3.2 Novel Complexes of Bulky Diphosphine Ligands with an Unsymmetric Backbone
3.2.5 Formation and Isolation of Pd(II) Hydride Species
The catalytically active species of the methoxycarbonylation is considered to be a Pd(II) hydride species, which is formed for the benchmark complex 58-(OTf)2 upon addition of methanol to the [(P^P)Pd(OTf)2] complexes within several minutes at room temperature.[18,20,22,63] In order to further elucidate whether the differences in catalyst productivity and selectivity are due to a slower and incomplete conversion of the catalyst precursors to the catalytic hydride species, the formation of hydride species for each complex upon addition of methanol was studied.
Complexes 51-(OTf)2-57-(OTf)2 and 58-(OTf)2 were dissolved in a mixture of CD2Cl2/MeOH (2/3, v/v) and 1H and 31P{1H} NMR spectra were recorded. Complete conversion of 51-(OTf)2, 54-(OTf)2, 57-(OTf)2 and 58-(OTf)2 is evidenced by 31P{1H} NMR spectroscopy. Evidence for the formation of hydride species arises from the appearance of 1H resonances at around -10 ppm with the typical coupling patterns of diphosphine Pd(II) hydride species. Figure 16 exemplarily illustrates the formation of the hydride 54-(H)(MeOH) from 54-(OTf)2 in CD2Cl2/MeOH solution.
Figure 16. 31P{1H} NMR spectrum of the in-situ formation of the Pd(II) hydride species 54-(H)(MeOH) in CD2Cl2/MeOH (2/3,v/v) at 25 °C. Insert: 1H (Pd-H) resonance.
In the 1H NMR spectrum a hydride signal at -9.53 ppm (dd with 2J(P,H)trans = 187.6 Hz and
2J(P,H)cis = 28.7 Hz) is observed. The 31P{1H} NMR spectrum shows two doublets at 34.2 and 82.3 ppm for the two inequivalent phosphorus atoms with 2J(P,P)cis = 24.2 Hz (Table 7). Notably, only a single isomer of the hydride was observed in methanol solution. By 2D 1H, 31P NMR correlation spectroscopy the signal at 34.2 ppm could be assigned to the aryl phosphorus atom (P1), whereas the signal at 82.3 ppm displayed a 2J(P,H) coupling to the benzyl protons and was therefore identified as the benzyl phosphorus atom (P2).
The hydride species generated from 54-(OTf)2 and 57-(OTf)2 are remarkably stable representatives of Pd(II) hydride complexes with a chelating diphosphine ligand and a weakly coordinated solvent molecule. For instance, these hydrides can be stored in methanol at room temperature for 4-5 days before notable decomposition occurs. Hydride species generated from 51-(OTf)2 (in-situ NMR experiments in CD2Cl2 / MeOH, 3:2, v/v) at room temperature showed a slow conversion into a
bridged binuclear hydride species [(P^P)Pd(-H)-(-L)Pd(P^P)]+OTf- 61 (with P^P = 29) within 3 days. Characteristic NMR resonances of such a bridged complex are: 1H NMR (400 MHz, CD2Cl2): -7.67 (qi), 2J(P,H) = 55.05 Hz for -H; 31P{1H} NMR (162 MHz, CD2Cl2): 65.0, P2(benzyl), 63.4, P1(aryl) d, each 2J(P,P) = 34.7 Hz.
Table 7. 31P{1H} NMR data for Pd(II) hydride complexes 51-(H)(MeOH), 54-(H)(MeOH) and 57-(H)(MeOH) in CD2Cl2/MeOH at 25 °C.
Entry Pd(II)hydride complex δ P2 / ppm δ P1 / ppm 2J(P2-P1) / Hz
1 51-(H)(MeOH) (tBu-tBu) 83.8 34.6 24.5
2 54-(H)(MeOH) (Ad-tBu) 82.3 34.2 24.2
3 57-(H)(MeOH) (Ad-Ad) 82.1 33.0 24.3
Using methanol-d4 as a solvent the respective deuteride complex 54-(D)(CD3OD) is formed (Figure 17). The coupling patterns in the 31P NMR showed the P2 being cis (dt, 2J(P,P)cis = 24.2 Hz and 2J(P,D)cis = 4.5 Hz) and the P1 being trans (dt, 2J(P,P)cis = 24.2 Hz, 2J(P,D)trans = 29.0 Hz) to the deuteride. Similar observations were made with complexes 51-(OTf)2, 57-(OTf)2 and 58-(OTf)2.
Figure 17. 31P{1H} NMR spectrum of in-situ formation of the Pd(II) hydride species 54-(D)(CD3OD) in methanol-d4 at 25 °C.
By contrast, addition of methanol to complexes 52-(OTf)2, 53-(OTf)2 and 55-(OTf)2 did not result in complete conversion of the triflate complexes to a single hydride species (52-(H)(MeOH), 53-(H)(MeOH), 55-(H)(MeOH)). In case of 55-(OTf)2 rather up to 70 % of the unreacted triflate complex was still detected. The observation of several 1H NMR resonances around -5 to -10 ppm with different coupling patterns for all three complexes suggests the formation of various different hydride species. For complexes 52-(H)(MeOH) and 53-(H)(MeOH) a quintet was observed among others, which could also indicate the formation of a binuclear hydride species.[22] No formation of any hydride species was observed for complex 56-(OTf)2 under these conditions.
For this reason, a different pathway for the synthesis of the hydride complex 56-(H)(PtBu3) from complex 56-(OTf)2 was chosen (Scheme 27). From work of Rünzi et al. it is known, that a combination of [Pd(0)(dba)2] and zwitterionic, P-protonated phosphine benzenesulfonic acid [(k2 -P,O)-(o-MeOC6H4)2P(C6H4SO3)] generates an active hydride complex for copolymerization of
ethylene and acrylates.[64] It is likely that the Pd(II) hydride species is formed upon oxidative addition of the protonated phosphine to Pd(0).
Dissolving ligand 34 in diethyl ether and subsequent addition of 1 equiv. of trifluoromethane-sulfonic acid led to the formation of the protonated ligand 62 as white precipitate, which was isolated via centrifugation. The protonation takes place at the benzyl phosphorus atom (P2), which reflects the higher basicity of benzyl-P2 versus Aryl-P1. For P2 a doublet arise (31P NMR (162 MHz, CD2Cl2): 32.8 ppm) due to the P-H (1J(P,H) = 455.3 Hz) coupling and was identified in the NMR spectra. The P-H proton could also be assigned to the broad doublet in the 1H NMR at 6.28 ppm with a coupling of 1J(P,H) = 455.3 Hz.
Scheme 27. Generation of the active hydride species from complex 56-(OTf)2 according to [64].
The isolated protonated ligand 62 was dissolved in benzene and 1 equiv. of Pd(P(tBu)3)2 was added.
The obtained mixture was stirred for 3 hours at 60 °C, evaporated under reduced pressure, the resulting red powder was washed with cold diethyl ether and centrifuged. Analysis of the obtained red solid by NMR indeed shows signals for cis and trans coordinated hydride complexes 56-(H)(PtBu3) in a ratio of 10 to 1(cis to trans) in the typical region of -7 ppm, even though in less than 5 % yield as evidenced by in-situ 1H NMR. This hydride complex is not stable at room temperature and decomposes rapidly. Increased reaction times at 60°C did not lead to the desired
enhancement of the conversion of 62 to the hydride complex 56-(H)(PtBu3). It can be suggested that the low catalyst productivity and selectivity of complex 56-(OTf)2 has its origin in the formation of hardly any catalytically active hydride species in methanol. Even the hydride species 56-(H)(PtBu3) being stabilized with PtBu3 is only generated up to 5 % and extremely instable.
Nevertheless, in principle the formation of a hydride complex via oxidative re-addition of a Pd(0) species with a protonated diphosphine ligand succeeds. A common decomposition pathway (Scheme 28) of phosphine based transition metal catalysts is the formation of protonated ligands as well as Pd black by reductive elimination leading to a diminished catalyst activity, responsible for a decreased methyl oleate consumption.[65] It is desirable to overcome the problem of catalyst deactivation. In this regard, the oxidative re-addition of the protonated ligand to Pd(0) is obviously a viable strategy to re-generate the active hydride species.
Scheme 28. Decomposition pathway of the hydride species according to [65].
Therefore, additional experiments were undertaken to investigate the general possibility of catalyst re-activiation with a soluble Pd(0) precursor and monoprotonated diphosphine ligands 63 (tButBu), 64 (tBuCy) as well as 65 (AdtBu) (Scheme 29).
By mixing the monoprotonated ligands 63, 64 and 65 as well as Pd(dba)2 (1:1) in a CD2Cl2 / MeOH (3:2) mixture re-formation of the MeOH coordinated hydride complexes is observed (Scheme 29).
Although oxidative re-addition of the protonated ligands did not result in complete conversion to the hydride complexes, but, as a proof of principle, these experiments clearly show that the a re-activation of a soluble molecular Pd(0) source to the catalytically active hydride complex in the reaction mixture is kinetically feasible. Consequently, it can be suggested that further addition of protonated phosphine ligands to the methoxycarbonylation experiment might enhance the conversion towards the diesters due to oxidative re-addition of protonated phosphines to precipitated Pd black by re-formation of the catalytically active hydride complex. This might be the basis for future investigations towards the Pd black re-activation during the reactor experiments.
Scheme 29. Re-generation of MeOH coordinated hydride complexes starting from mono-protonated ligands and Pd(dba)2.
All these experiments demonstrate that it is reasonable to assume that sterically more demanding substituents (Ad and tert-butyl groups, complexes 51-(OTf)2, 54-(OTf)2 and 57-(OTf)2) at phosphorus not only lead to a better generation of the corresponding hydride complexes, but also stabilize the hydride complex. Thus, complete conversion of complexes 51-(OTf)2, 54-(OTf)2 and 57-(OTf)2 into the corresponding MeOH coordinated hydride species occurs, which results (under given conditions) in an almost complete conversion of methyl oleate to the diesters. Hence, productivity goes along with ease of entering the catalytic cycle by the hydride complex formation.
The selectivity towards the formation of -diester seems better for those complexes with at least one adamantyl group attached to phosphorus (54-(OTf)2, 55-(OTf)2, 57-(OTf)2). Probably this is due to stablility reasons of the hydride complexes as induced by the adamantyl rest.
In order to obtain some additional structural information of the Pd(II) hydride complexes, they were isolated by using PPh3 as a strongly coordinating ligand, because the MeOH complexes could not be isolated for stability reasons (Scheme 30). Thus, addition of PPh3 to complexes
51-57-(OTf)2 and 58-(OTf)2 in methanol leads to formation of the corresponding [(P^P)Pd(II)(H)PPh3] species (51 to 57-(H)(PPh3)).
Scheme 30. Formation of the PPh3 stabilized cis-and trans-hydride complexes 51-57-(H)(PPh3).
Notably, a mixture of cis- and trans-isomers of 51-57-(H)(PPh3) in different ratios was observed from the corrresponding solvent coordinated species 51-57-(H)(MeOH) after addition of one equiv. of PPh3. For complexes 51-(OTf)2, 54-(OTf)2, 57-(OTf)2 and 58-(OTf)2 virtually full conversion to the PPh3 coordinated hydride complexes was observed, whereas in the case of 52-(OTf)2, 53-(OTf)2 and 55-(OTf)2 incomplete formation of the hydride complexes was detected by NMR. Signals in the NMR spectra could be assigned to the starting triflate complexes. Addition of a second equiv. PPh3 to 52-(OTf)2, 53-(OTf)2 and 55-(OTf)2 in methanol shifts the equilibrium to the corresponding hydride complexes and increasing amount of PPh3 shifts the equilibrium even more to the desired hydride complexes in 70-95 % yield (Table 8).
For hydride complexes 52-(H)(PPh3), 53-(H)(PPh3) and 55-(H)(PPh3), in which a bulky P2tBu2
or P2Ad2 moiety as well as less steric bulky P1Cy2 and P1iPr2 groups are present, the cis-isomer (PPh3 is cis situated to the less steric bulky P1 moiety) clearly predominates over the trans-isomer (Table 8, entry 2, 3 and 5). For complexes 54-(H)(PPh3) and 57-(H)(PPh3) with comparable bulky P1 and P2 moieties (tBu and Ad) also a preference for the cis coordinated hydride complex is observed although in a smaller ratio (Table 8, entry 4 and 6). In-situ time dependent experiments demonstrate that the isomer ratios of complexes 52-(H)(PPh3), 53-(H)(PPh3), 55-(H)(PPh3) and 57-(H)(PPh3) do not change in time and therefore the kinetic cis/trans ratios obtained after 5
minutes are identical with the thermodynamic cis/trans ratios (Table 8, entry 2, 3, 5 and 6).
However, for complex 51-(H)(PPh3) a ratio of 3:2 cis- to trans-isomers is obtained after 15 minutes, which indicates the kinetical preference for the cis hydride complex. After 24 hours the ratio has changed and the thermodynamically favored mixture cis/trans in a ratio of 1:1 is detected, which remains stable for at least 7 days (Table 8, entry 1).
Table 8. Conversion and thermodynamic cis/trans ratios of the hydride complexes.
Entry PPh3-hydride complex Conversion [%] cis : trans
A closer look into the time dependency of the cis/trans ratio of complex 54-(H)(PPh3) in solution shows a kinetical preferred cis/trans-isomer mixture of 1:3 after several minutes. A shift from the kinetical to the thermodynamical isomer preference to a cis to trans ratio of 7:3 (Table 8, entry 4) could be detected within 24 hours. Table 8 shows the thermodynamically favored isomeric cis/trans ratios of all hydride complexes 51-(H)(PPh3)-58-(H)(PPh3).
A typical 31P{1H} NMR spectrum is shown in Figure 18. All signals could be assigned to Pd(II) hydride complexes.
Figure 18. 31P{1H} NMR spectrum of a mixture of 54a-(H)(PPh3) and 54b-(H)(PPh3) formed by dissolution of 54-(OTf)2 and 1 equiv. of PPh3 in CD2Cl2/MeOH (2/3, v/v) at 25 °C. Insert: Pd-H resonances.
The hydride complexes 51-(H)(PPh3) to 58-(H)(PPh3) were isolated by extraction of the methanol solution with pentane, which resulted in precipitation of the desired complexes. Crystals suitable for X-ray diffraction were obtained for 51b-(H)(PPh3), 54b-(H)(PPh3), 55a-(H)(PPh3) and 58-(H)(PPh3)[a] by layering a methylene chloride solution of the respective compound with pentane at room temperature (Figure 19). Bond lengths (d(H-Pd) = 1.51(5) Å - 1.54(3) Å) and angles (Table 9) of 51b-(H)(PPh3), 54b-(H)(PPh3), 55a-(H)(PPh3) and 58-(H)(PPh3) are in the typical range for such complexes.
a Crystals suitable for X-ray diffraction were kindly provided by Dr. Etienne Grau
Table 9. Selected bond lengths [Å] and angles [°] for 51b-(H)(PPh3), 54b-(H)(PPh3),
1) The twist angles were measured between two calculated planes P1-Pd-P2 and H-Pd-PPh3
In 51b-(H)(PPh3) and 54b-(H)(PPh3) the palladium-bound hydrogen is oriented trans with respect to P2 while in 55a-(H)(PPh3) it is situated cis. The different stereochemistry most probably reflects the minimization of steric hindrance between the PPh3 ligand and the different steric demands of the R2P1 and R’2P2CH2 donors. With R = R’ = tBu (51b-(H)(PPh3)) the PPh3 ligand is oriented cis with respect to the more flexible tBu2P1CH2 substituent. The same applies for 54b-(H)(PPh3) (tBu2P1 versus Ad2P2CH2) but even a simple inspection of the molecular structure (Figure 19) reveals noticeable distortions arising from the larger steric bulk of the adamantyl substituents. The tilt of the two molecular planes P1-Pd-P2 and P3-Pd-H in 54b-(H)(PPh3) with respect to each other (13.0°) is more substantial as compared to complex 51b-(H)(PPh3) (7.5°). Finally, in 55a-(H)(PPh3) the ligand PPh3 prefers a cis orientation with respect to the (less bulky) Cy2P1 group rather than the Ad2P2CH2 moiety, again with severe distortions reflecting the large steric bulk of the groups involved (tilt between the molecular planes P1-Pd-P2 and H-Pd-PPh3: 11.0°). In 58-(H)(PPh3) the Pd-H bond length is slightly longer (1.59(2) Å) as compared to 51b-(H)(PPh3), 54b-(H)(PPh3) and 55a-(H)(PPh3).
Figure 19. Molecular structures of complexes 51b-(H)(PPh3), 54b-(H)(PPh3), 55a-(H)(PPh3) and 58-(H)(PPh3) in the solid state. Hydrogen atoms (except for Pd-H) and triflate counter ions were omitted for clarity. Pd-H was located in the electron density map. Displacement ellipsoids are shown at the 50 % probability level.
Upon crystallization of 51b-(H)(PPh3), which led to the molecular structure shown in Scheme 32, crystals of a second species 51-(CM)(PPh3) (CM: cyclometallated) are formed. Its X-ray structure determination (Scheme 32) revealed the formation of a four-membered pallada-phospha-heterocycle possibly formed by C-H activation[66] of the methyl group of a tBu substituent at P1 by
the adjacent palladium center with formation of a Pd-C bond (2.120(2) Å), and net expulsion of molecular hydrogen. The preference of a tBu substituent at P1 for C-H activation over that of a tBu2P2CH2 group might again reflect the larger steric rigidity of the R2P1 moiety as compared to the more flexible R2P2CH2 backbone. Probably more important, however, is the cis orientation, and thus closer special proximity, of the Pd-bound hydrogen atom in 51-(CM)(PPh3) with respect to the activated methyl group of the tBu substituent at P1 (d(Pd-C15) = 2.120(2) Å). This suggests the direct involvement of the Pd-bound hydrogen atom in the C-H activation process.
Similar examples in literature describe the formation of six- as well as five-membered palladaycles via intramolecular C-H activation from square-planar N-alkyl -diimine Pd(II) and Ni (II) complexes.[66] In analogy to this studies, the formation of 51-(CM)(PPh3) could take the following form (Scheme 31): Intramolecular C-H activation occurs through initial formation of an agostic interaction between the metal center and the -C-H bond of the tert-butyl groups attached to aryl-phosphorus atom. This mechanism requires displacement of PPh3 (i) in order to open up a free coordination site for an agostic interaction (ii) resulting in the formation of palladaycle 51-(CM)(PPh3) and the loss of molecular hydrogen (iii).[66]
Scheme 31. Possible mechanism leading to 51-(CM)(PPh3).
As complex 51-(CM)(PPh3) exhibits a four-membered palladacycle, which is unusual due to the ring strain, it would be interesting to investigate the complex stability as well as its catalytic
properties in the isomerizing alkoxycarbonylation of methyl oleate. Many five- and six-membered palladacycles are important intermediates during the catalytic processes, for example in many cross coupling reactions.[67] On the other hand, the formation of a pallada-phospha-heterocycle might also be a deactivation process of the catalyst occurring during the catalytic reaction under pressure reactor conditions, responsible for catalyst decay.[65a]
However, all attempts to selectively synthesize and isolate the pallada-phospha-heterocycle 51-(CM)(PPh3) failed so far, as the C-H activation process followed by reductive elimination could not be forced via the synthetic routes lined out in Scheme 32 (vide infra).
The first approach towards the palladacycle was direct heating (12 hours) of an in-situ formed PPh3-hydride complex 51-(H)(PPh3) in methanol and ethanol solution (i), respectively. The PPh3
coordinated hydride complexes (in methanol and ethanol) were stable under these conditions and did not undergo the desired C-H activation. Heating of the in-situ generated MeOH hydride complex (ii) neither generates the desired pallada-phospha-heterocycle nor reacts to protonated ligand PH+ and Pd black via reductive elimination. Mainly one palladium species is formed, which possibly can be assigned to a Pd-methoxy 51-(OMe)2 or di-MeOH coordinated Pd complexes 51-(MeOH)2. Two sets of doublets appeared in the 31P{1H} NMR: 83.4 (d, 2J(P,P) = 37.8 Hz, P(benzyl)), 42.3 (d, 2J(P,P) = 38.9 Hz, P(aryl)), which show couplings to the methanol molecule.
However, it could be not clearly confirmed from 1H and 31P{1H} NMR studies if a Pd-methoxy complex 51-(OMe)2 is formed, because no H2 signal could be detected in the 1H NMR spectrum.
Scheme 32. Synthetic attempts towards palladacycle 51-(CM)(PPh3) and molecular structures of 51-(CM)(PPh3) and 51-(dbaH+) in the solid state. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms (exept O-H), CO-H was located in the electron density map. Non coordinating triflate counterions, and interstitial solvent molecules have been omitted for clarity.
Another approach towards the cyclometallated complex would be the activation process, in what the zerovalent Pd-dba complex 44 reacts with one equiv. of trifluoromethanesulfonic acid to give probably the catalytically active hydride complex by an intramolecular transfer of the proton to the metal center followed by CH-activation at high temperature (Scheme 32, iii). We have not been able to find evidence for this in the protonation of complex 44. Formation of the palladacycle
51-(CM)(PPh3) from the dba-complex 44 (iii) requires not only the co-presence of an acid, a primary or secondary alcohol, but also an oxidant (BQ) in order to generate first the Pd(II) hydride species.[63a,68] The addition of triflic acid to the dba complex 44 and subsequent heating without any oxidant results in the protonation of the dba-oxygen atom. A mixture of two possible conformers of the stable complex 51-(dbaH+) was assigned by four sets of doublets in the
31P{1H} NMR spectrum (162 MHz, C6D6, 300 K): δ 68.7 (d, J(P,P) = 61.7 Hz), 60.1 (d, J(P,P) = 54.0 Hz), 53.3 (d, J(P,P) = 61.7 Hz), 46.9 (d, J(P,P) = 54.0 Hz). Single crystals suitable for X-ray structure determination were obtained for the minor s-cis,trans-isomer (Scheme 32, molecular structure of 51-(dbaH+)).
3.2.5.1 Mechanism of the Hydride Formation
Pd(II) hydride complexes are of special interest due to their exceptional relevance to the alkoxycarbonylation as catalytically active species. After previous investigations regarding the synthesis and stability of the hydride complexes, we focused our attention on the mechanism of the hydride complex formation. It is rationalized[63a] that the mechanism follows a -hydride elimination from an alcohol molecule (methanol or ethanol) coordinated to the metal resulting in the hydride species and one equivalent of an aldehyde. Oxidation of methanol and ethanol leads to the formation of formaldehyde 66 or acetaldehyde 67, respectively (Scheme 33).
Scheme 33. Proposed mechanism of the hydride complex formation.
Faced with the fact that the catalytically active hydride complex is quite reactive and therefore difficult to isolate and given the problems associated with direct “in-situ” observation of intermediates leading to the hydride complex at realistic concentrations and conditions of pressure
and temperature, evidence for the particular mechanistic pathway to the hydride species relies on indirect methods.[69] During the in-situ synthesis of the PPh3 coordinated hydride complexes in methanol we have found a phosphine decomposition side reaction (Scheme 34). Analizing the crude reaction mixture gave rise for the generation of phosphonium by-product 68. It can be assumed that nucleophilic attack of PPh3 at the formed formaldehyde 66 (never detected per se) resulted in the formation of the phosphonium salt 68, which provides evidence for solvent (methanol) involved hyride complex generation. In addition, the PPh3 coordinated hydride complex 51-(H)(PPh3) was generated in ethanol and in this case the by-product 69 was identified in the crude NMR spectrum, obtained from the reaction of PPh3 and acetaldehyde, which in turn is formed by the oxidation of a coordinated ethanol molecule.
Scheme 34. Formation of by-products 68, 69 and 70 of the eliminated aldehyde.
In order to proof our hypothesis regarding the formation of the by-products (68, 69) both phosphonium salts were synthesized separately and isolated by reaction of one equiv. of PPh3 with formaldehyde and acetaldehyde, respectively in the presence of one equiv. of trifluoromethane-sulfonic acid. Note that compound 69 is formed as a racemate. The NMR data from the crude reaction mixture and the isolated compounds ( Exp. Section, 1H/13C NMR; 1H, 13C-HSQC; 1H, 13 C-HMBC) match together and confirm the possibility that the phosphonium salts are formed by nucleophilic attack at the generated aldehydes.
Further side reactions derived from the generated formaldehyde include the formation of methyl formate 70, as is evident in the crude NMR spectrum (1H NMR (400 MHz, CD2Cl2): δ 8.00 (s, 1H), 3.65 (s, 3H) and 13C{1H} NMR (101 MHz, CD2Cl2): δ 162.7 (s, HC(O)OMe), 51.0 (s, HC(O)OMe) of the in-situ generated MeOH coordinated hydride complex 51-(H)(MeOH) in a MeOH/CD2Cl2
mixture. Methyl formate 70 could be possibly generated through acid catalyzed esterfication of formaldehyde in methanol.[70] If a small amount of formaldehyde is added to the mixture of the in-situ generated MeOH coordinated hydride complex 51-(H)(MeOH) the signals for the methyl formate arose.
These experimental findings underline the common perception that the hydride formation occurs via irreversible -hydride elimination from coordinated methanol (or another primary or secondary alcohol), the alcohol serving as a hydride donor. As a consequence, the choice of solvent is an important issue for the hydride complex formation.
3.2.6 Methoxycarbonylation of Methyl Oleate with the Pd(II) Hydride Species as a Catalyst Precursor
We further wanted to probe if the isolated PPh3 coordinated hydride complexes can be used as model compounds for the highly instable solvent coordinated hydride species. Therefore the possible catalytic activity of complexes 51-(H)(PPh3) and 58-(H)(PPh3) as a catalyst precursor was investigated (Table 10). Due to the steric demand and at the same time strong coordination of the PPh3 ligand it is reasonable to assume that the MeOH coordinated hydride complexes are more active and thus lead to better productivities in the catalytic reaction. Similar reaction conditions as for the methoxycarbonylations with 51-(OTf)2 and 58-(OTf)2 were employed (20 bar CO, ratio (MO/Pd) = 125, 10 mL of methanol, 90 °C, 120 hours).
However, it was found that the catalytic performances of the isolated hydride species 51-(H)(PPh3) and 58-(H)(PPh3) are virtually identical to those of the respective triflate complexes 51-(OTf)2
and 58-(OTf)2 (Table 10).
Table 10. Comparison of the isomerizing methoxycarbonylation of methyl oleate with complexes
This result is rather unexpected but still advantageous, because PPh3 does not seem to decrease the activity of the complexes significantly. The similar catalytic activities and productivities indicate
This result is rather unexpected but still advantageous, because PPh3 does not seem to decrease the activity of the complexes significantly. The similar catalytic activities and productivities indicate