Metal hydrides, addition alkenes

Once metal hydride addition (alkene insertion) has taken place, for example (3) —> (4), -elimination (4) - (3) and readdition can occur (Scheme 2). Accordingly, alkene isomerization can take place in the hydroformylation process (equation 3). 

This stereochemistry is a result of syn metal hydride addition across the alkene followed by CO insertion with retention of configuration at the carbon bound to the metal (e.g. steps 3 — 4 and 5 — 6 Scheme 2). 

All these ligands have extensive chemistry here we note only a few points that are of interest from the point of view of catalysis. The relatively easy formation of metal alkyls by two reactions—insertion of an alkene into a metal-hydrogen or an existing metal-carbon bond, and by addition of alkyl halides to unsaturated metal centers—are of special importance. The reactivity of metal alkyls, especially their kinetic instability towards conversion to metal hydrides and alkenes by the so-called /3-hydride elimination, plays a crucial role in catalytic alkene polymerization and isomerization reactions. These reactions are schematically shown in Fig. 2.5 and are discussed in greater detail in the next section. 

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. 

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. 

Manganese.—Elimination of transition-metal hydride from metal alkyls and addition of metal hydrides to alkenes are usually considered to be cA-processes. Since acylmanganese compounds undergo stereospecific reversible decarbonylation, thermal decomposition of (eryrAro-2,3-dimethylpentanoyl)(pentacarbonyl)manga-nese(i) should allow the determination of the stereochemistry of elimination of [MnH(CO)8] (Scheme 4). However, both the erythro and a mixture of the erythro and threo acyl complexes decompose thermally to give the same mixture of cis- and trans-3-methylpent-2-ene and 3-methylpent-l-ene under conditions which do not isomerize these alkenes. It is suggested that the mechanism involves interconversion of... 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

More recently homogeneous hydrogenation catalysts, such as RhCl(Ph3P)3, have been developed which are soluble in the reaction medium. These are believed to transfer H to an alkene via a metal hydride intermediate they, too, lead to a considerable degree of SYN stereoselectivity in hydrogen addition. 

The addition of an R-M moiety to the triple bond gives the corresponding vinylmetal intermediate 241, which is activated enough to react with the alkene moiety. Depending upon the nature of the R1 group, several options are open. In the case of an initial hydridometallation by a metal hydride, which is most often formed in situ through the oxidative addition to acetic acid (R-R1 = H-OAc), the resulting cyclization product 243 will liberate its metal component by... 

Recently, another type of catalytic cycle for the hydrosilylation has been reported, which does not involve the oxidative addition of a hydrosilane to a low-valent metal. Instead, it involves bond metathesis step to release the hydrosilylation product from the catalyst (Scheme 2). In the cycle C, alkylmetal intermediate generated by hydrometallation of alkene undergoes the metathesis with hydrosilane to give the hydrosilylation product and to regenerate the metal hydride. This catalytic cycle is proposed for the reaction catalyzed by lanthanide or a group 3 metal.20 In the hydrosilylation with a trialkylsilane and a cationic palladium complex, the catalytic cycle involves silylmetallation of an alkene and metathesis between the resulting /3-silylalkyl intermediate and hydrosilane (cycle D).21... 

The alkene inserts either in the metal hydride bond or in the metal silyl bond. The latter reaction leads to alkenylsilyl side products and also alkane formation may occur. Similar reactions have been observed for hydroboration, the addition of R2BH to alkenes. (R2 may be the catechol dianion). 

Alkynes show the same reaction and again the product obtained is the anti isomer. After a suitable elimination from the metal the alkene obtained is the product of the anti addition. Earlier we have seen that insertion into a metal hydride bond and subsequent hydrogenation will afford the syn product. If we use BH4 as the nucleophile we can accomplish anti addition of a hydride. Thus, with the borohydride methodology and the hydrogenation route either isomer can be prepared selectively. 

It can be inferred from additional examples (Table 3) that the stereoselectivity and stereochemical outcome of the reaction strongly depend on the type of metal hydride and the leaving group. Furthermore, the reaction temperature is important in several cases67. The scope of the method is broad, however, varying amounts of isomeric alkyne isomers are formed as byproducts, sometimes accompanied by the corresponding alkenes. 

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