Cationic Symmetrically and Unsymmetrically Substituted Diboranes and Bis(diboranes) with Direct Boron-Boron Bond: Synthesis by Substitution,Stability and Properties

Starting with diboranes with two electron-rich bridging bicyclic guanidinate substituents, we report in this work the rational synthesis of new dicationic symmetrically- and unsymmetrically-substituted diboranes in S_N1-type substitution reactions in which triflato or bromo substituents are replaced by neutral Lewis bases. The scope of such substitution reactions and their rate are analyzed with different pyridine derivatives of variable Lewis basicity. The first substitution step, leading to a monocationic diborane with one anionic substituent (triflate or bromide) and one neutral Lewis base, proceeds much faster than the second substitution step leading to a dicationic diborane with two neutral Lewis bases. The different time scales for the substitution steps could be used to conveniently synthesize in one-pot reactions several dicationic, unsymmetrically-substituted diboranes with two different neutral Lewis bases.     

Isomerization of monohydroperfluoroalkenes

Isomerization of monohydroperfluoro-1-alkenes HC(CF₂)_nCF=CF₂ (n = 2 to 8) catalyzed by Lewis bases or acids (CsF, KHF₂, and SbF₅) under conditions of thermodynamic control affords equilibrium mixtures of all of the possible isomers resulting from migration of the double bond along the carbon chain. Under conditions of kinetic control, isomerization through the action of SbF₅ gives -H-perfluoro-2-alkenes. The substantially higher proportion ofcis-isomers in the resulting monohydroperfluoroalkenes than in their perfluorinated analogs has been attributed to the effect of an intramolecular hydrogen bond.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1614–1619, September, 1994.     

Iridium‐Catalyzed Asymmetric Isomerization of Primary Allylic Alcohols

Nothing to sm(Ir)k at : Under appropriate reaction conditions, iridium hydride catalysts promote the isomerization of primary allylic alcohols. The best catalysts, like (R)‐ 1 (P green, O red, N blue, Ir yellow), deliver the desired chiral aldehydes with excellent enantioselectivity and good yields. Mechanistic hypotheses have been developed on the basis of preliminary investigations.

     

Synthesis of Heterocycles through Transition‐Metal‐Catalyzed Isomerization Reactions

Metal‐catalyzed isomerization of N‐ and O‐allylic systems is emerging as an effective method to form synthetically useful iminium and oxocarbenium intermediates. In the presence of tethered nucleophiles, several recent examples illuminate this approach as a powerful strategy for the synthesis of structurally complex and diverse heterocycles. In this Concept article, we attempt to cover this area of research through a selection of recent versatile examples.     

Iridium‐Catalyzed Regio‐ and Enantioselective Hydroarylation of Alkenyl Ethers by Olefin Isomerization

Iridium‐catalyzed hydroarylation of alkenyl ethers, such as allylic and homoallylic ethers, by C−H bond activation gave high yields of the corresponding addition products, where the aryl groups were selectively installed at the α‐carbon atom to the alkoxy group. The reaction involves an isomerization of the alkenyl ethers into the corresponding 1‐alkenyl ethers, which then undergo the regio‐ and enantioselective hydroarylation.     

廉价过渡金属催化烯烃异构反应研究进展

过渡金属催化的烯烃异构反应在有机化合物合成、日用化学品合成、原料油应用和天然产物合成中都有着举足轻重的作用。廉价过渡金属由于其在资源、价格、后处理等方面的优势,日益受到研究者的重视。本文主要综述了近几十年来廉价过渡金属铁、钴、镍在催化烯烃异构反应方面的研究进展,详细阐述了不同的催化体系在催化活性、反应选择性、底物适用性及反应机理方面的特点。虽然目前的催化体系已经表现出优异的性能与应用价值,但在烯烃异构的立体选择性、区域选择性机制等方面,仍然需要更深入的研究。     

Diphosphino-Functionalized 1,8-Naphthyridines: a Multifaceted Ligand Platform for Boranes and Diboranes

A 1,8-naphthyridine diphosphine (NDP) reacts with boron-containing Lewis acids to generate complexes featuring a number of different naphthyridine bonding modes. When exposed to diborane B₂Br₄, NDP underwent self-deprotonation to afford [NDP-B₂Br₃]Br, an unsymmetrical diborane comprised of four fused rings. The reaction of two equivalents of monoborane BBr₃ and NDP in a non-polar solvent provided the simple phosphine-borane adduct [NDP(BBr₃)₂], which then underwent intramolecular halide abstraction to furnish the salt [NDP-BBr₂][BBr₄], featuring a different coordination mode from that of [NDP-B₂Br₃]Br. Direct deprotonation of NDP by KHMDS or PhCH₂K generates mono- and dipotassium reagents, respectively. The monopotassium reagent reacts with one or half an equivalent of B₂(NMe₂)₂Cl₂ to afford NDP-based diboranes with three or four amino substituents.     

Convergent Synthesis of Fluorene Derivatives by a Rhodium-Catalyzed Stitching Reaction/Alkene Isomerization Sequence

A convergent synthetic method for the synthesis of fluorene derivatives has been developed by devising a rhodium-catalyzed stitching reaction/alkene isomerization sequence. The reactions proceed smoothly under mild conditions for a variety of substrate combinations, and extended π-conjugation systems are also readily accessible by utilizing this synthetic method. Optical properties of the obtained fluorene derivatives have also been examined.     

Palladium‐Catalyzed Isomerization and Hydroformylation of Olefins

A novel selective palladium catalyst system based on bidentate 2,2′‐heteroarylarylphosphines and p‐TsOH has been developed for hydroformylation reactions (see scheme). By applying optimal conditions good to excellent regioselectivity is obtained for the hydroformylation of aliphatic and aromatic olefins. It is shown that a low acid concentration is crucial for obtaining high degrees of the linear isomer.

     

水溶性钌-氢配合物催化1-已烯双键异构化反应研究

研究了水溶性钌-氢配合物RuHCl(TPPTS)3在水/有机两相体系中催化1-己烯双键异构化反应.考察了反应温度、时间、膦配体浓度、相转移催化剂CTAB浓度以及底物与催化剂摩尔比等对转化率和产物选择性的影响.在最佳条件下1-己烯转化率达到82.4%,2-己烯选择性21.2%,3-己烯61.2%,没有发现骨架异构化.催化剂可重复使用5次.     

DFT法研究分子筛催化trans-2-丁烯的双键异构

利用一个3T簇模型模拟分子筛催化剂的酸性位, 采用密度泛函理论(DFT)的 B3LYP/6-31G(d, p)方法, 研究了分子筛催化1-丁烯双键异构为trans-2-丁烯的反应机理. 对反应各驻点进行了全优化, 经过零点能校正后, 得到了反应的活化能. 研究表明, 反应分三步进行：物理吸附→化学反应→物理脱附. 分子筛的酸性位OH基团首先吸附1-丁烯的双键形成了π配位复合物, 然后按协同反应机理发生双键异构反应, 生成吸附态的trans-2-丁烯, 最后脱附成产物. 计算得到的表观活化能为57.1 kJ•mol^-1, 与实验结果接近.     

Oxidative Addition of Water and Methanol to a Dicationic Trivalent Phosphorus Centre

Our recently synthesised phosphorus dication is observed to activate water (and methanol) under reactions conditions atypical for other systems containing a non‐metal centre. This particular activation described as oxidative addition is quite rare and has been reserved exclusively for a couple of metal‐based compounds.     

Unique Structural Properties of 2,4,6‐Tri‐tert‐butylanilide: Isomerization and Switching between Separable Amide Rotamers through the Reaction of Anilide Enolates

Herein, we report a unique structural property of 2,4,6‐tri‐tert‐butylanilide, which can be separated into its amide rotamers at room temperature. Interconversion between the rotamers of anilide enolates occurs readily at room temperature and their reaction with electrophiles gives mixtures of the rotamers in a ratio that depends on the reactivity of the corresponding electrophile. That is, the reaction of the 2,4,6‐tri‐tert‐butylacetanilide enolate with reactive electrophiles, such as allyl bromide or protic acids, gives mixtures of the anilide rotamers in which the E rotamer is the major component, whereas less‐reactive electrophiles, such as 1‐bromopropane and 2‐iodopropane, yield mixtures of the rotamers in which the Z rotamer is the major component. The rotameric ratio of the product is also strongly dependent on the reactivity of the anilide enolate. Switching between the anilide rotamers can be achieved through protonation of a less‐reactive enolate by a less‐reactive protic acid and thermal isomerization of the anilide.     

Enantioselective Synthesis of Chiral Cyclopent‐2‐enones by Nickel‐Catalyzed Desymmetrization of Malonate Esters

The enantioselective synthesis of highly functionalized chiral cyclopent‐2‐enones by the reaction of alkynyl malonate esters with arylboronic acids is described. These desymmetrizing arylative cyclizations are catalyzed by a chiral phosphinooxazoline/nickel complex, and cyclization is enabled by the reversible E/Z isomerization of alkenylnickel species. The general methodology is also applicable to the synthesis of 1,6‐dihydropyridin‐3(2H)‐ones.     

氯化亚铜催化3-氯-1-丁烯异构为1-氯-2-丁烯研究

采用一价铜盐为催化剂、二甲基甲酰胺为溶剂,在均相体系中催化3-氯-1-丁烯异构化生成1-氯-2-丁烯.考察了溶剂、反应温度、催化剂种类和加入量对反应的影响,研究发现反应温度和催化剂的加入量对异构化反应有较大影响.在最优条件3-氯-1-丁烯1 mL,二甲基甲酰胺9 mL,CuCl 0.10 g,60℃反应5 h,产物和原料的浓度比为3.88 mmol L-1.采用在线红外光谱对反应过程进行监测,检测到有红外吸收峰在反应过程中先增加后减少的变化过程,提出了可能的反应机理.     

Selective Isomerization of Epoxides to Allylic Alcohols Catalyzed by TiO2‐Supported Gold Nanoparticles

ReacTiO ₂ ns for rings : Gold nanoparticles supported on TiO₂ are used as a novel heterogeneous catalyst for the isomerization of epoxides to allylic alcohols by a concerted mechanism (see scheme). The reaction proceeds in high yields and the product selectivity is often remarkable.

     

Mechanistic Study on Oxorhenium‐Catalyzed Deoxydehydration and Allylic Alcohol Isomerization

The reaction mechanism of 1,2×n‐deoxydehydration (DODH; n=1, 2, 3 …) reactions with 1‐butanol as a reductant in the presence of methyltrioxorhenium(VII) catalyst has been investigated by DFT. The reduced rhenium compound, methyloxodihydroxyrhenium(V), serves as the catalytically relevant species in both allylic alcohol isomerization and subsequent DODH processes. Compared with three‐step pathway A, involving [1,3]‐transposition of allylic alcohols, direct two‐step pathway B is an alternative option with lower activation barriers. The rate‐limiting step of the DODH reaction is the first hydrogen transfer in methyltrioxorhenium(VII) reduction. Moreover, the increase in the distance between two hydroxyl groups in direct 1,2×n‐DODH reactions for C4 and C6 diols results in a higher barrier height.     

Iridium‐Catalyzed Asymmetric Allylic Dearomatization by a Desymmetrization Strategy

A desymmetrization strategy was developed involving iridium‐catalyzed allylic dearomatization of indoles. The six‐membered‐ring spiroindolenines contain three contiguous stereogenic centers, including an all‐carbon quaternary center, and were obtained in up to 99 % yield with 99 % ee and >95:5 d.r. When treated with a catalytic amount of tosylic acid, six‐membered spiroindolenine undergoes an unprecedented six‐to‐seven‐membered ring expansion, affording the corresponding hexahydroazepino[4,5‐b ]indole.