5542
G. Vo-Thanh et al. / Tetrahedron 60 (2004) 5539–5543
allow water removal (Scheme 1). The reaction mixture was
submitted to MW irradiation (monomode system: Synthe-
wave 402 Prolabo or CEM Discover) at 120 8C for 60 min.
The temperature is measured by IR detection and controlled
by an optical fiber (after calibration of the IR detector). It
remains at a constant value by modulation of the emitted
power using a computered system. After cooling, the
reaction mixture was diluted with diethyl ether and dried
Scheme 3. Mechanism of the transimination of 4 with 2-amino-3-picoline.
ground state due to charge development and therefore more
sensitive to MW dipole–dipole stabilisation. Consequently,
MW irradiation can accelerate the transimination step by a
greater factor than could be achieved by classical means
over anhydrous MgSO . After filtration and evaporation, the
4
crude product was purified by distillation under reduced
pressure to afford the imine as a colourless liquid. These
imines have already been reported in the literature.
1
4
(
Scheme 3).
3.2. General procedure for ortho-alkylation
Non-thermal MW specific effects are based in qualitative
polarities between ground and transition states. These
values have been quantitatively determined by theoretical
calculations with the help of Hyperchem 5.1 system.
Hessian calculations were carried out after fully location
of the transition structures by PM3 semi-empirical method.
In fact, the calculated values of the electric dipole
moment are: m¼1.23, 1.96 and 6.37 Debyes for the imine
Imine (2 mmol), alkene (10 mmol), and Wilkinson’s
catalyst (0.04 mmol) were mixed in the absence of any
organic solvent and then exposed to MW irradiation for a
certain period of time and at adequate temperature (see
Tables 2–4). Two kinds of systems were used, either a
domestic oven (Samsung, RE-431H, 700 W) or a mono-
mode reactor (Synthewave 402 Prolabo or CEM Discover)
with temperature measurement and power modulation
between 15 and 300 W. After cooling, the reaction mixture
was extracted using CH Cl (3£20 mL). The organic phase
4
, 2-amino-3-picoline and the transiminative transition
state, respectively. The values show clearly that the
transition state is more polar than the ground state during
this transformation as expected. The optimised transition
state structure is shown in Figure 2.
2
2
was then washed with saturated NaCl solution, dried over
anhydrous MgSO , and concentrated under reduced press-
4
ure. The crude product was purified through flash chroma-
tography (ethyl acetate/pentane¼3/7) to afford the
corresponding ketone. All products have already been
2
described in the literature.
Acknowledgements
This work was supported in part by the National Research
Laboratory Program (NRL, 2000-N-NL-010C-271)
administered by the Ministry of Science and Technology
and CBMH. We thank sincerely Dr Ren e´ e Paugam (Paris-
South University, Laboratoire des Carbocycles) for skilled
assistance in theoretical calculations section of the polarity
of the transition state.
Figure 2. Optimised transition state for transimination of 4 with 2-amino-3-
˚
picoline (dN–C: 1.55 A).
In conclusion, we have developed an efficient procedure for
the synthesis of ortho-alkylated aromatic ketones through a
chelation-assisted Rh (I) catalysed ortho-alkylation reaction
of aromatic imines using solvent-free reactions and
microwave activation under ‘green chemistry’ conditions.
We have also reported on a new method for the synthesis of
aromatic imines from aromatic ketones under these
conditions. Efforts to extend this method to the preparation
References and notes
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3
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