Acetates, 4 + 2-dimerization

Silyi enol ethers can be dimerized to symmetrical 1,4-diketones by treatment with Ag20 in DMSO or certain other polar aprotic solvents." The reaction has been performed with R , R = hydrogen or alkyl, though best yields are obtained when r = r = H. In certain cases, unsymmetrical 1,4-diketones have been prepared by using a mixture of two silyi enol ethers. Other reagents that have been used to achieve either symmetrical or cross-coupled products are iodosobenzene-Bp3-Et20," ceric ammonium nitrate," and lead tetraacetate." If R =0R (in which case the substrate is a ketene silyi acetal), dimerization with TiCU leads to a dialkyl succinate (34, r =0R)." ... 

The oxidation of phenol, ortho/meta cresols and tyrosine with Oj over copper acetate-based catalysts at 298 K is shown in Table 3 [7]. In all the cases, the main product was the ortho hydroxylated diphenol product (and the corresponding orthoquinones). Again, the catalytic efficiency (turnover numbers) of the copper atoms are higher in the encapsulated state compared to that in the "neat" copper acetate. From a linear correlation observed [7] between the concentration of the copper acetate dimers in the molecular sieves (from ESR spectroscopic data) and the conversion of various phenols (Fig. 5), we had postulated [8] that dimeric copper atoms are the active sites in the activation of dioxygen in zeolite catalysts containing encapsulated copper acetate complexes. The high substratespecificity (for mono-... 

A qualitative and preliminary picture (Fig. 11.16) of the mechanism of oxidation that emerges from our studies is the following Under the reaction conditions (pH = 6.5), the phenols exist in the phenolate form. Two phenolate ions coordinate to the two Cu(II) ions of the copper acetate dimer, reducing them to the Cu(I) oxidation state. Next, dioxygen reacts with the copper-phenolate adduct. The latter undergoes an 0-0 bond scission concomitant with the hydroxylation of the substrate. The acetate... 

FIGURE 11.16. A possible mechanism for the ortho-hydroxy aXion of phenol over copper acetate dimers. 

Galal AM, Gul W, Slade D, Ross SA, Feng S, Hollingshead MG Alley MC, Kaur G ElSohly MA. (2009) Synthesis and evaluation of dihydroartemisinin and dihydroartemisitene acetal dimers showing anticancer and antiprotozoal activity. Bioorg Med Chem 17 741-751. 

The coals used were PSOC 1098 Illinois 6 and Beulah-Zap North Dakota lignite from the Argonne coal bank. The analytical data of these coals are shown in Table I. The ratio of catalyst to coal was approximately 0.6 mmoles of metal per gram of coal. The organometallic catalysts were molybdenum(II) acetate dimer, Mo2(OAc)4, obtained from Strem, molybdenum(II) allyl dimer Mo2(OAc)4, was prepared by die method of Cotton and Pipal (25). The NiMo supported catalyst was prepared by addition of bis(l,5-cyclooctadiene) Ni(0) (Strem) to sulfided Mo on alumina (. Cp2Mo2( l-SH)2(p.-S)2, referred to as MoS2(OM), was prepared by modification of method of Dubois et al. (26), and Cp2Mo2Co2( i3-S)2(li4-S)(CO)4, CoMo(OM) was prepared by the method of Curtis et al. (27). Pentacarbonyl iron was obtained from Aldrich,... 

The problem with the ether and ester linkage is that they are often hydrolytically unstable, and so there have been approaches to replace the CIO acetal linkage with more chemically robust linkages. Posner s group prepared a series of CIO olefinic non-acetal dimers and CIO saturated dimers that showed good antiproliferative activities . Dimers 108-110 were aU especially potent and selective at inhibiting leukaemia and colon cancer in the NCI assay (Scheme 38) . ... 

Saturated non-acetal dimers, such as 153 and 154, can be prepared from 151 via aluminium acetylide condensations or Friedel-Crafts reactions. Although the acetylenic dimer 153 is /3-linked to C-10 of the artemisinin skeleton, the aryl dimer 154 is a-linked <1999JME4275>. 

A 1 2 mixture of isomeric cis-3,4a-H- and fra s-3,4a-H-3-methyl-2,3,4, 4a,5,6-hexahydro-l//-pyrido[l,2-a]quinazolines gave a 20 1 mixture in the presence of rhodium(II) acetate dimer and triphenylphosphine in ethyl acetate at 80°C for 20 h (94AJC1061). Epimerization probably occurs via a 10-membered ring intermediate (26). 

Rhodium-catalyzed hydroformylation of 2-amino-/V-(but-3 -enyl)- and -A-(3 -rnethylbut-3 -enyi)benzylamines (381) in the presence of rho-dium(II) acetate dimer and triphenylphosphine in deoxygenated ethyl acetate gave mixtures of 5,5a,6,7,8,9-hexahydro-llH-pyrido[2,l-b]quinazo-line (382), isomeric 6-methyl-5,5a,6,7,8,10-hexahydropyrrolo[2,l-b]quina-zolines (383), and 6-methyl-6,7,8,10-tetrahydropyrrolo[2,l-ft]quinazoline (384), as well as a stereoisomeric mixture of 7-methyl-5,5a,6,7,8,9-hexahy-dro-ll//-pyrido[2,l-b]quinazolines (385) and 15% of 7-methyl-6,7,8,9-tetrahydro-llH-pyrido[2,l-fr)quinazolme (386), (95AJC2023). When the bulky tricyclohexylphosphine was used instead of triphenylphosphine, a 3 7 mixture of compounds 382 and 383 and a 3 1 mixture of isomeric 385 were formed. 

Rhodium-catalyzed hydroformylation of -(substituted amino)benzyl-amines (387, X = H2) and -(substituted amino)benzamides (387, R = H, X = O) in the presence of rhodium(II) acetate dimer and triphenylphos-phine in deoxygenated ethyl acetate gave a 7 3 mixture of 1,2,3,4,4 ,5-hexahydro-6//-pyrido[l,2-a]quinazolines (388, X = H2,0) and isomeric 3-methyl-l,2,3,3fl,4,5-hexahydropyrrolo[l,2-a]quinazolines (389, X = H2, O) (94AJC1061). The methyl derivative of benzylamine 387 (R = Me, X = H2) afforded a mixture of diastereoisomers 390 and 391 (X = H2). Their ratio depended on the reaction time. Longer reaction times gave more 391 (X = H2), containing the methyl group in an equatorial position. Compound 390 isomerized into 391 (X = H2), under the aforementioned conditions. The benzamide derivative (387, R = Me, X = O) yielded only one isomer (391, X = O), independent of the reaction period. 

It is well-known that an extensive chemistry of mononuclear chromium(II) exists in addition to the dimeric chemistry relevant to this review. Furthermore, oxidation of chromous acetate with a variety of oxidants has been shown to produce monomeric chromium(III) species. The rate law for disappearance of the chromous dimer has been interpreted as indicating that dissociation of the acetate dimer precedes the oxidation step (45). 

The reaction of Mo(II) complexes such as the acetate dimer with dithio-ligands seems to preserve the dimeric Mo(II) structure with ligands that are... 

The loss of stereospecificity in the addition of bis(sulfone) anions to cyclohexenyl allylic acetates was attributed to a scrambling of the stereochemistry of the starting acetate. The ability of Pd° catalyst to effect this epimerization was confirmed in the absence of added nucleophile. This epimerization was attributed to the ability of the acetate to return to add to the ir-allyl complex via attack at the metal center (equation 177).167 This suggestion was confirmed by treatment of a preformed allylpalladium acetate dimer with CO, which resulted in cis migration of the acetate from Pd to the allyl ligand (equation 178).164... 

The cyclopropanation of alkenes, alkynes, and aromatic compounds by carbenoids generated in the metal-catalyzed decomposition of diazo ketones has found widespread use as a method for carbon-carbon bond construction for many years, and intramolecular applications of these reactions have provided a useful cyclization strategy. Historically, copper metal, cuprous chloride, cupric sulfate, and other copper salts were used most commonly as catalysts for such reactions however, the superior catalytic activity of rhodium(ll) acetate dimer has recently become well-established.3 This commercially available rhodium salt exhibits high catalytic activity for the decomposition of diazo ketones even at very low catalyst substrate ratios (< 1%) and is less capricious than the old copper catalysts. We recommend the use of rhodium(ll) acetate dimer in preference to copper catalysts in all diazo ketone decomposition reactions. The present synthesis describes a typical cyclization procedure. 

The direct substitution of the added nucleophile was described (76JA5581) for 114 with 124 as the final product two authors postulated that the dissociation of the primary adduct takes place with the intermediate formation of cation 30. Heating of the pseudobase 114 without nucleophiles leads to the acetal dimer 125. 

On this basis one can deduce the course of cupric acetate hydrogenation during any single experiment, the rate again being assumed to be proportional to the concentration of cuprous acetate dimer. Figure 6 shows the agreement obtained between the theoretical curve (solid curve) and the observed data (circles) for an experiment in which the hydro-... 

To a 10-mL round-bottomed flask fitted with a nitrogen balloon was added sulfide catalyst (0.2 equiv), anhydrous dioxane (4.0 mL), rhodium(II) acetate dimer (2 mg, 0.01 equiv), substrate (0.5 mmol), benzyl triethylammonium chloride (23 mg, 0.2 equiv), and tosylhydrazone sodium salt (1.5 equiv). The reaction mixture was stirred vigorously at 40 °C for 48 h. Work-up consisted of the sequential addition to the reaction mixture of water (5 mL) and ethyl acetate (5 mL). The aqueous layer was washed with ethyl acetate (2.5 mL) and the combined organic phases dried (MgSC ), filtered, and concentrated in vacuo. The crude products were analyzed by aH NMR to determine the diastereomeric ratio, and then purified by FC to afford the corresponding cyclopropane. 

Triethylamine, dimethyl malonate, and rhodium(ll) acetate dimer were purchased from Aldrich Chemical Company, Inc., and used without further purification. 

Rhodium(ll) acetate dimer Rhodium, tetrakis(acetato-0,0 )di-, (Rh-Rh) (9) (46847-37-4)... 

Also isolated from the acetic acid reaction is K[Tc2(OAc)4C12], the structure of which shows a distinctly longer Tc-Tc bond distance of 2.1260(5) A, with two axial chlorides at 2.589(1) A 167c). The effective magnetic moment for the three acetate dimers is 1.78 0.05 BM and the EPR spectra are consistent with the unpaired electron equally shared by the two Tc centers in the 5 (blu) antibonding molecular orbital (168,169). 

A stirred solution of tetrahydrothiophene (0.066 mol), rhodium(II)acetate dimer (0.003 mmol), benzyltriethylammonium chloride (0.066 mmol) and benzaldehyde (0.33 mol) dissolved in 1ml acetonitrile was added to the product from Step 1 (0.495 mmol). The mixture was heated to 45 °C 3 hours, cooled, and 1 ml EtOAc/water, 1 1, added. The organic phase was dried, the epoxide purified by chromatography on silica with CH2Cl2/petroleum ether, 0-25%, and the product isolated. 

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