Preformed traps

The results of Baxendale and Wardman, Kenney-Wallace and Jonah, and Gangwer et al. are all consistent with the idea that there must be preformed traps in the solution [16,24-26]. Further evidence consistent with this view arises from experiments of Dimi-trijevic et al., who measured the yield and the spectrum of the solvated electron as a... 

Isocyanate trapped as formed at 100° (Procedure A) or isocyanate preformed at 110° and trapped at 25° (Procedure B). 

A method resulting in improved encapsulation of aqueous phase by MLV is the so-called dehydration-rehydration procedure (Kirby and Gregoriadis, 1984 Shew and Deamer, 1985). The lipid (usually preformed liposomes) is dried (by either lyophilization or evaporation) in the presence of the aqueous solute to be entrapped, thus forming a mixed film with solute trapped between layers. Subsequent gradual rehydration with a minimum of aqueous phase leads to the formation of MLV with a high entrapment of the aqueous solutes added. 

Several examples of conjugate addition of carbanions carried out under aprotic conditions are given in Scheme 2.24. The reactions are typically quenched by addition of a proton source to neutralize the enolate. It is also possible to trap the adduct by silylation or, as we will see in Section 2.6.2, to carry out a tandem alkylation. Lithium enolates preformed by reaction with LDA in THF react with enones to give 1,4-diketones (Entries 1 and 2). Entries 3 and 4 involve addition of ester enolates to enones. The reaction in Entry 3 gives the 1,2-addition product at —78°C but isomerizes to the 1,4-product at 25° C. Esters of 1,5-dicarboxylic acids are obtained by addition of ester enolates to a,(3-unsaturated esters (Entry 5). Entries 6 to 8 show cases of... 

When triflic anhydride is added to a preformed mixture of glycosyl sulfoxide and acceptor alcohol, it seems apparent that the first formed oxacarbenium ion is directly trapped by the alcohol, without the need for the implication of glycosyl triflates [75,280,323],... 

Entrapment Within a Porous Matrix Cells are allowed to diffuse into preformed porous matrices such as bricks, cordierite, and pore glass, in which they will grow and be trapped. The main advantages of this method are that the preformed support materials are more resistant to disintegration in packed beds or stirred vessels than other support materials, and the entrapment is not usually harmful to the cells. However, it is difficult to reach a high cell concentration due to the limited pore volume usually available for entrapment within typical support materials. 

With the exception of a nickel -benzyne complex (13) [Eq. (5)],32,33 which is probably formed by trapping the free intermediate with the coordinatively unsaturated metal,34 all complexes of nickel and platinum have been prepared by reduction of preformed (2-haloaryl)halometal complexes, which in turn have been synthesized by oxidative addition of M(0) to o-diha-loarenes.2 35-37 Typical of this procedure is preparation of the bis(triethyl-phosphine)nickel-benzyne complexes 19 shown in Eq. (6). 

The most widely used alternative approach in layer construction involves trapping components in a preformed matrix by saturation techniques. In this approach, the matrix of choice is saturated vdth a solution of desired components or reagents. Commonly used preformed matrices include paper, woven fabrics, nonwoven fabrics, and a variety of porous membranes. The availability... 

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