Reaction of Phosphorus Trihalides with Methyl Triflate. Molecular and Supramolecular Structure of Methyltrichloro- and M
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tion of Phosphorus Trihalides with Methyl Triflate. Molecular and Supramolecular Structure of Methyltrichloroand Methyltribromophosphonium Triflates A. T. Gubaidullina, V. F. Mironova,*, and I. A. Litvinova a A.E. Arbuzov Institute of Organic and Physical Chemistry, Federal Research Center “Kazan Scientific Center of the Russian Academy of Sciences,” Kazan, 420088 Russia *e-mail: [email protected]
Received April 20, 2020; revised April 20, 2020; accepted April 26, 2020
Abstract—Prolonged (4 years) keeping of mixtures of phosphorus trichloride or tribromide with methyl triflate in dark at 20–25°C has led to the formation of crystalline methyltrichloro- and methyltribromophosphonium triflates with a content of 10–11% in the reaction mixture and yield of 5–6%. Structure of the triflates has been elucidated by means of X-ray diffraction analysis. Keywords: phosphorus tribromide, phosphorus trichloride, methyl triflate, quasi-phosphonium salt, molecular and supramolecular structure
DOI: 10.1134/S1070363220090078 Phosphorus trichloride and tribromide readily react with alkyl halide only in the presence of strong Lewis acids (such as AlCl3, AlBr3, etc), resulting in alkylphosphonium salts 1 (Scheme 1), which can be further hydrolyzed into phosphonic dihalides 2 (with moderate yield) or phosphonic acids 3 (the Clay–Kinner–Perren reaction) [1–3]. Despite the difficulties of aluminum species separation, this reaction is among the most important approaches to the compounds containing a phosphorus– carbon bond, phosphonates, which have been regarded as formal analogs of phosphates not subject to enzymatic hydrolysis [4, 5], therefore exhibiting strong biologic activity. Hence, the interest to novel synthetic approaches to these compounds has emerged [6, 7]. Mechanism of the Clay–Kinner–Perren reaction is complex and has not been elucidated yet. It has been suggested that phosphorus trihalide acts as a nucleophile attacking the alkyl halide activated via the interaction with a Lewis acid. In the presence of oxygen, direct for-
mation of phosphonic acids 2 derivatives is possible in the phosphorus trihalide–alkyl halide systems (oxidative halophosphinylation) [8]. Herein we demonstrated for the first time that such weak nucleophiles as phosphorus trichloride and tribromide (4, 5) can react with a sufficiently strong electrophile (methyl triflate 6) under kinetic control (20°С, sealed ampoule, ~4 years) affording methylphosphonium salts 7, 8 (Scheme 2), their content in the reaction mixture being 10–11%. That remarkable experimental fact can be a basis of novel technology of preparation of methylphosphonic acid derivatives. Phosphonium salts 7 and 8 were partially converted into coarse-crystalline precipitates upon prolonged keeping and could be isolated via filtration under argon atmosphere. Their structure was elucidated by means of X-ray diffraction analysis (Table 1). Accounting for extreme hydrolytic instability of those compounds, the experiments were performed at low temperature using
Scheme 1.
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REACTION OF PHOSPH
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