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Intramolecular Photocycloaddition of N N-Alkylenedimaleimides.

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carbon atomr21. It is thence concluded that inversion of at
least one nitrogen atom is a relatively slow process; lower
limiting values of 21 kcal,'mole can be calculated for the free
enthalpies of activation AG?. It was therefore predicted [2J
that it should be possible to separate the 'invertomers' (diastereoisomers) of suitably substituted diaziridines and
similarly also of oxaziridines. Broisl-ll and Felix and Eschenmoser 141 have isolated the isomers of 1-chloroaziridines; the
presence of optically active oxaziridines 6 1 has been recently
proved [**I; but stable invertomers of diaziridines have not
previously become known.
Compounds ( 4 a ) and (46) behaved analogously: pure (401,
m.p. 27-28 OC, crystallized from a solution of their mixture[*]
in pentane at -10 "C. Repeated removal of (4a) by crystallization and thin-layer chromatography afforded 90%-pure
(46) with m.p. < 5 "C. Under the experimental conditions
R ~ ( 4 b=
used for (30) and (3b) the values R F ( ~ =~ 0.4,
0.3, AG(4,) = 27.1, and AG(4b) =- 26.3 kcal/mole were obin the
tained [91; thus one isomer preponderates (75.3
equilibrium mixture and we attribute the structure ( 4 0 ) t o it
for steric reasons. The assignments of (3a) and (3bi could
only be made on the basis of this finding.
Since no sign of by-products appeared during any of the
equilibrations carried out. we assign a low probability to
mechanisms in which the three-membered ring is opened; it
is more likely that the isomers are interconverted by consecutive inversions at the nitrogen atoms~lol.
Received: January 13, 1969
[Z 935 IE]
German version: Angew. Chem. 81, 224 (19691
( I ) , R = H o r CHB
A limiting value for 4 G 5 can also be obtained for (2) having
equal substituents on the nitrogen atoms but different groups
o n the ring carbon atom. Compound ( 2 ) was prepared with
m.p. 24-26.5 O C from benzyl methyl ketone, methylamine,
and N-methylhydroxylamine-0-sulfonicacid by a procedure
analogous t o that described by Schmitz [61. The N-methyl
signals, of equal intensity, at T = 7.43 and 7.59 (in diphenyl
ether at 28 "C) are not broadened at 159 'C, whence a lower
limit of 22 kcal/mole is obtained for AG?. Furthermore the
spectrum contains, besides the multiplet for the phenyl
protons, only the C-methyl singlet at T = 8.92 and the AB
absorption of the methylene group at 7.17 and 7.35 ( J =
13.9 Hz). These findings can be accommodated only by the
trans-structure ( 2 ) .
We have realized the predicted separation in the case of (3a)
and (36). The mixture obtained in the synthesis from benzyl
methyl ketone, benzylarnine, and N-methylhydroxylarnine0-sulfonic acid gave two non-crystalline products (3n) and
(3bi when twice subjected to preparative thin-layer chromatography at 2OoC on silica gel (Merck's HF7.54) with 9:l
benzene/ethanol as eluant:
[*] Priv.-Doz. Dr. A. Mannschreck and W. Seitz
Institut fur Organische Chemie der Universitlt
69 Heidelberg, Tiergartenstrasse (Germany)
[l] Part I1 of Proton-Resonance Studies of the Inversion a t Tervalent Nitrogen Atoms. - Part I: [2].
[2] A . Mannschreck, R . Rndeglia, E. Griindemann, and R . Ohme,
Chem. Ber. 100, 1778 (1967).
[3] S.J . Brois, J. Amer. chem. SOC.90, 506, 508 (1968); cf. J . M .
Lehn and J . Wagner, Chem. Commun. 1968, 148.
141 D. Felix and A . Eschenmoser, Angew. Chem. 80, 197 (1968);
Angew. Chem. internat. Edit. 7, 224 (1968).
[5] D . R. Boyd, Tetrahedron Letters I968, 4561 ; F. Montanari,
I. Moretti, and G. Torre, Chem. Commun. 1968, 1694; J . M .
Lehn, private communication.
[**I Note added in proof (Feb. 17, 1969): Very recently we have
concentrated the two invertomers (diastereoisomers) of 2.3-dimethyl-3-benzyloxaziridine. At 75 "C no sign of equilibration
was observed over a period of 3.5 days, i.e. AG* > 30 kcal/
mole. A. Mnnnschreck and J . Linss, unpublished results.
161 E. Schmitz: Dreiringe mit zwei Heteroatomen. SpringerVerlag, Berlin 1967, p. 75.
[7] For details of the method see A. Mannschreck, A. Mattheus,
and G. Rissmann, J. molecular Spectroscopy 23, 15 (1967).
[8] C. J . Pager and C. S . Davis, J . med. Chem. 7, 626 (1964).
[9] It has not yet been possible to exclude with certainty an influence of NH prototropy on these two inversion barriers.
[lo] For this reason a transmission coefficient of 0.5 was used in
calculating all the AG+ values. Cf. J . E. Anderson and J . M .
Lehn, J. Amer. chem. SOC.89, 81 (1967).
Intramolecular Photocycloaddition of
N,N'-Alkylenedimaleimidesf * *I
By F. C. De Schryver, I. Bhardwaj, and J . Put [*I
1 1 I 1
7.06; 1.29
( J = 13.9 Hz)
7.15; 7.28
( J = 13.9 Hz)
The T values (in CC14 at 28 "C) agree with those of known
diaziridinesrzl. The (3a) obtained showed no signals of the
other isomer, but the experimental samples of (3b) still
contained 6 % of (3a). Elemental analyses and molecular
weights accorded with expectation; so did the IR spectra,
which differed only slightly, namely, in the region 1080 to
1130 cm-1; both products exhibited the oxidizing puwer
characteristic of diaziridines An equilibrium containing
49.8 .? 0.4% of (3a) was established with a half-life of 431
min in CzCI4 solutions at 70.0 "C, whether (3a) or (3b) was the
starting material. Thence the rate constant k(3=) k(36) ==
1.34 Y,
sec-1 and A G ( j a )
mole were calculated 171.
0.2 kcal,'
Angew. Chem. internat. Edit. / Yol. 8 (1969)/ No. 3
Maleic anhydride and maleimide derivatives can undergo
photoaddition reactions with other double or triple bonds or
with themselves [IJ]. A large number of intramolecular
photocycloadditions of this kind are known
but, as stated
by Eaton[41, "the proximity of the condensing systems in
these examples might have special significance in the ease of
the cycloaddition reaction".
In most hitherto known intramolecular photodimerizations,
C4 cycloadditionf2I results in formation of a five- or sixmembered ring. We wish to report the photocyclomerization
of dimaleimides ( f i , in which the reacting double bonds are
separated by an alkyl chain, to the tetracyclic compounds (2).
The reaction is carried out in CH3CN or CH2C12. The
structure of the product is confirmed by the IR and N M R
spectral data, and especially by the mass spectra (see Table).
The absorption characteristics of the -CH=CH- system i n
( 1 ) are no longer present in ( 2 ) , but have been replaced by
bands that can be attributed to a cyclobutane ring. The ultraviolet spectra of the products (2) show no absorption due to
a -CH=CH-CO system at about 300 nm.
such formal diradicals 121. Two novel procedures for accomplishing this task are reported in this communication.
Consider trimethylenemethane, a typical carbon diradical,
which in a D3h geometry possesses a degenerate pair of nonbonding z orbitals. These are to be occupied by a pair of
electrons, and theoretically [31 and experimentally 141 the
3085, 680. 840
3094, 690, 840
3080, 695, 835
3090, 695, 835
144 [dl
3080, 690, 860
129.5 [el 3100, 698, 840
6.75 x 98
[c I
(v, cm-1)
( "C)
29 I
[a1 In KBr. [b] Resonance signals of the four cyclobutane protons, all signals are singlets. All spectra recorded
in CDCIJ except for I l b ) and (2bl which were recorded in [D,+DMSO; / 2 a ) and (2f) are not sufficiently soluble.
[cl Molecular w:ight me-sureli with a Mechrolab 301 A vapor pressure osmometer in CHCI,. [d] 136-141 "C [ 5 ] .
Iel 128-132" [6].
Long-range intramolecular cyclomerizations of this kind also
seem to occur during UV irradiation of dinucleotides containing pyrimidine bases 171.
As expected, when the two maleimide moieties are joined by
a rigid phenyl ring, as in m-phenylenedimaleimide, n o intramolecular cyclization occurs but a polymeric material is
The cyclodimerization of N,N'-hexamethylenedimaleimide
( I d ) to 5.1 2-diazatetracyclo[l0.2.1.12~~.03
13.15,16-tetraone (Zd) was studied in more detail. A tenfold
increase of the concentration of ( I d ) (5 x 10-3 mol 1-1 to
5 Y 10-2mol 1-1) resulted in a small decrease in the yield
from 95 % to 85 %, the remainder being polymeric material.
Addition of benzophenone as photosensitizer also decreased
the yield t o 80%, while the presence of oxygen in no way
inhibited the reaction. Most surprising is the ease and speed
of the reaction. Under the same conditions and in the absence of a sensitizer, N-butylmaleimide yields only 1 5 % of
the expected dimer.
Received: January 29, 1969
[Z 936 I € ]
German version: Angew. Chem. 81, 224 (1969)
I*] Dr. F. C. De Schryver, Dr. 1. Bhardwaj, and J . Put Lic. Sc.
Laboratorium voor Macromoleculaire Scheikunde
Universiteit te Leuven
Naamse straat 96, Leuven (Belgium)
["'I The authors are indebted to the Center of High Polymer
Research and Gevaert-Agfa for supporting this research and to
the National Science Foundation for a fellowship (J.P.).
[I J G. 0. Schenek and R. Steinmetz, Bull. SOC.chim. belges 71,
781 (1962).
[2] R. Steinmetz, Fortsch. chem. Forsch. 3, 445 (1967).
[3] W . L . Dilling,Chem. Reviews 66, 373 (1966).
141 P. E. Eaton, Accounts chem. Res. I, 52 (1968).
[5] R. J. Cotter, C . K . Sauers, and J . M . Whelan, J. org. Chemistry 26, 10 (1961).
[6] P. 0. Tawney, R . H. Snyder, E. P . Conger, K . A . Leibrand,
C. H. Stiteler, and A . R. Wifliams, J. org. Chemistry 26,15 (1961).
[7] J . G . Burr, Advances in Photochemistry 6, 193 (1968), and
references cited therein.
Formal Diradicals Which Should Have Singlet
Ground States(1 1
By R . Gleiter and Roald HoffmannI *I
Molecules whose every classical valence structure is at best a
diradical one generally emerge theoretically and experimentally as triplet ground states. We have for some time been
interested in the problem of stabilizing the singlet states of
ground state of the system appears to be a triplet. Of the two
nonbonding orbitals A and B, A has density at C1, whereas
orbital B by symmetry does not (51.
Substitution in trimethylenemethane of an oxygen atom for
CH2 at the C1 position would be anticipated to break the
degeneracy of A and B by stabilizing A and leaving B unaffected. The resulting species is oxyallyl ( I ) , the ring-opened
zwitterionic form of cyclopropanone. A previous calculation
which we carried out o n this molecule[*bl showed that the
lowest singlet was considerably stabilized, but that a triplet
remained the ground state of the molecule by 0.1 eV. In order
to obtain diradicals with singlet ground states we scrutinized
the molecular orbitals of a large number of carbon diradicals,
keeping an eye out for those molecules in which one nonbonding molecular orbital possessed electron density at one
or more carbon atoms which did not appear in the other
nonbonding orbital. After substituting the appropriate
number of oxygens in these differentiated positions we carried
out Pariser-Parr-Pople calculations with extensive configuration interaction [61.
We discovered three species, ( 2 ) - ( 4 ) , which were definitely
singlet ground states, and a number of other borderline cases.
The position relative to the ground state of the first two
excited states of ( 2 ) - ( 4 ) is listed below the corresponding
We have drawn diradical valence structures for ( 2 j - ( 4 ) , but
the calculations in each case showed final charge distributions
characteristic of the expected polar resonance structures,
such as ( 2 a ) and ( 2 6 ) .
Each of these molecules would be expected to be highly
colored. Substituted derivatives of ( 2 ) are known 17.81, and
are strong chromophores.
Angew. Chem. internat. Edit.
1 Vol. 8
No. 3
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alkylenedimaleimides, intramolecular, photocycloaddition
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