Allyl enol carbonates palladium-catalyzed

Palladium-Catalyzed Asymmetric Tsuji Allylation of Allyl Enol Carbonates... 

The use of palladium(II) 7i-allyl complexes in organic chemistry has a rich history. These complexes were the first examples of a C-M bond to be used as an electrophile [1-3]. At the dawn of the era of asymmetric catalysis, the use of chiral phosphines in palladium-catalyzed allylic alkylation reactions provided key early successes in asymmetric C-C bond formation that were an important validation of the usefulness of the field [4]. No researchers were more important to these innovations than Prof. B.M. Trost and Prof. J. Tsuji [5-10]. While most of the early discoveries in this field provided access to tertiary (3°) stereocenters formed on a prochiral electrophile [Eq. (1)] (Scheme 1), our interest focused on making quaternary (4°) stereocenters on prochiral enolates [Eq. (2)]. Recently, we have described decarboxylative asymmetric allylic alkylation reactions involving prochiral enolates that provide access to enantioenriched ot-quatemary carbonyl compounds [11-13]. We found that a range of substrates (e.g., allyl enol carbonates,... 

Allyl /3-keto carboxylates and allyl enol carbonates undergo a palladium-catalyzed decarboxylation-dehydrogenation to yield o, S-unsaturated ketones in usually high chemical yield and with good selectivity. Following this approach, it was possible to obtain 2-methyl-2-cyclopentenone in two steps from diallyl adipate in a procedure that could be convenient for large-scale preparations (eq 62). ... 

Scheme 5.30 Mechanistic pathways in the palladium-catalyzed decarboxylative allylic alkylation, starting from allyl p-keto esters 91 or allyl enol carbonates 92.

Tsuji J, Minami I, Shimizu I. Palladium-catalyzed allylation of ketones and aldehydes via allyl enol carbonates. Tetrahedron Lett. 1983 24 1793-1796. 

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). 

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. 

This methodology has been used to provide efficient protocols for the asymmetric allylic alkylation of p-keto esters, ketone enolates, barbituric acid derivatives, and nitroalkanes. Several natural products and analogs have been accessed using asymmetric desymmetrization of substrates with carbon nucleophiles. The palladium-catalyzed reaction of a dibenzoate with a sulfonylsuccinimide gave an advanced intermediate in the synthesis of L-showdomycin (eq 3). ... 

The research group of Muzart and Henin studied extensively the palladium-catalyzed EDP of allyl- or benzyl-carboxylated compounds. Mainly two types of substrates, prochiral enol carbonates A and racemic (3-keto esters B, were used to afford enols C as transient species [25]. In the presence of a chiral proton source, asymmetric protonation/tautomerization of enols led to enantioenriched ketones D... 

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. 

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. ... 

Scheme 5.44 Enantioselective formation of butenolides 133 by palladium-catalyzed decarboxylative allylic alkylation of furan-derived enol carbonate 132.

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. 

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. ... 

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. 

The first gold-catalyzed addition reactions of carbon nucleophiles to allenes were only first disclosed in 2006, and the number of examples is still small. Toste and co-workers showed that allenic silyl enol ethers undergo a 5-endo- ng cyclization to hexahydroindenone derivatives in the presence of a cationic gold catalyst (Scheme 4-10). In these transformations, water or methanol is used as an external proton source for protodeauration of an intermediate vinylgold species. In an analogous manner, cyclopentenes were obtained in good yields from allenic P-ketoesters. In the presence of a palladium catalyst and an allylic halide, these substrates afford functionalized 2,3-dihydroflirans. 

---