Pinacol-type dimer

Moreover, no aldol or pinacol-type dimerization of the carbonyl compound was observed, even in the case of easily reducible benzophenone or benzaldehyde derivatives. As observed previously, the nature of the solvent is an important factor for the success of the reaction. Indeed, no homoallylic alcohols were formed in DMF. 

The 4-acyl-l,2,3-triazole (96) gave an interesting pinacol-type dimer (97) by photoreduction in isopropanol118 [Eq. (6)]. When a heterocycle has an exocyclic C=C bond, a photodimer of type B2 can be formed for example, the dimer (99) was obtained1183 from the pseudo-oxazolone (98) [Eq, (7)]. 

We have found that crystalline 4,4 -dimethylbenzophenone (1 ) undergoes photocoupling, albeit a low maximum conversion, to give solely (eq 1) (10). A pinacol-type dimer, which is a main product in the solution phase photolysis, is not formed. The distances d and d are 3.32 A and 3.87 A, respectively, and the angles a and 8 are 80 and 92 , respectively. By contrast,... 

A pinacol-type silylative dimerization of various aromatic aldehydes promoted by a cationic thiolate-bridged diruthenium complex has been reported by Flidai and co-workers (Equation (28)).344 l,2-Diaryl-l,2-disiloxyethanes 153 were isolated as the major products along with the corresponding arylmethyl silyl ethers as minor products. 

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. 

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]). 

On one-electron rednction, aldehydes and ketones give anion-radicals. It is the carbonyl group that serves as a reservoir for the unpaired electron Ketones yield pinacols exclusively. Thus, acetophenone forms 2,3-diphenylbutan-2,3-diol as a result of electrolysis at the potential of the first one-electron transfer wave (nonaqueous acetonitrile as a solvent with tetraalkylammonium perchlorate as a supporting electrolyte) (van Tilborg and Smit 1977). In contrast, calculations have shown that the spin densities on the carbonyl group and in the para position of the benzene ring are equal (Mendkovich et al. 1991). This means that one should wait for the formation of three types of dimer products head-to-head, tail-to-tail, and head-to-tail (cf. Section 3.2.1). For the anion-radical of acetophenone, all of the three possible dimers are depicted in Scheme 5.21. 

Careful consideration of Scheme 3.9 allows one to make reasonable suppositions about how to control further reactions of 16. Once radical 16 is formed, the outcome of the reaction depends upon the relative stoichiometry of other components present in the reaction system. The use of a proton donor such as trifluoroethanol leads to rapid conversion of either dimeric or monomeric ketyl to intermediate 18. Rapid production of 18 leads to dimerisation and formation of pinacol 19. If protonation of the ketyl is slow, reduction of 18 by excess ketyl or Sml2 followed by protonation leads to alcohol 21 through a traditional House-type mechanism (Scheme 3.10).9... 

These dimerizations are analogous to those of the radical anions R2C 0 which are intermediates in the reduction of ketones to pinacols. Indeed, in the presence of magnesium amalgam, pyridine condenses with acetophenone to give alcohol 396 by oxidation of the intermediate dihydropyridine. In a similar reaction type, pyridine with zinc and acetic anhydride or ethyl chloroformate yields (397 R = Me or OEt, respectively). 

Another type of cleavage during electrolysis occurs as a slow step in the reduction of certain )6-diketones in DMSO [19] the primarily formed radical dimerizes to a pinacol that, catalyzed by the base generated during the electrolysis, is cleaved in a reverse aldol condensation to an a-diketone. 1,3-Diphenyl-1,3-propanedione is thus reduced in DMSO according to ... 

The example shows the dimerization of acetone to give a diol (2,3-dimethylbutane-2,3-diol) whose trivial name, pinacol, is used as a name for this type of reaction using any ketone. Sometimes pinacol reactions create new chiral centres in this example, the two diastereoisomeric diols are formed in a 60 40 mixture. If you want to make a single diastereoisomer of a diol, a pinacol reaction... 

The reductive coupling of a,p-unsaturated carbonyl compounds can lead to three types of dimerization products pinacols (1,2-coupling), 1,6-diones (1,4-coupling), and y-hydroxycarbonyl compounds (mixed coupling). These couplings are further complicated by the ability of the initially formed dimerization products to undergo further reactions and complex product mixtures can sometimes result. ... 

Some studies mention the cleavage of dianion n acceptors, which looks like a homolytic cleavage by cathodic activation. When electron-poor bonds are considered, this type of scission should not be confused [73] with retrodimerization of pinacols (or any dimer with labile hydrogens) in strongly basic media ... 

Pinacols are vicinal carbinols with two OH groups on tert-C atoms. They are available by Mg-promoted reductive dimerization of ketones involving radical-type... 

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