Steric interactions gauche

Studies have established that the partition between transition states 3 and 4 depends on the nature of the diol unit bound to boron and on the steric and electronic effects of the a-sub-stituent X23. The data shown below demonstrate that the reactions of2-(l-methyl-2-propenyl)-4,4,5,5-tetramethyl-l,3,2-dioxaborolane proceed with a moderate preference for transition state 3 with the C2 methyl group in an axial position. Selectivity diminishes with 2-(l-methyl-2-propenyl)-l,3,2-dioxaborolane and reverses with dimethyl (l-methyl-2-propenyl)boronale, suggesting that steric interactions (gauche interactions in the case of the tetramethyl-1,3,2-diox-aborolane) between X and the diol unit on boron are capable of destabilizing transition state 4 relative to 3. 

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... 

The preference for the civ-isomer has been explained by a cyclic transition state in which the steric interactions of the a-substituent of the iminc arc minimized by placing it gauche to the lactone. 8 shows the proposed transition state involved in the addition of phthalide enolates to imines24. 

The rationalization for the outcome of these cyclization reactions was based on minimizing steric interactions in the transition state leading to benzothiazine formation. For example, at least two possible transition states 101 and 102 could give rise to product 92 (or a diastereomer). Transition state 102 has gauche steric interactions that appear to be absent in the staggered transition state 101. This favors the latter and leads to the observed products 92 (Figure 8). 

It follows from the above X-ray data that betaines I and II have some structural peculiarities. Two main peculiarities are especially pronounced for betaines I. These compounds have the sterically strained gauche-conformation of the main chain due to the intramolecular Coulomb interaction of the cationic and anionic centers and noticeable distortions of the bond lengths in it. In Section 5 we discuss how these peculiarities of the betaine structure reflect their reactivity. 

Up to this point our discussion of y-gauche effects has demonstrated that the transmission mechanisms are not yet well understood and still open to speculation. The original concept of a steric interaction is highly controversial, and as long as there is no convincing explanation, the use of the deep-rooted term steric compression shift must be discouraged. 

R3 R2 and R2 Ri gauche interactions however, for the same set of substituents, an increase in the steric requirements of either Rj or R3 will influence only one set of vicinal steric interactions (Rj R2 or R3 R2). Some support for these conclusions has been cited (eqs. [6] and [7]). These qualitative arguments may also be relevant to the observed populations of hydrogen- and nonhydrogen-bonded populations of the aldol adducts as well (see Table 1, entries K, L). Unfortunately, little detailed information exists on the solution geometries of these metal chelates. Furthermore, in many studies it is impossible to ascertain whether the aldol condensations between metal enolates and aldehydes were carried out under kinetic or thermodynamic conditions. Consequently, the importance of metal structure and enolate geometry in the definition of product stereochemistry remains ill defined. This is particularly true in the numerous studies reported on the Reformatsky reaction (20) and related variants (21). 

Chemical shifts for monohydroxy-steroids may be used additively to predict those for diols, except in cases where the two functional groups are in a 1,2-relationship or in certain 1,3-relationships. Steric interactions of hydroxy-groups in 1,2-gauche or 1,3-diaxial relationships are thought to explain some of the observed deviations from additivity, which closely resemble those asociated with... 

It is a long-known fact that y-steric interactions of gauche substituents are most important for an evaluation of conformational equilibria of diastereomers with equal constitution378- 3ai. 

Stress-temperature coefficients are determined for cross-linked networks of PE and polyisobutylene elongated in the amorphous state. Interpretation of the indicated temperature coefficient of 0 for PE according to the three-fold potential model for rotation around the C—C bonds is consistent with an energy difference of 2.1 kJ mol-1 between gauche and trans states. The small temperature coefficient for isobutylene is due to steric interactions affecting bond rotations. 

The relative reactivities of alkyl vinyl ethers have been assessed in a number of chemical reactions and structure/activity correlations made via several NMR studies [for bibliography see Ref. (80)]. From the polymerisation data it appears that steric interaction between the incoming polymeric electrophile and monomer is the major factor controlling reactivity, rather than electronic effects. Vinyl ethers are known to exist in either a planar s-cis or a planar s-trans (or gauche) conformation. Infra-red absorption spectroscopy shows that methyl vinyl ether almost certainly exists largely in the s-cis form at room temperature (106), and it seems most likely that /(-chloroethyl vinyl ether also has an energetically favourable planar s-cis form as a result of a favourable gauche interaction of Cl and O atoms. The other alkyl vinyl ethers studied exist predominantly in either planar... 

The first precise evaluation of the anomeric effect was realized by Descotes and co-workers in 1968 (22). These authors have studied the acid catalyzed isomerization of the cis and trans bicyclic acetals 6 and 6 and found that, at equilibrium, the mixture contains 57% ci s and 43% trans at 80°C. The cis isomer is therefore more stable than the trans by 0.17 kcal/mol. The cis isomer 5 has one (stabilizing) anomeric effect whereas the trans isomer 6 has none. Steric interactions in cis acetal 5 were estimated tobel.65 kcal/mol (one gauche form of ri-butane, 0.85 kcal/mol and an OR group axial to cyclohexane, 0.8 kcal/mol). By subtracting an entropy factor (0.42 kcal/ mol at 80°C) caused by the fact that the cis acetal S exists as a mixture of two conformations (cis decalin system), they arrived at a value of 1.4 kcal/mol for the anomeric effect. 

While the steric polarization model may be called into question by the considerations presented above, it would be hard to reject the obvious parallels of steric interaction and the y-gauche shielding effect. A case in point is the highly strained molecule [14] (compound 480). The effects... 

Certain value judgments not built into the rules need to be applied. Thus, the envelope conformation accepted for the D-ring (43) suggests that a y-interaction should be allowed for the calculation of C-15 and C-18 but not for C-16 since the hydrogens on the latter are tilted away from C-18. Similarly, there is a gauche interaction between C-18 and C-20 in the side chain which influences the chemical shift of both carbons. It is likely that the difficulties at C-17 and C-21 arise from steric interactions which are difficult to account for without assigning a specific conformation to the side chain. The result at C-10 is more difficult to rationalize. 

For the pronounced endo selectivity in the reaction between aromatic aldehydes and 2,3-dihydrofuran, we can consider the two biradical conformers 10 and 11 to be responsible, with the alkyloxy substitutent localized in a pseudo-equatorial position and 10 being more populated because of fewer steric interactions (Scheme 3.17). When a methyl group is present, the increasing gauche... 

What is the reason for this apparent discrepancy It is a solvent effect. In aqueous solution (Figure 9.6), the OH group at the anomeric C atom of the glucose becomes so voluminous due to hydration that it strives for the position in which the steric interactions are as weak as possible. Thus, it moves into the equatorial position—with a AG° value of approximately -1.6 kcal/mol—to avoid a gauche interaction with the six-membered ring skeleton. (Remember that axially oriented substituents on the chair conformer of cyclohexane are subject to two gauche interactions with the two next-to-nearest C. —C bonds. They therefore have a... 

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