Metal-alkyl complex

The use of weakly coordinating and fluorinated anions such as B(C6H4F-4)4, B(C6F5)4, and MeB(C6F5)3 further enhanced the activities of Group 4 cationic complexes for the polymerization of olefins and thereby their activity reached a level comparable to those of MAO-activated metallocene catalysts. Base-free cationic metal alkyl complexes and catalytic studies on them had mainly been concerned with cationic methyl complexes, [Cp2M-Me] +. However, their thermal instability restricts the use of such systems at technically useful temperatures. The corresponding thermally more stable benzyl complexes,... 

Bochmann M, Lancaster SJ (1995) Cationic group IV metal alkyl complexes and their role as olefin polymerization catalysts The formation of ethyl-bridged dinuclear and heterodinuclear zirconium and hafnium complex. J Org Chem 494 55-59... 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

Structure determinations of secondary metal alkyl complexes are relatively rare, yet they provide an opportunity to assess interactions of the metal with the /3-atoms of the alkyl. The angles (excluding hydrogen) about C(24) all exceed 109°, ranging from 111.7° to 115.1°. There is no evidence for any Re 0 interaction (compare V), this distance exceeding 3 A. Both the /3-carbon, C(25), and its attached hydrogens are over 3 A from rhenium. The hydrogen on the a-carbon, C(24J, is 2.76 A from rhenium. 

Grigg, R. Sridharan, V. Transition Metal Alkyl Complexes Multiple Insertion Cascades. In Comprehensive Organometallic Chemistry II Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds. Elsevier Oxford, 1995 Vol. 12, pp 299-321. 

Selection of those metals where the metal alkyl complexes are stable with respect to hydride and liberated alkene. For the metals on the left-hand side of the periodic table, the Early Transition Metals and the Lanthanides, the alkyls are relatively stable. Therefore it is not surprising that the best alkene polymerisation catalysts are found amongst these metals. 

Steric hindrance may hamper the correct stereochemistry required for 13-elimination, and perhaps this can be used to stabilise our metal alkyl complex. In the modem polymerisation catalysts for polypropene this feature has actually been observed, which leads to higher molecular weight polymers. This now forms part of the design of new catalysts. 

In the last decade an enormous revival of late transition catalysts for the polymerisation of alkenes has taken place [45] (remember that the first discovery of Ziegler for ethene polymerisation also concerned nickel and not titanium). The development of these catalysts is due to Brookhart in collaboration with DuPont (Figure 10.28) [46], Detailed low-temperature NMR studies have revealed the mechanism of the reaction [47], Interestingly, the resting state of the catalyst is the ethene-metal-alkyl complex and not the metal-alkyl complex as is the case for the ETM catalysts. For ETM catalysts the alkene complex intermediates are never observed. Thus, the migratory insertion is the rate-determining step (the turnover limiting step , in Brookhart s words) and the reaction rate is independent of the ethene concentration. 

As a general guideline it can be stated that a-deprotonation of transition metal alkyl complexes will be easy when the metal is in a high oxidation state, the complex is positively charged, and the additional ligands are poor 7t-donors. 

A further possibility of inducing the elimination of alkanes from transition metal alkyl complexes is photolysis [398,424-427]. Two examples of photolytic a-eliminations leading to non-heteroatom-substituted alkylidene complexes are shown in Figure 3.7. 

Numerous examples have been reported of transition metal alkyl complexes which can be converted into carbene complexes by a-hydride abstraction [429-431], This process can also proceed intramolecularly by oxidative insertion of the metal into the a-C-H bond. Figure 3.8. shows some illustrative examples of iron, rhenium, and... 

Alkyl Hgands are preferred as precursors since the surface reaction leads to the release of only alkane molecules, which cannot be adsorbed on the support Therefore, it is worth analyzing the reactivity of metal-alkyl complexes as a funchon of the metal center. Since the first reports on the reachon of [Zr(C3H5)4] with the... 

The carbene complexes can also be formed by direct oxidative addition of ze-rovalent metal to an ionic liquid. The oxidative addition of a C-H bond has been demonstrated by heating [MMIM]BF4 with Pt(PPh3)4 in THF, resulting in the formation of a stable cationic platinum carbene complex (Scheme 15) (189). An effective method to protect this carbene-metal-alkyl complex from reductive elimination is to perform the reaction with an imidazolium salt as a solvent. 

In the same vein, reports have appeared suggesting the formation of 3 1 lithium-to-metal alkyl complexes (XXXV) with a structure resem-... 

From the practical point of view //-hydride elimination might also be an obstacle. In reactions that involve metal-alkyl complexes as early intermediates one has to block //-elimination to increase the lifetime of the intermediate and enable subsequent transformations on the complex. A reaction, which proved elusive partially for this very reason, is the coupling of alkyl halides. A set of conditions, which allowed for the Negishi coupling of primary alkyl halides and even tosylates with alkyzinc halides is shown in Equation 1.5.20 The recent work of Fu and others showed that the careful... 

Metal alkyl complexes, 655-657 Metal carbonyl complexes, 630-649... 

While some metal-alkyl complexes react violently with molecular oxygen and others are inert, there are a few well-documented examples of reactions which lead to the formation of alkoxide... 

Hydrocarbonylation reactions are multistep catalytic processes (14). For this reason our first problem was to determine in which step asymmetric induction takes place, since models could be very different depending on the step in which asymmetric induction is determined (e.g. in the metal alkyl complex formation or in the final reductive elimination step). 

Table 10 as well as the Tables 11 and 12 show the differences in energy of the transition states responsible for enantiomeric and regioisomeric excess. They have been calculated on the basis of the enantiomeric and isomeric ratios. They correspond to the difference in free activation energies (AAG ) for a single-step formation of the metal-alkyl complex intermediate from the substrate and the catalyst complex. 

Therefore, for either antipode, the diastereomeric activated complex controlling optical yield could be either the one corresponding to the formation of the x-complex or the one corresponding to the olefin insertion into the metal-hydrogen bond. In the case of rhodium, it appears from the results of the hydroformylation of 1,2-dimethylcyclohexene and of 2-methylmethylidencyclohexane, that the second case is more probable 10). In the case of platinum, the fact that isomerization of the substrate, which is very likely to occur via metal alkyl-complex formation, proceeds at a rate similar to or even higher than the hydroformylation rate seems to indicate that the same situation can also be assumed. 

In any case, if the step corresponding to the metal alkyl-complex formation is indeed the first irreversible step in asymmetric hydroformylation, both enantiomeric excess and isomeric ratio are very likely to be regulated, in the case examined, in this reaction step and should be considered together in any attempt to explain asymmetric induction. 

Possible Mechanism for Asymmetric Induction in the Step Corresponding to the Formation of the Metal Alkyl Complex... 

The model we have used for the description of the transition states implies that also the corresponding n-olefin complexes contain an asymmetric metal atom and that changes of configuration at the metal occur more slowly than the intramolecular transformation of the 7i-complex into the metal-alkyl complex. 

Figure 3.12 Generic equation for [S-hydride elimination from a transition metal-alkyl complex.

Coupling of metal hydride and metal alkyl complexes... 

Fig. 21 Stoichiometric reaction of C02 with rare-earth metal alkyl complexes to produce carboxylate dimeric catalysts...

W. Scherer, G.S. McGrady, Agostic interactions in dO metal alkyl complexes. Angew. Chem. Int. Ed. 43, 1782-1806 (2004)... 

Hydrogen transfer is stereospecifically cis and occurs in a stepwise manner,2 passing through a metal alkyl complex (species H in Fig. 2), the so-called half-hydrogenated state. 

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