Carbon-14 labelled acetylenes

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... 

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. 

Another example involves the soxurce of acetylene as a major product from the reaction of recoiling carbon atoms and cyclopropane (18). MacKay and Wolfgang attributed the formation of acetylene from carbon-atom reactions with alkanes to an insertion into a C—bond followed by fragmentation of the intermediate carbene (18). The hi yield of labeled acetylene from cyclopropane was in accord with this mechanism, since the intermediate cyclopropylmethylene was believed to cleave into two stable molecules ethylene in addition to acetylene. 

A major question which remains undecided here is whether the carbon which appears in carbon dioxide passes through an acetylide step. Probably this could be decided by experiments in which the surface complex was formed from labeled acetylene and this was then allowed to react with unlabeled acetylene, testing the effluent gas in the second stage for the presence of radioactive carbon dioxide. 

The only obvious thermal isomerization of acetylene is transposition of the atoms at either end of the triple bond. This type of dyotropic process was observed in the parent molecule using deuterium and carbon labeling. Thus, flash vacuum pyrolysis of [l- C,l- H]acetylene at temperatures above 700°C resulted in protium appearing at Cl of the recovered acetylene (Scheme 3.1). 

Evidence for this pathway came first from the observation of a decreased rate of reaction with alkyl substitution at Cl rather than a rate increase due to increased substitution. This rate retardation results from preventing the 1,3-hydrogen shift to make the vinylidene, resulting in slower formation of acetylene via the pathway described in Scheme 4.2. Further, pyrolysis of carbon-labeled 1,3,3-trimethyl-cyclopropene gave one acetylene, which could be derived from either pathway (Scheme 4.2 or 4.3), and nearly equal amounts of another acetylene, which could only be derived by Scheme 4.3 (Scheme 4.4). ... 

Propane-l,2- C, specific activity of about 110 mCi mmol" has been synthesized from doubly labelled acetylene in reaction series shown in equation 12. This synthesis included preparation of doubly labelled acetylene from barium carbonate-according to Cox and Warne, nearly quantitative hydrogenation of acetylene to ethylene, addition of hydroiodic acid to the latter to form iodoethane, preparation of ethylmagnesium iodide followed by carbonation to yield propionic acid and reduction o n-propanol with 70-75% yield. The latter yielded a tosylate which was finally reduced to doubly labelled propane 2 with sodium borohydride, and purified by gas chromatography on alumina or silica. Its specific activity was close to the maximal possible specific activity of acetylene (i.e. 124.9 mCi mmol " ). 

Synthetic chemical methods for the preparation of carbon-14-labeled chemicals utilize a number of basic building components that are prepared from barium carbonate ( " C). These are versions of carbon dioxide, acetylene, benzene, sodium acetate (1- and 2- " C), methyl iodide, methanol, sodium cyanide, and urea. Many complicated labeled compounds may be synthesized from these materials. 

The possibilities of labeling P-ionone have been reviewed by Isler et al, (1960). Total synthesis of P-ionone via citral, developed by Kimel et al. (1957), could be used to introduce carbon-14 in all positions by such basic building blocks as labeled acetylene, acetone, or acetoacetic ester. Preparation of all-rran5-retinoic-6,7- C2 acid, although unpublished, has been referred to in this review (Isler et al., 1960). The methodology was also used to synthesize 13-cw-retinoic-6,7- C acid and is discussed in Section II,A,2,a,i. 

Although unlabeled (or C-labeled) acetylene has been employed in various two-carbon extension reactions of [ " C]alkyl halides and sulfonates, only one application has been reported so far for [ C2]acetylene. In this case the monoanion reacted with Br(CH2)3Cl to afford l-chloro[4,5- C2]pent-4-yne (11) (Figure 8.4). Subsequent cyano-dechlorination, hydrolysis of the cyano function and a second alkylation with l-chloro-2,5-tetradecadiyne gave the multiply unsaturated carboxylic acid 12. ... 

The structure of the major aggregate was identified by labeling studies. Since the major set has two equal intensity 6Li signals, these signals could be assigned as a 1 1 complex 68 of lithium acetylide and lithium alkoxide or a dimer (such as 69) of the 1 1 complex 68 shown in Figure 1.9. Both structures have two different Li species. In order to discriminate between 68 and 69, a terminal acetylene carbon of 37 was labeled with 13C. In the case of 68, both lithium signals will be a doublet... 

Of greater relevance catalytically is that the combined use of l3C enrichment and 13C nutation NMR spectroscopy can distinguish between proposed rival mechanisms for the Ziegler-Natta catalyzed polymerization of acetylene. In the four-center insertion mechanism the enriched acetylene (HC =C H) is incorporated as shown in Scheme 6. It is to be noted that the, 3C—13C bond label is here incorporated into a carbon-carbon double bond, the length of which is significantly smaller than that of a carbon-carbon single bond, which is how the enriched acetylene would be incorporated in the two-center mechanism shown in Scheme 7. The results of nutation experiments leave little doubt that the Ziegler-Natta polymerization of acetylene proceeds by a four-center mechanism. 

Fig. 11. Adsorption of acetylene on Si(OOl). The experimental STM scans show three different adsorption configurations, labeled I-III (A). They are due to the possibility of restructuring of the carbon bond to either a double bond (configurations (1) and (2) in frame (B)), or to a single bond (configurations (3) and (4)) in frame (B)). The resulting STM images (frame (C)) in the simulation agree quite well with three of the configurations found in the experiments ((A), features I, II, and III). The experimental images were taken from [58].

Based on a 13C labeling study, the /3-acetylenic carbon atom C3 is more deshielded than the a-acetylenic carbon atom C2.l7a... 

The first of these was a study by Shevlin and Wolf who showed that photochemical addition of carbon suboxide to cyclopropene resulted in the formation of vinyl acetylene and acetylene. They postulated the intermediacy of bicyclobutanylidene as a source of the former and tetrahedrane as a source of the latter (Figure 65). The postulated intermediacy of tetrahedrane was supported by two labeling studies. In the first cyclopropene-3,3-d2 containing 87.1 3.2% deuterium was used. The resulting acetylene was analyzed for... 

Then, in a second experiment, carbon suboxide labeled at the central atom with was photolyzed in the presence of cyclopropene-3,3-d2. The resulting acetylenes were reduced to the corresponding ethylenes with chromous chloride and these were then separated on the basis of deuterium content by gas chromatography. The content of each ethylene was then analyzed and, again, found to be very close to that expected for a tetrahedrane intermediate (Table 3). 

The value of the charge density at a bond critical point can be used to define a bond order (Bader et al. 1983 Cremer and Kraka 1984). The molecular graphs for ethane, ethylene, and acetylene are shown in Fig. 2.8. In each case the unique pair of trajectories associated with a single (3, — 1) critical point is found to link the carbon nuclei to one another. Multiple bonds do not appear as such in the topology of the charge density. Instead, one finds that the extent of charge accumulation between the nuclei increases with the assumed number of electron pair bonds and this increase is faithfully monitored by the value of p at the bond critical point, a value labelled p, . For carbon-carbon bonds, one can define a bond order n in terms of the values of Ph using a relationship of the form... 

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). 

In a later study. Swanson and coworkers [81] studied the cure of acetylene-terminated poly(imide)s selectively labelled at various positions with nuclei. Curing of the sample, labelled at the imide carbonyl group, confirmed the completion of the imidization reaction on heating. The product of addition onto the carboxyl group was not observed. Four new peaks were identified in the spectrum of the cured sample labelled at the Ci-acetylene group, while a similar result was obtained for the sample labelled at the C2-acetylene position. Analysis of these results rules out the participation of coupling reactions and the biradical mechanism, which would produce triple-bond structures, but confirms the presence of the product of cyclotrimerization and Friedel-Crafts reactions. The latter mechanism is confirmed from the presence of small peaks due to aliphatic carbons in the spectra of the materials labelled at the acetylene groups. 

Since no intermediates could be detected in the in vitro studies of the oxidation of acetylenic derivatives, it was assumed that very short-lived intermediates were probably formed. In order to further characterize this pathyway, studies with deuterium and labeled biphenylacetylene (acetylenic hydrogen and internal acetylenic carbon, respectively) were initiated. The mass and nuclear magnetic resonance spectra of the resulting biphenyl acetic acid derivative showed that ... 

The second pathway, D-ring homoannulation, seems to be of a more general nature than the deethynylation. In this pathway, the ethynylcyclopentanol D-ring of the steroid is converted to a cyclohexanone ring, accompanied by formation of carbon dioxide. The latter is derived from the terminal acetylenic carbon, as shown in experiments with labeled steroids. The mechanism suggested for this pathway is depicted in Scheme 8. In the case of norgestrel (for structure, see Table 1 A), the acid was isolated under neutral conditions and identified following esterification with diazomethane. ... 

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