Metal-Free Ring-Opening Metathesis Polymerization with Hydrazonium Initiators.

The ring-opening metathesis polymerization (ROMP) of cyclopropenes using hydrazonium initiators is described. The initiators, which are formed by the condensation of 2,3-diazabicyclo[2.2.2]octane and an aldehyde, polymerize cyclopropene monomers by a sequence of [3+2] cycloaddition and cycloreversion reactions. This process generates short chain polyolefins (Mn ≤ 9.4 kg/mol) with relatively low dispersities (Đ ≤ 1.4). The optimized conditions showed efficiency comparable to that achieved with Grubbs' 2nd generation catalyst for the polymerization of 3-methyl-3-phenylcyclopropene. A positive correlation between monomer to initiator ratio and degree of polymerization was revealed through NMR spectroscopy.

Reactions were monitored by thin-layer chromatography (TLC) on EMD Silica Gel 60 F254 plates under UV light (254 nm) or visualized with I2. Flash chromatography was performed using silica gel 60 (230-400 mesh) from SilicaFlash on a Biotage Isolera One system. Organic solutions were concentrated under reduced pressure on a Büchi rotary evaporator R-210. Proton nuclear magnetic resonance ( 1 H NMR) spectra and carbon nuclear magnetic resonance ( 13 C NMR) spectra were recorded on Varian Magnet System 600 MHz, Bruker Magnet System 500 MHz, Varian Magnet System 400 MHz. All chemical shifts are reported in parts per million (ppm) downfield from tetramethylsilane. Proton resonances are referenced to residual protium in the NMR solvent (7.26 ppm for CHCl3, 3.31 ppm for d3-methanol, and 1.94 ppm for d3-acetonitrile). Carbon resonances are referenced to the carbon resonances of the NMR solvent (77.2 ppm for CDCl3, 49.0 ppm for d4-methanol, and 1.3 ppm and 118.3 ppm for d3-acetonitrile). Data are represented as follows: chemical shift, multiplicity (br = broad, s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), coupling constants in hertz (Hz), integration. Mass spectral (MS) data were obtained on Advion CMS Mass Spectrometer equipped with an APCI (Atmospheric Pressure Chemical Ionization) module and oligomer mass spectral data were obtained on Waters ® Micromass ® MALDI micro MX ™ (MALDI-TOF). All gel permeation chromatograms (GPC) traces were obtained from Tosoh EcoSec HLC 8320GPC system with two SuperHM-M tandem columns. The eluent solvent was tetrahydrofuran and the flow rate was set at 0.350 mL/min. The number-average molecular weights (Mn), weight-average molecular weights (Mw), and dispersities (Đ) were calculated from the f time against polystyrene standards. Thermogravimetric analysis (TGA) analysis was done on a Q500 TGA (TA Instruments; New Castle, DE). Sample was heated at 10 ºC / min up to 600 ºC in a nitrogen atmosphere with a sample purge flow rate of 60 mL/min. Analysis was performed using TA Instruments' Universal Analysis software. DSC analysis was performed on a DSC Auto 2500 (TA Instruments; New Castle, DE). Sample was encapsulated in Tzero © aluminum sample pans. Sample was heated to 225 ºC at 10 ºC / min to erase previous thermal history, cooled to 25 ºC at 5 ºC / min, and heated a second time to 225 ºC. Glass transition temperature (Tg) was analyzed using the second heating curve at the point of curve inflection. Temperature was calibrated using an indium metal standard. After being charged with H2, the mixture was stirred vigorously under an atmosphere of H2 for 16 h. The reaction mixture was filtered through Celite directly into a 50% solution of tetrafluoroboric acid in diethyl ether (1.1 mL, 9.08 mmol). The mixture was then concentrated in vacuo, yielding a white solid, which was recrystallized with Et2O/MeOH to furnish the title compound as white needles (673.4 mg, 52% yield). 1  , and acetonitrile (347 µL, 1M), were added to a 1-dram vial equipped with a stirbar. The mixture was heated to 90 ºC with stirring for 15 hours. The mixture was then cooled and concentrated in vacuo. The resulting solid was triturated with ether (0.5 mL x 2) and recrystallized in Et2O/MeOH to furnish the title compound as fine white needles (59.3 mg, 59% yield). 1  Cyclopropene synthesis 2,2-dibromo-1-methylcyclopropyl)benzene (18): The title compound was prepared according to the reported procedure. 1 To a 100 mL round bottom flask equipped with a stirbar were added 10 M aqueous sodium hydroxide (338 mmol, 33.8 mL) and cetrimonium bromide (3.39 g, 9.3 mmol). A solution of ɑ-methylstyrene (17) (11 mL, 84.5 mmol) in DCM (3.4 mL) was slowly added to the rapidly stirring solution. A solution of bromoform (15.1 mL, 169 mmol) in DCM (17 mL) was added dropwise over 1 hour from a 50 mL addition funnel into the rapidly stirring solution. The mixture was stirred vigorously at 25 ºC overnight. The layers were separated, and the aqueous layer was extracted with DCM (100 mL x 3). The combined organic layers were washed with brine (50 mL x 2), dried over MgSO 4 , and concentrated in vacuo. The crude residue was purified by flash column chromatography with a 0-1% ethyl acetate:hexanes gradient. The fractions were combined and concentrated in vacuo to afford the title compound as a white solid (32.1 g, quant.). The characterization data matched that previously reported. 2 (2-bromo-1-methylcyclopropyl)benzene (19): The title compound was prepared according to the reported procedure. 1 A flamed dried 1 L round bottom flask equipped with a stir bar and a 500 mL addition funnel was charged with a solution of 2,2-dibromo-1-methylcyclopropyl)benzene (18) (71.0 g, 245 mmol), Ti(OiPr) 4 (7.3 mL, 24 mmol), and THF (245 mL). The flask was cooled to 0 ºC using an ice bath. A 3 M solution of ethylmagnesium bromide in ether (122 mL, 367 mmol) was added dropwise over 1 hour to the rapidly stirring solution. The solution was then left to warm to room temperature and stirred for another 2 hours. The flask was cooled to 0 ºC before the reaction mixture was quenched by slow addition of water (100 mL) followed by 1 M HCl (120 mL). The aqueous layer was extracted with ether (100 mL x 4). The combined organic layers were washed with saturated sodium bicarbonate (50 mL) and brine (50 mL), dried over MgSO 4 , and concentrated in vacuo. The crude residue was purified by flash column chromatography with a 0-1% ethyl acetate:hexanes gradient to afford the title compound as a colorless oil (29.5 g, 57 % yield). The characterization data matched that previously reported. 2

3-methyl-3-phenyl-cyclopropene (3):
The title compound was prepared according to the reported procedure. 1 To a 250 mL round bottom flask equipped with a stir bar were added potassium tertbutoxide (2.76 g, 24.6 mmol) and DMSO (38.3 mL, 0.4 M). Then, (2-bromo-1-methylcyclopropyl)benzene (19) (3.22 g, 15.2 mmol) was added dropwise over 15 minutes. The reaction mixture was stirred overnight at 55 °C, before being quenched by the slow addition of water (100 mL) with rapidly stirring while the flask was cooled in an ice bath. After 10 minutes, pentane (50 mL) was S7 added, and the flask was allowed to warm to room temperature. The aqueous layer was extracted with pentane (50 mL x 4). The combined organic layers were washed with brine (50 mL x 2), dried over MgSO4, and concentrated in vacuo. The crude residue was purified by flash column chromatography (pentane) to furnish the title compound as a colorless oil. This oil was further dried over 4 Å molecular sieves overnight and then vacuum distilled at 35 ºC using a short-path distillation head (1.21g, 61% yield). The characterization data matched that previously reported. followed by 1 M HCl (60 mL). The aqueous layer was extracted with ether (50 mL x 4). The combined organic layers were washed with saturated sodium bicarbonate (25 mL) and brine (25 mL), dried over MgSO 4 , and concentrated in vacuo. The crude residue was purified by flash column chromatography with a 0-1% ethyl acetate:hexanes gradient to furnish the title compound as a colorless oil (5.9 g, 75 % yield.). The characterization data matched that previously reported. 3

3-methyl-3-p-fluorophenylcyclopropene (8):
The title compound was prepared according to the reported procedure. 1 To a 250 mL round bottom flask equipped with a stir bar were added potassium tert-butoxide (4.6 g, 41.3 mmol) and DMSO (65.0 mL, 0.4 M). Then, 1-(2-bromo-1-methylcyclopropyl)-4-fluorobenzene (21) (5.92 g, 41.3 mmol) was added dropwise over 15 minutes. The reaction mixture was stirred overnight at 55 °C, before being quenched by the slow addition of water (100 mL) with rapid stirring while the flask was cooled in an ice bath. After 10 minutes, pentane (50 mL) was added and the flask was allowed to warmed up to room temperature. The aqueous layer was extracted with pentane (50 mL x 4). The combined organic layers were washed with brine (50 mL x 2), dried over MgSO4, and concentrated in vacuo. The crude residue was purified by flash column chromatography (pentane) to furnish the title compound as a colorless oil. This oil was further dried over 4 Å molecular sieves overnight and then vacuum distilled at 35 ºC using a short-path distillation head (3.0 g, 78% yield). The characterization data matched that previously reported. 1

7-methylenetridecane (22):
The title compound was prepared according to the reported procedure. 4 To a 250 mL round bottom flask equipped with a stir bar were added methyltriphenylphosphonium bromide (18.0 g, 50.4 mmol) and THF (75.6 mL, 0.67 M). After being cooled to 0 °C in an ice bath, 2.5 M n-BuLi solution in hexanes (36.3 mL, 90.8 mmol) was added dropwise to the flask over 1 hour period. The reaction mixture was stirred for 2 hours at room temperature. Subsequently, 7-tridecanone (10.0 g, 50.4 mmol) dissolved in anhydrous THF (7.6 mL, 6.7 M) was added dropwise to the reaction mixture at 0 °C via a syringe. After stirring at room temperature for 24 hours, the mixture was quenched with 20 mL of saturated NH 4 Cl and then diluted with 15 mL water. The mixture was extracted with ethyl acetate (3 x 15 mL). The combined organic phases was washed with brine (2 x 15 mL), dried over MgSO 4 , and concentrated in vacuo. The crude residue was dissolved in hexanes and filtered through a cotton plug to remove solid triphenylphosphine oxide. The filtrate was concentrated in vacuo and then purified by flash column Me F 8 n-hex n-hex chromatography with a 0-1% ethyl acetate:hexanes gradient to furnish the title compound as a colorless oil (7.0 g, 71% yield). The characterization data matched that previously reported. 5 1,1-dibromo-2,2-dihexylcyclopropane (23): The title compound was prepared according to the reported procedure. 6 To a 50 mL round bottom flask equipped with a stir bar were added 10 M aqueous sodium hydroxide (61.1 mmol, 6.1 mL) and hexadecyltrimethylammonium bromide (0.61 g, 1.7 mmol). A solution of 7-methylenetridecane (22) (3.0 g, 15.3 mmol) in DCM (0.6 mL) was slowly added to the rapidly stirring solution. A solution of bromoform (2.7 mL, 30.6 mmol) in DCM (3.1 mL) was added dropwise over 1 hour from a 50 mL addition funnel into the rapidly stirring solution. The mixture was stirred vigorously at 25 ºC overnight, before being extracted with DCM (50 mL x 3). The combined organic layers were washed with brine (25 mL x 2), dried over MgSO 4 , and concentrated in vacuo. The crude residue was purified by flash column chromatography with a 0-1% ethyl acetate:hexanes gradient to furnish the title compound as a colorless oil (5.5 g, 97% yield). The characterization data matched that previously reported. 6

2-bromo-1,1-dihexylcyclopropane (24):
The title compound was prepared according to the reported procedure. 6 To a flame-dried 500 mL round bottom flask equipped with a 50 mL addition funnel and a stir bar were added 1,1-dibromo-2,2-dihexylcyclopropane (23) (15.0 g, 41.0 mmol), Ti(OiPr) 4 (1.2 mL, 4.1 mmol), and THF (41.0 mL). The flask was cooled to 0 ºC with an ice bath. A 3 M solution of ethylmagnesium bromide in ether (20.5 mL, 61.5 mmol) was added dropwise over 1 hour to the rapidly stirring solution. The solution was then left to warm to room temperature while stirring for another 2 hours. The flask was cooled to 0 ºC before being quenched by the slow addition of water (16 mL) followed by 1 M HCl (25 mL). The aqueous layer was extracted with ether (25 mL x 4). The combined organic layers were washed with saturated sodium bicarbonate (15 mL) and brine (15 mL), dried over MgSO 4 , and concentrated in vacuo. The crude residue was purified by flash column chromatography with a 0-1% ethyl acetate:hexanes gradient to furnish the title compound as a colorless oil (6.7 g, 56 % yield.

3,3-dihexylcyclopropene (13):
The title compound was prepared according to the reported procedure. 6 To a 250 mL round bottom flask equipped with a stir bar were added potassium tertbutoxide (5.5 g, 49.4 mmol) and DMSO (77.0 mL, 0.4 M). Then, 2-bromo-1,1-dihexylcyclopropane (24) (8.9 g, 30.6 mmol) was added dropwise over 15 minutes. The reaction mixture was stirred overnight at 55 °C before being quenched by the slow addition of water (100 mL) with rapid stirring while the flask was cooled in an ice bath. After 10 minutes, pentane (50 mL) was added, and the flask was allowed to warm to room temperature. The aqueous layer was extracted with pentane (50 mL x 4), and the combined organic layers were washed with brine (50 mL x 2), dried over MgSO4, and concentrated in vacuo. The crude residue was purified by flash column chromatography (pentane) to furnish the title compound as a colorless oil (1.7 g, 27% yield). The characterization data matched that previously reported.   After being purged with nitrogen, the reaction was stirred at 90 ºC for 18 hours. The reaction mixture was diluted with DCM (1 mL) and then neutralized with Et3N (10 µL, 0.08 mmol). The mixture was washed with water (1 mL). The aqueous layer was further washed with DCM (2 x 2 mL). The combined organic layers were concentrated in vacuo, and the crude residue was purified by column chromatography with a 0-15% hexanes/ethyl acetate gradient to furnish the title compound as a light yellow liquid (15.5 mg, 42 % yield). 1  General procedure for ROMP ɑ-Phenyl-ω-carboxaldehyde-poly(3-methyl-3-phenylcyclopropene) (5): A flame-dried 1dram vial equipped with a stir bar was charged with hydrazonium 6 (5.0 mg, 0.017 mmol) and 3methyl-3-phenylcyclopropene (3) (181.8 mg, 1.4 mmol). The mixture was stirred for 18 hours at 90 ºC. Subsequently, the vial was cooled to room temperature before acetonitrile (0.5 mL), trifluoroacetic acid (3 µL, 0.035 mmol), and water (6 µL, 0.35 mmol) were added. The resulting mixture was stirred at 90 ºC for 2 hours. After the vial was cooled to room temperature, a brown yellow precipitate was observed. The acetonitrile supernatant was removed and the solid was triturated with acetonitrile (0.5 mL x 2) before being dissolved in DCM (0.2 mL) and precipitated with isopropanol (2 mL). A pale-yellow gel was collected by centrifuge (5 min at 2000 x g RCF).
The gel was re-dissolved in DCM (0.2 mL) and precipitated with acetonitrile (2 mL). A white powder was collected by centrifuge (10 min at 2000 x g RCF). The supernatant liquid was collected and concentrated in vacuo and the above procedure for the isolation of the white powder with DCM/acetonitrile solvent combination was repeated three more times. The combined white powders were dried in vacuo to furnish the title polymer (45.3 mg, 25% yield). 1

ɑ-p-Fluorophenyl-ω-carboxaldehyde-poly(3-methyl-3-phenylcyclopropene) (9):
A flamedried 1-dram vial equipped with a stir bar was charged with hydrazonium 12 (5.3 mg, 0.017 mmol) and 3-methyl-3-phenylcyclopropene (3) (181.8 mg, 1.4 mmol). The reaction was stirred for 18 hours at 90 ºC. Subsequently, the vial was cooled down to room temperature before acetonitrile (0.5 mL), trifluoroacetic acid (3 µL, 0.035 mmol), and water (6 µL, 0.35 mmol) were added. The resulting mixture was stirred at 90 ºC for 2 hours. After the vial was cooled to room temperature, a brown yellow precipitate was observed. The acetonitrile supernatant was removed and the solid was triturated with acetonitrile (0.5 mL x 2) before being dissolved in DCM (0.2 mL) and precipitated with isopropanol (2 mL). A pale-yellow gel was collected by centrifuge (5 min at 2000 x g RCF). The gel was re-dissolved in DCM (0.2 mL) and precipitated with acetonitrile (2 mL). A white powder was collected by centrifuge (10 min at 2000 x g RCF). The supernatant liquid was collected and concentrated in vacuo and the above procedure for the isolation of the white powder

Computational Data
All DFT calculations were performed with the Gaussian 09 program package. 7 The geometry optimization of all the minima and transition states involved was carried out at the M06-2X level of theory 8 with the 6-31G(d) basis set. 9 The vibrational frequencies were computed at the same level to check whether each optimized structure is an energy minimum or a transition state and to evaluate its zero-point vibrational energy (ZPVE) and thermal corrections at 298 K. Solvent effects in toluene were computed at the M06-2X/6-311+G(d,p) level using the gas-phase optimized structures. Solvation energies were evaluated by a self-consistent reaction field (SCRF) using the PCM model.