Alkenes synthesis from halides

The stereodefined alkenyl halides are of prime importance due to the recent developments of di- or trisubstituted alkene synthesis by cross-coupling reactions between organometallics and alkenyl halides catalyzed by transition metal compounds 171). These alkenyl halides can be conveniently obtained from alkenylboranes or alkeneboronic acids. B-Alk enylcatecholboranes undergo rapid hydrolysis when stirred with excess water at 25 °C (Eq. 109)102). The alkeneboronic acids are usually crystalline solids of low solubility in water and can be easily isolated and handled in air without significant deterioration. 

Cor jug ated dienes are generally pri pared by the methods previously discus for alkene synthesis. The base-induced eiiminaticn vf HK from an allylic halide is one such reaction. 

The common metathesis reactions for the preparation of metallocenes, treating a metal salt MX2 with NaCp, are hampered in the case of ruthenium by the lack of suitable Ru salts. (Rul2 is commercially available, but is still not commonly used in the synthesis of rathenocene.) Thus, ruthenocene has been obtained from Ru(acac)3 and NaCp in very low yield and later from RuCb and NaCp in 50-60% yield. It has now become apparent that alkene polymers, in particular [Ru(nbd)Cl2]x, but also [Ru(cod)Cl2]x and hydrazine derivatives (Section 3.1), can serve as Ru precursors. Equally successful in many cases is reductive complexation of cyclopentadiene in ethanol in the presence of Zn (Section 3.2), which furnishes the metallocene in about 80% yield. Decamethylruthenocene (82) was first obtained by the Zn reduction route in 20% yield, but can now be prepared conveniently from halide complexes [Cp RuCl2]2 or [Cp RuCl]4, a common method for the preparation of symmetrical and unsymmetrical sandwich compounds of ruthenium featuring one alkyl-substituted ligand. 

Another interesting alkene synthesis starts from a cyclopropyl-substituted alcohol, which, on reaction with magnesium halides, is converted to a haloalkene MesSiX has also been used in this type of reaction (Scheme 22). ... 

One other experimental result from the Corey et al. study is important for trisubstituted alkene synthesis. When 55=58 is quenched with formaldehyde, the stereochemistry of C—C bond formation remains the same as before. However, the regiochemistry of the elimination step no longer favors the second aldehyde added, and the major product is now the allylic alcohol 64 (54). This experiment suggests that both oxaphosphetanes 63 and 62 are in equilibrium with the lithium halide adduct 61a. Decomposition is controlled by the nature and degree of oxaphosphetane substitution as well as by stereochemistry. In the formaldehyde reaction, these factors combine to favor the trisubstituted alkene (via 62) over the disubstituted alkene that would be formed via 63 (R"=H). Several examples of trisubstituted alkene synthesis using Corey s method are summarized in Table 10 without further comment because the origins of stereochemistry are not understood in detail, but Corey s model 58 is consistent with the available evidence. 

Coupling reactions of alkyl boranes, formed by hydroboration of alkenes, with unsaturated halides (or triflates or phosphonates) is possible, and this reaction is finding increasing use in synthesis. For example, coupling of the alkyl borane derived from hydroboration (with 9-borobicyclo[3.3.1]nonane, 9-BBN) of the alkene 200 with the alkenyl iodide 201 gave the substituted cyclopentene 202, used in a synthesis of prostaglandin Ei (1.205). This type of B-alkyl Suzuki coupling reaction is very useful for the synthesis of substituted alkenes. 

This reaction was first reported by Nenitzescu in 1931. It is the formation of an a,p-unsaturated ketone directly by aluminum chloride-promoted acylation of alkenes with acyl halides. Therefore, it is known as the Darzens-Nenitzescu reaction (or Nenitzescu reductive acylation), or Nenitzescu acylation. Under such reaction conditions, Nenitzescu prepared 2-butenyl methyl ketone from acetyl chloride and 1-butene and dimethylacetylcyclohex-ene from acetyl chloride and cyclooctene. However, in the presence of benzene or hexane, the saturated ketones are often resolved, as supported by the preparation of 4-phenyl cyclohexyl methyl ketone from the reaction of cyclohexene and acetyl chloride in benzene, and the synthesis of 3- or 4-methylcyclohexyl methyl ketone by refluxing the mixture of cycloheptene and acetyl chloride in cyclohexane or isopentane. This is probably caused by the intermolecular hydrogen transfer from the solvent. In addition, owing to its intrinsic strain, cyclopropyl group reacts in a manner similar to an oleflnic functionality so that it can be readily acylated. It should be pointed out that under various reaction conditions, the Darzens-Nenitzescu reaction is often complicated by the formation of -halo ketones, 3,)/-enones, or /3-acyloxy ketones. This complication can be overcome by an aluminum chloride-promoted acylation with vinyl mercuric chloride, resulting in a high purity of stereochemistry. ... 

Two reactions which should lend themselves to the synthesis of pyrrolidines with a wide variety of substituents have been published this year. In the first method Blake and co-workers have improved the versatility of a well known pyrrolidine synthesis from arylcyclopropyl ketones. By reaction with formamide in the presence of MgCl2, alkylcyclopropyl ketones (105) give good yields of pyrrolidines (106). The second method details the reaction of Schiff s bases of trimethylsilyl methylamine with acyl halides in the presence of alkenes and... 

Primary alkylamines are also available from alkenes in a simple two-step phosphoramidomercuration-demercuration sequence, and from halides via iV-carbalkoxyphosphoromonoamidates in an alternative to the Gabriel synthesis. The synthesis of N-labelled primary alkylamines has also been described. ... 

An improved procedure for the synthesis of alkyl iodides from organoboranes (and thus ultimately from alkenes), involving the use of iodine and sodium meth-oxide, has been shown to entail inversion at the C-B bond, and is employed in a new route to optically active iodides from alkenes e.g. Scheme 30). A similar sequence for conversion of alkenes to primary halides via hydrozirconation (Scheme 31) has been reviewed. Zr also catalyses addition of lithium aluminium hydride to alkenes under mild conditions, and the alanes so formed can likewise be converted into alkyl halides (Scheme 32). 

In some cases where a reaction involving a radical species occurred within cobalt porphyrin complexes, it has been possible to trap transient cobalt porphyrin hydride species. This was indeed observed during the synthesis of organocobalt porphyrin that resulted from the reaction of cobalt(n) porphyrin and dialkylcyanomethylradicals with alkenes, alkynes, alkyl halides, and epoxide. A transient hydride porphyrin complex was also involved in the cobalt porphyrin-catalyzed chain transfer in the free-radical polymerization of methacrylate. The catalytic chain transfer in free-radical polymerizations using cobalt porphyrin systems has been extensively investigated and will not be treated in this section. Gridnev and Ittel have published a comprehensive overview of the catalytic chain transfer in free-radical polymerizations. ... 

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