Bradford P. Mundv and Rodney D. Otzenberger Montana State University Bozeman, 59715
II
Textbook Errors, 103
I
Stereochemical Course of the Pinacol Rearrangement
I
The stereochemistry of organic compounds is frequently very important for mechanistic considerations. The classical "nonclassical" carbonium ion controversy has its origin in the exo and endo stereochemistry of nonbornyl derivatives and the resulting different chemistry observed ( 1 ) . Atany rearrangements follow definite stereochemical paths such that products can frequently be predicted (2). One important rearrangement which has had only limited success for stereochemical considerations is the pinacol rearrangement (8). Curtin has been able to demonstrate that the similar semipinacolonic deamination requires a definite stereochemistry for aryl migrating groups (4).
In an attempt to look a t the stereochemical requirements of a bona fide pinacol rearrangement, the cisSuggestions of material suitable for this column and columns suitable for publication directly should be sent with as many details as possible, and particularly with reference to modern textbooks, to W. H. Eberhardt, School of Chemistry, Georgia Institute of Technology, Atlanta, Ga. 30332. Since the purpose of this column is to prevent the s p m d and continuation of e m r s and not the evaluation of individual texts, the sources of errors discussed will not be cited. In order to be presented, an ermr must occur in a t least two independent recent standard books.
and trans-l,2-dimethylcyclohexane-l,%diolswere investigated (6). The results of this work have been frequently cited as being the example of steric control in the pinacol rearrangement.
It has been shown, however, that the early work was not correct (67, both the cis- and trans-glycol result primarily in ring contraction. By simply protonating the equatorial hydroxyl group for the cis-glycol, one can accomodate the migrating group being trans to the leaving group.
Although this arrangement may be sufficient to rationalize why the products are the same for the cis and trans isomers, it is important to realize that there are several problems inherent with any discussion of the pinacol rearrangement. Three of the most important for this question will be discussed.
Volume 48, Number 7, July 1971
/
431
Products from o Pinocol Reorrongement or Related to Conditions ( 9 )
Conditions POCh/Pyridine 20% HsSO, 50% HpS04,25°C Conc. HBOs 0°C BFJEI,her, HOAc, 2ii°C RF./Et,her. 2 5 T
1) The carbonium ion is assumed to be the same for both the cis and trans glycol. This assumption is reasonable in view of ~ the observation that the products are appmximately t b same for both glycoh. For example, wing 3.12 M perchloric aoid
--
100 76 76.3
... 60 .. .
Again looking at the specific example of the cisand trans-1,2-dimethylcyclohexane-1,2-diols several mechanistic conclusions may be possible (6).
...
...
20.1 1.5.1 43.4 19.5 21.7
3.9
8.6
56.6 20.5 78.3
Glycol Isomerization. It has been shown that the cis- and trans-1,2-dimethylcyclohexane-1,2-diolsare isomerized in acid (6). Although there is no accurate measure of whether this isomerization occurs with every rearrangement, it is certainly a problem to be considered in any mechanistic examination. We have also noted isomerization in the simple cis- and trans-cyclohexane-1,2-diols (7). Product Isomerization. I n the acid conditions necessary for the rearrangement, initially formed products can be isomerized to more stable isomers. As an extreme example, it has been demonstrated that aldehydes are particularly sensitive to further rearrangement (8).
Mole Trans d i d Cis dial
Yoof
I-methyl-1-acetylqclopenlane 59.7'C, 97'%; 1OOoC,92% 59.7'C, 9397,; 100DC,91%
2) Consideration of the two possible carbonium ion conformations (A and B) suggests that ring contraction could occu~from both, but methyl migration a l y fvmn B. Since A might be expected to be stabilized by hydroxyl psrtieipation, this would he expected to result primarily in ring contracted products, as observed.
A
3) Since the rate of rearrangement of the t~ans-glycol is about twice that for the cis isomer, it may he possible to suggest the cis-glycol is more stable than the trans in aquews solution. This could reasonably be rationalized by a lower ground state for the cis-glycol in the aqueous acid.
As an epilog to this analysis, i t is important to realize that the chapter on steric effects in pinacol rearrangements is not finished. Fort bas demonstrated that the carbonium ions generated from the solvolysis of the cis and trans- decalyl paranitrobenzoate derivatives are different (10).
ci; H
Ketones are not necessarily resistant to rearrangement, since spiro[3,5]nonan-5-one has been demonstrated t o be converted rapidly to the more stable spiro [4,4]nonan-1-one (7).
-
I
-
Reaction Conditions. Changes in the reaction conditions. have been demonstrated to influence profoundly the products resulting from the reaction (9). This is best exemplified by the data in the table.
432
/
Journal of Chemical Education
B
H
Thus, perhaps i t requires only a new trapping technique to see a bona fide sterkeffect in pinacol rearrangements. Literature Cited (1) GneAaa. G.E.,Rev.Pure Appl. Chcm., 16, 25 (1966). (2) DEMAYO.P. (editor),"Molecular Rearrangement," Yol. 1. Intemoienoe Publishers. New Yark. 1963. (3) Comma. C. J., Quart. Rm.,14,357 (1960). (4) C U ~ T I ND. , Y.. A N D Cnmw, M. C., J . Amar. Chem. Soc., 77, 354 (1955). ( 5 ) BARTLETT,P. D., AND POOREL, I., J . AmW. Chcm. Sot., 59, 820 (1937). C. A,. A N D C*R% M. D., J . Chem. Soc., 5854 (1963),and (6) BUNTOW, references in footnote 8 oi their paper. (7) Unpublished results of B. P. mu no^, and R . D. O~zmssnam. (8) BOTTERON, D. G., A N D WOOD, O., J . 010. Chem., 30, 3871 (1965). ~ , KRAPOHO. A. P., A N D PIETRASANT. F.. B d l . SOC.Chim. (9) C x n r s ~ a H., Fr. I I &"SO 11 OR41 . ~ (10) FORT.JR., R. C., H O R N I ~ R H.. E., A N D LIARS. G. A,, J . Amer. Chem. Soe., 92, 7558 (1970).
