Dimerization reactions ketones

The dimerization of ketones to 1,2-diols can also be accomplished photochemi-cally indeed, this is one of the most common photochemical reactions. The substrate, which is usually a diaryl or aryl alkyl ketone (though a few aromatic aldehydes and dialkyl ketones have been dimerized), is irradiated with UV light in the presence of a hydrogen donor such as isopropyl alcohol, toluene, or an amine. In the case of benzophenone, irradiated in the presence of 2-propanol, the ketone molecule initially undergoes n — k excitation, and the singlet species thus formed crosses to the T, state with a very high efficiency. 

The amine-catalyzed Diels-Alder dimerization reaction of a, 3-unsaturated ketones in water was developed by Barbas et al. to form cyclohexanone derivatives (Eq. 12.12). They believe that the reaction proceeds via the in situ formation of 2-amino-1,3-butadiene and iminium-activated enone, as the diene and dienophile, respectively. 

Scheme 17.13 Alternative mechanism for the cyclization—dimerization reaction of an a-allenyl acid and a ketone.

The dimerization of ketones to 1,2-diols can also be accomplished photochemically indeed, this is one of the most common photochemical reactions.688 The substrate, which... 

The photoactivated intermolecular dimerization reaction of aziridinyl ketone 18 leading to heterocycle 67 after the initial oxidation of piperazine 66 has also been described [84] (Scheme 1.17). 

Dimerization of ketone enolates. Frazier and Harlow1 have noted that ketone enolates (LDA or KH) couple to 1,4-diketones when oxidized by dry FeCl3 in DMF. Yields range from 25 to 70%. Paquette et al.2 have extended this reaction to intramolecular coupling of a bisenolate to a cyclopropane. Thus addition of a... 

Reductive dimerization of ketones Aromatic -unsaturated ketones are reduced and dimerized to 1,5-hexadienes by sequential reaction with LiAIH4, Fe3(CO)i2, and HCl. The effectiveness of iron carbonyls shows the order Fe3(CO), 2 > Fe2(CO)y > FeiCO). ... 

Jauch, J. Stereochemistry of the Rubottom oxidation with bicyclic silyl enol ethers synthesis and dimerization reactions of bicyclic a-hydroxy ketones. Tetrahedron 1994, 50,12903-12912. 

Carbon attack of the carbonyl group of cyclopropyl ketones has also been achieved in dimerization reactions under McMurry and photochemical conditions. Conceivably, the reactions take place via radical anion intermediates. Photodimerization does not occur if structural features of the substrate facilitate intramolecular reactions such as ring opening, ring expansion, and ring closure. Radical... 

Anhydro-5-hydroxyoxazolium hydroxides are attacked by electrophiles at position 4 cf. 229). If this position is free, substitution ensues, as in the dimerization reaction of Scheme 21. Other examples are uncatalyzed acylations (equation 55) and diazo coupling (equation 56). The key step in the Dakin-West reaction, i.e. the formation of a-acetamido ketones from a-amino acids and acetic anhydride in the presence of pyridine, is acetylation of an intermediate betaine (230 Scheme 23). 

Conjugate addition-dimerization reactions were also successfully accomplished within the tin(IV) enolate methodology, albeit in modest yields. Activated alkenes using two enone precursors formed conjugate addition products (Scheme 10). These conjugate additions with enone Michael acceptors provide a regiocontrolled route to bicyclic products. Because the enolate forms on the alkene side of the ketone, the reaction outcome can be predicted with some confidence. 

Hybrid linear tri- and tetra-oligomers possessing combinations of furan, thiophene or pyrrole have been obtained (2000TL2919, 2001T7323) by BFs-catalysed reactions of diols 18, and 22 with pyrrole, etc. The diols are obtained from reactions of bisanions of the parent heterocycles or their dimers with ketones. 

Biosynthesis In contrast to steroids and terpenes, there is no equivalent to the isoprene rule for alkaloids as a useful aid for structure determination and biogenetic investigations. It is generally accepted today that A. are formed in cyclization, condensation, and dimerization reactions from amino acids and biogenic amines with biogenic aldehydes and ketones. 

So far we have considered only self-condensations —dimerization reactions of a single carbonyl compound. These form only a tiny fraction of known aldol reactions. Those that occur between two different carbonyl compounds, one acting as a nucleophile in its enol or enolate form, and the other as an electrophile, are called cross-condensations. They are more interesting than self-condensations, but working out what happens needs more thought. We shall start with an example that works well. The ketone PhCOMe reacts with 4-nitrobenzaldehyde in aqueous ethanol under NaOH catalysis to give a quantitative yield of an enone. 

Although prolinamides have shown their versatility in the aldol reaction between ketones and aldehydes, their application towards the reaction between the cross or homo-aldol reaction between aldehydes have been scarcely studied. The homo-aldol dimerization reaction between neat propionaldehyde (5, R =Me and 2, R =Et in Scheme 4.23) catalyzed by (S)-prolinamide (40, 20 mol%) in the presence of... 

The enolate ion is nucleophilic at the alpha carbon. Enolates prepared from aldehydes are difficult to control, since aldehydes are also very good electrophiles and a dimerization reaction often occurs (self-aldol condensation). However, the enolate of a ketone is a versatile synthetic tool since it can react with a wide variety of electrophiles. For example, when treated with an unhindered alkyl halide (RX), an enolate will act as a nucleophile in an Sn2 mechanism that adds an alkyl group to the alpha carbon. This two-step a-alkylation process begins by deprotonation of a ketone with a strong base, such as lithium diisopropylamide (LDA) at -78°C, followed by the addition of an alkyl halide. Since the enolate nucleophile is also strongly basic, the alkyl halide must be unhindered to avoid the competing E2 elimination (ideal RX for Sn2 = 1°, ally lie, benzylic). 

Soft carbon nucleophiles, such as activated methylene or methyne compounds 11, react with butadiene to give a mixture of the mono- (12) and di-(2,7-octadienyl) adducts (13) in the case of = H (activated methylenes), and to give the monoadduct 12 in the case of activated methynes, as expected (Scheme 3). The addition reactions with various carbon nucleophiles, such as j8-keto esters, /S-diketones, malonate and malononitrile, a-formyl ketones and esters, a-cyano ketones and esters, cyanoacetoacetamide, phenyl-sulfonyl acetate,nitroalkanes, and an enamine, are summarized in Table 1. PdCl2(PPh3)2-NaOPh is often used as a catalyst for the dimerization reaction. Complexes... 

On the basis of the observed products and literatures on the oxidation of phenolic conqpounds by laccase, the reaction scheme for 8-hydroxybentzon oxidation by laccase is proposed as shown in Fig.5. Initially the phenol moiety of 8-hydroxybentazon is oxidized by laccase and formed phenoxy radicals. Further reactions of these phenoxy radicals are cross coupled with each other and formed dimeric products. Ketone moiety of 8-hydroxybentazon-derived dimers are converted to enol in acidic condition. The coupling reactions most likely occur via C-C coupling at para positions because of their electronic (resonance and inductive) and satiric effects. Proposed reaction pathway leading to the formation of co-polymerization product appears to be initiated by die coupling of two catechol radicals, and subsequent oxidation of that dimmer, resulting in catechol-derived benzoquinone dimer. It appears that a tetrameric copolymerization product is formed by the non-enzymatic addition of two 8-hydroxybentazon to a catechol-derived benzoquinone dimer (Fig. 5). 

As previously described, the function of the enzyme is to produce the phe-noxy radicals under mild reaction conditions. The resulting phenoxy radicals are able to form polymers via recombination processes if a suitable reaction medium (solvent mixture composition), pH value (buffer systems), and kind of substrate are chosen. Some phenols, for example, are known to react preferentially via oxidation to form orfho-diketones [68] or Pummerer ketones [52], and are therefore not suitable for polymerization. Other phenols are known to prefer dimerization reactions, for instance caffeic acid [71]. 

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