Cu-Catalyzed Alkyne Hydrocarbation of

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Stereoselective Access to (E)‑1,3-Enynes through Pd/Cu-Catalyzed Alkyne Hydrocarbation of Allenes Louis Jeanne-Julien,† Guillaume Masson,† Remy Kouoi,† Anne Regazzetti,† Greǵ ory Genta-Jouve,† Vincent Gandon,*,‡,§ and Emmanuel Roulland*,† †

C-TAC, UMR 8038, CNRS-Université de Paris, Faculté de Pharmacie, 4, avenue de l’Observatoire, 75006, Paris, France Institut de Chimie Moléculaire et des Matériaux d’Orsay, CNRS UMR 8182, Université Paris-Sud, Université Paris-Saclay, Bâtiment 420, 91405 Orsay cedex, France § Laboratoire de Chimie Moléculaire (LCM), CNRS UMR 9168, Ecole Polytechnique, IP Paris, route de Saclay, 91128, Palaiseau cedex, France

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S Supporting Information *

ABSTRACT: PdII and CuI cooperate in catalyzing the alkynes hydrocarbation of allenes (AHA) giving (E)-1,3-enynes with high yields, atom economy, and high regio-/stereoselectivities. We devised new efficient conditions and expanded the substrate scope. Experimental and computational studies support a nonorthodox PdII/PdIV catalytic cycle involving an oxidative addition triggered by a stereodeterminant H+ transfer. This reaction is leveraged in a new strategy of stereoselective synthesis of 1,3-dienes.

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This three-step strategy capitalizes on the ability of AHA to stereoselectively assemble even advanced complex fragments (Step A), giving (E)-1,3-enynes derivatizable into 1,3-dienes by regio-/stereoselective hydrosulfuration (Step B),16 and subsequent Pd-nanoparticle-catalyzed methylation, a method we designed for this aim (Step C).17 Various enyne derivatizations are imaginable, offering thus a versatile and greener alternative to traditional strategies relying on olefination reactions (Wittig, HWE, etc.), and/or transition-metal catalyzed cross-couplings (Suzuki, Sonogashira, etc.).18 Various AHA protocols8 were explored on butynol (±)-1a and penta-3,4- dien-1-ol 2a19 that give enyne 3aa mimicking the synthesis of the tiacumicin B C11−C17 region. Ultimately, Grigg’s protocol8c proved highly promising, as it gave good yields, with apparently total regio-/ E-stereoselectivity, under functional-group tolerant conditions. However, reproducibility issues called for various adjustments: Pd(OAc)2 was kept as the Pd source (Table 1, entry 1); the electron-poor P(furan-2-yl)3 was replaced by the electron-rich (p-MeOC6H4)3P, CuI by CuCl, toluene by THF; and the reaction was carried out at rt instead of 110 °C. Variations around the reaction parameters gave instructive insights: A decrease in catalysts loadings resulted in a slower and less clean reaction (entry 2). Using CuI altered the yield (entry 3), while CuOAc increased the reaction rate, but with byproduct formation (entry 4). In the absence of Cu salt (Trost’s conditions),8a 3aa was formed in 44% yield because of a known concurrent reaction involving hydrido-Pd which led to

ransition-metal-catalyzed addition of a terminal alkyne to a nonpolarized π-system acceptor is a direct, atomeconomic way of forging C(sp)−C(sp2) or C(sp)−C(sp3) bonds from readily accessible reagents. The nonpolarized πsystem acceptors can be alkynes,1,2 1,3-dienes,3 norbornadienes,4 norbornenes,5 styrenes,6 cyclopropanes,7 or allenes.8,9 The hydrocarbation of allenes gives 1,3-enynes that, in addition to the fact they are present in natural products10 or in materials,11 are also potential precursors of polysubstituted 1,3-dienes.12 The stereoselective synthesis of 1,3-dienes, which are ubiquitous in natural products, remains a challenging endeavor, so the development of new strategies is of paramount importance.13 The complex structure of tiacumicin B, a naturally occurring antibiotic drug,14 inspired us to develop an innovative strategy for the construction of the (E,E)-1,3-dimethyl-butadienylidene motif it contains (Scheme 1):15 Scheme 1. Alkyne Hydrocarbation of Allene (AHA): Its Use To Access Tetrasubstituted (E,E)-Dienes and Its Mechanistic Study

Received: March 6, 2019

© XXXX American Chemical Society

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DOI: 10.1021/acs.orglett.9b00828 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 2. Scope Explorationa

Table 1. Variations from the Optimized AHA Conditions

entry

variations from the optimized reaction conditions

yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

No variation Catalytic loadings divided by two CuI instead of CuCl CuOAc instead of CuCl Without Cu salt Without Pd salt Pd(OCOCF3)2 instead of Pd(OAc)2 PdCl2(PPh3)2 instead of Pd(OAc)2/2(p-MeOC6H4)3P 1.3 equiv of 1a instead of 1 equiv 1.3 equiv of 2a instead of 1 equiv Toluene instead of THF PPh3 instead of (p-MeOC6H4)3P P(furan-2-yl)3 instead of (p-MeOC6H4)3P Pd2(dba)3 (2.5 mol %) instead of Pd(OAc)2 (5 mol %)

79 66 67 59a,b 44c 0d 0d,e 0b 100 79 25e 64 7e 38d,f,g

a

Yields of isolated products. bVarious byproducts were formed. cNo starting materials recovered. dt = 40 °C.

a

Completed in 3.5 h. bUnidentified byproducts. c29% of 4. Unreacted starting materials recovered. e17 h of reaction. f5 days of reaction. gNo reaction occurred if 1 phosphine is added per Pd instead of 2.

dienes were investigated: 3aa gave vinylsulfide 5 through OHdirected hydrosulfuration16 and then, using our Kumada− Corriu conditions,17 tetrasubstituted 1,3-diene 6 (Scheme 3). Alternatively, the reaction of 3aa with RedAl, and then with allyl bromide in the presence of CuCl,20 led to compound 7, while an H2O quench gave ubiquitous diene 8.

d

allene dimer 4 in 29% yield (i.e., 58% of 2a was transformed) (entry 5). Without the Pd salt, the reaction failed to proceed (entry 6). These two experiments illustrate the benefit of bimetallic Pd/Cu catalysis in the case of AHA. The nature of the Pd counterions proved fundamental, as no AHA occurred with CF3CO2− and Cl− (entries 7, 8). Side reactions of allene 2a were bypassed by using a sacrificial excess of alkyne 1a, which delivered 3aa quantitatively (entry 9). Conversely, the use of an allene excess did not improved the yield (entry 10). The reaction was very slow in toluene at rt (entry 11), while in refluxing THF numerous byproducts were formed. Replacing (p-MeOC6H4)3P by Ph3P decreased yields (entry 12), while (furan-2-yl)3P led to poor conversions (entry 13). With Pd2(dba)3 (entry 14), a low level reactivity was observed (5 days, 38%) suggesting that Pd0 does not participate in the catalytic cycle. The scope was explored under our optimized reaction conditions. We studied electron-rich or electron-deficient substrates, where most of the possible combinations were explored (21 examples), so together with the previous work,8c there are now almost 40 examples of AHA described, illustrating well the efficiency of this reaction (Scheme 2). 3ba and 3ea were obtained with 78% and 67% yields, while former reaction conditions gave no product. Although trimethylsilylacetylene gave 3ca in 47% yield, tri-iso-propylsilylacetylene gave enyne 3fa in 81% yield. Electron-poor ethylpropiolate gave no 3ka. These reaction conditions are mild and neutral, so 3af was isolated with no alkene/carbonyl reconjugation. Amide 3ja was obtained in 54% yield, and free amines proved to be competent reaction partners (3ha: 76%; 3la: 69%). Electron-deficient allenes (2c), or electron-rich ones (2n), reacted equally well giving 3ac, 3dc, 3gc, and 3an in good yields. 1-Bromo- and 1-methoxypropa-1,2-diene failed to give enynes 3aj and 3ak. Enynes 3ad, 3ai, 3ll,15a and 3lm,15b were obtained in good yields. The two latter were key intermediates of our two syntheses of the tiacumicin B aglycon.15 Various derivatizations of 1,3-enynes to give 1,3-

Scheme 3. Examples of Enyne Derivatization

Involving Pd0, hydrido-Pd, and π-allyl-Pd species, the initially proposed AHA mechanism is certainly orthodox, but not demonstrated,8c and the following observations tend to disqualify the involvement of such species. First, Yamamoto reported that Pd2(dba)3-catalyzed hydrocarbation of 2n by malonitrile led exclusively to a linear product which proved the involvement of a π-allyl-Pd here.21 Contradictorily, while a similar mechanism was postulated, AHA gives only branched products (even with 2n) and requires a PdII source, Pd2(dba)3 being not an adequate Pd source (Table 1, entry 14). Second, while TEMPO is known to trap hydrido-Pd complexes,22 its addition had no impact on the course of the 1a/2a crosscoupling.23 Third, the 1a/2i cross-coupling gave enyne 3ai exclusively (Scheme 2), while an ideally positioned OH should have given tetrahydrofuran 9 by trapping postulated π-allyl-Pd intermediates (eq 1).24

When a THF solution of Pd(OAc)2 + 2 (p-MeOC6H4)3P that was stirred for 1.5 h was treated with 1a, 2a, and CuCl, no B


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bonding with PdII. The maximum electron density in the Pd··· Cu area is ρmax = 0.023 e·Å, which is typical of weak but nonnegligible metal−metal bonding.31 By locking both acetates, one can understand how the CuI atom impairs the PdII/Pd0 reduction (Scheme 4, green arrows).26 The overall conclusion to draw is that this AHA likely involves neither Pd0 nor hydrido-Pd, nor π-allyl-Pd, and that the catalyst is likely a Pd/ Cu heterobimetallic complex. With such conclusions, it was deemed necessary to propose a new reaction mechanism. We carried out DFT computations to screen the various possibilities. Alkyne 1a and allene 2a were used as substrates, and Ph3P was used as the model phosphine. We originally proposed a mechanism that began with the formation of a (R−CC−)(H)PdIV species, and we also considered Grigg’s π-allyl-Pd pathway; however, the first appeared unlikely and the second proved incompatible with experimental observations.27 Ultimately, the sole viable mechanism issuing from these calculations is shown in Scheme 5.32 In the absence of an exogenous base, the formation of a

AHA occurs. In contrast, when a THF solution of Pd(OAc)2 + 2 (p-MeOC6H4)3P + CuCl was preincubated for 3 h, and then treated with 1a and 2a, the reaction goes to completion with the usual rate and yield.25 We noticed that while CuCl is insoluble in THF in the presence of (p-MeOC6H4)3P, a THF solution combining CuCl, Pd(OAc)2, and (p-MeOC6H4)3P is obtained in minutes, giving a catalytically active deep-orange/ brown solution. This raises the question of the nature of the reaction catalyst. Amatore and Jutand have shown that mixing Pd(OAc)2 + 2 PPh3 at rt in THF resulted in a slow reduction of PdII into Pd0 by one of the phosphines (t1/2 ∼45 min), a reaction even slower with electron-rich (p-MeOC6H4)3P.26 To make a blank, we carried out 31P NMR experiments on Pd(OAc)2 + 2 (p-MeOC6H4)3P (0.015 M/THF).27 As expected, a signal at 24.2 ppm (p-MeOC6H4)3PO, as well as other expected signals, appeared progressively attesting to the reduction of PdII into Pd0.26b Then, 31P NMR experiments were carried out on the catalytically active Pd(OAc)2 + 2 (pMeOC6H4)3P + CuCl mixture.27 Surprisingly, the spectra recorded were very different from that of the blank: After 3 min all (p-MeOC6H4)3P had already disappeared, and instead signals at δP 11.9 and 16.7 ppm appeared corresponding to Pd(OAc)2[(p-MeOC6H4)3P]2 and to an unknown complex α. After 3 h, only the α complex signal remained. The PdII/Pd0 reduction is slow, so α that forms almost instantaneously is likely centered on a PdII and is symmetrical since it displays one single 31P NMR peak. MS experiments were performed on the same sample.27 Electrospray ionization gave signals at m/z 869.1626 ([L 2 PdOAc] + ) and 845.1157 ([L 2 PdCl] + ). [L2PdCl]+ shows that Pd2+ and Cu+ can exchange their counterions, a clue to the Pd/Cu heterobimetallic nature of α. Then, we shifted to atmospheric pressure photoionization (APPI), a milder method involving radical processes, and welladapted to organometallic complexes.28 Using toluene as a photoionization dopant, we detected reproducibly a signal at m/z 933.1808 that corresponds to [L2PdOAc + Cu + H]+ which could form through the pathway drawn in Scheme 4, confirming the heterobimetallic nature of α.

Scheme 5. Proposed Mechanism Based on DFT Calculations

Scheme 4. Structure of α Deduced from MS, RMN, and DFT Cu acetylide seems unlikely. However, coordination of CuCl to the alkyne moiety of 1a could assist the formation of a Pd acetylide. Indeed, from complex α and 1a we could reach intA, a step endergonic by 13.7 kcal/mol, and we could model a concerted metalation deprotonation (CMD) with a low free energy of activation (ΔG‡298 = 5.9 kcal/mol). Note that in contrast, CMD of 1a with PdII but without CuI displays a far higher free energy of activation (ΔG‡298 = 15.3 kcal/mol), showing thus the benefit of having here a bimetallic catalysis.33 The transformation of intA is exergonic by 3.2 kcal/mol and delivers complex intB, exhibiting a noncovalent interaction between the acidic hydrogen and the acetylide carbon. The reaction of Pd acetylide intB with allene 2a then gives simple adduct intC, lying at 5.7 kcal/mol on the potential energy surface. A H+ transfer on the allene34 from the protonated acetate ligand then follows to give intD. This complex was actually optimized as a vinylic carbocation35 (as revealed by inspection of its LUMO), which lies 4.8 kcal/mol above reference compounds intA and 2a. The protonated allene is held by two H-bonds with the acetate ligands. The formation

Ultimately we used density functional theory (DFT) to model complex α.29 Calculations converged toward a complex displaying a 2.90-Å-long PdII−CuI contact, a length typical of group 10/group 11 heterobimetallic complexes involving a PdII.30 The PdII atom is square planar, while the CuI atom shows a trigonal planar geometry with an additional apical C


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Organic Letters *E-mail: [email protected].

of the Pd−C bond from intD to give the pentavalent vinyl PdIV species18,36 intE requires only 1.0 kcal/mol of free energy of activation and is appreciably exergonic by 17.7 kcal mol−1. It is noteworthy that a similar oxidative addition initiated by protonation of a nonpolarized π-system has been recently reported in a case of rhodium catalysis.37 The reductive elimination transition state (TS) is also easily accessible (6.3 kcal/mol of free energy of activation from intE), and this C−C bond forming step releases 45.3 kcal/mol of free energy. The final PdII complex intF displays the experimentally observed stereochemistry. Calculations were reproduced to obtain unobserved (Z)-3aa. In that case, the R2 chain in the TS has a steric clash with an acetate ligand, and one stabilizing Hbond is missing (Frame, Scheme 5). The free energy of activation is 3.3 kcal/mol higher than in E series, which is perfectly enough to ensure virtually complete diastereoselectivity at 20 °C as observed experimentally. This mechanistic model accurately describes our experimental observations; however, it is also predictive, which validates it. Thus, calculations show that the pathway from intC to intD is rate determining and that intD is stabilized by H-bonds involving Pd-counterions. Our model predicted that the reaction rate depends on the pKa of the Pd counterions, since H-bond strength and the pKa of counterions are linked together. Stronger H-bonds are provided by anions from weak acids; therefore, higher reaction rates are expected with them.24 We calculated that the free energy of activation to go from intC to intD is 15.7 kcal/mol with acetates, 16.2 kcal/mol with benzoates, 17.1 kcal/mol with 3-fluorobenzoates, and with trifluoroacetate no value could be found. To our delight, DFT predictions were experimentally confirmed: the reaction rate decreases well with the pKa (PivO− (5.03) ≈ AcO− (4.76) > PhCOO− (4.20) > 3-fluoro-benzoate (3.86), and trifluoroacetate (0.23) allowed no reaction).38 This re-exploration of the Pd/Cu-catalyzed AHA led to devising new reliable conditions, to enlarge the substrate scope, to evaluate functional tolerance, and to devise an innovative strategy of synthesis of 1,3-dienes that proves remarkably useful to total synthesis. A series of observations were not in agreement with the originally proposed mechanism. So, supported by DFT calculations, NMR, MS, and chemical experiments, we propose a mechanism which proved nonorthodox but coherent. It involves a PdII/PdIV catalytic cycle, TSs tightly organized by H-bonds with the Pd counterions, and an oxidative addition initiated by a stereodeterminant H+ transfer. We also suggest that the catalyst is a heterobimetallic complex. We hope that this mechanism will help in the design of new reactions and that our strategy of synthesis of 1,3dienes will find more applications in synthesis.

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ORCID

Grégory Genta-Jouve: 0000-0002-9239-4371 Vincent Gandon: 0000-0003-1108-9410 Emmanuel Roulland: 0000-0002-8012-7946 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the ANR (ANR-14-CE16-0019-02), the CNRS, the Université de Paris, the Université Paris-Saclay, and the Institut Universitaire de France (IUF) for financial support. We thank Pr. Olivier Laprevote (C-TAC, UMR 8038) for the fruitful discussions about mass spectroscopy.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00828. Experimental details and characterization data for all new compounds, 31P NMR experiments, MS experiments, and DFT calculations (PDF)

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REFERENCES

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Cu-Catalyzed Alkyne Hydrocarbation of

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