Copolymerization of ethene with cyclic and other sterically hindered olefins

Sterically hindered olefins like norbornene, dimethanooctahydronaphthalene (DMON), 4‐methylpentene, and 3‐methylbutene can be copolymerised with ethene by metallocene/MAO catalysts. Different C₂‐, C_s‐ and C₁‐symmetric and meso‐zirconocenes were used. Only isolated and alternating norbornene sequences but no norbornene blocks are formed by substituted [Me₂C(Cp‐R)(Flu)]ZrCl₂ catalysts. The alternating microstructure leads to melting points up to 270°C for ethene‐norbornene copolymers and up to 380°C for the semi‐crystalline alternating copolymer of ethene and DMON. Other sterically hindered olefins such as 3‐methylpentene build more blocky structures with high glass transition temperatures. The mechanism for the insertion reaction of the different catalysts is discussed.     

Polymerization of 4‐methylpentene and vinylcyclohexane by amine bis(phenolate) titanium and zirconium complexes

The polymerization of the substituted olefins 4‐methylpentene and vinylcyclohexane by dibenzyl titanium and zirconium complexes of three amine bis(phenolate) ligands is reported. The ligands featured a dimethylamino side‐arm donor and either electron‐withdrawing (Cl and Br) or methyl phenolate substituents. After activation with B(C₆F₅)₃, the zirconium catalysts exhibited a higher activity than the titanium catalysts toward these bulky olefins. Very high weight‐average molecular weight poly(4‐methylpentene) was obtained with the zirconium catalysts. The zirconium catalysts were employed in 1‐hexene polymerization, and their activity was found to be the highest ever reported for catalysts of the amine bis(phenolate) family. The catalysts featuring methyl phenolate substituents showed a higher activity toward these substituted olefins than the electron‐poor catalysts; this trend was opposite to their activity toward 1‐hexene. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1136–1146, 2006     

Studies with condensed amino‐thiophenes: Further investigation of reactivity of amino‐thieno‐coumarines and amino‐thieno‐benzo[h]coumarines toward electron‐poor olefins and acetylenes

The reaction of 3‐amino‐5‐oxa‐2‐thia‐cyclopenta[a]naphthalene‐4‐one 2b with substituted acetylenes afforded C‐1 alkylation products. On the other hand, reaction of 17‐amino‐15‐methyl‐11‐oxa‐16‐thiacyclopenta[a]phenanthrene‐12‐one 5 with substituted acetylenes and electron‐poor olefins afforded the condensed thienopyridine derivatives 7 and 11a – c . The reaction of 5 with acrylonitrile and with 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione afforded compounds 13 and 21 with loss of H₂S via the expected [4 + 2] cycloaddition sequence. © 2004 Wiley Periodicals, Inc. Heteroatom Chem 15:502–507, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20047     

Pd(0)‐Catalyzed Tandem One‐Pot Reaction of Biphenyl Ketones/Aldehydes to the Corresponding Di‐substituted Aryl Olefins

Synthesis of di‐substituted aryl olefins via a Pd(0)‐catalyzed cross‐coupling reaction of biphenyl ketones/aldehydes, tosylhydrazide, and aryl bromides (or benzyl halides) was developed. This methodology was achieved by one‐pot two‐step reactions involving the preparation of N ‐tosylhydrazones by reacting tosylhydrazide with biphenyl ketones/aldehydes, followed by coupling with aryl bromides (or benzyl halides) in the presence of Pd(PPh₃ )₄ and lithium t ‐butoxide to produce various di‐substituted aryl olefins in moderate to good yields.     

Ionic‐Liquid‐Influenced Expeditious and Stereoselective Synthesis of Olefins

1‐Butyl‐3‐methylimidazolium chloroaluminate, [bmim]Cl·.AlCl₃ (molar fraction, N=0.67), ionic liquid has been used in combination with metallic Zn for the reductive coupling of carbonyl compounds to synthesize symmetrical olefins, with the Z‐isomer formed predominantly. The ionic liquid played a dual role of Lewis acid catalyst and solvent.     

Aromatic‐Amide‐Derived Olefins as a Springboard: Isomerization‐Initiated Palladium‐Catalyzed Hydrogenation of Olefins and Reductive Decarbonylation of Acyl Chlorides with Hydrosilane

A highly efficient catalytic protocol for the isomerization of substituted amide‐derived olefins is presented that successfully uses a hydride palladium catalyst system generated from [PdCl₂(PPh₃)₂] and HSi(OEt)₃. The Z to E isomerization was carried out smoothly and resulted in geometrically pure substituted olefins. Apart from the cis–trans isomerization of double bonds, the selective reduction of terminal olefins and activated alkenes was performed with excellent functional group tolerance in the presence of an amide‐derived olefin ligand, and the products were obtained in high isolated yields (up to >99 %). Furthermore, the palladium/hydrosilane system was able to promote the reductive decarbonylation of benzoyl chloride when a (Z)‐olefin with an aromatic amide moiety was used as a ligand.     

The stereochemistry of sodium dithionite reductions of cyclic ketones

The stereochemistry of reduction of several substituted cyclohexanones, norcamphor, and camphor by sodium dithionite (sodium hydrosulfite, Na₂S₂O₄) in aqueous DMF solution has been studied. The cyclohexanones yield mainly equatorial alcohols while the bicyclic ketones give mainly endo alcohols.     

Conversion of Carbonyl Compounds to Olefins via Enolate Intermediate

A general and efficient protocol to synthesize substituted olefins from carbonyl compounds via nickel catalyzed C—O activation of enolates was developed. Besides ketones, aldehydes were also suitable substrates for the presented catalytic system to produce di‐ or tri‐ substituted olefins. It is worth noting that this approach exhibited good tolerance to highly reactive tertiary alcohols, which could not survive in other reported routes for converting carbonyl compounds to olefins. This method also showed good regio‐ and stereo‐selectivity for olefin products. Preliminary mechanistic studies indicated that the reaction was accomplished through nickel catalyzed C—O activation of enolates, thus offering helpful contribution to current enol chemistry.     

Selective Palladium‐Catalyzed Aminocarbonylation of Olefins to Branched Amides

A general and efficient protocol for iso‐selective aminocarbonylation of olefins with aliphatic amines has been developed for the first time. Key to the success for this process is the use of a specific 2‐phosphino‐substituted pyrrole ligand in the presence of PdX₂ (X=halide) as a pre‐catalyst. Bulk industrial and functionalized olefins react with various aliphatic amines, including amino‐acid derivatives, to give the corresponding branched amides generally in good yields (up to 99 %) and regioselectivities (b/l up to 99:1).     

The Cp2ZrCl2-catalyzed cycloalumination of functionally substituted olefins with triethylaluminum

Reactions of functionally substituted olefins (allylamines, sulfides and ethers, homoallylic alcohols and amines, as well as vinyl ethers) with Et₃Al in the presence of Cp₂ZrCl₂ as a catalyst were studied. Cycloalumination of allylamines occurs with high regioselectivity to furnish after subsequent deuterolysis 4-deutero-2-(deuteromethyl)butyl-substituted amines. Cycloalumination of alkyl allyl sulfide is accompanied by a side process of the C-S bond cleavage. In the case of allyl and vinyl ethers, no cycloalumination products are formed under the reaction conditions. However, the reactions with homoallylic alcohol and amine after deuterolysis gave the corresponding dideutero-containing compounds in good yields.     

Tetra‐n‐butylammonium Hydroxide (TBAH)–Catalyzed Knoevenagel Condensation: A Facile Synthesis of α‐Cyanoacrylates, α‐Cyanoacrylonitriles,and α‐Cyanoacrylamides

Tetra‐n‐butylammonium hydroxide (TBAH) has been utilized as a novel and efficient catalyst for the Knoevenagel condensation of aldehydes with acidic methylene compounds such as methyl‐ and ethylcyanoacetate, malononitrile, and cyanoacetamide to afford substituted olefins.     

Regioselective Hydrosilylation of Olefins Catalyzed by a Molecular Calcium Hydride Cation

Chemo‐ and regioselectivity are often difficult to control during olefin hydrosilylation catalyzed by d‐ and f‐block metal complexes. The cationic hydride of calcium [CaH]⁺ stabilized by an NNNN macrocycle was found to catalyze the regioselective hydrosilylation of aliphatic olefins to give anti‐Markovnikov products, while aryl‐substituted olefins were hydrosilyated with Markovnikov regioselectivity. Ethylene was efficiently hydrosilylated by primary and secondary hydrosilanes to give di‐ and monoethylated silanes. Aliphatic hydrosilanes were preferred over other commonly employed hydrosilanes: Arylsilanes such as PhSiH₃ underwent scrambling reactions promoted by the nucleophilic hydride, while alkoxy‐ and siloxy‐substituted hydrosilanes gave isolable alkoxy and siloxy calcium derivatives.     

Free Radical Chemistry of Phosphasilenes

Understanding the characteristics of radicals formed from silicon‐containing heavy analogues of alkenes is of great importance for their application in radical polymerization. Steric and electronic substituent effects in compounds such as phosphasilenes not only stabilize the Si=P double bond, but also influence the structure and species of the formed radicals. Herein we report our first investigations of radicals derived from phosphasilenes with Mes, Tip, Dur, and NMe₂ substituents on the P atom, using muon spin spectroscopy and DFT calculations. Adding muonium (a light isotope of hydrogen) to phosphasilenes reveals that: a) the electron‐donor NMe₂ and the bulkiest Tip‐substituted phosphasilenes form several muoniated radicals with different rotamer conformations; b) bulky Dur‐substituted phosphasilene forms two radicals (Si‐ and P‐centred); and c) Mes‐substituted phosphasilene mainly forms one species of radical, at the P centre. These significant differences result from intramolecular substituent effects.     

Synthesis of Indolines via a SmI2 Promoted Domino Nitro Reduction–Intramolecular aza‐Michael Reaction

A simple and straightforward synthesis of substituted indolines based on a domino nitro reduction intramolecular aza‐Michael reaction is described. The reaction employs Samarium diiodide under mild conditions for the addition of dibromoacetic acid to substituted 2‐(2‐nitrophenyl) acetaldehyde derivatives and their subsequent cyclization upon nitro group reduction to provide corresponding indoline heterocycles in good yields. This “one pot” strategy also permitted the expeditious synthesis of a 1,2,3,4‐tetrahydroquinoline, whereas the seven‐membered 2,3,4,5‐tetrahydrobenzoazepines compounds were not formed under these reaction conditions.     

Ag Loaded on SiO2 as an Efficient and Recyclable Heterogeneous Catalyst for the Synthesis of Chloro‐8‐substituted‐9H‐purines

Ag support on silica has been used as an effective heterogeneous catalyst for the facile synthesis of chloro‐8‐substituted‐9H‐purine derivatives via the one‐pot reaction of 6‐chloro‐pyrimidines and substituted acids. The title compounds were formed as excellent yields with short reaction time under eco‐friendly conditions. The prepared catalyst (Ag/SiO₂) can be reused for a number of times with insignificant loss in its activity. This route has the advantage of being a cost‐effective, readily available, easy workup procedure.     

Iodoamination of Alkenyl Sulfonamides by Potassium Iodide and Hydrogen Peroxide in Aqueous Medium

A procedure for the iodoamination of unfunctionalized olefins tethered to a tosyl‐protected NH‐group has been developed. The combined use of KI and H₂O₂ in aqueous medium was effective for the preparation of iodomethyl‐substituted nitrogen‐containing heterocycles. The selective exo‐trig iodocyclization provided 1,2‐bifunctional 5‐, 6‐, and 7‐membered cyclic skeletons.     

Copper/P(tBu)3‐Mediated Regiospecific Synthesis of Fused Furans and Naphthofurans

A novel [3+2] cycloaddition between a variety of cyclic ketones and diverse olefins or alkynes can be effectively promoted by copper in combination with the tri‐tert ‐butylphosphine [P(t Bu)₃] ligand. This protocol exhibits excellent selectivity and provides an exemplary set of fused heterocycles in good to excellent yields. Present strategy also represents an extremely simple and atom‐economic way to construct substituted fused furans and naphthofurans from readily available starting materials under mild reaction conditions. The utility of the method is further demonstrated by the synthesis of chiral furans from (R )‐(−)‐carvone and (S )‐(+)‐carvone. A plausible mechanism involving the oxidative radical cyclization has been suggested based on experimental observations.     

Synthesis of Mesoionic 4‐(para‐substituted Phenyl‐5‐2,4‐dichlorophenyl)‐1,3‐4‐thiadiazolium‐2‐aminides by Direct Cyclization via Acylation of Thiosemicarbazides

The synthesis of ten novel mesoionic 4‐[para‐substituted (H, CH₃, OCH₃, NO₂, Cl, Br, OH, t‐C₄H₉, C₆H₅, C₄H₉) phenyl‐5‐2,4‐dichlorophenyl]‐1,3‐4‐thiadiazolium‐2‐aminides, as hydrochlorides, are described. The synthesis strategy utilized the corresponding para‐substituted isothiocyanates as starting materials to obtain the thiosemicarbazides through reaction with phenylhydrazine (61–98%), which were then submitted to acylation with 2,4‐dichloro benzoyl chloride and direct cyclization to generate the desired substituted mesoionic compounds in good yields (ca. 80%).     

Nitropyridines,their synthesis and reactions

Reaction of pyridine and substituted pyridines with N₂O₅ in an organic solvent gives the N‐nitropyridinium ion. When this is reacted with SO₂/HSO₃‐ in water, 3‐nitropyridine is obtained (77 % yield). With substituted pyridines the method gives good yields for 4‐substituted and moderate yields for 3‐substituted pyridines. The reaction mechanism is not an electrophilic aromatic substitution but one in which the nitro group migrates from the 1‐position to the 3‐position by a [1,5] sigmatropic shift. From 3‐nitropyridine, 5‐nitropyridine‐2‐sulfonic acid is formed in a two step reaction. From this, a series of 2‐substituted‐5‐nitropyridines has been synthesized. 3‐Nitropyridine and 4‐substituted‐3‐nitropyridines have been substituted with ammonia and amines by the vicarious nucleophilic substitution (VNS) method with ammonia and amines and by the oxidative substitution method in the position para to the nitro group. High regioselectivities and yields have been obtained in both cases to afford a series of 4‐substituted‐2‐alkylamino‐5‐nitropyridines. The VNS method has also been used in alkylation reactions with 3‐nitropyridines to form dichloromethyl‐and alkoxycarbomethyl‐β‐nitropyridines. From the appropriate substituted nitropyridines imidazopyridines and azaindoles have been formed.     

Chemoselective Synthesis of Substituted Imines,Secondary Amines,and β‐Amino Carbonyl Compounds from Nitroaromatics through Cascade Reactions on Gold Catalysts

Substituted imines, α,β‐unsaturated imines, substituted secondary amines, and β‐amino carbonyl compounds have been synthesized by means of new cascade reactions with mono‐ or bifunctional gold‐based solid catalysts under mild reaction conditions. The related synthetic route involves the hydrogenation of a nitroaromatic compound in the presence of a second reactant such as an aldehyde, α,β‐unsaturated carbonyl compound, or alkyne, which circumvents an ex situ reduction process for producing the aromatic amine. The process is shown to be highly selective towards other competing groups, such as double bonds, carbonyls, halogens, nitriles, or cinnamates, and thereby allows the synthesis of different substituted nitrogenated compounds. For the preparation of imines, substituted anilines are formed and condensed in situ with aldehydes to provide the final product through two tandem reactions. High chemoselectivity is observed, for instance, when double bonds or halides are present within the reactants. In addition, we show that the Au/TiO₂ system is also able to catalyze the chemoselective hydrogenation of imines, so that secondary amines can be prepared directly through a three‐step cascade reaction by starting from nitroaromatic compounds and aldehydes. On the other hand, Au/TiO₂ can also be used as a bifunctional catalyst to obtain substituted β‐amino carbonyl compounds from nitroaromatics and α,β‐unsaturated carbonyl compounds. Whereas gold sites promote the in situ formation of anilines, the intrinsic acidity of Ti species on the support surface accelerates the subsequent Michael addition. Finally, two gold‐catalyzed reactions, that is, the hydrogenation of nitro groups and a hydroamination, have been coupled to synthesize additional substituted imines from nitroaromatic compounds and alkynes.