7 Dinuclear PyrBOX Allyl Palladium Complexes
7.1 Cyclohexenyl Palladium Complexes
7.1.3 NMR-Investigations of the Dynamic Behavior of 48a-d
For the methallyl system it was shown, that three isomers are present in solution, which rapidly interconvert via apparent allyl rotation at room temperature. As the cyclohexenyl moiety sterically and electronically resembles this system, complementary behavior was anticipated. First, 1H-NMR spectra were recorded for all compounds at room temperature in CDCl3 solution (Figure 7.3).
Figure 7.3: 1H-NMR spectrum (300 MHz, 298 K, CDCl3) of 48a-d; X marks residual solvent signals (Et2O, CHCl3).
Broad signals for all cyclohexenyl groups were observed indicative of a rapid dynamic process. Despite this interconversion, the signals of the oxazoline ring and the pyrazole protones are relatively sharp. This indicates that the coalescence temperature TC for these protons is lower, thus showing the average of all isomers (T > TC). In order to observe coalescence, an intermediate exchange has to be present. It follows for the exchange rate k and the maximum peak separation of the corresponding signals Δν (in Hertz) [175]
k ≈ 2Δν
Due to this relationship, higher exchange rates are necessary for a higher peak separation.
Although, the same activation barrier has to be overcome, higher temperatures are therefore necessary in order to fulfill the aforementioned equation and observe the coalescence phenomena. While the resonances of the ligand scaffold should be hardly affected by the rotational process, this is most likely not the case for the cyclohexenyl protons. This results a higher peaker separation and coalescence temperature, thus, the signals of the allyl moiety generally appear relatively broad.
As the pyrazole substituent in 4-position does hardly influence the magnetic resonance frequency of all distal protons, by switching from hydrogen to a phenyl backbone substituent, the corresponding chemical shift for each proton should remain almost identical. Upon this replacement (Ph against H) a broadening, when comparing the line-width for the cyclohexenyl protons, can be evidenced. This leads to the qualitative assumption, that the rotational processes are significantly slower for the 4-phenyl pyrazolate complexes. For the complex 48d, the coalescence temperature should be relatively high, as at room temperature the line-shape analysis of the cyclohexenyl protons already suggests the presence of different isomers. In the following, the discussion of the isomerization of these intermediates will be exemplified for 48a. All three possible η3-allyl isomers and their stepwise interconversion are shown in Scheme 7.4.
Scheme 7.4: Three possible isomers of cyclohexenyl palladium complex 48a and potential steric interactions.
Due to steric repulsion of the cyclic systems with the oxazoline substituent, destabilization of all exo arrangements would be expected. Based on this assumption, the major isomer should be endo/endo configured, while the destabilization should lead to a lower population for the exo/exo arrangement. However, in case of the methallyl complexes opposite stabilities were observed. In order to verify these presumptions, variable temperature NMR experiments were carried out (Figure 7.4).
Upon cooling from 298 K to 273 K, line sharpening was observed and two isomers could be detected. However, the second most abundant species did not yet give well-resolved signals, indicative for an underlying fluxional process (e.g. Figure 7.4, δ ≈ 4.25 ppm). Decreasing the temperature further to 248 K resulted in separation of these signals to yield in total three different isomers (for complete assignment see Figure 7.5).
The major species A (50%) and the least abundant species C (7%) show one set of signals, as expected for a C2-symmetric isomer (endo/endo or exo/exo). Unfortunately, no unambiguous NOE-contacts were observed, therefore it cannot be directly distinguished between the two forms.
Figure 7.4: Variable temperature 1H-NMR spectra (CDCl3, 400 MHz) of complex 48a; above:
full spectrum below: oxazoline and allyl region; labeled signals belong to allylic termini for the different isomers A,B and C representing the fluxional behavior of 48a.
Figure 7.5: 1H-NMR spectrum (400 MHz, 248 K, CDCl3) of 48a: below: 0–2.2, above: 4.0–
5.6 ppm; labelling follows inset general notation scheme, the relative configuration of the β-oxazoline protons with respect to the substituent are abbreviated with a (anti) and s (syn); for C1-symmetric isomer B the two diastereotopic halves were additionally labeled with * for the cyclohexenyl moiety and + and # for the oxazoline ring.
However, one might derive the nature of these species due to the anisotropic shielding provided by the Ph-oxazoline substituents considering the solid state structure (Figure 7.6).
Figure 7.6: Potential anisotropic shielding effects for 48a; top exo/endo halves from the molecular structure of 48a, relevant hydrogen atoms for anisotropic shielding are indicated.
In this context, the unique allyl signals for B and C at δ ≈ 4.2 ppm have to be mentioned. The observed large separation (Δδ ≈ 1 ppm) of the chemical shift of the two allylic termini, if explained by such as shielding effects, would indicate an endo arrangement for the latter resonances (Figure 7.6, right). The large upfield-shift for one aliphatic cyclohexenyl protons (labeled as A(cy-H6/4)) to δ ≈ 0.2 ppm might represent the according analog effect of the aromatic ring system on species A (Figure 7.6, left).
The absence of this effect for the later discussed iso-propyl substituted oxazoline in complexes 48c and 48d, supports this model which would be in good agreement with the observation made for the methallyl systems. Consequently, steric interaction of the aliphatic substituents with the ligand scaffold cannot be the major decisive force for determining relative ground state stabilities.
The second most abundant species B (43%) can be unambiguously assigned to an exo/endo isomer, as due to its C1-symmetry two sets of signals for all oxazoline and cyclohexenyl protons were observed. The different electronic situation for the exo and endo arrangements becomes evident in the two types of resonances observed for B, thus it may be described as a hybrid of A and C. Here, the exo-half (indices: +, *) resonates analogous to A, whereas the signals of endo conformer (index # for oxazoline) closely resemble species C.
In order to quantify the exchange process for 48a, EXSY experiments were conducted at 248 K (Figure 7.7).
rate constants [s-1] kAB = 1.487 kBA = 1.762
A:B = 1.18 : 1 kBC = 0.319 kCB = 2.299
B:C = 7.20 : 1 A:B:C = 8.49: 7.20 :1
Figure 7.7: Exchange process for 48a: top: Scheme of exchange process, left: Rate constants calculated with EXSYCalc.[176] and corresponding kinetic ratios, right: part of an EXSY spectrum (400 MHz, 248 K, CDCl3, d8 = 0.5 s) of 48a.
The obtained exchange rates are in good agreement with the relative ratios obtained by integration (A:B:C = 7.14 : 6.28 : 1). The rate constant kCB is higher than the kAB and kBA, thus, a higher coalescence temperature might explain why species C could only be detected at 248 K. Another reason would be the larger separation of the relevant resonances. Further, no direct exchange between A and C could be observed, conforming a stepwise process.
Comparison of the methallyl and cyclohexenyl rotations yields an unexpected result (Table 7.2). First, the discrimination between the different isomer A and B is less pronounced, only C is strongly destabilized. Second, the obtained rate constants are one order of magnitude higher than for the methallyl system. Thus, neglecting the steric influence of the 2-methyl group of the methallyl moiety for the rotation, explanation of the relative stabilities and exchange rates exceeds plain and simple steric considerations.
ratio of isomers by integration [%] rate constants apparent allyl rotation [s-1]a
complex A B C kAB kBA kBC kCB
[L1Pd2(C6H9)2]BF4 50 43 7 1.5b 1.8b 0.32c 2.3c
[L1Pd2(C4H7)2]BF4 57 28 15 0.1d 0.2d 0.12d 0.2d
Table 7.2: Comparison of isomers ratios and rate constants for 48a and 46a (400 MHz, 248 K, CDCl3);a) determined by EXSY experiments; b) exchange of one βs-oxazoline resonance was used for the calculation; c) exchange of one cyclohexenyl terminus resonance was used for the calculation. d) see [101].
With the rate constants at hand, the free activation energy ΔG‡ for a given transformation can be derived, using the Eyring equation:[177]
Here, kB is the Boltzmann constant, T the temperature in Kelvin, h is Planck’s constant and R is the universal gas constant. This most common form can be rewritten as:
From the obtained rate constants, the corresponding free activation enthalpies were calculated (Table 7.3). As the three ground states differ in their relative thermodynamic
calculated from the reaction rate constants in Table 7.2 by the Eyring equation, a) see [101].
From the obtained data is can be concluded that the cyclohexenyl seems to stabilize the transition state (up to 5 kJ/mol) and/or destabilize the ground states of the apparent allyl rotation. One possible explanation might be the more pronounced electron donation of the cyclohexenyl to the palladium center, resulting in a weaker ligand-palladium bond (compare Chapter 7.1.1, Table 7.1). Based on these findings, a higher degree of participation of a dissociative mechanism for the rotation seems plausible.
Figure 7.8: 1H-NMR spectrum (400 MHz, 248 K, CDCl3) of 48a-d; X marks residual solvent signals (Et2O, CHCl3).
For the purpose of determining the critical influences on the stabilities of the stereoisomers, the relative abundancies for 48b-d were determined by integration from 1H-NMR spectra recorded of the CDCl3 solutions at 248 K (for 48b at 223 K) and are displayed Figure 7.8. The determined ratios for 48b-d as well as for the corresponding methallyl system are compiled in Table 7.4.
complex A [%] B [%] C [%] determined by integration of the allylic protons at δ ≈ 5.3-5.2 ppm, an was subsequently A calculated by subtraction of appropriate signals.
As expected due the similar steric demand, comparable isomer distributions result with respect to the corresponding methallyl systems. A slightly lower population for C is found for all ligands which might reflect the relevance of interallylic interactions. The A:B ratio varies between 2:1 and 1:1 with a maximum for 48c, as determined for the methallyl complex 46c.
The stabilities generally do not follow any obvious trends, indicating various competing factors influencing the equilibrium.
Since in general, the freezing out of two conformers of the cyclohexenyl ring (boat-like vs.
chair-like) is plausible, the temperature was reduced further to 223 K (Figure 7.9). However, only further line-broadening could be observed. This is in agreement with earlier findings for cyclohexenyl systems, where even at 143 K no signal separation of both conformers was evident.[178]
Figure 7.9. Variable temperature 1H-NMR spectra (CDCl3, 400 MHz) of complex 48c; X marks residual solvent signals (Et2O).
To sum up, for all of the investigated cyclohexenyl complexes, a stepwise fluxional process of all three possible stereoisomers could be detected. By comparison of the rate constants to the earlier investigated methallyl system, for the cyclohexenyl complexes acceleration of the process becomes evident. A possible explanation for this might be the higher degree of electron donation to the palladium center.
Furthermore, only weak ground state discrimination can be derived from the measured isomer ratios. Thus, only a low asymmetric induction would be anticipated for the subsequently investigated catalytic transformations. Nonetheless, these considerations do not take into account the inherent reactivity of the allylic termini. In combination with the experimental results, a more sophisticated evaluation of the enantioselection process for the cyclohexenyl substrates might become possible.