Double-gauche effects

Medium-sized and large ring systems often show complicated conformational interconversions involving pseudorotations in one or even more conformational families. This makes stereochemical assignments in diastereomers rather difficult. Thus, very few systematic studies have been published. The situation is improved if such rings are embedded in polycyclic systems, or if they contain double bonds, which leads to restricted conformational mobility. An example is the differentiation of diastereomeric 2,3-dihydro-lf/-benzo[6]azepines 1 on the basis of y-gauche effects and on d(13C) and 3/H H values640. 

Figure 1.61 Conformational freedom is heavily restricted in nucleic acids owing to lack of free rotation about 0 —P and P—0 bonds, (a) Nucleic acid equivalent of the Ramachandran plot illustrating the theoretically allowed angles of C and a. (b) Free rotation is primarily damped owing to the gauche effect in which lone-pair-o- orbital overlaps in phosphodiester links generate double bond character in 0—P bonds that restrict free rotation (adapted from Govil, 1976 [Wiley]).

The diaxial preference is further strengthened in 1,4-difluorocyclohexane whereas the ee-isomer is more stable in the respective 1,3-dihalocyclohexanes. This difference suggests that the unusual conformational preference of 1,4-isomer is likely to be stereoelectronic (note the unfavorable double hyperconjugation pattern where two o CF acceptors compete for the same donor - more details in Chapter 9). The conformational preferences in 1,2-difluorocyclohexane can be understood as an extension of the conformational preferences of 1,2-difluoroethane (the gauche effect). 

Various more serious attempts have been made to explain the y gauche effect by MO computations. The most recent are those of Barfield and Yamamura [4], using an ab initio method with a double-C basis set. Although this level of computation is a little less than that required to give the best available absolute shieldings, it reproduces very adequately a wide range of steric shifts in hydrocarbons (i.e. those dependent solely upon variations in stereochemistry) and also... 

Another class of mthenium aUcene complexes contains those derived from the hexaaqua ion [Ru(H20)6] +. The thermodynamically stable complex [(cod)Ru(H20)4] + (74) forms directly from [Ru(H20)6] + and cod in alcohol at ambient temperature (equation 14). In (74), the redox potential of Ru has shifted more positive for the oxidation to Ru and more negative for the reduction to Ru or Ru°, so as to impose a high stability towards disproportionation see Disproportionation) (in contrast to the readily disproportionating aqua ion [Ru(H20)6] +). The X-ray crystal structure see X-ray Crystallography) of the Tosylate (Ots) salt disclosed quite different R-OH2 distances of 2.095(2) and 2.156(2) A for water gauche or trans to the alkene double bond, showing the structural trans effect see Trans Effect) of the latter on a a- (and tv-) donor ligand trans... 

In certain instances, however, Lewis-acid-mediated Michael additions show a slight dependence on the geometry of both the donor and acceptor (vide supra). Hence, the first-order analysis must be modified to include the differential effects induced by the double-bond substitution patterns. By consideration of these effects and by minimization of the adverse gauche-type interactions, trends in Lewis-acid-mediated additions where the conjugate addition is likely to be the actual product-determining step can be rationalized. 

In view of the preference of 24 for the s-gauche form, it seemed worthwhile to examine its behaviour in [4-1-2] cycloadditions of the Diels-Alder type. Howev, until very recently all reactions with olefins, acetylenes, and the C=0 bond of ketones have remained unsuccessful. The effective shielding of the two double bonds of 24 by the bulky aryl groups and, above all, the large 1,4-separation between the terminal silicon atoms appear to be responsible for the failures. 

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