Catalytic Reactions of Terminal Alkynes Using Rhodium(I) Complexes

Perspective Cite This: ACS Catal. 2018, 8, 6127−6137

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Catalytic Reactions of Terminal Alkynes Using Rhodium(I) Complexes Bearing 8‑Quinolinolate Ligands Fumitoshi Kakiuchi,* Shotaro Takano, and Takuya Kochi Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan ABSTRACT: 8-Quinolinolate is a monoanionic, hard, strongly σ-donating ligand that can form strong chelates with various metals. However, studies of the catalytic activities of soft transition metal complexes such as greater than second row, lowvalent, late-transition metals having 8-quinolinolate ligands had not been well explored until recently. In recent years, several research groups including our own have studied and developed various reactions of terminal alkynes using rhodium(I) catalysts containing an 8-quinolinolate ligand. In this Perspective, we surveyed the transformations of terminal alkynes using 8-quinolinolato rhodium(I) catalyst such as additions of alcohols, amines, and thiols as well as polymerization, trimerization, dimerization, and alkyne/alkene [2 + 2] cycloaddition. Mechanistic studies based on theoretical calculations of hydroalkoxylation of alkynes and alkyne/alkene [2 + 2] cycloaddition reactions have also been described. KEYWORDS: 8-quinolinolate ligands, terminal alkynes, hydroalkoxylation, hydroamination, hydrothiolation, anti-Markovnikov addition, alkyne/alkyne coupling, alkyne/alkene coupling transition metals,3−6 but have not often been used as a ligand for soft transition metals, such as second- and third-row lowvalent late transition metals (Figure 1).7,8 In principle, it is

1. INTRODUCTION Homogeneous transition-metal-catalyzed transformations of organic compounds have become indispensable to modern organic synthesis because they provide highly efficient, selective, and unique routes to a variety of organic molecules. A large number of catalytic reactions have been developed to provide various valuable compounds.1 In many of the reactions catalyzed by transition metal complexes, ligands on the metal centers play important roles to achieve high reaction selectivity as well as great reactivity and robustness of the catalysts. For example, if an oxidative addition step is turnover-limiting in a catalytic cycle, electron-donating ligands generally offer high catalytic activity. On the other hand, if reductive elimination is the turnover-limiting step, the use of π-accepting ligands may be preferred. Organometallic and organic chemists usually tune the electronic properties and steric environments of the ligands to facilitate the desired transformation considering the characteristics of the reactions and the transition metals.2 Various types of ligands including phosphines, pyridines, amines, carbon monoxide, isonitriles, alkenes, cyclopentadienyl derivatives, and arenes have often been employed as spectator ligands, and extensive studies have been conducted on how to optimize the reactivity of the catalysts by choosing the suitable class of ligands and fine-tuning their electronic and steric properties. However, progress toward expansion of the available classes of spectator ligands are still desired as evidenced by how the emergence of N-heterocyclic carbenes (NHCs) have led to the development of a variety of practical transformations over the past two decades. 8-Quinolinolate, which can be readily prepared by deprotonation of a hydroxyl group in 8-hydroxyquinoline, is a wellknown ligand for Lewis acidic metals, such as Al and first-row © XXXX American Chemical Society

Figure 1. Metal complexes bearing the 8-quinolinolate ligand.

difficult to achieve stable coordination of hard, anionic, strongly σ-donating ligands to soft, low-valent transition metals, which would even tolerate the conditions used for catalytic reactions, but we envisioned that the use of 8-quinolinolate ligands for complexes of soft, low-valent metals may be achieved because of their strong chelating ability. If the nitrogen atom of the 8quinolinolate ligand coordinates to a soft metal, the hard, anionic oxygen must present around the metal center because there is no freedom of rotation around the bonds connecting the nitrogen and oxygen atoms. Therefore, there is high potential for formation of chelates robust enough for catalytic reactions even if it goes against the HSAB rule, particularly when it is assisted by the use of cationic metal centers to create electrostatic interactions. In addition, we speculated that this mismatched metal/ligand combination would provide unique catalytic activities that were not observed with well-studied matched combinations. Received: April 2, 2018 Revised: May 20, 2018 Published: May 22, 2018 6127

DOI: 10.1021/acscatal.8b01286 ACS Catal. 2018, 8, 6127−6137

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available for the anti-Markovnikov addition of alcohols to unactivated terminal alkynes with high product selectivity. In 2011, our group reported on the Z-selective antiMarkovnikov addition of alcohols to terminal alkynes using a rhodium(I) complex, (2-Me-Q)Rh(CO)2 (1a), which possesses a 2-methyl-8-quinolinolate ligand (abbreviated to 2-Me-Q) and two CO ligands as a catalyst.23 It is important to choose a ligand, solvent, and reaction temperature appropriately to achieve high selectivity and efficiency. When the reaction of phenylacetylene (2a) with an excess amount of methanol (3a) was carried out in the presence of 5 mol % 8-quinolinolato rhodium(I) complex 1a at 70 °C in DMA, anti-Markovnikov addition product 4aa was obtained in 80% yield with a 90:10 Z/E ratio (Table 1, entry 1). Substituents on the quinolinolate

Our group has been studying the reactivity of the 8quinolinolato rhodium(I) complexes with terminal alkynes. In addition, other groups have also reported on the reactivities of the 8-quinolinolato rhodium(I) complexes toward terminal alkynes and the theoretical calculations of the catalytic reactions. In this Perspective, we briefly survey studies on 8quinolinolato-rhodium(I)-catalyzed reactions of terminal alkynes up to the year 2017. It is important to note that syntheses and reactivities of 8-quinolinolato rhodium(I) complexes have been studied for a long time,7 but their application to catalytic reactions had been limited until recently. 8-Quinolinolato rhodium(I) catalysts have also been used in the reactions of substrates other than terminal alkynes such as Giuseppe, Castarlenas, and Oro’s β-selective H/D exchange of styrene derivatives9 as well as alkene oxidation,10 hydroformylation,7p,11 and carbene polymerization,12 but these reactions are not covered in this paper.

Table 1. Anti-Markovnikov Addition of Methanol to Terminal Alkynesa

2. ADDITION OF ALCOHOLS, AMINES, AND THIOLS TO ALKYNES Transition metal-catalyzed addition of heteroatom nucleophiles to alkynes is a useful strategy for the synthesis of alkenes possessing heteroatom substituents. There have been reported various transition metal-catalyzed reactions to construct C(sp2)-O, -N, and -S bonds using alcohols,13 carboxylic acids,14 amines,15 and thiols.16 Recently, 8-quinolinolato rhodium(I) complexes were found to function as catalysts for the addition of heteroatom nucleophiles to form enol ethers, enamines, and alkenyl thioethers. 2.1. Synthesis of Enol Ethers by Addition of Alcohols to Terminal Alkynes. Enol ethers are valuable intermediates in organic synthesis and are used for various transformations.17 The reaction of alkynes with alcohols is one of the most straightforward, atom-economical strategies to access enol ethers and has been developed previously.18−22 The use of strong bases for the reaction is a simple method to synthesize enol ethers, but harsh reaction conditions are required (eq 1).18

entry

2

R

4

yield of 4b

Z/Ec

1 2d 3f 4 5 6 7 8g 9 10h

2a 2a 2a 2b 2c 2d 2e 2f 2g 2a

C6H5 C6H5 C6H5 p-F3CC6H4 p-NCC6H4 p-MeO2CC6H4 p-O2NC6H4 p-MeOC6H4 Ph3C C6H5

4aa 4aa 4aa 4ba 4ca 4da 4ea 4fa 4ga 4ab

80% 67% not detected 92% 85% 82% 73%c 64% 69% 72%

90/10 −e −e 94/6 95/5 94/6 94/6 87/13 100/0 91/9

a

Reaction conditions: 1 mmol 2a, 1 mL of 3a, 0.02 mmol 1a, 1 mL of DMA, 70 °C, 48 h. bGC yield of the mixture of Z- and E-isomers. c Determined by 1H NMR. d(Q)Rh(CO)2 1b was used instead of 1a. e Not determined. fUsing 1c instead of 1a. gSeven days. hPerformed with ethanol (3b) and 5 mol % 3a for 72 h.

ligand (abbreviated to Q) affected the catalytic activity, and the use of (Q)Rh(CO)2 (1b), which has a parent quinolinolate ligand Q, decreased the yield to 67% (entry 2). A phosphine complex, (2-Me-Q)Rh(CO)(PPh3) (1c), did not show any catalytic activity under the reaction conditions (entry 3). The addition reaction using arylacetylenes bearing an electronwithdrawing group at the para position proceeded smoothly to give the corresponding enol ethers in high yields (entries 4−7) with excellent Z/E ratios. In contrast, the reaction of pmethoxyphenylacetylene (2f) needed an extended reaction time to obtain 64% yield of product 4fa (entry 8). Tritylacetylene (2g) reacted with 3a to give the corresponding enol ether 4ga with complete Z selectivity (entry 9). This addition reaction is sensitive to steric bulkiness of alcohols. The addition of ethanol (3b) in the presence of 5 mol % methanol required 72 h to achieve full conversion of 2a (entry 10). Wang and co-workers studied the mechanism of the hydroalkoxylation reaction based on DFT calculations and proposed the catalytic cycle shown in Figure 2.24 Coordination of an alkyne to 1a gives rhodium complex 5, which then tautomerizes to vinylidene rhodium complex 6 via an indirect 1,2-hydrogen shift. Nucleophilic attack of a methanol (3a)

Although transition metal-catalyzed intramolecular hydroalkoxylation of alkynes has been studied extensively,19 the corresponding intermolecular version, especially the addition to terminal alkynes, is still difficult to achieve.20−22 A PdMo3cluster catalyzed addition of alcohols to electron-deficient terminal alkynes gives anti-Markovnikov addition products (eq 2).21 A ruthenium(II)-catalyzed reaction of a terminal alkyne with allyl alcohol provides a mixture of the corresponding antiMarkovnikov hydroalkoxylation product and its Claisen rearrangement product (eq 3).22 However, no method was 6128


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enamines without loss of any atoms.26−28 Although various intermolecular coupling reactions of alkynes with primary amines to form imines have been developed using transition metal catalysts,26,27 the corresponding reactions with secondary amines to prepare enamines are still limited.26,28 In 2011, our group reported on the 8-quinolinolato rhodiumcatalyzed anti-Markovnikov hydroamination of terminal alkynes using secondary amines to give E-enamines.29 When the reaction of phenylacetylene (2a) with piperidine (11a) was conducted with 10 mol % (Q)Rh(cod) (12) and 20 mol % P(pMeOC6H4)3 (13a) at room temperature, anti-Markovnikov addition of 11a to 2a proceeded efficiently to give enamine 14aa in 85% yield (eq 4, entry 1). Both cyclic and acyclic

secondary amines can be used for this hydroamination reaction. The reactions using N-methylpiperazine (11b) and N-benzylN-methylamine (11c) provided the corresponding enamines 14ab and 14ac in 82 and 73% yields, respectively (entries 2 and 3). Only E-isomers of the hydroamination products were observed, probably because they are thermodynamically more stable than the Z-isomers. When the reaction of 2a with 11a was conducted using (Q)Rh{P(p-MeOC6H4)3}2 (15a), prepared by the reaction of 12 with 13a, as a catalyst, enamine 14aa was obtained in 62% yield. This result suggests that the COD ligand in 12 was exchanged with 13a under the reaction conditions. Because it was suggested that a phosphine ligand is essential to achieve high efficiency in the 8-quinolinolato rhodiumcatalyzed hydroamination, we synthesized rhodium(I) complexes bearing a tridentate ligand containing both a phosphine and an 8-quinolinolate moiety (abbreviated to PNO). The reaction of [Rh(OMe)(cod)]2 (16) with tridentate phosphinequinolinol 17 in benzene at room temperature afforded dinuclear rhodium(I) complex 18 in 61% yield (eq 5).30

Figure 2. Proposed mechanism of the (2-Me-Q)Rh(CO)2-catalyzed hydroalkoxylation of terminal alkynes based on the DFT calculation.

oxygen to the electrophilic α-carbon of the vinylidene ligand provides α-methoxyalkenyl complex 8. In this step, the oxygen atom of the quinolinolate ligand directs the nucleophilic attack of 3a through hydrogen bonding (7 → 8 → 8′). Alkenyl complex 8 is converted to Fischer carbene complex 9 by proton transfer from the 8-quinolinolate oxygen atom, and subsequent rate-determining 1,2-hydrogen shift provides π-bound enol ether complex 10. Ligand exchange between the π-bound enol ether and 2a produces product 4aa with regenerating catalytically active species 5. In their proposed catalytic cycle, the oxygen atom of the quinolinolate ligand is suggested to play two important roles: a base to direct the nucleophilic attack of methanol to the vinylidene ligand (6 → 7 → 8) and an acid to form α-methoxy carbene intermediate 9. The observed Zselectivity is considered to originate in the steric repulsion between the 8-quinolinolato rhodium framework and the phenyl group in the transition state of the rate-determining 1,2hydrogen shift step. Messerle and co-workers reported on a similar hydroalkoxylation of terminal alkynes using a rhodium catalyst bearing an anionic N−N bidentate ligand, 5-phenyldipyrrinate, but their reaction proceeded with E-selectivity.25 On the basis of Wang’s calculation, the product formation in the 5-phenyldipyrrinato-rhodium-catalyzed reaction occurs via reductive elimination rather than 1,2-hydrogen shift, and the observed E-selectivity can be explained by the steric repulsion between the alkoxy group and the phenyl group.24b 2.2. Synthesis of Enamines by Addition of Secondary Amines to Terminal Alkynes. Transition metal-catalyzed hydroaminations of alkynes with primary and secondary amines are convenient methods for the synthesis of imines and

Complex 18 can be used as a precursor for synthesis of (PNO)Rh(L) complexes (19) having various monodentate ligands (L) (Scheme 1). Generation of dinuclear complex 18 in situ by reacting 16 with 17 at room temperature followed by treatment with phosphine 13a afforded (PNO)Rh(P(4MeOC6H4)3) (19a) in 89% yield. Other (PNO)Rh(L) 6129


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Single-crystal X-ray diffraction analysis was also conducted for complex 21i to show that the obtained structure was consistent with the results of the NMR analysis. The bridging-vinylidene ligand can exchange with a free terminal alkyne in the presence of pyridine or 4-N,N-dimethylaminopyridine (DMAP). The reactions of complex 21h with piperidine (11a) and isoindoline (11d) at 90 °C for 24 h afforded (PNO)Rh(aminocarbene) complexes 23ha and 23hd in 88 and 82% yields, respectively (eq 8). The structure of 23hd was

Scheme 1. Synthesis of (PNO)Rh(L) Complexes

complexes (19b−e) were similarly synthesized in 25−98% yields by the reaction of in situ-generated 18 with PPh3 (13b), P(4-CF3C6H4)3 (13c), pyridine, and CO. Examination of the 8-quinolinolato rhodium complexes as catalysts for the hydroamination showed that the complex having π-accepting CO ligand 19e catalyzed the addition of 11a to 1-octyne (2h) with higher activity than 18 and 19a (eq 6). Both aliphatic (2h) and aromatic (2a,i) terminal alkynes are applicable to the hydroamination catalyzed by 19e.

confirmed by single-crystal X-ray diffraction analysis. The 13 C{1H} NMR spectrum of 23hd showed a signal corresponding to a carbene carbon coupled with a rhodium and a phosphorus nuclei at 263.6 ppm (dd, J = 43.2, 13.5 Hz) as is consistent with its solid-state structure. Simple heating of in situ-generated (PNO)Rh(aminocarbene) complex 23ha provided hydroamination product 14ha only in a low yield, but the use of additives facilitated the formation of 14ha. In particular, the addition of ligands with good π-accepting character such as CO, P(4CF3C6H4)3 (13c), P(3,5-(CF3)2C6H3)3 (13d), and P(OPh)3 (13e) effectively accelerated the generation of 14ha (eq 9).

For gaining insight into the reaction mechanism, the reactivity of the (PNO)Rh complexes toward the substrates were examined using a stoichiometric amount of (PNO)Rh dimer complex 18. The reaction of in situ-generated 18 with 1octyne (2h) gave vinylidene-bridged dirhodium complex 21h, which is considered to be generated via nucleophilic attack of mononuclear (PNO)Rh complex 22 to the carbene carbon in the vinylidene ligand of 20h (eq 7). Similar reactions with

Figure 3 shows the proposed mechanism of the (PNO)Rhcatalyzed hydroamination based on results of stoichiometric reactions in addition to monitoring of the catalytic reactions. The reaction of 18 and/or 19 with alkyne 2 yields alkyne complex 24, which subsequently isomerizes to mononuclear vinylidene rhodium complex 20. Complex 20 reacts with 22 to give vinylidene-bridged dirhodium complex 21. The reaction of 21 with amine 11 provides aminocarbene complex 23 via mononuclear complex 20. The reaction of 23 with a ligand (L) such as triarylphosphines leads to the formation of hydroamination product 14 with regenerating catalytically active (PNO)Rh(L) species 18 and/or 19. The formation of hydroamination product 14 from aminocarbene complex 23 is considered to be rate-determining in this reaction. Many types of anti-Markovnikov addition reactions have been considered to proceed via formation of vinylidene intermediates, followed by nucleophilic attack, direct evidence of the presence of vinylidene intermediates and their adduct with

ferrocenylacetylene (2i), cyclohexylacetylene (2j), 3-cyclohexyl1-propyne (2k), and tert-butyl acetylene (2l) also afforded the corresponding vinylidene-bridged dirhodium complexes 21i−l in 33, 78, 82, and 74% yields, respectively. The structures of complexes 21 were determined by various NMR spectra. 6130


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with P(4-CF3C6H4)3 (13c) afforded enamine 14ha, which was detected by 1H NMR spectrum. Subsequently, CuBr, alkyne 2h, and Et3N were added to the reaction mixture containing enamine 14ha, and heating of the resulting mixture at 90 °C for 2 days yielded propargylamine 25ha in 43% yield (eq 12). On the bsis of these observations, the 2:1 coupling reaction is considered to proceed via two steps: (PNO)Rh(PPh3)catalyzed anti-Markovnikov hydroamination of terminal alkyne 2 with secondary amine 11 to generate enamine 14 and the formation of propargylamine 25 by the reaction of 14 with a copper acetylide species generated in situ from CuBr, alkyne 2, and triethylamine (Figure 4). 2.4. Synthesis of Alkenyl Aryl Sulfides by Markovnikov Addition of Thiols to Terminal Alkynes. Castarlenas and co-workers reported on the alkyne hydrothiolation catalyzed by 8-quinolinolato rhodium complexes bearing an NHC ligand. The reaction of [Rh(μ−OH)(IPr)(coe)]2 (26)

Figure 3. Proposed mechanism of (PNO)Rh(L)-catalyzed antiMarkovnikov hydroamination of terminal alkynes with secondary amines.

nucleophiles were not shown in most cases.31 The experimental results obtained using (PNO)Rh complexes were the first structural determination of both vinylidene and aminocarbene intermediates in catalytic hydroamination reactions. Oro and co-workers have also studied the anti-Markovnikov hydroamination of terminal alkynes catalyzed by in situgenerated 15a using [Rh(OMe)(cod)]2 (16), 8-quinolinol (H-Q), and P(4-MeOC6H4)3 (13a).32 When the reaction of 2a with 11a was carried out using in situ-generated 15a at room temperature for 24 h, the corresponding anti-Markovnikov hydroamination product 14aa was obtained in 51% yield (eq 10).

2.3. Synthesis of α-Monosubstituted Propargylamines from Terminal Alkynes and Secondary Amines Using a (PNO)Rh(L)/CuBr Tandem Catalyst System. While studying (PNO)Rh(L)-catalyzed hydroamination of terminal alkynes with secondary amines, we found that propargylamines were formed from two molecules of a terminal alkyne and one molecule of a secondary amine in the presence of CuBr. When the reaction of 1-octyne (2h) with piperidine (11a) was conducted using (PNO)Rh(PPh3) (19b) and CuBr as catalysts in toluene at 100 °C for 2 h, a 2:1 coupling reaction of 2h and 11a proceeded to give propargylamine 25ha in 89% yield (eq 11).33 Various functional groups such as ether and ester groups tolerated the reaction conditions. The stoichiometric reaction of (PNO)Rh(aminocarbene) complex 23ha

Figure 4. Proposed mechanism of the formation of propargylamines from terminal alkynes and secondary amines by the 19b/CuBr tandem catalyst system. 6131


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pyridine to the β-alkenyl rhodium complex rather than sterically more hindered α-alkenyl rhodium complex retarded the reductive elimination from the β-alkenyl complex and led to the selective product formation from the α-alkenyl complex.

with 8-quinolinol such as H-Q and 2-methyl-8-quinolinol at room temperature gave 8-quinolinolato rhodium complexes (Q)Rh(IPr)(coe) (27) and (2-Me-Q)Rh(IPr)(coe) (28) (eq 13).34 These NHC complexes were catalytically active for the

3. ALKYNE OLIGOMERIZATION/POLYMERIZATIONS, ALKYNE/ALKENE CYCLOADDITION, AND THEIR RELATED REACTIONS 8-Quinolinolato rhodium complexes were found to exhibit catalytic activities for alkyne oligomerization/polymerization to give polymers, cyclotrimerization products, and head-to-head and head-to-tail dimers. The complexes can also catalyze an alkyne/alkene coupling reaction to form cyclobutene derivatives. In this section, 8-quinolinolato rhodium-catalyzed alkyne oligomerization/polymerization, alkyne/alkene coupling, and their related reactions are discussed. Polymerization of 2a was achieved using in situ-generated 8quinolinolato rhodium complexes. The reaction of 2a using a catalyst generated from [Rh(OMe)(nbd)]2 with 8-quinolinol (H-Q) led to complete conversion to poly(phenylacetylene) 31 with a high molecular weight (Mw = 167000, Mw/Mn = 2.10) (eq 15).32

When the reaction of 2a in EtOH was carried out at 60−70 °C using (Q)Rh(CO)(coe) (32a) as a catalyst, 1,2,4triphenylbenzenes (33a) was formed in 29.1% yield (eq 16).35 The use of (Q)Rh(CO)(AsPh3) (32b) as a catalyst

hydrothiolation of 2a with thiophenol. In the presence of pyridine additive, Markovnikov addition product 29 was obtained with high selectivity (97% selectivity) along with a small amount of anti-Markovnikov addition product 30 (eq 14). Studies of the reaction mechanism by NMR experiments and DFT calculations suggested that the hydrothiolation took place via oxidative addition of PhS−H bond to the rhodium(I) center to give a rhodium(III) hydride complex and 2,1insertion of 2a to the Rh−H bond followed by reductive elimination provided the C−S bond formation product (Figure 5). The authors proposed that selective coordination of

afforded 1,2,4- and 1,3,5-triphenylbenzenes (33a and 33b) in 43.1 and 10.3% yields, respectively. 8-Quinolinolato complex 32a showed higher catalytic activity in the alkyne trimerization reaction than structurally similar (acac)Rh(CO)(coe) and provided only a trace amount of 33. The additional ligand of 8-quinolinolato rhodium complexes, (Q)Rh(L), largely affected catalytic properties. When (Q)Rh(cod) (12) and P(4-MeOC6H4)3 (13a) were used as catalysts for the reaction of 2a in toluene-d8 at 60 °C for 4 h, head-to-tail dimer 34 and head-to-head dimer 35 were obtained in 56 and 8% yields, respectively (eq 17).36 This result is in sharp contrast

with the reaction described in eq 16, as the use of CO and AsPh3 ligands led to the formation of trimer 33. Although

Figure 5. Proposed mechanism of the (Q)Rh(IPr)-catalyzed hydrothiolation of phenylacetylene (2a). 6132


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reaction conditions, cyclobutene 39fa was obtained in 24% yield along with 1:2 addition product 40 in 28% yield (eq 21).

various catalyst systems have been developed to form head-tohead dimers of terminal alkynes,37 few studies of selective headto-tail dimerization have been reported.38 The catalyst system consisting of 12 and 13a is useful for the synthesis of unstable head-to-tail dimers of arylacetylenes. When the alkyne dimerization was carried out in the presence of dimethyl malonate, cyclopentene derivative 36a was formed by 2:1 coupling of 2a and dimethyl malonate and obtained in 35% yield (eq 18).36 The reactions with several

Extension of the reaction time to 48 h increased the yield of 40 to 42% and decreased the yield of 39fa to 3%. Therefore, 1:2 addition product 40 is considered to be formed by 1,4-addition of 39fa to another molecule of acrylate 38a. Our group proposed the catalytic cycle of the [2 + 2] cycloaddition in the original paper,39 and Fang and co-workers investigated the reaction mechanism further by DFT calculations to suggest the mechanism shown in Figure 6.40 Coordination of fluoride to the rhodium center, followed by dissociation of COD, generates catalytically active species 41 in which F− weakly interacts with Cs+ and binds to the coordination site trans to the quinoline nitrogen (12 → 41). The alkyne coordinates to the rhodium center (41 → 42), and

arylacetylenes afforded the corresponding 2:1 cycloaddition products, albeit in moderate yields. The 2:1 coupling reaction using dimethyl methylmalonate afforded allene-containing product 37, which was formed by rhodium-catalyzed nucleophilic attack of dimethyl methylmalonate to 34, generated in situ by head-to-tail dimerization of 2a (eq 19). 8-Quinolinolato rhodium complexes can also catalyze the [2 + 2] cycloaddition of terminal alkynes with electron-deficient alkenes.39 The reaction of n-butyl acrylate with 3 equiv of 1octyne (2h) using a 12/13c/CsF catalyst system in DMA at 80 °C selectively gave butyl 3-hexyl-2-cyclobutenecarboxylate (39ha) in 99% isolated yield (eq 20). Various aliphatic

terminal alkynes having cyclohexyl, ester, cyano, and ether groups were applicable to the [2 + 2] cycloaddition, and in all cases, the reaction afforded the corresponding cyclobutenes with complete regioselectivity. Other electron-deficient alkenes such as isopropyl and tert-butyl acrylates and acrylonitrile (38b−d, respectively) reacted with 1-octyne to give the corresponding cyclobutenes 39hb−hd in 28−99% yields. Arylacetylenes can also be applied to the [2 + 2] cycloaddition reaction, but their reactivity is different from aliphatic alkynes. When the reaction of alkyne 2f with n-butyl acrylate (38a) (3 equiv) was conducted under the standard

Figure 6. Proposed mechanism of the formation of cyclobutenes from terminal alkyne and acrylates by the (Q)Rh(cod)/phosphine/CsF catalyst system based on the DFT calculations. 6133


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Culture, Sports, Science and Technology, Japan (16H04150 and 24350050). T.K. is also grateful for support by JSPS KAKENHI Grant Number 18H04271 (Precisely Designed Catalysts with Customized Scaffolding).

deprotonation at the terminal position gives alkynylrhodium intermediate 43. Nucleophilic attack of the β-carbon of the alkynyl ligand onto the β-carbon of the acylate produces rhodium vinylidene intermediate 44. Cyclization by nucleophilic attack of the α-carbon of the acrylate to the vinylidene ligand led to the formation of cyclobutenyl intermediate 45. Protonation of the Rh−C bond (45 → 46) and dissociation of cyclobutene 39ha from the rhodium center regenerates the catalytically active species 41. The theoretical studies indicate that the electron-withdrawing groups such as ester and cyano groups stabilize the anionic character of the α-carbon in intermediate 44, and the cyclization process is the ratedetermining step.

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(1) (a) Transition Metals in the Synthesis of Complex Organic Molecules, 3rd ed.; Hegedus, L. S., Söderberg, B. C. G., Eds.; University Science: CA, 2010. (b) Organotransition Metal Chemistry: From Bonding to Catalysis; Hartwig, J. F., Ed.; University Science: CA, 2010. (c) Applied Homogeneous Catalysis; Behr, A., Neubert, P., Eds.; Weily-VCH Verlag, Weinheim, 2012. (d) Catalytic Asymmetric Synthesis, 3rd ed.; Ojima, I., Ed.; Weiley, NJ, 2010. (e) Applied Homogeneous Catalysis with Organometallic Compounds, 3rd ed.; Cornils, B., Herrmann, W. A., Beller, M., Paciello, R., Eds.; WileyVCH Verlag GmbH: Weinheim, 2018. (2) (a) Ligand Platforms in Homogeneous Catalytic Reactions with Metals; Yamaguchi, R., Fujita, K.-I., Eds.; Wiley, NJ, 2015. (b) Catalysis, 2nd ed.; Rothenberg, G., Ed.; Wiley-VCH Verlag GmbH: Weinheim, 2017. (3) For Al complexes, see: (a) Khan, R. U. A.; Kwon, O.-P.; Tapponnier, A.; Rashid, A. N.; Günter, P. Supramolecular Ordered Organic Thin Films for Nonlinear Optical and Optoelectronic Applications. Adv. Funct. Mater. 2006, 16, 180−188. (b) Murphy, L.; Williams, J. A. G. Luminescent Platinum Compounds: From Molecules to OLEDs. Top. Organomet. Chem. 2010, 28 (Molecular OrganometallicMaterials for Optics), 75−111. (4) For Cu complexes, see: (a) Nicholas, D. D.; Henry, W. P.; Vasishth, R. C. The Role of Copper in Wood Preservation. In Handbook of Copper Compounds and Applications; Richardson, H. W., Ed.; Marcel Dekker: New York, 1997; pp 163−176. (b) Kamdem, D. P. Copper-based Synthesis for Exterior Residential Applications. In Development of Commercial Wood Preservatives; Schultz, T. P., Holger, M., Michael, H. F., Goodell, B., Nicholas, D. D., Eds; ACS Symposium Series 982; American Chemical Society: Washington, DC, 2008; pp 427−439. (5) For Fe complexes, see: (a) Vogler, A. Intraligand fluorescence of M(III)(oxinate)3 under ambient conditions with M = Fe, Ru, Os and oxinate = 8-quinolinolate. Inorg. Chem. Commun. 2017, 84, 131−133. (b) Al-Noor, T. H.; Karam, N. H.; Ghanim, F. H.; Kindeel, A. S.; AlDujaili, A. H. Synthesis, characterization and liquid crystalline properties of novel benzimidazol-8-hydroxyquinoline complexes. Inorg. Chim. Acta 2017, 466, 612−617. (6) For Co complexes, see: Abu-Hussen, A. A. A. Synthesis and spectroscopic studies on ternary bis-Schiff-base complexes having oxygen and/or nitrogen donors. J. Coord. Chem. 2006, 59, 157−176. (7) (a) Ugo, R.; La Monica, G.; Cenini, S.; Bonati, F. Rhodium(I) and iridium(I) carbonyl derivatives with anionic chelating ligands. J. Organomet. Chem. 1968, 11, 159−166. (b) Varshavskii, Yu. S.; Knyazeva, N. N.; Cherkasova, T. G.; Ivannikova, N. V.; Ionina, T. I. Reaction of the product of rhodium carbonylation by dimethylformamide with 8-hydroxyquinoline and 8-mercaptoquinoline. Zh. Neorg. Khim. 1970, 15, 715−722. (c) Varshavskii, Yu. S.; Cherkasova, T. G.; Buzina, N. A.; Knyazeva, N. N.; Ionina, T. I. Analogs of βdiketonatedicarbonyl derivatives of rhodium(I). Zh. Neorg. Khim. 1970, 15, 2294−2296. (d) Varshavsky, Ju. S.; Kiseleva, N. V.; Cherkasova, T. G.; Buzina, N. A. Dimethylformamide carbonylation of platinum group metals under mild homogeneous conditions. Reactions of carbonyl-containing complexes. J. Organomet. Chem. 1971, 31, 119−122. (e) Kuz'mina, L. G.; Varshavskii, Yu. S.; Bokii, N. G.; Struchkov, Yu. T.; Cherkasova, T. G. Crystal structure of 8hydroxyquinolinato(triphenyl-phosphine)rhodium carbonyl and 8hydroxyquinolinatorhodium dicarbonyl. Zh. Strukt. Khim. 1971, 12, 653−660. (f) Varshavskii, Y. S.; Cherkasova, T. G.; Buzina, N. A. Action of halogens on rhodium(I) carbonyl complexes containing bidentate singly charged ligands. Zh. Neorg. Kim. 1972, 17, 2208− 2214. (g) Varshavsky, Yu. S.; Cherkasova, T. G.; Buzina, N. A.; Kormer, V. A. Mixed carbonyl cyclooctene complexes of rhodium(I).

4. CONCLUSIONS AND OUTLOOK In this Perspective, we briefly surveyed catalytic reactions of terminal alkynes using rhodium(I) complexes having 8quinolinolates, which can be regarded as monoanionic, hard, strongly σ-donating ligands. Unique features of 8-quinolinolate ligands and catalytic activities of rhodium(I) complexes bearing these ligands were discussed. Reaction developments using a mismatched soft metal/hard ligand catalyst system have not been pursued extensively, but several characteristic features have been revealed for the 8-quinolinolato rhodium catalysts as described in this Perspective. The oxygen in the quinolinolate ligands is considered to function as a coordinating group as well as a base in the hydroalkoxylation of terminal alkynes. In addition, the phenoxide oxygen was suggested to weakly interact with cesium cation in the [2 + 2] cycloaddition of terminal alkynes with acrylates. These features may become useful to the design of the catalytic reactions using 8-quinolinolato transition metal complexes. Installation of a phosphine moiety on the quinolinolate framework facilitated isolations and observations of key intermediates in the catalytic alkyne hydroamination, such as vinylidene-bridged dirhodium and (aminocarbene) rhodium complexes, and reversibility of the vinylidene formation was also suggested by the stoichiometric reaction using (PNO)Rh(L) complexes. These findings may provide valuable information for future studies of transition metalcatalyzed anti-Markovnikov addition to terminal alkynes. The chemistry of 8-quinolinolato rhodium catalysts toward terminal alkynes described in this Perspective is just an example of the fields explored by mismatched soft metal/hard ligand catalyst systems, and a variety of reactions have been discovered by unique combinations of metals and ligands as catalysts. We believe that further exploration of the use of hard ligands in soft metal catalysis will lead to developments of unprecedented organic reactions.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Fumitoshi Kakiuchi: 0000-0003-2605-4675 Takuya Kochi: 0000-0002-5491-0566 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support by a Grant-in-Aid for Scientific Research (B) from the Ministry of Education, 6134


Perspective

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Catalytic Reactions of Terminal Alkynes Using Rhodium(I) Complexes

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