Tin ( IV )-Porphyrin Tetracarbonyl Cobaltate : An E ffi cient Catalyst for the Carbonylation of Epoxides

Cationic tin(IV) porphyrins with tetracarbonyl cobaltates were synthesized, exhibiting bifunctional catalytic reactivity. The Lewis acidic tin-porphyrin center activated epoxides; concurrently, cobalt carbonyl anions efficiently opened epoxides and delivered carbonyl moieties. Thus, a series of β-lactones with a high synthetic value were obtained. This catalytic system showed excellent efficiency exceeding a turnover number of one thousand with a broad substrate scope. In addition, the presented tin porphyrin-based catalyst exhibited exclusive chemoselectivity to terminal epoxides over internal ones. The selective carbonylation of di-epoxides demonstrated the usefulness of these catalysts in the synthesis of complex molecular structures.

In a continuous effort to expand tin-based catalytic systems, we investigated novel tin complexes with a noninnocent tetracarbonyl cobaltate anion, which produced β-lactones through the selective mono carbonylation of epoxides.The direct conversion of epoxides to the corresponding β-lactones is a clean, one carbon homologation method with a 100% atom-economy [35].A list of catalytic systems has been developed based on the combination of Lewis acid and metal carbonyl for the carbonylative ring expansion of epoxides (Scheme 1) [36][37][38][39].Compared to the previous examples, our catalytic system consisting of Sn(IV)-centered porphyrins exhibited comparable reactivity with a turnover number (TON) of over 1000 and exclusive selectivity to β-lactones, without any side products or double carbonylations.In addition, highly selective carbonylation of terminal epoxide over internal epoxide allowed chemoselective lactone formation.The details of our findings are presented herein.
Catalysts 2019, 9, x FOR PEER REVIEW 2 of 13 catalytic systems has been developed based on the combination of Lewis acid and metal carbonyl for the carbonylative ring expansion of epoxides (Scheme 1) [36][37][38][39].Compared to the previous examples, our catalytic system consisting of Sn(IV)-centered porphyrins exhibited comparable reactivity with a turnover number (TON) of over 1000 and exclusive selectivity to β-lactones, without any side products or double carbonylations.In addition, highly selective carbonylation of terminal epoxide over internal epoxide allowed chemoselective lactone formation.The details of our findings are presented herein.

Results and Discussion
The syntheses of Sn(IV) porphyrins are outlined in Scheme 2. The conventional condensation reaction between benzaldehyde and pyrrole produced the porphyrins [40].Subsequent reaction of porphyrins with Sn(II) chloride produced Sn(IV) porphyrin dichloride [41].The X-ray crystal structure of 2a shows a chloride anion tightly bound to the Sn center (Figure S1).Finally, sodium tetracarbonyl cobaltate, NaCo(CO)4 reacted with Sn(IV) porphyrin dichloride precursors and produced the desired catalysts (3a-3d).

Results and Discussion
The syntheses of Sn(IV) porphyrins are outlined in Scheme 2. The conventional condensation reaction between benzaldehyde and pyrrole produced the porphyrins [40].Subsequent reaction of porphyrins with Sn(II) chloride produced Sn(IV) porphyrin dichloride [41].The X-ray crystal structure of 2a shows a chloride anion tightly bound to the Sn center (Figure S1).Finally, sodium tetracarbonyl cobaltate, NaCo(CO) 4 reacted with Sn(IV) porphyrin dichloride precursors and produced the desired catalysts (3a-3d).
Catalysts 2019, 9, x FOR PEER REVIEW 2 of 13 catalytic systems has been developed based on the combination of Lewis acid and metal carbonyl for the carbonylative ring expansion of epoxides (Scheme 1) [36][37][38][39].Compared to the previous examples, our catalytic system consisting of Sn(IV)-centered porphyrins exhibited comparable reactivity with a turnover number (TON) of over 1000 and exclusive selectivity to β-lactones, without any side products or double carbonylations.In addition, highly selective carbonylation of terminal epoxide over internal epoxide allowed chemoselective lactone formation.The details of our findings are presented herein.

Results and Discussion
The syntheses of Sn(IV) porphyrins are outlined in Scheme 2. The conventional condensation reaction between benzaldehyde and pyrrole produced the porphyrins [40].Subsequent reaction of porphyrins with Sn(II) chloride produced Sn(IV) porphyrin dichloride [41].The X-ray crystal structure of 2a shows a chloride anion tightly bound to the Sn center (Figure S1).Finally, sodium tetracarbonyl cobaltate, NaCo(CO)4 reacted with Sn(IV) porphyrin dichloride precursors and produced the desired catalysts (3a-3d).
First, catalyst 3a (0.3 mol %) with electron withdrawing chlorine substituents was tested for carbonylation of propylene oxide (4a, 2 mmol) with carbon monoxide (50 atm) in tetrahydrofuran (THF, 1 mL) at 90 • C.After 16 h, the desired β-butyrolactone was formed quantitatively (Scheme 3).To elucidate the reactive catalytic species, we tested NaCo(CO) 4 and Sn(IV) porphyrin dichloride 2a independently.Complex 2a with the tightly bound chloride anions did not promote carbonylation, and epoxide 4a remained unreacted.Interestingly, NaCo(CO) 4 was solely active under the same CO pressure, but the efficiency was relatively low (8%).The complex 3a prepared in situ from the mixture of NaCo(CO) 4 (0.6 mol %) and 2a (0.3 mol %) showed the same reactivity as isolated 3a.These results imply that the reaction between NaCo(CO) 4 and 2a produces a more reactive Lewis acidic species, which is not exactly identified by analytical methods.However, currently, the formation of NaCl indicates that the Sn(IV) porphyrin cationic complex with an empty coordination site is the active catalytic species as a Lewis acid for epoxide activation.First, catalyst 3a (0.3 mol %) with electron withdrawing chlorine substituents was tested for carbonylation of propylene oxide (4a, 2 mmol) with carbon monoxide (50 atm) in tetrahydrofuran (THF, 1 mL) at 90 °C.After 16 h, the desired β-butyrolactone was formed quantitatively (Scheme 3).To elucidate the reactive catalytic species, we tested NaCo(CO)4 and Sn(IV) porphyrin dichloride 2a independently.Complex 2a with the tightly bound chloride anions did not promote carbonylation, and epoxide 4a remained unreacted.Interestingly, NaCo(CO)4 was solely active under the same CO pressure, but the efficiency was relatively low (8%).The complex 3a prepared in situ from the mixture of NaCo(CO)4 (0.6 mol %) and 2a (0.3 mol %) showed the same reactivity as isolated 3a.These results imply that the reaction between NaCo(CO)4 and 2a produces a more reactive Lewis acidic species, which is not exactly identified by analytical methods.However, currently, the formation of NaCl indicates that the Sn(IV) porphyrin cationic complex with an empty coordination site is the active catalytic species as a Lewis acid for epoxide activation.
Further optimizations were undertaken (Table 1), and the influence of the electronic nature of the catalyst was investigated.A shift to electron rich porphyrins by substitution of Cl (3a, entry 1) with H (3b, entry 2), methyl (3c, entry 3), and methoxy (3d, entry 4) groups exhibited a gradual decrease in catalytic turnover rates.The diminishing Lewis acid character was directly indicated by the catalytic efficiency.Additionally, the effect of reaction media was significant.THF and 2-methyl THF exhibited the same quantitative conversion with 0.3 mol % catalyst loading (entries 1 and 5), but another ethereal solvent 1,4-dioxane gave a lower conversion (entry 6).Another coordinating polar solvent, dimethyl formamide (DMF, entry 7) [42] and a protic polar solvent, methanol (MeOH, entry 8) showed complete catalyst inhibition.In noncoordinating and nonpolar solvents, 3a showed poor performance (entries 9-11).In their study with Cr-porphyrin catalysts, Coates and coworkers noted that the coordination of solvents such as THF is very important for high efficiency [43].The transaddition of THF to a metal facilitates the alkoxide departure from the metal center allowing the alkoxide to attack the carbonyl group and form β-lactone (Scheme 1).We believe that a similar solvent effect is valid in the Sn-porphyrin system as well, thus the use of a noncoordinating solvent could not promote a β-lactone formation step.
Next, the variations in temperature and carbon monoxide pressure were evaluated.The catalytic efficiency of 3a is sensitive to both temperature and CO pressure.Decreasing the reaction temperature to 70 °C gave 61% conversion under otherwise identical conditions (entry 12).At 50 °C, the conversion dropped to 20% (entry 13).The change in pressure to 30 atm maintained good reactivity with 95% conversion to β-lactone (entry 14), but a notable loss of conversion was observed at 10 atm of CO (63%, entry 15).Further optimizations were undertaken (Table 1), and the influence of the electronic nature of the catalyst was investigated.A shift to electron rich porphyrins by substitution of Cl (3a, entry 1) with H (3b, entry 2), methyl (3c, entry 3), and methoxy (3d, entry 4) groups exhibited a gradual decrease in catalytic turnover rates.The diminishing Lewis acid character was directly indicated by the catalytic efficiency.Additionally, the effect of reaction media was significant.THF and 2-methyl THF exhibited the same quantitative conversion with 0.3 mol % catalyst loading (entries 1 and 5), but another ethereal solvent 1,4-dioxane gave a lower conversion (entry 6).Another coordinating polar solvent, dimethyl formamide (DMF, entry 7) [42] and a protic polar solvent, methanol (MeOH, entry 8) showed complete catalyst inhibition.In noncoordinating and nonpolar solvents, 3a showed poor performance (entries [9][10][11].In their study with Cr-porphyrin catalysts, Coates and coworkers noted that the coordination of solvents such as THF is very important for high efficiency [43].The trans-addition of THF to a metal facilitates the alkoxide departure from the metal center allowing the alkoxide to attack the carbonyl group and form β-lactone (Scheme 1).We believe that a similar solvent effect is valid in the Sn-porphyrin system as well, thus the use of a noncoordinating solvent could not promote a β-lactone formation step.
Next, the variations in temperature and carbon monoxide pressure were evaluated.The catalytic efficiency of 3a is sensitive to both temperature and CO pressure.Decreasing the reaction temperature to 70 • C gave 61% conversion under otherwise identical conditions (entry 12).At 50 • C, the conversion dropped to 20% (entry 13).The change in pressure to 30 atm maintained good reactivity with 95% conversion to β-lactone (entry 14), but a notable loss of conversion was observed at 10 atm of CO (63%, entry 15).
When the catalyst loading was decreased, higher temperature or pressure was required to maintain good β-lactone production.With 0.2 mol %, a longer reaction, 24 h, was needed to reach complete conversion (entries 16 and 17).When 0.1 mol % was used, changing the time and pressure to 36 h and 70 atm of CO, respectively, could achieve a TON of 1000 (entry 19).With the use of 0.05 mol %, 3a exhibited 63% conversion at 110 • C, with a TON of 1260 [44].When the catalyst loading was decreased, higher temperature or pressure was required to maintain good β-lactone production.With 0.2 mol %, a longer reaction, 24 h, was needed to reach complete conversion (entries 16 and 17).When 0.1 mol % was used, changing the time and pressure to 36 h and 70 atm of CO, respectively, could achieve a TON of 1000 (entry 19).With the use of 0.05 mol %, 3a exhibited 63% conversion at 110 °C, with a TON of 1260 [44]. 1 Unless otherwise indicated, the reaction was carried out for 16 h. 2Single experiment was performed on each entry. 3Yields were determined by 1 H NMR. 4 TON (turnover number) = Moles of product formed/moles of catalyst. 1Unless otherwise indicated, the reaction was carried out for 16 h. 2Single experiment was performed on each entry. 3Yields were determined by 1 H NMR. 4 TON (turnover number) = Moles of product formed/moles of catalyst.
The direct carbonylation of epoxides provided a series of β-lactones with high efficiencies (Table 2).A substituent on the terminal epoxides did not hamper the carbonylation performance of 3a.Variations in the chain length (5a-c) and size (5d and e) of substituents showed no influence and quantitative β-lactones were obtained.Interestingly, the aromatic substituent exhibited positional sensitivity.Styrene oxide, surprisingly, did not react with catalyst 3a at all (5f); however, adding a methylene bridge restored its reactivity (5g).Various kinds of coordinating functional groups were introduced that could behave as potential catalyst inhibitors.Epoxides with alkene (5h), ether (5i), and nitrile (5j) groups were smoothly converted to the corresponding β-lactones with a slight increase in catalyst loadings.Moreover, ester and amide functionalities were well tolerated (5k-m).Dual carbonylations of di-epoxide were also successful (5n).
Next, sterically more congested epoxides with 1,1-and 1,2-substitution were investigated.1,2-Epoxy-2-methylpropane (4o) gave only 27% of NMR yield under a high amount of catalyst loading.In the case of 1,2-substitution, such as with cyclohexene oxide (4p), carbon monoxide insertion did not occur and the starting material remained intact.This dramatic reactivity dependence on substitution patterns allowed for chemoselective carbonylation (Scheme 4).The di-epoxide (4q) has two distinctive epoxides: one has a single substitution and the other epoxide is internal and di-substituted.Catalyst 3a successfully discriminated the difference and only the terminal epoxide was converted to the corresponding β-lactone (5q).The other di-epoxide 4r also exhibited the same terminal selectivity to produce 5r with both epoxide and β-lactone.The use of these products in selective polymerization is currently under investigation. 1 Unless otherwise indicated, the reaction was carried out for 16 h. 2Single experiment was performed on each entry. 3Yields were determined by 1 H NMR. 4 TON (turnover number) = Moles of product formed/moles of catalyst.
The direct carbonylation of epoxides provided a series of β-lactones with high efficiencies (Table 2).A substituent on the terminal epoxides did not hamper the carbonylation performance of 3a.Variations in the chain length (5a-c) and size (5d and e) of substituents showed no influence and quantitative β-lactones were obtained.Interestingly, the aromatic substituent exhibited positional sensitivity.Styrene oxide, surprisingly, did not react with catalyst 3a at all (5f); however, adding a methylene bridge restored its reactivity (5g).Various kinds of coordinating functional groups were introduced that could behave as potential catalyst inhibitors.Epoxides with alkene (5h), ether (5i), and nitrile (5j) groups were smoothly converted to the corresponding β-lactones with a slight increase in catalyst loadings.Moreover, ester and amide functionalities were well tolerated (5k-m).Dual carbonylations of di-epoxide were also successful (5n).
Next, sterically more congested epoxides with 1,1-and 1,2-substitution were investigated.1,2-Epoxy-2-methylpropane (4o) gave only 27% of NMR yield under a high amount of catalyst loading.In the case of 1,2-substitution, such as with cyclohexene oxide (4p), carbon monoxide insertion did not occur and the starting material remained intact.This dramatic reactivity dependence on substitution patterns allowed for chemoselective carbonylation (Scheme 3).The di-epoxide (4q) has two distinctive epoxides: one has a single substitution and the other epoxide is internal and disubstituted.Catalyst 3a successfully discriminated the difference and only the terminal epoxide was converted to the corresponding β-lactone (5q).The other di-epoxide 4r also exhibited the same terminal selectivity to produce 5r with both epoxide and β-lactone.The use of these products in selective polymerization is currently under investigation.
1 Single experiment was performed on each entry. 2 Yields were determined by 1 H NMR. Numbers in parentheses are isolation yields. 3

General Considerations.
All manipulations for air and moisture sensitive compounds were carried out under dry nitrogen using either a glove box (MBRAUN UNIlab, Garching, Germany) or standard Schlenk line Scheme 4. Chemoselective carbonylations of di-epoxides.

Catalyst Synthesis
General procedure for synthesis of 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin dichlorotin(IV) (2a): 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin (1.00 g, 1.33 mmol) synthesized from pyrrole and 4-chlorobenzaldehyde by a reported procedure was dissolved in 200 mL of pyridine, and tin(II) chloride dihydrate (0.805 g, 3.57 mmol) was added, after which the mixture was refluxed for 3 h.The product was precipitated by adding excess water.The precipitate was filter-washed sequentially with water, 1.0 M hydrochloric acid solution, and then again with water.The collected product was dried to give 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin dichlorotin(IV) 2a (1.00 g, 80%) as a purple solid.An X-ray quality crystal was obtained from THF/benzene. 1   (20 mL) in a nitrogen atmosphere.The solution was stirred at room temperature, wrapped with aluminum foil for 48 h, and then allowed to settle.The resulting mixture was filtered and the solid was washed with additional THF.The filtrate was concentrated to 7-8 mL and was layered with hexane.The solid was filtered and washed with hexane and vacuum dried.Purple-greenish solid was obtained (0.18 g, 64%). Similarly

General Procedure for the Carbonylation of Epoxides
In a nitrogen glove box at room temperature, the appropriate amount of catalyst (3a) was added into a stainless-steel reactor.The solvent THF was added, followed by the addition of epoxide.The reactor was sealed properly and removed from the glove box.Then, the reactor was immediately pressurized to 50 atm pressure and stirred at 90 • C in a preheated heat bath.After the indicated time, the reactor was cooled at room temperature and with ice to a temperature of 0 • C. The excess CO gas was slowly vented.The lactone formed was determined by 1 H NMR spectroscopy.The mixture was filtered through a short column packed with silica gel using diethyl ether.The lactones were further purified by flash column chromatography using hexane/ethyl acetate (4/1) as the eluent.