P(i-PrNCH2CH2)3N: An Effective Lewis Base Promoter for the Allylation of Aromatic Aldehydes with Allyltrimethylsilane

Full text of DOI:10.1021/jo990225w

J . Org. Chem. 1999, 64, 6459-6461
6459
Notes
P (i-P r NCH2CH2)3N: An Effective Lew is
Ba se P r om oter for th e Allyla tion of
Ar om a tic Ald eh yd es w ith
Allyltr im eth ylsila n e
1 is due to transannular bonding that can occur as, for
example, in the conjugate Lewis acids H1a + and H1b+.6
Here we report that Lewis base 1b acts as a promoter
for the allylation of a substantial number of aldehydes
with allyltrimethylsilane at room temperature or 40 °C.
Zhigang Wang, Philip Kisanga, and J ohn G. Verkade*
Department of Chemistry, Iowa State University,
Ames, Iowa 50011-3111
Received February 8, 1999
The allylation of carbonyl compounds with allylsilanes
under Lewis acidic conditions has been extensively
investigated owing to its importance in the formation of
C-C bonds in organic synthesis.1,2 Recently, the allyla-
tion of aldehydes with allylsilanes promoted by Lewis
bases has also attracted considerable attention because
of the mild reaction conditions and the high regio- and
stereoselectivity encountered.3,4 Allylsilanes used in these
reactions, such as allyltrichlorosilane and allyltrifluo-
rosilane, are easily activated by Lewis bases since the
silicon atom bears strong electron-withdrawing groups,
thereby facilitating smooth aldehyde allylations. How-
ever, we found only one report on the reaction of allyl-
trimethylsilane with aldehydes in the presence of a Lewis
base promoter, namely, fluoride ion.5 In that report,
refluxing THF was required. In our continuing explora-
tion of the synthetic utility of exceedingly strong nonionic
bases of type 1 first synthesized in our laboratories,6 we
have discovered that 1a and 1b are very useful in a
variety of both stoichiometric7 and catalyzed reactions.8
It is noteworthy that the strong basicity of bases of type
Initially, P(NMe2)3 (HMPT) and 1a were chosen as
candidate catalysts for comparison in the allylation of
benzaldehyde with allyltrimethylsilane. However, HMPT
showed no activity in this reaction, and only a trace of
the corresponding homoallylic alcohol was observed in
the complicated reaction mixture when 1a was used. On
the other hand, when 1b (0.5 equiv) was employed, the
desired homoallylic alcohol was isolated in 74% yield. The
reaction was monitored by TLC which showed that the
reaction was quite clean. A comparable yield of 72% was
achieved even when the amount of 1b was decreased to
0.2 equiv, but it dropped to 62% when the amount of 1b
was further decreased to 0.1 equiv. Thus, reaction 1 with
a variety of aromatic aldehydes was investigated in the
presence of 20 mol % 1b, and the results are listed in
Table 1.
As shown in this table, aldehydes bearing an electron-
donating group possess lower reactivity. Hence, p-di-
methylaminobenzaldehyde, p-methoxybenzaldehyde, and
p-methylbenzaldehyde gave 44-57% yields even at 40
°C for extended time periods (entries 4-6). Except for
p-fluorobenzaldehyde, which displayed considerable re-
activity leading to a 77% yield (entry 7), some aldehydes
bearing electron-withdrawing groups, such as p-Cl, un-
expectedly showed reduced reactivity. When p-cyano- or
p-nitrobenzaldehyde was added to a solution of 1b in
THF, no corresponding homoallylic alcohol was detected.
However, the color of the reaction solution rapidly turned
deep red in both cases, which is attributed to the
(1) (a) Hosomi, A.; Sakurai, H. Tetrahedron Lett. 1976, 17, 1295.
(b) Tietze, L. F.; Schiemann, K.; Wegner, C.; Wulff, C. Chem. Eur. J .
1996, 2, 1164. (c) Gauthier, D. R.; Carreira, E. M. Angew. Chem., Int.
Ed. Engl. 1996, 35, 2363.
(2) (a) Majetich, G. Organic Synthesis, Theory and Application; J AI
Press: Greenwich, CT, 1989. For reviews see: (b) Yamamoto, Y. Chem.
Rev. 1993, 93, 137. (c) Masse, C. E.; Panek, J . S. Chem. Rev. 1995, 95,
1293. (d) Cozzi, P. G.; Tagliavini, E.; Umani-Ronchi, A. Gazz. Chim.
Ital. 1997, 127, 247. (e) Chan, T. H.; Wang, D. Chem. Rev. 1992, 92,
995.
(3) (a) Kira, M.; Hino, T.; Sakurai, H. Tetrahedron Lett. 1989, 30,
1099. (b) Kira, M.; Kobayashi, M.; Sakurai, H. Tetrahedron Lett. 1987,
28, 4081. (c) Kobayashi, S.; Nishio, K. Tetrahedron Lett. 1993, 34, 3453.
(d) Kobayashi, S.; Nishio, K. J . Org. Chem. 1994, 59, 6620. For reviews
on hypervalent silicates, see: (e) Sakurai, H. Synlett 1989, 1. (f) Chuit,
C.; Corriu, R. J . P.; Reye, C.; Young, J . C. Chem. Rev. 1993, 93, 1371.
(4) (a) Denmark, S. E.; Coe, D. M.; Pratt, N. E.; Griedel, B. D. J .
Org. Chem. 1994, 59, 6161. (b) Iseki, K.; Kuroki, Y.; Takahashi, M.;
Kobayashi, Y. Tetrahedron Lett. 1996, 37, 5149. (c) Iseki, K.; Kuroki,
Y.; Takahashi, M.; Kishimoto, S.; Kobayashi, Y. Tetrahedron 1997, 53,
3513. (d) Wang, Z.; Wang, D.; Sui, X. Chem. Commun. 1996, 2261. (e)
Iseki, K.; Mizuno, S.; Kuroki, Y.; Kobayashi, Y. Tetrahedron Lett. 1998,
39, 2767. (f) Nakajima, M.; Saito, M.; Hashimoto, S. J . Am. Chem. Soc.
1998, 120, 6419.
(7) (a) Tang, J . S.; Verkade, J . G. J . Org. Chem. 1994, 59, 7793. (b)
Mohan, T.; Arumugam, S.; Wang, T.; J acobson, R. A.; Verkade, J . G.
Heteroatom Chem. 1996, 7, 455. (c) Tang, J . S.; Verkade, J . G. J . Org.
Chem. 1996, 61, 8750. (d) Arumugam S.; Verkade, J . G. J . Org. Chem.
1997, 62, 4827. (e) Arumugam, S.; Mcleod, D.; Verkade, J . G. J . Org.
Chem. 1998, 63, 3677. (f) Wang, Z.; Verkade, J . G. Tetrahedron Lett.
1998, 39, 9331. (g) Wang, Z.; Verkade, J . G. Heteroatom Chem. 1998,
9, 687.
(8) (a) Tang, J . S.; Verkade, J . G. Angew. Chem., Int. Ed. Engl. 1993,
32, 896. (b) Tang, J . S.; Mohan, T.; Verkade, J . G. J . Org. Chem. 1994,
59, 4931. (c) D’Sa, B.; Verkade, J . G. J . Org. Chem. 1996, 61, 2963. (d)
D’Sa, B.; Verkade, J . G. J . Am. Chem. Soc. 1996, 118, 12832. (e) D’Sa,
B.; Mcleod, D.; Verkade, J . G. J . Org. Chem. 1997, 62, 5057. (f) D’Sa,
B.; Kisanga, P.; Verkade, J . G. J . Org. Chem. 1998, 63, 3961. (g)
Kisanga, P.; D’Sa, B.; Verkade, J . G. J . Org. Chem. 1998, 63, 10057.
(f) Kisanga, P.; Verkade, J . G. J . Org. Chem. 1999, 64, 4298.
(5) (a) Hosomi, A.; Shirahata, A.; Sakurai, H. Tetrahedron Lett. 1978,
19, 3043. (b) Asao, N.; Shibato, A.; Itagaki, Y.; J ourdan, F.; Maruoka,
K. Tetrahedron Lett. 1998, 39, 3177.
(6) Verkade, J . G. Acc. Chem. Res. 1993, 26, 483.
10.1021/jo990225w CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/16/1999

6460 J . Org. Chem., Vol. 64, No. 17, 1999
Notes
Sch em e 1
Ta ble 1. Rea ction s of Ald eh yd es w ith
Allyltr im eth ylsila n ea
nate further with an aldehyde oxygen to form a hexaco-
ordinate allylsilicate, thus allowing completion of this
reaction through a six-membered ring transition state.3a-c,4
Another possibility is that the C-Si bond of the pentac-
cordinate silicon intermediate cleaves to form an allylic
anion, which then adds to the aldehyde to give R- or
γ-addition products.3a,b To determine which pathway was
followed, crotyltrimethylsilane (E:Z ) 88:12)11 was re-
acted with benzaldehyde under the same conditions listed
in Table 1 (reaction 2). Although the reaction was very
reaction conditions
entry
1c Ph
Ph
3d Ph
4
5
6
7
8
9
3,4 R′
T, °C
t, h
yield, %b
rt
rt
rt
40
40
40
rt
rt
rt
72
72
72
84
84
84
72
84
72
72
72
74
72
62
57
52
44
77
59
80
73
73
2
p-Me2NC6H4
p-MeOC6H4
p-MeC6H4
p-FC6H4
p-ClC6H4
2-thiophenecarboxaldehyde
2-furaldehyde
2-naphthaldehyde
10
11
rt
rt
a
Reactions were carried out under argon in a ratio of 2:3:1b )
1:1:0.2 mmol. THF was freshly distilled from Na and stored over
4 Å molecular sieves. Homoallylic alcohol. c 2:3:1b ) 1:1:0.5.
b
d
2:3:1b ) 1:1:0.1.
formation of a complex between 1b and these aldehydes.9
Both reaction mixtures were EPR silent, suggesting that
an ionic complex is formed. This was further substanti-
ated by 31P NMR analysis in which only a 31P signal
associated with H1b+ at -10.7 ppm was observed. In the
reaction of p-cyanobenzaldehyde, an 81% yield of an
epoxide was isolated.10 The formation of epoxides in
similar reactions has recently been observed in ourlabo-
ratories and will be reported in due course.10 The reaction
of p-nitrobenzaldehyde, on the other hand, afforded a
complicated reaction mixture although 31P NMR spec-
troscopy revealed only a single peak at -10.7 ppm
indicative of H1b+ formation. As shown in Table 1,
2-thiophenecarboxaldehyde, 2-furaldehyde, and 2-naph-
thaldehyde gave respectable yields of product (entries
9-11).
The reaction of allyltrimethylsilane with aldehydes in
the presence of 1b is assumed to proceed in a manner
similar to that of other allylation reactions promoted by
Lewis bases.5a Thus, 1b can be expected to coordinate
with allyltrimethylsilane to form a pentacoordinate
silicon species that also features a pentacoordinate
phosphorus. The latter possibility, which would be
achieved via at least partial Nax f P transannulation,
receives support from an observation that HMPT does
not promote this reaction under the same reaction
conditions. One option for the reaction of the pentacoor-
dinate allylsilicate with aldehyde is for silicon to coordi-
sluggish, both R- and γ-addition products in a ratio of
47:5312 (6 and 7, respectively) were isolated as a mixture.
For 7, the syn:anti ratio was 48:52. This strongly suggests
that an allylic anion was formed during the reaction as
illustrated in Scheme 1. According to this scheme, 1b
coordinates with silicon to form a pentacoordinate allyl-
silicate intermediate probably featuring at least partial
transannulation of the phosphorus cage. The Si-C bond
is then broken, and an allylic anion is formed. The
cationic intermediate which can then be stabilized by
stronger transannulation is analogous to that character-
ized earlier by us in the silyl protection of alcohols
catalyzed by 1a .8d,e The allylic anion then reacts with the
aldehyde to afford a silyl ether, thus releasing 1b in
completion of the catalytic cycle. The silyl ether is then
finally hydrolyzed in workup to give the homoallylic
alcohol. The relatively high yield of product in the case
of p-FC6H4CHO compared with its p-NMe2, p-OMe and
p-Cl analogues (Table 1) is attributable to the greater
+R effect of the latter substituents which is expected to
reduce the electrophilicity of the carbonyl carbon to
nucleophilic attack by the allyl group in what is probably
the slow step. A similarly low product yield is expected
from the hyperconjugation and the +I effect of the p-CH3
group.
(11) (a) Hagen, G.; Mayr, H. J . Am. Chem. Soc. 1991. 113, 4954. (b)
Alberts, V.; Cuthbertson, M. J .; Hawker, D. W.; Wells, P. R. Org. Magn.
Reson. 1984, 22, 556.
(12) The ratio was determined by comparison of the 1H NMR
spectrum (300 MHz) of crude product with that in the literature
(Takuda, M.; Satoh, S.; Suginome, H. J . Org. Chem. 1989, 54, 5608).
(9) Yang, J . C.; Verkade. J . G. Manuscript in progress.
(10) Liu, X.; Verkade, J . G. Manuscript in progress.

Notes
J . Org. Chem., Vol. 64, No. 17, 1999 6461
the solvent was removed in a vacuum to afford the crude product.
Purification of the crude product was achieved by flash chro-
matography (hexane:ethyl acetate ) 30:1) to give the homoallylic
alcohol.
Exp er im en ta l Section
Reactions were carried out under argon. THF was freshly
distilled over sodium and stored over 4 Å molecular sieves under
argon. The aldehydes (Aldrich) were used as received.
Typ ica l P r oced u r e for th e Allyla tion . To a solution of 1b
(0.2 mmol) in dry THF (2 mL) was added the allyltrimethylsilane
(1.0 mmol) by syringe at room temperature under argon. The
reaction solution was then stirred at room temperature for 0.5
h followed by addition of the aldehyde (1.0 mmol). After the
reaction conditions stated in Table 1 had been met, the reaction
was quenched with saturated aqueous NaHCO3 (3 mL) and ether
(10 mL). The mixture was separated after stirring for 0.5 h, and
the aqueous layer was further extracted with ether (3 × 20 mL).
After the organic extracts were combined and dried with MgSO4,
Ack n ow led gm en t. We thank the donors of the
Petroleum Research Fund, administered by the Ameri
can Chemical Society, for grant support.
Su p p or tin g In for m a tion Ava ila ble: NMR spectral data
for compounds in this study. This material is available free of
charge via the Internet at http://pubs.acs.org.
J O990225W