Double bond distortions pyramidalization

The geometry about the P=C double bond for the Class 2 non-stabilized ylides is similar to the structure of the Class 1 ylides. The P=C distances range from 1.618 to 1.727 A. The phosphorus atom is a slightly distorted tetrahedron and the carbon atom is slightly pyramidal. One of the phosphonium substituents bisects the plane formed by the ylidic carbon and its two substituents, and this plane is bent toward the perpendicular P substituent, as shown in Figure 3. 

All X3PO molecules have a pyramidal PX3 group, with the oxygen atom occupying the fourth position to complete a distorted tetrahedron. The P—O bond lengths are about 1.55 A, consistent with the existence of double bonds. Several mixed phosphoryl halides, phosphoryl pseudohalides, as well as X3PS and X3PSe compounds are also known. All of these compounds are prone to ready hydrolysis. 

The reduction of Cr with certain tertiary a-hydroxy-carboxyUc acids has yielded Cr complexes, which are stable in aqueous solution. The stmctme of K[0Cr(02CC(Et)(Me)0)2] (14) showed a distorted square pyramid with a short apical Cr double bond (155.4 pm). The facile preparation and stability of these complexes have led to their use in the study of the mechanism of oxidations involving Cr. A similar complex containing two perfluoropinacolate ligands has also been prepared. 

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]

OC)4Fe=SiMe2 (4) has C2v symmetry as shown in Fig. 1. The double bond between Fe and Si is weakened in the donor adduct 1. The pyramidal distortion of the substituents around the silicon atom allows better interaction between the empty p-orbital and the donor atom (see Fig. 1). The removal of the stabilizing ligand is accompanied by a rearrangement of the CO ligands om Cs symmetry to the arrangement shown in Fig. 2. 

According to MNDO and ab initio results for the addition of BH3 to ethylene (1), the reaction is highly exothermic and proceeds via a reactant-like transition state with the double bond elongated to 138-140 pm and little pyramidalization (254). It therefore seems unlikely that release of strain will be noticeable in hydroborations of distorted double bonds. The low regioselectiv-ity observed in hydroboration of 69 and the structurally related 282 may therefore reflect similar hybridizations of both carbon atoms of the double bond. 

In 1972, Mock considered double-bond reactivity and its relationship to torsional strain, by which he understood the strain imposed on a double bond in medium-ring fra 5-cycloalkenes or by steric compression of large cis substituents [28]. He argued that the loss of 7t overlap due to a torsion about the double bond can be partially compensated by rehybridization in these two situations, leading, respectively, to syn and anti pyramidalization of the double bond consequently, such bonds will favor different modes of addition (cis and trans). The proposition was supported by examples of X-ray structures of strained olefins, STO-3G energy calculations for the twisted and pyramidalized ethylene geometries, and by analysis of the out-of-plane vibrational frequencies of ethylene. Mock concluded that small ground-state distortions may produce sizable effects in the transition states. 

Other distortions ofalkenes Pyramidalization and bending of double bonds can also be used to enhance reactiv-... 

The reaction of norbornene was also found to have a lower activation barrier to form the metallacyclobutane than the reactions of cyclopentene, cycloheptene, and Z-cyclooctene. This can be attributed to the pre-distorted double bond of norbornene, which requires less distortion to achieve the pyramidalized geometry in the transition state than that needed for planar olefins [43]. 

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