Reactions of homoallylstannanes with carbon electrophiles: Stereoselective cyclopropylmethylation based on the γ-effect of tin

Article abstract of DOI:10.1039/a900228f

Homoallylstannanes reacted with carbon electrophiles such as acetals, acid chlorides and aldehydes in the presence of Lewis acids to give the cyclopropylmethylated products with high stereoselectivities.

Full text of DOI:10.1039/a900228f

Reactions of homoallylstannanes with carbon electrophiles: stereoselective
cyclopropylmethylation based on the g-effect of tin
Masanobu Sugawara and Jun-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University,
Kyoto 606-8501, Japan. E-mail: yoshida@sbchem.kyoto-u.ac.jp
Received (in Cambridge, UK) 7th January 1999, Accepted 29th January 1999
Homoallylstannanes reacted with carbon electrophiles such
as acetals, acid chlorides and aldehydes in the presence of
Lewis acids to give the cyclopropylmethylated products with
high stereoselectivities.
increased. In the case of homoallylstannane 2a, however, the
energy level of the p orbital should to be similar to that for
normal carbon–carbon double bonds, because the interaction
between the p orbital of the carbon-carbon double bond and the
carbon–tin s orbital seems to be very weak, due to the flexibility
of the conformation.8
Enormous advances have been made in the study of the b-effect
of silicon, and a number of reactions have been developed based
on this effect and utilized for a variety of synthetic applica-
tions.1 The g-effect of silicon, however, seems to be less
effective and has not been embraced as a viable principle in
organic synthesis.2 On the other hand, studies on the g-effect of
tin revealed the effectiveness of this effect3 and have uncovered
a rich variety of synthetic applications in recent years.4
Previously, we have reported that the g-effect of tin is stronger
than the b-effect of silicon in intramolecular competition and
that cationic cyclopropanation of ‘tin carbenoids’ with alkenes
can be achieved based on the g-effect of tin.5 We have also been
interested in the intermolecular competition between the g-
effect of tin and the b-effect of silicon. During the course of this
study we found a cyclopropylmethylation reaction using
homoallylstanannes that would be useful in organic synthesis.6
We report here the preliminary results of these studies.
The next question which naturally emerged in our mind is
whether the reactivity of the carbon–carbon double bond of the
homoallylstannane is the same as those of simple terminal
alkenes. Thus, we examined the intermolecular competition
between the homoallylstannane and normal terminal alkenes. A
1:1 mixture of homoallylstannane 2a and dodec-1-ene 6 was
allowed to react with 1a in the presence of TMSOTf. Although
it required a slightly higher temperature, the reaction of 1a and
2a took place to give the cyclopropylmethylated product 4a in
60% yield. Most of 6 was recovered unchanged (Scheme 2).
This result indicates that the homoallylstannane is definitely
more reactive toward electrophiles than terminal alkenes.
Therefore, there seems to exist some interaction between the
carbon–tin s orbital and the p orbital of the carbon–carbon
double (B in Fig. 1) even if it is not particularly strong. This led
us to a new aspect of the g-effect of tin.
The intermolecular competition between the g-effect of tin
and the b-effect of silicon was examined using an electrophilic
reaction of a homoallylstannane and an allylsilane. Thus, a 1:1
mixture of homoallyltributylstannane 2a and allyltrimethylsi-
lane 3 in CH2Cl2 was treated with 1 equiv. of benzaldehyde
dimethyl acetal 1a in the presence of TMSOTf (Scheme 1).
Most of 2a was recovered unchanged and only allylated product
5 was obtained in 87% yield. The cyclopropylmethylated
product 4a was not detected.
The following mechanism seems to be plausible. The
oxonium ion is generated by the reaction of acetal 1a with
TMSOTf and then adds to the carbon–carbon double bond of
homoallylstannane 2a to give the g-carbocation to tin. The
facile g-elimination of tin to form a cyclopropane ring takes
place to give the final product 4a.
The successful reaction of homoallylstannane 2a and acetal
1a prompted us to develop new reactions of homoallylstanannes
with carbon electrophiles. As shown in Table 1, acetals, acid
This result indicates that 3 is much more reactive than 2a
toward the oxonium ion (PhCHNOMe+) generated from 1a and
suggests that the b-effect of silicon is stronger than the g-effect
of tin in intermolecular competition. In other words, the b-silyl
group activates the carbon–carbon double bond more effec-
tively toward the electrophilic reaction than the g-stannyl group.
This difference can be explained in terms of molecular orbital
considerations. In the case of allylsilane 3, the energy level of
the p orbital of the carbon–carbon double bond is increased by
interaction with the neighboring carbon–silicon s orbital (s–p
interaction).7 Therefore, the reactivity toward electrophiles is
OMe
+
+
SnBu3
C10H21
Ph
OMe
1a
2a
2a
6
i
OMe
+
+
6
Ph
(60%)
(33%)
(89%)
4a
OMe
SiMe3
Scheme 2 Reagents and conditions: i, TMSOTf (1.05 equiv.), CH2Cl2,
250 °C, 3 h.
+
+
SnBu3
Ph
OMe
1a
2a
3
Sn
i
Si
OMe
OMe
+
2a
+
Ph
Ph
(0%)
(87%)
(93%)
B
4a
5
A
Scheme 1 Reagents and conditions: i, TMSOTf (1.05 equiv.), CH2Cl2,
278 °C, 3 h.
Fig. 1 s–p Interactions in the allylsilane (A) and the homoallylstannane
(B).
Chem. Commun., 1999, 505–506
505

Table 1 Reactions of homoallylstannanes with carbon electrophiles in the
was more effective than TMSOTf. The corresponding cyclopro-
pylmethylated products 4 were obtained in good yields together
with small amounts of homoallylated by-products 7.2
presence of Lewis acidsa
Lewis
acid
The diastereoselectivity of the present reaction is remarkable.
For example, the erythro product was obtained selectively
(97:3) starting from 1e and (E)-pent-3-enyltributylstannane
(E:Z = 88:12).9 Presumably, the addition to the carbonyl
group took place in an antiperiplanar fashion.10 The reaction of
1e and (Z)-pent-3-enyltributylstannane (E:Z = 1:99), how-
ever, resulted in a decrease of the selectivity (68:32). The low
selectivity of the cis isomer might be explained in terms of the
competition between the antiperiplanar transition state and the
synclinal transition state.11
In summary, intermolecular competition revealed that a b-
silyl group activates carbon–carbon double bonds toward
electrophilic reactions more effectively than a g-stannyl group.
It was also found that the g-stannyl group activates the carbon–
carbon double bond slightly more effectively than alkyl groups.
We also found that the reactions of homoallylstannanes with
carbon electrophiles such as acetals, acid halides and aldehydes
took place smoothly in the presence of Lewis acids. These
reactions serve as a stereoselective method for cyclopropylme-
thylation of carbonyl compounds. Further mechanistic studies
on the evaluation of the g-effect of tin and its synthetic
applications are under investigation.
E+
E
+
SnBu3
E
+
–Bu3Sn+
1
2
4
7 (minor)
Yield (%)
Electrophile Homoallylstannane
4
4 (erythro:threo)c
7
OMe
OMe
SnBu3
<1d
12
Ph
OMe
87
Ph
1ab
2a
4a
O
(1.3 equiv.)
O
O
2a
87
Ph
Cl
Ph
Ph
(1.0 equiv.)
1b
4b
OH
2a
Ph
H
7
76
86
(1.2 equiv.)
1c
1c
4c
OH
SnBu3
10
7
Ph
Ph
4d
2b (1.3 equiv.)
Notes and references
OH
1 For example, W. P. Weber, Silicon Reagents for Organic Synthesis,
Springer-Verlag, Berlin, Heidelberg, 1983, p. 173; J. B. Lambert,
Tetrahedron, 1990, 46, 2677 and references cited therein.
2 H. Sakurai, T. Imai and A. Hosomi, Tetrahedron Lett., 1977, 4045; Y.
Hatanaka and I. Kuwajima, Tetrahedron Lett., 1986, 27, 719.
3 I. Fleming and C. Urch, J. Organomet. Chem., 1985, 285, 173; J. B.
Lambert, L. A. Salvador and J.-H. So, Organometallics, 1993, 12, 697.
See also ref. 4 and 5.
4 M. Pereyre, J.-B. Quintard and A. Rahm, Tin in Organic Synthesis,
Butterworth, London, 1987, p. 235; A. G. Davies, Organotin Chemistry,
VCH, Weinheim, 1997 and references cited therein. See also ref. 5.
5 M. Sugawara and J. Yoshida, J. Am. Chem. Soc., 1997, 119, 11986; M.
Sugawara and J. Yoshida, Synlett., 1998, 1057.
6 Cyclopropylmethylations with hetero-electrophiles by homoallylstan-
nanes have been reported: D. J. Peterson and M. D. Robbins,
Tetrahedron Lett., 1972, 2135; D. J. Paterson, M. D. Robbins and J. R.
Hansen, J. Organomet. Chem., 1974, 73, 237; K. C. Nicolaou, D. A.
Claremon, W. E. Barnette and S. P. Seitz, J. Am. Chem. Soc., 1979, 101,
3704; Y. Ueno, M. Ohta and M. Okawara, Tetrahedron Lett., 1982, 23,
2577; J. W. Herndon and J. J. Harp, Tetrahedron Lett., 1992, 33,
6243.
87
(97:3)
SnBu3
1c
4e
2c (E:Z = 88:12)
(1.1 equiv.)
O
OH
2c
C7H15
H
3d
C7H15
83
(>99:1)
(1.1 equiv)
1d
4f
O
OH
2c
H
87
(97:3)
5d
(1.1 equiv.)
1e
1e
4g
4g
SnBu3
7 J. C. Giordan, J. Am. Chem. Soc., 1983, 105, 6544.
<1d
8 H. C. Clark and R. C. Poller, Can. J. Chem., 1970, 48, 2670; R. S.
Brown, D. F. Eaton, A. Hosomi, T. G. Traylor and J. M. Wright,
J. Organomet. Chem., 1974, 66, 249.
78
(68:32)
2d (E:Z = 1:>99)
(1.2 equiv.)
9 The erythro stereochemistry was determined by comparison with an
authentic sample, which was prepared by the cyclopropanation of the
corresponding homoallylalcohol.
10 M. Santelli and J.-M. Pons, Lewis Acids and Selectivity in Organic
Synthesis, CRC Press, Boca Raton, 1996, p. 91.
a Reaction conditions; 0.50 mmol of 1, 1.0–1.3 equiv. of 2 and 1.1 equiv. of
TiCl4 in CH2Cl2 (1.8 ml) at 278 °C for 3 h. b TMSOTf (1.05 equiv.) was
used instead of TiCl4, at 240 °C, 3 h. Determined by GLC. The
homoallylated product 7 was not fully characterized.
c
d
11 T. Hayashi, K. Kabeta, I. Hamachi and M. Kumada, Tetrahedron Lett.,
1983, 24, 2865.
halides and aldehydes were found to be effective as carbon
electrophiles. In the case of acid halides and aldehydes, TiCl4
Communication 9/00228F
506
Chem. Commun., 1999, 505–506