Article pubs.acs.org/joc
Thermal Rearrangement of Sulfamoyl Azides: Reactivity and Mechanistic Study Xiaodong Zou, Jiaqi Zou, Lizheng Yang, Guigen Li, and Hongjian Lu* Institute of Chemistry and BioMedical Sciences, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China S Supporting Information *
ABSTRACT: The rearrangement of sulfamoyl azides under thermal conditions to form a C−C bond while breaking two C−N bonds is reported. Mechanistic study shows that this reaction goes through a Curtius-type rearrangement to form a 1,1-diazene, then which rearranges possibly through both a concerted rearrangement process and a stepwise radical process. This rearrangement could be used in the synthesis of complex biologically active molecules, such as sterols, and piperine derivatives.
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INTRODUCTION
RESULTS AND DISCUSSION Sulfamoyl azides are chemically stable even in strong basic or acidic conditions10 and are thermally stable up to at least 100 °C in the absence of solvent.11 They have recently been characterized as a new class of effective nitrene sources for metal-catalyzed C−H amination11,12 as well as a new type of azide for applications in click chemistry.13 In comparison with the widely used alkylsulfonyl and arylsulfonyl azides, the potential applications of sulfamoyl azides in organic synthesis have not been fully explored and it is of interest to examine the rearrangement of this sulfamoyl nitrene. Sulfamoyl azide (1a), prepared from N-benzyl-1-(4-methoxyphenyl)methanamine in one step (see Supporting Information (SI)), was used as the starting material (Table 1). The rearrangement of 1a was performed by heating a solution in methanol (entry 1) and it was found that the hydrocarbon product (2a) was formed at 80 °C. No evidence was found of products formed by reaction with methanol. The decomposition temperature of sulfamoyl azides in solvent is much lower than that of arylsulfonyl azides.5g Different solvents were screened (Table 1, entries 1− 9) and dioxane was found to be optimal. No C−H abstraction or olefin aziridination products were obtained in toluene and dioxane.5 This process may be thermodynamically favored because it generates two equivalents of N2 and one of SO2. The yield was slightly decreased in the absence of 4 Å molecular sieves (MS) or without the protection of nitrogen (entries 10− 12). When the reaction was decreased to 60 °C, no rearrangement product (2a) was observed (entry 13). Upon increasing the temperature to 100 °C, the yield of the reaction was slightly decreased as side reactions were observed (entry 14).
The Curtius rearrangement of an acyl azide into an isocyanate via an acyl nitrene intermediate is a reliable method for the synthesis of amines and their derivatives.1 It has been widely used in many aspects of chemical industry from synthetic chemistry, medicinal chemistry, to material sectors.2 There have been however very few reports of Curtius-type rearrangements via other α-oxo nitrene species, such as phosphoryl nitrenes3 or sulfonyl nitrenes.4 Consequently, development of an efficient Curtius-type rearrangement via other species in place of acyl nitrenes to enlarge the applications of this methodology is of interest. The sulfonyl nitrene species is often thought to be involved as transient intermediates in many organic reactions,5,6 such as aziridination reactions and C−H amidation reactions, and can even react with different common solvents, such as cyclohexane5g and benzene,5h prior to rearrangement. Due to its high reactivity, the sulfonyl nitrene species was thus thought to be unlikely to be involved in a Curtius-type rearrangement.7 and indeed, little experimental evidence of a Curtius-type rearrangement of sulfonyl azides has been reported.8,9 An example is the formation of the product PhNHSO2OMe in methanol,8 and another one is observed directly using matrix isolation techniques.5e,9 There appears to be no report of Curtius rearrangements via sulfonyl nitrenes used synthetically because the intermediate sulfonyl nitrene species reacts readily with solvents. In this article, we report the thermal decomposition of sulfamoyl azides R1R2NSO2N3, in which a Curtius-type rearrangement occurs under relatively mild thermal conditions to form a nitrogen stabilized nitrene, a 1,1-diazene, which then efficiently rearranges to generate a new C−C bond simultaneously breaking two C−N bonds. © 2017 American Chemical Society
Received: February 9, 2017 Published: April 17, 2017 4677
DOI: 10.1021/acs.joc.7b00308 J. Org. Chem. 2017, 82, 4677−4688
Article
The Journal of Organic Chemistry Table 1. Optimization of Reaction Conditionsa
entry
solvent
temp (°C)
4 Å MS
yield (%)b
1 2 3 4 5 6 7 8 9 10d 11d 12 13d 14d
MeOH DMF t BuOH PhCF3 H2O DCE MeCN toluene dioxane dioxane dioxane dioxane dioxane dioxane
80 80 80 80 80 80 80 80 80 80 80 80 60 100
add add add add add add add add add add no no add add
31 38 41 37 38 64(62c) 32 37 80(80c) 85(85c) 70(71c) 50
