Allyl ethers
T. W. Green, P. G. M. Wuts, Protective Groups in Organic
Synthesis,
Wiley-Interscience, New York, 1999, 67-74, 708-711.
Stability
H2O: | pH < 1, 100°C | pH = 1, RT | pH = 4, RT | pH = 9, RT | pH = 12, RT | pH > 12, 100°C |
Bases: | LDA | NEt3, Py | t-BuOK | Others: | DCC | SOCl2 |
Nucleophiles: | RLi | RMgX | RCuLi | Enolates | NH3, RNH2 | NaOCH3 |
Electrophiles: | RCOCl | RCHO | CH3I | Others: | :CCl2 | Bu3SnH |
Reduction: | H2 / Ni | H2 / Rh | Zn / HCl | Na / NH3 | LiAlH4 | NaBH4 |
Oxidation: | KMnO4 | OsO4 | CrO3 / Py | RCOOOH | I2, Br2, Cl2 | MnO2 / CH2Cl2 |
General
The allyl group is a commonly used protecting group for alcohols, with relative stability towards both acidic and basic conditions that permits orthogonal protection strategies. Isomerization to the more labile enol ether employing KOtBu, with subsequent mild acidic hydrolysis, is one of the most common deprotection methods. However, KOtBu can only be used, when the substrate is not base sensitive.
The properties of the allylic double bond may be exploited to effect a one-step deprotection, by activation of the double bond by a palladium catalyst with subsequent reduction or SET (single electron transfer), or by selective oxidation. Some of these newer methods are highlighted below.
Protection of Hydroxyl Compounds
M. Ishizaki, M. Yamada, S.-I. Watanabe, O. Hoshino, K. Nishitani, M. Hayashida,
A. Tanaka, H. Hara, Tetrahedron, 2004, 60, 7973-7981.
M. Ishizaki, M. Yamada, S.-I. Watanabe, O. Hoshino, K. Nishitani, M. Hayashida,
A. Tanaka, H. Hara, Tetrahedron, 2004, 60, 7973-7981.
Deprotection
A mixture of trihaloboranes triggers a regioselective cleavage of
unsymmetrical alkyl ethers to generate alkyl alcohol and alkyl bromide products.
This conversion exhibits improved regioselectivity and yield compared with BBr3
alone.
B. J. P. Atienza, N. Truong, F. J. Williams, Org. Lett.,
2018, 20, 6332-6335.
tert-Butyllithium induces a cleavage of allylic ethers to the
corresponding alcohols or phenols in excellent yield at low temperatures in n-pentane.
The reaction produces 4,4-dimethyl-1-pentene as a coproduct and most likely
involves a SN2' process, in which the organolithium attacks the allyl
ether.
W. F. Bailey, M. D. England, M. J. Mealy, C. Thongsornkleeb, L. Teng,
Org. Lett., 2000, 2, 489-491.
A new one-pot method is described for the removal of O- and N-allyl
protecting groups under oxidative conditions at near neutral pH. The allyl group
undergoes hydroxylation and subsequent periodate scission of the vicinal diol.
Repetition of this reaction sequence on the enol tautomer of the aldehyde
intermediate releases the deprotected functional group.
P. I. Kitov, D. R. Bundle, Org. Lett., 2001, 3, 2835-2838.
Pd(0)-catalyzed deprotection of allyl ethers using barbituric acid
derivatives in protic polar solvent such as MeOH and aqueous 1,4-dioxane
proceeds at room temperature without affecting a wide variety of functional
groups. Control of the reaction temperature allows selective and successive
cleavage of allyl, methallyl, and prenyl ethers.
H. Tsukamoto, T. Suzuki, Y. Kondo, Synlett, 2007,
3131-3132.
A mild deprotection strategy for allyl ethers under basic conditions in the
presence of a palladium catalyst allows the deprotection of aryl allyl ethers in
the presence of alkyl allyl ethers. These conditions are also effective in the
deprotection of allyloxycarbonyl groups.
D. R. Vutukuri, P. Bharathi, Z. Yu, K. Rajasekaran, M.-H. Tran, S. Thayumanavan,
J. Org. Chem.,
2003, 68, 1146-1149.
Deprotection of allyl ethers, amines and esters to liberate hydroxyl, amino
and acid groups is achieved under mild conditions. The reagent combination
employed for this transformation is polymethylhydrosiloxane (PMHS), ZnCl2
and Pd(PPh3)4.
S. Chandrasekhar, R. Reddy, R. J. Rao, Tetrahedron, 2001, 57,
3435-3438.
A combination of a Ni-H precatalyst and excess Brønsted acid enables a
deallylation of O- and N-allyl functional groups. Key
steps are a Ni-catalyzed double-bond migration of the O- and N-allyl
group followed by Brønsted acid induced O/N-C bond hydrolysis. This deprotection
method tolerates a broad range of functional groups.
P. M. Kathe, A. Berkefeld, I. Fleischer, Synlett, 2021,
32,
1629-1632.
Selective cleavage of unsubstituted allyl ethers is provided by SmI2/H2O/i-PrNH2
in very good yields. This method is useful in the deprotection of alcohols
and carbohydrates.
A. Dahlen, A. Sundgren, M. Lahmann, S. Oscarson, G. Hilmersson, Org. Lett.,
2003, 5, 4085-4088.
Allyl aryl ethers can be easily cleaved by the use of 10% Pd/C under mild and
basic conditions. The present reaction would involve a SET process rather than a
π-allyl-palladium complex. The scope and limitation of this new deprotective
methodology are also described.
M. Ishizaki, M. Yamada, S.-I. Watanabe, O. Hoshino, K. Nishitani, M. Hayashida,
A. Tanaka, H. Hara, Tetrahedron, 2004, 60, 7973-7981.
A selective deallylation of o-Allyloxyanisoles by treatment with sec-
or tert-butyllithium at low temperature proceeds through a tandem
intermolecular carbolithiation-β-elimination process.
R. Sanz, A. Martínez, C. Marcos, F. J. Fañanás, Synlett, 2008,
1957-1960.
Conversion of Allyl Ethers
B. Schmidt, Eur. J. Org. Chem., 2003, 816-819.
An efficient Z-selective oxidative isomerization process of allyl ethers
catalyzed by a cobalt(II) (salen) complex using N-fluoro-2,4,6-trimethylpyridinium
trifluoromethanesulfonate (Me3NFPY•OTf) as an oxidant provides
thermodynamically less stable Z-enol ethers in excellent yields with high
geometric control. Diallyl ethers can also be isomerized at room temperature.
G. Huang, M. Ke, Y. Tao, F. Chen, J. Org. Chem., 2020, 85,
5321-5329.