Examination of the Isomer Distribution for 54d

After the relative conformation in solid state could be determined for 54d, the influence of the bulky phenyl substituents on the fluxional behavior and speciation in solution will be initially exemplified using this complex. Therefore, crystalline material of 54d was dissolved in CDCl3 and analyzed via 1D- and 2D-NMR spectroscopy at different temperatures. Part of the ¹H-NMR spectrum of 54d recorded at 273 K is shown in Figure 7.14. Based on the integration of the signal intensities, combined with the ¹H-COSY-spectrum, in total three different species (described in the following as A, B and C) could be associated with the different resonances.

Figure 7.14: ¹H-NMR spectrum (400 MHz, 273 K, CDCl3) of 54d. above: 0.1–2.6, below: 3.2–

6.5 ppm; labelling follows inset general notation scheme, the relative configuration of the β-oxazoline protons with respect to the substituent or more elaborateassignement of the different sets of signals was not performed, due to the lack of reliable NOE-contacts; for C1 -symmetric isomers B and C the two diastereotopic halves were additionally labeled with * for the allyl moiety and + and # for the oxazoline ring.

Of the three different species present, only one possesses C2-symmetry based number of equiintense signals (isomer A). The remaining two asymmetric species (B and C) therefore show two sets of signals, for each of their chemically inequivalent side arms and their allyl groups.

In order to identify the present isomers, stabilities of the different configurations are qualitatively assessed. First, since highly pure material was used, it has to be assumed that every observed species consists of two allyl palladium species and one PyrBOX ligand.

Second, the syn/syn orientation of the phenyl substituents should be more stable than any syn/anti or anti/anti arrangement due to the steric repulsion associated with the latter two forms.

Two possibilities exist for C2 symmetric isomer A, the exo/exo and the endo/endo form (Scheme 7.11). The endo/endo isomer should be disfavored because of the steric repulsion between the phenyl groups and the oxazoline substituents. This would be in good agreement with the presumptions for the enantiocontrol (Chapter 6) that the exo/exo configuration, which was found in solid state, is the most stable isomer in solution as well (61% relative population).

Scheme 7.11: Possible syn/syn isomers in solution with proposed assignment of A and B for 54d.

The second most populated species B (34%), however, cannot be assigned to the endo/endo isomer, as previously proposed, because this is C2-symmetric and would accordingly only give one set of signals. Thus, it can be concluded, that the steric repulsion between the phenyl and the oxazoline substituents is quite severe. This leads, in contrast to the allyl systems lacking sterically demanding syn-substituents, to a total suppression of the endo/endo rotamer.

Instead, the observed intensities for the remaining two other isomers can only be explained by two asymmetric species. As for A, large coupling constants of ³J = 10-12 Hz, supporting a syn-syn-configuration, can be found for B, although the value of the coupling constant alone may not be a definite proof.^[192] Isomer B was consequently assigned to the endo/exo form,

which can be formed via a single apparent allyl rotation of A. A distinct feature of B is its strong upfield shift for the resonance of one inner allyl proton (δ = 2.45 ppm), which could originate either from interallylic anisotropic shielding of the phenyl rings belonging to different allyl moieties or the palladium center.

For the least prominent isomer C (5%), only speculations can be made. As two sets of signals are observed, it must possess an asymmetric nature as well. This leads to the conclusion that a syn/anti arrangement for at least one allyl moiety is likely. Evidence therefore might be the doublet of doublets observed for one of the central allylic protons (marked with C(a2) in Figure 7.14). The two different coupling constants of ³J = 12.4 and 7.6 Hz, are indicative for a syn/anti allyl system.^[193] For the other allyl moiety in C, a regular triplet for the central allylic proton is observed (³J = 11.3 Hz), and thus a syn/syn configuration can be assumed.

These findings are somewhat surprising, as syn/anti isomers are generally destabilized because of unfavorable anti-Ph-anti-H steric interactions. Normally, these isomers are found for P-donor ligands and rarely for oxazoline ligands.^[194] Thus, taking a syn/anti arrangement into account for C, the number of possible isomers rapidly multiplies to eight (Figure 7.15).

Figure 7.15: Schematic diagram for all possible isomers C in a syn/anti arrangement.

Depending on the most restricting steric interaction, assumptions on the nature of C might be made. As the two palladium centers are pulled apart due to the small bite angle of the ligand, interallylic interaction can be assumed to be of minor importance, as indicated by the nature of B. Thus, steric interaction between the oxazoline substituent and the outer phenyl rings should be one of the determining factors, leading to the exclusion of C6, due to the twofold interaction with these substituents.

If the allyl-allyl interactions are considered, C1 and C7 would be the most discriminated species. Another argument against these species is that no unique upfield shifted resonance for one allylic terminus, as for B, was found. Taking the obtained structural information (Chapter 7.2.2) into account, uniquely strong interallylic anti-Ph-syn-Ph repulsion can be assumed for C4 and C6.

Further it might be assumed, that for any exo-rotamer the repulsion of an outer anti-Ph is significantly larger, as the substituent would be tilted directly towards the oxazoline substituent. From the latter assumption, the presence of C3, as well as previously ruled out C7 would be unlikely. However, as later investigations of the triphenyl allyl system revealed, such an arrangement might not be as destabilized as initially anticipated.

This scenario would leave three possible alternatives (C2, C5 and C8), for which destabilization by ligand-allyl interactions can be expected. For C5 anisotropic shielding by phenyl substituents on the oxazoline ring should lead to upfield shifts of approximately Δδ ≈ 1 ppm, if nearly identical geometric parameters are to be found as for the cyclohexenyl systems. As only a small shift of approximately Δδ ≈ 0.3 ppm is observed for 54a (= [L¹Pd2(1,3-Ph2C3H3)2]BF4) (see upcoming section), this isomer might be ruled out. The two remaining options C2 and C8 should only marginally differ in their stabilities due to comparable interallylic repulsion. If again, interallylic shielding effects would be considered, for both isomers a distinct upfield shift for the syn-H-protons should be observed. Based on this assumption, C3 would become the most likely isomer as analogous arrangements are found for the later discussed triphenylallyl systems.

In summary, the true nature of isomer C cannot be unambiguously specified, but the latter discussion might give an indication or at least rule out some of the possible isomers.

Additional computational experiments on the stabilities of the various possible isomers might be beneficial and could support the previous assumptions.

DOSY experiments (Figure 7.16) of the isomer mixture gave rise to similar diffusion constants (slightly smaller for B than A) for the two major isomers. Although a slightly larger hydrodynamic radius for B is found, these findings support that both species correspond to stereoisomer of 54d. The signal intensities for C were unfortunately too low to pass judgement on its nature.

Figure 7.16: DOSY-NMR spectrum (400 MHz, 298 K, THF-d8) of 54d.

In order to correlate the solid state structure of 54d to species solution, a deep temperature NMR experiments were carried out. Therefore, crystalline material of 54d was put in a NMR tube which was cooled to 195 K and solvent (CD2Cl2) was slowly added in order allow for thermal equilibration. The directly recorded ¹H-NMR spectra at 193 K, interestingly, initially not only the resonances of A, but also signals for B (δ = 0.54-0.48 and 0.20-0.13 ppm) were observed (Figure 7.17, a)). The determined A:B ratio of 3:1 remained principally constant within the investigated time frame (20-30 min, Figure 7.17, b)). However, after the sample was allow to warm up to room temperature for 1 h an measured again at 193 K (Figure 7.17, c)), a higher abundance for B could be determined (A:B = 2:1). Later experiments (Figure 7.17, d)) would suggest that this ratio corresponds to the thermodynamic equilibrium under these conditions.

Figure 7.17: Part of the ¹H-NMR spectrum of 54d, (400 MHz, 193 K, CD2Cl2), a) start, ratio A:B

≈ 3:1; b) after 45 min at 193 K, ratio A:B ≈ 3:1; c) directly after b) than 1 h at room temperature and again at 193 K, ratio A:B ≈ 2:1; d) at room temperature for several days, than 12 h at 195 K, measured at 193 K, ratio A:B ≈ 2:1, signals at δ = 0.05-0.10 and 0.23-0.20 ppm correspond to isomer C.

Repetition of the experiment using different batches of crystalline material or sample preparation (using liquid nitrogen) gave nearly identical results, thus, the investigated samples must contain isomer B prior to the measurement. Isomer A is furthermore significantly enriched in the solid material with respect to B, while C is initially not present and forms only slowly (Figure 7.17, d)). As further evidence, that C is an isomer of 54d and not a decomposition product, might count its constant percentage of about 5% over several days and at various concentrations.

Henceforth, it will be focused on the fluxional processes primarily of the major isomers A and B and its dependence on conditions such as solvents and temperature.