Full Papers
different in entries 3 and 8 of Table 1, we cannot give a definite
reason for this difference. However, we demonstrated that
SnCl ·5H O dissolved in 1,4-dioxane and anhydrous SnCl did
ratio was 4, whereas the highest yield of 3 was observed for
+
119
a H /Sn ratio of 5. In addition, we performed Sn NMR spec-
troscopy and confirmed that the most active species was a six-
coordinate SnCl (H O) complex. From these results, we sug-
4
2
4
not. On the basis of this experimental result, we predicted that
4
2
2
anhydrous SnCl dissolved slowly in 1,4-dioxane with heat and
gested the chelate compound 10 with tin metal as the inter-
mediate in the reaction pathway (Scheme 3).
4
then HCl could be evolved effectively as an activator in com-
parison with the use of SnCl ·5H O. The spectrum of
Compound 2 is not easy to produce but it could become
commercially available through this coupling between 1 and
formaldehyde. This would be a huge step forward for the es-
tablishment of wood biomass as a source for important chemi-
cal entries. Furthermore, the results presented here could be
important for the further development of more sustainable
4
2
SnCl ·2H O is shown in Figure 4d, and a peak at d=
2
2
1
19
À244 ppm (^) is observed. The
Sn NMR spectrum of
SnCl ·2H O combined with two equivalents of HCl is shown in
2
2
Figure 4e. After heating to 808C for 3 h, the peak at d=
À244 ppm (^) is completely absent and the peak at d=
[12]
À637 ppm (
&
), which is also seen in the spectrum of
heterogeneous Sn catalysts. In particular, the immobilization
[13]
[3e]
SnCl (H O) , is observed exclusively. These results indicate that
of Sn in zeolites or on silica could be a solution to regen-
erate the Sn catalyst.
4
2
2
the combination of SnCl and two equivalents of HCl generates
2
the six-coordinate complex SnCl (H O) , which is the optimum
Glucose or fructose, which are abundant resources, could be
utilized as environmentally friendly alternative starting points
for the production of 1. Therefore, the establishment of the
process for the retro-aldol degradation of glucose is a worth-
while future challenge.
4
2
2
catalytic species in this reaction system. Furthermore, as HCl
was added to SnCl ·2H O in [D ]CD CN, a new peak at d=
2
2
3
3
À660 ppm (*) was observed (Figure 4 f), and the addition of
four equivalents of HCl led to a single peak (Figure 4 g). On
[11c]
the basis of the above results and a previous report,
conclude that the peak at d=À660 ppm (*) might be derived
from the six-coordinate complex SnCl6
We attempted to follow the catalytic behavior during a reac-
we
2À
.
Experimental Section
Catalytic tests
119
tion. The Sn NMR spectrum of SnCl ·5H O in [D ]dioxane is
4
2
8
presented in Figure S9a, which has characteristic peaks at d=
À635 (*) and À661 ppm (I). Then 1 was added to the reac-
tion mixture and heated to effect the dehydration of 1. How-
ever, the spectrum of this reaction mixture still shows two
peaks at d=À635 (*) and À661 ppm (I), and other reaction
species, such as the chelated Sn intermediate, are not detected
Experiments were performed in 50 mL autoclave reactors. Typically,
1
(1.25 mmol, 113 mg, dimeric form, Aldrich) and paraformalde-
hyde (1.31 mmol, 39.4 mg, TCI) were combined with Sn catalyst
0.171 mmol), for example, SnCl ·5H O (60.0 mg, Wako chemicals),
(
4
2
in 1,4-dioxane (4.0 mL). If anhydrous catalysts were used, water
0.86 mmol) was added. A magnetic stirring bar was then added,
(
the reaction atmosphere was purged with Ar, and the autoclave
was closed with a stainless-steel cap. The vessel was then placed in
an oil bath and allowed to react for a certain amount of time. In re-
actions to which HCl was added, a 4m HCl in 1,4-dioxane solution
(
Figure S9b). The spectrum of SnCl ·5H O combined with
4 2
1
and paraformaldehyde and heated to 1008C for 6 h also
shows peaks at d=À635 (*) and À661 ppm (I; Figure S9c).
This result shows that tin chlorides remain active in the reac-
tion mixture. On the basis of this experimental result, we
added fresh 1 and paraformaldehyde to the reaction mixture
after an initial run, and 2, 3, and 4 were afforded in almost the
same yields as in the first run. These results also support the
assertion that the Sn catalyst continues to behave as a catalyst
without deactivation through multiple uses.
(Aldrich) was used. Mesitylene (20.0 mg) was used as the internal
standard.
Time profile
Time profile measurements were performed in a 20 mL Schlenk
flask. All substrates were used according to the procedure reported
in the Catalytic Tests section. A magnetic stirrer bar was added, the
reaction atmosphere was purged with Ar, and the vessel was
placed in an oil bath. The mixture was allowed to react for a certain
amount of time (5 min to 24 h). To better observe the change of
product distributions, the reaction was slowed by lowering the
usual reaction temperature from 140 to 1008C. In this experiment,
naphthalene (20.0 mg) was used as the internal standard.
Conclusions
We investigated the specific catalytic activity of homogeneous
tin chloride to convert dihydroxyacetone (1) and formaldehyde
into a-hydroxy-g-butyrolactone (2). Isotope experiments, sub-
strate screening, and time profile measurements allowed us to
propose a detailed reaction pathway supported by GC, MS,
and NMR spectroscopy. This transformation involves a tautome-
rization and a dehydration of 1 to pyruvic aldehyde (9), an
aldol reaction between tin-activated enolate and formalde-
hyde, and finally cyclization to form 2.
119
Sn NMR spectroscopy
Procedure to obtain the spectrum shown in Figure 4e: the experi-
ment was performed in a 20 mL Schlenk flask. Sn catalyst
(0.171 mmol) was combined with 4m HCl in 1,4-dioxane solution
To investigate the activated species of this reaction, the
manner in which the balance between Brønsted and Lewis
acid affected the reaction preference for 2, 3, and 4 was exam-
(0.34 mmol) and deuterated acetonitrile (1.00 mL, Kanto Kagaku
Co.) at RT. A magnetic stirring bar was added, and the flask was
placed in an oil bath. After stirring at 808C for 3 h under ambient
air, the mixture was cooled to RT and inserted directly into an NMR
+
ined. The highest yields of 2 and 4 were seen if the H /Sn
ChemSusChem 0000, 00, 0 – 0
7
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ