Organic Electrosynthesis without a Potentiostat Selectivity in the Stepwise Reduction of Trichloromethylphosphonate: A Representative Experiment Jean-Christophe Le Menn and Andre T a l k Laboratoire d'Electrochimie, Unit6 Associee au C.N.R.S. no 439, Universite de Rennes, Campus Beaulieu, F-35042 Rennes, France Jean Sarrazin Laboratoire d'Electrochimie et Chimie Analytique. Universite des Sciences et Techniques du Languedoc, E.N.S.C.M., 8 rue de I'Ecole Normale, F-34075 Montpellier, France Several organic electrochemical experiments were recently published in this Journal. Most of them are dealing with cyclic voltammetry ( I ) , due to the increasing interest of electrochemistry as an analytical tool in the field of organic and organometallic chemistry. Organic electrosynthesis experiments (2) are less numerous, probably because they are often believed to require sophisticated and onerous equipments. Actually, selectivity is often obtained by running the experiments under controlled potential conditions: in the electrochemical reduction of trichloroacetic acid proposed a few years ago (2a) the reaction affords good yields but requires a long time since the current intensity decreases exponentially. In this paper, we propose two short experiments that illustrate a means of ~erforminaeasilv and selectivelv the stepwise reduction of a polyhalophosphonate in aqueous media: selectivity is governed by the acidity of the chosen medium. Reactions Diethyl trichloromethylphosphonate (EtO)>P(OJCCI3( I ) is easily available from the Arbusw reactiun between triethylpbosphite and tetrachloromethane (3):
T h e two-electron reduction of 1 is known to lead (4, 5) t o diethyl dichloromethylphospbonate carbanion (Et0)2P(0)CC12-, which is quantitatively protonated in protic media (5). thus affording diethyl dichloromethylphosphonate 2: (EtO),P(O)CCl, + 2e-
+ Ht
-
1
+
(EtO),P(O)CHCl, CI- (2) 2
and 2 can be further reduced, leading to diethyl chloromethylphosphonate 3: (EtO),P(O)CHCI, + 2e- + H i 2
-
(EtO),P(O)CH,Cl+ C1- (3) 3
The electrochemical synthesis of 2 and 3 is of particular interest since these two phosphonates are useful synthetic intermediates and are difficult to prepare by chemical routes. Analytical Detennlnatlon of Electrolysls Condltlons
The experimental conditions of the macroscale electrolyses can be determined by a previous analytical study. Polarograms of 1 have then been registered a t different pH values. In a mixture of ammoniacal buffer (pH ..;9.3) and ethanol (Ill), the polarogram shows two well-separated reduction of waves (Fig. l a ) situated a t -0.7 and -1.4 VSCE. As i t is
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Fioure 1. Polaroorarns of 1 MI in 111 mixlures of 95% ethanol and " aqueous electrolytes: (a) NHdNH,CI buffer (0.5 M): (b) CH3C02H/CH3C02NH, buffer (2 M): (c)H,S04 (0.5 M). ~
~
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~
usually the case for alkyl halides, the shape of the waves is not well-defined. The first one corresponds to the bielectronic reduction of 1 to 2 (eq 2) and the second one to the bielectronic reduction of 2 to 3 (eq 3). A third wave, corresponding to the reduction of 3 into diethyl methylphosphonate. can hardlv be distineuished from the solvent reduction a t adout -1.8 GEE. " Lowerine the nH of the medium shifts the water reduction potential & les'negative values, whereas the reduction potentials of the halides are unaffected. In acetic buffer (pH * 4.7)lethanol mixture (Fig. Ib) the second reduction wave can still be observed on the polarogram a t the foot of the solvent barrier. In very acidic media (0.5 M H~S0~Iethanol111) (Fig. lc), the second wave of the halide is totally masked. Preparation of 2 through the electroreduction of 1 requires that the potential of the cathode cannot become as negative as thevalues corresponding to the second wave. Aqueous sulfuric acid is a suitable medium for carrying out such a synthesis: the reduction of water, acting as a "potential barrier", should prevent thereduction of the halides to 3. In the same manner. the use of acetic buffered media insures that the electroreduction cannot be performed up ro the diethsl methvl~husohonatesteD since the third reduction wave is full; hidden by the reduction of the solvent; synthesis of 3 should he performed in such pH conditions. Remark: a complete reduction of 1 is not of syn~~~~
Volume 68
Number 6 June 1991
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513
Diethyl trichloromethylphosphonate Twenty-five milliliters of triethyl phosphite and 125 mL of CC4 were refluxed overnight in a 250-mL round-bottomed flask equipped with a water-cooled condenser. The excess of CC14 was then removed in a rotary evaporator, and the remaining oil was distilled under reduced pressure of the water pump, affording a colorless liquid: b.p. = 125-130 "C (1 mm Hg); yield: 82%. Electrochemical Cell (Fig. 2) The electrochemicalcell is made of a 100-mLbeaker covered by a plastic cap with four holes for the passage of the anodic compartment (aglass cylinder, 3-em diameter ended by asintered glass),the olatinum contact of the cathode. and the nitroren inlet and outlet tubes. The cathode isa mercury pool in which the platinum contact must he fully immersed.Theanode ir n carhm rud d l -cm diameter. ~~
FIgu~e2. Electrochemical cell.
t h e t i c i n t e r e s t because d i e t h v l m e t h v l u h o s ~ h o n a t e (E~O)ZP(O)CHS i.9 readily availabie by a n ~ r b u s o v r e a c t i o n between triethyl phosphite and iodomethane (6). Working under controlled potential conditions is not necessary since further reduction of the compounds to be synthesized is prevented by water reduction. Electrolyzing 1 under controlled current with an efficient agitation in sulfuric acid media produces 2 selecti\dy. T h e electrolysis is stopped when the theoretical duration corresponding to twofuradays per mole is reached. Evolvement of hydrogen can be observed at the end of the experiment since water is easier to reduce than 2 at the chosen pH value. Electrolyzing 1 in a n acetic buffered medium. under controlled current with a n efficient agitation affords 3 after the theoretical duration corresponding to the consumption uf 4 F , m d Hydrogen evolvement also occurs at the mercurv cathode a t t h e k n d of the experiment. T h e reduction products can easily h e analyzed b y NMR spectroscopy after extraction. Remark: N M R spectra of compounds 2 and 3 present two t w e s of couuliue: 'H-'H and lH-3lP. On t h e suectra reeisteied at 60 M ~ G , ' p r o t o n sot'the alkyl moiety g;ve a doublet ('H-.>lP coupling) and protons of the methylene moiety of thealcoxy groupsgiveaquinruplet that isin fact adoublet of quadruplet ('H-'H and 'H- "l'cuupling).
Safety Precautions The synthesis of 1 must be performed under a well-ventilated hood (stench!). Also nathine is known about the comnound's haz. ards; the use of protective gloves is recommended in order to avoid skin cmtact, and the electroch~miralcell during the preparative ~lertrolysesshould he placed under a ventilated hmd. ~~
514
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Journal of Chemical Education
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Diethyl dichlommethyphosphonate 2 Two and fifty-fivehundredths grams of 1 (0.01mol) was dissolved in 30 mL of a 111 mixture of 0.5 M HnSOn aqueous solution/95% ethanol. This solution was poured into the beaker over mercury. The anodic compartment was filled with 10 mL of 0.5 M HzSO+Nitrogen was bubbled through the catholyte, and the current value was set at 0.27 A. Since the reduction needs two electrons per molecule, the theoretical amount of electricity is Q = 0.01 X 2 X 96500 = 1930 C, and the theoretical time: t = Qli = 1930/(0.27 X 3600) = 2 h. W o r k - u ~Mercury . was carefully separated from the catholyte and the water-ethanol solution was remuved inn rotary evaporator. Fifty milliliterr of C'HCI. was rhrn added, and the organlc phase was washed with 20 ml. water. I t was dried nwr Mg30;, and the sdvent wasevsporat~d,affording an almost pure colorless oil: Ibdg (yreld:83'1; N\'\IH 'H60 hlHn: 1.14 r t , 6, J = 7 H,1;4.13 dq, 4, Jllll = 7 H z : . / ~ .=~ 7 Hz); 3.: (dl ,Ii H = 2 Hz). Diethyl chloromethylphosphonate 3 Two and fifty-five hundredths grams of 1 (0.01 mol), 2.31 g of CHBC02NH4(0.03 mol), and 1.8 g of CHG02H (0.03 mol) were dissolved in 30 mL of e 111 mixture of water/95% ethanol, and the solution was poured into the beaker over the mercury pool. Two and thirty-one hundredths grams of CHaC02NH4dissolved in 10 mL of a 111 mixture of water19590 ethanol were ~ o u r e dinto the anodic compartment. The current value was set at0.27 A for 4 h. Wark-up. Mercury was carefully separated from the catholyte, and the waterethanol solution was removed in a rotary evaporator. Fifty milliliters of CH2CI2was then added, and the organic phase was washed twice with 10 mL saturated aqueous NanCOa. It was dried over MgSOr, and the solvent was evaporated affording an almost pure colorless oil: 1.50g (yield: 80%)NMR'H 60 MHz: 1.4 (t, 6, J =7Hz); 3.62 (d, 2, JPH = 10 HZ);4.33 (dq, 4, JHH = 7 HZ,JPH =7 Hz). Llteralure Cited 1. (a) Klssingcr, P. T.; Heineman, W. R. II Chem. Educ. 1983. 60, 702: (bl Desbene1987,M,S6:(el Carriedo, ~ ~ ~ ~ ~ ~ ~ ~ , A . : J.;Oeabmo,P. B e r t h e iL.~Jt. Chsm.Edur. , G.A. JChem.Edue. 198S,65,1020:(dlPun~emy,R.S.;Dentan.M.B.:Armstmng,N. R.J. Chern.Educ. 1989.66.877. 2. (a) lversen. P. E. J. Chem. Edue. 1971,48.136:lb) Pouliquen, J.: Heint8.M.: S a k . 0 . : Troupel, M. J . Cham.Educ. 1986.63,1013. 3. Koso1aooff.G.M. J. Am. Cham.Sac. 1947.69.1002;Bakkss,S.'Julltard.M.;Chanon,M.
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