O-alkylphosphonates

Fachbereich Chemie der Martin-Luther-Universitat Halle-Wittenberg, D-06120 Halle/Saale, Germany 

We found that dichloro diorgano silanes are suitable reagents for the synthesis of diorgano bis(0-alkyl)phosphonates. A different reactivity of both chlorine atoms of the silanes is observed in the dealkylation reaction of dialkylphosphonates in contrast to Me3SiBr [2]. 

Me(Ph)SiCl2 reacts with dimethyl phosphonate in a 1 1-mixture at 100°C to the O.O-disilylated phosphonate. In contrast to this observation the O-alkyl O-silyl phosphonate is formed solely by reaction of both compounds in acetonitrile at 80°C (line 7,8). An excess of the corresponding phosphonate as polar solvent leads to the mixed ester 2 only. The excess of the dialkyl phosphonate is removed from the mixture by vacuum distillation (line 9). 

The enhanced reactivity of the chlorine atom in the dichloro diorgano silanes can also be observed in Me2(MeO)SiCl 4 or Me(EtO)2SiCl 5. In both cases the O-alkyl O-silylphosphonates (6, 7) are formed in high yields after reaction of 4 or 5 with dialkyl phosphonates. 

Dialkoxydichloro silanes exhibit a higher nucleophilicity compared to the chlorosilanes described before. Diethoxysilyl-bis(0-alkyl)phosphonates of type 8 are formed in 90% yield besides 10% of disilylated phosphonates in the reaction of dialkyl phosphonates with dichlorodiethoxy silanes at 50-80°C. 

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Figure 4.41. Schematic representations of the hydrogen bonds made by the anionic functional groups in the diastereomeric salts of a) carboxylates, (b) sulfonates, (c) O-alkylphosphonates, and (d) O-alkylphosphonothioates. The dotted lines represent hydrogen bonds.

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