Selective Alkene Insertion into Inert Hydrogen–Metal Bonds Catalyzed

Article Cite This: Organometallics XXXX, XXX, XXX-XXX

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Selective Alkene Insertion into Inert Hydrogen−Metal Bonds Catalyzed by Mono(phosphorus ligand)palladium(0) Complexes Nobuyuki Komine,* Ryo Ito, Hiromi Suda, Masafumi Hirano, and Sanshiro Komiya† Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16, Nakacho, Koganei, Tokyo 184-8588, Japan S Supporting Information *

ABSTRACT: Isolated mono(phosphorus ligand)palladium(0) complexes catalyzed alkene insertions into hydrogen−tungsten bonds. These insertions using WHCp(CO)3 with ethyl acrylate and dimethyl fumarate smoothly gave the corresponding alkyltungsten complexes. Kinetic studies involving the stoichiometric reactions and DFT calculations suggest the following steps: (i) formation of a mono(phosphorus ligand)mono(alkene)palladium(0) species, (ii) subsequent reaction of a metal hydride with the palladium(0), (iii) insertion of the coordinated alkene into the resulting palladium hydride, and (iv) reductive elimination between the alkyl and metal on the palladium center to release the alkylmetal species with regeneration of a palladium(0) by a reaction with alkene.

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INTRODUCTION Alkene insertion into a hydrogen−metal bond, called hydrometalation, is an interesting and important reaction related to alkene transformation.1 Hydrometalation with a main-groupmetal hydride, such as hydrosilylation and hydroboration, is a useful reaction in synthetic organic chemistry. Similarly, hydrometalation with a transition-metal hydride is one of the most well established, important, and fundamental reaction steps found in many transition-metal catalyses such as hydrogenation, hydroformylation, and dimerization. However, several hydride complexes of transition metals are inert to the insertion reactions. For example, molybdenum and tungsten hydrides MHCp(CO)3 (M = Mo, W) do not cause insertions of alkenes and alkynes unless they contain strongly electron withdrawing groups. In fact, Stone and co-workers2 reported the addition of tungsten hydrides WHCp(CO)3 into tetrafiuoroethylene, whereas ethylene or acrylonitrile remained unreacted. Trans addition to alkynes containing strongly electron withdrawing groups such as acetylenedicarboxylate and cyanoacetylene were also reported.3 In contrast, the reaction with methyl propionate in refluxing CHCl3 gave the insertion product with anti-Markovnikov selectivity but the product yield remained only 10%.4 On the other hand, during the course of our studies concerning the reactivity of hydridopalladium heterodinuclear complexes,5 we found that a zerovalent palladium complex effectively catalyzes alkene insertion into inactive metal hydrides. In the presence of a zerovalent palladium complex such as tetrakis(triphenylphosphine)palladium(0), smooth catalytic insertions of alkene and alkyne into inactive transition-metal bonds such as molybdenum−, tungsten−, and manganese−hydrogen bonds have proceeded with Markovnikov and syn selectivity.6 Among the zerovalent palladium complexes we have screened, mono(phosphine)palladium(0) complexes Pd(η2:η2-diallyl © XXXX American Chemical Society

ether)(PR3) (1-R), whose preparation was originally documented by Pörschke and co-workers,7 were found to be the best catalysts for alkene insertion into inactive transition-metal bonds such as molybdenum−, tungsten−, and or manganese− hydrogen bonds. In this paper, we report a full account of the details of the stoichiometric and catalytic studies involving the mechanism on the basis of kinetic studies and DFT calculations.

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RESULTS AND DISCUSSION Alkene Insertion into Hydrogen−Metal Bonds Assisted by Mono(phosphorus ligand)palladium(0) Complexes. When a mono(triphenylphosphine)palladium complex having a diallyl ether, Pd(η2:η2-diallyl ether)(PPh3) (1-Ph) (1 mol %), was used as a catalyst, dimethyl fumarate quickly inserted into the W−H bond in WHCp(CO)3 at 30 °C to give the alkyl complex in 88% yield within 12 min (eq 1 and Figure 1).

Since WHCp(CO)3 was completely inert for the hydrometalation in the absence of catalyst under these conditions, the catalytic activity of 1-Ph was compared with that of some palladium(0) triphenylphosphine complexes in this reaction. When the reaction was performed with Pd(PPh3)4 (2-Ph) (1 mol %), insertion did not proceed even after 24 h (eq 1 and Figure 1). With the bis(triphenylphosphine)complex Pd(η2dimethyl fumarate)(PPh3)2 (3-Ph), the reaction proceeded slowly and the reaction took more than 6 h for completion. Received: August 2, 2017

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DOI: 10.1021/acs.organomet.7b00593 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

discrete catalytic activity was observed for 1-OPh and 1-OXyl. In these systems, the palladium(0) complexes with one phosphorus ligand having moderate electron-donating properties and moderate cone angles are considered to be effective catalysts.8 The scope of alkenes in the hydrometalation with WHCp(CO)3 catalyzed by 1-Ph or 1-Cy is given in Table 1. The insertion into acrylonitrile proceeded quickly to give the corresponding alkyl tungsten complex in 100% yield within 6 min in the presence of 1 mol % of 1-Ph (entry 1). Similarly, insertions of ethyl acrylate, dimethyl fumarate, and dimethyl maleate into tungsten hydride took place immediately (entries 2, 8, and 9). No insertion took place for methyl vinyl ketone, vinyl acetate, t-BuCHCH2, styrene, and methyl crotonate (entries 3, 4, 6, and 7). 1-Hexene did not produce the insertion product but gave 2-hexenes in C6D6 after 3 days at 30 °C (entry 5). Since the present reaction is limited to electron-deficient alkenes, the electronic nature of the hydride in M−H may play an important role in this reaction rather than the reaction involving a hydrogen radical.9 The scope of some transition-metal hydrides is also shown in Table 1. When MoHCp(CO)3 or MnH(CO)5 were treated with methyl acrylate and acrylonitrile in the presence of 1 mol % of 1-Ph or 1-Cy at 30 °C, selective and instant alkene insertion also took place to give the corresponding alkyl complexes (entries 11, 12, 18, and 19). However, similar reactions using WHCp*(CO)3 and FeHCp(CO)2 were sluggish. For example, after treatment of FeHCp(CO)2 with methyl acrylate in the presence of 1-Ph (1 mol %) for 35 min at room temperature, the corresponding insertion product was obtained only in 19% yield, but the product was eventually increased to 73% after 275 min (entry 22). When the reaction of WHCp*(CO)3 with methyl acrylate was carried out, the yield of the insertion product remained at only 35% yield (entry 24). The insertion of dimethyl fumarate and dimethyl maleate into molybdenum hydride also took place immediately (entries 15 and 16). However, the insertion of MnH(CO)5 was much slower than that of molybdenum or tungsten hydride (entries 20 and 21). The reaction of FeHCp(CO)2 with dimethyl fumarate gave dimethyl succinate, the hydrogenated product, instead of the insertion product (entry 23). Since the BDEs for active hydride complexes for alkene insertion such as Ni−H (58 kcal/mol) or Co−H (46 kcal/mol) (and also for the BDEs for Ni−Me and Co−Me) are significantly larger than that for Mn−H (30 kcal/mol),10 the BDE seems to have little reaction relationship for the insertion. A more likely explanation is the reactivity depends on the pKa value of M−H. In fact, the more acidic hydride tends to proceed by the insertion reaction of methyl acrylate at a faster rate in comparison to a less acidic hydride such as FeHCp(CO)2.11 Mechanistic Study for Alkene Insertion into a Hydrogen−Tungsten Bond Catalyzed by Mono(phosphine)palladium(0) Complexes. As described in the previous section, mono(phosphine)palladium(0) complexes having diallyl ether (1-R) show high catalytic activity for the hydrometalation of alkenes with transition-metal hydride complexes. In order to clarify the reaction mechanism of hydrometalation of alkenes catalyzed by a mono(phosphine)palladium(0) complex, stoichiometric reactions, kinetic studies, and deuterium labeling experiments12 were carried out. Stoichiometric Reactions in Relation to the Catalytic Reaction. Stoichiometric reactions of various mono(phosphorus ligand)palladium(0) complexes having diallyl ether (1-R) with dimethyl fumarate were investigated. As a result, they immediately gave a mixture of the mono(dimethyl fumarate)bis(phosphorus

Figure 1. Time−yield curves for insertion of dimethyl fumarate into WHCp(CO)3 by palladium(0) triphenylphosphine catalysts: Pd(0) cat = Pd(diallyl ether)(PPh3) (1-Ph, black ■), Pd(PPh3)4 (2-Ph, blue ■), Pd(dimethyl fumarate)(PPh3)2 (3-Ph, red ■). Conditions: [dimethyl fumarate]0 = 36 mM, [WHCp(CO)3]0 = 36 mM, [Pd(0) complex] = 0.36 mM, solvent C6D6, temperature 30 °C.

Complex 1-Ph shows the highest catalytic activity among these catalysts. With 1-Ph as an active catalyst for the hydrometalation in hand, we screened the catalytic activity of various mono(phosphorus ligand)palladium(0) complexes (eq 2 and Figure 2). In the

presence of 1 mol % of palladium(0) complexes having various tertiary phosphorus ligands, Pd(η2:η2-diallyl ether)(PR3) (1-R), the time−yield curves for the catalytic reactions insertion of dimethyl fumarate into the tungsten hydride complex are shown in Figure 2.

Figure 2. Time−yield curves for insertion of dimethyl fumalate into WHCp(CO)3 catalyzed by Pd(η2:η2-diallyl ether)(PR3): R = Me (1-Me, ●), Et (1-Et, ▲), Ph (1-Ph, ■), iPr (1-iPr, ◆), Cy (1-Cy, ○), OiPr (1-OiPr, △), OPh (1-OPh, □), OXyl (1-OXyl, ◇). Conditions: [dimethyl fumarate]0 = 36 mM, [WHCp(CO)3]0 = 36 mM, [Pd(diallyl ether)(PR3)] = 0.36 mM, solvent C6D6, temperature 30 °C.

In the scope of mono(phosphorus ligand)palladium complexes, 1-Ph and 1-OiPr are the catalysts best suited for this reaction. In contrast, the catalytic activity of trialkylphosphine complexes such as 1-Me, 1-Et, 1-iPr, and 1-Cy is diminished, and very B

DOI: 10.1021/acs.organomet.7b00593 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Table 1. Catalytic Insertion of Alkenes into an M−H Bond Promoted by Pd(η2:η2-diallyl ether)(PPh3) (1-Ph)a

entry

MHLn

R1

R2

R3

time/min

yield/%

1 2 3 4 5 6b 7b 8 9 10 11b 12 13 14 15 16c 17 18c 19 20 21 22 23 24

WHCp(CO)3 WHCp(CO)3 WHCp(CO)3 WHCp(CO)3 WHCp(CO)3 WHCp(CO)3 WHCp(CO)3 WHCp(CO)3 WHCp(CO)3 WHCp(CO)3 MoHCp(CO)3 MoHCp(CO)3 MoHCp(CO)3 MoHCp(CO)3 MoHCp(CO)3 MoHCp(CO)3 MoHCp(CO)3 MnH(CO)5 MnH(CO)5 MnH(CO)5 MnH(CO)5 FeHCp(CO)2 FeHCp(CO)2 WHCp*(CO)3

H H H H H H H H CO2Me H H H H H H CO2Me H H H H CO2Me H H H

H H H H H H H CO2Me H Me H H H H CO2Me H Me H H CO2Me H H CO2Me H

CN CO2Me COMe OCOMe n-Bu t-Bu Ph CO2Me CO2Me CO2Me CN CO2Me t-Bu Ph CO2Me CO2Me CO2Me CN CO2Me CO2Me CO2Me CO2Me CO2Me CO2Me