2- benzaldehyde, dimerization

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)). ... 

Fig. 26 Left. A benzaldehyde dimer at 5 A separation. Right. CD of the dimer computed with TDDFT for the dimer supermolecule, and using a coupled TDDFT frozen-density embedding subsystem approach [266], Figures courtesy of Dr. J. Neugebauer...

That is, the reaction of primary alcohols or ethers with a calculated amount of BTMA Br3 in carbon tetrachloride-water in the presence of Na2HP04 at 60°C gave dimeric esters in good yields. In the case of benzyl alcohol, the only oxidation product was benzaldehyde (Fig. 20). 

Salen ligands have also been used in the titanium-catalyzed trimethylsilyl-cyanation of benzaldehyde. The complexes were immobilized by substitution of a chloride with a surface silanol from the support. In the first study on this reaction [38], the most efficient ligand was the non-symmetrical salen Im (Fig. 11) (94% ee), whereas the selectivity obtained with the symmetrical ligand la was significantly lower (72% ee). In a recent paper, the immobilization of different titanium species, including monomeric and dimeric systems with... 

Incorporation of a flavin electron donor and a thymine dimer acceptor into DNA double strands was achieved as depicted in Scheme 5 using a complex phosphoramidite/H-phosphonate/phosphoramidite DNA synthesis protocol. For the preparation of a flavin-base, which fits well into a DNA double strand structure, riboflavin was reacted with benzaldehyde-dimethylacetale to rigidify the ribityl-chain as a part of a 1,3-dioxane substructure [49]. The benzacetal-protected flavin was finally converted into the 5 -dimethoxytri-tyl-protected-3 -H-phosphonate ready for the incorporation into DNA using machine assisted DNA synthesis (Scheme 5a). For the cyclobutane pyrimidine dimer acceptor, a formacetal-linked thymine dimer phosphoramidite was prepared, which was found to be accessible in large quantities [50]. Both the flavin base and the formacetal-linked thymidine dimer, were finally incorporated into DNA strands like 7-12 (Scheme 5c). As depicted in... 

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). 

Benzaldehyde gave modest yields of the coupled product and 15% of trans-stilbene. By using large excesses of zinc and trimethylchlorosilane, the stilbene yield was increased to 36%. Thus far, only 1-indanone has produced high yields, 83%, of the unsaturated dimer(29) ... 

In a related study involving structurally similar chiral methylzinc anisyl fencholates, both chiral amplification and depletion were observed in the catalytic alkylations of benzaldehyde.209 Thus, methylzinc anisyl fencholates, bearing sterically small substituents in the ortho-position of the anisyl group, crystallized preferentially as homochiral dimers, as shown for the methyl-substituted anisyl group in Scheme 91. Because of the greater stability of the homochiral dimers, scalemic mixtures of both enantiomers of the ligand showed a chiral depletion of the benzyl alcohol. 

Perhaps due to oxidizing quinoid type electronic structure of benzotriazol-2-yl derivatives, some of their properties are completely different from those of isomeric benzotriazol-l-yl derivatives. Thus, anions derived from 2-alkylben-zotriazoles 388 are rapidly converted to appropriate radicals that undergo coupling to form dimers as mixtures of racemic 289 and meso 390 forms <1996LA745>. When the reaction mixture is kept for an extended period of time at —78 °C, (Z)- 391 and (E)- 392 alkenes are formed. When benzophenone is added to the reaction mixture, alcohols 387 are obtained in good yields however, benzaldehyde does not react under these conditions (Scheme 63). 

In a direct comparison of the reactivity of 1-alkyl- and 2-alkylbenzotriazoles, compound 393 was lithiated in the presence of benzophenone with 1 equiv of LDA to give a mixture of alcohol 394 and dimer 395 (Equation 12) <1996LA745>. No reaction was detected at the carbon adjacent to the benzotriazol-l-yl moiety. When benzaldehyde was used instead of benzophenone, only dimer 395 was obtained. This suggests that a-benzotriazol-2-yl carbon radical reactions are much faster than those of a-benzotriazol-l-yl) carbanions. 

Another example of homogeneous catalysis in aqueous solution is the dimerization of benzaldehyde catalyzed by cyanide ion, CN- (Wilkinson, 1980, p.28) ... 

There is an equilibrium between the dimer and monomer, and molecular orbital study suggests that the heterochiral dimer is more stable than the homochiral isomer. The existence and behavior of the dimeric species were well confirmed by experiments such as cryoscopic molecular weight and NMR measurement. In the NMR study of a DAIB-catalyzed dialkylzinc addition reaction, noticeable changes were observed in the spectrum of the homochiral dimer on the addition of benzaldehyde, while the spectrum of the heterochiral complex remained the same. This may imply that the heterochiral complex is very stable and does not react, and the homochiral dimer leads to the reaction product. 

The reactions of complex 2a with ketones and aldehydes show a strong dependence on the substituents. With benzophenone, substitution of the silyl-substituted acetylene leads to the r]2-complex 58, which is additionally stabilized by a THF ligand. This complex can serve as an interesting starting material for other reactions. With benzaldehyde and acetophenone, the typical zirconadihydrofuran 59, akin to 2c, is obtained from a coupling reaction. This complex is unstable in the case of benzaldehyde and dimerizes, after elimination of bis(trimethylsilyl)acetylene, to yield 60. In this respect, it is similar to the above discussed complex 2c, since both of them show a tendency to eliminate the bis(trimethyl-silyl)acetylene. The reaction of methacrolein with complex 2a depends strongly on the solvent used [40]. 

As noted in Section 2.2.5, the effect of dimerization may also be seen on the second wave, the wave that corresponds to the reduction of the radicals formed at the first wave. The example presented in Figure 2.35 shows the cyclic voltammetry of benzaldehyde in basic ethanol.26 The second wave represents the reduction of the benzaldehyde anion radicals formed at the first wave that have escaped dimerization. In other words, Scheme 2.29 should be completed by Scheme 2.30. 

The generation of phenylcarbene (512) and p-tolylcarbene at 250 °C/40Torr in the gas phase brought about only the ring expansion to give 5 and 10, respectively and their dimerization yielding 522 and 557 (Scheme 6.113). The co-thermolysis of the sodium salts of benzaldehyde and p-methylbenzaldehyde tosylhydrazone also gave rise to the mixed heptafulvalene 556. With a ratio of 1 2 1, the amounts of 522, 556 and 557 verified the expectation based on statistics [212]. 

The dimerization of aldehydes to form esters is a completely atom-efficient process known as the Tishchenko reaction, which involves no net oxidation or reduction. Suzuki, Katoh, and coworkers have used complex 77 to catalyze the Tishchenko reaction of a range of aldehydes, including dihydrocinnamaldehyde 91 and benzaldehyde 14 (Scheme 22) [81]. The same catalyst has been used for an intramolecular variant of the reaction, where keto-aldehyde 92 isomerizes to lactone 93 via an intramolecular Tishchenko reaction. The oxidized product is formed as a by-product,... 

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

The Ponzio reaction provides a useful route to gem-dinitro compounds and involves treating oximes with a solution of nitrogen dioxide or its dimer in diethyl ether or a chlorinated solvent. The Ponzio reaction works best for aromatic oximes where the synthesis of many substituted aryldinitromethanes have been reported. Compound (56), an isomer of TNT, is formed from the reaction of dinitrogen tetroxide with the oxime of benzaldehyde (55) followed by mononitration of the aromatic ring with mixed acid. Yields are usually much lower for aliphatic aldoximes and ketoximes. " The parent carbonyl compound of the oxime is usually the major by-product in these reactions. 

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