Table 1. Fe-Catalyzed Liberation of Alcohols: Influence of the
Allyl Scavengera
Table 2. Fe-Catalyzed Liberation of Alcohols: Influence of the
Solventa
entry
scavenger
conv (%)b
entry
solvent
DMSO
conv (%)b
1
2
3
4
5
6
NaBH4
28
À
1
2
3
4
5
6
7
8
9
77
17
69
99
93
55
80
À
BH3*Et3N
PhSiH3
DMF
À
CH3CN
[NH4][HCO3]
PhSH
À
EtOH
96
97
1,4-Dioxane
THF
iPrSH
MTBE
a All reactions were performed on a 1.0-mmol scale in the presence of
allyl alkyl carbonates (1.0 mmol), scavenger (2.0 mmol), TBAFe (0.05
mmol), and PPh3 (0.06 mmol) in THF (1 mL) under a N2 atmosphere.
b Determined by GC-integration using dodecane as external standard.
Dichloromethane
Toluene
78
a All reactions were performed on a 1.0-mmol scale in the presence of
allyl alkyl carbonates (1.0 mmol), iPrSH (2.0 mmol), TBAFe (0.05
mmol), PPh3 (0.06 mmol) in the given solvent (1 mL) under a N2
atmosphere. b Determined by GC-integration using dodecane as exter-
nal standard.
hydrosilylations,9 we wondered whether it would be pos-
sible to develop this type of nucleophilic catalysis further
into a novel deprotection method for allyl alkyl carbonates
applying an iron-based alternative10 to the established
palladium-based systems.4,5 Moreover, a practical, timely
deprotection reaction is characterized by traceless removal
of the deprotection side products. Hence, a special empha-
sis was placed upon the identification of a volatile allyl
scavenger that delivers a volatile allylation product, which
allowsfor easy removal ofall impurities and byproducts by
simple evaporation of the crude reaction mixture. Various
allyl scavengers have been employed in the corresponding
Pd-catalyzed deallylation reaction. Hence, at the outset of
our investigations different scavengers were tested in the
deprotection of sterically hindered menthol derived carbo-
nate 1 under the reaction conditions that proved successful
in the allylic substitutions (Table 1).7
From these results it is obvious that thiols are most
suitable for the deprotection. For reasons of convenience
we selected isopropyl thiol as the scavenger of choice for
further optimizations for three main reasons. Apart from
the acceptable smell, the volatility of the scavenger and
allylated scavenger plus its low price fulfilled all the criteria
for an applicable deprotection protocol. Since both reac-
tion temperature and catalyst perfomance needed to be
optimized we subsequently turned our attention toward a
screening of solvent effects (Table 2).
The reaction proceeds in quantitative conversions in
polar solvents. Both ethanol and 1,4-dioxane (entries 4
and 5, Table 2) gave full conversion after 12 h at 60 °C. At
this point the influence of ligands was investigated. A
variety of different monodentate ligands were tested. Bulky
electron-rich phosphines were the most potent ones. Using
either PCy3 or PMes3 (Mes = 2,4,6-trimethylphenyl), full
conversion was obtained using 2.5 mol % TBAFe, 2.8 mol %
ligand at 40 °C. At this temperature only ethanol allowed for
good conversion rates (conditions A, Scheme 1). Recently
we disclosed a related study in which we were able to
show that TBAFe in the presence of an excess thiol is
transferred into electron-rich binuclear Fe-complexes of
the type [Bu4N]2[(NO)2Fe(SR)]2.11 These carbonyl-free
complexes allow for a regioselective sulfenylation already
at temperatures of 40 °C under ligand-free conditions.
Alternativley, the acitve species can be prepared in situ
starting from the readily available oxidized complex
[(NO)2Fe(SR)]2, potassium hydride, and TBABr. We en-
visioned complexes of this type to be formed under the
given deprotection conditions. To our delight [Bu4N]2-
[(NO)2Fe(SR)]2 is equally capable ofdeprotecting menthol
derivative 1 quantitatively even in the absence of any
ligand (conditions B, Scheme 1).
(8) (a) Magens, S.; Ertelt, M.; Jatsch, A.; Plietker, B. Org. Lett. 2008,
10, 53. (b) Magens, S.; Plietker, B. J. Org. Chem. 2010, 75, 3715. (c)
Magens, S.; Plietker, B. Chem.;Eur. J. 2011, 17, 8807.
(9) (a) Dieskau, A.; Begouin, J. M.; Plietker, B. Eur. J. Org. Chem.
2011, 5291. (b) Shaikh, N. S.; Enthaler, S.; Junge, K.; Beller, M. Angew.
Chem. 2008, 120, 2531. Angew. Chem., Int. Ed. 2008, 47, 2497.
(10) For recent reviews on Fe-catalysis, see: (a) Advances in Sustain-
able Metal Catalysis: Iron Catalysis (Topics in Organometallic
Chemistry); Plietker, B., Ed.; Springer-Verlag: Heidelberg, 2011. (b) Iron
Catalysis in Organic Synthesis; Plietker, B., Ed.; Wiley-VCH, Weinheim,
2008. (c) Correa, A.; García Manchano, O.; Bolm, C. Chem. Soc. Rev. 2008,
37, 1108. (d) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004,
104, 6217. (e) Dieskau, A.; Plietker, B. Eur. J. Org. Chem. 2009, 775.
With the optimized conditions in hand we subsequently
turned our attention toward an exploration of the scope
and limitation of this process (Table 3).
(11) Holzwarth, M. S., Frey, W.; Plietker, B. Chem. Commun. 2011,
DOI:10.1039/C1CC14599A.
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