Isotope effects acetaldehyde

The hydroxo complex [D] is assumed to rearrange to the cr-bonded compound [A] since only a secolidary isotope effect is observed with C2D4, and if the n complex went directly to acetaldehyde Henry claims that the hydride shift involved would have produced a primary deuterium effect. Although the hydroxo complex [D] would have been expected to be traris, it was suggested that kinetically significant amounts of the cis isomer are present. 

The excellent agreement with the experimental and calculated isotope effect (calculated for formaldehyde, 3.22, and for acetaldehyde, 3.3 experimental value 2.9) supports the computational approach. This suggests that the computed transition structure for hydride transfer in the reaction of the lithium enolate of acetone with acetaldehyde (Figure 30) is realistic. 

TABLE 6. Computed reaction and activation energies and corresponding isotope effects for the sequence of reactions between lithium vinyloxide (LiEn) and formaldehyde and between the hthium enolate of acetone (AcCH2Li) and acetaldehyde. Reproduced with permission from Reference 29. Copyright 1998 American Chemical Society... 

Based on secondary kinetic deuterium isotope effects and some ab initio calculations, Alston et al.47 calculated that the transition state in the deprotonation of acetaldehyde by HO is imbalanced in the sense shown in Equation (3). 

Two types of inhibitors, pyrazoles and imidazoles (with E-NAD+) and iso-butyramide (with E-NADH), form tight ternary complexes with E-coenzyme, allowing single turnover to be observed (through photometry at 290 nm or fluorescence caused by NADH) and thus titration of the active sites (see Section 9.2.3.). Pyrazole and isobutyramide are kinetically competitive with ethanol and acetaldehyde, respectively. If the reaction E + NADH + aldehyde is run in the presence of a high concentration of pyrazole, the complex E-NAD+ formed by dissociation of alcohol immediately binds pyrazole for a single turnover only. Under favorable conditions, a single NADH oxidation can be observed by stopped-flow techniques to find a kcat of about 150 s 1 and a deuterium isotope effect kD 4 as expected (see Section 9.2.5). 

No metastable peak has been observed for loss of a methyl group following El of acetaldehyde, CH3CHO however, the metastable ion decomposition to lose CD3 has been seen with CD3CDO [695], This intermolecular isotope effect has been interpreted in terms of the isomerisation (CH3CHO)f - (CH2=CHOH)t occurring in the unlabelled molecule and precluding methyl loss [695]. 

TABLE 13. Computed reaction and activation energies (kcalmoD ) and corresponding kinetic (KIE) and equilibrium (ElE) isotope effects a reaction sequence between lithium acetaldehyde enolate (Lien) and formaldehyde... 

The ratios of the two deuterated acetaldehydes, which is a measure of the isotope effect, is easily measured by mass spectrometry. The value of h/ d was found to be 1.7 which is higher than the experimentally determined value of This is strong evidence that hydroxypallada-... 

The existence of a free carbonium ion such as VII in a strongly solvating medium is highly improbable. Only if VII could exist in association with the palladium could decomposition to vinyl acetate be expected to occur with a reasonable degree of frequency, in competition with the reaction with acetate to form ethylidene diacetate. Similar results have been reported in the Wacker acetaldehyde synthesis when D2O is used as the solvent (25). Stern (54) has reported results in which 2-deuteropropylene was used as substrate in the reaction. Based on assumed /J-acetoxyalkylpalladium intermediates, on the absence of an appreciable isotope effect in the proton-loss step, and on the product distribution observed, excellent agreement between calculated (71%) and observed (75%) deuterium retention was obtained. Several problems inherent in this study (54) have been discussed in a recent review (I). Hence, considerable additional effort must be expended before a clear-cut decision can be made between a simple / -hydrogen elimination and a palladium-assisted hydride shift in this reaction. 

Isotope effects can be used to choose the most likely path. When ethylene is oxidized in deuterated water, the acetaldehyde contains no deuterium hence, all four hydrogens in the acetaldehyde must come from the ethylene. Thus, if the slow step of the reaction involves the formation of acetaldehyde, the activated complex for this slow step would involve a hydride transfer, and a primary isotope effect would be expected when deuterated ethylene is used. Actually, the isotope effect kn/ko was found to be only 1.07. In Paths 1 and 3, the slow step is, respectively, the decomposition of a 7r-complex and a a-complex to product, and they would be expected to display a primary isotope effect. However, in Path 2, the rate-determining step is the rearrangement of a 7r-complex to a (T-complex. Since no carbon-hydrogen bonds are broken, no primary isotope effect would be expected. Thus, Path 2 is consistent with all the experimental facts. Paths involving oxypalladation adducts, first suggested by the Russian workers (32), are now generally accepted (19, 28, 32). 

Indicate the kind of equilibrium isotope effect one would expect and predict the magnitude of the isotope effect on the reaction of acetaldehyde-di (110) with water to give the hydrate 111 (Figure 6.87). 

Phenylacetylene and l-phenylprop)T e hydrate in solutions containing about 50% sulfuric acid in water, and even acetylene can be hydrated to acetaldehyde in sufficiently acidic solutions. A solvent deuterium isotope effect on the acid-catalyzed hydration of 1-phenylpropyne indicates that proton transfer occurs during the rate-determining step. The data are therefore consistent with the intermediacy of a vinyl cation, as illustrated for 1-phenylpropyne in equation 9.73. 

A deuterium kinetic isotope effect of 6.1 was observed in the oxidation of acetaldehyde by permanganate in aqueous HC104, implying a hydride... 

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