Isotopic labelling studies addition reactions

The remaining results are concerned with redox reactions /t-superoxo-/<-peroxo redox reactions have been reported for rhodium complexes The complex [(NC)sCo( -02)Mo(0)(OH2)(CN)5] decomposes on standing in oxygen to [(>7 02)Mo(0)(CN)4] and isotopic labelling studies show that the dioxygen in this complex does not originate from [(NC)5Co(u-02)Mo(0)(OH2)(CN)5] °). The details of this reaction are not clear but the final product could conceivably arise from the formation of a Mo(IV) complex on breakdown of the rj complex followed by addition of dioxygen. 

Kinetic and isotopic labeling studies by Long and Pritchard on the hydrolyses of ethylene oxide, propylene oxide and isobutylene oxide to their corresponding glycols provide additional insight on the hydrolysis mechanisms of simple aliphatic epoxides.23 In each case, three kinetically distinct mechanisms for their reactions in water solutions were observed acid-catalyzed hydrolysis, pH-independent hydrolysis and hydroxide-catalyzed hydrolysis. Thus, the rate law for hydrolyses of these simple... 

The catalytic system consisting of [Pd(OAc)2] and P(t-Bu)2Me was applied to the arylation of a primary alkyl tosylate with 9-phenyl-9-BBN (Equation 5.7) [11]. An isotope-labeling study revealed that the substitution reaction proceeded with an inversion of configuration at the stereogenic carbon center as the net result of an inversion during the oxidative addition step and a retention during the subsequent reductive elimination step (Equation 5.8). 

The proposed mechanism of this transformation (Scheme 39), as supported by GC/MS and isotope labeling studies, involves removal of the hydroxide proton of ethanol by a base, thus giving a molecule of water and ethanolate (Step 1). This anion rapidly reacts with water to give H2, acetaldehyde, and hydroxide ion (Step 2). Reaction of the hydroxide ion with the carbonyl carbon of acetaldehyde (via nucleophilic addition) forms alkoxide which is then deprotonated and gives a dianionic Cannizzaro intermediate, that subsequendy reacts with another molecule of acetaldehyde to give the final product (sodium acetate) and a molecule of ethanol (Step 3). 

Studies conducted since the mid-1990s have lent additional support to the reaction network shown in Scheme 3.1 and emphasized the interesting class of heterogeneous-homogeneous radical nature of the system.539 Isotope labeling experiments have demonstrated that at small conversions levels most of the C02 is derived from methane, whereas at high conversion levels required for practical applications, ethylene is the dominant source of C02.543... 

Among the classic methods for the extension of the aldose chain by one carbon atom from the reducing end [9J, the Kiliani-Fischer cyanohydrin synthesis [10] is a milestone in carbohydrate chemistry. However after 110 years from discovery and numerous applications [11], including the preparation of carbon and hydrogen isotopically labeled compounds for mechanistic and structural studies [12], there are still several drawbacks that make the method impractical. These are the low and variable degree of selectivity and the harsh reaction conditions that are required to reveal the aldose from either the aldonic acid or directly from the cyanohydrin. Synthetic applications that have appeared in recent times confirmed these limitations. For instance, a quite low selectivity was registered [13] in the addition of the cyanide ion to the D-ga/acfo-hexodialdo-l,5-pyranose derivative 1... 

It should be stressed that it is a prerequisite of successful flavor precursor studies that the contribution of the odorant under investigation to a food flavor or off-flavor has been established. Sometimes the structure of a precursor can be assumed on the basis of structural elements in the odorant. In such cases, additions of the respective isotope-labelled precursor to the food system is commonly used to elucidate the precursor and to clarify reaction pathways governing the formation of the odorant. This method has been frequently applied, especially, in studies on the enzymatic generation of odor-active aldehydes (e.g., (Z)-3-hexenal in tea leaves) or alcohols (e.g., l-octen-3-oI in mushrooms) [cf. reviews in 84, 85] as well as lactones [86] from unsaturated fatty acids. 

In addition to information compiled elsewhere 1-7> the preparation of N-trimethylborazine has been described in Inorganic Syntheses 20> and the standard procedures have been used to make several isotopically labeled species of borazine 21> and N-methylborazine 22.28). Recently, the direct synthesis of N-organoborazines by the interaction of nitriles with diborane or alkali metal hydroborates, rather than the two-step procedures involving the reduction of the corresponding B-chloroborazi-nes, has received much attention. Besides the extensive study by Wade et al. 24>, it has been shown 25> that borazines can be prepared in high yield according to the reaction depicted in [Eq. (4)]. 

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