Avoiding Toxic Catalysts Toxic Acids and Solvents

These examples include using a dye and a light in green oxidations and an alternate to the Friedel-Crafts reaction.

The use of nontoxic dyes as catalysts in oxidation reactions that were previously carried out with toxic compounds of metals such as cadmium, lead, mercury, nickel, and chromium are being explored by Epling. His strategy uses a dye to absorb visible light and then transfer an electron efficiently and selectively to cause a reaction. Some of these reactions are shown in Figure 3.6.7.

In this figure, with light as a reagent and dye as a catalyst, deprotection of organic functional groups proceeds under neutral conditions without the use of heavy metals or chemical oxidants. The reaction at the top of the figure shows deprotection of dithianes. Shown in the middle, the benzyl ether protecting group is often used to protect an alcohol during organic synthesis. The usual ways to remove this blocking group—catalytic hydrogenation or al-kalimetal reduction—involve conditions that can result in additional, unwanted reductions in the alcohol molecule. Using visible light and a dye catalyst, Epling has achieved excellent yields of alcohol. The bottom reaction shows 1,3-oxathianes, used in stereo-controlled synthesis routes, which often are deprotected by oxidative methods that involve mercuric chloride in acetic acid, mercuric chloride with alkaline ethanolic water, or silver nitrate with N-chlorosuccinimide. In addition to the carbonyl product, Epling obtains the nonoxidized thioalcohol, allowing the chiral starting material to be recovered while avoiding the generation of toxic pollutants. Eosins (yellow), erthrosin (red), and methylene blue are examples of dyes that have worked well (Illman 1993).

Kraus is studying a photochemical alternative to the Friedel-Crafts reaction. The Friedel-Crafts reaction uses Lewis acids such as aluminum chloride and tin chloride, as well as corrosive, air-sensitive, and toxic acid chlorides and solvents such as nitrobenzene, carbon disulfide, or halogenated hydrocarbons. Kraus' photochemical alterna-


£^ch=o tive exploits the reaction between a quinone and an aldehyde and is initiated by a simple lamp. Some of the products produced with this alternative are shown in Figure 3.6.8.

This figure show that no restrictions appear to exist for functional groups meta and para to the formyl group of the benzaldehyde. Ortho groups that are compatible with the reaction conditions include alkoxy groups, alkyl groups, esters, and halogens. Many substituted benzo-quinones react with aromatic and aliphatic aldehydes according to this scheme.

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