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Formal Diradicals Which Should Have Singlet Ground States.

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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
I LJ
(2)
IR
M.P.
C
d
e
f
174.5
3085, 680. 840
206
3094, 690, 840
107
3080, 695, 835
3090, 695, 835
144 [dl
3080, 690, 860
85.5
129.5 [el 3100, 698, 840
100
6.73
6.89
85
6.75 x 98
6.74
95
6.76
80
100
6.81
[c I
fa1
a
b
M.W.
(v, cm-1)
( "C)
350
350
349
29 I
269
320
3040
3020
3010
3000
2980
3040
-
3.80
3.80
3.80
3.80
-
234
248
262
276
290
236
264
280
(decornp.)
[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
formed.
The cyclodimerization of N,N'-hexamethylenedimaleimide
( I d ) to 5.1 2-diazatetracyclo[l0.2.1.12~~.03
14lhexadecane-4,
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
214
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.
A
B
(1)
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
formula.
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
(1969)
No. 3
0
0
OQ
A remaining unanswered question is whether these planar
molecules a r e in fact more stable than t h e nonplanar bicyclic
isomers [e.g. (2) +(2c)]. Some extended Hiickel calculations
indicate that (2) is more stable than (2c). but we d o n o t consider t h e result very reliable in this instance. T h e stabilization
of (3) is of great interest, since it suggests t h e possibility of
achieving a C o p e rearrangement with negative activation
energy.
[6] The details of our particular variant are given in reference
[2a]. We always included all singly and doubly excited configurations, and many higher excited configurations as well. The final
results for the species ( 3 ) , for example, are for the 70 most significantly contributing configurations selected from a total of some
800 exa.mined. Pariser-Parr-Pople calculations are normally
quite reliable for x-electron spectra; however we have no calibration points for these diradicals, and we consider the results o n l y
qualitatively informative.
[7] A . Treibs and K. Jacob, Angew. Chem. 77, 680 (1965);Angew. Chem. internat. Edit. 4, 694 (1965);Liebigs Ann. Chem.
699,153(1966);H.-E. Sprengerand W. Ziegenbein,Angew. Chem.
78, 937 (1966); Angew. Chem. internat. Edit. 5, 894 (1966);
Angew. Chem. 79, 581 (1967);Angew. Chem. internat. Edit. 6.
553 (1967); Angew. Chem. 80, 541 (1968); Angew. Chem. internat. Edit. 7, 530 (1968); W. Ziegenbein and H.-E. Sprenger,
Angew. Chem. 78, 937 (1966);Angew. Chem. internat. Edit. 5,
893 (1966);G. Manecke and J . Gauger, Tetrahedron Letters 1967,
3509; 1968,1.339.
[8] D. G. Farnum, 5. Webster, and A . D. War, Tetrahedron
Letters 1968,5003.
[9] E. Heilbronner and P. A . Srraub: Hiickel Molecular Orbitals.
Springer-Verlag. Berlin-Heidelberg-New York 1966.
A related class of molecules is exemplified by ( 5 ) .T h e classical
valence structure (5a) implies a diradical; however, the
resonance structure (5b) clearly suggests a closed shell, and
The Thermal Oxidation of N-Vicinal Methylene
Groups in Polyamides, a Reaction Dependent on
Conformation
By H. Krussmtmn, G . Valk, G . Heidemann, a n d S. Dligali*l
In the thermal oxidation of polycaprolactam ( I ) , a s in the
thermal oxidation of low-molecular weight alkyl amides, the
N-vicinal methylene group (arrowed) is the preferred site of
attackrl.21.
the simple Hiickel calculation gives a gap of 0.704 p between
occupied a n d unoccupied levels [91. Nevertheless, according
t o our S C F calculations t h e ground state o f ( 5 ) is a triplet,
some 1.1 eV below t h e lowest singlet - t h e energy g a p is thus
n o t large enough for stabilization of t h e singlet state.
W e noticed, however, t h a t the highest occupied MO of ( 5 )
was a n essentiaily locaiized nonbonding ally1 orbital a t C-1
a n d C-3, whereas the lowest unoccupied MO was localized in
t h e three-membered ring. Replacement of t h e methylene
groups a t C-l a n d C-3 by oxygens produced t h e necessary
selective stabilization of the highest occupied MO.
r o
This group is distinguished by a n interaction (hyperconjugation) which takes place between its protons a n d the 4a electrons of t h e carboxamide group (31; a n interaction which
should b e particularly intensive if t h e N-vicinal methylene
group is in t h e trans-conformation t o the amide group (Fig. 1).
T h e ground state of t h e cyclopropenylium carboxylate (6) is
a singlet, with a 3B2 state 0.97 eV above it. Similarly we find a
singlet ground state for the cycloheptatrienylium carboxylate
( 7 ) , with a 3B2 state 0.97 eV above it.
Received: January 27, 1969
[Z 937 I € ]
German version: Angew. Chem. 81, 225 (1969)
[*I Dr.
R. Gleiter and Prof. Dr. Roald Hoffmann
Cornell University
Department of Chemistry
Ithaca, N.Y. 14850 (USA)
[l]The SCF-CI program we used was written by A . Imamura.
We are grateful to the National Science Foundation, the National
Institutes of Health, and the Chevron Research Corporation for
financial support of this work.
[ 2 ] a) R. Hoffmann, A . Imamura, and G. D. Zeiss, J. Amer.
chem. SOC.89,5215 (1967); b) R . Hofmann, ibid. 90, 1475 (1968);
c) R . Hoffmann, A . Imamura, and W. J . Hehre, ibid. 90, 1499
(1968); d ) R . Cl2iter and R. Hoffmann, ibid. 90, 5457 (1968);
e) R. Gleiter and R . Hoffnzann, Tetrahedron 24, 5899 (1968).
1
L-
(4
--- - - ,
ib)
Fig. I
irand (a) and cauchr (b) conformations of the N-vicinal C H z
group. The dotted parallelogram indicates the plane of the amide group.
Proof of this hypothesis was afforded by thermally oxidizing
compounds ( I ) a n d 1,8-diazacyclotetradecane-2,9-dione
[cyclo-bis(~-aminocaproyl)](2) a t 200-220 'C. ( I ) a n d ( 2 )
consist of t h e same fundamental units; in contrast to ( I ) ,
compound ( 2 ) contains a-methylene groups in t h e gauche
position only 149 51.
131 D. P. Chong and J . W. Linnett, J. chem. SOC.(London) 1965,
1798; W. T . Borden, Tetrahedron Letters 1967,259.
[4] P. Dowd, J. Amer. chem. SOC.88, 2587 (1966); P. Dowd and
K . Sachdev ibid. 89, 715 (1967).
[5] A convenient basis, symmetry adapted for reduction of symmetry from D j h to C P ~is, chosen for the degenerate orbital.
Angew. Chem. internat. Edit.
/ Vol. 8 (1969) / No. 3
215
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