Silica fluoroboric acid

Immobilization of fluoroboric acid (HBF4) on silica was also successfully performed,191 192 and the resulting material has shown good characteristics in catalytic transformations. 

Fluoroboric acid supported on silica (HBF4-silica) has recently been found to be a highly efficient catalyst in the protection of various functional groups. Structurally diverse alcohols, phenols, thiophenols, and anilines can be acylated under solvent-free conditions at room temperature.669 Even acid-sensitive tertiary alcohols (1-alkylcyclo-hexanols) and sterically hindered compounds, such as endo-borneol, give the acylated products in high yields. A triflic acid-silica catalyst also shows high activity in the (9-acetylation with Ac20 of alcohols and phenols.359... 

Fluoroboric acid supported on silica is also highly efficient for the synthesis of 1,5-benzodiazepines under solvent-free conditions723 [Eq. (5.272)]. 

Alcohols can be O-methylated with diazomethane in the presence of a protic acid such as fluoroboric acid 210 Meerwein and Hinz211 showed that zinc chloride was an effective catalyst in 1930 and trifluoroborane later found favour 212 An alcohol adsorbed onto neutral silica gel undergoes O-methylation213 and the method can be used on a moderate scale [Scheme 4.117]214 though the perils of preparing diazomethane on a moderate scale must be the cause of some trepidation When tin(Il) chloride is used as the catalyst selective mono-0-methyla-tion of a l 2-diol can be achieved [Scheme 4 118] 215... 

In recent years, the use of heterogeneous catalysis has received useful applications in various organic transformations due to several advantages over conventional homogeneous catalysis. Several solid acid catalysts [108] such as Amberlyst [109], solid-supported fluoroboric acid [110], polyaniline sulfate [111], polystyrene-supported sulfonic acids [112], sulfated zirconia [113], and silica [114] in conjunction with other greener techniques such as MW, US, environmentally benign solvents, solvent-free conditions, and so on have been screened, evaluated, and compared with respect to yields, reaction time, reaction temperature, ease of purification, reusability, toxicity, and other hazards for sustained applications. Many of these efforts have found applications in the medium ring-sized heterocycles such as oxepines, azepines, diazepines, oxazepines, thiazepines, and so on. 

B.R Bandgar, A.V. Patil, O.S. Chavan, Silica supported fluoroboric acid as a novel, efficient and reusable catalyst for the s)mthesis of 1,5-benzodiazepines rmder solvent-free conditions, J. Mol. Catal. A Chem. 256 (2006) 99-105. 

Amides can also be methylated with diazomethane in the presence of silica gel however, the reaction requires a large excess of diazomethane (25-60 equiv, eq 16). The reaction primarily provides 0-methylated material however, in one case a mixture of O-and A-methylation was reported. Thioamides are also effectively methylated with this procedure to provide 5-methylated compounds. Finally, amines have been methylated with diazomethane in the presence of BF3 etherate, fluoroboric acid, or copperfi) salts however, the yields are low to moderate, and the method is not widely used. 

To a dichloromethane solution (5 ml) containing triphenylbismuth difluoride (239 mg, 0.50 mmol) and 4-methylphenylboronic acid (68 mg, 0.50 mmol) was added boron trifluoride diethyl etherate (65 p.1, 0.50 mmol) at 0°C, and the resulting mixture was stirred at room temperature for 2 h. An aqueous solution (20 M) of sodium fluoroborate (500 mg, 4.55 mmol) was then added and the two-phase mixture was vigorously stirred for 30 min. The water phase was extracted with dichloromethane (5 ml X 2), and the combined extracts were dried over MgS04, and passed through a short silica gel column. Evaporation of the solvent under reduced pressure left an oily residue, which was crystallized from ether-dichloromethane (10 1) to yield (4-methylphenyl)triphenylbismuthonium tetrafluoroborate (300 mg, 97%) as a colorless solid [980M4332]. 

The smoke suppressing effect is attributed to several metal compounds, mainly oxides, hydroxides and salts, but it has also been observed with other chemicals, such as carboxylic acids, aldehydes, alcohols, fluoroborates, sulphur, and silica. Some commercial smoke suppressor additives are listed in Table 5.16. 

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