1
102
Chemistry Letters Vol.35, No.10 (2006)
Facile and Recyclable Method for the Prins Reaction Using Hafnium(IV)
Bis(perfluorooctanesulfonyl)amides in Fluorous Biphase System
Ã
Ã
Xiuhua Hao and Nobuto Hoshi
The Noguchi Institute, 1-8-1 Kaga, Itabashi-ku, Tokyo 173-0003
(Received July 10, 2006; CL-060779; E-mail: haoxiuhua@noguchi.or.jp, hoshi.nb@noguchi.or.jp)
In fluorous biphase system, Hf[N(SO2C8F17)2]4 has been
ties in FBS and hence shows better catalytic activity. Further-
more, Hf[N(SO2C8F17)2]4 in FBS gives significantly good
recyclability than Hf(OSO2C8F17)4 or Hf(OSO2CF3)4, i.e.,
Hf[N(SO2C8F17)2]4 could be immobilized in the lower fluorous
phase and directly reused more than 17 times, whereas
Hf(OSO2C8F17)4 and Hf(OSO2CF3)4 could be reused only for
3 to 4 times owing to their distribution in the upper 1,2-dichloro-
ethane phase. In addition, the use of formaldehyde aqueous solu-
tion (35 wt %) instead of paraformaldehyde thus resulted in
much slow reaction rates, unlike previously reported processes
found to be an efficient catalyst for the Prins reaction of ꢀ-meth-
yl styrene with aldehydes at low catalytic loading. The reaction
proceeds smoothly and affords the corresponding 1,3-dioxanes
in good yields. The catalyst is selectively soluble in lower fluo-
rous phase and can be recovered simply by phase separation.
Furthermore, the recovered fluorous phase containing catalyst
can be recycled 17 times with yields consistently above 80%.
9
The Prins reaction is a fundamental reaction for carbon–car-
bon bond formation and is one of the most straightforward meth-
od for the synthesis of 1,3-dioxanes, which are widely used in
in the gas phase. To the best of our knowledge, this is the
first example for fluorous activity comparison of amide with
sulfonate complexes.
1
,2
organic synthesis as solvents or intermediates. Some tradition-
al Lewis acids (e.g., BF3, SnCl4) and mineral acids (e.g., H2SO4,
H3PO4) can promote this transformation, but most of these meth-
ods involve the use of corrosive or toxic reagents and high reac-
tion temperature resulting in low to moderate yield of products
As an approach to fragrance industrial application, the recy-
clable Hf[N(SO C F ) ] -catalyzed fluorous biphase Prins
2
8 17 2 4
reaction system was used to investigate the production of 2,4,6-
trimethyl-4-phenyl-1,3-dioxanes, employed for more than 20
a
Table 1. Reaction of ꢀ-methyl styrene with paraformaldehyde
3
due to the polymerization of starting materials. Although some
improvements have been made using 5 mol % Bi(OSO2CF3)3 in
acetonitrile4 and 10 mol % 2,6-di-tert-butylphenoxy(difluoro)-
O
O
Catalyst (0.5 mol %)
(HCHO)n
5
1,2-dichloroethane 1.5 mL
GALDEN SV 135 1.5 mL
borane in 1,4-dioxane, but high catalyst loading was required
and no catalyst recycle was given. Recently, Yadav et al. report-
ed 10 mol % InBr3 catalyzed the Prins reaction of styrene with
paraformaldehyde in dialkylimidazolium-based ionic liquids
and have showed that the recovered ionic liquid containing cat-
35 oC, 0.5 h
1
mmol
2 mmol
Entry
1
Catalyst
Hf[N(SO2C8F17)2]4
Cycle
Conversion/%b
Yield/%c
1
2
3
4
5
95
97
87
87
85
85
86
81–85
80
73
74
50
77
82
74
72
65
75
84
86
58
0
6
alyst can be recycled in 3 to 4 subsequent runs. On the other
hand, fluorous biphase system (FBS), as a phase separation
and catalyst immobilization technique, can be shown to be one
97
96
100
94–100
95
7
of the environment-benign technical candidates.
6
–16
7
Our previous works in FBS have found that metal (e.g., Hf,
Sn, Yb) complexes with bis(perfluorooctanesulfonyl)imide
ponytails are excellent active and recyclable catalysts in the
fluorous immobilized phase for the Baeyer–Villiger oxidation,
esterification, and the Friedel–Crafts acylation, far superior to
the corresponding metal trifluoromethanesulfonates.8
In this letter, we will describe our new approach to FBS for
the Prins reaction, and industrial application possibilities of this
FBS technique.
1
18
19
1
94
93
2
3
Hf(OSO2C8F17)4
77
93
2
3
95
89
4
5
91
81
6
Hf(OSO2CF3)4
1
99
92
The potential of Hf[N(SO2C8F17)2]4 in FBS was first stud-
2
IV
ied and compared with other Hf -based sulfonate complexes,
using the Prins reaction of ꢀ-methyl styrene and paraformalde-
hyde under the mild condition of 35 C for 0.5 h. A catalyst
3
92
85
4
ꢀ
4
None
1
0
a
loading of 0.5 mol % turned out to be ideal. The use of a lower
catalyst/ꢀ-methyl styrene ratio led to longer reaction time
or to incomplete conversion. As summarized in Table 1, no
product was obtained in the absence of catalyst (Entry 4).
Hf[N(SO2C8F17)2]4 (Entry 1) was found to be more active than
Hf(OSO2C8F17)4(Entry 2) or Hf(OSO2CF3)4 (Entry 3) in FBS.
It is reasonable to speculate that the stronger electron-withdraw-
ing ligands of Hf[N(SO2C8F17)2]4 renders it super Lewis acidi-
Paraformaldehyde (60 mg, corresponding to 2 mmol equivalents of
Ò
formaldehyde) was used as formaldehyde source; GALDEN SV
135 (CF3{[O-CF(CF3)-CF2]n-(O-CF2)m}-O-CF3) was purchased from
Solvay Solexis K.K.; The partition coefficients (1,2-dichloroethane/
GALDEN SV 135 solvent) is (<1)/(>99) for Hf[N(SO2C8F17)2]4
and (>30)/(<70) for Hf(OSO2CF3)4, which were determined by atomic
emission spectrometry. Conversion of ꢀ-methyl styrene. Yields were
determined by calibrated quantitative GC and GC/MS analysis using
n-tridecane as internal standard.
Ò
b
c
Copyright Ó 2006 The Chemical Society of Japan