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Diradical-Type Behavior of Tri-tert-butylcyclobutadiene.

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[2] a) F . Schodeii, Chem. Ber. 106,2084 (1973); b) W Simon and H . Giucobho,
Chem.-1ng.-Tech. 37, 709 (1965); c) G . Schuden, Adv. Mass Spectrom.
6, 93 ( I 974).
[3] J . F. King, P. deMayo, C . L. Mclntosh, K. Piers, and D. J . H . Smith,
Can. J. Chem. 48, 3704 (1970).
[4] C . C . Chuen and S. W Fmton, J. Org. Chem. 23, 1538 (1958).
[S] K . Hafner, R . DBnges, E. Goedecke, and R . Kaiser, Angew. Chem. 85,
362 (1973): Angew. Chem. Int. Ed. Engl. 12, 337 (1973).
161 M. Zanderand W H . Franke, Chem. Ber. 101,2404 (1968), and references
cited therein.
Diradical-Type Behavior of Tri-tevt-butylcyclobutadiene[‘l [**I
0
%
110)
+
0
0
i71
was found, by comparison of its UV, IR, and mass spectrum
and of its gas chromatogram with those of authentic materito be indeno[2,1-a]indene (9). Small amounts of 5,lOdihydroindeno[2,1-~]indene (10) could also be detected. In
addition, a slight amount of the primary pyrolysis product
5H-benzo[a]fluoren-5-one ( 8 ) was isolated. The compound
likewise afforded (9) on Curie point pyrolysis.
Thus a simple entry is now available to indeno[2,1-a]indene
(dibenzopentalene) ( 9 ) , which was formerly accessible only
with difficulty. The method also suggests a new synthetic
pathway to other molecules derived from pentalene or to
the parent compound[51.
By Giinther Maier and Wolfgang SauerF”]
The geometry of cyclobutadienes and the multiplicity of
the parent hydrocarbon and its alkyl derivatives in the ground
state have not yet been elucidated. The two aspects are interrelated, for most calculations require that square cyclobutadiene
should have a triplet ground state, while a cyclobutadiene
with a lower symmetry is predicted to have a singlet ground
state.
The IR spectrum of unsubstituted cyclobutadiene suggests
an “effectively square” structure[’]. Assuming a dynamic JahnTeller effect it becomes understandable why this molecule
exhibits singlet character instead of the triplet character predicted on theoretical grounds[*].
In this communication we show for the first time that
the converse behavior is also possible: tri-tevt-butylcyclobutadiene
for which a rectangular structure is to be
can undergo reactions typical of a triplet molecule.
In spite of pronounced steric hindrance, tri-tert-butylcyclobutadiene ( I ) is capable of not only synchronous cycloadditions but also ionic additions in which it proves to be an
exceptionally good nucleophile and a strong base[51.We have
classed the autoxidation of ( I ) to give an isolable dioxetane
derivativeE6I as a radical addition, but only with reservation
because this unusually fast reaction does not appear to be
affected by the necessary change of multiplicity. Meanwhile
we have found that the autoxidation has parallels in the
addition of tetrahalomethanes, which already proceeds at
room temperature in the dark and without addition of radical
initiators.
+
CXY3
(a),X
(b),X
(c),
= Y = C1
= B r , Y = C1
X = Y = Br
121
Procedure
Chrysened,l2-dione (7)[@ (100mg) was passed at 5 x
torr through a quartz tube (12mm diameter, 70mm length)
filled with quartz wool and heated to 960°C. The product
was condensed in a trap cooled with liquid nitrogenr3].Chromatography of the pyrolysis mixture (33 mg) on A1203(activity
grade 3, basic) with light petroleum/benzene afforded 12 mg
(1 5 ”/.) of pure ( 9 ) .
Received: October 27, 1976 [Z 593 IE]
German version: Angew. Chem. 89, 50 (1977)
CAS Registry numbers:
(I), 90-44-8; (2), 86-73-7; ( 3 ) , 82-05-3; ( 4 ) , 206-44-0: (S), 548-39-0; (6),
208-96-8; (7), 19301-82-7; (a), 60921-06-4; ( 9 ) , 248-55-8; (10), 6543-29-9
~ . _ _-
[I]
Flow Pyrolysis and Spectroscopy of Unstable Compounds, Part 4.-Presented in part during lectures at a conference held by the Arbeitsgemeinschaft Massenspektrometrieat Willingen, April 23, at the 7th International
Mass Spectrometry Conference at Florence, August 31, 1976, and at
the 3rd International Symposium on Analytical Pyrolysis at Amsterdam,
September 9,1976. This work was supported by the Deutsche Forschungsgemeinschaft.-Part 3: ref. [3aJ
Aiigew.
Chem. I r i r . Ed. Engl. 16 (1977) N o . I
f 41
If cyclobutadiene ( I jC31
prepared by matrix irradiation of
2,3,4-tri-tert-butylcyclopentadienoneis allowed to react after
thawing with an excess of carbon tetrachloride the product
is found to contain no dimer of ( 1 but exclusively the adduct
[”I
Prof. Dr. G . Maier and Dipl.-Chem. W. Sauer
Fachbereich Chemie der Universitat
Lahnberge, D-3550 Marburg (Germany)
I**] This work was supported by the Fonds der Chemischen Industrie
and the Deutsche Forschungsgemeinschaft.
51
( 3 a ) ['H-NMR (Ccl4): z=6.09 (1 H), 8.60 (18H), 8.83 (9H);
I3C-NMR (CDCIJ: 6=156.9ppm (Cl or C2), 149.5 (C1 or
C2), 101.7 (C5), 82.2 (C3), 68.0 (C4, off resonance: d), 41.7,
34.5, 33.8, 31.4, 30.4, 28.4 (tert-butyl groups)].
The regiospecificity of the addition can be seen from the
structure of the secondary products ( 4 a ) and (Sa). The alcohol ( 4 a ) is formed from ( 3 a ) on standing in air. Heating
or chromatography of ( 3 a ) or ( 4 a ) leads to the stable end
product ( 5 a ) [m.p. 96 "C; 'H-NMR: 5 = 3.68 (1 H), 8.69 (1 8 H),
8.84 (9H); I3C-NMR: 6=213.5ppm, 149.8, 136.2, 133.2 (off
resonance: d), 125.5,44.5,37.7,34.4,32.9,32.3,29.5;
IR (CC14):
1680cm-' ; correct elemental analysis].
The adducts formed by the action of bromotrichloromethane and tetrabromomethane"] (3 b ) and (3 c) are even more
sensitive than ( 3 a)"]. On hydrolysis and subsequent ring
cleavage, (3 b) affords ( 4 a ) and ( 5 a). The CBr4 adduct ( 3 c)
is transformed particularly readily via the alcohol (4 c) into
the acyclic ketone ( 5 c ) [m.p. 103°C; 'H-NMR: ~ = 3 . 1 9(1 H),
8.70 (9H), 8.73 (9H), 8.86 (9H); 13C-NMR: 6=213.3ppm,
149.3, 141.6 (off resonance: d), 138.5, 96.9, 44.7, 37.4, 34.4,
32.8, 32.5, 29.7; IR: 1680cm ; correct elemental analysis].
Thus tri-tert-butylcyclobutadiene( 1 ) undergoes reactions
expected of a diradical. A possible mechanism, formulated
for the reaction with CC14, is seen in the radical chain proceeding via the intermediate (6).
x
x
Since the rectangular singlet and the square triplet state
of ( 1 ) are of similar energy and CCI4 can accelerate the
interconversion of these species by a heavy atom effect"],
it is possible that the reactions observed might involve the
triplet molecule (1 ,ST) in equilibrium with the rectangular
singlet cyclobutadiene ( 1 ,RS). However, the explanation may
lie in the extremely good donor properties['] of ( I ). In analogy
with similar reactions of amines"'] and electron-rich olefinsr1'], an electron transition in (7) with formation of the
P
G. Muier and A . AliPrrecu, Angew. Chem. 85, 1056 (1973); Angew.
Chem. Int. Ed. Engl. 12, 1015 (1973).
G. Lauer, C. Muller, K.-W Schulte, A . Schweig, G. Muier, and A . AlzPrrecu, Angew. Chem. 87, 194 (1975): Angew. Chem. Int. Ed. Engl. 14,
172 (1975).
G. Muier and W Saner, Angew. Chem. 87, 675 (1975); Angew. Chem.
Int. Ed. Engl. 14, 648 (1975).-The "C-NMR spectrum has meanwhile
been recorded: It displays three signals at &=15[.4ppm (C1 +C3),
144.7 (C2), and 126.2 (C4) for the olefinic C atoms in 2,2-dimethylbutane / rz-pentane / 2-methylbutane / n-butyltris(trideuteriomethy1)silane.
Neither the "C-NMR signal for the positions C1 and C3, nor the
'H-NMR signal for the terr-butyl groups located there show any broadening on lowering the temperature t o - 140°C. We have determined
the "C-IH coupling constant as 176.8 Hz.
G. Muier, Angew. Chem. 86, 491 (1974); Angew. Chem. Int. Ed. Engl.
13,425 (1974).
The identity of the compounds of type (3) and ( 4 ) , which could
not be isolated in analytically pure state. is established by their spectral
data ('H-NMR, "C-NMR, IR, MS).
Preliminary experiments suggest that an analogous radical addition
of the two Fragments B r . and .CHBr2 occurs on reaction of ( I ) with
bromoform. In contrast, chloroform does not react; the dimer of ( I )
is obtained on addition thereof.
P . G. Gassman and R . L . Cryherg, J. Am. Chem. SOC.91, 5176 (1969).
have discussed such an effect in connection with the transition of singlet
to triplet nitrenes.~-We are indebted to Professor R . Xeese, Bern,
for this information.
K . G. Hancock and D. A . Dickinson, J. Org. Chem. 39, 331 (1974);
J. R . L. Smirh and Z . A . Malik, J. Chem. Soc. (B) 1970, 920.-We
are grateful t o Dr. J. W. Scheeren, Nijmegen, for drawing our attention
t o these papers.
Very pronounced in tetraaminoethylenes: N . Wiberg, Angew. Chem.
80,809(1968);Angew.Chem.
Int. Ed. Engl. 7,766 (1968); R . W Hojfmunn,
Angew. Chem. 80, 823 (1968); Angew. Chem. Int. Ed. Engl. 7, 754
( I 968).
Only phosphanes also add CCll in the absence of radical initiators:
R . Rahinowiti and R . Murctrs, J. Am. Chem. Soc. 84, 1312 (1962).
Tetraaxial Conformation of
1,2,3,4-Tetra-0-benzoyl-~-~-xylopyranose~**]
By Peter Luger, Gabriele Kothe, and Hans Paulsen[']
During our studies"] on the conformation of P-D-XylOpyranosyl halides in solution and in the crystalline state we
had observed a 1,3-diaxial arrangement of two 0-benzoyl
groups to be particularly favorable from an energetic standpoint. The 3,3-diaxial interaction between two 0-acyl groups
is probably much smaller[21than hitherto
Estimatesc4]based on model compounds belonging to the 1,sanhydropentitol series indicate the interaction between two
0-benzoyl groups to be 0.2 to 0.6 kcal/mol smaller than that
between two 0-acetyl groups; the reason for this difference
has yet to be explained.
H
(81
radical (8) and the trichloromethyl radical is to be regarded
as initiation reaction. Even if this explanation is valid, cyclobutadiene still occupies a special place since the electronic transition does not require additional excitation[12' by light or
metal ions, in contrast to the case of amine/CC14complexes" ".
Received: October 26, 1976 [Z 592 IE]
German version: Angew. Chem. 89,49 (1977)
CAS Registry numbers:
(l),51067-53-9;(2~),56-23-5;(2b),15-62-1; (2~),558-13-4;(3~),60921-41
7; f 3 b ) , 60921-42-8: ( 3 c ) , 60921-43-9; ( 4 u ) , 60921-44-0: ( 4 ~ ) 60921-45-1
,
( 5 a ) , 60921-46-2; ( 5 c ) , 60921-47-3
[l] Small Rings, Part 2O.-Part
19: ref. 121.
[2] G. Muier and H . P . Reisenuuer, Tetrahedron Lett. 1976, 3591
52
[*I Priv.-Doz. Dr. P. Luger and Dip].-Chem. G. Kothe
Institut f i r Kristdllographie, Fachbereich 21 (Chemie) der Freien Universitat
Takustrasse 6, D-1000 Berlin 33 (Germany)
Prof. Dr. H. Paulsen
lnstitut fur Organische Chemie und Biochemie der Universitat
Martin-Luther-King-Platz 6, D-2000 Hamburg 13 (Germany)
[**I Conformational Analysis, Part Ig.-Part 17: P . Luger and H . Putchen,
Carbohydr. Res., in press.
Angew. Chem. Int. Ed. Engl. 16 (1977) N o . 1
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