Photo-allylation and photo-benzylation of carbonyl compounds using organotrifluoroborate reagents

Article abstract of DOI:10.1016/j.jorganchem.2009.08.011

Allyl- and benzyl-trifluoroborates can be applied to the photoreaction of carbonyl compounds to afford the corresponding alcoholic adducts in acceptable yields via a photo-induced single electron transfer pathway. The results were confirmed from the react

Full text of DOI:10.1016/j.jorganchem.2009.08.011

Journal of Organometallic Chemistry 694 (2009) 3837–3839
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Photo-allylation and photo-benzylation of carbonyl compounds
using organotrifluoroborate reagents
*
Yutaka Nishigaichi , Takayuki Orimi, Akio Takuwa
Department of Material Science, Faculty of Science and Engineering, Shimane University, 1060 Nishikawatsu-cho, Matsue, Shimane 690-8504, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2009
Received in revised form 5 August 2009
Accepted 10 August 2009
Available online 14 August 2009
Allyl- and benzyl-trifluoroborates can be applied to the photoreaction of carbonyl compounds to afford
the corresponding alcoholic adducts in acceptable yields via a photo-induced single electron transfer
pathway. The results were confirmed from the reaction selectivity and the negative free energy change
for the electron transfer process.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Photoreaction
Allylation
Benzylation
Organotrifluoroborate
Carbonyl compound
Electron transfer
1. Introduction
[5,6], we employed these substrates for a photoreaction with
potassium allyl- and benzyl-trifluoroborate 1 and 2 (Scheme 1). Ta-
Organotrifluoroborates are known as air- and water-stable re-
agents and thus can be stored and handled easily [1]. In addition,
some of them are either commercially available or can be readily
prepared. They are currently used in a wide variety of applications
in synthetic chemistry, transition metal catalyzed coupling reac-
tions [2] (e.g., Suzuki–Miyaura coupling [3]), Lewis acid promoted
allylation and other C–C bond formation reactions [1].
The stability of the organotrifluoroborates is due to the tetra-
coordination of the boron atom, which is coordinatively saturated,
and thus they possess an electron-rich character. Therefore, we
anticipated its effectiveness as an electron-donating reagent in
reactions via a photoinduced electron transfer (PET) process [4].
Previously, we reported photochemical allylation of carbonyl com-
pounds via PET from allyltin reagents [5]. More recently, we used it
in a similar photoreaction with silicon-based reagents [6] wherein
extra coordination to the silicon atom (penta- or hexa-coordina-
tion) was the key to the effective PET. Here, we report that the
photo-induced allylation and benzylation of carbonyl compounds
can be further applied successfully to organotrifluoroborates [7].
ble 1 summarizes the results.
In all reactions conducted here, more than 80% of the substrates
were consumed and the corresponding allyl- and benzyl-adducts
were obtained in various yields. Methyl benzoylformate 3a affor-
ded the corresponding adducts in low yields (Entries 1 and 6).
We did not obtain any products derived from the ester moiety.
Aromatic diketones 3b and 3c afforded the corresponding adducts
in good yields. Interestingly, phenylpropanedione 3b, a non-sym-
metrical diketone, preferentially reacted with the aromatic ketone
moiety (Entries 2 and 7), which is less reactive in the thermal reac-
tion [5c]. Quinones 3d (acenaphthenequinone) and 3e (9,10-phen-
anthrenequinone) afforded higher yields of the corresponding
adducts (Entries 4, 5, 9 and 10). These reactions proceeded photo-
chemically, confirmed by the fact that no reaction occurred with-
out photoirradiation at room temperature. Considering the
reduction potentials of the carbonyl compounds (Table 2) [8,9],
the more strongly electron-accepting substrate afforded higher
yield of the product. Therefore, the PET mechanism is the most
probable.
As stated in our previous reports, that
agents preferentially afforded the -adduct, which was derived
from the reaction at the less crowded terminal of the allylic radical
c-substituted allylic re-
a
2. Results and discussion
intermediate, is a characteristic feature of the photo-allylation
Because 1,2-dicarbonyl compounds are good substrates for a
photoreaction via the PET mechanism with tin and silicon reagents
[5a,10]. If the thermal allylation proceeded, the
c-adduct would
be preferentially obtained as in the allylborane chemistry [11].
To confirm the mechanism and to expand the scope, the photore-
* Corresponding author.
E-mail address: nishigai@riko.shimane-u.ac.jp (Y. Nishigaichi).
action of
c-substituted allyltrifluoroborates were attempted.
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.08.011

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Y. Nishigaichi et al. / Journal of Organometallic Chemistry 694 (2009) 3837–3839
Table 3
Regioselectivity in photo-allylation with c
-substituted allyl-trifluoroboratesa
.
Entry
Borate
Carbonyl comp.
Yields of adducts (a/c)/%
1
2
3
4
5
6
8
3c
3d
3e
3c
3d
3e
44/9
47/17
85/tr
65/17
55/33
67/tr
Scheme 1. Photo-induced allylation and benzylation of 1,2-dicarbonyl compounds.
9
Table 1
Photo-induced allylation and benzylation of 1,2-dicarbonyl compounds.a
a
Reaction conditions: hm (k > 400 nm), CH3CN, N2, 5 h.
Entry
Borate
Carbonyl comp.
Yield of 4, 5 (6, 7)/%
1
2
3
4
5
6
7
8
9
10
1
3a
3b
3c
3d
3e
3a
3b
3c
3d
3e
21
49 (19)
58
78
91
34
62 (21)
74
90
90
2
Scheme 2. Reaction of benzophenone.
a
Reaction conditions: h
m
(k > 400 nm), CH3CN, N2, 5 h.
Table 2
Reduction potentials, excitation energies, and free energy changes for the PET.a
Carbonyl comp.
E
red/Vb
D
ET/kcal molÀ1
D
G/kcal molÀ1
3a
3b
3c
3d
3e
10
À1.23
À1.24
À1.13
À0.91
À0.65
À1.59
66 [9a]
À13.5
À1.7
À4.0
À6.8
À12.1
À7.7
54.4 [9b]
54.2 [9c]
51.8 [9d]
51.3 [9d]
68.6 [9c]
Scheme 3. Proposed reaction path.
nucleophile such as the substrate and/or the solvent most likely
cleaved the radical species of the borate reagent, and the resulting
organo-radical coupled with the ketyl radical to give the adduct.
The results shown in Table 1 (Entries 2 and 7) and Table 3 are very
consistent with this reaction path. The higher reactivity of the ben-
zoyl moiety indicates that the preferred resonance stabilization of
the intermediate via the PET path determined the major product,
and the a-selectivity of the allyl moiety implies the prior C–B bond
cleavage to the C–C bond formation. The fact that a small amount
of bibenzyl was observed in the reactions of 2 further supported
the radicallic path. Rather low yields of the adduct for the reactions
of 3a and 10 could be attributed to their high triplet energies (Ta-
ble 2) [9]; these substrates might also have undergone other pho-
toreactions such as hydrogen abstraction [14].
For PET with 1 (Eox = +1.10 V).
vs. SCE.
a
b
Mono-substituted cinnamylborate 8 and di-substituted prenyl
borate 9 were employed as shown in Table 3.
Evidently,
in good yields. The present
served in the reactions with tin and silicon reagents. Interestingly,
in the case of the substrate 3e, only -adducts were obtained in
a
-adducts were preferentially obtained as expected
a/c
ratios were similar to those ob-
a
both cinnamyl and prenyl additions (Entries 3 and 6).
The photo-allylation and photo-benzylation was also applied to
an aromatic mono-ketone, however the efficiency appreciably de-
creased. As shown in Scheme 2, photoirradiation (k > 330 nm) of
benzophenone 10 with 1 in CH3CN afforded 21% of the correspond-
ing allyl adduct. Photo-benzylation with 2, in turn, gave 25% of the
benzyl-adduct. In both reactions, considerable amount of pinacol
was also obtained as a by-product.
Scheme 3 depicts the plausible reaction pathway. As mentioned
above, the PET process should be crucial. A photo-excited carbonyl
compound is more reducible from its excitation energy than its
ground state. Then a PET toward it from an organotrifluoroborate
3. Conclusion
Allyl- and benzyl-trifluoroborate reagents are found to be ap-
plied successfully to the photo-induced allylation and benzylation
reactions of aromatic carbonyl compounds. We are currently work-
ing to extend the scope of this photoreaction to other substrates
and to apply natural product synthesis, taking advantage of their
ionic characters.
can occur exothermally because the free energy change
DG for
the PET was estimated to be negative by the Rehm–Weller equa-
tion [12] given the oxidation potentials of the borates [13]. Table
4. Experimental
2 also summarizes the calculated values of
of borate 1, which had less electron-donating potential than 2. A
D
G for the reactions
A mixture of a carbonyl compound (3, 0.2 mmol) and allyl- and
benzyl-trifluoroborate (1 and 2, 0.3 mmol) in dry acetonitrile

Y. Nishigaichi et al. / Journal of Organometallic Chemistry 694 (2009) 3837–3839
(f) A. Takuwa, J. Shiigi, Y. Nishigaichi, Tetrahedron Lett. 34 (1993) 3457;
3839
(10 mL) was degassed by bubbling nitrogen in a Pyrex test tube,
which was irradiated for 5 h with a high pressure mercury lamp
(300 W) through an appropriate filter solution under a nitrogen
atmosphere at room temperature. After concentrating the reaction
mixture under reduced pressure, the residue was chromatographed
on a silica gel TLC plate to isolate the corresponding products.
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