Hybrid super electron donors - Preparation and reactivity

Article abstract of DOI:10.3762/bjoc.8.112

Neutral organic electron donors, featuring pyridinylidene-imidazolylidene, pyridinylidene-benzimidazolylidene and imidazolylidene-benzimidazolylidene linkages are reported. The pyridinylidene-benzimidazolylidene and imidazolylidene-benzimidazolylidene hyb

Full text of DOI:10.3762/bjoc.8.112

Hybrid super electron donors – preparation
and reactivity
Jean Garnier, Douglas W. Thomson, Shengze Zhou, Phillip I. Jolly,
Leonard E. A. Berlouis and John A. Murphy*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 994–1002.
WestCHEM, Department of Pure and Applied Chemistry, University of
Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, United Kingdom
doi:10.3762/bjoc.8.112
Received: 09 April 2012
Accepted: 06 June 2012
Published: 03 July 2012
Email:
John A. Murphy* - john.murphy@strath.ac.uk
* Corresponding author
Associate Editor: M. P. Sibi
Keywords:
© 2012 Garnier et al; licensee Beilstein-Institut.
License and terms: see end of document.
aryl iodide; electron transfer; hybrid donors; reduction
Abstract
Neutral organic electron donors, featuring pyridinylidene–imidazolylidene, pyridinylidene–benzimidazolylidene and imidazolyli-
dene–benzimidazolylidene linkages are reported. The pyridinylidene–benzimidazolylidene and imidazolylidene–benzimidazolyli-
dene hybrid systems were designed to be the first super electron donors to convert iodoarenes to aryl radicals at room temperature,
and indeed both show evidence for significant aryl radical formation at room temperature. The stronger pyridinylidene–imidazolyli-
dene donor converts iodoarenes to aryl anions efficiently under appropriate conditions (3 equiv of donor). The presence of excess
sodium hydride base has a very important and selective effect on some of these electron-transfer reactions, and a rationale for this is
proposed.
Introduction
Alkenes that are substituted by four heteroatoms are notable for Neutral organic donors that can reduce aryl halides have been
their ease of oxidation. Whereas tetrathiafulvalenes and termed “super-electron-donors”. Our recent research has exam-
analogues [1-5] have principally found widespread applications ined the remarkable chemical reactivity of such donors 1 and 2
in materials science, tetraazaalkenes and related compounds are as well as the related electron-donors 4 and 5 (Figure 1), with
much more reactive and are of potential or actual interest as organic substrates [17-27]. Benzimidazole-derived donor 1
reagents in synthesis [6-33]. Among the tetraazaalkenes, those converted aryl iodides to aryl radicals by transfer of a single
that are converted to aromatic molecules upon oxidation, e.g., electron at 110 °C [17], and was the first neutral organic
tetraazafulvalenes 1 and 2, are extremely reactive, and their ground-state molecule to achieve this. Later, the more powerful
electrochemical properties have been studied in some reagents 2 [18], 4 [20] and 5 [21] and related compounds [25]
depth [13-16].
afforded aryl anions from the same substrates by transfer of two
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Beilstein J. Org. Chem. 2012, 8, 994–1002.
Figure 1: Super-electron-donors and related compounds.
electrons at room temperature, and also cleaved selected sulfon- prime candidates, as the driving force for the loss of their first
amides [19], bis-sulfones [19], Weinreb amides [22], acyloin electron should exceed that for the loss of their second. The
derivatives [24], triflate esters and triflamides [26]. Most electrochemical properties of some hybrid imidazolium–benz-
recently, we announced the synthesis of the unstable compound imidazolium-derived donors have been reported [15]. For com-
3 [16,27].
parison, donor 11 is also of interest, although it features two
"stronger" donor components. As indicated below, our work has
Cyclic voltammetry (CV) studies showed that the more found remarkable effects of excess base in reactions of some
powerful donors, e.g., 2 [15,16,18] and 4 [19], lose their second hybrid donors.
electrons at almost the same potential as their first electron
(Figure 2 includes the CV of 4, showing a single two-electron Results and Discussion
redox wave, while Figure 4a includes that of 2). The differen- Compounds 9–11 were adopted as targets for synthesis. Of
tial strengths of the donors (2, 4 versus 1) correlated with the these, 10 and 11 are imidazolylidenes derived from an imida-
expected relative driving force resulting from aromatisation zolium salt. Donors derived from imdazolium salts are highly
following the loss of electrons. In the case of the respective reactive and unstable; CV studies in MeCN have shown [15]
oxidised forms, i.e., the dications 6–8, the newly aromatic rings that two-electron reduction of 13, bearing a single trimethylene
are represented in blue in Figure 1, while the pre-existing bridge, which was intended to afford 3, does not lead to a stable
aromatic rings in 6 are represented in red. The driving force for product; moreover, Taton and Chen [16] did not observe forma-
oxidation arising through aromatisation is greater for the imida- tion of 3 (by deprotonation of 14 in DMSO or by reduction of
zole- and pyridine-derived motifs 7 and 8, which are associated 13), reporting instead the formation of bis-carbene 15. In the
with the strongest donors, than it is for the benzimidazole- recent synthesis of 3 [27], its decomposition was noted over a
derived motif, 6, which marks a weaker donor.
period of hours in ultradry conditions under argon. Within the
series of imidazole-derived tetraazafulvalenes, Taton and Chen
To extend the capabilities of such reagents, we now set out to established [16] that the only member that remained stable on
design the first neutral, organic ground-state donor that could, at storage under inert conditions was the bis-trimethylene bridged
room temperature, reduce aryl iodides to aryl radicals (thereby donor 2, and its greater stability was attributed to the two
acting as a single-electron donor) as opposed to aryl anions. trimethylene tethers. As compounds 10 and 11 are derived from
Hybrid organic electron donors incorporating a "stronger" donor imidazolium precursors, we were keen to explore their
component and a "weaker" component, e.g., 9 or 10 would be reactivity.
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Beilstein J. Org. Chem. 2012, 8, 994–1002.
The redox properties of the donors were first measured by analysis. (Anion exchange to bis(hexafluorophosphate) salts
cyclic voltammetry. Either the electron donors or their oxidized was required since the iodide anions within diiodide salts would
salts could, in principle, be used as a starting point for the CV be electrochemically active in CV studies). To verify the inter-
studies; however, the oxidised disalts were routinely chosen as mediacy of 11, its formation from 19 was repeated in DMF-d7,
they can be conveniently weighed out under air, while the and the 1H and 13C NMR spectra of 11 were determined. The
donors are extremely air-sensitive. The oxidized salts, derived 1H NMR spectrum showed the characteristic upfield shift of
from the donors, were prepared as shown in Scheme 1. Reac- proton signals for nonaromatic electron-rich donors.
tion of 1-(3-bromopropyl)-4-dimethylaminopyridinium bro-
mide (16) [34] with N-methylimidazole (17) afforded disalt 19. Figure 2 shows the cyclic voltammogram of 21 (blue trace) and
Deprotonation with NaH (15 equiv) in DMF then afforded the a comparison with 8 (X = PF6) (red trace). As seen, 21 under-
electron donor 11 in situ; this was reacted with iodine to afford goes reversible redox chemistry [E1½ (DMF) = −1.75 V, E2½
the oxidised diiodide salt, and this was subjected to anion (DMF) = −1.63 V versus Fc/Fc+; this corresponds to E1½
exchange to afford the bis(hexafluorophosphate) salt 21 for (DMF) = −1.30 V, E2½ (DMF) = −1.18 V versus SCE]. The
cyclic voltammogram, together with the NMR determination
above, shows that 11 is a stable imidazole-derived donor (i.e., it
does not decompose under the conditions used for its formation)
[15], and so its capability as an electron donor was tested.
Figure 2: Cyclic voltammograms in DMF of 8/4 (red) and 21/11 (blue).
Current plotted vs V (relative to Fc/Fc+ as standard).
To test reactivity, donor 11 was prepared in situ and treated
with the substrates 28 and 30 at room temperature (Scheme 2).
Simple substrate 28 [35] was added to 11, prepared by adding
disalt 19 (1.5 equiv) to excess sodium hydride (15 equiv). As
expected, it behaved as a strong donor, affording 29 [20] in
74% yield. (A blank experiment, in which substrate 28 was
treated at room temperature for 16 h with NaH in DMF, led to
quantitative recovery of 28). Substrate 30 [35] was designed to
test whether a single electron or two electrons are transferred to
an iodoarene; single electron transfer would afford an aryl
radical that would undergo cyclisation efficiently [17], while
two-electron transfer to afford an aryl anion would afford an
aryl anion that would not cyclise in DMF as solvent [18]. The
reaction with 30 was conducted under slightly different condi-
tions than with 28. Donor 11 was prepared by using disalt 19
(3 equiv) added to the excess sodium hydride (15 equiv), and
Scheme 1: Preparation of the oxidised disalts.
the resulting donor solution was filtered to remove excess NaH
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Beilstein J. Org. Chem. 2012, 8, 994–1002.
Scheme 2: Reductive reactions of donor 11 [17,18].
before substrate 30 was added. (Previous experience had raised In situ generation of 9 from 20 (1.5 equiv, Scheme 3) and reac-
suspicion that the excess NaH could deprotonate the aliphatic tion with iodoarenes 28 and 30 was again carried out at room
side-chain of allyloxy substrates). The sole isolated product, temperature. As for the reactions with donor 11, the excess NaH
i.e., the de-iodinated but uncyclised compound 31 (59%) [36], was filtered prior to the addition of substrate 30. Reaction of
is consistent with 11 donating two electrons.
iodoarene 28 led to an inseparable mixture of 29 and 28 in a 2:1
ratio; based on the mass recovered, this corresponded to 29
Donor 9 was prepared by a route analogous to that used for 11, (46%) and 28 (22%). By comparison, reaction with aryl iodide
and was then oxidised and converted to the bis(hexafluorophos- 30, again at room temperature, afforded a mixture of 32 [35],
phate) salt 22. Cyclic voltammetry, starting with its oxidised the product of aryl radical cyclization, (74%), together with
disalt 22 (Figure 3, blue trace) shows that its redox activity recovered 30 (6%) and deiodinated but uncyclised product 31
occurs as two separate steps at potentials intermediate between (12%). This is the first observation of efficient aryl radical
those for compounds 6 and 8. [E1½ (DMF) = −1.54 V, E2½ generation at room temperature from a super-electron-donor.
(DMF) = −1.42 V versus Fc/Fc+; this corresponds to E1½ For comparison, less than 1% yield of 32 was observed when
(DMF) = −1.09 V, E2½ (DMF) = −0.97 V versus SCE]. The repeating the reaction with donor 1, also generated in situ.
cyclic voltammetry studies on oxidised disalt 22 show two re- Hence 9 reacts at room temperature with iodoarenes and func-
versible one-electron transitions on its reduction to donor 9. The tions as the strongest known neutral organic ground-state one-
redox potential in the oxidation trace for the removal of the first electron donor to iodoarenes.
electron from 9 shows that the molecule is not as strong a donor
as 4, while the transfer of the second electron occurs at a more Hybrid donor 10 was next prepared from the known chloro-
negative potential than for the first electron from 1.
propylbenzimidazole 23 [37] (Scheme 1), then oxidised and
converted to its bis(hexafluorophosphate) salt, 27, for analysis.
Cyclic voltammetry on 27 is shown in Figure 4a (blue trace), in
comparison with salts 7 and 6. Looking at the blue trace in
Figure 4a, it is immediately clear that the oxidative sweep
provides a very low current relative to the initial reductive
sweep, suggesting decomposition of the reduced species on the
timeframe of the CV studies. Repeating the experiment at
different scan rates (Figure 4b) shows that at low scan speeds
the effect is even more pronounced. Note that the CV traces
reproduced by Ames et al. [15] demonstrate the same effect.
This instability suggested that it should be difficult to obtain
complete reaction when using 26 as a precursor of 10 in the
preparative-scale reduction of aryl iodides. The standard two
iodides 28 and 30 were tested under slightly different condi-
tions, as mentioned above for donors 9 and 11. Here a
Figure 3: Cyclic voltammograms in DMF of 8/4 (red), 6/1 (green) and
22/9 (blue). Current plotted vs V relative to Fc/Fc+.
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Beilstein J. Org. Chem. 2012, 8, 994–1002.
Scheme 3: Reductive reactions of donor 9.
surprising outcome was seen. Complete reduction was observed
for iodide 28, affording a good isolated yield of 29 (70%)
(Scheme 4). However, iodide 30 was reduced by 10 to give the
products 32/31//30 in a 2:1.4:1 ratio (1H NMR) with a poor
overall recovery of 54%.
In both cases, the donor had been prepared in situ by reacting
the precursor salt 26 (1.5 equiv) with excess sodium hydride (15
equiv). However, whereas 28 was simply added to the resulting
mixture, which included residual excess base, the excess base
had been removed by filtration prior to addition of 30. This led
us to question whether excess base could be helpful in such
reactions and, if so, could the reported instability of other
imidazole-based electron donors [15] also be addressed in the
presence of base?
To test this, mono-trimethylene precursor 14 [38], was
prepared. This compound is the precursor of donor 3. However,
earlier CV studies to prepare 3 by reduction of 13 showed that 3
was not a stable compound, as discussed above [16,27].
Treating 14 with excess NaH, and then adding 28 to this reac-
tion mixture pleasingly provided 29 (74%) exclusively
(Scheme 5). However, repeating the same reaction, but filtering
the excess NaH prior to addition of substrate 28 gave only 11%
of reduced product 29, together with starting substrate 28
(84%). The same outcome was seen with a second substrate, 33
[39]. In the presence of excess NaH, reduced product 34 was
isolated in 86% yield, whereas when the substrate was added
after removal of excess NaH, a lower yield of 34 (9%) was
isolated, together with starting substrate 33 (85%).
Figure 4: (a) c.v. in DMF of 7/2 (red), 6/1 (green) and 27/10 (blue); (b)
c.v. in DMF of 27/10 at different scan rates. Current plotted vs V rela-
tive to Fc/Fc+.
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Beilstein J. Org. Chem. 2012, 8, 994–1002.
Scheme 4: Use of hybrid donor 10 in reduction of iodoarenes.
Scheme 5: Reductive chemistry from disalt 15.
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Beilstein J. Org. Chem. 2012, 8, 994–1002.
Scheme 6: Rationalisation of effect of excess NaH base.
How can the base be assisting these reactions? Scheme 6 takes of 38 to form reduced arene 39, consistent with our previous
disalt 14 as an example.
studies on deprotonation of pyridinium salts [21]. However, 38
is a dication, and, to attain neutrality, could lose two protons.
Treatment of 14 with two equivalents of NaH would afford Compound 3 could be a strong base (in support of this, we have
donor 3 as shown. This should then react with an iodoarene 37 witnessed complete conversion of the analogous donor 2 to
to afford the dication salt 38 featuring an aryl anion and an form 40 by rapid exchange in CD3CN as solvent; see
iodide as counterions. In these circumstances, we suggest that Supporting Information File 1). We also note that in the previ-
the aryl anion can abstract a proton rapidly from the periphery ously reported electrochemical studies, irreversible behaviour
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Beilstein J. Org. Chem. 2012, 8, 994–1002.
was always observed in acetonitrile, consistent with a role of Conclusion
this solvent as a proton donor in the decomposition, whereas it Hybrid organic super-electron-donors have been prepared, and
was much more rarely reported in the much less acidic solvent their reactivity with aryl iodides tested. The donors show evi-
DMF [14,15], and if the experiment were conducted with no dence for transfer of one electron or two electrons, dependent
excess of NaH base, 3 could itself act as a base. Protonation of on their structure. Excess sodium hydride has a very beneficial
3 would afford 36, capable of undergoing spontaneous fragmen- effect on yields of products in certain cases, and a rationale for
tation to 35 [40-43] thereby lowering the concentration of this has been proposed.
donor. However, excess sodium hydride can inhibit the protona-
tion of 3 by competing for protons. (Notably, in earlier studies
Supporting Information
on the reversibility of formation of imidazoline-based donors,
Liu and Lemal inhibited dissociation by adding KH as base
Supporting Information File 1
[43]).
Experimental and computational details.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-112-S1.pdf]
In the cyclic voltammetry case, 3 would be generated from
disalt 38. As 3 starts to be generated, it can deprotonate 38,
lowering the concentrations of 3 and therefore lowering the
cathodic current in the CV, as observed for couple 27/10 in
Figure 4.
Acknowledgements
We thank AstraZeneca, EPSRC and WestCHEM for funding,
and the EPSRC National Mass Spectrometry Service Swansea
for mass spectra.
It would then remain to explain why some imidazole-derived
donors, e.g., 2 and 11, apparently are not affected, or are much
less affected by this problem. Protonation of 2 leads to 41, and
it is likely that the equilibrium fragmentation of this compound
to 42 is less favourable than the fragmentation of 36 to 35
because of the restriction imposed by the second trimethylene
bridge [16]. (Compound 41 has not previously been reported,
but its existence is clear from its preparation here by deprotona-
tion of 12 with one equivalent of NaH (see Supporting Informa-
tion File 1). For protonated forms of other tetraaza donors, see
[44,45]).
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