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I. Chiarotto, M. Feroci / Journal of Organometallic Chemistry 691 (2006) 2589–2592
A dramatic change in the mechanism occurred by
the electrolysis was carried out on a solution of 2-iodothi-
ophene (0.50 mmol), in 30 dm3 of solvent containing n-
Bu4NBF4 (TBAF, 0.20 mol dmꢀ3) as supporting electrolyte,
in the presence of Pd(OAc)2 (0.05 mmol), formic acid
(1.50 mmol) and a tertiary amine (0.10 mmol), under 1
atmosphere of carbon monoxide at ꢀ1.2 V vs. SCE. A
graphite electrode, of apparent area 3 cm2, was used as
working electrode. The counter-electrode was a Pt wire
and the reference was a saturated calomel electrode (SCE).
At first, we have applied to 2-iodothiophene the same reac-
tion conditions used with arylhalides, i.e., DMF/0.2 M
TBAF as solvent, Pd(OAc)2 as catalyst, CO and HCOOH
(Table 1, entry 1). Unlike aryliodides, 2-iodothiophene gave
poor yields in aldehyde (25%), so we tried to vary some
parameters in order to increase the yields. Many solvents
were used, with no or little increments in the yield of thio-
phene-2-carbaldehyde (Table 1, entries 1–5), so we screened
several amines with different sterical bulk as ligands. The cat-
alytic activity increases with increasing bulk around the
nitrogen atom (even if the results suggest other factors
may be involved in the activity of these amines; in fact, it can-
not be excluded an action as a base). For example, 4-dimeth-
ylaminopyridine (DMAP), N,N-diisopropylethylamine
(DIPEA) and 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU)
showed no activity in the reaction (Table 1, entries 6–8),
while the bulkier 1,4-diazabicyclo-[2,2,2]-octane (DABCO)
gave the best result. A synergistic effect of DABCO as ligand
and DMF (in which HCOOꢀ is highly soluble) as solvent
results in 81% yield of aldehyde for 2-iodothiophene (Table
1, entry 9). The position of the iodine atom on the molecule
of thiophene seems not to be important; in fact, starting from
3-iodothiophene, the corresponding aldehyde was obtained
in 82% yield (Table 1, entry 10).
The reactivity of 5-butyl-2-iodofuran is slightly lower
than iodothiophenes. When this reaction was carried out
on this oxygenated compound, the results confirmed the
hypothesis that DMF/DABCO system gives the best result,
with 72% yield (Table 1 entries 11–13).
Because of their p-electron deficient nature, halopyri-
dines normally react slowly to the insertion of carbon mon-
oxide into arylpalladium(II) complexes (formed by initial
oxidative insertion of palladium(0) into the carbon–halo-
gen bond) [9]. Generally, high pressures of carbon monox-
ide are required to suppress side-reactions. In fact, not
significant yields are obtained for this class of heterocycles.
However, using our conditions, under atmospheric pres-
sure of carbon monoxide we obtained moderate yield of
3-pyridinecarbaldehyde (35%, Table 1, entry 17). This
result is very significant because the b-position results to
be less reactive to oxidative addition of Pd(0) [11].
switching to formic acid. As a matter of fact, the synthesis
of aryl aldehydes from aryl halides, CO and formic acid is
formally an hydride transfer from HCOOꢀ. Formate ions
cannot be supplied directly from formic acid; in fact, the
dissociation constant of formic acid in DMF is too low
to allow it to be the direct source of formate ions [8]. More-
over, formic acid cannot be the direct source of formate
ions by electrochemical reduction of its protons. Indeed,
the electrolyses were carried out at a potential of ꢀ1.2 V
vs. SCE, whereas the electrochemical reduction of formic
acid under the same conditions, in the absence of palla-
dium, occurred at ꢀ2.0 V vs. SCE. It is apparent that under
our electrolytic conditions, palladium must play a second
crucial role in supplying the reaction medium with formate
ions. In fact, the reaction between Pd(0)(Ph3P)3 (that
derives from a dissociation equilibrium of Pd(0)(PPh3)4)
and formic acid – presumably via the low ligated complex
Pd(0)(Ph3P)2 – affords a formate anion and the cationic
palladium(II) hydride (Eq. (1)).
Pdð0ÞðPh3PÞ þ HCO2HꢀHPdðIIÞðPh3PÞþ þ HCOꢀ þ PPh3
3
2
2
ð1Þ
ð2Þ
HPdðPh3PÞþ2 þ 1eꢀ ! Pdð0ÞðPh3PÞ2 þ 1=2H2
On the basis of its reduction potential (ꢀ0.84 V vs. SCE),
HPdðPh3PÞþ is reducible under our electrolytic conditions
2
(Eq. (2)). Consequently, the equilibrium sketched in Eq. (1)
is continuously displaced to its right-hand side, generating
a small but constant flux of formate ions. The result is that
formate ions are generated at an adequate rate via a combi-
nation of chemical and electrochemical steps, at a potential
where the direct reduction of formic acid to formate and
H2 cannot occur [8]. When the electrochemical reduction
of aryl halides was carried out under 1 atm of CO, in the pres-
ence of catalytic amount of PdCl2(PPh3)2 and formic acid,
aromatic aldehydes were isolated in high yields [9].
On the basis of these results, we report a Pd(OAc)2 and
amine catalytic system for formylation of some aromatic
heterocycles like iodofuran, iodothiophene and iodopyri-
dine, in order to test the effectiveness of this electrochemi-
cal method.
2. Results and discussion
Thiophene was used as a model compound to study sol-
vents and ligands influence on the reaction. The process is
outlined in the following reaction (Scheme 1).
The following general procedure was used: in a cell with
three separated compartments [10], kept at 50.0 0.1 °C,
The proposed mechanism of the electrosynthesis of alde-
hydes is outlined in Scheme 2 [9].
CHO
+ CO + 1/2 H + I
I
Pd cat/ligand
-
-
+ CO + HCOOH + e
2
2
DMF, 50˚C
-1.2 V vs SCE
X
X
3. Conclusions
1
2
X=O,S
We have obtained in moderate to good yields heteroaro-
matic aldehydes from iodothiophens, iodofuran and
Scheme 1.
Chiarotto, Isabella
