Chapter III.2 Electrocatalytic Reactions with Electron Carriers

231

C h a p t e r 111.2

ELECTROCATALYTIC REACTIONS WITH ELECTRON CARRIERS

SIGERU TORI1 Department of Applied Chemistry, Faculty of Engineering, Okayama University, Okayama 700, Japan

ABSTRACT Much attention has been paid to electron carriers as electrontransfer catalysts in redox systems which play an important role in recent electroorganic synthesis. This paper reviews some current topics on electrochemical reactions with electron carriers in which each of the redox complexes (cerium, ruthenium, and selenium redox complexes and/or organic redox systems for the functionalization of complex molecules and palladium, cobalt, tin and lead metal complexes for carbon-carbon bond formation) mediates an electron transfer in key reaction processes. INTRODUCTION Recently, electrocatalytic reactions with reactive metal complexes and organic redox systems as electron carriers have become potential methods not only for functionalization in complex molecules, but also for new carbon-carbon bond formation. Today, various electron-transport systems with a variety of reactive metal complexes and organic redox systems have brought new possibilities for developing synthetic methodology (ref. 1). This review deals with, first, the reactions of cerium, ruthenium, selenium and tellurium redox complexes and as organic oxoammonium redox systems for the functionalization of complex molecules and, second, the reactions of palladium, cobalt, tin and lead metal redox complexes for carbon-carbon bond formation. The recycled use of the electron-carrier systems for the functionalization of complex molecules involves electrochemically activated metal redox complexes such as tetravalent cerium, octavalent ruthenium, divalent selenium and divalent tellurium complexes together with organic redox systems. On the other hand, the metal complexassisted carbon-carbon bond-making reactions involve the recycled use

232 of divalent palladium, cobalt and lead metal complexes together with tetravalent tin complexes.

FUNCTIONALIZATION OF COMPLEX MOLECULES WITH ELECTRON CARRIERS Indirect electrooxidation (an ex-cell method) of the side-chain of alkylbenzenes has been performed by the recycled use of (NH4)2Ce(N03)6 (CAN) in various solvent systems (ref. 2). The side-chain oxidation of p-methoxytoluene with CAN in methanol proceeds smoothly, yielding anisaldehyde in 94% yield (Scheme 1).

1

-n Oil-Water separation

63 ,

&” 8 Electrolysis

Scheme 1.

Electrooxidation of p-methoxytoluene by an ex-cell method.

Electrooxidation of the recovered cerium(II1) salts is performed smoothly in methanol by passing 1.1-1.2 F/mol of electricity to give a reddish methanol solution of CAN, which can be used for the subsequent oxidation of p-methoxytoluene. The 10-fold repeated use of CAN produced the desired aldehyde in over 90% yields. A double mediatory system consisting of RuOq/Ru02 and [Cl+]/Clredox couples has been developed for the indirect electrooxidation of alcohols and aldehydes (ref. 3 ) . The reaction proceeds in the following manner (see Scheme 2) : (1)

233

ffi

PHASE

IC PROCESS ] A

E PROCESS: Elechpn Transfer Process C PROCESS: Chemical Reaction Process

Scheme 2. Double mediatory system of Ru(VnI)/Ru(IV)-[Cl'~Ci redoxes in two phase solution.

R-R

Scheme 3

0

oxidation of the substrate with ruthenium tetraoxide (RuO4) in the organic layer [C PROCESS], (2) regeneration of ruthenium tetraoxide from ruthenium dioxide (RuO2) with active chlorine species (C12, NaOC1, C120, etc., providing [Cl+]) [ C PROCESS] and ( 3 ) oxidation of chloride ion to [Cl+] on the anode in the aqueous layer [ E PROCESS]. The range of applicability of the procedure is clarified by oxidations of (1) secondary alcohols to ketones, (2) primary alcohols and aldehydes to carboxylic acids, (3) 1,n-diols to lactones and keto

234

acids and (4) carbohydrate derivatives. Typical examples of the RuOq-mediated transformations are summarized in Scheme 3. An oxyselenation-oxidative deselenation sequence provides doublebond transpositioned allylic alcohols and ethers from olefins (ref. 4a). One-step preparation of allylic derivatives ( 4 ) from isoprenoids (1) (Scheme 4) is characterized by (1) electrochemical generation and recycled use of selenation reagents, PhSeOH, from a catalytic amount of diphenyl diselenide, (PhSe)2 and ( 2 ) deselenation through further electrooxidation ( 2 to 3) in situ, leading to allylic alcohols ( 4 ) ( R = H). The electrochemical conversion of citronellol ( 5 ) to rose oxide ( 7 ) , involving the oxyselenationdeselenation process ( 5 to 6 ) , is shown in Scheme 5.

LyT -T yLy [ PhSeOH]

oJSePh

SePh

3

2

1 Scheme 4 (SePh)p

-e ROH

(PhSeOR)

Po. Scheme 5

4

1

EG acid OH M.

5

RO

6

CH,CI,-LiCI0,-

2

-fii;

EtNClOr-(pt) 02%

7 cis I trans (74 126)

[af: -52: CHCI,

Electrochemically produced aryl selenides (ArSe-) and aryl tellurides (ArTe-) can be used as reducing reagents for the removal of heteroatoms, i.e., OR, OMS, SeAr, TeAr, etc., attached to the a-

235

position of carbonyl functions (ref. 4b). The electron-transfer flow from the cathode to a substrate ( 8 ) , leading to 9 via the selenide 10, mediated by electron carriers (ArSe-, ArTe-) is shown in The procedure can be extended to a,P- epoxy ketone Scheme 6 .

r

2 Ar-S;

Scheme 6.

1

(or Ar-Te- )

Ar-SeSe-Ar (or Ar-TeTeAr)

Ar-Se- and Ar-Te as mediators (electron carriers).

systems. The ring opening of the epoxy ketone 11 proceeds first by attack of the phenylselenide anion on the CL-position of 11, producing the oxyse phenylselenyl group P-hydroxy ketone 13 ring opening of the

enide 12, and the subsequent elimination of the is induced by a phenyl selenide anion to give the (Scheme 7). The PhSe--catalysed electroreductive steroid epoxide 14 in an MeOH-NaClOq-(Pt/Pt)

system affords the orresponding 1-hydroxy-3-keto derivative (15) in 7 2 8 yield (Scheme 8).

11

Scheme 7.

12

13

Ar-Se- and Ar-Te- as mediators for ring opening of a, p - epoxyketones.

236

....J\/cOOMe

'---WCWMe [ PhSe- ]

72% 15

14 14 (1 mml), PhSeScF'h (0.02mmol), C&(CO&t), (5 rnmol) Md)H - NaCIO, (0.2M) - (PvPt), 3V. 4.5 F/ml

Scheme 8.

Electroreductive ring opening of steroid epoxide.

In a similar manner, the reductive removal of a methanesulphonyl group at the a-position of the y-lactone 16 gives the corresponding Good results are phenylselenide (17) in 82% yield (Scheme 9). obtained for the removal of the chlorine atom of the &U-dichloro-ylactone 18 when the phenylselenide or phenyltelluride anion is employed as an electron carrier in an MeOH-NaC104(0.2 M)-(Pt) system (Scheme 10).

17

16 16 (Immol). (PhSe), (0.6mmol). McOH-NaCIO,(O.Z M)-(PI),3V

Scheme 9.

Selenation of D-ribnolactone.

Conditions: 18 (1 mmol), McOH NaCIQ (0.2 M) - (R),3V. divided cell, calalfic mount of WSch or (PhTch ~

Scheme 10.

Dechlorination of a,a-dichloro-?lactone.

The two chlorine atoms attached to 18 are reductively removed by attack of the phenyltelluride anion to give 20 in 76% yield, b u t the phenylselenide anion can eliminate only one of the chlorine atoms to lead to the corresponding monochloro-r-lactone (19). Most recently,

237 indirect electrooxidation of thioamides with organotellurium as a mediator has been reported (ref. 5). Few organic molecules have been utilized in electrochemical mediatory systems (ref. 1). Their study is regarded as a promising field in electroorganic synthesis. A single mediatory system with the oxoammonium ion 21 has already been

developed as

'tL0

R

/

'CHOH

R" 21

23

> L O

\

22

24

Scheme 11. Double mediatory system with oxoammonium ion.

TABLE 1 Oxidation of Alcohols with N+=OBr+/N-O' - [Br+l/Br- Redoxes Alwhol

C7H150H

W

O

H

Fmol

Yield. %

22

89

2.4

90

2.8

87

2.0

87

4.4

88

wH Alwhol

F/mol Yield, %

3.0

87

ooH 2.8 90

b

OH

3.0 87

Alcohol ( I mmol), N - 0 compound (0.01mmol). CH2C12(3 ml)- 25% aq. NaBr (5 mi)-Cpt) system. 20 mA/cmZ, undivided cell.

238

a means for the oxidation of alcohols (23) to the corresponding aldehydes and ketones ( 2 4 ) (ref. 6). More recently, an efficient double mediatory system with the oxoammonium ion 21 has been developed, in which the oxidation of alcohols with 21 is accomplished by use of an electrochemically produced oxidizing agent, [Brf], leading to an N-oxyl radical (22 to 21) (Scheme 11) (ref. 7 ) . Some typical data are given in Table 1. CARBON-CARBON BOND MAKING REACTIONS WITH ELECTRON CARRIERS An efficient electroreductive coupling of aryl bromides and iodides into biaryls has been performed by electrolysis with Pd(0) and/or Pd(I1) catalysts in a DMF-Et4NOTs-(Pb cathode) system (Scheme 13) (ref 8).

It is interesting that the cross-coupling between two different

Scheme 12

pq(78%)

Scheme 13.

N

W

(91%)

Pd(0)-promoted aryl-aryl coupling.

Q-4J (91%)

239

aryl iodides (25 and 27) (R

=

p-Me2N and p-t-Bu; X

=

I) leading to 28

could be achieved by using stoichiometric amounts of the Pd(0) complex in an electrolysis medium (Scheme 12). The results suggest that an aryl-Pd complex 26 formed i n s i t u would be accumulated and coupled with iodide 27 via two-electron reduction in a highly selective manner. Although the mechanism of the Pd(0)-catalysed electroreductive aryl-aryl coupling has not been clarified, it is likely that, in the early stage of the reaction, oxidative addition of Pd(0). complexes to aryl halides ( 2 9 ) would take place, affording 30, which would, in turn, suffer from discharge at the cathode (to 31, see Scheme 13) to give the biaryl 33 v i a an organopalladium complex (32). 4-Bromo- and 2-bromopyridines were also converted into the corresponding bipyridyls. Recently, electrooxidative functionalization of olefins in a palladium diacetate-benzoquinone double mediatory system has been reported (ref. 9). A variety of olefins can be converted to the corresponding ketones by electrooxidation in an MeCN (or DMS0)Et4NBF4 system in the presence of palladium diacetate ( 5 mol%) and benzoquinone (30 mol%) . Vitamin B12 derivatives and vitamin B12 model compounds have been used as recycling electrocatalysts for the reduction of alkyl halides. For example, a P-haloethyl protecting group of an acid can be readily deprotected by reductive elimination (ref. 10a). A novel Michael-type addition of alkyl halides to a,eenones has been realized by using a cobalt(II1) complex as an electrocatalyst (ref. lob). Cyclization by trapping of a free radical with an internal x-bond system is a promising strategy for the construction of carbo- and hetero-ring molecules under mild conditions. The electrochemically regenerated cobaloxime (Co(1)) 37 from cobaloxime (Co(111)) has been exploited as a mediator for the reductive cleavage of C-Br bonds of bromo compounds (34) to afford 36 v i a 35 (Scheme 14) (ref. 11). No cyclization was observed in the absence of the cobalt catalyst. An interesting feature of the present radical cyclization is that the reaction can be carried out in methanol, in contrast to the trialkyltin hydride- promoted radical reaction in aprotic nonpolar solvents Cobalt(1)-mediated electroreduction of 38 in methanol results in

.

240

36

35

Cathode Scheme 14. Co(1)-mediated intramolecular radical cyclization.

,".

0

"0

-

-0..

-74

38

39

Scheme 15.

37

40

Comparison of radical cyclizations with &(I) or B%SnH.

the formation of 40 in 73% yield, whereas treatment of 38 with trin-butyltin hydride in benzene produces predominantly 39

(Scheme 15).

Interestingly, depending on the structure and stability of the radical intermediate, two reaction modes, namely normal hydrogen abstraction or concomitant addition and elimination of the cobalt complex 37, are available.

241

The ring enlargement of a-(bromomethy1)cycloalkanones via an acyl rearrangement of a radical intermediate promoted by cobalt complexes has been investigated (ref. 12). The electrochemically produced low-valent cobalt complexes derived from the chloropyridine 9 cobaloxime 37 can be used for this purpose. The electrocatalytic rearrangement of 4 1 in an MeOH-EtqNOTs/KOH-(Pt) system in the presence of 3 7 affords the large ring enlarged cycloalkenones 42 in 51-74% yields (Scheme 16).

Scheme 16.

42

43

74%

17%

44

45

50%

11%

Co(I)-mediated alkyl radical associated acyl rearrangement.

In contrast, the reduction with triphenyltin hydride gives the Recently, catalytic carbon saturated product 4 4 in 50% yield. skeleton rearrangements with a simple vitamin B12 model complex as an electron carrier in an electrolysis system have been investigated (ref 13). Electrochemical allylation of aldehydes and ketones in methanol is achieved by the electroreductive regeneration of diallyltin reagent in the presence of a catalytic amount of tin, affording the corresponding homoallyl alcohols in 72-918 yields (Scheme 17)(ref. 14). For example, diallyltin dibromide ( 4 9 ) , prepared from metallic tin and ally1 bromide in methanol, reacts immediately with benzaldehyde to provide tin complex of 47 which undergoes alkoxy exchange with methanol, affording the desired homoallyl alcohols ( 4 7 ) and allyldibromotin monomethoxide ( 4 8 ) . In contrast to the reaction in THF, the reaction rate of allyltin dibromide ( 4 9 ) with

242

benzaldehyde in methanol is so fast as to avoid loss of the tin reagent by electroreduction. Meanwhile, 4 8 is susceptible to electroreduction so that it can be converted electrochemically to di and/or zerovalent tin, which reacts smoothly with ally1 bromide to regenerate diallyltin dibromide ( 4 9 ) .

MeO(BrhS -n

Scheme 17.

TABLE 2

Tin-promoted allylation.

Allylation of Aldehydes and Ketones with Allyltin Complexes

Carbonyl Compound

-

H d c H o

H

Allylated Product

Yield, %

O

81

G

OH

O

V

H

O

-

"p 80

Another interesting feature of this reaction is that after extraction of 47 with hexane, the residual methanol solution can be used further nine times for the allylation to provide 47 in 9 0 - 9 5 8 yields by repeated extraction and electrolysis, demonstrating that

243

the tin reagent is satisfactorily recycled in this electrolysis system. Some results are shown in Table 2.

I'''@[

50 R-CHO

R

52

53

Scheme 18

Electroreductive regeneration of low valent metals has attracted much attention as a promising procedure for the metal recycle system. A new PbBr2-mediated electrolysis system involving both Pb(O)/Pb(II) and Br-/Br+ redox processes (Scheme 18) has been found to promote the allylation of carbonyl compounds, together with halofunctionalization of olefins in a single compartment cell (ref. 15). The electrolysis is carried out in a beaker-type undivided cell fitted with two platinum electrodes. For instance, a mixture of the aldehyde 50 ( R = Ph; 1 mmol), allyl bromide (51) (2 mmol) and cyclohexene (2 mmol) in a DMF/AcOH-PbBr2 system is electrolysed under a constant current (10 mA/cm2, 3.5 F/mol) to afford the alcohol 52 (R

=

Ph; 90%)

together with 1,2-dibromocyclohexane ( 5 3 ) (70% based on 5 0 ) . The presence of cyclohexene in the electrolysis media is essential for the successful allylation of the aldehydes 50, as its absence results in the complete recovery of 5 0 . Electroreductive allylation of the imine 54 takes place in a metal redox mediatory system. The allylation of 54 with allyl bromide ( 3 equiv.) was carried out in a THF-PbB~(0.05 equiv.)/BuqNBr(O.l M) system in an undivided cell fitted with an A1 anode and a Pt cathode

244

to give the corresponding homoallylamine 55 in 9 8 % yield (Scheme 19) (ref. 16). In place of PbBr2, several metal salts, such as PbC12, Tic14 and BiC13, can be used successfully, but ZnC12 and SnC12 are less effective (Table 3 ) . These metal salts would work as mediators for producing allyl metal reagents as illustrated in Scheme 20. The cathodic reduction of the metal salts probably gives low-valent metals, M ( O ) , which, in turn, react with allyl bromide ( 5 6 X = Br) On the other hand, it is to give the allyl metal complexes ( 5 7 ) . worth noting that the A 1 anode, as a sacrificial anode, releases Al(II1) salts which associate with the imine 5 8 to form an iminium ion intermediate. The latter would react with allyl metal reagent to give a homoallylamine ( 5 9 ) . Recently, electrogenerated bismuth metal assisted allylation of aldehydes in an H20/CH2C12-NaBF4-(Pt) two-phase system has also been developed (ref. 17).

55

54 Scheme 19 TABLE 3

Mediator

Electroreductive Allylation of Imine

Electricity (Flrnol)

Yield of55 (“A) Recoveryof 54 (%)

-

PbBr2

0.8

98

PbCh

1.4

a4

TiCId

1.6

98

BiCl,

2.0

88

ZnCL,

2.0

57

-

SnCI2

2.0

46

26

Conditions: Medialm (0.05 mmol). Imine (1 mmol). AUyl Bromide (3 mmol). BhNBr (0.3 mmol), THF (6ml). undividedcell. ZOmA/cm*.

245

Another application of the Pb(O)/Pb(II) redox mediatory system is The electrolysis is carried out hydrocoupling of imines (ref. 18). in an undivided cell fitted with two Pt electrodes. A 1:l mixture of an imine (60) and trifluoroacetic acid in a THF-PbBr2 (0.05 equiv.) system is allowed to electrolyse at 20 mA/cm2 until most of The electrolysis affords the imine has been consumed (1.8 F/mol). the corresponding 1,2-diamine (61) in 80% yield. Some results are A plausible reaction mechanism is shown in shown in Table 4 . Scheme 22. The presence of trifluoroacetic acid is essential, otherwise little 1,2-diamine (61) is detected.

Anode

M=AI, Zn

58

56 M=Pb, Sn, Bi, Ti

Scheme 20. M(0) / M(n) - mediated electroreductive allylation of an imine.

N/Bn

R'

PbBtj / F A - THF - (Pt)

D1 c

HE;"Bn

R2 60

Scheme 2 1

61

246

TABLE 4

Electroreductive Coupling of Imines

R2

(mmol)

Electricity (F/mol)

Yield of 61,%

1

H

1.O

1.8

80

2

H

1.5

1.8

R

H

1.o

1.9

90

60

Envy

R'

3

Me

TFA

Conditions: Imine (1 mmol). PbBrz (0.05 mmol), THF (6 ml), (Pt) - (Pt), undivided cell. 20 mA/cmz.

Anode

Scheme 22.

Cathode

Pb(0) / Pb(I1) - mediated electroreductive coupling of an imine.

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2 3

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247

10

11

12 13 14 15

16

17 18

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