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promoted by light and catalyzed by Bi2O3. The results of this
study have been summarized in Table 2.
Four different ATRA donors were tested (diethyl and dimeth-
yl bromomalonate, ethyl bromodifluoroacetate, and carbon
tetrabromide). Among them, diethyl and dimethyl bromomalo-
nate displayed the highest reactivity in the ATRA reaction pro-
moted by light and Bi2O3. For instance, the reaction of diethyl
bromomalonate with a variety of functionalized terminal ole-
fins led the expected adducts (3a–g) in good to excellent
yields (up to 95%) in relative short reaction time (<24 h). Inter-
estingly, the highest yields in the preparation of some of these
products were recorded when the limiting reagent was the
ATRA donor (olefin/ATRA donor=1.1), a fact not observed in
the preparation of 3a. Alkyne partners could also be used in
combination with this ATRA donor, resulting in the formation
of compound 3h in a roughly 60:40 mixture of Z/E isomers.[17]
On the other hand, the use of dimethyl bromomalonate as
ATRA donor appears to involve slightly longer reaction times
and leads to somewhat lower yields (compare 3a and 3i).
It is worth mentioning that the ATRA additions of 2a per-
formed using this procedure can be readily scaled up. Thus,
3a could be prepared on the 5 mmol scale (1.52 g, 90% isolat-
ed yield) with the same experimental setup and in the same
reaction time by linearly scaling the amounts of reactants, sol-
vent, and catalyst. As a further example of the robustness of
this methodology, the preparation of 3d was also performed
with daylight promotion (71% yield). Remarkably enough, the
reactions performed in this manner involved shorter reaction
times (4 vs. 24 h) than with simulated sunlight (23 W lamp).
Thus, the use of Bi2O3 as a photocatalyst effectively allows per-
forming ATRA reactions promoted by costless sunlight energy.
Optimal conditions for the ATRA reaction of ethyl bromodi-
fluoroacetate (2c) involved working with a slight excess of or-
ganobromide (2c/1=1.2), as established in the initial screen-
ing (Table 1). Thus, while the formation of 3j took place in
69% yield under these conditions, a significant yield decrease
was noted (to 50%) when 2c and 1 were used in a 1:1.1 molar
ratio. A parallel behavior was observed for the formation of 3k
and 3l.
Figure 1. Proposed mechanism for the Bi2O3-catalyzed, visible-light-induced
ATRA reaction.
As anticipated, this provoked complete inhibition of the addi-
tion reaction (Scheme 2).
This fact, along with the semiconducting properties of the
photocatalyst, enables us to propose the tentative mechanism
shown in Figure 1.[8b,14,18] According to it; the incident photons
promote the photoexcitation of electrons on the surface of the
semiconductor from the valence to the conduction band with
generation of positive holes (h+). The photoexcited electrons
induce reductive cleavage of the organobromide to generate
C
the electrophilic radical R (I). Then, this photogenerated radical
undergoes addition to the partner olefin, giving rise to the rad-
ical intermediate II. From this point, two routes are possible. In
route a (a radical–polar crossover), the radical intermediate II
delivers an electron to the semiconductor to neutralize a posi-
tive hole and to provide a carbocation intermediate, which ul-
timately reacts with bromide leading to the ATRA product 3. In
route b, a radical chain propagation pathway is proposed. Rad-
ical II subtracts a bromine atom from the starting material,
leading directly to 3 and regenerating radical I that continues
the chain. Likewise, routes a and b operate in a concomitant
manner. While route b is well established in the context of the
ATRA reaction,[8b,14] the nature of Bi2O3 (solid semiconductor
particles) could importantly favor route a.[19]
Carbon tetrabromide (2d) could also be used as an ATRA
donor for a variety of terminal olefins (3m–3r). As with 2c, the
reactions proceeded better when the ATRA donor was used in
slight excess with respect to the olefin partner (2d/1=1.2).
With the aim of verifying whether the reaction catalyzed by
Bi2O3 takes place through radical intermediates, the known
In conclusion, a simple catalytic system composed of a non-
toxic, commercially available Bi2O3 powder operating at low
loading (1%) under visible light irradiation in DMSO displays
excellent performance in the atom transfer radical addition
(ATRA) reaction between a variety of olefins and organobro-
mides. The photocatalytic reaction works specially well for dia-
lkyl bromomalonate ATRA donors and allows using
radical
scavenger
(2,2,6,6-tetramethylpiperidin-1-yl)oxyl
(TEMPO; 1.2 mmol per mmol of 1a) was used as an additive in
the preparation of 3a under otherwise optimized conditions.
costless daylight to promote the process. Interesting-
ly, the present methodology does not require the use
of any additive for the reaction to proceed and offers
advantages in cost and atom economy over previ-
ously reported methods involving the use of expen-
sive metals or large amounts of organic materials.
Scheme 2. Inhibition by TEMPO of the visible-light-induced ATRA reaction.
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