Acetone reductive dimerization

The octacyclic dimer (+)-94 could be obtained in short order from the tetracyclic bromide (+)-93 via a Co(I)-mediated reductive dimerization protocol first implemented in our prior syntheses of (+)-chimonanthine (7), (+)-folicanthine (8), and (—)-calycanthine (9) [7]. Simple exposure of intermediate (+)-93 to tris (triphenylphosphine)cobalt(I) chloride [48] in acetone under anaerobic conditions rapidly afforded dimer (+)-94 in 46 % yield. While higher yields (52 % yield) could be obtained in tetrahydrofuran on small scale, performing the reaction in acetone reproducibly afforded higher yields on gram scales. Notably, the product was obtained in similar efficiency on multi-gram scale (43 % yield on 8-g scale)... 

Other authors obtained such a diol by reductive dimerization using sodium [128]. The internal chain can be longer thus Loeb et al. [129] developed the condensation of acetone and hexafluoroacetone to produce ... 

Kinetic studies have revealed that aliphatic ketyl radical anions are very shortlived compared with aromatic (half life of acetone " in aqueous 2-propanol is 72 ps, whereas that for acetophenone " is 1.5 ms) [253]. The reductive dimerization of simple aromatic aldehydes has been studied in aprotic solvents, with the second order rate constant being larger in acetonitrile than in DMF, because of ion-pair effects [254]. Electron-withdrawing substituents reduce the speed of dimerization (benzaldehyde " k = 2.4x 10 m" s", p-cyanobenzaldehyde k = 5 M s" ) [255], whereas protic solvents lead to protonation before dimerization [256]. 

C in aprotic solvents such as acetone or, more smoothly, in -PrOH, with 80% yield to 1,3,7-octatrienes (1). The presence of COj greatly enhances the catalytic activity of [Pd(PR3) ] (n = 2-4) and causes isomerization of 1 to 2,4,6-octatrienes. In formic acid, reductive dimerization proceeds and, depending on catalyst and cocatalyst, either... 

Reduction of triphenyltin piperidyldithiocarbamate in acetone was shown by polarography and voltammetry to consist of two diffusion-controlled peaks and two peaks which seem to reflect adsorption142. Apparently, a dithiocarbamate group dissociates and triphenyltin radical forms by reduction. The latter partly dimerizes and partly reduces to triphenyltin anion. 

The higher catalytic activity of the cluster compound [Pd4(dppm)4(H2)](BPh4)2 [21] (20 in Scheme 4.12) in DMF with respect to less coordinating solvents (e.g., THF, acetone, acetonitrile), combined with a kinetic analysis, led to the mechanism depicted in Scheme 4.12. Initially, 20 dissociates into the less sterically demanding d9-d9 solvento-dimer 21, which is the active catalyst An alkyne molecule then inserts into the Pd-Pd bond to yield 22 and, after migratory insertion into the Pd-H bond, the d9-d9 intermediate 23 forms. Now, H2 can oxidatively add to 23 giving rise to 24 which, upon reductive elimination, results in the formation of the alkene and regenerates 21. 

An example of a photolabile mask for alcohols and thiols employing o-benzoylbenzoate esters is reported by Porter and co-workers [100], Primary and secondary alcohols as well as thiols can be easily masked via the formation of the corresponding 2-benzoylbenzoate esters and converted back into alcohols/ thiols under PET-reductive conditions. Irradiation in isopropanol/benzene 1 1 leads to 3-phenylphthalide dimers as the coproduct (together with acetone), whereas more potent electron donors (e.g., amines) result in the monomers and the corresponding imine. Yields generally range from 60% to 90% (Scheme 55) [100]. 

Steroidal n-alfylpalladium complexes. These complexes of unsaturated steroids are prepared conveniently by reaction in chloroform or acetone with this Pd(II) complex. Ergosterol (1) is converted in this way into the dimeric complex 2, with loss of the hydroxyl group. Two interesting new reactions of the complex 2 are reported reduction (o a triene (3) and oxidation to a tricnol (4). 

Carbanions, such as the enolate ions of acetone, acetylmorpholine and others were un-reactive toward 1-AdI in liquid ammonia under photostimulation, and only the reduction product AdH and the dimer (1-Ad)2 were formed. It has been proposed that even though the 1-Ad radicals are formed under photostimulation, they do not couple with carbanions to give the S l product, at least at a competitive rate with other reactions121. 

As in the case of electrochemical reduction, the photochemical transformation of 5-fluorouracil derivatives differs from that of the other 5-halogeno uracils. The primary photoproduct of 5-fluorouracil, its glycosides and poly(5-FU) is the photohydrate. However, at shorter wavelengths of irradiation, e.g. 254 nm where the photohydrate exhibits absorption, there is elimination of HF from the 5,6 bond and formation of barbituric acid 129 13I>. There is also some evidence for acetone photosensitized formation of cyclobutane dimers of 5-fluorouracil132), as well as dimer formation in irradiated poly(5-FU)133>. 

The methoxycarbonylmethyl radical I is obtained by Kolbe electrolysis from methyl-malonate [Eq. (4, path a)], and in homogeneous solution by reductive fragmentation of the hydroperoxide of dimethyl acetone dicarboxylate [Eq. (4, path b)]. Radical I forms with styrene the adduct II, which reacts by disproportionation to III and IV, by coupling with I to V and by dimerization to VI. The yields and product ratios for the electrolysis and the reductive fragmentation correspond surprisingly well, and no extensive polymerization was found for the homogeneous reaction. 

As a representative example, treatment of bicyclo[1.1.0]butane (1) with Zeise s dimer in acetone at 23°C for 20 minutes gives butadiene in quantitative yield. At — 45°C (48 hours) a 1 1 complex is formed in 97% yield, which upon further treatment with pyridine gives 2-[bis(pyridine)dichloroplatina]bicyclo[l.l.l]pentane (2) in 94% yield. Thermal decomposition of this complex 2 unselectively leads to a mixture of the starting hydrocarbon 1, cyclobutene (3), butadiene (4), 2-methylcyclopropene (5) and methylene cyclopropane (6). Reduction of the metallacyclic intermediate 2 with lithium aluminum deuteride leads to a bisdeuterated methylcyclopropane 7 accompanied by a bisdeuterated cyclobutane 8 and the starting material 1. ... 

However attempts to isolate this complex were unsuccessful with the [Coj Cyjdtc) ] dimer being the only product. These E° values suggest a Co "-oxidized ligand formalism and the monomer is apparently unstable. The first one-electron reduction to give [Co(R2dtc)3]- is also not easy ( ,/2 = - 0.80 V (benzyl), —1.13V (cyclohexyl), acetone solvent) but shows considerable chemical reversibility at Pt and Hg in acetone" or DMSO." Further reduction leads to Co (equation 188). Both the electrochemistry and the stability of the oxidized and reduced species are sensitive to the nature of the R group, the solvent and the redox procedure." ... 

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