Nickel-catalyzed generation of 2-methylhex-5-enyl ethers from allyl ethers with
trimethylaluminium
Takahiko Taniguchi and Kunio Ogasawara*†
Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980-8578, Japan
Allyl ethers have been converted into the corresponding
2-methylhex-5-enyl ethers in one step on treatment with
trimethylaluminium in the presence of a catalytic amount of
(dppp)NiCl2.
deallylated diol in the presence of 2.6 equiv. of trimethylalu-
minium (Scheme 3).
The other simple allyl ethers 1c–l, those that gave the
corresponding alcohols 2c–l with DIBAL-H and (dppp)NiCl2,
were, therefore, subjected to the same reaction with trimethyl-
aluminium in the presence of (dppp)NiCl2 and it was found that
all but the aryl ether gave the corresponding products having the
same ether moiety in addition to the corresponding deallylated
products. The aryl ether gave the phenol quantitatively as the
sole product. Most importantly, neither the coupling reaction
nor the cleavage reaction occurred at all in the absence of
We recently found that allyl ethers 1 are cleaved facilely and
selectively to give the alcohols 2 with expulsion of propene on
treatment with DIBAL-H in the presence of a catalytic amount
of
dichloro[bis(diphenylphosphino)propane]
nickel
[(dppp)NiCl2] in an aprotic solvent1 (Scheme 1). The reaction
may be applied to allyl acetals, such as 1a,b, using triethylalu-
minium in place of DIBAL-H to give the same hemiacetal
mixture 2a/b, in good yield, without causing reduction of the
hemiacetal functionality, although the fate of the allyl function-
ality was uncertain1 (Scheme 2). We report here an unprece-
dented type of C–C bond formation reaction which was
observed during the examination of the reaction of the allyl
acetals 1a,b with trimethylaluminium in place of triethylalu-
minium in the presence of the same nickel catalyst.
O
O
*
BnO
Me3Al
1a,b
(dppp)NiCl2
toluene
O
O
3a (α-H)
b (β-H)
Thus, when the a-H allyl acetal 1a was treated with
trimethylaluminium (1.3 equiv.) in the presence of (dppp)NiCl2
(2 mol%) in toluene at 0 °C to room temperature for 0.5 h, an
inseparable mixture of 3a‡ consisting of two diastereomers was
generated in a substantial yield as well as the expected
hemiacetal mixture 2a/b. On the same treatment, the b-H
epimer 1b gave the diastereoisomeric mixture 3b consisting of
two diastereomers and the same hemiacetal mixture 2a/b above.
The sugar moiety, including the hemiacetal stereochemistry, of
the products was recognized to be unchanged during the
O
O
O
O
O
O
5
+
O
O
O
Me3Al
1
(dppp)NiCl2
toluene
reaction by examination of H NMR and mass spectra which
O
O
O
O
O
indicated that the change occurred in the allyl moiety of 1 with
an increment of 56 mass numbers attributed to one Me on a
tertiary stereogenic center and one allyl unit. It was also
concluded that the reaction that occurred was essentially a
dimerization and, therefore, the yields of 3a,b were calculated
as 61 and 60%, respectively. The same reaction occurred with
another sugar derivative 4 having a bis-allyl ether moiety in the
molecule to give the bis(2-methylhex-5-enyl) ether 5 and the
mono-2-methylhex-5-enyl ether 6 both as diastereomeric mix-
tures in yields of 10 and 40% as well as 34% of the double
HO
4
O
O
O
6
Scheme 3
Table 1 Reaction of allyl ethers 1 with Me3Al and (dppp)NiCl2
Entry
Substrate 1 Product 3
R
Yielda
DIBAL
ROCH2CH=CH2
ROH
+
MeCH=CH2
(dppp)NiCl2
aprotic solvent
1
2
3
4
5
6
7
8
9
a
b
c
d
e
f
ab
bb
c
d
e
Scheme 3
Scheme 3
HO(CH2)2O
HO(CH2)3O
4-MeOPh
61c
60c
27
27
0
45c
42c
44c
30c
39c
42c
48c
1
2
R = alkyl, allyl, aryl
Scheme 1
f
PhCH2
g
h
i
g
h
i
PhCH2CH2
AcO(CH2)4
Ph2CH
Me2CNCHCH2O(CH2)4
(S)-PhCHMe
THP
O
O
O
OH
*
Et3Al
BnO
BnO
(dppp)NiCl2
10
11
12
j
j
aprotic solvent
k
l
k
l
O
O
O
O
1a (α-H)
b (β-H)
2a/b: 78% (from α-H)
2a/b: 80% (from β-H)
a
b
Isolated yield after SiO2 column chromatography. Obtained as a
diastereomeric mixture. c Yield was calculated based on consumption of 2
equiv. of the substrate per product.
Scheme 2
Chem. Commun., 1998
1531
Taniguchi, Takahiko
