Stereoretentive introduction of (E)- And (Z)-γ-alkoxyallyl groups into carbonyl compounds via light-promoted reaction with γ-alkoxyallylstannanes

Article abstract of DOI:10.1039/a804459g

The light-promoted condensation between carbonyl compounds and (E)- or (Z)-γ-alkoxyallylstannanes affords predominantly the linear homoallylic alcohols, with retention of the double bond geometry of the γ-alkoxyallyl moieties.

Full text of DOI:10.1039/a804459g

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Stereoretentive introduction of (E)- and (Z)-g-alkoxyallyl groups into carbonyl
compounds via light-promoted reaction with g-alkoxyallylstannanes
Akio Takuwa,*† Yutaka Nishigaichi, Seiji Ebara and Hidetoshi Iwamoto
Department of Material Science, Faculty of Science and Engineering, Shimane University, Matsue, 690-8504, Japan
The light-promoted condensation between carbonyl com-
pounds and (E)- or (Z)-g-alkoxyallylstannanes affords
predominantly the linear homoallylic alcohols, with reten-
tion of the double bond geometry of the g-alkoxyallyl
moieties.
the a-product (Z)-3a (66%) and the regioisomer 4a (23%)
(entry 2). The E/Z olefin geometry of a-product 3a could be
deduced from the 12.7 and 6.3 Hz coupling constants of the
vicinal vinyl protons in the 1H NMR spectrum.‡ Double
allylation of the two carbonyl groups in the diketone 2a did not
occurred even under prolonged irradiation. The photoallylation
also proceeded even in less polar solvents such as benzene and
hexane, but slightly less efficiently. (E)- and (Z)-g-[(tert-
Butyldimethylsiloxy)allyl]tributylstannanes [(E)- and (Z)-1b]
also reacted photochemically with 2a to give again the desired
a-products predominantly, with complete retention of the
double bond geometry of the allylic moiety of the tin reagent 1b
It is well-known that both (E)- and (Z)-g-methoxyallylstannanes
add to aldehydes at the g-position in the presence of BF3•OEt2
to produce the vicinal diol monomethyl ether (g-product).1 The
a-ethoxyallylstannane also afforded the g-product via a double
allylic shift (one at the reagent level and the second in the
addition reaction).2 In addition, Thomas,3 Marshall4 and Gung5
have reported that a-alkoxy or a-silyloxy substituted crotyl-
stannanes also reacted at the g-position with aldehydes or
ketones under both thermal and Lewis acid-promoted condi-
tions to afford the homoallyl alcohols having an alkoxy or
silyloxy group at the terminus of the allylic carbon (i.e. vinyl
ethers) and that the geometry of the vinyl ether unit in the
products is generally Z or an E/Z mixture depending on the
reaction conditions and on the substrates. Thus, the stereor-
etentive introduction of (E)- and (Z)-g-alkoxyallyl groups at the
a-position of carbonyl compounds has yet to be developed. We6
and Fukuzumi7 have found recently that the light-promoted
reaction between Group 14 organometallic compounds and
electron accepting carbonyl compounds is one of the most
useful methodologies for selective carbon–carbon bond form-
ing reactions. In connection with our interest in the light-
promoted reaction of allylstannane, we studied the photoallyla-
tion of carbonyl compounds with g-methoxy- and
g-siloxy-allylstannanes. Reported here is the first example of
the regioreversed and stereoretentive introduction of (E)- and
(Z)-g-alkoxyallyl groups via light-promoted reaction (Scheme
1).
in both cases (entries
3
and 4). When the
g-methoxyallylstannane had a bulky tert-butyl group at the
b-position (1c), it showed complete a-regioselectivity [eqn.
(1)]. Thus, the a-regioselectivity depended upon the steric
crowding around the g-position of the allylic stannanes.
O
OMe
hν
OH
Bu3Sn
+
2a
(1)
Ph
96%
Ph
OMe
The photoallylation of p-cyanobenzaldehyde 2b with
g-alkoxyallylstannanes was also examined. Irradiation of the
absorption band of aldehyde 2b in MeCN containing (E)- or
(Z)-1a with light of wavelength longer than 320 nm for 5 h at 0
°C gave two regioisomeric homoallylic alcohols in which the
g-methoxyallyl group was again introduced preferentially at the
a-position (entries
5
and 7). Similar but higher
a-regioselectivity was observed in the photoallyltion with the
siloxyallyltin reagents (E)- and (Z)-1b. However, the
g-alkoxyallylic groups in both the tin reagents 1a,b were
Irradiation (hn > 400 nm) of an MeCN solution containing
(E)-g-(methoxyallyl)tributylstannane [(E)-1a, 24 mmol dm23
]
and benzil (2a, 20 mmol dm23) for 2 h at 0 °C gave two addition
products. These were identified as the desired a-product (E)-3a
(74%) and the regioisomeric g-product 4a (18%) (Table 1, entry
1). Irradiation of the diketone 2a with the geometrically
isomeric tin reagent (Z)-1a under the same conditions afforded
Table 1 Light-promoted condensation of (E)- and (Z)-g-alkoxyallyltributyl-
stannanes with carbonyl compounds
Entry
1
2
T/°C
Products (% yield; E:Z)a,b
O
1
2
3
4
5
6
7
8
(E)-1a
(Z)-1a
(E)-1b
(Z)-1b
(E)-1a
(E)-1a
(Z)-1a
(Z)-1a
(E)-1b
(E)-1b
(Z)-1b
(Z)-1b
(E)-1a
(Z)-1a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2b
2b
2b
2c
2c
0
0
0
0
0
3a (74; 98:2), 4a (18)
3a (66; 2:98), 4a (21)
3b (78; 99:1), 4b (9)
3b (74; 1:99), 4b (14)
3c (36; 91:9), 4c (14)
3c (37; 96:4), 4c (13)
3c (32; 14:86), 4c (17)
3c (42; 8:92), 4c (21)
3d (39; 88:12), 4d (8)
3d (37; 92:8), 4d (6)
3d (36; 13:87), 4d (12)
3d (47; 8:92), 4d (15)
3e (52; 96:4), 4e (15)
3e (46; 9:91), 4e (19)
Bu3Sn
OR1
+
Ar
R2
α
γ
1a R1 = Me (E)
b R1 = Me (Z)
c R1 = TBDMS (E)
d R1 = TBDMS (Z)
2a Ar = Ph, R2 = Bz
b Ar = p-NCC6H4, R2 = H
c Ar = R2 = Ph
278
0
278
0
9
10
11
12
13
14
278
0
hν
278
278
278
R2 OH
R2 OH
+
OR1
Ph
Ph
OR1
a
E:Z ratios were determined by 1H NMR analysis of the crude mixture.
The g-products consisted of a 1:1 mixture of syn and anti diaster-
3
4
b
Scheme 1
eomers.
Chem. Commun., 1998
1789

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CH2Cl2, 1:2, v/v). Selected data for a-product (E)-3a: oil; dH(CDCl3, 270
MHz) 2.7 (dd, J 7.7, 15.2, 1H), 3.0 (dd, J 7.7, 13.2, 1H), 3.4 (s, 3H), 4.1 (s,
1H), 4.4 (ddd, J 12.7, 13.2, 15.2, 1H), 6.2 (d, J 12.7, 1H), 7.2–7.8 (m, 10H);
nmax(CHCl3)/cm21 3534 (OH), 1679 (C§O). For a-product (Z)-3a: oil;
dH(CDCl3, 270 MHz) 2.9 (dd, J 7.0, 14.4, 1H), 3.2 (dd, J 7.0, 15.4, 1H), 3.4
(s, 3H), 4.4 (ddd, J 6.3, 14.4, 15.4, 1H), 4.5 (s, 1H), 6.0 (d, J 6.3, 1H),
7.2–7.8 (m, 10H); nmax(CHCl3)/cm21 3542 (OH), 1673 (C§O). All other
compounds gave satisfactory spectral data. The g-product 4a consisted of a
1:1 mixture of syn and anti diastereomers, which supports the proposed
reaction mechanism showing the products being formed by the combination
of allylic and ketyl radicals.
§ To a solution of the tin reagent (E)- or (Z)-1a (0.3 mmol) in CH2Cl2 (6 ml)
under N2 at 278 °C was added BuSnCl3 (0.3 mmol) in CH2Cl2 (3 ml). After
stirring the mixture at that temperature for 10 min, the substrate 2b (0.2
mmol) was added slowly and the resulting mixture was stirred for 2 h at that
temperature. The reaction mixture was quenched with a 10% aq. KF and
worked up as usual. The (Z)-a-product was obtained in 19 and 71% yield,
respectively, along with a very small amount of g-product [(Z)-3a:4a =
91:9 from (E)-1a; 99:1 from (Z)-1a].
introduced with partial loss of the geometry of the allylic double
bond. On the other hand, when the irradiation was carried out at
278 °C in propionitrile, the geometry of the g-alkoxyallylic
group of the tin reagents 1a,b was almost completely main-
tained in the a-products (entries 6 and 8). Benzophenone 2c
could also be allylated with (E)- and (Z)-1a to give preferen-
tially the a-product with retention of the geometry of the
g-methoxyallyl moiety (entries 13 and 14).
The present allylation is initiated by an electron transfer from
the tin reagent 1 to the photoexcited carbonyl compound 2 to
give the g-alkoxyallyltributyltin radical cation (1•+)-ketyl
radical anion pair (2•2), followed by the dissociation of the
(g-alkoxyallyl) CUSn bond of 1•+ into a g-alkoxyallyl radical
and a tributyltin cation.6,7 The resulting g-alkoxyallyl radical
couples with 2•2 preferentially at the less crowded a-position to
yield the linear homoallylic alcohol. The configuration of the
g-alkoxyallyl unit is maintained without E/Z-isomerization
during all these processes to give the a-product stereospecif-
ically.
When the BuSnCl3-mediated transmetallation method8 was
employed for the reaction of the tin reagents (E)- and (Z)-1a
with aldehyde 2b, the a-product 3a was also produced from
both the tin reagents, but the olefin geometry of the product 3a
was Z regardless of the geometry of the starting tin reagents.§ In
addition, it has been reported that alkoxyallyl carbanion
reagents (ROCH§CHCH2M: R = alkyl or SiMe3; M = Li)
react with electrophiles including carbonyl compounds to afford
(Z)-linear vinyl ethers.9
1 M. Koreeda and Y. Tanaka, Tetrahedron Lett., 1987, 28, 143; V.
Gevorgyan and Y. Yamamoto, J. Chem. Soc., Chem. Commun., 1994,
59.
2 J. P. Quintard, B. Elissondo and M. Pereyre, J. Org. Chem., 1983, 48,
1559.
3 A. J. Pratt and E. J. Thomas, J. Chem. Soc., Perkin Trans. 1, 1989, 1521;
J. Chem. Soc., Chem. Commun., 1982, 1115.
4 J. A. Marshall and W. Y. Gung, Tetrahedron, 1989, 45, 1043; J. A.
Marshall and G. S. Weimaker, Tetrahedron Lett., 1991, 32, 2101.
5 B. W. Gung, D. T. Smith and M. A. Wolf, Tetrahedron Lett., 1991, 32,
13; B. W. Gung, A. J. Peat, B. M. Snook and D. T. Smith, Tetrahedron
Lett., 1991, 32, 453.
6 A. Takuwa, Y. Nishigaichi and H. Iwamoto, Chem. Lett., 1991, 1013; A.
Takuwa, Y. Nishigaichi, T. Yamaoka and K. Iihama, J. Chem. Soc.,
Chem. Commun., 1991, 1359; A. Takuwa, J. Shiigi and Y. Nishigaichi,
Tetrahedron Lett., 1993, 34, 3457.
In conclusion, we have found that the present light-promoted
reaction provides the first example of a stereoretentive introduc-
tion of (E)- and (Z)-g-alkoxyallyl groups from
g-alkoxyallylstannanes into carbonyl compounds at the
a-position.
This work was supported by a Grant-in-aid for Scientific
Research (No. 09640636) from the Ministry of Education,
Science, Sports and Culture, Japan.
7 S. Fukuzumi and T. Okamoto, J. Am. Chem. Soc., 1994, 116, 5503; S.
Fukuzumi, Bull. Chem. Soc. Jpn., 1997, 70, 1.
8 H. Miyake and K. Yamamura, Chem. Lett., 1992, 1369.
9 W. C. Still and T. L. Macdonald, J. Org. Chem., 1976, 41, 3620; W. C.
Still and T. L. Macdonald, J. Am. Chem. Soc., 1974, 96, 5561; D. A.
Evans, G. C. Andrews and B. Buckwalter, J. Am. Chem. Soc., 1974, 96,
5560.
Notes and References
† E-mail: takuwa@riko.shimane-u.ac.jp
‡ After irradiation, the products were separated into individual regioisomers
(3a and 4a) by TLC (SiO2, hexane–Et2O, 1:1, v/v). Each geometrical isomer
could be purified by carefully repeating the TLC separation (hexane–
Received in Cambridge, UK, 12th June 1998; 8/04459G
1790
Chem. Commun., 1998