Alkyne electron density

Addition funnel pressure reactor, 201 Adjustable pressure relief valve, 200 Aerial oxidation, 64 Aerobic product transfer, 193 Aerosol pressure vessel, 198 Air-sensitive materials decomposition, 147 HPLC analysis, 24 recovering, 193 synthesis and handling, 34 Alkyne electron density, 287 Alkyne ligand, 282 Alkyne it donor orbitals, 287 Alkyne levels, 285 Ambient pressure flow cell, 238-244 Ammonia synthesis, 182 Anaerobic column chromatography, 17-18/ Anaerobic transfer, 144 Anionic polymerization, 182 Apparatus design philosophy, 117 Arc lamp... 

AgN02, MeOH, H2O, 24°, cool to 0°, add KCN, then HCl, 96% yield." The reduced electron density of the propargylic alkyne directs the electrophilic silver to the other alkyne and activates it for cleavage. 

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

Now consider the alkynes, hydrocarbons with carbon-carbon triple bonds. The Lewis structure of the linear molecule ethyne (acetylene) is H—O C- H. To describe the bonding in a linear molecule, we need a hybridization scheme that produces two equivalent orbitals at 180° from each other this is sp hybridization. Each C atom has one electron in each of its two sp hybrid orbitals and one electron in each of its two perpendicular unhybridized 2p-orbitals (43). The electrons in the sp hybrid orbitals on the two carbon atoms pair and form a carbon—carbon tr-bond. The electrons in the remaining sp hybrid orbitals pair with hydrogen Ls-elec-trons to form two carbon—hydrogen o-bonds. The electrons in the two perpendicular sets of 2/z-orbitals pair with a side-by-side overlap, forming two ir-honds at 90° to each other. As in the N2 molecule, the electron density in the o-bonds forms a cylinder about the C—C bond axis. The resulting bonding pattern is shown in Fig. 3.23. 

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

This makes the acetylene molecule linear, i.e. bond angles of 180°, and there are two n bonds with electron density either side of this axis. The properties of an alkyne, like acetylene, are also special in that the Jt bonds are again much more reactive than the a bond. 

In the preceding chapters we have seen how new bonds may be formed between nucleophilic reagents and various substrates that have electrophilic centres, the latter typically arising as a result of uneven electron distribution in the molecule. The nucleophile was considered to be the reactive species. In this chapter we shall consider reactions in which electrophilic reagents become bonded to substrates that are electron rich, especially those that contain multiple bonds, i.e. alkenes, alkynes, and aromatics. The jt electrons in these systems provide regions of high electron density, and electrophilic reactions feature as... 

Alkynes have two v and two it orbitals that can potentially interact with metal orbitals, and in some instances, it is thought that all of these are involved at the same time in a mononuclear complex. An extended Hiickel calculation on Mofmexo-tetra-p-tolylporphyrinXHC=CH) supports this view (Fig. I5.25).8" Thus both bonding orbitals of the alkyne (6, and at) can donate electron density to molybdenum to form (he 16, and lo, MOs. and both anti bonding orbitals (b2 and a2) can accept electron density to form the lf>2 and Ui2 MOs. Notice that both it bonding orbitals (a, and bt) of acetylene interact significantly with metal d orbitals of the same symmetry. 

Electron-withdrawing m-substituents will decrease the electron density at the aromatic terminus of the [3,3]-rearrangement, thereby retarding the reaction. This allows polymerization of the terminal alkyne to compete successfully with the cyclization. However, the presence of a m -substituent has a much more significant effect the cyclization of m-substituted aryl propargyl ethers can lead to two isomeric products (46a and b). 

To cite some newer work on Fukui functions it was claimed that if one accepts negative values of the function (apparently previously shunned), one can understand reactions in which oxidation of an entire molecule leads to reduction of a part of it (removing electrons from alkynes can increase the electron density in the CC bond) [162] the Fukui functions concept has been extended beyond the local... 

Alkenes and alkynes are nucleophilic and they react with electrophiles in a reaction called electrophilic addition. The nucleophilic centre of the alkene or alkyne is the double bond or triple bond(Following fig.). These are areas of high electron density... 

The primary adducts (156) and (157) of oxazoles with alkenes and alkynes, respectively, are usually too unstable to be isolated. An exception is compound (158), obtained from 5-ethoxy-4-methyloxazole and 4,7-dihydro-l,3-dioxepin, which has been separated into its endo and exo components. If the dienophile is unsymmetrical the cycloaddition can take place in two senses. This is usually the case in the reactions of oxazoles with monosubstituted alkynes with alkenes on the other hand, regioselectivity is observed. Attempts to rationalize the orientation of the major adducts by the use of various MO indices, such as 7r-electron densities or localization energies and by Frontier MO theory (80KGS1255) have not been uniformly successful. A general rule for the reactions of alkyl- and alkoxy-substituted oxazoles is that in the adducts the more electronegative substituent R4 of the dienophile occupies the position shown in formula (156). The primary adducts undergo a spontaneous decomposition, whose outcome depends on the nature of the groups R and on whether alkenes or alkynes have been employed. 

Milet, Gimbert and co-workers used DFT calculations to study the formation of the cobaltacycle and calculated activation energies and energies for the orbitals involved.96 From their calculations, they were able to deduce that the 7i -orbital (LUMO) of the alkene is very important in the PK reaction. On coordination of the alkene to the ( 42-alkyne)Co2(CO)5 fragment, possessing a vacant coordination site, the ji -orbital of the alkene is able to accept electron density from the d -orbital (HOMO) of the Co2(CO)5 fragment. This orbital is also involved in the formation of the new C C bond (Fig. 4). 

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