Steric and electronic considerations

Factors influencing the macrocyclic hole size. The hole size of a macrocyclic ligand is a fundamental structural parameter which will usually influence, to a large degree, the properties of resultant metal complexes relative to those of the corresponding non-cyclic ligands. The large number of X-ray diffraction studies now complete for macrocyclic systems makes it possible to define many of the parameters which affect hole size 

What is different about macrocyclic ligand complexes  

Macrocycles may also promote the formation of less common coordination geometries for particular metal ions because of increased macrocyclic ring strain on coordination. Such an effect is illustrated by the variation in the structures of the nickel complexes of the 14-, 16-, 18-, and 20-mem-bered tropocoronand macrocycles of type (14) (Imajo, Nakanishi, 

Donor atom to metal-ion bond lengths which are shorter or longer than expected as well as unusual angular relationships between such bonds have all been documented in macrocyclic complexes. Such effects can be a prime cause of the unusual properties mentioned previously. Thus the 

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C6H5—C—Co— and that steric and electronic considerations indicate... 

Steric and Electronic Considerations. Electronically, the N-substituted macrocycle is a monoanion in its deprotonated form. For-... 

Fig. 14.10). Due to steric and electronic considerations, the usual arrangement is head-to-tail, so that the pendant groups are usually on every other carhon atom in the chain. 

Lithium enolates exist as large aggregates and their approach to the electrophile is restricted by steric and electronic considerations, as well as by the relative geometry of the molecule. Despite the structural complexity remarkably good predictions for reactivity and diastereoselectivity can be made based on the steric requirements that would be present in a monomeric system. For example, in most reactions of enolate anions the electrophile will be delivered to the less hindered face of the enolate to give the major product. In all models used to describe reactivity (secs. 9.5.A.iii-9.5.A.v), a monomeric enolate will be shown but the facial and orientational bias of the enolate is clearly influenced by the state of aggregation in solution. 

Note that were we to react unobstructed cholesterol with NBS, bromination could occur at either Ci or C7, resulting in a mixture of 4- and 7-dehydrocholesterol as final products. Therefore, PhCOO- is first substituted for the hydroxy group on C3, making the 4-bromination unfavorable due to steric and electronic considerations. 

The complex formed by treatment of 2-methyl-5-vinylpyridine with 9-BBN (1 1 molar ratio) gets dissociated readily at 25 or 65 °C due to 2-methylsubstitu-ent, and hydroboration proceeds smoothly to place the boron atom predominantly at the (5-carbon [38]. The electron-withdrawing property of pyridine nitrogen here is transmitted weakly as compared with vinylpyridine. The comparable hydroboration with BMS, 9-BBN, Clix BH, and SiajBH for their steric and electronic consideration along with their molar ratio studies are presented in Table 5.10 [38]. 

While the incorporation of carborane cages into maaomolecules is desirable, steric and electronic considerations make the synthesis of dendritic and symmetrical star-shaped molecules containing closely placed multiple carborane clusters a difficult task (Craciun et al., 2004). There have been several approaches to the incorporation of carboranes into star-shaped molecules. 

The reactivity of different alkenes toward mercuration spans a considerable range and is governed by a combination of steric and electronic factors.24 Terminal double bonds are more reactive than internal ones. Disubstituted terminal alkenes, however, are more reactive than monosubstituted cases, as would be expected for electrophilic attack. (See Part A, Table 5.6 for comparative rate data.) The differences in relative reactivities are large enough that selectivity can be achieved with certain dienes. 

With reference to hosts and a guest, molecular assemblies have to conform to certain circumstances, generally called complementary relationships. They involve both steric and electronic terms. The objects may be achieved by the use of properly chosen sensor groups and by a suitably tailored basic skeleton as exemplified by the present scissor- or roof-shaped host molecules. From the point of view of the introductory thoughts of this chapter (cf. Sect. 3.1), it is a matter of consideration to see how consistent the scissor or the roof simile is in the light of crystal structures. 

To date, the most popular and successful designs for PN-ligands have incorporated an achiral phosphine with a substituted non-aromatic heterocyde as the primary chiral unit, largely due to the ease by which the P and N modules may be altered, and the ready availability of enantiopure, amino add-derived starting materials.1281 Considerable variation of the predse steric and electronic environment imposed on a chelated metal centre has been achieved by the synthesis of a substantial library of... 

Equipped with these reference trends for steric and electronic effects, one is prepared to survey more general classes of electrophilic aromatic substitution on benzocycloalkenes. Such reactions include nitration, halogenation, sulfonation, and alkylation. Each has its own mechanistic peculiarities, but their product distributions can be rationalized by consideration of the appropriate reference. 

The highest ee value (57%) was obtained with ligand 2, but here again there should be considerable scope for improvement by variation of the steric and electronic properties of the two ligand functions [8]. 

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