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木虫 (小有名气)

应助: 4 (幼儿园)
金币: 2352
帖子: 235
在线: 52.9小时
虫号: 1300191
注册: 2011-05-19
性别: GG
专业: 有机合成

[求助] 求翻译下面一段化学合成内容请帮忙英文或麻烦翻译下第1--3页

Initially o-phenylenediamine and benzaldehyde were used in a
1:1 molar ratio in water for the titled transformation, but it failed
to produce the desired 1,2-disubstituted benzimidazole 3a as a
major product, instead a mixture of products was isolated (Scheme
1). Increase in the reaction time, temperature, and variation in the
molar proportion of the reactants did not make any influence on
the ratio of the product distribution. At this juncture and based
on the literature report46,47 on SHFP (sodium hexafluorophosphate)
we undertook the above study by using 1 mmol of anhydrous
SHFP in the above reaction mixture of o-phenylenediamine
and benzaldehyde (1:2 mmol). This revised protocol after a very
simple work-up gave only a single isolated product in 76% yield.
However, considering the recent emphasis on the development
of greener protocols we further worked on the above transformation
to replace with a suitable greener catalyst the use of SHFP,
which has been reported to produce even more hazardous waste
due to its decomposition to phosphorus oxides and toxic hydrofluoridric
acid.47 Working in this direction and looking into the recent
literature of resin bound greener catalyst we used PF6
ion bound Amberlite 900 (Cl) as a potential catalyst for the above
transformation.
Polymeric resin bound hexafluorophosphate ion (PHP) was prepared48
(Scheme 2) by washing Amberlite 900 resin (Cl) packed in
a column with 10% aqueous sodium hexafluorophosphate solution
repeatedly until the washing gave a negative response for the chloride
ion (Scheme 6). Finally the solid was washed several times
with water and then dried under vacuum. The binding of hexafluorophosphate
ion on the surface of Amberlite 900 resin was confirmed
by IR spectroscopy49 in the following way. IR spectrum of
PHP was taken in KBr discs (Fig. 1). PF6
belongs to Oh point group,
where six kinds of vibration modes are included in the molecule,
whose basic frequency number is 15. From the characteristics of
Oh point group, it can be calculated that for PF6
two energy levels
belonging to F1u have infrared activity which results from the flexing
vibration and bending vibration of F–P bonds, respectively.
Therefore, two strong absorption peaks of PF6
should appear at
820–860 and 550–565 cm1. In the IR spectra of PHP, strong
absorption peak appeared at 557 and 832 cm1, respectively
(Fig. 1). Appearance of the above said peaks did confirm the binding
of PF6
ion on polymeric resin (Amberlite 900).
This resin bound PF6
ion (PHP) was then applied in place of
SHFP as the catalyst and it also showed excellent chemoselectivity
by giving 3a as the major product in water at room temperature
(Scheme 3). In a bid to increase the yield of the desired product,
we performed the same reaction in other green solvents, such as
ethanol, methanol, and mixtures of both water–methanol and
water-ethanol in varying proportions at room temperature as well
as in 50 C (Table 1). Of the entire solvent systems studied, methanol
gave similar results as water, but ethanol performed poorly
(entry 3). A further study with mixed solvent systems indicated
that a 1:1 mixture of water and methanol gave an excellent yield of
3a within 1 h at ambient temperature (Table 1, entry 13); increase
in reaction temperature had no effect on the isolated yield of the
reaction (entry 14).

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