The addition of β-ketoester 8 or allylenol carbonate 6 resulted in complete disappearance of the resonance at δ = 18.8 ppm and produced a single long-lived resonance at δ = 30.9 ppm (Spectra 1.B and 2.B, in figure 2.1). The allylic alkylation reaction of silyl enol ether 4 by 31P NMR had many similarities to the NMR reactions of allyl β-ketoester 8 and allyl enol carbonate 6. The resonance at δ = 31.3 ppm remained even after the reaction was complete, as determined by the disappearance of silyl enol ether 4 via TLC (Spectrum 1.E, in figure 2.2).5.
One of the most notable differences between the silyl enol ether and allyl enol carbonate and allyl β-ketoester reactions is that the catalyst does not return to the species responsible for the resonance at δ = 18.8 ppm when the reaction is complete. 1.E and 2.E were obtained 30 min after the silyl enol ether 4 could no longer be observed by TLC for the reactions initiated in 1.D and 2.D, respectively. Integration with respect to the free PHOX ligand at δ = -5.95 ppm suggests that the sum of the species represented at δ and 31.3 ppm for the silyl enol ether variant and δ = 18.8 and 30.9 ppm for the allyl carbonate enol and allyl β-ketoester variants are constant throughout the course of their respective reactions.
This species was identified as the major intermediate corresponding to the 31 P NMR resonance at δ = 30.9 ppm (Figure 2.1 on page 25). This still leaves questions about the structures of the equivalent intermediates seen by 31 P NMR at δ = 30.9 ppm for the decarboxylative allylic alkylation of allylenol carbonate 6 and at 29.4 and 31.3 ppm for silylenol ether 4.
Role and Significance of the Intermediates
Although the similarity of these intermediates to palladium allyl carboxylate 31 suggests that they may be palladium carbonates 33 and 34 (Figure 2.3 left, on page 34), the DFT calculation predicts a tetragonal pyramidal structure with a η3 allyl ligand and an apically coordinated carbonate ligand (carbonates 35 and 36 in Figure 2.3 right).1a Similar neutral palladium η3 allyl complexes with tetragonal pyramidal structures are known in the literature.22. To investigate the role of carboxylate 31 in the catalytic cycle, it was subjected to reaction-like conditions in THF at 24°C in the presence of free dba ligand, and the reaction was monitored by 31 P NMR. During this time, the resonance at δ = 18.8 ppm appeared in the 31 P NMR observed at the end of the catalytic reaction and after 40 minutes became the only 31 P resonance observable.
This reaction demonstrates that complex 30 is regenerated at the end of an allylic alkylation reaction, making it the resting state of the catalyst in the absence of allyl electrophiles. In the first run only 30 were isolated, and in the second run only the yield and ee of 9 were determined. To determine whether carboxylate 31 was part of the catalytic cycle or merely an unproductive resting state, its decomposition was monitored in the absence of the dba ligand, both in THF and neat.
In THF solution at 24 °C in the absence of dba allyl carboxylate 31 decomposes to form an increasingly complex mixture of phosphorus-containing species by 31P NMR, none of which can be observed during the course of the catalytic reaction. Under these conditions, the allylic alkylation product 9 appears to be the only metal- or phosphine-free product formed in the reaction, and 9 can be isolated in 85% ee from solution (Scheme 2.8 on page 37).
Conclusions
Experimental Procedures
The solution was mixed by hand with a pipette until all the material was dissolved, forming a clear colorless solution. The solution was mixed manually by pipetting for just under 1 min until all material was dissolved. The solution was allowed to stand and stirred by diffusion for 30 min during which time it turned an orange color.
The solution was stirred manually with a pipette for 1 min until all solids were dissolved. The solution was mixed manually with a pipette for 1 min until all material was dissolved. The NMR tube was allowed to stand for 10 hours at 24°C, during which time the solution turned orange.
The solution was then concentrated under a stream of nitrogen to a small amount of yellow oil. The solution was stirred for approx. 5 minutes until it turned a dark purple-brown color. The solution was vortexed manually by hand for 1 min and then allowed to stand and mix by diffusion for 1.5 h.
After cooling, the solution was moved to a pre-cooled aluminum block at -36°C in the glove compartment. The solution was then diluted with hexanes (12 mL) that had been pre-cooled to -20 °C and then the mixture was concentrated in vacuo in a -20 °C glovebox freezer for ca. 10 ml. The solution was mixed manually by pipette for two minutes and then allowed to stand and mix by diffusion for 20 minutes.
The solution was filtered through a pipette with glass filter paper into five separate 1 dram vials evenly (1.6 ml each). The solution was then carefully layered with hexanes (18 mL) and allyl acetate (200 µL) and left to crystallize via layer diffusion in a −20 °C glovebox freezer, resulting in The solution was left to stir for 8 h, giving a dark orange color with a small amount of black precipitate.
C revealed intermediate 31 as the only detectable phosphorus-containing compound. 3 The tube was left at 24 °C for 40 min, after which a 31P NMR spectrum indicated complete conversion to precatalyst complex 30 as the only detectable phosphorus-containing compound. The solution was mixed manually via pipette for approx. 10 s until all material was dissolved, resulting in the formation of a yellow solution. The solution was transferred via pipette to the half-dram vial containing intermediate 31, where it was mixed manually via pipette for 10 s until all the material was dissolved, resulting in the formation of a yellow-green solution.
This is consistent with both resonances δ = 29.4 and 31.3 ppm being a form of palladium carbonate complex, and strongly suggests that neither resonance corresponds to a palladium enolate complex like 27. 6) The decarboxylative asymmetric allylic alkylation reaction is very sensitive to oxygen and many NMR experiments show some oxidative degradation at their end despite precautions to exclude air.
