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Organocatalytic Coupling of Bromo-Lactide with Cyclic Ethers and Carbonates to Chiral Bromo-Diesters: NHC or Anion Catalysis? Jian-Bo Zhu, Xiaoyan Tang, Laura Falivene, Lucia Caporaso, Luigi Cavallo, and Eugene Y.-X. Chen ACS Catal., Just Accepted Manuscript • Publication Date (Web): 03 May 2017 Downloaded from http://pubs.acs.org on May 3, 2017
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Organocatalytic Coupling of Bromo-Lactide with Cyclic Ethers and Carbonates to Chiral Bromo-Diesters: NHC or Anion Catalysis? Jian-Bo Zhu,† Xiaoyan Tang,† Laura Falivene,*,‡ Lucia Caporaso,§ Luigi Cavallo, ‡ and Eugene Y.-X. Chen*,† †
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States King Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering Division, Kaust Catalysis Center, Thuwal 23955-6900, Saudi Arabia § Dipartimento di Chimica e Biologia, Università di Salerno, Via Papa Paolo Giovanni II, I-84084, Fisciano, Italy ‡
ABSTRACT: In the presence of a N-heterocyclic carbene (NHC) in THF, Br-substituted L-lactide (Br-LA) unexpectedly undergoes exclusive coupling with THF to form a chiral ω-bromo-α-keto-diester. This coupling reaction is completely selective (in a precise 1:1 fashion), readily scalable (> 20 g scale), and extremely efficient (with only 50 ppm NHC loading). Other cyclic ethers and carbonates can also undergo similar coupling with Br-LA, thus offering a class of Br-functionalized chiral diesters with various functions and chain lengths. Combined experimental and computational studies led to a coupling mechanism that proceeds through an anion (bromide)–mediated catalytic cycle, rather than an apparent NHC-catalyzed cycle.
KEYWORDS: organocatalysis, N-heterocyclic carbene, coupling, chiral diesters, bromo-lactide he rapidly growing field of organocatalysis1 has fueled the development of organopolymerization2 into a preferred method for polymer synthesis when metal-free products or processes are of primary concern. Thanks to their inherently high Brønsted basicity and nucleophilicity while also being good leaving groups, N-heterocyclic carbene (NHC) based catalysts3 are particularly attractive and have brought about unique reactivity and selectivity observed in many different types of organic reactions and polymerization processes.4,5 The most prominent example in the NHC-mediated organopolymerization is the ring-opening polymerization (ROP) of lactide2,4 as the controlled and rapid method for the synthesis of relatively high molecular weight poly(lactide) (PLA).6 The need for functionalized PLA materials with enhanced physical and mechanical properties7 has led to an effective strategy to derivatize PLA, which is through (3R,6S)-3-bromo-3,6dimethyl-1,4-dioxane-2,5-dione (Br-LA), readily prepared via bromination of L-lactide (L-LA), which undergoes subsequent HBr elimination by Et3N to produce monomer exo-methylenelactide (MLA).8,9 MLA can be radically polymerized to vinyladdition polymer products8,10,11,12 which can be postfunctionalized.11,12 Derivatization of MLA via the Diels-Alder cycloaddition with dienes led to bifunctional monomers that can be polymerized into various functionalized PLA materials.9,13,14 All of the above approaches to the modified PLA go through Br-LA, followed by further downstream modifications. However, there was no report on the direct use of Br-LA as a functional monomer or reagent.15 In our efforts to explore the utility of this readily available and renewable reagent, we discovered that, unlike the ROP observed for the parent L-LA in the present of an NHC catalyst such as 1,3-bis(2,4,6trimethylphenyl)imidazol-2-ylidene (IMes),2 Br-LA unexpectedly underwent exclusive coupling with the solvent molecule (THF) to form a chiral ω-bromo-α-keto-diester (1, Scheme 1).
T
a)
R
Br
N
O
O
R
O
Br
+ N R R N O O O
O
N
O
O O IMes
IMes (R = Mes)
O Br
O
O
IMes (50 ppm)
O O
O
O O
O
1
Br-LA b)
Br
IMes IMes-LA + Br O Br O O O
Br
O > 20 g (~100%)
Br O O
O Br
O O
O
Scheme 1. Unexpected coupling of Br-LA with THF by NHC to chiral ω-bromo-α α-keto-diester 1 and proposed two pathways: a) NHC (IMes) vs b) anion (Br–) catalysis. Two possible mechanistic pathways can be envisioned. The first proceeds via NHC catalysis, involving the NHC-bound acylazolium3 intermediate resulted from the initial ringopening event, followed by elimination of Br–; such an activated intermediate could be intercepted by nucleophilic THF to produce the coupling products (Scheme 1a). The second proceeds via anion catalysis, involving the “free” Br–, generated from the reaction of Br-LA and NHC (c.f. Scheme 2), acting as the catalyst promoting the ring-opening of Br-LA to form Br– plus the acyl bromide intermediate; the subsequent reaction with THF resembles the known reaction of acetyl bromide with THF16 to give the coupling product 1 while regenerating the catalyst Br– (Scheme 1b). Our combined experimental and computational studies described herein showed the anion catalysis is responsible for the observed unique 1
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O
O
O
O
O
O
S2 O
O
O
O
O
Br
O
1 (100%) O
O O
6 (93%)
O
O
O O 7' (72%)
O
Br
O
O O
S9 O
O O
Br
5 (82%)
4 (75%)
Br
S
O O
O O
O
S8 O
O
3 (66%)
Br
S7"
O O
O O
O
S7' Br
O
2 (93%)
Br
S6 O
O
O
O O
O
O
O O
Br
O O
O
S S5
O S4
S3
O
O
O
O S1
O O
Br-LA + Substrates:
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O O
7" (51%)
O O 8 (83%)
O
O O Br
O 9 (95%)
Br
Chart 1. Scope of the coupling of Br-LA with cyclic ethers and carbonates to chiral diesters (isolated yields). coupling between Br-LA and THF or related cyclic ethers and were adjusted for each coupling partner (Table S2) and also occasionally optimized. For instance, the coupling of Br-LA carbonates. with 1,4-dioxane by IMes gave not only the desired diester 4 Using IMes either alone or in combination with an initiator but also the competing elimination product MLA (Table S3). (ROH), no apparent Br-LA conversion was observed when Through reaction optimization, the reaction with 0.5 mol% carried out in toluene under various conditions (varied BrIMes at 100 °C for 15 h gave the highest 4/MLA ratio of 92/8, LA/IMes/ROH ratios and temperature), but mixing Br-LA which resulted in an isolated yield of 75% for the pure product with IMes in a 1:1 ratio under various (solvent and tempera+ – 4. Two cyclic carbonates were examined, ethylene carbonate ture) conditions led to HBr elimination to form [IMesH] Br , and propylene carbonate, also underwent facile coupling reacplus some unknown minor species (not MLA). Surprisingly, tion with Br-LA, but accompanied by decarboxylation, therequantitative conversion of Br-LA was observed when the reacby leading to 6 and 7, respectively. Diester 6 was conveniently tion of Br-LA with a catalytic amount of IMes (0.005 mol% to isolated in a multi-gram scale with 0.5 mol% IMes in ≥ 90% 1 mol%) was carried out in THF at room temperature or 60 yield, while 7 can also be prepared from the coupling between °C, while no obvious reaction was observed when using either Br-LA and propylene oxide, which produced the same product Br-LA or THF alone in the presence of IMes. Careful analyses as that from the reaction of Br-LA and propylene carbonate of the cleanly formed colorless liquid with spectroscopic (Fig(Chart 1, Figure S23). Noteworthy is the high-yield synthesis ures S3-S7) and analytical methods (HRMS and elemental of unsaturated diester 9 containing a reactive double bond, analysis, see the Supporting Information) conclusively showed which is derived from the coupling between Br-LA and 2,5that the product is chiral ω-bromo-α-keto-diester 1, derived dihydrofuran in 95% isolated yield. Reactions of Br-LA with from the direct coupling of Br-LA with THF. Specific rotation 4-methylmorpholine and β-butyrolactone afforded no coupling of 1, [α]D28.2 = –30.8°, is insensitive to the Br-LA/IMes/ROH products. ratio used for the synthesis and rather similar to that of Br-LA, Additional studies confirmed that this coupling reaction, [α]D30.7 = –27.0°. The same reaction when carried out in DMF proceeds through incorporating both substrates in a precise 1:1 (which functions as a base) led to elimination of HBr and formolar ratio into the coupling product, regardless of whether mation of MLA. Br-LA or THF is added in excess. For instance, all the followWith the structure of the coupling product being estabing reactions, including the reaction in THF (as both the sublished, we examined the scope of the organic catalysts (Table strate and solvent) as well as the 1:1, 2:1, and 1:2 (BrS1), which revealed IMes to be the best precatalyst. For examLA:THF) ratio reactions in toluene, led to formation of the ple, in a Br-LA/IMes/Ph2CHOH ratio of 5000/1/1 in THF at o same coupling product 1, although the conversion of Br-LA 60 C, 100% conversion of Br-LA to 1 was achieved in 24 h. varied, depending on the ratio of the two reagents employed Reducing IMes loading to 0.01 mol%, a high conversion of (see Figure S28). 95% can still be achieved. Employing IMes alone (without Based on the above experimental observations, we perROH) was even more effective, achieving 100% Br-LA conformed a DFT study17 to shed light on the mechanism of this version to 1 with only 50 ppm (Br-LA/IMes = 20,000/1) IMes new coupling reaction of Br-LA with THF. At the outset, we loading in 24 h. A scale-up reaction with 50 ppm IMes preexplored the general reactivity of the typical basic and nucleopared 21.1 g of the pure 1 in quantitative yield. To more accuphilic NHC (IMes) towards Br-LA. As a base, IMes can abrately assess the rate of the coupling, the reaction was stopped stract a proton from the CH moiety of Br-LA, with a calculatat different times and the conversion-time plot gave a high ed energy barrier of 10.3 kcal/mol, leading to [IMesH]+Br– and maximum turnover-frequency of 3,430 h-1 (Figure S27). Other a zwitterionic cyclic species (X) in equilibrium with the reaccommon NHCs were also evaluated and found to be less or tants (Scheme 2). Removal of the proton from a CH3 group to much less effective (Table S1). Some base catalysts and seform [IMesH]+Br– and MLA is kinetically disfavored by 5.6 lected metal catalysts were also effective, although much less kcal/mol, even though it leads to very stable product Y, 26.6 effective than IMes, while Lewis and Brønsted acids such as kcal/mol below the reactants. Conversely, in the case of a typB(C6F5)3, triflic acid, and diphenylphosphoric acid showed no ical amine (e.g., NMe3), proton abstraction from the CH3 activity for this coupling reaction. group leading to formation of MLA is preferred by 5.2 Next, we examined the scope of cyclic ether substrates, unkcal/mol over proton abstraction from the CH group (Scheme covering a broad range of cyclic ethers and carbonates that can S1). As a nucleophile, IMes can attack both carbonyl groups in be readily coupled with Br-LA into chiral bromo-diesters (1– Br-LA. The preferred attack, addition of IMes to the less steri9), with isolated yields varying from 51% to ~100% (Chart 1). cally hindered carbonyl (marked in red, Scheme 2) with a Reaction conditions (catalyst loading, temperature, and time) 2
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trans orientation of the attacked C=O and of the Br atom, leads to a concerted ring opening and Br– elimination from the neighboring carbon atom, with an energy barrier of only 6.9 kcal/mol. The resulting stable acylazolium bromide Z is at 21.4 kcal/mol below the reactants, and dissociation of this ion pair requires 7.4 kcal/mol. The other disfavored modes of attack at C=O were outlined in Scheme S2 of the SI. O N
R N
R
R
O O
N
H
O
R
R
Br
O
X -1.1
A-X 10.3
O Br
N H
O
O
+
R N
N
O
Br
ylazolium is attacked by Br– (see transition state B-C in Scheme 3), with an overall barrier of 25.2 kcal/mol. Ringopening of THF leads to the tetrahedral zwitterionic intermediate C, followed by elimination (regeneration) of NHC and formation of the coupling product 1 at almost 11 kcal/mol below the unreacted Br-LA and THF. In the bromide cycle the “free” Br– acts as the catalyst promoting the ring-opening of Br-LA via transition state D-E, at 17.2 kcal/mol, forming Br– plus the acyl bromide intermediate E (Scheme 4). The subsequent reaction of E with THF proceeds via transition state E-1 with a barrier of 15.1 kcal/mol (relative to E) to give the coupling product 1 while regenerating the catalyst Br–.
N
O
Br
H N R
O
A 0.0
O
O
H 2C
O
O
Br
Y -26.6
R
O
N R
N
R N
A-Z/A-B 6.9
O
N
O O
R
O
+
Br
O O
Z -21.4
O
Br
N
O
O
R
A 0.0
O
Br
1 -11.1
+ N R R N O
O R N
O O
Br O
C-1 2.2
O
R N
R O
N N R
O
B -14.0 O
+ N R R N
+ Br
O
O
O + N R R N O O O
B-C 11.2
Br
O O O
O
Br
O
O
O
Br
O
O
O
O R N
D-E 17.2
O Br
O
E-1 18.0
O
O
O
O
O
B -14.0
O
O
O
O
Scheme 2. Possible reactions of Br-LA with IMes and associated energy profiles: favored pathway shown in solid line arrows and disfavored ones in dashed line arrows (numbers in red are free energies in kcal/mol).
Br
Br
O
N
O
N
Br
O
1 -11.1
O
R
O
O
D 0.0
Br Br
R
O
A-B 6.9
Br O
O O
O O
O R
Br
O
O
Br-LA IMes IMes-LA +
R 4N +
O
H
O
O
N
R N
A-Y 15.9
Br
R 4N +Br -
O
R
O
Br O
C -4.2
O
O
O
E O 2.9
Scheme 4. Proposed bromide cycle for the coupling of BrLA with THF, showing this cycle is more kinetically competent than the NHC cycle for this coupling reaction. Comparison of the two catalytic cycles indicates that the largest energy window in the NHC cycle, 25.2 kcal/mol from B to B-C, is 7.2 kcal/mol higher in energy than the largest energy window in the Br– cycle, 18.0 kcal/mol from D to E-1. This result suggests that the real catalytically competent species is Br–, with IMes only involved in the reactivity with BrLA to liberate Br–. Consequently, non-NHC compounds, such as common ammonium or imidazolium bromide salts should also be effective in promoting this coupling reaction. Gratifyingly, experimental tests indicated that bromide salts such as n Bu4N+Br– and [IMesH]+Br– are indeed effective catalysts for this coupling reaction, although less active than IMes, with a general activity trend following the order of IMes > [IMesH]+Br– > nBu4N+Br– (Table S4). This activity trend is well reproduced by the trend in the G of dissociation of the respective ion pairs, that is 7.4, 8.1 and 9.4 kcal/mol for Z, [IMesH]+Br–, and nBu4N+Br–, respectively. This mechanism also explains the decarboxylation observed in the coupling reaction of Br-LA with carbonates S6 and S7′′, as shown in Scheme 5. O
O O
O
Scheme 3. Proposed NHC cycle for the coupling of BrLA with THF.
Br R
O O
Next, the reactivity with THF was explored starting both from the acylazolium, within the “NHC cycle” (Scheme 3), and from Br–, within the “bromide cycle” (Scheme 4). In the proposed NHC cycle, the THF molecule activated by the ac-
Br
O
Br
O -CO 2
O
O O
O
Br + Br R
R = H (6), Me (7)
Scheme 5. Decarboxylation occurred in the coupling of Br-LA with carbonates.
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In conclusion, we have discovered the unique organocatalytic coupling reaction between Br-LA and cyclic ethers or carbonates to form Br-functionalized chiral diesters with various functions and chain lengths. In the presence of an NHC (e.g., IMes), Br-LA, behaves differently than the parent L-LA, which is well known to proceed through ROP to produce PLA; it displays no ROP activity but undergoes exclusive and quantitative 1:1 coupling with the solvent molecule (THF), leading to the chain-extended diester. This new coupling reaction was shown to proceed through the catalytic cycle catalyzed by the Br– anion, rather than the apparent NHC-catalyzed cycle. This coupling reaction exhibits several notable features. First, it is completely selective with the precise 1:1 molar ratio of Br-LA and THF incorporated into the diester product, regardless of whether one or the other reagent was present in excess. Second, it is extremely efficient, requiring only 50 ppm NHC loading to achieve quantitative conversion of the substrate and nearly quantitative isolated yield of the diester product. Third, it is performed with convenient experimental procedures and also readily scalable. Fourth, it can be extended to other cyclic ethers and cyclic carbonates, thereby offering a rapid entry to a class of new chiral diesters with tailored functionalities and chain lengths. Research towards other brominated substrates and possible stereoselective coupling reactions catalyzed by the bromide anion is currently underway.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Experimental details and characterization data (PDF).
AUTHOR INFORMATION Corresponding Author
*
[email protected] *
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported by the US National Science Foundation (NSF-1300267) and the W. M. Keck Foundation for the study carried out at Colorado State University, and by the funding from King Abdullah University of Science and Technology (KAUST) for the study performed at KAUST. The computing resources and the related technical support used by the University of Salerno have been provided by CRESCO/ENEAGRID High Performance Computing infrastructure and its staff.
REFERENCES (1) Selected recent reviews on organocatalysis: (a) James, T.; van Gemmeren, M.; List, B. Chem. Rev. 2015, 115, 9388−9409; (b) Akiyama, T.; Mori, K. Chem. Rev. 2015, 115, 9277−9306; (c) Volla, C. M. R.; Atodiresei, I.; Rueping, M. Chem. Rev. 2014, 114, 2390−2431. (d) Liu, D.; Chen, E. Y.-X. Green Chem. 2014, 16, 964–981. (e) Grondal, C.; Jeanty, M.; Enders, D. Nat. Chem.
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2010, 2, 167−178. (f) Marcelli, T.; Hiemstra, H. Synthesis, 2010, 1229−1279. (2) Selected reviews on organopolymerization: (a) Sardon, W. N. H.; Mecerreyes, D.; Vignolle, J.; Taton, D. Prog. Polym. Sci. 2016, 56, 64−115; (b) Sardon, H.; Pascual, A.; Mecerreyes, D.; Taton, D.; Cramail, H.; Hedrick, J. L. Macromolecules 2015, 48, 3153−3165. (c) Fuchise, K.; Chen, Y.; Satoh, T.; Kakuchi, T. Polym. Chem. 2013, 4, 4278−4291. (d) Kiesewetter, M. K.; Shin, E. J.; Hedrick, J. L.; Waymouth, R. M. Macromolecules 2010, 43, 2093−2107. (e) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Chem. Rev. 2007, 107, 5813−5840. (3) Selected recent reviews on NHCs and NHC-mediated organic reactions: (a) Flanigan, D. M.; Romanov-Michailidis, F.; White, N. A.; Rovis, T. Chem. Rev. 2015, 115, 9307−9387. (b) Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. Nature 2014, 510, 485–496. (c) Nelson, D. J.; Nolan, S. P. Chem. Soc. Rev. 2013, 42, 6723−6753. (d) Ryan, S. J.; Candish, L.; Lupton, D. W. Chem. Soc. Rev. 2013, 42, 4906−4917. (4) Selected reviews on NHC-mediated organopolymerization: (a) Matsuoka, S.-I. Polym. J. 2015, 47, 713–718. (b) Naumann, S.; Dove, A. P. Polym. Chem. 2015, 6, 3185–3200. (c) Naumanna, S.; Buchmeiser, M. R. Catal. Sci. Technol. 2014, 4, 2466–2479. (d) Fèvre, M.; Pinaud, J.; Gnanou, Y.; Vignolle, J.; Taton, D. Chem. Soc. Rev. 2013, 42, 2142−2172. (5) Selected recent examples: (a) Nakano, Y.; Lupton, D. W. Angew. Chem., Int. Ed. 2016, 55, 3135−3139; (b) Hong, M.; Tang, X.; Falivene, L.; Caporaso, L.; Cavallo, L.; Chen, E. Y.-X. J. Am. Chem. Soc., 2016, 138, 2021‒2035. (c) Brown, H. A.; Chang, Y. A.; Waymouth, R. M. J. Am. Chem. Soc. 2013, 135, 18738−18941. (d) Zhang, Y.; Schmitt M.; Falivene L.; Caporaso L.; Cavallo L.; Chen, E. Y.-X. J. Am. Chem. Soc. 2013, 135, 17925‒17942. (e) Shin, E. J.; Brown, H. A.; Gonzalez, S.; Jeong, W.; Hedrick, J. L.; Waymouth, R. M. Angew. Chem., Int. Ed. 2011, 50, 6388−6391. (6) (a) Stanford, M. J.; Dove, A. P. Chem. Soc. Rev. 2010, 39, 486– 494. (b) Thomas, C. M. Chem. Soc. Rev. 2010, 39, 165–173. (c) Platel, R. H.; Hodgson, L. M.; Williams, C. K. Polymer Rev. 2008, 48, 11–63. (d) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147–6176. (7) Anderson, K. S.; Schreck, K. M.; Hillmyer, M. A. Polym. Rev. 2008, 48, 85–108. (8) Scheibelhoffer, A. S.; Blose, W. A.; Harwood, H. J. Polym. Prepr. 1969, 10, 1375–1380. (9) Jing, F.; Hillmyer, M. A. J. Am. Chem. Soc. 2008, 130, 13826– 13827. (10) Miyake, G. M.; Zhang, Y.; Chen, E.Y.-X. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1523−1532. (11) Britner, J.; Ritter, H. Macromolecules 2015, 48, 3516−3522. (12) Mauldin, T. C.; Wertz, J. T.; Boday, D. J. ACS Macro Lett. 2016, 5, 544–546. (13) Fiore, G. L.; Jing, F.; Young Jr., V. G.; Cramer, C. J.; Hillmyer, M. A. Polym. Chem. 2010, 1, 870–877. (14) Castillo, J. A.; Borchmann, D. E.; Cheng, A. Y.; Wang, Y.; Hu, C.; García, A. J.; Weck, M. Macromolecules 2012, 45, 62–69. (15) Br-LA was used as initiator: Coulembier, O.; Moins, S.; De Winter, J.; Gerbaux, P.; Leclère, P.; Lazzaroni, R.; Dubois, P. Macromolecules 2010, 43, 575–579. (16) (a) Krim, J.; Taourirte, M.; Engels, J. W. Molecules 2012, 17, 179–190. (b) Schneider, D. F.; Viljoen, M. S. Syn. Commun. 2002, 32, 721–728. (c) Coles, S. J.; Costello, J. F.; Draffin, W. N.; Bursthouse, M. B.; Paver, S. P. Tetrahedron 2005, 61, 4447– 4452. (d) Fitch, J. W.; Payne, W. G.; Westmoreland, D. J. Org. Chem. 1983, 48, 751–753. (17) Geometries were optimized using the BP86 functional and the SVP basis set using the Gaussian 09 package on the CRESCO platform. Free energies were built through single point energy calculations with the M06 functional and the TZVP basis set. Solvent effects were included with the PCM model using THF as the solvent. Further details were provided in the SI.
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Table of Contents (TOC) graphics O
O
Br O
IMes O n
O
Br
O O
ROP O Br-LA
O
IMes (50 ppm)
O O
O
Br
O
Br
O > 20 g (100%) 10 examples
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