General and convenient TsOH-induced allylboration of ketones

Article abstract of DOI:10.1016/j.tetlet.2009.02.226

TsOH-induced allylation of various ketones with air- and moisture-stable potassium allyltrifluoroborate is described.

Full text of DOI:10.1016/j.tetlet.2009.02.226

Tetrahedron Letters 50 (2009) 2320–2321
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Tetrahedron Letters
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General and convenient TsOH-induced allylboration of ketones
Hiroaki Matsuoka a, Kazuhiro Kondo a,b,
*
a Graduate School of Medical Sciences, Nagoya City University, 1 Azakawasumi, Mizuho-ku, Nagoya 467-8601, Japan
b Graduate School of Pharmaceutical Sciences, Nagoya City University, 3-1 Tanabe-dori, Mizuho-ku, Nagoya 467-8603, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
TsOH-induced allylation of various ketones with air- and moisture-stable potassium allyltrifluoroborate
Received 12 February 2009
Revised 23 February 2009
Accepted 24 February 2009
Available online 6 March 2009
is described.
Ó 2009 Elsevier Ltd. All rights reserved.
The allylation of ketones is one of the most important carbon–
carbon bond-forming reactions, because homoallylic alcohols pos-
sessing a tetrasubstituted carbon are potentially useful synthetic
intermediates.1,2 Among various allylmetal reagents, potassium
allyltrifluoroborate3 is preferred given the recent demand for safe
and sustainable organic synthesis, because this reagent is air-sta-
ble. As part of our research on the development of catalytic syn-
thetic reactions in which the catalyst is composed of elements of
a high Clarke number,4,5 we herein report the general and conve-
nient TsOH-induced allylation of ketones by potassium allyltri-
fluoroborate.
Since it has been reported that a Lewis acid such as BF3ꢀEt2O
immediately converts potassium allyltrifluoroborate into allyl
difluoroborane (1), the air- and moisture-sensitive trivalent borane
(Scheme 1),3 we undertook allylation of ketones with the combina-
tion of Brønsted acids and potassium allyltrifluoroborate.6,7 The ef-
fect of Brønsted acids in the allylboration8 using the test substrate
1-naphthoaldehyde was first investigated. As shown in Scheme 2,
TsOHꢀH2O was found to be the most effective catalyst among the
acids employed.
BF3•Et2O
BF3K
BF2
1
Scheme 1.
BF3K
HO
CHO
(2 mol equiv)
TsOH•H2O (10 mol%)
dioxane, time, rt
BINOL: 24 h, y. <5%
CH3CO2H: 24 h, y. <5%
TsOH•H2O: 0.5 h, y. >99%
Scheme 2. Effects of Brønsted acids in allylboration.
a
,b-unsaturated ketone 10 was allylated in only the 1,2-mode,
Encouraged by the result with TsOHꢀH2O, we turned our atten-
tion to the allylboration of ketones (Table 1). The allylation of ke-
tone 2 with 2 mol equiv of potassium allyltrifluoroborate and
20 mol % of TsOHꢀH2O at 40 °C was carried out in several solvents
(entries 1–5): using 10 mol % of TsOHꢀH2O or lowering the temper-
ature from 40 °C to rt resulted in a sluggish reaction. All the sol-
vents evaluated produced the desired allylated products. Toluene
was found to exhibit a high reaction rate (entry 5: 3 h, y. 90%).
The generality of this allylboration under the optimized reaction
conditions was then tested with a variety of other ketones. Acyclic
3–5, easily enolizable 6, and cyclic ketones 7 and 8 gave good
yields of the corresponding allylated products (entries 6–11). Ace-
tophenone derivatives (9a–c) could also be used, giving the corre-
sponding allylated products in good yields (entries 12–14). The
and no 1,4-addition product was observed (entry 15).
A
11B NMR (CD3CN, 40 °C) study was performed on the mixture of
potassium allyltrifluoroborate and TsOHꢀH2O. CD3CN was chosen as
the solvent for 11B NMR measurements because of the solubility for
potassium allyltrifluoroborate: no 11B NMR data were available be-
cause of low solubility of potassium allyltrifluoroborate in toluene-
d8. The chemical shift (d = ꢁ7.10 ppm, q, J = 61.7 Hz) of the mixture
compared to that of the potassium allyltrifluoroborate9 alone
showed no change in the chemical shift.10 Taking these results into
consideration, a possible active nucleophile appears to be potassium
allyltrifluoroborate (not the trivalent borane 1) as shown in Figure 1,
although further investigation is needed.
In summary, we developed a generally and conveniently semi-
catalytic allylation to various ketones of potassium allyltrifluorob-
orate using TsOHꢀH2O.11 Investigation of a catalytic asymmetric
allylboration of ketones with a novel chiral sulfonic acid is in
progress.12
* Corresponding author.
E-mail address: kazu@gakushikai.jp (K. Kondo).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.226

H. Matsuoka, K. Kondo / Tetrahedron Letters 50 (2009) 2320–2321
2321
Table 1
HOTs
R2
Allylboration to ketones with TsOHꢀH2O
O
BF3K
R1
O
(2 mol equiv)
BF3K
HO
R2
R2
R1
R1
TsOH•H2O (20 mol%)
solvent, time, 40 ºC
Figure 1. Possible reaction mechanism.
Entry
1
Ketone
Solvent
Time (h)
14
Yield (%)
59a
Acknowledgment
O
We thank Professor Toyohiko Aoyama (Graduate School of
Pharmaceutical Sciences, Nagoya City University) for his very
important support in various ways.
Dioxane
2
2
3
4
5
2
2
2
2
PrOH
CH3CN
DMF
24
24
24
3
19a
68a
23a
90
References and notes
Toluene
1. Only one example of synthetically useful and general catalytic allylation of
ketones (achiral version): (a) Yamasaki, S.; Fujii, K.; Wada, R.; Kanai, M.;
Shibasaki, M. J. Am. Chem. Soc. 2002, 124, 6536–6537; For other catalytic
allylations of ketones (achiral version), see: (b) Hosomi, A.; Shirahata, A.;
Sakurai, H. Tetrahedron Lett. 1978, 3043–3046; (c) Ishihara, K.; Hiraiwa, Y.;
Yamamoto, H. Synlett 2001, 1851–1854; (d) Barker, T. J.; Jarvo, E. R. Org. Lett.
2009, 11, 1047–1049.
MeO
O
6
Toluene
4
85
3
2. (a) Yamamoto, Y.; Asao, N. Chem. Rev. 1993, 93, 2207–2293; (b) Denmark, S. E.;
Fu, J. Chem. Rev. 2003, 103, 2763–2793.
O
7
8
Toluene
Toluene
Toluene
Toluene
Toluene
Dioxanec
6
82
3. For a recent review, see: Darses, S.; Genet, J. P. Chem. Rev. 2008, 108, 288–325.
4. Ni-Catalyzed arylation to aldehydes of arylboron reagents: (a) Arao, T.; Kondo,
K.; Aoyama, T. Tetrahedron Lett. 2007, 48, 4115–4117; (b) Yamamoto, K.;
Tsurumi, K.; Sakurai, F.; Kondo, K.; Aoyama, T. Synthesis 2008, 3585–3590; (c)
Sakurai, F.; Kondo, K.; Aoyama, T. Chem. Pharm. Bull., in press.
5. N-Heterocyclic carbene as nucleophilic organocatalyst: (a) Fukuda, Y.; Maeda,
Y.; Ishii, S.; Kondo, K.; Aoyama, T. Synthesis 2006, 589–590; (b) Fukuda, Y.;
Maeda, Y.; Kondo, K.; Aoyama, T. Chem. Pharm. Bull. 2006, 54, 397–398; (c)
Fukuda, Y.; Maeda, Y.; Kondo, K.; Aoyama, T. Synthesis 2006, 1937–1939; (d)
Fukuda, Y.; Kondo, K.; Aoyama, T. Synthesis 2006, 2649–2652.
6. For Lewis acid- or PTC-catalyzed allylations to aldehydes using potassium
allyltrifluoroborate: (a) Batey, R. A.; Thadani, A. N.; Smil, D. V. Tetrahedron Lett.
1999, 40, 4289–4292; (b) Batey, R. A.; Thadani, A. N.; Smil, D. V.; Lough, A. J.
Synthesis 2000, 990–998; (c) Thadani, A. N.; Batey, R. A. Org. Lett. 2002, 4, 3827–
3830.
4
O
5
85
5
O
6
9
6
81
O
7
10
11
12
6
91b
82
7. With allyl-9-BBN and no catalyst, allylboration of
a-amono ketones: Pace, R. D.;
Kabalka, G. W. J. Org. Chem. 1995, 60, 4838–4844.
8. Catalytic enantioselective allylboration of benzylic and allylic ketones with the
combination of 2,20-di-Br-BINOL and allyldiisopropoxyboron, the trivalent
borane, has been reported: Lou, S.; Moquist, P. N.; Schaus, S. E. J. Am. Chem. Soc.
2006, 128, 12660–12661.
O
20
24
8
O
9. The NMR data of potassium allyltrifluoroborate in CD3CN at rt have been
reported.6b
73a
10. 11B chemical shifts were referenced to BF3 (d = ꢁ8.70 ppm) in CDCl3.
11. Representative procedure for the allylboration of benzylacetone (2) (entry 5): To a
stirred solution of TsOHꢀH2O (7.6 mg, 0.0400 mmol) in toluene (0.60 mL) were
added potassium allyltrifluoroborate (59.2 mg, 0.400 mmol) and benzyl-
X
9: X= OMe (a)
acetone (2) (30 lL, 0.200 mmol) at rt. The reaction mixture was stirred for
13
14
9b: X = H
9c: X = Cl
Toluene
Toluene
24
24
81
90
3 h at 40 °C, allowed to cool, diluted with aq NaHCO3, and extracted with
EtOAc. The organic extracts were washed with brine, dried (Na2SO4), and
concentrated. Purification by SiO2 column (pretreated with 1% Et3N in hexane,
hexane/EtOAc = 15:1) afforded 3-methyl-1-phenylhex-5-ene-3-ol (35.3 mg,
90%) as a colorless oil.
O
15
Dioxanec
14
84
12. Successful examples for catalytic enantioselective allylation of ketones: (a)
Wada, R.; Oisaki, K.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2004, 126, 8910–
8911; (b) Kim, J. G.; Waltz, K. M.; Garcia, I. F.; Kwiatkowski, D.; Walsh, P. J. J.
Am. Chem. Soc. 2004, 126, 12580–12585; (c) Wadamoto, M.; Yamamoto, H. J.
Am. Chem. Soc. 2005, 127, 14556–14557; (d) Ref. 8.; (e) Casolari, S.; D’Addario,
D.; Tagliavini, E. Org. Lett. 1999, 1, 1061–1063.
10
a
Remainder of mass balance was the starting ketone.
Diastereoselectivity is 17:1, although the stereochemistry was not determined.
The use of toluene in place of dioxane as a solvent gave less satisfactory results.
b
c