Mechanism, metal catalyzed aldehydes

The reactions of aldehydes at 313 K [69] or 323 K [70] in CoAlPO-5 in the presence of oxygen results in formation of an oxidant capable of converting olefins to epoxides and ketones to lactones (Fig. 23). This reaction is a zeolite-catalyzed variant of metal [71-73] and non-metal-catalyzed oxidations [73,74], which utilize a sacrificial aldehyde. Jarboe and Beak [75] have suggested that these reactions proceed via the intermediacy of an acyl radical that is converted either to an acyl peroxy radical or peroxy acid which acts as the oxygen-transfer agent. Although the detailed intrazeolite mechanism has not been elucidated a similar type IIaRH reaction is likely to be operative in the interior of the redox catalysts. The catalytically active sites have been demonstrated to be framework-substituted Co° or Mn ions [70]. In addition, a sufficient pore size to allow access to these centers by the aldehyde is required for oxidation [70]. 

The mechanisms for metal-catalyzed and organocatalyzed direct aldol addition reactions differ one from another, and resemble the mode of action of the type 11 and type I aldolases, respectively. Some metal-ligand complexes, for example, 1-4 and 9 are considered to have a bifunctional character [22], embodying within the same molecular frame a Lewis acidic site and a Bronsted basic site. Whereas base would be required to form the transient enolate species as an active form of the carbonyl donor, the Lewis acid site would coordinate the acceptor aldehyde carbonyl, increasing its electrophilicity. By this means, both transition state stabilization and substrates preorganization would be provided (see Scheme 5 for a proposal). 

The importance of the electrophilic character of the cation in organo-alkali compounds has been discussed by Morton (793,194) for a variety of reactions. Roha (195) reviewed the polymerization of diolefins with emphasis on the electrophilic metal component of the catalyst. In essence, this review willattempt to treat coordination polymerization with a wide variety of organometallic catalysts in a similar manner irrespective of the initiation and propagation mechanisms. The discussion will be restricted to the polymerization of olefins, vinyl monomers and diolefins, although it is evident that coordinated anionic and cationic mechanisms apply equally well to alkyl metal catalyzed polymerizations of polar monomers such as aldehydes and ketones. 

Yet another transition metal-catalyzed route to indoles involves the use of aminoalcohol precursors, for instance 94, which could be efficiently converted to 6-chloroindole 95 (Equation 24). A plausible mechanism seems to feature an oxidation of the alcohol to an aldehyde functionality, which undergoes intramolecular condensation with the amino group <20020L2691>. 

It is known that insertion of carbon monoxide to form an acyl complex is reversible, in which results depend on the pressure of carbon monoxide and temperature. If the above-mentioned mechanisms are correct, then acyl halides and aldehydes should be decarbonylated to form olefins provided that an acyl-palladium bond is formed by the oxidative addition of acyl halides or aldehydes to metallic palladium. This proved to be the case. When acyl halide was heated with a catalytic amount of metallic palladium or palladium chloride at 200°C. in a distilling flask, carbon monoxide and hydrogen halide were evolved rapidly, and olefin was collected in a good yield. This reaction is a new and useful preparative method of olefins. In the same way, aldehydes can be decarbonylated smoothly, but in this case, both olefin and the corresponding paraffin Were obtained. The latter probably arises by the hydrogenation of the olefin. Decarbonylation of certain aldehydes has been reported by several workers (3, 6), but no reasonable mechanism has been known. The mechanism of the palladium-catalyzed aldehyde formation discussed above gives clear explanation for the palladium catalyzed decarbonylation of aldehydes. 

Intramolecular transition metal-catalyzed hydro acylation reactions have opened up a new area of synthesizing cyclic ketones. This reaction can also be extended to intermolecular addition reactions. Miller et al. found the first example of an intermolecular hydroacylation of an aldehyde with an olefin giving ketones, when they were studying the mechanism of the rhodium-catalyzed intramolecular cyclization of 4-pentenal using ethylene-saturated chloroform as the solvent (Eqs.46,50) [112]. 

Advances in the development of metal-catalyzed Mukaiyama aldol addition reactions have primarily relied on a mechanistic construct in which the role of the Lewis acidic metal complex is to activate the electrophilic partner towards addition by the enol silane. Alternate mechanisms that rely on metallation of enol silane to generate reactive enolates also serve as an important construct for the design of new catalytic aldol addition processes. In pioneering studies, Bergman and Heathcock documented that transition-metal enolates add to aldehydes and that the resulting metallated adducts undergo silylation by the enol silane leading to catalyst turnover. 

The mechanism for these metal-catalyzed, 02 promoted oxidations was proposed to be metal-catalyzed 02 oxidation of the aldehyde 55 to form a peroxy radical species 56 which adds to ketone 60 to provide a radical variant of a Criegee intermediate, 57. This intermediate would then extract a proton from the aldehyde 55 to provide the normal Criegee intermediate 58. Alternatively, the generated peroxy radical intermediate 56 abstracts a proton from an aldehyde molecule providing peracid 59 which attacks the ketone 60 to provide the Criegee intermediate 58. 

Two possible mechanisms were proposed. One is transition metal-catalyzed hetero-Pauson-Khand process. Another pathway was shown in Scheme 7.6, which was initiated by the oxidative addition of an aldehyde C-H bond to ruthenium. 

Epoxidation reactions have been widely utilized for over 100 years with peradds, peroxides and, more recently, metal catalysts [7]. However, direct metal-catalyzed aerobic epoxidations are rare and generally require an aldehyde coreductant. In this case, the metal is proposed to catalyze radical formation (A-C, Scheme 5.2) followed by O2 insertion to form acyl peroxide D. Metal-catalyzed aerobic oxidation of aldehydes to peradds has previously been observed [8]. With the formation of species D, either an outer-sphere path similar to a peracid-type oxidation occurs (Path 1) or an inner-sphere metal-catalyzed oxidation in which the metal-based oxidant and substrate interact during oxygen transfer (Path 2 or 3). Mu-kaiyama and coworkers were the first to report an aerobic epoxidation of olefins catalyzed by transition metals using either a primary alcohol or an aldehyde as coreductants [9]. The role of the metal was probed through parallel studies of peracid and metal-catalyzed epoxidations of 2 which yielded different stereochemical outcomes. Therefore, a metal-centered mechanism for olefin epoxidation was proposed which implicates an oxygenase system. Path 2 or 3 (Table 5.1) [10]. 

The mechanism of [3 + 2] reductive cycloadditions clearly is more complex than other aldehyde/alkyne couplings since additional bonds are formed in the process. The catalytic reductive [3 + 2] cycloaddition process likely proceeds via the intermediacy of metallacycle 29, followed by enolate protonation to afford vinyl nickel species 30, alkenyl addition to the aldehyde to afford nickel alkoxide 31, and reduction of the Ni(II) alkoxide 31 back to the catalytically active Ni(0) species by Et3B (Scheme 23). In an intramolecular case, metallacycle 29 was isolated, fully characterized, and illustrated to undergo [3 + 2] reductive cycloaddition upon exposure to methanol [45]. Related pathways have recently been described involving cobalt-catalyzed reductive cyclo additions of enones and allenes [46], suggesting that this novel mechanism may be general for a variety of metals and substrate combinations. 

The cyclic mechanism of free radicals generation in oxidation of aldehydes catalyzed by transition metal ions was demonstrated C. Bawn and J. Williamson [65]... 

Judging from these findings, the mechanism of Lewis acid catalysis in water (for example, aldol reactions of aldehydes with silyl enol ethers) can be assumed to be as follows. When metal compounds are added to water, the metals dissodate and hydration occurs immediatdy. At this stage, the intramolecular and intermolecular exchange reactions of water molecules frequently occur. If an aldehyde exists in the system, there is a chance that it will coordinate to the metal cations instead of the water molecules and the aldehyde is then activated. A silyl enol ether attacks this adivated aldehyde to produce the aldol adduct. According to this mechanism, it is expected that many Lewis acid-catalyzed reactions should be successful in aqueous solutions. Although the precise activity as Lewis acids in aqueous media cannot be predicted quantitatively... 

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