Cationic routes, synthesis

In this paper we will discuss the synthesis of a, urhydroxybutyl terminated polydimethylsiloxane oligomers by cationic routes and a,u-aminopropyl terminated poly(dimethy1-diphenyl)siloxane oligomers by anionic methods respectively. Detailed procedures for the synthesis of aminopropyl, carhoxypropyl and glycidoxypropyl terminated polydimethylsiloxane oligomers have already been described elsewhere(1 1). ... 

Synthesis of organized mesoporous aluminas is based on the same approaches as those successfully used for the synthesis of mesoporous molecular sieves ( anionic , cationic , and neutral ) using aluminum alkoxide as the source of aluminum and in the absence of any silicon source. In contrast to the synthesis of siliceous MCM-41, the cationic route to organized mesoporous aluminas using hexadecyltrimethylammonium cations is the least applied and understood. Cabrera et al. [68] described the possibility to tailor the pore dimensions from 3.3 to 6.0 nm by modifying the ratio of surfactant, water and triethanolamine however, this synthesis route seems to be less reproducible compared with anionic and neutral routes. 

The polymerization of cyclotrisiloxane with mixed siloxane units was expected to allow for a synthesis of siloxane-siloxane copolymers with a uniform distribution of units. These copolymers are not accessible by the kinetically controlled copolymerization of two cyclic trisiloxane comonomers, which leads to a microsequential order of siloxane units and a gradient arrangement of monomer units along the chain 16J7), So far the sequencing in poly siloxane, obtained from cyclotrisiloxane having two kinds of siloxane units, have been studied only for the polymerization in the presence of anionic initiators (16,18,19). The use of cationic routes for a controlled synthesis of copolymers with a uniform distribution of units is, therefore, of interest. 

The first synthesis of a P-amyrin derivative was accomplished by a convergent route which depended on cation-olefin cyclization to form the critical central ring. The plan of synthesis was largely guided by the selection of SM goals for the A/B and E ring portions of the target. 

A more versatile synthesis of 11.1 (and the selenium analogue) involves the cyclocondensation of trisilylated amidines with sulfur dichloride or SeCl2 generated in situ (Eq. 11.3). This route can be used to prepare the prototypal systems [HCNEEN]" (E = S, Se). It is also readily extended to the synthesis of multi-dichalcogenadiazolium cations such as 1,3- or 1,4-C6H4(CNEEN)2] (H-2, E = S, Se), °... 

The synthesis, structures, and reactivity of neutral and cationic mono- and bis(guanidinato)zirconium(rV) complexes have been studied in detail. Either salt-metathesis using preformed lithium guanidinates or carbodiimide insertion of zirconium amides can be employed. Typical examples for these two main synthetic routes are illustrated in Schemes 73 and 74. Various cr-alkyl complexes and cationic species derived from these precursors have been prepared and structurally characterized. 

The (pentamethylcyclopentadienyl)zirconium amidinate unit also served as a platform for the synthesis and characterization of remarkable cationic and zwitterionic allyl zirconium complexes derived from trimethylenemethane (TMM). A direct synthetic route to the neutral precursors was found in the... 

In this progress report we have reviewed the latest developments in the large area of cationic low-coordinated species and their coordination with Lewis donors. It is clear that these species are of a broad interest, in particular for catalysis. In some cases, e. g. the methylene phosphenium cation, the donor adducts also open new routes for synthesis. Regarding the mechanism for the diverse donor-addition reactions, the structural details are only poorly understood and need a better classification. In particular the variation of the Lewis-donor has to be established. Hitherto in most cases iV-donation is studied. It includes amines or pyridines. Obviously the effect of other donors, such as phosphines, thioethers needs to be studied as well. The siliconium cation for which these effects are better known could provide an understanding for further investigations within this field. 

Hydrosilylation of dienes accompanied by cyclization is emerging as a potential route to the synthesis of functionalized carbocycles. However, the utility of cycliza-tion/hydrosilylation has been Umited because of the absence of an asymmetric protocol. One example of asymmetric cycUzation/hydrosilylation has been reported very recently using a chiral pyridine-oxazoUne ligand instead of 1,10-phenanthroline of the cationic palladium complex (53) [60]. As shown in Scheme 3-21, the pyridine-oxazoUne Ugand is more effective than the bisoxazoUne ligand in this asymmetric cyclization/hydrosilylation of a 1,6-diene. 

From Free Radicals RR R"E This last synthetic route, involving the one-electron oxidation of the free radicals RR R"E with an appropriate Lewis acid such as PhjC, is one of the best methods for the extremely fast and clean formation of the element-centered cations RR R"E+. Although this approach requires the presence of the radical species as readily available starting materials, the recent synthesis of stable silyl-substituted radicals of the type (r-Bu2MeSi)3E (E = Si, Ge, Sn) (see Section 2.2.4.1.2) made such an approach a rather attractive and easily accessible synthetic route to the stable and free (r-Bu2MeSi)3E+ cations (Scheme 2.6)... 

Complexation studies with bidentate phosphine ligands showed that stable cationic complexes of Tc(V), Tc(III), and Tc(I) are easily accessible. The influence of reaction conditions on reaction route and products is well demonstrated by the reaction of pertechnetate with the prototype 1,2-bis(dimethylphosphino)-ethane (dmpe) (Fig. 16). Careful control of reduction conditions allows the synthesis of [Tc02(dmpe)2]+, [TCl2(dmpe)2]+, and [Tc(dmpe)3]+, with the metal in the oxidation states V, III, and I [120,121]. This series illustrates the variety of oxidation states available to technetium and their successive generation by the action of a 2-electron reducing agent. 

The double diphenylethylenes (DDPE) approach was first reported by Hocker and Latterman [172] and was later developed mainly by the Quirk (anionic) [173,174] and Faust (cationic) groups [175,176]. Representative reactions for the synthesis of the A2B2 4//-star by the anionic route are shown in Scheme 91. 

Finally, Crich [31] and Danishefsky [32] demonstrated that enantioselective hexahydropyrroloindole formation could precede C3-quatemary center formation. The carbon-carbon bond formation could be accomplished through a stereoretentive cationic or radical process as demonstrated through the reverse-prenylation reaction in Danishefsky s synthesis of amouramine [32a, b] or the allylation reaction en route to Crich s synthesis of (+)-debromofhistramine [33], respectively. 

Hydroamination of olefins has received considerable attention this year as a route to functionalized piperidines and spiropiperidines, particularly in regard to the investigation of new catalysts. In the synthesis of spiro-piperidines, two new mild and more general intramolecular hydroamination protocols were developed this year. One protocol uses a cationic gold-phosphine complex (Au[P(fBu)2(o-biphenyl)]Cl) as the catalyst... 

Triafulvene 69 was synthesized by an interesting route starting with diphenyl cyclopropenium cation and lithio ethyl acetate72. Although widely used in calicene chemistry (see later) this reaction principle — like the Wittig reaction — did not find general application in methylene cyclopropene synthesis. 

Analogously the ethoxy cation 75 was found to be valuable for the synthesis of calicenes when combined with cyclopentadienyl anions93 Hexaphenylcalicene (100) was prepared by this route from tetraphenylcyclopentadienyl-Li94,76) and the highly polar dicyanocalicene 10195 from the tetramethylammonium salt of dicyano-cyclopentadiene. 

Hydrosilylation of 1,6-dienes accompanied by cyclization giving a five-membered ring system is emerging as a potential route to the synthesis of functionalized carbocycles.81,81a,81b 82 As its asymmetric version, diallylmalonates 86 were treated with trialkylsilane in the presence of a cationic palladium catalyst 88, which is coordinated with a chiral pyridine-oxazoline ligand. As the cyclization-hydrosilylation products, //ww-disubstituted cyclopentanes 87 were obtained with high diastereoselectivity (>95%), whose enantioselectivity ranged between 87% and 90% (Scheme 25).83 83a... 

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