Weakly coordinating counterions

The proposed mechanism of the hydrovinylation is supported by available evidences, but so far no study has established clearly all the reaction intermediates. RajanBabu has proposed a mechanism of nickel-catalyzed hydrovinylation, which seems to be one of the most efficient processes, involving a cationic nickel hydride species 144 complexed with a weakly coordinated counterion (Scheme 40). The active catalyst species can be generated through... 

In the way it became clear that at least one or two of the criteria (1) - (6) had to be fulfilled to expect any success in the quest for free silylium cations in solution, one systematically started to use solvents with weak nucleophilic character as well as weakly coordinating counterions such as 33 or 35 shown in Scheme 5. The lead in this research was taken by Lambert, but Reed made also several important contributions and, therefore, one can say that both Lambert and Reed pushed forward the issue of silylium cations in solution despite scepticism and criticism on the usefulness of such work for general chemistry. Benzene and toluene were chosen as the ideal solvents and in 1993 Lambert and Zhang reported on trialkyl substituted silyl cations in aromatic solvents at the presence of TPFPB as a counterion. [7] From measured 29Si NMR chemical shifts (80 - 110 ppm), Lambert and Zhang concluded that they had obtained R3Si+ (R = Me, Et, Pr, Me3Si) ions with reduced electrophilic interactions with solvent or anion and, therefore, with nearly free cationic structure. 

The synthesis of 61 by Lambert and Zhao [135] proofs that it is possible to generate free silylium cations in solution. The success of this work is based on fulfilling the requirements listed in section 8, namely (1) internal stabilization of the cation, (2) steric blocking of the Si+ center, (3) use of a weakly nucleophilic solvent with (4) not too small solvent molecules, and (5,6) use of bulky, weakly coordinating counterions with delocalized anionic charge. 

The lanthanides form a range of complexes with terpy, with stoichiometries of 1 1, 1 2, and 1 3, although the 1 1 compounds are the most widely investigated. A number of workers have reported difficulties in the preparation of the 1 2 complexes, particularly with the lighter lanthanides, and it seems to be essential that only weakly coordinating counterions are present. 

Interestingly, the monodentate phosphine-modified Ni and Pd catalysts respond differently to variation of the counterion the best results for the Ni catalysts are obtained in the presence of weakly coordinating counterions such as OTf whereas the analogous Pd catalysts require the presence of a noncoordinating counterion such as BAr4 or SbFe". The latter are also the preferred anions for Ni-catalysts modified by 1 or the hemilabile ligand 27 [75-77, 80]. 

In contrast, addition of H0S02CF3, providing a weak-coordinating counterion, leads to [tra s,trara-ReH(C0)2(N0)(PR3)2+][S03CF3 ]. Electrophilic NO can also be obtained by electrochemical reduction, as shown in the generation of adsorbed Mb(NO-).113... 

If 4,4 -bipyridine is mixed with (dppp)Pd(II) or (dppp)Pt(II) triflates (dppp = bis-(diphenylphosphine) propane), the two reactants spontaneously self-assembly [53] to the metallo-supramolecular square shown in Fig. 5.24, because the weakly coordinating counterions leave two coordination sites open at the metal complex. By virtue of the dppp ligand, the coordination angle of 90° is fixed. It is rather difficult to ionize these squares as intact but completely desolvated entities, but under the... 

The mechanism of the reaction may involve a cationic nickel hydride associated with a weakly coordinated counterion (4, Scheme 1) as the true catalyst. This species is formed by the Lewis acid-assisted dissociation of the Ni-X bond from the 16-electron phosphine complex 3, coordination of ethylene (or styrene), coupling of the allyl and vinyl moieties, and subsequent 3-hydride elimination [11a]. Insertion of the vinylarene into the Ni-H bond gives a benzylic complex 5, which can be stabilized as an rj -intermediate 5. The coordinately unsaturated 5 can react with ethylene (and possibly not another vinylarene, if the phosphine is sufficiently bulky) to give 6, which can undergo an insertion followed by P-hydride... 

Similar trends were seen in a study of N1 versus C3 alkylation of indole under phase transfer conditions. These conditions involve a weakly coordinating counterion so that iV-alkylation should be preferred. Benzyl chloride gives a higher N C ratio than benzyl bromide, reflecting the generalization that more reactive alkylating agents will tend to favor C-alkylation <90H(31)447>. 

In 2010, Migsowicz et al. used chiral Cr(iii) complexes 11 and 12 (Figure 10.8) in the HDA reaction of aldehydes with Danishefsl s diene. The catalyst used was a tridentate Schiff base, which was previously seen to exhibit good enantioselectivity in its dimeric form compared to its monomeric form. Additionally, the nature of the counter ion affected the product. When a weakly coordinating counterion was introduced, the product yield... 

A new route to urethanes avoids phosgene and isocyanates by reacting amines and alcohols directly with CO over a catalyst in the presence of an acid. The urethanes 332 hitherto prepared in this way have been designed to be cracked to isocyanates 333, and a new BASF patent application covers both the synthesis of urethanes and their thermolysis to isocyanates [215]. The catalyst consists of a cationic Group VIII metal (e.g. Pd) with weakly coordinating counterions and methylene-bis-phosphine ligands. 

The abundant literature data on the chain propagation reaction itself shows a defined and consensual accepted reaction mechanism. After activation of a precursor or chain termination, the active species is created, which possesses either a Pd-C(=0)R (acyl group) 87 or a Pd-carbon (alkyl group) 89 (Scheme 22). Due to the strength of the CO coordination to the metal, the vacant coordination site will usually be occupied by CO molecules (alternative coordination of ethene, solvent molecules, or weakly coordinating counterions). 

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