Lone-pair bond-weakening effect

Valence Bond Analysis of the Lone Pair Bond Weakening Effect for the X—H Bonds in the Series XH = CH4, NH3, OH2, FH. 

Scheme 7 Lone-pair bond-weakening effect (LPBWE) in the ionic structure for halogen (X) transfer reactions in (a) and the lack of LPBWE in the ionic structure of H transfer reactions in (h). Adapted from [9] with permission of Wiley-VCH...

Finally, transition states for hydroxide attack and methoxide departure, corresponding to each of the rotameric TBP intermediates [56]-[58] were partially geometry-optimized. Population analysis of these putative penta-coordinate transition states does indeed demonstrate that the bond-weakening effects observed in the intermediate and ground state are accentuated in the transition state . The percentage decreases in overlap population in P—O bonds subject to an app lone-pair interaction in the ground, intermediate and transition states are 4-5%, 6-8% and 7-12% respectively. 

This anomeric (n->CT ) interpretation, employing an electron-delocalization model, is the basis of the phosphorus app lone-pair hypothesis (PAPH). This hypothesis is completed by integration of two observations from the theoretical calculations discussed above (I) bond-weakening effects are calculated to be greater for apical than equatorial P—O bonds and (2) effects on conformational energy and electron distribution are amplified in pentacoordinate transition-state structures. 

Dinitromethide ions add to methyl acrylate in the Michael fashion. When the remaining hydrogen of the methide ions is replaced by fluorine, the Michael addition proceeds with considerable rate enhancement (61). The phenomenon has been attributed to anion destabilization through C-F bond weakening by the more electronegative trigonal carbon atom and the repulsion of the delocalized p electrons by the fluorine lone-pair electrons. The effect can simply be viewed as a consequence of the inharmonious union of the hard fluorine with the soft, delocalized carbanion center. 

Another example of a stereoelectronic effect is observed in amines. Amines in which a C—H bond is oriented antiperiplanar to the nitrogen lone pair show a shift in the C—H bond stretching frequency that corresponds to a weakening of the bond by about... 

Stereoelectronic effects in chemical reactivity The bond-lengthening and -weakening influence of an antiperiplanar lone pair leads to strong stereoelectronic effects on chemical reactivity.97 In molecule 28a with lone-pair-bearing atom D adjacent to an A—B bond, a vicinal nD—s-cab hyperconjugative interaction can be associated (cf. Example 1.4 and Section 3.3.1) with a partial admixture of the alternative resonance structure 28b,... 

Substitution reactions taking place in water solution can often be accelerated by the presence of an acid or base. If the coordinated leaving group (X) has lone pairs whicb can interact with H+ or metal ions such as Ag+ or Hg2, the M—X bond may be weakened and loss of X facilitated.25 This effect is seen in the aquation of [Cr(H2OUF]2+ ... 

The chemical shift of the methine proton in orthoamide 223 is 2.3 ppm. It is therefore at a much higher field than that of orthoamide 122. This remarkable difference of 2.7 ppm can be ascribed to a dramatic stereoelectronic effect. The origin of the unusual spectroscopic properties of orthoamide 123 presumably is the antiperiplanar relationship of the central C-H bond to the three lone pairs. This arrangement permits mixing of the lone pair orbital with the antibonding orbital of the central C-H bond (a ). As a result, the electron density at the methine hydrogen increases and the central C-H bond is weakened. Indeed, this hydrogen has a notably small chemical shift. 

Molecular orbital calculations have also provided theoretical justification for these stereoelectronic effects in tetracovalent and pentacovalent phosphorus species (2-7). As has been shown in molecular orbital calculations on the X -P-X2 (X = 0,N) structural fragments, the X.-P bond is strengthened (as indicated by an increase in the Mulliken overlap population) while the P-X3 bond is weakened when the X atom lone pair is app to the P-X3 bond. Thus, in the g,t conformation of dimethyl phosphate (Structure ll the overlap population for the trans P-0 bond is. 017 electron lower than the overlap population for the gauche P-0 bond. As shown for g,t dimethyl phosphate one lone pair (shaded in 1) on the gauche bond oxygen is app to the trans bond, while no lone pairs on the trans bond oxygen are app to the gauche bond. Thus, the weakest X.-P bond has one app lone pair and no lone pairs on X. app to the P=X2 bond. 1... 

The effect is naturally more pronounced if the substituent is charged. The best known example is the so-called anionic oxy-Cope reaction,54 where an O- at position 3 induces a rate acceleration of 1010-1017-fold, a result generally attributed to the 3-4 bond weakening.47 55 The lone pairs of HN" lie at even higher energies than those of O-. The 3-4 bond breaking then becomes so easy that the anionic amino-Cope reaction occurs by a stepwise mechanism.56... 

As indicated in Scheme VII/32, cyclononanone (VII/165) is transformed into hydroperoxide hemiacetal, VII/167, which is isolated as a mixture of stereoisomers. The addition of Fe(II)S04 to a solution of VII/167 in methanol saturated with Cu(OAc)2 gave ( )-recifeiolide (VII/171) in quantitative yield. No isomeric olefins were detected. In the first step of the proposed mechanism, an electron from Fe2+ is transferred to the peroxide to form the oxy radical VII/168. The central C,C-bond is weakened by antiperiplanar overlap with the lone pair on the ether oxygen. Cleavage of this bond leads to the secondary carbon radical VII/169, which yields, by an oxidative coupling with Cu(OAc)2, the alkyl copper intermediate VII/170. If we assume that the alkyl copper intermediate, VII/170, exists (a) as a (Z)-ester, stabilized by n (ether O) —> <7 (C=0) overlap (anomeric effect), and (b) is internally coordinated by the ester to form a pseudo-six-membered ring, then only one of the four -hydrogens is available for a syn-//-elimination. [111]. This reaction principle has been used in other macrolide syntheses, too [112] [113]. 

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