Alkyl halides carbon monoxide addition

Carbon monoxide rapidly inserts into the carbon—zirconium bond of alkyl- and alkenyl-zirconocene chlorides at low temperature with retention of configuration at carbon to give acylzirconocene chlorides 17 (Scheme 3.5). Acylzirconocene chlorides have found utility in synthesis, as described elsewhere in this volume [17]. Lewis acid catalyzed additions to enones, aldehydes, and imines, yielding a-keto allylic alcohols, a-hydroxy ketones, and a-amino ketones, respectively [18], and palladium-catalyzed addition to alkyl/aryl halides and a,[5-ynones [19] are examples. The acyl complex 18 formed by the insertion of carbon monoxide into dialkyl, alkylaryl, or diaryl zirconocenes may rearrange to a r 2-ketone complex 19 either thermally (particularly when R1 = R2 = Ph) or on addition of a Lewis acid [5,20,21]. The rearrangement proceeds through the less stable... 

Carbonylation procedures have been successfully used for C—C bond forming radical strategies. Alkyl halides could be carbonylated under moderate pressure of CO (15-30 atm) in benzene at 80 °C in the presence of (TMS)3SiH and AIBN [16], Reaction (7.8) shows the effect of the CO pressure on the carbonylation of a primary alkyl bromide. These radical chain reactions proceed by the addition of an alkyl radical onto carbon monoxide, which generates... 

Carbon monoxide will also add to aromatic coumpounds such as benzene and toluene. As the product of such an addition is an aldehyde and as aromatic aldehydes readily polymerize under the conditions necessary for the addition of carbon monoxide, the simple addition product is not obtained. These reactions have been performed in the author s laboratory using a technique similar to the addition to alcohols and alkyl halides. The products obtained are the same shellac-like resins that are obtained by treating the theoretically expected aldehyde with hydrogen fluoride under the same conditions. 

Other additions, such as addition of alkyl halides and carbonyl compounds, are discussed in Chapter 5, whereas Chapter 7 covers addition reactions involving carbon monoxide (hydroformylation, carboxylations). Hydrogen addition is discussed in Chapter 11. The nucleophilic addition of organometallics to multiple bonds is of great significance in the anionic polymerization of alkenes and dienes and is treated in Chapter 13. 

Addition of nucleophiles to a carbon monoxide ligand of pentacarbonyliron provides anionic acyliron intermediates which can be trapped by electrophiles (H+ or R—X) to furnish aldehydes or ketones [18]. However, carbonyl insertion into alkyl halides using iron carbonyl complexes is more efficiently achieved with disodium tetracarbonylferrate (Collman s reagent) and provides unsymmetrical ketones (Scheme 1.2) [19, 20]. Collman s reagent is extremely sensitive towards air and moisture, but offers a great synthetic potential as carbonyl transfer reagent. It can be prepared by an in situ procedure starting from Fe(CO)5 and Na-naphthalene [20]. 

An unusual synthesis of acyldienes from conjugated dienes, carbon monoxide, and alkyl or acyl halides using cobalt carbonylate anion as a catalyst should be mentioned here (57). The reaction apparently involves the addition of an acylcobalt carbonyl to a conjugated diene to produce a l-acylmethyl-7r-allylcobalt tricarbonyl, followed by elimination of cobalt hydrocarbonyl in the presence of base. The reaction can thus be made catalytic. Since the reaction was discussed in detail in the recent review by Heck (59), it will not be pursued further here. 

The reaction sequence in the vinylation of aromatic halides and vinyl halides, i.e. the Heck reaction, is oxidative addition of the alkyl halide to a zerovalent palladium complex, then insertion of an alkene and completed by /3-hydride elimination and HX elimination. Initially though, C-H activation of a C-H alkene bond had also been taken into consideration. Although the Heck reaction reduces the formation of salt by-products by half compared with cross-coupling reactions, salts are still formed in stoichiometric amounts. Further reduction of salt production by a proper choice of aryl precursors has been reported (Chapter III.2.1) [1]. In these examples aromatic carboxylic anhydrides were used instead of halides and the co-produced acid can be recycled and one molecule of carbon monoxide is sacrificed. Catalytic activation of aromatic C-H bonds and subsequent insertion of alkenes leads to new C-C bond formation without production of halide salt byproducts, as shown in Scheme 1. When the hydroarylation reaction is performed with alkynes one obtains arylalkenes, the products of the Heck reaction, which now are synthesized without the co-production of salts. No reoxidation of the metal is required, because palladium(II) is regenerated. 

RX RCHO. Alkyl halides can be converted directly into aldehydes in moderate to high yield by reaction with carbon monoxide (1-3 atm.) and tri-n-butyltin hydride catalyzed by the palladium(O) complex. The reaction involves insertion of carbon monoxide to form an acyl halide, which is known to be reduced to an aldehyde under these conditions (10, 411). Direct reduction of the halide can be minimized by slow addition of the tin hydride to the reaction and by an increase in the carbon monoxide pressure. 

Vanhoye and coworkers [402] synthesized aldehydes by using the electrogenerated radical anion of iron pentacarbonyl to reduce iodoethane and benzyl bromide in the presence of carbon monoxide. Esters can be prepared catalytically from alkyl halides and alcohols in the presence of iron pentacarbonyl [403]. Yoshida and coworkers reduced mixtures of organic halides and iron pentacarbonyl and then introduced an electrophile to obtain carbonyl compounds [404] and converted alkyl halides into aldehydes by using iron pentacarbonyl as a catalyst [405,406]. Finally, a review by Torii [407] provides references to additional papers that deal with catalytic processes involving complexes of nickel, cobalt, iron, palladium, rhodium, platinum, chromium, molybdenum, tungsten, manganese, rhenium, tin, lead, zinc, mercury, and titanium. 

M(C0)5X , and H2 in the presence of a general base to provide anionic metal hydrides. This process was shown to be first-order in both metal complex and dihydrogen and was not inhibited by addition of carbon monoxide. Consistent with the rds in catalysis being formation of the metal hydride intermediate, the metal catalyzed reaction of RX/CO2/H2 to provide HCOOR is not inhibited by CO. The well-established formation of metalloformate, M(C0)s02CH", from M(C0)5H and CO2 is followed by a less facile process involving the reaction of the metalloformate with RX. This latter reaction is first-order in both metal complex and alkyl halide and is inhibited by carbon monoxide. 

Ryu and Sonoda have extended this strategy to free radical carbonylation reactions [27], Under low pressures of carbon monoxide an alkyl halide, such as iodooctane (37) reacts with ally Itri butyl tin to form a y ,y-unsaturated ketone. The initially formed alkyl radical undergoes addition to carbon monoxide to form an acyl radical. This acyl radical then adds to allyltributyltin to form the final product (Scheme 7). It is interesting to note that the slow rate of addition of an alkyl radical to allyltributyltin allows this reaction to be carried out with much lower carbon monoxide pressures than are necessary with analogous reactions using tin hydrides as radical mediators. Ryu and Sonoda have also taken this reaction one step further, successfully intercepting the acyl radical with an electron-deficient alkene before addition to allyltributyltin [28]. In these reactions, three carbon-carbon bonds are formed and the products are y9-functionalized, (5, -unsaturated ketones. 

Another important process involving oxidative addition of alkyl halide is found in the hrst step in Monsanto process, an important industrial process to convert methanol into acetic acid with carbon monoxide at 150-200°C under 30-60 bar of CO [24]. In the Monsanto process methyl iodide is formed first from methanol and hydrogen iodide, added as the promoter to the catalytic system. The subsequent reaction of methyl iodide with a rhodium(I) species forms a methylrhodium(III) iodide species hy oxidative addition (Eq. 1.2). 

The reactions of benzylic halides with carbon monoxide and alcohols form esters in good yields. However, the reactions of alkyl halides are more limited for two reasons. First, the oxidative addition of alkyl halides occurs less readily to palladium complexes than the oxidative addition of aryl halides. This difference was noted in Chapter 7. Second, the intermediate alkylpalladium halide can undergo P-hydrogen elimination. As noted in Chapters 9 and 10, these hurdles have been overcome in some cases, and cross-coupling... 

Halide-promoted heterogeneous catalysts for carbonylation are analogues to homogeneous carbonylation catalyzed by metal carbonyls, that is, the Reppe reaction (11). The first step of the Reppe reaction involves the oxidative addition of alkyl halide promoter to carbonyl metal, for example, Rh(I) complex (Fig. 1). This step is followed by methyl migration, bonding of carbon monoxide to give a coor-dinatively saturated Rh(III) complex, and subsequent decomposition of this complex in the presence of methanol to yield a carbonylated product and regenerate the promoter and the catalyst. 

The reactions of some optically active benzyl halides have provided further evidence for the nucleophilicities of the [Pd(PR3)j] species (Lau et al., 1974, 1976 Wong et al., 1974 Stille and Lau, 1976). The salient features of these studies are summarised in Scheme 6. The stereochemistry of the addition is established by reaction of the palladium-alkyl complex with carbon monoxide (this insertion is known to take place by an intramolecular migration process, with retention of configuration in the migrating alkyl group), and subsequent formation of an ester from this acyl complex. 

Condensation of aldehyde with amide affords 14, whose hydroxyl group is converted into halide 15. Oxidative addition of alkyl halide 15 to Pd(0) gives (x-alkylpalladium complex 16. Then insertion of carbon monoxide into the carbon-Pd bond of 16 gives 17, and then nucleophilic attack by H2O occurs to give amino acid derivative 13. 

In spite of the usefulness of these complexes, it is generally not possible to cause the satisfactory reaction with transition metals in the metallic state [86] under mild conditions due to their poor reactivity. We have reported that activated metallic nickel, prepared by the reduction of nickel halide with lithium, underwent oxidative addition of benzylic halides to give homocoupled products [45]. We reported that carbonylation of the oxidative adducts of benzylic halides to the nickel proceeded smoothly to afford symmetrical 1,3-diarylpro-pan-2-ones in moderate yields, in which the carbonyl groups of alkyl oxalyl chlorides served as a source of carbon monoxide [43] see Equation 7.5. 

One possible mechanistic sequence for the present reaction is shown in Scheme 7.1. The carbon monoxide insertion into the carbon-metal o bond of alkyltransition metal complexes is well known [88]. Thus, the oxidative addition of benzyl halide to metallic nickel gives benzylnickel (II) halide 4, and the insertion of carbon monoxide, which is formed by decarbonylation of alkyl oxalyl chloride into the benzyl-nickel bond of complex 4, would afford arylacetyl (II) complex 5. The metathesis of complexes 4 and 5 seems to give (arylacetyl)benzylnickel complex 6, which undergoes reductive elimination to yield l,3-diarylpropan-2-one, 3. The formation of 1,2-diarylethane may be explained by the reductive elimination of bisbenzylnickel complex 7 formed by metathesis of benzylnickel complex 4 [89]. It is also possible that the reaction of benzyl halide with complex 4 or 5 gives homocoupled product or ketone, respectively. 

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