Distorted Alkenes

Helical atropisomers are produced if sterically demanding substituents are fixed at the double bond, which cause a twisting. 

Attempts to construct the highly twisted double bond of 66 have not been successful to date Nevertheless, the less sterically strained tetraneopentylethene 67 has 

The sterically twisted bomanylidenes 69, 70 and norbornylidenes 71, 72 have been separated into the enantiomers and their chiroptical properties have been studied (see Table 7). 

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Strained and sterically hindered alkenes are of special interest in physical organic chemistry. By their very nature these structures are difficult to synthesize and the McMurry reaction represents the only viable route to many such compounds (see Table 2). Torsionally distorted alkenes, capable of exhibiting optical activity without a chiral center, have also been prepared. If the reaction is carried out in a chiral solvent the ( ) isomer is formed selectively (equation 82). 

Figure 2.24 Jrans-cyclooctene - a highly distorted alkene.

The geometry of bicyclic rings can also cause distortion of the alkene bond from coplanarity. An example is bicyclo[2.2.1]hept-l-ene ... 

Other metals can also be used as a catalytic species. For example, Feringa and coworkers <96TET3521> have reported on the epoxidation of unfunctionalized alkenes using dinuclear nickel(II) catalysts (i.e., 16). These slightly distorted square planar complexes show activity in biphasic systems with either sodium hypochlorite or t-butyl hydroperoxide as a terminal oxidant. No enantioselectivity is observed under these conditions, supporting the idea that radical processes are operative. In the case of hypochlorite, Feringa proposed the intermediacy of hypochlorite radical as the active species, which is generated in a catalytic cycle (Scheme 1). 

Apart from type 62, which is only slowly convergent to the optimised geometry, the other centres are well described by the ROHF method. Polyhedral views of the three type a structures are shown in Fig. 6. These all illustrate the change of hybridisation at the point of muonium attachment and at the adjacent carbon atom where the unpaired electron is effectively localised as expected from addition to an alkene. The bi and c defects (Fig. 7) are quite different. The expected hybridisation change to sp is clearly present for the atom bonded to muonium, but other significant distortions are not obvious. This is consistent with the prediction from resonance theory (Fig. 8) that the unpaired electron for these structures is delocalised over a large number of centres. 

The carbene route to bridgehead olefins is a well established reaction and has been developed to a major method for the generation of bridgehead alkenes. The field has been recently reviewed by one of the main contributors to this area.1 The reversed reaction, the formation of carbenes from distorted olefins, has also been known for a long time. Distortion of the tt bond in alkenes is easily affected by photoexitation. In connection with those reactions, carbene chemistry has been observed and the field has also been reviewed some years ago.2... 

Most stannylenes, R2Sn , that are two coordinate in solution are in equilibrium with, and separate out as, the distannenes, R2Sn=SnR2. These are the distanna analogs of the alkenes, R2C=CR2, but the nature of the Sn=Sn bond is different from that of the C=C bond, and can be represented by the mutual coordination of the electron pair in the 5sp2 orbital of one stannylene unit into the vacant 5p orbital of the second stannylene unit, as illustrated in Equation (179). 5 The molecules are usually trans-bent (9 = 9 = ca. 40°, 0 = 0°), but are easily distorted, and the... 

We know that in alkenes a double bond consists of a o and a n bond. The / -orbital tends to overlap as much as possible to make the bond strong because the greater the overlap of the two p orbitals, the stronger would be the bond. Maximum overlap occurs when the molecule becomes planar because in this condition the two p orbitals are parallel. Any distortion from the planar structure leads to decreasing overlap of the orbitals and a consequence of the weakening of the bond. The picture is represented as follows ... 

Apparently the geometry of the transition state for adsorption is approximately that of a ir-complexed olefin in that its structure seems to be only slightly distorted from that of the isolated alkene. However, this does not necessarily mean that the adsorbed state which is formed in the elementary reaction to which the stereochemistry refers is a tt complex, because the same geometry also represents a stage in the progression of olefin to the eclipsed 1,2-diadsorbed alkane. Hopefully other experi-... 

In all the complexes shown in Table 9 (with one exception) the nickel atom is four-coordinated by two phosphorus atoms and by two carbon atoms in a distorted planar arrangement (24a). The plane containing the nickel and phosphorus atoms and the plane containing the nickel and the coordinated carbon atoms form a dihedral angle which varies between 4° and 27° (24b), depending on the coordinated alkene. In the [Ni(p3)(C2F4)2] complex (25) the nickel atom is five-coordinated by three phosphorus atoms of the tridentate ligand and two carbon atoms of tetrafluoroethylene. 

An alkene complexed to platinum(II) is only slightly modified on coordination, but complexation to platinum(O) causes major changes. Platinum(O) alkene complexes show both weakening and lengthening of the carbon-carbon bond, as well as distortion of the plane of the double bond away from the platinum. In platinum(ll) alkene complexes the double bond lies approximately perpendicular to the square plane of platinum(II), but in platinum(O) complexes there is only a small dihedral angle between the platinum and alkenic planes. For platinum(II) the energy barrier to free rotation of the alkene about the platinum(D)-alkene bond is only about 40-65 kJ mol-1, whereas no rotation is observed with platinum(O) alkene complexes. Alkenes bonded to platinum(ll) exert a large trans effect but only have a small trans influence. 

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