Alkenes rotation

Table 2. Propene insertion for the benzoxantphos system, energies and geometrical parameters of transition states. Energies in kJ.mol"1, distances in A and angles in degrees, (a) Dihedral angle Hhvdndc-Rh-Cali.mc-C llktr . for defining alkene rotation.

Surprisingly, the critical experiment has been done infrequently over the last one-half of a century The requirements for an experiment that truly speaks to the issue at hand are that one be able to see the results of addition of both spin states of a single carbene, and these requirements rarely have been met. For example, the direct irradiation of methyl diazomalonate leads to the stereospecific addition expected of a singlet carbene, whereas the photosensitized decomposition of the diazo compound leads to formation of the triplet carbene and loss of the stereochemical relationship originally present in the reacting alkene. Rotational equilibration in the intermediate seems to be complete, as it makes no significant difference whether cis or trans alkene is used as starting material (Scheme 7.9). ... 

The reverse reaction corresponds to intramolecular C—H bond formation. It requires that the alkene rotate from its equilibrium perpendicular orientation toward the less favored parallel orientation. The barrier hindering the rotation of the alkene may be partially or totally offset by the incipient agostic interaction. If the alkene is unsym-metrical, the question of regioselectivity of hydride transfer arises. In the case of an unsymmetrical alkene with an X or C substituent, the donor orbital is polarized away from the substituent (Figure 13.9) and the metal lies closer to that end. Conversely,... 

Steric factors probably prohibit simultaneous rotation of the olefin and alkyne C2 units which would crowd all four metal-bound carbons into the same plane. Separate rotation of each unsaturated ligand was explored theoretically using the EHMO method. Rotation of the olefin destroys the one-to-one correspondence of metal-ligand tt interactions. Overlap of the filled dxz orbital with olefin n is turned off as the alkene rotates 90°, creating a large calculated barrier for olefin rotation (75 kcal/mol). Alkyne rotation quickly reveals an important point the absence of three-center bonds involving dir orbitals allows the alkyne to effectively define the linear combinations of dxy and dyz which serve as dn donor and dir acceptor orbitals for 7T and ttx, respectively. Thus there should be a small electronic barrier to alkyne rotation (the Huckel calculation with fixed metal... 

Signficant retrodonation (in addition to steric factors) may contribute to a barrier for alkene rotation about the metal-alkene axis (Table 1.4). 

The nomenclature has not been consistent in attempts to describe deviations from the idealized configurations. We will generally refer to a deviation in 0 simply as partial rotation. The term twist has been used for this, but has often been used to describe rotational distortion about the C=C axis. Partial rotation about the remaining axis has been called skewing. The stereochemically nonrigid process of alkene rotation primarily involves changes in 9, but would also... 

Alken-Rotation In Ubergangsmetall-Alken-Komplexen stellt sich die Frage, ob eine Rotation des Alkens um die C-C-Bindungsachse oder senkrecht zu ihr stattfindet. 

The standard Heck reaction involves the reaction of an aryl halide with an alkene, commonly acrylate, to give a styrene (cinnamate) in the presence of a catalytic amount of palladium (often less than 1 mol %). The sequence involves (i) oxidative addition of the halide to Pd(0) followed by (ii) 1,2-insertion into the alkene rotation then occurs to produce a species with hydrogen syn to palladium, then (iii) /3-hydride elimination gives the styrene and regenerates Pd(0), which rejoins the catalytic cycle and can take part in a second oxidative addition, and so on. 

Theoretical studies on the insertion of ethylene into d [MMeL2] (M = Ni, Pd, Pt) catalysts provide the relative potential energy plot in Fig. 6.5. The process consists of a coordination of the alkene perpendicular to the coordination square plane, followed by an easy alkene rotation to an in-plane coordination and a more difficult insertion. The rate-determining step of these reactions is the migratory insertion of the alkene and these calculations show that the stabilization of the system upon coordination of the alkene increases in the order Ni < Pd < Pt, whereas the insertion transition state changes very little. This explains the catalytic activity Ni > Pd > Pt generally observed, which is associated to insertion barriers in the range of 10 kcal/mol for Ni, 16 kcal/mol for Pd, and 25 kcal/mol forPt[l],... 

The pentamethylcyclopentadienyl niobocene (f complexes [Cp 2NbH(CH2CHR)], in which the alkene is coordinated to the metal center cis to the metal hydride, with a planar arrangement of the alkene carbon-carbon bond and the metal-hydrogen bond, are ideally suited for insertion without interference of the need for alkene rotation, and have been studied thoroughly [46]. In solution only the endo hydrido-alkene tautomer (Scheme 6.9) is observed by... 

Molecules react because they move. Atoms have (limited) movement within molecules— you saw in Chapter 3 how the stretching and bending of bonds can be detected by infrared spectroscopy, and we explained In Chapter 4 how the a bonds of alkanes (but not the n bonds of alkenes) rotate freely. On top of that, in a liquid or a gas whole molecules move around continuously. They bump into each other, into the walls of the container, maybe into solvent... 

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