Benzylidene dibromide

Rate differences observed between the same bromophenylcarbene (241) when prepared by two different routes, diazirine photolysis and the reaction of benzylidene dibromide with potassium r-butoxide, vanish when a crown ether is added to the basic solution in the latter experiment. In this case the complexing potassium bromide is taken over by the crown ether, and selectivity towards alkenes reaches the values of the photolytic runs (74JA5632). 

The formation of quaternary salts from benzyl halides and related compounds occurs readily and has been known for many years. More recently, Krohnke and co-workers, who have studied the reactions of many heterocyclic quaternary salts, reported the formation of 5 from pyridine and benzylidene dibromide on heating the reactants together for 1 hr at 100°. The salt is sufficiently stable to be recrystallized from methanol containing a trace of hydrogen bromide. Isoquinoline gives a similar salt. 

In reaction with benzaldehyde or its dimethyl acetal, or benzylidene dibromide, sucrose undergoes conversion into its 4,6-acetal in moderate yield (28-35%).160,161 The transacetalation reaction of dialkyl acetals prepared from unsaturated or aromatic aldehydes can take place under very mild acidic conditions. For example, sucrose-derived monomers or surfactants (variously substituted 4,6-(9-methylidenesucroses) have been prepared146,162 and 4,6-monoacetals incorporating masked aromas or fragrances derived from p-citral or a- or p-ionone were prepared directly from sucrose in high yields.163,164 These latter molecules, whose unsaturated carbonyl system is very susceptible to oxidation, are thus temporarily protected and are readily released under very mild conditions (Scheme 13). 

In 1908, while working at University of Heidelberg, Auwers and Muller described the transformation of 4-methyl-2-cumaranone (3) to flavanol 6. Thus aldol condensation of 3 with benzaldehyde gave benzylidene derivative 4, which was brominated to give dibromide 5. Subsequent treatment of 5 with alcoholic KOH then furnished 2-methylflavonol 6. In the following years, Auwers published more extensively on the scope and limitations of this reaction. ... 

Phase transfer catalysis has been used in the methylenation of carbohydrates methyl 4,6-0-benzylidene-glucosides and galactosides gave the /raw -fused 2,3-acetals in about 60% yield with methylene dibromide and alkali. Methyl jS-D-ribofuranoside gave the fully substituted dimeric compound (1), and its 5-trityl derivative afforded the monomeric 2,3-acetal as anticipated, ... 

Bromine in dry chloroform added dropwise with stirring at room temp, during 5 hrs. to a soln. of benzylidene-p-nitraniline in the same solvent, and kept 6 hrs. at 4° dibromide (Y 71%) treated 6 hrs. at 35° with thiourea in 90%-ethanol benzylidene-p-nitraniline. Y 63%. Also bromine elimination by other nucleophiles such as iodide, thiosulfate, thiophenolate, and thiocyanate ions, or thio-cresol s. Nasir-ud-Din et al., Ghem. Ind. 1967, 1875. 

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