Palladium-Catalyzed Coupling of Allenylphosphine Oxides with N

Nov 29, 2016 - Ling Zhang , Jie Zhu , Jing Ma , Lei Wu , and Wei-Hua Zhang. Organic Letters 2017 ... Kai Luo , Ling Zhang , Jing Ma , Qiang Sha , and ...
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Palladium-Catalyzed Coupling of Allenylphosphine Oxides with N-Tosylhydrazones Toward Phosphinyl [3]Dendralenes Mao Mao, Ling Zhang, Yao-Zhong Chen, Jie Zhu, and Lei Wu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02972 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

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Palladium-Catalyzed Coupling of Allenylphosphine Oxides with N-Tosylhydrazones Toward Phosphinyl [3]Dendralenes Mao Mao,†,§ Ling Zhang,†,§ Yao-Zhong Chen,† Jie Zhu,† and Lei Wu*,†,‡ †

Jiangsu Key Laboratory of Pesticide Science and Department of Chemistry, College of Sciences, Nanjing Agricultural University, Nanjing 210095, China ‡ Beijing National Laboratory for Molecular Sciences and Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China Dedication to Professor Henning Hopf Supporting Information Placeholder ABSTRACT: A palladium-catalyzed coupling of allenylphosphine oxides with N-tosylhydrazones leading to phosphinyl [3]dendralenes is established. The coupling reaction can be catalyzed by bis(triphenylphosphine)palladium chloride with sodium pivalate as a key additive, presumably via π-allyl-Pd-carbene intermediates. This protocol provides an expedient synthesis of unprecedented multi-substituted phosphinyl [3]dendralenes with a broad substrate diversity and dominant Z-stereoselectivity depending on the substitution positions. X-ray crystallographic analyses confirm the relative stereochemistry of products and reveal that the multiple double bonds in phosphinyl [3]dendralenes array with a "fan-like" dimensional orientation. Further applications of phosphinyl [3]dendralenes to intramolecular cyclization and selective oxidation demonstrate the differentiated reactivities of double bonds. KEYWORDS: Phosphinyl [3]dendralenes, Palladium, Allenylphosphine oxide, N-tosylhydrazones, Carbene

Conjugated carbon-carbon double bonds in molecules can be assembled in six varieties to construct distinct families of unsaturated hydrocarbons including conjugated polyolefins, annulenes, radialenes, fulvenes, dendralenes, and cumulenes. The importance of these unsaturated hydrocarbons has been exemplified by their existence and as motifs in rapid synthesis of natural products.1 Despite of these achievements, dendralenes have long been neglected since their discovery in 1955, in part because that the compounds were assumed to be unstable on benches, which restricted their further study.1a However, in recent years, dendralenes have witnessed growing interests due to their unique structure and electronic phenomenon with applications extended to polymer chemistry,2 theoretical chemistry,3 electrochemistry4 and synthetic chemistry.5 It’s noticed that a synthetic breakthrough was reported by Sherburn and co-workers in 2009,6 who disclosed a practical synthesis of dendralenes, and found that the dendralenes are stable after all, along with surprisingly reactivities depending on the odd or even numbers of double bonds in molecules. So far, dendralenes have been synthetically engaged in dienetransmissive Diels-Alder reactions,7 synthesis of ivyane family,8 vinylogous Nazarov reactions,9 and organocatalytic domino cyclization.10 The high value of dendralenes necessitates their synthetic developments against intrinsic challenges, with a handful of catalytic methods reported.1b,6,11 For representative studies, Sherburn et al. extensively developed various strategies through Stille coupling and thermolysis,11c successive Kumada-Tamao-Corriu and Negishi couplings,6 two-fold SuzukiMiyaura couplings (Scheme 1, a)11d to realize the synthesis of multi-conjugated dendralenes bearing 3 to 12 double bonds,

respectively. Another option on the synthesis of dendralenes was developed by Glorius and coworkers. They revealed the first RhIII-catalyzed alkenyl C-H activation and coupling with allenyl carbinol carbonates to access substituted [3]dendralenes (Scheme 1, b).11e To be noted, these transformations require the building blocks containing one or more C=C bonds, either with preinstalled directing groups. In view of the importance of dendralenes and their scattered synthetic protocols in sharp contrast, it’s still desirable to develop highly efficient strategies for rapid construction of novel substituted dendralenes. Intriguingly, the incorporation of substituents on dendralenes dramatically influences the properties of crossconjugated systems in terms of stability and reactivity, depending on the nature of substituents and the positions attached.12 It’s well-known that phosphorus substituents regulate important biological, medicinal, material functions,13 and most importantly to organic chemists, they can probably alter the electronic nature of dendralenes toward various regioselective synthetic applications. Nevertheless, to the best of our knowledge, phosphinyl dendralenes have never been reported. Over the past decades, N-tosylhydrazones have emerged as versatile building blocks to afford functionalized C=C bonds in palladium-catalyzed reactions.14 In general, the reaction intermediates undergo migratory insertion of palladium carbenes, which might be mechanistically compatible with the allenes chemistry.15 Although many achievements have been devoted to converting N-tosylhydrazones into allenes,15a,16 the transition-metal catalyzed coupling involving N-tosylhydrazones and allenes in sharp contrast, is rather rare.17 For an elegant study in 2013, Wang and co-workers discovered a

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new method for synthesis of 1,3-dienes through threecomponent coupling of allenes, aryliodides and N-tosylhydrazones, where a π-allyl-Pd-carbene intermediate was proposed.17a In their study, aryliodides were crucial to facilitate the generation of π-allyl-Pd species. From this point of view, the two-component coupling of N-tosylhydrazones with allenes has not been established actually. Inspired by the aforementioned pioneering researches on dendralenes and Ntosylhydrazones, we envisioned that the co-existence of allenylphosphine oxide, N-tosylhydrazones and palladium complexes would lead to the formation of π-allyl-Pd-carbene intermediate, followed by migratory insertion and β-hydride elimination to finalize a novel family of phosphinyl [3]dendralenes (Scheme 1, c). As part of our continuing interest in organophosphorus chemistry and palladium catalysis,18 herein, we report, for the first time, a palladium-catalyzed synthesis of multi-substituted phosphinyl [3]dendralenes based on the couplings of allenylphosphine oxides with N-tosylhydrazones.

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best additive with the combination of potassium carbonate to enable the coupling, 92% of 3aa formed without detection of the byproduct (entry 8). The observations indicated that metal precursor, base and additive would play key roles in achieving the high efficiency and suppressing the byproduct. Table 1. Palladium-catalyzed coupling of allenylphosphine oxide (1a) with N-tosylhydroazone (2a): conditions screening.a

entry

catalyst

base/solvent/additive

yield (3aa/3’,%)b

(5 mol%) 1

Pd(PPh3)4

K2CO3/THF/-

21/10

2

Pd(PPh3)2Cl2

K2CO3/THF/-

trace/42

3

Pd(OAc)2

K2CO3/dioxane/-

11/32

4

Pd(BINAP)Cl2

K2CO3/dioxane/-

24/10

5

Pd(DPPF)Cl2

K2CO3/dioxane/-

38/10

6

Pd(PPh3)2Cl2

K2CO3/dioxane/ CH3COONH4

72/0

7

Pd(PPh3)2Cl2

K2CO3/dioxane/ NH4H2PO4

52/0

8

Pd(PPh3)2Cl2

K2CO3/dioxane/ tBuCOONa

92/0

9

Pd(PPh3)2Cl2

-/dioxane/

60/0

tBuCOONa

Scheme 1. Representative catalytic synthesis of dendralenes and this work. The study was initiated by examining the coupling of allenylphosphine oxide (1a) with 1-(1-(4-methoxyphenyl) ethylidene)-2-tosylhydrazine (2a) catalyzed by tetrakis(triphenyl phosphine)palladium in the presence of potassium carbonate in refluxing THF. For a preliminary result, phosphinyl [3]dendralenes 3aa was detected with isolated yield of 21% (entry 1), along with a 10% yield of byproduct 3’, which might be generated from the nucleophilic attack of TsN=Nonto intermediates. Systematically screenings of the conditions were performed after then. Increasing the reaction temperature reduced the byproduct efficiently. However, the yields of 3aa exhibited relatively insensitive to the bases and solvents used, albeit the byproduct was suppressed in some cases. Other metal precursors including nickel and rhodium complexes led to the formation of byproduct 3’ as the major product (see more details in Supporting Information). To our delight, as shown in Table 1, salt additives fortuitously changed the pathway to higher yields of 3aa, spontaneously avoiding the generation of 3' (entries 6-9). Utilizing 2 equivalents of ammonium acetate in the presence of Pd(PPh3)2Cl2 (5 mol%) improved the yield of phosphinyl [3]dendralenes up to 72%. Eventually, sodium pivalate was considered to be the

a

Reaction conditions: allenylphosphine oxide (1a, 0.3 mmol), N-tosylhydrazone (2a, 0.6 mmol), catalyst (5 mol%), base (0.9 mmol), additive (0.6 mmol) in 3 mL refluxing solvent for 18 hours. b Isolated yield by column chromatography.

With the optimized conditions in hand, we investigated the substrate scopes of various allenylphosphine oxides (1a-1f) with substituted N-tosylhydrazones (2a-2m). As depicted in Scheme 2, the couplings toward phosphinyl [3]dendralenes gave yields ranged from medium to excellent, relying on the structure and electronic properties of substituents. For instance, allenylphosphine oxides with endmost symmetrical alkyl, cyclic or aromatic substitutions afforded products 3aa-3af with good to excellent yields. Otherwise, N-tosylhydrazones bearing electron-donating groups, including p-methoxyl, p-methyl, m-methyl, p-dimethylamine, proceeded smoothly to give structural diversed phosphinyl [3]dendralenes (3af-3cf, 3ef). The coupling is quite insensitive to the steric effect, where substrate with ortho-methyl group (2d) furnished 3df in a good yield of 89%. Moderate yields were achieved for electronic neutral substrates of 2f and 2i toward 3ff and 3if, respectively. Moreover, the X-ray structure of 3if revealed a "fan-like" dimensional orientation of phosphinyl [3]dendralenes and its strong affinity with water.19 Electron-

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withdrawing groups, such as p-F (2g), and p-Cl (2h), dramatically diminished the coupling efficiency with medium yields of products isolated (3gf, 3hf). Unfortunately, bromosubstituted substrate undertook sluggishly giving very complicated results because of the competing homolytic coupling and Suzuki-Miyaura coupling of aromatic bromide with allenes, with only trace amount of target product detected. Furanyl substituted N-tosylhydrazones and cyclic ones were also applicable but with comparably lower yields (3jf-3mf).

PPh2 R

MeO

1

R

1O

3aa, R1 = CH3, 92% 3ab, R1 = Et, 73% 3ac, R1 = n-Pr, 76%

PPh2 O

MeO

R2 Ph

PPh2 O Ph

aliphatic N-tosylhydrazones exhibited superior stereoselectivities over those generated from aromatic ones and allenylphosphine oxides.

PPh2 O Ph

Ph

3if, 67% (X-ray)19

3af, R2 = p-CH3O, 82% 3bf, R2 = p-CH3, 83% 3cf, R2 = m-CH3, 88% 3df, R2 = o-CH3, 89% 3ef, R2 = N,N-dimethyl, 88% 3ff, R2 = H, 48% 3gf, R2 = p-F, 51% 3hf, R2 = p-Cl, 54%

n 3ad, n = 1, 89% 3ae, n = 2, 63%

O

Ph

PPh2 O Ph

3jf, 41%

O P Ph2 Ph 3kf, 48%

Ph

O P Ph2 Ph 3lf, 74%

Ph

Ph

PPh2 O Ph

3mf, 36%

a

Reaction conditions: allenylphosphine oxide (1a, 0.3 mmol), N-tosylhydrazone (2a, 0.6 mmol), Pd(PPh3)2Cl2 (5 mol%), K2CO3 (0.9 mmol), sodium pivalate (0.6 mmol) in 3 mL refluxing 1,4-dioxane for 18 hours. b Isolated yield by column chromatography. Scheme 2. Palladium-catalyzed coupling of allenylphosphine oxides (1) with N-tosylhydrazones (2): substrate scope. It is worthy to mention that the variation of R1, R2 and R4 groups would produce stereodiversity depending on the substitutions of double bonds (Scheme 3). Hence, a wide range of allenylphosphine oxides and substituted N-tosylhydrazones was prepared to investigate the stereochemistry of this reaction. On the one hand, when unsymmetrical allenylphosphine oxides (1g-1k) were allowed to react with 2a, the coupling products gave slight preferences of Z-selectivity with ratios of 1.11.8:1 over E-isomers. Quite interestingly, the stereoselectivity could be fully controlled for the reaction of 2m with 1g, Zisomer was detected as dominant configuration for product (3mg) in 40% yield (Z:E>20:1). On the other hand, when the stereochemically diversity generated from N-tosylhydrazones instead of allenylphosphine oxides, exclusively Z-isomers were obtained (3nf-3uf), albeit with acceptable yields ranged from 19% to 60%. Meanwhile, the relative stereochemistry of Z-isomers were assigned based on the X-ray crystallographic analysis of 3aj and 3uf (Figure 1, [a], [b]).19 For more complicated systems derived from both allenylphosphine oxides and N-tosylhydrazones, the stereoselectivities can be efficiently controlled with Z,Z-selectivity (3qg, 3og, 3qi). In general,

a

Reaction conditions: allenylphosphine oxide (1, 0.3 mmol), Ntosylhydrazone (2, 0.6 mmol), Pd(PPh3)2Cl2 (5 mol%), K2CO3 (0.9 mmol), sodium pivalate (0.6 mmol) in 3 mL 1,4-dioxane, reflux, 18 hrs. b Isolated yield by chromatography. c The Z/E ratios were also confirmed by crude 1HNMR spectra.

Scheme 3. Palladium-catalyzed coupling of various allenylphosphine oxides (1) with N-tosylhydrazones (2): stereochemistry studies. [a]

[b]

Figure 1. X-ray structures of [3aj] (Z-isomer, [a]) and [3uf] (Z-isomer, [b]).19

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Finally, we sought to demonstrate the synthetic applications of the phosphinyl [3]dendralenes product (Scheme 4). 3ac was initially treated with 10 mol % BF3·Et2O, in which the internal C=C bond cyclized with electron-rich aromatic ring to afford a new heavily substituted indenyl product (4) in 88% yield. Subsequently, while an attempt was made to epoxidize 3ac with 5 mol% manganese acetate and aqueous hydrogen peroxide, the electron-rich terminal double bond was broken selectively leading to the formation of a novel oxo-phosphinyl dendralenes (5).

Scheme 4. The synthetic applications of phosphinyl [3]dendralenes and X-ray structure of compound 4. Based on the palladium catalyzed allene chemistry, N-tosylhydrazones and previous reports,15a,16,17a,18d,20 a plausible mechanism is proposed in Scheme 5. Initially, the oxidative addition and cleavage of C(sp3)-O(Ar) bond lead to the formation of π-allyl-palladium species A. Subsequently, the carbene species decomposed from N-tosylhydrazones is in-situ captured by A to form a π-allyl-palladium carbene species B, meanwhile, the parthway to allenic byproduct via carbene species B' is not observed. Migratory insertion of the allylphosphine oxide moiety to the carbenic carbon centre affords the intermediate D. Eventually, β-hydride elimination of D generates the final coupling product 3, along with the recycle of the Pd(0) catalyst. The stereoselectivity for the formation of (Z)-phosphinyl [3]dendralenes 3 can be explicated on the basis of the transition state of syn-β-hydride elimination from the intermediate D (Scheme 5): Four different palladium complexes could be formed after the migratory insertion. With an unsymmetrical allenylphosphine oxide as substrate, the R1 (R1>R2, R4=H) groups prefer to eclipse with the less bulky methyl group, and the Z-isomers form after β-hydride elimination; Otherwise, for the stereochemically diversities generated from N-tosylhydrazones (R1=R2, R4≠H), eclipsing strain dominates toward Z-isomers exclusively. In summary, we disclosed, for the first time, a palladiumcatalyzed coupling of allenylphosphine oxides with Ntosylhydrazones. The coupling reactions catalyzed by bis(tri-

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phenylphosphine)palladium chloride with sodium pivalate as an additive, are stereoselective depending on the substrate structure, and show a broad substrate diversity and functional group tolerance. Moreover, the coupling products as a novel serials of phosphinyl [3]dendralenes were synthesized, characterized and applied to intramolecular cyclization and selective oxidation reactions. X-ray crystallographic analysis revealed that the multiple double bonds in phosphinyl [3]dendralenes array with a "fan-like" dimensional orientation. We expect this new protocol to enrich the emerging methodology of dendralenes chemistry and provide novel scaffolds to build potential bioactive compounds.

R1>R2, R4=H

O P Ph2

R3 R2

R1

PdLn

R2

PdLn

R2

R1

R1

O

O

Z-isomer

O P Ph2

R3 R3

R3 P Ph2 less steric

R1

R2

E-isomer

P Ph2 higher steric

R1=R2, R4

O

R3

P Ph2 R1

PdLn

R1

R4

PdLn

R4

R2

R2

O P Ph2

R3

R4 R3

R3

R2

Z-isomer

R1

O

P Ph2

favored

O R4 P Ph2

R1

R2

E-isomer

disfavored

Scheme 5. Proposed mechanism and rationale for the stereoselectivity.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author * Email: [email protected].

Author Contributions § These two authors contributed equally.

Notes The authors declare no competing financial interest.

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Supporting Information

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Experimental procedures and spectral data for all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT We are grateful for financial support from the National Natural Science Foundation of China (NSFC, Grant No. 21372118), the Foundation Research Project of Jiangsu Province (the Natural Science Foundation No. BK20141359), the Fundamental Research Funds for the Central Universities (Grant No. KYTZ201604) and “333 High Level Talent Project” of JiangSu Province.

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