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Kinetic Spectrophotometry of Relaxation Systems Containing 7 8-Polyphenyl-Substituted o-Quinodimethane Derivatives.

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(3)
Substances whose concentration remains constant throughout
the reaction (e.g. the solvent) can be accounted for formally
by assignment of a reaction coefficient of zero.
E and E; depend upon the wavelength. They have the values
E, and E,,~ and E2 and E ; , ~ at the wavelengths A , and h, respectively:
(4)
Eq. (4) is the parametric representation of a straight line from
which X can be eliminated:
Thus it is established that E, and E2 are connected by a linear
relation.
In practice the converse of this proof is more important, i.e.
deduction of the number of reaction variables from the linearity
of the observed function. The converse applies when the slope of
the line is negative. Provided that the spectra are continuous,
this slope is a continuous function of h, and A,. If, e.g., h , approaches h,, the slope assumes all values between a negative
amount and one, and must therefore be zero at one wavelength
at least. However, a straight line of zero slope corresponds to
an isosbestic point whose existence is thus proved indirectly.
a) Simultaneous extinction measurements at two wavelengths.
Suitable spectrophotometers are commercially available.
b) Alternate measurements at two wavelengths with interpolation of the missing values. This method has proved very
effective in our laboratory. About 10 alternations are sufficient
for the half-life of a reaction. With a Beckman DB-G instrument it is possible to switch over once every second provided
that the two wavelengths are not too far apart. The interpolation
can be carried out graphically or numerically.
c) Stopping the reaction. This method is of greatest interest
for photochemical reactions where samples can be taken at
regular intervals from the irradiation vessel, the reaction being
interrupted in the samples themselves.
d) Repetition of the reaction. The extinction is first measured
at one wavelength, and then at a second wavelength for a reaction mixture that is as similar as possible to the first. This is
not recommended in the case of slow reactionssince kinetic data
are often not very accurately reproducible and poorer results
are obtained than with method b) for doubie the experimental
effort. We have successfully employed the technique for fast
reactions which can be reproduced several times in a short period and for which it is impossible to switch over the monochromator.
Recewed: November 26, 1970 [Z 348 IE]
German version: Angew. Chern. 83, 2i4 (1971)
[*I
Dr. Ch. Chylewski
Ciba-Geigy Photochemie AG
CH-1701 Fribourg (Switzerland)
[I] G. Kortum;Kolorimetrie, Photometrie und Spektrometrie.Springer,
Berlin 1962, p. 32.
[2] H. L. Schlaferand 0. Kling, Angew. Chem. 68, 667 (1956)
[3] Xhas the dimensions of a concentration.
Kinetic Spectrophotometry of Relaxation Systems
Containing 7,8-Polyphenyl-Substitutedo-Quinodimethane Derivatives[']
By Karl-Heinz Grellmann, Joachim Palm0 wski, and Gerhard
Quinkertyl
o-Quinodimethanes have only a young history as experimentally
confirmed intermediates. Kinetic[*] and ~tereochemical[~]
investigations require, and low temperature (< - 180°C) electronic ~ p e c t r a [ ~ establish,
>'~I
their part as kinetically unstable
seco isomers of benzocyclobutenes of the appropriate constitution and configuration. Results of flash photolysis experiments at temperatures between - 53 and 23 O d 6 1 provide
information about other modes of formation, lifetimes, and reactions of these species.
-200
006
[
Z
m
008
010
012
014
Ex,-
Fig. Example of a reaction that proceeds in two stages, each having one
rate-determining reaction step (reduction of methyl orange by 1,2-dihydro-2,3-dimethylquinoxalinein excess at pH 0).
Ph
If the line observed has a positive slope, this suggests, but does
not prove, that the reaction has only one reaction variable. The
line is particularly uninformative if its extrapolation passes close
to or through the origin. In such a case it is highly probable
that it is mainly the same species that is absorbing at both the
wavelengths chosen. A straight line then clearly results purely
on the basis of Beer's law, independently of the stoichiometry
of the reaction.
Ph
Ph
Ph
Measuring technique and applications: When the parameter to
be varied is not the time, the successive measurement at two
wavelengths gives rise to no difficulties. On the other hand,
four possible methods are available for a kinetic study:
196
Angew. Chrm. Internat. Edii.
I
Vol. I0 (1971) / N o . 3
Thermal cyclization of 7,7,8,8-tetraphenyl-o-quinodimethane
affords, depending on the temperature used, the cyclo isomers
(I) or (3), which have been interpreted as being the thermodynamically or kinetically determined products, respecti~ely[~I.
Flash photolysis of (1) and (3) (Fig. 1) reveals the presence
of the common intermediate (2); (2) rapidly cyclizes to (3)
[ r = 3 . 6 ~lo-% at 23°C; Arrhenius activation parameters: E =
15.3 kcal/mole, A = 3 . 2 ~ 1 0 s'-~
l (measured between -53
and 23"C)], while the formation of (1) from (3) is relatively
i
ai
bi
1cm
w
ai
bl
1cm
Cl
w
Fig. 2. Oscilloscope traces after flashing a 2.67
(5) in methylcyclohexane at 2 1 6 ° C [7a].
----t---'-i-i
dl
Fig. 1. Oscilloscopic traces after flashing a 1.66 X
M solution of ( I )
in methylcyclohexane [7a].
a) ( T = 23°C; f = 0 . 5 m s / c m ; I = 4 5 0 n m ; S = 1.0Vicm):Theprimary
product (2)formed by light-induced cycloiseco isomerization, which,
unlike the reactant ( I ) absorbs at 450 nm [4], decays in a thermally
induced first order seco/cyclo isomerization to give the secondary product (3) (k(2)+c3) = 2.8 x 104s-') which does not absorb above
300 nm [4].
b) (7= 23°C. t = 0.05 ms/cm: h = 4 0 0 nm; S = 5.0 V/cm): Light
induced cycloiseco isomerization of the reactant ( I ) leads to the primary
product (2) which absorbs weaker at 400 nm and which undergoes a
thermally induced first-order secoicyclo isomerization to give (3)
(k(21,c3) = 2.8 x lo4 SKI).
c) ( T = 2 3 T , t = 1000 msicm; h = 400 nm, S = 0.5 V/cm): The secondary product (3) is completely reconverted in a two-stage first order
reaction via (2) into the original reactant (1); k(3)-(I) = 0.385 SKI.
d) ( T = -35°C: t = mslcm; A = 400 nm; S = 0.5 Vlcm): Compound
(3) prepared from (1) by irradiation is kinetically unstable at room temperature [4] and is converted in a light-induced cycloiseco isomerization
into (2) which absorbs at 400 nm and shows the same spectral data [7b]
as the primary product formed from (i) by light-induced cyclolseco
isomerization, and which regenerates (3) in a thermally induced firstorder seco/cyclo isomerization (k(2),,i3) = 57 s -I).
According to their electronic absorption spectra the light-induced ring opening of the configurationally isomeric benzocyclobutene derivatives (4) and (5) (with 254-nm light) at
- 189°C roceeds non-stereospecifically, although stereoselectivelyf51.All three configurationally isomeric o-quinodimethane derivatives (6), (7), and (8) participate in the reaction[*];
their relative proportions depend on the viscosity of the
medium. Flash photolysis of (4) and (5) at 21.6"C (Fig. 2)
reveals three distinct decay processes. A comparison of the absorption spectra of those o-quinodimethanes ']obtained either
directly using low-temperature techniques at -186°C or by
point by point scanning at 21.6"C using kinetic spectrophotometry indicates that (6) (t = 19 ms; E = 15 kcalimole) and
(7) (T = 690 ms; E = 17 kcal/mole) cyclize to the benzocyclobutenes (5) and (4)lsl in order of their decreasing rates;
at21.6"C(6)and(7)areforrnedfrom(4)or
(5)in 1 3 a n d 8 7 %
or 26 and 74% yield, respectively['OJ.
Electronic spectroscopic control experiments show[13]that the
o-quinodimethane derivatives (6) and (7) also occur in the
photodecarbonylation of the confi urationally isomeric 2-indanone derivatives (9) and (10)[12fat - 189°C. According to
flash photolysis experiments, the same is true for the lightinduced CO elimination at 21.6"C; (6) and (7) are formedfrom
(9) or (10) in the ratios of 54:46 or 47:53 respectively['*]
(Fig. 3).
Angew. Chem. internat. Edit. / Vol. I 0 (1971) / N o . 3
M solution
X
of
a) ( t = 10 msicm; h = 440 nm; S = 1.OVicm): Llght-inducedcycloiseco
isomerization of (5) [l l a ] leads to formation of (6) which rapidly undergoes first order decay (k = 52.5 s - l ) [ l l b ] .
b) ( t = 500 msicm; h = 440 nm, S = 0.5 Vicm): Compound (7), which
also decays with first order reaction kinetics, but slower ( k = 1.5 s - I )
than (6), is also formed [llb].
c) ( t = 500 mslcm; h = 250 nm: S = 0.5 V/cm): A third decay process
of minor importance takes place which again proceeds via a first order
reaction: the species formed, which cannot be detected by flash spectroscopy, has a halflife of about 20 s.
1cm
ai
f
f
bl
Ci
Fig. 3. Oscilloscope traces after flashing a 2.44 X
M solution of
M solution of (9) (c) in methylcyclohex(10)(a and b) and a 3.49 X
ane at 21.6"C [7a].
a) ( I = 10 msicm; h = 440 nm; S = 0.5 Vlcm): Decay curve (first order)
of (6) formed from (10) by photodecarbonylation (k = 52.5 s-'; E =
15 kcalimole; Amax = 450 nm).
b) ( t = 500 msicm, h = 440 nm; S = 0.2 V i m ) : First order decay of
(7) which is formed in practically the same amount as (6) ( k = 1.5 S - I ;
E = 17 kcal/mole; Amax = 445 nm).
c) ( t = 10 msicm; h = 440 nm; S = 1.0 V/cm): First order decay of
(6) formed from (9) by photodecarbonylation ( k = 52.5 s-': E = 15
kcalimole; I,,, = 450 nm) [14].
Received. December 23, 1970 [2 344a I€]
German version: Angew. Chern. 83, 209 (1971)
["I Dr. K. H. Grellmann
Max-Planck-Institut fur Spektroskopie
34 Gottingen, Bunsenstrasse 10 (Germany)
Prof. Dr. G. Quinkert and Dr. J. Palmowski
Institut fur Organische Chemie der Universitat
6 FrankfurrlM., Robert-Mayer-Str. 7-9 (Germany)
197
[ l ] This work was supported by Farbwerke Hoechst AG, the Deutsche
Forschungsgemeinschaft, and the Fonds der Chemischen Industrie.
[Z] R. Huisgen and H. Seidel, Tetrahedron Lett. 1964, 3381.
131 G. Quinkerr, K. Opitz, W.-W. Wiersdorff, and M. Finke, Tetrahedron Lett. 1965, 3009; Liebigs Ann. Chem. 693, 44 (1966).
[a] G. Quinkerr, W.-W. Wiersdorff, M. Finke, K . Opitz, and E-G. von
der Haar, Chem. Ber. 101, 2302 (1968).
(a), R = H
(b), R = CH,
(c), R = D
[ 5 ] G. Quinkert, M. Finke, 1.Palmowski, and W.- W. Wiersdorff, Mol.
Photochem. 1, 433 (1969).
&CH3
[6] Performance and evaluation see K. H. Grellmann, E. Heilbronner,
P. Seiler, and A. Weller, J. Amer. Chem. SOC.90, 4238 (1968).
(8)
Ph H
[7a] The first portion of the curve shows the position of the base line,
upward (downward) deviations correspond to a decrease (increase) in
the transparency. 7 = temperature; t = sweep rate of trace; h =
wavelength at which decay curve was recorded; S = sensitivity ( Vo =
10 V);T = l/k. - The time-dependent photomultipliersignals Vo(before
flashing)and V = vf (at time f after flashing) are proportional to the intensity [of the signal. The change in optical density A E = A E ( f )is A E ( f )
= log IolJ = log vo/
v,.
[7b] Low-temperature spectroscopic data [4]: h,,, (E) = 284 (30600)
relative
and 519 nm ( 9 1 7 0 ) , ~ ~ =~ 3.34;flashspectroscopicdataof
,/~~~~
= 290 and 505 nm, E
~ ~= 3.3.
~ / E ~ ~
intensity: h,,
[8] See footnote 14 in IS].
191 The photo product obtained from (4) at -189°C [5] has an absorption maximum at 465 nm (E= 10600); warming to -178°C results
in an irreversible shift of the maximum to 455 nm without change in
intensity. This observation concerns that portion of the o-quinodimethane derivatives able to photocyclize and is compatible with a Z / E
isomerization of (8) to (7).
[lo] The quantitative results are based on the observation that the absorption of (6) at 455 nm has almost twice the intensity (20100) of that
of the other configurational isomers.
[ 1 la] The flash photolysis of (4) shows- apart from a slight participation
of (6) - analogous behavior.
[ 1Ib] According to low-temperature spectroscopic (flash spectroscopic)
data recorded in methyicyclohexane/3-methylpentane,4 : 1 (methylcyclohexane), the long wavelength absorption maximum of the thermally
most labile o-quinodimethane derivative (6) unable to photocyclize [5]
appears at 455 nm 191 (450 nm at 21.6"C); (7) shows a maximum at
455 (445) nm.
112) G. Quinkert, H.-P. Lorenz, and W.-W. Wiersdorff, Chem. Ber.
102, 1597 (1969).
~
Educt
Conversion Hydrocarbon fraction [a] Ketone fraction [a]
(Yo)
Ratio of isomers (Ye)
Ratio of isomers (%)
[13] G. Quinkerf, J. Palmowski, H.-P. Lorenz, W.-W. Wiersdorff, and
M. Finke, Angew. Chem. 83,210 (1971); Angew. Chem. internat. Edit.
(2aJ
Non-Cheletropic Photodecarbonylation of 1,3Diphenyl-Substituted 2-Indanone Derivatives"]
By Gerhard Quinkert, Joachim Palmowski, Hans-Peter Lorenz,
Walter- Wielant Wiersdorff, and Manfred Finkel']
Unconjugated cyclopentenones undergo smooth decarbonylation, both in their electronic ground state['] and after x * c n
t o give butadiene derivatives. If the thermal
decarbonylation proceeds in o n e step, the correlation diagrams
demand a stereospecific, in the present case disrotatory,
c o ~ r s e [ ~ ~T h~e~ ]stereochemistry
.
of the corresponding
photodecarbonylation, which has been discussed under different, in part contradictory, premises, is a przori unclear. Two
shortcomings a r e responsible for this, vjz. insufficient knowledge of the photoreactive state and lack of a conclusive theory
of light-induced reactions.
so so
R Ph
Ph R
(3)
(4)
R
198
( l a ) : 87, (2aJ: 13
( l a / .58; @a) 42
( l a ) 36; (20) 64
(3aJ. 8; (4a): 92
13a) : 10; (4aJ-90
m a ) : 11; (5a): 89
10, 198 (1971).
[ 141 In this case compound (7), which occurs in the same way, was de= 450 nm).
termined only flash spectroscopically (A,
70
&
1 ;;
100 Ib]
100 [b]
(3a) : 18; (4a). 82
(30) : 17; (4a). 83
(3a): 13; (40). 87
(3b) : 69;
(3b) : 70;
(36). 69;
(36) : 29;
exclusively (20)
(4b). 31
(46): 30
(461: 31
(4b): 12; (8J.59
(3bJ . 8; (46): 92
(36) : 10; (4b): 90
exclusively ( l b )
1
exclusively (Zb)
I
[a] The irradiation products were analyzed by gas chromatography IS]
and by NMR spectroscopy [ 121after treatment with an excess of LiAIH,
and chromatographicseparation of the alcohols from the hydrocarbons;
no effect o n the conversion
the presence of piperylene ( 0 . 7 5 ~ showed
)
and the product composition.
[b] Irradiation at -78°C.
T h e Table contains data of the decarbonylation of (la), (lb),
(a), and (Zb)[" in a merry-go-round apparatus[']. In each case
a mixture of configurational isomers of four-membered ring
hydrocarbons (3) and (4) occur, the composition of which
remains unaffected by the presence of the ketone triplet quencher piperylene. Simulation of a trivial non-stereospecificity by
photostereomutation of the benzocyclobutene derivatives does
not occur in direct irradiation or sensitization["]; in contrast
to ( l a ) a light-induced configurational isomerization of the ketonic reactants (lb), (Za), and (Zb) does not come into consideration. Although it has been established, for example, that (3a)
Angew. Chem. internat. Edit. / Voi. 10 (1971) / N o . 3
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containing, quinodimethane, kinetics, polyphenyl, system, relaxation, substituted, spectrophotometry, derivatives
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