Valorization of CO2: Preparation of 2-Oxazolidinones by MetalâLigand

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Valorization of CO2: Preparation of 2-Oxazolidinones by Metal / Ligand Cooperative Catalysis with SCS Indenediide Pd Complexes Paul Brunel, Julien Monot, Christos E. Kefalidis, Laurent Maron, Blanca Martin-Vaca, and Didier Bourissou ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00209 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 7, 2017

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ACS Catalysis

Valorization of CO2 : Preparation of 2Oxazolidinones by Metal / Ligand Cooperative Catalysis with SCS Indenediide Pd Complexes

Paul Brunel,a Julien Monot,a Christos E. Kefalidis,b Laurent Maron,b Blanca Martin-Vaca,*a and Didier Bourissou*a

a

Université de Toulouse, UPS, LHFA, UMR 5069 CNRS, 118 route de Narbonne, F-31062

Toulouse, France. b

Université de Toulouse, INSA, UPS, LPCNO, UMR 5215 CNRS, 135 avenue de Rangueil,

31400 Toulouse, France.

ACS Paragon Plus Environment

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ABSTRACT.

The capture and utilization of CO 2 to prepare high-value compounds is very attractive chemically and highly desirable socially. Indenediide-based Pd SCS pincer complexes are shown here to promote the carboxylative cyclization of propargylamines leading to 2-oxazolidinones under mild conditions (0.5-1 bar of CO 2 , DMSO, 40-80°C, 1-5 mol% Pd loading). The indenediide Pd complex is competitive with known catalysts. It proved successful for a wide range of propargylamines, including hitherto challenging substrates such as secondary propargylamines bearing tertiary alkyl groups at nitrogen, primary propargylamines and propargylanilines. Thorough experimental (NMR) and computational (DFT) investigations were undertaken to gain mechanistic insights. Accordingly, (i) the resting state of the catalytic cycle is a Pd DMSO complex, (ii) the indenediide backbone and Pd center act in concert to activate the carbamic acid intermediate and promote its cyclization, (iii) proton shuttling is essential to lower the activation barriers of the initial amine carboxylation as well as of the proton transfers between the ligand backbone and the organic fragments at Pd.

Keywords: CO 2 functionalization; palladium; metal-ligand cooperative catalysis; proton shuttling; catalytic cycle


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Introduction Over the last decades, the valorization of CO 2 as an abundant and non-toxic C1 feedstock has garnered increasing interest from chemists and the whole scientific community.1 The chemical transformation of CO 2 has the advantage of creating added-value from a raw material. It is very complementary to sequestration2 in order to reduce the deleterious greenhouse effect of everincreasing CO 2 emission. To chemically transform CO 2 , several approaches involving reduction and/or functionalization have been envisioned and undertaken. Reduction into formic acid or methanol (as a mean to store H 2 ) are undoubtedly the most intensively investigated CO 2 transformations. A number of organic and organometallic catalysts have been reported to efficiently mediate these reductions.3 In addition, more and more studies have been performed over the last decade on the reductive functionalization of CO 2 leading the formation of new C–N, C–O, C–C bonds. Very promising results have been obtained along this line too.4 Great interest has also been devoted to the use of CO 2 as raw material to prepare functional molecules (and polymers) in which the CO 2 moiety is not reduced,5 such as carbonates, carbamates or urethanes (and polymers thereof). Among them, 2-oxazolidinones (fixe-membered cyclic carbamates) stand as prominent compounds with a broad variety of applications ranging from organic synthesis (chiral auxiliaries,6 synthons7), agrochemistry and pharmaceutical chemistry,8 as well as polymer science.9 Different strategies have been developed for the preparation of 2-oxazolidinones using CO 2 as a C1 building block and aziridines,5b,10 haloalkylamines11 or propargylamines5,10 as nucleophilic reaction partners. Due to their ready availability and wide structural diversity, propargylamines are particularly appealing starting materials and their carboxylative cyclization leading to alkylidene-oxazolidinones (whose exocyclic C=C double bond can be subsequently


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reduced or further derivatized) is a very attractive transformation combining CO 2 capture and utilization. The search for efficient catalysts started with the seminal discovery of Mitsudo and Watanabe in 1987 using [Ru(COD)(COT)] (COD = 1,5-cyclooctadiene, COT = 1,3,5-cyclooctatriene) / PPh 3 .12 However, this catalytic system and those reported in the 1990’s operate under relatively harsh conditions (~ 100°C, ≥ 10 bar of CO 2 ) and a strong base is often needed.13 Over the last decade, the search for efficient catalytic systems working under milder conditions, in particular low pressure of CO 2 , has stimulated much interest and significant progress has been achieved. Most noticeable contributions (Figure 1a) are those using silver salts (AgNO 3 , AgOAc) in the presence of an excess of DBU (1,8-diazabicycloundec-7-ene), as initially reported by Yamada in 2009 (these are among the most, if not the most, versatile catalysts known to date for this transformation),14,15

and

the

NHC

gold(I)

complex

(IPr)AuCl

[IPr

=

1,3-bis(2,6-

diisopropylphenyl)-imidazol-2-ylidene] described by Ikariya in 201316 (which performs particularly well with N-alkyl propargylamines bearing an internal alkyl-substituted alkyne).17 In addition, C. Nevado reported in 2016 an efficient Pd-catalyzed synthesis of oxazolidinones, combining the carboxylative cyclization of propargylamines with C sp 2-C sp 2 cross-coupling (Figure 1b).18 This one-pot two-step transformation gives access to highly-substituted alkylideneoxazolidinones in good to excellent yields. Thanks to all these studies, the efficiency and operating conditions of the title transformation have been significantly improved, but there are still issues to overcome in order to extend its scope and strengthen its synthetic interest. In particular, the following subclasses of substrates are still challenging: (i) secondary propargylamines featuring tertiary alkyl groups at the nitrogen atom, (ii) primary propargylamines (their nitrogen atom is less nucleophilic and the resulting N-H oxazolidinone are less stable),19 (iii) propargylanilines (their


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nitrogen atom is also less nucleophilic and the corresponding N-aryl oxazolidinones are usually prepared in two steps) and (iv) homopropargylamines whose carboxylative cyclization would afford direct access to six-membered cyclic carbamates, namely 1,3-oxazin-2-ones.

O Catalyst

N

O

(a) R2

R4

+ CO 2

R1 R3

N R3

R2

H

R1

R4

1 bar

O Ar I [PdCl2(dppf )] . (cat ) . NaOtBu (1 5 eq)

R1 N R3

O

(b) R2

R4 Ar

Figure 1. Schematic representation of the catalytic carboxylative cyclization of propargylamines; comparison of the efficiency and scope of the key metal complexes recently shown to promote this transformation under mild conditions, including the indenediide SCS Pd complex studied in this work.

Over the last few years, we have developed new SCS (and NCN) tridentate ligands deriving from indene.20 The corresponding Pd and Pt complexes were shown to efficiently promote the cycloisomerization of alkynoic acids and N-tosyl akynylamides (Chart 1).21,22 Even very challenging substrates were successfully transformed and a variety of alkylidene-lactones and lactams, including 7-membered rings were thereby obtained in good yields and with high


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selectivities.21 Detailed mechanistic investigations performed on alkynoic acids revealed the active participation of the electron-rich indenediide moiety which temporarily fixes a proton of the carboxylic acids, while a second molecule of substrate acts as a proton shuttle.23

O

PR2 R2P S [Pd/Pt] S 1 5 mol %

XH n

R

X = O, NTs n = 1-3

R O

X

X

O

n

n

exo

endo

R

Chart 1. Cycloisomerization of alkynoic acids and N-tosylalkynylamides catalyzed by indenediide-based palladium or platinum pincer complexes.

Taking into account that the carboxylative cyclization of propargylamines involves as key intermediates carbamic acids, whose structure is reminiscent to that of alkynoic acids, we became interested in exploring the behavior of our indenediide-based Pd and Pt complexes in this transformation. We wondered about the compatibility of these pincer complexes with CO 2 fixation and their ability to catalyze the formation of alkylidene-oxazolidinones. As reported hereafter, indenediide-based Pd complexes were indeed found to promote efficiently the carboxylative cyclization of propargylamines under mild conditions. The scope of the transformation has been enlarged to include secondary propargylamines with bulky alkyl and electron-rich aryl substituents at N, as well as primary propargylamines. Mechanistic studies substantiating the resting state of the catalytic cycle, the contribution of the indenediide backbone (metal-ligand cooperativity) and the key role of proton shuttling are also reported.


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Results and Discussion Catalytic evaluation of SCS indenediide pincer complexes in the carboxylative cyclization of propargylamines and homopropargylamines. The catalytic activity of indenediide-based SCS pincer complexes was first evaluated in the carboxylative cyclization of N-methylpropargylamine (1a) as benchmark reaction (Table 1). The reaction conditions were optimized and the activities of the four complexes (I-IV) were compared. Initial tests were carried out with the dimeric Pd complex I, which performed best in the cycloisomerization of alkynoic acids and alkynyl amides.21 The influence of the solvent was investigated first.24 The catalytic runs were directly performed in deuterated solvents and monitored by 1H NMR spectroscopy, using 5 mol% loading of Pd, working at 50°C and atmospheric pressure of CO 2 . No reaction occurred in acetone (entry 1) and only low conversions of 1a (7-19%, entries 2-6) were observed in chloroform, dimethylformamide, tetrahydrofurane or methanol. By far, the best results were obtained in dimethylsulfoxide (DMSO), full conversion of 1a being reached within 20 minutes (entry 7). Thus the solvent has a marked influence on the reaction, and it is clearly not simply the solubility of carbon dioxide which governs the efficiency of the transformation since CO 2 is >2 times more soluble in acetone and THF (1.95 and 2.7 mol%, respectively, at 25°C and 1 atm) than in DMSO (0.9 mol%).25 Such carboxylative cyclizations typically proceed faster in DMSO, as reported by T. Yamada with Ag+/DBU15 and by C. Nevado with PdCl 2 (dppf).18 In stark contrast, T. Ikariya noticed that the gold(I) complex (IPr)AuCl performs best in methanol, a protic solvent being required in this case to promote the final protodeauration step.16b The lower conversion we observed in methanol most likely results from the low solubility of complex I in this solvent. Increasing the reaction temperature substantially speeded up the transformation, allowing the catalyst loading to be reduced to 1 mol% Pd in DMSO


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(Figure S1).24 Accordingly, full conversion of 1a could be achieved within only ten minutes at 80°C (entry 8). Conversely, working at higher CO 2 pressure (2 bar) did not increase significantly the reaction rate,24 confirming that CO 2 solubility is not a limiting factor. Thus the reaction can be efficiently and conveniently performed at low pressure of CO 2 (0.5-1 bar).26 The impact of the catalyst structure was then evaluated in DMSO at 70°C and 1 bar of CO 2 using 1 mol% Pd. The chloropalladate II showed similar activity as the neutral dimer I (entries 9 and 10), indicating that the chloride co-ligand at Pd and the ammonium counter-cation have no noticeable influence. By contrast, alkylation of the indenediide backbone drastically decreased the catalytic activity. Indeed, the 1-methylindenyl-based palladium complex III proved to be only poorly active, giving 10% of conversion after 10 min of reaction (entry 11) vs. 55% with I. This result supports an active role of the indenediide backbone (see mechanistic discussions below), in line with what we previously reported for cycloisomerization reactions.21,23 Finally, the Pt complex IV was evaluated and found to display moderate activity (20% conversion after 10 min, entry 12). Thus, the Pd complex I afforded the best results and it was selected for the following catalytic studies.


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Table 1. Reaction conditions and indenediide-based SCS pincer complexes I-IV evaluated in the benchmark carboxylative cyclization of N-methylpropargylamine 1a. . . 0 5 5% mol [Pd] . CO2 0 5 1 bar solvent, T (°C), t (h)

HN Me

1a

n

i Pr

S

Pd S

i Pr

2P

i Pr

Pi Pr2

2P

Pd S Pi Pr2 I

Entrya Cat.

Pd Cl

N

O

2a

Bu4N

Pi Pr2

2P

S

S

O Me

i Pr

S

Pi Pr2

2P

S

II

Pt S

Me Ph2P S Pd Cl

Solvent

i Pr

2P

PPh2

S Pt S Pi Pr2 IV

S III

T mol% Time Conv (°C) Pd (h) (%)b

1

I

(CD 3 ) 2 CO

50

5

6

-

2

I

CD 3 CN

50

5

4.5

7

3

I

DMF-d 7

50

5

4.5

17

4

I

THF-d 8

50

5

4.5

19

5c

I

CDCl 3

50

5

6

13

6

I

CD 3 OD

50

5

1

11

7

I

DMSO-d 6

50

5

0.33

99

8

I

DMSO-d 6

80

1

0.16

99

9

I

DMSO-d 6

70

1

0.16

55

10

II

DMSO-d 6

70

1

0.16

53

11

III

DMSO-d 6

70

1

0.16

10

12

IV

DMSO-d 6

70

1

0.16

20

(a) All catalytic tests were performed starting from 0.8 mmol of N-methylpropargylamine (1 M solutions), at 1 bar of CO 2 and 5 or 1 mol% Pd loading. (b) Conversions were determined by 1H NMR.


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The substrate scope of the reaction was then explored using DMSO as solvent (1 M solutions) and working at 1 mol% Pd loading, 70°C and 1 bar of CO 2 , as standard conditions (Table 2). Secondary propargylamines 1a-h were investigated first.24 Clean and quantitative reactions were observed in all cases (entries 1-8). As expected, the steric demand of the N-substituent strongly influences the reaction rate. Full conversions were achieved within only 10 min with the Nmethylated and N-benzylated substrates 1a and 1d (entries 1 and 4), whereas 12 h of reaction were needed for the N-isopropyl substrate 1b (entry 2). Remarkably, complex I was also efficient in promoting the carboxylative cyclization of the N-tert-butyl substrate 1c. Full conversion could be achieved after 90 h and the corresponding oxazolidinone was isolated in 88% yield (entry 3). To the best of our knowledge, this is only the second example of such a transformation with a propargylamine featuring a tertiary alkyl group at nitrogen. The other one was reported recently by C. Nevado with PdCl 2 (dppf)18 but this carboxylative cyclization / cross-coupling reaction seems to be more sensitive to steric hindrance since only 38 % yield was obtained after heating for 3 days. The substitution of the alkyne, terminal or internal, was then studied and it proved to also have a clear impact on the reaction rate. Still, very efficient conversions could be achieved in all cases. Terminal alkynes 1a,b,d were converted more rapidly than their substituted counterparts 1e-h (entries 5-8). At 70°C, reaction times were typically in the range of 12 hours for terminal alkynes vs. 1-3 days for internal alkynes. Note also that complex I converted the Ph- and Mesubstituted alkynes 1g,h with similar rates (entries 7 and 8).27 But the nature of the alkyne substituent, aryl or alkyl, has an impact on the regioselectivity of the cyclization. The phenylsubstituted substrates 1e-1g underwent fully selective 5-exo cyclization, whatever the alkyl group at nitrogen (entries 5-7), while some 6-endo cyclization was observed with the methyl-substituted


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alkyne 1h (entry 8). Indeed, a 8.25/1 mixture of 5-exo/6-endo oxazolidinones was obtained in this case, similarly to what was observed by Yoshida with Ag+/DBU.15c

Table 2. Substrate scope of the carboxylative cyclization of propargyl and homo-propargylamines with I. Entrya

Substrate

T (ºC)

Product

t (h)

Conv (%)b

Entrya

Substrate

HN Me 1a

N

O

80

. 0 16

2a

>99 (90)

Bn

9d e

N

N Bn H 1i

2i

2 1b

HN iPr

N

70

O

12

>99 (91)

Bn

, 10d e

1j

2b tBu

HN tBu 1c

N

70

O

90

>99 (88)

H N Bn

,

11d e Ph

40

144

40

48

>99 (36)f

O

2j

Bn N

Ph

>99 (37)f

>99g

40

1

N Bn O

70

160

>99 (95)

O

40

33

>99 (74)

O

40

36

>99 (85)

O

40

6

>99 (85)

O

40

21

>99 (85)

4k

1k

2c

O

N

N Bn H

O 3

Conv (%)b

O

O iPr

t (h)

O ,

Me 1

T (ºC)

Product

O

Ph

O Bn

4 1d

HN Bn

N

80

O

. 0 16

>99 (83)

Ph

12

2d

1l

N Bn H

4l

O 5

Me

Ph 1e

N

HN Me

Ph 1f

N

HN iPr

Bn 1g

HN Bn

PMP PMP N H PMP = p(MeO)C6H4

90

>99 (85)c

, 14d e

N

N

60

>99 (97)c

15d

HN 1o

NH2 2o O

Me

O Bn

2n

O

2h HN Bn

70 Ph

O

8 Me

O

O O

N

N

2m

HN NH2

Ph

2g Bn

1h

13e

1n 2f

Ph

70

O O

7

>99 (98)c

Ph

O 6

20

1m

2e iPr

70

O

70

60

exo/endo 8.25/1

O

>99 (74)c

HN

16d 1p

NH2 2p

Me 3h

(a) General conditions: the catalytic runs were performed starting from 0.8 mmol of the Npropargyl amine (1 M solutions), at 1 bar of CO 2 and 1 mol% Pd loading. (b) Conversions were determined by 1H NMR using 1,3,5-trimethoxybenzene as internal standard. Isolated yields are given in brackets. (c) Full Z stereoselectivity was attributed according to literature data, see Supp. Info. (d) 0.1 M solution of the propargyl amine. (e) 5 mol% Pd loading. (f) Spectroscopic yield. Compound 2i was isolated in 30% yield. (g) Low stability of the product prevented isolation.


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The good results obtained with secondary propargyl amines prompted us to assess the ability of I to catalyze the carboxylative cyclization of more challenging substrates such as homopropargylamines (leading to oxazinones), propargylanilines and primary propargylamines. The homopropargylamine 1i was prepared24 and reacted with CO 2 in the presence of I using the same conditions as for propargylamines (DMSO, 1 M, 70°C, 1 bar of CO 2 , 1 mol% Pd loading). Complete conversion of 1i was observed after a few hours of reaction but 1H NMR spectroscopy indicated the formation of a complex mixture of compounds containing only a small amount of carboxylative cyclization product 2i. Working under milder conditions (40°C, 0.1 M) and increasing the catalytic loading to 5 mol% Pd allowed to decrease the proportion of secondary products and the desired oxazinone 2i could be thereby obtained in 30% isolated yield.28 Seeking to favor the cyclization process by Thorpe-Ingold effect, the propargylamine 1j featuring a methyl group in the position α to nitrogen was then synthesized24 and subjected to the carboxylative cyclization (entry 10). Unfortunately, only a minor improvement was observed and the corresponding oxazinone 2j was obtained again in modest yield (37%). With the aim to gain more insight into the undesirable processes competing with carboxylative cyclization, the propargylamines 1a and 1e were heated for 48 h in the presence of the Pd complex I (40°C, 0.1 M, 5 mol% of Pd) but in the absence of CO 2 . While the internal alkyne 1e remained unchanged, the terminal alkyne 1a was converted into a mixture of unidentified products. This observation suggests that side-reactions are more likely to occur with substrates featuring terminal alkynes, spurring us to prepare and test substrate 1k, an homo-propargylamine with an internal (phenylsubstituted) alkyne moiety. As hoped for, a clean cyclization reaction occurred with 1k (entry 11) but 1H NMR and MS analyses revealed that CO 2 was not incorporated. In fact, 5-endo cyclohydroamination leading to the corresponding dehydro-pyrrole 4k was favored over the


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carboxylative cyclization. An attempt was made to fully carboxylate the amine of 1k before addition of the catalyst, but the outcome was the same, no carboxylation could be observed. Cyclohydroamination was also observed with substrate 1l featuring an o-phenylene spacer between the amine and the alkyne, and the resulting indole 4l was isolated in 95% yield (entry 12). Note that carboxylative cyclization of 1l was reported by Yamada using Ag+/DBU as catalyst and working at 10 atm of CO 2 .15d,29 The results obtained with homopropargylamines can be summed up as follows. Complex I allows carboxylative cyclization of 1i,j into alkylidene-oxazinones 2i,j. Although modest yields were obtained, this is the first time such compounds have been prepared this way.30 In addition, cyclo-hydroamination (5-endo cyclization) has been identified as a competitive process to the carboxylative cyclization (6-exo cyclization). We then turned our attention to N-propargylanilines, which are extremely scarce substrates for this type of reaction due to the low nucleophilicity of the nitrogen atom.31 No reaction was observed from the parent N-propargylaniline HC≡CCH 2 NHPh, but compound 1m featuring a para-methoxy group on the phenyl ring led to the desired oxazolidinone 2m in good yield (74%, entry 13). Thus, complex I enables straightforward and efficient access to oxazolidones featuring electron-rich aromatic rings at the nitrogen atom, a subclass of oxazolidinones actually known for their biological activities,8b after suitable derivatization of the alkylidene moiety. Primary propargylamines are also quite challenging substrates and not surprisingly, their carboxylative cyclization is much less developed than that of secondary propargylamines. Sidereactions can arise from the NH 2 group and lower yields are typically obtained when compared with related secondary amines.19,32 Using complex I as catalyst, propargylamines 1n-p (entries 1416) led to complex mixtures when the reactions were operated under our standard conditions (1 M, 70°C). As for homopropargylamines, working under milder conditions (0.1 M, 40°C) proved


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highly beneficial. Indeed, NMR monitoring indicated clean and quantitative formation of the desired N-H oxazolidinones 2n-p which were isolated in 85% yield. Such unsubstituted oxazolidinones are particularly interesting as direct precursors for N-arylated oxazolidinones.32,33 Accordingly, carboxylative cyclization of primary propargylamines followed by catalytic Narylation represents an alternative and complementary route to the carboxylative cyclization of Npropargylanilines discussed above.

Mechanistic considerations The commonly accepted catalytic cycle for these carboxylative cyclizations involves carboxylation of the propargylamine to form a carbamic acid, activation of the C≡C triple bond by side-on coordination to the metal center, cyclization by outer-sphere nucleophilic attack of the carbamic acid and protodemetallation. In order to get more mechanistic insight into the reaction catalyzed by the Pd complex I, and in particular to shed light on the contribution of the indenediide ligand, experimental and computational investigations were undertaken. Reaction monitoring, stoichiometric reactions. The outcome of the Pd complex I was first probed by monitoring some catalytic runs by

31

P

NMR spectroscopy. In all cases, a unique signal at δ 73.9 ppm was observed. This signature markedly differs from that of the starting complex, which adopts a dimeric structure in solvents such as CDCl 3 , giving two 31P NMR signals at δ 77.8 and 71.5 ppm for the inequivalent bridging and non-bridging P=S groups.21b It is very likely that the signal observed at δ 73.9 ppm in the catalytic runs results from the dissociation of the dimeric structure of I by coordination of DMSO to Pd (Scheme 1). This hypothesis was confirmed by the observation of the same unique 31P NMR signal upon dissolution of complex I in DMSO (in the absence of substrate and CO 2 ). The affinity


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of DMSO for the indenediide Pd fragment was then quantitatively assessed. To do so, increasing amounts of DMSO were added to a CDCl 3 solution of I and the relative proportions of the dimer and DMSO complexes were measured by 31P NMR (Fig S2). After addition of 2 eq of DMSO per Pd center, a small peak was observed at δ 73.9 ppm. The two signals associated with the dimeric structure completely disappeared upon addition of 60 eq of DMSO. The equilibrium constant K (which corresponds to dimer dissociation and DMSO coordination) was estimated to 2.7x10-2.24 Similar experiments were carried out with the analogous Pt complex IV. In this case, the dimeric structure was completely splitted upon addition of only 2 eq of DMSO per Pt center (Figure S3),24 indicating a significantly higher affinity of DMSO for Pt than for Pd.34 The strong affinity of the indenediide Pt fragment for DMSO may contribute to the lower activity of IV in the carboxylative cyclization of propargylamines, as the reaction requires the substrate to coordinate to the metal and thus to displace DMSO. We then examined the ability of model alkynes and amines to displace DMSO from the corresponding Pd adduct. No reaction was observed upon addition of up to 100 eq of phenylacetylene, but a stoichiometric amount of methylbenzylamine was enough to induce quantitative transformation, as apparent from

P NMR spectroscopy: the signal at δ 73.9 ppm

31

disappeared and a new signal raised at δ 77.7 ppm.24 The latter signal can be confidently attributed to the Pd amine complex, by analogy with the related indenediide Pd dicyclohexylamine we previously reported.20a


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PhC CH i Pr

2P S

½

Pi Pr2 Pd S

i Pr

2P

S Pd S Pi Pr2

DMSO

i Pr

No reaction

Pi Pr2

2P

S

Page 16 of 32

Pd

S

DMSO

BnMeNH

i Pr

Pi Pr2

2P

S

Pd

S

Bn N H Me

I

Scheme 1. Splitting of the Pd dimer I upon DMSO coordination and subsequent displacement of DMSO by N-methylbenzylamine.

Based on these studies, we propose that the resting state of the catalytic cycle is the Pd DMSO complex, whose DMSO molecule is labile enough to be displaced by the propargylamine or the corresponding carbamic acid so that the C≡C triple bond can be activated and cyclization can take place. In addition, the very low activity of the related indenyl Pd complex III supports an active participation of the indenediide moiety (in the activation of the carbamic acid moiety, see the computational studies below), as in the case of the cycloisomerization reactions we investigated previously.21,23 To complete the experimental study, we examined the very first stage of the transformation, ie the carboxylation of propargylamines. Accordingly, compound 1a was found to spontaneously and quantitatively form the corresponding carbamic acid when reacted with CO 2 (1 bar) in DMSO at room temperature. Most diagnostic is the deshielding of the 1H NMR signal associated with the NCH 2 group (from δ 3.23 ppm in the propargylamine 1a, to δ 4.01 ppm in the corresponding carbamic acid, see Figure S6).24 Interestingly, the carbamic acid remained unchanged after 3 freeze-pump cycles, but CO 2 was slowly released and 1a was recovered upon heating at 80°C (decarboxylation was complete after ca 12 h).35


16

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Computational studies. DFT calculations were performed at the same level of theory (B3PW91) than the one we used previously for the cycloisomerization of alkynoic acids.23 However, to account for the striking effect of DMSO observed experimentally, solvent effects were included by means of the SMD model (see computational details).24 The carboxylation of the propargylamine 1a was investigated first. The indenediide Pd complex, which is not expected to play any role in this step, was not considered and consistently, a lowenergy path was identified for the reaction of 1a with CO 2 (Figure 2). It requires the inclusion of a second molecule of amine 1a, in line with that reported by Yuan and Lin for the carboxylation of H 3 CC≡CCH 2 N(CH 3 )CO 2 H in the coordination sphere of gold.36 Interaction of the amine with CO 2 first gives a weak adduct (ad1) which is slightly downhill in energy (by 4.4 kcal.mol-1 in enthalpy).35,37 Nucleophilic addition of the nitrogen atom to CO 2 is then assisted by the second molecule of amine, which mediates the proton transfer from the attacking nitrogen atom to one of the oxygen atom of CO 2 (TS ad12 ). The formation of the N−CO 2 bond (1.58 Å) and the bending of the CO 2 fragment (133.6°) are quite advanced at the transition state. The corresponding activation barrier is accessible (2.6 kcal.mol-1 in enthalpy from ad1, 22.4 kcal.mol-1 in Gibbs energy from the initial reactants) and much lower than that computed for the direct carboxylation of 1a without the assistance of a second amine molecule (∆H≠ 28.7 kcal.mol-1, ∆G≠ 36.5 kcal.mol-1, see Figure S8).24 This emphasizes the role of proton shuttling, which is optimal in TS ad12 (six-membered cyclic transition state vs. four-membered TS for the direct reaction). Following the intrinsic reaction coordinate, ad2, an adduct between the carbamic acid and amine molecules is obtained. Dissociation of the hydrogen bond is relatively demanding in energy (∆H 11.3 kcal.mol-1) but nevertheless thermodynamically favored (∆G −1.5 kcal.mol-1) due to the large entropic factor


17

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(formation of two molecules from one). These computational results are consistent with spontaneous and reversible reaction of the propargylamine 1a with CO 2 , and therefore corroborate our experimental observations (formation of the carbamic acid at RT under CO 2 atmosphere and release of CO 2 upon heating at 80°C).

Enthalpy (Gibbs free energy) in kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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HC HC

C 2

CH2 Me

HC C

+ CO2

C +

CH2

N H

. 00 . (0 0)

HC C

CH2 N

O C O

H Me

C

CH

N H

- . 73 . 3 ( 7)

ad2 - . 18 6 . 5 2) ( e

M

HC C

H

Me CO2H

Me N

N

Me

TSad12 - . 18 . 22 ( 4)

ad1 - . 44 . (16 9)

CH2

C H2

N C

O

O

CH

H H

C N Me

Reaction Coordinate

Figure 2. Reaction energy profile in enthalpy at room temperature (Gibbs free energy at room temperature in brackets) for the reaction of propargylamine 1a with CO 2 in the presence of a second propargylamine molecule in DMSO (O in red, N in dark blue).

The

intramolecular

cyclization

of

the

formed

propargylic

carbamic

acid

HC≡CCH 2 N(CH 3 )CO 2 H was then studied at the same level of theory (B3PW91) including solvent effects (DMSO, PCM model) and considering the actual indenediide Pd complex (the iPr groups at P were retained). The corresponding reaction profile is depicted in Figure 3 (for clarity, the substituents at P are omitted). It is discussed hereafter in parallel and comparison with the reaction


18

Page 19 of 32

profiles computed i) by us for the cycloisomerization of alkynoic acids catalyzed by the same indenediide Pd complex23 and ii) by Yuan and Lin for the carboxylative cyclization of the propargylamine H 3 CC≡CCH 2 NH(CH 3 ) catalyzed by the NHC gold complex (IPr)AuCl.36,38 A second molecule of carbamic acid was included in the calculations to mediate proton transfers and lower the barriers of the key steps.

Enthalpy (Gibbs f ree energy) in kcal/mol

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TSB . 21 8 . (22 4)

CH C

Cat . 00 . (0 0)

TSA . 82 . (5 7)

CH2 Me N O Me C O H C N O O CH2 H S P C Pd C H S P

int A . 60 . (3 2)

TSC - . 37 - . ( 0 9)

CH C CH2 Me N C O O H P

O Me C H O N CH2 S P C Pd C H S

int B CH C CH2

Me

N

O H P

C O S S P Pd S

H C C O O H2C N Me

Reaction Coordinate

int C - . 25 0 - . ( 30 7) cyc lic pro duc t

acid Cat - . 31 6 - . 31 8) (

Figure 3. Reaction energy profile in enthalpy at room temperature (Gibbs free energy at room temperature

in

brackets)

for

the

cyclization

of

the

propargylic

carbamic

acid

HC≡CCH 2 N(CH 3 )CO 2 H mediated by the indenediide Pd complex (the iPr groups at P are omitted


19

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for clarity) in the presence of a second carbamic acid molecule in DMSO (Pd in light blue, S in yellow, P in orange, O in red, N in dark blue).

The reaction starts by side-on coordination of the substrate to the Pd center. In the active species Cat, the triple bond is bound in an η2 fashion and oriented perpendicular to the SCS coordination plane of Pd. The carbamic moiety is engaged in O−H∙∙∙O hydrogen bonding with the second molecule of carbamic acid whose acidic proton weakly interacts with the electron-rich backbone of the SCS ligand. The first step of the reaction is the protonation of the indenediide moiety. Thanks to the network of hydrogen bonds, the corresponding activation barrier is low (8.2 kcal.mol-1 in enthalpy). The second molecule of carbamic acid acts as a proton shuttle and mediates the proton transfer from the side-on coordinated substrate to the ligand backbone. Following the intrinsic reaction coordinate, intermediate int-A is obtained. Then, cyclization occurs by nucleophilic attack of the carbamic acid moiety to the π-activated triple bond with concomitant proton transfer to the external molecule of carbamic acid. The outer-sphere approach of the nucleophile controls the stereochemistry of the reaction, Z products being selectively obtained from substrates bearing internal alkynes. The corresponding transition state TS B is the highest point of the reaction profile in energy, making this 5-exo cyclization process the rate-determining step of the overall transformation.39 The corresponding activation barrier was estimated to 21.8 kcal.mol-1 in enthalpy (22.4 kcal.mol-1 in Gibbs energy) in line with a relatively fast reaction under mild heating, as observed experimentally.40 In the ensuing Pd species int-B, the oxazolidinone ring is formed and bound to palladium via a σ Pd–C bond. The external molecule of carbamic acid is weakly hydrogen-bonded to the ligand backbone and Pd center, ready to promote the back transfer of a proton from the indenyl moiety to the cyclized substrate. It acts again as a proton


20

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shuttle and makes this protonolysis a low-activation barrier process (TS C , 12.9 kcal.mol-1 in enthalpy). This step regenerates the indenediide pincer complex and yields the final oxazolidinone product which remains weakly bound to the palladium center via side-on coordination of the exocyclic double bond. Then, the oxazolidinone is released and replaced by another substrate molecule, making turnover possible. Overall, the cyclization of the carbamic acid is highly exothermic / exergonic (∆H and ∆G values of about 31 kcal.mol-1). These calculations confirm the participation of the indenediide moiety (by temporary fixation of a proton) in this transformation and highlight the key role of a second external molecule of carbamic acid, which acts as a proton relay and mediates proton transfers from the carbamic acid substrate to the ligand backbone and after cyclization has occurred, from the ligand backbone to the vinyl Pd moiety.41 This scheme parallels that we reported previously for the cycloisomerization of alkynoic acids and N-tosyl alkynylamides.23 The carboxylative cyclization of propargylamines generalizes and extends the scope of metal-ligand cooperativity with such SCS indenediide pincer complexes. The computational study also emphasizes and explains the influence of DMSO. A much higher activation barrier was computed in the gas phase for the rate-determining step of the reaction profile (∆H≠ 33.4 kcal.mol-1).24,42 The high polar character of DMSO apparently plays a key role in stabilizing the key transition state TS B . Consistently, a huge dipolar moment was computed for TS B (18.6 D) as to compare with the reactants (3.4 D for Cat).

Conclusion In summary, we have shown in this study that indenediide-based Pd SCS pincer complexes are competitive with known catalysts to promote the carboxylative cyclization of propargylamines. The bench-stable Pd dimer I enables straightforward, atom-economic and high-yield preparation


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of alkylidene-oxazolidinones under mild conditions (0.5-1 bar of CO 2 , DMSO, 40-80°C, 1-5 mol% Pd loading). It works with a wide range of propargylamines, including hitherto challenging substrates such as secondary propargylamines bearing tertiary alkyl groups at nitrogen, primary propargylamines and propargylanilines. Valuable information on the catalytic cycle have been gained thanks to detailed NMR and DFT studies. Accordingly, DMSO, the very preferred solvent for this transformation, was found to readily split the dimer I and coordinate to Pd to form a labile adduct which is actually the resting state of the catalytic cycle. The high polarity of DMSO also comes into play to lower the activation barrier of the rate-determining cyclization step. Furthermore, the indenediide backbone of the ligand actively participates in the transformation by temporarily fixing a proton and facilitating thereby the nucleophilic attack of the carbamic acid to the side-on coordinated C≡C triple bond (metal-ligand cooperation). Last but not least, proton shuttling appears essential at all stage of the transformations, from the initial amine carboxylation (which is assisted by a second molecule of amine) to the back and forth proton transfers between the ligand backbone and the organic fragment at Pd (which are assisted by a second molecule of carbamic acid).43 By analogy and extension of what we reported for the cycloisomerization of alkynoic acids,23 it is conceivable and very attractive to use H-bond donors as external additives acting as proton relays. However the high polarity of DMSO and its propensity to form relative strong H-bonds may mitigate the impact of such additives. The preliminary tests we performed in this regard support this view. Indeed, the conversion of the sterically hindered propargylamine 1c into the corresponding oxazolidinone 2c with the Pd dimer I was significantly speeded up upon addition of 5 mol% of pyrogallol (97% conversion was reached after 20 h at 1 mol% Pd loading and 70°C, instead of 90 h in the absence of additive).24 However, less effect was observed with


22

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other substrates such as 1h (a secondary propargylamine bearing an internal alkyne) in terms of activity as well as selectivity.24 Future work from our group will seek to develop further the chemistry of indenediide-based pincer complexes and extend their catalytic applications, with special interest for transformations involving metal/ligand cooperativity.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Any funds used to support the research of the manuscript should be placed here (per journal style).


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ASSOCIATED CONTENT Supporting Information. Electronic Supplementary Information (ESI) available: Experimental and computational details, characterization data including NMR spectra (PDF), optimized structures (XYZ). This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was supported financially by the Centre National de la Recherche Scientifique, the Université de Toulouse, the Agence Nationale de la Recherche (ANR CE6-CYCLOOP) and the COST Action CM1205 CARISMA (Catalytic Routines for Small Molecule Activation). The authors are grateful to CalMip (CNRS, Toulouse, France) for calculation facilities. L. M. thanks the Institut Universitaire de France. Dr. J. Babinot is acknowledged for the preparation of substrates 1j,k. This manuscript is in memory of Prof. José Barluenga.

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S.; Tsubo, T.; Yamada, T. Org. Lett. 2013, 15, 848–851. (e) Song, Q.-W.; He, L.-N. Adv. Synth. Catal. 2016, 358, 1251–1258. 16. (a) Hase, S.; Kayaki, Y.; Ikariya, T. Organometallics, 2013, 32, 5285–5288. (b) Hase, S.; Kayaki, Y.; Ikariya, T. ACS Catal. 2015, 5, 5135–5140. 17. (a) For a rare example of metal-free conditions, see the recent report of Ma and Han using the protic ionic liquid [DBUH][MIm] (MIm = 2-methylimidazolide) as catalyst and solvent. Very good results were obtained with N-alkyl propargylamines bearing an internal aryl-substituted alkyne, see: Hu, J.; Ma, J.; Zhu, Q.; Zhang, Z.; Wu, C.; Han, B. Angew. Chem. Int. Ed. 2015, 54, 5399–5403. (b) For another recent example using copper(II) substituted polyoxometalate-based ionic liquids, see: Wang, M.-Y.; Song, Q.-W.; Xie, J.N.; He, L.-N. Green Chem. 2016, 18, 282–287. 18. García-Dominguez, P.; Ferh, L.; Rusconi, G.; Nevado, C. Chem. Sci. 2016, 7, 3914–3918. 19. Ishida, T.; Kobayashi, R.; Yamada, T. Org. Lett. 2014, 16, 2430–2433. 20. (a) Oulié, P.; Nebra, N.; Saffon, N.; Maron, L.; Martin-Vaca, B.; Bourissou, D. J. Am. Chem. Soc. 2009, 131, 3493–3498. (b) Nebra, N.; Lisena, J.; Ladeira, S.; Saffon, N.; Maron, L.; Martin-Vaca, B.; Bourissou, D. Dalton Trans. 2011, 40, 8912–8921. (c) Oulié, P.; Nebra, N.; Ladeira, S.; Martin-Vaca, B.; Bourissou, D. Organometallics 2011, 30, 6416–6422. (d) Nebra, N.; Saffon, N.; Maron, L.; Martin- Vaca, B.; Bourissou, D. Inorg. Chem. 2011, 50, 6378–6383. (e) Nebra, N.; Ladeira, S.; Maron, L.; Martin-Vaca, B.; Bourissou, D. Chem. Eur. J. 2012, 18, 8474–8481. (f) Nicolas, E.; Martin-Vaca, B.; Mézailles, N.; Bourissou, D.; Maron, L. Eur. J. Inorg. Chem. 2013, 4068–4076.


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21. (a) Nebra, N.; Monot, J.; Shaw, R.; Martin-Vaca, B.; Bourissou, D. ACS Catal. 2013, 3, 2930–2934. (b) Espinosa-Jalapa, N. Á.; Ke, D.; Nebra, N.; Le Goanvic, L.; Mallet-Ladeira, S.; Monot, J.; Martin-Vaca, B.; Bourissou, D. ACS Catal. 2014, 4, 3605–3611. (c) Ke, D.; Espinosa, N. Á.; Mallet-Ladeira, S.; Monot, J.; Martin-Vaca, B.; Bourissou, D. Adv. Synth. Catal. 2016, 358, 2324–2331. 22. An example of imine allylation catalysed by a SCS indolyl Pd pincer complex has also been reported, see: Lisena, J.; Monot, J.; Mallet-Ladeira, S.; Martin-Vaca, B.; Bourissou, D. Organometallics 2013, 32, 4301–4305. 23. Monot, J.; Brunel, P.; Kefalidis, C. E.; Espinosa-Jalapa, N. Á.; Maron, L.; Martin-Vaca, B.; Bourissou, D. Chem. Sci. 2016, 7, 2179–2187. 24. See supporting information. 25. Hansen, C. M. Hansen Solubility Parameters: A User’s Handbook, CRC Press, Boca Raton, 2007, 177 pp. 26. Clean and quantitative cyclization of 1a could also be achieved at 0.5 bar of CO 2 using 1 mol% Pd loading.24 27. Ikariya observed significantly faster reactions with alkyl than aryl-substituted alkynes.16b Based on mechanistic studies, this difference was attributed to the faster protodeauration of the vinyl gold intermediates when substituted by alkyl rather than aryl groups. 28. The spectroscopic yield of 2i is 36%. MS analysis suggest the formation of oligomeric products, but the exact structure of the by-products could not be further established.


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29. Increasing the pressure of CO 2 to 8 bar at 40°C did not change the outcome of the reaction with our Pd complex I, only cyclo-hydroamination was observed. 30. Transformation of the corresponding N-Boc homopropargylamines catalyzed by a gold(I) catalyst leads to the same products under mild conditions but with less atom-economy. See: (a) Robles-Machín, R.; Adrio, J.; Carretero, J. C. J. Org. Chem. 2006, 71, 5023–5026. (b) Alcaide, B.; Almendros, P.; Quirós, M. T.; Fernández, I. Beilstein J. Org. Chem. 2013, 9, 816–826. (c) Sánchez-Roselló, M.; Miró, J.; del Pozo, C.; Fustero, S. J. Fluorine Chem. 2015, 171, 60–66. 31. Carboxylative cyclization of N-propargylanilines has been reported only once, using electrochemical conditions: Feroci, M.; Orsini, M.; Sotgiu, G.; Rossi, L.; Inesi, A. J. Org. Chem. 2005, 70, 7795–7798. 32. Efficient preparation has been reported from the corresponding N-Boc propargylamines catalyzed by a gold(I) catalyst, see: Lee, E.-S.; Yeom, H.-S.; Hwang, J.-H.; Shin, S. Eur. J. Org. Chem. 2007, 3503–3507. 33. (a) Cacchi, S.; Fabrizi, G.; Goggiamani, A.; Zappia, G. Org. Lett. 2001, 16, 2539–2541. (b) Ghosh, A.; Sieser, J. E.; Riou, M.; Cai, W.; Rivera-Ruiz, L. Org. Lett. 2003, 13, 2207– 2210. 34. This conclusion was further supported by a competitive experience in which DMSO was added to an equimolar mixture of I and IV in CDCl 3 (Figure S4). After addition of 2 eq of DMSO per metal center, the Pt dimer was fully converted into the corresponding


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monomeric DMSO complex, while complex I remained unchanged, only the two signals associated with the dimeric structure being observed.24 35. For reversible fixation of CO 2 with N compounds, see: (a) Hampe, E. M.; Rudkevich, D. M. Chem. Commun. 2002, 1450–1451. (b) Heldebrant, D. J.; Jessop, P. G.; Thomas, C. A.; Eckert, C. A.; Liotta, C. L. J. Org. Chem. 2005, 70, 5335–5338. 36. Yuan, R.; Lin, Z. ACS Catal. 2015, 5, 2866–2872. 37. For recent references dealing with N adducts of CO 2 , see: (a) Dell’Amico, D. B.; Calderazzo, F.; Labella, L.; Marchetti, F.; Pampaloni, G. Chem. Rev. 2003, 103, 3857– 3897. (b) Villiers, C.; Dognon, J.-P.; Pollet, R.; Thuéry, P.; Ephritikhine, M. Angew. Chem. Int. Ed. 2010, 49, 3465–3468. (c) Murphy, L. J.; Robertson, K. N.; Kemp, R. A.; Tuononen, H. M.; Clyburne, J. A. C. Chem. Commun. 2015, 51, 3942–3956. 38. For DFT calculations on the carboxylative cyclization of propargyl alcohols, see: Kakuchi, S.; Yoshida, S.; Sugawara, Y.; Yamada, W.; Cheng, H.-M.; Fukui, K.; Sekine, K.; Iwakura, I.; Ikeno, T.; Yamada, T. Bull. Chem. Soc. Jpn. 2011, 84, 698–717. 39. This contrasts with the gold-catalyzed process studied computationally by Yuan and Lin. In the latter case, the activation barrier of the cyclization is quite low and the ratedetermining step is the final protodeauration.36 40. Computations have been performed on key stationary points with Pt. Accordingly, the activation barrier for the rate-determining step, namely the cyclization reaction, was found to be 23.8 kcal/mol with Pt. This barrier is only 2 kcal/mol higher that than computed for


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Pd, and it is likely that the difference in catalytic activity mainly arises from the different affinity for DMSO as mentioned above. 41. The amine substrate and methanol used as a solvent were also proposed to assist proton transfer in the carboxylative cyclization catalyzed by (IPr)AuCl.16,36 42. DMSO also likely plays a role in the carboxylation of the propargylamine. The formation of the carbamic acid is probably favored both kinetically and thermodynamically in a polar solvent such as DMSO. 43. For a recent review on computational mechanistic studies dealing with pincer complexes of non-innocent pincer ligands and discussing proton shuttling, see: Li, H.; Hall, M. B. ACS Catal. 2015, 5, 1895–1913.


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SYNOPSIS. Graphical Table of Content.


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Valorization of CO2: Preparation of 2-Oxazolidinones by MetalâLigand

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Valorization of CO2: Preparation of 2-Oxazolidinones by MetalâLigand