Alkyl electronic influence

It may be concluded that the steric bulk of the alkyl substituents (R) is an extremely important factor in stabilizing the various coordination modes and thus steers the course of the reactions. However, the tendency to form metal-metal bonds and the electronic influence of R may also be important. In this respect it is of interest that in the case of the analogous iron systems the number of products is much smaller. For example Fe2(CO)9 reacts with R-DAB according to equations (8) and (9). 

The electronic influence of R, i.e. the substitution of R = alkyl by R = aryl, on the product formation will be demonstrated in the following subsection. 

The electronic properties of both alkyl [5] and aryl alcohols [6] play a clearly definable role in ester formation, with formation constants decreasing with increase in electron withdrawing ability of the ligand. For both types of ligands, the electronic influences are quite small, but the resonance effects found with the aromatic ligands indicate there are 71-electron contributions to the empty d orbitals of vanadate [6], The influences of the electronic properties of ligands on coordination mode and geometry are discussed in detail in Chapter 9. 

How substituents in the alkylating agent influence the rate constants of SN2 reactions can be explained by means of the transition state model developed in Section 2.4.3. This model makes it possible to understand both the steric and the electronic substituent effects. 

Fused benzene rings, aryl and alkyl groups, and other groups with relatively weak electronic effects have a relatively small electronic influence. 

The fragment CpTi(acac)S2 is considered as an S-donor metalloligand that supports the binuclear moiety Ir2(CO)4 in the complex CpTi(acac)(/i3-S)2[Ir(CO)2]2, which is obtained upon carbonylation of CpTi(acac)(//3-S)2[Ir(diolefin)2]2. In this Tilr2 system the titanium fragment, as a metalloligand, exerts important steric and electronic influences on the reactivity of the Ir metal toward electrophiles. Thus, the reactivity of this complex with iodine, alkyl iodides, and activated acetylenes has been studied.1798... 

The question we have to ask ourselves while looking at the data compiled in Table 5.4 is this Is this system a valid model for the monitoring of the cone angle influence, and, in particular, did we select the right substituents The answer is clearly no. By changing the substituent R from alkyl to aryl and back to alkyl, we in fact change the ligand class of the phosphane and thus introduce electronic influences that obscure the steric influence that we want to measure. 

Alkyl-substitution influences the absorption spectra in thin films mainly by their influence on the co-planarity of the molecules, particularly in the excited state, and only to a minor extent by their electron pushing (inductive) effect (see [181]). The latter should result in a red shift which is not observed in absorption spectra of thin films but can be detected in the fluorescence spectra. In absorption spectra, major blue shifts of the absorption peaks occur if larger differences in the torsion angle between different rings are induced by the substituents. As long as the co-planarity is not distorted, quite similar absorption spectra are observed if compared to unsubstituted oligothiophenes [13,182]. 

In the ligand series synthesized, l,l -bis(diphenylphosphino)cobaltocenium hexafluorophosphate (5) proved to be particularly suitable. In [BMIM]]PF,5], high catalyst activity (810 h ), high selectivity for aldehydes (94%) and for the n-product (n/i = 16.2), and above all no detectable catalyst leaching were demonstrated. The analogous cobaltocenium (6), in which an alkyl bridge was placed between the cyclopentadienyl and the phosphorus atom, yields much less positive effects (e.g., TOF = 66 h ). This result can be explained by the electronic influence of the central... 

PP (65,67,70,71,344). The observed enhancement has been explained by a combination of steric and electronic influence on the chain-termination reaction. Enhanced rigidity of the ligand framework combined with a direct steric interaction of the alkyl group with the growing polymer chain decreases the rate of P-elimination. Electron donation from the alkyl substituent may further decrease the local Lewis acidity of the central metal atom, thus reducing its tendency for /3-H abstraction. 

An important method for construction of functionalized 3-alkyl substituents involves introduction of a nucleophilic carbon synthon by displacement of an a-substituent. This corresponds to formation of a benzylic bond but the ability of the indole ring to act as an electron donor strongly influences the reaction pattern. Under many conditions displacement takes place by an elimination-addition sequence[l]. Substituents that are normally poor leaving groups, e.g. alkoxy or dialkylamino, exhibit a convenient level of reactivity. Conversely, the 3-(halomethyl)indoles are too reactive to be synthetically useful unless stabilized by a ring EW substituent. 3-(Dimethylaminomethyl)indoles (gramine derivatives) prepared by Mannich reactions or the derived quaternary salts are often the preferred starting material for the nucleophilic substitution reactions. 

---