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虫号: 2070225
注册: 2012-10-18
专业: 化学反应工程

[求助] 求助翻译啊，关于核磁共振分析的，急用！！

The 13C NMR spectra indicated that glucose was in the forms
of a-pyranose (2) and b-pyranose (1; Scheme 1) in [BMIM]Cl at
room temperature in the absence of GeCl4 (Figure 9 a). However,
when 1 equivalent of GeCl4 was added into the mixture, a
new peak at d=76.4 ppm appeared (Figure 9b). The ability of
heavier main-group elements to accommodate more than
eight electrons in their valence shells, thus forming so-called
hypervalent or hypercoordinate compounds, is well established.[
20] The cations of Group 14 elements can be stabilized
thermodynamically by introduction of auxiliary NR2 or OR
groups, the lone electron pairs of which could, in principle, be
capable of bonding to the empty p orbital of the Group 14 element.[
21] Therefore, we speculated that GeIV interacted with the
two O atoms of 1,2-dihydroxy group of a-glucose to form a
stable intermediate 3 (Scheme 1). When the 13C NMR spectrum
was recorded at 508C, the relative intensity of the peak at d=
76.4 ppm decreased, and two new peaks at d=152.1 and
103.6 ppm appeared. The intensity of the signal at d=
152.1 ppm increased when the reaction was held at higher
temperatures (Figure 9d ande) and also increased over time
(data not shown). Therefore, this signal was assigned to the
13C-labeled carbon of HMF at the 2-position. Assignment of the
signal at d=103.6 ppm was more tricky. Because this signal
was found only at 508C, it should be associated to a relatively
stable intermediate with a short lifetime along the reaction
pathway to HMF. In other words, at higher temperature, the
turnover of this intermediate was greatly accelerated. Therefore,
we suppose it was from fructose,[22] which indicated that
the intermediate fructose was formed through glucose isomerization.
Moreover, the signal at d=76.4 ppm was missing at
808C, indicating that the conversion of 3 was fast. As proposed
by zhao et al., fructose was formed from glucose through an
enediol intermediate catalyzed by GeCl4.
[10] Fructose then lost
one water molecular to form the fructofuranosyl oxocarbenium
ion.[18] Due to the basicity and nucleophilicity of the halide ion
(X), the oxocarbenium ion lost HX to form the enolic species
through the nucleophile or the base pathway, followed by loss
of two water molecules to form HMF.
Our results also suggested that increasing the reaction temperature
would enhance the interaction of glucose and GeCl4,
and thus promote a ring-opening step of the intermediate 3
to form the straight-chain intermediate 4 through hydrogen
transfer, which could be enolated to form 5, followed by the
release of GeCl4 to give fructose
(Scheme 1). At 80 8C, three new
signals at d=56.1, 110.9 and
124.3 ppm appeared, which
were attributed to C6, C4 and
C3, respectively, of HMF. A new
peak at d=80.1 ppm was also
observed at 80 8C and 100 8C,
which was attributed to a byproduct
formed during the reaction.

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