Monodentate Phosphorus Ligand-Enabled General Palladium

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

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Monodentate Phosphorus Ligand-Enabled General PalladiumCatalyzed Allylic C−H Alkylation of Terminal Alkenes Lian-Feng Fan,† Pu-Sheng Wang,*,† and Liu-Zhu Gong*,† †

Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry, University of Science and Technology of China, Hefei, 230026, China

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S Supporting Information *

ABSTRACT: Monodentate phosphorus ligands have been found to enable the palladium-catalyzed allylic C−H alkylation reaction of terminal alkenes with a wide variety of carbon nucleophiles. Moreover, an asymmetric allylic C−H alkylation of terminal alkenes with pyrazol-5-ones has been established in the presence of chiral phosphoramidite ligand and chiral phosphoric acid as co-catalyst. Mechanistic studies suggest that a ternary Pd(0) complex, coordinated with a monodentate phosphorus ligand, benzoquinone, and alkene, is most likely to be an active species.

T

ransition-metal-mediated C(sp3)−C(sp3) bond formation is one of the fundamental issues for building up structurally complex molecules.1 Among the versatile approaches, palladium-catalyzed allylic C−H alkylation2 of readily accessible olefins can enable efficient assembly of carbon−carbon bond via oxidative coupling of C(sp3)−H bond with minimal preoxidation and manipulation of functionalities. Although a stoichiometric palladium-mediated stepwise allylic C−H alkylation was first achieved by Trost in 1973,3 a one-pot catalytic version had not been developed until White4 and Shi5 independently disclosed a significant allylic C−H alkylation with active methylene compound via Pd(OAc)2-bis(sulfoxide) catalysis in 2008 (see Scheme 1a). This

catalyst system has exhibited extraordinarily excellent activity toward an extensive set of terminal alkenes, including activated allylarenes and unactivated aliphatic alkenes.6 In recent years, Trost and our group found that phosphorusbased ligands7 were also able to promote the palladiumcatalyzed allylic C−H functionalization of activated terminal alkenes, such as allylarenes,8 1,4-dienes,9 and allyl ethers10 (see Scheme 1b). Because the coordination of phosphorus ligands principally erodes the electrophilicity of Pd(II) toward allylic C−H cleavage via TS1,11 this type of Pd complex does not seem amenable to the allylic C−H functionalization of unactivated terminal alkenes. However, based on experimental and computational studies, we recently disclosed a concerted proton and two-electron transfer process for the allylic C−H cleavage of 1,4-dienes with Pd(0)-phosphoramidite catalyst. Considering that the overall barrier of this transition state (TS2) was only 16.5 kcal/mol,9e we posited that this activation mode might also be applicable to cleave inert allylic C−H bonds of unactivated terminal alkenes, thereby generating a reactive π-allyl palladium intermediate toward nucleophilic substitutions (Scheme 1c), providing an opportunity to replace traditional allylic halides (or pseudo-halides) involved in the allylation process12 by using readily available olefins as allylating reagents (Scheme 1c). The validation of our hypothesis started with investigating a palladium-catalyzed reaction of 2-benzylmalononitrile (1a) and 1-heptene (2a) by using triphenylphosphine (PPh3) as ligand,

Scheme 1. Palladium-Based Catalyst Systems for Allylic C− H Alkylation

Received: July 5, 2019

© XXXX American Chemical Society

A

DOI: 10.1021/acs.orglett.9b02325 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 2. Scope with Respect to the Nucleophilesa

in the light of our previous work on the allylic C−H cleavage, especially the fact that the presence of phosphorus-based ligands significantly enhanced the activity of palladium catalyst.7−10 A careful screening of oxidants and solvents was initially conducted (see the Supporting Information). To our delight, the desired alkylation product was smoothly afforded in 80% NMR yield in the presence of 2.5 mol % of Pd2(dba)3, 5 mol % of PPh3, and 1.1 equiv of 2,5-di-tert-butyl-pbenzoquinone (2,5-DTBQ) in 1,4-dioxane at 50 °C for 12 h. To further verify the ligand effect on the catalysis, a range of structurally different monodentate phosphorus ligands were evaluated (Figure 1). The presence of either electron-donating

a

Figure 1. Screening of monodentate phosphorus ligands.

Reaction conditions: 1 (0.20 mmol), 2a (0.40 mmol), Pd2(dba)3 (0.005 mmol), L11 (0.01 mmol), 2,5-DTBQ (0.22 mmol), 1,4dioxane (1.0 mL), 50 °C, 24 h. Isolated yield based on 1 and the isolated yield of the reaction using PPh3 is given in parentheses. bThe reaction was carried out at 70 °C. c120 mol % K2CO3 was added. d L12 was used.

or electron-withdrawing substituents at the para-positions of triphenylphosphine derivatives (L1−L6) provided similar yields. meta-Substituted triarylphosphines (L7 and L8) could also enable the reaction to deliver high yields, whereas bulkier tri(o-tolyl)phosphine (L9) and tri(1-naphthyl)phosphine (L10) ligands provided much lower conversion, implying that the efficiency of palladium catalyst is highly sensitive to the steric hindrance of phosphine ligands and more sterically demanding feature of the ligand would be able to diminish catalytic efficiency of palladium. Interestingly, a phosphoramidite ligand13 (L11) derived from 2,2′-biphenol could render the reaction to proceed in a high chemical yield, and 3,3′-substituted biphenol-based phosphoramidite ligand (L12) was still able to maintain the catalytic activity of palladium complex. With the optimal conditions in hand, the generality of the allylic C−H alkylation reaction of 1-heptene (2a) for carbon nucleophiles (1), using either phosphoramidite ligand L11 or PPh3 as the ligand, was then explored (see Scheme 2). Generally, the use of L11 exhibited superior performance than that of PPh3, and could smoothly tolerate a wide scope of acidic carbon nucleophiles. For example, in the presence of L11, substituted malononitriles (1a and 1b) and αcyanoacetates (1c and 1d) were nicely tolerated to give quaternary alkylation products with yields of 92%−95%, while the use of PPh3 only provided yields of 31%−92%. For α-nitro ketone (1e), L11 still provided 55% yield while PPh3 only gave 7% yield. Other active methylene compounds, including ethyl cyanoacetate (1f), ethyl acetoacetate (1g), ethyl nitroacetate (1h), and ethyl (benzenesulphonyl)acetate (1i) smoothly

participated in the desired reactions to yield tertiary alkylation products in yields of 55%−92% when L11 was used, while PPh3 only gave yields of 11%−64%. Moreover, in the presence of L11, cyclic 1,3-dicarbonyl compounds (1j−1l) were compatible with the reaction conditions, delivering the desired products in yields of 48%−70%, while PPh3 only gave yields of 38%−63%. Notably, N-heterocyclic compounds, such as azlactone (1m) and N-Boc-3-phenyl-2-oxindole (1n), were also well tolerated when L11 was used, providing alkylation products in good yields. Although pyrazol-5-one (1o) under the optimized conditions resulted in unsatisfied yield, the use of a bulkier phosphoramidite ligand L12 to replace L11 successfully allowed the reaction to proceed in good conversion. However, in contrast, the use of PPh3 for substrates 1m and 1o only resulted in a trace amount of alkylation products. Then, the substrate scope, with respect to various terminal alkenes, was examined under the optimized reaction conditions, using either phosphoramidite ligand L11 or PPh3 as a ligand (see Scheme 3). In the presence of L11, the protocol was amenable to a wide scope of alkenes. Significantly, propene 2b, a ten-million ton-scale chemical produced annually in the petroleum industry, was able to undergo allylic C−H alkylation with 1b. This success actually explicated the great potential of this protocol in industrial production. Other unactivated terminal alkenes, bearing either a cyclohexyl, phenyl, alkynyl, ether, or ester group (2c−2i), were nicely tolerated to give some densely functionalized alkylation products in high yields. Highly reactive functional groups, such as terminal epoxide (2j), aldehyde (2k), and alkyl B

DOI: 10.1021/acs.orglett.9b02325 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 3. Scope with Respect to the Terminal Alkenesa

a

Reaction conditions: 1b (0.20 mmol), 2 (0.40 mmol), Pd2(dba)3 (0.005 mmol), L11 (0.01 mmol), and 2,5-DTBQ (0.22 mmol) were reacted in 1,4-dioxane (1.0 mL) at 50 °C for 24 h. Isolated yield based on 1b and the isolated yield of the reaction using PPh3 is given in parentheses. bWith 9 bar of propene.

chloride (2l), were also well-tolerated. Notably, terminal alkenes bearing either a hydroxyl (2m) or a t-butyl carbamate (2n) functionality underwent the desired alkylation reaction in good yields. The reaction involving alkenes incorporated with heteroaromatic moieties (2o and 2p) also proceeded smoothly. In addition, a broad spectrum of activated alkene substrates, including allyl cyanide (2q), tert-butyl 3-butenoate (2r), allylbenzene (2s), and 1,4-diene (2t), were also able to participate in the alkylation reaction. Significantly, this allylic C−H alkylation protocol was amenable for the late-stage functionalization of complex molecules. For instance, a nerol derivative (2u), bearing multiple allylic C−H bonds, regioselectively underwent the allylic C−H alkylation at the terminal double bond. Medicinally relevant chiral compounds bearing terminal alkene handles (2v−2x) appeared to be excellent substrates and delivered good to excellent yields. Note that the multiple functionalities in these substrates have no obvious effect on reaction efficiency. This unique feature actually implies the synthetic significance of this protocol, namely, the direct alkylation of allylic C−H bonds with minimal preoxidation and manipulation of functionalities. In contrast to the high efficiency of L11, PPh3 exhibited inferior performance in terms of yields toward functional group tolerance, as exemplified by 2e, 2n, 2p, 2w, and 2x. Notably, a gram-scale reaction of 1b and 2a also proceeded cleanly in the presence of 0.75 mol % of Pd2(dba)3 and 1.5 mol % of L11, to furnish the desired alkylation product in a 95% yield (Scheme 4a). Interestingly, in the presence of 3 equiv of 1-heptene (2a) and 2.2 equiv of 2,5-DTBQ as an external

Scheme 4. Scale-Up Reaction and Pd-Catalyzed Double Allylic C−H Alkylations

oxidant, a double allylic C−H alkylation between 1f and 2a occurred to generate a synthetically valuable 1,6-diene derivative (4) in excellent yield (see Scheme 4b). In comparison with previous Pd-catalyzed allylic C−H alkylation of α-alkenes using other ligands,4−6 the phosphoramidite ligand acceleration that is observed in this case actually provides a much larger space to develop an enantioselective version, considering that the easily tunable structure of chiral binols, biphenols, and diols can build up a huge library of phosphoramidite ligands for screening. As a showcase, an asymmetric allylic C−H alkylation of unactivated terminal alkenes with pyrazol-5-ones14,9b was investigated. To our delight, in the presence of 2.5 mol % Pd2(dba)3 and 5 mol % chiral phosphoramidite ligand L13, the desired allylic alkylation product 3oa was smoothly afforded in a 63% yield and with moderate regioselectivity and enantioselectivity (see C

DOI: 10.1021/acs.orglett.9b02325 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Table 1, entry 1). Although the presence of an achiral Brønsted acid as a co-catalyst was beneficial to the control of the

Scheme 5. Palladium-Catalyzed Enantioselective Allylic C− H Alkylation of Terminal Alkenes with Pyrazol-5-onesa

Table 1. Optimization of Reaction Conditionsa

entry

H−B

yieldb (%)

l/bc

enantiomeric ratio, erd

1 2 3 4 5 6 7 8 9e 10e,f 11e,f,g

− OFBA (PhO)2PO2H A1 A2 A3 A4 A5 A5 A5 A5

63 14 13 9 18 12 12 25 53 63 71 (68h)

4.5:1 >20:1 >20:1 >20:1 17:1 >20:1 >20:1 14:1 15:1 >20:1 >20:1

82:18 87.5:12.5 88:12 87:13 87:13 87.5:12.5 90.5:9.5 91.5:8.5 87.5:12.5 94:6 93.5:6.5

a

a

Reaction conditions: 1o (0.1 mmol), 2a (0.15 mmol), Pd2(dba)3 (2.5 mol %), L13 (5 mol %), 2,5-DTBQ (0.11 mmol), toluene (1.0 mL), 50 °C, 16 h. bThe yields were determined by 1H NMR analysis of the crude products, based on 1,3,5-triacetylbenzene as the internal standard. cDetermined by 1H NMR analysis of the crude products. d Determined by chiral HPLC analysis. e10 mol % Pd(dba)2 and 10 mol % L13 were used, and the reaction time was prolonged to 36 h. f 5a was used. g5 Å (60 mg) was added. hIsolated yield.

Reaction conditions: 5 (0.1 mmol), 2 (0.15 mmol), Pd(dba)2 (10 mol %), L13 (10 mol %), A5 (10 mol %), 2,5-DTBQ (0.11 mmol), 5 Å (60 mg), toluene (1.0 mL) at 50 °C for 48 h. > 20:1 l/b, determined by 1H NMR analysis of the crude products. The enantiomeric ratio (er) was determined by chiral HPLC analysis. Ar1 = 4-OMePh. b15:1 l/b. cThe absolute configuration of 6m was determined by comparing the optical rotation with that reported in the literature.9b

regioselectivity and enantioselectivity, a significantly diminished yield was obtained (Table 1, entries 2 and 3). Examination of chiral Brønsted acids revealed that H8− BINOL derived phosphoric acid A5 was able to further enhance the enantioselectivity (Table 1, entries 4−8). Prolonging the reaction time and increasing the chiral catalyst loading resulted in a much enhanced yield, but with a slightly decreased enantioselectivity (Table 1, entry 9). Substituted pyrazol-5-one 5a showed better reactivity and furnished the corresponding alkylating product in 63% yield and with an enantiomeric ratio (er) of 94:6 (Table 1, entry 10). Moreover, the presence of 5 Å molecular sieves led to slightly increased yield and maintained enantioselectivity (Table 1, entry 11). Under the optimized reaction conditions, the generality of the asymmetric allylic C−H alkylation reaction was explored (Scheme 5). Pyrazol-5-ones, bearing a substituted benzyl or methyl group at the C4 position, were nicely tolerated to afford the corresponding products (6b−6f) in moderate to good yields and with high levels of enantioselectivity. Gratifyingly, a variety of functional groups, such as ether, ester, silyl ether, amide and alkyl chloride, tethered to terminal alkenes, were also well-tolerated to undergo the reaction with high enantioselectivities of up to 96:4 er (6g−6l). Allylbenzene was also a successful substrate for this protocol to give the desired product 6m in good yield and with an excellent enantioselectivity (95:5 er). To understand the reaction mechanism, a series of experiments were conducted. A significant kinetic isotope

effect (KIE, kH/kD = 4.3) was observed in the reaction of 1b and 2d(d2) (Scheme 6a), suggesting that the allylic C−H cleavage was the rate-limiting step. The effect of the ratio of ligand to palladium then was investigated (Scheme 6b). The variation of the ratio of phosphoramidite L11 to Pd from 1.0 to 2.5 did not exhibit an obvious influence on the yield. However, in the case that was catalyzed by the palladium complex of PPh3, the reaction efficiency generally trends toward lower values as the molar ratio of ligand to Pd increases. In particular, the presence of 2.5 equiv of PPh3 to Pd completely inhibited the catalytic activity of the palladium complex. These results were consistent with the concerted proton and two-electron transfer process9e (Scheme 1), in which a Pd(0) complex that coordinated with a monodentate phosphorus ligand, a benzoquinone, and an alkene was convinced to be a reactive species for the allylic C−H cleavage. Therefore, excess amounts of PPh3 would compete with alkene or benzoquinone toward the coordination to palladium, presumably leading to the formation of inactive bis(triphenylphosphine) Pd(0) complex. Indeed, the direct use of bis(triphenylphosphine)(p-toluquinone) Pd(0) complex 715 as a catalyst under the optimal conditions led to trace amounts of the desired allylating product (Scheme 6c). In contrast, the phosphoramidite L11 is structurally bulkier and shows lower Lewis basicity than PPh3; thus, it is not strong enough to impede the coordination of olefin to the palladium. As a consequence, the addition of large excess amounts of L11 is unable to exert D

DOI: 10.1021/acs.orglett.9b02325 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Notes

Scheme 6. Mechanistic Investigations

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from MOST (973 Project No. 2015CB856600), the NSFC (Nos. 21672197 and 21602214), and the Chinese Academy of Sciences (Grant No. XDB20020000).

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detrimental effect on the catalytic activity of palladium complex. This interesting phenomenon was also observed in other palladium-catalyzed allylic alkylation reactions16 and Heck reactions.17 In conclusion, a highly efficient and general allylic C−H alkylation reaction of terminal alkenes proceeding under mild conditions has been established by palladium-phosphoramidite catalysis. A wide range of terminal alkenes and carbon nucleophiles are tolerated, allowing for the late-stage functionalization of medicinally important chiral compounds. Mechanistic studies suggest that a ternary Pd(0) complex, coordinated with a monodentate phosphorus ligand, benzoquinone, and alkene, is most likely to be an active species. More importantly, the chiral phosphoramidite ligand is able to significantly enhance the catalytic activity of palladium complex and to accelerate the allylic C−H alkylation process, providing a successful approach for the creation of asymmetric allylic C−H functionalization of unactivated terminal alkenes with pyrazol-5-ones.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b02325. Complete experimental procedures and characterization data for the prepared compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.-S. Wang). *E-mail: [email protected] (L.-Z. Gong). ORCID

Pu-Sheng Wang: 0000-0002-7229-3426 Liu-Zhu Gong: 0000-0001-6099-827X E

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