Bromodecarboxylation of (E)-3-Aryl-2-(diethoxyphosphoryl)acrylic Acids: A Facile Route to Diethyl Arylethynylphosphonates

Abstract

Bromodecarboxylation of (E)-3-aryl-2-(diethoxyphosphoryl)acrylic acids leading to diethyl (Z)-2-aryl-1-bromovinylphosphonates has been conducted for the first time. The products have been shown to be useful for the synthesis of diethyl arylethynylphosphonates.Decarboxylation of carboxylic acids accompanied by simultaneous replacement by a halogen, known in the literature as the Hunsdiecker reaction, constitutes an important and useful method for the synthesis of halogenated organic compounds containing one fewer carbon atom than the original acid. 1 It is well established that simple alkanoic acids can be converted into the corresponding alkyl bromides by reaction of their silver salts with elemental bromine. Unfortunately, the salts of a,b-unsaturated acids were found not to be useful in this reaction. 1 Recently, the Hunsdiecker reaction of cinnamic acids has been shown to proceed efficiently in the presence of various reagents (NBS/iodosylbenzene, 2a NBS/LiOAc, 2b NBS/LiOAc with irradiation, 2c tetrabutylammonium trifluoroacetate/N-halosuccinimides, 2d NaX/Oxone, 2e KBr/ Selectfluor, 2f KBr/Na 2 MoO 4 ·2H 2 O/H 2 O 2 , 2g Dess-Martin periodinane/TEAB, 2h LiBr/CAN 2i ) giving access to a range of vinyl halides.Halogen-substituted vinylphosphonates are masked acetylenic compounds as they can be easily transformed into the corresponding alk-1-ynylphosphonates. 3 Moreover, 1-bromoalk-1-enylphosphonates have been successfully utilized for the preparation of the corresponding 1-arylalk-1-enylphosphonates and phosphorylated dienes by means of transition-metal-catalyzed arylation and alkenylation. 4 1-Bromoalk-1-enylphosphonates are commonly prepared through a sequence of reactions involving addition of elemental bromine to alk-1-enylphosphonates and base-catalyzed b-elimination of the resulting 1,2-dibromoalkylphosphonates. 4,5 These reactions lead to the formation of particular products as mixtures of E-and Zisomers. For this reason, methods to obtain such compounds in a stereoselective manner are desirable.Alk-1-ynylphosphonates have found widespread application in organic synthesis. 6 Stereoselective partial reduction of the triple bond of dibutyl prop-1-ynylphosphonate to give dibutyl (Z)-prop-1-enylphosphonate constitutes a key step in the first racemic synthesis of the antibiotic fosfomycin. 7 Hydration of alk-1-ynylphosphonates provides an effective and general approach for the preparation of 2-oxoalkylphosphonates, versatile synthetic intermediates. 8 Additionally, alk-1-ynylphosphonates have been utilized in [2+2]-, [3+2]-, and [4+2]-cycloaddition reactions for the synthesis of complex organophosphorus compounds. 6 Alk-1-ynylphosphonates are also useful precursors for the preparation of arylidenebisphosphonates providing a new access to the P-C-P backbone. 9 Recently, the reagent system Cp 2 ZrCl 2 /2 EtMgBr/2 AlCl 3 was shown to convert alk-1-ynylphosphonates into cyclopropylmethylphosphonates in good isolated yields. 10 As a consequence considerable effort has been directed towards the development of metho...

Decarboxylative Halogenation of Organic Compounds

“…We previously described the utilization of simple alkali salts of halides as a source of halogen in the presence of relatively strong oxidizers, for the synthesis of aryl halides. A similar approach, employing different oxidizing agents with sodium halides, proved efficient for the halodecarboxylation of α,β-unsaturated acids. − Here, we would like to discuss some representative examples.…”

“…Stoichiometric combinations of Oxone (KHSO 5 )/NaCl, NaBr or NaI in the presence of Na 2 CO 3, afforded ( E )-β-halostyrenes (Scheme a) and 2-aryl-1-bromophosphonates (Scheme b) from the corresponding ( E )-α,β-unsaturated acids. , While bromo- and chloroalkenes were obtained in useful yields, iododecarboxylation by this approach was less effective (yields 0–45%). The stereoselectivity varies from high to moderate, which might impose certain limitations on the method.…”