The frans-Effect

Like the isoelectronic Pd2+ and Pt2+, Au3+ exhibits both trans-effects and frans-influence. Table 4.13 (above) lists structural data for a number of complexes AuL3L, showing how the disparity in Au-X distances between cis-and trans-X depends on the position of L in the frans-effect series for the compounds listed, the effect is least noticeable in AUCI3NH3 as these two ligands are proximate in the series. 

The photoaquations of X in fra .v-[Ir(X)2(en)2]+ provide a useful route for the rapid and clean preparation of various fraM.T-[Tr(X)(Y)(en)2] + and trans-[lr(X )2 (en)2]+ complexes (reactions 67-70).229,237 The products obtained were those expected on the basis of the frans-effect behaviour in such systems. 

Based upon the concept of the frans-effect, Chemyaev was able to explain each of these rules. Thus, referring to Scheme 1.2, when an amine reacts with K2[PtCl4], any one of the four, equivalent, halide ligands can undergo substitution. Scheme 1.6 ... 

Ring-opening of the frans-cyclobutene isomer shown takes place at much lower temperature than a similar ring-opening of the cis-cyclouiitciie isomer. Explain the temperature effect, and identify the stereochemistry of each reaction as either conrotatory or disrotatory. 

Having developed effective synthetic methodology for the construction of seven-membered cyclic ethers, we were confident that the problem of the frans-fused bis(oxepane) system could now be addressed on a solid foundation. It was our hope that the breve-toxin-type bis(oxepane) system could be assembled by a stepwise strategy utilizing both photochemical dithioester and reductive hydroxy ketone cyclization methods. 

The axial P-Os-P angle is 166°, so the three atoms are not quite colinear (Os—P 2.311 A) the unique Os-P bond is slightly longer at 2.347 A, showing a frans-effect of hydride. Os-H bonds average 1.663 A [164]. 

Theoretical explanation of the trans-effect (and /rans-influence) has centred on two theories, one based on cr-bonding the other on 7r-bonding. The tr-bonding argument considers two frans-ligands sharing a metal p orbital (Figure 3.83). 

Hall et al.1 s estimated the conformational equilibrium for the structural units in the polymer of 2 using the numerical parameters determined for carbohydrates16. For a frans-l,3-tetrahydropyranoside, conformer 8 is calculated to be more stable than 7 by 9.2 kJmol-1 and would therefore occur almost exclusively (ca. 98%) at equilibrium. For a m-1,3-tetrahydropyranoside unit, the anomeric effect favors con-former 9, but its severe syn-axial interaction between alkoxy and alkyl groups would highly favor 10 (ca. 99%). 

The infrared absorption frequencies of frans-3-substituted methyl acrylates gave a significant correlation with eq. (2). The value of p obtained is 75, which indicates predominance of the resonance effect. Three sets of nmr chemical shifts also were studied. All three sets gave significant correlation with eq. (2). The values of p obtained in two of the three sets were 70 and 78. Again, the resonance effect appears to predominate. 

No useful comparisons can be made with other systems in so far as the magnitude of the electrical effect is concerned, because unfortunately, no data are available for a set of /rans-substituted ends. The values of a for the frans-heterovinylene sets can be correlated graphically with eq. (21) with b i2 = 1 12 = 2.5. The values of /3 do not seem to be a function of eq. (22). 

Data are extant in the literature for four tra s-cyclopropylene sets. Of these, two are disubstituted (sets 39-9 and 39-11). Positions trans-2 and tranS 3 are completely equivalent in cyclopropanes bearing the reaction site at position 1. These sets have been correlated with eq. (30). The other two sets have been correlated with eq. (2). Three of the four sets studied gave significant correlations. The fourth set had only four points. The results obtained clearly show a significant resonance effect. They clearly demonstrate that the frans-cyclopropylene system does involve a resonance interaction between the substituent and the cyclopropane group. The rra s-cyclopropylene system again... 

The frara-effect in co-ordination chemistry is well known however, what has not been examined in any detail is the effect of trons-molecules in heterogeneous catalytic hydrogenation. In this paper we will show that trans molecules hydrogenate more slowly than other isomers and can poison reactions of species that would be expected to be more strongly bound. However, if a c/s/frans-mixture is used this strong adsorption can be disrapted. 

The same reaction but with trimethylacetaldehyde results in the frans-adduct exclusively in 100% yield and in 97% ee. The use of the gold catalyst is essential for the high selectivity silver and copper catalysts are much less effective. The length of the side chain between the diethylamino group and the ferrocene nucleus is also an important factor in the selectivity. 

V) [37], frans- Fe(NCR)(depe)2 2+ (electron-poorer, 5 = 1-7-2.0 V) [35], with a N2, CNR, or NCR ligand. The strong electron-release from the trans bromo ligand in the first center, in contrast with the appreciable r-electron acceptance by the trans- 2, CNR, or NCR ligands in the other centers, hampers the electron-donation from the cyanamides, whereas the opposite effect occurs for the other metal sites. 

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