Acetylene isotopic labelling

The acetylene-insertion reaction presumably occurs by the following mechanistic sequence (1) insertion of Pd(0) into the SCB, (2) regioselective yy -silylpalladation of the acetylenic compounds to provide seven-membered l-pallada-4-silacyclic intermediate, and (3) reductive elimination of Pd(0) to afford silacyclohexene. Alternatively, /3-hydride elimination would open the palladacycle, generating a vinylpalladium hydride species that would undergo reductive elimination to yield the ring-opened allylvinylsilane. Isotopic labeling studies provided evidence in support of this mechanistic hypothesis (Scheme 47). 

Laureni and co-workers have studied the photochemical decomposition of 1,2,3-selena and 1,2,3-thiadiazoles in argon or nitrogen matrix. In each case they could identify the products as the ethynylselenol or thiol (136) and the seleno-or thioketene (137). In addition, in the case of 1,2,3-selen-adiazole acetylene was also detected. Using isotopically labelled substrates, they demonstrated that a major portion of the ethynylthiol formed from 1,2,3-thiadiazoles must have undergone an equilibriation of the carbons, probably through the symmetrical thiirene intermediate (138). In the case of the selenium compound, however, their results showed that the selenirene is not on the route to the ethynylselenol (136, X = Se). 

The most important methods of measuring denitrification are (1) the detection of totally produced gas by incubation, (2) isotope-labeling methods with and N, and (3) the acetylene (C Hj) inhibition technique. 

J. C. Lavellay and J. Saussey, Synthesis and uses of isotopically labelled acetylenes , in The Chemistry of the Carbon-Carbon Triple Bond (Ed. S. Patai), Wiley, Chichester, 1978, pp. 957-976. 

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). 

The laboratory synthesis of unsaturated fatty acids has been pursued extensively. Whilst a few acids are easily isolated from appropriate natural sources (Section 4.9) it is necessary to resort to chemical synthesis when the acid occurs only in obscure sources or at low levels, or when an isotopically labelled sample is needed. Many synthetic procedures have been employed but those based on the reactivity of acetylene (ethyne) and its derivatives are the most common. The topic has been reviewed by Kunau (1973) and by Sprecher (1977, 1979). The Wittig reaction which is also useful in this connection and the synthesis of isotopically labelled compounds are covered in Sections 7.2 and 7.3 respectively. 

Heavy water is the cheapest available deuterium source. However, it has one inconvenience. Being inevitably employed in excess, it necessitates extraction or at least drying of the reaction mixture as part of the workup protocol. In contrast, deuterium chloride in diethyl ether can be employed in stoichiometric doses. But also this reagent suffers from a drawback. It reacts, if slowly, with the ethereal solvent. Therefore, the indefinitely storable phenylacetylene-ry-c may often become the deuteration reagent of choice. Due to its high intrinsic acidity and its needle-like slim shape, this terminally deuterated acetylene transfers the isotope label most effectively and in general irreversibly. 

A number of hydrocarbon radicals having multiple bonds at the radical centre have also been trapped in inert matrices and studied by IR spectroscopy. Thus, ethynyl radical was obtained by vacuum UV photolysis (9) of matrix-isolated acetylene (Shepherd and Graham, 1987) as well as when acetylene and argon atoms excited in a microwave discharge were codeposited at 12 K (Jacox and Olson, 1987). An appearance of diacetylene bands was observed when the matrices were warmed up, while the absorptions of the radical C2H disappeared. Detailed isotopic studies of D-and C-labelled ethynyl radicals showed a surprisingly low frequency of the C=C bond stretching vibration at 1846 cm instead of c.2100cm for a true C=C triple bond (the band at 2104 cm was attributed to the... 

The reaction between sodium acetylide and alkyl halides in liquid ammonia is the most general method of preparing substituted acetylenes. It has very often been used to prepare labelled alkynes . Appropriately labelled alkyl halides have to be prepared first (a review of the preparation of the most common ones has already been given ), but more and more are now commercially available. The yield of the condensation in liquid ammonia is about 75% and, under experimental conditions, no isotopic exchange has been found the isotopic purity of the final alkynes is the same as that of the starting alkyl halides. 

The yield of labelled acetylene is 90-100%. A variant, using labelled barium carbonate and excess of barium to make labelled BaC., then hydrolysis, has been used to prepare and . The isotopic yield is excellent if all traces of paraffin... 

Before using labelled acetylenes, it is necessary to check their isotopic purity. In the following, we shall summarize some separation and spectrometric methods which are most often employed and also consider isotope effects. Since general accounts of isotope effects have already been given, for instance by Halevi or Laszlo and Welvart ", and brought up to date by Thomas, in this chapter we shall mainly consider the effects related to the presence of the triple bond. 

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