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Download fileReactivity of N-Phenyl-1-Aza-2-Cyano-1,3-Butadienes in the Diels−Alder Reaction
journal contribution
posted on 1996-05-31, 00:00 authored by Nicholas J. Sisti, Irina A. Motorina, Marie-Elise Tran Huu Dau, Claude Riche, Frank W. Fowler, David S. GriersonIt is found that N-phenyl-2-cyano-1-azadiene
4, prepared via a two-step, one-pot, sequence
from
acrylanilide, undergoes efficient [4 + 2] cycloaddition with a
complete range of electron rich, electron
poor, and neutral dienophiles under remarkably mild thermal conditions
(90−120 °C for 20−48 h).
Regiospecific formation of the α-cycloadduct wherein the
dienophile substituent is α to nitrogen is
observed for vinyl ethers and styrene, whereas the Diels−Alder
reactions with methyl acrylate
and methyl vinyl ketone (MVK) produce α/β mixtures in which the
α-cycloadduct is the major
regioisomer (approximately 4−5:1). An essentially identical
reaction pattern was observed in the
Diels−Alder reaction of
N-(p-methoxyphenyl)-2-cyano-1-azadiene
18 and the 4-methyl-substituted
azadiene 27. For compound 19 derived from
cycloaddition of 18 with ethyl vinyl ether,
facile
conversion to the dihydropyridine 21 through loss of EtOH on
brief acid treatment was also noted.
The 2,4-cis-disubstitution pattern confirmed by X-ray
diffraction for the major cycloadduct 29
isolated from the reaction of 27 with styrene provides
evidence for the endo mode of cycloaddition
in the Diels−Alder reaction of
N-phenyl(aryl)-2-cyano-1-azadienes. Calculation of
the frontier orbital
energies and coefficients, as well as the transition state geometries
for the [4 + 2] cycloaddition of
N-phenyl-2-cyano-1-azadiene 4 with methyl vinyl
ether, styrene, and MVK were carried out at the
RHF AM1 level (MOPAC, Version 5.0). The FMO treatment indicates
that the reaction of 4 with
methyl vinyl ether occurs under LUMOdiene control, whereas
in contrast, the corresponding
cycloaddition with MVK occurs preferably under HOMOdiene
control. A high degree of asynchronicity
is observed in the calculated transition states for reaction of
4 with the three representative
dienophiles. In all cases the transition states leading to the
α-cycloadducts are lower in energy
than those giving the β-products. However, at the AM1 level the
exo cycloaddition mode is found
to be the preferred, this result contrasting with experimental results
for azadiene 27.