Allylsilylation and stannylation of 1,2-diketones using bifunctional allylsilane-allylstannane reagents via photoinduced electron transfer reaction

Article abstract of DOI:10.1039/a906620i

Bifunctional allylsilane-allylstannane and allylstannane-allylstannane reagents react photochemically with 1,2-diketones to give α-ketohomoallyl alcohols having an allylsilane moiety and α-ketohomoally alcohols having an allylstannane moiety respectively

Full text of DOI:10.1039/a906620i

Allylsilylation and stannylation of 1,2-diketones using bifunctional
allylsilane–allylstannane reagents via photoinduced electron transfer reaction
Akio Takuwa,* Hiroyuki Saito and Yutaka Nishigaichi
Department of Material Science, Faculty of Science and Engineering, Shimane University, Matsue 690-8504, Japan.
E-mail: takuwa@riko.shimane-u.ac.jp
Received (in Cambridge, UK) 16th August 1999, Accepted 31st August 1999
Bifunctional allylsilane–allylstannane and allylstannane–
allylstannane reagents react photochemically with 1,2-dike-
tones to give a-ketohomoallyl alcohols having an allylsilane
moiety and a-ketohomoally alcohols having an allylstan-
nane moiety respectively in excellent yields, and the
remaining allylstannane moiety further reacts effectively
with another diketone in the presence of Mg(ClO4)2 to afford
a bis-a-ketohomoallyl alcohol.
(Scheme 1) afforded an a-ketohomoallyl alcohol having an
allylsilane moiety (4a). The 1+1 adduct 4a was isolated in 90%
yield by preparative TLC (silica gel, hexane–Et2O, 4+1 v/v) and
the structure was determined from its spectral properties and
elemental analysis.† The allylsilane unit in the product 4a does
not react further with benzil even under prolonged irradiation or
even under irradiation in the presence of Mg(ClO4)2 (vide
infra).
The bifunctional allylsilane–allylstannane reagent 1 also
reacted photochemically with other diketones 3 to produce the
corresponding a-ketohomoallyl alcohols containing allylsilane
moieties in moderate to good yields (Table 1, entries 1–4).
On the other hand, the light promoted reaction of 3 with
3-triphenylstannyl-2-[(triphenylstannyl)methyl]propene8 2 af-
forded two types of addition products, an a-ketohomoallyl
alcohol with an allylstannane moiety 5 (1+1 adduct) and a bis-a-
ketohomoallyl alcohol 6 (1+2 adduct), depending on the ratio of
The addition of allylsilane1 and allylstannane2 with various
carbonyl compounds has been widely used as one of the most
useful procedures for C–C bond formation. The process
generally requires promoters such as Lewis acids which activate
the electrophilic function of carbonyl compounds towards
nucleophilic attack by the allylmetal reagents.1–3 On the other
hand, light-promoted addition via photoinduced electron trans-
fer (PET) from allylstannane4 and allylsilane5 to the photo-
excited carbonyl compounds has recently attracted considerable
interest from both synthetic and mechanistic viewpoints. In
these photoadditions, allylstannanes are good electron donors
compared with the corresponding allylsilanes. For example, we
found that the reaction of allylstannanes with benzophenones
gave homoallylic alcohols via PET,4 whereas allylsilanes
behaved as an ordinary alkene to produce oxetanes and
hydrogen abstraction products6 without PET reaction. This
observation prompted us to examine the photoadditon reactions
of bifuntional allylsilane–allylstannane reagent 1 and allyl-
stannane–allylstannane reagent 2 with 1,2-diketones. We ex-
pected that the differences in the reactivity of allylstannane and
allylsilane would permit their differentiation in the bifunctional
reagents under photochemical conditions.
Irradiation ( > 400 nm) of the absorption band of benzil 3a
(0.02 mol dm23) in a N2-purged MeCN–benzene (5+1 v/v)
solution containing 3-tributylstannyl-2-[(trimethylsilyl)methyl-
]propene7 1 (0.03 mmol dm23) for 3 h at ambient temperature
Scheme 1 Reagents and conditions: i, hn ( > 400 nm), MeCN–benzene (4+1,
v/v).
Table 1 Photoaddition reaction of the bifunctional allylmetal reagents with diketones
Ratio
Entry
Allylmetal
Diketone 1 (or 2)+3
t/h
Products
(% yield)a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3a
3b
3c
3d
3a
3a
3a
3a
3b
3b
3c
3c
3c
3e
1.5+1
1.5+1
1.5+1
1.5+1
1.5+1
1.5+1
0.5+1
0.5+1
1.5+1
0.5+1
1.5+1
0.5+1
0.5+1
1.5+1
3
3
3
3
3
4a (90)b
4b (92)b
4c (78)b
4d (57)
5a (96)
5a (86)
5a (25)
5a (4)
5b (98)
5b (14)
5c (98)
5c (17)
5c ( < 1)
5e (61)
6a (0)
6b (0)
6c (0)
6d (0)
6a (0)
6a (8)
6a (56)d
6a (90)d
6b (0)
6b (67)d
6c (0)
6c (71)d
6c (91)d
6e (0)
c
3c
12
3c
3
12
3
12
3c
3
a
b
Isolated yields. A small amount of butylated product, 1,2-diaryl-2-hydroxyhexan-1-one, was also produced. Irradiation was carried out in the
presence of 5 equiv. of Mg(ClO4)2. d Diastereomer ratio was 1+1.
Chem. Commun., 1999, 1963–1964
This journal is © The Royal Society of Chemistry 1999
1963

amount (8%) of 6 was also produced in the presence of the metal
salt (entry 6). These results show that the first step of the PET
reaction is faster than the second one and that Mg2+ should
catalyze the present PET reaction.
In conclusion, the light-promoted addition reaction of
bifunctional allylmetal reagents to 1,2-diketones yielded a-
ketohomoallyl alcohols bearing allylsilane or allylstannane
groups in high yields via single electron transfer reaction. These
products are unlikely to be obtained from well known Lewis
acid-promoted reactions, as they have both nucleophilic
(allylmetal group) and electrophilic (carbonyl) functions in the
same molecule which are sensitive to Lewis acids.11 In addition,
bis-a-ketohomoallyl alcohols were effectively produced by
stepwise photoinduced electron transfer reaction between
allylstannane–allylstannane reagent and 1,2-diketones. More
detailed studies on the bifunctional reagents under photo-
chemical conditions and the intramolecular cyclization reaction
of the resulting a-ketohomoallyl alcohols are in progress in our
laboratory.
Scheme 2 Reagents and conditions: i, hn ( > 400 nm), MeCN-benzene (4+1,
v/v); ii, hn ( > 400 nm), Mg(ClO4)2, MeCN-benzene (4+1, v/v).
the bifunctional reagent 2 to the 1,2-diketones (Table 1). Thus
irradiation of benzil in the presence of a 1.5-fold molar excess
of the reagent 2 for 3 h produced the 1+1 adduct 5a in very high
yield (96%),‡ while irradiation in the presence of 0.5 equiv. of
2 gave both 5a and 1+2 adduct 6a, where the ratio of 5a+6a
gradually decreased with increasing irradiation time; these two
adducts were isolated respectively in 25 and 56% yield after
irradiation for 12 h (entry 7). These results show that the
addition reactions proceed stepwise. Other diketones 3 also
gave the 1+1 adduct 5 and/or the 1+2 adduct 6 in good yields,
depending on the ratio of the substrates and the reagent as
shown in Table 1.
The present allylsilylation and stannylation may be initiated
by PET from the bifunctional reagents to the photoexcited
diketones to generate the organometalic radical cation (1•+ or
2•+)-ketyl radical anion (3•2) pair. The selective cleavage of the
Sn–C(allyl) bond of the radical cation 1•+ produces b-
[(trimethylsilyl)methyl]allyl radical because of the Si–C bond
being stronger than the Sn-C bond.9 The resulting allyl radical
couples with 3•2 to give allylsilylated product 4. In the case of
the reaction using allylstannane–allylstannane reagent 2, the b-
[(triphenylstannyl)methyl]allyl radical formed by the fragmen-
tation of 2•+ couples with the ketyl radical 3•2 to yield the
allylstannylated product 5.
This work was supported by a Grand-in-aid for Scientific
Research (No 11640535) from the Ministry of Education,
Science, Sports and Culture, Japan.
Notes and references
† Selected data for 4a : needles, mp 77–79 °C; dH(CDCl3, 270 MHz) 20.02
[s, Si(CH3)3, 9H], 1.16 (d, J 13.2, CHHSiMe3, 1H), 1.39 (d, J 13.2,
CHHSiMe3, 1H), 2.86 (d, J 13.4 , CHH, 1H), 3.18 (d, J 13.4, CHH, 1H),
4.08 (s, OH, 1H), 4.56 (s, CHHN, 1H), 4.72 (s, CHHN, 1H) and 7.26-7.86 (m,
ArH, 10H); nmax(KBr)/cm21 3487, 1679, 1249, 1234 and 842; m/z 233 [(M
2 PhCO)+], 105 (PhCO+) and 73 (Me3Si+).
‡ Selected data for 5a: prisms, mp 110–111 °C; dH(CDCl3, 270 MHz) 2.19
(d, J 11.0, CHHSnPh3, 1H), 2.30 (d, J 11.0, CHHSnPh3, 1H), 2.77 (d, J 13.7,
CHH, 1H), 3.02 (d, J 13.7, CHH, 1H), 4.10 (s, OH, 1H), 4.47 (s, CHHN, 1H),
4.87 (s, CHHN, 1H) and 7.21–7.74 (m, ArH, 25H); nmax(KBr)/cm21 3515,
3059, 1671, 1428, 1239, 1073, 729 and 701.
1 I. Fleming, J. Dunogues and R. Smithers, Org. React., 1989, 37, 57.
2 Y. Yamamoto and N. Asao, Chem. Rev., 1993, 93, 2207.
3 Y. Nishigaichi, A. Takuwa, Y. Naruta and K. Maruyama, Tetrahedron,
1993, 49, 7395.
4 A. Takuwa, H. Tagawa, H. Iwamoto, O. Soga and K. Maruyama, Chem.
Lett., 1987, 1091; A. Takuwa, Y. Nishigaichi, T. Yamaoka and K.
Iihama, J. Chem. Soc., Chem. Commun., 1991, 1359; A. Takuwa, Y.
Nishigaichi, S. Ebara and H. Iwamoto, Chem. Commun., 1998, 1789.
5 S. Fukuzumi and T. Okamoto, J. Am. Chem. Soc., 1994, 116, 5503.
6 A. Takuwa, N. Fujii, H. Tagawa and H. Iwamoto, Bull. Chem. Soc. Jpn.,
1986, 59, 2959.
7 D. L. J. Clive, C. C. Paul and Z. Wang, J. Org. Chem., 1997, 62,
7028.
8 S. Chandrasekhar, S. Latour, J. D. Wuest and B. Zacharie, J. Org.
Chem., 1983, 48, 3810.
9 E. Butcher, C. J. Rhodes, M. Standing, R. S. Davidson and R. Bowser,
J. Chem. Soc., Perkin Trans. 2, 1992, 1469.
10 S. Fukuzumi, Bull. Chem. Soc. Jpn., 1997, 70, 1.
11 G. E. Keck and A. Palani, Tetrahedron Lett., 1993, 34, 3223; G.
Majetich, H. Nishidie and Y. Zhang, J. Chem. Soc., Perkin Trans. 1,
1995, 453.
Next we examined the subsequent addition step from 1+1
adduct 5 to 1+2 adduct 6. Irradiation of an MeCN–benzene
solution containing benzil (0.01 mmol dm23) and isolated 5a
(0.01 mmol dm23) for 3 h gave the 1+2 adduct 6a in 55% yield
(Scheme 2).
Interestingly, when the irradiation was carried out in the
presence of 5 equiv. of Mg(ClO4)2 under otherwise identical
conditions, the yield of 6a increased to 90%. These results
suggest that the second addition reaction also proceeds via PET
and Mg2+ accelerates the PET reaction, probably due to the
higher reducing ability of the Mg2+–benzil complex compared
to that of uncomplexed benzil.10 Therefore, the photochemical
reaction of the bifunctional reagent 2 with 1,2-diketones in the
presence of Mg(ClO4)2 was examined. The 1+2 adduct 6 was
obtained in very high yield ( > 90%) from the reaction with 0.5
equiv. of 2 in the presence of Mg(ClO4)2 (entries 8 and 13).
Even in the reaction of benzil with an excess amount of 2 a small
Communication 9/06620I
1964
Chem. Commun., 1999, 1963–1964