First examples of hypervalent enhancement of photoallylation by allylsilicon compounds via photoinduced electron transfer

Article abstract of DOI:10.1016/j.tetlet.2005.05.124

Hypervalency (pentacoordination) of silicon atom enhanced photoallylation of 1,2-diketones with allylsilicon reagents, while normal tetracoordinated ones could not. This reaction seems to proceed via photoinduced electron transfer from the silicon reagent

Full text of DOI:10.1016/j.tetlet.2005.05.124

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 5149–5151
First examples of hypervalent enhancement of photoallylation by
allylsilicon compounds via photoinduced electron transfer
Yutaka Nishigaichi,* Akira Suzuki, Takahiro Saito and Akio Takuwa
Department of Material Science, Faculty of Science and Engineering, Shimane University, 1060 Nishikawatsu-cho, Matsue,
Shimane 690-8504, Japan
Received 30 April 2005; revised 30 May 2005; accepted 30 May 2005
Available online 14 June 2005
Abstract—Hypervalency (pentacoordination) of silicon atom enhanced photoallylation of 1,2-diketones with allylsilicon reagents,
while normal tetracoordinated ones could not. This reaction seems to proceed via photoinduced electron transfer from the silicon
reagent to the photoexcited ketone judged from its reactivity and selectivity.
Ó 2005 Elsevier Ltd. All rights reserved.
Allylation of carbonyl compounds is one of the funda-
mental synthetic reactions. Thermal reactions using an-
ionic and thus strongly basic reagents such as lithium
and Grignard reagents have been mostly employed.
More recently, less basic allylmetals including those in
Lewis acid mediated reactions have also been utilized
to achieve various selectivities.1 In contrast to such ther-
mal allylations, photochemically promoted allylation
has potential advantages that include neutral reaction
conditions and clean acceleration by light. One success-
ful example is the reaction between unsaturated carbonyl
compounds and allyltin reagents via photoinduced elec-
tron transfer (PET).2 As shown in Scheme 1, a quantita-
tive yield of the allyl adduct (5) was realized in the
reaction of allyltrimethyltin (2) with benzil (1). However,
toxicity of organotin reagents is a serious drawback, thus
analogous silicon reagents can be a promising candidate
for their substitute. Actually, allyltrimethylsilicon (3) is
known to behave as an electron donor in the photoallyl-
ation of iminium ions,3 aromatic cyanides,4 and aromatic
imides,5 but it is far less reactive toward carbonyl com-
pounds6 and acts as an ordinary alkene to give oxetane
(6) as a major product2 (Scheme 1). Therefore, we have
investigated into the activation of such silicon reagents
toward photoallylation of carbonyl compounds and
found that it could be realized by hypercoordination of
the silicon atom, which is described herein.
Though hypercoordinated allylsilicon reagents such as
fluorosilicates, biscatecholatosiliconates, and silatranes
have been applied to the thermal carbonyl allylation,7
their photochemical application has not yet been reported
as far as we know. Such silicon reagents are considered
to be more electron-rich than tetracoordinated ones
Scheme 1. Photochemical allylation with tin and silicon reagents.
Keywords: Photoallylation; Allylsilicon; Hypervalency; Electron transfer.
*
Corresponding author. Tel.: +81 852326415; fax: +81 852326429; e-mail: nishigai@riko.shimane-u.ac.jp
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.05.124

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Y. Nishigaichi et al. / Tetrahedron Letters 46 (2005) 5149–5151
Table 1. Photoreaction of benzil with 7 or 8
Entry
Reagent
Adduct (yield/%) [a/c]
1
2
3
4
5
6
7
8
9
7a
8a
7b
8b
7c
7d
8d
7e
8e
5a (6)
5a (22)
5b (18)
5b (73)
5c (26) [72/28]
5d (46) [83/17]
5d (72) [85/15]
5e (15)
5e (29)
due to the extra ligandÕs electron donation. Thus, reduced
oxidation potential (i.e., hypervalent activation) of silicon
reagents was expected and we attempted their application
toward photoreaction of 1,2-diketones.
63%, respectively, under similar reaction conditions
(Scheme 2).
Substituted allylic reagents, 7b–d and 8b,d were also pre-
pared and applied to the photoreaction with benzil. The
results are also summarized in Table 1. Again, catecho-
lates 8 gave the higher yields. Especially, 8b and 8d
afforded rather high yields up to 70%. Interestingly,
a-regioselectivity was observed in the cases of 7c,d and
8d, which is very similar to the photoreaction with allylic
tin reagents and is characteristic to the reaction via PET.
First, addition of Bu4NF (TBAF) to the reaction be-
tween 1 and 3 was employed and was found to give
the allyl adduct 5 in a moderate yield. But this was
proved to be due to the thermal reaction because the
same yield was obtained in the dark.8 Thus, we next
tried less thermally reactive and more Lewis acidic allyl-
triethoxysilicon (4) with fluoride ion. Then, the photo-
allylation was actually realized for the first time by the
hypercoordinated allylsilicon reagent though the yield
was less than 20% (Scheme 1).
Table 2. Oxidation potentials of silicon and tin reagents
Entry
Silicon reagent
Eox/Va
DGET/kcal molÀ1 b
1
2
3
4
5
6
7
Allyltrimethylsilicon (3)
7a
1.64
1.26
1.12
1.06
1.46
1.20
1.10
8.4
À0.4
À3.6
À5.0
4.2
Encouraged by this result, we next attempted stable and
readily preparable pentacoordinated silicon reagents
such as silatrane 7 and catecholatosiliconate 8. Unsub-
stituted allyl reagents 7a and 8a were allowed to react
with 1 in acetonitrile under photochemical conditions
(Table 1, entries 1 and 2). Though 7a afforded only
6% of allylated product, 8a gave 22% after 7 h of irradi-
ation.9 More reducible substrates, phenanthrenequinone
and acenaphthenequinone, with 8a afforded the corre-
sponding allyl adducts in much higher yields, 84% and
8a
Allyltrimethyltin (2)
Benzyltrimethylsilicon
7e
8e
À1.8
À4.1
a Values were measured in acetonitrile versus SCE.
b DGET = 23.06(Eox À Ered À DE0–0) + Ecoul, where Ered is the reduc-
tion potential of benzil (À1.13 V), DE0–0 is the excitation energy of
benzil (2.35 eV), and Ecoul is the electrostatic term for acetonitrile
(À1.3 kcal molÀ1).
Scheme 2. Photoallylation of o-quinones with 8a.

Y. Nishigaichi et al. / Tetrahedron Letters 46 (2005) 5149–5151
5151
Scheme 3. Plausible reaction mechanism for photoallylation with a hypervalent silicon reagent.
It is also noteworthy that benzylsilicon reagents 7e and
8e, which hardly react thermally, also underwent
photobenzylation.
effective photoreaction systems is in progress in our lab-
oratory and will appear in due course.
References and notes
Activation of silicon reagents by the hypervalency for
PET was supported by the measurement of oxidation
potentials of 7 and 8. Selected data are listed in Table
1. Yamamoto, Y.; Asao, N. Chem. Rev. 1993, 93, 2207.
2. In the photoreaction of aromatic ketones such as benzo-
phenone and benzil, allyltrimethysilicon acts as a usual
alkene to undergo [2+2] cycloaddition (oxetane formation)
and hydrogen abstraction. (a) Takuwa, A.; Tagawa, H.;
Iwamoto, H.; Soga, O.; Maruyama, K. Chem. Lett. 1987,
1091; (b) Takuwa, A.; Nishigaichi, Y.; Yamashita, K.;
Iwamoto, H. Chem. Lett. 1990, 639; (c) Takuwa, A.;
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1990, 1761; (d) Takuwa, A.; Nishigaichi, Y.; Iwamoto, H.
Chem. Lett. 1991, 1013; (e) Takuwa, A.; Nishigaichi, Y.;
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1991, 1359; (f) Takuwa, A.; Shiigi, J.; Nishigaichi, Y.
Tetrahedron Lett. 1993, 34, 3457; (g) Nishigaichi, Y.;
Yoshida, N.; Matsuura, M.; Takuwa, A. Chem. Lett.
1999, 803.
2
with those of the corresponding trimethylsilyl
reagents. In addition, the tin reagent 2 for which photo-
allylation is well known, is compared. All these hyper-
valent reagents show significantly lower values than
the corresponding tetravalent silicon reagents. By means
of the Rehm–Weller equation,10 the tetravalent ones
give positive DGET values (entries 1 and 5), which mean
unfavorable PET. In contrast, the hypervalent ones
show negative values, which mean PET can occur
exothermally and approximately parallel to their reac-
tivity. This trend is consistent with the reactivity of the
tin reagent (entry 4).2b These results point to the electron
transfer mechanism of the present photoreaction.
3. Ohga, K.; Yoon, U. C.; Mariano, P. S. J. Org. Chem.
1984, 49, 213; Fukuzumi, S.; Fujita, M.; Noura, S.;
Ohkubo, K.; Suenobu, T.; Araki, Y.; Ito, O. J. Phys.
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4. Mella, M.; Fasani, E.; Albini, A. J. Org. Chem. 1992, 57,
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From the above results, the following reaction scheme
can be depicted as shown in Scheme 3. This is very sim-
ilar to that with the corresponding tin reagents. Electron
transfer from silicon reagents to the excited ketone is
facilitated by the hypervalency, that is, extra donation
of electrons from the ligand toward the silicon atom.
In the allylation with allylsilicon reagents via the single
electron transfer, C–Si bond fission of the cation radical
is a very important step as well, because the back elec-
tron transfer (BET) would be preferred if the fission is
slow. Therefore, it is probable that the extra ligand also
facilitates the C–Si fission as an intramolecular nucleo-
phile.11 Then, the ketyl radical and the allyl radical are
coupled to result in the adduct formation, where the ste-
rically less hindered a-position of the allyl radical is
preferred.
5. Kubo, Y.; Imaoka, T.; Shiragami, T.; Araki, T. Chem.
Lett. 1986, 1749.
6. Photoallylation of an aromatic aldehyde with 3 was
promoted by magnesium ion. Fukuzumi, S.; Okamoto,
T.; Otera, J. J. Am. Chem. Soc. 1994, 116, 5503.
7. Thermal reaction of hypervalent allylic silicon reagents:
Cerveau, G.; Chuit, C.; Corriu, R. J. P.; Reye, C.
J. Organomet. Chem. 1987, 328, C17; Kira, M.; Sato, K.;
Sakurai, H. J. Am. Chem. Soc. 1988, 110, 4599; Sakurai,
H. Synlett 1989, 1; Hosomi, A.; Kohra, S.; Ogata, K.;
Yanagi, T.; Tominaga, Y. J. Org. Chem. 1990, 55, 2415.
8. Hosomi, A.; Shirahata, A.; Sakurai, H. Tetrahedron Lett.
1978, 3043.
9. For the reactions with 7 and 8, it was confirmed that no
reaction proceeded in the dark.
10. Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259.
11. Dockery, K. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J.
L.; Gould, I. R.; Todd, W. P. J. Am. Chem. Soc. 1997, 119,
1876.
In conclusion, the photoallylation of aromatic diones
with silicon reagents could be enhanced by the hyperva-
lency of the silicon atom for the first time. These results
suggest the hypervalency is an efficient method to pro-
mote photoinduced electron transfer. As the ligands em-
ployed here can be modified, development of the more