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Low-Temperature Photolysis of Cyclopentadiene.

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Low-Temperature Photolysis of Cyclopentadiene
By Dr. E. H. Gold and Prof. Dr. D. Ginsburg
Department of Chemistry,
Israel Institute of Technology, Haifa (Israel)
Dedicated to Professor K . Freudenberg on his 80th birthduy
The photolysis of cyclopentadiene at ca. -1OOC in the
presence of ketonic sensitizers is reported [21 to afford the
products (I)-(3) in equal proportions. We found that photolysis at -78OC with either acetone or benzophenone as
sensitizer [31 yields in addition to (1)-(3), the compounds
(4) and (6), and various polymers of cyclopentadiene.
between C-2 and C-3. This is the structure to be expected
according to the Biichi mechanism [61.
( 5 ) : IR: 961 cm-1; N M R : T = 5.05 (q, H-5.H-1, H-2), 7.42
(9, H-3), ca. 8.2 (m, H-4), 8.61 (s, CH3), 8.93 (s, CH3),
relative areas: 0.96: 0.96: 12.1.
( 6 ) : UV: hmax = 248 m p (E = 2730) in ethanol; N M R : T =
3.63-4.37 (m, olefinic H,) 6.3 (m, allylic tert. H), 7.18 (m,
cyclopentadienyl H), 7.5-8.6 (m, aliphatic H), relative areas:
4.8:0.95:2.2:4.0. Hydrogenation of (6) over Pt (3 moles of
H2) affords only bicyclopentyl. With tetracyanoethylene, (6)
forms only one adduct (yield 70-80 %), the N M R spectrum
of which shows four olefinic and two tertiary allylic hydrogen atoms.
Received: October 7th, 1965
[ Z 103/931 IEI
German version: Angew. Chem. 78, 207 (1966)
Publication delayed until n o w at the authors' request
Irradiation[3bl of 30 g of a mixture consisting of cyclopentadiene plus 10 % by weight of acetone under Hammond's
conditions [zal (ca. -10 "C, 2 h) yields the compounds
(1)-(4) and (6) in a weight ratio of [ ( I ) (2) (3)]:(4) :(6)
= 1 1 : 11 : 1, whereas at ca. -78 "C [31 the ratio is 3 : 14: 1. On
irradiation of a 1 : 1 mixture of cyclopentadiene and acetone
at cu. -78 "C, the product ratio is 2.4: 8.5 : 1.
+
+
[I] This work was supported in part by Grant No. F2-MH-10,498
of the National Institutes of Mental Health of the Department
of Health, Education and Welfare of the U.S.A.
[2] a) G. S. Hammond, N. J. Turro, and S. H . Liu, J. org. Chemistry 28, 3297 (1963); b) N. J. Turro and G. S. Hammond, J.
Amer. chem. SOC.84, 2841 (1962).
[3] a) Hanau Q 81 high-pressure lamp with pyrex filter; b) Hanovia 450 watt high-pressure lamp with corex filter.
[41 Determined by gas chromatography.
[ 5 ] H . D.Scharfand F. Korte, Chem. Ber. 97, 2425 (1965).
[61 G. Buchi, C . G . Inman, and E. S. Lipinsky, J. Amer. chem.
SOC.76,4327 (1954).
Bis(2,5-di-t-butylphenyl)methyl,
a Non-associating Diarylmethyl
By Prof. Dr. W. Theilacker and Dipl.-Chem. F. Koch
Institut fur Organische Chemie
der Technischen Hochschule Hannover (Germany)
Dedicated to Professor K . Freudenberg on his 80th birthday
A temperature effect is also observed on photolysisr3al of
cyclopentene/acetone (1 : 1 w/w): at -78 "C the oxetane (5)
is formed in 28 % yieldr41, whereas the yield is only 16 "/,
at room temperature [3ap51.
When 35 g of cyclopentadiene is irradiatedc3bl with 5 g of
benzophenone for 2.5 h at ca. -78"C, no oxetane (4) is
(3)]:(6) = 2: 1
formed, and the product ratio is [ ( I ) + (2)
(cu. 13 % overall reaction). This ratio increases upon further
irradiation with concurrent polymer formation. That these
polymers are formed from (6) is also shown by the fact that
the ratio of olefinic to aliphatic hydrogens (NMR) is 1 :2 for
both types of compounds. At ca. -1O"C, the product ratio
[ ( I ) + (2) (3)]:(6) = 11.5:l in good agreement with the
results of Hammond, who obtained 130 g of the mixture
(1)-(3) from 140 g of cyclopentadiene.
Neither acetone nor bsnzophenone undergo pinacol formation under these conditions.
The structures of the products (4)-(6) follow from their
ultraviolet, infrared, and N M R spectra:
(4): b.p. 38"C/17 mm; 1R: 955 cm-1; N M R : T = 4.06 (m,
H-2), 4.30 (m, H-3), 4.99 (m, H-5), 6.81 (m, H-l), 7.63
(m, H-4), 8.53 ( s , CH3), 8.94 ( s , C H 4 , relative areas:
1.0: 1.0: 1.0: 1.0:2.1:3.0:2.9. - The fact that the chemical
shift of the signal of H-5 in oxetane (5) is the same as in
( 4 ) whereas the signal of H-1 in oxetane (5) is shifted upfield
relative to (4) is in accord with the double bond in (4) being
+
+
246
Whereas, unlike hexaphenylethane, 1,1,2,2-tetraphenylethane
does not dissociate in solution into free radicals, such dissociation does occur at elevated temperature for 1,1,2,2-tetraphenylethanes that carry methyl groups at all ortho-positions "1, and at room temperature 1,1,2,2-tetra-(o-t-butyIphenyl)ethane dissociates into radicals to about the same
extent as hexaphenylethane [21. Considerable dissociation is
C31,and
shown also by 1,1,2,2-tetrakis(2,6-diethylphenyl)ethane
1,1,2,2-tetrakis(2,6-diisopropylphenyl)ethaneappears to be
fully dissociated into radicals [31, which however are stable
only at low temperature.
We have now found that, in distinction to 1,1,2,2-tetra-(o-tbutylphenyl)ethane, 1,1,2,2-tetrakis(2,5-di-t-butylphenyl)ethane as a 6 % solution in benzene is completely dissociated at
room temperature into the deep yellow bis(2,5-di-t-butylphenyl)methyl ( I ) . Whereas 27 h after preparation from the diarylmethyl chloride and thallium amalgam only about 70 %
of radicals is found with the magnetic balance (35
after
150 h), this can be traced to disproportionation: if one
extrapolates to half the time of preparation, 100 % of
radicals is found. That complete dissociation occurs is
confirmed by the fact that increase in temperature causes no
increase but rather an accelerated decrease in radical content.
Angew. Chem. internat. Edit.
Vol.5(1966) 1 No. 2
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