Enolates, palladium® acetate

Diazonium salts react with oximes to give aryl oximes, which are easily hydrolyzed to aldehydes (R = H) or ketones." A copper sulfate-sodium sulfite catalyst is essential. In most cases higher yields (40-60%) are obtained when the reaction is used for aldehydes than for ketones. In another method for achieving the conversion ArNj —> ArCOR, diazonium salts are treated with R4Sn and CO with palladium acetate as catalyst. In a different kind of reaction, silyl enol ethers of aryl ketones, Ar C(OSiMe3)=CHR, react with sohd diazonium fluoroborates, ArNj BF4, to give ketones, ArCHRCOAr. " This is, in effect, an arylation of the aryl ketone. 

An interestingly short total synthesis of quadrone was developed by Kende and coworkers who made application of Pd(II)-mediated cycloalkenylation of silyl enol ethers (Scheme LV) Their point of departure was 609 which was converted directly to 610, Reaction of this silyl enol ether with palladium acetate in acetonitrile gave predominantly 6JI which could be cyclized to 612. From this intermediate, it was possible to prepare the known keto acid. 

Enantioselective deprotonation can also be successfully extended to 4,4-disubstituted cyclohexanones. 4-Methyl-4-phenylcyclohexanone (3) gives, upon reaction with various chiral lithium amides in THF under internal quenching with chlorotrimethylsilane, the silyl enol ether 4 having a quaternary stereogenic carbon atom. Not surprisingly, enantioselectivities are lower than in the case of 4-tm-butylcyclohexanone. Oxidation of 4 with palladium acetate furnishes the a./i-unsaturated ketone 5 whose ee value can be determined by HPLC using the chiral column Chiralcel OJ (Diacel Chemical Industries, Ltd.)59c... 

In contrast, the closely related palladium acetate-promoted intramolecular alkylation of alkenes by tri-methylsilyl enol ethers (Scheme 4)6,7 has been used to synthesize a large number of bridged carbocyclic systems (Table 1). In principle, this process should be capable of being made catalytic in palladium(II), since silyl enol ethers are stable to a range of oxidants used to carry the Pd° -> Pd11 redox chemistry required for catalysis. In practice, catalytically efficient conditions have not yet been developed, and the reaction is usually carried out using a full equivalent of palladium(II) acetate. This chemistry has been used in the synthesis of quadrone (equation 2).8 With the more electrophilic palladium(II) trifluoroace-tate, methyl enol ethers underwent this cyclization process (equation 3).9... 

A nonclassical substrate for the Heck reaction is 2,3-epoxycyclohexanone. The reactivity of this molecule under Heck coupling conditions is most likely attributed to its in situ isomerization to 1,2-cyclohexanedione. The 1,2-diketone subsequently reacts with aryl bromides as an olefin via the enol tautomer. Thus, within 5 to 30 min of directed microwave heating of the aqueous PEG mixture, up to 13 different C3-arylations were conducted using less than 0.05 mol % palladium acetate and no phosphine ligand (Scheme 12) [51]. 

The palladium acetate addition to 1-olefins in acetic acid solution is predominately of the Markovnikov type producing ketone enol esters 14>. 

The arylation of enol esters has also been improved (95). Previously a wide range of products were produced including j8-aryl carbonyls, arylated enol esters, styrene, and stilbene derivatives (96). It has also been found that arylated enol esters can be obtained as major products if the reactions are carried out with stoichiometric amounts of aryl mercuric acetate and palladium acetate in anhydrous acetonitrile or in excess enol ester solution. The products are those arising from addition of the phenyl group to the carbons not containing the ester. Thus, with vinyl acetate and phenyl mercuric acetate, the product is the enol acetate of phenyl acetaldehyde ... 

Genet has studied the enantioselective allylation of acyclic Shiff bases. Given the discussion and results above, one would not expect significant selectivity to result from use of simple chiral hgands. However, a respectable ee of 57% was determined for the reaction at -60 °C using chiral DIOP as hgand with the hthium enolate of the imine, Eq. (11) [42]. Further work indicated that use of two moles of DIOP vs. Pd, use of palladium acetate as catalyst precursor, and hthium hex-amethyldisilazide instead of LDA as base provided an increase in ee to 68% [43,... 

Even simple enols have substantial lifetimes, provided that bases or acids are completely excluded173. Thus, an aromatic enol 4 is prepared in situ by Norrish-type fragmentation of 2. If (-)-ephedrine is present in the reaction mixture, the enol reverts enantioselectively to (/ )-2-rnethy 1 -1 -indanone (3). With as little as 0.01 mol % catalyst, 45% ee is obtained176. The crucial enol 4 has also been generated from either the benzyl enol ester 5 by palladium on charcoal and hydrogen or from the allyl ester 6 by palladium acetate, triphenylphosphine and ammonium formate. In the presence of a chiral 1.2-hydroxyamine, e.g., ephedrine, substantial stereogenic induction in 2-methylindanone 3 was observed175. 

With this bicyclic intermediate available in sizeable amounts, ready advance to 111 could be conveniently accomplished prior to annulation of the second five-membered ring (Scheme XIV). 1,3-Carbonyl transposition was realized by complete eradication of the original carbonyl by Ireland s method [60] followed by allylic oxidation. Application of the Piers cyclopentannulation protocol [61] to 111 made 113 conveniently available. Introduction of a methyl group into ring B was brought about by treatment of the kinetically derived enol triflate [62] with lithium dimethylcuprate [63], Hydrolysis of 114 gave the dienone, which was directly transformed into 115 by oxidation of its silyl enol ether with palladium acetate in acetonitrile [64],... 

In view of related couplings of enol triflates, it is perhaps not surprising that triflates (304) derived from phenols undergo smooth carbonylation in the presence of palladium acetate and methanol to... 

The same transformations can he effected by reaction of the trialkylsilyl enol ether of the aldehydes or ketones with palladium acetate. For example, treatment of the trimethylsilyl enol ether of cyclooctanone with 10 mol% Pd(OAc)2 in DMSO under one atmosphere of oxygen at 25 °C for 12 h gave cyclooctenone (82% yield). 

Takasago Perfumery Ltd. manufactures optically pure (Jl)-muscone from the racemic compound by way of its silyl enol ether which is dehydrosUylated with palladium acetate to the pure (Z)-enone. [202] The enantioselective hydrogenation with ruthenium-BINAP catalysts finally gives the enantiomericaUy pure product. [203]... 

Conversion of silyl enol ethers into corresponding oc,(3-eneones using stoichiometric amounts of palladium acetate ... 

The silyl enol ethers (42), in the presence of palladium acetate, cyclize to give the a,/8-unsaturated ketones (44), the reaction probably proceeding via the oxo-ir-allylpalladium complex (43)/ The reaction proceeds readily at room temperature in the presence of stoicheiometric amounts of Pd(OAc)2, and is particularly useful for the synthesis of 3-methylcyclopent-2-enone derivatives (45). 

The first case of a tetrahedral palladium(O) tetraolefin complex (more exactly, Pd(diolefin)2) has been isolated in the course of the Saegusa oxidation of a silyl enol ether, aimed at the synthesis of alkaloids. Palladium acetate was used as oxidant in this reaction, and a brown compound separated from the solution, which was characterized by X-ray diffraction as 16 (Equation (5)). It decomposed upon heating to give the expected product of oxidation. This supports the accepted mechanism of Saegusa oxidation. ... 

Enol esters (126) can be formed in >70% yields during condensations between thioesters (125) and aldehydes (R CH2CHO) in the presence of caesium fluoride. An alternative approach to esters (126) is to react vinylmercurials, e.g. (127), with mercury carboxylates [(R C02)2Hg] and a catalytic quantity of palladium acetate. A wide range of the organometallic reagents (127) can be obtained from acetylenes by a hydroboration-mercuration sequence. 

The preparation of enol esters in a regio- and stereo-specific manner is important, as they are particularly valuable synthetic intermediates. A room-temperature reaction of vinylmercurials, mercury carboxylates, and a catalytic amount of palladium acetate provides a novel and convenient stereospecific route to a variety of enol carboxylates. ... 

The protection of the hemiacetal hydroxyl in step C is followed by a purification of the dominant stereoisomer. The C-6 methyl group is introduced in step C by conjugate addition of dimethylcuprate. The enolate is trapped as the silyl enol ether and oxidized to the enone by palladium acetate. The enone from step D is then subjected to a Wittig reaction. As in several of the other syntheses, the hydrogenation in step E is used to establish the configuration at C-4 and C-6. 

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. 

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