Transannular effects

If the substituents are, however, electron-donating, the first electron transfer must take place at the double bond bearing the substituents. Hence, it is impossible to observe the transannular effect in this case. 

The observed tt-IP s were compared with calculated values, obtained from four 7t-electron models. On the whole the agreement between calculated and observed k-IP s was good, indicating that transannular effects are not present. The authors ascribe the absence of such an effect in helicenes to the greater flexibility of these molecules, compared with cyclophanes. From and INDO-MO-calculation on hexa-helicene 121) it was concluded that the four HOMO S have predominantly Jt-character, in contrast to the other occupied MO s. A linear combination of the four MO S... 

Most of the pKa values for the phosphonium salts have been determined by potentio-metric measurements in the case of / -ketophosphonium salts562 (Table 13). The acidity of these phosphonium salts depends not only on the R and R groups of the carbonyl chain, but also on the other substituents linked to the phosphorus562 564. Particularly for the cyclic phosphonium salts, the pKa value can vary significantly depending on the different steric and electronic (even transannular) effects on the phosphorus atom564 ... 

The reactivity of the allylic sulfide double bond with electrophiles is largely masked by the presence of the heteroatom. However, on oxidation to the sulfone the alkene is found to be surprisingly inert toward such species, and this has been taken as further evidence for a transannular effect of the —SO2— unit greater than that seen for the sulfur atom in tetrahydrothiopyrans (77MI22502). 

Stereoselective radical cyclization of 1,5-cyclooctadienes. Winkler and Sridar14 have noted transannular effects on the stereochemistry of cyclization of these octadienes (equation I). Thus radical cyclization of cis-1 and of the corresponding ketone (3) is highly frans-selective and is more rrans-selective than that of the monosubstituted diene 5. Cyclization of a 3,8-disubstituted cyclooctene is... 

However, the transannular effect of the substituents was shown to operate essentially through an inductive mechanism. The effects were studied most extensively with the redox potentials of the X-substituted ferrocene versus the X-substituted ferricinium system. The potentials of the ferrocene derivatives offer a parallel to the activity of the ferrocene nucleus as a function of the substituents. Alkyl ferrocenes are oxidized more readily than ferrocene and they are more active in their electrophilic substitutions. Halogens, COOH, COOCHj, NHCOCH3, HgCl, CH OH, C Hj, to say nothing about RC(0) and CN groups, are electron acceptors... 

When a pentavalent bismuth dichloride 54 is refluxed in dichloromethane, reductive elimination takes place smoothly to give chlorobismuthine (52e) quantitatively together with 4-chlorotoluene. This process is considered to be facilitated by the transannular effect. 

Finally, mention should be made of the increasing application of the intramolecular Nuclear Overhauser Effect (NOE) which has played an important role not only as an aid to structural elucidation but also in the conformational determination of some of the germacrane sesquiterpenes. These results have a significant bearing on the understanding of transannular effects and chemical reactivity. 

The splitting of the four-electron step was attributed37 to a transannular effect rendering, after the first scission, the cathodic cleavage of the second N—Ts bond less easy by increasing the electron density on it. 

The absolute configurations of daphmalenines A (20) and B (21) were determined on X-ray diffraction by using the Flack parameter and computational methods, respectively. Daphnioldhamine A (22) is the first Daphniphyllum alkaloid with transannular effect and is easily tautomerized imder acidic or alkaline conditions (Figure 5). 

The unusual chemical shifts associated with carbonium ions carry over to H NMR as well. Eq. 14.65 shows the novel bridged structure obtained when cyclodecanol is exposed to the highly acidic environment associated with stable ion media. The remarkable upheld shift seen for one proton confirms its bridging nature. This system is a beautiful example of the types of transannular effects discussed in Section 2.3.2. 

E, Z,E,Z,Z-isomer suffers transannular H.H-repul-sion. Solution enthalpy of hydrogenation measurements are perhaps plausible to suggest, but catalyti-cally facilitated interconversion of E,Z,E,Z,Z-[10]-annulene into its valence isomer 4a,8a-dihydronaph-thalene (bicyclo[4.4.0]deca-2,4,7,9-tetraene) is also expected. In addition, the transannular effect in both the annulene and cyclodecane hydrogenation product... 

Another possible route for the development of total synthesis is the use of suitably selected monocyclic precursors with rings of medium size (20) the synthesis of which has been fairly well developed in recent years. Compounds of type (20) may subsequently be converted into polycyclic products (21) by using transannular effects. In this connection it must be mentioned that modern methods of total synthesis use ring closure by the formation of the internal bonds C5-C10, C3-C9, and C13-C14 shown in formula (21) by heavy lines only in isolated cases (Schemes 26 and 78). 

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