Acetals, acid catalyzed dimerization

Under other conditions,9 aldehydes give dimers and even tetramers. The tetramer of acetaldehyde, made by the action of a trace of acid at about 0°, has been shown by X-ray diffraction analysis to be tetramethyl-tetraoxacyclooctane (2).10 The acid-catalyzed dimerization of mercapto-aeetaldehyde acetal HSCH2CH(OEt)2 gives 2,5-diethoxy-l,4-dithian (3), which can be converted to 1,4-dithiadiene by heating at 300° over alumina.11... 

The use of acetic acid as solvent promotes the second ort/io-metalation step. Thus after 1 h of reaction of rhodium acetate and P(o-BrCsF4)Ph2, two new products, 5 and 6 were obtained in addition to the previously described compounds 3 and 4. This was the first direct evidence that acetic acid catalyzes the ort/ o-metalation reaction. Both 5 and 6 are bimetalated Rh(II) dimers. Compound 6 has a close structural resemblance to 1, with two metalated ligands. In this case the PCBr ligands assume an, / -mode of ligation. In 5 one of the axial positions was found to be occupied by a molecule of water. P NMR spectroscopy of the reaction mixture shows that this site is initially occupied by a molecule of acetic acid which is displaced by water during the subsequent chromatography. 

The most common oxidatiou states and corresponding electronic configurations of rhodium are +1 which is usually square planar although some five coordinate complexes are known, and +3 (t7 ) which is usually octahedral. Dimeric rhodium carboxylates are +2 (t/) complexes. Compounds iu oxidatiou states —1 to +6 (t5 ) exist. Significant iudustrial appHcatious iuclude rhodium-catalyzed carbouylatiou of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to -butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

The reaction of thiirane 1-oxides with water or methanol is usually acid-catalyzed and gives /3-substituted sulfenic acids which dimerize to thiolsulfinates (54 Scheme 70) (72JA5786). If acetic acid is used a mixture of disulfide (55) and thiolsulfonate (56) is obtained. Treatment of thiirane 1,1-dioxides with hydroxide ion may involve attack on carbon as well as on sulfur as exemplified by 2-phenylthiirane 1,1-dioxide (Scheme 71). 

A kinetic study of nitrous acid-catalyzed nitration of naphthalene with an excess of nitric acid in aqueous mixture of sulfuric and acetic acids (Leis et al. 1988) shows a transition from first-order to second-order kinetics with respect to naphthalene. (At this acidity, the rate of reaction through the nitronium ion is too slow to be significant the amount of nitrous acid is sufficient to make one-electron oxidation of naphthalene as the main reaction path.) The reaction that initially had the first-order in respect to naphthalene becomes the second-order reaction. The electron transfer from naphthalene to NO+ has an equilibrium (reversible) character. In excess of the substrate, the equilibrium shifts to the right. A cause of the shift is the stabilization of cation-radical by uncharged naphthalene. The stabilized cation-radical dimer (NaphH)2 is just involved in nitration ... 

The trimerization of cyclopentadiene (6) is catalyzed by a homogeneous bifunctional palladium-acid catalyst system.7 The reaction gives trimers 7 and 8 as a 1 1 mixture in 70% yield with bis(acetylacetonato)palladium(II) [Pd(acac)2] or with bis(benzylideneacetone)-palladium(O) as the palladium component of the catalyst. As the phosphorus component, phosphanes like trimethyl-, triethyl-, or triphenylphosphane, and triisopropylphosphite or tris(2-methylphcnyl)phosphite, are suitable. A third component, an organic acid with 3 < pK < 5, is necessary in at least equimolar amounts, in the reaction with cyclopentadiene (6), as catalytic amounts are insufficient. Acids that can be used are acetic acid, chloroacetic acid, benzoic acid, and 2,2-dimethylpropanoic acid. Stronger acids, e.g. trichloroacetic acid, result in the formation of poly(cyclopentadiene). The new catalyst system is able to almost completely suppress the competing Diels-Alder reaction, thus preventing the formation of dimeric cyclopentadiene, even at reaction temperatures between 100 and 130°C. 

In the presence of oxygen, reaction (186) is replaced by reaction (187). The rate of reaction is first order in Co(III) under these conditions.253,258 Other workers241 254"256 have observed, however, a second-order dependence on Co(III) concentration, which is more difficult to explain. A rate law containing both second-order and half-order terms in Co(III) has also been reported257 for the cobalt-catalyzed autoxidation of toluene in acetic acid. The mixed kinetic expression was explained by the participation of reactions of both a Co(III) monomer and a Co(III) dimer with the substrate. 

The Sonogashira coupling can be considered a special case of catalytic alkyne activation. Interestingly, it is also possible to conduct alkyne activation under oxidative conditions in the presence of Pd catalysts without oxidative dimerization. Here, Costa and coworkers [139] have developed a Pd-catalyzed sequential carboxylation-alkoxycarbonylation of acetylenic amines in the presence of oxygen to give mixtures of Z- and -configurcd 2-oxo-oxazolidin-5-ylidene]-acetic acid methyl ester 193 and 194 in good to excellent yields (Scheme 75). 

Some of the first catalytic model systems for the simulation of the function of methane monooxygenase comprise monomeric as well as dimeric iron-containing model complexes bearing hydro-tris(pyrazolyl)borate ligands [6]. These complexes, e.g. 3, catalyze the oxidation of aromatic and aliphatic carbon-hydrogen bonds in the presence of oxygen (1 atm), acetic acid and zinc powder at room temperature (Scheme 2). 

A new synthetic method for steroids has been developed using a butadiene dimer (66) as a building block and the palladium-catalyzed oxidation as the key reaction. 3-Acetoxy-l,7-octadiene (66), prepared by the palladium-catalyzed reaction of butadiene with acetic acid, is hydrolyzed and oxidized to... 

A new synthetic method for steroids has been developed using a butadiene dimer (66) as a building block and the palladium-catalyzed oxidation as the key reaction.3-Acetoxy-l,7-octadiene (66), prepared by the palladium-catalyzed reaction of butadiene with acetic acid, is hydrolyzed and oxidized to l,7-octadien-3-one (67) in high yield. The enone (67) is a very useful reagent for bisanellation because its termiiud double bond can be regarded as a masked ketone which can be readily unmasked by the palladium catalyst to form the l,S-diketone (68) after Michael addition at the enone moiety of (67 Scheme 20). Thus, the enone (67) is the cheapest and most readily available bisanellation reagent, permitting a simple total synthesis of steroids. 

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