Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Electrosynthesis of (E)‑Vinyl Thiocyanates from Cinnamic Acids via Decarboxylative Coupling Reaction Shun-Ming Yang, Tian-Jun He, Dian-Zhao Lin, and Jing-Mei Huang* Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China
Downloaded via EAST CAROLINA UNIV on March 14, 2019 at 19:49:48 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: Thiocyanate compounds are key intermediates in the synthesis of pharmaceuticals and other sulfur-containing organic compounds. Herein, we first report an electrochemical protocol to synthesize vinyl thiocyanates from decarboxylative coupling of cinnamic acids with NH4SCN in aqueous solution. This method provides thiocyanation products with broad functional group tolerance under ambient conditions. Scheme 1. Methods for the Synthesis of E-Vinyl Thiocyanates
O
rganic thiocyanates serve as useful precursors for many functionalized compounds with groups containing the sulfur atom and heterocycles.1 They also exhibit important drug properties and biological activities, such as antifungal, antibacterial, and antiparasitic.2 In the past few decades, great progress had been made to synthesize alkyl and aromatic thiocyanates.3 However, the reports on the construction of Cvinyl−SCN bonds were limited. In 1980, Kumada reported the use of (E)-alkenylpentafluorosilicates as substrates to prepare vinyl thiocyanates by the reaction with copper(II) thiocyanate.4a Then some graceful methods to synthesize Cvinyl−SCN compounds were successively reported by Taniguchi,4b Chen,4c and Kawabata4d with the use of activated olefins (Scheme 1, eq a). At the same time, alkynes and their derivatives were demonstrated to be able to prepare vinyl thiocyanates by Moriarty,5a Jiang,5b Xu,5c and Chen5d (Scheme 1, eq b). However, these methods exposed one or more disadvantages, such as unwanted byproducts, complicated raw material, low Z/E selectivity, the involvement of halogen or transition-metal, and limited substrate scope. Hence, efficient and environmentally friendly methods are in high demand. Oxidative coupling reactions have been demonstrated to be efficient methods to construct C−C or C−heteroatom bonds.6 In particular, the oxidative decarboxylation of carboxylic acids has been employed to access many valuable compounds.7,8 The decarboxylative coupling of vinyl carboxylic acids with the SCN salts is envisioned as an appealing process for the construction of Cvinyl−SCN bonds. However, this strategy has not been reported. On the other hand, electrochemistry has demonstrated its advantages and environmentally friendly characteristics for the synthesis of organic compounds.9,10 In continuation of our interest in electroorganic synthesis,11 herein we report the first electrochemical decarboxylative coupling reaction of cinnamic acids with NH4SCN to provide the vinyl thiocyanates in good yields. To start our investigation, 2-methylcinnamic acid (1a) and NH4SCN (2) were chosen as starting materials. Under constant current at 5 mA in an undivided cell, the reaction © XXXX American Chemical Society
of 1a and 2 in the presence of CH3COONH4 (3 equiv) and NaHCO3 (1 equiv) afforded the desired product 3a in 82% yield (Table 1, entry 1). It was worth noting that the addition of water to the reaction (MeCN/H2O = 7:1) was vital because no desired product was obtained when the reaction was conducted with dry MeCN (Table 1, entry 2). Subsequently, a series of common solvents were investigated, and the yield decreased significantly when MeCN was replaced by THF or DMSO (Table 1, entries 3 and 4; see Table S1 for more results). In addition, increasing or decreasing the proportion of Received: December 27, 2018
A
DOI: 10.1021/acs.orglett.8b04136 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Condition Optimizationa
Scheme 2. Substrate Scopeab
entry
variation from the standard conditions
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11c 12d 13c 14d 15
none MeCN instead of MeCN/H2O (7:1) THF/H2O (7:1) instead of MeCN/H2O (7:1) DMSO/H2O (7:1) instead of MeCN/H2O (7:1) MeCN/H2O (5:1) instead of MeCN/H2O (7:1) MeCN/H2O (10:1) instead of MeCN/H2O (7:1) CH3COONa instead of CH3COONH4 Na2CO3 instead of NaHCO3 Cs2CO3 instead of NaHCO3 NH4HCO3 instead of NaHCO3 RVC (+) instead of Pt (+) Ni (+) instead of Pt (+) RVC (−) instead of Pt (−) Ni (−) instead of Pt (−) no electric current
82 0 47
