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Increase in Selectivity of Photoelimination Products on Replacing the Liquid Phase by the Crystalline Phase.

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and (4a) are formed stereospecifically['21 from the corresponding seco isomers also detected a t room temperature"'], a thermal and/or photo configurational isomerization of the intermediate compounds under these conditionscannot b e ruled out.
T h e observed non-stereospecificity is therefore irrelevant for
the course of the reaction in question. However, the fact that
photostereornutation and photodecarbonylation in (la) result
from one and the same electronically excited singlet state verifies the stepwise, noncheletropic character of the light-induced
CO elimination in the case of the cyclopent-3-en-1-one system
investigated" '1.
Received December 23, 1970 [Z 344b IE]
German version: Angew. Chem. 83, 210 (1971)
Prof. Dr. G. Quinkert [**I, Dr. J. Palmowski [**I,
Dr. H.-P. Lorenz [***I, Dr. W.-W. Wiersdorf [****I,
and Dr. M. Finke [*****I
Institut diir Organische Chemie der Technischen Universitat Braunschweig
[**I Present address: Institut fur Organische Chemie der Universitat
6 Frankfurt/M., Robert-Mayer-Str. 7-9 (Germany)
[***I Present address: Schering AG, Bergkamen (Germany)
[****I Present address: Badische Anilin- und Soda-Fabrik AG, Ludwigshafen (Germany)
[*****I Present address: Farbwerke Hoechst AG, Frankfurt-Hoechst
(Germany)
[I] This work was supported by Farbwerke HoechstAG, the Deutsche
Forschungsgemeinschaft, and the Fonds der Cbemischen Industrie.
[2] a) C E H. Allen, Chem. Rev. 37, 209 (1945); 62, 653 (1962);
b) J. E. Baldwin, Can. I. Chem. 44, 2051 (1966).
[3] a) L. D. Hess and 1.N. Pitts, jr., J. Amer. Chem. SOC.89, 1973
(1967); b) G. Quinkert, K. Opitz, W.-W. Wiersdodf, and J. Weinlich,
Tetrahedron Lett. 1963, 1863; c) B. Fuchs, Israel J. Chem. 6,511, 517
(1968); J. Chem. SOC.C 1968, 68.
[4] a) R. B. Woodward and R. Hoffmann, Angew. Chem. 81, 797
(1969); Angew. Chem. internat. Edit. 8, 781 (1969); Science 167, 825
(1970); b) D. M. Lemal and S. D. McGregor, J. Amer. Chem. SOC.88,
1335 (1966); c) H.-P. Lorem, Dissertation, Technische Universitat
Braunschweig 1968.
[*I
tion, of molecular motion. Investigations carried o u t a t the
Weizmann Ir1stitute[*,~1suggest that the crystalline phase is a
suitable medium for photoelirninations as it allows entry of light
and escape of gaseous products.
A considerable increase of the cage effect, and consequently
of the constitutional selectivity, is observed for the
by
photodecarbonylation of 1,1,3-triphenyl-2-pr0panone[~~l
substituting the crystalline phase for the (benzene) solution. T h e
extent of the cage effect given by the ratio:
a=
2M[1,1,2-triphenylethane]
-
By Gerhard Quinkert, Taka0 Tabata, Eckhard A. J. Hickmann,
and Walter Dobrati']
T h e observation that photodecarbonylation of 1,3-diphenyl
substituted 2-indanone derivativesr2] does not take place in a
single step['] indicates a n intermediacy of biradicals, which, because of internal rotations and inversion a t radical centers comprise whole groups of subspecies. Experience has shown that in
such cases the preparatively as well a s mechanistically useful
product selectivity increases with increasing viscosity of the
medium[4]. T h e crystal lattice of a solid is expected t o have a
particularly strong influence o n the rate, and possibly the direc-
Angew. Chem. internat. Edit. / Vol. 10 (1971) / N o . 3
mde equivalent.
Ph -R
R Ph
R
Ph
(4)
(3)
(a), R = H
(b). R = CH,
:CH,
.Ph
(c), R = D
H
Table. Data of the decarbonylation of (la), (lb), (Za), and (2b)in solution and in the crystalline phase.
Percentage composition in
u
Solution [a]
Cryst. phase [b]
I
(3a): 13
(4a) : 87
Increase in Selectivity of Photoelimination Products on Replacing the Liquid Phase by the Crystalline Phase[']
=
increases from t h e one limiting value a = 0 in solution
(1,2-diphenylethane, 1,1,2-triphenylethane, and 1,1,2,2-tetrat o the other limiting
phenylethane occur in the ratio 1 :2 :
value a = 1 in t h e solid (1,1,2-triphenylethane is formed exclusively). T h e constitutional selectivity for the photodenitrogenation of diphenylmethylazotoluene changes less drastically;
practically only 1,1,2-triphenylethane occurs in the light-induced N, elimination in the crystalline phase a t -196"C['],
while o n irradiation of a benzene solution a t 27°C a falls only
t o 0.28[81. 'This finding is in accordance with t h e higher kinetic
stability of an acyl radical relative to the isoelectronicdiazoalkyl
radical[9].
[ S ] G. Quinkert, H.-P.Lorenz, and W.- W. Wiersdorff, Chem. Ber. 102,
1517 (1969).
[6] G. Quinkert, M. Finke, J. Palmowski, and W.- W. Wiersdodf, Mol.
Photochem. 1, 433 (1969).
[7] For the analysis of the irradiation product regarding photocyclizable
and non-photocyclizable o-quinodimethane derivatives see [6].
[Sl See footnote 14 in Ref. [6).
[9] B. Bronsrerr, Diplomarbeit, Technische Universitat Braunschweig
1968.
[lo] Irradiation of mixtures of ( l c ) and (3a) or of (lc) and (4)with
313-nm light does not result in stereomutation of the hydrocarbonsused.
[ 111 K. H. Grellmann, J. Palmowski, and G. Quinkert, Angew. Chem.
83, 209 (1971); Angew. Chem. internat. Edit. 10, 196 (1971).
[I21 G. Quinkerf, K. Opitz, W.-W. Wiersdorff and M. F i n k , Liebigs
Ann. Chem. 639,44 (1966).
[13] Premise for positive conclusions from quencher experiments: If
the energy hypersurface contains several triplet conformations they all
must be quenchable.
1; M
M[reactantl
Oaj. 5
(4a) 95
(36) : 69
(4b) : 31
061.90
(4bj: 4
(5): 6
(3b): 9
146) .91
(36) : 14
(4bj: 86
[a] The products obtained by irradiation of an ethereal solution at
13i 2°C with 313-nm light were analyzed by gas chromatography I121
and by NMR spectroscopy[Zb] after treatment with an excess of LiAlH4
and subsequent chromatographic separation of the alcohols from the
hydrocarbons.
[b] Irradiation was carried out in a Rayonet reactor (300-nm lamp;
Southern Ultraviolet Co., Middleton, Conn.) using afilter (2.5 % aqueous
solution of potassium hydrogen phthalate; layer thickness 8 mm) in a
Duran vessel of wall-thickness 1 mm at 13C 2°C. For a detailed analysis
of the irradiation products see [a]; in none of the cases was a stereomutation of the educt observed.
199
As a result of the restricted internal mobility of the intermediate
species formed from the confi urationally isomeric ketone pairs
(la)l(Za) and (1b)i(2b)1’zf on UV irradiation, the configurational selectivity of the subsequently isolated benzocyclobutene derivatives (3) and (4) increases considerably
on changing from the solution to the crystalline phase as a reaction system and in one case even changes its direction.
Received: December 23, 1970 [Z 344c IE]
German version: Angew. Chem. 83, 212 (1970)
[‘I
Prof. Dr. G. Quinkert [**I, Dr.T. Tabata, Dr. E. A. J. Hickmann [**I,
and Dr. W. Dobrat
Institut fur Organische Chemie der Technischen Universitat
Braunschweig (Germany)
[“‘I Present address: Institut fur Organische Chemie der Universitat
6 FrankfudM., Robert-Mayer-Str. 7-9 (Germany)
[I] This work was supported by Farbwerke Hoechst AG, the Deutsche
Forschungsgemeinschaft, and the Fonds der Chemischen Industrie.
[2] a) G. Quinkert, K . Opitz, W.- W. Wiersdorft and J. Weinlich, Tetrahedron Lett. 1963, 1863; b) G. Quinkert, K. Opitz, W.-W. Wiersdorff,
and M. Finke, Liebigs Ann. Chem. 693, 44 (1966); c) G.Quinkerf,
W.- W. Wiersdorff, M. Finke, K. Opitz, and F.-G. von derHaar, Chem.
Ber. 111, 2302 (1968).
[3] G. Quinkert, J. Palmowski, H.-P. Lorenz, W.-W. Wiersdorff. and
Finke, Angew. Chem. 83,210 (1971); Angew. Chem. internat. Edit.
10, 198 (1971).
[4]P. D. Barfleft and 1.M.McBride, Pure Appl. Chem. 15,89 (1967).
[ 5 ] G. M. 1.Schmidt in: Proceedings of the Thirteenth Conference on
Chemistry at the University of Brussels, October 1965; Reactivity of
the Photoexcited Organic Molecule. Interscience Publ., New York 1967.
[6] M. D. Cohen, Pure Appl. Chem. 9, 567 (1964)
[7] Low temperatures were chosen in order to guarantee the crystalline
character of the irradiated sample; at -26°C an additional compound
was formed ( 4 9 ) which was not further investigated.
181 a) No cage effect is observed to take place in the thermolysisof diphenylmethylazotoluene at 120°C. either in the melt or in benzene solution.
b) The result of a nuclear spin polarization investigation by G. L. CIoss
and A. D. Trifunac[J. Amer. Chem. SOC. 91, 4554 (1969)] indicates a
factor a = 0.2.
[9] Stepwise decarbonylation is typical even for constitutionally symmetrical ketones [lo]; stepwise denitrogenation has been detected only occasionally in the case of constitutionally asymmetrical azo compounds [ l l ] .
[lo] W. K. Robins and R. H. Eastman, J. h e r . Chem. SOC.92, 6077
(1970).
1111 S. Seltzerand F. T. Dunne, J . Amer. Chem. SOC.87,2628 (1965).
[12] G. Quinkerf, H.-P. Lorenz, and W.- W. Wiersdorff, Chem. Ber.
10.7, 1517 (1969).
CONFERENCE REPORTS
Present Trends in Polyurethane Chemistry
By Helmut Piechota[*]
The discovery of the diisocyanate polyaddition process for the
production of macromolecular materials has led to a wide variety of valuable synthetic materials within a little more than
30 years: PUR elastomers, which can be cast or sprayed or,
alternatively, injection molded o r extruded, are commonly
encountered nowadays. The same applies to urethane lacquers and varnishes, to PUR coatings for fabrics, to PUR synthetic leather, to adhesives, and to PUR synthetic fibers and
foams. (PUR has become an accepted abbreviation for polyurethane.)
The production of PUR raw materials (polyisocyanates, polyethers, polyesters), which is nowadays carried out on a full industrial scale, requires an accurate knowledge of the structural
parameters that determine the properties of the resulting polyurethane. The starting materials are made to react in one step
or by the prepolymer process under strictly controlled conditions to give polyurethane. By suitable choice of raw materials
and control of the reaction’it is possible to determine the properties of the PUR product in advance: polyurethanes are “tailor
made” plastics.
with a core of rigid PUR foam, will become increasingly important and many examples of its industrial application can
already be given. Increasing use in building construction raises
the question of fire prevention: Recent developments have resulted in the trimerization of polymeric diphenylmethane diisocyanate with simultaneous foaming. The polyisocyanurate
foams thus produced are flame retardant and comply with the
German industrial standard DIN 4102.
PUR structural foams are produced by foaming in place. By
taking particular chemical and process-engineering measures
it is possible to produce, from a single reaction mixture in a onestep operation, a molded article having a massive skin, which
is gradually transformed towards the microporous core. Soft
and semirigid structural PUR foams are widely used for automobile safety fittings (e.g. arm rests, crash pads, bumpers) and
also for lightweight shoe soles that can be foamed directly onto
the shoe shaft. Owing to their sandwich structure, rigid PUR
structural foams represent novel materials of construction that
are suitable for the production of large-surface, high-volume
moldings. These materials have acquired an important position
in the furniture industry, in the manufacture of casings, and in
the sports equipment industry.
The most important FUR products are foams, which can be
produced in all degrees of hardness. In 1970 world production
exceeded lo6 tons. Flexible PUR foams in the form of continuously produced slabs or foamed in place are used as upholstering materials, for mattresses, automobile seats, etc. Recent
developments include foams that are self-extinguishing under
defined test conditions, as well as “cold foams”. The latter are
used in the manufacture of molded materials having properties
resembling those of latex foams. They have been rapidly accepted, e.g. for the production of automobile seats and arm
chairs.
There is probably no other field in the plastics industry where
chemistry and process engineering are so closely linked as in
the polyurethanes. From the chemical point of view increasing
attention is being devoted to other reactions of the isocyanate
group (e. g. trimerization), while in process engineering new
molding techniques are being sought (e.g. structural foams).
This leads to new materials with novel properties for new applications (e. g. agricultural foams). 30 years of PUR chemistry
clearly represents not a final goal but a milestone on the way
of continuing progress.
Rigid PUR foams are excellent insulating materials. Practically
all European household refrigerators are insulated with such
materials, as are Jso refrigerated vehicles, cold stores, district
heating supply pipes, flat roofs of buildings, erc. In the future,
the sandwich technique, i e . the combination of rigid facings
[“I
200
Lecture at Dortmund on December 15, 1970 ,[VB266 IE]
Germanversion- Angew. Chem. 83, 179 (1971)
Dr. H. Piechota
Farbenfabriken Bayer A G
SO9 Leverkusen (Germany)
Angew. Chem. internat. Edit. / Vol. 10 (1971) / N o . 3
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