Distortion of the Double Bond

Wilkinson s catalyst, it underwent smooth hydrogenation (usually tetrasubsti-tuted double bonds are not reduced with this catalyst).  

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A simple rule for the occurrence of trans-h A distorted structures 2 at homopolar double bonds was derived from an elementary molecular orbital model treating a-jt mixing and a valence bond treatment . The relation between the singlet-triplet separation (A st) of the constituent ER2 and a +jt bond energy Ea+ was used as a criterion for determining the expected structure of R2E=ER2. The trans-b ai geometry 2 occurs when l/AEa+ < A sT < l/2Ea+n- The first part of the inequality determines the irawi-bending distortion of the double bond, while the second part determines the existence of a direct E=E link. 

Aspects of the chemistry of trans-cyclo-octene have been reviewed. Evidence from the H n.m.r. spectrum of truus-cyclo-octene suggested that the major component of strain was out-of-plane distortion of the double bond, and that trans-cyclo-octene has a twist conformation in solution. The strain in trans-cyclo-octene is reflected in its increased reactivity in, for example, cycloaddition and epoxidation. On heating... 

Compare geometries of the cis and trans cycloalkenes. Are the double bonds incorporated into the trans compounds significantly more distorted than those incorporated into the analogous cis cycloalkenes Consider carbon-carbon bond lengths and the twisting and/ or puckering of the double bond. Are any distortions greater in trans-cycloheptene than in trans-cyclooctene ... 

Another sort of evidence supporting the bent-bond structure of the, double bond is provided by the information about restricted rotation, The bent-bond structure for propylene leads to the expectation that the potential function hindering the rotation of the methyl group would be nearly the same as in ethane, but with the barrier a little smaller than in ethane because two of the bonds on the adjacent carbon Atom (the bent bonds) are distorted the stable orientation would be the stag-... 

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. 

The classical Dewar-Chatt-Duncanson model for metal-alkene bonding has been revisited with a combination of X-ray structural data (see Diffraction Methods in Inorganic Chemistry) and DFT calculations (see Molecular Orbital Theory), particularly on complexes of the type (acac)Rh(alkene)2. These indicate the existence of distortions from idealized geometry involving a twist (127), where the axis of the double bond is no longer perpendicular to the molecular plane and a roll (128), where the line... 

Bicyclo[3.3.0]oct-l(5)-ene 178 (Scheme 4.55) is a stable compound with a flattened alkene fragment and exhibits a regular pattern of reactivity. Computational studies revealed, however, that installation of a short 3,7-bridge should lead to noticeable pyramidalization of the double bond. Compounds like 179-181 were synthesized to check this prediction. Tricyclic hydrocarbon 179, with the smallest possible bridge, was generated as a transient species from diiodide 182. The formation of 179 is implicated by the isolation of its cyclodimer 183 (or respective Diels-Alder adduct if the reaction is carried out in the presence of a 1,3-diene). The next member of this series, 180, is more stable. In fact, the formation of 180 was ascertained not only from the structure of the final products (as was done for 179), but also by its matrix isolation and analysis of spectral data. The selenium derivative 181 was found to be stable at ambient temperature in the absence of oxygen. X-ray data confirmed a noticeable pyramidalization of the double bond in 181 but the distortion was different [Pg.372]

The crystal structure of (s-ds-PhCH =CHCH =CHPh)Mg(THF)3 [Compound 40] has shown that the organic group chelates to the Mg atom and has a distorted trigonal bipyramidal structure [40]. The interesting feature comes from the presence of the double bond which gives the ligand a partial coordination to the Mg atom. 

The equilibrium distributions of cycloolefins also involve predictable entropy terms. Nonbonded H-H interaction energies are not yet predictable because of the distortion introduced into the familiar chair form of the cyclohexane ring by introduction of the double bond. The cis hydrogens attached to the double bond are eclipsed and, owing to the shorter bond length, may interact more strongly than the eclipsed hydrogens in ethane. 

It was mentioned earlier that the overall distortion of a C=C double bond is in most cases a combination of more than one type of deformation. Comparison of 24a with 24b (Table 3) reveals that introduction of an ethyl group in both adamantylidene moieties leads to twist as well as symmetrical oop bending. Also, the contribution of oop bending to the overall deformation of the double bond in the trans-cyclooctene derivative 37b should be mentioned here. 

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