Antiperiplanar influence

The stereoselectivity of these intermolecular reactions between 1-alkoxyallylstannanes and aldehydes induced by boron trifluoride-diethyl ether complex is consistent with an open-chain, antiperiplanar transition state. However, for intramolecular reactions, this transition state is inaccessible, and either (Z)-.yyn-products are formed, possibly from a synclinal process105, or 1,3-isomerization competes113. Remote substituents can influence the stereoselectivity of the intramolecular reaction114. 

The diastereoselectivity observed can be explained by a synclinal transition state, probably influenced by chelation and/or stereoelectronic effects of the developing cation38. The minor product is formed via an antiperiplanar transition state. All compounds obtained are useful precursors for several spirocyclic natural products, such as terpenes like lubimine or acoradi-ene. 

In general, the Henry reaction proceeds in a non-selective way to give a mixture of anti (erythro) and syn (threo) isomers. Ab initio calculations on the Henry reaction suggest that free nitronate anions (not influenced by cations) react with aldehydes via transition states in which the nitro and carbonyl dipoles are antiperiplanar to each other. This kind of reaction yields anti-nitro alcohols. The Henry reaction between lithium nitronates and aldehydes is predicted to occur via cyclic transition states yielding syn-nitro alcohols as major products (Eq. 3.64).108... 

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,... 

Hie coupling constants values of H-16 allowed the determination of the configuration at C-16. In 189, H-16 must be axially oriented, antiperiplanar with axial H-17, because one of the coupling constants is 9.5 Hz. On the other hand, H-16 in 190 must be equatorially oriented. The different configuration at C-16 deeply influences the chemical shift of H-15a, which is strongly shielded in 189 with respect to 190 (0.37 ppm versus 1.13 ppm). 

Of all substituent effects, that caused by y-anti substituents has been the hardest to understand in terms of transmission mechanisms, notably because the resulting shifts are sometimes upfield and sometimes downfield. So, for example, y -SCS(OH) values range from about +6 (208) to —6 (209). Correspondingly, the discussion about transmission mechanisms is prone to speculation and controversy. At present it is impossible to arrive at a consistent interpretation of this substituent effect there is no choice but to compile a number of molecular properties that apparently affect the transmission of substituent influences on antiperiplanar carbon atoms. 

In the case of the antiperiplanar conformer of 1,2-difluoropropane (7) we studied again the influence of the basis for the carbon atom in the methyl group. For all hydrogen atoms we used the minimal basis set apart from the reference calculation where the cc-pVDZ basis set was employed on hydrogens 5H , 6H and 7H . The result, shown in Fig. 7, is the same as for trans-1,2-difluoropropene (3). The electron density around the methyl carbon, although it is out of the F-F bonding path, is not properly described with the minimal basis set. The OP, SD and FC terms are all not well reproduced by this calculation... 

For steric reasons, top attack on A, C, D and E will give rise to high-energy transition states. Assuming that cycloheptanone is flexible enough for antiperiplanarity to exert its influence fully, we expect that the more adaptable conformer will lead to the most stable transition state. The best choice would then be B. The second best is more difficult to pick. A is more flexible (carbonyl is located at the stem) than C (carbonyl at the bow), but has only one CC bond antiperiplanar to the incoming nucleophile. D is certainly less favorable than C because its vertical CC bonds are tilted more to the left, so that more distortion is required in order to reach antiperiplanarity. Finally, E is believed to be the least favorable conformer as it is less flexible and has only one antiperiplanar CC bond. 

There are emerging examples in which dppf functions as a unidentate ligand. This coordination mode has been postulated as the key intermediate in some catalytic processes (see Sect. 1.5.2.1). Diphosphines acting as a unidentate ligand were once thought to be unstable based on entropy arguments. In this mode of coordination, the expected antiperiplanar orientation of the pendant phosphine is rarely observed. The approximate anticlinal (eclipsed) conformation of most known unidentate complexes (t 122 — 123°) [42, 47, 51, 53] possibly reflects the higher influence of the... 

The results obtained from the cyclization of model 5 indicated that the size of the Lewis acid-aldehyde complex influences the selectivity of the reaction. For model system 10 it appears that the steric bulk of the Lewis acid does not play a significant role in determining the stereochemical outcome of the reaction. In model system 10 no external methylene unit exists which could interact with the Lewis acid-aldehyde complex. In fact, the silane is fixed in an anti orientation with respect to the approaching aldehyde (anti Se ). The cyclization of model system 10 with fluoride ion affords primarily the distal product resulting from cyclization through an antiperiplanar transition structure. Thus, the antiperiplanar transition structure is accessible, but is not favored in reactions with the Lewis acids. 

The results of Brosch and Kirmse undoubtedly have had a broad influence on mechanistic interpretations of the stereochemistry of deaminations. An example is the investigation of Monera et al. (1989) on the decomposition products of oc-tanediazonium ion (7.31) in aqueous buffer solutions at pH 2, 4, 8, and 10. In accord with Streitwieser (1957) and Maskill et al. (1965), they assumed that the bimolecular displacement of the diazonio group by water starts from the conformation of 7.31 that is likely to be the most stable. As the diazonio group at C(l) and the alkyl chain (R) at C(2) are in antiperiplanar conformation in 7.31 A, an ANDN-like displacement, as found by Brosch and Kirmse, is, in the opinion of the present author, more likely than a displacement from the two other conformations (7.31 B and 7.31 C). ... 

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