Benzaldehydes, para-substituted

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

Through the use of a tin(iv) enolate with benzaldehyde it was possible to generate the anti A diastereomer 47 with high selectivity (Entry 5). With tin(n) etiolates a highly substituent-dependent outcome was observed. Low selectivities resulted with para-substituted aromatic aldehydes, but good selectivities were observed for ortho-substituted aromatic aldehydes (Entries 7-9). Simultaneous re-... 

A Hammett plot for para-substituted benzaldehydes showed that electron-rich aldehydes gave higher ees (r = -0.4). As in Shibuya s related results (Section 5.3.3.1 above), this indicates that aldehyde coordination is important in enantiodifferentia-tion, but the lower rvalue (compared to Shibuya s r = -1.30) suggests a weaker electronic influence, probably due to the relative Lewis acidities of A1 and La. For ortho-substituted aldehydes, lower ees were observed, presumably due to steric effects. Although Al-Cl and Al-triflate complexes 29-30a-b did not catalyze the reaction, they... 

The Claisen-Schmidt condensation of 2 -hydroxyacetophenone and different chlorinated benzaldehydes over MgO has been investigated through kinetic and FTIR spectroscopic studies. The results indicate that the position of the chlorine atom on the aromatic ring of the benzaldehyde substantially affects the rate of this reaction. In particular, the rate increases in the following order p-chlorobenzaldehyde < m-chlorobenzaldehyde < o-chlorobenzaldehyde. The difference between the meta and para-substituted benzaldehyde can be attributed to electronic effects due to the difference in the Hammett constants for these two positions. Steric effects were found to be responsible for the higher rate observed with the o-chlorobenzaldehyde. 

Raja and Perumal reported the synthesis of novel 2,6-diaryl-3-(arylthio)piperidin-4-ones via a four-component reaction consisting of arylthioacetones, 2-substituted aromatic aldehydes and methylamine or ammonium acetate <06CPB795>. Further elaboration of this four component reaction to a novel five component tandem Mannich-enamine-substitution sequence involving the reaction of ethyl 2-[(2-oxopropyl)sulfanyl]acetate, two equivalents of a substituted aromatic aldehyde, and two equivalents of ammonium acetate is shown below <06T4892>. When this five-component tandem reaction involves para-substituted benzaldehydes, the cis (193) and trans (194) diastereomers of thiazones are obtained. Alternatively, orf/zo-substituted benzaldehydes form only the trans (194) diastereomer along with an air-oxidized product 195. 

The benzylideneanilines of Table IX are prepared by refluxing a mixture of 0.010 mole each of the appropriate para-substituted benzaldehyde and aniline in 150 ml of benzene containing 0.1 gm of benzenesulfonic acid for 2-4 hr. A Dean-Stark trap is used to collect the water and then the solvent is removed under reduced pressure. The residue is recrystallized from hexane or other... 

One step or two-step transfer Another major question about dehydrogenases is whether the hydrogen atom that is transferred moves as a hydride ion, as is generally accepted, or as a hydrogen atom with separate transfer of an electron and with an intermediate NAD or NADPH free radical. In one study para-substituted benzaldehydes were reduced with NADH and NAD2H using yeast alcohol dehydrogenase as a catalyst.30 This permitted the application of the Hammett equation (Box 6-C) to the rate data. For a series of benzaldehydes for which o+ varied widely, a value... 

Reaction Steps 3a and 3b also can be used to rationalize the observed para-substituent effects presented in Table III the more electron-releasing, para-substituted benzaldehydes retard the rate of oxidative addition (18) for RhCl(PPh3)3. Therefore, p-methyl- and p-methoxybenzaldehyde are expected to be decarbonylated slower than the unsubstituted benzaldehyde, as is observed in Table III. (This argument requires that Reaction 3a be saturated to the right, which is expected, in neat aldehyde solvent with electron-releasing, para-substituted benzaldehydes.) The unexpected slower rate for p-chloro-benzaldehyde could be accounted for ifK for this aldehyde is small and saturation of equilibrium in Equation 3a is not achieved. Note that fcobs is a function of K and k (see Equation 4b) under this condition. It is also possible that the rate-determining step is different for this aldehyde. Present research includes a careful kinetic analysis using several aldehydes so that K and k can be determined independently. 

Of the aromatic aldehydes the hydrogenation of para-substituted benzaldehydes was studied on supported Pd, Pt and Rh catalysts prepared with different types of Ti02 (equation 24)333-334. 

First, it was observed that substrates of lower basicity were hydrosilated by Ph3SiH much more rapidly than substrates of higher basicity. Thus, for the substrates benzaldehyde, acetophenone and ethylbenzoate, observed turnover numbers were 19, 45 and 637 h 1, respectively, while the measured equilibrium constants for adduct formation of these substrates with B(C6F5)3 were 2.1 X 104, 1.1 X 103 and 1.9 X 102. A similar inverse correlation between turnover number and equilibrium constant was observed for a series of para-substituted acetophenone derivatives, where much faster hydrosilation rates were observed for substrates with strongly electron withdrawing groups in the para position. Clearly, if activation of the substrate via adduct formation is important in the hydrosilation reaction, the opposite correlation between TON and Keq should be observed. 

Although significant improvements have been made in the synthesis of phenol from benzene, the practical utility of direct radical hydroxylation of substituted arenes remains very low. A mixture of ortho-, meta- and para-substituted phenols is typically formed. Alkyl substituents are subject to radical H-atom abstraction, giving benzyl alcohol, benzaldehyde, and benzoic acid in addition to the mixture of cresols. Hydroxylation of phenylacetic acid leads to decarboxylation and gives benzyl alcohol along with phenolic products [2], A mixture of naphthols is produced in radical oxidations of naphthalene, in addition to diols and hydroxyketones [19]. 

This rare combination is represented by only one type of example. Thus a-tosylaminomalononitrile (317) and benzaldehyde (318), in methanolic sodium acetate at 20-25°C for 20 h, gave 3, 6-diphenyl-2,2,5,5-piperazinetetracarbonitrile (319) as the major product (48% yield) 834 several para-substituted phenyl and other analogs were made similarly, most in comparable yields.834... 

Fio. 7. Logarithms of second-order rate constants (in units of min"i) for the hydrolysis of a series of para-substituted benzaldehyde diethyl acetals in aqueous solution (lower line) and in the presence of sodium dodecyl sulfate (upper line) plotted against the Hammett substituent constants (Dimlap et cU., 1969). 

Second-order rate coefficients, kH (= rate/[S] [H30+]), for the hydrolyses of some typical acetals, ketals, and orthoesters in purely aqueous solutions are collected in Table 12. In a compilation of data from one single source [162], ftH values can be found for the reactions of a large number of diethyl acetals and ketals in 50 % dioxane—water at 25 °C. In more recent studies, kH values have been determined for the hydrolyses of substituted benzaldehyde diethylacetals [163] and benzophenone diethyl-ketal [164] in the same solvent (Table 13). The hydrolyses of para-substituted methyl orthobenzoates have been studied in 70 % methanol-water [169]. A large amount of other work is concerned with various special examples. 

In the case of aryl aldehydes and ketones, benzaldehyde afforded benzyl alcohol as the major product, but acetophenone and its para-substituted derivatives carrying such groups as OMe, Cl or OH provided ethylbenzene derivatives in good yields. As with Clemmensen reduction, the alcohol produced in this reduction cannot be further reduced, and the alcohol is not therefore an intermediate. Still uncertain in the reaction mechanism of electrolytic reduction, however, is the role of adsorbed hydrogen. ... 

The effective catalyst 20o ((—)-DFPE) was examined for the ethylation of other aldehydes (Table 3-7). Para-Substituted benzaldehydes, ( )-cinnamaldehyde, 2-furaldehyde, and 2-naphthaldehyde, which possess n electrons adjacent to the carbonyl group, afforded the corresponding secondary alcohols in high enantiomeric purity (entries 1 — 5). In addition, cyclohexanecarboxaldehyde, 2-ethylbutyralde-hyde, and pivalaldehyde, which are branched a. to the carbonyl group, were ethylated in >98% ee (entries 6—11). On the other hand, ethylation of isovaleraldehyde, 3-phenylpropionaldehyde, and n-butyraldehyde, which lack a substituent a to the carbonyl group, proceeded with low selectivity (entries 12 — 14). 

The IL effects can be explained with solvophobic interactions that generate an internal pressure, which promoted the association of the reactants in a solvent cavity during the activation process and showed an acceleration of the multicomponent reactions (MCRs) in comparison to conventional solvents. The reaction proceeded very efficiently with benzaldehyde and electron releasing and electron-withdrawing ortho-, meta-, and para-substituted benzaldehydes. IL was easily separated from the reaction medium by washing with water and distillation of the solvent nnder vacnnm and it can be reused for subsequent reactions and recycled. IL showed no loss of efficiency with regard to reaction time and yield after four successive runs. 

Oxazolidines are subject to ring-chain tautomerism. A variety of substituted oxazolidines in the solid state exist in the chain form, based on C NMR experiments <85X5919,92X4979). In solution, the two forms are in equilibrium, the position of which depends on the solvent and the substituents. Oxazolidines prepared from meta- and para-substituted benzaldehydes and 2-amino-2-methyl-propanol, norephedrine, norpseudoephedrine, and serine esters all give good linear plots for the equation log -l-log Ax=h where a are the Hammett-Brown values <93X6701,93JOC1967). 

Kinetic and spectroscopic measurements support the hypothesis that the substrate binds directly to the catalytic zinc atom. Dunn and Hutchison (336) have produced evidence using a chromophoric aldehyde as substrate that the carbonyl oxygen of the reaction intermediate is coordinated to zinc. They concluded that zinc acts as a Lewis acid catalyst. Similar conclusions have been reached by McFarland and co-workers from the spectral properties of the enzyme complex with 4- (2 -imidazolyl-azo) benzaldehyde (337), from the observed small electronic substituent effect of para-substituted benzaldehydes (335), and from the absence of a large pH effect in the hydride transfer step (338). Assuming the mechanisms and the subunit structures to be essentially similar in YADH and LADH, a magnetic resonance study of coenzyme and substrate binding to YADH (339) also support the hypothesis of direct binding of substrate to zinc. 

Carbon-13 shifts, measured by the double-resonance technique, of the carbonyl group of weto-substituted benzaldehydes correlate with Hammetts parameter. In the case of the para-substituted benzaldehydes, there was no correlation between the chemical shift and a, presumably because, in contrast to substituted benzenes, resonance contributions for both electron-attracting and releasing groups will not significantly affect the electron density at the carbonyl carbon. 

Jacobs et al. (86) have compared the electronic substituent effect on the rate of sodium borohydride reduction vis a vis the LADH-catalyzed reduction (under transient-state kinetic conditions) for a series of para-substituted benzaldehydes. In contrast to the large electronic substituent effect observed in the sodium borohydride reaction (the rate ratio ftp-ci/ p-ocHs is 100), the LADH catalyzed reaction was found to show almost no electronic substituent effect ( p-ci/ p-ocHs —... 

Berson and co-workers have further studied the kinetics of the cycloaddition of 2,5-dimethyl-3,4-dimethylenethiophene with para-substituted styrenes and benzaldehydes in order to understand... 

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