Epoxidation Processes by Pyridoxal Dioxomolybdenum(VI) (Pre)Catalysts Without Organic Solvent

Molybdenum(VI) pyridoxal thiosemicarbazonato complexes have proved to be efficient catalysts in the olefinic epoxidation of cyclooctene by aqueous TBHP in the absence of an organic solvent. High activity and selectivity have been obtained with a catalyst loading of just 0.05% molybdenum in the case of one mononuclear (pre)catalyst (TOF 3360 h⁻¹) under solvent‐free conditions. The influence of methanol on the activity and selectivity has been studied.     

Grafting of Molecularly Ordered Mesoporous Phenylene‐Silica with Molybdenum Carbonyl Complexes: Efficient Heterogeneous Catalysts for the Epoxidation of Olefins

Arenetricarbonyl complexes, or the general formula  C₆H₄Mo(CO)₃ , were incorporated into crystal‐like mesoporous phenylene‐silica by liquid‐phase deposition of molybdenum hexacarbonyl [Mo(CO)₆]. By adjusting the reaction conditions, different molybdenum loadings of 1.5 and 5.9 wt% were obtained, which correspond to 3% and 14% of the phenylene contents. The texture properties of the materials as well as the nature of the surface‐fixed complexes were characterized by powder X‐ray diffraction, transmission electron microscopy (TEM), N₂ adsorption, FT‐IR, UV‐vis and MAS (¹³C, ²⁹Si) NMR spectroscopy. The derivatized organosilicas were examined as catalyst precursors for the liquid‐phase epoxidation of cis‐cyclooctene, 1‐octene, trans‐2‐octene and (R)‐(+)‐limonene at 55 °C, using tert‐butyl hydroperoxide as the oxidant. For each olefin the corresponding epoxide was the only product detected. In the case of cyclooctene, the intrinsic reaction rates per surface molybdenum atom were similar for both Mo loadings (TOF∼1150 mol mol_Mo⁻¹ h⁻¹), suggesting that the resultant materials act as single site epoxidation catalysts. Leaching tests and metal analyses of reaction solutions showed that the catalytic activity stemmed from the immobilized species and not from the leaching of active species into solution. The oxidation of limonene gave limonene oxide as the only product in 95% yield at 3 h, which reveals an outstanding regioselectivity to the epoxidation of the endocyclic double bond.     

Chiral Metallosalen Complexes: Structures and Catalyst Tuning for Asymmetric Epoxidation and Cyclopropanation

Optically active metallosalens are currently one of the most widely used catalysts for asymmetric synthesis. This is mainly due to the following reasons: various salen [N,N′‐ethylenebis(salicyldeneaminato)] ligands form complexes with many metal ions and the resulting metallosalens adopt versatile structures, therefore, metallosalens show a variety of catalytic performances. Thus, selection of a metal ion serving the aimed reaction and regulation of ligand conformation to be suitable for the reaction are indispensable for achieving metallosalen‐catalyzed asymmetric reactions. Recently, several factors controlling ligand‐conformation in metallosalens have been clarified. This article provides selected examples of tuning the structures of metallosalens in compliance to the aimed reaction, by taking asymmetric oxene or carbene transfer reaction as the instance.     

烯烃环氧化钼系催化剂的多相化研究进展

介绍了以有机聚合物和无机硅酸盐为载体，负载钼催化剂及其对烯烃环氧化反应催化性能的研究进展；分析了硼酸树脂、乙烯-丙烯橡胶、螯合树脂、苯并吡咯树脂、聚酰亚胺树脂、聚硅氧烷树脂、阳离子交换树脂和聚苯乙烯等有机聚合物为载体的研究情况；评价了以无机硅酸盐为载体，采用溶胶-凝胶、离子交换、同晶取代等方法负载钼对催化性能的影响；展望了多相化烯烃环氧化钼系催化剂的发展前景。     

C3‐Symmetric Titanium(IV) Triphenolate Amino Complexes for a Fast and Effective Oxidation of Secondary Amines to Nitrones with Hydrogen Peroxide

The efficient catalytic oxidation of secondary amines to nitrones using hydrogen peroxide as primary oxidant is described. The titanium(IV) complex 2 bearing a C₃‐symmetrical triphenolate amino ligand has proved to be an air‐ and water‐tolerant complex that efficiently catalyzes secondary amine oxidations at 60 °C without previous activation [catalyst loading as low as 0.01%, yields up to 99%, turnover numbers (TONs) up to 8000 and turnover frequencies (TOFs) up to 11000 h⁻¹).     

Silica Gel Anchored Chiral Mn(III)Salen Complexes for Enantioselective Epoxidation of Unfunctionalised Olefins

Heterogeneous Mn(III) chiral salen complexes are prepared through covalent attachment of salen ligand on silica gel via chloropropyl spacer and subsequent complexation with manganese. The complexes are well characterized by IR, UV/VIS, TGA and elemental analysis. Epoxidation of unfunctionalised prochiral olefins by Mn(III) chiral salen complexes using iodosobenzene and m-CPBA as the terminal oxidants and NMO as a co-oxidant was achieved with good yields albeit low enantiomeric excess.     

Asymmetric Epoxidation of Olefins by Manganese(III) Complexes Stabilised on Nanocrystalline Magnesium Oxide

The asymmetric epoxidation of unfunctionalised olefins to epoxides is realised by using manganese(III) complexes stabilised on nanocrystalline magnesium oxide in the presence (1R,2R)‐(−)‐diaminocyclohexane as a chiral ligand in good yields and up to 91 % enantiomeric excess.     

Air Stable Iron(II) PNP Pincer Complexes as Efficient Catalysts for the Selective Alkylation of Amines with Alcohols

A series of well‐defined iron(II) complexes of the types [Fe(PNP)Br₂] and [Fe(PNP)(CO)Br₂] with PNP pincer ligands based on triazine and pyridine backbones were prepared and fully characterized. These complexes were tested as catalysts for the alkylation of amines by alcohols. The high‐spin complexes [Fe(PNP)Br₂] are catalytically inactive. The low‐spin complexes [Fe(PNP)(CO)Br₂] bearing a carbonyl co‐ligand efficiently and selectively convert primary alcohols and aromatic and benzylic amines selectively into mono‐N‐alkylated amines in good to excellent isolated yields. A mechanistic proposal is given.

     

An Efficient and Selective Epoxidation of Olefins with Novel Methyltrioxorhenium/(Fluorous Ponytailed) 2,2′‐Bipyridine Catalysts

Novel complexes between methyltrioxorhenium (MTO) and bis(fluorous‐ponytailed) 2,2′‐bipyridines (bpy‐F_n) were synthesized and used for the oxidation of alkenes with hydrogen peroxide under fluorous catalysis. High conversions and yields of the corresponding epoxides were obtained.     

Tungsten and Molybdenum 2,4,6‐Trimethylbenzylidyne Complexes as Robust Pre‐Catalysts for Alkyne Metathesis

A series of 2,4,6‐trimethylbenzylidyne tungsten and molybdenum complexes was prepared from the tribromides mer‐[MesCMBr₃(dme)] ( 9a , M=W; 9b , M=Mo, dme=1,2‐dimethoxyethane). Successive reaction of complexes 9 with lithium or potassium hexafluoro‐tert‐butoxide and lithium 1,3‐di‐tert‐butylimidazolin‐2‐imide, (Im^t‐BuN)Li, afforded the imidazolin‐2‐iminato complexes [MesCM{OC(CF₃)₂Me}₂(Im^t‐BuN)] ( 10a , M=W; 10b , M=Mo), whereas the reactions of 9 with three equivalents of LiOSi(O‐t‐Bu)₃ or KOC(CF₃)₃ gave [MesCM{OSi(O‐t‐Bu)₃}₃] ( 11a , M=W; 11b , M=Mo) and [MesCM{OC(CF₃)₃}₃] ( 12a , M=W; 12b , M=Mo), respectively. For comparison, the benzylidyne complex [PhCMo{OSi(O‐t‐Bu)₃}₃] ( 7b ) was also prepared, and its molecular structure together with those of 10a , 10b , 11b , 12a and 12b were established by X‐ray diffraction analysis. Complexes 10 and 11 were employed as pre‐catalysts for the alkyne metathesis of the test substrate 3‐pentynyl benzyl ether ( 13 ) at low catalyst loadings (1 mol%) in the presence of molecular sieve (5 Å). Comparative studies of these 2,4,6‐trimethylbenzylidyne species (MesCM) with their benzylidyne analogues (PhCM) revealed that the increased steric bulk renders the former more stable and manageable in air in solid form for shorter periods of time, but at the expense of a slower initiation, which requires higher temperatures or longer reaction times.

     

Catalytic Synthesis of Peptide‐Derived Thiazolines and Oxazolines using Bis(quinolinolato)dioxomolybdenum(VI) Complexes

Bis(2‐ethyl‐8‐quinolinolato)dioxomolybdenum(VI) ( 9 ) (1 mol %) shows remarkable catalytic activity for the dehydrative cyclization of cysteine‐containing dipeptides 1 to give the corresponding thiazolines 2 with less than 6 % epimerization at the C2‐exomethine position. For the dehydrative cyclization of threonine‐containing dipeptides 4 , 1 mol % of bis(2‐phenyl‐8‐quinolinolato)dioxomolybdenum(VI) ( 10 ) gives the corresponding oxazolines 5 with retention of configuration at the 5‐position.     

Chelating Bis(1,2,3‐triazol‐5‐ylidene) Rhodium Complexes: Versatile Catalysts for Hydrosilylation Reactions

NHC‐rhodium complexes (NHC=N‐heterocyclic carbenes) have been widely used as efficient catalysts for hydrosilylation reactions. However, the substrates were mostly limited to reactive carbonyl compounds (aldehydes and ketones) or carbon‐carbon multiple bonds. Here, we describe the application of newly‐developed chelating bis(tzNHC)‐rhodium complexes (tz=1,2,3‐triazol‐5‐ylidene) for several reductive transformations. With these catalysts, the formal reductive methylation of amines using carbon dioxide, the hydrosilylation of amides and carboxylic acids, and the reductive alkylation of amines using carboxylic acids have been achieved under mild reaction conditions.

     

不对称钴配合物的合成及其催化苯乙烯环氧化

将合成的N-取代的吡啶-2-醛亚胺制备成2∶1型不对称吡啶亚胺钴(Ⅱ)配合物(CoL2n,n=1,2…6),并通过红外光谱、元素分析、核磁共振氢谱、热重分析等对相关化合物进行表征。以分子氧为氧源,考察催化剂的种类、溶剂、反应温度、反应时间对苯乙烯环氧化反应性能的影响。研究表明,配合物CoL42是最有效的催化剂;配合物中亚氨氮上取代基的供电子性和空间位阻有利于提高配合物的催化活性;在优化的反应条件下,苯乙烯的转化率达99.9%,环氧苯乙烷的选择性为62.4%。     

Molybdenum‐Catalyzed Asymmetric Allylic Alkylation Reactions Using Mo(CO)6 as Precatalyst

Molybdenum‐catalyzed asymmetric allylic alkylation reactions were carried out using the inexpensive, air‐stable, and readily available Mo(CO)₆ as precatalyst. In situ IR was used to determine the required activation time and temperature required to achieve the maximum concentration of the ‘‘active’’ catalyst from the molybdenum‐precatalyst and chiral ligand. Results from comparison studies are consistent with the notion that the same active catalyst is generated regardless of molybdenum‐precatalyst.     

Novel [Ruthenium(substituted‐tetramethylcyclopentadiene) (2‐quinolinecarboxylato)(allyl)] Hexafluorophosphate Complexes as Efficient Catalysts for Highly Regioselective Nucleophilic Substitution of Aliphatic Allylic Substrates

Stable [ruthenium(R‐substituted‐tetramethylcyclopentadiene)(2‐quinolinecarboxylato)(1‐R′‐substituted‐allyl) hexafluorophosphate (R=Me, R′=H, Me, n‐Pr, Ph; R=t‐Bu, R′=Me) and [ruthenium(pentamethylcyclopentadiene)(2‐quinolinecarboxylato)(1‐n‐propylallyl)] tetrafluoroborate ( 4′a ), as allylruthenium(IV) complexes, have been synthesized in one step, starting from [ruthenium(R‐substituted‐tetramethylcyclopentadiene)tris(acetonitrile) hexafluorophosphate or tetrafluoroborate complexes, quinaldic acid, and allylic alcohols. Single stereoisomers are obtained and the X‐ray single crystal structure determinations of 3b (R=t‐Bu, R′=Me) and 4′a allow one to specify the preferred arrangement. Complexes 3a (R=R′=Me) and 3b are involved as precatalysts favoring the formation of branched products in regioselective nucleophilic allylic substitution reactions, starting from ethyl 2‐(E)‐hexen‐1‐yl carbonate and chlorohexene as unsymmetrical aliphatic allylic substrates. Phenols, dimethyl malonate, and primary (aniline) and secondary (pyrrolidine, piperidine) amines have been used as nucleophiles under mild basic conditions. For the first time, the regioselectivity in favor of the branched product obtained from purely aliphatic allylic substrates is close to the high regioselectivity previously reached starting from cinnamyl‐type substrates in the presence of ruthenium catalysts.