The cis-effect

Early studies of the trans-effect led to the conclusion that the lability of a ligand is affected not only by the ligand in the trans-position, but also by the ligands in the czs-positions. Indeed, Grinberg noted the possibility of the existence of a cis-effect in his 1945 monograph.21 

Grinberg and Kukushkin established experimentally the cis-effects of chloride and ammonia ligands on the substitution kinetics of chloride 

In the absence of any cis-effect the rates of ligand substitution in these complexes would be approximately equal because substitution occurs trans to chloride in both cases. Further, as the ions have different charges, identical rates would not be anticipated but, once this difference is accounted for, the cis-effect becomes evident. 

In general, the higher the fra s-effect of a ligand, the lower is its cis-effect. However, the series are not completely reversible. For example, dimethyl sulfoxide, coordinated to platinum(II) through sulfur, exhibits both a very high frans-effect (comparable with that of ethylene) and also a high cis-effect. A cis-effect series based upon comparable data is  

The cis-effects of ligands are an aid to understanding and predicting some properties of coordination compounds, but they are not as important in planning syntheses as the trans-eHects. 

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Scheme 20 HOMO-LUMO interaction in the perepoxide quasi-intermediate for the cis-effect and the regioselectivity (percent) of the hydrogen abstractions...

Recently techniques have been developed for x02 oxidations in zeolite cavities.174 The photosensitizer is absorbed in the zeolite and generation of l02 and reaction with the alkene occurs within the cavity. The reactions under these conditions show changes in both regiochemistry175 and stereoselectivity. The cis effect is reduced and there is a... 

Despite this fact it is interesting that there is quite a good correlation between the C(10)-chemical shift and the position of the /9-band1) in the electronic spectrum (134). This correlation, shown in Fig. 32, is somewhat unexpected because, as mentioned above, the electronic transmission in the cis-effect seems to be quite complicated. 

The kinetics and mechanism of ligand substitution reactions of square-planar platinum(II) dimethyl sulfoxide complexes have been exhaustively studied (173), and these workers conclude that the cis and trans influences and the trans effects of Me2SO and ethylene are similar in magnitude whereas the cis effect of Me2SO is about 100 times as large as that of ethylene. The results for reaction (5), where the stability constants, Kt, are reported to be 1.5 x 108 (L = S-Me2SO) and 4.5 x 108 (L = ethylene) corroborate this analogy (213). 

The operation of the cis effect of the porphyrin may be explained according to Case D (Fig. 1) the more basic the porphinato ligand, the more electron density it induces at the metal ion, and the more will the d -electrons at the metal tend to flow away to the 7r-acceptor ligand X, reinforcing the M—X bond (90). An alternative ra-... 

Fig. 8. Electronic transmission paths illustrating the cis effects in osmochromes Os(OEP)L2 (see Table 9)...

The compounds, however, are sensibly distinguished by their Os-O-stretching frequencies (i>0s0) which allow us to state a quite different trans effect, although the cis effects of the ON-Os-OMe and N = Os-OMe entities are so similar. The OsVI compound, [25c], has Vq 0 at the rather low value of 410 cm-1, which is due to the repulsive interaction of the two rr-donors, N3e and OMe (see Sect. 5.2 and Table 6). 

Table 12 shows the cis effect of the axial ligand X or L on the optical spectrum of cobalt(III) etioporphyrin derivatives Co(Etio-I)LX [32a-h] which behave similarly to the corresponding rhodium(III) complexes (Table 4). Analogous trends are found in the Co(TTP)LX series (133). The only notable feature is the large bathochromic shift caused by benzimidazole in [32h] obviously this ligand is far more different from pyridine than imidazole, the condensed ring system acting as a stronger 7r-acceptor.

Table 15. The cis effect of the porphyrin ligand (P) on the CO-stretching frequencies in some series of Fe11 or Ru11 porphyrins M(P)CO(L) (for abbreviations, see Table 2)...

These intrazeolite singlet oxygen ene reactions have synthetic potential because the cis effect observed in solution is suppressed in the zeolite [13]. Consequently, allylic hydroperoxides which are inaccessible by other routes may be available via this new technology. For example, photo-oxidations of aryl-substituted alkenes, 7, in sensitizer-doped NaY react to generate the allylic hydroperoxides as the major or exclusive product [17]. In contrast, in solution, the hydroperoxides are formed in only 5-20% yields, with 2-1-2 and 4-1-2 adducts dominating the reaction mixtures. In the case of 2-methyl-5-phenyl-2-hexene, 8, the regio-selectivity for 8b and 8c improved from 47% to 94% and the diastereoselectivity from 10% to 44% as the reaction is moved from solution into the zeolite [18] ... 

The cis effect was rationalized by examination of the possible transition states leading to the major and the minor ene adduct (Scheme 2). In transition state 1 (TSO, the interactions between the incoming oxygen and the two allylic hydrogens are more favourable than those in transition state 2 (TS2) where only one allylic hydrogen is present (Scheme 2). 

The regioselectivity for the hydrogen atom abstraction from each of two geminal methyl groups (twin or twix) in trisubstituted alkenes such as 34, 35, 39 and 122 (Table 17) was studied by specific deuterium labeling. Independent studies revealed that the cis effect selectivity found in solution no longer operates within the zeolite. As seen in Table 19, for the case of 124-127, the twin methyl group reactivity increases up to 14 times (see substrate 126) by zeolite confinement. 

The cis effect observed also in trisubstituted enol ethers [74,75] is well rationalized by a similar mechanism (Scheme 15). In this case, the electron-donating group that stabilizes the partially charged double-bond carbon of the perepoxide in the syn transition state is the alkoxy moiety. Therefore, the favorable interactions of the singlet oxygen with the phenyl or alkoxy substituents direct the orientation of the perepoxide intermediate. 

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