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The Potential of Microwave in Organophosphorus Syntheses

György Keglevich

Department of Organic Chemistry and Technology, Budapest University of Technology and Economics, 1521 Budapest, Hungary – E-mail: gkeglevich@mail.bme.hu

Abstract: The spread of professional microwave (MW) reactors has brought about a revolutionary change in synthetic organic chemistry. This environmentally friendly methodology exerted also a positive influence on organophosphorus chemistry making possible new reactions, or just making the reactions more efficient in respect of rate, selectivity and yield. In special cases, MW irradiation may substitute catalyst.

Key Words: microwave-assisted organic synthesis, organophosphorus chemistry

 
 

The use of the microwave (MW) technique in organic syntheses spred gradually in research laboratories, and after more than two decades it knocks at the door of industry. At the beginning, only domestic MW ovens were available, but later, different variations of professional MW equipment were developed and utilized in a variety of syntheses, such as substitutions, additions, eliminations, condensations, acylations, esterifications, alkylations, C–C coupling reactions, cycloadditions, rearrangements and the formation of heterocycles [1].

The main problem with industrial application is scale-up [2,3]. On the one hand, there is a problem with the structural material, as the batch reactors may be made of only teflon or glass. On the other hand, the limited penetration depth of MWs into the reaction mixtures prevents the construction of bigger size batch reactors. Presently, the only possibility for a certain degree of scale-up is the use of continuous-flow reactors [2,4]. A batch MW reactor (CEM) may be supplied with a flow cell where the mixture is moved by HPLC pumps. In another variation, a continuous tube reactor with a diameter of up to 6–9 mm was elaborated (Milestone) that makes possible the processment of ca 300 l/day [5]. A capillary microreactor consisting of four parallel capillary tubes was also described. The above equipment may be used well in industrial laboratories. The only criterion of the application is that the reaction mixtures must not be too viscous and heterogeneous. The author of this paper believes that “bundle of tubes” reactors incorporating a number of glass tubes with a diameter of several mm-s may bring a breakthrough in the industry. Another good accomplishment is to apply an assembly line-type equipment that transports the solid reaction components placed in suitable vessels into a tunnel, where the irradiation takes place [6].

The most common benefits from MW irradiation is the considerable shortening of reaction times and the increase in the selectivities. However, the most valuable benefit is when a reaction can be accomplished that is otherwise impossible under traditional thermal conditions. This may be the consequence of a so-called special MW effect [7]. There are, of course, other advantages as well that will be shown below within the discipline of organophosphorus chemistry. Organophosphorus chemistry is a dynamically developing field within organic chemistry. Organophosphorus compounds including P-hetereocycles find applications in the synthetic organic chemistry as reactants, solvents (ionic liquids), catalysts and P-ligands and, due to their biological activity, also as components of drugs and plant protecting agents [8–10]. The utilization of MW irradiation in organophosphorus chemistry is a relatively new field [11,12]. In this article, the attractive features of the application of the MW technique in organophosphorus syntheses are summarized. The chain of thoughts are grouped around three points.

 

1.) Reactions otherwise impossible under thermal conditions

The most common way to prepare esters (2) is the acid catalyzed direct esterification of carboxylic acids (1) with alcohols (1). The reaction is reversible, hence the alcohol should be applied in excess and/or the water formed should be removed by distillation, in most cases, in the form of binary or ternary azeotropes.



However, it is well-known that phosphinic acids (3) do not undergo esterification with alcohols to afford phosphinates (4) ((2)/A). For this, the esters of phosphinic acids (4) are synthesized by the reaction of phosphinic chlorides (5) with alcohols in the presence of a base ((2)/B) [8]. An alternative possibility is preparation by the Arbusov reaction ((2)/C) [8].



The generally applied esterification method ((2)/B) has the drawback of requiring the use of relatively expensive P-chlorides (e.g. 5). Beside this, hydrogen chloride is formed as the by-product, that must be bond by a base. Hence, the method is not atomic efficient and is not environmentally friendly.

It was a challenge for us to try the direct esterification of phosphinic acids with alcohols under MW conditions. To our surprise, it was found that a series of phosphinic acids underwent esterification with alcohols with longer chain at around 200 °C on MW irradiation (3) [13–15].



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