Substrate Self-Assisted Secondary Bond Activation of Allylic

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Substrate Self-Assisted Secondary Bond Activation of Allylic Alcohol in a Tsuji−Trost Reaction Revealed by NMR Methods Xiantao Ma,*,† Jing Yu,† Qiuju Zhou,*,‡ Ran Yan,† Lingyun Zheng,‡ and Lingling Wang‡ †

College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang, Henan 464000, China Analysis & Testing Center, Xinyang Normal University, Xinyang, Henan 464000, China

‡

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S Supporting Information *

ABSTRACT: The first experimental evidence for the palladium-catalyzed secondary bond activation of allylic alcohols in a Tsuji−Trost reaction was provided by NMR methods, such as variable-temperature 1H NMR, diffusionordered spectroscopy (DOSY), Job’s method, 1H NMR titration, and nuclear Overhauser enhancement spectroscopy (NOESY). The experimental results revealed that the substrate self-assisted activation of allylic alcohols is probably performed via a 1:1 binding six-membered-ring complex, which are formed by the formation of the secondary bonds, the hydrogen bond and P···O noncovalent bond between allylic alcohol and phosphonium ylide.

T

Scheme 1. Mechanism Study Methods for Activator-Free Tsuji−Trost Reactions of Allylic Alcohols

he Tsuji−Trost reaction is a powerful tool to construct C−C and C−X (X = O, N, S, P, etc.) bonds. The reactive allylic halides and allylic alcohol derivatives such as allylic ethers or esters were traditionally employed as allylic partners.1−4 In recent decades, the direct use of the cheap and easily available allylic alcohols as the alkylating reagents to develop an atom-economic and environmentally friendly Tsuji−Trost reaction has received much attention, since the reactions with allylic alcohols usually give water as the sole byproduct.5−8 However, owing to the poor leaving character of the hydroxy group, a stoichiometric or catalytic amount of an acid activator such as As2O3, B2O3, BEt3, or Ti(O-iPr)4 is generally required.6−8 Therefore, it is still highly desired to develop an activator-free Tsuji−Trost reaction of allylic alcohol. Ozawa reported the first activator-free Tsuji−Trost reactions of allylic alcohols for the waste-free synthesis of allylic amines,9 in which the activation of allylic alcohol involving a palladium hydride intermediate was well documented by both theoretical calculations and experimental approaches (Scheme 1A).10−13 Afterward, the reaction mechanisms involved in secondary bond activation such as hydrogen bonding, supported by theoretical calculations,14−18 were proposed in some activatorfree Tsuji−Trost reactions of allylic alcohols (Scheme 1B).19−25 However, probably owing to the elusory character of the secondary bond, experimental evidence is still rare.26 Very recently, we developed an extra activator-free Tsuji− Trost reaction of allylic alcohols with stabilized phosphonium ylides in water media (Table 1, run 1).27 Control experiments using diffusion-ordered spectroscopy (DOSY) NMR and ESIHRMS techniques indicated that a hydrogen bond is probably formed between water and allylic alcohols and therefore leads © 2019 American Chemical Society

to the activation of allylic alcohols. Hence, one may hypothesize that aprotic solvents may hinder the reaction. However, to our surprise, the reaction of cinnamyl alcohol 1a and stabilized phosphonium ylide 2a heated at 60 °C in cyclohexane, a typical nonpolar and aprotic solvent, could still afford the desired product 3a in 65% isolated yield (run 2). Likewise, the reaction in toluene and CHCl3 could also give Received: March 2, 2019 Published: May 15, 2019 7468

DOI: 10.1021/acs.joc.9b00616 J. Org. Chem. 2019, 84, 7468−7473

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The Journal of Organic Chemistry Table 1. Solvent Effects of the Reactiona

run

solvent

yield/%b

1 2 3 4

H2O cyclohexane toluene CHCl3

92 65 60 10

a Unless otherwise noted, the mixture of 1a (0.36 mmol), 2a (0.3 mmol), [Pd(ally)Cl]2 (2.5 mol %), and dppf (5 mol %) in a solvent (0.5 mL) was sealed under N2 in a Schlenk tube and heated at 60 °C for 10 h; then HCHO (37% w/w in water, 3 equiv) was added. The mixture was stirred at rt for another 6 h and monitored by TLC and/ or GC−MS. b Isolated yield based on 2a. dppf: 1,1′-bis(diphenylphosphino)ferrocene

the desired 3a in moderate to low yields (runs 3−4). Thus, a new activation mechanism may be involved. With our continuous interest in alkylation with alcohols as the synthons and NMR studies,28−33 herein, we investigated the activation mechanism of allylic alcohols with stabilized phosphonium ylides by using NMR techniques. According to our results, we put forward a new mechanism (Scheme 1C), in which a substrate self-assisted activation of allylic alcohols is achieved via a binding six-membered-ring complex, which are formed by the formation of the secondary bonds, a hydrogen bond and P···O noncovalent bond between allylic alcohol and phosphonium ylide. Variable-temperature 1H NMR, DOSY, the Job’s method, 1 H NMR titration experiments, and two-dimensional nuclear overhauser enhancement spectroscopy (NOESY) were conducted to investigate the possible activation mechanism in the activator-free Tsuji−Trost reaction of allylic alcohol with a stabilized phosphonium ylide.34 Variable-temperature 1H NMR experiments35,36 for the mixture of cinnamyl alcohol 1a and stabilized phosphonium ylide 2a were initially carried out to investigate the possible formation of an intermolecular hydrogen bond. The signals of a hydroxy group in cinnamyl alcohol 1a were carefully examined. The results showed that the chemical shift of the OH group moves upfield from 4.88 (293 K) to 4.71 (333 K) and to 4.38 (393 K), when the test temperature was increased, revealing the possible formation of an intermolecular hydrogen bond between cinnamyl alcohol 1a and stabilized phosphonium ylide 2a (Figure 1). The possible intermolecular hydrogen bond between cinnamyl alcohol 1a and stabilized phosphonium ylide 2a was then investigated by DOSY NMR techniques.37,38 Equation 1 indicates that the self-diffusion coefficient (D) of a given molecular complex under controlled conditions has a negative relation with its hydrodynamic radius (RH).37 As shown in Figure 2, the diffusion coefficient of cinnamyl alcohol 1a in CDCl3 is 1.98 × 10−9 (sample 1, see spectra highlighted in red), while it significantly decreased to 1.61 × 10−9 m2/s, when phosphonium ylide 2a was added (sample 2, see spectra highlighted in green). These results suggested that a much bigger complex might be formed by binding cinnamyl alcohol 1a with phosphonium ylide 2a and that a hydrogen bond

Figure 1. Variable-temperature NMR analysis.

Figure 2. DOSY NMR analysis.

might be formed between cinnamyl alcohol 1a and phosphonium ylide 2a. D=

KT 6πηRH

(1)

More detailed host−guest binding interactions of cinnamyl alcohol 1a and stabilized phosphonium ylide 2a were further investigated by Job’s method and 1H NMR titration experiments.39−43 The Job’s method of continuous variation to the NMR results for the binding complex in rapid exchange was employed to confirm the expected 1:1 binding stoichiometry for cinnamyl alcohol 1a and stabilized phosphonium ylide 2a. According to the Job’s method, the total concentration, [1a] + [2a], is constant in all solutions; therefore, the concentration of the possible 1:1 binding complex will reach a maximum value when [1a] = [2a]. As shown in Figure 3, the maximum in the curve for phosphonium ylide 2a is at XH = 0.5, indicating that the stoichiometry between cinnamyl alcohol 1a and stabilized phosphonium ylide 2a is 1:1 as expected.44 Moreover, the theoretical fitting of the 1H NMR titration experimental results gave an association constant of Ka = 153 ± 7469

DOI: 10.1021/acs.joc.9b00616 J. Org. Chem. 2019, 84, 7468−7473

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The Journal of Organic Chemistry

Figure 6. Test sample: 1b and 2b. Figure 3. Job’s method.

6 M−1 with a 0.93 ± 0.01 binding ratio at 298 K in CDCl3,44 which further indicated the possible 1:1 binding complex (Figure 4). These experiment results suggested that a substrate

Figure 7. 2D NOESY spectrum of the mixture of 1b and 2b.

interact to give rise to the NOE effect among them. According to the dependence between longer range NOEs and internuclear separation, it could be inferred that the internuclear separation among these protons (H1, H2, and H3) is within ca. 0.5 nm.46 Hence, a six-membered ring is probably formed by the formation of the secondary bonds, hydrogen bond and P···O noncovalent bond (Figure 6). Based on these control experiments and the literature reports,5−8,14−18,27 a possible reaction mechanism was depicted in Scheme 2. As to this novel extra activator-free Tsuji−Trost reaction of allylic alcohols 1 with stabilized phosphonium ylides 2, two activation paths might be involved in the activation of allylic alcohols 1 (Scheme 2). When the reaction is carried out in water media, allylic alcohols 1 may be mainly activated by water via a hydrogen bond (path a), followed by the cleavage of the C−O bond gives π-allyl palladium intermediate 6. When the reaction is carried out in a nonpolar aprotic solvent such as cyclohexane, the substrate self-assisted activation of allylic alcohols via six-membered-ring complex 4 may be the main path (path b), thus giving π-allyl palladium intermediate 6. Then a nucleophilic attack of π-allyl palladium intermediate 6 by stabilized phosphonium ylides 2, followed by a proton transfer of phosphonium salt intermediate 7, gives a new stabilized phosphonium ylide 8, liberating Pd(0) to continue the catalytic cycle and a molecule of water. Finally, a one-pot Wittig reaction of phosphonium ylide 8 with formaldehyde gives the desired product 3. In conclusion, variable-temperature 1H NMR, DOSY, the Job’s method, and 1H NMR titration and 2D NOESY experiments were employed to investigate the activation mechanism of allylic alcohols with stabilized phosphonium ylides in a Tsuji−Trost reaction, and the results revealed that a substrate self-assisted activation of allylic alcohols probably

Figure 4. 1H NMR titration experiments.

self-assisted secondary bond activation of allylic alcohol via a 1:1 binding six-membered-ring complex might be present (Figure 5). The rigid six-membered-ring complex is possibly

Figure 5. Possible 1:1 binding six-membered-ring complex.

formed by the formation of a hydrogen bond and P···O noncovalent bond between cinnamyl alcohol 1a and stabilized phosphonium ylide 2a (Figure 5). Finally, the two-dimensional NOESY method, which could provide detailed information on internuclear spatial proximity,45,46 was employed to further confirm the possible formation of the P···O noncovalent bond between the allylic alcohol and stabilized phosphonium ylide (Figure 5).47 In order to obtain a clear and unambiguous result, the mixture of 2-methylprop-2-en-1-ol (1b) and stabilized phosphonium ylide (2b) was selected as the test sample (Figure 6). As shown in Figure 7, the key cross-peaks (H1/H2, δ 1.72/3.75, marked with a triangle; H1/H3, δ 1.72/7.60, marked with a rectangle) revealed that the dipolar couplings between allylic alcohol 1b and stabilized phosphonium ylide 2b may be present. These results suggested that the formation of a P···O noncovalent bond brings these nuclei close enough, and therefore, they can 7470

DOI: 10.1021/acs.joc.9b00616 J. Org. Chem. 2019, 84, 7468−7473

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Materials and Methods for the Variable-Temperature NMR Analysis. A mixture of cinnamyl alcohol 1a (0.0156 mmol, 2.1 mg), stabilized phosphonium ylide 2a (0.010 mmol, 3.8 mg), and DMSOd6 (0.50 mL) was prepared as the test sample. Then variabletemperature NMR experiments (293, 333, and 393 K) were conducted. Materials and Methods for the DOSY NMR Analysis. Two samples were prepared: a mixture of cinnamyl alcohol 1a (0.0075 mmol, 1.0 mg) and CDCl3 (0.50 mL) (sample 1), a mixture of cinnamyl alcohol 1a (0.0075 mmol, 1.0 mg), stabilized phosphonium ylide 2a (0.0079 mmol, 3.0 mg), and CDCl3 (0.50 mL) (sample 2). Then the DOSY NMR spectra were recorded by using a simple Carr− Purcell spin−echo sequence (diffusion time = 0.08 s, FG pulse width = 0.002 s, relaxation time = 1 s, points = 16, BASE = 2, and scans = 8) at 298 K. Materials and Methods for the Investigation on the Host− Guest Binding Interactions by the Job’s Method. A stock solvent of cinnamyl alcohol 1a (0.026 mmol, 3.5 mg) in 2.2 mL of CDCl3 was prepared, and the concentration of cinnamyl alcohol 1a is 0.012 mmol/mL. A stock solvent of stabilized phosphonium ylide 2a (0.026 mmol, 9.9 mg) in 2.2 mL of CDCl3 was prepared, and the concentration of stabilized phosphonium ylide 2a is 0.012 mmol/mL. The solutions of the cinnamyl alcohol 1a (host) and stabilized phosphonium ylide 2a (guest) were mixed to NMR tubes according to certain proportions. The CH2 chemical shift of free host is 4.337. Then the host−guest binding interactions was studied by the Job’s method at 293 K. Materials and Methods for the 1H NMR Titration. Cinnamyl alcohol 1a (0.0186 mmol, 2.5 mg) was dissolved in 0.55 mL of CDCl3, and the concentration was 0.034 mmol/mL. A certain amount of phosphonium ylide 2a was added to the above solution. All 1H NMR titration experiments were performed at 293 K. Materials and Methods for the 2D NOESY Analysis. A mixture of allylic alcohol 1b (0.060 mmol, 4.3 mg) and stabilized phosphonium ylide 2b (0.013 mmol, 4.7 mg) was dissolved in CDCl3 (0.50 mL) for 24 h. Then 2D NOESY spectrum was recorded with a spectral width of 15 ppm, 1 K data points in the t2 time domain, and 256 t1 increments, a relaxation delay of 1.5 s, and a mixing time of 2.5 s.

Scheme 2. Possible Activation Model of Cinnamyl Alcohol under Extra Activator-Free Conditions

performed via the 1:1 binding six-membered-ring complex. Notably, to our best knowledge, this is the first time investigation of the palladium-catalyzed secondary bond activation of allylic alcohols by using NMR techniques and it may provide a practical experimental approach to investigate the secondary bond in mechanism studies to scientifically design and explore new reactions.

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EXPERIMENTAL SECTION

General Information. The 1H NMR, DOSY, the Job’s method, and NOE spectra were recorded on a JNM-ECZ600R/S3 (Jeol, Japan) (600 MHz) using tetramethylsilane as an internal reference. Chemical shifts (δ) and coupling constants (J) were expressed in ppm and Hz, respectively. The following abbreviations are used in reporting NMR data: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet. Mass spectra were measured on an Agilent GC−MS5890A/5975C Plus spectrometer (EI). All air-sensitive manipulations were conducted under a nitrogen atmosphere by standard Schlenk techniques. Phosphonium ylides were prepared from α-halo carbonyl compounds according to known procedures.48 Chemicals were purchased from the Energy and Tansoole Chemical Reagent Co. and used as received. Typical Procedure for the Activator-Free Tsuji−Trost Reaction of Cinnamyl Alcohol 1a and Stabilized Phosphonium Ylides 2a. A mixture of cinnamyl alcohol 1a (0.36 mmol, 48.2 mg), phosphonium ylide 2a (0.30 mmol, 114.0 mg), [Pd(allyl)Cl]2 (2.8 mg, 2.5 mol %), and dppf (8.3 mg, 5 mol %) in cyclohexane (0.50 mL) was heated under nitrogen at 60 °C for 10 h. After the mixture cooled down to room temperature, formalin (37% in water, w/w, 0.068 mL, 0.90 mmol) was added, and the resulting mixture was stirred for 6 h. Then the solvent was evaporated, and the residue was purified by silica gel chromatography, eluting with ethyl acetate/ petroleum ether (0/100−1/20), to give compound 3a in 65% yield. (E)-2-Methylene-1,5-diphenylpent-4-en-1-one (3a): 27 colorless oil (48.4 mg, 65% yield); 1H NMR (600 MHz, CDCl3) δ 7.79− 7.75 (m, 2H), 7.58−7.52 (m, 1H), 7.44 (t, J = 7.8 Hz, 2H), 7.36 (d, J = 7.2 Hz, 2H), 7.30 (t, J = 7.2 Hz, 2H), 7.21 (t, J = 7.2 Hz, 1H), 6.51 (d, J = 15.6 Hz, 1H), 6.30 (dt, J = 15.6, 7.2 Hz, 1H), 5.94 (s, 1H), 5.71 (s, 1H), 3.38 (d, J = 7.2 Hz, 2H); 13C {1H} NMR (150 MHz, CDCl3) δ 197.8, 146.6, 137.8, 137.4, 132.6, 132.3, 129.6, 128.6, 128.3, 127.4, 126.9, 126.7, 126.3, 35.5; MS (EI) m/z 248, 233, 219, 215, 203, 190, 178, 169, 152, 143, 128, 115, 105, 91, 77.

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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.joc.9b00616.

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Details of the control experiments for mechanistic studies, NMR spectra and titration experiments, and host−guest interactions (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiantao Ma: 0000-0002-0012-6944 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Foundation of Key Scientific and Technological Project of Henan Province (192102310031 and 182400410166), the Key Research Programs in Universities of Henan Province (19B150018 and 18A150049), and the Nanhu Scholars Program for Young Scholars of XYNU and Young Core Instructor Program of XYNU (2018GGJS-05). 7471

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