Huang et al. Sustain Chem Process (2015) 3:13 DOI 10.1186/s40508-015-0040-7 RESEARCH ARTICLE Open Access Butadiene sulfone as ‘volatile’, recyclable dipolar, aprotic solvent for conducting substitution and cycloaddition reactions Yong Huang1,3†, Esteban E. Ureña‑Benavides1,3†, Afrah J. Boigny1, Zachary S. Campbell1, Fiaz S. Mohammed1,3, Jason S. Fisk4, Bruce Holden4, Charles A. Eckert1,2,3, Pamela Pollet2,3* and Charles L. Liotta1,2,3* Abstract Butadiene sulfone has been employed as a “volatile”, recyclable dipolar, aprotic solvent in the reaction of benzyl halide with metal azides to form benzyl azide (1) and the subsequent reaction of benzyl azide with p-toluenesulfonyl cyanide (3) to produce 1-benzyl-5-(p-toluenesulfonyl)tetrazole (2). Comparisons are made with the solvent DMSO and an analogous sulfolene solvent—piperylene sulfone. In addition, recycling protocols for butadiene sulfone and piperylene sulfone are also presented. Keywords: Butadiene sulfone, Piperylene sulfone, Sulfolenes, Recyclable dipolar aprotic solvent, Tetrazoles, Sustainable, Green, DMSO Background undergoes a smooth reversal at 110 °C while butadiene Dimethylsulfoxide (DMSO) is an outstanding solvent sulfone requires temperatures in the 135–140 °C range. for conducting a wide variety of organic reactions. Its In both cases the gaseous diene and SO2 can be captured specific dipolar, aprotic properties allow for the dissolu- by condensing at low temperatures (−76 °C) and react- tion of a range of organic molecules and ionic species. ing at room temperature to reform the original sulfolene Unfortunately, the isolation of reaction products and solvent. the recyclability of the solvent is often times difficult as Several reports have employed sulfolenes as primary well as economically expensive. Recently a couple of sul- solvents for conducting various organic reactions along folene solvents have been proposed as possible recycla- with the subsequent recycling of the solvent. Vinci et al. ble substitutes for DMSO [1–6]. Piperylene sulfone is a [5] reported the substitution reactions and associated liquid at room temperature and butadiene sulfone is a rates of a wide variety of nucleophiles with benzyl chlo- liquid at 64 °C. Both possess similar properties to that of ride in both DMSO and in piperylene sulfone solvent. In DMSO. Unlike DMSO, however, each of these solvents general the reactions conducted in DMSO proceeded at can undergo a thermally promoted reversible retro- faster rates than those in piperylene sulfone. It was dis- cheletropic process to form SO2 and the respective diene covered, however, that the addition of trace quantities (Fig. 1). This reversible characteristic provides a strategy of water (1–3 %) added to piperylene sulfone increased for both solvent removal from the products of reaction the rates of the nucleophilic substitution reactions. Fur- as well as solvent recovery and reuse. Piperylene sulfone thermore, the reaction of benzyl chloride with thiocy- anate ion in piperylene sulfone resulted in a 96 % isolated yield of benzyl thiocyanate upon reversal of piperylene *Correspondence: [email protected]; charles.liotta@ chemistry.gatech.edu sulfone to gaseous SO2 and piperylene. The reformation †Yong Huang and Esteban E. Ureña-Benavides contributed equally as and recovery of piperylene sulfone solvent was also dem- first-authors onstrated with 87 % efficiency [5]; a clear demonstration 3 Specialty Separations Center, Georgia Institute of Technology, Atlanta, GA 30332, USA of the sulfolene’s advantage over its DMSO counterpart. Full list of author information is available at the end of the article Ragauskas et al. [2] reported the TEMPO oxidation of © 2015 Huang et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Huang et al. Sustain Chem Process (2015) 3:13 Page 2 of 10 Matheson (Montgomeryville, PA, USA). p-Toluenesulfo- nyl cyanide (>95 %) was purchased from AK Scientific, Inc (Union City, CA, USA) and Accel Pharmtech, LLC (East Brunswick, NJ, USA). Benzyl bromide (98 %), ben- zyl chloride (98 %), cesium azide (99.99 %), sodium azide (99.5 %) and dimethyl sulfoxide (99.9 %) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Butadiene sul- fone (98 %) and all other chemicals were purchased from VWR International (Suwanee, GA, USA). All compounds were used as received. Authentic samples of benzyl azide were prepared in lab batches (see Additional file 1). Fig. 1 Thermally induced retro‑cheletropic switch enabling the recy‑ clability of piperylene sulfone (top) and butadiene sulfone (bottom). Synthesis of piperylene sulfone (PS) Piperylene sulfone melts at 12 °C, making it a liquid at room tem‑ − Piperylene sulfone was synthesized in large quantities perature while butadiene sulfone is a stable solid that melts at 64 °C (200–500 mL) from piperylene (cis- and trans- mix- ture) (200–630 mL) and sulfur dioxide (12 eq.) using 8-anilino-1-naphthalenesulfonic acid hemi-magnesium substituted benzyl alcohols to benzaldehydes in piper- salt (0.012 eq.) as a polymerization inhibitor [3, 7]. The ylene sulfone. Not only were the product yields as high inhibitor was weighed and added to an Ace-Glass 5 L as the reactions conducted in DMSO but, in addition, the glass reactor. The experimental apparatus was then turn-over frequencies (TOF) were greater. purged with N2. The reactor was filled with 2 atm of Herein is reported a reaction sequence which exempli- vapour SO2 and purged to remove N2, this process was fies the potential superiority of sulfolene solvents over repeated three times. Liquid SO2 was allowed to flow into DMSO. Specifically, the synthesis of 1-benzyl-5-(p-tol- the reactor while keeping the temperature at −30 °C or uenesulfonyl) tetrazole (2) in piperylene sulfone (PS) and less. Once, all the desired amount of SO2 was introduced, butadiene sulfone (BS) via a two-step process is reported the piperylene was added into the reactor using an air (Scheme 1). The first step involves the reaction of azide tight syringe. The reactor was then sealed and allowed to with benzyl chloride or benzyl bromide to form the cor- warm up to room temperature around 21 °C. responding benzyl azide. The second step involves the The reaction was carried for at least 15 h after which cycloaddition reaction of benzyl azide with p-toluene- the excess SO2 was vented and collected in a bubbler con- sulfonyl cyanide (TsCN, 2). Each of these reactions was taining 2.2 L of saturated potassium carbonate (K2CO3) investigated individually and in tandem in both DMSO solution, yielding an orange slurry product mixture. The and a sulfolene solvent. In addition, a detailed protocol mixture was sparged with N2 to further remove residual is presented for the recycling of the sulfolene solvents SO2. Water saturated with sodium chloride was added (piperylene and butadiene sulfones). to the reactor and the aqueous phase was extracted with dichloromethane three times. Ethyl ether (1/3, v/v) was Experimental added to the combined organic phase as an anti-solvent Materials to precipitate the inhibitor. The resulting liquid was dried Piperylene (cis- and trans- mixutres) (97 %) was pur- over MgSO4, and then filtered. A clear yellow liquid was chased from TCI America (Portland, OR, USA). Sulfur obtained after evaporating the ethyl ether and dichlo- dioxide (>99.9 %) was purchased from Airgas (Kenne- romethane under reduced pressure, affording 78 % yield saw, GA, USA), Sigma-Aldrich (St. Louis, MO, USA) and of PS based on the trans isomer content. The resulting O N S N N X N3 O N O N S M+ N - 3 3 O Sulfolene Sulfolene 1 2 Scheme 1 Overall reaction sequence in the synthesis of 1‑benzyl‑5‑(4‑toluenesulfonyl)tetrazole (2) Huang et al. Sustain Chem Process (2015) 3:13 Page 3 of 10 piperylene sulfone was characterized by 1H and 13C NMR Table 1 Recycling results of sulfolene solvents to verify nearly pure production. Entry Solvent Scale (mL) SO2/diene Cold bath Recovered molar ratio (°C) solvent (%) Recycling of sulfolenes 1 PS 5 8 55 89 2 − ± The recycle of sulfolene solvents was demonstrated by 2 PS 20 6 60 98 0 beginning with a certain quantity of sulfolene, thermally − ± 3 BS 20 6 76 95 1a decomposing it, and subsequently reforming it. The differ- − ± a ence between the initial and final weights was designated Two replicates, one of them is added with inhibitor as the percent recovery. The experiments were conducted in the prototype apparatus described in Fig. 2. Reactors clogging from solid BS reforming inside the tube. When R-1 and R-2 are Ace-Glass pressure tubes which could be reactor R-1 was completely empty, 100 mL/min of N2 were easily removed and reattached to the setup. Both reactors allowed to enter from V-1 to further transport volatile were weighted before recycling. A desired amount of sul- products to R-2; this final wash was performed for at least folene was first added to R-1 (1 % by weight hydroquinone 30 min. Reactor R-2 was then sealed and allowed to warm with regard to BS was added to R-2 for recycling of BS); to room temperature. the system was then purged with N2. Liquid SO2 was intro- The reformation reaction was carried for at least 40 h. duced into R-2, which was kept cold using a dry ice/isopro- During that time a pressure relief valve (V-7) ensured that panol bath. When the desired amount of liquid SO2 was the entire system was held under the pressure rating for introduced, the extra SO2 was released through the base the glass reactors.