CarbonylâOlefin Metathesis Catalyzed by Molecular Iodine | ACS

Research Article Cite This: ACS Catal. 2019, 9, 912−919

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Carbonyl−Olefin Metathesis Catalyzed by Molecular Iodine Uyen P. N. Tran,† Giulia Oss,† Martin Breugst,‡ Eric Detmar,‡ Domenic P. Pace,† Kevin Liyanto,† and Thanh V. Nguyen*,† †

School of Chemistry, University of New South Wales, Sydney, New South Wales 2052, Australia Department für Chemie, Universität zu Köln, Greinstraße 4, 50939 Köln, Germany

‡

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S Supporting Information *

ABSTRACT: The carbonyl−olefin metathesis reaction could facilitate rapid functional group interconversion and allow construction of complicated organic structures. Herein, we demonstrate that elemental iodine, a very simple catalyst, can efficiently promote this chemical transformation under mild reaction conditions. Our mechanistic studies revealed intriguing aspects of the activation mode via molecular iodine and the iodonium ion that could change the previously established perception of catalyst and substrate design for the carbonyl−olefin metathesis reaction. KEYWORDS: carbonyl−olefin metathesis, olefination, iodine, iodonium, catalysis, metal-free

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INTRODUCTION The carbonyl−olefin metathesis (COM) reaction,1 an analogue of the olefin−olefin metathesis reaction,2 is considered to be very useful for direct carbon−carbon bond forming transformations and functional group interconversions. There had been a number of developments on experimental protocols3 and catalysts4 for the COM reaction. Over the last five years, it has become increasingly attractive after interesting studies from the Lambert group using hydrazine5 and the Franzén group using tritylium salts6 as organocatalysts for this reaction (Scheme 1). More recently, the Schindler group7 and the Li group8 reported elegant studies in which they utilized salts of iron(III) and gallium(III) to promote COM reactions. Earlier this year, the Tiefenbacher group also published an interesting report on the cooperative effect of the hexameric resorcinarene assembly and Brønsted acid to catalyze COM reactions.9 A recent study from our laboratory showed that the tropylium ion10 could also be used as an organocatalytic promoter for the COM reaction (Scheme 1).11 The COM reaction is, however, still much less investigated than the olefin−olefin metathesis,12 most likely due to a lack of practicability and general knowledge about rational catalyst and substrate design for the reaction. Herein, we demonstrate that elemental iodine, a very simple catalyst, can efficiently promote the COM reaction on a broad range of substrates under mild conditions with excellent outcomes. The inspiration for our use of iodine as a promoter for the COM reaction came from previous reports of iodine being able to activate carbonyl substrates for a range of chemical reactions via halogen-bonding interaction.13 Elemental iodine can also serve as a precatalyst for iodonium activation of alkenes14 or alkynes15 in isomerization and cyclization reactions. We predicted that iodine could activate either the carbonyl moiety or the alkene moiety or cooperatively activate both functional © XXXX American Chemical Society

groups for the carbonyl−olefin metathesis reaction. Thus, we set out to test our hypothesis by using I2 as catalyst to promote the intramolecular COM reaction of substrate 1a1 (Scheme 2).

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RESULTS AND DISCUSSION Gratifyingly, our preliminary investigation immediately showed very promising outcomes: within 24 h at 50 °C using 5 mol % of I2 as catalyst, substrate 1a1 was converted to the cyclized COM product 2a and its isomerized product 2a′ in 62% total yield (∼90% conversion).16 Subsequent optimization studies (see page S3 in the SI) revealed that the reaction could be carried out smoothly and cleanly at ambient temperature and atmosphere in solvent-free conditions using 10 mol % I2 catalyst, giving the products 2a and 2a′ in 93% total yield after purification.16 The optimal conditions were effective for a wide range of substrates (Scheme 2). Most of the products 2 were obtained in moderate to excellent yields from generally clean intramolecular COM reactions when substrates 1 bear the isopropylidene moiety (i.e., when acetone is formed as a byproduct, Scheme 2; see pages S5−S17 in the SI for the specific structures of COM precursors 1).16 Substrates with active olefin moieties bearing different substituents (such as Ph or H) also cyclized via the COM reactions, albeit with lower yields (Scheme 2, comparison of entries 1a1 to 1a2−1a3 and entries 1s1 to 1s2). This phenomenon has been previously observed with transition metal Lewis acid catalysts7a−d,8 and our own tropylium catalytic system. This new protocol can be practically carried out on gram scale with very high efficiency, as demonstrated for substrates 1a1 and 1s1 to form 1.60 g of 2a Received: September 19, 2018 Revised: December 13, 2018 Published: December 14, 2018 912

DOI: 10.1021/acscatal.8b03769 ACS Catal. 2019, 9, 912−919

Research Article

ACS Catalysis

reactions (Scheme 2; see products 2q, 2t, and 2u, 25 mol % I2 was used).16 Apart from the driving force due to the formation of acetone as a byproduct, a stabilizing effect from the conjugation of the aromatic ring to the carbonyl group is also essential for this iodine-catalyzed COM reaction, as nonaromatic substrate 1l was only converted to bicyclic product 2l in poor yield (see page S12 in the SI for more unproductive nonaromatic substrates).16 When the distance between the carbonyl and the olefin moieties was varied with different carbon chain linkers (1′, Scheme 3a), the reaction outcomes changed vastly. γ,δ-Olefin ketone (n = 1) gave the 3,4-dihydro-2H-pyran 4, presumably via a 6-endo-trig cyclization process similar to what was reported by Schindler and co-workers,17 while δ,ε-olefin ketone (n = 2) gave the normal COM products (2a and 2a′). ε,ζ-Olefin ketone (n = 3), on the other hand, did not convert to any cyclized products (see page S19 in the SI for more unproductive ε,ζ-olefin ketone substrates).16 This newly developed I2-catalyzed protocol was subsequently tested on the intermolecular COM reactions between aromatic aldehydes 5 and isopropylidenyl olefins 6 (Scheme 3b). This type of reaction was reported previously by the Franzén group and our group using tritylium6a and tropylium11 salts as catalysts, respectively. In both cases, only low to moderate efficiencies were obtained, most probably because the intermolecular COM reaction is not as entropically favored as the intramolecular reaction. Our reactions afforded 31−50% of the intermolecular COM adducts (7a−7d, Scheme 3b), which were comparable results to previous studies. 6a,11 The intermolecular ring-opening COM reactions with 1-methylcyclopentene, identified as the best substrate for this type of reaction in our previous study,11 also gave low product yields (7e and 7f). Nevertheless, these reaction outcomes are comparable with similar reactions promoted by our tropylium catalyst11 or Schindler’s gallium(III) catalyst,7e which potentially paves the way for future development in this field based on the simplicity of the iodine catalytic system. At this stage, it became very interesting to us to understand how such a simple catalytic system like iodine can promote this previously considered challenging COM reaction. In principle, there are four potential pathways18 of how this iodine-promoted reaction can proceed:13a,b,e (i) Brønsted acid-catalyzed9 pathway with HI formed in situ from I2;13e (ii) radical-initiated pathway from homolytic cleavage of I2 (by light or air);19 (iii) halogen-bonding assisted pathway with molecular iodine I−I;18a (iv) Lewis acid-catalyzed pathway with iodonium ion (I+) formed in situ by the heterolytic cleavage of I2.18a A halogenbond activation was recently proposed for the iodine-catalyzed Michael addition to α,β-unsaturated carbonyls as both Brønsted acid and iodonium ion catalysis were unlikely under the reaction conditions.18 Thus, we carried out a range of mechanistic studies to get further insights into the mode of activation by iodine (Table 1; see page S24 in the SI for more details).16 Replacement of elemental iodine with HI or KI resulted in nonproductive reactions (entries 2−3, Table 1), suggesting that iodide anion and Brønsted acid have no significant role in this reaction. Carrying the reaction with I2 catalyst in the dark (entry 4) or with a radical scavenger (BHT, see entry 8) still gave full conversions of 1a1 to the products while BHT itself has no effect on the reaction (entry 9), hinting that the radical pathway can also be ruled out. A series of reactions with I2 catalyst, where we excluded the presence of moisture and air (entry 5) or just air (entries 6−7), was met with significant reductions in the conversions of 1a1.

Scheme 1. Catalytic Carbonyl−Olefin Metathesis Reaction

and 1.51 g of 2s, respectively (Scheme 2).16 The most notable feature of this protocol is that it only required short reaction time at ambient temperature to achieve similar results to the COM reactions promoted by other catalytic systems7a−e,8,9,11 at harsher conditions. As expected with the benign nature of the iodine catalyst and the mild reaction conditions, this COM protocol tolerates several types of functional groups. Most of the non-nitrogen containing substrates (1a−1p) went through the iodinecatalyzed COM reaction to give a mixture of two regioisomeric products. When the functional group at the α-position of the newly formed C−C double bond is an alkoxycarbonyl group, the expected COM products (2a−2d and 2h) were produced as major components. When this functional group is an acyl group, phenyl ring, or methyl group, the isomerized olefins were formed as the major products (2e′, 2g′, and 2o′). The formation of these isomerized products can be attributed to isomerization of the C−C double bond, generated after the COM reactions, presumably due to trace amounts of Brønsted acids present in the reaction mixture (Table 1, vide infra). The Tiefenbacher group also reported similar observations in their Brønsted acidcatalyzed COM reactions.9 These regioisomeric ratios changed with modifications of reaction conditions (Scheme 2), and we observed the isomerization (2a to 2a′; see page S31 in the SI for more details) when subjecting the pure product 2a to the same I2-catalyzed COM reaction conditions. Substrates bearing Brønsted-basic nitrogen-centers8 did not undergo this type of isomerization (Scheme 2, 2q−2v). They worked smoothly to give cyclized COM products exclusively in high to excellent yields (2q−2v), although substrates without substitution on the carbon between the carbonyl and the nitrogen centers required higher catalyst loading for efficient 913


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ACS Catalysis Scheme 2. Iodine-Catalyzed Intramolecular Carbonyl−Olefin Metathesis Reactions16

This demonstrated that air did participate in promoting the reaction, most likely through the oxidation of molecular iodine

or iodide ion to higher oxidation states. However, when we tried to recreate this oxidative environment in a controlled manner 914


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ACS Catalysis Table 1. Mechanistic Studies of the I2-Catalyzed COM Reaction with Substrate 1a116

a

Reaction conditions: substrate 1a (0.5 mmol) and catalyst/additive were stirred for 24 h at rt. bAir means the reaction was carried out without exclusion of air. Normal lab light means fumehood lighting (4 × 13 W fluorescent lamps). cNIS was recrystallized. dConversions in the parentheses were recorded after 72 h.

using m-CPBA (entry 10) or hydrogen peroxide (entry 11), they only resulted in low conversion of 1a1 to the product. Two other iodonium sources, namely NIS and ICl, were also tested as catalysts for these COM reactions (entries 12 and 15).

These reactions gave very positive outcomes, supporting the iodonium-catalyzed pathway with I2 catalyst. That it was not the succinimide component catalyzing the reaction is confirmed by entry 14 where NBS could not convert any of 1a1 to the 915


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products. Interestingly, when we added DMSO (20 mol %, 2 equiv to NIS catalyst, entry 13, Table 1), a very good iodoniumbinding solvent,20 to the reaction, it completely turned off the catalytic activity of NIS. Similar results were observed with the addition of DMSO (20 mol %) to the original I2 (10 mol %)-catalyzed reaction conditions (entry 16, compared to entry 1), which was not too surprising as DMSO can also coordinate to I2.21 The addition of potassium iodide (10 mol %), which could suppress the formation of iodonium ion (I+) and combine with molecular iodine to form triiodide (I3−), also negatively affected the reaction outcomes (entry 17). These experiments once again confirmed the possibility of the iodonium ion being the actual active catalyst in this COM reaction. Due to the presence of moisture, it is possible that some hypoiodous acid (HOI) could form from the disproportionation of the molecular iodine and play some roles in this reaction.22 However, under oxidative conditions such as in entry 11, which should favor the formation of HOI,23 the reaction outcomes were rather poor. We further examined this possibility by carrying out the COM reaction with 10 mol % I2 in the presence of 10 mol % KOH (entry 18, Table 1), which could form potassium hypoiodite (KOI) in situ. This reaction gave a messy mixture with only trace amounts of the wanted COM products, suggesting that a hypoiodous/hypoiodite pathway is unlikely. On the other hand, although we cannot completely rule out the possibility of activation by direct halogen-bonding interaction from molecular iodine based on the experimental investigations,18a entries 19−22 in Table 1 were indicative that halogenbonding activation is unlikely to be the driving force for this type of reaction. Indeed, the use of catalytic or stoichiometric amounts of pentafluorophenyl iodide (entry 19), a frequently used halogen-bond donor,24 did not result in any reaction. Similarly, using triphenylphosphine diiodine (entry 20) and

Scheme 3. (a) COM Reactions with Different Chain Lengths; (b) Intermolecular COM Reactions

Scheme 4. Gibbs Free Energy Profile for the Uncatalyzed and the Iodonium Ion-Catalyzed Carbonyl−Olefin Metathesis

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ACS Catalysis Huber’s monodentate or bidentate iodoazolium salts25 (entries 21−22), all very good halogen-bonding donors but not iodonium sources, did not lead to formation of the COM products. We furthermore carried out kinetic studies to determine the reaction order with respect to the iodine catalyst. The kinetic studies were performed by 1H NMR spectroscopy at different catalyst loadings (8 to 14 mol %), as the reaction was too slow or too fast outside this range.16 For higher iodine loadings (>10 mol %), we observed a good linear correlation with a slope of 1 in double-logarithmic plots. The data point for low catalyst loading (8 mol %) deviates slightly from the above, and if it was taken into account, a slope close to 2 was obtained (see page S26 in the SI). These outcomes were reproducible from a triplicate. On the basis of the kinetic data, it is rather unlikely that I3− or any other higher iodine aggregate is involved in the rate-limiting transition state. Arguably, the reaction order in the catalyst is around 1, and the iodine dissociation probably occurs only “partially”, e.g., through the formation of very tight ion pairs. On the basis of these mechanistic studies, we hypothesized a mechanistic pathway in which the I+ ion acted as a Lewis acid to activate the carbonyl moiety and trigger the oxetane formation event (1a1 to 10, Scheme 4). Presumably, the oxetane intermediate 10 subsequently rearranges to the cyclized product 2a similar to how it works with the other acidic catalysts.6a,7a−d,8,9,11 To further probe the reaction mechanism, we additionally employed DFT calculation at the M06-2X/aug-ccpVTZ/IEFPCM//M06-2X/6-311+G(d,p)/IEFPCM with the aug-cc-pVTZ-PP for I level to validate this proposed mechanism (Scheme 4). In line with the experimental observation that the reverse reaction of a mixture of 2a and 3a (in excess) did not proceed to form 1a1 under the same catalytic conditions (see page S31 in the SI),16 the calculations predict a substantial thermodynamic driving force for the COM reaction (Scheme 4, ΔG = −11.9 kcal mol−1). Furthermore, the computational investigations indicate that a putative halogen-bond activation by I2 requires an activation free energy of 51 kcal mol−1. Compared to the uncatalyzed reaction, this pathway is still favorable (ΔΔG‡ = 6.7 kcal mol−1) but very unlikely due to the high activation free energy (see the computational SI for more details). Our calculations on an iodonium ion pathway are summarized in Scheme 4. While our calculations predict a very endergonic splitting of molecular iodine due to unfavorable charge separation (see the computational SI for more details), experimental data suggest equilibrium constants between 1.4 × 10−5 (ΔG = +6.5 kcal mol−1) and 1.8 × 10−10 (ΔG = +13.3 kcal mol−1) for this process.26 As different solvation models (IEFPCM,27a refined SMD,27b,c and CPCM27d−g) resulted in very similar energies, this deviation is most likely caused by charge separation and only to a small extent by an insufficient solvation model. Consequently, we decided to use the latter experimental value for our calculations. As a consequence, the calculated free energy differences will constitute an upper limit for these processes and the real number might even be smaller. On the basis of our calculations, the rate-limiting transition state is the nucleophilic attack of the alkene onto the activated carbonyl (TS1, Scheme 4), i.e., the first step of the [2 + 2] cycloaddition. All subsequent transition states are significantly lower in energy. This energy profile is therefore slightly different from that obtained by Schindler and co-workers for the FeCl3catalyzed reaction where all transition states were comparable in energy.7a Our calculations furthermore indicate that the

addition of iodine to the double bond is thermodynamically unfavorable (ΔG = +6.6 kcal mol−1) and is consequently not observed under the reaction conditions. Therefore, the computational data support the picture of an iodonium ion pathway as the origin of the catalytic activity of molecular iodine in COM.

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CONCLUSION We have developed a new practical protocol for the intramolecular carbonyl−olefin metathesis reactions using elemental iodine as catalyst. This method is easy to set up with very mild reaction conditions and involves an inexpensive and benign catalyst while offering comparable outcomes to previously reported procedures. Preliminary mechanistic studies revealed the important role of the in situ generated iodonium ion, which might promote the COM reaction via the activation of the carbonyl moiety. Further applications of this new method in organic synthesis are ongoing and will be reported shortly.

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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/acscatal.8b03769. Experimental procedures, characterization data, and NMR spectra (PDF) Details of the computational investigations, comparison of the calculated energy profiles, uncatalyzed reaction, halogen-bond pathway, iodonium ion pathway, and thermodynamics of the products (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Martin Breugst: 0000-0003-0950-8858 Thanh V. Nguyen: 0000-0002-0757-9970 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by Australian Research Council (grant DE150100517 to T.V.N.). Additional support from the Fonds der chemischen Industrie (Liebig scholarship to M.B.) and the DFG (BR 5154/2-1) is gratefully acknowledged. We are grateful to the Regional Computing Center of the University of Cologne for providing computing time of the DFG-funded High Performance Computing (HPC) System CHEOPS as well as for their support.

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REFERENCES

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