Steric effects alkene rotation

The stereochemistry of the resulting cyclopropane product (.s vn vs anti) was rationalized from a kinetic study which implicated an early transition state with no detectable intermediates. Approach of the alkene substrate perpendicular to the proposed carbene intermediate occurs with the largest alkene substituent opposite the carbene ester group. This is followed by rotation of the alkene as the new C—C bonds begin to form. The steric effect of the alkene substituent determines... 

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... 

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. 

Steric effects can also explain why carbocyclic radicals (which are not able to undergo free rotation about the C-C bonds) can add to alkenes stereo-selectively. The introduction of an adjacent chiral centre can make the two faces of the radical non-equivalent and this can lead to the alkene preferentially adding to the less hindered face (Scheme 2.3). 

A more recent study of series of W(CO)4(ene)2 complexes by NMR showed that from the highest barrier, for 1-pentene rotation (A = 45.8 kj moR ), to the lowest barrier, for cyclohexenone (AG = 38.8 kJ moR ), there was only a difference of 7 kJ mol It appears that the different steric effects of the alkenes in these complexes have little effect on the barrier to rotation. 

The alkene ligands in rra -[M(CO)4(methylacrylate)2] (M = Mo or W) are mutually perpendicular and the two diastereomers 3 and 4 are not interconvertible by alkene rotation. Rotational barriers are somewhat higher for W than Mo and are different for the two diastereomers 69.4 2.0 (3 M = W) and 81.5 2.0 kJ mol (4 M = W). This illustrates the dangers in accounting for differences in barriers in simple terms in this case steric effects must be essentially the same in 3 and 4 while electronic differences must be rather subtle. Alkyne rotation is observed in c/5-[W(CO)(Me2NCS2)2(alkyne)] (5) to occur... 

Nelsen and coworkers determined a barrier to inversion through the planar form in 2 and 3 to be approximately 2 kcalmol-1 by variable temperature ESR spectroscopy [59]. Gerson and coworkers found, also by ESR spectroscopy, that the frequency of electron exchange between the two sites in 4, which is equivalent to rotation about the central bond, can vary between < 106 and > 109 s-1 depending the degree of steric hindrance to planarity [60]. Recent calculations also provide very small barriers to inversion through the planar form [56,57]. It is apparent, therefore, that for most synthetic purposes most alkene radical cations can be considered as essentially planar with effective delocalization over the two sp2-hybridized C atoms, and they will be considered as such in this chapter. 

The importance of steric interactions with cis ligands can be more clearly seen in alkene substituent effects. The barrier increases significantly in the PtCl(acac)(ene) series where ene = substituted ethylene compared to ene = ethylene. The c/s-Cl2(DMSO)Pt(alkene) series (6)-(9), studied by Boucher and Bosnich, is particularly instructive. The rotation in the ethylene begins to produce broadening about —47°C, whereas the propene conformers broaden at —20°C and... 

The effect of introducing sp -hybridized atoms into open-chain molecules has been discussed previously, and it has been noted that the torsional barriers in 1-alkenes and in aldehydes and ketones are smaller than those in alkanes. Similar properties carry over to incorporation of sp centers in six-membered rings. Whereas the free energy of activation for ring inversion in cyclohexane is 10.3 kcal/mol, the barrier is reduced to 7.7 kcal/mol in methylenecyclohexane, and to 4.9 kcal/mol in cyclohexanone." The decrease in activation energy is related to the lower torsional barriers for rotation about sp -sp bonds, and to the decreased steric requirements of a carbonyl or methylene group. 

---