Hydrocarbyl Eliminations

A more straightforward example of p-alkyl elimination is illustrated by the cationic neopentyl zirconocene and hafnocene complexes in Equation 10.20. - ° This p-alkyl elimination occurs in high yield to form the corresponding methyl complex and free isobutylene, even at -15 °C. A comparative study of the rates of reaction of the Zr and Hf complexes show that the two complexes react with similar rates and that the differences in the enthal-pic and entropic parameters offset each other. 

In addition to these reactions of discrete alkyl complexes of tP metals, much evidence for p-alkyl elimination has been gained from end-group analysis of polymers generated from olefin polymerization catalyzed by d metal complexes. These data imply that 3-alkyl elimination could be a major pathway for chain transfer in the absence of any added chain-transfer agents, such as hydrogen.  

Cleavage of the C-C bond to the 3-carbon of late-metal-alkyl complexes is slower and less common than that of d early metal complexes. However, a few examples of 3-aU yl elimination from late metal complexes are known. The reversible intramolecular insertion of an olefin into the platinum-alkyl complex described in Chapter 9 involves an early example of 3-alkyl elimination from a late-metal-alkyl complex. In addition, mild 3-aIkyl elimination has been reported to occur from a ruthenacylobutane complex. In this case the product is a stable alkyl allyl species. -  

Like 3 hydrogen eliminations, 3-alkyl eliminations require an open coordination site. This site is generated in the scandocene system by dissociation of PMej and in the zirconocene and hafnocene complexes by dissociation of the borate from the zwitterionic species. - The open coordination site is generated in the platinum system by abstraction of chloride and is generated in the ruthenium complex by dissociation of the monodentate phosphme. - The mild conditions for 3-methyl elimination from the ruthenium metalla-cycle is surprising, considering that it would seem to require the propellane-type transition state shown in Equation 10.21. 

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A hydrocarbyl elimination approach is used to produce the Zr(rv) dibenzyl complex incorporating a tridentate bis(amido) silylether [iV, 0,(V ] complex 18261 (Equation (13)). The molecular structure of 182 features a distorted tbp geometry with an approximately linear ZrN20 unit and the two amido nitrogen atoms occupying approximately axial positions.One benzyl group is -coordinated. When activated with MAO, complex 182 shows moderate activity for ethylene polymerization. 

Amine or hydrocarbyl elimination was also employed to prepare the following t -mono-Cp-silylamido derivatives (Scheme 106), including zirconium bis(diethylamido) complexes 469 with variations on the ring and amido substitutions,330 zirconium dibenzyl complex 4 70,331 t -mono-Ind-silylamido zirconium complex 471, 0 isodicy-clopentadienyl zirconium complexes 472,332 and enantiomerically pure zirconium bis(dimethylamido) and dichloro complexes 473333 with the R or S -CH(Me)Ph group attached to the amido nitrogen the last two complexes of this... 

A vacant coordination site is created during the forward insertion reaction of Equation 9.1, and a vacant coordination site must be present for the reverse reaction to occur. This mechanism makes it necessary for coordinatively saturated (18-electron) metal-alkyl complexes to dissociate a ligand prior to 3-hydrogen and 3-hydrocarbyl elimination reactions. 

Hydrocarbyl Complexes. Stable homoleptic and heteroleptic uranium hydrocarbyl complexes have been synthesized. Unlike the thorium analogues, uranium alkyl complexes are generally thermally unstable due to P-hydride elimination or reductive elimination processes. A rare example of a homoleptic uranium complex is U(CH(Si(CH2)3)2)3, the first stable U(I11) homoleptic complex to have been isolated. A stmctural study indicated a triganol... 

Oxidative additions involving C-H bond breaking have recently been the topic of an extensive study, usually referred to as C-H activation the idea is that the M-H and M-hydrocarbyl bonds formed will be much more prone to functionalization than the unreactive C-H bond. Intramolecular oxidative additions of C-H bonds have been known for quite some time see Figure 2.15. This process is named orthometallation or cyclometallation. It occurs frequently in metal complexes, and is not restricted to "ortho" protons. It is referred to as cyclometallation and is often followed by elimination of HX, while the metal returns to its initial (lower) oxidation state. 

The disproportionation activity in the supported species is parallel to the increased activity of ethylene polymerization on supported catalysts. Many of the steps in the reaction may be identical for example, the initial coordination of olefin to the metal center will be common to both systems. Indeed, some of these catalysts are also ethylene polymerization catalysts (see Table IV) although their activities are much less than the corresponding zirconium derivatives. A possible intermediate common to both disproportionation and polymerization could be the hydrocarbyl-olefin species (Structure I). Olefin disproportionation would result if the metal favored /3-hydrogen elimination to give the diolefin intermediate (Structure II) which is thought to be necessary for olefin disproportionation. Thus, the similarity between the mechanism and activation of olefin disproportionation and polymerization is suggested. 

Metallacyclobutene complexes of both early and late transition metals can, in some cases, be prepared by intramolecular 7-hydrogen elimination, although the intimate mechanism of the reaction varies across the transition series. For low-valent late metals, the reaction is generally assumed to proceed via the oxidative addition of an accessible 7-C-H bond (Scheme 28, path A), but for early metals and, presumably, any metal in a relatively high oxidation state, a concerted cr-bond metathesis is considered most probable (path B). In this process, the 7-C-H bond interacts directly with an M-X fragment (typically a second hydrocarbyl residue) to produce the metallacycle with the extrusion of H-X (i.e., a hydrocarbon). Either sp3- or spz-hybridized C-H bonds can participate in the 7-hydrogen elimination. 

The transient zirconocene butene complex, 105, has proved to be useful in a number of organic transformations. For example, butene substitution of zirconocene alkene complexes with alkoxy-substituted olefins results in /3-alkoxide elimination to furnish the zirconocene alkoxy compounds (R = Me, 123 R = Bnz, 124) (Scheme 16).50,51 Addition of propargyl alcohols to the zirconocene butene complex, 105, affords homoallylic alcohols. These reactions are of limited utility owing to the lack of stereoselectivity or formation of multiple products. Positioning the alkoxide functional group further down the hydrocarbyl chain allows synthesis of cyclopropanes, though mixtures of the carbocycle and alkene products are obtained in some cases (Scheme 16).52... 

When the hydrocarbyl ligand is 1° alkyl, the reaction proceeds with overall retention of configuration to produce RHgX and L FeX. When R = IIP or benzyl, race-mization is the stereochemical outcome and the products are RX and L FeHgX. Apparently two pathways are operative in this example. Path e (Schemes 8.9 and 8.11) gives retention by a reductive elimination pathway. As R becomes substituted with groups that can stabilize a carbocation, path d (Schemes 8.9 and 8.11) takes over to produce free R+ that picks up X from HgX3. ... 

The conversion of the Ir(III) cyclohexyl hydride complex to an Ir/cyclohexane system involves a change in the formal oxidation state of Ir from + 3 to +1 (i.e., a formal two-electron reduction). As a result, this elementary reaction step is generally called a reductive coupling (Chart 11.4). From a metal hydrocarbyl hydride complex (i.e., M(R)(H)), the overall process of C H bond formation and dissociation of free hydrocarbon (or related functionalized molecule) is called reductive elimination (Chart 11.4). The reverse process, metal coordination of a C—H bond and insertion into the C—H bond, is called oxidative addition. Note Oxidative addition and reductive elimination reactions are not limited to reactions involving C and H.)... 

The key steps of a concerted three-center reductive elimination mechanism (Fig. 4, path b) are dissociation of the ligand L trans- to the hydrocarbyl R (step b-i), a concerted M-C bond cleavage and C-X bond formation (step h 2). and a displacement of the organic product R-Z by the ligand L (step b i). The reaction leads to the product of cis-elimination of R-Z with the retention of the configuration of the metal-bound carbon atom. 

As a result of the combined effect of the factors mentioned above, Pd monoaUcyl complexes are expected to be the least stable among hydrocarbyls (M = Pd, Pt). Such complexes are currently unknown, whereas monoaUcyl ft " complexes were among the first compounds characterized in C(sp )-0 reductive elimination reactions [16-19]. Among monoaryl Pd " derivatives only few were isolated to date aU of them are stabilized by fluoride ligands [26, 27]. No monohydrocarbyl Pd complexes supported by 0-donor ligand have yet been reported and characterized. The most studied O-ligated organopalladium(lV) complexes are diaryl dicarbox-ylates 13 shown in Fig. 7 [11, 12]. 

Since most of the important insertion// -elimination processes affect molecules where A=B is a carbon-carbon double or biple bond, the discussion is organized considering brst the different types of M-X bond involved, and the insertion of alkenes or alkynes. Less abundant cases are discussed later. The reader should have in mind that sometimes the word alkene is used in a more general way, meaning an unsaturated carbon-carbon bond not involved in aromaticity. Also the word alkyl can be used in a more general context meaning hydrocarbyl. 

An early theoretical study suggested that better 0 donors are eliminated more easily from cA-MR(R )L2 type complexes [8]. However, this prospect is now known to be applicable only to a dialkyl complex series [e.g., Me > Et > Pr > Bu]. Thus the rate of reductive elimination is rather sensitive to the other factors, especially to the orbital hybridization of hydrocarbyl ligands. In general, the reactivity decreases in the order [hydrido (s)alkenyl (sp ) > aryl (sp ) 3> alkynyl (sp) > alkyl (sp )]. While a Jt-allyl ligand ranks middle, its behavior in reductive elimination will be described separately in Section 9.4. 

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