Gold-Catalyzed Polymerization Based on Carbene

Article pubs.acs.org/Macromolecules

Gold-Catalyzed Polymerization Based on Carbene Polycyclopropanation Frida Nzulu,†,‡ Alexis Bontemps,†,‡ Julien Robert,†,‡ Marion Barbazanges,*,†,‡ Louis Fensterbank,*,†,‡ Jean-Philippe Goddard,*,†,‡,§ Max Malacria,†,‡ Cyril Ollivier,†,‡ Marc Petit,†,‡ Jutta Rieger,†,‡ and François Stoffelbach*,†,‡ †

Institut Parisien de Chimie Moléculaire, Sorbonne Universités, UPMC Univ Paris 06, UMR 8232, F-75252 Paris Cedex 05, France Institut Parisien de Chimie Moléculaire, CNRS, UMR 8232, F 75252 Paris Cedex 05, France

‡

S Supporting Information *

ABSTRACT: The first polymerization exploiting the carbenic reactivity of homogeneous gold catalysis has been devised. In the presence of a gold catalyst, monomers incorporating both a propargylic ester and an alkene moiety polymerized through a metallocarbene generation/cyclopropanation sequence to afford the corresponding macromolecules. This approach constitutes an unprecedented example of cyclopropanationbased polymerization and allows access to original macromolecule skeletons.

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INTRODUCTION Polymer chemistry is a continuously and actively expanding field where chemists, biologists, and physicists work together to tune properties of new materials.1 Some of the challenges in this field can be addressed by synthetic chemists who can opportunely devise new strategies to access original polymeric structures that have never been obtained before. Three approaches can be envisaged. The postfunctionalization of polymer may be considered as the most versatile method but generally suffers from a lack of control in terms of chemical reactivity.2 The modification of monomers is also an attractive strategy when specific functions have to be incorporated into a polymer.3 Finally, the discovery of new polymerization reactions appears very appealing from an academic and industrial point of view, especially when the two first approaches fail. New polymerization reactions would lead to new families of polymers and so to new properties.4,3a In this context, we have developed an original gold-catalyzed polymerization of propargylic esters that allowed to prepare polycyclopropane polymers. To the best of our knowledge, no cyclopropanation-based polymerization methods have been reported to date. Hitherto, such material have only been obtained by postfunctionalization of existing polymers5 or polymerization of monomers containing a cyclopropane unit.6 Our approach involves a metallacarbene addition to an olefin through a polycyclopropanation process (Scheme 1). Remarkably, this highly versatile transformation7 has never been successfully used in the context of polymerization. A very interesting attempt consisting in a copper-catalyzed carbene transfer and subsequent addition to an olefin using allyldiazoacetate as precursor was reported.8a Nevertheless, © XXXX American Chemical Society

Scheme 1. Polymerization by Carbene Addition to Olefin

the isolated polymers resulted from a C1 polymerization through poly(C−H) insertion (eq 1 in Scheme 1).9 Similarly, palladium8b,c and rhodium8d−f complexes have also been used to catalyze the polymerization of diazoesters that proceeds only through C−H insertion of carbene A to afford C1 polymerization material, even with a monomer bearing an olefin moiety.8c Thus, in order to favor the polycyclopropanation process, we decided to involve gold vinylcarbenes of type B. The later can be generated from the corresponding propargyl acetates 1 and have shown to add onto olefins following a Received: July 23, 2014 Revised: August 20, 2014

A

dx.doi.org/10.1021/ma501516s | Macromolecules XXXX, XXX, XXX−XXX

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cyclopropanation process,7 whether in intra-10 and intermolecular11,12 transformations. Importantly, in contrast to carbenes A, gold carbenes B do not undergo intermolecular C−H insertion.7,13 Gold catalysis has been sparsely used for polymerization but never through the intervention of gold carbenes.14 We surmised that if the carbene precursor and the olefin were located on the same monomer 1, no intramolecular cyclopropanation could be possible and iterative intermolecular cyclopropanations would afford vinylcyclopropane polymer 2 (eq 2 in Scheme 1).15 We thus selected a phenyl moiety, substituted in para position to avoid intramolecular cyclopropanation. A propargylic acetate was selected as the migratory group to evaluate the feasibility of the polymerization (monomer 1a in Table 1, preparation described in Supporting Information, section I).16

By using AuCl3, we succeeded in isolating a gray solid in a fair 40% yield and SEC analysis confirmed the formation of a polymer (Mw = 7.2 kg·mol−1, Đ = 5, Table 1, entry 5). Square planar pic-AuCl2 led to results similar to AuCl3, but with shorter chains (Mw = 2.2 kg·mol−1, Đ = 3, Table 1, entry 6). Further optimization of reaction conditions (solvent, temperature, catalyst loading; see Supporting Information, section IV−1) led us to select dichloromethane and/or acetonitrile as solvents, at room temperature, in the presence of 5 mol% of AuCl3 under vigorous stirring. We selected MeOH as the purification solvent as it affords good solubilization of the monomer and short oligomers but precipitates longer chains. Under these conditions, when scaling up the polymerization, the desired polymers 2a were isolated in 58% (in CH2Cl2, Mw = 15.2 kg·mol−1, Table 1, entry 7) and 46% yield (in MeCN, Mw = 19.2 kg·mol−1, Table 1, entry 8), respectively. Analysis of the 1H and 13C NMR spectra corroborates the postulated polymer structure and the vinylic chain end (Supporting Information, section IV−1.3, Figure SI− 3, SI−4, and SI−5). The nature of the propargylic acetate chain end was secured by 2H and 13C isotopic labeling (Supporting Information, sections II and IV-2). The isolated polymers (entries 7−8) were also characterized by MALDI−TOF mass spectrometry (MS), which confirmed that the repeat unit corresponds to the monomer unit, without mass loss (200.2 m/z). Moreover, the experimental molar mass value correlates with the expected Na+-ionized polymer, thus confirming the postulated chain ends (Figure SI−6). When the reaction was carried out in CH2Cl2 (entry 7), a very minor population corresponding to monodeacetylation of one vinylacetate moiety (−42.01 m/z) was observed (Figure SI−6a). Supported by literature data, a mechanism can be suggested for the polymerization reaction (Scheme 2).10−12c,15 The electrophilic gold complex activates the carbon−carbon triple bond in 1 (C intermediate) that triggers the nucleophilic attack of the adjacent ester. Two migrations can be envisaged at this stage, a priori in equilibrium:12c a 1,3-migration, leading to the corresponding allenyl ester or a 1,2-migration, affording the

Table 1. Catalyst Screening

entry 1 2 3 4 5 6 7 8g

[AuI]+ [AuI] [AuIII]

[Au]

yield (%)a

[LAu][SbF6] [Ph3PAu][SbF6] AuCl AuCN AuCl3 pic-AuCl2 AuCl3 AuCl3

65d 95d,e 10d 0d 40d 58d 58f,g 46f,h

Mwb

Mnb

Đc

15.8 − 2.8

0.7 − 0.7

24 − 4

7.2 2.2 15.2 19.2

1.4 0.7 2.5 3.0

5 3 6 6.5

DPnb 3 − 4 7 4 12 15

a

Determined after precipitation. bDetermined by SEC analysis, polystyrene (PS) calibration in kg·mol−1. cĐ = Mw/Mn d40 mg scale; the crude reaction mixture was purified by precipitation in Et2O. e Insoluble solid. f200 mg scale; the crude reaction mixture was purified by precipitation in MeOH. g15 h. hMeCN was used as solvent (72 h). [LAu][SbF6] = [(4-C6H4−Ph)(t-Bu)2PAu][SbF6]; pic-AuCl2 = dichloro(2-pyridinecarboxylato)gold.

Scheme 2. Proposed Mechanism

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RESULTS AND DISCUSSION On the basis of the intermolecular studies from literature,11 we first tested cationic gold(I) catalysts for this polymerization reaction. A catalytic amount of [(4-C6H4−Ph)(t-Bu)2PAu][SbF6] in CH2Cl2 (5 mol%, room temperature) succeeded in performing the polymerization of monomer 1a. However, under these conditions, polymer 2a was obtained with a very broad molar mass distribution (Mw = 15.8 kg·mol−1, Mw/Mn = Đ = 24, Table 1, entry 1) and this catalyst was thus abandoned. When classical [Ph3PAu][SbF6] was used, an insoluble solid was formed in 95% yield (Table 1, entry 2).17 We hypothesized that this cationic gold(I) may be too reactive, leading to high molar mass and rigid macromolecules which are insoluble in consequence. We thus switched to neutral gold(I) catalysts. When AuCl was used, 10% of a gray solid was isolated by precipitation of the crude residue in Et2O. Size exclusion chromatography (SEC) analysis of the latter disclosed oligomeric structures (Mw = 2.8 kg mol−1, Đ = 4, Table 1, entry 3). By contrast, no reaction was observed in the presence of AuCN as the catalyst. Gold(III) catalysts were then assessed. B


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productive oxocarbenium D.12d Subsequent rearrangement provides gold carbene E that can undergo cyclopropanation with an olefin to lead to cyclopropyl derivative G and regenerate the catalyst. In our polymerization system, both propargylic ester and olefin partners are present on the monomer. Thus, following the same catalytic cycle, the propagation occurs following a polycyclopropanation pathway. In order to understand the selectivity of the propagation step, carbene intermediate E (Scheme 2) derived from 1a has been intermolecularly trapped by styrene, known to be a highly reactive carbene acceptor (Scheme 3).11 In optimized

Table 2. Monomer Screening

entry 1 2 3 4 5

Scheme 3. Carbene Trapping Experiment

6 7 8 9 10 11 12 13 14g

monomer (R) 1b (t-Bu) 1b 1c (Ph) 1c 1d (MeOCH2) 1d 1e (C2H5) 1e 1f (n-C5H11) 1f 1g (n-C15H31) 1g 1g 1g

cond.a yieldb (%)

Mwc

Mnc

Đ c, d

DPnc

A B A B A

42e 47e 63e 42e 44e

5.5 6.8 3.0 7.4 7.7

2.8 3.4 1.5 1.5 2.6

2 2 2 5 3

11 16 5 6 13

B A B A B A C B A

3e 21e 25e 53e 52e 77f 48f 57f 73f,g

4.1 12.1 6.6 5.4 15.6 10.8 14.1 30.2 122.8

0.6 1.5 1.7 1.4 1.6 3.6 4.7 3.0 12.3

7 8 4 4 10 3 3 10 10

3 7 9 6 6 11 11 7 29

a

Experimental conditions: A (reaction in CH2Cl2, 15 h), B (reaction in MeCN, 72 h), C (reaction in CH2Cl2/MeCN 1/1 v/v, 72 h). b Determined after precipitation. cDetermined by SEC analysis, polystyrene (PS) calibration in kg mol−1. dĐ = Mw/Mn. ethe crude reaction mixture was purified by precipitation in MeOH. fThe crude reaction mixture was purified by precipitation in EtOH. gCationic [Ph3PAu][SbF6] was used as the catalyst.

conditions, the reaction of 1a with 5 mol% of AuCl3 and 20 equiv of styrene afforded 2′a in 86% yield as a mixture of diastereoisomers. The composition of this mixture was determined by 1H NMR of the crude sample and each diastereoisomer was identified by comparison with literature data, NOESY, 1H and 13C NMR experiment (see Supporting Information, Section III-1). (E)-cis, (Z)-cis, (E)-trans, and (Z)trans isomers were formed in a ratio of 22/6/6/1 respectively. This indicates that the cyclopropanation step is mainly cisselective (cis/trans: 4/1) while the formation of the carbene is E-selective (E/Z: 4/1). This result does not rigorously prejudge the stereoselectivity of each iterative cyclopropanation reaction but one can anticipate that the polymerization would mainly afford a (E)-cis polymer 2a. To assess the presence and activity of a vinyl and a propargyl ester groups at the α- and ω-chain ends and to create longer polymer chains, the isolated polymer 2a (Table 1, entry 8) was thus reactivated in a second polymerization step, by treatment with AuCl3 in the presence of 1a. However, under these conditions, an insoluble polymer was isolated, presumably due to the formation of long and rigid chains. We thus screened various migratory ester groups to probe the scope of the reaction and increase the solubility of the obtained polymer.16 Both CH2Cl2 and MeCN were tested as the polymerization solvent and all polymers were characterized by 1H and 13C NMR, as well as by MALDI−TOFs MS, that confirmed the postulated structures (Supporting Information, section V). Introduction of a pivaloyl ester led to low molar mass dispersity but fairly low Mw (Table 2, entries 1−2). Switching to a benzoyl ester afforded similar results (Table 2, entries 3−4). Introduction of a methoxymethyl substituent led to a dramatic modification of the reactivity: the polymer 2d was isolated as a complex mixture of macromolecules appreciable by NMR and MALDI−TOF MS and/or low yield depending on the solvent used (Table 2, entries 5−6). We then decided to increase the aliphatic ester side chain length, from C1 to C2 (Table 2, entries 7−8), C5 (Table 2, entries 9−10), and finally C15 that

led to increased polymer solubility and higher Mw (Table 2, entries 11−13). In view of these results, palmitate monomer 1g (entries 11− 14) was retained for its improved solubility and for the number of monomer units incorporated per chain during the polymerization. Analysis of the 1H (Figure 1) and 13C NMR spectra

Figure 1. 1H NMR (CD2Cl2, 300 MHz) of polymer 2g (entry 11).

(Figure SI−25) confirmed the postulated cyclopropyl-vinylester-phenyl polymer structure. It also corroborates a numberaverage degree of polymerization DPn of 10−11, based on the integration of the different massifs relative to the vinylic chain end. The polymer was also characterized by MALDI−TOF MS that confirmed that the repeat unit corresponds to the monomer unit without mass loss (396.6 m/z) (Figure 2). C


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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures and compound characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(M.B.) E-mail: [email protected]. *(L.F.) E-mail: [email protected]. *(J.-P.G.) E-mail: [email protected]. *(F.S.) E-mail: francois.stoff[email protected].

Figure 2. MALDI−TOF mass spectrum of polymer 2g (entry 11).

Present Address

For example, when n = 5, the theoretical molar mass value of the Na+ -ionized polymer is 2006.02 g·mol −1 and the experimental molar mass value is 2006.15 g·mol−1. A minor population fits with monodeprotection of the same polymer (−238.4 m/z). Moreover, whereas cationic gold complex [Ph3PAu][SbF6] led to an insoluble polymer when 1a was used (Table 1, entry 2), a successful polymerization was this time achieved by using monomer 1g (Table 2, entry 14), certainly due to the superior solubility of 2g vs 2a. Under these conditions, a large increase of the DPn was observed by SEC and NMR analysis (Supporting Information, section V−6.2). Finally, we demonstrated that it was possible to access a second completely different polymer structure by simple ester cleavage of 2g. Polymer 9, possessing a totally unprecedented α-cyclopropylketo unit, was recovered after treatment of 2g (Table 2, entry 11) with KOH in a MeOH/THF mixture (Đ = 2.0, Mw = 8.2 kg·mol−1, PS calibration) (Scheme 4).18 NMR

§

Université de Haute-Alsace, Ecole Nationale Supérieure de Chimie de Mulhouse, Laboratoire de Chimie Organique et Bioorganique EA 4566, 3 rue Alfred Werner, F-68093 Mulhouse Cedex, France Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank E. Caytan for NMR spectroscopic studies. This work was supported by the CNRS, UPMC, IUF, and LabEx MiChem (ANR-11-IDEX-0004-02), which we gratefully acknowledge.

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REFERENCES

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Scheme 4. Ester Cleavage toward New Polymer 9

analysis corroborated the complete ester cleavage. Characterization by MALDI−TOF MS confirmed that the chain ends and the repeating unit correspond to the monomer unit (158.2 m/z) (Figure SI−32). In conclusion, we have performed the first carbene-to-olefin addition polymerization, yielding novel polymers possessing an original cyclopropyl−vinylester−phenyl skeleton. We have shown that the degree of polymerization and the solubility of the resulting macromolecules depended mainly on the nature of the migrating ester function. We also demonstrated that the utilization of a cationic gold catalyst increased the chain length. Characterization by NMR and MALDI−TOF MS, as well as isotopic labeling study, confirmed the postulated structure. An intriguing feature of these polymers is the presence of a cyclopropyl unit which is an element of conjugation19 that may provide interesting properties. D


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Gold-Catalyzed Polymerization Based on Carbene

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