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Low Temperature Electronic Spectroscopic Observations on Light-Induced Cyclobutanone-Tetrahydrofurylidene Rearrangement.

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diradicals the configuration-preserving effect of through-bond
interaction is overcompensated[*]by the configuration-changing effect caused by formation of the ally1 system.
The stereoisomers (9) and (10) were prepared by conventional
methods and their configuration established non-empirically
by NMR spectroscopy of suitable derivatives[''. UV irradiation of their methanolic solutions at room temperature yields
a binary mixture of acetals in both cases. Chromic acid oxidation of the heterocyclic photoproduct ( 1 I), obtained from
(91, affords exclusively (13) and that of the acetalic photoproduct (12), derived from ( l o ) , yields exclusively ( 1 4 ) . The
configuration of the two y-Iactones follows from the formation
of(l.?)[together with (f5)] from ( 9 ) and of(14) [in addition
to ( I 6)] from ( 1 0 ) on Baeyer-Villiger oxidation.
H\ ,H
C
0
CH3
(fl)
0
Since the four-membered rings of both (9) and (10) are
subject to stereospecific expansion and undergo neither configurational nor constitutional isomerization we conclude that
the resulting "oxucarbenes""ol are formed directly from the
electronically excited four-membered cyclic ketones und not
via intermediate I ,4-alkyl/acyl diradicals.
Received: September 4, 1973:
revised: November 2, 1973 [ Z 959a IF.]
German version: Angew Chem. 86. 197 (1974)
[Sj Depending on the bond lengths used, EH model calculations predict
the allyl-stabilized diradical to be 0.7 to 0.8 eV more stable than the throughbond coupled diradical.
[9] For experimental details see P. Jocobs, Dissertation, Technlsche Universitat Braunschweig 1973.
[lo] The designation "oxacarbenes" has become accepted for cyclic organooxycarbenes.
Low Temperature Electronic Spectroscopic Observations on Light-Induced Cyclobutanone-Tetrahydrofurylidene Rearrangement"]
By Gerhard Quinkert, Klaus H . Kaiser, and WolfDieter
StohrerI'[
Electronically excited cyclobutanones undergo cycloelimination to ethylene and ketene (or their derivatives), decarbonylation to cyclopropyl compounds, and/or isomerization to ringexpanded oxacarbenes (tetrahydro-2-furylidenes)lZ1.
The kinetically unstable oxacarbenes can be trapped by alcohols, olefins, or oxygen to give the usual secondary products[31and
thus constitute experimentally well substantiated transients.
As C, retains the spatial orientation of its ligands on migrating
from the C atom to the 0 atom of the original carbonyl
c h r ~ m o p h o r e [ ~1,4-alkyI/acyl
~,
diradicals (a-0x0-a,w-alkylenes) can be ruled out as intermediates during light-induced
cyclobutanone-tetrahydrofurylideneisomerization; they do,
however, play a plausible role in the genetic interpretation
of the structure of cycloelimination and decarbonylation products provided their formation goes with or follows oxacarbene
formation. Elucidation of the complex reaction mode of electronically excited four-membered cyclic ketones requires the
availability of an experimental method permitting proof of
the existence and of subsequent reactions of the oxacarbenes.
Since these transients should absorb in the same region as
the isoelectronic azo compounds, we have carried out a UV
spectroscopic transient analysis using a low temperature
technique that has proved of value in other cases[51.
It should be noted that 2,2-bis(trifluoromethyl)-3-phenylcyclobutanone ( I ) gives neither the corresponding acetal at room
temperaturel61 nor a photoproduct exhibiting maximum electronic absorption around 360 nmc7]at - 186°C in 2-methyltetrahydrofuran (MTHF). On the other hand, many cyclobutanone derivatives which are at least partially converted into
acetals on irradiation of their alcoholic solutions lead to a
photoproduct, on exposure to light of wavelength > 300nm
in MTHF (and sometimes in other solvent glasses) at - 186 'C,
whose electronic absorption spectrum corresponds to that
of (Z)-azo compounds[71.
[ I ] 15th Communication on Light-Induced Reactions. This work was supported by Farbwerke Hoechst AG, the Deutsche Forschungsgemeinschaft,
and the Fonds der Chemischen lndustrie. P. J . is grateful to Schering AG
for a grant.-l4th Communication: G. Quinkrrt, B. Bronstert, and K . R
Shmirder, Angew. Chem. 84, 638 (1972): Angew. Chem. internat. Edit. 11,
637 (1972).
W - D . Stohrer, P. Jacohs, K . H . Kaiser, G. Wiwh, and G. Quinkrrt,
Fortschr. Chem. Forsch. 46, I S 1 (1974).
[2]
[3] G . Quinkrrt, Angew. Chem. 77, 229 (1965): Angew. Chem. internat. Edit.
4, 21 1 (1965); for unequivocal detection of 1,m-alkyl/acyl diradicals see ref.
~41.
[4j G. L c'loss and C E . Doubleday, J. Amer Chem. Soc. 94, 9248 (19721,
95, 2735 (1973): R . Kaptein, personal communication.
[ 5 ] G. Quinkrrt, G . Cimbollrk, and G . Buhr, Tetrahedron Lett. 1966, 4573;
N . J . Turro and D. M . McDanid, J. Amer. Chem. Sac. 92, 5727 (1970):
D . M . McDanirl and N . J . Turro,Tetrahedron Lett. 1972. 3035.
[6j D. R . Morton and N . J . Xrrro, J. Amcr. Chem. Soc. YS. 3947 (1973).
and further literature cited therein.
[7] R . Hoffmann, Accounts Chem. Res. 4, 1 (1971): and further literature
cited therein.
198
The species concerned ought to be oxacarbenes whose kinetic
stability depends on the substituents present and on the viscosity of the glass matrix. In some
the primary
tetrahydrofurylidene undergoes thermal conversion into a likewise kinetically unstable secondary intermediate. For
example, 2,2,4,4-tetramethylcyclobutanone is rapidly converted into a colorless primary phototransient [Amax= 357 nm
in ethanol/isopentane/ether (EPA)= 5 : 2 :5 ; h,,,=363 nm in
methylcyclohexane/3-methylpentane] by 302 nm light at
[*] Prof. Dr. G. Quinkert, DipLChem. K. H. Kaiser, and Doz. Dr. W.-D.
Stohrer
Institut fur Organische Chemie der Universitat, Laboratorium Niederrad
6 Frankfurt am Main, Theodor-Stern-Kai 7 (Germany)
Angew. Chem. internat. Edit.
Vol. 13 (1974) / No. 3
- 186”C,which slowly transforms into a red secondary thermo
transient (A,,, =437 nm in EPA). The colorless Fluorolube
glass permits observation of a broad ESR signal and the
red solution shows a well structured signal. Further studies
should reveal whether the thermo transient is the 1,4-alkyl/acyl
diradical formed from the oxacarbene.
Received: September 4, 1973:
revised: November 2, 1973 [Z 959b IE]
German version: Angew. Chem. 86,198 (1974)
[ I ] 16th Communication o n Light-Induced Reactions. This work was supported by Farbwerke Hoechst AG, the Deutsche Forschungsgemeinschaft,
and the Fonds der Chemischen Industrk- 15th Communication: ref. [4].
[2] J . C . Dalton and N . .I.
Turro, Annu. Rev. Phys. Chem. 21, 499 (1970).
The designation “oxacarbenes” has become accepted for cyclic organooxycarbenes.
[3] P. Yures, Pure Appl. Chem. 16, 93 (1968).
(41 G. Quinkert, P. Jacobs, and W-D. Stohrer, Angew. Chem. 86, 197 (1974);
Angew. Chem. internat. Edit. 13, 197 (1974).
[S] G. Quinkrrr, Pure Appl. Chem. 33, 285 (1973): and further literature
cited therein.
[6] N. J Turro and D. R . Morton, J. Amer. Chem. SOC. 93, 2569 (1971).
171 W - D . Srohru, P Jacobs, K . H. Kaiser, G. Wiech, and G. Quinkrrt,
Fortschr. Chem. Forsch. 46, 181 (1974).
[8] K . H. Kaiser, Dissertation, Technische Universitat Braunschweig 1973.
in the molecular plane whereas orbital (b2 is transformed
into orbital p at C“. On cr-cleavage in the n*,n state the
ketone having the configuration zx,242xT transforms into
the acyl radical of configuration nY2nz2x:and into the alkyl
radical of configuration p. The antibonding character in the
C”-0 region of orbital (bz, which is occupied by one electron
along the entire reaction coordinate, withstands concerted
oxacarbene formation as a competing reaction to a-cleavage.
In the four-membered cyclic ketone special conditions prevail
in that the two C atoms are not decoupled on stretching
of the C‘-C4 bond: above a certain C’-C4 distance the
initially predominating through-space interaction is replaced
].
of through-bond
by through-bond c o ~ p t i n g [ ~Dominance
coupling is characterized by reversal of the original energy
sequence of orbitals Q 2 and 43l4]. This means that the two
orbitals tend to cross on cleavage of the C’-C4 bond, a
true crossing being forbidden by the pseudosymmetry of the
system (see Fig. 2).
The Peculiar Behavior of Electronically Excited Cyclobutanones: A Photochemical Example of the Consequence of Through-Bond Interaction“. 1’
By WowDieter Stohrer, Gerhard Wiech, and Gerhard Quinkerf[*]
x*,n-Excited common-sized cycloalkanones (m= 5, 6, 7)
undergo ring cleavage to 1 ,m-alkyllacyl diradicals (a-cleavage),
while x*,n-excited cyclobutanones (m= 4) generally undergo
ring expansion to isomeric oxacarbenes (tetrahydro-2-furylid e n e ~ ) [ ~Inl . both cases the photoreaction is initiated by
stretching of the C’-C“ bond. Why d o four-membered cyclic
ketones behave differently from their higher homologs?
Fig. I. Correlation diagram for the cr-cleavage of a cycloalkanone; the cr
orbitals @,, & , +3 of the ketone which participate in the cc-cleavage (or oxacarbene formation) are contructed in a conventional manner from the cr and
cr* orbitals of the CC bond to be broken and the n orbital of the oxygen “lone”
pair.
The correlation diagram for the a-cleavage of “normal”
ketones is depicted in Figure 1. The 0 orbitals $1 and 4 3
of the ketone correlate with the acyl orbitals x, and x; lying
[*] Doz. Dr. W.-D. Stohrer, DiplLChem. G. Wiech, and Prof. Dr. G. Quinkert
Institut fur Organische Chemie der Universitat, Laboratorium Niederrad
6 Frankfurt am Main, Theodor-Stern-Kai 7 IGermany)
Angew. Chem. internat. Edit.
/ Vol. 13 11974) No. 3
Fig. 2. Correlation diagram for the cleavage of the Cl--C4 bond in cyclobutanone
For a small stretching distance C’-C4, the highest occupied
0 orbital, as in the “normal” ketones (Fig. l), has 42 character:
the region C’-C4 is bonding while C4-O is antibonding.
For a large stretching distance C1-C4 the highest occupied
0 orbital has $ 3 character: the area important for oxacarbene
formation, C4-0, is bonding and C1-C4 is antibonding.
These perturbational considerations are confirmed by EH
calculations[s1for the correlation diagram of Figure 21h1.Thus
extension of the C’-C4 bond in an electronically excited
cyclobutanone creates a situation in which the highest (in
the x*,n state singly) occupied u orbital, in contrast to “normal
ketones”, has bonding character in the region C4-O and
therefore favors oxacarbene formation.
This qualitative M O analysis neither requires a 1,4-alkyl/acyl
diradical as an intermediate nor rules such a species out,
in contrast to stereochemical studiesf3];everything will depend
on the structural scenery in which the transition from a higher
to the lowest energy surface takes place (see ref.”]).
_-_
Received: September 4, 1973:
revised. November 2. 1973 [Z 959, IE]
German version: Angew. Chem. 86. 199 (1974)
[ I ] 17th Communication on Light-Induced Reactions. This work was supported by Farbwerke Hoechst AG, the Deutsche Forschungsgemeinschaft,
and the Fonds der Chemischen Industrie. The calculations were performed
a t the Computing Center of Frankfurt University. - 16th Communication:
G. Qirinkcrl, K . H . Kaiser, and W-D. Srohrer. Angew. Chem. X6, 198 (1974):
Angew. Chem. internat. Edit. 13,198 (1973).
199
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spectroscopy, rearrangements, induced, low, electronica, observations, temperature, cyclobutanones, light, tetrahydrofurylidene
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