Dialkyl 2-alkenylphosphonates

Gerber, K.P, Modro, T.A., Muller, E.L., and Phillips, A.M., Chemistry of dialkyl alkenylphosphonates. Synthetic implications. Phosphorus, Sulfur Silicon Relat. Elem., 75, 19, 1993. 

Dialkyl arylphosphonates and alkenylphosphonates are prepared by the coupling of halides or triflates with the dialkyl phosphonate 783[64l-643]. 

Han and Tanaka reported that Pd(0) complexes catalyze the addition of dialkyl phosphites to terminal alkynes to give alkenylphosphonates (Scheme 5-17, hydrophosphorylation) [15]. 

Zirconocene dichloride has been reported to serve as catalyst for the addition of dialkyl phosphorochloridates across the triple bond of terminal alkynes.159 DIBAL is used with the terminal alkyne system to generate the dialkyl 1-alkenylphosphonate products. 

Unfortunately, the subsequent step using the Lindlar catalyst met with little success, and nonstereoselective partial reduction of the triple bond is observed." It was reported that most catalytic hydrogenations of dialkyl 1-alkynylphosphonates in EtOH using 5% Pd/CaCOj poisoned with quinoline gave mixtures comprising cis- and fra i-l-alkenylphosphonate and starting material, from which the predominant cis isomer was isolated. 

Several groups have demonstrated that desilylation of dialkyl l-(trimethylsilyl)methylphosphonates initiated by means of fluoride ion (CsF, KF, or TBAF) is an effective process for the transfer of carbanions to electrophilic centers. Some examples involving the fluoride-induced formation of a-phosphonylated carbanions from dialkyl l-(trimethylsilyl)methylphosphonates containing a fragile C-Si bond have been described. Cleavage of the carbon-silicon bond under these conditions offers the advantages of neutral conditions and contributes to obtain better yields from sensitive substrates than those obtained under basic conditions. This procedure, which appears operationally simpler and cleaner than traditional protocols, has been applied to the preparation of alkenes " and 1-alkenylphosphonates" by Homer-Wadsworth-Emmons and Peterson reactions, respectively. 

An interesting feature of the Peterson olefination reaction concerns the subsequent transfonna-tion of dialkyl 1-alkenylphosphonates into new functionalized phosphonates. For example, diethyl... 

Pyrolysis (in boiling toluene) of the sulphoxide obtained from a dialkyl (l-phenylth> ioalkyl)phosphonate (339) and 3-chloroperoxybenzoic acid affords an alkenephosphonic diester, and subsequent work showed that the sequence was adaptable to the production of chiral esters of alkenylphosphonic diester with optical purities of not less than 93%. Use of the ( S )p-phosphonic amide 340 afforded a mixture of the (E)-(S)p- and (Z)-(S)p-stereoisomers (341), separable by chromatographic methods. ... 

The additions of thiols and sulphenyl chlorides to alkenylphosphonic derivatives to yield (2-alkylthioethyl)phosphonic compounds are reactions which have already been noted" Dialkyl (alkylthiomethyl)phosphonates yield a-chloro derivatives when treated with ncs in CCl/° the resultant dialkyl (l-alkylthio-l-chloromethyl)phosphonates undergo Friedel-Crafts arylation with benzene, alkylbenzenes or other activated aromatics in the presence of SnCl4 or TiCl/ yields are said to be good. 

Insertion of alkynes into Pd-P bonds give alkenylpalladium intermediates 2 and 3. Photonolysis of these intermediates by diaUcyl H-phosphonate results in aUcenylphospho-nates 4 and 5 and regenerates the active catalyst 1. Indirect evidence for the insertion of alkynes into Pd-P bonds comes from the isolation of trace amounts of alkynyl phospho-nate 6, which can only result from reductive elimination of the alkenylpalladium intermediate 3. The second hydrophosphorylation reaction proceeds via the insertion of geminal alkenylphosphonate 4 into the Pd-H bonds, followed by reductive elimination of the product 7. This reaction pathway can explain the failure of 5 to react with dialkyl H-phosphonate because of the steric hindrance of the potential insertion intermediate. These bisphosphonates are evalnating as ligands for radionuclides such as and for... 

P-H oxidative addition followed by alkyne insertion into a Pd-P bond gives the re-gio-isomeric alkenyl hydrides 15 and 16. Protonolysis with dialkyl phosphite regenerates hydride 17 and gives alkenylphosphonate products 18 and 19. Insertion of alkene 18 into the Pd-H bond of 17 followed by reductive elimination gives the bis-products, but alkene 19 does not react, presumably for steric reasons. P-Hydride elimination from 16 was invoked to explain formation of trace product 20. 

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