Hydride Addition—Elimination

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

Although the reaction responsible for the generation of the hydride is not specified, it is assumed that it arises from a disproportionation of iron carbonyl complexes. The hydride presumably adds after ir-complexing to form the c-bonded complex which then splits out the metal hydride in either direction. The ir-complexed olefin may then be displaced by another olefin or undergo another hydride addition-elimination sequence. The second path involves olefin complexing with the deficient Fe(CO)3 species and formation of a jr-allyliron hydride intermediate ... 

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. ... 

This elimination is reminiscent of the last step in the aqueous palladium chloride oxidation mentioned above and this reaction also may involve multiple hydride addition-elimination steps. Minor amounts of the normal products and Markovnikov products are also generally found in these reactions. Cupric chloride can be used as a reoxidant although the yields are generally lower than with an all acetate, non-catalytic reaction. 

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 square planar complex 73 undergoes P-elimination via liberation of L to form a transient equilibrium of the iminium-rhodium hydride o-complex 74a and the Jt-complex 74b, Eq. 16. These complexes 74a and 74b represent a unique nitrogen-triggered mechanism that is different from either the hydride addition-elimination pathway or the 7i-allyl mechanism resulting in the intramolecular 1,3-hydrogen shift. 

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. 

This reaction has provided strong evidence that the hydride addition-elimination pathway is utilized. 

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. ... 

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