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Competing Cycloadditions in the Reaction of Acetylazobenzene with Diphenylketene.

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by cleavage of the central o-bond; comparison with ( 1 2 )
and ( 1 3 ) rules out a primary cyclobutene ring opening as
(proposed['') alternative mechanism in the case of (I). Annelated cyclobutane or cyclobutene rings such as those in (3)
and (1) therefore have practically no influence on the Arrhenius parameters of (11).
flli
113,
/I21
Table 1. Kinetic data. G=gas phase, L=liquid phase.
Compound
(1)
L
(3)
L
(11)
(12)
(13)
L
L
G
E, [kcal/mol]
log.4
24.5
24.1
25.5
31.7
32.6
32.4
12.0
11.9
12.1
13.6
14.0
14.2
Ref
[I1
this communication
this communication, [4]
this communication
PI
[61
Received: March 23, 1976;
in abridged form: April 1, 1976 [Z 452 IE]
German version: Angew. Chem. 88,447 (1976)
CAS Registry numbers:
( I ) , 49542-83-8;( 2 ) , 59247-30-2;( 3 ) , 59247-31-3;( 4 ) , 59247-32-4;
( S ) , 2166-14-5;(?), 59247-33-5;(81, 840-65-3;( 9 ) , 59247-34-6;
(101, 59247-35-7;(If), 31947-22-5;( 1 2 ) , 3097-63-0;
cyclobutadienetricarbonyliron, 12078-17-0
H.-D. Martin and M. Hekman, Angew. Chem. 85, 615 (1973);Angew.
Chem. Int. Ed. Engl. 22,572 (1973).
The fragmentation of a 1,4-cyclohexanediyl into two ally1 radicals is
endothermic to the extent of about 25 kcal/mol; however, annelation
of two four-membered rings (strain energy 52 kcal/mol) can lead to
exothermic cleavage. The formation of an intermediate transpans-diene
from (3) as a precursor of ( 1 0 ) has been suggested as an interesting
alternative by H . Schmid.
L. A. Paquette, M . R. Short, and 1. F. Kelly, J. Am. Chem. SOC.93,
7179 (1971);W R. Roth cited in J . K . Kochi: Free Radicals. Vol. I,
Wiley-Interscience 1973,p. 229; W R . Roth and M. Martin, Tetrahedron
Lett. 1967, 3865. The equilibration of intermediate nitrogen-free fragments, as observed in the case of diazabicyclo[2.2.2]octenes, possibly
does not occur here.
The kinetics of thermolysis of ( 1 1 ) was investigated in collaboration
with Dr. Daniel B e h f , Basel.
H. M. Frey, H.-D. Martin, and M. Hekman, J. Chem. SOC.Chem. Commun.
1975, 204.
R. S. Case, M . J . S . Dewar, S . Kirscltner, R. Pettit, and W Sleyier,
J. Am. Chem. SOC.96, 7581 (1974).
[l]
[2]
[3]
[4]
[5]
[6]
spite of the considerable difference in electronic character
of the substituents attached to the azo group both regioisomers
(3) and ( 4 ) should be formed. In contrast, the comparable
reactions of a r y l a z ~ n i t r i l e sand
~ ~ ~ethyl benzeneazocarboxylate[2.4l with (1) were found to give only one 1,2-diazetidinone
of type (3) in each case.
The exothermic reaction of ( I ) with (2) is complete within
ca. 1 h; the 'H-NMR spectrum of the crude solution shows
it to proceed quantitatively. Compounds (3) and ( 4 ) are
isolated by fractional crystallization from ,methanol. Pure ( 5 )
and (6) can be obtained by rearrangement of (3)['1 and
( 4 ) , respectively, under the influence of trifluoroacetic acid.
Crystalline (5) and (6) are also obtained on seeding the
reaction solution after extensive removal of (3) and ( 4 ) .
The structures of ( 3 ) and (5) have been proved by independent
syntheses[*r61,and the structures of ( 4 ) and (6) by IR, 'HNMR, and I3C-NMR spectra (Table I).
Table 1. Cycloadducts from diphenylketene (1) and acetylazobenzene (2).
All compounds gave correct CHN analyses. The product composition was
determined from theintegrals of the methyl signals in the 'H-NMR spectrum
of the crude solution.
Cpd.
131
(4)
(5)
(6)
Competing Cycloadditionsin the Reaction of Acetylazobenzene with Diphenylketene
By Sven Sommer"]
Among the few cycloadditions undergone by aryl azo compounds of the type Ar-N=N-X
[X=Ar, CH(CH&,
N(CH3)2, C 0 2 C 2 H 5 , C=N] are to be found the reactions
with diphenylketene (1 )['- 3 1 . Only [2+2] cycloadducts (1,2diazetidinones) have so far been isolated as reaction products.
In contrast, the reaction of acetylazobenzene (2) with (I)
(1 : 1 in benzene at room temperature) leads to the two [ 2 21
cycloadducts (3) and ( 4 ) , to a [4+2] cycloadduct ( 5 ) , and
to a product (6) formally arising by [4+2] cycloaddition
and subsequent 1,3-H shift.
No mutual interconversion of the products under the reaction conditions could be detected. It is remarkable that in
+
[*] Dr. S. Sommer
Organisch-chemisches Institut der Technischen Universitat
Arcisstrasse 21, 8000 Miinchen 2 (Germany)
432
'H-NMR
M.p.
["CI
171 [bl
169-170
167-168
136-137
136f61
182-183
SCH3
IR [cm-']
W r l
vC=O (Ring)
vC=O
[%I
I.[
Proportion
1.65
1792
1709
60
[2]
[b]
[b]
2.50
2.08
1796
1678
1725
(C=N
26
10
[c]
2.67
1680
(SNH
(sNH
= 8.43)
= 3280)
= 1660)
1715
4
[a] &values [ppm], in CDCI3 with TMS as internal standard.
[h] From methanol.
[c] Dissolved in ethyl acetate and precipitated with methanol.
Received: March 26, 1976 [Z 453 IE]
German version: Angew. Chem. 88,449 (1976)
CAS Registry numbers:
( I ) , 525-06-4;(2), 13443-97-5;( 3 ) , 59231-00-4;(41, 59231-01-5;
( S ) , 59231-02-6;( 6 ) , 59231-03-7
[l] R. C. Kerbrr, 7: J . Ryan, and S . D.Hsu, J. Org. Chem. 39, 1215 (1974).
C.W Bird, J. Chem. SOC.1963, 674.
131 C . W Bird, J. Chem. SOC.1964, 5285.
[4] On the basis of a shoulder at ca. 1820cm-' in the 1R spectrum of
the crude solution, Kerber et al. [l] suspect the formation of small
amounts of the regioisomer of type ( 4 ) ; confirmatory evidence and
an isolation have yet to be reported.
[5] The rearrangement ( 3 ) + ( 5 ) has already been performed with the aid
of concentrated HCI. Under these conditions ( 5 ) is formed together
with two other products in only 23 % yield [2]. In trifluoroacetic acid
the rearrangements ( 3 ) + ( 5 ) and ( 4 ) + ( 6 ) are quantitative.
161 J. van Alphen, Rec. Trav. Chim. Pays-Bas 48, 163 (1929).
121
Angrw. Chem. Int. Ed. Enyl. J Vol. 15 (1976) No. 7
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