Abstract
Synthetic methods for the preparation of phosphinic derivatives (esters and amides) are summarized. The basic method is, when phosphinic chlorides are reacted with alcohols or amines. These reactions take place under mild conditions, but utilize expensive chlorides, need a solvent, and a base has to be added to remove the HCl formed. On conventional heating, the phosphinic acids resist undergoing derivatizations by nucleophiles. However, on microwave (MW) irradiation, the phosphinic acids could be esterified by alcohols. The direct esterification does not require an extra solvent, it is atomic efficient, but needs a relatively higher temperature. The similar amidations were reluctant. Phosphinic acids may also be esterified by alkylation that may be promoted by combined phase transfer catalysis and MW irradiation. It is also possible to convert the phosphinic acids by different reagents (e.g. by the T3Pi reactant) to a more reactive intermediate that is ready to react with alcohols or amines. Other methods, such as preparation by the Arbuzov reaction or by fragmentation-related phosphorylation are also discussed.
Highlights
Phosphinic derivatives are important building blocks in synthetic organic chemistry [1]
Alkylation Esterification of 1-Hydroxy-3-Phospholene 1-Oxides: The senior author of this chapter, together with others studied the alkylating esterification of cyclic phosphinic acids under phase transfer catalytic (PTC) and solvent-free microwave (MW) conditions using alkyl halides in the presence of potassium carbonate [109,116]
Phosphinates by P-Ligand-free P-C Coupling: An environmentally friendly approach for the synthesis of arylphosphinates by the Hirao reaction of phenyl-H-phosphinate with bromobenzene involves the use of P-ligand-free Pd(OAc)2 catalyst under solvent-free MW conditions as this was observed by the senior author of this chapter together with a co-worker (Scheme 46). [117,120]
Summary
Phosphinic derivatives (acids, chlorides, esters, amides etc.) are important building blocks (starting materials and intermediates) in synthetic organic chemistry [1]. Hindered unsymmetrical diarylphosphinates were synthesized starting from hypophosphorous acid and applying an esterification step by orthoacetate that was followed by a palladium-catalyzed P-C bond formation step [56] Another example is a one-pot process including an esterification step by aminoalkoxysilanes followed by a P-C coupling step giving rise to phenyl-H-phosphinates [57]. The most general method for the preparation of phosphinic amides is the reaction of phosphinic chlorides with primary or secondary amines In these cases, the hydrochloric acid formed can be removed by a second equivalent of the amine (Scheme 18) [1,8,9,10,13,96]. A 3-ethoxy-phosphabicyclo[3.1.0]hexane 3-oxide was converted to the corresponding phosphinic amide in two steps via the P-chloride (Scheme 22) [102]
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