Allyl rotation

The mechanism of this interconversion is known as apparent allyl rotation and results in a switching of the allylic termini in addition to the movement of the central allyl carbon from one side of the coordination plane to the other although, for situations with symmetrically substituted allyl units, only this latter change is noticeable As the name suggests, the process is non-trivial and, in reality, the mechanism is dependent on the specific conditions.1161 Nevertheless, in most cases, the interchange is rapid compared with the rate of nucleophilic attack and, therefore, the product outcome does not necessarily depend on the intermediate isomer ratio (Curtin-Hammett principle). 

CpMo(CO)2( ] -allyl) complexes show a planar allyl rotation analogous to that found for the diene in [CpMo(CO)2( ]" -diene)]+ complexes. Endo-exo isomerism is observed and interconversion occurs rapidly at room temperatme... 

There are also other isomerization processes that can occur by a ti-o-ti mechanism, e.g., racemization (or epimerization, if the Pd complex is derived from a chiral ligand, see Sect. 3.3) or a rearrangement of the allyl system which is often referred to as apparent allyl rotation. 

As the name suggests, apparent allyl rotation probably does not involve a simple rotation around the Pd-allyl axis. Such a process seems energetically unfavorable because it would require simultaneous breaking of two Pd-C bonds or a severe geometrical distortion of the preferred planar coordination geometry. Several mechanisms of apparent allyl rotation have been proposed [19,23,24]. One of them involves k-G-k isomerization with concomitant rotation around the Pd-C bond in the -intermediate as shown in Scheme 5. 

Rotation of the allyl group, in which syn-anti interconversion does not occur, is now known in many instances for complexes with coordination numbers greater than four (see Table III). NMR spectra consistent with allyl rotation in most square palladium complexes have been shown to result from ligand exchange (47). 

The stereodynamics of rf -diene and if -dX y complexes are likely to be of importance in understanding their reactivity, as these complexes are used in many synthetic strategies. Although an 7] -cyclopropenyl may be considered to rotate about a metal-centroid axis, rearrangements of conformers in rf -diene complexes, such as 15 and 16 (Schemes 10 and 11), may be considered as motions involving a pseudo-rotation of a five-coordinate or seven-coordinate metal complex with a bidentate diene. This can account for presence of an allyl rotation in 17... 

For the mechanism of apparent it-allyl rotation, see Gogoll, A., Ornebro,... 

Allyl and benzyl radical are substantially stabilized, as anticipated from the resonance structures (see Section 1.3.6). Comparing the BDEs of propene and toluene to an appropriate reference such as ethane suggests resonance stabilization energies of 12.4 and 14.1 kcal / mol, respectively. An alternative way to estimate allyl stabilization is to consider allyl rotation barriers (Eq. 2.12). Rotating a terminal CH2 90° out-of-plane completely destroys allyl resonance, and so the transition state for rotation is a good model for an allylic structure lacking resonance. For allyl radical the rotation barrier has been determined to be 15.7 kcal / mol, in acceptable agreement with the direct thermochemical number. 

With Tj -allyl complexes the two major types of fluxionality operating are allyl rotation about the M-allyl axis and interconversion. 

The 7j -7j -Tj path is characterized by an associated exchange of the syn and anti substituents at the allyl terminus, while for allyl rotation this exchange cannot take place. The complexes [M2(allyl)4] (M = Cr or Mo) have two bridging and two terminal rj -allyl ligands with the minor (8) and major (9) configurations both present in solution. Their rates of interconver-... 

Exo-endo interconversions in [Mo(C5H5)(NO)I(T7 -allyl)] occur by an i7 -77 -T7 mechanism while allyl rotation is faster for [MofCsHs)-(CO)2(T7 -allyl)] and [Mo(C5H5)(NO)(CO)(T7 -allyl)]. Resolution of the enantiomers of the the iodide where allyl = cycloctenyl has been achieved spontaneously by crystallization. The endo-exo interconversion of the (R) isomer shown in Schemes could occur by clockwise or anti clockwise... 

In the previous section we have already considered some examples of the 17 mechanism as an alternative to allyl rotation. This is a special... 

G. Doyle, J. OrganometalUc Chem, 1977, 132, 243. Allyl rotational conformers of(MCl(diketo-nate)( /3.aIlyl)(CO)2]. 

---