Reaction Conditions and Scope

In order to initiate the polymerization reaction, catalyst pretreatment is necessary. Various methods have been suggested, e.g. warming the solution to 100-110° C in the presence of acetylenes, or agitating the reaction mixture by stirring (2, 67). The main reaction during the catalyst development is the evolution of carbon monoxide. Evidently a labile nickel-phosphine-acetylene complex is formed. With this pretreated catalyst the trimerization 

Less reactive 1-alkynes yield linear oligomers more readily than benzene derivatives. The chain lengths of the products varied from two to seven. Usually mixtures of several isomers are obtained, e.g., 

Co-trimerization of the unreactive disubstituted acetylenes with acetylene proved possible in some cases. For instance, acetylene with 2-butyne produced benzene, o-xylene (72), tetramethylbenzene (67), and styrene (72, 67). Acetylene and divinylacetylene yielded mainly o-divinylbenzene (13, 74). The co-trimerization of acetylene with vinylacetylene yielded styrene (75). This reaction probably also accounts for formation of styrene during cyclotrimerization of acetylene, because small amounts of vinyl-acetylene are usually present or are formed from acetylene under the reaction conditions. 

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Sinha s group has reported a Heck-Decarboxylation-Heck (HDH) strategy of 4-halophenols with acrylic acid, leading to hydroxylated stUbenoid compounds with COj as the only by-product [61]. The reaction conditions and scope of this procedure are given in Figure 1.13. 

Ritter et ah, reported on the electrophilic fluorination of atyl stannanes mediated hy silver(i). The reaction was demonstrated to he general with respect to substrate scope and practical because it is performed using commercially available reagents. However, the major drawback for pharmaceutical applications is the use of toxic stannanes and the difficulties often encountered in removing tin residues after the reaction. The reaction conditions and scope are shown in Scheme 15.63. 

The authors determined the optimal reaction conditions and illustrated the scope of the method with 32 different starting compounds including alkenyl-, alkynyl-conjugated and 2,2-disubstituted 1,1-dibromo-l-alkenes. 

The introduction of these alternative approaches has permitted the direct trans-ferral and smooth integration of many versatile solution phase reagents from mainstream chemistry catalogues straight into practical protocols for use in com-hinatorial parallel hbrary generation thus broadening the amenable chemistry base. As with aU procedures it is the flexibihty and synthetic scope provided by these simple operations that has enhanced their utility as alternative reaction conditions and purification strategies. 

Mild reaction conditions and excellent selectivity provide a large scope of potential acylating agents that include a variety of alkyl and aryl methyl esters [133,136]. As a further advantage over traditional methods, acid sensitive esters readily undergo transesterification in quantitative yield (Table 21, entry 2). 

The asymmetric Mannich addition of carbon nucleophiles to imines catalyzed by the cyclohexane-diamine catalysts has developed significantly in the past decade. List and co-workers reported the asymmetric acyl-cyanantion of imines catalyzed by a cyclohexane-diamine catalyst [103], Using a derivative of Jacobsen s chiral urea catalyst, the authors optimized reaction conditions and obtained chiral iV-acyl-aminonitriles in high yield and enantioselectivities (Scheme 51). The scope of the reaction was explored with both aliphatic and aromatic imines, providing good to high selectivities for a variety of substrates. 

Meyers and Shimano further expanded the scope of this methodology to include lithium amides as the nucleophile. The authors meticulously optimized the reaction conditions and determined the scope of the amide addition. Selected examples are listed in Table 8.32 (Scheme 8.163). The best results were obtained when THF was used as the solvent together with a stoichiometric amount of HMPA, relative to the lithium amide. The reaction was quite sensitive to the steric demand of the amide. Thus, lithium diethylamide give no product whereas lithium methyl n-pentylamide and lithium piperidide gave efficient reaction. Primary amides also failed to react. 

Curtin-Hammett principle, 23 industrial applications, 8, 26 mechanism, 21 phosphine ligands, 7, 18 reaction conditions, 18 scope and limitations, 27 Wilkinson complex, 17 Rhodium-catalyzed olefin isomerization ab initio calculations, 110... 

The electronic nature of a nitrogen centered radical, dictated by reaction conditions and/or the radical precursor employed, is crucial to the mode of reaction, to the ability to undergo efficient intramolecular cyclizations or intermolecular additions, and to the products isolated from the radical reaction. The types of radicals discussed in this review include neutral aminyl radicals, protonated aminyl radicals (aminium cation radicals), metal complexed aminyl radicals, and amidyl radicals. Sulfonamidyl and urethanyl radicals are known (71S1 78T3241), but they are not within the scope of this chapter. 

Aryl-bound functional groups which are tolerated in Pd-mediated arylations include ortko-alkynyl [63], ortho-vinyl [64], ortho-nitro[65], and ortho-formyl groups [66]. Some examples of Pd-mediated cross-coupling reactions are depicted in Schemes 8.4 and 8.5, to illustrate the required conditions and scope of these reactions. 

The formal [2 + 2+1] cycloaddition between an alkyne, an alkene and carbon monoxide has become commonly known as the Pauson-Khand (PK) reaction and has undergone extensive investigation since its initial discovery.4 7 Recent improvements in the reaction conditions and an increase in substrate scope has led to the reaction becoming an important method for the preparation of cyclopentenones. 

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