CO2 Adducts of Phosphorus Ylides: Highly Active Organocatalysts for

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CO2 Adducts of Phosphorus Ylides: Highly Active Organocatalysts for Carbon Dioxide Transformation Hui Zhou, Guo-Xu Wang, Wen-Zhen Zhang, and Xiao-Bing Lu ACS Catal., Just Accepted Manuscript • Publication Date (Web): 30 Sep 2015 Downloaded from http://pubs.acs.org on October 6, 2015

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CO2 Adducts of Phosphorus Ylides: Highly Active Organocatalysts for Carbon Dioxide Transformation Hui Zhou,* Guo-Xu Wang, Wen-Zhen Zhang and Xiao-Bing Lu* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China ABSTRACT: A series of phosphorus ylide (P-ylide) CO2 adducts were synthesized and firstly used as organocatalysts for CO2 transformation. Detailed studies on the cycloaddition reaction of CO2 with terminal epoxides show that P-ylide CO2 adducts are efficient metal-free and halogen-free organocatalytsts to mediate this reaction under ambient conditions (25 oC, 1 atm CO2). More importantly, the reactions proceeded with a broad scope, high efficiency, and functional group tolerance and the corresponding cyclic carbonate products were obtained in good to excellent yield (46-99%). Meanwhile, the kinetic study by in-situ FTIR method suggested an intermolecular cooperation effect for effectively accelerating the ring-opening of terminal epoxides. Furthermore, from the investigation on the catalytic diversity of Pylide CO2 adducts, CO2 also could be availably converted to functionalized cyclic α– alkylidene carbonates, oxazolidinone, N-methylated and N-formylated amines by organocatalytic reactions. Keywords: CO2 adduct, phosphorus ylide, organocatalyst, cycloaddition, CO2 transformation

INTRODUCTION The development of efficient catalytic processes for carbon dioxide (CO2) transformation into desirable, economically competitive products has been a long-standing goal for chemists, since CO2 is an inexpensive, abundant, nontoxic and renewable C1 feedstock. Up to now, more than 20 reactions involving CO2 as a starting material have been developed in recent decades.[1] Although the quantity of consumed CO2 in these processes is likely a very small fraction (0.5%) of the total CO2 generated from human activity (about 37Gt). However, these strategies potentially provide more environmentally benign routes to producing chemicals otherwise made from the reagents detrimental to the environment. It is generally known that CO2 is a thermodynamically inert molecule, the end product in any carbon-based combustion process, thus relatively high-energy reagents are often used to facilitate its transformation. In these processes, the activation of CO2 is pivotal for its effective transformation. CO2 is a non polar molecule, due to the linear geometry, though the two carbon-oxygen bonds are polar and a net partial charge is present on carbon and oxygen atoms. The oxygen atoms show a Lewis base character, while the carbon atom can play the role of a Lewis acid center. Since the electrophilicity of carbon is higher than the nucleophilicity of the oxygen atoms, CO2 is a

better acceptor than donor of electron density. As a consequence, electron-rich nucleophiles (such as metals in a low oxidation state, Lewis bases, etc.) will most likely interact with CO2 by binding to the C-atom, while electrophiles (such as metal ions in high oxidation state, electron deficient molecules, etc.) will attack one or two of the Oatoms.[2] The first CO2-based complex, Ni(PCy3)2(CO2), was reported by Aresta and co-worker in 1975.[3] The X-ray singlecrystal analysis revealed that the CO2 ligand was coordinated through the C-atom and one of the O-atoms and thus possessed bent geometry with a O−C−O angle of 133°, being distinct from the linear structure of free CO2 at ground state. Similarly, a bent structure with a O−C−O angle of 132° was also observed in [Co(I)(nPrsalen)K(CO2)(THF)], in which CO2 is anchored to the nucleophilic cobalt(I) ion through a Co−C σ bond, while the O-atoms interact with the alkali cation in a polymeric structure.[4] Following these studies, Herskovitz et al. revealed the structure of Rh(diars)2Cl(CO2), in which CO2 trans to the chloride is formally η1-bound toward Rh(I).[5] Since CO2 prevalently behaves as an electrophile, strong Lewis bases such as the amidines and guanidines containing nitrogen heterocyclic have been reported to react with CO2, expectantly affording zwitterionic adducts. The representative example is TBD-CO2 adduct from the reaction of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) with CO2,

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bearing a O−C−O angle of 128.59°.[6] The stability of TBDCO2 adduct can be ascribed to the H-bond effect between N−H of TBD and O-atom of the carboxylate anion. Besides, both CO2 adducts of N-heterocyclic carbene (NHC)[7] and N-heterocyclic olefin (NHO)[8] were clearly characterized by single crystal X-ray crystallography, all bearing bent geometries with a O−C−O angle of 127−131°. Notably, the two kinds of CO2 adducts were shown to be an efficient organocatalyst for chemical fixation of CO2 to organic compounds.[8,9] It is noteworthy that frustrated Lewis pairs were found to straightforwardly sequester CO2 binding into frustrated Lewis acid and Lewis base centers.[10] Unfortunately, these systems showed very poor catalytic activity for CO2 transformation.[11] 127.7 o O (C6H5)3P

O

R1 CHCOOH

1a

R2

P(C6H5)3

Matthews, 1966 (ref. 13)

Bestmann, 1974 (ref.15) CO2 stoichiometric transformation

(a) R1 Ph3P R2

O Activation C O (c)

R3

R1

H

COOSiMe3

Bestmann, 1992 (ref. 16) CO2 stoichiometric transformation

(b) R1 Ph3P

Fixation

R2 O

O O

(d) X

O

O

H

CH3

N 2 1 N 2 R R R R1 R2 R1 X= N, O This work: CO2 catalytic transformation

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chemical transformation. In the present contribution, various P-ylide-CO2 adducts were synthesized and carefully characterized, especially with a focus on the difference in thermal stability in comparison with the previously studied NHC-CO2 and NHO-CO2 adducts. In addition, these P-ylide-CO2 adducts were found to be very efficient in catalyzing some reactions for CO2 transformation to useful chemicals at mild conditions (Scheme 1. d).

RESULTS AND DISCUSSION Synthesis and Characterization. Firstly, a series of Pylide-CO2 adducts 1b-1g (Figure 1) were prepared in good yields according to the reported literatures.[15, 17] Among them, air and moisture-stable difluoromethylene phosphobetaine 1b was synthesized from the reaction of triphenylphosphine with BrCF2CO2K, which was recently reported by Xiao group.[17] P-ylide-CO2 adducts 1c-1g were formed by the reaction of the corresponding P-ylides with CO2 (Scheme 1). Although 1d and 1e are the reference examples which were early reported by Bestmann’s group,[15] there is no any experimental data to prove the actual structures. In this paper, all P-ylide-CO2 adducts were characterized by 1H NMR[18], 13C NMR, 31P NMR, IR spectroscopy and Mass spectrometry in detail (See supporting information). 13C NMR data of 1b-1g show strong carboxylate carbon signals at 162.1-174.4 ppm. Meanwhile, the characteristic carbonyl absorption band of 1b-1g at FTIR spectra changes from 1630-1650 cm-1. Moreover, [P-ylideCO2 adducts + H+] are clearly observed by HRMS analyses in the positive ion mode.

Scheme 1. P-ylide CO2 adducts reported in the literatures and those described in this work. (a) The first example of P-ylide CO2 adducts; (b) Synthesis of carboxylic acids from P-ylide CO2 adducts; (c) Synthesis of α, βunsaturated silyl esters from P-ylide CO2 adducts; (d) Pylide CO2 adducts catalyzed CO2 chemical transformation. It is well known that phosphorus ylides are powerful and versatile nucleophilic reagents in organic synthesis.[12] The first CO2 adduct of P-ylides was obtained from the nucleophilic additionby Matthews and coworkers in 1966.[13] A O–C–O angle of 127.7o was observed by X-ray single crystal analysis (Scheme 1. a).[14] The hydrolysis of P-ylide-CO2 adducts under alkaline and acidic conditions predominantly afford carboxylic acid derivatives (Scheme 1. b).[15] P-ylide-CO2 adducts containing trimethylsilyl (TMS) group on the ylidic carbon atom were chemically unstable. This is ascribed to the intermolecular migration of TMS to an oxygen atom in CO2 molecule to generate new P-ylides functionalized by silyl ester group, and thus easily reacts with additional aldehydes to afford α, β-unsaturated silyl esters (Scheme 1. c).[16] However, no report regards P-ylide or its CO2 adduct as organocatalyst for CO2 activation and

Figure 1. Investigated organocatalysts 1b-1g for CO2 chemical transformation. The nature of the substituent groups on the ylidic carbon shows differences in nucleophilic ability of P-ylides to free CO2 and then affects stability of P-ylide-CO2 adducts. 1b as a P-ylide precursor, which smoothly decarboxylated at 80 oC for 4 h in 1-methyl-2-pyrrolidinone (NMP) solution, has been detailed researched in Wittig difluoroolefination. Introducing the electro-donating group on the ylidic carbon will increase the nucleophilicity of P-ylides and enhance the thermal stability of P-ylide-CO2 adducts. When 1e was used in the above mentioned reaction under the same experimental conditions, the olefination of benzaldehyde showed much lower reactivity. Even prolonged

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reaction time to 48 h, only 20 % desired olefinated product was isolated (Detailed procedure, see supporting information). Further, thermal stability of 1e was studied by means of in situ FTIR method with monitoring the v(CO2) region of infrared spectra using a temperature-controlled high pressure liquid cell (HPL-TC). Figure 2 shows that 1e is very stable even at 80 oC in CH2Cl2 solution, and no obvioulsy change in intensity at 1636 cm-1 was observed in 4 h. Compared with NHC-CO2 and NHO-CO2 analogues,[8,9b] P-ylide-CO2 adducts are more thermal stability. These special properties promote us to further assess the catalytic competency of P-ylides-CO2 adduct for CO2 chemical transformation.

temperature results in decreased yield of PC. It is interesting to note that when reaction temperature decreased from 100 0C to 25 °C, the 36 % yield of PC was delightedly observed in 4 h (entry 8) and higher yield (70%) could be obtained by the prolonged reaction time for 8 h (entry 9). Further increase in the catalyst loading also improves the yield of PC. When the catalyst loading was increased from 0.5 mol% to 5 mol% (entries 10-12), yield of PC significantly reach to 98% in 4 h at 25 oC. Additionally, the influence of the CO2 pressure on the reactivity was also investigated. By decreased the CO2 pressure from 2.0 MPa to 0.1 MPa, a good yield of 71% of PC was observed (entry 13), and the yield was increased up to 90% by the prolonged reaction time for 6 h (entry 14). Table 1. Cyclic addition of CO2 with PO catalyzed by Pylide CO2 adducts a

Entry

Cat. (mol%)

T (oC)

Solvent

P (MPa)

t (h)

1

1b (0.5)

100

DCM

2.0

6

99% ee). This indicated that 1e attacked the β-carbon atom of terminal epoxides (less hindered) with high regioselectivity during the ringopening step. Internal epoxides as the challenging substrates, also were investigated in this reaction. Although cyclohexene oxide only was converted in 8% to the carbonate 2m under optimized conditions, the high yield could be obtained by increasing the reaction temperature to 100 oC. In-Situ FTIR Study. The controlled experiments were also investigated through monitoring of v(CO2) region utilizing in-situ infrared spectroscopy to further understand the mechanism clearly, as shown in Figure 3. Figure

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3A displays the stability of 1e in CH2Cl2 at room temperature. The results show that 1e is very stable, and no observable change in the absorption intensity at 1636 cm-1 was found within 6 hours. The presence of PO does not affect the stability of 1e and no PC was observed in this process, as shown in Figure 3B. When free CO2 was introduced in this system, the carbonyl peak of PC at 1800 cm1 gradually increased (Figure 3C). The results show that no cycloaddition reaction of P-ylide-CO2 adducts and terminal epoxides occur in the absence of free CO2, and Pylide-CO2 alone could not open the epoxide ring due to the steric hindrance around P atom (Figure 3B). Whereas in the presence of CO2, P-ylide-CO2 firstly activate free CO2 substrate to formed more nucleophilic intermediate[10, 27] and subsequently open the epoxide ring by synergistic effects with phosphonium moiety of another catalyst to smoothly generate PC (Figure 3C).

Scheme 2. Cycloaddition of CO2 with functionalized epoxides catalyzed by 1ea a

Reaction conditions: neat, 10.0 mmol epoxide, Cat. 1e 5 mol%, 25 oC, CO2 balloon, 6 h. Yield and selectivity determined by 1H NMR of the crude reaction mixture. b ee is the enantiomeric excess of the resulting cyclic carbonates, which is determined by chiral GC. c 100 oC.

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Figure 3. The resulting three-dimensional stack plot of the IR spectra collected every 20 seconds. Reaction conditions: (A) 0.6 mmol Cat. 1e, 2 mL CH2Cl2, 25 oC, 6 h. (B) 0.6 mmol Cat. 1e, 0.6 mmol PO, 2 mL CH2Cl2, 25 oC, 6 h. (C) 0.6 mmol Cat. 1e, 0.6 mmol PO, 2 mL CH2Cl2, CO2 balloon, 25 oC, 6 h. 1636 cm-1 and 1800 cm-1 are attributable to the v(C=O) vibration of cat. 1e and PC respectively.

Figure 4. Logarithmic plots of the initial rate of the cycloaddition of PO with CO2 catalyzed by 1e versus catalyst concentration.

Figure 5. Logarithmic plots of the initial rate of the cycloaddition of PO with CO2 catalyzed by 1e versus PO concentration.

Kinetic Studies. In order to clarify the possible mechanism, a detailed kinetic study was performed using in situ FTIR measurements. A reaction order of 1.93 of catalyst concentration was obtained from 1e catalyzed cycloaddition of PO with CO2 under ambient conditions by altering catalyst concentration, as shown in Figure 4 and Figure S1-5 in the Supporting Information. The experimental results show a second-order dependence on catalyst concentration, which means that the intermolecular catalysis obviously exist to effectively activate substrates by an ambiphilic dual activation of P+ with O- moeities. Kinetic investigation regarding the PO concentration was also undertaken (Figure 5 and Figure S6 in the Supporting Information) and a reaction order of 1.27 was obtained. The reaction order between 1 and 2 indicates that the reaction process for simultaneously activating two molecular substrates exist during the ring-opening of epoxides. Proposed mechanism. On the basis of the above experimental results and previous work [10, 19-27], a possible mechanism was proposed, as shown in Scheme 3. Firstly, the acetate anion of P-ylide CO2 adducts acts as a Lewis base to activate free CO2[10, 27] and new CO2 complex A is formed, which make new acetate anion fragment more nucleophilic than P-ylide CO2 due to reduced steric hindrance. Then, the epoxide is activated by its coordination to the central P+ unit on the phosphorus ylide, while acetate anion of intermediate A simultaneously attacks the activated epoxide at the less substituted C−O bond to form intermediate B. Finally, the desired cyclic carbonates is generated by intermolecular cyclization and Pylide CO2 adducts are released for the next catalytic cycle. The Catalytic Diversity of P-ylide CO2 Adducts. Encouraged by the successful results of P-ylide CO2 adducts catalyzed cycloaddition of epoxide and CO2 under ambient conditions, we further explored the application of these organocatalysts into other reactions with CO2 as a starting material.9g,9i,9k To our delight, the catalytic behaviors of P-ylide-CO2 adducts are diverse, as shown in

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Scheme 4. When using 1e as organocatalyst for the cycloaddition of aziridines with CO2, 1-butyl-2phenylaziridine effectively reacted with CO2 to form 5substituted regioisomer 3a in 95% isolated yield with high regioselectivity at 25 oC for 6 h under an atmospheric pressure of CO2 (Scheme 4. a). These results showed that the ring opening of N-butyl aziridine highly regioselectively occurred at the most substituted carbon.19m Meanwhile, P-ylide-CO2 adducts were found to be efficient in catalyzing the carboxylative cyclization of propargylic alcohols with CO2 to selectively give the corresponding cyclic α–alkylidene carbonates. Treatment of 2-methy-4phenylbut-3-yn-2-ol with 2.o MPa CO2 in the presence of 5 mol% of 1c at 80 oC could smoothly provide the desired cyclic carbonates 4a and 4b in excellent yields (Scheme 4. b). Furthermore, P-ylide-CO2 adducts were applied to CO2 reduction to form N-methylated and N-formylated compounds. In the presence of 5 mol% of 1e as organocatalyst and 2.0 equiv. of 9-BBN as reducing agent, the corresponding N-methylated 5aa and N-formylated amines 5ab were generated in 35% and 5 % isolated yield respectively, with 24 h at 90 oC under 2.0 MPa CO2 (Scheme 4. c). When PhSiH3 was used as reductant, near

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quantitative conversions of substrates to formamides and methylamides were achieved under the same reaction conditions (Scheme 4. d).

Scheme 3. Proposed mechanism for synthesis of cyclic carbonates catalyzed by P-ylide CO2 adducts.

Scheme 4. P-ylide CO2 adducts as organocatalyts for other CO2 transformation. Reaction conditions: (a) aziridine (10 mmol), 1e (5 mol%), CO2 (0.1 MPa), 25 oC, 6h. (b) Propargylic alcohol (0.15 mmol), 1c (5 mol%), 80 oC, 12 h (according to ref. 8). (c) amine (2.0 mmol), 1e (5 mol%), 9-BBN (2.0 equiv.), CO2 (2.0 MPa), THF (1 mL), 90 oC, 24 h (according to ref. 9k). (d) amine (2.0 mmol), 1e (5mol%), PhSiH3 (2.0 equiv.), CO2 (2.0 MPa), CH3CN (1 mL), 100 oC, 24 h (according to ref. 9i). portant insight into the reaction mechanism, in which a synergistic effect contributes the ring-opening of terminal epoxides.

CONCLUSION In summary, we have demonstrated that P-ylide-CO2 adducts are efficient organocatalysts for CO2 transformation to useful fine chemicals at mild conditions. Cycloaddition of terminal epoxides with CO2 to the corresponding cyclic carbonates could smoothly proceed with 100% regioselecitivity under very mild conditions (25 oC, PCO2 = 0.1 MPa, solvent-free, halogen-free). The catalytic system is tolerant to a wide range of functional groups, including alkyl, alkenyl, alkynyl, phenyl, halide, ether, amino and ester groups to give the corresponding cyclic carbonates in moderate to good yields. In-situ FTIR study provided im-

Further investigations of the catalytic diversity show that P-ylide-CO2 adducts could successfully catalyze CO2 transformations to cyclic α–alkylidene carbonates, oxazolidinone, N-methylated and N-formylated amines.

EXPERIMENTAL SECTION General Procedure for the synthesis of P-ylide-CO2 adducts 1c-1g.

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In a glove box, a 50 mL Schlenk flask containing a stir bar was charged with the phosphonium salt (o.50 mmol) and 10 mL of THF were added. When the solution was cooled to -35 oC, nBuLi (o.32 mL, 1.6 M in nHexane, 0.50 mmol) was added over 2 min via a microsyringe, instantly resulting in a red solution, which was allowed to warm to room temperature for 1 hour under continuous stirring. Then, the Schlenk flask was sealed with a septum, and transferred from glovebox to an argon/vacuum double manifold to cool to -78 oC. The Schlenk flask was evacuated and allowed to warm to room temperature. After backfilled with highly pure CO2 gas, the reaction system was stirred for 30 minutes at room temperature. The resultant white precipitate was collected via filtration, washed with nhexane (3×10 mL) and then dried under high vacuum to afford the desired product. 1f. White solid, 95% yield. 1H NMR (400 MHz, CD3OD): δ 7.64‒7.86 (m, 15H), 1.66 (d, J = 18.4 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 174.4 (s), 134.8 (d, J = 8.8 Hz), 134.0 (d, J = 3.0 Hz), 129.61 (d, J = 12.0 Hz), 121.2 (d, J = 82.8 Hz), 24.3 (s). 31P NMR (162 MHz, CDCl3): δ 33.3 (s). IR vC=O: 1641 cm-1 (vs). HRMS (ESI): calcd for C22H21O2P: 348.1274 [M]. Found: 349.1365 [M+H]+. Representative experimental procedure for the cycloaddition of epoxide and CO2 to cyclic carbonate. In a glovebox, a 5 mL vial containing a stir bar was charged with the terminal epoxide(10 mmol) and catalyst 1e (0.18 g, 0.5 mmol). Then the vial was sealed with a septum and immediately transferred from glovebox into a 100 mL Schlenk flask with a CO2 balloon. The reaction was carried out at 25 oC for 6 hours with continuous stirring. Then, the yield and selectivity were determined by 1 H NMR of the crude reaction mixture. 2g. 1H NMR (400 MHz, CDCl3): δ 4.82 (m, J = 8.1, 6.1, 3.9 Hz, 1H), 4.48 (t, J = 8.4 Hz, 1H), 4.37 (t, J = 8.4, 6.1 Hz, 1H), 4.27 – 4.14 (m, 2H), 3.74 (m, J = 10.9, 3.9 Hz, 2H), 2.46 (t, J = 2.4 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 154.96 (s), 78.67 (s), 75.57 (s), 74.93 (s), 68.52 (s), 66.18 (s), 58.74 (s) (reported in ref.20k).

ASSOCIATED CONTENT Supporting_Information Experimental procedures, characterization data, NMR spectra of P-ylide-CO2 adducts and products. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (Grant No. 21402021), the Start-Up Foundation of Dalian University of Technology (Grant No. DUT13RC(3)84), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13008). X.-B. Lu gratefully acknowledges the Chang Jiang Scholars Program (No. T2011056) from Ministry of Education, People’s Republic of China.

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