PHOTOCHEMICAL
REARRANGEMENT REACTIONS OF 2-n-PROPYLCYCLOPENTANONE
3145
Photochemical Rearrangement Reactions of 2-n-Propylcyclopentanone
by R. Srinivasan and Sheldon E. Cremer IBM Watson Research Center, Yorktown HeiQhts,.New Yorlc 10698 (Received April 19, 1966)
Photolysis of 2-n-propylcyclopentanone in the vapor phase at about 140" with 3130-A. radiation gave rise to carbon monoxide, ethylene, propylene, l-pentene, a C7hydrocarbon, cyclopentanone, and trans-4-octenal. The production of carbon monoxide, along with Cz, C6,and C7hydrocarbons, and the formation of trans-4-octenal represent well-known primary processes in cyclic ketones. However, the formation of cyclopentanone and propylene in equivalent amounts represents the first observation of the Norrish type I1 process in an alicyclic ketone. The quantum yield for this process has been determined to be 0.08. The reaction was essentially unaffected by adding oxygen or by carrying it out in the liquid phase. Photolysis of 2-(n-propyl-2,2-d~)cyclopentanoneshowed, as in other instances, the specific nature of the hydrogen atom that underwent intramolecular transfer.
Introduction The occurrence of an intramolecular hydrogen migration process on the irradiation of aliphatic ketones with at least one proton on the y-carbon atom has long been recognized.' The mechanism of this process was postulated to jnvolve a six-membered cyclic intermediate (i) in which the bonds break along the broken
ii
hexanone has failed to give positive result^.^ This study was undertaken to see if the type I1 process would occur in a cyclic ketone which had hydrogen atoms on a y-carbon along a side chain rather than in the ring itself.
Experimental i lines to give an olefin and the enol form of the ketone.2 Indirect evidence for this mechanism has been obtained by deuterium-labeling experiments,a while very recently the enol form of the ketone that is obtained transiently has been directly observed by infrared spectroscopy.' Although cyclic ketones with at least six carbons in the ring may possess hydrogen atoms attached to the y-carbon atom, the analog of the type I1 process has not been observed. Thus this process in cyclohexanone would be expected to give 5-hexen-2-one via the intermediate ii. A careful search for this product in both the vapor phase and liquid phase photolysis of cyclo-
2-n-Propylcyclopentanone (K & K Laboratories) was fractionated on a 24-plate spinning-band column. A narrow middle cut was collected and used. 2-(nPropyl-2,2-&)cyclopentanone was synthesized by the condensation of propyl bromide-2,2-dz with 2-carbethoxycyclopentanone, followed by hydrolysis and decarboxylation of the condensate. The propyl bromide had an isotopic purity of over 98 atom % ac(1) For a review see, J. N. Pitts, Jr., J . Chem. Educ., 34, 112 (1957). (2) W. Davis, Jr., and W. A. Noyes, Jr., J . Am. Chem. SOC.,69, 2153 (1947). (3) R.Srinivasan, ibid., 81, 5061 (1959). (4) G.R. McMillan, J. G. Calved, and J. N. Pitts, Jr., ibid., 86, 3602 (1964). (5) R. Srinivasan, unpublished work.
Volume 69,Number 0 September 1966
R. SRINIVASAN AND SHELDON E. CREMER
3146
cording to the manufacturer (Merck Sharp and Dohme, Montreal). This purity could not have changed during the course of the synthesis of the 2-n-propylcyclopentanone. Photolyses were carried out in a quartz cell 3.0 cm. long and 5.0 cm. in diameter, which was fitted with a break-seal to remove the volatile products for analysis. The light source was a Hanovia 5-100 medium pressure mercury arc filtered by 2 mm. of Pyrex glass (Corning 0-53). It transmitted -5% at 2800 8. and -36% at 3000 8. In each experiment, a measured quantity of the ketone was introduced into the cell, which was then evacuated, degassed, and sealed, The cell was placed in a furnace, allowed to come up to temperature, and irradiated. Analyses were conducted by a combination of vapor phase chromatography and mass spectrometry,
Results Products. In a typical photolysis at 147”, the products that were measured and their relative amounts were : carbon monoxide, 1.70; ethylene, 1.05; propylene, 5.43 ; cyclopentanone, 5.3; trans4-octena1, 3.2. The identity of the cyclopentanone was established by infrared analysis and by comparison with the authentic material. 1-Pentene was identified qualitatively while a CTH14 product that was found was taken to be n-propylcyclobutane. The aldehyde was found to be isomeric to the starting materid. Its infrared spectrum showed strong absorptions at 2720,1735, and 975 cm. -l. The intense absorption at 975 cm.-l and the lack of any strong absorption below 850 cm.-1was indicative of the exclusive formation of the trans product.6 The nuclear magnetic resonance spectrum showed a proton distribution of 1:2 :6 :5 at the 7-values 0.22, 4.55, 7.85, and 8.9. However, this in itself does not distinguish b e tween the structures
cyclopentanone (within experimental error) was observed in all of the experiments in the vapor phase. The stoichiometry of the reactions is given by
-+ CO
hH2CH,CH2bHC3H,
c, H~+ ~H,CH,CH,CH,~O
-C
(2) (3)
CH,CH,CHZ CH=CHCHZCHzCHO (4)
Quantum Yields. The light source was calibrated with a diethyl ketone actinometer at 120” (aco = 1). The quantum yields for carbon monoxide, cyclopentanone, and 4-octenal were 0.03, 0.08, and 0.05, respectively. These values, which were obtained by averaging the results from at least four runs, were reproducible to *lo%. Since the yield of ethylene was 0.62 of that for CO, the quantum yields for reactions 1and 2 are 0.02 and 0.01, respectively. In the presence of 11.2 mm. of oxygen, the quantum yield of cyclopentanone was 0.08; that of propylene was not determined although its formation was observed. In the photolysis of 2-n-propylcyclopentanone as a liquid, the quantum yields for cyclopentanone and trans-4-octenal were 0.19 and 0.38, respectively. These values may be uncertain by as much as *50%, as the absorbed intensity was calculated from the geometry of the optical train. No effort was made to look for emission of radiation from 2n-propylcyclopentanoneon excitation. Deuterium-Labeling Studies. The results of the photolysis of 2-(n-propyl-2,2-dz)cyclopentanone
&CH2CD2C&
CHsCH&H&H=CHCH2CH&HO
iii CHaCH&H=C€€CH&H&H&HO iv both of which are possible. It was observed’ that photolysis of 2-methylcyclopentanone gave only trans4-hexenal (no CH=CH2 grouping in the infrared spectrum) , while 2-ethylcyclopentanone gave trans-4-heptenal (proton distribution of 1:2 :6 :3 in the manges 0-1, 4-5, 7-8.5, and 9 rather than a distribution of 1:2 :7 :2) exclusively. By analogy, the single compound that was formed in the present instance was taken to be trans-4-octenal. The equivalence between the yields of propylene and Tha Journal of P h y a Chsmistry
as a function of reaction conditions are given in Table I. The procedure that was used to coat the walls with deuterium oxide has been described.a In one experiment photolysis of the “light” ketone was carried out in a cell that had previously been coated with deuterium oxide. The isotopic composition of the cyclopentanone that was obtained in this experiment was: do 99.2%; dl 0.8%. The amount of cyclopentanone-dl is less than the uncertainty in the analysis and is hence not significant. (6) R. Srinivasan and S. E. Cremer, J. Am. Chem. SOC.,87, 1647 (1966). (7) R. Srinivasan and S. E. Cremer, unpublished work.
PHOTOCHEMICAL
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REARRANGEMENT REACTIONS O F 2-’7Z-PROPYLCYCLOPENTANONE
Table I : Photolysis of 2-(n-Propyl-2,2-d2)cyclopentanone Isotopic composition of -products-
Conditions
“Untreated” quartz cell W‘ntreated” quartz cell, repeat of above Quartz cell, flamed out and then coated with DzO Quartz cell from above, coatedagainwithD20 Uncoated cell, unfiltered arc
Time, min.
% pro- % oyclopentanone” do di di pylene-dl
150
loob
88.8 11.2 0.0
150
loob
90.0
9.1 0.0
180
loob
92.3
7.7 0.0
150 15
loob
82.4 17.6 0.0 93.0 7.0 0.0
loob
Not corrected for small (-2%) quantity of unlabeled ketone Refers to same isotopic content aa ~tsrt-
in starting material. ing material.
Discussion The equivalence in the yields of propylene and cyclopentanone, the lack of any effect on their formation on the addition of oxygen, and the equal facility with which these products are formed, both in the vapor and liquid phases, leave no doubt that reaction 3 is an example of the Norrish type I1 process. This appears to be the first instance in which the reaction has been observed to involve an alicyclic ketone. The exclusive formation of propylene-& in the photolysis of the deuterium-labeled ketone strongly favors a mechanism analogous to that observed in the aliphatic ketones, i.e., one that involves a six-membered intermediate
While it may seem that the formation of cyclopentanone-& rather than cyclopentanone-dl as the major product is a point of difference between the present system and the earlier resultsa with 2-hexanone-5,5-dzJ it should be borne in mind that both the temperature and the pressures used were quite different in the two systems. The temperature was more than 100”higher,
which would tend to decrease the amount of water adsorbed on the walls. The pressures were tenfold greater, which would slow down the diffusion of active species to the walls. Under these circumstances the rearrangement of the enol form of the ketone that is formed in reaction 5 to the keto form is probably not a wall reaction but a homogenous reaction in the gas phase for the most part. Since the starting material could not be freed entirely from HzO, the rearrangement with HzO as the other reactant would invariably proceed with loss of labeling. The results in Table I merely point out that attempts to “ i e e this exchange by coating the walls with DzO proved to be futile. The most significant conclusion that can be drawn from the present study is the importance of the geometry of the proton which undergoes transfer with respect to the carbonyl group. Two different requirements ariseJ depending on the abstracting atom.* In I
the Norrish type I1 process the oxygen of the carbonyl group is the abstracting atom, and its bonding orbital lies in the plane of the cyclopentanone ring. As a result it can easily abstract only a proton in the side chain and not one on the ring (there is no 7-carbon in cyclopentanone but the argument is applicable to cyclohexanone). The proton migration which leads to trans-4octenal involves the carbon of the carbonyl group as the abstracting atom. The bonding orbital on this carbon is perpendicular to the ring so that only a proton on a carbon in the ring can successfully undergo bonding. This would readily explaii why the product is exclusively 4-octenal and not 5-octenal. The selectivity in the protons that undergo transfer implies that the ring is virtually intact a t the instant of the migration. ~~
(8) The authors wish to thank Dr. Edwin F. Ullman, who first suggested this interpretation.
Voluma 69, Number 9 September 1966
