Pinacol-type coupling

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. 

Samarium metal, in the presence of various additives such as ammonium chloride or bromide, induces reductive dimerizations of aromatic ketones to give 1,2-diols, with some diastereoselectivity, and with some alcohol (i.e. reduced ketone) by-product.164 The syn-selectivity observed in many cases may be due to samarium chelation of the 

Synthetically useful ft, y-8, e-doubly-unsaturated quaternary carbinols have been prepared enantioselectively via regioselective reductive coupling of 1,3-enynes with 

Samarium(II) iodide mediates diastereo- and enantio-selective pinacol coupling of chiral a-ketoamides.166 

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Similar stereoselectivities are achieved in the allylation of enantiomerically pure proline-derived a-oxoamides47. l-Bromo-3-methy]-2-butcne reacts with clean allylic inversion. Since pinacol-type coupling products are also produced under the reaction conditions, this was taken as evidence for a radical addition mechanism47. 

SELECTIVE PINACOL-TYPE COUPLING USING ZINC, CHLOROSILANE, AND CATALYTIC AMOUNTS OF Cp2VCl2 dl-1,2-DICYCLOHEXYLETHANEDIOL... 

Samarium diiodide is another powerful one-electron reducing agent that can effect carbon-carbon bond formation under appropriate conditions.257 Aromatic aldehydes and aliphatic aldehydes and ketones undergo pinacol-type coupling with Sml2 or SmBr2. 

From a stereochemical point of view, the pinacol-type coupling is much more interesting, because two new stereogenic centers are established at the former carbonyl C atoms. When the coupling reactions described above were carried out with (R)-2 at 0 °C the pinacol product 19 (Fig. 1.5.4) was obtained as one out of three possible stereoisomers. 

Racemic aldehyde 2 under similar reaction conditions yielded two diastereo-meric pinacols the configuration of all the products was assigned by X-ray diffraction. On the basis of these results a mechanistic scheme could be proposed to explain the stereochemical course of the coupling reactions [19]. It is assumed that the reaction proceeds via an intermediate titanaoxirane by insertion of a molecule of aldehyde 2 into the Ti—C bond - a proposal that has been put forward recently on the basis of DFT calculations [20]. The pinacol-type coupling products could be employed as bidentate P,P chelate ligands to Mo(CO)4 fragments however analogous experiments carried out with the olefinic derivatives 18 were unsuccessful [19]. 

The pinacol-type coupling of aliphatic aldehydes, in the presence of niobium(IH) salts, occurs, with a high anti diastereoselectivity (equation 131)512. In the case of aromatic aldehydes and ketones the alkene product is sometimes formed513. In both cases the cyclic acetals may also be formed. 

Esters undergo pinacol-type coupling the acyloin reaction... 

A speculative biosynthetic pathway for trehazolin (2) has been proposed,42 as shown in Scheme 1. Two molecules of the glucosylamine (i) could react with carbon dioxide to give the carbodiimide ii, which could be transformed into iii. Subsequent regioselective oxidation to iv, followed by a stereoselective pinacol-type coupling could afford trehazolin (2). The biosynthesis of trehalose has also been the subject of several subsequent chapters.43-48... 

When a,j9-unsaturated aldehydes were exposed to a stoichiometric or a catalytic amount of Cp2TiCl2 and s-BuMgCl, pinacol-type coupling products were isolated in good yields [Eq. (179) 423,424]. The excellent stereoselectivity found in the products is noteworthy. 

There are a few reports on the synthesis of N,N -dimethyl-1,2-diphenyl-ethylenediamine by pinacol couplings3 8 or by N-methylation of the primary diamine.9,10 Pinacol type couplings of imines have been reported for the synthesis of other C2 symmetrical N-substituted or unsubstituted 1,2-diarylethylenediamines using various reducing metals in different solvents.11 30 However, the present synthesis of N,N -dimethyl-1,2-diphenylethylenediamine is the most suitable for large scale preparation. There are no expensive or hazardous starting materials, and the procedure does not require tedious filiations or chromatographic separation of diastereomers. 

The coupling of carbonyl compounds " to give 1,2-diols, known as the pinacol coupling, has been carried out in aqueous media. Clerici and Porta extensively studied the aqueous pinacol coupling reactions mediated by Ti(III). Schwartz reported a stereoselective pinacol coupling with a cyclopentadienetitanium complex. Pinacol-type couplings were also developed by using a Zn-Cu couple, Mn, °... 

A soln. of benzaldehyde and 5% SmClj in DMF electrolyzed (apparent current density 0.4-0.5 A/dm ) in a one-compartment cell fitted with a nickel or stainless steel cathode and a sacrificial Mg anode at 20° for 5-18 h, and hydrolyzed with acid - product. Y SOVo. This is the first use of a samarium salt in catalytic amount for pinacol-type coupling the method is applicable to aliphatic and aromatic oxo compds. F.e. inch intramolecular coupling s. E. Leonard et al., J. Chem. Soc. Chem. Commun. 1989, 276-7. 

Pinacol Type Coupling Reaction Reduction of Ketones... 

High stereoselectivity for the d,l product diastereomer has been found in the photochemically induced pinacol-type coupling of aldehydes by tristannanes (Scheme 35). The stannadioxolans (55) produced may be worked-up with acetyl chloride (or trimethylsilyl chloride) to give derivatives of 1,2-diols. 

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