Alkene rotational barrier

However, the barrier to rotation does not always predict the regioselectivity of the ene reaction of O2 with alkenes. As shown latef, it is the non-bonded interactions in the isomeric transition states that control product formation and barriers to rotation are rather irrelevant. The calculated rotational barrier values, with the HF-STO-3G method, for the allylic methyls in a series of trisubstituted alkenes, as well as the experimentally observed ene regioselectivity of a series of selective substrates, are shown in Table 9. ... 

For alkenes ll- and 11-Z, the syn methyl groups have lower rotational barriers than the corresponding anti ones by 0.5 kcalmoG. This is in the opposite direction to the proposed theoretical model. However, for alkene 4 there is a correlation between rotational barriers and ene reactivity. Alkenes 12 and 59 also demonstrate impressively that there is... 

The perpendicular orientation of the alkene in such complexes is favored because it maximizes the overlap of the bond with the LUMO (dx2 — y2, Figure 13.7) and minimizes 4e repulsive interactions with the HOMO (ndz2). The in-plane orientation is not expected to be strongly disfavored, however, because of the secondary interaction between the orbital and the dxy orbital. The rotational barrier of ethylene in Zeise s anion was theoretically estimated to be 55 kj/mol [282], within the range 42-63 kj/mol measured by NMR for related complexes [286]. 

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,... 

There are relatively few cis-trans forms of 1,2,3-alkatrienes known. They appear to interconvert readily on mild heating, which suggests that one of the double bonds has a lower rotational barrier than is normal for an alkene double bond. 

For example, molecular mechanics calculations (MM2) showed that the methyl group geminal to the neopentyl group in 2,3,5,5-tetramethyl-2-hexene (51, Scheme 18) has the lowest rotational barrier and is the most reactive [79], Furthermore, the ethyl group in 2,3-dimethyl-2-pentene (52) has a much higher rotational barrier (5.76 kcal/mol) than the methyl groups and is totally inactive to 102. Similar trends hold with 2-methyl-2-butene (4). However, the barrier to rotation does not always predict the regioselectivity of the ene reaction of 102 with alkenes. As it was shown later [62], it is the nonbonded interactions in the isomeric... 

The calculated rotational barrier values for the allylic methyls, with the HF-STO-3G method [62], in a series of trisubstituted alkenes and the experimentally observed ene regioselectivity of a series of selective substrates are shown in Table 7. 

For alkenes 15-E and 15-Z, the syn methyl groups have lower rotational barriers than the corresponding anti ones by 0.5 kcal/mol. This is in the opposite... 

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... 

This resonance representation correctly predicts a planar amide nitrogen atom that is sp2 hybridized to allow pi bonding with the carbonyl carbon atom. For example, formamide has a planar structure like an alkene. The C—N bond has partial double-bond character, with a rotational barrier of 75 kJ/mol (18 kcal/mol). 

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

MO calculations suggest that exceptional care mnst be taken in the interpretations and extrapolations from the barrier to rotation obtained by NMR and a detailed description of the real potential function for rotation of an alkene. The barrier to rotation (AG ) can be readily determined and perhaps may best be viewed as a very usefiil correlation of interconversion rates as a function of temperatme. 

The most effective approach to interpreting the barriers for a wide range of compounds lies in the consideration of the relative interactions within the Dewar, Chatt, Ducanson model of metal alkene bonding. An extended Hiickel MO approach has explored the interactions of the valence orbitals and examined the important interactions. A comprehensive extended Hiickel MO treatment of ethylene bonding and rotational barriers by Albright, Hoffmann et a/. presents an excellent analysis and the reader is referred to their paper for further discussiou. We have found that the following approach, which considers oifly three orbitals on the metal and the n and y orbitals of the alkene, provides the essential elements for understanding the barriers to rotation. Naturally, steric effects and secondary interactions with other orbitals modulate these primary iuteractious. 

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