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Alkynes metal hydrides

Two mechanisms are conceivable for the reaction. One involves the hydrometalation of the metal hydride at alkyne followed by reductive elimination.15 Alternatively, phosphinometalation followed by protiode-metalation can occur.16 To gain insight into the mechanism, a carbonylation... [Pg.495]

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. [Pg.45]

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. [Pg.547]

The inertness of ordinary double bonds toward metallic hydrides is quite useful, since it permits reduction of, say, a carbonyl or nitro group, without disturbing a double bond in the same molecule (see Chapter 19 for a discussion of selectivity in reduction reactions). Sodium in liquid ammonia also does not reduce ordinary double bonds,275 although it does reduce alkynes, allenes, conjugated dienes,276 and aromatic rings (5-10). [Pg.775]

The following discussion deals not only with this reaction, but related reactions in which a transition metal complex achieves the addition of carbon monoxide to an alkene or alkyne to yield carboxylic acids and their derivatives. These reactions take place either by the insertion of an alkene (or alkyne) into a metal-hydride bond (equation 1) or into a metal-carboxylate bond (equation 2) as the initial key step. Subsequent steps include carbonyl insertion reactions, metal-acyl hydrogenolysis or solvolysis and metal-carbon bond protonolysis. [Pg.913]

In certain other systems, there is compelling evidence for the insertion into a metal-caiboxylate complex (equation 37). For example, in the synthesis of a-methylene-y-lactones from alkynic alcohols,70,71 no double bond rearrangement to a butenolide occurs, a reaction shown to take place in the presence of transition metal hydrides. The source of the vinyl proton (deuterium) on the a-methylene group is indeed the alcohol function. Finally, palladium carboxylate complexes containing alkynic (equation 40) or vinyl tails (equation 41) can be isolated and the corresponding insertion reaction can be observed. [Pg.937]

The following compounds with H—C and II—M bonds undergo oxidative addition to form metal hydrides. This is examplified by the reaction of 6, which is often called ortho-metallation, and occurs on the aromatic C—H bond at the ortho position of such donar atoms as N, S, 0 and P. Reactions of terminal alkynes and aldehydes are known to start by the oxidative addition of their C—H bonds. Some reactions of carboxylic acids and active methylene compounds are explained as starting with oxidative addition of their O—H and C—H bonds. [Pg.11]

The insertion of alkene to metal hydride (hydrometallation of alkene) affords the alkylmetal complex 34, and insertion of alkyne to an M—R (R = alkyl) bond forms the vinyl metal complex 35. The reaction can be understood as the cis carbometallation of alkenes and alkynes. [Pg.15]

The reactions of type II proceed by transmetallation of the complex 5. The transmetallation of 5 with hard carbon nucleophiles M R (M = main group metals) such as Grignard reagents and metal hydrides MH generates 8. Subsequent reductive elimination gives rise to an allene derivative as the final product. Coupling reactions of terminal alkynes in the presence of Cul belong to Type II. [Pg.200]

Addition reactions of three kinds of main group metal compounds, namely R—M X (carbometallation, when R are alkyl, alkenyl, aryl or allyl groups), H—M X (hydrometallation with metal hydrides) and R—M —M"—R (dimetallation with dimetal compounds) to alkenes and alkynes, are important synthetic routes to useful organometallic compounds. Some reactions proceed without a catalyst, but many are catalysed by transition metal complexes. [Pg.277]

Addition of hydride bonds of main group metals such as B—H, Mg—H, Al—H, Si—H and Sn—H to alkenes and alkynes to give 513 and 514 is called hydrometallation and is an important synthetic route to compounds of the main group metals. Further transformation of the addition product of alkenes 513 and alkynes 514 to 515,516 and 517 is possible. Addition of B—H, Mg—H, Al—H and Sn—H bonds proceeds without catalysis, but their hydrometallations are accelerated or proceed with higher stereoselectivity in the presence of transition metal catalysts. Hydrometallation with some hydrides proceeds only in the presence of transition metal catalysts. Hydrometallation starts by the oxidative addition of metal hydride to the transition metal to generate transition metal hydrides 510. Subsequent insertion of alkene or alkyne to the M—H bonds gives 511 or 512. The final step is reductive elimination. Only catalysed hydrometallations are treated in this section. [Pg.284]


See other pages where Alkynes metal hydrides is mentioned: [Pg.1933]    [Pg.1933]    [Pg.463]    [Pg.184]    [Pg.224]    [Pg.147]    [Pg.7]    [Pg.8]    [Pg.69]    [Pg.300]    [Pg.280]    [Pg.48]    [Pg.609]    [Pg.537]    [Pg.789]    [Pg.790]    [Pg.117]    [Pg.167]    [Pg.110]    [Pg.108]    [Pg.496]    [Pg.138]    [Pg.122]    [Pg.104]    [Pg.177]    [Pg.519]    [Pg.248]    [Pg.94]    [Pg.348]    [Pg.254]    [Pg.104]    [Pg.1116]    [Pg.2636]    [Pg.69]    [Pg.100]    [Pg.393]    [Pg.406]   
See also in sourсe #XX -- [ Pg.2 , Pg.2 , Pg.7 , Pg.8 ]

See also in sourсe #XX -- [ Pg.2 , Pg.4 , Pg.6 , Pg.7 , Pg.7 ]

See also in sourсe #XX -- [ Pg.2 , Pg.2 , Pg.5 , Pg.7 , Pg.8 , Pg.12 ]




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Addition of metal hydrides to alkenes and alkynes

Alkyne, reaction with metal hydride

Alkynes insertions into metal hydrides

Alkynes metalated

Alkynes metallation

Alkynes with metal hydrides

Metal alkynes

Metalation alkynes

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