Microwave Oven Reaction Enhanced (MORE) preparation of substituted
The Department of Chemistry, University of the West Indies, Mona,
Kingston 7, JAMAICA.
Email addresses: email@example.com and firstname.lastname@example.org
Transition metal complexes formed from stilbenediamines have been
of interest for some time. From the early work on the unusual
solvent effects on the nickel complexes known as Lifschitz's salts to
more recent studies where manganese complexes have been utilised in the
chiral epoxidation of alkenes.
In an effort to shorten the preparation time of the compounds,
the condensation of benzaldehyde with ammonium acetate has been
re-examined under microwave conditions.
It was observed by Trippett in 1957 (l), that when
benzaldehyde was refluxed with ammonium acetate or ammonium propionate
for three to four hours, a highly crystalline neutral compound,
N-benzoyl-N-benzylidene meso-1,2-diamino-1,2-diphenylethane (C28H24N2O),
was produced. Hydrolysis of this compound with 70% sulfuric acid over
three hours was reported to yield
(stilbenediamine or stien).
More recently, there has been considerable interest in the use of the
optical isomers of stien (either R,R or S,S) as precursors to catalysts used for
enantiometric epoxidation of olefins (2) or as chiral
Lewis acids to catalyse enantioselective Diels-Alder reactions.(3)
The difference between these forms can be seen in Figure 1.
Figure 1. (enlarged GIF,
New synthetic approaches to the optically active forms have been
reported recently. These include the hydrolysis of a chiral imidazoline (4), and a new preparation of the racemate (3)
which was resolved using existing methods (5).
As part of our studies (6) (7) on MORE
(Microwave Oven Reaction Enhanced) chemistry (8) (9) we have repeated the reaction of Trippett in a
microwave oven using benzaldehyde as well as p-chloro and
m-nitrobenzaldehyde as substrates. We have been able to produce
the meso compounds in two steps, with heating times reduced to
approximately one sixtieth of those in the original paper.
Products analogous to those formed in the benzaldehyde
reaction are formed rapidly from p-chloro- and m-nitrobenzaldehyde.
It is thought that formation of a 1,2-diphenylethylenediamine derivative
under such mild conditions may be due to a benzoin-type condensation.
This view is supported by the effect of substituents on the course of
Alternatively, the product could be formed by a condensation reaction
between the aldehyde and ammonia (generated in situ) to give the
imine which could undergo radical dimerisation. Further reaction of the
resulting diradical with two molecules of benzaldehyde would lead to the
adduct as shown in Figure 2.
Figure 2. A possible mechanism (16K .tgf file).
The products obtained from the the microwave reactions were identical to
those obtained by classical reflux methods.
The yields in forming the adduct compared well with the reflux method,
and ranged from 55% to as high as 81% which compares to 60% as reported
However, the hydrolysis to form the diamine generally gave yields
under 50%. Using the original conditions of 70% aqueous sulfuric acid,
charring was often evident and so the acid strength was diluted to 50%.
This necessitated extending the reaction time.
The lower yields of the diamine could possibly be due to
incomplete hydrolysis of the adduct.
The times used for the hydrolysis reactions were between three and four
minutes, except for the p-chlorostilbenediamine when the reactants were
irradiated for five minutes.
Aspects of this work formed the basis of several undergraduate projects,
for which the authors gratefully acknowledge the assistance of:
Noelle Finlay, Llewellyn Jarrett, Floyd Beckford, Roy Porter and Maureen
In general the methods of preparation of the compounds were similar.
All reactions were carried out in Teflon bombs in a commercial 600
Watt microwave oven (Sharp[TM] Carousel). The Teflon bombs were obtained
from Savillex Corp. (60 cm3).
Melting points were determined on a Kofler-type micro hot stage block
and are uncorrected.
Infrared spectra were recorded as potassium bromide (KBr) discs
using either a Perkin Elmer 60 series or FTIR 1600 model spectrometer.
The mass spectra were recorded at 70 eV via solid insertion in a
Finnigan GC/MS quadrupole spectrometer. Information from the 20 most
prominent peaks was manually converted into JCAMP-DX format file for presentation.
Proton NMR spectra were recorded on either a Bruker AC200 or Varian T60 and
were for deuterochloroform solutions. Tetramethylsilane served as the internal
Benzaldehyde (15.1cm3, 15g, 140 mmol) and
ammonium acetate (30.12 g, 390 mmol) were added to a Teflon bomb which was
sealed and placed in a microwave oven.
The vessel was heated for three minutes, cooled and the mixture filtered.
The product was washed with ethanol, air dried then recrystallized
from toluene. M.P. 253-255 C (lit. 258-259 C). Yield 14.32g, 56%. The Mass Spectrum showed m/e at 404.
IR Spectrum (56 K GIF file).
The 200 MHz proton NMR (85K
JCAMP-DX file) gave signals at 8.30 (s),7.79-7.24 (m), 6.71 (d) 5.55
(dd), 4.95 (d) and 1.55 (s).
The adduct (5g, 12 mmol) along with 50% sulfuric acid (25 cm3) were
added to a Teflon bomb and placed in a microwave oven. The
vessel was heated for 3.5 minutes and then cooled. The mixture was
extracted with ethyl acetate (3 times 30 cm3). The organic layer, which
contained unreacted starting material and other impurities was discarded.
The aqueous layer was made alkaline (pH~10) using KOH and further
extracted with ethyl acetate. The organic extract was washed
with water, dried with sodium sulfate and filtered.
The filtrate was evaporated in vacuo. The product was recrystallized
from methanol. M.P. 119-120 C (lit.120.5 121.5 C). Yield 2.62g, 24%.
The Mass Spectrum showed m/e at 212.
IR Spectrum- (53K GIF file)
The 200 MHz proton NMR (85K
JCAMP-DX file) gave signals at 7.41 (m), 5.41 (s) and 1.47
Preparation of adduct from p-chlorobenzaldehyde.
p-Chlorobenzaldehyde (5.04 g, 36 mmol) and ammonium acetate (10.06 g, 130
mmol) were added to a Teflon bomb which was sealed and
placed in a microwave oven. The vessel was heated for 2.5 minutes,
cooled and the mixture filtered. The product was washed with ethanol and
air dried. The product was recrystallized from butan-l-ol. M.P.
251-253 C (lit. 249 C). Yield 1.5g, 31%.
The 200 MHz proton NMR (85K
JCAMP-DX file) gave signals at 8.26 (s), 7.77-7.12 (m), 6.62 (d),
5.52 (dd), 4.96 (d), 1.69 (broad-s) and 1.28 (s).
The adduct (0.9 g, 2 mmol) and 40% sulfuric
acid (4.5 cm3) were added to a Teflon bomb which was sealed and placed in a
microwave oven. The vessel was heated for 5 minutes,
cooled and the mixture extracted with ethyl acetate (3 times 30 cm3)
which was discarded. The aqueous portion was made alkaline (pH~9) using KOH
and further extracted with ethyl acetate. The organic extract was washed
with water, dried with sodium sulfate and filtered. The filtrate was
evaporated using a rotary evaporator and the product recrystallized from
methanol. M.P. 140-141 C (lit 137-138 C). Yield 0.24g, 51%.
The 60 MHz proton NMR (8K
GIF file) gave signals at 7.25 (Ar-H), 3.9 (N-C-H) and 1.5 (N-H).
Preparation of adduct from m-nitrobenzaldehyde.
m-Nitrobenzaldehyde (5.14 g, 34 mmol) and ammonium acetate
(10.14 g, 132 mmol) were added to a Teflon
bomb which was sealed and heated in a microwave oven for 3 minutes.
The vessel was then cooled and the mixture filtered. The crystals were
washed with ethanol and the product recrystallized from butan-l-ol. M.P.
300 C (sublimes) Yield 4.01g, 81%.
The adduct (4.01 g, 7 mmol) and 50% sulfuric acid (23 cm3)
were added to a Teflon bomb which was sealed and heated in a microwave
oven for 4 minutes. The vessel was then cooled and the organic portion
extracted with ethyl acetate (3 times 30 cm3) which was then discarded.
The aqueous portion was made alkaline (pH~10) using KOH and further
extracted with ethyl acetate. The organic extract was washed with water,
dried with sodium sulfate and filtered. The filtrate was evaporated under
reduced pressure and the product was recrystallized from methanol. M.P.
184-187 C (lit. 189-190 C). Yield 0.63g, 30%.
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http://wwwchem.uwimona.edu.jm:1104/lab_manuals/MORE.html or here.
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