Mechanism, metal hydride addition-elimination

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 327). Electrocyclic and sigmatropic rearrangements are treated at 8-29 to 8-37. Double-bond migrations have also been accomplished photochemically,67 and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts.68 In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the metal hydride addition-elimination mechanism ... 

The two mechanisms may result in substantial and characteristic differences in deuterium distribution. The metal hydride addition-elimination mechanism usually leads to a complex mixture of labeled isomers.195 198 208-210 Hydride exchange between the catalyst and the solvent may further complicate deuterium distribution. Simple repeated intramolecular 1,3 shifts, in contrast, result in deuterium scram-bling in allylic positions when the ir-allyl mechanism is operative. ... 

Isomerization of allylic alcohol to ketone has been extensively studied [13], and two different pathways have been established, including tt-allyl metal hydride and the metal hydride addition-elimination mechanisms [5,14]. McGrath and Grubbs [ 15] investigated the ruthenium-catalyzed isomerization of allyl alcohol in water and proposed a modified metal hydride addition-elimination mechanism through an oxygen-functionality-directed Markovnikov addition to the double bond. 

The two established pathways for transition metal-catalyzed alkene isomerization are the jr-allyl metal hydride and the metal hydride addition-elimination mechanisms. The metal hydride addition-elimination mechanism is the more common pathway for transition metal-catalyzed isomerization. In this mechanism, free alkene coordinates to a metal hydride species. Subsequent insertion into the metal-hydride bond yields a metal alkyl. Formation of a secondary metal alkyl followed by y3-elimination yields isomerized alkene and regenerates the metal hydride. The jr-allylhydride mechanism is the less commonly found pathway for alkene isomerization. Oxidative addition of an activated allylic C-H bond to the metal yields a jr-allyl metal hydride. Transfer of the coordinated hydride to the opposite end of the allyl group yields isomerized alkene. 

The fundamental differences between these two mechanisms are that 1) the jr-allyl metal hydride mechanism involves a 1,3-hydrogen shift while the metal hydride addition-elimination mechanism involves a 1,2-hydrogen shift and 2) the hydrogen shift in the Jt-allylhydride mechanism proceeds in an intramolecular fashion while that in the metalhydride addition-elimination mechanism proceeds in an intermolecular fashion. 

The crossover product, propionaldehyde-l,3-d-3- C 12, clearly demonstrated that the isomerization occurred via intermolecular 1,3-hydrogen shift. These results are consistent with a modified metal hydride addition-elimination mechanism which involves exclusive 1,3-hydrogen shift through oxygen-directed Markovnikov addition of the metal hydride to the carbon-carbon double bond (Scheme 12.2). The directing effect of functional groups on the selectivity of transition metal catalysis is well presented [9], and an analogous process appears to be operative in the isomerization of allylamines to enamines [10]. 

The most attractive mechanism for this isomerization is a metal hydride addition-elimination mechanism (Fig. 22). Initial complexing occurs between an olefin and the metal complex, which is followed by the addition of a hydrogen causing a tt-o- rearrangement. Next, elimination of an hydrogen occurs in the opposite direction by a jS-interaction with eventual release of the isomerized olefin. 

Wagener has used deuterium-labeUed substrates to probe alkene isomerization processes that occur during metathesis reactions. The observation of a 1,2-deuterium shift as well as a 1,3-deuterium shift provided evidence for a metal hydride addition/elimination process as opposed to a 7t-aUylru-thenium hydride mechanism, as the latter would be expected to yield a net 1,3-deuterium shift only (Scheme 2.58). In addition, complete deuteration next to the oxygen suggested that this isomerization was irreversible, otherwise H/D exchange at this position would have been expected. 

Isomerization of olefins by transition-metal complexes is one of the most important goals in organometallic chemistry [6, 7]. For the topic considered here [8], two principal mechanisms can be distinguished (Scheme 5.2) (a) metal hydride addition-elimination mechanism (alkyl mechanism) [9], and (b) reaction via a it-allyl metal hydride intermediate (allyl mechanism) [10]. 

Various CpRh( -1,4-diene) complexes have been prepared, and isomerize thermally to the CpRh( -l,3-diene) complex the results are in accord with a metal hydride addition-elimination mechanism. A crystal-structure determination of (cod)(benzoyl-l,l,l-trifluoroacetonato)Rh has been reported. ... 

Some experimental evidences are in agreement with this proposed mechanism. For example, coordinating solvents like diethyl ether show a deactivating effect certainly due to competition with a Lewis base (149). For the same reason, poor reactivity has been observed for the substrates carrying heteroatoms when an aluminum-based Lewis acid is used. Less efficient hydrovinylation of electron-deficient vinylarenes can be explained by their weaker coordination to the nickel hydride 144, hence metal hydride addition to form key intermediate 146. Isomerization of the final product can be catalyzed by metal hydride through sequential addition/elimination, affording the more stable compound. Finally, chelating phosphines inhibit the hydrovinylation reaction. 

Ti o.9i jji most cases the organometallic undergoing insertion is formed in situ by a metal hydride addition (insertion) with an olefin rather than by the exchange reaction. The olefin reacting with the hydride to form the alkyl may be the same one that undergoes the insertion with the metal alkyl, or a different one. This sequence with a single olefin produces olefin dimers after the final / -hydride elimination. A mechanism for the RhClj-catalyzed dimerization of ethylene is ... 

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

A mechanism similar to Scheme 10 was proposed, involving CO addition, followed by H20 addition (in lieu of hydroxide anion) to form a metallocarboxylic acid complex. Then, decomposition to C02 and a metal hydride was proposed, followed by hydride elimination. Table 15 provides data from reaction testing in the temperature range 140 to 180 °C. In later testing, they compared Rh and Ir complexes for the reduction of benzalacetone under water-gas shift conditions. 

As briefly discussed in section 1.1, and shown in Figure 1, the accepted mechanism for the catalytic cycle of hydrogenation of C02 to formic add starts with the insertion of C02 into a metal-hydride bond. Then, there are two possible continuations. The first possibility is the reductive elimination of formic acid followed by the oxidative addition of dihydrogen molecule to the metal center. The second possible path goes through the a-bond metathesis of a metal formate complex with a dihydrogen molecule. In this section, we will review theoretical investigations on each of these elementary processes, with the exception of oxidative addition of H2 to the metal center, which has already been discussed in many reviews. 

The first mechanism appears to be the better basis for describing most of the results referred to by Cramer (56). It will, however, be noted that the addition-elimination mechanism requires that the metal catalyst be supplied as a metal hydride. Where the catalyst has not been supplied in this form, the reaction has usually been carried out in the presence of reagents known to convert transition metal compounds to hydrides (e.g. protonic acids, alcohols or hydrogen). These substances are known as co-catalysts and, where they have been used, induction periods have been encountered which are consistent with hydride formation as required in mechanism (a), but which would not be expected for (b). 

Mechanistic studies of the rearrangement activity of the ring-opening metathesis polymerization catalyst [Ru(H20)6]2+ were reported for unfunctionalized alkenes (112). The mechanism was found to be intermolecular, the alkene isomerization proceeding through an addition-elimination mechanism with a metal hydride catalytic species. This interpretation was... 

Scheme 19 shows a general mechanism for C—H bond activation. In principle, any donor groups, including olefinic bonds, carbanions, heteroatom anions, neutral heteroatoms, for example, can activate their adjacent C—H bonds through coordination with appropriate transition metal centers. The metal hydride complexes formed by oxidative addition or /3-elimination, undergo unique chemical transformations. 

The homogeneous hydrogenation of alkenes is explained by two mechanisms. The first is the dihydride mechanism, in which the dihydride 1 is formed by oxidative addition of H2, and the hydrogenation proceeds by the insertion of alkene to the metal hydride bond, followed by reductive elimination (Scheme 10.1). The other hydrogenation is explained by the formation of the monohydride 2 (Scheme 10.2). Insertion of alkenes... 

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