2542
ELLIOTAT.M A R V E L L [CONTRIBUTIOX FROM THE
AND E U G E N E .htAGOON
DEPARTMENT OF
CHEMISTRY, OREGON STATE COLLEGE]
The Dienone-Phenol Rearrangement. 111. Rearrangement of 6,6-Dimethyl-2,4-cyclohexadienone BY ELLIOTN. MARVELL AND EUGENE MAGOON RECEIVED SEPTEMBER 20, 1954 A new dienone, F,6-dimethyl-2,4-c~clohexadienone, has been prepared, characterized and rearranged. A pheiioljc proclu r t was isolated in 75% yield and 2,3-dimethylphenol was identified in that product by preparation of a phenylurethan. The formation of 2,3-dimethylphenol is in accord with the Arnold mechanism for the dienone-phenol rearrangement.
It has been shown in earlier papers of this series? that the rearrangement of l-oxo-2,2-dialkyl-l,2dihydronaphthalenes does not proceed in complete accord with the Arnold mechanism3 for the dienonephenol rearrangement. Failure t o follow the normal (Arnold) course for the dienone-phenol rearrangement in such cases was accounted for by the suggestionZathat one of the intermediates in that path is an o-quinoid structure (I) of high energy. The mechanism for the alternate pathzb explains the means by which the rearrangement may occur without passing such an energy barrier. However,
the reaction t o proceed. Therefore, 2,2-dimethylcyclohexanone was prepared by two methods: first by the unequivocal procedure employed originally by hleerweinI6in order to obtain comparison samples. Later large-scale production was carried out by direct methylation of 2-methylcyclohexanone. The product reacted normally with elementary bromine giving 2-bromo-6,~-dimethylcyclohexanone as a light gray solid. Introduction of a double bond in the cu,(?-position was effected smoothly by treatment with y-collidine at 135-1.20'. The yield of 6,6-dimethyl-2-cyclohexenonefell off considerably when the reaction was carried out a t the boiling OH point of the collidine solution. The product was identified as 6,G-dimethyl-2-cyclohexenonesince it absorbed one mole of hydrogen when treated with Adarns catalyst and hydrogen a t atmospheric pressure to give 2,2-dimethylcyclohexanone,identithis energy barrier will be present only as long as fied as the 2,4-dinitrophenylhydrazone.According the dienone ring is fused to an aromatic system so to Woodward's rules7 an a,@-unsaturatedketone of that one bond of the diene is also a part of the aro- this type should exhibit maximum absorption in thc matic system. Accordingly if the sole reason for ultraviolet at 225 mp. This ketone exhibited a maxfailure to follow the Arnold path lies in the high- imum at 225.8 mp. energy o-quinoid intermediate, then an orthoA bromine was introduced onto the 4( ?)-position blocked dienone of the cyclohexane ring system with N-bromosuccinimide. No attempt was made might be expected to follow that path. to identify this product but it was dehydrogenated Since all of the dimethylphenols are known 6,O- directly with either -y-collidine or quinaldine. Best dimethyl-2,4-cyclohexadienonewas considered the results were obtained by working in an atmosphere ideal case for study. Although G-dichloromethyl- of nitrogen, using quinaldine at 140'. The pressure (i-methyl-2,4-cyclohexadienonehas been prepared was reduced to 40 mm. and the dienone distilled and studied4 there appears in the literature no rec- from t.he reaction mixture as fast as it was formed. ord of the preparation of G,G-dialkyl-2,4-cyclohexa- The ti,G-dimethyl-2,4-cyclohexadienone is a clear dienones. The straightforward approach to the mobile liquid when first isolated. On storage, even synthesis of this system has been studied in part by in the cold and dark, for short periods of time it Bennet and Burger5 who prepared 6,6-diphenyl-4- changes to a thick viscous substance which eventubromo-2-cyclohexenone. Although one of the pos- ally deposits some solid material. It is possible sible products of rearrangement of GIG-diphenyl- that the cyclohexadienone reacts with itself in a 2,4-~yclohexadienoneis not known, the advantage Diels-Alder condensation. This is a characteristic of dealing with a previously studied series appeared behavior of cyclohexadiene8 and cyclopentadiene$ sufficient to warrant the preparation of that di- and appears to occur with extreme ease with phenyl enone. Accordingly we repeated the work of and chloro-substituted cyclopentadienones. lo Rennet and Burger, but found that the 4-bromo The mobile liquid first isolated absorbs two moles compound could not be dehydrohalogenated to the of hydrogen forming 2,3-dimethylcyclohexanone. desired product by treatment with either ycolli- It gives a bright red 2,4-dinitrophenylhydrazone dine or quinaldine. which melts with decomposition a t 27.5'. The bright Discouraging as this failure to dehydrohalogen- red color of the original 2,4-dinitrophenylhydrazorie ate was, it seemed possible to attribute this reluc- contrasts beautifully with the orange derivative of tance to steric hindrance. According to this assump- A,G-dimethyl-2-cyclohexenone and the yellow onc tion the smaller methyl substituent might permit from 2,2-dimethylcyclohexanone. The liquid prod(1) Published with t h e a p p r o v a l of t h e M o n o g r a p h s Publications uct also shows an absorption peak in the ultraviolet C o m m i t t e e , Oregon S t a t e College, a s Research P a p e r N o . 268. Schoril of Science, D e p a r t m e n t of Chemistry. ( 2 ) (a) E. N. Marvel1 a n d A. 0. Geiszler, T H I SJ O U R W A L , 7 4 , 1259 f l q . 5 2 ) ; (h) E. N. hfarvell a n d E. SIagoon, i b i d . . 76, 5118 (1954). f:i!R. T. Arnold, J . S. B u c k l e y a n d J . R i c h t e r , ibid., 69, 2322 ( 1 9 1 7 ~ . ( 4 ) K. Auwers a n d G . Keil, B w . , 36, 4207 (1902). (:) 1%'. H. Bennet a n d 4 . Burger, THISJ O U R N A L . 76, 84 (1933).
( G ) H . Meerrvein a n d W.Unkel, A P I L , 376, 132 f1910). (71 I
