Ytterbium(II) as a mediator in organic electrosynthesis—possibilities and limitations

Electrochimica Acta 48 (2003) 1065 /1071 www.elsevier.com/locate/electacta

Ytterbium(II) as a mediator in organic electrosynthesis* possibilities and limitations /

Raquel Andreu, Derek Pletcher * Department of Chemistry, The University, Southampton SO17 1BJ, UK Received 21 October 2002; received in revised form 10 December 2002

Abstract The kinetics of the Yb(III)/Yb(II) couple in aprotic solvents are rapid at several electrode materials and the reducing power of the Yb(II) may be modified substantially by choice of solvent and electrolyte. Electrogenerated Yb(II), when present in stoichiometric amounts, allows the stereoselective reduction of 1,3-dibenzoylpropane to the cis isomer of a cyclic diol. Ytterbium(III) is not, however, a straightforward ‘mediator’ because, after the reduction of 1,3-dibenzoylpropane by the Yb(II), the Yb(III) is bound to the organic product. Both use of an aluminium anode or addition of trimethylsilylbromide lead to release of the Yb(III); then the ytterbium acts as a catalyst. Such procedures, however, lead to loss in the stereoselectivity of the reduction and the reactions are slow so that the regeneration of the Yb(III) does not enhance the current density. The current density is always low, limited by mass transport of the catalyst. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Mediated electrolysis; Ytterbium(III)/(II); 1,3-Dibenzoylpropane

1. Introduction Divalent rare earth ions, especially samarium(II), have now become very popular reducing agents in organic synthesis [1 /3]. A particularly attractive feature of such reagents is the ability to tune their formal potentials by the choice of anion (iodide, bromide, chloride, triflate) [4 /6], solvent [7] or complexing agent [8 /11] and this has often been demonstrated using cyclic voltammetry. The metal(II) reagents are, however, relatively expensive. They are also air sensitive and often have to be used in more than stoichiometric amounts. Hence, it is not surprising that there is interest in procedures that commence from metal(III) salts or where the metal(II) species can be employed in catalytic quantities. For

* Corresponding author. Tel.: /44-2380-593-519; fax: /44-2380593-781. E-mail address: [email protected] (D. Pletcher).

example, methods using samarium metal [12 /14], magnesium [15,16] or zinc amalgam [17] as reducing agents to generate/regenerate Sm(II) have been described. Electrolysis is another obvious way to reduce the cheaper metal(III) reagents to the active metal(II) species. Indeed, some 10 years ago, a series of papers by Pe´richon and coworkers reported a number of electrosyntheses in aprotic solvents employing a consumable anode and catalytic quantities of Sm(III); reactions reported included the coupling of aldehydes and ketones [18], esters [19] and organic halides [20], the cleavage of allyl ethers [21], the cyanomethylation of esters [22], the allylation of ketones [23], and the synthesis of g-lactones [24]. Surprisingly, these do not seem to have been followed up and, certainly, electrolytic recycling of rare earth reducing agents has not achieved any popularity with synthetic chemists. More recently, Parrish and Little [6] have described an electrochemical cyclisation reaction using Sm(II) or Yb(II) as the active intermedite produced by reduction of the commercially available triflate salts. They, how-

0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0013-4686(02)00844-7

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ever, used a mercury cathode and only reported reactions where a stoichiometric quantity of the rare earth reagent was employed. This paper coincided with some unsuccessful attempts in our laboratory to carry out some Sm(II) mediated electrosyntheses and this has led us to investigate the reactions in more detail with the aim of developing procedures that are easy to transfer to synthetic laboratories. In fact, we have focussed on systems involving the Yb(III)/Yb(II) couple. Preliminary studies of the voltammetry of this couple in aprotic solvents have been reported by Bulhoes and Rabockai [25] as well as Parrish and Little [6].

2. Experimental The solvents, electrolytes, ytterbium(III) triflate and the 1,3-dibenzoylpropane were all obtained from Aldrich Chemicals. The acetonitrile was distilled from calcium hydride but the other materials were used as supplied. Cyclic voltammograms were recorded in a glass cell with disc working electrodes and a Pt spiral secondary electrode in the same compartment. The reference electrode was a commercial, aqueous saturated calomel reference electrode (SCE) within a separate compartment with a Luggin capillary that was placed approximately 1 mm from the surface of the working electrode. Electrolyses were also carried out in glass cells with a working electrode compartment requiring approximately 50 cm3 of solution. In some cases, a divided cell was used; the working and counter electrodes were separated by a glass sinter but the detailed designs of the cells were different for mercury and solid cathodes. In others, the cell was undivided; with the solid cathodes, the anodes were rods placed at the centre of a cylindrical cathode while with the mercury pool, the anode was a rod placed above the pool. In most cases, the reference electrode was again within a separate compartment connected to the working electrode compartment via a Luggin capillary. Some convection was introduced into the electrolytes by using a magnetic stirrer and/or a stream of nitrogen. The experiments were all carried out at room temperature and solutions were all thoroughly deoxygenated with a stream of nitrogen before and during the experiment. The cells were controlled with a HiTek potentiostat type DT2101 and function generator, type PPR1. Voltammograms were recorded on a Bryans series 60000 x/y recorder and charges were measured with a home-built integrator. After an electrolysis, the working electrode solution was poured into 200 cm3 of 1 M aqueous HCl and the organic products were extracted with 2 /200 cm3 volumes of ether. The combined ether phase was washed twice with brine and then dried over MgSO4 and filtered before the ether was removed under vacuum. The

products were examined by hplc and NMR spectroscopy. The hplc analysis was carried out on a Hewlett Packard series 1100 instrument with an UV detector. The sample was dissolved in acetonitrile, injected onto a Phenomenex column and eluted with a gradient programme employing acetonitrile/trifluoroacetic acid mixtures. The product were detected at 220 nm and gave well resolved peaks, identified by comparing retention times with standard samples. 1H and 13C NMR were recorded on a Bruker 300 MHz spectrometer using CDCl3 as solvent and tetramethylsilane as internal standard. The ratios of cis :trans isomers, II:III were estimated by taking the ratio of the integrated responses in the ranges 6.98 /7.10 to 7.15 /7.28 ppm; these are the signals for the aromatic protons in the cis and trans isomers, respectively.

3. Results and discussion 3.1. Cyclic voltammetry Cyclic voltammograms were recorded for solutions of ytterbium triflate (1 /20 mM) at several electrode materials, in two solvents and with three tetrabutylammonium halides as the electrolyte. In all media, the solutions were thoroughly deoxygenated with a stream of N2 bubbles. Fig. 1 illustrates typical cyclic voltammograms in DMF/Bu4NBr (0.1 M) at (a) vitreous carbon and (b) nickel disc electrodes. At vitreous carbon, a single, wellformed reduction peak is observed at/1400 mV versus SCE and the cyclic voltammogram has all the characteristics of a reversible 1e  reaction; for example, the peak separation is 60 mV and the peak current is proportional to the square root of the potential scan rate. The diffusion coefficient for Yb(III) calculated from the peak current densities is 3/106 cm2 s 1. Table 1 reports data for the Yb(III)/Yb(II) couple in six media, all obtained at a vitreous carbon disc electrode. In acetonitrile as a solvent, it can be seen that there is a substantial negative shift in the formal potential when the halide electrolyte is changed from iodide to bromide to chloride and, indeed, with chloride the reduction of Yb(III) is not seen positive to /2000 mV. This indicates that the halides complex Yb(III) and the stability constant for the complexes increases along the series I  B/Br  B/Cl  and that Yb(II) is a significantly stronger reducing agent in the bromide medium compared to the iodide medium. These trends are similar to those reported previously with THF as the solvent [4,5]. Surprisingly, however, when the solvent is DMF, the formal potentials are much closer together and this implies that DMF must be a ligand able to compete with the halides in bonding within the Yb(III) species.

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Fig. 1. Cyclic voltammograms for a solution of Yb(CF3SO3)3 (2 mM) in DMF/Bu4NBr (0.1 M) at (a) a vitreous carbon disc (b) a nickel disc. Potential scan rate 100 mV s 1.

Table 1 Data from voltammograms of Yb(III) at a vitreous carbon disc in two solvents Electrolyte

I Br  Cl 

DMF

CH3CN

Ee (mV) vs. SCE

DEp (mV)

Ip/c (mA cm 2 mM 1)

Ee (mV) vs. SCE

DEp (mV)

Ip/c (mA cm 2 mM 1)

/1355 /1370 /1520

90 60 240

0.12 0.14 0.13

/670 /1080 No peak

160 380 /

0.15 0.15 /

The electrolytes were tetrabutylammonium halides (0.1 M). Potential scan rate 100 mV s 1. The formal potentials, Ee were estimated as the mean of the cathodic and anodic peak potentials.

Table 2 reports data for the Yb(III)/Yb(II) couple in DMF/Bu4NBr at four different electrode materials. The response at an amalgamated gold electrode is very similar to that at vitreous carbon. On the other hand,

Table 2 Influence of the electrode material on the voltammetry of Yb(III) in DMF/Bu4NBr (0.1 M) Electrode

Ecp (mV) vs. SCE

Vitreous C /1400 HgAu /1400 Ni /1450 Pb /1450

DEp (mV) Ip/c (mA cm 2 mM 1) 60 70 200 200

0.14 0.16 0.14 0.15

at nickel and lead, the reverse, anodic peak is less wellformed and is shifted to more positive potentials, see Fig. 1(b). Moreover, the cathodic peak is more drawn out and there is current for a competing electrode reaction at potentials just negative to the peak. It appears that the kinetics of electron transfer are slower at these metals and they promote other reactions. Coulometry carried out at a mercury pool electrode confirmed that the reduction of Yb(III) was a 1e  reaction. In acetonitrile, the solid electrodes were found to passivate rapidly. This was not the case in DMF and this is the reason that DMF was used for all later experiments. When the medium is DMF/Bu4NBr, the reduction of Yb(III) to Yb(II) was accompanied by a change in the solution from colourless to yellow.

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The model reaction used in this study was the cyclisation of 1,3-dibenzoylpropane. This reaction was reported by Parrish and Little [6] and can lead to two isomers

other hand, there was no increase in the peak current density for the reduction of Yb(III) to Yb(II). Indeed, even with a large excess of 1,3-dibenzoylpropane, there is little increase in the cathodic peak height. This is

Hence, cyclic voltammograms were recorded for solutions of 1,3-dibenzoylpropane in DMF and at all electrode materials, it gave an irreversible reduction peak at around /2000 mV versus SCE. No reduction was observed in the potential range where the Yb(III) was found to reduce. Voltammograms were also recorded for various mixtures of Yb(III) and 1,3-dibenzoylpropane, mainly at a vitreous carbon disc in DMF/ Bu4NBr but the observations were the same for all combinations of electrode material/solvent/halide investigated. An equal concentration of Yb(III) and 1,3dibenzoylpropane gave voltammograms where the Yb(III)/Yb(II) couple had become completely irreversible, i.e. the Yb(II)/substrate reaction is rapid. On the

illustrated by the voltammograms shown in Fig. 2, in fact recorded at a mercury pool electrode in the small preparative cell immediately prior to an electrolysis. With a tenfold excess of 1,3-dibenzoylpropane, the reduction peak current only doubles. Hence, although the reaction of Yb(II) with the 1,3-dibenzoylpropane is rapid, regeneration of Yb(III) does not occur on the same timescale. This is not a desirable conclusion for mediated electrosynthesis. The maximum current density will be observed when the electrode reaction is mass transfer controlled with respect to the organic reactant and this will only be observed when the whole catalytic cycle is fast and the Yb(III) is regenerated is completely reformed rapidly. A reaction that is mass transport

Fig. 2. Cyclic voltammograms for a solution of Yb(CF3SO3)3 (5 mM) in DMF/Bu4NBr (0.1 M) at a mercury pool electrode (a) before and (b) after the addition of 50 mM 1,3-dibenzoylpropane. Potential scan rate 100 mV s 1.

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Table 3 Controlled potential electrolyses of 1,3-dibenzoylpropane in a divided cell with Hg cathode and Pt anode Mole ratio, I:Yb(III)

b

12.5:0 12.5:25c 10:1c a b c

E vs. SCE (mV)

/2000 /1200 /1200

Chargea (F)

3 3 0.1

Conversion (%)

100 98 10

Product yield (%) II

III

67 100 10

33 0 0

Charge for current to decay to B/5% of initial value. Based on I. Medium DMF/Bu4NBr (0.1 M). Medium AN/Bu4NBr.

controlled with respect to the mediator present in catalytic quantities will inevitably be limited to a much lower current density 3.2. Preparative electrolyses In the first set of experiments, see Table 3, a series of controlled potential electrolyses were carried out in a divided cell with a mercury pool cathode and a Pt anode. When there was no ytterbium(III) in the catholyte, the direct reduction of 1,3-dibenzoylpropane, I, required the use of a rather negative potential, /2000 mV versus SCE. After extraction, the electrolysis gave the cyclopentane product in good yield but both cis and trans isomers, II and III were formed. When Yb(III) was added to the catholyte, electrolysis becomes possible at less negative potentials; /1200 mV was selected so that the reduction of Yb(III) is mass transfer controlled but the direct reduction of I is quite impossible. With a stoichiometric amount of Yb(III) (the only conditions reported by Parrish and Little [6]), the reduction of I again goes to completion and a quantitative yield of the cyclopropane product is also found. Interestingly, however, the mediation by Yb(II) introduces stereoselectivity into the reaction and only the cis isomer, II, is formed (as also reported by Parrish and Little [6]). On the other hand, when only a catalytic quantity of Yb(III) is used, the reduction stops at a very low conversion although only one product, the cis isomer, II is isolated. Both electrolyses took place with the initial current density expected for the mass transport controlled reduction of the Yb(III) present and the current density Table 4 Controlled potential electrolyses of 1,3-dibenzoylpropane at /1400 mV vs. SCE in an undivided cell with Hg cathode and Al anode Mole ratio of I:Yb(III)

% Conversiona

20:1 10:1 5:1 1:1

26 55 100 100

Medium DMF/Bu4NBr (0.1 M). a Conversion when current drops to B/5% of initial value.

decayed during the electrolysis. These observations suggest that the ytterbium(III) reformed in the Yb(II)/ 1,3-dibenzoylpropane reaction is not in an electroactive form, rather it is bound to the product. It is not released prior to the aqueous work-up.

Two methods to make the reactions catalytic were therefore investigated. Pe´richon et al [18 /24] have proposed the use of a sacrificial anode in an undivided cell to generate a metal ion capable of a transmetallation reaction to release the Yb(III). A series of electrolyses were carried out in an undivided cell with a mercury pool cathode and an aluminium rod anode. These electrolyses were all carried out at /1400 mV in DMF/Bu4NBr and the mole ratio of I:Yb(III) was varied, see Table 4. It can be seen that in this cell configuration, the electrolyses are catalytic in Yb(III) but not so indefinitely. Above 10% Yb(III) is necessary for complete conversion of the 1,3-dibenzoylpropane, I, to product. Moreover, while the product is formed in good yield, it is a 1:1 ratio of the cis and trans isomers. This is a surprising result and it implies that the presence of Al(III) in solution influences the mechanism before the stereochemistry is fixed, i.e. the carbon /carbon bond is formed. For example, the reduction of 1,3dibenzoylpropane by Yb(II) could lead to anion radical/ Yb(III) ion pair or covalently bonded intermediate; a transmetallation reaction could then lead to displacement of the Yb by Al at this stage with consequent loss in the stereoselectivity. The complexity of the these reactions is also illustrated by a similar electrolysis

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carried out with a 5:1 mole ration of I:Sm(III) in DMF/ Bu4NI, the reaction again went to conclusion but the product isolated was only the cis isomer, II. The difference between the Sm/I and Yb/Br systems has not yet been investigated further. The other approach to making the reactions catalytic in Yb(III) used a trimethylsilylhalide [26]. Preliminary experiments showed that trimethylsilylchloride could not be used because chloride ion was formed from the reaction between the ytterbium intermediate and the silyl chloride. This chloride then complexed the Yb(III) released and shifted the potential for its reduction to a more negative value. Hence, the Yb(III) did not reduce further at the potential of the electrolysis. Moreover, both trimethylsilylchloride and trimethylsilylbromide were found to reduce at the mercury cathode at the potential of the electrolysis. Hence, an electrolysis was carried out in the divided cell with mercury cathode and platinum anode at /1400 mV. The electrolyte was in DMF/Bu4NBr and the mole ratio of I:Yb(III) was 5:1. After the current had dropped to zero (1e  per Yb(III)), an aliquot of Me3SiBr (20 mol% of initial I) was added. The current returned to the initial value and the electrolysis was recommenced until the current again dropped to zero. Then a further aliquot of Me3SiBr was added. This sequence was repeated eight times to give a 80% conversion of the 1,3-dibenzoylpropane without any indication that ytterbium was becoming inactive or lost from solution; the current still returned to its initial value. After work up, the product was a mixture of the two isomers, II and III but from this procedure the ratio of cis :trans was 1.6:1.0. Many of the Sm(II) mediated reactions reported by Pe´richon and coworkers [18 /24] employed a high surface area, nickel foam cathode. This is attractive since it avoids the use of mercury and should also allow the reactions to go to completion more rapidly. Hence, a nickel cathode was investigated for the reduction of 1,3dibenzoylpropane. Electrolyses were carried out in an undivided cell with a cylindrical nickel foil cathode and an aluminium rod anode. It was again possible to carry out the reactions with less than a stoichiometric quantity of Yb(III) (see Table 5) confirming that the use of an Al anode does allow completion of the catalytic cycle and regeneration of Yb(III). Furthermore, the electrolyses Table 5 Controlled potential electrolyses of 1,3-dibenzoylpropane in an undivided cell with Ni cathode and Al anode. Mole ratio of I:Yb(III) E vs. SCE (mV) Charge (F)a % Conversion 5:1 2.5:1

/1800 / /1700

Medium DMF/Bu4NBr (0.1 M). a Based on I.

2.4 3.0 4.0

50 70 90

again led to the formation of a mixture of the cis and trans isomers of the substituted cyclopentane, II and III. It was found essential to pass charges in excess of that equivalent to 2e per molecule of I. This confirms the observation made during the cyclic voltammetry that there is a competing reaction associated with reduction of the solvent and/or electrolyte with a nickel cathode and at potentials negative to the Yb(III) reduction peak. It was, however, noted that for much of the electrolyses, the current was lower than expected and passivation of the nickel surface was suspected. Certainly, it was possible to observe solid material deposited on the surface and scraping away this solid led to an increase in current. It was also noted that the direct reduction of 1,3-dibenzoylpropane was possible at a nickel cathode; an electrolysis at /2000 mV versus SCE also gave a good yield of the two isomeric products. The mediated reduction was therefore not attempted using constant current electrolysis (as reported by the French Group [18 /24] for the samarium mediated reactions) since one could not then distinguish the direct and mediated reductions with certainty.

4. Conclusions It is confirmed that the formal potential of the Yb(III)/Yb(II) couple can be optimised by selection of the aprotic solvent and the halide used as electrolyte. The Yb(III)/Yb(II) couple also has a high rate constant for electron transfer at clean electrode surfaces. In addition, ytterbium(II) undoubtably is a powerful reducing agent with potential applications in organic synthesis and it may readily be generated by electrolysis. Unfortunately, the ytterbium(III) precursor it is most fruitfully used in stoichiometric quantities and there are clear problems associated with using it as a mediator in electrosynthesis. Firstly, the ytterbium(III) is not regenerated in an electroactive form during reduction of the organic substrate. Rather, it remains complexed to the reduced organic product. While two procedures for breaking this complex and regenerating the Yb(III) in an electroactive form have been demonstrated, they lead to the loss of the stereospecificity noted for reactions employing stoichiometric amounts of Yb(III) and the reactions are not fast enough to allow the mediated electrosyntheses to be carried out at a high current density. The achievable current density is limited by mass transport of the electroactive Yb(III) species not the mass transport of the organic substrate. Secondly, solid electrodes suffer passivation during extended electrolysis and mercury would be the recommended cathode material.

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Acknowledgements The authors would like to thank the Secretarı´a de Estado de Educacio´n y Universidades, Spain for financial support for a Fellowship for R.A.

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