Tetrabutylammonium, ligand structure

Ligand 92 was readily prepared by reaction of (+)-pinocarvone with 1-phenacylpyridinium iodide. The authors similarly prepared corresponding 5,6-dihydro-1,10-phenanthrolines derived from (+)-pinocarvone and a tetrahydroquinolone (structure 93, [127]) and obtained up to 81% in the palladium-catalyzed test reaction. Chelucci et al. [ 128] reported the synthesis of chiral Ci-symmetric 1,10-phenanthrolines incorporated in asteroid backbone. Structure 94 derived from 5o -cholestan-4-one in Scheme 51, allowed very high yield and up to 96% ee using BSA and tetrabutylammonium fluoride to generate the malonate anion. 

In some systems it is necessary to add a large amount of salts to obtain polymers with low polydispersities. This happens when salts participate in ligand/anion exchange (special salt effect) and when they enhance ionization of covalent compounds through the increase of ionic strength. The special salt effect may either reduce or enhance ionization. Strong rate increases observed in the polymerization of isobutyl vinyl ether initiated by an alkyl iodide in the presence of tetrabutylammonium perchlorate or triflate can be explained by the special salt effect [109]. The reduction in polymerization rate of cyclohexyl vinyl ether initiated by its HI adduct in the presence of ammonium bromide and chloride can be also ascribed to the special salt effect [33]. The breadth of MWD depends on the relative rate of conversion of ion pairs to covalent species and is affected by the structure of the counterions. 

The catalytic performance of NHC-Pd complexes in this reaction is however usually less satisfactory than that of catalytic systems based on other ligands, such as, for example, phosphane hgands, and poly-NHC Pd complexes make no exception to this general trend. Consequently, although reports on the catalysis of the Heck reactions by these complexes have been numerous, they are often hmited to the reaction of aryl iodide or electron-poor, activated aryl bromide substrates, whereas electron-neutral or electron-rich aryl bromides require high temperatures for satisfactory yields. Reactivity of aryl chlorides is confined to electron-poor substrates and is observed only under very drastic conditions (T> 150 °C with tetra-alkylammonium salts as promoters). " An exception is represented by the work of Huynh and Guo on dipalladium(II) complexes with the Janus-type ditz ligand of structure 68." In this case, Heck reactions of activated aryl chlorides were possible already at 120 °C without addition of promoters the authors ascribe this result to the cooperative effect of the two metal centers. Poly-NHC complexes of type 69 have been found to catalyze also the less common diarylation (i.e., double Heck reaction) of ethyl acrylate with aryl bromides, albeit at 120 °C and with tetrabutylammonium bromide as additive. " Simple Pd species such as Pd(OAc)2 and PdCl2 were found to be quite ineffective under these conditions. 

Recently, the preparation and crystal structure of [(C6Hs)3PNP(C6Hs)3]-[TcOCU], a five-coordinate technetium(V) complex have been achieved. The tetrabutylammonium salt, (Bu4N)[TcOCl]4, can be easily prepared and isolated from the reduction of pertechnetate(VII) ion in 12IV hydrochloric acid in nearly quantitative yield. It is soluble in a variety of polar organic solvents and reacts easily with a variety of ligands to form 0x0 technetium(V) complexes. ... 

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