Showing papers on "Lewis acids and bases published in 1986"
TL;DR: In this paper, it was shown that the two species cannot coexist and undergo transmetalation reactions at low temperature, but they exhibit new and unique reactivities and selectivities: in conjugate additions, e.g., not only activation but also high regio-, stereo-, and chemo-selectivity as well as very high asymmetric induction.
Abstract: Organometallic compounds with pronounced nucleophilicity such as organocopper and organolithium compounds, and powerful electrophiles, such as BF3 and AlCl3, are generally thought to be incompatible under normal, homogeneous reaction conditions. As a matter of course, it is anticipated that the two species cannot coexist and undergo transmetalation reactions. To our surprise, however, RCu and BF3 are compatible at low temperature. RCu·BF3 and related organocopper-Lewis acid reagents exhibit new and unique reactivities and selectivities: in conjugate additions, e.g., not only activation but also high regio-, stereo-, and chemo-selectivity as well as very high asymmetric induction is observed; allylic derivatives are alkylated regioselectively, and reaction towards carbonyl groups, imines, epoxides, aziridines, ethers, acetals, orthoformates, and pyridinium salts is in each case found to be selective. RLi·BF3 and related organolithium-Lewis acid reagents also exhibit noteworthy selectivities and reactivities. These complex reagents are utilized in the key steps of the total synthesis of many important natural products, such as certain terpenes, steroids, and alkaloids.
213 citations
TL;DR: In this article, a complexe de W de type W(CH−t−Bu)(NR)(OR') 2, OR'=OC(Me)(CF 3 ) 2
Abstract: Preparation et activite d'un complexe de W de type W(CH−t−Bu)(NR)(OR') 2 , OR'=OC(Me)(CF 3 ) 2
207 citations
154 citations
138 citations
129 citations
TL;DR: The structure of the benzaldehyde/boron trifluoride adduct has been determined by X-ray crystallography and precise structural information concerning complexes of aldehydes with the above-mentioned Lewis acid remains to be presented.
Abstract: The structure of the benzaldehyde/boron trifluoride adduct (6) has been determined by X-ray crystallography. Accordingly, the Lewis acid BF3 is complexed anti to the phenyl group in benzaldehyde. The B-OC-C fragment lies essentially in a common plane. Anti complexation also pertains in solution, as shown by a heteronuclear Overhauser experiment. MNDO calculations of the acetaldehyde/BF, adduct show that anti complexation does indeed lead to the lowest energy species. However, the syn adduct lies only 1.8 kcal/mol higher in energy. The linear form does not represent a minimum on the energy surface but rather the lowest energy transition state for intramolecular anti * syn isomerization. The calculations of CH,CHO/BF,, of 6, and of the free aldehydes clearly point to LUMO lowering and to an increased positive charge at the carbonyl carbon atom upon complexation. The results are discussed in light of Lewis acid mediated aldehyde additions involving allyl and enolsilanes, stannanes, and cyanotrimethylsilane as well as such processes as Diels-Alder, ene, and Grignard reactions. A wide variety of C-C bond-forming reactions of carbonyl compounds are mediated by Lewis acids such as BF3, AICI,, EtAlCI,, TiCI4, SnC1,. and ZnX,. They include carbonyl additions of allylsilanes, enosilanes, cyanotrimethylsilane, and other silyated' and stannylated* carbon nucleophiles, as well as ene reaction^,^ Diels-Alder addition^,^ and hetero-Diels-Alder cycloconden~at ions.~ It is generally accepted that the Lewis acid activates the carbonyl component by forming an adduct prior to C-C bond formation. Whereas X-ray crystallographic data of a TiCI4 adduct of a chiral acrylic acid ester has recently been reported and discussed with regard to stereoselective Diels-Alder reactions,6 precise structural information concerning complexes of aldehydes with the above-mentioned Lewis acid remains to be presented. N M R , UV, and I R data of common aldehyde/Lewis acid complexes are available, but they do not answer the question of anti vs. syn complexation (1 vs. 2; MX, = Lewis acid).' A great deal of experimental and theoretical work concerning the interaction of formaldehyde with Li+ and other Lewis acids has accumulated ' Philipps-Universitat. 'Universitat Essen. R H KMX"
128 citations
TL;DR: The optically active Lewis acid 6 has been used in the enantioselective addition of enolsilanes and Me3SiCN to aldehydes as discussed by the authors.
119 citations
TL;DR: In this article, N-fluoropyridinium salts having non-nucleophilic counter-anions and their derivatives were synthesized by treating pyridine-F 2 adducts with strong Bronsted acids, their salts or trimethylsilyl esters, or Lewis acids.
105 citations
TL;DR: In this paper, the concentration and properties of the Bronsted and Lewis acid sites in samples of zeolite Y dealuminated with silicon tetrachloride vapour at elevated temperatures were examined using infrared spectroscopy with temperature-controlled sorption/desorption of pyridine and ammonia.
103 citations
TL;DR: In this article, the isocyanates conduisent aux oxazolidones-2 and aux imines de dioxolanne-1,3ones -2.
Abstract: Cycloadditions en presence d'halogenures de trialkyletain/bases de Lewis: les isocyanates conduisent aux oxazolidones-2 et aux imines de dioxolanne-1,3ones-2; les isothiocyanates et carbodiimides conduisent respectivement aux imines d'oxathiolanne-1,3ones-2 et d'oxazolidones-2
97 citations
TL;DR: In this article, the authors used colloidal alumina, alumina-coated silica and chlorhydrated complexes of zirconium and aluminum to produce pillared bentonites, which, upon calcination at 400 °C, formed microporous two-dimensional structures with 200m 2 g −1 Langmuir surface area and basal spacing between 17 and 18 A.
TL;DR: The Sakurai reaction of allyltrimethylsilane with dial-dose derivatives was studied in this article, under catalysis by boron trifluoride etherale or titanium tetrachloride, with high stercosclectivity, providing simple routes to the corresponding allyl carbinols.
TL;DR: In this article, the acid properties of 0.5 and 1.0 wt% iron supported by adsorption of pyridine were probed by gravimetric measurements and infrared spectroscopy.
TL;DR: Pyridine adsorption measurements showed that addition of Fe on MgO did not generate acidity, whereas iron produced a small number of sites on Al2O3 and iron addition caused a selective poisoning and strengthening of the acid sites on TiO2 as mentioned in this paper.
TL;DR: In this paper, the ground state conformations of thioester silylketene acetals were evaluated using molecular mechanics and an acyclic transition state model was hypothesized in order to account for the observed selectivity.
TL;DR: Allylation des phenyloxalyl-1 prolinates par des allyl-silanes, stannanes and magnesium bromure in presence d'acides de Lewis (induction asymetrique-1,4) as mentioned in this paper.
Abstract: Allylation des phenyloxalyl-1 prolinates par des allyl-silanes, stannanes et magnesium bromure en presence d'acides de Lewis (induction asymetrique-1,4)
TL;DR: In this paper, the spectroscopic properties and photoisomerization reactions of several (E and Z)-cinnamic esters, bis cinnamates, and model esters and lactones in the presence and absence of Lewis acids have been investigated.
Abstract: The spectroscopic properties and photoisomerization reactions of several (E)- and (Z)-cinnamic esters, bis cinnamic esters, and model esters and lactones in the presence and absence of Lewis acids have been investigated. The use of Lewis acids such as BF/sub 3/ or EtAlCl/sub 2/ results in enhanced photoisomerization efficiency and a shift in the photoequilibrium toward the thermodynamically less stable Z isomer. Enhanced E ..-->.. Z photoisomerization results from selective excitation of ground-state ester-Lewis acid complexes. These complexes have been characterized by /sup 1/H NMR, ultraviolet, and fluorescence spectroscopies. The equilibrium constants for complexation are dependent upon both the electron donor strength of the ester and its conformational mobility. These factors also determine the magnitude of the red shifts in the electronic absorption spectra observed upon complexation. Enhanced E ..-->.. Z photoisomerization upon complex formation is a consequence of selective excitation of the E vs. Z complex, more efficient isomerization of the excited E vs. Z complex, and larger equilibrium constants for complexation of E vs. Z esters. The photoequilibria obtained for bis cinnamic esters are highly enriched in the Z,Z and Z,E isomers in accord with independent isomerization of the two cinnamate groups; however, in the case of 1,3-trimethylenebis(cinnamate), two-bondmore » isomerization of the E,E to Z,Z isomer is observed at low conversions.« less
TL;DR: In this paper, the polymerization of propene in the presence of MgCl2-supported catalysts was studied and new results were obtained when polymerizing at variable ethyl benzoate (EB)/alkyl aluminium and alkyl aluminium/Ti mole ratios.
Abstract: As a part of a study on the polymerization of propene in the presence of MgCl2-supported catalysts, new results were obtained when polymerizing at variable ethyl benzoate (EB)/alkylaluminium and alkylaluminium/Ti mole ratios. The pre-treatment of MgCl2 with the Lewis base is essential to achieve high stereoregularities without an unacceptable decrease of polymerization rate, apparently because it prevents the formation of nonstereospecific active sites of low Lewis acidity, which require high EB/alkylaluminium mole ratios in the co-catalyst in order to be poisoned. The data may be explained in the framework of a model of catalytic sites preliminarily presented in Part 1 of this series.
Patent•
05 Dec 1986TL;DR: In this paper, the color-forming di- and triarylmethane compounds possessing certain S-containing ring-closing moieties, namely, or thiolactone, dithiolacton or thioether ring closing moiety, were converted to their chromophore color.
Abstract: This invention relates to novel recording materials which employ color-forming di- and triarylmethane compounds possessing certain S-containing ring-closing moieties, namely, or thiolactone, dithiolactone or thioether ring-closing moiety and to a method of forming color by contacting these dye precursor compounds with a Lewis acid material capable of opening the thiolactone, dithiolactone or thioether ring-closing moiety whereby the compound is rendered colored, that is, converted to its chromophore color.
TL;DR: Preparation of dicetones and cetoacides: 1,5 acycliques possedant 2 ou 3 stereocentres as discussed by the authors, 1.
Abstract: Preparation de dicetones-1,5 et de cetoacides-1,5 acycliques possedant 2 ou 3 stereocentres
TL;DR: In the presence of SnCl 4 de nitro-1 nonadiene-1,7 and n-oxide-2 decadiene -2,8: obtention d'oxydes-3 dhexahydro-4a,5,6,7,8,8-8, 8a benzooxazines-2,3 as mentioned in this paper.
Abstract: Cycloadditions stereoselectives intramoleculaires en presence de SnCl 4 de nitro-1 nonadiene-1,7 et de nitro-2 decadiene-2,8: obtention d'oxydes-3 d'hexahydro-4a,5,6,7,8,8a benzooxazines-2,3
TL;DR: In this article, a new modification of the silicon-directed Nazarov cyclization is described which involves the cyclization of divinyl ketones incorporating an allylsilane as the control unit.
TL;DR: In this paper, a general method for the preparation of 1,5-dicarbonyl compounds and six membered ring annelation is described, which involves the reaction of hemiacetal vinylogs with enol ethers 2 or 3 in the presence of a Lewis acid.