Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Silver-Catalyzed Carboxylative Cyclization of Primary Propargyl Alcohols with CO2 Saumya Dabral,†,∥ Bilguun Bayarmagnai,†,∥ Marko Hermsen,‡ Jasmin Schießl,§ Verena Mormul,‡ A. Stephen K. Hashmi,†,§ and Thomas Schaub*,†,‡ †
Catalysis Research Laboratory (CaRLa), Im Neuenheimer Feld 584, Heidelberg 69120, Germany BASF SE, Carl-Bosch-Str. 38, Ludwigshafen 67056, Germany § Institute of Organic Chemistry, Heidelberg University, Im Neuenheimer Feld 270, Heidelberg 69120, Germany Downloaded via MACQUARIE UNIV on February 21, 2019 at 01:25:02 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: By using silver complexes with bulky ligands such as DavePhos or N-heterocyclic carbenes, propargylic alcohols are smoothly converted with CO2 into a unique class of unexplored cyclic alkylidene carbonates. These systems, for the first time, also achieve the direct carboxylative cyclization of primary propargylic alcohols. The silver-DavePhos catalyst is further applied for the biscarboxylative cyclization of primary propargyl derivatives, thereby providing an effective route to a series of previously inaccessible and industrially relevant αalkylidene cyclic carbonates.
E
Scheme 1. Synthetic Routes to 4-Methylene-1,3-dioxolan-2one (2)
xovinylene carbonates (EVCs) are an important class of cyclic carbonates that find broad applications as biological compounds, pharmaceuticals, and agrochemicals.1 They are attractive chemical intermediates or precursors in organic synthesis.2 The five-membered secondary and tertiary EVCs also form polymerizable building blocks for the synthesis of poly(β-hydroxyurethane)s and poly(carbonates)s.1,3 Substantial efforts have been made to add CO2 to propargylic alcohols, to produce the corresponding cyclic carbonates with exocyclic double bonds.4,5 Despite a broad series of catalytic systems being reported over the recent years,6−12 their scope is still limited to easily cyclizable, secondary9 and tertiary6−12 propargylic alcohols. To the best of our knowledge, only one example is known, where a primary propargylic alcohol was cyclized with CO2 to the corresponding exovinylene carbonate, but in only 8% yield.13 Further insights into the mechanism of these carboxylative cyclizations were reported by Inoue and Yamada.4,10 The synthesis of unsubstituted EVCs is very attractive, owing to their desirable properties in material science. For example 4-methylene-1,3-dioxolan-2-one (2) (Scheme 1) has already been investigated as an additive that improves the high temperature performance of lithium ion batteries.11 While the synthesis of the iodomethylene derivative of 2 is reported using stoichiometric amounts of tBuOI,14 unfortunately, due to the reduced reactivity displayed by the unsubstituted propargylic alcohols,9,12 until now there exist only three methods to access 2 in isolable yields (Scheme 1, right). The first synthesis published by Trost et al. (1983) is based on the ozone mediated elimination reaction of 4-(phenylselenenyl)-methyl© XXXX American Chemical Society
1,3-dioxolan-2-one, (Scheme 1, right top).15 The other two routes, reported by Gagosz et al. (2006)16 and Nishizawa et al. (2006),17 follow a gold or an undesirable mercury catalyzed rearrangement of propargylic tert-butyl carbonate respectively (Scheme 1, right, middle and bottom). These approaches are neither very atom-economic nor sustainable: First propargylic tert-butyl carbonate is preformed in an additional step starting from alkynyl bromides and di-tert- butyl dicarbonate, followed by the loss of the tert-butyl group as isobutene in the successive cyclization reaction. These relatively expensive routes have so far prevented the industrial use of 2, despite its interesting properties. Received: January 14, 2019
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DOI: 10.1021/acs.orglett.9b00156 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters To develop a simpler route to the α-unsubstituted class of exovinylene carbonates, we investigated their synthesis directly from primary propargylic alcohols and CO2 (see Scheme 1, left). Simultaneously, we were also interested in the synthesis of unique and easily accessible bis-exovinylene carbonates which could potentially serve as monomers for materials. Propargyl alcohol (1) and its derivatives such as 1,4-butynediol are produced industrially on a multithousand-ton scale by reacting acetylene with formaldehyde.18 Assuming that the cyclization of a α-substituted secondary or tertiary propargyl alcohol was accelerated as a result of the Thrope−Ingold effect,19 we were convinced that a bulky donor ligand in combination with silver(I) salt (vide infra) could induce a similar angle compression effect together with a higher population of the conformer suitable for cyclization to initiate the successful carboxylative cyclization of the primary propargylic alcohols. Based on this assumption, we tested the so-called Buchwaldphosphine-ligands in combination with AgOAc and obtained the desired product 2 in satisfying yields of 60−75%. Linear carbonate 3 was formed as the only byproduct, due to the ring opening reaction of 2 with the unreacted alcohol 1 (Table 1,
importance of the basicity of the counterion. For these experiments, we could also show the principal recyclability of the Ag-DavePhos catalyst (see section 2.5 of the Supporting Information). The scope of this reaction was investigated using MeCN as the common solvent for better substrate solubility. As determined by the NOE experiments, the internal propargylic alcohols exclusively afforded the Z-alkylidene cyclic carbonates (Scheme 2). Furthermore, according to the DFT calculations, the thermodynamic preference for these Z alkylidene carbonates was found to be low (only 0.1/0.6 kJ/mol for products 4/5; see Supporting Information), which suggests that the Z isomers are kinetically favored products. Both the Ag-DavePhos and [(IPr)AgOAc] catalytic systems afforded EVC 4 in comparable 1H NMR yields of 78% (isolated yield: 65%) and 70% respectively. Additionally, a range of monosubstituted alkyl and aryl derivatives were synthesized by taking an excess amount of 1,4-butynediol and hence surpassing the second −OH group protection/deprotection step (see section 3 of the Supporting Information). As the DavePhos ligand, owing to its commercial availability, is more easy to access than the IPrAgOAc catalyst,21 the rest of the substrate scope was investigated using the Ag-DavePhos catalytic system. With the system in hand we were able to synthesize a series of new exovinylenecarbonates, based on the bulk-chemical 1,4butyndiol (intermediate in the industrial production of tetrahydrofuran) as compounds 4−9, 14, and 15. The functional and/or protecting groups such as acetyl, carbonate, benzyl, methacrylic, and O-carbamoyl were well tolerated and afforded compounds 5−9 in moderate to high yields. This methodology was also effective in converting the α-substituted alcohols into their respective carbonate products in very good yields (Scheme 2, compounds 10−13). To our delight, extending the methodology to substrates having two primary reactive sites such as 1,4-dibenzyl, toluene diisocyanate (TDI), and triethylene glycol derivatives resulted in their smooth transformation and afforded the corresponding bis-carboxylated cyclic carbonates also in excellent yields (Scheme 2, compounds 14−16). Surprisingly neither 3-methyl and 3phenyl substituted propargyl alcohols nor pent-3-yn-1-ol showed reactivity toward the formation of 17, 18, and 19 under the given reaction conditions (see Supporting Information, part 8 for details). Since the exocyclic olefinic groups of the new difunctional EVCs form potential monomers via selective ring-opening reactions by various bifunctional nucleophiles such as diamines or diols,3c preliminary studies were focused in this direction. Although not optimized, pleasantly, the treatment of the benzyl protected EVC 7 with an alcohol (EtOH in combination with catalytic amounts of DBU) and a secondary amine (pyrrolidine) afforded the linear products 17 and 18 in very high yields of 95% and 89%, respectively (see Supporting Information section 9). These results are indicative for the future applicability of the above formed EVCs (Scheme 2), which are initially based on the cheap bulk chemical 1,4butynediol. To gain a deeper mechanistic understanding, the reaction was followed by 31P NMR. A mixture of propargyl alcohol 1, AgOAc, DavePhos, and CO2 in CDCl3 was found to contain the [(DavePhos)Ag(OAc)] complex (peaks 16 and 25 ppm). When the preformed [(DavePhos)Ag(OAc)] complex22 was treated with propargyl alcohol 1 no change could be observed
Table 1. Optimization of Reaction Conditionsa
entry
[M]
ligand
2 [%]b
1 2 3 4c 5d 6e 7 8 9 10 11
AgOAc AgOAc AgOAc AgOAc AgOAc IPrAgOAc AgF AgBF4 AgNO3 AgTFA Ag2CO3
JohnPhos MePhos DavePhos DavePhos DavePhos − DavePhos DavePhos DavePhos DavePhos DavePhos
60 70 75 95 95 (86) 99 93 0 20 18 72
a
Propargyl alcohol 1 (5 mmol), Ag (1) salt (10 mol %), and ligand (10 mol %) in anhydrous DCM (2 mL) were pressurized with CO2 (8 bar) and stirred for 16 h. bYields were determined by 1H NMR spectroscopy using anisole as an internal standard. Isolated yields in parentheses. c5 mol % catalyst loading in 5 mL of DCM. d1 mol % catalyst loading. e1 (0.35 mmol), IPrAgOAc (1 mol %), CD3CN (0.7 mL), 20 bar of CO2.
entries 1−3). Moreover, decreasing the catalyst loading from 10 mol % to 5 mol % and increasing the solvent amounts from 2 to 5 mL resulted in an overall increase in the yield of 2 to 95% (Table 1, entry 4). Moderate to high yields of EVC 2 were also achieved with other DavePhos-like phosphine and bulky N-heterocylic carbene ligands.20 The amount of silver (Ag) and DavePhos ligand could be further decreased to 1 mol % without a loss in the yield (Table 1, entry 5). Excellent selectivity for 2 was also achieved with the isolated NHC− Ag(I) complex, [(IPr)AgOAc], at 1 mol % catalyst loading21 (Table 1, entry 6). Among the Ag(I) sources tested (Table 1, entries 7−11), only AgF (pKa of the conjugate acid = 3.2) gave yields comparable to those with AgOAc (pKa of the conjugate acid = 4.7) (Table 1, entry 7), thereby highlighting the B
DOI: 10.1021/acs.orglett.9b00156 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 2. Scope of the Cyclizative Carboxylation of Propargyl Alcoholsa
a
AgOAc (2 mol %) and DavePhos (2 mol %). Yields reported after column chromatography.
by 31P NMR: This indicates that no acetate−propargyl alcoholate ligand exchange took place. The reaction was further studied by in situ IR spectroscopy. New peaks at 1836 cm−1 (ν(CO) stretching mode) and at 1695 cm−1 (ν(C C) stretching mode) were observed which are in line with the formation of 2 (see section 7 of the Supporting Information). Within 3 h the intensity of the peaks no longer increased, and indeed the 1H NMR confirmed the full conversion of the reaction. We further studied the mechanism of the carboxylative cyclization of 1 by DFT (see Scheme 3). The silver(I)
replaces the acetate on complex A to form intermediate B and acetic acid. The intermediate B is lower-lying compared to the hemiester of the propargyl alcohol and is relatively more stable by 22.9 kJ/mol. As mentioned earlier (see Table 1), this step is only possible for certain counterions. For silver salts of acids with a lower pKa, the deprotonation of the hydrogen carbonate is hindered and lower yields are observed. Furthermore, B can rearrange to the alkyne coordinating complex C which has side-on coordination of the carbonate and is endergonic by 36.0 kJ/mol with respect to A and CO2/propargyl alcohol. Intermediate C is the structural precursor which undergoes cyclization, via TS-D or TS-E, leading to either the E or Z vinyl silver isomer. The latter is preferred by 1.2 kJ/mol, which is in agreement with our experimental results, where only the Z isomer is obtained when using internal alkynes (see Scheme 3). Note that the correct diastereoselectivity is only obtained when including solvent effects of acetonitrile within the COSMO solvation model (see Supporting Information). Lastly, complexes F/G irreversibly release the vinylene carbonate upon proto-demetalation through acetic acid to regenerate the initial complex A. This step is exergonic by 66.1 kJ/mol. In conclusion, we established a system that allows, for the first time, the straightforward synthesis of a wide range of unsubstituted mono- and di-α-alkylidene cyclic carbonates from their corresponding alkynols. The reaction conditions are mild and, at room temperature, afford the desired products in 65−95% yield. The broad substrate scope can also be extended to the unsubstituted α-alkylidene cyclic carbonates. The low catalyst loading and recyclability of the catalyst system add to the key features of this new methodology and are further supported by DFT calculations. The development for ringopening polymerization reaction of the newly formed αalkylidene cyclic carbonates by various nucleophiles is currently underway in our laboratories.
Scheme 3. Catalytic Pathway of Carboxylative Cyclization of Propargyl Alcohol with CO2 Using Ag(I)-DavePhosa
a
All energies indicated are free enthalpies in kJ/mol.
DavePhos acetate complex A was taken as the starting point for the calculations. A variant of this reaction with ZnI2/NEt3 has been extensively investigated by Ma et al.23 The selectivity between the five- and six-membered carbonates and the effect of solvents as explained by them was also researched by Yamada et al. for the silver-catalyzed reactions with DBU as the base.5 According to our calculations, the formation of propargyl hydrogen carbonate from CO2/propargyl alcohol is endergonic by 53.6 kJ/mol. This is generally expected, as the organic hydrogen carbonate esters are not stable. The intermediary formed propargyl hydrogen carbonate, however,
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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.orglett.9b00156. Experimental, characterization, and computational details (PDF) C
DOI: 10.1021/acs.orglett.9b00156 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
A. Stephen K. Hashmi: 0000-0002-6720-8602 Thomas Schaub: 0000-0003-2332-0376 Author Contributions ∥
S.D. and B.B. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS CaRLa (Catalysis Research Laboratory) is cofinanced by the Heidelberg University and BASF SE. REFERENCES
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