Aldehydes interconversions

The commercially important normal to branched aldehyde isomer ratio is critically dependent on CO partial pressure which, in propylene hydroformylation, determines the rate of interconversion of the -butyryl and isobutyryl cobalt tetracarbonyl intermediates (11). 

Enols aie related to an aldehyde or a ketone by a proton-transfer equilibrium known as keto-enol tautomerism. (Tautomerism refers to an interconversion between two structures that differ by the placement of an atom or a group.)... 

Methods of the first type have been used for both qualitative and quantitative investigation. An important limitation is that the rates of interconversion of the tautomeric forms must be small as compared with those of the test reaction (s). The method is further complicated since the test reactions are sometimes complex and it is difficult to be certain that only one tautomer is reacting. An even more fundamental objection is that much chemical evidence is based on incorrect reaction mechanisms. Thus, the formation of condensation products (30) with aldehydes has repeatedly been quoted as evidence for structures of type 31 and against type 32,. whereas if 31 does react with an aldehyde it must either first tautomerize to 32 or ionize to 33. 

Another interesting biooxygenation reaction with alkenes, recently identified, represents an enzymatic equivalent to an ozonolysis. While only studied on nonchiral molecules, so far, this cleavage of an alkene into two aldehydes under scores the diversity of functional group interconversions possible by enzymatic processes [121,122]. 

Isomerization has been observed with many a,j3-unsaturated carboxylic acids such as w-cinnamic 10), angelic, maleic, and itaconic acids (94). The possibility of catalyzing the interconversion of, for example, 2-ethyl-butadiene and 3-methylpenta-l,3-diene has not apparently been explored. The cobalt cyanide hydride will also catalyze the isomerization of epoxides to ketones (even terminal epoxides give ketones, not aldehydes) as well as their reduction to alcohols. Since the yield of ketone increases with pH, it was suggested that reduction involved reaction with the hydride [Co" (CN)jH] and isomerization reaction with [Co (CN)j] 103). A related reaction is the decomposition of 2-bromoethanol to acetaldehyde... 

These catalysts facilitate the interconversion of isomeric compounds and include racemases, optimerases, cis-trans isomerases, intramolecular oxidoreductases and intramolecular transferases. Scheme 10.14 shows the conversion of an aldehyde to a ketone by triose phosphate isomerase. 

Dipolar cycloaddition of diazomethane to aldehydes can successfully be used for the preparation of tetrahy-drooxadiazole derivatives. Photochemical interconversion of 3-acylamino-l,2,5-oxadiazole derivatives leads to 1,3,4-oxadiazoles, though the method suffers from lack of selectivity. Many reports concentrate only on the synthesis and applications of new 1,3,4-oxadiazoles substituted with a wide variety of groups without introducing much of new chemistry. 

Furthermore, a base-catalyzed transformation by OH from the reaction medium between glycerate and hydroxypyruvate aldehyde (or hydroxypyruvic acid) could be excluded, while hydroxyacetone and glyceraldehyde interconversion was possible (Scheme 11.11). The existence of two major routes, of which hydroxyacetone and glyceric aldehyde are the primary oxidation products and glycolic and oxalic acid are the end-members, respectively, is now firmly established. Clearly, rapid oxidation of glyceraldehydes favors glyceric acid rather than hydroxyacetone formation. 

An irreversible consecutive reaction as a driving force to shift an unfavorable Cope rearrangement equilibria in the needed direction can be illustrated by the Cope-Claisen tandem process used for the synthesis of chiral natural compounds243. It was found that thermolysis of fraws-isomeric allyl ethers 484 or 485 at 255 °C leads to an equilibrium mixture of the two isomers in a 55 45 ratio without conversion into any other products (equation 184). Under the same conditions the isomer 487 rearranges to give the Cope-Claisen aldehyde 491 (equation 185). Presumably, the interconversion 484 485 proceeds via intermediate 486 whose structure is not favorable for Claisen rearrangement. In contrast, one of the two cyclodiene intermediates of process 487 488 (viz. 490 rather than 489) has a conformation appropriate for irreversible Claisen rearrangement243. 

D-xylose was converted into 2-furaldehyde in acidified, tritiated water, no carbon-bound isotope was detected. This suggested that the 1,2-enediol (2) reacted immediately, as otherwise, tritium would have been detected at the aldehydic carbon atom of 2-furaldehyde, as a result of aldose-ketose interconversion.An acidic dehydration performed with d-[2- H]xylose showed that an intramolecular C-2-C-1 hydrogen transfer had actually occurred. Thus, these data indicated that an intramolecular hydride shift is more probable than the previously accepted step involving a 1,2-enediol intermediate. 

There is a distinct relationship between keto-enol tautomerism and the iminium-enamine interconversion it can be seen from the above scheme that enamines are actually nitrogen analogues of enols. Their chemical properties reflect this relationship. It also leads us to another reason why enamine formation is a property of secondary amines, whereas primary amines give imines with aldehydes and ketones (see Section 7.7.1). Enamines from primary amines would undergo rapid conversion into the more stable imine tautomers (compare enol and keto tautomers) this isomerization cannot occur with enamines from secondary amines, and such enamines are, therefore, stable. 

As the last example in equation 44 demonstrates, synthetically interesting enantioen-richments can be achieved from configurationally labile 1-phenylselanylalkyllithium compounds . It has been shown that the e.r. in aldehyde addition roughly corresponds with the d.r., and it has been concluded that the interconversion of diastereomers is slower than the addition step " . 

A small number of examples is available for the synthesis of E and Z isomers of oximes. In many cases, E isomers were obtained either from the Z forms (by the hydrochloride method) or isolated by column chromatography. Often, the reagents that have been used for oximation of aldehydes and ketones also catalyze the interconversion of Z and E isomers. The rate of equilibration of a mixture of Z and E isomers and the position of the equilibrium is temperature-dependent ". In 2001, Sharghi and Sarvani reported a convenient method for controlling the stereochemistry of the reaction of hydroxylamine hydrochloride with aldehydes or ketones in the solid state. The highly stereoselective conversion of aldehydes and ketones to their corresponding oximes... 

Lithium Enolates. The control of mixed aldol additions between aldehydes and ketones that present several possible sites for enolization is a challenging problem. Such reactions are normally carried out by complete conversion of the carbonyl compound that is to serve as the nucleophile to an enolate, silyl enol ether, or imine anion. The reactive nucleophile is then allowed to react with the second reaction component. As long as the addition step is faster than proton transfer, or other mechanisms of interconversion of the nucleophilic and electrophilic components, the adduct will have the desired... 

FIGURE 7-6 Formation of the two cyclic forms of D-glucose. Reaction between the aldehyde group at C-l and the hydroxyl group at C-5 forms a hemiacetal linkage, producing either of two stereoisomers, the a and fi anomers, which differ only in the stereochemistry around the hemiacetal carbon. The interconversion of a and fi anomers is called mutarotation. 

IR data established a clear distinction between the predominant rhodium species in the absence and in the presence of amine, i.e., in reaction systems producing aldehyde and alcohol, respectively. Rhodium carbonyl anions, normally absent, are formed on addition of amine. No significance is attached to the differences between monomer and polymer behavior since a facile interconversion among the various carbonyl anions is well established (18). 

Griffiths and Gutsche (23) recently studied the interconversion of deuterated mandelaldehyde dimer and 2-hydroxyacetophenone in pyridine to obtain information concerning the glyceraldehyde-dihydroxy-acetone rearrangement. Their results support an enolization mechanism requiring a base and an acid catalyst. They found a deuterium isotope effect of ca. 1.3 for the transformation of the aldehyde to the ketone. When they corrected this for the apparently differing amounts of the aldehyde form in equilibrium with the proteo dimer and the deuterio dimer, they obtained a value of 3.9. By the Swain-Schaad equation (26) ... 

The enol tautomers of many ketones and aldehydes, carboxylic acids, esters and amides, ketenes, as well as the keto tautomers of phenols have since all been generated by flash photolysis to determine the pH rate profiles for keto-enol interconversion. Equilibrium constants of enolization, KB, were determined accurately as the ratio of the rate constants of enolization, kE, and of ketonization, kK, Equation (1). 

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