Antiperiplanar interactions

Similar effects apply to aromatic ethers and related compounds. Chein and Corey discovered an interesting stereoelectronic preference for heteroaromatic ethers to orient the OR group in a way that minimizes lone pan-repulsion of the exocyclic substituent o-lone pair and the lone pair of nitrogetf but maximizes the anomeric interactions between OR lone pairs and endocyclic C-X bond. The antiperiplanar interactions of this type are likely to contribute as well, more so in the six-membered cycles where the interacting orbitals can easily adopt the ideal parallel geometry (Figure 6.121). 

Not only do the antiperiplanar interactions define the conformational profile in these compounds but they play an important role in their reactivity as well (Figure 7.28). cis-Difluorodiazene is highly reactive and the cw-isomers of dichloro- and dibromodiazenes are extremely unstable. This reactivity is also a consequence of stereoelectronic interactions because n interactions weaken the N-X bonds and stabilize... 

Figure 7.48 Stereoelectronic interactions with hyperconjugative acceptors assist in directing anionic cycliza-tions to the endo-paths. The antiperiplanar interactions between carbanion and the a C-Nu bond, that exist for both the exo- and the endo- products are not considered as a factor in determining the relative stability of the two cycUzation products.

The synclinal conformation (sc) is appropriate for overlap of an oxygen nonbonded pair with the a C—Cl orbital. The preferred ap relationship, requires an antiperiplanar alignment of a lone-pair orbital with the bond to the electronegative substituent. Because of the donor-acceptor nature of the interaction it is enhanced in the order F < O < N for the donor (D) atom and N < O < F for the acceptor (A) atom. 

In cyclic systems such as 1, the dominant conformation is the one with the maximum anomeric effect. In the case of 1, only conformation lA provides the preferred antiperiplanar geometry for both oxygens. Antiperiplanar relationships are indicated by including lone pairs in the oxygen orbitals. Other effects, such as torsional strain and nonbonded repulsion, contribute to the conformational equilibrium, of course. Normally, a value of about 1.5 kcal/mol is assigned to the stabilization due to an optimum anomeric interaction in an acetal. 

Since equatorial attack is roughly antiperiplanar to two C-C bonds of the cyclic ketone, an extended hypothesis of antiperiplanar attack was proposed39. Since the incipient bond is intrinsically electron deficient, the attack of a nucleophile occurs anti to the best electron-donor bond, with the electron-donor order C—S > C —H > C —C > C—N > C—O. The transition state-stabilizing donor- acceptor interactions are assumed to be more important for the stereochemical outcome of nucleophilic addition reactions than the torsional and steric effects suggested by Felkin. 

Butanes are chosen as the simplest models for the normal and branched isomers. Both branched and normal isomers contain a C-C bond (2 ) interacting with the terminal C-H bonds (2 and 2 ) (Scheme 26a). The cyclic -aj-a2 -a3 a2- interaction (Scheme 26b) occurs in the polarization of the middle C-C a-bond by the interactions with the antiperiplanar C-H a-bonds. The orbital phase is continuous in the branched isomer and discontinuous in the normal isomer (cf Scheme 4). The branched isomer is more stable. The basic rule of the branching effects on the stability of alkanes is ... 

Gronert [42] and Schleyer [43] are not aware of our theory [41]. Branched alkanes are stabilized by the C-C bond polarization by two antiperiplanar C-H bonds. The polarization is favored by the orbital phase continuity. We can predict the relative stabilities of alkanes only by counting the number of the vicinal bond trios. Neither the Gronert nor the Schleyer model contains any vicinal interactions. 

The carbonyl n face of the adamantan-2-one with an electron-withdrawing group at the 5-position is nnsynunetrized by interaction of the P o bonds antiperiplanar to the C-H bonds and to the C-R bond. The orbital phase environment of the carbonyl n orbital (7) is nnsynunetrized by the more electron-donating orbitals at the P-position, which is consistent with the observed syn preference. 

Synclinal and antiperiplanar conformations of the TS are possible. The two TSs are believed to be close in energy and either may be involved in individual systems. An electronic tt interaction between the stannane HOMO and the carbonyl LUMO, as well as polar effects appear to favor the synclinal TS and can overcome the unfavorable steric effects.161bi 162 Generally the synclinal TS seems to be preferred for intramolecular reactions. The steric effects that favor the antiperiplanar TS are not present in intramolecular reactions, since the aldehyde and the stannane substituents are then part of the intramolecular linkage. 

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,... 

Finally, it is worth mentioning that 15N-NMR spectra of yohimbine (74), reserpine (109), and isoreserpine (514) also have been investigated (319). The 15N chemical-shift differences between C/D-cis- and trans-fused compounds could be explained by a hyperconjugative interaction between the antiperiplanar C—H bonds and the nitrogen lone pair characteristic for trans-fused compounds only. 

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... 

This is attributed to the unfavourable steric interactions which arise in the transition state that is required for antiperiplanar migration of the exocyclic substituent.143 Some examples of synthesis of alcohols by hydroboration-oxidation are included in Scheme 4.8. More vigorous oxidizing agents such as Cr(VT) reagents effect replacement of boron... 

It remains to explain why the Felkin transition states are the most stable. During the reaction, the major interaction occurs between the nucleophile s HOMO and the substrate s LUMO. Therefore, the most reactive conformation of the substrate is that with the lowest LUMO. This corresponds to the geometry in which the C2-L bond is parallel to the tt system, as there is then a good overlap between the orbital and the lowlying ac2-L orbital, leading to a stabilization of the LUMO. The nucleophile may attack this conformer in an antiperiplanar or synperiplanar stereochemistry. The latter is disfavoured for two reasons ... 

Li+ interacts with both alkyl chains is more stable, compared to the structure where it sits at the antiperiplanar position, by 3.0 kcalmoG. The two binding trends of the alkali metal cations to alkenes were rationalized, taking into account the polarization of the double bond by the cation, polarization of the alkyl chains and steric effects as well. Recently, Gal and coworkers reported a similar alkyl chain-Li+ attraction in alkylbenzenes, namely the scorpion effect ... 

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