Experimental...recipe for success
To a stirred solution of 3.30 g (4.55 mmol) of 20, 416mg Pd2(dba)3 (0.46mmol) and 427mg P(o-tolyl) (0.91 mmol) in 23ml CH3CN was added to 1.89ml (CH2CH3)3N (13.6 mmol) at room temperature and the solution was stirred for 1 hour at 85 ºC. Then the solvent was removed in vacuum filtration, and the residue was purified with flash column chromatography (20% ethyl acetate in n-hexane) to afford 21.
Vacuum filtration is used primarily to collect a desired solid, for instance, the collection of crystals in a recrystallization procedure. Vacuum filtration uses either a Buchner or a Hirsch funnel. Vacuum filtration is faster than gravity filtration, because the solvent or solution and air is forced through the filter paper by the application of reduced pressure. The reduced pressure requires that they be carried out in special equipment:
- Büchner or Hirsch funnel
- heavy-walled, side arm filtering flask
- rubber adaptor or stopper to seal the funnel to the flask when under vacuum
- vacuum source
Flash column chromatography
Column chromatography in chemistry is a method used to purify individual chemical compounds from mixtures of compounds. It is often used for preparative applications on scales from micrograms up to kilograms. The main advantage of column chromatography is the relatively low cost and disposability of the stationary phase used in the process. The latter prevents cross-contamination and stationary phase degradation due to recycling.
The classical preparative chromatography column is a glass tube with a diameter from 5 mm to 50 mm and a height of 5 cm to 1 m with a tap and some kind of a filter (a glass frit or glass wool plug – to prevent the loss of the stationary phase) at the bottom. Two methods are generally used to prepare a column; the dry method, and the wet method.
- For the dry method, the column is first filled with dry stationary phase powder, followed by the addition of mobile phase, which is flushed through the column until it is completely wet, and from this point is never allowed to run dry.
- For the wet method, a slurry is prepared of the eluent with the stationary phase powder and then carefully poured into the column. Care must be taken to avoid air bubbles. A solution of the organic material is pipetted on top of the stationary phase. This layer is usually topped with a small layer of sand or with cotton or glass wool to protect the shape of the organic layer from the velocity of newly added eluent. Eluent is slowly passed through the column to advance the organic material. Often a spherical eluent reservoir or an eluent-filled and stoppered separating funnel is put on top of the column.
The individual components are retained by the stationary phase differently and separate from each other while they are running at different speeds through the column with the eluent. At the end of the column they elute one at a time. During the entire chromatography process the eluent is collected in a series of fractions. The composition of the eluent flow can be monitored and each fraction is analyzed for dissolved compounds, e.g. by analytical chromatography, UV absorption, or fluorescence. Colored compounds (or fluorescent compounds with the aid of an UV lamp) can be seen through the glass wall as moving bands.
Many essential biological chemicals are chelates. Chelates play important roles in oxygen transport and in photosynthesis. Furthermore, many biological catalysts (enzymes) are chelates. In addition to their significance in living organisms, chelates are also economically important, both as products in themselves and as agents in the production of other chemicals.
A chelate is a chemical compound composed of a metal ion and a chelating agent. A chelating agent is a substance whose molecules can form several bonds to a single metal ion. In other words, a chelating agent is a multidentate (ligands with more than one bonded atom) ligand. Pd2(dba)3 (Tris-(benzylideneacetone)-dipalladium (0)) used as a catalyst in the reaction to give Molecule 21 is an example of a chelating agent.