Showing papers on "Lewis acids and bases published in 1991"

Metal complex compounds

Abstract: A monocyclopentadienyl or substituted monocyclopentadienyl metal complex containing compound useful as a polymerization catalyst corresponding to the formula: CpMX.sub.n.sup.+ A.sup.- wherein: Cp is a single η 5 -cyclopentadienyl or η 5 -substituted cyclopentadienyl group optionally covalently bonded to M through a substituent; M is a metal of Group 3-10 or the Lanthanide Series of the Periodic Table bound in an η 5 bonding mode to the cyclopentadienyl or substituted cyclopentadienyl group; X each occurrence independently is selected from the group consisting of hydride, halo, alkyl, aryl, silyl, germyl, aryloxy, alkoxy, amide, siloxy, neutral Lewis base ligands and combinations thereof having up to 20 non-hydrogen atoms, and optionally one X together with Cp forms a metallocycle with M; R is alkyl or aryl of up to 10 carbons; n is one or two depending on the valence of M; and A is a noncoordinating, compatible anion of a Bronsted acid salt.

Ionic metallocene catalyst compositions

Abstract: Ionic catalyst compositions can be prepared by combining two components. The first component is a bis(cyclopentadienyl) Group IV-B metal complex containing at least one ligand which will combine irreversibly with the second component or at least a portion thereof such as a cation portion thereof. The second component comprises a cation which will irreversibly react with at least one ligand on the Group IV-B metal complex and a non-coordinating anion. The combination of the two components produces an ionic catalyst composition comprising a cationic bis(cyclopentadienyl) Group IV-B metal complex which has a formal coordination number of 3 and a 4+ valence charge and the aforementioned non-coordinating anion. The anion is (i) labile and can be displaced by an olefin, diolefin or acetylenically unsaturated monomer; (ii) has a molecular diameter about or greater than 4 angstroms; (iii) forms stable salts with reducible Lewis acids and protonated Lewis bases; (v) has a negative charge delocalized over the framework on the anion or within the core thereof; (v) is not a reducing or oxidizing agent; and (vi) is a relatively poor nucleophile. These ionic catalyst compositions can be used to polymerize α-olefins, diolefins and/or acetylenically unsaturated monomers, either alone or in combination, to polymers and copolymers.

Aluminum-free monocyclopentadienyl metallocene catalysts for olefin polymerization

Abstract: The invention is a catalyst system including a Group IV-B transition metal component and an activator component which may be employed to polymerize olefins to produce a high molecular weight polymer. The Group IV-B transition metal component contains a single cyclopentadienyl ligand and a single heteroatom ligand, which ligands may be bridged one to another; the activator component comprises a cation, which may be a Bronsted acid capable of donating a proton (hereafter denoted as L'-H wherein L' is a neutral Lewis base and H is hydrogen), and a compatible noncoordinating anion.

Acyclic diene metathesis (ADMET) polymerization

Abstract: 1,9-Decadiene has been converted quantitatively to poly(octenylene), and 1,5-hexadiene has been converted to exclusively 1,4-polybutadiene by acyclic diene metathese polymerization, using a catalyst free of Lewis acids. Poly(octenylene) exhibits a minimum Mw of 108000 and is more than 90% trans in its stereochemistry. Exclusively 1,4-polybutadiene is of a minimum Mw of 28000 and is more than 70% trans in its stereochemistry. The stereochemistry appears to be controlled thermodynamically due to the equilibrium nature of the polymerization

Three-coordinate iron complexes: x-ray structural characterization of the iron amide-bridged dimers [Fe(NR2)2]2 (R = SiMe3, C6H5) and the adduct Fe[N(SiMe3)2]2(THF) and determination of the association energy of the monomer Fe{N(SiMe3)2}2 in solution

Abstract: The X-ray structural characterization of the iron (II) amides [Fe{N(SiMe 3 ) 2 } 2 ] 2 (1) and [Fe(NPh 2 ) 2 ] 2 (2) and the Lewis base adduct Fe[N(SiMe 3 ) 2 ] 2 (THF) (3), as well as the syntheses of the last two compounds, is described. These complexes are rare examples of the coordination number 3 for iron. Compounds 1 and 2 are both dimeric in the solid state, with each trigonal-planar ion bound to one terminal and two bridging amides. They closely resemble the corresponding Mn(II) and Co(II) compounds. Compound 3 is monomeric in the solid state, with one THF and two amides arranged in a trigonal-planar fashion

Chiral Lewis acids in catalytic asymmetric reactions

Abstract: This review examines the recent advances in the use of chiral Lewis acids, usually generated in situ from a Lewis acid and a chiral auxiliary, in the Diels-Alder reaction, the ene reaction, the hydrocyanation of aldehydes, the ring-opening of epoxides and various miscellaneous reactions Structural studies of the complex formed between carbonyl compounds and Lewis acids are also reported The Sharpless epoxidation is not covered by this review 1 Introduction 2 Asymmetric Diels-Alder Reactions 21 Chiral Titanium Reagents 22 Chiral Aluminum Reagents 23 Chiral Boron Reagents 24 Chiral Ruthenium Reagents 3 Asymmetric [2+2] Cycloaddition Reactions 4 Asymmetric Ene Reaction 5 Asymmetric Hydrocyanation of Aldehydes 6 Asymmetric Ring Opening of Epoxides 7 Miscellaneous Asymmetric Reactions 8 Structural Studies on the Complexes of Carbonyl Compounds and Lewis Acids

Catalytic asymmetric aldol reactions. use of a chiral (acyloxy)borane complex as a versatile lewis acid catalyst

Abstract: In the presence of 20 mol% of a chiral (acyloxy)borane (CAB) complex prepared from (2R,3R)-2-O(2,6-diisopropoxybenzoyl)tartaric acid and borane-tetrahydrofuran, various allyltrimethylsilanes react with achiral aldehydes to afford the corresponding homoallylic alcohols in good yields with high diastereo- and enantioselectivities. Furthermore, the reactivity of allylation can be improved without reducing the enantioselectivity by using 10-20 mol% of the CAB complex prepared from 3,5-bis(trifluoromethyl)phenylboronic acid and chiral tartaric acid derivative. The observed selectivities and re-face attack of nucleophiles on the carbonyl carbons of aldehydes imply that the extended transition-state model is applicable

Lanthanide Trifluoromethanesulfonates as Stable Lewis Acids in Aqueous Media. Yb(OTf)3 Catalyzed Hydroxymethylation Reaction of Silyl Enol Ethers with Commercial Formaldehyde Solution

Abstract: Silyl enol ethers smoothly react with commercial formaldehyde solution to give the corresponding hydroxymethylated adducts in high yields by the promotion of a catalytic amount of ytterbium(III) triflate (Yb(OTf)3), which is a stable Lewis acid in aqueous media. Other lanthanide triflates are also found to be effective.

A highly stereoselective synthesis of (E)-alkene dipeptide isosteres via organocyanocopper-Lewis acid mediated reaction

Abstract: A stereoselective synthesis of protected (E)-alkene dipeptide isosteres by the reaction of the mesylates of homochiral δ-aminated γ-hydroxy (E)-α,β-enoates with either RCu(CN)Li-BF 3 or RCu(CN)MgX-BF 3 reagent is described. The degree of diastereoselectivity has been found to be uniformly high except for the serine- and threonine-derived acetonides 77 and 81. The synthesis permits the introduction of sterically hindered appendages such as isopropyl and tert-butyl groups at the α position to the ester group. This methodology provides a new route to a wide range of modified (E)-alkene peptide mimics that may have biological importance

Short, enantiogenic syntheses of (-)-indolizidine 167B and (+)-monomorine

Abstract: The enantiogenic syntheses of (−)-indolizidine 167B (1) and (+)-monomorine (2) are described. D-Norvaline and L-alanine are converted into their 1-pyrrole derivatives by reaction with 2,5-dimethoxytetrahydrofuran. Thereafter, Arndt-Eistert homologation of the N-alkanoic acid substituent, followed by rhodium (II) acetate catalyzed decomposition of its α-diazo ketone derivative, provides the relevant bicyclic precursors, the vested chirality of which directs catalytic hydrogenation affording 1 and 2. Provision for the 5-butyl side chain in 2 is made by prior Lewis acid catalyzed rearrangement of the mixed anhydride obtained from butyryl chloride and the pyrrole analogue of L-alanine

A Facile Synthesis of Cyanohydrin Trimethylsilyl Ethers by the Addition Reaction of Trimethylsilyl Cyanide with Aldehydes under Basic Condition

Abstract: In the presence of a catalytic amount of Lewis base such as amine, phosphine, arsine or antimony, trimethylsilyl cyanide (TMS-CN) smoothly reacts with aldehydes to afford the corresponding cyanohydrin trimethylsilyl ethers in excellent yields.

The Prins and Carbonyl Ene Reactions

Abstract: The addition of aldehydes and ketones to alkenes in the presence of Bronsted acids is usually called the Prins reaction.1,2 Addition of the protonated carbonyl compound to the alkene gives a β-hydroxy carbocation which reacts with a nucleophile such as chloride, water or acetate to give (1), adds to a second molecule of aldehyde to give (2) or loses a proton to give homoallylic alcohol (3; Scheme 1). Formaldehyde and electron-deficient carbonyl compounds undergo concerted thermal ene reactions with alkenes at temperatures ranging from 100 to 200 °C to give homoallylic alcohols (equation 1; R1 = H, CCl3, or CO2R).3–7 A wide variety of carbonyl compounds undergo Lewis acid catalyzed reactions with alkenes to give homoallylic alcohol ene adducts in excellent yield with high selectivity, although in some cases γ-chloro alcohols are formed as by-products.8,9 Recent investigations suggest that at least some of these Lewis acid catalyzed ene reactions are stepwise, rather than concerted, but that the intermediate bears more resemblance to a π-complex than to the open carbocation intermediate in the Prins reaction.

Review: dehydration catalysts for the methanol/dimethyl ether reaction

Abstract: Catalysts for the methanol/dimethyl ether reaction are reviewed with particular emphasis on the acid/base properties of the catalysts. The focus is on temperatues less than around 300°C and atmospheric pressure conditions, which correspond to potential automotive applications. Mechanisms including both dual site (acid/base) Langmiur-Hinshelwood and single site Rideal-Eley mechanisms have been postulated, with both Bronsted and Lewis acid sites involved in the dehydration reaction. Temperatures higher than about 300°C, as well as a strong surface acidity, tend to promote the further dehydration of DME to form C2-Q5 olefins and eventually hydrocarbon products in a series type reaction sequence. Methods exist, particularly in zeolites, to control the strength and type of surface acidity, and therefore the selectivity to DME.

The lithium perchlorate-diethyl ether rate acceleration of the Diels-Alder reaction: Lewis acid catalysis by lithium ion

Abstract: We present evidence that the rate acceleration of the Diels-Alder reaction in lithium perchlorate-diethyl ether may be due to Lewis acid catalysis, with lithium ion functioning as the Lewis acid

Stereoselectivity of cyclization of substituted 5-hexen-1-yllithiums: regiospecific and highly stereoselective insertion of an unactivated alkene into a carbon-lithium bond

Abstract: Substituted 5-hexen-1-yllithiums, which were prepared in solutions of n-C 5 H 12 -Et 2 O (3:2 by vol) by low-temperature lithium-iodine exchange between t-BuLi and the appropriate iodide, undergo clean, 5-exo-trig cyclization upon warming to give substituted (cyclopentyl)methyllithiums, in good yield and with a high degree of stereocontrol. In each case, the major product is the same isomer as that observed in studies of the isomerization of analogously substituted 5-hexen-1-yl radicals, but the organolithium cyclizations are invariably much more stereoselective than radical-mediated processes. Lewis base additives such as THF, TMEDA, and PMDTA serve to increase the rate of cyclization of the substituted 5-hexen-1-yllithiums, but such additives do not reduce the high stereoselectivity of the process. The observed regioselectivities and stereoselectivities of the intramolecular addition of a C-Li bond to an unactivated alkene suggest that the closure of the anion proceeds via a transition state that resembles a chair cyclohexane in which substituents preferentially occupy pseudoequatorial positions

Site-specific reversible scission of hydrogen bonds in polymers: an investigation of polyamides and their Lewis acid-base complexes by infrared spectroscopy

Abstract: The scission and re-formation of N―H...O=C hydrogen bonds in seven aliphatic polyamides and one aromatic polyamide is investigated. Complete scission of the intermolecular hydrogen bonds in polyamides was achieved by the 1:1 Lewis acid complexation of the basic oxygen sites of the carbonyl groups in the polymers, thus «liberating» the N―H groups from hydrogen bonding. Facile decomplexation of the polyamide-Lewis acid complexes resulted in the reformation of the hydrogen bonds and, consequently, the characteristic solid-state structure and properties of the polyamides. Thus, the hydrogen bonds in polyamides function like a molecular zipper

Syntheses of Charge-Compensated Dicarbollide Ligand Precursors and Their Use in the Preparation of Novel Metallacarboranes.

Abstract: A series of charge-compensated, boron-substituted carborane cage derivatives, [nido-9-L-7,8-C 2 B 9 H 11 ] or [nido-10-L-7,8-C 2 B 9 H 11 ] (L=nitrogen-containing ligand), were prepared by the ferric chloride-promoted oxidative coupling reaction of K[nido-7,8-C 2 B 9 H 12 ] with the Lewis base, L. Another series of neutral boron-substituted carborane cage derivatives, [nido-10-L-7,9-C 2 B 9 H 11 ], were synthesized directly from the reaction of [closo-1,8-C 2 B 9 H 11 ] with the nitrogen-containing ligand, L

Thermogravimetric and infrared study of the desorption of butylamine, cyclohexylamine and pyridine from Ni- and Co-exchanged montmorillonite

Abstract: A B S T R A C T : The acidity of_Ni 2+- and Co 2+-exchanged montmorillonite has been probed using the diagnostic bases n-butylamine, cyclohexylamine and pyridine. Derivative thermograms for the desorption of pyridine from Ni 2+- and Co2+-exchanged montmorillonite exhibited strong maxima at 40 ~ 90 ~ and 360~ together with a weak maximum near 170~ The desorption maximum at 360~ is usually attributed to desorption of base from Bronsted acid sites. However, IR spectra of pyridine adsorbed on Ni 2+- and CoZ+-exchanged montmorillonite, at 250~ were dominated by intense bands near 1450 and 1607 cm 1 which are diagnostic of Lewis-bound pyridine. Consequently, the desorption maximum at 360~ must, in this instance, be attributed to desorption of pyridine from Lewis acid centres. Other bases, including cyclohexylamine and butylamine, also desorb at temperatures which have previously been attributed to the desorption of protonated base. Ni 2+- and Co2+-exchanged clay contained predominatly Lewis acid or electron accepting sites, which is in marked contrast to the behaviour of trivalent cation exchanged clays.

Importance of Lewis acid mediated electron transfer in Mukaiyama-Michael reaction of ketene silyl acetals

Abstract: The mechanism of the Mukaiyama reaction has never been examined from the standpoint of electron transfer. We conclude here that the Lewis mediated electron transfer from hindered ketene silyl acetals to α-enones plays an essential role in connecting quaternary carbon centers. The conclusion sheds light on a novel facet of the ketene silyl acetal chemistry, and consequently relevant reactions should be reexamined along this line, particularly in terms of stereochemistry