Immobilized stoichiometric reagent

Floiv Reactions with an Immobilized Stoichiometric Reagent 1159... 

A stoichiometric forerunner of this approach was the wolf and lamb reaction, reported by Patchornik and co-workers in 1981 [17]. Two reagents that in solution react with each other quickly to give an undesired product (hence wolf and lamb ) were immobilized on solid polymeric supports (two insoluble and mutually immiscible polymers, Scheme 5.1), rendering them unreactive towards each other. This allowed the formation of the desired product through the use of a messenger reagent. 

In this stoichiometric forerunner, the use of a polymeric support demonstrated the concept of using an immobilization method to prevent reagents from reacting with each other in an undesired manner, permitting a reaction to occur that is not normally possible. By analogy, there are possibilities where various immobilization methods (in this case, we are interested in nanoencapsulation methods) are used to enable two incompatible catalysts to work concomitantly in an otherwise impossible reaction. 

Oxidation of alcohols to carbonyl compounds is an important reaction. Stoichiometric oxidants such as chromates, permanganates and MO4 (M = Ru, Os) are the commonly used reagents [19a,59,60]. However, they are going out of favour increasingly because they create heavy metal wastes . In view of this, development of environmentally friendly heterogeneous catalysts for alcohol oxidation is very important. In the use of catalytic amounts of transition metal salts or complexes as homogeneous catalysts for the oxidation of alcohols [61-64], separation of the catalyst from the reaction mixture and its subsequent recovery in active form is cumbersome. Heterogeneous catalysts for this kind of reaction are therefore necessary [65]. Clearly, encapsulation and/or immobilization of known... 

Despite the unique synthetic possibilities offered by organotin reagents, their use in the chemical industry has steadily declined in recent years as a result of perceived toxicity and environmental concerns associated with disposal of tin wastes.30-31 Consequently, appreciable efforts have been made to develop heavy metal-free alternatives based on silicon, germanium, or phosphorus chemistry. Other strategies have been applied to immobilize the tin reagents or to reduce their usage to less than stoichiometric amounts. 

High enantioselectivity and good yields have been obtained in asymmetric epoxidation of enones. Roberts modification of the Julia epoxidation using an immobilized polyleucine catalyst now represents a simple, practical method for enone epoxidation. Of the metal-based systems, the most economical and practical method is probably Enders protocol, despite the fact that it uses stoichiometric amounts of metal and hgand, as all the reagents are commercially available and cheap. It is difficult to compare the polypeptide-based catalysts with the metal based catalysts in terms of overall efficiency. 

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