Synthesis of Trans-Stilbene
The purpose of this experiment was to perform a wittig reaction, the horner-emmons wittig specifically, reacting an aldehyde with an ylide to make an alkene. This particular variation of the wittig reaction has several advantages: It gives only the trans product; it uses a much milder base that is easier to handle; and it gives a water soluble byproduct which is easy to separate from the product. The reason that these advantages occur is a change in the structure of the ylide. Instead of a tripheylphosphine ylide, we use a diethylphosphonate ylide.
The protons are much more acidic and its byproduct is negatively charged. The reason why we chose to create trans-stilbene is become of its many practical applications. Stilbene exists as two possible isomers. The first is trans-1,2-diphenylethylene, called trans-stilbene. The second is cis-1,2-diphenylethylene, called cis-stilbene which is sterically hindered and less stable because the steric interactions force the aromatic rings out-of-plane and prevent conjugation. Z-stilbene has a melting point of 5°C to 6°C, while (E)-stilbene melts around 125°C, illustrating the two compounds are quite different.
These two isomers can interconvert, however, under the influence of light. There are many ways the trans compound can be utilized. Trans-stilbene is related to the treatment of individuals afflicted with cancer or a precancerous condition, inflammatory disease or condition, and/or stroke or other ischemic disease or condition. The name stilbene was derived from the Greek word stilbos, which means shining, which is interesting considering that it is used to manufacture dyes and optical brighteners.
It is also used as a gain medium in dye lasers. As a compound, stilbene is a diarylethene, or a conjugated alkene. This is a hydrocarbon consisting of an trans ethene double bond substituted with an phenyl group on both carbon atoms of the double bond. Please see outline of reaction and drawing of trans-stilbene below: When the trans (E) geometry is desired, as in this week’s experiment, a variation known as the “phosphonate” approach is used, the horner-wittig reaction.
This modification is similar to a standard Wittig reaction in that the first step, which has already been done for you, is the reaction of a trialkyl phosphite with a suitable alkyl halide as shown below in two steps: In the first step the trialkyl phosphate acts as a nucleophile and, in a typical Sn2 reaction, forms a phosphonium salt. The salt is unstable and a halide ion X displaces R in the Sn2 manner to form a dialkylphosphonate. It is the phosphonate that, in the presence of base, is converted to a Wittig-like reagent.
Normally the Wittig reagent is an ylid and neutral, but the modified Wittig is analogous to the carbanion of an aldol intermediate. Due to its resonance forms, the phosphonate anion is able to attack the carbonyl much like acarbanion in an aldol reaction to give an oxyanion species. This is where the analogy with the aldol reaction fails. The oxyanion undergoes a reaction analogous to nucleophilic substitution at an unsaturated center to form the olefin, normally as the E isomer, and a water soluble phosphonate anion.
In this particular experiment, diethyl benzylphosphonate is used with benzaldehyde as the carbonyl component. Since phase transfer conditions are used, we can use a weaker base, the hydroxide ion. The reactivity o the anion formed is very high, resulting in excellent yields of trans-stilbene. The trans form of Stilbene is more favored than the sterically hindered cis form. Although synthesis of the trans form of Stilbene takes longer, it is more stable than the cis form. The phase transfer catalyst for this experiment is a commercial product, Aliquat 336, tricaprylmethylammonium chloride.
This methodology is utilized because of its many industrial applications. Some of these include: Elimination of dangerous, inconvenient, and expensive reactants; high reactivity of the active species, high yield and purity of products, and simplicity of the procedure. The phase transfer catalyst facilitates the migration of a reactant in a two-part system from one phase to the other where the main reaction is taking place. During this reaction the PTC brought the OH anion (from KOH in aqueous layer) into contact with the two organic wittig reagents in the hexane layer.
The phase transfer catalyst works by holding the ion within a hydrophobic exterior. The PTC for anion reactants are usually quaternary ammonium salts and the catalysts for cations are commonly known ethers. Removal of the PTC was carried out by recrystallization and washings of cold water and cold ethanol. Both washings were needed to take care of both kinds of contaminants.
When the crude stilbene is ready to be recrystallized from EtOH, when the cooling begins, it is necessary to carry out the procedure slowly because if his is done too quickly impurities will be trapped within the crystals. This error will become apparent when one takes the melting point of the product, which is a good method to test the purity of the product. The melting point of trans-stilbene is 122.