Studying Supported Reagents

Weik and Rademann have described the use of phosphoranes as polymer-bound acylation equivalents [65]. The authors chose a norstatine isostere as a synthetic target and employed classical polymer-bound triphenylphosphine in their studies (Scheme 7.54). Initial alkylation of the polymer-supported reagent was achieved with bromoacetonitrile under microwave irradiation. Simple treatment with triethyl-amine transformed the polymer-bound phosphonium salt into the corresponding stable phosphorane, which could be efficiently coupled with various protected amino acids. In this acylation step, the exclusion of water was crucial. 

Diffuse refleetance infrared spectroscopy (DRIFTS) provides useful information about the degree of ineorporation and nature of the immobilized eomplex in supported reagents. We have studied the supports as well as the supported reagents containing the immobilized eomplexes by this form of infrared spectroscopy where samples in the solid state are examined to determine the nature species under examination. We discuss here the infrared speetral evidenee of two supported materials - Co(III)-CMS3, prepared by the in-situ method and Co(III)-CMS4 whieh has been prepared by the ligand substitution route. 

The interlaboratory studies supported the beliet that, if a laboratory performs well with the methods using distilled water, it should be able to obtain good results with surface waters and industrial waste waters. On the basis of these studies, the multilaboratory regression equations for accuracy and single-analyst overall precision for distilled or reagent water have been incorporated into the quality assurance and quality control provisions of Methods 601, 602, 604-613, 624, and 625. These provisions will be discussed later. 

The mechanism of the enantioselective 1,4-addition of Grignard reagents to a,j3-unsaturated carbonyl compounds (Scheme 5 R1 = alkyl R2 = alkyl, OR3), promoted by copper complexes of chiral ferrocenyl diphosphines (180), has been explored using kinetic, spectroscopic, and electrochemical analysis. The roles of the solvent, copper halide, and the Grignard reagent have been thoroughly examined. Kinetic studies support a reductive elimination as the rate-limiting step, in which the chiral catalyst,... 

The stereoselective synthesis of 1,4-disubstituted-l,3-dienes proceeds by head-to-head oxidative coupling of two alkynes with formation of an isolable metallacyclic biscarbene ruthenium complex [23], as shown in Scheme 6. Several key experiments involving labeled reagents and stoichiometric reactions and theoretical studies support the formation of a mixed Fischer-Schrock-type biscarbene complex which undergoes protonation at one carbene carbon atom whereas the other becomes accessible to nucleophilic addition of the carboxylate anion (Scheme 6) [23]. 

The polymer-supported organotin dihydride 70 was shown to be an efficient reducing agent for aldehydes and ketones, but substantial loss of activity was observed after regeneration. More recently, various polymer-supported butyltin reagents (71, 72) were studied as reagents for the acetylation of sucrose158. 

Similarly, the Grignard reagent shown as an intermediate reacts with a variety of lithioanilines to form styrene derivatives (Equation 2). Deuteration studies support an MAI-type mechanism and by inference a cyclic Grignard intermediate <2005T10262>. 

A nonpolar solvent favors conformation A, whereas conformation B is favored by more polar solvents (e.g. dimethylformamide, hexamethylphosphoric triamide) because the cation is more solvated (cf. Table 9, entries 1 and 2). However, this solvent effect is absent when BujP Cu" is used as counterion. Conformation A is more favored by relatively small counterions, such as the lithium and sodium ion, as compared to the larger potassium ion, due to the higher degree of association of the former. Steric strain between ASG and ASG is minimized in conformation B. Conformations A and B lead to trans- and c -substituted cyclopropanes, respectively. A study of cyclopropane esters, -in which the stereoselectivity of the reaction of polymer-supported reagents was compared with molecules of low-molecular weight, made clear that the steric and polar microenvironment of the polymer-supported reaction is not different enough in bulk to influence the selectivity substantially. Nevertheless, a specific influence of the solid phase can be observed at low temperatures. 

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