Isotope labeling olefins

Considerable work has been published concerning the incorporation of radioactive-isotope-labeled olefins in hydrocarbons during Fischer-Tropsch reactions. The pioneering work of Kummer and Emmett [89] and of Hall et al. [88] suggested that ethylene acted as a chain initiator over iron catalysts. The same results were obtained over cobalt catalyst by Eidus et al. [131]. 

Figure 11.25 Catalysts used in the enantioselective oxidation of isotopically labeled olefinic substrates...

The oxidation reactions were performed in a 25 mL ronnd bottom flask. In a typical reaction the catalyst (0.5 % Fe mol) was added to 0.125 M olefin solntion in acetone then dry TBHP (3.5 M in CH2CI2 or in PhCl) was added in one step, and the reaction mixture was stirred and heated in an oil bath at 40°C for 7 h. For the allyhc oxidation of cyclohexene with isotopically labelled oxygen ( 02) the following procedure was carried out the suspension of the catalyst (0.5% Fe mol) in cyclohexene (4 mL, 0.125 M) was frozen and the air in the reactor was evacuated and replaced by an oxygen (21% mol) - argon (79% mol) mixture. Then, the suspension was allowed to warm at room temperatnre and 1.3 mmol of degasified TBHP was added to the solution and the reaction mixtnre was stirred at 40°C for 3 h. 

Therefore, another analogous reaction was studied with a more reactive olefin, viz. methyl acrylate, which reacts with (+)-methylneophylphenyltin deuteride (86) at room temperature and yields after 18 h again an optically inactive adduct which is reduced with lithium aluminum hydride to give racemic isotopically labeled (55) 44). After 18h in the presence of AIBN at room temperature, (86) only loses 30% of its optical activity in benzene. The fact that the obtained adduct is optically inactive might be due to the nucleophilicity of methyl acrylate, which might be important enough to cause the racemization of (56). 

Tebbe and co-workers reported the first olefin metatheses between titanocene-methyli-dene and simple terminal olefins [13]. They showed cross-metathesis between isotopically labeled isobutene and methylenecyclohexane to be catalyzed by titanocene-methylidene. This process is referred to as degenerate olefm metathesis as it does not yield any new olefin (Scheme 14.6). The intermediate titanacyclobutane has been isolated and characterized [14], and its stability [15] and mechanism of rearrangement [16] have been investigated. 

I-Ph, or LNiIH-0-NiIUL) have been proposed as the active oxidant (92). In the reaction, E olefins are more reactive than the corresponding Z isomers, and a strong correlation was observed between the electron-donating effect of the para substituents in styrene and the initial reaction rate (91). Isotope labeling studies have shown that the epoxide oxygen is derived from PhIO. 

From the observation that the generally exothermic addition reactions do not occur readily we may infer that they require appreciable activation energies. These latter range from 17 to 23 kcal. for the additions of HI to olefins53 to 30 to 40 kcal. for the additions of HBr or HC1 to olefins, f It has been demonstrated by Okabe and McNesby24 that if sufficient energy is available in the molecule (e.g., by vacuum UV photolysis) then direct elimination of CH4 or H2 is possible from alkanes. In these cases, isotopic labeling experiments have shown that the two H atoms may come from adjacent C atoms to form the olefin, or even from the same C atom to form the isomeric carbene ... 

Isotopic labelling experiments have demonstrated that for the reaction of propene on MoC VTiC h or on photoreduced Mo03/TiC>2 the chain carrier for the degenerate metathesis is [Mo]=CHMe rather than [Mo]=CH2149-151. As a general rule in olefin metathesis a substituted carbene is less reactive than an unsubstituted carbene, and so the former tends to build up to a higher steady state. 

Therefore, without isotopic labeling, it is impossible to distinguish between azide-products and nitrene-products. That is the reason why only one NCN-olefin reaction (the thermolysis of NCN3 in cyclooctatetraene) was studied in detail. Cyclooctatetraene 49) reacts with NCN3 very slowly at room temperature and leads exclusively to the alkylidenecyanamide. Thermolysis of NCN3 in diluted (ethylacetate) cyclooctatetraene at 78 °C afforded 50, 51, 52 02.93). 

Watanabe reported that the addition of C-H bonds in aldehydes to olefins takes place efficiently ivith the aid of Ru3(CO)i2 under a CO atmosphere at 200 °C (Eq. 9.44) [62]. In the case of the reaction with 1-hexene, a mixture of linear and branched ketones was obtained in 35% and 12% yields, respectively. The use of a CO atmosphere is the key to accomplishing this reaction in a catalytic manner. These authors revealed the role of CO by means of isotope-labeling experiments using CO. The presence of CO is essential for suppressing the decarbonylation of aldehydes and for stabilizing the active catalyst species. Interestingly, the reaction using 1,3-dienes as an acceptor of the C-H bond proceeds in the absence of CO (Eq. 9.45) [63]. Aromatic and heteroaromatic aldehydes can also be used in this reaction. 

Several studies of soluble transition metal complexes have clarified the mechanism of olefin insertion and polymerization. Isotopic labeling experiments have shown that insertion of ethylene into a cobalt-methyl bond (Scheme 1) occurs by 1,2-insertion and not by a elimination. 

Other rhodium complex also catalyzed the addition of the C-H bond in aldehyde to olefins [115-117]. The use of paraformaldehyde results in the formation of aldehydes [115]. Marder et al. proposed the reaction mechanism of CpRh(eth-ylene)2-catalyzed addition of C-H bond in aldehyde to ethylene by the use of isotope-labeling experiments [117]. They suggested that insertion of ethylene to the Rh-H bond must take place rapidly and reversibly, and this equilibrium must be established significantly faster than either aldehyde reductive elimination or product formation (Scheme 4). 

We have studied binary blends dxx/hx2 of random olefinic copolymers x=(Ex x EEx)n, with one blend constituent protonated (hx) and the other deuterated (dx). The blends examined were grouped in four pairs of structurally identical mixtures xx/x2 but with a swapped isotope labeled component (dxx/hx2 and hxx/dx2). For such blend pairs the bulk interaction parameter % (and hence also the critical point Tc) has been found (see Sect. 2.2.3 and references therein) to be higher when the more branched (say xx>x2) component is deuterated, i.e., X(dx /hx2)>x(hx /dx2) or Tc(dxx/hx2)>Tc(hxx/dx2) (see Fig. 9). An identical pattern is exhibited here by the force driving the segregation at the free surface. This is illustrated in Fig. 26a,b where the composition vs depth profiles of the more branched (xx) component are shown for blend pairs with swapped isotope... 

Subsequently, the isotope labeling experiment using water, H2 O (95% O), was performed during olefin epoxidation using czs-stilbene. cis-Stilbene is a useful substrate when seeking mechanistic information from isotope labeling... 

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