Benzaldehyde, pinacolization

Catalytic enantioselective synthesis of vzc-diols is a challenging issue. Chiral induction using chiral ligands is difficult to achieve. The moderately enantioselective pinacolization of benzaldehyde is demonstrated to be performed by the chiral titanocene catalysts 15 and 16 [42,43]. 

This suggestion appears reasonable, and accounts for the increased preference for formation of the dl pinacol in alkali, since the hydrogen bond in the transition state leading to 92 should be stronger than in 88. Propiophenone and benzaldehyde exhibit behavior analogous to that observed with acetophenone 98). 

Intermolecular coupling Many papers on hydrodimerization of aromatic carbonyl compounds have appeared indicating the importance of this reaction. The rac/meso ratio for the pinacolization of acetophenone in aqueous ethanol ranges between 0.9 and 1.4 in acidic medium and between 2.5 and 3.2 in basic medium. The diastereoselectivity is independent of the cathode material mercury, tin, or copper. Electrolysis conditions such as current density, potential, or current-controlled electrolysis also do not influence the diastereoselectivity. The same holds for propiophenone. For benzaldehyde, the rac/meso ratio is 1.1 to 1.2 in acidic as well as in basic media [283]. In the presence... 

Work at Coventry has shown the same switch from a two-electron to a one-electron pathway is occuring at higher insonation frequency in that the electroreduction of benzaldehyde at 800 kHz gives almost entirely the pinacol. A major benefit of the use of the higher frequency however is that there is little weight loss of the lead cathode compared to lower frequency (20 kHz) where it is substantially abraded away. 

Reduction of aromatic aldehydes to pinacols using sodium amalgam is quite rare. Equally rare is conversion of aromatic aldehydes to alkenes formed by deoxygenation and coupling and accomplished by treatment of the aldehyde with a reagent obtained by reduction of titanium trichloride with lithium in dimethoxyethane. Benzaldehyde thus afforded /ra/is-stilbene in 97% yield [206, 209]. 

As described above in Eq. 43, simple allylboronates can be transformed into more elaborated ones using olefin cross-metathesis. " Treatment of pinacol allylboronate 31 with a variety of olefin partners in the presence of Grubbs second-generation catalyst 142 smoothly leads to formation of 3-substituted allylboronates 143 as cross-metathesis products (Eq. 104). Unfortunately, these new allylic boronates are formed as mixtures of geometrical isomers with modest E/Z selectivity. They are not isolated but rather are treated directly with benzaldehyde to give the corresponding homoallylic alcohol products in good yields (Table A). 

Treatment of benzaldehyde with TiCU (lequiv.) and magnesium (l.Sequiv.) in TFIE at 25°C for 30min gives a mixture of 1,2-diphenylethene (43% yield) and dl- and OT ro-l,2-diphenylethane-l,2-diols (29% combined yields, dljmeso = AZiS ). The reaction conducted at 80°C produces the diphenylethene in 62% yield selectively. However, when catechol (1 equiv.) is added to the reaction mixture, pinacols are produced selectively in 76% yield dljmeso = 54/46) even at 80 °C. " Ultrasonic irradiation of a mixture of TiCU (15% in dilute HCl solution, 2 equiv.) and magnesium (4 equiv.) in EtOH accelerates the pincaol formation, but the diastereomeric ratio remains at a low level (dljmeso = 68/32). ... 

Table 1 Pinacol coupling reactions of benzaldehyde promoted by titanium reagents... 

Benzaldehyde acetal of the desired pinacol was produced in 64% yield. 

Addition of HMPA to Sml2 in THE changes the reaction course of the benzaldehyde dimerization. Although samarium(ii) iodide promotes pinacol coupling of benzaldehyde, use of 2.8equiv. of HMPA leads to the formation of, in addition to the pinacol (10% yield), a dimer (60% yield) that results from the connection of a carbonyl carbon and a phenyl para-c3.rbon (Equation (31)). ... 

Diastereomeric ratios of pinacol dimers obtained by coupling of benzaldehyde and acetophenone with samarium reagents are summarized in Tables 3 and 4, respectively. In general, the stereochemical control of pinacol coupling reactions with a stoichiometric or catalytic samarium(ll) reagent is poor, giving nearly a 1 1 dljmeso mixture. 

Table 3 Pinacol coupling of benzaldehyde with samarium reagents...

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. 

Pinacol rearrangement driven by the release of the ring strain of a cyclobutane ring has been employed in an extremely efficient manner to form cyclopentanone derivatives. Experimentally. the Lewis acid mediated aldol condensation of benzaldehyde with l,2-bis(trimethyl-siloxy)cyclobutcne at —78 C gave the pinacol 1 in its silylated form.35,36 Subsequent treatment of this pinacol with trifluoroacetic acid at room temperature afforded 2-phenyl-cyclopentane-l,3-dione (2) in 97% yield.35,36... 

In 1983, Kagan35 reported that benzaldehyde is conveniently converted into its pinacol dimer by reactions with Sml2 in 95% yield (cll/meso 54 44, Scheme 8). Ten years later, Kagan also reported that pinacolization of a variety of aldehydes and ketones could also be achieved with SmBr2 in virtually quantitative yield36. 

Jenevein and Stocker 132,404) jiave studied the ratio of d,l- to meso-pinacols in the hydrodimerization of benzaldehyde and substituted acetophenones at different cathodes (Hg, Cu, Sn), current densities, cathode potentials and pH. The yields ranged in acid as well as basic medium between 40% to 90%. The mesojd.l ratio for acetophenone pinacol is independent of all parameters but... 

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