Nonbonding interactions steric

Ibis hindered rotation of >f2-H2 is governed by various forces, which can be divided into bonded (electronic) and nonbonded interactions ( steric effects). The direct electronic interaction between M and H2 results from overlap of the appropriate molecular orbitals. Nonbonded interactions such as van der Waals forces between the q2-H2 atoms and the other atoms on the molecule may vary as 2- 2 rotates. Intermolecular interactions should not contribute much to the barrier to rotation of q2-H2, as the metals are far apart. However, they may have a minor effect on the coordination geometry about M, which could in turn affect M-H2 binding. 

Try to explain the conformational preference in terms of steric repulsion. Which ring atom(s) in the higher-energy conformer approach the CH3 group most closely (Make sure that you find all significant nonbonded interactions.) Which of these interactions are absent in the lower-energy conformer Can interactions that appear in both conformers account for the conformational preference ... 

Examine space-filling models for the two conformers and identify any likely unfavorable nonbonded interactions. Based on steric effects, which conformer would you anticipate would be the more stable Compare energies of anti-1,2-ethanediol and gauche-1,2-ethanediol to see if you are correct. Is this the same ordering of conformer energies as seen for n-butane (see Chapter 5, Problem 3)7... 

Stereoelectronic effects and nonbonded interactions are non-cooperative in the reactions of (E)-allylboronates and x-heteroatom-substituted aldehydes. Thus, while transition state 8 experiences the fewest nonbonded interactions (gauche pentane type, to the extent that X has a lower steric requirement than R3), transition state 9 is expected to benefit from favorable stereoelectronic activation (Felkin-type)58f. This perhaps explains why the reaction of 2,3-[iso-propylidenebis(oxy)]propanal and ( >2-butenylboronate proceeds with a modest preference (55%) by way ol transition state 9. This result is probably a special case, how ever, since C-3 of 2.3-[isopropylidenebis(oxy)]propanal is not very stcrically demanding in 9 owing to the acetonide unit that ties back the oxygen substituent, thereby minimizing interactions with the... 

First, one of the strongest piece of evidence in support of the existence of a thianorcaradiene intermediate is the steric effect of the substituents at C-2 and C-7 of a thiepin. Substantial stability gained by 2,7-di-tert-butyl substitution on thiepin implies that these groups force the nonbonding interaction in the thianorcaradiene structure to be large and hence the thiepin structure will be favored (see Section 4-1, 4-3 and 4-4). 

The sigma nonbonded interaction between the two substituents fall into pattern d of Scheme 1. Here, unlike the case of 1,2-difluoroethane, we conclude that there will be a preference for the syn conformation due to the sigma nonbonded interaction of the pi systems of the substituents. This will be counteracted by the inherent preference of any ethane molecule for the staggered geometry and a compromise is expected to be reached in the gauche conformation, barring adverse steric effects. 

Now, since sigma nonbonded interactions are weaker than pi nonbonded interactions, there will be an electronic bias in favor of Css. However, steric effects favor the Cee conformation. Hence, steric effects may make it appear that sigma nonbonded interactions dominate pi nonbonded interactions. 

Nonbonded interactions may obtain in halosubstituted benzenes. For example, consider the model compound o/T/io-difluoro-benzene which can exist in either a planar geometry or a distorted geometry in which the C-F bonds are bent, alternately above and below the mean plane as a result of severe dipolar and steric repulsion. 

On the basis of a previous discussion, we predicted that pi nonbonded interactions will produce an order of stability of gauche > tram > cis, assuming appreciable Ci—C4 pi overlap in 1,3-butadiene. However, if sigma nonbonded interactions dominate pi interactions the order of stability will be cis > gauche > tram. Finally, in the event that steric effects are the controlling factor the order of stability will become trans > gauche > cis. 

In other words, the apparent order of stability of 1,3-butadiene is trans > gauche > cis. We conclude, therefore, that the preferred conformation of 1,3-butadiene is dictated by steric effects which overwhelm attractive pi and sigma nonbonded interactions. 

We can extend the above theoretical approach to isoconjugate molecules such as acrolein and glyoxal. Arguing as before, we conclude that the relative order of conformational stability will be gauche > tram > cis if pi nonbonded interactions dictate the preferred conformation, cis > gauche > trans if sigma interactions are dominant, and trans > gauche > cis if steric effects are the most important factor. 

Property Sigma conjugative effect Nonbonded interaction effect Electrostatic or steric effect Ab initio... 

Dominant Ri R2 steric control elements are predicted to disfavor transition state T and promote enolization to give the (Z)-geometry, whereas dominant R2 L nonbonded interactions should disfavor transition state C and promote enolization to afford the ( )-enolate geometry. As summarized below in Table 10, under conditions of apparent kinetic control, esters and thioesters afford largely ( > enolates (transition state T ), and the dialkylamides exhibit predominant to exclusive (Z)-enolization (transition state C ). 

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