Allyl carbonates palladium enolates

Derivatives of Y-hydroxyacetylenic acids, which are useful intermediates in the synthesis of butenolides, are prepared from propiolic acid and ester anions. Alk-2-ynoic and 2-allenic esters are prepared by the oxidation of 3,4-disubstituted 2-pyrazolin-5-ones with lead(iv) tetra-acetate in the absence and presence of BF3 respectively. Py-Unsaturated esters are produced in high yield by the palladium-catalysed decarboxylation-carbonylation of allylic carbonates. Magnesium enolates of esters react with nitriles to give (Z)-3-amino-alk-2-enoates. Enol lactones react with diethyl methoxycarbonylmethylphosphonate to give cyclic unsaturated keto-esters (Scheme 66). ... 

Silyl enol ethers are other ketone or aldehyde enolate equivalents and react with allyl carbonate to give allyl ketones or aldehydes 13,300. The transme-tallation of the 7r-allylpalladium methoxide, formed from allyl alkyl carbonate, with the silyl enol ether 464 forms the palladium enolate 465, which undergoes reductive elimination to afford the allyl ketone or aldehyde 466. For this reaction, neither fluoride anion nor a Lewis acid is necessary for the activation of silyl enol ethers. The reaction also proceed.s with metallic Pd supported on silica by a special method[301j. The ketene silyl acetal 467 derived from esters or lactones also reacts with allyl carbonates, affording allylated esters or lactones by using dppe as a ligand[302]... 

Silyl enol ethers and ketene acetals derived from ketones, aldehydes, esters and lactones are converted into the corresponding o/i-unsaturated derivatives on treatment with allyl carbonates in high yields in the catalytic presence of the palladium-bis(diphenylphosphino)ethane complex (32). A phosphinc-free catalyst gives higher selectivity in certain cases, such as those involving ketene acetals. Nitrile solvents, such as acetonitrile, are essential for success. 

Ceric ammonium nitrate promoted oxidative addition of silyl enol ethers to 1,3-butadiene affords 1 1 mixtures of 4-(/J-oxoalkyl)-substituted 3-nitroxy-l-butene and l-nitroxy-2-butene27. Palladium(0)-catalyzed alkylation of the nitroxy isomeric mixture takes place through a common ij3 palladium complex which undergoes nucleophilic attack almost exclusively at the less substituted allylic carbon. Thus, oxidative addition of the silyl enol ether of 1-indanone to 1,3-butadiene followed by palladium-catalyzed substitution with sodium dimethyl malonate afforded 42% of a 19 1 mixture of methyl ( )-2-(methoxycarbonyl)-6-(l-oxo-2-indanyl)-4-hexenoate (5) and methyl 2-(methoxycarbonyl)-4-(l-oxo-2-indanyl)-3-vinylbutanoate (6), respectively (equation 12). 

Palladium(0)-catalyzed a-allylation of silyl ethers is a reaction which can be carried out with ketones as well as with aldehydes91. It is highly regiospecific when applied to ketones. a-Allylations can also be performed with enol acetates using allyl carbonates in the presence of catalytic amounts of palladium(O) complexes and (tributyl)methoxytin92,93. The steric course of the reaction has not been studied systematically but a high level of diastereoselectivity is expected and possibilities for asymmetric induction by the use of chiral auxiliaries are envisaged. 

In 1999 Trost and Schroder reported on the first asymmetric allylic alkylation of nonstabilized ketone enolates of 2-substituted cyclohexanone derivatives, e.g. 2-methyl-1-tetralone (45), by using a catalytic amount of a chiral palladium complex formed from TT-allylpaUadium chloride dimer and the chiral cyclohexyldiamine derivative 47 (equation 14). The addition of tin chloride helped to soften the lithium enolate by transmetala-tion and a slight increase in enantioselectivity and yield for the alkylated product 46 was observed. Besides allyl acetate also linearly substituted or 1,3-dialkyl substituted allylic carbonates functioned well as electrophiles. A variety of cyclohexanones or cyclopen-tanones could be employed as nucleophiles with comparable results . Hon, Dai and coworkers reported comparable results for 45, using ferrocene-modified chiral ligands similar to 47. Their results were comparable to those obtained by Trost. 

Najera and coworkers introduced a new class of cyclic alanine templates (227, equation 59), the structure of which was anchored on Schollkopf s bislactim ether . Palladium-catalyzed allylations of the chiral pyrazinone derivative 227 with allylic carbonates (228) as substrates led to the formation of y,i5-unsaturated amino acids (229a-c) under very mild and neutral reaction conditions, whereas the required base for enolate preparation has been generated in situ from the allylic carbonate during jr-allyl complex formation. With this protocol in hand, the alkylated pyrazinones 229 were obtained with excellent regio- and diastereoselectivities (>98% ds). Finally, hydrolysis with 6 N aqueous HCl under relatively drastic conditions (150 °C) led to the free amino acids. 

Palladium(0)-catalyzed a-allylations of TMS enol ethers can be carried out cleanly with allylic carbonates. These reactions are highly regioselective, e.g. the mtne- and less-substituted TMS enol derivative of 2-methylcyclohexanone cf. Scheme 37) gave 2-allyl-2-methylcyclohexanone and 2-methyl-6-allylcyclohexanone, respectively. Allylations of aldehyde silyl enol ers occur similarly. Allylations of enol acetates occur with allyl carbonates in the presence of catalytic amounts of palla-dium(0) complexes and tri-n-butyltin methoxide. ... 

In chapter 19 you will meet palladium allyl cations as useful reagents and Evans and Robinson38 have combined the Pauson-Khand reaction with allyl cation complexes using rhodium as a compromise between Pd and Co. The enolate 132 combines with the Rh(I) cation complex from the allylic carbonate 133 to give the enyne 134 that gives the Pauson-Khand product 135 in 87% yield with the same catalyst but at higher temperatures. 

Ito and Sawamura showed that the use of rhodium and palladium in the presence of the TRAP-type ligand generates an effective catalyst combination for the reaction of an allyl carbonate with a cyanopropionamide [128]. The palladium-TRAP complex is proposed to generate a cationic Jt-allyl species. In addition, a rhodium-TRAP species complexes the cyano group of the nucleophile and induces formation of the enolate. Reaction of the enolate with the Tt-complex in assembly I generates the observed product. Scheme 45. The notion that enoliza-tion is caused by complexation to the cyano group is based on previous results in the enantioselective rhodium-catalyzed Michael addition. 

Allyl y3-keto carboxylates 563 undergo facile Pd-catalyzed decarboxylation to form either jr-allylpalladium enolates 565 or a-palladaketone 564. Also rr-allyl-palladium enolates are generated from enol carbonates 566. As summarized below, several transformations to afford 567-573 are possible under different but proper conditions depending on the substituents R [199]. In addition to allyl j6-keto carboxylates, other allyl esters such as allyl malonates, cyanoacetates and nitroacetates undergo similar transformations. With these Pd-catalyzed reactions, a new generation of j6-keto esters and malonate chemistry has been developed. 

Nucleophilic attack on the central allyl carbon atom of (T) -allyl) palladium and platinum compounds was employed in the synthesis of cyclopropanes from allylic electrophiles and silyl enolates. Treatment with base of (r) -allyl) palladium and platinum complexes bearing a methoxymethoxy group at the 2-position afforded the corresponding oxodimethylenemethane complexes in contrast to the formation of 2-hydroxysubstitutcd-(Ti5-allyl) complexes which was observed under acidic conditions. 

Trost and Runge announced in 1981 the use of simple allyl carbonates and described even earlier the Pd(0)-catalyzed rearrangement of allyl ethers of enolic /3-ketoesters into the isomeric a-allyl-/S-ketoesters (Scheme 5). Allyl ethers of enolic /3-ketoesters are vinylogous of allyl carbonates In 1980 Tsuji and co-workers described a similar rearrangement,declaring that the palladium-catalyzed rearrangement of a similar cyclic ether has been presented in a lecture by B.M. Trost at the 1st Intern. Kyoto Conf. Org. Chem., Dec., 6,1979. ... 

Transition Metal-Catalyzed Allylie Alkylation. Chelated amino acid ester enolates were found to be suitable nucleophiles for palladium-catalyzed allylie alkylations (eq 25). They were conveniently prepared by deprotonation of a glycine derivative with LHMDS followed by transmetallation with zinc chloride. The palladium-catalyzed allylie alkylation then takes place in the presence of allyl carbonates to produce the desired anti amino acid derivative. ... 

The role of palladium in organic synthesis continues to be explored and exploited. Enol stannanes are monoalkylated by allylic acetates in the presence of tetrakis(triphenylphosphine)palladium, Enol stannanes give higher selectivity for monoalkylation than enolate ions or silyl enol ethers. High regioselec-tivity is observed for alkylation at the less substituted end of the allyl moiety. Olefins, after complexation to palladium(ll), alkylate enolate anions. The organopalladium product may be converted into saturated ketones, or into enones by /3-elimination, or acylated with carbon monoxide (Scheme... 

A complementary functional cyclopropane assembly relies on the utilization of the Tsuji-Trost reaction [101], A highly enantio and diastereoselective cou-pling/cyclopropanation sequence of acyclic amides 85 with allyl carbonates 86 is illustrated in Scheme 5.30 [102], In this reaction, a scarcely described addition of the nucleophilic enolate intermediate onto the central carbon of the i-allyl palladium is involved, which affords the corresponding cyclopropane. 

The proposed catalytic cycle [42] is analogous to that shown in Scheme 5.6, except for the additional release of an enolate anion due to the fluoride-induced desilylation. Oxidative addition of allyl carbonates leads to the formation of the allyl complex 78, COj, and an alkoxide RO . The fluoride source and the alkoxide RO are capable of liberating an enolate anion by desilylation. This explains why substoichiometric amounts of Bu4NPhgSiF2 are sufficient to maintain the catalytic cycle that is displayed in Scheme 5.25 for the allylation of 2-methylcyclohexanone through the silyl enol ether. The carbon-carbon bond-forming step is assumed to occur by a collapse of the ion pair 79 consisting of the cationic allylpalladium complex and the enolate anion. Aside from these ionic species, covalently bound palladium enolates were also discussed. 

Tsuji J, Minami I, Shimizu I. Palladium-catalyzed aUyla-tion of ketones and aldehydes with allylic carbonates via silyl enol ethers under neutral conditions. Chem. Lett. 1983 1325-1326. 

Allenes also react with aryl and alkenyl halides, or triflates, and the 7r-allyl-palladium intermediates are trapped with carbon nucleophiles. The formation of 283 with malonate is an example[186]. The steroid skeleton 287 has been constructed by two-step reactions of allene with the enol trillate 284, followed by trapping with 2-methyl-l,3-cyclopentanedione (285) to give 286[187]. The inter- and intramolecular reactions of dimethyl 2,3-butenylmalonate (288) with iodobenzene afford the 3-cyclopentenedicarboxylate 289 as a main product) 188]. 

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