Transmetalation route

A third route to allenyltin halides involves transmetallation of isolable allenyltri-butyltin compounds, as exemplified by the reaction of allenyltributyltin with Bu2SnCl2 [68]. The resulting mixture of allenyl- and propargyldibutyltin chlorides reacts with various aldehydes to afford mixtures of propargyl- and allenylcarbinols (Eqs. 9.78 and 9.79). The yields of these additions are uniformly high, but the selectivity depends on the nature of the aldehyde substituent. The transmetallation route to allenyltin and -indium halides will be discussed in more detail in a later section. 

Bipyridine-derived cycloaurated complexes have also been synthesised through the transmetallation route. Conversion of 6-phenyl-2,2 -bipyridine to its organomercury derivative 58, followed by reaction with Na[AuCl4] gave the cycloaurated complex 59 in 38% yield as its [AuC14] salt.39... 

An alternative transmetallation route found during our investigations is based on the fact that organoboron compounds readily transfer their organic groups to (alkoxo)palladium(II) complexes under neutral conditions (Scheme. 2-20). 

A useful metalation approach that has little precedent in cyclic carbene chemistry [18] is the use of 2-chloroamidinium or chloroiminium ions as precursors for acyclic carbene ligands. Fiirstner and coworkers prepared cationic Pd complexes of acyclic diamino-, aminooxy-, aminoarjd, and aminothiocarbenes by oxidative addition of chloroiminium precursors to Pd(PPhs)4 (route d. Scheme 16.1), an approach that was also effective for ADC-Ni complexes [19]. This route permits complexation of sterically nonhindered acyclic carbenes that would not be stable in the free state. Chloroamidinium precursors can be meta-lated without a change in metal oxidation state via lithium-halogen exchange followed by transmetalation (route e). This strategy has been successfully employed with Pd", Rh, and Ir [20]. 

Nickel ate complexes of composition NiR4 have been known for a long time, especially the tetraalkynyl derivatives, which are readily preparable compounds. A recent example is shown in Equation (4). Tetraalkyl and tetraaryl nickelate compounds are very air sensitive, but thermally stable. The stability of these compounds is improved when the alkyl moiety forms a metallacyclic structure. Some of these compounds have been prepared by conventional transmetallation routes, although the choice of the starting material (NiGp2, KNi(NPh2)s) seems to be important for the success of the reaction (Equation (5)). As shown in Equations (6)-(8), their stability is further... 

As above mentioned, the step with the highest energy barrier in all the transmetalation routes is the one that involves the j -coordination of 3 to Pd and the substitution of one of the N ligands by one of the OH ligands of boron (TSl, Table 6.1). Consequently, the route A-anti that has the lowest energy for that step (23.9 kcal moP ), is expected to be the most favorable route for transmetalation. 

Table 6.1 Calculated Gibbs energy barriers in toluene (AGj , kcal - mol" ) at 298 K for the transition states corresponding to TBa-TSl (i.e. TSl) and TBa-TS2 (i.e. TS2) in the different transmetalation routes. The highest-energy transition states within these routes are shown in bold...

The last step in the catalytic cycle that leads to the final coupling product with the defined stereochemistry and the concomitant regeneration of the catalytic species, is the reductive elimination. Thus, for the sake of completeness, the reductive elimination reactions starting from the products obtained in the four analyzed transmetalation routes (Fig. 6.10) were investigated. In particular, for each of these intermediates, the corresponding transition state and product involved in this process were computed. As expected, in all the calculated transition states for the different reductive eliminations, the formation of a three-membered ring between Pd and the two carbon... 

Fig. 6.12 Representation of the steric interactions (in red) that govern the stereochemical outcome of the reductive elimination reaction from the intermediates obtained in the four transmetalation routes. Favored rotations of the tail of 3 are shown in blue. Only the Me group of 2 is displayed using a ball-and-stick representation for clarity...

The application of NHC ligands in this reaction has naturally been extended to asymmetric processes. In general, the chiral induction of NHCs was low, probably due to the rapid internal rotation of the chiral substituents around the C-N axis. Also, metalation procedures involving deprotonation of a chiral azolium precursor with strong bases may cause loss of chirality in the desired complex. In contrast, the silver transmetalation route generally yields the optically active target complex. Since the first reports by Herrmann et al., and shortly after by Enders et al. in 1996-1997 on the synthesis and applications of chiral NHC-Rh complexes in the hydrosilylation of carbonyl compounds, increased efforts were made in this area. ... 

The reactions of polynuclear Cu(II) substrates with the labile Zn(II), Ni(II), and Co(II) complexes of substituted hydrazine-carbodithioates, [M(NS)2], are very effective transmetalation routes to the production of mixed polymetallic complexes. Modeling these processes by the reaction with [Cu(acac)2] to establish possible details of the transmetalation mechanism, Davies and co-workershave measured the equilibrium constants and stopped flow and conventional rate kinetics. For Ni(II), the kinetics indicate formation of two labile precursors, with formation constants which have quite different temperature dependences,and each of which convert to successor complexes at rates which can be resolved the successor complexes then dissociate to the transmetalated product when subjected to the subsequent chromatographic separation. Data are accumulating to build free-energy, enthalpy, and entropy profiles for the process and to identify the shifts in these for different [M(NS)2] reagents. 

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