Alkynes with metal hydrides

All of the proposed mechanisms for the reduction of alkynes with metal hydride-transition metal halide combinations involve an initial hydrometallation of the ir-system by the transition metal hydride, formed by the reaction of the original metal hydride with the transition metal halide, to form the vi-nylmetallic intermediate (99 equation 38). For the reduction of alkenes, similar alkylmetallic intermediates are implied to be formed. In the case of the reduction of alkenes with NaBH4 in the presence of Co" in alcohol solution, the hydrometallation reaction appears to be reversible as evidenced by the incorporation of an excess of deuterium when NaBD4 was used in the reduction. ... 

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. 

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. 

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. 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

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. 

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. 

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. 

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