Current project is aimed at the investigation of in situ electrochemical reactions of white phosphorus with organic substrates, such as carbonyl compounds (aliphatic, cyclic and aromatic ketones, aliphatic and aromatic aldehydes). These reactions lead to the formation of new phosphorous intermediates which can be important for synthesis of new organometallic compounds with P-C and P-H bonds. The proposed methods are based on previously investigated phosphorus compound – phosphine oxide H3PO, which can be prepared directly from white phosphorus P4 by mild anodic oxidation of electrochemically generated phosphine PH3 as it has been shown in our previous works.
Scheme 1. Electrochemical reactions of white phosphorus
We used the conditions of electrochemical generation of phosphine oxide in the study of the electrochemical reaction of white phosphorus with carbonyl compounds. The relevance of the study is in the development of environmentally friendly synthetic pathways of organophosphorus compounds preparation avoiding the toxic process of chemical synthesis of phosphine.
We are developing new method for preparation of phosphorus compounds in the frames of the current project. The experimental conditions for selective electrochemical preparation of phosphorous compounds directly from white phosphorus in undivided electrochemical cell are being elucidated: lead cathode, electrochemically soluble metal anode (zinc and aluminum), alcohols as solvents and electrolyte (hydrochloric acid in different concentrations). We use quantum chemical computations to study thermal stability of formed products.
The reactivity of phosphine oxide H3PO electrochemically generated in situ from white phosphorus P4 towards various aliphatic ketones (acetone, methyl ethyl ketone, methyl i-propyl ketone, methyl n-propyl ketone, methyl tert-butyl ketone, diethyl ketone) and cyclic ketones (cyclopentanone, cyclohexanone, cycloheptanone, cyclooctanone) was studied. We have found that this interaction gives formation of mono- and bis-(α-hydroxyalkyl)phosphine oxides RR’C(OH)P(O)H2 (1) and (RR’C(OH))2P(O)H (2) where R = Me; R’ = Me, Et, n-Pr, i-Pr, t-Bu; R = R’ = Et; -(R-R’)- = -(CH2)4-, -(CH2)5-, -(CH2)6-, -(CH2)7- (Scheme 2).
Scheme 2. Electrochemical generation of H3PO and reactivity of with ketones
Thermal properties of the formed primary and secondary phosphine oxides have been studied and quantum chemical calculations of thermodynamic stability of one of these reactions compounds were performed. We have undertaken quantum chemical computations within the framework of density functional theory (DFT). DFT computations were applied to the simplest process of obtaining the products from acetone, since this reaction proceeds in the absence of specifically added ethanol. The formation of product 1 (primary phosphine oxide), ΔG (gas) = -3.0 kcal•mol-1 is more thermodynamically favorable than formation of product 2 (secondary phosphine oxide), ΔG (gas) = -1.3 kcal•mol-1 (Figure 1). This explains the reasons why we see that secondary phosphine oxide disappear with time after the reactions with most of the used ketones, especially after the reaction mixtures are heated at the temperatures higher than 70°.
Figure 1. DFT computations for reactivity of phosphine oxide Н3РО with acetone