Communication pubs.acs.org/JACS
Cascade Polymerization via Controlled Tandem Olefin Metathesis/ Metallotropic 1,3-Shift Reactions for the Synthesis of Fully Conjugated Polyenynes Cheol Kang, Hyeon Park, Jin-Kyung Lee, and Tae-Lim Choi* Department of Chemistry, Seoul National University, Seoul 08826, Korea S Supporting Information *
with an adjacent carbon−carbon triple bond, known as a metallotropic 1,3-shift (Scheme 1b). More interestingly, the Lee group reported a ring-closing metathesis reaction followed by a metallotropic 1,3-shift from Ru-alkylidenes13 to prepare various complex oligoenynes13b and natural products14 (Scheme 1c). Although there are many examples of using alkynyl transition metal carbenes in organic synthesis, no macromolecules have been prepared by this interesting transformation. Herein, we report the synthesis of a new class of conjugated polyenynes via tandem catalysis by combining olefin metathesis and the metallotropic 1,3-shift using a Grubbs catalyst, which we now call cascade metathesis and metallotropy (M&M) polymerization. In contrast to previously reported olefin metathesis polymerizations (such as ring-opening metathesis polymerization (ROMP), acyclic diene metathesis (ADMET) polymerization,15 and cyclopolymerization (CP)16) and their tandem olefin metathesis polymerizations,17,18 M&M polymerization exploits two fundamentally different transformations, olefin metathesis, and a nonmetathesis reaction (metallotropic 1,3-shift), with the two transformations occurring in a perfectly alternating sequence. As a result, unique conjugated polyenynes containing specific sequences of three double bonds (Z-E-Z) and one triple bond were prepared in a highly selective manner by solution polymerization. Furthermore, we even achieved living cascade polymerization, making this a very rare example of forming new triple bonds in a conjugated backbone by chain-growth polymerization.19 To realize successful tandem/cascade reactions, high efficiency of the whole process, as well as excellent selectivity of each reaction, is crucial. Especially for cascade polymerization, if each reaction deviates from the intended perfect relay as a result of nonselective transformations or side reactions, the formation of ill-defined polymers is inevitable. Therefore, understanding how to control the whole cascade sequence and designing appropriate monomers is the key to success. For this cascade M&M polymerization, we designed a series of monomers (a−d) that could potentially undergo both olefin metathesis and metallotropic 1,3-shift reactions (Scheme 2). The simplest diyne structure a was expected to form a polydiacetylene, but the desired M&M polymerization failed (Table S1). Moreover, another monomer containing a terminal diyne, b, also showed low polymerization efficiencies (Table S2). We suspected that the resulting propagating species from a, consisting of a 1,1-
ABSTRACT: We demonstrate the first example of cascade polymerization by combining olefin metathesis and metallotropic 1,3-shift reactions to form unique conjugated polyenynes. Rational design of monomers enabled controlled polymerization, and kinetic investigation of the polymerization mechanism was conducted.
C
onjugated polyenynes, such as polydiacetylenes, are wellknown for their intriguing optoelectronic properties, exhibiting characteristic color transitions in response to external stimuli, which makes them excellent chemical or biological sensors.1 However, despite extensive studies on polydiacetylenes, most synthetic routes rely on topochemical polymerization,2 in which transient alkynyl carbenes generated upon light irradiation or heating promotes polymerization only when the diacetylene monomers are strictly aligned in proximity (Scheme 1a).3 Scheme 1. Previous Reactions Involving Alkynyl Carbenes
Therefore, only certain monomers containing specific functional groups underwent successful polymerization in the solid state.4 Moreover, the resulting polymers generally showed limited solubility, making solution fabrication challenging. To overcome these limitations, there have been efforts to make polyenynes via Sonogashira5 or Glaser−Hay coupling reactions6 or alkyne metathesis reaction.7 In contrast to the transient alkynyl carbenes generated by light or heat, transition metal complexes can form stable yet reactive alkynyl carbenes.8 In particular, various alkynyl carbenes of Rh,9 Cr,10,11 Mo,10 W,10 and Ru12 undergo a unique rearrangement © XXXX American Chemical Society
Received: May 12, 2017
A
DOI: 10.1021/jacs.7b04913 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society Scheme 2. Mechanism of Cascade Olefin Metathesis/Metallotropic 1,3-Shift (M&M) Polymerization
disubstituted Ru carbene, became less active for the desired cascade polymerization, analogous to terminal alkyne polymerization,20 or new alkynyl carbenes (propagating species for a or b) were not reactive enough for further propagation.13c,14a Unfortunately, structure c decomposed rapidly. Finally, we designed monomer d, tetradeca-1,6,8,13-tetrayne, as the symmetric and stable internal diyne moiety should produce a uniform polyenyne backbone. Based on this design principle, we prepared various monomers, M1−M6, by 3−5 simple synthetic steps, and tested M&M polymerization using a fast-initiating third generation Grubbs catalyst (G3), which was the optimal catalyst for successful CP (Table 1).16d−h First, monomer M1 containing a dimethyl malonate moiety was polymerized in dichloromethane (DCM) with a monomer to initiator ratio (M/I) of 25, and the reaction solution rapidly turned red. Complete conversion was achieved within 1 h at room temperature, giving a high yield (95%) of P1. This polymer was highly soluble in common organic solvents, such as chloroform, DCM, and tetrahydrofuran (THF), and its molecular weight (Mn = 14.8 kDa) and dispersity (Đ = 1.64) were measured by SEC in THF (Table 1, entry 1). At M/I = 50, P1 with a higher Mn (26.6 kDa) was obtained in 84% conversion (entry 2). With M2, which contained larger isopropyl side chains, the polymerization efficiency increased slightly to 91% conversion at M/I = 50 (Mn = 33.9 kDa, entry 3). Monomer M3, with an even bulkier di-tert-butyl malonate moiety, showed the best polymerization efficiency, giving 98% conversion at M/I = 50 (Mn = 34.9 kDa, entry 4). The molecular weight of this polymer further increased to 42.7 kDa at M/I = 75 (80% conversion, entry 5). Encouraged by the positive influence of the steric effect, we maximized the size by introducing an adamantyl group (M4), but the conversion decreased to 76% at M/I = 50 (entry 6), suggesting that steric factors only improved the polymerization efficiency to a certain degree. Another monomer containing triethylsilyl (TES) ether groups (M5) was polymerized successfully, resulting in >99% and 95% conversion at M/I = 25 (Mn = 15.8 kDa, entry 7) and M/I = 50 (Mn = 22.0 kDa, entry 8), respectively. Furthermore, monomer M6, containing methyl ester and gemdimethyl tert-butyldimethylsilyl (TBS) ether groups, also showed good polymerization, giving >99% and 84% conversion at M/I = 25 (Mn = 20.0 kDa, entry 9) and M/I = 50 (Mn = 27.0 kDa, entry 10), respectively. However, P1−P6 (in Table 1, entries 1−10) generally showed broad SEC traces and Đ values due to the chain transfer reaction and the catalyst decomposition. Based on the mechanistic similarity between cascade M&M polymerization and CP, we also tried several strategies that have been effective for improving the control of CP to suppress carbene decomposition during polymerization.16g First, polymerization at a lower temperature (0 °C) was tested with the best monomer, M3, but despite a narrower Đ (1.45), the conversion was lower (89% at M/I = 50, entry 11). Second, THF, the optimal solvent for CP,16d,g was used as a weakly coordinating solvent, but the result was even less satisfactory (83% conversion at M/I = 50, Đ = 1.84, entry 12).
Table 1. Cascade M&M Polymerization of Various Monomers
entry
monomer
M/I
time (h)
conv (%)a
yield (%)
Mn (kDa)b
Đb
1 2 3 4 5 6 7 8 9 10 11c 12d 13e 14e 15e 16e 17e 18e 19e 20e 21e 22e
M1 M1 M2 M3 M3 M4 M5 M5 M6 M6 M3 M3 M3 M3 M3 M3 M1 M2 M2 M5 M5 M6
25 50 50 50 75 50 25 50 25 50 50 50 50 10 25 40 25 25 50 25 50 25
1 2 1 2 4 3 1 2 2 4 4 2 3 0.33 0.83 2.5 0.42 0.67 1.5 0.83 3 0.83
>99 84 91 98 80 76 >99 95 >99 84 89 83 97 99 98 97 97 >99 91 >99 >99 90
95 79 82 94 74 74 96 88 86 84 81 76 93 95 98 91 93 98 91 97 98 84
14.8 26.6 33.9 34.9 42.7 28.0 15.8 22.0 20.0 27.0 31.3 33.7 36.4 8.4 22.2 30.6 12.8 16.8 35.8 18.1 31.2 18.0
1.64 2.33 1.69 1.62 1.77 1.99 1.45 1.90 1.45 1.75 1.45 1.84 1.39 1.11 1.15 1.29 1.30 1.15 1.32 1.14 1.38 1.33
a
Calculated from 1H NMR. bDetermined by THF size-exclusion chromatography (SEC) calibrated using polystyrene standards. c Reaction conducted at 0 °C. dTHF was used as solvent. e20 mol % of 3,5-dichloropyridine was added.
Gratifyingly, with a weakly coordinating ligand, 3,5-dichloropyridine, as an additive,16f,g P3 with a high conversion of 97% (M/I = 50) and the lowest Đ of 1.39 was obtained (entry 13) because the additive effectively hindered the catalyst decomposition and also chain transfer reaction (Figure S2). Furthermore, with this additive, cascade M&M polymerization proceeded via clear firstorder kinetics, as observed by in situ NMR analysis (Figure S1), indicating that both cyclization and the metallotropic 1,3-shift occurred quickly, with the intermolecular propagating reaction as the rate-determining step. More importantly, this result suggests the possibility of controlled polymerization. Indeed, under the optimized conditions, the Mn of P3 increased linearly for M/I of 10−50 with excellent conversions and yields. Further, narrow Đ B
DOI: 10.1021/jacs.7b04913 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
(Figure 1d). Analogous to the conclusions from previous mechanistic studies on CP,16f,g,j the weakly coordinating ligand improved the stability of the propagating carbene during M&M polymerization. To synthesize conjugated block copolymers, we combined this new controlled cascade M&M polymerization with other living polymerization methods. First, ROMP of norbornene derivative M7 was conducted using G3 and 3,5-dichloropyridine (M/I/Add = 50/1/10) to prepare the first block, and then M3 (25 equiv) was added to the same reaction pot to successfully produce a block copolymer with an increase of Mn from 14.8 to 36.1 kDa and a narrow Đ of 1.10 (Figure 2a). Furthermore, a new fully
Figure 1. (a) Mn vs M/I and corresponding Đ values for M3, and conversions and carbene changes monitored by in situ NMR analysis during M&M polymerization (M/I = 20) in CD2Cl2 using (b) M1, (c) M3, and (d) M3 with 3,5-dichloropyridine (10 equiv to G3).
values were obtained at M/I = 10 (1.11), 15 (1.15), and 40 (1.29) (Table 1, entries 14−16, Figure 1a, Figure S2). Gratifyingly, under the same conditions, other monomers (M1, M2, M5, and M6) also underwent such controlled polymerization to give excellent yields (up to 98%) and narrow Đ (
