Isotopic-Stereochemical Experiments

Combined Isotopic-Stereochemical Experiments Credit for first mention of the principle that compounds whose structures are superimposable with their mirror images can still maintain asymmetry probably should go to Ogston (1948), who suggested 

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Phosphate esters, particularly AMP, ADP and ATP, have vital biological functions and this fact has generated intense interest in their reaction mechanisms. Subtle stereochemical experiments, such as the use of isotopically chiral compounds, have been important and, since all biological phosphorylation reactions appear to involve metal ion catalysis, the stereochemistry of phosphate ion coordination has also been subject to much attention.229,230 Apart from its biological significance, this work has revealed some interesting contrasts with the stereochemistry of ligand systems in which saturated carbon units link the donor atoms. 

Unlike the Rh-based hydrogenation of a-(acylamino)acrylates, the corresponding Ru chemistry has not been studied extensively. Ru complexes of (S)-BINAP and (S,S)-CHIRAPHOS catalyze the hydrogenation of (Z)-a-(acylamino)cinnamates to give the protected ( -phenylalanine with 92% ee [74] and 97% ee [75], respectively. It is interesting that the Rh and Ru complexes with the same chiral diphosphines exhibit an opposite sense of asymmetric induction (Scheme 1.6) [13,15,56,74,75]. This condition is due primarily to the difference in the mechanisms the Rh-catalyzed hydrogenation proceeds via Rh dihydride species [76], whereas the Ru-catalyzed reaction takes place via Ru monohydride intermediate [77]. The Rh-catalyzed reaction has been studied in more detail by kinetic measurement [78], isotope tracer experiments [79], NMR studies [80], and MO calculations [81]. The stereochemical outcome is understandable by considering the thermodynamic stability and reactivity of the catalyst-enamide complexes. 

A classic example of a CKlII) photosubstitution is the well-studied complex, Cr(NH3)5Clp , for which NHj photoaquation is the principal result of excitation of ligand-field bands, and Cl aquation represents only about 2% of the overall total . The stereochemical origin of the labilized NHj is trans to the coordinated Cl as shown by isotope-labeling experiments, although the final reaction product has the cis configuration ... 

A. The Deamination of 1,2,2-Triphenylethylamine The use of carbon-14 labels in two different positions of the molecule led to identification of open carboniiun ion intermediates during the reaction of several 1,2,2-tiiphenylethyl derivatives, and these experiments have already been discussed in Section II, A, 4 of this chapter. We shall now present some completely independent experiments of combined isotopic stereochemical nature which verify the results of the double-labeling experiments, and which leave little doubt that bridged, non-classical carbonium ions cannot explain either the stereochemical results or the isotope position rearrangements which take place upon deamination of 1,2,2-triphenylethylamine. 

For instance, it has been claimed on the basis of isotopic labelling experiments that lutein (14) was converted to (3S,3 5)-astaxanthin (28) in fish (95, 111, 112). The route proposed which did not involve an epi-merization at C-3 is not compatible with the absolute configurations later established for (14) and (28). However, recent evidence for the 3 -S chirality of a-doradexanthin (96) from goldfish (43 a) has led to the plausible suggestion that the biosynthetic conversion of lutein (14) proceeds via 3 -didehydrolutein- (3 5 )-epilutein (58)- (3 S)-oc-doradexanthin (96) - (3S,3 5)-astaxanthin (28). The reported conversion of alloxanthin (31) to 7,8,7, 8 -tetradehydroastaxanthin (34) in goldfish (55) is compatible with stereochemical data. But this would also be true for a conversion of (3S,3 S)-astaxanthin (28) to its 7,8,7, 8 -tetra-dehydroderivative (28). 

Just as in the preceding examples, early indications of tunneling in enzyme-catalyzed reactions depended on the failure of experiments to conform to the traditional expectations for kinetic isotope effects (Chart 3). Table 1 describes experimental determinations of -secondary isotope effects for redox reactions of the cofactors NADH and NAD. The two hydrogenic positions at C4 of NADH are stereochemically distinct and can be labeled individually by synthetic use of enzyme-catalyzed reactions. In reactions where the deuterium label is not transferred (see below), an... 

If no thermodynamic isotope effect is operative, the relative stability of the diastereomeric [U-Mj ]" " and [U-Ms]" " complexes can be represented approximately by their relative peak intensity, i.e., the IRIS value (IRIS = [U-Ms] /[U-Mx]+). Usually, isotope effects on non-covalent binding are small. However, both stereochemical and isotope effects can easily be separated by performing a control experiments using the other enantiomer of the host under the same conditions. 

Today, the perepoxide intermediate proposed earlier by Sharp [35] seems to be the most popular mechanism that found support from results derived from kinetic isotope effects, stereochemical studies, trapping experiments [36-38], and theoretical calculations [39 43],... 

The elucidation of reaction mechanisms is a central topic in organic chemistry that led to many elegant studies emphasizing the interplay of theory and experiment as demonstrated, for example, by the seminal contributions of the Houk group to the understanding of the Diels-Alder and other pericyclic reactions.38 This reaction class is rather typical for the elucidation of reaction mechanisms. On the experimental side, the toolbox of solvent, substituent and isotope effect studies as well as stereochemical probes have been used extensively, while the reactants, products, intermediates and transition structures involved have been calculated at all feasible levels of theory. As a result, these reactions often serve as a success story in physical organic chemistry. 

Classical feeding experiments with both stable and radioactive isotopic labels (7) enabled the biosynthetic origin of the polyethers to be elucidated and for a general stereochemical model to be proposed (1). More recent work on this class of compounds has focused on a genetic approach, and unusual and interesting genes specific to polyether biosynthesis have been isolated from these clusters. 

Table 2 lists phosphatases for which the stereochemical outcome has been determined by using chiral phosphomonoester substrates. In such experiments the phosphoryl group is made chiral using lfiO, 170, and lsO, or, a phosphorothioate substrate is used that has a sulfur atom and two isotopes of oxygen in the nonbridging positions. Summaries of results for additional enzymes that catalyze phosphoryl transfer are available in other reviews.20,76,77... 

The Grubbs Stereochemical Isotope Experiment The Green-Rooney Mechanism... 

One of the critical aspects of this approach is that two different experiments have to be performed between which the particular instrument conditions must be carefully kept constant in order not to affect the intensity ratios. This problem can be overcome by the enantiomer-labeled guest method [47]. It is based on the mass spectrometric examination of one enantiomer of the host with a pseudo-racemic mixture of the guest. In order to be able to detect both diastereomers separately, one enantiomer of the guest must be isotopically labeled, usually with deuterium. In the same experiment, both diastereotopic complexes are formed and their intensities can be compared directly. However, the stereochemical effect might additionally be superimposed by an unknown isotope effect. A way to separate stereochemical and isotope effects is to perform the same experiment with the second host enantiomer [4B]. In one experiment both stereochemical and isotope effects disfavor the same complex and thus work in the same direction. In the other experiment, they partly cancel each other. If both experiments have been performed, one can use the two experimental values for the intensity ratios of both diastero-meric complexes to deconvolute both effects [49]. 

The enzyme also catalyzes a /3,y-elimination reaction with homoserine (in the absence of cysteine) to form a-ketobutyrate, a reaction having no apparent physiological significance. The results of various isotope-labeling studies on both the normal substitution and aberrant elimination reactions have given rise to a reasonably clear picture of the stereochemical features of this interesting reaction. Experiments on the enzyme from Salmonella by Flavin and co-workers, as well as others, demonstrated that the /8,y-elimination reaction involves significant... 

The reaction of 17 with 18 is essentially irreversible, but the reaction of 17 with acyclic olefins is reversible and leads to the expected metathesis reactions, for example, the cis/trans isomerization of HDC=CMePh (Lee, J.B. 1982). Many isotopic labelling and kinetic experiments have been carried out in an attempt to discover whether a titanium-carbene-olefin complex plays a significant kinetic role in these reactions. The general conclusion is that this is unlikely and it is thought that complete dissociation to Ti(=CH2)Cp2 must occur before reaction takes place with an olefin or acetylene (Gilliom 1986a Anslyn 1987 Hawkins 1988, see ref 4 therein). If such a complex does have a finite existence, it probably corresponds only to a very shallow minimum in the energy profile for the reaction. Stereochemical evidence for this conclusion comes from a study of the isomerization reaction (17). 

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