Synthesis of (Imido) niobium (V)–Alkylidene Complexes That Exhibit

Aug 24, 2016 - of 2a−c with (CF3)3COH (1.0 equiv) in n-hexane afforded the corresponding dialkyl .... relationship between ln[NBE]t/[NBE]0 and the p...
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Synthesis of (Imido)niobium(V)−Alkylidene Complexes That Exhibit High Catalytic Activities for Metathesis Polymerization of Cyclic Olefins and Internal Alkynes Kritdikul Wised and Kotohiro Nomura* Department of Chemistry, Faculty of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami Osawa, Hachioji, Tokyo 192-0397, Japan S Supporting Information *

ABSTRACT: The (imido)niobium(V)−alkylidene complexes Nb(CHSiMe3)(NR)[OC(CF3)3](PMe3)2 (R = 2,6-Me2C6H3, 2,6-iPr2C6H3, 1-adamantyl), which could be isolated from the dialkyl analogues by αhydrogen elimination in the presence of PMe3, exhibited remarkable catalytic activities for ring-opening metathesis polymerization (ROMP) of norbornene, and the polymerizations proceeded in a living manner. Metathesis polymerizations of (unstrained) disubstituted acetylenes also took place, affording polymers with uniform (narrow) molecular weight distributions.

H

equiv) in n-hexane afforded corresponding the (imido)niobium(V)−trialkyl analogues Nb(NR)(CH2SiMe3)3 (2a−c, Scheme 1), in moderate yields (68−76%), and 2a−c were

igh-oxidation-state early-transition-metal alkylidene complexes have attracted considerable attention,1,2 because they play essential roles as catalysts in olefin metathesis,1−4 as demonstrated especially by molybdenum.1,2a,b,e,3a,d In contrast to many successful examples demonstrated by molybdenum− alkylidene catalysts, examples in synthesis of niobium− alkylidene complexes5,6 and their application to olefin metathesis have been limited so far.7,8 Moreover, examples for complex catalysts for metathesis polymerization of (unstrained) internal alkynes9 have also been limited,8 whereas Nb and Ta catalyst systems (such as NbCl5 or TaCl5 with Ph4Sn or nBu4Sn and Nb(O-2,6-Me2C6H3)nCl5−n(THF) with tBuMgCl or AlEt3)10 exhibited unique reactivity toward polymerization of disubstituted acetylenes.9c,f,10,11 Since several (imido)vanadium(V)−alkylidene complexes12 were known as being highly active for ring-opening metathesis polymerization (ROMP) of norbornene (NBE) and its derivatives,12e,g,h we thus had an interest in exploring the possibility of “olefin metathesis active” Nb(V)−alkylidene complex catalysts. In this paper, we wish to communicate that the (imido)niobium(V)−alkylidene complexes Nb(CHSiMe3)(NR)[OC(CF3)3](PMe3)2 (R = 2,6Me2C6H3, 2,6-iPr2C6H3, 1-adamantyl (Ad)) exhibited remarkable catalytic activities for (living) ROMP of NBE and its derivatives, and the complexes also exhibited catalytic activity for metathesis polymerization of internal alkynes (disubstituted acetylenes) to afford polymers with uniform molecular weight distributions.13 (Imido)niobium(V) complexes containing perfluorinated alkoxo ligands have been chosen in this study, on the basis of our results with the (imido)vanadium(V)−alkylidene complexes.12h (Imido)niobium(V) trichloride complexes Nb(NR)Cl3(dme) (R = 2,6-Me2C6H3 (1a), 2,6-iPr2C6H3 (1b), Ad (1c)) were prepared according to the reported procedures for syntheses of 1b,c.14,15 Reactions of 1a−c with LiCH2SiMe3 (3.0 © XXXX American Chemical Society

Scheme 1. Synthesis of (Imido)niobium(V)−Alkylidene Complexes 4a−c

identified by NMR spectra and elemental analysis.15 Reaction of 2a−c with (CF3)3COH (1.0 equiv) in n-hexane afforded the corresponding dialkyl complexes Nb(NR)(CH2SiMe3)2[OC(CF3)3] (3a−c),15 and these complexes were also identified by NMR spectra and elemental analysis. The reaction of 2a with 2,6- t Bu 2 C 6 H 3 OH also afforded Nb(N-2,6-Me 2 C 6 H 3 )(CH2SiMe3)2(O-2,6-tBu2C6H3) (5).15 However, the reactions of 2a with C6F5OH, 2,6-F2C6H3OH, 2,6-iPr2C6H3OH, 2,6Ph2C6 H3OH, and with (CF 3) 2(CH3)COH in n-hexane Received: July 12, 2016

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DOI: 10.1021/acs.organomet.6b00560 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Ring-Opening Metathesis Polymerization (ROMP) of Norbornene (NBE) using Nb(CHSiMe3)(NR)[OC(CF3)3](PMe3)2 (R = 2,6-Me2C6H3 (4a), 2,6-iPr2C6H3 (4b), 1-Adamantyl (Ad, 4c))a

run

cat. (amt/μmol)

temp/°C

time/min

yield/%

TONb

TOF/h−1

Mnc × 10−5

Mw/Mnc

cisd/%

1 2 3 4 5 6 7 8 9

4a (0.5) 4a (0.3) 4a (0.3) 4a (0.3) 4b (1.0) 4b (1.0) 4c (2.0) 4c (1.0) 4c (1.0)

25 25 25 50 25 50 25 25 50

5 10 5 2 5 2 5 5 3

97 49 66 90 72 95 >99 48 55

4110 3480 4640 6350 1530 2030 1060 1020 1170

49300 20900 55700 191000 18300 60800 12700 12200 23400

7.72 34.2 15.3 60.9 11.7 12.3 5.79 6.26 5.70

1.19 1.60 1.14 1.61 1.07 1.09 1.13 1.07 1.09

70 70 59 41

a

Conditions: toluene 4.8 mL (run 2:9.6 mL), NBE 200 mg (2.12 mmol), initial NBE concentration 0.44 mmol/mL (run 2, 0.22 mmol/mL). bTON (turnovers) = NBE reacted (mmol)/Nb (mmol); TOF = TON/time. cGPC data in THF vs polystyrene standards. dEstimated by 1H NMR spectra.

apparently higher than those conducted at 25 °C, as demonstrated by the (imido)vanadium(V)−alkylidenes.12a,h The activities by 4a,b decreased upon addition of PMe3;15 these trends are opposite those with the certain vanadium(V)− alkylidenes.12h The activity by 4a−c was also affected by the initial NBE concentration charged (runs 2 and 3 and the Supporting Information).15,18 As suggested from the above results (at 25 °C), good linear correlations between TON (turnover number, polymer yield on the basis of Nb) and Mn value along with low PDI (Mw/Mn) values were observed in the ROMP of NBE by 4a−c (Figure 1,

conducted under the same conditions afforded a mixture of several complexes. The results seem an interesting contrast to those in the reactions of V(NR)(CH2SiMe3)3, because the reactions gave the corresponding monophenoxides/-alkoxides in high yields12b,e,h and the reaction with 2,6-tBu2C6H3OH did not take place even on excess addition.12b Note that the niobium(V)−alkylidene complexes Nb(NR)(CHSiMe3)[OC(CF3)3](PMe3)2 (4a−c) could be obtained by α-hydrogen elimination, when the dialkyl complexes 3a−c were heated in n-hexane at 70 °C in the presence of PMe3 (excess, 12−20 equiv), and the complexes were identified by NMR spectra (1H, 13C, 19F, and 31P spectra, shown in the Supporting Information) and elemental analysis (Scheme 1).15,16 In contrast, an attempted isolation of the corresponding alkylidene from 5 in the presence of PMe3 (excess) in C6D6 (at 70 °C) failed due to low conversion.15 Two resonances (large and small) ascribed to the alkylidene proton were observed at 14.13 (t, major, 94%) and 12.43 (t, minor) ppm in the 1H NMR spectrum of 4a;17 two resonances (−74.17 (major) and −74.46 (minor) ppm) were also observed in the 19F NMR spectrum. The ratio did not change upon addition of PMe3, suggesting that the complexes were present as a mixture of syn/anti forms, as observed in the (imido)vanadium(V)−alkylidenes;12a,e,h,17 the syn/anti ratios were assigned by JC−H values of the alkylidene protons.17 Moreover, two broad resonances, ascribed to methyl protons in the isopropyl groups in 4b, were observed in the 1H NMR spectrum at 25 °C and only tiny broad resonance was observed in the 31P NMR spectrum.15 The 1H and 31P VT-NMR spectra with/without addition of PMe3 (shown in the Supporting Information)15 suggest that this would be due to a fast equilibrium between coordination and dissociation of PMe3 in solution, as observed in the (imido)vanadium(V)−alkylidenes.12e,h Note that, as shown in Table 1, these niobium(V)− alkylidene complexes (4a−c) exhibited remarkable catalytic activities for ring-opening metathesis polymerization (ROMP) of norbornene (NBE), and the activity increased in the order (initial NBE concentration 0.44 mmol/mL, 25 °C, 5 min) TOF = 12200 (4c, run 8) < 18300 (4b, run 5) < 49300 (4a, run 1), 55700 (4a, run 3). The resultant polymers prepared at 25 °C possessed ultrahigh molecular weights with narrow, uniform molecular weight distributions (Mn = (5.79−15.3) × 105, Mw/ Mn = 1.07−1.19, initial monomer concentration 0.44 mmol/ mL). Moreover, the catalytic activities by 4a−c at 50 °C were

Figure 1. Plots of Mn (◆) and Mw/Mn (◇) vs turnover numbers (TON, polymer yield) in the ROMP of NBE by 4c. Conditions: 4c 4.0 μmol, NBE 8.48 mmol in toluene, NBE 0.22 mmol/mL at 25 °C.

exemplified by 4c; more data are shown in the Supporting Information).15 Moreover, the reaction rate had a first-order dependence on the NBE concentration, since a linear relationship between ln[NBE]t/[NBE]0 and the polymerization time was observed ([NBE]0 and [NBE]t are the initial NBE concentration and that at time t, respectively).15 These facts strongly suggest a possibility of living polymerization without deactivation in this catalysis.15 Reported examples for the fast living ROMP yielding ultrahigh-molecular-weight polymers have been limited,12h and this is thus also one of the unique characteristics in these catalyses. The resultant polymers prepared by 4a−c possessed a cis/trans mixture of olefinic double bonds, but rather high cis content (70%) was observed in the polymer prepared by 4a.15 B

DOI: 10.1021/acs.organomet.6b00560 Organometallics XXXX, XXX, XXX−XXX

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Organometallics It also turned out that the ROMPs of norbornene derivatives (Scheme 2) such as 5-vinyl-2-norbornene (VNBE), 5-ethyl-

(Mo-F6), known as being a highly active catalyst, afforded polymer with bimodal molecular weight distributions (runs 20 and 21), and the attempted polymerizations by Mo(CHCMe2Ph)(N-2,6- iPr2C6H 3)(OtBu) 2 (Mo-F0) and V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6F5)(PMe3)2 (V) did not take place (runs 22 and 23). Since reported examples for metathesis polymerization of disubstituted acetylenes (internal alkynes) have been limited,8,9c,f,11 this would be one of the unique characteristics for using Nb(V)−alkylidenes as olefin metathesis catalysts. In contrast, the reaction of 1-hexyne with 4a afforded two products, and these products were identified by NMR, GPC, and GC-MS spectrometry. The resultant polymers (isolated as an MeOH-insoluble fraction) possessed rather high molecular weights with uniform molecular weight distributions, whereas the reaction also gave a mixture of 1,2,4- and 1,3,5tributylbenzene by cyclotrimerization. We have shown that the present (imido)niobium(V)− alkylidene complexes, Nb(CHSiMe 3 )(NR)[OC(CF 3 ) 3 ](PMe3)2 (R = 2,6-Me2C6H3 (4a), 2,6-iPr2C6H3 (4b), Ad (4c)), are highly effective for olefin metathesis polymerization, and 4a showed the highest activity. The ROMPs of NBE proceeded in a living manner, affording ultrahigh-molecularweight polymers with low PDI (Mw/Mn) values. As far as we know, this is the f irst demonstration of “olef in metathesis active” Nb−alkylidenes. Moreover, 4a polymerized terminal and internal alkynes; the resultant poly(1-hexynes) possessed uniform (narrow) molecular weight distributions. This is also a rare successful example of metathesis polymerization of unstrained internal alkynes (second examples, followed by Ta8). We are exploring more details, including the synthesis of a series of alkylidene complexes and the scope and limitation of this catalysis.

Scheme 2. ROMP of Norbornene Derivatives and Internal Alkynes by Nb(CHSiMe3)(N-2,6Me2C6H3)[OC(CF3)3](PMe3)2 (4a)

idene-2-norbornene (ENBE), and tetracyclododecene (TCD) by 4a afforded polymers with uniform molecular weight distributions (at 25 °C, data shown in the Supporting Information).15 The activity in the ROMP of norbornadiene was much higher than that in the ROMP of NBE (4a 0.10 μmol, conversion 99% after 1 min, TOF 1290000 h−1), but as observed in the ROMP by V(CHSiMe3)(N-2,6-Me2C6H3)[OC(CH3)(CF3)3](PMe3)2,12h the resultant polymer became insoluble for ordinary GPC analysis.15 Note that 4a polymerized 2-hexyne, affording highmolecular-weight polymers with narrow molecular weight distributions (Mn = 11600−23600, Mw/Mn = 1.05−1.09, after 30 min; runs 17−19, Table 2). The activity on the basis of polymer yield seems to increase upon an increase in the 2hexyne concentration (runs 17−19), and the Mn values also increased upon an increase in the polymer yields when the reactions were conducted for longer times (runs 15−17). Polymerization of 2-butyne by 4a also proceeded to afford polymers, but as reported previously for the Ta complex and others,8,19 the resultant polymers were insoluble for ordinary GPC runs.15 For comparison, the polymerization of 2-hexyne by Mo(CHCMe2 Ph)(N-2,6-Me 2C 6H 3)[OC(CH 3)(CF 3) 2]2

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00560.

Table 2. Polymerization of Alkynes by Nb(CHSiMe3)(N-2,6-Me2C6H3)[OC(CF3)3](PMe3)2 (4a)a run

cat.

monomer

[M]0b

time/h

yieldc/% e

10 11 12 13 14 15 16 17 18 19 20

4a 4a 4a 4a 4a 4a 4a 4a 4a 4a Mo-F6f

1-hexyne 1-hexyne 1-hexyne 2-butyne 2-butyne 2-hexyne 2-hexyne 2-hexyne 2-hexyne 2-hexyne 2-hexyne

2.0 1.0 0.5 4.0 2.0 2.0 2.0 2.0 1.0 3.0 2.0

0.25 1.0 24 0.5 4.0 4.0 2.0 0.5 0.5 0.5 4.0

11 (17) 9 (20)e 12 (32)e 10 54 31 23 13 22 17 46

21

Mo-F6f

2-hexyne

2.0

0.5

29

22 23

Mo-F0g Vh

2-hexyne 2-hexyne

2.0 2.0

0.5 4.0

trace

Mnd

Mw/Mnd

2700 2910 2060 insoluble insoluble 43000 35900 19400 11600 23600 37900 3750 38000 2730

1.34 1.32 1.41

1.42 1.53 1.06 1.09 1.07 l.43 1.43 1.27 1.24

Reaction conditions: catalyst 20 μmol (run 12, 10 μmol), monomer 2.0 mmol (run 19, 3.0 mmol; runs 12 and 18, 1.0 mmol), toluene 1.0−2.0 mL, 25 °C. bInitial monomer concentration in mmol/mL. cIsolated as methanol-insoluble fraction. dGPC data in THF vs polystyrene standards (MeOH insoluble fraction). eIsolated as MeOH soluble fraction (containing trace amount of polymer). fMo-F6: Mo(CHCMe2Ph)(N-2,6-Me2C6H3)[OC(CH3)(CF3)2]2. gMo-F0: Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)(OtBu)2. hV: V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6F5)(PMe3)2.12h a

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2003; Vol. 3. (c) Metathesis Polymerization; Buchmeiser, M., Ed.; Springer-Verlag: Berlin, Heidelberg, Germany, 2005. (d) Nomura, K.; Abdellatif, M. M. Polymer 2010, 51, 1861. (e) Leitgeb, A.; Wappel, J.; Slugovc, C. Polymer 2010, 51, 2927. (f) Buchmeiser, M. R. Macromol. Symp. 2010, 298, 17. (g) Mutlu, H.; de Espinosa, L. M.; Meier, M. A. R. Chem. Soc. Rev. 2011, 40, 1404. (h) Atallah, P.; Wagener, K. B.; Schulz, M. D. Macromolecules 2013, 46, 4735. (i) Schulz, M. D.; Wagener, K. B. Macromol. Chem. Phys. 2014, 215, 1936. (j) Handbook of Metathesis, 2nd ed.; Grubbs, R. H., Khosravi, E., Eds.; Wiley-VCH: Weinheim, Germany, 2015; Vol. 3. (5) Synthesis of Nb(V)−alkylidene complexes containing a cyclopentadienyl fragment: (a) Schrock, R. R.; Messerle, L. W.; Wood, C. D.; Guggenberger, L. J. J. Am. Chem. Soc. 1978, 100, 3793. (b) Rupprecht, G. A.; Messerle, L. W.; Fellmann, J. D.; Schrock, R. R. J. Am. Chem. Soc. 1980, 102, 6236. (c) Antiñolo, A.; Otero, A.; Fajardo, M.; García-Yebra, C.; Gil-Sanz, R.; López-Mardomingo, C.; Martín, A.; Gómez-Sal, P. Organometallics 1994, 13, 4679. (d) Duncalf, D. J.; Harrison, R. J.; McCamley, A.; Royan, B. W. J. Chem. Soc., Chem. Commun. 1995, 2421. (e) Antiñolo, A.; del Hierro, I.; Fajardo, M.; Garcia-Yuste, S.; Otero, A.; Blacque, O.; Kubicki, M. M.; Amandrut, J. Organometallics 1996, 15, 1966. (f) Mashima, K.; Kaidzu, M.; Nakayama, Y.; Nakamura, A. Organometallics 1997, 16, 1345. In situ generation of Nb−methylidene species [Cp*Nb(CH2)(η4C4H6)]: (g) Chan, M. C. W.; Cole, J. M.; Gibson, V. C.; Howard, J. A. K.; Lehmann, C.; Poole, A. D.; Siemeling, U. J. Chem. Soc., Dalton Trans. 1998, 103. (h) Mashima, K.; Matsuo, Y.; Tani, K. Organometallics 1999, 18, 1471. (6) Other examples of the synthesis of Nb(IV)− and Nb(V)− alkylidene complexes: (a) de Castro, I.; de la Mata, J.; Gómez, M.; Gómez-Sal, P.; Royo, P.; Selas, J. M. Polyhedron 1992, 11, 1023. (b) Etienne, M.; White, P. S.; Templeton, J. L. Organometallics 1993, 12, 4010. (c) Biasotto, F.; Etienne, M.; Dahan, F. Organometallics 1995, 14, 1870. (d) Kleckley, T. S.; Bennett, J. L.; Wolczanski, P. T.; Lobkovsky, E. B. J. Am. Chem. Soc. 1997, 119, 247. (e) Caselli, A.; Solari, E.; Scopelliti, R.; Floriani, C. J. Am. Chem. Soc. 1999, 121, 8296. (f) Veige, A. S.; Wolczanski, P. T.; Lobkovsky, E. B. Angew. Chem., Int. Ed. 2001, 40, 3629. (g) Kilgore, U. J.; Tomaszewski, J.; Fan, H.; Huffman, J. C.; Mindiola, D. J. Organometallics 2007, 26, 6132. (h) Searles, K.; Keijzer, K.; Chen, C. H.; Baik, M. H.; Mindiola, D. J. Chem. Commun. 2014, 50, 6267. (i) Kurogi, T.; Carroll, P. J.; Mindiola, D. J. J. Am. Chem. Soc. 2016, 138, 4306. (j) Searles, K.; Smith, K. T.; Kurogi, T.; Chen, C.-H.; Carroll, P. J.; Mindiola, D. J. Angew. Chem., Int. Ed. 2016, 55, 6642. (7) ROMP by Ta−alkylidene complexes Ta(CHtBu)(OAr′)3(THF) (Ar′ = 2,6-Me2C6H3, 2,6-iPr2C6H3, 2,4,6-iPr3C6H2, etc.) and the metallacyclobutane formed by treatment with norbornene (NBE): (a) Wallace, K. C.; Schrock, R. R. Macromolecules 1987, 20, 448. (b) Wallace, K. C.; Liu, A. H.; Dewan, J. C.; Schrock, R. R. J. Am. Chem. Soc. 1988, 110, 4964. (c) Mashima, K.; Kaidzu, M.; Tanaka, Y.; Nakayama, Y.; Nakamura, A.; Hamilton, J. G.; Rooney, J. J. Organometallics 1998, 17, 4183. ROMP of NBE by Cp*Ta( CHPh)[η4-o-(CH2)2C6H4] (one example at 65 °C for 30 h, TON 8). The precursors (dialkyl complexes) showed higher activities for the ROMPs (e.g. TON 94 after 30 h at 65 °C). (8) Polymerization of 2-butyne and 1-pentyne by Ta(V)−alkylidene catalysts: Wallace, K. C.; Liu, A. H.; Davis, W. M.; Schrock, R. R. Organometallics 1989, 8, 644. (9) Examples (review, book chapter) in alkyne metathesis reactions/ polymerizations: (a) Bunz, U. H. F. Acc. Chem. Res. 2001, 34, 998. Acyclic diyne metathesis (ADIMET) polymerization: (b) Fürstner, A. In Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 2, p 432. (c) Masuda, T.; Sanda, F. In Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 3, p 375. (d) Wu, X.; Tamm, M. Beilstein J. Org. Chem. 2011, 7, 82. (e) Fürstner, A. Angew. Chem., Int. Ed. 2013, 52, 2794. (f) Masuda, T.; Zhang, A. In Handbook of Metathesis, 2nd ed.; Grubbs, R. H., Khosravi, E., Eds.; Wiley-VCH: Weinheim, Germany, 2015; Vol. 3, p 375. References 9c and 9f give details of the polymerization of substituted acetylenes by transition-

Experimental details, synthesis, identification of Nb(NR)Cl3(dme) (1a−c), Nb(NR)(CH2SiMe3)3 (2a−c), Nb(NR)(CH 2 SiMe 3 ) 2 [OC(CF 3 ) 3 ] (3a−c), Nb(CHSiMe 3 )(NR)[OC(CF 3 ) 3 ](PMe 3 ) 2 (R = 2,6Me2C6H3 (4a), 2,6-iPr2C6H3 (4b), 1-adamantyl (4c)), and Nb(N-2,6-Me2C6H3)(CH2SiMe3)2(O-tBu2C6H3) (5) and selected NMR spectra, NMR spectra of 4a in the presence of PMe3, VT-NMR spectra of 4b (with/ without addition of PMe3), living ROMP of NBE and additional ROMP data (NBE and NBE derivatives), NMR spectra monitoring the reaction of 2a and 5 in the presence of PMe3 in C6D6, NMR spectra of poly(1hexyne), 1,2,4-tributylbenzene, 1,3,5-tributylbenzene, poly(2-hexyne), poly(NBE), and polymers from ROMP of NBE derivatives, and GC-MS data of 1,2,4tributylbenzene and 1,3,5-tributylbenzene (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for K.N.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.W. acknowledges the Tokyo Metropolitan government (Asian Human Resources Fund) for a predoctoral fellowship, and the project was partially supported by the advanced research program (Tokyo Metropolitan government). This project was partially supported by a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS, No. 15H03812). The authors express their thanks to Profs. S. Komiya, A. Inagaki, K. Tsutsumi, and S. Sueki (Tokyo Metropolitan University) for discussions. K.N. also expresses his thanks to Prof. Toshio Masuda (emeritus, Kyoto University) for helpful comments.

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REFERENCES

(1) For examples, see: (a) Schrock, R. R. Acc. Chem. Res. 1990, 23, 158. (b) Schrock, R. R. In Alkene Metathesis in Organic Synthesis; Fürstner, A., Ed.; Springer: Berlin, Germany, 1998; p 1. (c) Schrock, R. R. In Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 1, p 8. (d) Pietraszuk, C. In Olefin Metathesis: Theory and Practice; Grela, K., Ed.; Wiley: Hoboken, NJ, USA, 2014; p 371. (e) Schrock, R. R. Acc. Chem. Res. 2014, 47, 2457. (f) Schrock, R. R. In Handbook of Metathesis, 2nd ed.; Grubbs, R. H., Wenzel, A. G., Eds.; Wiley-VCH: Weinheim, Germany, 2015; Vol. 1, p 1. (2) For examples, see: (a) Schrock, R. R. Acc. Chem. Res. 1979, 12, 98. (b) Schrock, R. R. Chem. Rev. 2002, 102, 145. (c) Mindiola, D. Acc. Chem. Res. 2006, 39, 813. (d) Mindiola, D.; Bailey, B.; Basuli, F. Eur. J. Inorg. Chem. 2006, 2006, 3135. (e) Schrock, R. R. Chem. Rev. 2009, 109, 3211. (f) Nomura, K.; Zhang, W. Chem. Sci. 2010, 1, 161. (3) Olefin metathesis applications in organic synthesis: (a) Fürstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012. (b) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. (c) Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 2. (d) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42, 4592. (e) Samojłowicz, C.; Bieniek, M.; Grela, K. Chem. Rev. 2009, 109, 3708. (f) van Otterlo, W. A. L.; de Koning, C. B. Chem. Rev. 2009, 109, 3743. (g) Vougioukalakis, G.; Grubbs, R. H. Chem. Rev. 2010, 110, 1746. (h) Handbook of Metathesis, 2nd ed.; Grubbs, R. H., O’Leary, D. J., Eds.; Wiley-VCH: Weinheim, Germany, 2015; Vol. 2. (4) For examples of olefin metathesis polymerization, see: (a) Buchmeiser, M. R. Chem. Rev. 2000, 100, 1565. (b) Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, Germany, D

DOI: 10.1021/acs.organomet.6b00560 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics metal catalyst systems (metal halides, organometallic reagents, etc.) and reported alkylidene complexes. (10) For example:9c,f (a) Masuda, T.; Matsumoto, T.; Yoshimura, T.; Higashimura, T. Macromolecules 1990, 23, 4902. (b) Kouzai, H.; Masuda, T.; Higashimura, T. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 2523. (c) Kouzai, H.; Masuda, T.; Higashimura, T. Polymer 1994, 35, 4920. (d) Aoki, T.; Shinohara, K.; Kaneko, T.; Oikawa, E. Macromolecules 1996, 29, 4192. (e) Teraguchi, M.; Masuda, T. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 4546. (11) For example:9c,f (a) Katz, T. J.; Lee, S. J. J. Am. Chem. Soc. 1980, 102, 422. Polymerization by WBr(CPh)(CO)4 (probably metathesis mechanism). (b) Katz, T. J.; Ho, T. H.; Shih, N.-Y.; Ying, Y.-C.; Stuart, V. I. W. J. Am. Chem. Soc. 1984, 106, 2659. Polymerization by W(CPh2)(CO)5. (12) (a) Yamada, J.; Nomura, K. Organometallics 2005, 24, 2248. (b) Nomura, K.; Onishi, Y.; Fujiki, M.; Yamada, J. Organometallics 2008, 27, 3818. (c) Zhang, W.; Nomura, K. Organometallics 2008, 27, 6400. (d) Zhang, S.; Tamm, M.; Nomura, K. Organometallics 2011, 30, 2712. (e) Nomura, K.; Suzuki, K.; Katao, S.; Matsumoto, Y. Organometallics 2012, 31, 5114. (f) Hatagami, K.; Nomura, K. Organometallics 2014, 33, 6585. (g) Nomura, K.; Bahuleyan, B. K.; Tsutsumi, K.; Igarashi, A. Organometallics 2014, 33, 6682. (h) Hou, X.; Nomura, K. J. Am. Chem. Soc. 2015, 137, 4662. (13) Part of these data (especially synthesis of alkylidenes and ROMP of NBE) were presented at the following: 21st International Symposium on Olefin Metathesis and Related Chemistry (ISOM XXI), Graz, Austria, July 2015; Asian Polyolefin Workshop 2015 (APO2015), Tokyo, November 2015; Pacifichem 2015, Honolulu, Hawaii, December, 2015. (14) Korolev, A. V.; Rheingold, A. L.; Williams, D. S. Inorg. Chem. 1997, 36, 2647. (15) Experimental descriptions including synthesis and identification of complexes, attempted reactions, polymerization procedure, and additional polymerization results and selected NMR spectra (complexes, polymers) are given in the Supporting Information. (16) The structure of complex 4a is also supported by X-ray crystallographic analysis, and the CIF file was deposited as a Personal Communication to the Cambridge Structural Database (CCDC 1500202). Some bonds (OC(CF3)3 ligand and CHSiMe3 ligand) possess disorder even after refinement, and the crystal contains solvent (toluene) (R1 = 0.1429; wR2 = 0.3370; GOF = 1.094). (17) Assignments of resonances ascribed to anti/syn alkylidene protons were made by measurement of JC−H values in 1H NMR spectra, on the basis of a previous report on Mo−alkylidenes:3d Oskam, J. H.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 11831. The JC−H values at 14.13 (t, major, 94%) and 12.43 (t, minor) ppm in 4a were 120.7 Hz (anti) and 92.1 Hz (syn), respectively; these also correspond to the reported fact that the anti isomer appears 1−2 ppm downfield of the syn isomer. The anti/syn ratios (%) were 94/6 (4a), 88/12 (4b), and 35/65 (4c), respectively. (18) The cis contents in the resultant polymers prepared by 4a were lower than those prepared by the analogous (imido)vanadium− alkylidene V(CHSiMe3)(N-2,6-Me2C6H3)[OC(CH3)(CF3)2](PMe3)2 (85%).12h Moreover, the cis percentage in the resultant polymer by 4c (containing the small adamantylimido ligand) was lower than those by 4a,b (containing arylimido ligands); it thus seems likely that electronic factors (not only steric factors) play a role in the control of cis/trans regularity. This should be a future subject by modification of ligand substituents. (19) As described previously,8,11 the resultant poly(2-butynes) are hardly soluble in THF even for ordinary GPC analysis (insoluble in CDCl3 or any other solvent ordinarily used for NMR measurements). This (limited solubility) is due to symmetrically disubstituted poly(acetylene) speculated previously8,11 in addition to high molecular weights (on the basis of the data for poly(2-hexynes)).

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DOI: 10.1021/acs.organomet.6b00560 Organometallics XXXX, XXX, XXX−XXX