Metal alkyls elimination

The reverse reaction (formation of metal alkyls by addition of alkenes to M-H) is the basis of several important catalytic reactions such as alkene hydrogenation, hydroformylation, hydroboration, and isomerization. A good example of decomposition by y3-elimination is the first-order intramolecular reaction ... 

Theoretical studies of catalytic alkane-dehydrogenation reactions by [(PCP )IrH2], PCP rf-C6H3(CH2P112)2-l, 3 and [cpIr(PH3)(H)]+, suggest that they proceed through similar steps in both cases namely (i) alkane oxidation, (ii) dihydride reductive elimination, (iii) /3-II transfer from alkyl ligand to metal, (iv) elimination of olefin.402 The calculated barriers to steps (i), (ii), and (iv) are more balanced for the PCP system than for cp(PH3). 

Metal alkyls can be prepared in a simple manner from the main group halides (X = Cl, Br, I) and the appropriate alkyl Grignard reagent (RMgl) or the alkyllithium salt (RLi), as shown for the cadmium alkyls (Equation (2)).13 The elimination of impurities from the precursor source is of great importance, as any remaining impurities are invariably carried over into the growing semiconductor layers. Incorporation of impurities, even at levels as low as 1015 free carriers per cubic centimeter (one part in ca. 107), can drastically affect the electronic properties of the... 

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... 

E) Sigma-bond metathesis. Dihydrogen is observed to react with transition-metal-alkyl bonds even when the metal lacks lone pairs. In this case the reaction cannot be explained in terms of the oxidative-addition or reductive-elimination motif. Instead, we can view this reaction as a special type of insertion reaction whereby the ctmr bond pair takes the donor role of the metal lone pair and donates into the cthh antibond. When the M—R bonds are highly polarized as M+R, the process could also be described as a concerted electrophilic H2 activation in which R acts as the base accepting H+. 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

The complex TpPtMeH2 was synthesized by reacting TpPtMe(CO) with water (66). While it is stable towards reductive elimination of methane at 55 °C, deuterium incorporation from methanol-c/4 solvent occurs rapidly into the hydride positions and subsequently, more slowly, into the methyl position (Scheme 15). The scrambling into the methyl position has been attributed to reversible formation of a methane complex which does not lose methane under the reaction conditions (75,76). Similar scrambling reactions have been observed for other metal alkyl hydrides at temperatures below those where alkane reductive elimination becomes dominant (77-84). This includes examples of scrambling without methane loss at elevated temperature (78). 

There is almost no reliable information about the strength of metal-hydrogen bonds in organometallic compounds. Earlier (Section 2.2.) it was established that M-H bonds formed by (spontaneous) -elimination from metal alkyls should be stronger that their precursor, but this is an unsatisfactory and imprecise position. The dissociation... 

Tandem procedures under hydroformylation conditions cannot only make use of the intrinsic reactivity of the aldehyde carbonyl group and its acidic a-position but they also include conversions of the metal alkyl and metal acyl systems which are intermediates in the catalytic cycle of hydroformylation. Metal alkyls can undergo -elimination leading to olefin isomerization, or couplings, respectively, insertion of unsaturated units enlarging the carbon skeleton. Similarly, metal acyls can be trapped by addition of nucleophiles or undergo insertion of unsaturated units to form synthetically useful ketones (Scheme 1). 

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. 

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. 

The mechanism of this elimination reaction may be regarded as being similar to that of the dehydrogenation reaction. Double-bond isomerization leads to formation of a species with both double bonds in the ring. This is then metalated and eliminates the alkyl group (A )-... 

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. 

Strong bases readily eliminate thiol from vinylic sulfides. 1,2-Bis(alkyl- or arylthio)ethynes for example, react with two equivalents of BuLi or alkali amide to give metallated alkyl- or arylthioacetylenes and thiolates [24] ... 

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... 

CO into a metal-hydrogen bond, apparently analogous to the common insertion of CO into a metal-alkyl bond (6). Step (c) is the reductive elimination of an acyl group and a hydride, observed in catalytic decarbonylation of aldehydes (7,8). Steps (d-f) correspond to catalytic hydrogenation of an organic carbonyl compound to an alcohol that can be achieved by several mononuclear complexes (9JO). Schemes similar to this one have been proposed for the mechanism of CO reduction by heterogeneous catalysts, the latter considered to consist of effectively separate, one-metal atom centers (11,12). As noted earlier, however, this may not be a reasonable model. 

Catalytic hydrogenolysis of light alkanes (propane, butanes, pentanes) with the exception of ethane has been accomplished under very mild conditions over silica-supported hydride complexes.502 The hydrogenolysis proceeds over (=SiO)3 ZrH,503 (=SiO)3HfH,504 and (=SiO)3TiH505 by stepwise cleavage of carbon-carbon bonds by P-alkyl elimination from surface metal-alkyl intermediates. 

Although of relatively weak acidity, amines will react with either carbanionic metal alkyls or hydridic metal hydrides to form amides with the elimination of alkane or hydrogen, respectively. The easiest and most exploited method for the synthesis of lithium and magnesium amides is to treat lithium alkyls or Grignard reagents (normally commercially available) with the corresponding amine (equations 25,47 264 and 2749). 

The final step in the catalytic cycle is the cleavage of the metal-alkyl bond with acid, which must take place faster in the hydrocarboxylation of alkenes than -elimination. 

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). 

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