Copper (II) Catalyzed Oxidative Alkoxylation of White Phosphorus. Communication 2

White phosphorus has been catalytically oxidized by oxygen in alcoholic solutions of copper (II) acetylacetonate, halides or carboxylates to yield dialkyl phosphites and trialkyl phosphates under mild reaction conditions. Trialkyl phosphite has been observed as unstable organophosphorus intermediate, which is being converted into the main reaction products. In the case of methanolic solutions, the derivatives of two step acidolysis of dimethyl phosphite, monomethyl phosphite and phosphorous acid, have been additionally detected among the reaction products. The influence of the copper (II) catalysts on the kinetics of accumulation and transmutation of organophosphorus products has been explored. It has been found that the Cu(II) compounds take a role of catalysts-electron-carriers from white phosphorus to oxygen. The indispensable molar ratio between catalyst and white phosphorus and the order of catalytic activity for the copper (II) compounds have been established. The major steps of the catalytic reaction including (i) the coordination of white phosphorus and alcohol to metal ion, (ii) the redox decomposition of this intermediate complex accompanied by reducing elimination of elementary copper and formation of organophosphorus product and (iii) the oxidation of the reduced form of catalyst by oxygen have been also suggested.


Introduction
The current technology for the synthesis of phosphorus esters is based on the oxidation of white phosphorus (P 4 ) by chlorine, followed by phosphorylation of alcohol by the phosphorus chloride. Evolution of the aggressive hydrogen chloride accompanies this latter reaction causing serious environmental problems. The increased commercial and environmental interests have prompted the search for technical protocols for converting white phosphorus into useful derivatives which are alternative to the presently used procedures. The direct oxidation of P 4 by such accessible technical oxidant as oxygen in alcoholic solution leads to the radical chain oxidation of P 4 b y O 2 which afford a dense white smoke of phosphorus oxides P 4 O 6 , P 4 O 10 and only traces of organophosphorus compounds. To accelerate the desired reaction of the oxidative P-O coupling of P 4 to alcohol, the catalysis by copper (II) compounds has been used [1,2]. The application of catalysts has allowed to substitute the toxic Cl 2 by harmless O 2 and to prevent the undesirable interaction of white phosphorus with oxygen. Nevertheless, this new catalytic protocol is in an early stage of development, so the rate and sequence of the organophosphorus compounds formation has not yet investigated. This contribution is aimed at expanding further the fundamental knowledge on the catalytic functionalization of white phosphorus. Special attention will be devoted to the kinetics of accumulation of organophosphorus products and to the molar ratio between catalyst and white phosphorus. communication 1 [3]. Oxygen was dried under CaCl 2 . The Cu(II) catalyzed oxidative alkoxylation of white phosphorus was carried out in a glass "jacket" reactor equipped with a dropping funnel, a sampling tube and connected to a thermostat to c o n t r o l t h e r e a c t i o n t e m p e r a t u r e , a n d t o a gasometric burette filled by oxygen for measurement of O 2 consumed. The alcohol and CuX 2 were added to the reactor in this order and the resulting solution was thoroughly purged with oxygen during 10-15 min. Then, an aliquot of the stock's arene solution of P 4 (2-4 mL, 0.18 -0.62 mmol) was added into the reaction solution through the dropping funnel connected with a glass tube reaching the reactor bottom. The reactions were conducted at partial pressure of oxygen (P O2 ) equal 0.6 or 1.0 atm. During experiment, the reaction velocity W O2 (M/min), equal to the mols of O 2 consumed by 1 L of the catalytic solution per 1 min, was continuously measured and the solution sample was periodically taken and subjected to GC analysis.
Caution. White phosphorus is flammable, extremely poisonous and volatile and particular care must be taken in handling it. It is strongly recommended to conduct its reactions under oxygen atmosphere behind shields.

Results and discussion
The intensive stirring of the alcoholic solution of Cu(II) acetylacetonate, carboxylates or halides (CuX 2 ) with white phosphorus dissolved in an arene at 40-60°C and the [CuX 2 ]/[P 4 ] molar ratio, equal 5, under oxygen atmosphere is accompanied by a rapid color change from green to brownishblack and by consumption of O 2 by this solution. As s o o n a s O 2 is steadily consumed, the brown sediment eventually dissolves and solution gradually lightens to restore the original green transparent color of the initial catalytic solution. The white smoke of the phosphorus oxides is not observed above catalytic solution. The progress of the reaction was monitored by GC at prescribed time intervals (5-10 min). Irrespectively on the alcohol and catalyst used, the oxidative alkoxylation of P 4 catalyzed by Cu(II) compounds gives a mixture of dialkyl phosphites 1a,b and trialkyl phosphates 2a,b according to the following general reactions: Blank control experiment, carried out in the absence of catalyst, did not produce organophosphorus derivatives, only dense white smoke of phosphorus oxides were observed in the reactor. The kinetic curve (the velocity vs. time) and kinetics of organophosphorus products accumulation (the yield vs. time) for the oxidative coupling of P 4 with n-butanol catalyzed by some The yield of dibutyl phosphite 1a also passes a maximum (30% at 30 min) while tributyl phosphate 2a is rapidly accumulated in the catalytic solution. In addition to the main reaction products, GC monitoring of the catalytic reaction reveals the intermediate tributyl phosphite P(OBu) 3 which passes a maximum (50% at 10 min) and disappears at 50 min because of a gradual transmutation into the main products 1a and 2a (see the reaction equations below). The complete P 4 conversion is achieved in 90 min, 1a and 2a are obtained in 16 and 84% yield, respectively (Table, entry 1). As one can see from the entry 1, the experimental molar ratio between O 2 c o n s u m e d a n d P 4 introduced in the reaction (O 2 /P 4 ) is close to 5, i.e. to the reaction stoichiometry depicted in the equation (2).
The copper(II) acetylacetonate exhibits more catalytic activity in the oxidative alkoxylation of white phosphorus as compared with chloride. The complete P 4 conversion is achieved in 40 min, the reaction velocity passes a maximum (8,5 10 -3 /min) at 3 (Fig. 2). During reaction of P 4 with n-BuOH and O 2 in the presence of Cu(acac) 2 , phosphite 1a passes a maximum (81%) at 20 min whereas phosphate 2a is slowly accumulated in the solution. The final product distribution is as follows: 78% yield of dibutyl phosphite 1a and 22% yield of tributyl phosphate 2a (Table, entry  In contrast to oxidative ability demonstrated by the copper(II) butyrate and bromide [3], the catalytic activity of Cu(C 3 H 7 CO 2 ) 2 is higher than one of CuBr 2 . In the presence of both copper (II) butyrate and bromide, tributyl prosphite 2a is predominantly obtained (Table, entries 3, 4). Copper (II) acetate, butyrate and bromide catalyze the oxidative P-O coupling of P 4 to MeOH at 50 o C and the molar ratio CuX 2 /P 4 , equal 5, to give trimethyl phosphate 2b in a moderate yield (Table, entries 5-7). Dimethyl phosphite 1b is not identified because it is strongly sensitive toward the cleavage by acid and water both generated during the catalytic reaction.
Thus the same products are obtained in the oxidative alkoxylation of P 4 under both inert [3] and oxygen atmosphere, i.e. oxygen does not directly react with P 4 and participate in formation of organophosphorus compounds. In both cases, white phosphorus is oxidized by CuX 2 in alcohol to yield organophosphorus products. Our results suggest the following major steps for the catalytic reaction of the oxidative alkoxylation of P 4 : the reduction of CuX 2 by white phosphorus in alcohol, which generates elementary copper together with the intermediate trialkyl phosphite (3), the oxidation of Cu by CuX 2 to CuX (4) and the regeneration of catalyst via acidic oxidation of CuX by dissolved oxygen (5 As have been shown in the previous communication 1 [3], trialkyl phosphite is dealkylated by an acid generated in situ to give dialkyl phosphite 1a, b (6) and oxidized by CuX 2 to yield trialkyl phosphate 2a,b (7). The proposed mechanisms of the reactions (3) and (7) have been earlier discussed [3]. The methanolic P(III) esters may undergo the stepwise acidolysis into monomethyl phosphite 3b (8) and phosphorous acid (9). In the catalytic reactions (1, 2), O 2 is formally considered as P 4 oxidant although white phosphorus transfers its electrons to oxygen through catalyst. Actually, oxygen is responsible only for reoxidation of the reduced form of catalyst (5). It is worthy to note that at a low concentration of the redox partner (CuX 2 ) in solution, white phosphorus vapors escape to the gas phase where they readily undergo a radical chain oxidation by O 2 which may be finished with explosion [4]. The occurrence of this extremely undesirable and dangerous side process may however be prevented by using a large [CuX 2 ]/[P 4 ] molar ratio, equal 4-6, which is requested as for safety as for productive reasons. Indeed, the rate of the catalytic reaction is dramatically decreased when a minor [CuX 2 ]/[P 4 ] molar ratio, equal 2-3, is used.
The addition of the benzene solution of P 4 to the solution of CuCl 2 in MeOH under oxygen atmosphere at 40°C and the [CuCl 2 ]/[P 4 ] molar ratio, equal 2,5, is accompanied by a rapid change from green solution to black sediment which does not consume oxygen. In contrast to CuX, the elementary copper is stable in the oxygen atmosphere. Some amount of white smoke (P 4 O 6 , P 4 O 10 ) is formed in the gas phase of the reactor. The black color of the reaction mixture together with an extremely low velocity of O 2 consumption remain for a long time (Fig. 3, the kinetic curve a). When a necessary amount of O 2 is at last consumed (after ~ 2 hours), the black sediment is converted into a brown and then a white one (CuCl) which is readily oxidized by O 2 to restore the original green transparent color of the methanolic CuCl 2 solution. Trimethyl phosphate 2b in 48% yield has been identified by GC in catalytic solution after 220 min (Table, entry 8). At rather small [CuX 2 ]/[P 4 ] molar ratio, the reduction of CuX 2 to Cu (3) proceeds much deeper and stage (4) is hampered due to a sharp disappearance of a free CuX 2 which needed to oxidize elementary copper. A long induction period at lower [CuX 2 ]/[P 4 ] molar ratio characterized by the extremely low velocity of O 2 consumption is explained by the extremely low concentration of CuX which needed to consume oxygen (5). At increasing the [CuX 2 ]/[P 4 ] molar ratio till 5 and 10, the reaction is accelerated and finished in 80 and 50 min (Fig. 3, curves b and c) to give trimetyl phosphate 2b in 67 and 78% yield, respectively (Table, entries 9, 10).
A large initial amount of catalyst for oxidative alkoxylation of white phosphorus can not be overwhelmed because the P 4 is admitted into the copper (II) alcoholic solution under oxygen atmosphere, and therefore the only way to prevent its direct interaction with O 2 is to use an excess of copper reagent which takes the role of electron receptor. A number of similar processes are known which proceed in the presence of a catalystselectron-carriers (CEC), such as transition metal compounds or their combinations [5][6][7][8][9][10]. These redox reactions are based on the reversible reduction / or oxidation of the CEC resulting in the oxidation / or reduction of organic or inorganic substrates. To hasten the reactions, a large amount of CEC is often used. A typical example of such reaction is the industrial process of the ethylene oxidation by O 2 to acetaldehyde in aqueous solution of the combined PdCl 2 /CuCl 2 catalytic system, which proceeds via t h e r e d u c t i o n o f P d ( I I ) b y ethylene to Pd, accompanied by the acetaldehyde formation, followed by the reoxidation of Pd by Cu(II) and of Cu(I) by O 2 [5]. The PdCl 2 /CuCl 2 system behaves as a CEC from ethylene to oxygen. The PdCl 2 concentration in the reaction solution is equal 0.02-0.2 , while CuCl 2 is usually taken in a large excess (100 mol CuCl 2 per mol PdCl 2 ) [11]. Although the redox steps sequence above mentioned includes no catalytic reactions from the conventional point of view, the PdCl 2 and CuCl 2 are considered as catalysts and this step sequence -as a catalytic one [5]. The redox catalysis of the oxidative alkoxylation of white phosphorus (1, 2) is realized via a sequence of the similar key steps (3)(4)(5)7) where the Cu(II) compounds carry 12 or 20 electrons from white phosphorus to oxygen to yield four molecules of P(III) or P(V) esters, respectively. After finishing the first catalytic cycle, the same catalytic solution can convert the second and following portions of P 4 solution practically with the same rate. As a result, the final molar ratio between initial CuX 2 and total P 4 oxidized may be considerably decreased. A series of preparative scale experiments with CuX 2 as catalyst have been carried out by this manner, the phosphorus esters have been isolated and characterized by NMR spectroscopy [12].

Conclusion
Under oxygen atmosphere, the oxidative alkoxylation of white phosphorus in the presence of copper (II) compounds proceeds in a catalytic fashion yielding the same set of products as under inert atmosphere. The nature of Cu(II) catalysts and alcohols used does not dramatically change the nature of reaction products. In all cases, dialkyl phosphites and trialkyl phosphates are formed, only the ratio of these products changes. Trialkyl phosphate is predominantly obtained in the presence of copper (II) halides and carboxylates, whereas acetylacetonate promotes the formation of dialkyl phosphite. Trialkyl phosphite is observed as unstable organophosphorus intermediate, which is being converted into the main reaction products. In the case of methanolic solutions, the monomethyl phosphite and phosphorous acid were additionally f o r m e d a s a r e s u l t o f t w o s t e p a c i d o l y s i s o f dimethyl phosphite. The catalytic activity of the tested Cu(II) compounds is decreased in the following order: Cu(acac) 2 > Cu(CH 3 CO 2 ) 2 Cu(C 3 H 7 CO 2 ) 2 > CuCl 2 CuBr 2 . The oxidative alkoxylation of white phosphorus in the presence of Cu(II) compounds under argon and oxygen atmosphere proceeds according to an identical mechanism. In both cases, organophosphorus compounds are formed as a result of the activation and P-O coupling of P 4 and ROH in coordination sphere of the copper center. The only difference is that under argon the reduced Cu(I) species are accumulated in the solution while under oxygen atmosphere they are regenerated by O 2 to return Cu(II) compounds back to the catalytic cycle. To avoid the side oxidation of P 4 by O 2 , a special condition should be kept in order to white phosphorus transferred its electrons to oxygen through catalyst CuX 2 . A rather large amount of catalyst, between 4 and 6 equivalents of catalyst CuX 2 f o r e a c h P 4 , i. e. between 1.0 and 1,5 equivalents for each phosphorus atom, is therefore needed for safety and productivity reasons. In fact, the Cu (II) compounds take a role of catalystselectron-carriers from white phosphorus to oxygen.