Facile Preparation of (2Z,4E)-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate

Summary

The ruthenium-catalyzed olefination of electron-deficient alkenes with allyl acetate is described here. By using aminocarbonyl as a directing group, this external oxidant-free protocol has high efficiency and good stereo- and regioselectivity, opening a novel synthetic route to (Z,E)-butadiene skeletons.

Abstract

Direct cross-coupling between two alkenes via vinylic C-H bond activation represents an efficient strategy for the synthesis of butadienes with high atomic and step economy. However, this functionality-directed cross-coupling reaction has not been developed, as there are still limited directing groups in practical use. In particular, a stoichiometric amount of oxidant is usually required, producing a large amount of waste. Due to our interest in novel 1,3-butadiene synthesis, we describe the ruthenium-catalyzed olefination of electron-deficient alkenes using allyl acetate and without external oxidant. The reaction of 2-phenyl acrylamide and allyl acetate was chosen as a model reaction, and the desired diene product was obtained in 80% isolated yield with good stereoselectivity (Z,E/Z,Z = 88:12) under optimal conditions: [Ru(p-cymene) Cl₂]₂ (3 mol %) and AgSbF₆ (20 mol %) in DCE at 110 ºC for 16 h. With the optimized catalytic conditions in hand, representative α- and/or β-substituted acrylamides were investigated, and all reacted smoothly, regardless of aliphatic or aromatic groups. Also, differently N-substituted acrylamides have proven to be good substrates. Moreover, we examined the reactivity of different allyl derivatives, suggesting that the chelation of acetate oxygen to the metal is crucial for the catalytic process. Deuterium-labeled experiments were also conducted to investigate the reaction mechanism. Only Z-selective H/D exchanges on acrylamide were observed, indicating a reversible cyclometalation event. In addition, a kinetic isotope effect (KIE) of 3.2 was observed in the intermolecular isotopic study, suggesting that the olefinic C-H metalation step is probably involved in the rate-determining step.

Introduction

Butadienes are widely occurring and are commonly found in many natural products, drugs, and bioactive molecules¹. Chemists have made intense efforts to develop an efficient, selective, and practical synthetic methodology for the synthesis of 1,3-butadienes^2,3_. Recently, direct cross-couplings between two alkenes via double vinylic C-H bond activation was developed, representing an efficient strategy for the synthesis of butadienes, with high atomic and step economy. Among them, the palladium-catalyzed cross-coupling of two alkenes has attracted much attention, providing (E,E)-configured butadienes via alkenyl-Pd species^4,5. For example, Liu's group developed a Pd-catalyzed butadiene synthesis by the direct cross-coupling of alkenes and allyl acetate (Figure 1 and Equation 3)⁴. Meanwhile, the functional group-directed cross-coupling between alkenes provided butadienes with excellent (Z,E)-stereoselectivity due to the olefinic C-H cyclometalation event, representing a complementary method⁶. To date, some directing groups, such as enolates, amides, esters, and phosphates, have been successfully introduced to the cross-coupling between alkenes, providing a series of valuable and functionalized 1,3-butadienes. However, the directed cross-coupling reaction has not been developed, as there are still limited directing groups in practical use. In particular, a stoichiometric amount of oxidant is usually required to maintain the catalytic cycle, which produces a large amount of organic and inorganic wastes. There are very limited examples using electron-rich alkenes as the coupling partner.

Allyl acetate and its derivatives have been deeply investigated in organic transformations as powerful allylation and olefination reagents, including catalyzed cross-coupling, Friedel-Crafts allylation of electron-rich arenes, and catalytic C-H activation of electron-deficient arenes (Figure 1 and Equation 1)⁷. More recently, the Loh group developed a rhodium(III)-catalyzed C-H allylation of electron-deficient alkenes with allyl acetates, creating 1,4-dienes (Figure 1 and Equation 2)⁸. Meanwhile, the Kanai group reported a dehydrative direct C-H allylation with allylic alcohols by using a Co(III) catalyst⁹. Interestingly, Snaddon and co-workers disclosed a novel cooperative catalysis-based method for the direct asymmetric α-allylation of acyclic esters¹⁰. Very recently, the Ackermann group reported several novel allylation examples using inexpensive Fe, Co, and Mn catalysts¹¹. These reports have made breakthroughs in allylation and olefination reactions, but double-bond migration and poor regioselectivity are usually inevitable and are not easily controlled. Hence, developing more efficient and selective reaction patterns of allyl acetates to construct valuable molecules is still highly desirable. With our interest in novel 1,3-butadiene synthesis via C-H olefination, we assumed that allyl acetate could be introduced to the directed allylation of electron-deficient alkenes, first delivering 1,4-diene. Then, the more thermodynamically stable 1,3-butadiene could be formed after the migratory isomerization of the C-C double bond⁷, forming the diene product that cannot be obtained by cross-coupling using electron-rich alkenes, such as propene, as coupling partner⁶. Here, we report an inexpensive Ru(III)-catalyzed olefinic C-H bond olefination of acrylamides with allyl acetates in the absence of any oxidant, which opens a novel synthetic route for the creation of (Z,E)-butadienes (Figure 1 and Equation 4)¹³.

Protocol

Caution: Please consult all relevant material safety data sheets (MSDS) before use. All cross-coupling reactions should be performed in vials under a sealed argon atmosphere (1 atm).

1. Preparation of Butadienes by the Olefination of Acrylamides with Allyl Acetate

1. Dry a screw-cap vial (8 mL) with a compatible magnetic stir bar in an oven at 120 °C for over 2 h. Cool the hot vial to room temperature by blowing on it with inert gas before use.
2. Use an analytical balance and weigh 3.7 mg (~3 mol %, ~0.005 mmol) of [Ru(p-cymene)Cl₂]₂ (brown powder) and 13.7 mg (20 mol%, 0.04 mmol) of AgSbF₆ (white solid) into the above reaction vial.
   NOTE: Since this is a new methodology, the cross-coupling reactions have been performed on a small scale for proof of concept to reduce waste formation. AgSbF₆ is used as an additive that may abstract chloride to generate a cationic ruthenium complex for electrophilic C-H bond activation¹³. Other silver salts, such as Ag₂CO₃, have also been tested, but no product was detected. The weight of the catalyst ([Ru(p-cymene)Cl₂]₂) is not very accurate and is in the range of 3.4-3.9 mg.
3. Add 1 mL of dry 1,2-dichloroethane to the reaction vial.
   NOTE: The amount of solvent is flexible-1 mL of 1,2-dichloroethane is just about enough to meet the minimal requirement of volume for the cross-coupling reaction. However, a little more (~0.1 mL) solvent is also permissible for a reaction of this scale. 1,2-dichloroethane was dried over a 3-Ȧ molecular sieve before its use.
4. Use an analytical balance and add acrylamide (0.2 mmol, 1.0 equiv; solid or oil) to the above reaction vial.
5. Use a micro-syringe to add 43 µL (0.4 mmol, 2.0 equiv) of allyl acetate (a colorless liquid) to the above reaction vial.
   NOTE: Here, an excess amount of allyl acetate is required to inhibit the homo-coupling of acrylamide and to ensure that the acrylamide is fully converted. The product yield decreases if less allyl acetate (1.5 equiv) is added. The addition of more allyl acetate (3.0 equiv) cannot further improve the yield. In practice, there is no observed homo-coupling of allyl acetate, and the residual allyl acetate could be recovered.
6. Gently blow on the reaction vial with argon gas and cover the vial with a compatible screw-cap as quickly as possible.
   NOTE: The vial should be covered with a screw-cap as quickly as possible because an inert atmosphere is crucial for the cross-coupling reaction. It is better to perform the above protocol in a glove box.
7. Stir the reaction mixture at room temperature for an additional 5 min.
8. Heat the reaction vial to 110 °C in an oil bath with stirring for 16-18 h.
   NOTE: Generally, a color change to dark red is an indication that the reaction is taking place.
9. After cooling the vial down, use ethyl acetate:petroleum ether (2:1 or 1:3) mixtures as the solvent to develop the thin layer chromatography (TLC) plates to monitor the progress of the reaction by comparing the mixture to an acrylamide standard.
   NOTE: Depending upon the nature of the starting materials, the reaction may not go to completion. Typical R_f values of the products and starting materials are in the range of 0.3 - 0.7. The acrylamide starting material has been observed as a lower running spot than the butadiene product.
10. Dissolve the crude product in a minimum of DCM and load it onto a silica column wet with petroleum ether. Separate the cross-coupling product via column chromatography using a mixture of ethyl acetate:petroleum ether (1:100 to 1:4) as the eluent.
    1. Collect the eluent in a separate flask, evaporate the solvent on a rotatory evaporator, and place it under a high vacuum for a minimum of 2 h.
    2. Obtain approximately 20-50 mg of product for characterization by NMR spectroscopy.
       NOTE: The reaction mixture should be applied to column chromatography for purification directly after reaction completion.

2. Characterization of Dienamides

1. .Characterize and assess the purity of the final product using ¹H and ¹³C NMR spectroscopy¹⁴. Typically, the chemical shift of the carbonyl carbon appears near 170 ppm on the ¹³C NMR spectrum. The three sp² protons of the butadiene functional group are represented by characteristic peaks near 6.0 and 5.6 ppm.
2. Use infrared spectroscopy¹⁴ to identify the characteristic carbonyl and C-C double-bond peak of the diene product.
3. Determine the molecular mass of the product and further validate the identity using high-resolution mass spectrometry (HRMS)¹⁴.
4. Determine the melting point of the solid products¹⁴.

Results

Our efforts were focused on the preparation of 1,3-butadiene from acrylamide and allyl acetate.

Table 1 illustrates the optimization of conditions, including the screening of various additives and solvents, using [Ru(p-cymene)Cl₂]₂ as the catalyst. After screening a series of representative solvents, we were pleased to find that the product yield dramatically improved to 80%, with ...

Discussion

[Ru(p-cymene)Cl₂]₂ is a cheap, easily accessible, air-stable, and highly active Ru-based catalyst with excellent functional group tolerance that efficiently operates under mild reaction conditions to give C-H/C-H coupling butadiene products. Silver salt AgSbF₆ was used as an additive that may abstract the chloride of [Ru(p-cymene)Cl₂]₂ to generate a cationic ruthenium complex for the following C-H bond activation. However, only α-subst...

Materials

Name	Company	Catalog Number	Comments
Allyl Acetate	TCI	A0020	> 98.0%(GC), 25 mL package
Dichloro(p-cymene)ruthenium(II) dimer	TCI	D2751	> 95.0%(T), 5 g package
Silver hexafluoroantimonate	TCI	S0463	> 97.0%(T),  5 g package
1,2-Dichloroethane	TCI	D0364	> 99.5%(GC), 500 g package
Rotavapor	EYELA	N-1200A	Use to dry solvent
Silica gel	Merck	107734	Silica gel 60 (0.063-0.2 mm), for column chromatoraphy