Loss of ethyne

The molecular ion of 3-phenylbenzo[l>] thiophene gives the rearranged 1471 ion 2 prior to fragmentation.97 The mode of fragmentation of the 2,3-dideutero derivative suggested that all of the hydrogens in benzo[fo]thio-phene were randomized prior to the loss of ethyne from the molecular ion.98 However, later studies on benzo[b]thiophene-2-[13C] have shown that this randomization is undoubtedly due to carbon scrambling.99... 

Example Indole molecular ions, m/z 117, preferably dissociate by loss of HCN (Fig. 6.52) [201]. The [C7He] fragment ion, m/z 90, then stabilizes by H loss to form an even-electron species, [CtHs] m/z 89, which deconposes further by loss of ethyne ... 

The destruction of DDT by ball milling with CaO resulted in substantial loss of chloride and produced a graphitic product containing some residual chlorine. In addition, an exceptional rearrangement occnrred with the formation of bis(4-chlorophenyl)ethyne that was identified by H NMR (Hall et al. 1996) (Figure 1.28). 

The hydrogens of the —CH2— group of 1,3-cyclopentadiene are acidic. In fact, they are considerably more acidic than the ethyne hydrogens of the 1-alkynes (Section 1 j-8). This means that 1,3-cyclopentadiene is at least 1030 times more acidic than the ordinary alkanes. The reason is that loss of one of the CH2 protons of cyclopentadiene results in formation of an especially stabilized anion ... 

Analogous complexes have not been observed in the case of platinum instead, a disilylplatinum complex (entry 120) adds ethyne with loss of hydrogen (but no Si-Pt bond cleavage) to give a platinadisilacyclopen-tene. A similar product (XIX) was claimed from the reactions of entry... 

Conversion of a terminal alkyne to its alkynylsilane prevents loss of the relatively acidic terminal hydrogen (pKa of ethyne c. 25) during later synthetic steps. For example, the terminal hydrogen of propyne was masked whilst its propargylic anion was used in a synthesis of Cecropia juvenile hormone, a chemical which plays ail important role in insect development (Figure Si5.2). 

The activated internal conversion, which is a hallmark of the photoexcited DNA bases, also occurs in a number of aromatic ethynes and nitriles, including dipheny-lacetylene (DPA) and 4-(dimethylamino)benzonitrile (DMABN). Thus, as in the case of the nucleobases, DPA exhibits an abrupt break-off (loss) of fluorescence in supersonic free jet [35], Figure 15-16, and the strong thermal quenching of... 

By the Darzens reaction. -ionone afforded a C14 aldehyde which as the diethyl acetal underwent addition to ethyl vinyl ether in the presence of boron trifluoride etherate to yield a Cl6 acetal. After hydrolysis, loss of ethanol and formation of the diethyl acetal as before, reaction under acidic condiions with ethyl propenyl ether gave the unsaturated Cl9 aldehyde after hydrolysis and removal of ethanol. Reaction of two moles with ethyne dimagnesiuro bromide produced the C40 chain and dehydration of the diol, selective catalytic hydrogenation followed by isomerisation completed a remarkable technical synthesis of i-carotene. Further variations have involved the use of two moles of the C14 aldehyde and a Cl2 divinyl ether. An independent approach (ref.29) has utilised vitamin A (32) converted to a phosphonium salt, thence to the corresponding phosphoran, autoxidation of which afforded s-carotene ( scheme 16). 

C-H bond fission and the production of ethynyl radicals. Butadiyne and vinyl acetate are formed when the T -shaped ethyne dimer is irradiated at 193 nm in argon or xenon. The dynamics of the photodissociation of propyne and allene have been studied. The H2 elimination from propyne is a minor route for propyne dissociation and the major path identified in this study is loss of the alkyne hydrogen. A study of the photodissociation dynamics of allene and propyne has been reported and this work has demonstrated that allene gives rise to a propargyl radical while propyne yields the propynyl radical. Other research has examined the photodissociation of propyne and allene by irradiation at 193 nm. ... 

While numerous applications of this methodology in heterocyclic synthesis are known, few applications to stereoselective synthesis have been reported. Cocyclotrimerization of ethyne with optically pure (.S )-2-methylbutanenitriIc (1) in the presence of (l,5-cyclooctadicne)(i 5-cy-clopentadienyI)cobalt gives optically active ( + )-(A )-2-(l-methylpropyI(pyridine (2) in >90% yield with 96% optical purity24, i.e.. there is only a small loss of chiral information. 

Ethylidyne (8) has been recognised on Pt/SiOa at 300 K using the SEDOR NMR technique applied to heavily C-labelled ethene the C—C bond length was 149 pm. This seemed to occur on large platinum particles, where areas of (111) face are most likely it was also seen by SIMS on platinum black, but on small particles vinylidene (17) predominated. Similar SEDOR experiments with ethyne showed 75% vinylidene and 25% ethyne as 12A or 15. " Adsorbed benzene was shown to rotate freely at 300 K, and cyclopropane was adsorbed, but not strongly, i.e. without loss of hydrogen. ... 

It will also become clear that the behaviour of alkynes closely resembles that of the 1,2- and 1,3-alkadienes already discussed. Their strong chemisorption was due to the simultaneous interaction of both double bonds in the latter case, and probably to the large release of strain that accompanied the opening of one of the double-bonds (or at least the disengagement of one set of tt orbitals) in the latter case. The heat of hydrogenation reflects the magnitude of the release of strain and hence the strength of the interaction with surface atoms for ethyne to ethene it is 172 kJ moU compared to 137 kJ moU for ethene to ethane. These reactions are thus quite exothermic, and in industrial use care must be taken to avoid temperature excursions, as these would lead to loss of selectivity. 

The data shown in Table 6.3 show no obvious trends that may shed light on the mechanism(s) of the metathesis reactions. In terms of overall relative reactivity (krei of Table 6.3), phenylacetylene reacts fastest and pent-l-yne is the most sluggish alkyne, while ethyne and the parasubstituted phenylacetylenes are unreactive under the experimental conditions used. When the branching ratios and the relative reaction rates are combined, the alkyne - nitrile chaimel is the most productive for propargyl alcohol and phenylacetylene and least productive for pent-2-yne and pent-l-yne. The sterically congested alkyne exhibits modest reactivity for the metathesis reaction. No obvious relationship exists between the structure of the alkyne substrate and the propensity for a metathesis reaction. With regard to which metathesis reaction is favored for unsymmetrical alkynes, while no regioselectivity operates for pent-2-yne and phenylacetylene other terminal acetylenes favor the loss of the more substituted nitrile (Eq. (6.131)). 

The formation of a substituted tropylium ion is typical for alkyl-substituted benzenes, hi the mass spectrum of isopropylbenzene (Fig. 4.24), a strong peak appears at miz = 105. This peak corresponds to loss of a methyl group to form a methyl-substituted tropyhum ion. The tropylium ion has characteristic fragmentations of its own. The tropylium ion can fragment to form the aromatic cyclopentadienyl cation (m/z = 65) plus ethyne (acetylene). The cyclopentadienyl cation in turn can fragment to form another equivalent of ethyne and the aromatic cyclopropenyl cation (m/z = 39) (Fig. 4.25). 

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