Aldehydes competition reaction

This strong influence of the steric environment of the carbonyl group on the rate of the addition reaction is also observed in the case of aldehydes (Scheme 13.10). It allows good results in various aldehyde-aldehyde competition reactions to be obtained. To the best of our knowledge, such a selectivity has never been described with other organometallics. 

On the basis of the dissociation constant values, it seems sensible to conclude that, in these moderately basic carbinolamines, the hydrogen atom of the hydroxyl group is suflQciently acid to be eliminated under the influence of an alkali and by its transfer to the nitrogen atom of the mesomeric anion, the formation of the amino-aldehyde form may result. Instead of the amino-aldehyde, however, the corresponding bimolecular ether (15a-c) can be obtained. " It can be concluded that the formation of the bimolecular ether (S l or 8 2 mechanism) and the formation of the amino-aldehyde (B-SeI or B-Se2 mechanism) are competitive reactions. It seems probable that where the first reaction can occur the latter one is pushed into the background. The triple tautomeric system postulated by Gadamer... 

Various competitive reactions can reduce the yield of the desired Michael-addition product. An important side-reaction is the 1,2-addition of the enolate to the C=0 double bond (see aldol reaction, Knoevenagel reaction), especially with a ,/3-unsaturated aldehydes, the 1,2-addition product may be formed preferentially, rather than the 1,4-addition product. Generally the 1,2-addition is a kinetically favored and reversible process. At higher temperatures, the thermodynamically favored 1,4-addition products are obtained. 

The concept of site isolation is important in catalysis. On metal particles one usually assumes that ensembles of metal atoms are necessary to activate bonds and to accommodate the fragments of molecules that tend to dissociate or to recombine. We present here three examples of such effects the dehydrogenation of decane into 1-decene, the dehydrogenation of isobutane into isobutene and the hydrogenolysis of acids or esters into aldehydes and alcohols. In most cases the effect of tin, present as a surface alloy, wiU be to dilute the active sites, reducing thereby the yield of competitive reactions. 

Oxidation of the hydroxyl group in 186 with pyridinium chlorochromate (PCC) in CH2CI2 affords the aldehyde 197. The reduction of 197 back to 186 is possible in EtOH in the presence of TiCl4, whereas upon treatment of 197 with diisobutyl-aluminium hydride a competitive reaction with the fullerene core was observed. 

Alternatively, acetaldehyde and acetic anhydride are fed directly to the cracking reactor where the same sulfonic acid can catalyze the condensation of the aldehvde-anhydride mixture to EDA and the subsequent thermal elimination forming vinyl acetate. The best results are obtained when acetic anhydride is present as solvent to inhibit the competitive elimination to acetaldehyde and anhydride (see reverse of equation 2). Aldehyde degradation reactions are minimal under these conditions. 

C Kinetic isotope effects (KIEs) of a xylose reductase-catalysed cinnamalde-hyde reduction have been determined by 13C NMR using competition reactions with reactants at natural 13C abundance. The primary KIEs indicated that the chemical reaction steps are only partly rate limiting during reduction of aromatic aldehydes and slow steps occur outside the catalytic sequence. The aldo-keto reductase-catalysed... 

In the preparation of dynamic nitroaldol systems, different aldehydes and nitroalkanes were first evaluated for reversible nitroaldol reactions in the presence of base to avoid any side- or competitive reactions, and to investigate the rate of the reactions. 1H-NMR spectroscopy was used to follow the reactions by comparison of the ratios of aldehyde and the nitroalcohols. Among various bases, triethylamine was chosen as catalyst because its reactions provided the fastest exchange reaction and proved compatible with the enzymatic reactions. Then, five benzaldehydes 18A-E and 2-nitropropane 19 (Scheme 9) were chosen to study dynamic nitroaldol system (CDS-2) generation, because of their similar individual reactivity and product stabilities in the nitroaldol reaction. Ten nitroaldol adducts ( )-20A-E were generated under basic conditions under thermodynamic control, showing... 

Carbonyls and acetals are converted to 1,3-dithianes and -dithiolanes upon treatment with 2-stanna-l,3-dithianes and -dithiolanes under catalysis by organotin tri-flates [147]. In these competition reactions, various types of carbonyl and acetal are differentiated. Aldehydes react preferentially over ketones (Eq. 104), but the preference is completely reversed in the competition reactions between the corresponding acetals and ketals (Eq. 105). The reactivity of aliphatic aldehydes is greater than that of the acetals of aliphatic aldehydes and ketones. Conversely, an aromatic acetal is... 

The relative reactivity of aldehydes and acetals toward a typical ketene silyl acetal in the presence of TiCU has been revealed by use of competition reactions (Eqs (22) [98, 99], (23) [98], and (24) [98]). Although yields are not necessarily high, perhaps because these experiments were conducted solely to compare the reactivity of the compounds, the results shown in these equations are quite informative (i) acetals are more reactive than the corresponding aldehyde, (ii) aliphatic aldehydes are more reactive than aromatic aldehydes, and (iii) electron-rich aromatic aldehydes are somewhat more reactive than the electron-deficient variety. 

Kobayashi et al. also reported interesting chemoselectivity of aldehydes and imines in the Yb(OTf)3-catalyzed addition reactions of silyl enol ether, allylstannane or trimethylsilyl cyanide [12]. In the competitive reactions between aldehydes and imines, the imines reacted faster than the aldehydes (Tables 4-6). This tendency is not unique to Yb as catalyst selectivity is similar for other Ln(OTf)3. Nuclear magnetic resonance (NMR) studies revealed selective formation of an imine-Yb(OTf)3 complex in the presence of an aldehyde. This preference was reversed when conventional Lewis acids (SnCE, TiCU, TMSOTf, and BF3 OEt2) were used. 

The same catalyst is also effective in three-component reactions between aldehydes, amines, and silylated nucleophiles, leading to amino ketone, amino ester, and amino nitrile derivatives, respectively (Eq. 30) [114]. It is reported that 103 can be recovered and that continuous use is possible without any loss of activity. More interestingly, in competitive reaction of aldehyde, aldimine and silyl enolate, the less reactive aldimine reacted exclusively with silyl enolate in the presence of 103. This unique selectivity was explained by the polymer effect [115]. 

One of the standard methods for the preparation of aldehydes involves the reduction of acid halides. A variety of stoichiometric reducing systems are available for this transfomiation, which include NaAlH(OBu-r)3, LiAlHfOBu-O.i, NaBHfOMe). Catalytic hydrogenation with H2 and Pd on carbon is also a popular method. In contrast, methods based on the radical reduction of acyl halides are synthetically less important. Radical reduction methods involve generation and subsequent hydrogen abstraction as key steps, which is complicated by decarbonylation of the intermediate acyl radicals. The first example in Scheme 4-1 shows that this competitive reaction is temperature dependent, where an acyl radical is generated from an acyl phenyl selenide via the abstraction of a phenylseleno group by tributyltin radical [5]. 

Maruoka et al. have developed the aluminum-based bidentate Lewis acid 14 for double electrophilic activation of carbonyl compounds (Scheme 10.9) [37]. The aldol addition of cyclohexanone TMS enolate to benzaldehyde is effected by the bidentate 14, whereas its monodentate counterpart 15 shows no evidence of reaction under similar conditions. In competitive reactions of aldehydes and acetals, 14 effects aldehyde-selective addition [38]. 

Cyanosilylation of imines (Strecker-type reaction) is efficiently promoted by conventional Lewis acids such as ZnX2, AlCl , and TiCLj [604]. Kobayashi et al. recently disclosed that Yb(OTf)3 has high catalytic activity in this cyanosilylation (Scheme 10.237) [622]. In the competitive reaction of aldehydes and the corresponding imines with TMSCN, Yb(OTf)3 activates imines to give only a-aminoni-... 

Attempted cyclization of 233 with alkali afforded 234 in addition to 238, resulting from the elimination of one molecule of water from the hydrazone residue and the quinoxalinone ring and simultaneous hydrolysis of the ester group. This indicated the presence of two competitive reactions under conditions of cyclization. The structure of 234 was proved by its preparation by hydrolysis of 235. The latter was prepared by the reaction of aldehyde 237 with the phosphorane. The structures were confirmed by studying their H-NMR and mass spectra. 

Likewise, the competitive reaction of aldehydes 23 and 24 with allyltributylstannane gives homoallylic alcohol (25) exclusively (Scheme 1.18). In contrast, nonfluorinated cycloadduct is produced mainly under the catalysis of BF3 etherate in methylene chloride. These results clearly demonstrate that fluorinated carbonyl compounds are more reactive however, there is a turnabout in the reactivity under the Lewis acid catalytic conditions because of the lower basicity of the carbonyl group than that of the nonfluorinated one. 

Scheme 10. A competition reaction between an aldehyde and an aldimine...

The competitive oxidation of substituted benzaldehydes in acetic anhydride solution, in which the peracids are transformed into acetyl benzoyl peroxides thus eliminating peracid—aldehyde interaction, enabled the relative reactivities of the benzoylperoxy radicals to be determined with respect to a series of aldehydes [47], In a mixture of two aldehydes, ther peroxidic radicals react according to the two pairs of competitive reactions... 

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