Acylation metal-catalyzed

The insertion of unsaturated molecules into metal-carbon bonds is a critically important step in many transition-metal catalyzed organic transformations. The difference in insertion propensity of carbon-carbon and carbon-nitrogen multiple bonds can be attributed to the coordination characteristics of the respective molecules. The difficulty in achieving a to it isomerization may be the reason for the paucity of imine insertions. The synthesis of amides by the insertion of imines into palladium(II)-acyl bonds is the first direct observation of the insertion of imines into bonds between transition metals and carbon (see Scheme 7). The alternating copolymerization of imines with carbon monoxide (in which the insertion of the imine into palladium-acyl bonds would be the key step in the chain growth sequence), if successful, should constitute a new procedure for the synthesis of polypeptides (see Scheme 7).348... 

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

N-Acyl-a-amino acids are important compounds in both chemistry and biology. They are easily obtained in a transition metal-catalyzed, three-component domino reaction of an aldehyde, an amide, and CO. Whereas cobalt was mainly used for this process, Beller and coworkers [159] have recently shown that palladium has a... 

Whereas metal-catalyzed decomposition of simple diazoketones in the presence of ketene acetals yields dihydrofurans 121,124,134), cyclopropanes usually result from reaction with enol ethers, enol acetates and silyl enol ethers, just as with unactivated alkenes 13). l-Acyl-2-alkoxycyclopropanes were thus obtained by copper-catalyzed reactions between diazoacetone and enol ethers 79 105,135), enol acetates 79,135 and... 

When Wacker-type reactions are performed under a CO atmosphere, the (3-H elimination pathway can be suppressed in favor of CO insertion and subsequent nucleophilic cleavage of the acyl metal species.399 This alkoxycarbonylation process has found widespread utility, particularly in the synthesis of five- and six-membered oxacyclic natural products. For example, the THF core of tetronomycin was prepared by the Pd-catalyzed alkoxycarbonylation of 4-alkenol derivatives (Equations (117) and (118)), where stereocontrol was achieved by utilizing either the directing ability of a free hydroxyl or the conformational bias imposed by a bulky silyl ether.420 Additional examples making... 

Recently, two new P- and C-chiral monodentate phosphines 13 were reported. The ligands were applied in a number of transition metal-catalyzed reactions, though ee-values in the rhodium-catalyzed hydrogenation of N-acyl dehydrophenylalanine were only moderate [37]. 

The use of ethylene adduct lb is particularly important when the species added to activate catalyst la is incompatible with one of the reaction components. Iridium-catalyzed monoallylation of ammonia requires high concentrations of ammonia, but these conditions are not compatible with the additive [Ir(COD)Cl]2 because this complex reacts with ammonia [102]. Thus, a reaction between ammonia and ethyl ciimamyl carbonate catalyzed by ethylene adduct lb produces the monoallylation product in higher yield than the same reaction catalyzed by la and [Ir(COD)Cl]2 (Scheme 27). Ammonia reacts with a range of allylic carbonates in the presence of lb to form branched primary allylic amines in good yield and high enantioselectivity (Scheme 28). Quenching these reactions with acyl chlorides or anhydrides leads to a one-pot synthesis of branched allylic amides that are not yet directly accessible by metal-catalyzed allylation of amides. 

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]. 

DKR of secondary alcohol is achieved by coupling enzyme-catalyzed resolution with metal-catalyzed racemization. For efficient DKR, these catalyhc reactions must be compatible with each other. In the case of DKR of secondary alcohol with the lipase-ruthenium combinahon, the use of a proper acyl donor (required for enzymatic reaction) is parhcularly crucial because metal catalyst can react with the acyl donor or its deacylated form. Popular vinyl acetate is incompatible with all the ruthenium complexes, while isopropenyl acetate can be used with most monomeric ruthenium complexes. p-Chlorophenyl acetate (PCPA) is the best acyl donor for use with dimeric ruthenium complex 1. On the other hand, reaction temperature is another crucial factor. Many enzymes lose their activities at elevated temperatures. Thus, the racemizahon catalyst should show good catalytic efficiency at room temperature to be combined with these enzymes. One representative example is subtilisin. This enzyme rapidly loses catalytic activities at elevated temperatures and gradually even at ambient temperature. It therefore is compatible with the racemization catalysts 6-9, showing good activities at ambient temperature. In case the racemization catalyst requires an elevated temperature, CALB is the best counterpart. 

Scheme 2 Metal-catalyzed anionic ROP of lactide (a) deprotonation (b) acyl cleavage...

Many such activated acyl derivatives have been developed, and the field has been reviewed [7-9]. The most commonly used irreversible acyl donors are various types of vinyl esters. During the acylation of the enzyme, vinyl alcohols are liberated, which rapidly tautomerize to non-nucleophilic carbonyl compounds (Scheme 4.5). The acyl-enzyme then reacts with the racemic nucleophile (e.g., an alcohol or amine). Many vinyl esters and isopropenyl acetate are commercially available, and others can be made from vinyl and isopropenyl acetate by Lewis acid- or palladium-catalyzed reactions with acids [10-12] or from transition metal-catalyzed additions to acetylenes [13-15]. If ethoxyacetylene is used in such reactions, R1 in the resulting acyl donor will be OEt (Scheme 4.5), and hence the end product from the acyl donor leaving group will be the innocuous ethyl acetate [16]. Other frequently used acylation agents that act as more or less irreversible acyl donors are the easily prepared 2,2,2-trifluoro- and 2,2,2-trichloro-ethyl esters [17-23]. Less frequently used are oxime esters and cyanomethyl ester [7]. S-ethyl thioesters such as the thiooctanoate has also been used, and here the ethanethiol formed is allowed to evaporate to displace the equilibrium [24, 25]. Some anhydrides can also serve as irreversible acyl donors. 

The generation and addition of acyl- and alkoxycarbonylcarbenes to alkenes is usually carried out by using the corresponding diazo derivatives under catalysis of metallic compounds. The metal-catalyzed cyclopropanation reactions with diazo compounds are described in detail by McKervey in Chapter 11, hence dealt with rather briefly in this chapter. The readers who are interested in the preparation of acyl- and alkoxycarbonyl-substi-tuted cyclopropanes are requested to refer also to Chapter 9. 

Transition-metal catalyzed transfer of acylcarbenes to nitriles leads to 1,3-oxazoles via nitrile ylide intermediates123. The corresponding nitrile ylide chemistry derived from acyl(silyl)carbenes still awaits a closer look, but it has been shown that the rhodium-catalyzed decomposition of 198 in the presence of methyl cyanoformate and benzaldehyde provides 1,3-oxazole 221 (equation 71) exclusively120. This implies that the carbene moiety has been transferred only to the nitrile but not to the aldehyde. 

Lin, M.-H. RajaBabu, T. V. Metal-catalyzed acyl transfer reactions of enol esters role of Y5(OiPr)i30 and (thd)2Y(OiPr) as transesterification catalysts. Org. Lett. 2000, 2, 997-1000. 

The hydroacylation of olefins with aldehydes is one of the most promising transformations using a transition metal-catalyzed C-H bond activation process [1-4]. It is, furthermore, a potentially environmentally-friendly reaction because the resulting ketones are made from the whole atoms of reactants (aldehydes and olefins), i.e. it is atom-economic [5]. A key intermediate in hydroacylation is a acyl metal hydride generated from the oxidative addition of a transition metal into the C-H bond of the aldehyde. This intermediate can undergo the hydrometalation ofthe olefin followed by reductive elimination to give a ketone or the undesired decarbonyla-tion, driven by the stability of a metal carbonyl complex as outlined in Scheme 1. 

Theoretically, in a simple kinetic resolution the ee value should not exceed 32 % at this specific conversion. In addition to the rhodium complex, this reaction requires acetophenone as stoichiometric hydride acceptor, phenanthroline as coligand and potassium hydroxide as base. An ee value of 98 % at 60 % conversion (theoretical value 67 %)is achieved with [Rh2(OAc)4] without an added base after 3 days. Surprisingly, the enzyme tolerates potassium hydroxide in amounts up to 20 mol% at elevated temperatures however, the enantiomeric excesses are somewhat lower than those obtained in an ordinary kinetic resolution. Unselective, base- or metal-catalyzed acylation might be the reason for the somewhat lower ee value. 

This concept was also recently extended by Reetz et al. to the resolution of phenylethylamine [12]. In this case, an immobilised lipase and ethyl acetate as acyl donor are used the non-acylated (S)-enantiomer of the amine is racemized in situ by palladium on charcoal. After 8 days -the metal catalyzed racemization is again likely to he the rate-determining step - (/ )-A-acetyl-phenylethylamine is isolated in 64 % yield and 99 % enantiomeric excess. 

The method of combined enzyme- and transition metal-catalyzed reactions widely applied to the DKR of secondary alcohols has also been applied to the DKR of a-hydroxy acid esters rac-1. The principle is based on the enantioselective acylation catalyzed by Pseudomonas species lipase (PS-C from Amano Ltd) using p-Cl-phenyl acetate as an acyl donor in cyclohexane combined with in situ racemization of the non-acylated enantiomer catalyzed by ruthenium compounds [7]. Under these conditions, various a-hydroxy esters of type 1 were deracemized in moderate to good yields and high enantioselectivity (Scheme 13.2). 

A new metal-catalyzed exocyclic carbonylation of cycloimino esters 184 was reported. The reaction proceeded without HI in relatively mild conditions affording the expected N-acyl oxazolidin-2-ones 185 <03OL3955>. An improved procedure for the palladium catalyzed oxidative carbonylation of P-aminoalcohols to oxazolidin-2-ones was published <03JOC601>. 

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