Mendeleev Commun., 2020, 30, 618–620
The method for the synthesis of dichloro(phenyl)acetaldehyde
differ for exo- and endo-derivatives because of stereospecific
dependence on the nitrogen lone pair arrangement.11 For exo-
and endo-isomers of 2b, the JC(3)H(3) values are 156.7 and
149.5 Hz, respectively. For the key NOE interactions, see Online
Supplementary Materials (Figure S1).
For isomeric amidoalkylated cyclopentadienes 3/3', the NOE
effect occurred involving NH protons and cyclopentadienyl ones
located in the positions 2 and 5 (for isomers 3) and in the
positions 1 and 3 (for isomers 3'), which corresponds to 1- or
2-substituted cyclopentadienyl derivatives, respectively.
In summary, dichloro(phenyl)acetaldehyde N-arylsulfonyl-
imines were explored for the first time in cycloaddition with
cyclopentadiene, which gave new functional derivatives of
2-azabicyclo[2.2.1]hept-5-enes. A possibility for synthesis of
amidoalkylated cyclopentadiene derivatives by reaction with
polychloroacetaldehydeN-arylsulfonylimineswasdemonstrated.
N-arylsulfonylimines 1d–f is based on the addition of N,N-
dichloroarenesulfonamides at phenylacetylene.10 This procedure
provides imines 1d–f as individual compounds. Therefore, their
further reaction with cyclopentadiene can be carried out in
toluene, which is a ‘classical’ solvent for imine cycloaddition. In
our experiments, imines 1d–f reacted with cyclopentadiene at
55–60 °C in toluene for 8 h to afford the corresponding [4+2]
cycloadducts, new azanorbornene derivatives 2d–f, in high
yields. Importantly, the reaction time for the synthesis of
compounds 2d–f can be reduced to 2 h by the use of microwave
(MW) activation.
1
Both in the cases of chloral (1a–c) and dichloro(phenyl)-
acetaldehyde (1d–f) imines, the cycloaddition proceeds
selectively to give exo-isomers 2. Apparently, this is due to a
higher thermodynamic preference of exo-isomers as compared
to endo-ones caused by steric factors. Nevertheless, minor
1
quantities of endo-isomers were fixed by H NMR in reaction
The main results were obtained using the equipment of Baikal
Analytical Center of Collective Using, Siberian Branch of the
Russian Academy of Sciences.
mixtures in some cases.
The corresponding amidoalkylated derivatives of cyclo-
pentadiene 3d,e/3'd,e (X = Ph) are formed under much harsher
reaction conditions than analogues 3a–c/3'a–c (X = Cl), namely,
boiling of cycloadducts 2d–e in bromoform for 72 h is required.
A tentative route to the formation of amidoalkylated
cyclopentadiene derivatives 3/3' likely involves the initial
cycloaddition to furnish [4+2] cycloadducts 2. The latter upon
heating would undergo the opening of the azanorbornene
fragment (Scheme 2). Probably, this opening proceeds hetero-
lytically across the N–C1H bond to deliver bipolar inter-
mediate A, which may exist in the form of several resonance
structures. Further stabilization of intermediate A leads to the
corresponding amidoalkyl-substituted cyclopentadienes 3/3'
existing in equilibrium in molar ratios close to 1:1. The third
thermodynamically less preferable isomer 3'' was never detected
both in the reaction mixtures and in the final products. Control
experiments have confirmed that heating of cycloadducts 2
undertheabovereactionconditions[80–90°Cintrichloroethylene
in the case of chloral derivatives 2a–c or ~145–150 °C in bromo-
form in the case of dichloro(phenyl)acetaldehyde derivatives
2d,e] really gave compounds 3/3'. It should be noted that
C-amidoalkylated derivatives 3a–c/3'a–c can be also prepared
byheating in toluene, DMF or DMSO.
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2020.09.022.
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1
with one of the protons of 7-CH2 group. In H NMR of 2b,
chemical shifts of H3 protons are different for exo- (4.05 ppm)
and endo- (4.88 ppm) isomers. Moreover, using 2D HSQC data
we have measured the direct coupling constants 1JC(3)H(3) which
Cl
Cl
–
N
Cl
X
H
Cl
X
N
ArO2S
ArO2S
3 + 3'
2
+
A
3"
unfavourable
Scheme 2
– 619 –