Pinacol Rearrangement of Cyclopentylcyclohexane-I ,I'-diol Revisited Richard D. Sands Alfred University, Alfred, NY 14802
Heating six-membered and larger cyclic diols with 25% sulfuric acid gives mainly dienes and only small amounts of spiro ketones via pinawl-pinacolone rearrangement (I, 2). In the present investigation two cyclic diols, 1 and 2, were treated with ice-cold boron trifluoride etherate instead of refluxing 25% sulfuric acid. The treatment with boron trifluoride made rearrangement instead of diene formation by far the major product. Furthermore, in the case of 2, where rearrangement can give two ketones, spiro[4.6] undecan-6-one (3) became the only ketone formed instead of the less favored of two ketones.
Discussion Cool boron trifluoride etherate forms relatively stable complexes with ketones (3) and could be expected to form less olefin from an alcohol than the strongly acidic, and hot, 25% sulfuric acid. a six-membered ring with all sp3 By I-strain theory (4,5) carbons is less strained than a five-membered ring with all sp3carbons and a five-membered ring with an sp2carbon is less strained than a six-membered ring with an spZcarbon. When cyc~opentylcyclopentane-1,l'-diol is heated with 25% sulfuric acid, one of its OH groups is lost and the strain of that ring is reduced. The strain of the other fivemembered ring can be reduced by expansion to the sixmembered ring of spiro[4.5ldecan-6-one. That diol, with two five-membered rings, therefore, gives 86% rearrangement and 14% diene (1). In the case of 1,however, loss of the OH from one of the six-membered rings increases the strain of that ring. Rearrangement would require the other ring to position its carbon number 2 or 6 behind the carbon that lost the OH and increase its strain by expanding to a seven-membered ring. In hot acid, loss of a proton on the ring that lost the OH group is usually forced before the unstrained sixmembered ring can position itself for rearrangement. With diol 2 in hot acid there isn't much preference for which OH is lost first even though loss of the OH from the five-membered ring would give greater strain relief. If the OH is lost from the five-membered ring, loss of a proton on the same ring is usually forced before the unstrained six-
membered ring can position itself for rearrangement. Diene formation is the main outcome; only a trace of 3 results. If the strainless six-membered ring of 2 loses its OH, then the strained five-membered ring can well rearrange (as was the case with cyclopentylcyclopentane-1,l'-diol) making spiro[5,5lundecan-16ne more abundant than 3. When either 1or 2 is treated with wld boron trifluoride etherate, however, an OH is lost from one ring and, under the milder conditions, the other ring can position itself for rearrangement. Rearrangement becomes the favored course of reaction. Loss of the OH on the five-membered ring of 2 is so much favored over loss of the OH on the six-membered ring that subsequent rearrangement gives 3 as the only ketone. Experimental Diols 1and 2 were prepared by pinawl reduction of the cyclic ketones, 1by the procedure of Barnette and Lawrence (6)and 2 by the procedure of Sands and Bottemn (I). The spiro ketones were prepared in about 20% yield by the alkylation of t h e cyclic ketones with dibromides by Christol's procedure (7).The dienes were obtained by refluxing the diols with 25% sulfuric acid, removing the ketones as semicarbazones, and then distilling. Boron Trifluoride Etherate Treatment
A solution of 1.359 g (7.39 mmol) of 2 (of about 90% purity) in 50 mL of methylene chloride was stirred a t room temperature, under nitrogen, with 1.0 g of anhydrous magnesium sulfate for a n hour, cooled in an ice bath, treated with 0.5 ml, boron trifluoride etherate, and stirred for another hour (81. Then 2.5 g of sodlum carbonate was added and stimne under nitroeen continued all nieht while the ice melted.-Filtration aGd evaporation gave-0.995 g of a yellowish oil. The oil was analyzed by gas chromatography using a polypropylene column. The products were identified bvretention times and their auantities determined bv w e i g h g the peaks. The identity bf3 was confirmed when the tlc retention time of the 2.4-dinitroohenvlhvdrazone of the suspect was the same as' that of 3 witi five different solvents. The oil from 2 gave 92.4% ketone 3 and 7.6% 1-(1cyclopentenyl)cyclohexene. The oil from 1gave 75.4% spiro [5.6ldodecan-7-one and 24.6% 1-(1-cyclohexenyl) cyclohexene. Literature Cited 1.Sanda, R. D.: Bottemn, D. G. J Ow. C h .1863.28.2690-2681. 2. Saods, R. D. lk(mhpdmn lW.21,887-890. 3. Lombard, R.;Stephan, J. P Bull. Soc Chrn h n c p 1967, 1569-1373.
4. Bm-, H. C.; Fletcher, R. S.; J o b e n , R. B. J h r Ckm.Soe 1051,73,212. 5. Bm-, H. C.; Breweter. J. H.; Shffhteq H. J . A m r C k m Soe.1954,76,461474. 6. BameUe, E,deB.;Lauorenee,C.A.J. C h .Soe.1956, llOP1107. 7.Moussemn, M.; Jacquier, R.; Chrhta1.H. C.R A d Sci P w & l964,239,1805-1807. 8. Razdan, R. K.; Dalzell, B . C.; Hand&, G. H. J. &I: Cbm.S a LIY14,96.586%
5865.
Volume 69 Number 8 August 1992
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