Alkyne relative energies

The isomerization, itself, originates from the a complex (B in Figure 3). However the total activation energy depends critically on the relative energy of A and B (Figure 3). An alkyne C=C triple bond binds more efficiently to a transition metal complex than a o C-H bond since the % C-C orbital is a better electron-donor and the 71 C-C orbital a better electron acceptor than the a and a C-H orbitals, respectively. However, the difference in energy between the two isomers is relatively low for a d6 metal center because four-electron repulsion between an occupied metal d orbital and the other n C-C orbital destabilizes the alkyne complex. This contributes to facilitate the transformation for the Ru11 system studied by Wakatsuki et al. 

DFT calculations confirmed the similarities with the alkyne/vinylidene transformation but have revealed that additional parameters were essential to achieve the isomerization [8, 20-23]. The hydride ligand on the 14-electron fragment RuHC1L2 opens up a pathway for the transformation similar to that obtained for the acetylene to vinylidene isomerization. However, thermodynamics is not in favor of the carbene isomer for unsubstituted olefins and the tautomerization is observed only when a re electron donor group is present on the alkene. Finally the nature of the X ligand on the RuHXL2+q (X = Cl, q=0 X = CO, q=l) 14-electron complex alters the relative energy of the various intermediates and enables to stop the reaction on route to carbene. 

Relative energies of conjugated, isolated, and cumulated dienes compared with alkynes, based on molar heats of hydrogenation. 

Figure 6.127 (a) Relative energies and barrier for rotation of the s-trans and skewed conformations of 1,3-butadiene, (b) Relative abundance of each isomer in a,fi-unsaturated ketones, (c) Energy of polyynes is insensitive to rotation due to the presence of two orthogonal n-systems in the alkyne moieties. 

We calculated the relative energies of the alkyne and vinylidene isomers at HF/II, HF/III, MP2/III, and MP3/III. The results are shown in Table 18. The additional f-type polarization function in basis set III has little effect on the relative energies of the isomers. Relative energies of the alkyne and vinylidene isomers are similar at the MP2 and MP3 level, so the predicted stability order should be quite reliable. The calculations predict that substitution of hydrogen by the more electronegative substituent fluorine increases the relative stability of the vinylidene isomer, which is in agreement with experimental evidence. In... 

The relative stabilities of 1-phenylvinyl cations can be measured by determining the gas-phase basicity of the corresponding alkynes. The table below gives some data on free energy of protonation for substituted phenylethynes and 1-phenylpropynes. These give rise to the corresponding Yukawa-Tsuno relationships. 

Carbon-13 shifts of alkynes (Table 4.13) [246-250] are found between 60 and 95 ppm. To conclude, alkyne carbons are shielded relative to olefinic but deshielded relative to alkane carbons, also paralleling the behavior of protons in proton NMR. Shielding relative to alkenes is attributed to the higher electronic excitation energy of alkynes which decreases the paramagnetic term according to eq. (3.4), and to the anisotropic effect of the triple bond. An increment system can be used to predict carbon shieldings in alkynes... 

The reactivity order also appears to correlate with the C-X bond energy, inasmuch as the tertiary alkyl halides both are more reactive and have weaker carbon-halogen bonds than either primary or secondary halides (see Table 4-6). In fact, elimination of HX from haloalkenes or haloarenes with relatively strong C-X bonds, such as chloroethene or chlorobenzene, is much less facile than for haloalkanes. Nonetheless, elimination does occur under the right conditions and constitutes one of the most useful general methods for the synthesis of alkynes. For example,... 

A large number of accurate rate constants are known for addition of simple alkyl radicals to alkenes.33-33 Table 2 summarizes some substituent effects in the addition of the cyclohexyl radical to a series of monosubstituted alkenes.36 The resonance stabilization of the adduct radical is relatively unimportant (because of the early transition state) and the rate constants for additions roughly parallel the LUMO energy of the alkene. Styrene is selected as a convenient reference because it is experimentally difficult to conduct additions of nucleophilic radicals to alkenes that are much poorer acceptors than styrene. Thus, high yield additions of alkyl radicals to acceptors, such as vinyl chloride and vinyl acetate, are difficult to accomplish and it is not possible to add alkyl radicals to simple alkyl-substituted alkenes. Alkynes are slightly poorer acceptors than similarly activated alkenes but are still useful.37... 

Indole-3-carboxylic acids 325,326, and 327 afford the cycloadducts 328, 329, and 330 by intramolecular reactions. The conditions needed to effect cyclization were not especially mild (232°C, 8 hours for 2a 210°C, 7 hours for 2b refluxing nitrobenzene, 3 hours for 325), and yields were better in the case of the alkynes 326 and 327. The relative ease of reaction and greater yield for 326 and 327 were due to the smaller HOMO/LUMO energy gap between the relatively electron-rich 2-vinylindole diene and the relatively electron-poor alkyne (compared to an olefin) (90H993). 

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