Dynamic Processes for Allylic Compounds

For allylic species, different interconverting isomers can frequently be found in solution.

Their equilibrium plays a crucial role in the catalytic transformation and is governed by the type of allyl moiety, as well as the nature of the coordinating ligand. In order to explain the fluxional behavior, two different main mechanistic models for the exchange process have evolved. These will be discussed in regard to their stereochemical impact. However, first a brief introduction into the stereochemical nomenclature for allylic complexes will be given.

5.2.1 Stereochemical Notations for Allylic Moieties

Due to substitution of the allyl moiety the number of possible isomers for the η³-complex is multiplied, resulting from the different possible relative configurations. These stereoisomers are traditionally named in relation to the substituent in the 2-position (R in Figure 5.1, a)).

Figure 5.1: Stereochemical nomenclature a) syn/anti and b) endo/exo nomenclature for allyl ligands.

The position of the substituents has geometric implications, as for allylic complexes, the central carbon atom is generally bend away from the palladium center. This can be explained by relevant orbitals involved in bonding to the transition metal (Figure 5.2). The most relevant interactions are those of the HOMO (2π, π-donation) and the LUMO (3π, π-back donation) of the allyl anion with transition metal orbitals of suitable symmetry.

Due to the nodal plane going directly through the central allylic carbon atom (see Figure 5.2;

2π-orbital), no orbital overlap can occurs, resulting in an overall tilt of the allyl moiety to maximize the π-interaction. Thus, all syn substituents are tilted towards the metal center, while all anti substituents are bent away.

Figure 5.2: Bonding for allyl ligands; left: relevant allyl molecular orbitals, right: energetic order of the molecular orbitals and suitable transition metal orbitals for bonding interactions.

Another important aspect is the endo/exo nomenclature (Figure 5.1, b)), which describes the relative orientation in space of the allyl moiety with respect to the chiral ligand (only for C1-symmetric binding site). Therefore, the allyl vector is defined, which originates at the central carbon atom ending at the substituent in 2-position. For exo-rotamers, the allyl vector is pointing towards the substituent at the chiral center of the ligand, while for endo-isomers it is pointing away in the opposite direction.

5.2.2 Isomerization through a η

-η

-η

-Pathway

As it has been pointed out, in solution dynamic exchange processes for the aforementioned isomers are often observed. The first postulated mechanism for the detected interconversion of syn and anti-substituents assumes a η³-η¹-η³ pathway (Scheme 5.4).^[141]

Scheme 5.4: Mechanism for syn/anti exchange in a (η³-allyl)palladium complex; L^A, L^B = bidentate ligand.

In this process, the outer allylic carbon atoms do not change place, instead the rotation around the C-C-single bond of the η¹-complex changes the position of the central C2-carbon atom. The preferred formation of one η¹-complex over the other can be explained either by the trans effect (strong σ-donor/stronger trans influence donor, favors σ-bond formation in cis position)^[142] or steric interactions.^[141,143] Further theoretical calculations have led to the

conclusion that a solvent molecule or an ancillary ligand may coordinate to the (η¹-allyl)palladium intermediate.^[144] In case R¹ ≠ R², the stereochemistry of this process has to be addressed. For the displayed example it follows, that the syn/anti exchange yields inversion of the absolute stereochemistry at the carbon center bound to R².

The η³-η¹-η³ exchange process is mainly found for allyl palladium complexes coordinated by phosphorous or N-heterocyclic carbene donors.^[145]

5.2.3 Isomerization via Apparent Allyl Rotation

On the contrary, for N-donor ligands another dynamic process is often proposed, which proceeds via a syn/syn, anti/anti exchange. Within this apparent allyl rotation, the outer carbon atoms switch their positions, while the C2-atom changes its geometric orientation by 180 degrees. This results in different diastereomers (exo/endo) for a C1-symmetric binding site, whereas for C2-symmetry both bidentate donors atoms are identical (L^A = L^B), thus the rotation process results again in the starting complex.

Instead of a simple rotation around the allyl-palladium bond, two other scenarios have been proposed.

(I) Associative mechanism:^[146] After coordination of an additional ligand (solvent molecules, counter anions or other molecules), a square-pyramidal intermediate is formed. For this five-fold coordinated transition state, the isomerization takes place via a Berry pseudorotation without cleavage of the Pd-N-bond of the bidentate ligand (Scheme 5.5).

Scheme 5.5: Mechanism of the apparent allyl rotation via ligand association; L^A, L^B = bidentate ligand, L^C = additional ligand.

(II) Dissociative mechanism:^[147] After partial dissociation of the bidentate ligand through Pd‒N bond cleavage, a 3-fold coordinated T-shaped complex is formed. The rotation around the remaining Pd-N bond explains the apparent allyl rotation. Stabilization effects for the unsaturated intermediate (by solvent molecules or anions) and correlations between the basicity of the N-donor atoms and the exchange rate have been reported.^[148] Generally, the more basic the ligand, the more hindered the rotation. Steric hindrance at the N-donor atoms furthermore facilitates the interconversion.^[149]

Scheme 5.6: Mechanism of the apparent allyl rotation via ligand dissociation. L^A, L^B = bidentate ligand.

In contrast to the η³-η¹-η³-pathway, epimerization does not occur for the apparent rotation (for R¹ ≠ R²).

5.2.4 Isomerization via other Processes

Apart from the previously discussed processes, in some cases a different exchange reaction has been observed.^[150,151] Here, the allyl moiety isomerizes via a SN2-type reaction of a palladium(0) complex with a palladium(II) allyl complex (Scheme 5.7).

Scheme 5.7: Mechanism of enantioface exchange through SN2 type reaction. L^A, L^B = bidentate ligand.

By this enantioface exchange process, the absolute stereochemistry of all three carbon atoms is inverted. For symmetrically substituted species (R¹ = R²), this can also be achieved by the aforementioned processes.

As a result of the fluxional nature of the allyl species and its impact on stereochemistry, efficient asymmetric induction becomes more elaborate. Nevertheless, several different successful concepts have evolved for various types of allylic substrates, which will be discussed in the next subchapter.