Olefins bond angles

It might be expected that the bond angle involving the centroids of the cyclopentadienyl rings and the metal center would anticipate the amount of space available to an incoming olefin and, therefore, reasonably expected to correlate with reactivity and/or selectivity of Ziegler-Natta processes. 

Figure 4.35 Potential energy diagram of the ground and excited states of the cis and trans isomers of an olefin. The energy is shown as a function of the bond angle...

Recently, density functional calculations were performed to determine the nature and stereochemistry of the olefin insertion into the Cu-B bond of (NHC)Cu boryl complexes (NHC = iV-heterocyclic carbene). The theoretical calculations confirm that the mechanism of insertion involves a nucleophilic attack of the boryl ligand on the coordinated olefin. Furthermore, the hyperconjugation of Cu-C (bond angles, which was also experimentally confirmed by the X-ray diffraction studies of these boryl-copper complexes <2007OM2824>. 

Since all the arene protons of 26 resonate at lower field than those of 25, the wider bond angle at sp2 carbon is assumed to keep the arene units in the olefin at a distance greater than in the CH2CH2-bridged phane ). 

The dependence of the relative reaction rates on olefin geometry can be discussed with reference to Equation 14. As pointed out by Murray and co-workers (25), the complex formation occurs with cis-olefins rather than with their trans isomers for steric reasons hence, Kc (cis) > Kc (trans). However, during the formation of the primary ozonide by either path the olefinic carbon atoms change in hybridization, from sp2 to spz. The bond angles thus decrease from 120° to 109° in the cis isomers, this results in a compression of the substituents van der Waals radii. The repulsion between the substituents is increased, and so is the activation energy. Consequently, ki (trans) > ki (cis) and k2 (trans) > k2 (cis). In the final analysis, the geometry of the olefin has opposite effects (a) onKc and (b) on ki and k2. Present results seem to indicate that for large substituents the effect on Kc predominates since k (cis) > k (trans). 

Structure of tosyl derivative 20 was determined by X-ray crystallography and revealed that the sum of the nitrogen s bond angles is 348.2°. This means that the nitrogen center of 20 is chiral and C(3)-C(4) and C(7)—C(8) olefinic moieties form chiral planes in the solid state <20060L963>. 

When the effects of 77-bonding are taken into account, it transpires that 77-accepting ligands have a preference for the equatorial sites of the trigonal-bipyramidal molecules and that a single-faced 77-acceptor such as an olefin should lie parallel to the equator 77-donor ligands, on the other hand, favor the axial sites. In square-pyramidal molecules, the preferences do not appear to be so clear-cut, especially with variations possible in the bond angles. 

This phenomenon is unique to fluoroethenes (10b and 10c) and the corresponding dichloroethene (lOd) shows much weaker p-n repulsion. This can be ascribed to the facts that (i) chlorine is a third-row element and its 3p lone pairs do not effectively interact with the 2p, olefinic 7r-electrons, and (ii) the C-Cl bond is considerably longer than the C-F bond (1.744A for lOd vs. 1.326 A for 10c, see also Table 1.1). Another unique structural feature of 10c is its unusually small F-C -F bond angle (109.5°), which is 10.5°... 

Similarly, the adsorbed state of 1,2-dienes may be represented in two ways, as shown in Structures (IV) and (V). Since the two 7r-electron systems in a 1,2-diene are mutally at right angles it follows that the two metal-olefin bonds in Structure (V) must also be at right angles to each other. The fission of one olefinic linkage and the subsequent... 

It is not possible to state whether the more favorable C-H bond angle or the lower strain energy of the allylic structure (after C-H bond rupture) is responsible for the lower selectivity of bicyclo[2,2,2]oct-2-ene. However, comparison of these two bicyclic olefins indicate that relatively small changes in structure can have dramatic effects on selectivities to corresponding epoxides. Prevention of allylic C-H bond breaking is critical for epoxide formation. 

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