Trigonal Electrophiles

It is also possible to account for the order of electrophilicity using only the energy of n Co, which is the LUMO for each of the carbonyl compounds. Using the arguments developed in Chapter 2, C-substituents lower the energy of the LUMO a little, X-substituents raise the energy of the LUMO, and Z-substituents substantially lower the energy of the LUMO. 

In one important case, stabilisation in the intermediate plays a larger role in determining electrophilicity than the differences in the energy of the starting materials. In contrast to the order of electrophilicity in alkyl halides in SnI, Sn2, El and E2 reactions (L Br Cl F ) aromatic 

A striking example of modified electrophilicity is provided by the imine 4.111.353 The conjugation of the Si—C bonds (Fig. 2.18) with the C=N n system raises the energy of the LUMO, making it less electrophilic than an ordinary imine. Furthermore, since the Si—C bonds are X-substituents, they raise the coefficient on the imine carbon atom in the HOMO and reduce it in the LUMO (Fig. 2.7), further decreasing the electrophilicity, since that depends upon the coefficient of the atomic orbital as well as on the energy. The net result is that the imine 4.111 is a stable compound, unlike other methylene imines, which normally polymerise before they can be isolated. 

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Similar diastereoselectivities have been observed for trigonal electrophiles [48] [for example, ratios from the E enolate and Z enolate respectively (E, 2) ... 

Most tin(II) compounds display structures with a trigonal pyramidal coordination. This is of course to be expected as the tin atom is in the first place electrophilic in order to complete its outer electron configuration (cf. Chapter 5 and 6). To illustrate the resemblance of this geometry between ionic and molecular compounds, the structure of NH4SnF3 (5) 31) is compared with that of the cage compound (Me3CN)3(Me3A10)Sn4 (6) 32). The coordination sphere of the tin atom is the same in 5 and 6 (for the complete structure of 6 see Sect. 6.5) ... 

As we have seen (Section 4, p. 191) the range of effective molarities associated with ring-closure reactions is very much greater than that characteristic of intramolecular general acid-base catalysis the main classification is therefore in terms of mechanism. By far the largest section (I, Tables A-D) gives EM s for intramolecular nucleophilic reactions. These can be concerted displacements (mostly at tetrahedral carbon), stepwise displacements (mostly addition-elimination reactions at trigonal carbon), or additions, and they have been classified in terms of the nucleophilic and electrophilic centres. 

A possible justification for frontside attack in electrophilic substitution is that ab initio molecular orbital calculations for the CH5+ cation, the species that would be formed if H+ attacked methane, indicate that the most stable structure would not be a trigonal bipyramid, in which carbon uses a p orbital to bond to two protons, but would be a relatively unsymmetrical structure that has a smallest H—C—H bond angle of about 37° (Figure 4.10).85 For further discussion of SB2 substitution on carbon, see Section 10.3.86... 

The pyramidalization of the trigonal centers (a common feature found in the X-ray structures of several related dioxinones) is probably responsible for the high selectivity.61 The nucleophile preferentially attacks the electrophilic center from that (convex) face into which it is pyramidalized, in accordance with the principle of minimization of torsional strain. A related example for asymmetric 1,4-addition to chiral dioxinones was reported.62... 

Baldwin concluded that the remarkable difference between these two cycliza-tions results from stereoelectronic control of the alkylation of the amhi-dent nucleophile, i.e. the enolate ion. For such an ion, carbon alkylation requires approach of the electrophile perpendicular to the plane of the enolate, whereas oxygen alkylation requires approach in the plane of the enolate. Consequently, in the five-membered ring case, the C-alkylation process 196A 198 (which can be considered as a 5-Endo-trigonal process) is sterically difficult, but not the 0-alkylation process 196B 197 (a 5-Exo-tetrahedral process). 

For example, the C02 complex of iron(O), Fe(C02)(depe)2, which has a trigonal bipyramidal geometry with a side-on bonded C02, reacts with electrophiles such... 

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