Stevens Experimental Procedure

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

Experimental Procedure 4.2.5. Ammonium Ylide Formation and Stevens Rearrangement Diethyl 2-Benzyl-2- [(ethoxycarbonyl)mefiryl](methyI)amino malo-nate [1241]... 

Despite its mathematical simplicity, the foregoing procedure may present some experimental difficulties, which normally result from (1) small values of some pre-exponential coefficients in the decays of A and/or B, (2) too close decay times (differing by less than a factor of two) that mix, or (3) insufficient time resolution. In most cases, these difficulties can be overcome by changing the experimental conditions (temperature, solvent viscosity and/or polarity, and concentration among others, e.g. pressure) and/or by coupling the results from time-resolved fluorescence with those obtained from steady-state experiments (Stern-Volmer [1] and/or Stevens-Ban [56] plots). 

Solvent extraction systems often display nonlinear equilibrium behavior. Pratt (1983) and Pratt and Stevens (1991) have illustrated methods in which results of sufficient accuracy are obtained by representing the equilibrium curve by means of two or three straight segments for both continuous as well as multicompartment columns. Additional simple and shortcut procedures have been described by Von Stockar and Lu (1991). The extent of backmlxing In the continuous phase in Karr reciprocating-plate extraction columns (Figure 8.1.35(d)), widely used In the petroleum, pharmaceutical and hydro-metallurgical Industries, has been studied experimentally by Stella et al. (2006). 

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