Boron trifluoride Diels-Alder reaction catalysts

Lewis acids such as zinc chloride, boron trifluoride, aluminum chloride, and diethylaluminum chloride catalyze Diels-Alder reactions.8 The catalytic effect is the result of coordination of the Lewis acid with the dienophile. The complexed dienophile is more electrophilic and more reactive toward electron-rich dienes. The mechanism of the cycloaddition is still believed to be concerted, and high stereoselectivity is observed.9 10 Lewis acid catalysts also usually increase the regioselectivity of the reaction. 

He and his students developed C-alkylation with quaternary ammonium salts and nucleophilic displacements on such salts, including the stereochemistry. His name is immediately associated with important innovations in the use of polyphosphoric acid for inter- and intramolecular condensations, cyclizations, and functional conversions in organic chemistry. He pioneered the use of boron trifluoride as an efficient catalyst in the Fischer indole synthesis and discovered new reactions of anils, including Diels-Alder reactions. He and his students delineated the requirements for disproportionation of tertiary amines. He developed the synthesis and chemistry of arylboranic acids. One of his fundamental ideas was for the incorporation of sufficient boron into organ-specific drugs that they could then be... 

Diel -AldeT catalyst. Kelly and Montury have examined the effect of three Lewis acids on the Diels-Alder reaction of perf-hydroxylated naphthoquinones with l-methoxy-3-methyl-l,3-butadiene magnesium iodide, boron trifluoride etherate, and boron triacetate. All three increase regioselectivity. BF, etherate and B(OAc)3... 

The synthesis of a soluble nonacenetriquinone based on the well-known Diels-Alder reaction of 1,3-diarylisoben-zofurans was developed by L.L. Miller and co-workers. The preparation of the 1,3-diarylisobenzofuran commenced with the Paai-Knorr furan synthesis. The substrate was an aromatic 1,4-diketone, which was treated with excess neat boron trifluoride etherate for almost two days to afford the desired 2,5-diarylfuran in almost quantitative yield. Interestingly, this cyclization could not be achieved efficiently by using the more traditional acid catalysts such as H2SO4 or PPA. 

The Diels-Alder reaction has been shown to be subject to catalysis by a wide range of solid catalysts (see Chapter 4 for some examples). Acidic mesoporous aluminosilicates can be used to catalyse selective Diels-Alder reactions such as that between cyclopentadiene with methyl acrylate. The zinc-exchanged version of the material is particularly effective and compares well to other more established solid acids such as the ion-exchanged clay Zn2+-K10 as well as homogeneous catalysts such as boron trifluoride (Table 2.7).50... 

Boron trifluoride is a highly moisture-sensitive gas (31). It is utilized in esterification, ether formation, Friedel-Crafts alkylation and acylation, and Lewis acid-catalyzed Diels-Alder reactions. A more widely used, easy-to-handle and convenient liquid source of BF3 is boron trifluoride etherate [BF3-0(C2H5)2] (32). Its main usage as catalyst is in the direct esterification of all types of acids, rearrangements, aldol condensation, and Lewis acid-catalyzed Diels-Alder reactions. It is the most frequently used acid in epoxide ring opening and rearrangement (33). 

Similar structures can be realised in low yield from the Friedel-Crafts reaction of 1,4-dimethoxybenzene with substituted phthalic anhydrides (R = Me, Et, i-Pr), which were obtained by the appropriate Diels-Alder synthesis. The acylations were effected in the presence of boron trifluoride etherate as catalyst, followed by demethylation of the product (ref.55). 

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