Dimerization carbonyl compounds

Electro-generated chromium(II) is also very effective in the pinacolization of otherwise unsatisfactory dimerizing carbonyl compounds (Table 5, No. 13)220-222) this case the chromium(II) ion does not act as redox agent but catalyzes the formation of a chromium(III) complex of the carbonyl compound which subsequently is reduced to the pinacol directly at the cathode (Eqs. (73)-(77)). 

The samarium-lV-bromosuccinimide combination reductively dimerizes carbonyl compounds.163 This pinacol-type coupling gives diols in 60-80% yield, with some diastereoselectivity the by-product from simple reduction (i.e. alcohol) is typically 5-10%. The conditions suggest a single electron transfer to give carbonyl radical anion, which then self-couples. Even congested ketones such as benzophenone and fluorenone worked well. 

Dimeric metal carbonyls that obey the 34-electron rule. These compounds have direct intermetallic bonds, and either may or may not have bridging carbonyl ligands between the metals. Examples of such dimeric carbonyl compounds are Mn2(CO)io, Co2(CO)s, and Fe2(CO)9. 

Most ozonolysis reaction products are postulated to form by the reaction of the 1,3-zwitterion with the extmded carbonyl compound in a 1,3-dipolar cycloaddition reaction to produce stable 1,2,4-trioxanes (ozonides) (17) as shown with itself (dimerization) to form cycHc diperoxides (4) or with protic solvents, such as alcohols, carboxyUc acids, etc, to form a-substituted alkyl hydroperoxides. The latter can form other peroxidic products, depending on reactants, reaction conditions, and solvent. 

Pyrazolones show a great variety of reactions with carbonyl compounds (B-76MI40402). For instance, antipyrine is 4-hydroxymethylated by formaldehyde and it also undergoes the Mannich reaction. Tautomerizable 2-pyrazolin-5-ones react with aldehydes to yield compound (324) and with acetone to form 4-isopropylidene derivatives or dimers (Scheme 8 Section 4.02.1.4.10). 

Two classes of charged radicals derived from ketones have been well studied. Ketyls are radical anions formed by one-electron reduction of carbonyl compounds. The formation of the benzophenone radical anion by reduction with sodium metal is an example. This radical anion is deep blue in color and is veiy reactive toward both oxygen and protons. Many detailed studies on the structure and spectral properties of this and related radical anions have been carried out. A common chemical reaction of the ketyl radicals is coupling to form a diamagnetic dianion. This occurs reversibly for simple aromatic ketyls. The dimerization is promoted by protonation of one or both of the ketyls because the electrostatic repulsion is then removed. The coupling process leads to reductive dimerization of carbonyl compounds, a reaction that will be discussed in detail in Section 5.5.3 of Part B. 

Oxaziranes are rapidly decomposed by the further action of peracids. As shpwn by Emmons and Krimm the oxazirane is converted into a carbonyl compound and a nitroso compound dimer. The reaction can be formulated via an oxazirane A -oxide intermediate [Eq. (30)]. 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

Ordinary aldehydes and ketones can add to alkenes, under the influence of UV light, to give oxetanes. Quinones also react to give spirocyclic oxetanes. This reaction, called the Patemo-BUchi reaction,is similar to the photochemical dimerization of alkenes discussed at 15-61.In general, the mechanism consists of the addition of an excited state of the carbonyl compound to the ground state of the alkene. Both singlet (5i) and n,n triplet states have been shown to add to... 

Nickel(O) complexes are extremely effective for the dimerization and oligomerization of conjugated dienes [8,9]. Two molecules of 1,3-butadiene readily undergo oxidative cyclization with a Ni(0) metal to form bis-allylnickel species. Palladium(O) complexes also form bis-allylpalladium species of structural similarity (Scheme 2). The bis-allylpalladium complexes show amphiphilic reactivity and serve as an allyl cation equivalent in the presence of appropriate nucleophiles, and also serve as an allyl anion equivalent in the presence of appropriate electrophiles. Characteristically, the bis-allylnickel species is known to date only as a nucleophile toward carbonyl compounds (Eq. 1) [10,11],... 

If, on the other hand, unsymmetrically substituted carbonyl compounds such as monosubstituted benzophenones (X = OCH3, CH3, Cl), tert-butyl methyl ketone, acetophenone, acetaldehyde, or benzaldehyde are used for trapping 39a, diastere-omeric mixtures are formed in each case they could all be resolved except for the products obtained with p-methoxybenzophenone and acetophenone 33>. An X-ray structure analysis has been performed for the E-isomer 57g 36) which, in conjunction with H-NMR studies, permitted structural assignment in cases 56 and 57e, g and h35>. Additional chemical evidence for the structure of the six-membered heterocycles is provided by the thermolysis of 56 a considered in another context (see Sect. 3.1). In general the reaction 39a- 56 or 57 is accompanied by formation of phosphene dimers, presumably via [4 + 4]- and via [4 + 2]-cycloaddition 35). 

Palladium(II) acetate was found to be a good catalyst for such cyclopropanations with ethyl diazoacetate (Scheme 19) by analogy with the same transformation using diazomethane (see Sect. 2.1). The best yields were obtained with monosubstituted alkenes such as acrylic esters and methyl vinyl ketone (64-85 %), whereas they dropped to 10-30% for a,p-unsaturated carbonyl compounds bearing alkyl groups in a- or p-position such as ethyl crotonate, isophorone and methyl methacrylate 141). In none of these reactions was formation of carbene dimers observed. 7>ms-benzalaceto-phenone was cyclopropanated stereospecifically in about 50% yield PdCl2 and palladium(II) acetylacetonate were less efficient catalysts 34 >. Diazoketones may be used instead of diazoesters, as the cyclopropanation of acrylonitrile by diazoacenaph-thenone/Pd(OAc)2 (75 % yield) shows142). 

Dimerizations of oc,/ -unsaturated carbonyl compounds are perhaps the most interesting reactions, and certainly are the subjects of the most wide-spread investigations. Many are photosensitized, including that of coumarin, Eq. 50. 123> Other reactions of simple enones also involve... 

More generally, double bonds between two carbons or one carbon and a heteroatom, possibly conjugated with other unsaturated moieties in the molecule, are eligible for two-electron/two-proton reactions according to Scheme 2.20. Carbonyl compounds are typical examples of such two-electron/two-proton hydrogenation reactions. In the case of quinones, the reaction that converts the quinone into the corresponding hydroquinone is reversible. With other carbonyl compounds, the protonation of the initial ketyl anion radical compete with its dimerization, as discussed later. 

Radical anion EGBs derived from aromatic carbonyl compounds are expected to be relatively weak bases but since the radical anions undergo dimerization, the more basic dimer dianions may be active as EGBs for substrates with pK values in the range 20 to 23. Aromatic carbonyl compounds have primarily been used as PBs in catalytic reactions in which the PB also functions as an electrophile (cf. Sect. 14.9.2). 

A number of electrocatalytic reactions have been reported in which the EGB is derived by initial reduction of an aldehyde or a ketone that at the same time functions as the electrophile in a coupling reaction [136-139]. It is Kkely that the actual EGB is a dimer dianion of the carbonyl compound or a dianion of the carbonyl compound formed by disproportionation. The general principle is outlined in Scheme 38. The reactions become catalytic when the product anion, P , is protonated by the weak acid, NuH, whereby the nucleophile, Nu , is regenerated. 

Scheme 140 Tungsten-catalyzed reductive dimerization of carbonyl compounds to alkenes.

Electrochemically reduced W species tend to transform carbonyl compounds into olefins after dimerization. For example, benzaldeliyde (387) can be quantitatively converted into stilbene (388) by electroreduction in a THF-BU4NCIO4/WCI6-(Al/Pt) system at a potential of —1.9 V (SCE) (Scheme 140) [501]. 

Probably the most familiar radical reactions leading to 1,2-D systems are the so called acyloin condensation and the different variants of the "pinacol condensation". Both types of condensation involve an electron-transfer from a metal atom to a carbonyl compound (whether an ester or an aldehyde or a ketone) to give a radical anion which either dimerises directly, if the concentration of the species is very high, or more generally it reacts with the starting neutral carbonyl compound and then a second electron is transferred from the metal to the radical dimer species (for an alternative mechanism of the acyloin condensation, see Bloomfield, 1975 [29]). 

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