Olefins rotational barriers

Complex 7 now possesses all the prerequisites for cw-ligand insertion reaction a close (cis) position to the interacting ligands, and activation of the r-bonded olefin by weakening the olefin-metal T-bond and the olefin rotation barrier through the trans effect of the tran -chloro ligand across the complex. 

Olefin rotational barriers are larger in the CpPt(PPh )(olefin) case, and crystal structure determinations show that olefin is coordinated perpendicular to the plane defined by Pt, P and the... 

A thorough study of olefin rotation barriers in [PtCl(acac)(olefin)] and [PtCl2L-(olefin)] (the olefins are monosubstituted, 1,1- or cis- or rron -disubstituted, or tri-substituted) reveal the expected upward trend with increasing bulk and also with electronegativity of the substituent. The orientation of the olefin in the ground state does not follow strictly the same trends, the most populated isomers being found for relatively small but electronegative substituents. ... 

The l3C NMR spectrum in CDC13 at ambient temperature displays two Mo—CO (<5 222.2 and 228.79 ppm) and two characteristic high-field coordinated C=C (<5 58.64 and 71.91 ppm) resonances, indicating that the solid state structure of the Mo-chelate complex is maintained in solution. Conformational rigidity is generally observed in many -complexes. Rotational barriers about the metal-olefin axes and conformational preferences in these complexes have been estimated using extended Huckel-type calculations283. 

NMR spectroscopic studies of two rhodium complexes 18 and 19 also indicate that the binding of the C2F4 group to rhodium is better pictured as a conformationally locked metallacyclopropane (59). The activation barrier for rotation of the C2H4 ligands around the Rh-olefin (centroid) vector in 19 was determined to be 15.0 0.2 kcal/mol. In contrast, the rotational barrier for the C2H4 ligand in 18 was demonstrated to be 13.6 0.6 kcal/mol... 

A related topic that was already discussed in these first DFT/MM works is that of branching. The scheme shown in Fig. 1 would always produce always a linear polymer if ethylene was used as olefin. But a simple process of / C-H oxidative addition/reductive elimination, coupled with olefin rotation, can produce a branched polymer, as shown in Fig. 4. Calculations on the branching process for cationic diimine Ni(II) complexes [36, 37] indicated a small increase between 0.9 and 2.5 kcal/mol in the barrier for this process, associated with the introduction of the bulky substituents in the catalysts. 

To calculate the insertion transition states from each propene adduct, the authors considered the olefin rotation in clockwise and counterclockwise fashion for these two intermediates, as previously described by Carbo et al [115]. For the equatorial-axial adduct, the barrier to propene insertion leading to the linear insertion product was predicted to be 2.8 kcal/mol smaller than the barrier for the insertion reaction leading to the branched product. For the equatorial-equatorial adduct, the barrier for the insertion leading to the branched product was predicted to be 1.4 kcal/mol lower in energy than the barrier for the reaction leading to the linear product. Therefore, it appears that for this type of catalyst there are two separate propene insertion reaction channels, one generating almost exclusively the linear product, and the other producing primarily the branched product. 

Carbon-carbon double bonds (olefins) present significantly higher rotational barriers—typically 25-65 kcalmol 1—than single bonds, providing kinetic stability of both cis and trans isomers. This stability, together with the possibility of their interconversion by photoisomerization, have been exploited in the construction of a wide variety of rotors—and even directional molecular rotary motors—in which the rotor and base are connected via an olefin. 

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... 

Furthermore, no exchange of the two independent olefin proton sites in either isomer was observed, indicating that olefin rotation is not ocurring at 77°C. Thus, the experimental results are consistent with molecular orbital expectations the barrier to olefin rotation is large while the barrier to alkyne rotation is small. 

Among the difference types of olefins known with barriers to rotation amenable to study by dynamic H NMR technique, the reported rotational barriers of push-pull ethylenes containing potentially heteroaromatic systems are rather low, ca. 50 kJ-mol [85MI1 88AHC(43)173], Moreover, Elguero and co-workers have studied the rotational barriers around the C—C interannular bond of several 2-(4-pyridyl)benzazoles and their pyri-dinium salts (areno-analogues of amides), since they are too low to measure by H NMR (60 MHz) at 173 K (77H911). 

The donor properties of the methyl group provide a preference for the substituted end of the olefin to be trans to one of the ligands. Thus the inherent instability of the endo isomer and the interaction of the methyl group with the ring are partially overcome. The rotational barriers of substituted olefin derivatives in the molybdenum complexes (AG —13-... 

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