Metal complexes branching points

The subsequent insertion of the alkyne into the metal-carbene bond affords the (r]1 r]3)-vinylcarbene complex D, which may exist either as a (Z)- or an ( )-metallatriene. This intermediate maybe considered as a branching point in the benzannulation reaction as three diverging routes starting from this point have been explored. 

One of the first results on the use of phosphine dendrimers in catalysis was reported by Dubois and co-workers [16]. They prepared dendritic architectures containing phosphorus branching points which can also serve as binding sites for metal salts. These terdentate phosphine-based dendrimers were used to incorporate cationic Pd centers in the presence of PPh3. Such cationic metalloden-dritic compounds were successfully applied as catalysts for the electrochemical reduction of C02 to CO (e.g. 9, Scheme 9) with reaction rates and selectivities comparable to those found for analogous monomeric palladium-phosphine model complexes suggesting that this catalysis did not involve cooperative effects of the different metal sites. 

In this proposed process, p-hydride elimination first yields a putative hydride olefin rc-complex. Rotation of the -coordinated olefin moiety about its co-ordination axis, followed by reinsertion produces a secondary carbon unit and therefore a branching point. Consecutive repetitions of this process allows the metal center to migrate down the polymer chain, thus producing longer chain branches. Chain termination occurs via monomer assisted p-hydrogen elimination, either in a fully concerted fashion as illustrated in Figure 2b or in a multistep associative mechanism as implicated by Johnson1 et al. 

Metal ions within organometallic dendrimers can be incorporated at the core, in the branches, or at branch points. Examples of dendrimers having metal-ion-containing cores included the dendritic metalloporphyrins [66,67] and related materials reported by Aida and Enomoto [68],Diederich et al. [69], Moore et al. [70], and Erechet et al. [71], dendritic terpyridine-ruthenium complexes report-... 

By employing coordination complexes as branch points, dendrimers can be synthesized that contain metal ions throughout their structure. The repetitive unit of such dendrimers contains M-C, M-N, M-P, or M-S bonds [53,62]. The metal ions act as supramolecular glue [63], in which the complexation chemistry directs the assembly and structure of the dendrimer [53]. One of the synthetic procedures used to prepare organometallic dendrimers with coordination centers in every layer is based on a protection/deprotection procedure in which two complexes are used as dendritic building blocks wherein one acts as a metal and the other as a ligand [64,76]. 

Another interesting class of metallodendrimers are zwitterionic dendrimers in which the positive charge on the metal ion is balanced by carboxylate moieties present in the dendritic branches.58ab Dendrimers 20 and 21 (Fig. 6.14) that contain four [Ru(tpy)2]2+ complexes along the branches and eight carboxylate units either in the internal branching points or at the periphery are examples of this class of zwitterionic dendrimers. 

The group of Van Leeuwen has reported the synthesis of a series of functionalized diphenylphosphines using carbosilane dendrimers as supports. These were applied as ligands for palladium-catalyzed allylic substitution and amination, as well as for rhodium-catalyzed hydroformylation reactions [20,21,44,45]. Carbosilane dendrimers containing two and three carbon atoms between the silicon branching points were used as models in order to investigate the effect of compactness and flexibility of the dendritic ligands on the catalytic performance of their metal complexes. Peripherally phosphine-functionalized carbosilane dendrimers (with both monodentate... 

CH Activation is sometimes used rather too loosely to cover a wide variety of situations in which CH bonds are broken. As Sames has most recently pointed out, the term was first adopted to make a distinction between organic reactions in which CH bonds are broken by classical mechanistic pathways, and the class of reactions involving transition metals that avoid these pathways and their consequences in terms of reaction selectivity. For example, radicals such as RO- and -OH readily abstract an H atom from alkanes, RH, to give the alkyl radical R. Also in this class, are some of the metal catalyzed oxidations, such as the Gif reaction and Fenton chemistry see Oxidation Catalysis by Transition Metal Complexes). Since this reaction tends to occur at the weakest CH bond, the most highly substituted R tends to be formed, for example, iPr-and not nPn from propane. Likewise, electrophilic reagents such as superacids see Superacid), readily abstract a H ion from an alkane. The selectivity is even more strongly in favor of the more substituted carbonium ion product such as iPr+ and not nPr+ from propane. The result is that in any subsequent fimctionalization, the branched product is obtained, for example, iPrX and not nPrX (Scheme 1). 

In this section, we will describe selected examples in which metal complexes constitute the core (Fig. 2a) or the branching points (Fig. 2b) of the dendritic architecture. Therefore, the den-drimer does not exist by itself as a ligand. 

To see physically the problem of motion of wavepackets in a non-diagonal diabatic potential, we plot in figure B3.4.17 a set of two adiabatic potentials and their diabatic counterparts for a ID problem, for example, vibrations in a diatom (as in metal-metal complexes). As figure B3.4.17 shows, if a wavepacket is started away from the crossing point, it would slide towards this crossing point (where Fj j = V22) where it would branch a part of it would continue on the same adiabatic state (i.e. shift to a different diabatic state) and the other part would jump to a different adiabatic state. 

More complex branching occurs in mechanisms where selectivities arise. Therefore, consider as an example the overall transformation of a symmetric alkene cyclopentene in the presence of CO and H2 to both cyclopentane carboxaldehyde and cyclopentane, facilitated by the addition of a Group 9 (or CAS systems VIIIB) metal hydride complex HML capable of performing both hydroformylation [27,28] and hydrogenation [29]. Upon addition of the metal hydride to the system, one would expect the steps HML —> HML i —> H(ji-cyclopentene)ML i CsH9ML i where the bold intermediates should be common to both unicycUc catalytic sequence of steps. At this point there is a branching in the network of intermediates, and one of either two paths is followed ... 

Although less studied, the hydrocyanation of alkynes in the presence of soluble transition metal complexes has also been reported. - The reactions conducted with nickel(O) catalysts occur with cis stereochemistry, high regioselectivity, and moderate-to-high yields. Again, both steric and electronic effects control the regioselectivity. These points are illustrated by the data in Equation 16.11. Terminal, straight-chain alkynes such as l-hex)me react to form predominantly the branched nitrile, whereas tert-butyl acetylene reacts to form mostly the terminal nitrile. Reactions conducted with DCN have shown that the addition occurs in a syn fashion. ... 

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