Pat Dussault, University of Nebraska-Lincoln

[email protected] August 1, 201

Introduction and leading references The following provides a brief introduction to the application of ozonolysis within academic labs. Ozonolysis remains among the most frequently used of methods for oxidative cleavage of alkenes. While best known as a means by which to introduce aldehydes and ketones, ozonolysis can also be used to generate other functional groups. An overview of some of the most common ozonolysis transformations illustrated in Figure 1; the reader is directed to a recent review for more information and leading references. 1 A number of useful technical documents are available online. ,2

Ozone reacts with most alkenes to form short-lived 1,2,3-trioxolanes, aka "primary ozonides." 1,3 These typically undergo very rapid cycloreversion to generate a pair consisting of an aldehyde or ketone and a short-lived carbonyl oxide. 4 The carbonyl oxide generally undergoes one of two reactions: 1) cycloaddition with a carbonyl to furnish "secondary" ozonides (favored under aprotic conditions); or 2) addition of a nucleophile, most often an unhindered alcohol to form hydroperoxyacetals and related addition products.1,10 Exceptions to this pattern of reactivity are often encountered with enones, allylic alcohols, and silyl/stannyl alkenes. 1,3,5 Caution: Ozonides and hydroperoxyacetals are typically decomposed to stable products without isolation (see "Workup" section). Although these peroxidic intermediates are frequently isolable, they are capable of self-accelerating decomposition reactions and must handled with care. The reader is directed to our web-published guide to handling of peroxides.6

Substrate reactivity: Electron density is the most significant predictor of rate, with bimolecular rate constants increasing by up to 105 in moving from electron-poor substrates such as acrylates to enol ethers and highly substituted alkenes.1 However, despite this, selective cleavage within polyunsaturated systems or in the presence of an electron-rich group can be challenging.1,7 Selectivity can be enhanced by optimizing reagent distribution (gas delivery device, rapid stirring, reaction dilution), by use of a Sudan dye as an ozone indicator (see below), and/or by reaction in the presence of small amounts of pyridine. 8 Typical Reaction Conditions: Small-scale reactions often employ a pipette or narrow tube to deliver a gas solution of O3/O2 onto or into chilled and stirred reaction solutions. The more effective dispersion made possible by a gas frit or similar device can be useful even on small scale and becomes essential on larger scales. Ozonolysis within flow reactors has also been reported.1 Reactions are most often conducted in dichloromethane (when ozonides are desired) or mixtures of dichloromethane/methanol (to form intermediate hydroperoxyacetals); see reference 1 for discussions of reactions in other solvents. Although most procedures describe reactions in a -78 °C bath, most reactions can be run uneventfully up to 0 °C. Warning: Solution ozonolyses should never be attempted at temperatures less than -95 °C as condensation (-112 °C) or freezing (-193 °C) of ozone in the presence of organics almost guarantees an explosion.

Monitoring: A control run against a known substrate will often allow one to establish the amount of O3 delivered per unit time under a given set of conditions. The pale blue of solubilized ozone (Fig. 3B) has been often used as an indicator for reaction completion. However, this tinting is only observed at low temperatures and for some solvents. Ozonesensitive Sudan dyes (compare 3C to 3D below) offer a more reliable indicator of endpoint and can be particularly valuable for selective consumption of an alkene in the presence of another reactive group. 7, 9

Work-up (see Figure 1). Most ozonolysis reactions are followed by a work-up reaction that destroys the ozonide or hydroperoxyacetal intermediates. A brief summary follows. For more detailed information, the reader is directed to leading references:1,10 Reduction to ketones and aldehydes: Selective reduction to generate aldehydes and ketones has been accomplished with a variety of reagents. 11 Hydroperoxyacetals are far more reactive than ozonides, and reduction is often accomplished with triphenylphosphine,12 thiourea, 13 dimethyl sulfide.14 Ozonides can sometimes be reduced using these same reagents but reactions are much slower, and removal of the ozonide should be verified (see "Monitoring") before solutions are concentrated.15, Ozonides are nearly always susceptible to rapid reduction by zinc/acetic acid or similar "dissolving metal" systems. 15, 16 Reduction of ozonides or hydroperoxyacetals to alcohols is possible with many common metal hydrides, most often NaBH4. 1, 17 "Reduction" via base-promoted fragmentation: Ozonides derived from terminal and 1,1- disubstituted alkenes undergo base-promoted fragmentation under mild conditions (e.g., Et3N) to directly furnish aldehydes or ketones.18 Reductive ozonolysis (no work-up): Aprotic ozonolyses conducted in the presence of Noxides or pyridine directly output anhydrous solutions of ketones and aldehydes without formation of peroxide intermediates; 19,20, the products can be directly applied as substrates for C-C bond-forming processes.21 Ozonolysis in the presence of solubilized water also provides moderate to good yields of ketones and aldehydes. 22 Oxidation: The direct conversion of terminal and 1,2-disubstituted alkenes to carboxylic acids, sometimes described as "oxidative ozonolysis", can through treatment of initial ozonolysis products with hydrogen peroxide or other oxidants.1,10,23 Heterolytic fragmentations and rearrangements:1,24 Acylation or sulfonation of hydroperoxyacetals containing an adjacent C-H results in dehydration to esters. 25 

Hydroperoxyacetals lacking an adjacent C-H can undergo C-to-O skeletal rearrangements upon acylation (Criegee rearrangement). 26 Homolytic fragmentation: Hydroperoxyacetals and ozonides are both decomposed by Iron salts (typically Fe+2 but sometimes Fe+3) to give "c-1" products through decarbonylation of an initially generated alkoxyl radical. 1, 27 Depending upon conditions, chlorides, bromides, alkenes, or dimeric products can be generated. Safety issues. Prior to performing an ozonolysis, experimenters must consider the reactivity and toxicity of ozone, the exothermicity of the reaction, and the potential of the intermediates and products to undergo self-accelerating and dangerously exothermic decomposition reactions. 28 Ozone is highly toxic and negative effects have been associated with long-term exposure to levels of 80 ppb; concentrations in the low ppm range are considered immediately dangerous to life and health. 29, As a consequence, most chemistry involving ozone is conducted in an exhaust cabinet or with reaction gases vented through some form of scrubber. Fortunately, most individuals can detect the odor of O3, sometimes described as "sharp" or "clean", at a concentration of 20 ppb. 30 An earlier section of this document warns against conditions that would result in condensation of liquid or solid ozone. Ozonolyses, which are often highly exothermic reactions conducted in organic solvents under an oxygen-rich atmosphere, are inherently hazardous. 28,31 The danger of fire has been addressed by removal of headspace oxygen,32 reaction in flow/microchannel systems,33 or reaction in nonflammable media. 34 For reactions on a small scale and conducted in low boiling solvents (e.g. CH2Cl2 or CH3OH/CH2Cl2) generated heat is typically is cancelled out by rapid evaporative cooling. However, reactions at scale and/or in higher-boiling solvents must be careful to avoid temperature rise and the associated self-accelerating decomposition of peroxide intermediates. 1135 As discussed above, it is imperative to avoid formation of liquid or solid ozone.  

The potential of the peroxide intermediates or products to undergo self-accelerating decomposition,28, 36combined with the exothermicity of typical work-ups,31, 37 makes thermal analysis of reactions and intermediates essential for reactions at even medium scales. For example, differential scanning calorimetry (DSC) has found ?H for decomposition of a terminal ozonide to be 70-80 kcal/mol, with maximum heat release occurring around 131 °C.38 The use of tools to predict the latent energy of unstable organic compounds has been described. 37 The undesired isolation of difficult-to-reduce ozonides be mitigated by reaction inn the presence of methanol to steer reactions towards formation of easily reduced hydroperoxyacetals, 15b, 39 or by use of a reductive" ozonolyses (described earlier).

1 Fisher, T.; Dussault, P. H. "Alkene Ozonolysis" Tetrahedron, 2017, 73, 4233; https://doi.org/10.1016/j.tet.2017.03.039.

2 a. http://www.ozonetech.com/about-ozone; b. http://www.lenntech.com/library/ozone/ozoneintroduction.htm; c. http://www.ozonesolutions.com/information/ozone-basics. (All accessed August 2018)

3 Bailey, P. S. Ozonation in Organic Chemistry, Volume 1: Olefinic Compounds, Academic Press, New York, 1978;

4 Bunnelle, W. H. “Preparation, properties, and reactions of carbonyl oxides” Chem. Rev. 1991, 91, 335.

5 Zvereva, T. I.; Kasradze, V. G.; Kazakova, O. B.; Kukovinets, O. S. "Ozonolysis of unsaturated carbonyl compounds and alcohols" Russ. J. Org. Chem. 2010, 46, 1431; doi: 10.1134/S1070428010100015

6 "Working with peroxides in the academic lab" Dussault, P. 2018. http://digitalcommons.unl.edu/chemistryperoxides/ (Accessed August 2018)

7 a. Evans, D. A.; Kværnø, L.; Mulder, J. A.; Raymer, Dunn, T. B.; Beauchemin, A.; Olhava, E. J.; Juhl, M.; Kagechika, K. “Total synthesis of (+)-azaspiracid-1. Part I: synthesis of the fully elaborated ABCD aldehyde” Angew. Chem. Int. Ed. 2007, 46, 4693, doi:10.1002/anie.200701515; b. Ireland, R. E.; Thaisrivongs, S.; Dussault, P. H. “An approach to the total synthesis of aplysiatoxin” J. Am. Chem. Soc. 1988, 110, 5768, doi: 10.1021/ja00225a031.

8 Slomp, G.; Johnson, J. L. “Ozonolysis. II. The effect of pyridine on the ozonolysis of 4,22- stigmastadien-3-one” J. Am. Chem Soc. 1958, 80, 915; doi: 10.1021/ja01537a041.

9 Veysoglu,. T.; Mitscher, L. A.; Swayze, J. K. “A convenient method for the control of selective ozonizations of olefins” Synthesis 1980, 807; DOI: 10.1055/s-1980-29214.

10 Ishmuratov, G. Y.; Legostaeva, Y. V.; Botsman, L. P.; Tolstikov, G. A. "Transformations of peroxide products of olefin ozonolysis" Russ. J. Org. Chem. 2010, 46, 1593; doi:10.1134/S1070428010110011

11 Kropf, H., in Houben Weyl Method der Organischen Chemie, v. E13, pt 2 (Organische Peroxo Verbindungen); Kropf, H., ed. George Thieme Verlag, Stuttgart, 1988, pp 1108-1114.

12 a. Griesbaum, K.; Kiesel, G. " Ozonolysen von 1-Methylcyclopenten, 1-Methylcyclobuten, 5- Hexen-2-on und Naturkautschuk im Beisein von Methanol." Chem. Ber. 1989, 122,145, doi: 10.1002/cber.19891220123.

13 Gupta, D.; Soman, R.; Dev, S. “Thiourea, a convenient reagent for the reductive cleavage of olefin ozonolysis products” Tetrahedron, 1982, 38, 3013; https://doi.org/10.1016/0040- 4020(82)80187-7.

14 Pappas, J. J.; Keaveney, W. P.; Gancher, E.; Berger, M. “A New And Convenient Method For Converting Olefins To Aldehydes” Tetrahedron Lett. 1966, 4273, https://doi.org/10.1016/S0040- 4039(00)76049-7.

15 a. Chen, L.; Wiemer, D. F. "Synthesis of a Carbon Analogue of N-Acetylmannosamine via Acetolysis on a Relatively Stable Ozonide." J. Org. Chem. 2002, 67, 7561, doi: 10.1021/jo020362k; b. Lavallee, P.; Bouthillier, G., "Efficient Conversion of (1R,5R)-( +)-aPinene to (1S,5R)-(-)-Nopinone" J. Org. Chem. 1986, 51, 1365; see footnote 27; doi: 10.1021/jo00358a041.

16 Dai, P.; Dussault, P. H.; Trullinger, T. K. “Magnesium/Methanol: An Effective Reducing Agent for Peroxides” J. Org. Chem., 2004, 69, 2851; DOI: 10.1021/jo00358a041.

17 a. Sousa, J. A.; Bluhm, A. L. "The reductive cleavage of ozonides to alcohols" J. Org. Chem. 1959, 25, 108; doi: 10.1021/jo01071a031; b. B-H reductants: Russell, A. T., Science of Synthesis, (2008) 36, 223; doi: 10.1055/sos-SD-036-00146.

18 Hon, Y.S., Lin, S.W., Lu, L.; Chen, Y.J. "The mechanistic study and synthetic applications of the base treatment in the ozonolytic reactions." Tetrahedron 1995, 51, 5019, https://doi.org/10.1016/0040-4020(95)98699-I.

19 Willand-Charnley, R.; Fisher, T.; Johnson, B.; Dussault, P. H. “Pyridine as an organocatalyst for the reductive ozonolysis of alkenes” Org. Lett.2012, 14, 2242. 10.1021/ol300617r.

20 Schwartz, C.; Raible, J., Mott, K.; Dussault, P. H. " 'Reductive Ozonolysis' via a new fragmentation of carbonyl oxides” Tetrahedron, 2006, 62, 10747; https://doi.org/10.1016/j.tet.2006.08.092

21 Willand-Charnley, R. ; Dussault, P.H. "Tandem C-C Bond- Forming Reactions Involving Reductive Ozonolysis." J. Org. Chem. 2013: 78, 42; doi: 10.1021/jo3015775 

22 “Ozonolysis in solvent/water mixtures; direct conversion of alkenes to aldehydes and ketones” Schiaffo, C. E., Dussault, P. H. J. Org. Chem. 2008, 73, 4688 – 4690. 10.1021/jo800323x

23 Examples of oxidative ozonolysis: a. Bailey, P. “Adipic acid by ozonolysis of cyclohexene” Ind. Eng. Chem., 1958, 50, 993–996; b. Pilar?ik, T.; Havli?ek, J.; Haji?ek, J. Tetrahedron Lett. 2005, 46, 7909; doi: 10.1016/j.tetlet.2005.09.098; c. Rodríguez R., G. H. Biellmann, J-F. J. Org. Chem., 1996, 61, 1822; doi: 10.1021/jo951063g

24 Yaremenko, I. A.; Vil’, V. A.; Demchuk, D. V.; Terent’ev, A. O. "Rearrangements of peroxides and related processes" Beilstein J. Org. Chem. 2016, 12, 1647; doi: 10.3762/bjoc.12.162.

25 Fisher, T. J.; Dussault, P. H. “Fragmentation of chloroperoxides: hypochlorite-mediated dehydration of hydroperoxyacetals to esters” Tetrahedron Letters, 2010, 51, 5615.

26 Schreiber, S. L.; Liew, W.-F. “Criegee rearrangement of α-alkoxy hydroperoxides. A synthesis of esters and lactones that complements the Baeyer-Villiger oxidation of ketones” Tetrahedron Lett. 1983, 24, 2363; https://doi.org/10.1016/S0040-4039(00)81926-7.

27 Cardinale, G.; Grimmelikhuysen, J. C.; Laan, J. A. M.; Van Lier, F. P.; Van der Steen, D.; Ward, J. P. “Reaction of alkoxy hydroperoxides with metal salts. Alkyl halide preparation” Tetrahedron, 1989, 45, 5971, and references within; https://doi.org/10.1016/S0040- 4020(01)89123-7.

28 Kula, J. Chem. Health Saf. 1999, 6, 21; doi: 10.1016/S1074-9098(99)00046-5

29 The OSHA-determined PEL (permissible exposure limit) is 100 ppb (8 h) or 300 ppb (15 min, The recommended Maximum Acceptable Concentration for exposure is 0.06 ppm (over 40 hr). Concentrations ≥ 4 ppm are considered immediately dangerous to health and health; "Ozone" in Kirk-Othmer Encyclopedia of Chemical Technology, 5th ed., Wiley: Hoboken, 2005; v. 17, 768. 30 http://www.ozonesolutions.com/info/ozone-safety (accessed August 2018)

31 Reaction enthaplies of 100-125 kcal/mole have been measured: Ragan, J. A.; am Ende, D. J.; Brenek, S. J.; Eisenbeis, S. A.; Singer, R. A.; Tickner, D. L.; Teixeira, J. J. Jr.; Vanderplas, B. C.; Weston. N. "Safe Execution of a Large-Scale Ozonolysis: Preparation of the Bisulfite Adduct of 2-Hydroxyindan-2-carboxaldehyde and Its Utility in a Reductive Amination" Org. Proc. Res. Dev. 2003, 7, 155-160. DOI: 10.1021/op0202235.

32 Zimmermann, C.; Seebauer, F.; Werenka, C.; Mayerhofer, J.; Schnellendorfer, M.; Wieltsch, U. “Process for the safe ozonolysis of organic compounds in flammable solvents” PCT Int. ppl. (2008), WO 2008077769 A1 20080703.

33 a) For an overview of this area, see Roydhouse, M. D.; Motherwell, W. B.; Constantinou, A.; Gavriilidis, A.; Wheeler, R.; Down, K.; Campbell, I. "Ozonolysis of some complex organic substrates in flow" RSC Adv., 2013, 3, 5076-5082, and references within.

34 a. Van Doorslaer, C.; Peeters, A., Mertens, P.; Vinckier, C.; Binnemans, K.; De Vos, D. “Oxidation of cyclic acetals by ozone in ionic liquid media” Chem. Commun., 2009, 6439–6441; b. Lundin, M. D.; Danby, A. M.; Akien, G. R; Binder, T. P.; Busch, D. H.; Subramaniam, B. ACS Sustainable Chem. Eng. 2015, 3, 3307; doi: 10.1021/acssuschemeng.5b00913.

35 Allian, A. D.; Richter, S. M.; Kallemeyn, J. M.; Robbins, T. A.; Kishore, V. “The Development of Continuous Process for Alkene Ozonolysis Based on Combined in Situ FTIR, Calorimetry, and Computational Chemistry” Org. Process Res. Dev. 2011, 15, 91–97; 

36 Castrantas, H. M.; Banerjee, D. K.; Noller, D. C. Fire and Explosion Hazards of Peroxy Compounds, ASTM STP 394; American Society for Testing and Materials: Philadelphia, PA, 1965. Castrantas, H. M.; Banerjee, D. K. Laboratory Handling and Storage of Peroxy Compounds, ASTM STP 471; American Society for Testing and Materials: Philadelphia, PA, 1970; doi: 10.1520/STP48361S.

37 Hida, T.; Kikuchi, J.; Kakinuma, M.; Nogusa, H. Org. Proc. Res. Dev. 2010, 14, 1485; doi: 10.1021/op100065d.

38 Cataldo, F. "Thermal stability, decomposition enthalpy, and Raman spectroscopy of 1-alkene secondary ozonides." Tetrahedron Lett. 2015, 56, 994; doi: 10.1016/j.tetlet.2015.01.056.

39 Steinfeldt, N.; Bentrup, U.; Jähnisch, K. Ind. Eng. Chem. Res. 2010, 49, 72; doi: 10.1021/ie900726s; DOI: 10.1021/ie900726s.