Alkyne electron distribution

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... 

Bearing in mind what you already know about the electron distribution in alkynes and also about the nature of the intermediates in the reaction pathway, suggest three reasons to explain why alkenes react faster with bromine than do alkynes. 

Veillard [19] covers a similar range of molecules but from the Hartree-Fock and post-HF view. The discussion is organised more in terms of molecular properties. Thus, he deals with metal carbonyls, carbides, cyanides, C02 complexes, alkyls, carbenes, carbynes, alkenes, alkynes and metallocenes under the headings of electronic states, electronic spectra, optimised geometries, binding energies, Ionisation Potentials and Electron Affinities, nature of M-L bonding and other properties (e.g. vibrational spectra, dipole moments and electron distributions). 

Why are Ugi s MCRs so efficient The answer hes in the low activation energies of all the elementary steps, which are either eqmlibrium processes or irreversible steps. This concept of energetically preferred reactions is discussed in Chap. 15, in relation to click-reactions of azides and alkynes. Low-energy elementary reactions either occur in concert and are pericychc or involve two reacting groups with highly matching electron distribution. 

The first example of a stable pyridinium yhde of type 80 is the pyridinium phenacylide (80, A = CH2COPh), obtained from the N-phenacylpyridinium ion (79, A = CH2COPh) by deprotonation with Na2C03 [97]- The reactivity of the pyridinium betaines is determined by their electron distribution (80a-c). Thus, they can be smoothly alkylated or acylated at the N-substituent ( 81) as 1,3-dipoles, they undergo dipolar cycloadditions with activated alkynes or alkenes [98] for example, the sequence 80 —> 82 83 establishes an efficient principle of indolizine synthesis (cf. p. 154). 

Dodelet, J.-P., Shinsaka, K., Kortsch, U., and Freeman, G.R., 1973, Electron ranges in liquid alkanes, dienes, and alkynes Range distribution function in hydrocarbons, Chem. Phys., 59 2376. 

The formation of trans-products is observed to a lesser extent in the reaction of 3-alkoxycarbonyl-substituted cyclohexenones, in the reaction with electron-deficient alkenes and in the reaction with olefinic reaction partners, such as alkynes and allenes, in which the four-membered ring is highly strained (Scheme 6.11). The ester 26 reacted with cyclopentene upon irradiation in toluene to only two diastereomeric products 27 [36]. The exo-product 27a (cis-anti-cis) prevailed over the endo-product 27b (cis-syn-cis) the formation of trans-products was not observed. The well-known [2 + 2]-photocycloaddition of cyclohexenone (24) to acrylonitrile was recently reinvestigated in connection with a comprehensive study [37]. The product distribution, with the two major products 28a and 28b being isolated in 90% purity, nicely illustrates the preferential formation of HH (head-to-head) cyclobutanes with electron-acceptor substituted olefins. The low simple diastereoselectivity can be interpreted by the fact that the cyano group is relatively small and does not exhibit a significant preference for being positioned in an exo-fashion. 

For reduction, relevant data from polarographic and cyclic voltammetric experiments are summarized in Tables 1 and 2, respectively. For the results in Table 1 the variety of solvents and reference electrodes used makes comparisons difficult. It is clear, however, that even with the activation of a phenyl substituent (entries 6,7,9-14) reduction occurs at very cathodic potentials. In this context it is worth noting that in aprotic solvents at ca. — 3 V vs. S.C.E.) it becomes difficult to distinguish between direct electron transfer to the alkyne and the production of the cathode of solvated electrons. Under the latter conditions the indirect electroreductions show many of the characteristics of dissolving metal reductions (see Section II.B). Even at extreme cathodic potentials it is not clear that an electron is added to the triple bond the e.s.r. spectra of the radical anions of dimesitylacetylene and (2,4,6,2, 4, 6 -hexa-r-butyldiphenyl)acetylene have been interpreted in terms of equal distribution of the odd electron in the aromatic rings . 

For a portion of their time, these molecules will be oriented with their Tt-bonds perpendicular to applied field, as illustrated in Figure 6. The current distribution of the 7i-electrons is such that a magnetic field is produced that opposes the applied field above and below the bond, but reenforces the applied field at the attached proton. An exception to this is the terminal alkyne group (Figure 6), where the proton is oriented in the same plane as the rt-bond. The major contribution comes when the 7t-bond is oriented parallel to the applied field. The current distribution from the 7i-electrons leads to an induced field that opposes the applied field at the proton. All of the above effects are additive in determining the final chemical shift of a proton and the position of resonances can be suggestive of specific function groups (see Table 1). 

---