Boron Trifluoride Etherate addition reactions

Symmetrical 1,5- diketones may be prepared by addition of a THF solution of a ketone lacking a1 - hydrogens to a solution of potassium in DMF - THF the central methylene appears to derive from the formal carbon of dimethylformamide [equation (42)].143 A more general preparation of 1,5- dicarbonyl compounds uses the boron trifluoride etherate catalysed reaction of silyl enol ethers with 3-methoxyallyl alcohols in nitromethane [equation (43)]. The... 

Polymer-type antioxidants have been prepared by Eriedel-Crafts reaction of -cresol andp- and/or y -chloromethylstyrene in the presence of boron trifluoride-etherate (198). The oligomeric product resulting from the alkylation of phenyl-a-naphthylamine using C12—15 propylene oligomer in the presence of AlCl or activated white clays is used as an antioxidant additive for lubricating oils (199). 

To a suspension of 73.9 g of 1 -methyl-5-nitro-3-phenylindole-2-carbonitrile in 1.5 liters of dry tetrahydrofuran Is added dropwise a solution of 126 g of boron trifluoride etherate in 220 ml of dry tetrahydrofuran with stirring for 2 hours. After addition, stirring is continued for an additional 3 hours. To the reaction mixture Is added dropwise 370 ml of water and then 370 ml of concentrated hydrochloric acid with stirring under ice-cooling. 

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. 

The behavior of members of the bicyclo[2.2.1]heptene family is also different from that of other common 1,2-disubstituted alkenes.230 The parent bicy-clo[2.2.1]heptene gives bicyclo[2.2.1]heptane in only 3.5% yield when it is treated with Et3SiH/TFA. The major product is reported to be a 2-bicyclo[2.2.1]heptyl trifluoroacetate of unspecified configuration (Eq. 70).230 The carbocation intermediate is presumably the 2-norbornyl cation. Addition of small amounts of boron trifluoride etherate to the reaction mixture causes the yield of hydrocarbon product to rise to 22% after a reaction time of 24 hours at room temperature. Further... 

They offer the advantage that reductions can be effected under conditions that permit the conversion of substrates that may be adversely sensitive to the presence of strong Brpnsted acids. For example, in the presence of a 10% excess of triethylsilane, addition of one-half equivalent of boron trifluoride etherate to octanal results, within one hour, in the formation of a 66% yield of dioctyl ether after a basic hydrolytic workup. Benzaldehyde provides a 75% yield of dibenzyl ether under the same reaction conditions. The remainder of the mass is found as the respective alcohol.70 Zinc chloride is also capable of catalyzing this reaction. With its use, simple alkyl aldehydes are converted into the symmetrical ethers in about 50% yields.330... 

The reactivity of acylzirconocene chlorides towards carbon electrophiles is very low, and no reaction takes place with aldehydes at ambient temperature. In the reaction described in Scheme 5.12, addition of a silver salt gave the expected product, albeit in low yield (22—34%). The yield was improved to 79% by the use of a stoichiometric amount of boron trifluoride etherate (BF3OEt2) (1 equivalent with respect to the acylzirconocene chloride) at 0 °C. Other Lewis acids, such as chlorotitanium derivatives, zinc chloride, aluminum trichloride, etc., are less efficient. Neither ketones nor acid chlorides react with acylzirconocene chlorides. In Table 5.1, BF3 OEt2-mediated reactions of acylzirconocene chlorides with aldehydes in CH2C12 are listed. 

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

Selenosulfonylation of olefins in the presence of boron trifluoride etherate produces chiefly or exclusively M products arising from a stereospecific anti addition, from which vinyl sulfones can be obtained by stereospecific oxidation-elimination with m-chloroper-benzoic acid134. When the reaction is carried out on conjugated dienes, with the exception of isoprene, M 1,2-addition products are generally formed selectively from which, through the above-reported oxidation-elimination procedure, 2-(phenylsulfonyl)-l,3-dienes may be prepared (equation 123)135. Interestingly, the selenosulfonylation of butadiene gives quantitatively the 1,4-adduct at room temperature, but selectively 1,2-adducts at 0°C. Furthermore, while the addition to cyclic 1,3-dienes, such as cyclohexadiene and cycloheptadiene, is completely anti stereospecific, the addition to 2,4-hexadienes is nonstereospecific and affords mixtures of erythro and threo isomers. For both (E,E)- and ( ,Z)-2,4-hexadienes, the threo isomer prevails if the reaction is carried out at room temperature. 

Cyclic acetals are useful and common protecting groups for aldehydes and ketones, especially during the course of a total synthesis [8]. The successful synthesis of acetals frequently relies on the removal of water, a by-product of the reaction between the carbonyl compound and the corresponding diol. A Dean-Stark trap is often used for the removal of water as an azeotrope with benzene, but this method is not suitable for small-scale reactions. In addition, the highly carcinogenic nature of benzene makes it an undesirable solvent. Many of the reported catalysts for acetal synthesis such as p-toluenesulfonic acid and boron trifluoride etherate are toxic and corrosive. 

To a dry three-neck 250-mL round-bottom flask equipped with a 100-mL addition funnel and nitrogen inlet is added boron trifluoride etherate (8.14 mL, 66.2 mmol) (Scheme 12). The flask is then chilled to -15°C in an ice-acetone bath. The aryl amine (4.5 g, 16.54 mmol) is dissolved in THF and added slowly over 20 min. tert-Butyl nitrite (6.9 mL, 57.9 mmol) in THF is then added to the mixture over 30 min. The reaction is stirred another 10 min at -15°C and then warmed to 5°C over 20 min. Hexane is added (300 mL) to the mixture upon which a solid precipitates. The solid is removed by filtration and washed with cold ether and dried in vacuo to give... 

The reaction of 3-(3,4-dimethoxyphenyl)propanoic acid with thallium(III) trifluoroace-tate in the presence of boron trifluoride etherate leads to a mixture of the dihydrocoumarin (574) and the spirolactone (572) (78JOC3632). It is suggested that these products arise through an initial one-electron oxidation to the radical cation, the fate of which may vary. Thus, intramolecular reaction with the carboxyl group gives the radical (571) and eventually the spirolactone. Alternatively, capture of the radical ion by solvent and further oxidation affords the radical (573), whereupon an intramolecular Michael addition to the carboxyl group and aromatization lead to the dihydrocoumarin (Scheme 218) (81JA6856). 

Mercury salts, such as mercury(II) acetate,521-525 mercury(II) oxide,524,526-528 metcury(II) trifluoroace-tate,529,530 mercury(II) sulfate524,531 and mercury(II) phosphate531 catalyze the addition of carboxylic acids to alkynes. Acetic anhydride in the presence of boron trifluoride etherate can also be effectively used in this reaction (equation 292).521,522 Alkynoic acids undergo mercury-catalyzed cyclization to lactones (equation 293).523,532,533... 

Upon addition of boron trifluoride etherate, acetoxylactam 8 eliminates an acetoxy group to produce A-acyliminium ion 9. The indo-lizidinone 10 is formed diastereomerically pure in an iminium-ion-ini bated cyclization reaction of the Overman type ending in a vinyl-silane.14... 

Similar results have been obtained in the reactions of 1 and sodium salts of p-cresol and - and /J-naphthols. Under SN2 conditions (DMF or DMSO solvent), the alkylation of sodium cresolate occurs exclusively at the oxygen atom. The addition of a protic solvent causes C-alkylation, though the yields of C-alkylated products are low. Thus in acetone-water or dioxane-water, the yield of C-alkylated products 251 and 252 increases only up to 2%. C-Alkylation has also been observed in the reactions catalyzed by trifluoroacetic acid or boron trifluoride etherate at room temperature. The observed C-alkylation in protic media may be a reflection of a mechanism that involves a protonated epoxide or a more polarized transition state than in an SN2 pathway. 

This borane reagent may be prepared as follows.6 In a dry 500-ml flask equipped with a condenser, a thermometer and a pressure-equalising dropping funnel are placed 80 ml of diglyme, 23.1 g of 2-methylbut-2-ene (0.33 mol) in 20ml of diglyme and 4.7 g of sodium borohydride (0.125 mol). The flask is immersed in an ice bath and 23.5 g of boron trifluoride-etherate (0.165 mol) is added dropwise over a period of 30 minutes. The semi-solid reaction mixture containing 0.165 mol of disiamylborane is permitted to remain an additional 15 hours at 0-5 °C, and then used for hydroboration. Examples of the use of disiamylborane are described in Sections 5.4.3 p. 542 and 5.7.1, p. 588. 

Borane, as a solution in tetrahydrofuran or generated in situ by the reaction of a metal hydride with boron trifluoride etherate, adds readily to alkenes to yield trialkylboranes. With a terminal alkene the reaction is highly (though not completely) regioselective and gives a primary trialkylborane, since the mode of addition results from the electrophilic character of the boron atom. 

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