Lone-pair stabilization

FIGURE 5. (a) Anomeric interactions in bisheteroatom-substituted amides (b) lone-pair stabilization through an ny NX anomeric interaction... 

The lone pair MO of the free amine and phosphine possesses at symmetry in Cjt, local symmetry. As anticipated, this MO undergoes considerable stabilization as coordination to the metal occurs. The lone pair stabilization energies of amines have been found to be greater than those of phosphines with respect to main group acceptors such as BH3, O, and S (32, 113, 210). The sulfide ligands feature two MOs with significant lone pair character. 

Electrophilic substitution of vinylsilanes may be combined with the fazarov cyclization to build five-membered rings, as depicted in Figures i5.11 and Si5.12 (LA = Lewis acid). Note that the reaction depicted in igure Si5.12 is anomalous because the vinylsilane is attacked by the lectrophile at the end of the double bond remote from silicon. This is ecause the sulphur lone pairs stabilize a carbocation more effectively than le silicon P effect. 

A closer analysis of the DFT results within the vibronic coupling model shows5 that the lone-pair stabilization—as has been emphasized before7,10—is a purely orbital effect. It is caused by the changes in overlap due to the electronic rearrangements, which accompany the nuclear Td—>C2v... 

Derivatives of Group V Elements.—Cyanogen, Cyanides, Cyanates, and Related Species. The number of papers published during the period of this Report which describe the chemistry of these species is markedly lower than that for previous Reports. Theoretical calculations of the electronic structures of HCN,225 HNCS,226 and the CN radical227 228 have been successfully completed. The electronic structure of HNCS has been compared with that of HNCO226 and it is concluded that (i) the 77-system in HNCS involves a nitrogen lone pair stabilized by a C—S 77-bond, whereas the 77-system in HNCO consists of a C—O 77-bond stabilized by the nitrogen lone pair, and (ii) the d -orbitals of sulphur accept electron density in a <7- rather than a 77-fashion. The electronic structure of the NCS ion has also been determined experimentally from the X-ray Kp fluorescence and K absorption spectra of the S atom in KSCN.229... 

All elimination routes are improved by this swap of oxygen for carbon (Fig. 4.29). The ElcB proton transfer step now creates a very stable oxyanion, an anion on a very electronegative oxygen atom. The slow step of the El, loss of the leaving group, is also sped up because the oxygen lone pair can stabilize the carbocation formed. All routes have formed a very strong C=0 bond. The primary determinant at this point, since all routes are reasonable, is the pH of the reaction medium. The ElcB is found in base because the oxyanion is basic (pA"abH is about 12 to 16). The El is found in acid because the lone pair stabilized carbocation is just a protonated carbonyl (pA a is near -7). The E2 is found in more neutral media. 

Line structure can be deceptive in eliminations because the important C-H bond is not explicitly drawn out, in this example, when there is a lone pair stabilized carbocation intermediate in the El. The loss of a formal charge may be difficult to notice in reactions that drop off a proton because the proton is commonly not drawn as one of the products. Since the proton lost was not drawn in the line structure on the left, it was simply forgotten. The arrow made a double bond in the right place but flowed the electrons the wrong way away from the positive center and not toward it. Besides the loss of the formal charge, the lone pair on nitrogen was lost it was assumed in the product, but the incorrect arrow did not form it. The right side shows the correct electron flow path Dg, deprotonation of a carbocation to form a pi bond. 

Acid catalysis goes via the lone-pair-stabilized carbocation. 

Simple pi bonds are usually not good enough nucleophiles to react with polarized multiple bonds. If a Br0nsted acid or a Lewis acid is added to improve the electron sink, then addition can occur via the lone-pair-stabilized carbocation as the sink. Figure 8.7 shows a mechanistic example from a short synthesis of the human hormone estrone. 

Friedel-Crafts—Aromatic Rings with Lone-Pair-Stabilized Carbocations as Sinks... 

The carbon-carbon bond-forming step of the acid-catalyzed aldol reaction has an enol (allylic source) attacking a protonated carbonyl (which is just a lone-pair-stabilized carbocation). With those hints, give a mechanism for the acid-catalyzed aldol reaction. 

The water buffer solvent is very polar. The water leaving group is p aHL= -7, and the carbocation formed has lone pair stabilization by resonance. The Dn step, and therefore the El route is reasonable. 

The stability of both path Dn product carbocations must be checked the lone-pair-stabilized tertiary cation is much more stable and therefore is favored over the unstabilized primary cation. Ionization of the leaving group (path Dn) creates a somewhat less stable cation than the protonated ketal, so ionization of the leaving group is uphill in energy. 

The lone-pair-stabilized tertiary cation is an excellent electron sink. The newly formed alcohol group and also the water solvent are both mildly basic and nucleophilic. However, nucleophilic attack on the carbocation by the alcohol group simply returns us to the previous intermediate, so let s go forward. 

Again, the reacting species enter the SnI/EI surface (the Sn2 was previously ruled out by the study). The protonated hemiketal can lose the leaving group in a process identical with that of the ketal (path D ) to give the lone-pair-stabilized tertiary cation. 

The lone-pair-stabilized cation can lose a proton from carbon to form the enol (path De),... 

Delocalization of an oxygen lone pair stabilizes the cation. All the atoms in B have octets of electrons, making it a more stable structure than A. Only six electrons are associated with the positively charged carbon in A. 

Lone-pair stabilization decreases as you go down the periodic table (e.g., O >... 

This interaction corresponds to the point we made above about adjacent lone pairs stabilizing carbenes via the delocalization shown in the margin. As these arrows suggest, carbenes that have heavUy electron-donating substituents are less electrophilic than other carbenes indeed, diamino carbenes can be quite nucleophilic. The division of carbenes into two types explains... 

Delocalization of the nitrogen lone pair stabilizes the carbonyl group and makes amides less reactive than other carboxylic acid derivatives toward nucleophilic acyl substitution. 

Oxygen lone pair stabilizes the radical, making it prone to oxidation... 

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