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Differing 1 3-Dipolar Activity of cis-trans Isomeric Azomethine Ylide Dicarboxylic Esters.

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In azomethine oxides (nitrones) the oxygen stabilizes the
pegative charge better than the carbon of the carbanion in
Table 1 . Steric course of some reactions of cis-jl) [viatrans-(2)l and
r r n s - ( l ) [via cis-(2)1 with dipolarophiles at IW-140 “C [a].
From &-(I)
Dipolarophile
a= b
azomethine ylides. This is responsible for the conversion of
oxaziranes into the open-chain nitrones.
Received: June 4, 1969
IZ 29a IEJ
German version: Angew. Chem. 81, 619 (1969)
[*] Prof. R. Huisgen, Dr. W. Scheer, and Dip1.-Chem. H. Mader
Institut fur Organische Chemie der Universitat
8 Miinchen 2, Karlstrasse 23 (Germany)
[I] R. Huisgen, W. Scheer, G. Szeimies, and H. Huber, Tetrahedron Letters 1966, 397.
121 R . Huisgen, W . Scheer, and H . Huber, 3. Amer. chem. SOC.
89, 1753 (1967).
[3] R. B. Woodward and R. Hoffmann, 5. Amer. chem. SOC.87,
395 (1965).
[4] C. Reichardt and K . Dimroth, Fortschr. chem. Forsch. 11, 1
(1968).
[ 5 ] R. E. Lutz and A . B. Turner, J. org. Chemistry 33, 516 (1968).
161 M. F. Hawthorne and R. D . Sfrahm, J. org. Chemistry 22,
1263 (1957).
Differing 1,3-Dipolar Activity of cis-trans
Isomeric Azomethine Ylide Dicarboxylic Esters[**I
By R . Huisgen, W. Scheer, H. Miider, and E. Brunn I*]
The stereoisomeric dimethyl azomethine ylide dicarboxylates,
cis-(2) and trans-(2), which arise from the dimethyl aziridinedicarboxylates trans-(I) and c i s - f l ) , combine stereospecifically with highly active dipolarophiles such as acetylenedicarboxylic ester [*I or tetracyanoethylene 121. If the dipolarophilic
activity of the multiple bond system a = b is reduced, cistruns isomerization of the azomethine ylides competes with
cycloaddition to a=b.
A characteristic difference between truns-(2) and cis-(2)
becomes apparent under the above conditions. Cycloadditiohs of trans-(2) remain stereospecific, even with “poor”
dipolarophiles (Table 1); rruns-(3) is the sole product detectable by NMR spectroscopy. With cis-(.?), on the other
CH302C
trms-
H
(I)
JT
R
cH
I
CH30zC.c~NSo,H
.f:C02CH3
I
H
~ CO, ~~ ~ ~ ‘ ~ , CCH,
O,
&
&(2)
trms-(2)
Tetracyanoethylene
Dimethyl
acetylenedicarboxylate
Diethyl
azodicarboxylate
Diethyl mesoxalate
Dimethyl fumarate
Tetraethyl ethylenetetracarboxylate
Norbornene
Cyclohexene
Phenanthrene
88
98
0100
96
6: 94
94
100
99
7 : 93
0:lOO
9: 91
25: 75
86
78
71
36
45: 55
83: 17
100: 0
[a1 Most of the adducts (3) listed in the Table are new and gave correct
analyses.
Ibl The ratio of fruns-(3) to cis-(3) was 1OO:O in all these cases,
hand, the lower the activity of the dipolarophile the greater the
amount of the sterically “wrong” adduct truns-(3). With
norbornene nearly half of the cis-(2) isomerizes to trans-(2)
before it enters into cycloaddition with a=b; the cyclohexene
adduct has 83% truns configuration with respect to
the carboxylate groups. Finally, with phenanthrene the sequence trans-(1) + cis-(2) + trans-(2) leads to the adduct2,5-trans-diester, trans-(3). No subsequent stereoisomerization of the adducts (3) could be detected under the reaction
conditions or during the work-up procedure.
What is the cause of this striking phenomenon? Initially we
suspected that the azomethine ylide equilibrium lies largely on
ki. However, the energy
the side of trans-(2), i.e. that k - i
profile of the ring opening and isomerization[21 does not
lead one to expect such an energy difference between truns(2) and cis-(2); differing rate constants for the cycloaddition,
namely k 2 > kz‘, must therefore be responsible. A molecular
model shows that strong van der Waals repulsion and restriction of rotation of the cis ester groups in cis-(3) can occur.
It is also possible that the transition state of the cycloaddition
- here the carbon atoms of (2) are intermediate between spz
and sp3 hybridization - is affected by such an overlapping of
the substituent radii as well. It is only with extremely active
dipolarophiles that kz’ remains large enough to preclude
competition from k-i.
>
Received: June 4, 1969
[Z 29b IE1
German version: Angew. Chem. 81, 621 (1969)
and Dr. E. Brunn
Institut fur Organische Chemie der Universitat
8 Miinchen 2, Karlstrasse 23 (Germany)
[**I This work was supported by the Fonds der Chemischen
Industrie.
[11 R. Huisgen, W. Scheer, and H. Huber, J. Amer. chem. Soc.
89, 1753 (1967).
121 R. Huisgen, W. Scheer, and H . Mader, Angew. Chem. 81,619
(1969); Angew. Chem. internat. Edit. 8, 602 (1969).
-7H
I
H
k-i
(3).
yield ( %)
[*I Prof. R. Huisgen, Dr. W. Scheer, Dip1.-Chem. H. Mader,
B
B
From trans-(I)
[bl
Configuration of a cis-Disubstituted Azomethine
Ylide
,
=
a
+
By R. Huisgen and H. Maderr*]
R
trans-(3)
604
R
cis-(3)
The conrotatory ring opening of dimethyl l-(rl-methoxyphenyl)aziridine-cis-2,3-dicarboxylate[I] to give the azomethine ylide trans-dicarboxylate is structurally unambiguous. In contrast, the conrotation of the trans-isomer ( I ) can
give the azomethine ylide dicarboxylate in the horse-shoe
form (2) or in the W form (3); in the latter form the van der
Angew. Chem. internut. Edit. 1 VoI. 8 (1969) I No.8
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esters, activity, ylide, transp, dipolar, dicarboxylic, isomeric, differing, azomethine, cis
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