August 1998
SYNLETT
857
Reductive Coupling of Aldimines to Vicinal Diamines Using Zn Powder in Aqueous Basic
Media
Manu P. Dutta, Bipul Baruah, Anima Boruah, Dipak Prajapati and Jagir S. Sandhu*
Regional Research Laboratory, Jorhat 785 006, Assam, India
Received 9 April 1998
Reductive coupling of aldimines into vicinal diamines has been
performed by the action of zinc powder and 10% sodium hydroxide
solution without using any organic solvents at ambient temperature in
high yields.
The reductive coupling reaction of carbonyl compounds leading to
vicinal diols or olefins is promoted by a variety of reductants including
1
low-valent transition metals . Similar reactions for the synthesis of
vicinal diamines involving carbon-nitrogen functions are less
2
investigated in spite of their key roles in medicinal chemistry ,
3
coordination chemistry and asymmetric synthesis . There are however,
very few general methods available for their synthesis from readily
4
available starting materials . Several approaches based on transition
metal mediated coupling of imines for the synthesis of symmetrical
5
diamines are reported . Other routes involve introduction of a protected
amine moiety and require a number of synthetic manipulations to
6
release the free diamine functionality . But these methods have their
own merits as well as limitations, as the majority of these rely on olefin
addition reaction that require several steps to arrive at the desired
7
exclusively. The reaction with magnesium in the presence of methanol
(15ml) was carried out in the same way and the corresponding diamines
were obtained in (65-72)% yields (Table).
diamino products . Recently two new methods, the nucleophilic
8
opening of a cyclic sulfate with secondary amines and a low valent
titanium reagent for the reductive coupling of imines have been
9
In conclusion, this new environmentally friendly method has many
advantages over other methods as it is a very convenient and mild
reaction, the vicinal diamines were obtained in excellent yields and is
completely devoid of side products. Moreover, optically pure derivatives
performed .
We report here an environmentally friendly and convenient one pot
procedure for the reductive coupling of aldimines using inexpensive
reagents. We have observed that aldimines undergo coupling in the
presence of zinc powder and 10% aqueous sodium hydroxide solution at
ambient temperature without using any organic solvents (Scheme 1).
The reaction proceeds efficiently and the progress is monitored by the
absence of starting material vide TLC. No significant change in dl:meso
ratio was observed when the molar ratio of RCH=N-R’: Zn: NaOH was
increased to 1:4:8 or 1:6:9. The observed diastereoselectivity (2/3 ratio)
12
of 2 have considerable potential in asymmetric synthesis as evidenced
by recent reports. Further, zinc has the advantage of being readily
available, inexpensive, stable, easy to handle and less toxic.
are in accordance with Smith’s postulated mechanism during the
10
preparation of N,N’-tetraarylethylenic diamines
.
No reduced
compound 4 (due to reduction of the C=N bond) was observed to have
been formed in these reactions. A mixture of products were obtained in
the case of 4-nitro substituted aldimines due to competing reaction of
the nitro group. This new reductive dimerisation system couples with
ease a wide variety of aldimines 1 directly to the corresponding vicinal
diamines 2/3 and there was no evidence for the formation of any side
products. Interestingly, ketimine Ph(CH )C=N-Ph did not afford any
3
detectable amount of dimers even after stirring for 24 hrs or heating to
reflux with 1:4:8 or 1:6:9 equivalent of Zn:NaOH for 10 hrs, showing
selectivity for the coupling of aldimines. When zinc is replaced by
magnesium, the coupling proceeds effectively when carried out in
methanol and the products were obtained in good yields. The reaction
did not proceed in the absence of methanol. The results are summarised
in the Table.
In a typical procedure, benzylidine aniline (1.81g, 10 mmol) was taken
11
in 10% aq NaOH solution (10 ml), to this zinc powder (1.95g, 30
mmol) was added gradually and the reaction mixture was stirred at room
temperature. After 30 mins the reaction mixture was filtered to remove
the zinc and water (40 ml) was added to the filtrate. It was then extracted
with chloroform (3x20 ml), and the extract was washed with water,
dried over anhydrous sodium sulfate, concentrated under reduced
pressure to afford 1,2-dianilino-1,2-diphenylethane in 90% yields
Dutta, Manu P.
