Conformational and Electronic Effects

Another example which shows how subtle changes in a protecting group can dramatically alter the ground state conformation of a molecule, and hence its reactivity, comes from the synthesis of dihydroerythronolide A by Stork and 

However, by manipulating the nature of the protection of the 3 hydroxyl protecting groups the problem was surmounted. Thus, changing from di-rerr-butylsi-lylene protection of the terminal 1,3-diol to di-isopropylsilylene protection of the internal 1,3-diol in 45.4 resulted in a restoration of reactivity and the desired product 45.5 was obtained in 82% yield. 

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In a completely different interpretation Zefirov (242) proposed a new concept of frontier-orbital mixing (243) to explain how conformational and electronic effects in monosubstituted cyclohexanes are transmitted to remote 8-carbon atoms (Scheme 36). The orbitals at C(l) and C(4) in 112 are considered to be equatorial (242). A perturbation at C(l) (H is replaced by X) produces an electron-density shift from H(4) toward C(4) (242), which is associated with an upheld shift of the latter s signal. Although this approach appears to be quite crude and does not account for axial substituents, it deserves fiirther attention. 

During our further studies of ketone catalysts, ketone 16 was found to be highly enantioselective for a number of acyclic and cyclic d.s-olefins (Table 10.6).73-74 It is important to note that the epoxidation is stereospecific with no isomerization observed in the epoxidation of acyclic systems. Ketone 16 also provides encouragingly high ee s for certain terminal olefins, particularly styrenes.74-75 In general, ketones 1 and 16 have complementary substrate scopes. In our subsequent study of the conformational and electronic effects of ketone catalysts on epoxidation, ketone 17, a carbocyclic analog of 16, was found to be highly enantioselective for various styrenes (Table 10.7).76... 

High-level nonempirical quantum chemical calculations of C- C coupling constants (/cc) of heteroaromatic ketoximes are well consistent with the experimental data, the effect of the nitrogen lone electron pair on /cc values being strongly predominant over conformational and electronic effects (07RJO872). 

The [Rh2(OAc)4] catalysed cyclization of a-diazo-a-(diethoxyphosphonyl)ac-etamides (415) led to a- and y-lactams (416) and (417). Conformational and electronic effects responsible for trans-stereochemistry of the y-lactam ring... 

Amide bonds ean be partieularly problematie. Difficulties with solubility, bioavailabihty, and stability are eommon. At the same time, this group is frequently essential for activity due to the H-bonding they enable as well as their conformational and electronic effects. Research on amide bond replacements for peptidomimetie compounds has a rich history going back many decades and is still an area of active interest today. A very recent example of the sueeessful use of a simple amide bond bioisostere can be seen in Figure 8.10. 

A conceptually different approach for Bergman cyclization of acyclic enediynes (and stable cyclic enediynes) relies on the use of metal ion chelation to allow the reaction to proceed at lower temperatures.Metal ion chelation requires that the enediyne contain heteroatoms at appropriate positions within the enediyne scaffold. Buchwald et al. reported an elegant use of this approach for the cyclization of a bisphosphane-1,2-diaryl dijme 19. In this case, was found to be the optimal ion and allowed the Bergman cyclization to proceed at 61 °C, instead of 243 °C in the absence of a metal ion. The change in reactivity of the enediyne 19 under metal ion chelation conditions was attributed to both conformational and electronic effects. 

In 1991, an important paper was published by Bock et a/.84 that described the steric and electronic effects on the formation of the dispiroketal dihexulose dianhydrides. The authors described the conformation of six dihexulose dianhydrides, as determined by X-ray crystallography or NMR spectroscopy. They concluded that these conformations are dictated by the anomeric and exo-anomeric effects. Thus, the dihexulose dianhydrides are disposed to adopt conformations that permit operation of these effects—even if this results in the dioxane ring having a boat conformation or all three substituents on one pyranose ring being axial. 

The rationalization of stereoselectivity is based on two assumptions. (1) The 1-arylthio-1-nitroalkenes adopt a reactive conformation in which the ally lie hydrogen occupies the inside position, minimizing 1,3-allylic strain. (2) The epoxidation reagent can then either coordinate to the ally lie oxygen (in the case of Li), which results in preferential syn epoxidation or in the absence of appropriate cation capable of strong coordination (in the case of K) steric and electronic effects play a large part, which results in preferential anti epoxidation (Scheme 4.7).52... 

Sterically (Mechanics), there is no significant energy difference between the competing transition states 22 and 23. We therefore assume that the difference is electronic, and that conformation 22 makes electron density more readily available from the target C-H bond than does conformation 23. This interplay between steric and electronic effects will be important throughout this discussion of rhodium-mediated intramolecular C-H insertion. 

For reviews of the cyclization of acyclic molecules, sec Nakagaki Sakuragi Mutai J. Phys. Org. Chem. 1989 2, 187-204 Mandolini Adv. Phys. Org. Chem. 1986, 22. Mil. For a review of the cyclization and conformation of hydrocarbon chains, see Winnik Chem. Rev. 1981,5/. 491-524. Fora review of steric and electronic effects in heterolytic ring closures, sec Valters Russ. Chem. Rev. 1982 51, 788-801. 

The same group has investigated 10-phenylphenoxazine (263) and -phenothiazine (264) cation-radicals and the influence of substituents in the phenyl group upon the spin distribution. The interest here lay in the effect of the conformation on electronic effects. Earlier studies on similarly shaped radicals had indicated that a twist of about 65° must exist in the V-phenyl bond (cf. phenylxanthenyl and isologs Sections II,B,3,a III,C,1 IV,B,1). This value may be calculated trigonometrically for a model of such radicals which has planar aromatic components, each C—C and C—H bond equal to those in benzene, and in which the phenyl ortho and heterocyclic 1- and 9-protons are at van der Waals separation. A rather similar angle has to be allowed in MO calculations in order correctly to reproduce experimental hyperfine splittings. 

High levels of enantioselectivity are achieved when the stereochemistry-inducing group is fixed in the transition state, thereby preventing one side of the molecule from attacking the carboxenium ion. Because of steric and electronic effects, the crotyl silane confor-m ations are stabilized when the dihedral angle between the C-Si bond and the ally lie double bond is about 120°. This structural element is still relevant in the transition state of the attack of 12 to a carboxenium ion such as 4 or 8. Therefore, attack cannot come fi om conformation III, but only from conformations I or II. Conformation II, however, is disfavored over conformation I because of 1,3-allylic strain. The bulky silyl group in conformation I then demands that the attack to the electrophile comes from the opposite si face. 

Nuclear magnetic resonance experiments indicate that both staggered and eclipsed conformations of substituted arene chromium tricarbonyls exist in solution 177, 235). The preferred conformations are attributed to both steric and electronic effects. 

Nucleophilic additions to cyclic carbonyl compounds differ greatly from those of acyclic systems. In acyclic systems, only the configuration at an adjacent (1,2-asymmetric induction) or nea y center (remote asymmetric induction) is usually considered in predicting the outcome of nucleophilic attack. In cyclic systems, the conformation of the entire molecule (which is in part determined by the individual substituents) must be considered when predicting the mode of nucleophilic attack. Furthermore, a number of other factors such as torsional and electronic effects also play a role in the stereochemical course of additions to cyclic substrates. The relative importance of all of these effects (as well as others) has been the subject of considerable debate in the literature, and has not as yet been adequately resolved. ... 

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