Insertion processes complexes

The potential of C-Si and Si-H insertion processes of platinum fragments like [(dtbpm)Pt(O)] and [(dcpm)Pt(O)] with respect to stoichiometric or catalytic applications is being evaluated in our group now. The chemistry of the nickel and palladium analogs of these intermediates and of their complexes [28, 29], in accord with theoretical expectations, has turned out to be comparably unusual and exciting. This will be reported elsewhere. 

Allylic C/H insertion accompanied by an allylic rearrangement has been observed for carbenoid reactions of ethyl diazoacetate with allylamines (Scheme 23)1S1). Apparently, metal-catalyzed isomerization 117 118 proceeds the C/H insertion process. Although mechanistic details have not yet been unraveled, T)3-allyl complexes... 

Since group 3 metallocene alkyls are isoelectronic with the cationic alkyls of group 4 catalysts they may be used as olefin polymerization initiators without the need for cocatalysts. The neutral metal center typically results in much lower activities, and detailed mechanistic studies on the insertion process have therefore proved possible.216-220 Among the first group 3 catalysts reported to show moderate activities (42 gmmol-1 h-1bar-1) was the yttrocene complex (77).221... 

Insertion of carbon monoxide and alkenes into metal-carbon bonds is one of the most important reaction steps in homogeneous catalysis. It has been found for insertion processes of platinum [16] that the relative positions of the hydrocarbyl group and the unsaturated fragment must be cis in the reacting complex [17], The second issue concerns the stereochemical course of the reaction, insertion versus migration as discussed in Chapter 2.2. 

Insertion of CO is therefore always kinetically controlled. When an alkyl palladium species has formed, the open site will be occupied by a coordinating CO molecule. Carbon monoxide coordinates more strongly to palladium than ethene, even when the palladium centre is cationic. The reason for this is steric the cone angle of ethene is much larger than that of CO and the steric hindrance in the ethene complex is therefore much larger. If the barriers of activation for the insertion processes of ethene and CO are of the same order of... 

Otherwise, the reactions of indenyl-ruthenium(II) allenylidenes [RuCty -CgHy) =C=C=C(R )Ph (PPh3)2][PF6] (R = H, Ph) with ynamines R C CNEtj (R = Me, SiMea) have been reported to yield the alkenyl(amino)allenylidene complexes 41 via insertion of the ynamine into the Cp=Cy allenylidene bond (Scheme 10) [52, 53], This insertion process involves an initial nucleophilic addition of the ynamine at Cy atom of the cumulene, which leads to the cationic alkynyl intermediate complexes 39. Further ring closing, involving the Cp atom, generates the [2+2]... 

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

As already briefly mentioned, the oxygen-atom insertion into Si—H bonds of silanes constitutes a selective method for the chemoselective preparation of silanols, which has been much less studied compared to the CH oxidation. This unique oxyfunctionalization of silanes is also highly stereoselective (equation 35) since, like the CH insertions, it proceeds with complete retention of configuration. A novel application of the SiH insertion process is the synthesis of the unusual iron complex with a silanediol functionality, in which selectively both Si—H bonds of the silicon atom proximate to the iron ligand are oxidized in the silane substrate (equation 36). ... 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

Bacteria have two classes of transposons. Insertion sequences (simple transposons) contain only the sequences required for transposition and the genes for proteins (transposases) that promote the process. Complex transposons contain one or more genes in addition to those needed for transposition. These extra genes might, for example, confer resistance to antibiotics and thus enhance the survival chances of the host cell. The spread of antibiotic-resistance elements among disease-causing bacterial populations that is rendering some antibiotics ineffectual (pp. 925-926) is mediated in part by transposition. 

Cyclotetramerization to form cyclooctatetraene occurs only with nickel.46,63 68 The best catalysts are octahedral Ni(II) complexes, such as bis(cyclooctatetraene) dinickel.46 Internal alkynes do not form cyclooctatetraene derivatives but participate in cooligomerization with acetylene. Of the possible mechanistic pathways, results with [l-13C]-acetylene81 favor a stepwise insertion process or a concerted reaction, and exclude any symmetric intermediate (cyclobutadiene, benzene). The involvement of dinuclear species are in agreement with most observations.46,82-84... 

Relatively little mechanistic work has been reported on the insertion reactions of C02. The mechanism seems to be established only for the insertion of C02 into the dialkylamides of the early transition metals (131). We will speculate on probable mechanisms for the various types of insertion reactions that follow. Future work will undoubtedly shed more light on these processes, leading to a better understanding of the reaction, and enabling a more rational design of catalyst complexes in order to incorporate the insertion process into an efficient catalytic cycle. 

The M-NMe2 moiety in these complexes, however, is planar, indicating that the nitrogen lone pair is involved in -bonding with the metal. It is worth noting that in neither the mechanism discerned by Chisholm nor in the mechanism speculated upon here is activation of C02 by the metal complex an integral part of the insertion process. 

Ultraviolet photolysis of the diphosphine complex [Os3(CO)io(bpcd)] (bpcd, Struct. 3) yields an orthometalated hydrido triosmium complex as the exclusive product (Fig. 17). The mechanism presumably involves initial CO loss with subsequent coordination of one phenyl of the diphenylphosphino unit followed by the insertion process [100]. 

An isotactic stereospecific polymerization arises essentially from the favored complexation of one prochiral face of the a-olefin, followed by a stereospecific process. The stereospecific insertion process and the stereospecific polymerization of racemic a-olefins giving isotactic polymers may be expected to be stereoselective whenever the asymmetric carbon atom is in an a- or /3-position relative to the double bond, and when the interaction between the chirality center of the olefin and the chiral catalytic site is negligible. 

Electrophilic attack at carbyne complexes may ultimately place the electrophile on either the metal or the (former) carbyne carbon, the two possibilities being related in principle by a-elimination/migratory insertion processes (Figure 5.39). The reactions of the osmium carbyne complex are suggestive of an analogy with alkynes. Each of these reactions (hydro-halogenation, chlorination, chalcogen addition, metal complexation see below) have parallels in the chemistry of alkynes. 

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