Pd-Catalyzed Asymmetric β-Hydride Elimination En Route to Chiral Allenes

Abstract

We wish to report our preliminary results on the discovery and development of a catalytic, asymmetric β-hydride elimination from vinyl Pd(II)-complexes derived from enol triflates to access chiral allenes. To achieve this, we developed a class of chiral phosphite ligands that demonstrate high enantioselectivity, allow access of either allene enantiomer, and are readily synthesized. The methodology is demonstrated on over 20 substrates and application to the formal asymmetric total synthesis of the natural product, (+)-epibatidine, is also provided.

Organic chemists are on a perpetual path of discovery to identify new methods for asymmetric synthesis. Historically, much of this effort has focused on the generation and control of point chirality (i.e. stereogenic centers) and as a result, approaches to engender axial chirality have not been as forthcoming. A perfect example of this unbalanced dichotomy is in the synthesis of chiral allenes. While allenes continue to be exploited as substrates in a multitude of synthetic methodologies,¹ as chiral ligands in asymmetric catalysis² and exist as structural motifs in ~150 natural products (usually in enantioenriched form),³ robust methods to synthesize chiral allenes directly from prochiral substrates with high stereochemical fidelity are few and far between.^4–9

Our own interest in this area was sparked by our ability to exploit stereodefined enol triflates as pluripotent substrates in organic synthesis.^10,11 In previous work, we reported the catalytic elimination/isomerization of enol triflates to 1,3-dienes using Pd(P(tBu)₃)₂ as the ideal catalyst.¹² A viable mechanism for this reaction involves an initial β-hydride elimination from a cationic vinyl Pd(II)-complex generating the corresponding allene intermediate that subsequently isomerizes to the 1,3-diene product (Scheme 1). We believed that it might be possible to interrupt this catalytic cycle under attenuated reaction conditions to allow the allene to be isolated as the terminal product in these reactions.

Scheme 1.

Pd-Catalyzed Pathways to 1,3-Dienes or Allenes from Enol Triflates.

More importantly, we envisioned that this pathway might lead to a viable approach to access enantioenriched allenes using chiral ligands associated with the Pd-catalyst via an asymmetric β-hydride elimination; a reaction that has no precedence in the literature as far as we are aware.¹³

Our initial experiments began with (E)-enol triflate 1 as our model substrate and Pd₂dba₃ as our preferred source of palladium (eq. 1). A preliminary survey of ~60 commercially available chiral phosphorous-based ligands to promote the asymmetric β-hydride elimination en route to chiral allene 2 provided only partial success with respect to enantioselectivity. Fortuitously, however, we were able to determine that several of these ligands were in fact able to induce moderate enantioselectively initially but competing racemization over the course of the reaction was responsible for the observed low enantioselectivity at higher conversions.¹⁴ Upon closer inspection, a general trend was noticed that the rate of racemization was slower with phosphite-based ligands than phosphine-based ligands. Thus, we turned our attention to the design of our own chiral phosphite ligands. After an extensive screen of ~50 novel phosphite ligands, we found ligands based on the BINOL scaffold in combination with menthol-derived alcohols proved most effective. A highly condensed summary of our optimization studies with a selected set of these ligands is provided in Table 1.¹⁵

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Table 1.

Selected Optimization Data for the Pd-Catalyzed Asymmetric β-Hydride Elimination of Enol Triflates to Chiral Allenes.

entry	R1	R2	mol % Pd2dba3 ligand	ee of 2a	yield of 2a
1	H		6%	25%	69%b,d,f
			12%
2	Ph	“	3%	54%	69%b,d,f
			6%
3		“	3%	63%	74%b,d,f
			6%
4		“	3%	65%	31%b,d,f
			6%	76%	59%c,d,f
5	“		5%		c,e,g
			10%
6	“		5%	h	c,e,g
			10%

^aEnantiomeric excesses (ee) and yields reported were determined by quantitative chiral HPLC analysis.

^bReaction performed in THF.

^cReaction performed in i-PrOAc.

^dReaction performed at 35 °C

^eReaction performed at r.t.

^f2 equiv of base.

^g4 equiv of base.

^hReaction performed using ligand based on (S)-BINOL instead of (R)-BINOL yielding the opposite enantiomer of 2.

With two chiral phosphite ligands capable of providing either allene enantiomer in high excess (Table 1, entries 5 and 6), we turned our attention to the robust synthesis of these ligands that would facilitate their utility in asymmetric β-hydride eliminations. Their respective syntheses are outlined in Scheme 2. Both routes developed are easily scalable providing multi-gram quantities of 4 and 5 in 80% and 23% overall yield (respectively) from commercially available starting materials.

Scheme 2.

Synthesis of Ligands 4 and 5.

The applicability of this methodology to a broad range of disubstituted (E)-enol triflates is shown in Table 2. The preferred reaction conditions include using isopropyl acetate as the solvent (0.2 M), 5 mol% Pd₂dba₃, 10 mol% chiral ligand, Hünig’s base (4 eq), and performing the reaction at room temperature. Under these conditions, the catalytic system tolerates multiple functionalities providing good yields of the corresponding chiral allenes in moderate to excellent enantioselectivities. Absolute configuration assignments in Table 2 are based on the observed optical rotation and application of the rules developed by both Brewster and Lowe for chiral allenes.¹⁶

Table 2.

Preliminary Scope of the Pd-Catalyzed, Asymmetric β-Hydride Elimination En Route to Chiral Allenes.^a,^b

^aReported are isolated yields with enantiomeric excess (ee) or diastereomeric excess (de) determined by chiral HPLC analysis.

^bAbsolute configuration assignments are based on the observed optical rotation and application of the rules developed by both Brewster and Lowe for chiral allenes (see ref. 16).

^cReaction performed with the enantiomer of ligand 4 provided (+)-6 in equal yield and ee (87% yield, 95% ee).

During the course of these studies we did observe differences with respect to substrate specificity between ligands 4 and 5 that we wish to highlight. In general, higher enantioselectivities are obtained using ligand 4 with enol triflates that contain bulky carbonyl groups (compare 6 and 7 for both ligands). In contrast, higher enantioselectivities are observed with ligand 5 with less hindered carbonyl groups (Table 1, entries 5 vs. 6). We cannot yet formulate a stereochemical model for these reactions that would rationalize the apparent substrate preferences between the two ligands. Nonetheless, we believe the identification of two highly selective, complementary ligands is an advantage that broadens the scope of viable substrates at this early stage of development.

There are several limitations of this methodology that we would like to disclose (Scheme 3). First, the corresponding (Z)-enol triflates do not participate in this reaction. Based on our previous work, we believe this lack of reactivity can be attributed to the internal chelation of the carbonyl group after stereospecific oxidative addition that attenuates the agostic interaction with the β-hydrogens in these cationic Pd(II)-complexes.¹² Even the addition of Lewis acids (i.e., TMSOTf) did not recover reactivity of (Z)-enol triflates as had been observed previously.¹² Second, fully substituted (E)-enol triflates such as 28 that would provide trisubstituted chiral allenes are poor substrates for this methodology. In this particular example, we were only able to observe ~30% conversion to the chiral allene 29 in 20% ee. While we view these as significant limitations that warrant disclosure, we are confident that further optimization will ultimately provide a solution for these substrates.

Scheme 3.

Known Limitations of the Methodology

The application of this methodology to the formal asymmetric total synthesis of (+)-epibatidine is shown in Scheme 4.¹⁷ Originally discovered in 1976 from the skin extract of the Ecaudorian poison frog, Epipedobates tricolor, interest in epibatidine did not peak until its structure was reported in 1992 along with its exquisite biological activity as a powerful analgesic (~200× over morphine).¹⁸ It is now known that epibatidine is a highly potent agonist at multiple neuronal nicotinic acetylcholine receptor (nAChR) subtypes.¹⁹ However, the lack of receptor specificity and dose-limiting toxicity have led to significant efforts to identify analogs that overcome these limitations that still continue today.²⁰

Scheme 4.

Formal Total Synthesis of (+)-Epibatidine

Our approach to (+)-epibatidine began with enol triflate (E)-30 readily available from the corresponding acetoacetate derivative.¹⁰ Catalytic, asymmetric β-hydride elimination using ligand 5 gave(+)-(S)-19 in 75% yield and 96% ee on a 3 mmol scale. Lewis acid catalyzed Diels-Alder reaction with excess N-Boc pyr-role gave the corresponding endo-product 31 in 57% yield.²¹ Low-pressure chemoselective hydrogenation followed by ozonolysis yielded β-keto ester 32 in 65% overall yield. Subsequent decarboxylation followed by reprotection gave ketone 33 in 70% yield and 89% ee. Previously, Merck has shown the conversion of racemic 33 to both enantiomers of epibatidine that completes the formal asymmetric total synthesis.²²

In summary, we have discovered a Pd-catalyzed, asymmetric β-hydride elimination from enol triflates to access chiral allenes using two optimized phosphite ligands that provide complementary substrate scope. Extension of this methodology to the formal total synthesis of (+)-epibatidine is also described. system to additional substrates is currently underway.