Hyperconjugative interactions

The vertical ionization potentials from the photoelectron spectra of some thiirane and thiirene derivatives are given in Table 3. A Walsh localized scheme of bonding is generally preferred. There is a strong hyperconjugative interaction in thiirene 1,1-dioxides between the occupied C=C tt-MO and the occupied SO2 pure sulfur d-AO. Thiirene oxides are suggested to be less aromatic than cyclopropenones and tropone. 

How do you interpret the values of p and r in these equations Which system is more sensitive to the aryl substituent How would you explain this difference in sensitivity Sketch the resonance, field and hyperconjugative interactions which you believe would contribute to these substituent effects. What, if any, geometric constraints would these interactions place on the ions ... 

Figure 6.12 Stabilization of the ethyl carbocation, CH3CH2+, through hyperconjugation. Interaction of neighboring C H

Semi-empirical molecular orbital calculations have been carried out on the model phosphorane HaPiCHa. Besides the expected transfer of charge, the inclusion of the phosphorus 2>d orbitals showed a significant hyperconjugative interaction between the CHg orbitals and a 2>d orbital of appropriate symmetry on phosphorus. Calculations on cyclopropylidene-phosphorane revealed a similar interaction between the Walsh orbitals of the ring and an in-plane phosphorus 2>d orbital. 

Other types of hyperconjugative interactions are also thought to exist. A recent example involves the compound R2A1A1R2 (R = —CH(SiMe3)2) whose planar structure has been accounted for in terms of a hyperconjugative interaction involving the orbitals of the C—Si bond.1618... 

The hydrogen atom migration observed on thermolysis of is reminiscent of 1,2-hydrogen atom migrations in carbene chemistry (45,46,47). The stereochemistry of such processes is now relatively well-understood and involves initial hyperconjugative interaction between a gauche C-H bond and the carbene unoccupied p atomic orbital, followed by a low activation energy 1,2 shift (eq.(6)) ( 7,48,49,50). 

Thermolysis of the syn and anti /J-acetoxy stannanes, obtained by addition of Bu3SnLi to tram- or cis- 1,2-dimethyloxirane and subsequent acetylation, led to (Z)- or (E)-2-butene, respectively, by a stereospecific anti process (equation 34).73. It is postulated that a hyperconjugative interaction of the C—Sn bond facilitates departure of the acetate group in these acyclic systems. The trimethylstannyl and triphenylstannyl analogues likewise undergo anti elimination. 

In this section we will describe some PES investigations on Group XIV element derivatives in which one or more sulphur or selenium atoms are bonded directly or indirectly to the metal120-132. The aspects investigated preferentially for these molecules were the electronic and conformational situation, the role played in the bonding by the chalcogen lone-pair orbitals and possible hyperconjugative interaction (n-cr or p -d ). 

Hyperconjugative interactions (Section 3.4.2) are another factor leading to bond bending. Such interactions are associated with the stereoelectronic influence of more remote bonds or lone pairs, particularly those anti (trans) to the bond of... 

From Table 3.22 one can see that typical hyperconjugative interactions of vinyl pi bonds with hydride bonds are rather weak (2 1 kcal mol-1), but those with lone pairs are considerably stronger (7-30 kcal mol-1). (Of course, a more polar pi bond... 

Figure 3.49 Orbital contours for hyperconjugative interactions in propene (a)

Figure 3.51 also contains a dissection of the total energy ( totai) into Lewis (ii(L)) and non-Lewis (ElSL>) components. The localized Lewis component E" corresponds to more than 99.3% of the full electron density, and so incorporates steric and classical electrostatic effects in nearly exact fashion. Yet, as shown in Fig. 3.51, this component predicts local minima (at 70° and 180°) and maxima (at = 0° and 130 ) that are opposite to those of the full potential. In contrast, the non-Lewis component E (NL) exhibits a stronger torsional dependence that is able to cancel out the unphysical behavior predicted by (L), leading to minima correctly located near 0° and 120°. Thus, the hyperconjugative interactions incorporated in E(SL> clearly provide the surprising stabilization of 0° and 120° conformers that counter the expected steric and electrostatic effects contained in ElL>. 

Still another way to picture the conformational preferences is to visualize the C=C double bond in terms of two equivalent banana bonds (Fig. 3.50). In this picture the preferred conformations are those with C—F in staggered orientation with respect to the three bonds (two banana bonds and one C—H bond) of the vinyl moiety, analogous to the preferred conformations of ethane. However, in using this ethane-like mnemonic one should recall that its essential origin lies in the hyperconjugative interactions of E(SL> rather than the steric and electrostatic interactions of (L). 

From comparison of Figs. 3.51 and 3.52 one can judge that Em,) includes contributions other than the six primary hyperconjugative interactions of Fig. 3.52. These omitted contributions are principally of secondary cr-a type, to be discussed in Section 3.4.2. However, these weaker hyperconjugative interactions do not significantly alter the preferred conformational angles established by the dominant six interactions of Fig. 3.52. 

A particularly dramatic and interesting example of the dominant role of hyperconjugative interactions over steric and electrostatic interactions is given by cis (27a) and trans (27b) isomers of 1,2-difluoroethylene ... 

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,... 

The general role of aCH orbitals and hyperconjugative interactions in methyl tilting was suggested much earlier on qualitative grounds H. P. Hamlow, S. Okuda, and N. Nakagawa, Tetrahedron Lett. 1964 (1964), 2553 and note 141. 

The corresponding hyperconjugative interactions in an open-shell singlet description of radical dissociation would similarly promote homolytic bond cleavage. 

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