Boron trifluoride, initiator

It was projected that compound 13 could be stereoselectively linked, through its free phenolic hydroxyl group, with the anomeric carbon of intermediate 12 under suitably acidic conditions (see Scheme 8). Gratifyingly, the action of boron trifluoride etherate on a mixture of 12 and 13 in CH2CI2 at -50 °C induces a completely stereoselective glycosidation reaction, providing the desired a-ano-mer 48 in an excellent yield of 95 % from 46. It is presumed that boron trifluoride initiates cleavage of the anomeric trichloroacetimi-... 

Optically active ( —)-(2R,35 )-( )-2-(buten-l-enyl)-3-methylcyclobutanone (34) was obtained from the boron trifluoride initiated regio- and stereoselective ring expansion of 1-(2-methyl-1-siloxycyclopropyl)pent-l-en-3-ol (33). A small amount of (Z)-alkene was also obtained. 

How does a polymer and a copolymer form side by side in boron trifluoride initiated polymerizations of dioxepane ... 

The above can be shown on boron trifluoride initiation of the polymerization of isobutylene ... 

Poly(N-ethyl-2-vinylcarbazole) (Structure 3a) has been prepared by free-radical polymerization, whereas poly(N-ethyl-3-vinylcarbazole) (3b) was synthesized by cationic polymerization with a boron trifluoride initiator [43]. The 2-isomer is reported to exhibit... 

Poly(ALethyl-2-vinylcarbazole) (formula 7a) has been prepared by free-radical polymerization, whereas poly(A -ethyl-3-vinylcarbazole) (7b) was synthesized by cationic polymerization with a boron trifluoride initiator [150], The 2-isomer is reported to exhibit higher carrier mobility than PVK, while that of the 3-isomer is lower [151], Similar polymers with optically active groups such as poly((5)-9-(2-methylbutyl)-2-vinylcarbazole) (formula 7c), poly((5)-9-(2-methylbu-tyl)-3-vinylcarbazole) and poly((5)-3-(2-methylbutyl)-9-vinylcarbazole) have been prepared by Chielini et al. [152-154], The UV-absorption, fluorescence emission, NMR, and circular dichroism spectra were reported in detail and used to establish a correlation between the photophysical behaviour and both primary and secondary structural features of the polymers,... 

Both boron and aluminium chlorides can be prepared by the direct combination of the elements. Boron trichloride can also be prepared by passing chlorine gas over a strongly heated mixture of boron trioxide and carbon. Like boron trifluoride, this is a covalent compound and a gas at ordinary temperature and pressure (boiling point 285 K). It reacts vigorously with water, the mechanism probably involving initial co-ordination of a water molecule (p, 152). and hydrochloric acid is obtained ... 

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

Methane sulfonic acid, trifluoroacetic acid, hydrogen iodide, and other Brmnsted acids can faciUtate 3 -acetoxy displacement (87,173). Displacement yields can also be enhanced by the addition of inorganic salts such as potassium thiocyanate and potassium iodide (174). Because initial displacement of the acetoxy by the added salt does not appear to occur, the role of these added salts is not clear. Under nonaqueous conditions, boron trifluoride complexes of ethers, alcohols, and acids also faciUtate displacement (87,175). 

Catalytic curing agents initiate resin homopolymerization, either cationic or anionic, as a consequence of using a Lewis acid or base in the curing process. The Lewis acid catalysts frequently employed are complexes of boron trifluoride with amines or ethers. 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

The oligomerization of cardanol with boron trifluoride etharate as the initiator was studied in detail by Antony et al. [171]. The reaction conditions were optimized by using gel permeation chromatography as 140°C with an initiator concentration of 1%. GPC data indicate conversion of all monoene, diene, and triene components into polymer except the saturated component, indicating participation of all the unsaturated components in polymerization. It is possible that the initiation of po-... 

The dehydrogenation of 2,3-dihydro- and 2,5-dihydro-l//-l-benzazepines to 3//-l-benz-azepincs with heterocyclic enamines in the presence of boron trifluoride diethyl ether complex has been achieved in moderate yields (30-35%).241 In contrast, electrochemical oxidation of 2,5-dihydro-1 H- -benzazepines in buffered acetic acid solution furnishes initially 5//-l-benz-azepines in 35-45% yield.242... 

Indole and dimethyl acetylenedicarboxylate yield 2-(indol-3-yl)-2,3-dihydro-l//-l-ben-zazepine (mp 240-242 C) by addition of indole to the initially formed l//-l-benzazepinc,21 whereas 1,3-dimethylindole (10, R = H) fails to react with the diester under a variety of conditions.145 However, in the presence of boron trifluoride-diethyl ether complex at room tem-... 

The stereoselectivity of the boron trifluoride induced reactions was initially discussed in terms of open-chain, antiperiplanar transition states66. However studies of Lewis acid induced intramolecular allylstannane-aldehyde reactions are supportive of a synclinal process56,67. 

The polymerization of 1,3,3-trimethyl-2,7-dioxabicyclo[2.2.1 Jheptane 35 was carried out in methylene chloride, toluene, and 1-nitropropane at temperatures between —78 and 0 °C32l Boron trifluoride etherate, triethyloxonium tetrafluoro-borate, antimony pentachloride, and iodine were used as initiators. Irrespective of the solvents and initiators employed, the products obtained at 0 °C were white powders with melting points of 50—55 °C, while those obtained at tower temperatures were sirups. The number average molecular weight of the unfractionated products ranged from 400 to 600. The molecular weight distribution of the oligomers prepared at 0 °C was broad, in contrast to the relatively narrow distribution of those obtained at -40 °C. 

Note It is reported that the use of chlorobenzene as solvent is essential when the reagent is to be used to detect aromatic amines [1]. In the case of steroids, penicillins, diuretics and alkaloids the reaction should be accelerated and intensified by spraying afterwards with dimethylsulfoxide (DMSO) or dimethylformamide (DMF), indeed this step makes it possible to detect some substances when this would not otherwise be possible [5,9-11] this latter treatment can, like heating, cause color changes [5,9]. Penicillins and diuretics only exhibit weak reactions if not treated afterwards with DMF [10, 11]. Steroids alone also yield colored derivatives with DMSO [9]. Tlreatment afterwards with diluted sulfuric acid (c = 2 mol/L) also leads to an improvement in detection sensitivity in the case of a range of alkaloids. In the case of pyrrolizidine alkaloids it is possible to use o-chloranil as an alternative detection reagent however, in this case it is recommended that the plate be treated afterwards with a solution of 2 g 4-(dimethyl-amino)-benzaldehyde and 2 ml boron trifluoride etherate in 100 ml anhydrous ethanol because otherwise the colors initially produced with o-chloranil rapidly fade [12]. 

It is much more likely that initiation involves transfer of a proton, or possibly some other cation, to the monomer. Thus, the mechanism proposed by Evans and Polanyi and others to account for the polymerization of isobutylene in the presence of boron trifluoride monohydrate is represented as follows ... 

A recent total synthesis of tubulysin U and V makes use of a one-pot, three-component reaction to form 2-acyloxymethylthiazoles <06AG(E)7235>. Treatment of isonitrile 25, Boc-protected Z-homovaline aldehyde 26, and thioacetic acid with boron trifluoride etherate gives a 3 1 mixture of two diastereomers 30. The reaction pathway involves transacylation of the initial adduct 27 to give thioamide 28. This amide is in equilibrium with its mercaptoimine tautomer 29, which undergoes intramolecular Michael addition followed by elimination of dimethylamine to afford thiazole 30. The major diastereomer serves as an intermediate in the synthesis of tubulysin U and V. 

Cages derived from oxime precursors. In the initial synthesis of a cage (Boston Rose, 1968), tris(dimethylglyoximato)cobalt(m) [where dimethylglyoximato (dmgH) = (147)] was reacted with boron trifluoride... 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

It is worth recalling that the phenomenon of co-catalysis (now more consistently renamed co-initiation ) was found first with boron trifluoride and with the tetrahalides of titanium and tin. It is well known that it can be interpreted by the reaction scheme (1)... 

Upon initial addition of boron trifluoride etherate, a slightly endothermic reaction takes place. 

The first species produced in cationic polymerizations are carbocations, and these were unknown as such prior to World War II. It is now known that pure Lewis acids, such as boron trifluoride and aluminum chloride, are not effective as initiators. A trace of a proton-containing Lewis base, such as water, is also required. The Lewis base coordinates with the electrophilic Lewis acid, and the proton is the actual initiator. Since cations cannot exist alone, they are accompanied by a counterion, also called a gegenion. 

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